HK1065665B - Method and apparatus for allocating resources in a multiple-input multiple-output (mimo) communication system - Google Patents

Description

Method and apparatus for allocating resources in a multiple-input multiple-output (MIMO) communication system

Background

FIELD

The present invention relates generally to data communications, and more specifically to techniques for allocating downlink resources within a multiple-input multiple-output (MIMO) communication system.

Background

Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on to multiple users. These systems may be based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), or some other multiple access technique.

Multiple Input Multiple Output (MIMO) communication systems using multiple (N)_T) Transmitting antenna and a plurality of (N)_R) The receive antennas are configured to transmit multiple independent data streams. In one common MIMO system implementation, data streams are transmitted to signal terminals at any given time. However, a multiple access communication system with a base station having multiple antennas may also communicate with multiple terminals simultaneously. In this case, the base station uses a plurality of antennas and each terminal uses N_RMultiple antennas to receive one or more of the multiple data streams.

The connection between a multi-antenna base station and a single multi-antenna terminal is called a MIMO channel. From these N_TA transmission sum N_RThe MIMO channel formed by the receiving antennas can be decomposed into N_CA separate channel of which N_C≤min{N_T，N_R}。N_CEach of the individual channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to one dimension. If these subchannels established by multiple transmit and receive antennas are usedWith additional dimensions, the MIMO system can provide improved performance (e.g., increased transmission capacity).

The MIMO channel between each base station and terminal typically has different link characteristics and is associated with different transmission capabilities, so the spatial subchannels available to each terminal have different effective capacities. If N is present_CThe available spatial subchannels are efficiently allocated such that data is transmitted on these subchannels to a set of "suitable" terminals within the MIMO system, efficient utilization of the available downlink resources (and high throughput) is obtained.

There is therefore a need in the art for techniques to allocate downlink resources in a MIMO system to provide improved system performance.

SUMMARY

Aspects of the present disclosure provide techniques for increasing downlink performance of a wireless communication system. In an aspect, data may be transmitted from a base station to one or more terminals using one of a plurality of different operating modes. In MIMO mode, all available downlink data streams are allocated to a single terminal using multiple antennas (i.e., a MIMO terminal). In the N-SIMO mode, a single data stream is assigned to each of a plurality of different terminals, each terminal using multiple antennas (i.e., SIMO terminals). In mixed mode, downlink resources may be allocated to a combination of SIMO and MIMO terminals while supporting both types of terminals. The transmission capacity of the system can be increased by transmitting data to multiple SIMO terminals, one or more MIMO terminals, or a combination thereof, simultaneously.

In another aspect, a scheduling scheme is provided to schedule data transmission to active terminals. The scheduler selects the best mode of operation to use based on various factors, such as, for example, the service requested by the terminal. In addition, the scheduler can perform additional layers of optimization by selecting a particular set of terminals for simultaneous data transmission and allocating available transmit antennas to the selected terminals to achieve high system performance and other requirements. Several scheduling schemes and antenna allocation schemes are provided, as described below.

Certain embodiments of the present invention provide a method of scheduling downlink data transmissions to a plurality of terminals within a wireless communication system. According to the method, one or more sets of terminals are formed for possible data transmission, each set comprising a unique combination of one or more terminals and corresponding to a hypothesis to be evaluated. One or more sub-hypotheses may be formed for each hypothesis, with each sub-hypothesis corresponding to a particular assignment of multiple transmit antennas to one or more terminals in the hypothesis. The performance of each sub-hypothesis is then evaluated, and one of the evaluated sub-hypotheses is selected based on their performance. The terminals within the selected sub-hypothesis are then scheduled for data transmission, after which data is transmitted to each scheduled terminal from the one or more transmit antennas assigned to the terminal.

Each transmit antenna may be used to transmit a separate data stream. To achieve high performance, each data stream may be coded and modulated according to a selected scheme, e.g., based on an estimate of the signal-to-noise-and-interference ratio of the pair of antennas used to transmit the data stream.

Terminals desiring data transmission (i.e., "active" terminals) may be prioritized based on a number of metrics and factors. The priority of the active terminals may be used to select which terminal to consider for scheduling and/or to assign the available transmit antennas to the selected terminal.

The present invention also provides methods, systems, and apparatus that implement various aspects, embodiments, and features of the present invention, as described in detail below.

Brief description of the drawings

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a multiple-input multiple-output (MIMO) communication system that may be designed and used to implement various aspects and embodiments of the present invention;

FIG. 2 is a flow diagram of a process for scheduling terminals for data transmission according to an embodiment of the present invention;

FIG. 3 is a flow diagram of a process for assigning transmit antennas to receive antennas using the "max-max" criterion according to an embodiment of the present invention;

fig. 4 is a flow diagram of a priority-based scheduling scheme in which scheduling of a set of one or more highest priority terminals is considered, according to an embodiment of the present invention;

fig. 5 is a block diagram of a base station and a plurality of terminals in a MIMO communication system;

FIG. 6 is a block diagram of an embodiment of a transmit portion of a base station capable of processing data for transmission to a terminal based on available CSI;

FIG. 7 is a block diagram of an embodiment of a receiving portion of a terminal;

FIGS. 8A and 8B are block diagrams of embodiments of a channel MIMO/data processor and an interference canceller, respectively, of a Receive (RX) MIMO/data processor at a terminal; and

FIG. 9 shows four transmit antennas (i.e., N) at each terminal for two different modes of operation_T4) and four receive antennas (i.e., N)_R4) average throughput of the MIMO communication system.

Detailed Description

Fig. 1 is a diagram of a multiple-input multiple-output (MIMO) communication system 100 that may be designed and used to implement various aspects and embodiments of the present invention. MIMO System 100 uses multiple (N)_T) A transmitting antenna and a plurality of (N)_R) The receive antennas are used for data transmission. The MIMO system 100 is effectively used for a multiple access communication system with a Base Station (BS) capable of communicating with a plurality of terminals (T)106 simultaneously. In this case, the base station 104 uses multiple antennas and represents multiple inputs (M) for downlink transmission from the base station to the terminalI)。

The set of one or more "communication" terminals 106 collectively represents a Multiple Output (MO) of the downlink transmission. As used herein, a communication terminal is a terminal that receives user-specific data from a base station, and an "active" terminal is a terminal that expects data to be transmitted at the next or future transmission interval. The active terminals may include terminals currently communicating.

MIMO system 100 may be designed to implement the standards and designs of any number of CDMA, TDMA, FDMA, and other multiple access techniques. CDMA standards include IS-95, CDMA2000, and W-CDMA standards, and TDMA standards include global system for mobile communications (GSM) standards. These standards are known in the art and are incorporated herein by reference.

MIMO system 100 may be used to transmit data over multiple transmission channels. Each terminal 106 communicates with the base station 104 over a MIMO channel. The MIMO channel may be decomposed into N_CA separate channel of which N_C≤min{N_T，N_R}。N_CEach of the individual channels is also referred to as a spatial subchannel of the MIMO channel. For MIMO systems that do not use Orthogonal Frequency Division Modulation (OFDM), there is typically only one frequency subchannel and each spatial subchannel may be referred to as a "transmission channel". And for MIMO systems using OFDM, the spatial subchannels of each frequency subchannel may be referred to as transmission channels.

For the example shown in fig. 1, base station 104 communicates with terminals 106a through 106d simultaneously (as indicated by the solid lines) via multiple available antennas at the base station and multiple available antennas at each terminal. Terminals 106e through 106h may receive pilot references and other signaling information from the base station (as indicated by the dotted lines), but not user-specific data from the base station.

Each terminal 106 within MIMO system 100 uses N_RAn antenna receives one or more data streams. Typically, the number of antennas at each terminal is equal to or greater than the number of data streams transmitted by the base station. However, the terminals in the system need not have the same number of receive antennas.

For MIMO system 100, terminal (N)_R) Is generally greater than or equal to the number of antennas at the base station (N)_T). In this case, for the downlink, the number of spatial subchannels is limited by the number of transmit antennas at the base station. Each transmit antenna may be configured to transmit an independent data stream that may be encoded and modulated according to a scheme supported by spatial subchannels associated with the MIMO channel between the base station and the selected terminal.

Aspects of the present invention provide techniques for increasing the performance of a wireless communication system. These techniques may be advantageously used to increase the downlink capacity of a multiple access cellular system. These techniques may also be used with other multiple access techniques.

In an aspect, data may be transmitted from a base station to one or more terminals using one of a plurality of different operating modes. In MIMO mode, the available downlink resources are allocated to a single terminal (i.e., a MIMO terminal). In the N-SIMO mode, downlink resources are allocated to multiple different terminals, each demodulating a single data stream (i.e., SIMO terminals). In mixed mode, downlink resources may be allocated to a combination of SIMO and MIMO terminals, supporting both types of terminals simultaneously on the same channel, which may be a time slot, code channel, frequency subchannel, etc. The transmission capacity of the system is increased by transmitting data to multiple SIMO terminals, one or more MIMO terminals, or a combination thereof, simultaneously.

In another aspect, a scheduling scheme is provided to schedule data transmission to active terminals. The scheduler selects the best mode of operation to use based on a number of factors, such as for example the service requested by the terminal. In addition, the scheduler can achieve an additional layer of optimization by selecting a particular set of terminals to be simultaneously data transmitted and allocating available transmit antennas to the selected terminals to achieve high system performance and other requirements. Several scheduling schemes and antenna allocation schemes are described in detail below.

For MIMO, multiple independent data streams may be transmitted from a base station via multiple transmit antennasTo one or more scheduled terminals. If the propagation environment is sufficiently scattered, it is possible to use MIMO receiver processing techniques at the terminal to efficiently use the spatial dimension of the MIMO channel to increase transmission capacity. MIMO receiver processing techniques may be used when a base station is communicating with multiple terminals simultaneously. From the terminal's perspective, the same receiver processing technique may be used to process the N to be transmitted to the terminal (i.e., a single MIMO terminal)_TA different signal or only N_TOne of the signals (i.e., SIMO terminals).

As shown in fig. 1, the terminals may be randomly distributed within the coverage area (or "cell") of the base station or may be co-located. For wireless communication systems, the link characteristics typically vary over time due to a number of factors, such as fading and multipath. N of a single terminal of a base station at a particular time_TA transmitting antenna and N_RThe array of individual receive antennas may be described by a matrix H whose elements are composed of independent gaussian random variables, as follows:

formula (1)

Where H is the channel response matrix for the terminal, and H_i，jIs the coupling between the ith transmit antenna of the base station and the jth receive antenna of the terminal.

As equation (1) shows, the channel estimation for each terminal may use N, which corresponds to the number of transmit antennas at the base station and the number of receive antennas at the terminal_T×N_RA matrix representation of the elements. Each element of the matrix H describing the space between the base station and a terminalThe response between the corresponding transmit receive antenna pair. To simplify the description, equation (1) is described in terms of a flat fading channel model (i.e., one complex value for the entire system bandwidth). In a practical operating environment, the channel may be frequency selective (i.e., the channel response varies across the system bandwidth) and may use a more detailed description of the channel characteristics (e.g., each element of the matrix H may comprise a set of values for different frequency sub-channels or time delays).

An active terminal in a MIMO system periodically estimates the channel response for each transmit-receive antenna pair. Channel estimation may be simplified in a number of ways, for example, using pilots and/or data decision directed techniques known in the art. The channel estimate may comprise a complex-valued channel response estimate for each transmit-receive antenna pair, as described in equation (1) above. The channel estimate gives information about the transmission characteristics of each spatial self-channel, i.e. what data rate can be supported on each subchannel with a given set of transmission parameters. The information given by the channel estimate may be extracted as a processed signal-to-noise-plus-interference ratio (SNR) estimate for each spatial subchannel, or some other statistic that enables the transmitter to select the appropriate transmission parameters for that spatial subchannel. In general, the derivation of the key statistics reduces the amount of data required to represent the channel characteristics. In both cases, the information represents a form of Channel State Information (CSI) that may be reported to the base station. Other forms of CSI may also be reported, as will be described in more detail below.

The set of CSI received from the set of terminals may be used to (1) select a "best" set of one or more terminals for data transmission, (2) assign available transmit antennas to the selected terminals in the set, and (3) select an appropriate coding and modulation scheme for each transmit antenna. With the available CSI, it is possible to design multiple scheduling schemes to maximize downlink performance by evaluating which specific combinations of terminal and antenna assignments provide the best system performance (e.g., highest throughput) under any system constraints and requirements. By using the spatial (possibly frequency) "signature" of the individual active terminals (i.e. their channel estimates), the average downlink throughput can be increased.

A terminal may be scheduled for data transmission based on various factors. A set of factors may be related to system limitations and requirements such as desired quality of service (QoS), maximum latency, average data rate, etc. Some or all of these factors may need to be met on a per-terminal basis (i.e., for each terminal) within a multiple-access communication system. Another set of factors may relate to system performance, which may be quantified by an average system throughput rate or some other indication of performance. These various factors are described in detail below.

The scheduling scheme can be designed to select the best set of terminals for simultaneous data transmission on the available transmission channels so that system performance is maximized while complying with system limitations and requirements. For simplicity, the following describes aspects of the invention for a MIMO system without OFDM, where a separate data stream may be transmitted by the base station from each transmit antenna. In this case, (up to) N_TThe independent data streams may be simultaneously transmitted from the base station to N_TTransmitting by transmitting antennas aimed at one or more terminals, each terminal being equipped with N_RA receiving antenna (i.e. N)_T×N_RMIMO) where N_R≥N_T。

For the sake of brevity, for much of the description below, the number of receive antennas is assumed to be equal to the number of transmit antennas (i.e., N)_R＝N_T). Since all analyses apply to N_R≥N_TThis is not a requirement.

The scheduling of data transmission on the downlink comprises two parts: (1) selecting one or more sets of terminals to be evaluated, and (2) assigning available transmit antennas to the terminals in each set. All or a subset of the active terminals may need to consider scheduling and these terminals may combine to form one or more sets (i.e., hypotheses) to be evaluated. For each hypothesis, the available transmit antennas may be assigned to terminals within the hypothesis according to any one of a plurality of antenna assignment schemes. The terminals within the best hypothesis may be scheduled for data transmission within the incoming time interval. The flexibility in selecting the best set of terminals for data transmission and assigning transmit antennas to the selected terminals enables the scheduler to optimize performance by using a multi-user diversity environment.

To determine the "optimal" transmission to the set of terminals, SNRs or some other sufficient statistics are provided for each terminal and each spatial subchannel. If the statistic is SNR, then for each set of terminals to be evaluated for data transmission within the incoming transmission interval, the hypothesis matrix Γ (defined below) for the "processed" SNRs for that set of terminals may be expressed as:

formula (2)

Wherein, γ_i，jIs the processed SNR for the data stream transmitted from the ith (hypothetically) transmit antenna to the jth terminal.

In N-SIMO mode, N within the hypothetical matrix Γ_TThe rows correspond to the slave N_TN of SNRs from different terminals_TAnd (5) vector quantity. In this mode, it is assumed that each row within the matrix Γ gives the SNR per transmitted data stream for one terminal. And in the hybrid mode, for a particular MIMO terminal designated to receive two or more data streams, the vector of SNRs for that terminal may be replicated such that the vector appears in as many rows (i.e., one row per data stream) as the number of data streams to be transmitted to the terminal. Alternatively, assuming that one row within the matrix Γ may be used for each SIMO or MIMO terminal, the scheduler may be designed to designate and evaluate these different types of terminals accordingly.

Within each terminal of the set to be evaluated, N_TN of a (hypothetical) transmitted data stream by a terminal_RA receiving antenna receives, and N_RThe received signals are processed by space or space-time equalization to separate out N_TThe transmitted data streams are described as follows. It is possible to estimate the SNR of the processed data stream (i.e., after equalization) and include the processed SNR of the data stream. For each terminal, N which may be received by the terminal_TProviding N for each data stream_TA set of processed SNRs.

If successive equalization and interference cancellation (or "successive cancellation") receiver processing techniques are used at the terminal to process the received signal, the post-processing SNR obtained at the terminal for each transmitted data stream depends on the order in which the transmitted data streams are detected (i.e., demodulated and decoded) to recover the transmitted data, as described below. In this case, multiple sets of SNRs may be provided to each terminal in multiple possible detection orders. Multiple hypothesis matrices may then be formed and evaluated to determine which particular combination of terminals and detection order provides the best system performance.

In either case, each hypothesis matrix Γ includes the processed SNRs for the particular set of terminals (i.e., hypotheses) to be evaluated. These post-processing SNRs represent SNRs available to the terminal and are used to evaluate the hypothesis.

Fig. 2 is a flow diagram 200 of a process for scheduling terminals for data transmission according to an embodiment of the present invention. For the sake of brevity, the overall process is described first, followed by a description of the details of some of the steps in the process.

Initially, at step 212, metrics used to select the best set of terminals for data transmission are initialized. Various performance metrics may be used to evaluate the set of terminals and some of these metrics are described in detail below. For example, a performance metric may be used that maximizes system throughput.

At step 214, a (new) set of one or more active terminals is then selected from all active terminals to be considered for scheduling. The set of terminals forms the hypothesis to be evaluated. Various techniques may be used to limit the number of active terminals to consider for scheduling, which may reduce the number of hypotheses to evaluate. For each terminal in the hypothesis, SNR vectors are obtained at step 216 (e.g.,). The SNR vectors for all terminals within a hypothesis form a hypothesis matrix Γ shown in equation (2).

To N_TA transmitting antenna and N_TEach terminal assumes a matrix Γ, and the assignment of transmit antennas to terminals has N_TFactorial possible combinations (i.e. N)_T| A Sub-hypotheses). Thus, one particular (new) combination of antenna/terminal assignments is selected for evaluation at step 218. The particular combination of antenna/terminal assignments forms the sub-hypothesis to be evaluated.

At step 220, the sub-hypothesis is then evaluated and a performance metric (e.g., system throughput) corresponding to the sub-hypothesis is determined (e.g., based on the SNRs of the sub-hypothesis). The performance metric is then used at step 222 to update the performance metric corresponding to the current best sub-hypothesis. In particular, if the performance metric of the sub-hypothesis is better than the performance metric of the currently best sub-hypothesis, the sub-hypothesis becomes the new best sub-hypothesis, and the performance metric and other terminal metrics corresponding to the sub-hypothesis are saved. The performance and terminal metrics will be described below.

It is then determined whether all sub-hypotheses for all current hypotheses have been evaluated at step 224. If all sub-hypotheses have not been evaluated, the process returns to step 218 to select a different and yet unevaluated combination of antenna/terminal assignments for evaluation. Steps 218 through 224 are repeated for each sub-hypothesis to be evaluated.

If all sub-hypotheses for a particular hypothesis have been evaluated at step 224, a determination is made at step 226 whether all hypotheses have been considered. If all the hypotheses have not been considered, the process returns to step 214 and a different set of terminals not considered is selected for evaluation. Steps 214 through 226 are repeated for each hypothesis to be considered.

If all assumptions are taken into account in step 226, the particular set of terminals scheduled for data transmission and their assigned transmit antennas in the incoming transmission interval are known. The processed SNRs corresponding to the terminal and antenna assignments may be used to select the appropriate coding and modulation schemes for the data streams to be transmitted to the terminal. At step 228, the scheduled transmission interval, antenna assignment, coding and modulation scheme, other information, and combinations thereof may be transmitted to the scheduled terminal (via the control channel). Alternatively, the terminal may implement "blind" detection and attempt to detect all transmitted data streams to determine which, if any, data streams are intended for them.

If the scheduling scheme requires that other system and terminal metrics be maintained (e.g., average data rate over the past K transmission intervals, latency of data transmission, etc.), the metrics are updated in step 230. Terminal metrics may be used to evaluate the performance of individual terminals, as described below. Scheduling is typically implemented for each transmission interval.

For a given hypothesis matrix Γ, the scheduler evaluates the combination of transmit antenna and terminal pairing (i.e., sub-hypothesis) to determine the best allocation for the hypothesis. Various allocation schemes may be used to allocate transmit antennas to terminals to achieve various system goals such as fairness, maximizing performance, etc.

In one antenna allocation scheme, all possible sub-hypotheses are evaluated according to a particular performance metric and the sub-hypothesis with the best performance metric is selected. For each hypothesis matrix Γ, there are possible N's to evaluate_TFactorial (i.e. N)_T| A ) A sub-hypothesis. Each sub-hypothesis corresponds to a particular assignment of each transmit antenna to a respective terminal. Each sub-hypothesis may therefore be represented by a vector of processed SNRs, which may be expressed as:

wherein gamma is_i，jIs the processed SNR from the ith transmit antenna to the jth terminal. And the subscripts { a, b.. and r } identify the particular terminal within the transmission/terminal pair of the sub-hypothesis.

Each sub-hypothesis is also associated with a performance metric R_sub-hypCorrelation, which may be a function of various factors. For example, the performance metric according to post-processing SNs may be expressed as:

R_sub-hyp＝f(γ _sub-hyp)

where f (-) is a specific positive real function of the parameter in parentheses.

Various functions may be used to form the performance metric equations. In one embodiment, all N of the sub-hypotheses may be used_TA function of the achievable throughput for the individual transmit antennas, which may be expressed as:

formula (3)

Wherein r is_iIs and at son falseThe throughput associated with the ith transmit antenna in the set, and may be expressed as:

r_i＝c_i·log₂(1+γ_i) Formula (4)

Wherein c is_iIs a normal quantity reflecting a part of the theoretical capacity obtained for the coding and modulation scheme selected for the data stream transmitted on the ith transmit antenna, and is gamma_iIs the processed SNR for the ith data stream.

The first antenna allocation scheme shown in fig. 2 and the specific scheme described above representing the evaluation of all possible transmit antenna to terminal allocation combinations. The total number of potential sub-hypotheses for each hypothesis to be evaluated by the scheduler is N_T| A Considering a large number of hypotheses N that may need to be evaluated_T| A It is still large. The first scheduling scheme implements an exhaustive search to determine the sub-hypotheses that provide "optimal" system performance in accordance with the system performance quantified by the performance metric used to select the best sub-hypothesis.

Various techniques may be used to reduce the complexity of the process of assigning transmit antennas. One of these techniques is described below, and others may be implemented and are within the scope of the invention. These techniques may also provide high system performance while reducing the amount of processing to allocate transmit antennas to terminals.

In a second antenna allocation scheme, a maximum-maximum ("max-max") criterion is used to assign transmit antennas to terminals within the evaluated hypothesis. Using this max-max criterion, each transmit antenna is assigned to the particular terminal that achieves the best SNR for the transmit antenna. Antenna allocation is achieved for one transmit antenna at a time.

Fig. 3 is a flow diagram 300 of a process for assigning transmit antennas to receive antennas using the "max-max" criterion according to an embodiment of the present invention. The process illustrated in fig. 3 is implemented for a particular hypothesis, which corresponds to a particular set of one or more terminals. Initially, the maximum processed SNR within the hypothesis matrix Γ is determined at step 312. The maximum SNR corresponds to a specificThe transmit/terminal pair is paired and the transmit antenna is assigned to the terminal at step 314. The transmit antennas and terminals are then removed from the matrix Γ, which is reduced to (N) by simultaneously removing the columns corresponding to the transmit antennas and the rows corresponding to the terminals just assigned at step 316_T-1)×(N_T-1) dimension.

At step 318, it is determined whether all transmit antennas within the hypothesis are assigned. If all transmit antennas are assigned, then antenna assignment is provided at step 320 and the process terminates. Otherwise, the process returns to step 312 to assign other transmit antennas in a similar manner.

Once the antenna assignments are made for a given hypothesis matrix Γ, as in equations (3) and (4), it is possible to determine a performance metric (e.g., system throughput) corresponding to the hypothesis (e.g., based on the SNRs corresponding to the antenna assignments). The performance metric is updated for each hypothesis. When all hypotheses have been evaluated, the best terminal and antenna allocation set for data transmission is selected within the incoming transmission interval.

Table 1 shows an example matrix r of processed SNRs derived within a 4x4 MIMO system, where the base station includes four transmit antennas and each terminal includes four receive antennas. For an antenna allocation scheme based on the max-max criterion, the best SNR (16dB) in the original matrix is obtained by the transmit antenna 3 and allocated to the terminal 1, as shown by the shaded box in the third row of the fourth column in the table. The transmitting antenna 3 and the terminal 1 are removed from the matrix. The best SNR (14dB) within the reduced 3x3 matrix is obtained for transmit antennas 1 and 4, which are assigned to terminals 3 and 2, respectively. The remaining transmit antennas 2 are assigned to terminals 4.

TABLE 1

Table 2 shows the antenna assignments for the example matrix Γ shown in table 1 using the max-max criterion. For the terminal 1, the best SNR (16db) is obtained when processing the signal from the transmitting antenna 3. The best transmit antennas for the other terminals are also indicated in table 2. The scheduler can use this information to select the appropriate coding and modulation schemes to use for data transmission.

TABLE 2

Terminal device	Transmitting antenna	SNR(dB)
			1	3	16
2	4	14
			3	1	14
4	2	10

The scheduling schemes depicted in fig. 2 and 3 represent specific schemes for evaluating a plurality of hypotheses for a plurality of possible sets of active terminals desiring data transmission within an incoming transmission interval. The total number of hypotheses to be evaluated by the scheduler may be large, even for a small number of active terminals. In practice, the total number of hypotheses can be expressed as:

formula (5)

Wherein N is_UIs the number of active terminals to consider for scheduling: for example, if N_U8 and N_TIf 4, then N_hyp70. An exhaustive search may be used to determine the specific hypotheses (and specific antenna assignments) that provide the optimal system performance, as quantified by the performance metrics used to select the best hypotheses and antenna assignments.

Other scheduling with reduced complexity is also possible and within the scope of the invention. One such scheduling scheme is described below. These schemes may also provide high system performance while reducing the amount of processing required to schedule terminals for data transmission.

In another scheduling scheme, the active terminals schedule data transmission according to their priority. The priority of each terminal may be derived based on one or more metrics (e.g., average throughput), system limitations and requirements (e.g., maximum latency), other factors, or a combination thereof, as described below. A list may be maintained for all active terminals desiring data transmission within an incoming transmission interval (also referred to as a "frame"). When a terminal desires to transmit data, it is added to the list and its metric is initialized (e.g., initially to zero). The metric for each terminal in the list is thereafter updated on each frame. Once the terminal no longer wishes to transmit data, it is removed from the list.

For each frame, all or terminals in the listA subset may be considered for scheduling. The particular number of terminals to consider may be based on a number of factors. In one embodiment, only N is selected_TAnd the terminal with the highest priority carries out data transmission. In another embodiment, consider N in the table_XScheduling of the highest priority terminals, where N_X>N_T。

FIG. 4 is a flow diagram 400 of a priority-based scheduling scheme in which N is considered, according to an embodiment of the invention_TScheduling of the set of highest priority terminals. At each frame interval, the scheduler checks all active terminals in the table and selects N, step 412_TA set of highest priority terminals. The remaining terminals in the table are not used for scheduling considerations. A channel estimate is then obtained for each selected terminal at step 414. For example, the processed SNRs for the selected terminal may be obtained and used to form the hypothesis matrix Γ.

Then at step 416, N is estimated from the channel and using any one of a plurality of antenna allocation schemes_TThe transmit antennas are assigned to the selected terminal. For example, the antenna allocation scheme may be based on the exhaustive search or the max-max criterion described above. In another antenna allocation scheme, transmit antennas are allocated to terminals such that their priorities are normalized as much as possible after the terminal metrics are updated.

The data rate and coding and modulation scheme for the terminal are then determined based on the antenna assignments at step 418. The scheduled transmission interval and data rate may be reported to the scheduled terminal. At step 420, scheduled (unscheduled) terminals within the table are updated to reflect the scheduled data transmissions (as well as non-transmissions), and system metrics are also updated.

Various metrics and factors may be used to determine the priority of the active terminals. In one embodiment, a "score" may be maintained for each terminal in the table and for each metric used for scheduling. In an embodiment, a score is maintained for each active terminal indicating the average throughput over a particular averaging time interval. In one embodimentOf the terminal n at frame k_n(k) Calculated as the linear average throughput obtained over some time interval, can be expressed as:

formula (6)

Wherein r is_n(i) Is the data rate (in bits/frame) of terminal n at frame i achieved and may be calculated as shown in equation (4). In general, r_n(i) From a specific maximum obtainable data rate r_maxAnd a certain minimum data rate (e.g., zero) definition. In another implementation, the fraction φ of terminal n within frame k_n(k) Is the exponential average throughput obtained over some time interval and can be expressed as:

φ_n(k)＝(1-α)·φ_n(k-1)+α·r_n(k)/r_maxformula (7)

Where α is the time constant of the exponential averaging, and α is somewhat larger for longer averaging intervals.

When a terminal desires to transmit data, it is added to the list and its score is initialized to zero. The score for each terminal in the list is updated successively on each frame. Whenever a terminal has no transmission schedule within a frame, its data rate for that frame is set to zero (i.e., r)_n(k) 0) andits score is updated accordingly. If a frame is received by the terminal in error, the effective data rate of the frame for the terminal may be set to zero. The frame error may not be immediately known (e.g., due to round-trip delay of the positive/negative acknowledgement (Ack/Nak) scheme used for the data transmission), but once one of the information is available, the score can be adjusted accordingly.

The priority of the active terminal may also be determined based in part on system limitations and requirements. For example, the priority of a terminal may be raised if the maximum latency of a particular terminal exceeds a certain threshold.

Other factors may also be considered in determining the priority of the active terminals. One such factor may be related to the type of data to be transmitted to the terminal. Delay sensitive data may be associated with a higher priority and delay insensitive data associated with a lower priority. The retransmitted data due to decoding errors of previous transmissions may be associated with a higher priority because other processes may be waiting for the retransmitted data. Other factors may be related to the type of data service provided to the terminal. Other factors may also be considered in determining priority and are within the scope of the present invention.

The priority of a terminal is thus a function of any combination of (1) the score maintained for each metric considered by the terminal, (2) the system limitations and other parameter values that need to be maintained, and (3) other factors. In one embodiment, the system constraints and requirements represent "hard" values (i.e., high or low priority, depending on whether the constraints and requirements are violated) and the scores represent "soft" values. For this embodiment terminals not meeting the system constraints and requirements are considered immediately and other terminals are considered according to their scores.

It is possible to design a priority-based scheduling scheme to obtain equal average throughput (i.e., equal QoS) for all terminals in the list. In this case, the active terminals are prioritized according to the average throughput they obtain, which may be determined as shown in equation (6) or (7). In this priority-based scheduling scheme, the scheduler prioritizes the terminals using scores for allocation to the available transmit antennas. The scores of the terminals are updated according to their assignment or non-assignment to the transmit antennas. The active terminals in the list may be prioritized such that the terminal with the lowest score is given the highest priority and the terminal with the highest score is given the lowest priority instead. Other methods of ranking the endpoints may also be used. Prioritization may also assign uneven weighting factors to the terminal scores.

For scheduling schemes where terminals are selected according to their priority and scheduled for data transmission, it may occasionally be found that poor terminals are grouped together. The "bad" set of terminals is the set that produces a similar channel correlation matrix H_kWhich results in similar and poor SNRs for all terminals given in the hypothesis matrix Γ over all transmit data streams. This then results in a lower overall throughput for each terminal in the set. When this happens, the priority of the terminal may not change anything over several frames. Thus, the scheduler may stall at that particular set of terminals until the priority changes sufficiently to cause member changes within the set.

To avoid the "aggregation" effect described above, the scheduler can be designed to recognize the situation before assigning the terminal to an available transmit antenna and/or to detect it once it occurs. It is possible to use a number of different techniques to determine the channel response matrix H_kDegree of linear correlation within. A simple method of detection is to apply a specific threshold on the hypothesis matrix Γ. If all SNRs are below the threshold, an aggregation condition exists. Upon detection of an aggregation condition, the scheduler can rearrange the terminals (e.g., in a random manner) in an attempt to reduce linear correlation within the hypothesis matrix. The scrambling scheme may also be designed to force the scheduler to select a set of terminals that produce a "good" hypothesis matrix (e.g., with minimal linear correlation).

Some of the scheduling schemes described above use techniques to reduce the amount of processing required to select terminals and assign transmit antennas to those selected terminals. These and other techniques may be combined to derive other scheduling schemesAnd this is within the scope of the invention. For example, N may be considered using any of the schemes described above_XScheduling of the highest priority terminal.

It is also possible to design more complex scheduling schemes that achieve a closer to optimal throughput. These schemes may require evaluation of a large number of hypotheses and antenna assignments to determine the best set of terminals and best antenna assignments. It is also possible to design other scheduling schemes to take advantage of the statistical distribution of data rates obtained by each terminal. This information may be useful in reducing the number of hypotheses to evaluate. In addition, for some applications, it may be possible to know which terminal groups (i.e., hypotheses) work well by analyzing performance in the time domain. This information may be stored, updated and used by the scheduler at future scheduling intervals.

The techniques described above may be used to schedule data transmissions for a terminal when using MIMO mode, N-SIMO mode, and mixed mode. Other considerations may also apply to each of these modes of operation, as described below.

MIMO mode

In MIMO mode, (up to) N_TThe independent data streams may be simultaneously transmitted from the base station to N_TA transmitting antenna transmits and sends to a carrier with N_RSingle MIMO terminal with multiple receive antennas (i.e., N)_T×N_RMIMO) where N_R≥N_T. The terminal may use spatial equalization (for non-dispersive MIMO channels with flat frequency channel response) or space-time equalization (for dispersive MIMO channels with frequency-based channel response) to process and separate N_TA transmitted data stream. The SNR (i.e., after equalization) of each processed data stream may be estimated and sent back to the base station as CSI, which may then use this information to select an appropriate coding and modulation scheme to use on each transmit antenna so that the target terminal can detect each transmitted data stream at a desired level of performance.

If all data streams are transmitted to one terminal, as is the case in MIMO mode, it is possible to use continuous at the terminalCancelling receiver processing techniques to process N_RA received signal to recover N_TA transmitted data stream. This technique processes N a number of times (or iteratively) in succession_RThe received signals are processed to recover the signals transmitted from the terminals, one signal at each iteration. For each iteration, the technique is at N_RLinear or non-linear processing (i.e., spatial or space-time equalization) is performed on the received signal to recover one of the transmitted signals and to cancel interference in the received signal due to the recovered signal to derive a "modified" signal with the interference component removed.

The modified signal is then processed for the next iteration to recover another transmitted signal. By removing the interference due to each recovered signal from the received channel, the SNR of the transmitted signal included in the modified but unrecovered signal is improved. The improved SNR improves the performance of the terminal as well as the system. Indeed, under certain operating conditions, the performance obtained using successive cancellation receiver processing in conjunction with Minimum Mean Square Error (MMSE) spatial equalization is comparable to full CSI processing. The technique of successive cancellation receiver PROCESSING is described IN detail IN U.S. patent application Ser. No. 09/854,235 entitled "METHOD AND APPARATUS FOR PROCESSING DATA IN A MULTIPLE-INPUT MULTIPLE-OUTPUT (MIMO) COMMUNICATION SYSTEM UTILIZING CHANNEL STATE INFORMATION", filed on 5/11/2001, assigned to the assignee of the present invention AND incorporated herein by reference.

In an embodiment, each MIMO terminal in the system estimates and sends back N_TN of transmitting antenna_TAnd a post-processing SNR value. The SNRs from the active terminals may be evaluated by the scheduler to determine which terminal to transmit to and when to transmit, and the appropriate coding and modulation schemes to use for each selected terminal on a per transmit antenna basis.

A MIMO terminal may be selected for data transmission based on certain performance metrics that are configured to achieve a desired system objective. The performance metric may be based on one or more functions and any number of parameters. It is possible to use multiple functions to construct a performance metric, such as a function of the throughput achievable by the MIMO terminal, as shown in equations (3) and (4) above.

N-SIMO mode

In N-SIMO mode, (up to) N_TThe independent data streams may be simultaneously transmitted from the base station to N_TMultiple transmit antennas transmitting simultaneously to (up to) N_TA different target SIMO terminal. To optimize performance, the scheduler may consider a large set of possible terminals for data transmission. The scheduler then determines the best N for simultaneous transmission on a given channel (i.e., time slot, code channel, frequency subchannel, etc.)_TA set of individual terminals. In multiple access communication systems, there are typically limitations on a per-terminal basis that meet certain requirements, such as maximum latency or average data rate. In this case, the scheduler can be designed to fit the best set of terminals for these constraints.

In one implementation of the N-SIMO mode, the terminal uses linear spatial equalization to process the received signal and provides the processed SNR for each transmit antenna to the base station. The scheduler then uses the information to select terminals for data transmission and assigns transmit antennas to the selected terminals.

In another implementation of the N-SIMO mode, the terminal uses successive cancellation receiver processing to process the received signal to obtain higher processed SNRs. For successive cancellation receiver processing, the post-processing SNRs of the transmitted data stream depend on the order of the detected (i.e., demodulated and decoded) data streams. In some cases, a particular S IMO terminal may not be able to cancel interference from a given transmit data stream intended for another terminal because the coding and modulation schemes used for the data stream are selected based on the post-processing SNRs of the other terminals. For example, the transmitted data stream may be targeted to terminal u_xAnd with the target terminal u_xWhere the available post-processing SNR (e.g., 10dB) is coded and modulated for proper detection, but another terminal u_yMay receive the same transmitted data stream with a poor post-processing SNR and therefore notThe data stream can be suitably detected. If a data stream intended for another terminal cannot be detected error-free, interference cancellation of the data stream is not possible. Successive cancellation receiver processing is only possible if the post-processing SNR of the corresponding transmitted data stream allows reliable detection.

In order for the scheduler to take advantage of the post-processing SNRs provided by SIMO terminals using successive cancellation receiver processing, each such terminal can derive post-processing SNRs corresponding to different possible detection orders for the transmitted data streams. Possibly N at SIMO terminal_TFactorial (i.e. N)_T| A ) Detecting N on a possibly sequential basis_TA transmitted data stream, and each sequence is associated with N_TThe individual processed SNR values are correlated. Thus, there may be N_T·N_T| A One SNR value is reported by each active terminal to the base station (e.g., if N_TEach SIMO terminal may report 96 SNR values ═ 4). The scheduler can then use the information to select terminals for data transmission and assign transmit antennas to the selected terminals.

The scheduler may also take into account the possible detection order of each terminal if successive cancellation receiver processing is used at the terminal. However, a large number of such orderings are generally ineffective because a particular terminal may not be able to correctly detect data streams transmitted to other terminals due to the lower processed SNRs of the data streams that cannot be detected at the terminal.

As described above, the transmit antennas may be assigned to selected terminals according to various schemes. In an antenna allocation scheme, the allocation of transmit antennas is to achieve higher system performance and is based on the priority of the terminal.

Table 3 shows an example of the processed SNRs derived by each terminal within the considered hypothesis. For terminal 1, the best SNR is obtained when detecting the data stream from transmit antenna 3, as shown by the shaded area in row 3 and column 4 of the table. The best transmit antennas for the other terminals in the hypothesis are also indicated by the shaded areas in the table.

TABLE 3

If each terminal identifies a different transmit antenna from which the best processed SNR is detected, the transmit antennas may be assigned to the terminals according to their best processed SNRs. For the example shown in table 3, terminal 1 may be assigned to transmit antenna 3 and terminal 2 may be assigned to transmit antenna 4.

If there is more than one terminal for the same transmit antenna, the scheduler can determine the antenna allocation based on a variety of criteria (e.g., fairness, performance metrics, and others). For example, table 3 indicates that the best processed SNRs for terminals 3 and 4 are from data streams transmitted from the same transmit antenna 1. If the goal is to maximize throughput, the scheduler may assign transmit antenna 1 to terminal 3 and transmit antenna 2 to terminal 4. However, if the antenna allocation is for fairness, transmit antenna 1 may be allocated to terminal 4 if terminal 4 has a higher priority than terminal 3.

Mixed mode

The above techniques may be generalized to handle hybrid SIMO and MIMO terminals. For example, if there are four transmit antennas at the base station, it may be possible to transmit four independent data streams to a single 4x4 MIMO terminal, two 2x4 MIMO terminals, four 1x4 SIMO terminals, one 2x4 MIMO terminal plus two 1x4 SIMO terminals, or any other combination of terminals receiving a total of four data streams. The scheduler can be designed to select the best combination of terminals based on the post-processing SNRs for a number of hypothesized sets of terminals, where each hypothesized set may include a mix of MIMO and SIMO terminals.

Any support for mixed mode traffic, the use of successive cancellation receiver processing by the terminal (e.g., MIMO) places additional restrictions on the scheduler due to the introduced correlation. These limitations may result in more sets of assumptions to be evaluated, since the scheduler must consider the demodulation of the data streams by the various orderings of each terminal in addition to considering the different sets of terminals. The allocation of transmit antennas and the selection of coding and modulation schemes then take these correlations into account to achieve improved performance.

Transmitting antenna

The set of transmit antennas at the base station may be a physically distinct set of "apertures," each of which may be used to directly transmit a respective data stream. Each aperture may be formed by a set of one or more antenna elements distributed in space (e.g., physically distributed at a single site or distributed at multiple sites). Alternatively, the antenna apertures may be placed after one or more (fixed) beam forming matrixing circuits, each for synthesizing a different set of antenna beams from the respective sets of apertures. In this case, the above description of the transmit antenna is similarly applicable to the transformed antenna beam.

Multiple fixed beamforming matrices may be predefined, and the terminal may evaluate the processed SNRs for each (or set of antenna beams) of the possible matrices and send the SNR vector back to the base station. Different performance (i.e., post-processing SNR) is typically obtained for different sets of transformed antenna beams, and this is reflected in the reported SNR vector. The base station may implement scheduling and antenna allocation (using reported SNR vectors) for each of the possible beamforming matrices and select a particular beamforming matrix and set of terminals and their best used antenna allocation to obtain available resources.

The use of a beamforming matrix provides additional flexibility in terminal scheduling and may also provide improved performance. For example, the following may be well suited for beamforming transformations:

● the correlation within the MIMO channel is so good that the best performance is possible with a few data streams. However, transmitting with only a subset of the available transmit antennas (and only their associated transmit amplifiers) results in a lower total transmit power. The transform may be selected to use most or all of the transmit antennas (and their amplifiers) when transmitting the data stream. In this case, a higher transmit power is obtained for the transmitted data stream.

● terminals that are physically dispersed may be isolated due to their location. In this case, the terminal may become a set of beams pointed at different azimuths through standard FFT-type transformation of horizontally spatially separated apertures.

Performance of

The techniques described above may be viewed as a particular form of Spatial Division Multiple Access (SDMA), in which each transmit antenna within an antenna array of a base station is used to transmit a different data stream using terminal-derived channel state information (e.g., SNRs or some other sufficient parameter to determine a supported data rate) within the coverage area. High performance is obtained on the basis of CSI, which is used to schedule terminals and process data.

The techniques described herein can provide improved system performance (e.g., higher throughput). Simulations have been performed to quantify the possible system throughput using some such techniques. In the simulation, the channel response matrix H of the transmit and receive antenna arrays coupled to the kth terminal_kIt is assumed to be gaussian random variables with equal variance, zero mean complex. Simulations were performed for MIMO and N-SIMO modes.

In MIMO mode, each implementation (e.g., each transmission interval) considers four MIMO terminals (each with four receive antennas) and selects the best terminal and schedules data transmission. Four separate data streams are transmitted to the scheduled terminals and processed using a successive cancellation receiver (with MMSE equalization) to process the received signals and recover the transmitted data streams. The average throughput of the scheduled MIMO terminals is recorded.

In the N-SIMO mode, four terminals each with four receive antennas are considered per implementation. The post-processing SNRs for each SIMO terminal are determined using MMSE linear spatial equalization (without successive cancellation receiver processing). The transmit antennas are assigned to selected terminals according to the max-max criterion. Four independent data streams are transmitted to the four scheduled terminals and each terminal uses MMSE equalization to process the received signal and recover the data streams. The throughput for each scheduled SIMO terminal is recorded separately and the average throughput for all scheduled terminals is also recorded.

FIG. 9 shows a four-antenna (i.e., N) antenna with four transmit antennas_T4) and four receiving antennas per terminal (N)_R4) average throughput of the MIMO communication system. The simulated throughput associated with each mode of operation is provided as a function of the average processed SNR. The average throughput for the MIMO mode is shown as curve 910 and the average throughput for the N-SIMO mode is shown as curve 912.

As shown in fig. 9, the simulated throughput associated with the N-SIMO mode using the max-max criterion antenna assignment shows better performance than that obtained for the MIMO mode. In MIMO mode, the MTMO terminal takes advantage of the continuous cancellation of receiver processing to achieve higher post-processing SNRs. In SIMO mode, the scheduling scheme can exploit multi-user selection diversity to achieve improved performance (i.e., higher throughput) even though each SIMO terminal uses linear spatial equalization. In effect, the multi-user diversity provided by the N-SIMO mode results in an average downlink throughput that exceeds that obtained by dividing the transmission interval into four equally spaced time slots and allocating each MIMO terminal to a corresponding sub-slot.

The scheduling scheme used in the simulation of the two modes of operation is not designed to provide suitable fairness and it will be observed that some terminals have a higher average throughput than others. When the fairness criteria is applied, the difference in throughput for the two modes of operation disappears. However, the ability to accommodate MIMO and N-SIMO terminals provides additional flexibility in providing wireless data services.

For simplicity, various aspects and embodiments of the present invention are described for a communication system in which (1) the number of receive antennas is equal to the number of transmit antennas (i.e., N)_R＝N_T) And (2) one data stream is transmitted from each antenna at the base station. In this case, the number of transmission channels is equal to the number of available spatial subchannels of the MIMO channel. For MIMO systems using OFDM, multiple frequency subchannels may be usedEach spatial subchannel is correlated and these frequency subchannels may be allocated to terminals according to the techniques described above. For dispersive channels, the matrix H represents a three-dimensional cube of the corresponding estimates of the channel for each terminal.

Each scheduled terminal may also have more receive antennas than the total number of data streams. Moreover, multiple terminals may share a given transmit antenna, and sharing may be achieved through time division multiplexing (e.g., assigning different portions of a transmission interval to different terminals), frequency division multiplexing (e.g., assigning different frequency subchannels to different terminals), code division multiplexing (e.g., assigning different orthogonal codes to different terminals), some other multiplexing techniques, and any combination thereof.

The scheduling scheme described herein selects terminals and allocates antennas for data transmission based on channel state information (e.g., post-processing SNRs). The post-processing SNRs for the terminal depend on the particular transmit power level used to transmit the data stream from the base station. For the sake of simplicity, it is assumed that all data streams have the same transmit power level (i.e., no transmit power control). However, by controlling the transmit power of each antenna, the available SNRs are adjusted. For example, by reducing the transmit power of a particular transmit antenna through power control, the SNR associated with a data stream transmitted from that antenna is reduced, the interference of that data stream with other data streams is also reduced, and other data streams may be able to achieve better SNRs. Therefore, power control is also possible to use with the scheduling scheme described herein and this is within the scope of the invention.

Scheduling terminals according to priority is described in U.S. patent No. 6745044 entitled "method and APPARATUS FOR detecting and using AVAILABLE transition power a wireless communication SYSTEM" and issued to Jack Holtzman et al on 6/1 of 2004. Data SCHEDULING FOR the downlink is also described in U.S. patent application serial No. 08798951, entitled "method and APPARATUS FOR FORWARD LINK speed SCHEDULING", filed on 11/2/1997. These applications are assigned to the assignee of the present invention and are incorporated herein by reference.

The scheduling scheme described herein includes a variety of features and provides a variety of benefits. These features and benefits are described below.

First, the scheduling scheme supports multiple modes of operation, including mixed modes, where it is possible to schedule data transmission for any combination of SIMO and MIMO terminals on the downlink. Each SIMO and MIMO terminal is associated with an SNR vector (i.e., one row in equation (2)). The scheduling scheme can evaluate any number of possible combinations of terminals for data transmission.

Second, the scheduling scheme provides scheduling for each transmission interval that includes a set of (optimal or near optimal) terminals that are "compatible" with each other, based on the spatial characteristics of the terminals. Mutual compatibility may mean that transmissions on the same channel and at the same time coexist given the data rate requirements, transmit power, link margin, performance between SIMO and MIMO terminals, and possibly other factors for the terminals.

Third, various data rate adaptations are supported according to the post-processing SNRs scheduling scheme obtained at the terminal. Each scheduled terminal may be informed when there is data to transmit, the assigned transmit antenna, and the data rate to use for the data transmission (e.g., on a per transmit antenna basis).

Fourth, the scheduler can be designed to consider a set of terminals with similar link margins. Terminals may be grouped according to their link margin characteristics. The scheduler can consider the combination of terminals within the same "link margin" group when searching for mutually compatible spatial characteristics. Grouping according to link margin may improve the overall spectral efficiency of the scheduling scheme compared to that obtained ignoring the link margin. Also, by scheduling terminals with similar link margins to transmit, it may be easier to implement downlink power control (over the entire set of terminals) to improve overall spectrum reuse. This may be seen as a combination of downlink adaptive reuse scheduling and SDMA for SIMO/MIMO. Scheduling according to link margin is described in U.S. patent No. 6493331, entitled "METHOD AND APPARATUS FOR CONTROLLING transmission OF messages OF a COMMUNICATIONS SYSTEM", issued to Walton et al on 12/10 OF 2002, AND in U.S. patent application serial No. 09/848937, entitled "METHOD AND APPARATUS FOR CONTROLLING transmission OF messages OF A WIRELESS COMMUNICATIONS SYSTEM", filed on 3/5 OF 2001, AND assigned to the assignee OF the present invention AND incorporated herein by reference.

MIMO communication system

Fig. 5 is a block diagram of a base station 104 and a terminal 106 within a MIMO communication system 100. At the base station 104, a data source 512 provides data (i.e., information) to a Transmit (TX) data processor 514. For each transmit antenna, TX data processor 514(1) encodes data according to a particular coding scheme, (2) interleaves (i.e., reorders) the encoded data according to a particular interleaving scheme, and (3) maps the interleaved bits to modulation symbols for one or more transport channels used for data transmission selection. The encoding increases the reliability of the data transmission. Interleaving provides time diversity of the coded bits so that data can be transmitted with the average SNR of the transmit antennas, against fading, and further removes correlation between the coded bits used to form each modulation symbol. Interleaving may also provide frequency diversity if the coded bits are transmitted on multiple frequency subchannels. In an aspect, the encoding and symbol mapping may be performed in accordance with control signals provided by scheduler 534.

Encoding, interleaving, and signal mapping may be achieved by various schemes. Some such schemes are described in the following documents: U.S. patent application Ser. No. 09/854235, U.S. patent application Ser. No. 09/816481, entitled "METHOD AND APPARATUS FOR UTILIZING CHANNEL STATIONATION FORMATION IN A WIRELESS COMMUNICATION SYSTEM", filed on 23/3/2001, AND U.S. patent application Ser. No. 09/776073, entitled "CODING SCHEME FOR A WIRELESS COMMUNICATION SYSTEM", filed on 1/2/2001, which are assigned to the assignee of the present invention AND are hereby incorporated by reference.

A TX MIMO processor 520 receives and multiplexes the modulation symbols from TX data processor 514 and provides a stream of modulation symbols, one modulation symbol for each time slot, for each transmission channel (e.g., each transmit antenna). TX MIMO processor 520 may also perform pre-processing of the modulation symbols for each selected transmission channel if full CSI (e.g., channel response matrix H) is available. MIMO and full CSI processes are further described in U.S. patent application Ser. No. 09/532492 entitled "HIGH EFFICIENCY, HIGHPERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTIPLE-CARRIER MODULATION", filed on 3/22/2000, assigned to the assignee of the present invention and incorporated herein by reference.

If OFDM is not used, a TX MIMO processor 520 provides a stream of modulation symbols for each antenna used for data transmission. And if OFDM is used, TX MIMO processor 520 provides a vector of modulation symbols for each antenna used for data transmission. And if full CSI processing is performed, TX MIMO processor 520 provides the pre-conditioned modulation symbols or pre-conditioned vector of modulation symbols for each antenna used for data transmission. Each stream is then modulated by a respective Modulator (MOD)522 and transmitted via an associated antenna 524.

At each scheduled terminal 106, a plurality of receive antennas 552 receives the transmitted signals and each provides a received signal to a respective demodulator (DEMOD) 554. Each demodulator (or front end unit) 554 performs the inverse of the operations at demodulator 522. The modulation symbols from all demodulators 554 are then provided to a Receive (RX) MIMO/data processor 556 and processed to recover one or more data streams transmitted to the terminals. An RX MIMO/data processor 556 performs inverse processing to TX data processor 514 and TX MIMO processor 520 and provides decoded data to a data sink 560. The processing of the terminal 106 is described in detail in each of patent application serial numbers 09/854235 and 09/776073.

At each active terminal 106, an RX MIMO/data processor 556 further estimates link conditions and provides CSI (e.g., post-processing SNRs or channel gain estimates). A TX data processor 562 then receives and processes the CSI and provides processed data indicative of the CSI to one or more modulators 554. A modulator 554 further conditions and transmits CSI back to base station 104 via the reverse channel. The CSI may be reported by the terminal using various signaling techniques (e.g., in full or differential or a combination thereof), as described in the aforementioned U.S. patent application serial No. 09/816481.

At base station 104, the transmitted feedback signals are received by antennas 524, demodulated by a demodulator 522, and provided to a RX data/MIMO processor 532. An RX data/MIMO processor 532 performs operations complementary to the TX data processor 562 and recovers the reported CSI, which is then provided to a scheduler 534.

Scheduler 534 uses the reported CSI to perform a number of functions, such as (1) selecting the best set of terminals for data transmission, (2) assigning available transmit antennas to the selected terminals, and (3) determining the coding and modulation schemes to be used by each assigned transmit antenna. Scheduler 534 may schedule terminals for high throughput, as described above, or according to some other performance criteria or metric. In fig. 5, a scheduler 534 is shown implemented within the base station 104. In other implementations, scheduler 534 may be implemented within some other element of communication system 100 (e.g., with a base station controller coupled to and interacting with multiple base stations).

Fig. 6 is a block diagram of an embodiment of a base station 104x that can process data for transmission to a terminal based on CSI available to the base station (e.g., reported by the terminal). Base station 104x is an embodiment of the transmitter portion of base station 104 of fig. 5. Base station 104x includes (1) a TX data processor 514x that receives and processes information bits to provide modulation symbols and (2) N_TTX MIMO processor 520x, which demultiplexes the modulation symbols for the transmit antennas.

In the particular embodiment shown in fig. 6, the TX data processor 514x includes a demultiplexer 608, N, coupled to a plurality of channel data processors 610_COne for each of the transport channels. Demultiplexer 608 receives and demultiplexes the aggregated information bits into multiple (up to N) bits_CA) data stream D₁...D_NCOne data stream per transport channel for data transmission. Each data stream is provided to a respective channel data processor 610.

In the embodiment shown in fig. 6, each channel data processor includes an encoder 612, a channel interleaver 614, and a symbol mapping element 616. Encoder 612 receives and encodes information bits within a received data stream according to a particular coding scheme to provide coded bits. Channel interleaver 614 interleaves the coded bits according to a particular interleaving scheme to provide time diversity. And symbol mapping element 616 maps the interleaved bits into modulation symbols for the transmission channel used to transmit the data stream.

Pilot data (e.g., data in a known form) may also be encoded and multiplexed with the processed information bits. The processed pilot data may be transmitted (in a Time Diversity Multiplex (TDM)) on all subsets of the transmission channel used to transmit the information bits. The pilot data may be used at the terminal to perform channel estimation.

As shown in fig. 6, data coding, interleaving, and modulation (or a combination thereof) may be adjusted according to the available CSI (e.g., reported by the terminal). In one coding and modulation scheme, adaptive coding is achieved by a fixed base code (e.g., a Turbo code for code rate 1/3), and puncturing is adjusted to achieve a desired code rate, as supported by the SNR of the transmission channel used to transmit the data. For this scheme, puncturing may be implemented after channel interleaving. In another coding and modulation scheme, different coding schemes may be used depending on the reported CSI. For example, each of the data streams may be encoded with a separate code. With this scheme it is possible to use a successive cancellation receiver processing scheme at the terminal to detect and decode the data stream to derive a more reliable estimate of the transmitted data stream, as will be described in detail below.

Symbol mapping element 616 may be designed to combine the interleaved sets of bits to form non-binary symbols and map each non-binary symbol to a point within the signal constellation corresponding to a particular modulation scheme (e.g., QPSK, MPSK, MQAM, or some other scheme). Each mapped signal point corresponds to a modulation symbol. The number of information bits that may be transmitted for a particular performance level (e.g., one percent Packet Error Rate (PER)) PER modulation symbol depends on the SNR of the transmission channel. Thus, the coding and modulation scheme for each transmission channel may be selected based on the available CSI. The channel interleaving may also be adjusted according to the available CSI.

The modulation symbols from TX data processor 514x are provided to a TX MIMO processor 520x, which is an embodiment of TX MIMO processor 520 in fig. 5. Within TX MIMO processor 520x, a multiplexer 622 is coupled from N_CMultiple channel data processor 61O receives (up to) N_CA modulation symbol stream S₁...S_NCAnd multiplexing the received modulation symbols into a plurality of (N)_T) Modulating the symbol stream V₁...V_NTOne stream per antenna used to transmit the modulation symbols. Each modulation symbol stream is provided to a respective modulator 522. Each modulator 522 will modulate a symbol V₁...V_NTConverted to analog signals and further amplified, filtered, quadrature modulated, and upconverted to generate a modulated signal suitable for transmission over a wireless link.

Transmitter designs implementing OFDM are described in the aforementioned U.S. patent application serial nos. 09/854235, 09/816481, 0/9776073, and 09532492.

Fig. 7 is a block diagram of an embodiment of a terminal 106x capable of implementing various aspects and embodiments of the invention. Terminal 106x is an embodiment of the receive portion of terminals 106a through 106n of fig. 5 and implements a successive cancellation receiver processing technique to receive and recover the transmitted signal. From (up to) N_TTransmitting signals from transmitting antennas N of antennas 552a to 552r_REach antenna receives and routes to a respective demodulator (DEMOD)554 (which is also referred to as a front-end processor). Each demodulator 554 conditions (e.g., filters and amplifies) the corresponding received signal, downconverts the conditioned signal to an intermediate frequency or baseband, and digitizes the downconverted signal to provide samples. Each demodulator 554 may further demodulate samples with received pilots to generateThe received modulated symbol streams are provided to an RX MIMO/data processor 556 x.

In the illustrated embodiment of fig. 7, RX MIMO/data processor 556x, which is an embodiment of RX MIMO/data processor 556 in fig. 5, includes multiple successive (i.e., cascaded) receiver processing stages 710, one for each of the transmitted data streams to be recovered by terminal 106 x. In a transmit processing scheme, one data stream is transmitted on each transport channel assigned to terminal 106x, and each data stream is independently processed (e.g., with its own coding and modulation schemes) and transmitted from a corresponding transmit antenna. For this transmit processing scheme, the number of data streams is equal to the number of assigned transmission channels, which is also equal to the number of transmit antennas assigned to terminal 106x for data transmission (which may be a subset of the available transmit antennas). For clarity, the transmit processing scheme is described with an RXMIMO/data processor 556 x.

Each receiver processing stage 710 (except for the last stage 710n) includes a channel MIMO/data processor 720 coupled to an interference canceller 730, with the last stage 710n including only a channel MIMO/data processor 720 n. For the first receive processing stage 710a, a channel MIMO/data processor 720a receives and processes N from demodulators 554a through 554r_RThe symbol stream is modulated to provide a decoded data stream (or first transmitted signal) for the first transmission channel. And for the second stage 710b to the last stage 710N, the channel MIMO/data processor 720 of that stage receives and processes N from the interference canceller 730 of the previous stage_RA modified symbol stream to derive a decoded data stream for the transport channel processed at that stage. Each channel MIMO/data processor also provides CSI (e.g., SNR) for the associated transmission channel.

For the first receiver processing stage 710a, interference canceller 730a receives signals from all N_RN from demodulator 554_RA stream of modulation symbols. And for each of the previous stages from the second to the last stage, the interference canceller 730 receives N from the interference canceller in the previous stage_RA modified symbol stream. Each interference canceller 730 also receives decoded data from channel MIMO/data processors in the same stageA stream of data and performs processing (e.g., encoding, interleaving, demodulation, channel response, etc.) to derive N_RA stream of remodulated symbols that are estimates of interference components of the stream of modulated symbols received from the demodulated data stream. The re-demodulated symbol stream is then subtracted from the received modulated symbol stream to derive N, which includes all but the subtracted (i.e., cancelled) interference components_RA modified symbol stream. Then N_RThe modified symbol stream is provided to the next stage.

In fig. 7, a controller 740 is coupled to RX MIMO/data processor 556x and may direct various steps in the successive cancellation receiver processing performed by processor 556 x.

Fig. 7 shows a receiver structure that may be used in a straightforward manner when each data stream is transmitted on a respective transmit antenna (i.e., one data stream for each transmitted signal). In this case, each receiver processing stage 710 may be configured to recover one transmitted signal and provide a decoded data stream corresponding to the recovered transmitted signal. For some other transmit processing schemes, the data streams may be transmitted over multiple transmit antennas, frequency subchannels, and/or time intervals that provide spatial, frequency, and time diversity, respectively. For these schemes, receiver processing initially derives a received modulation symbol stream for the signal transmitted on each transmit antenna for each frequency subchannel. Modulation symbols for multiple transmit antennas, frequency subchannels, and/or time intervals may be combined in a manner complementary to the demultiplexing performed at the base station. The combined modulation symbol streams are then processed to provide corresponding decoded data streams.

Fig. 8A is a block diagram of an embodiment of a channel MIMO/data processor 720x, which is an embodiment of channel MIMO/data processor 720 in fig. 7. In this embodiment, channel MIMO/data processor 720x includes a space/space time processor 810, a CSI processor 812, a selector 814, a demodulation element 816, a deinterleaver 818, and a decoder 820.

Spatial/space-time processor 810 implements N for non-dispersive MIMO channels (i.e., with flat fading)_RLinear spatial processing of received signals or N of dispersive MIMO channels (i.e. with frequency selective fading)_RSpace-time processing of the received signal. Spatial processing may be implemented using linear spatial processing techniques, such as Channel Correlation Matrix Inversion (CCMI) techniques, Minimum Mean Square Error (MMSE) techniques, and others. These techniques may be used to remove undesired signals or to maximize the received SNR of each constituent signal in the presence of noise and interference from other signals. The space-time processing may be implemented using linear space-time processing techniques such as an MMSE linear equalizer (MMSE-LE), a Decision Feedback Equalizer (DFE), a Maximum Likelihood Sequence Estimator (MLSE), and others. The CCMI, MMSE-LE, and DFE techniques are further detailed in the aforementioned U.S. patent application serial No. 09/854235. DFE and MLSE techniques are further detailed in the following documents: l. Ariyavistakul et al, in a title "optimal Space-Time Process sensors with diversity Interference: in the Unified Analysis and Required Filter Span, "IEEE trans. on Communication, Vol.7, No.7, month 7 1999, which is incorporated herein by reference.

CSI processor 812 determines the CSI for each transmission channel used for data transmission. For example, the CSI processor 812 may estimate a noise covariance matrix from the received pilot signals and then calculate the SNR for the kth transmission channel of the data stream to be decoded. The SNR can then be estimated in a single and multi-carrier system similar to conventional pilot-assisted, as is known in the art. The SNR for all transmission channels used for data transmission may include the CSI for that transmission channel reported back to the base station. CSI processor 812 also provides a control signal to selector 814 that identifies the particular data stream being recovered by the receiver processing stage.

Selector 814 receives the multiple symbol streams from spatial/space-time processor 810 and extracts the symbol streams corresponding to the data streams to be decoded, as indicated by the control signals from CSI processor 812. The decimated stream of modulation symbols is then provided to a demodulation element 816.

For the embodiment shown in fig. 6, where the data streams for each transmission channel are independently encoded and modulated according to the SNR of the channel, the recovered modulation symbols for the selected transmission channel are demodulated according to a demodulation scheme that is complementary to the modulation scheme used for the transmission channel (e.g., M-PSK, M-QAM). The demodulated data from demodulation element 816 is then deinterleaved by a deinterleaver 818 in a manner complementary to that implemented by channel interleaver 614, and the deinterleaved data is further decoded by a decoder 820 in a manner complementary to that implemented by encoder 612. For example, decoder 820 may use a Turbo decoder or a Viterbi decoder if Turbo or convolutional coding is implemented at the base station, respectively. The decoded data stream from decoder 820 represents an estimate of the recovered transmitted data stream.

Fig. 8B is a block diagram of an interference canceller 730x, which is an embodiment of interference canceller 730 of fig. 7. Within interference canceller 730x, the decoded data stream from channel MIMO/data processor 720 in the same stage is re-encoded, interleaved, and re-modulated by channel data processor 610x to provide re-modulation symbols, which are estimates of the modulation symbols prior to MIMO processing and channel distortion at the base station. The channel data processor 610x performs the same processing (e.g., encoding, interleaving, and modulation) as performed on the data stream at the base station. The remodulated symbols are then provided to a channel simulator 830, which processes the symbols with an estimated channel response to provide an estimate of interference due to the decoded data streamThe channel response estimate may be derived from the pilot and/or data transmitted by the base station and based on the description in the aforementioned U.S. patent application serial No. 09/854235. Interference vector due to symbol stream transmitted on k-th transmitting antennaInner N_ROne element for each N_RComponents of a received signal at a receive antenna. Each element of the vector represents an estimated component due to the decoded data stream within the corresponding received modulation symbol stream. These components are for N_RA received stream of modulation symbols (i.e., vectors)r ^k) The rest of (has not been examined yet)Detected) interference of the transmitted signal and is derived from the received signal vector by summer 832r ^kIs subtracted (i.e., canceled) to provide a modified vector with the component removed from the decoded data streamr ^k+1. Modified vectorr ^k+1Provided as input vectors to the next receiver processing stage 710x, as shown in fig. 7.

Various aspects of the successive cancellation receiver processing are further detailed in the aforementioned U.S. patent application serial No. [ attorney docket No. PA010210 ].

Receiver designs that do not use successive cancellation receiver processing techniques may also be used to receive, process, and recover the transmitted data stream. Some such receiver designs are described in the aforementioned U.S. patent application Ser. Nos. 09/776073 and 09/816481 and in U.S. patent application 09532492, entitled "HIGHEFFICIENCY, HIGHPERFORMANCE COMMUNICATIONS SYSTEMEMBRANE YNING MULTI-CARRIER MODULATION", filed on 3/22.2000, assigned to the assignee of the present invention and incorporated herein by reference.

For simplicity of description, various aspects and embodiments of the invention are described in which CSI comprises SNR. In general, the CSI may not include any type of information that indicates characteristics of the communication link. Various types of information may be provided as CSI, some examples of which are described below.

In one embodiment, the CSI includes a channel-to-noise-plus-interference ratio (SNR) that is derived as a ratio of signal power to noise-plus-interference power. The SNR is typically estimated and provided to each transmission channel (e.g., each transmit data stream) for data transmission, although one aggregate SNR may be provided for multiple transmission channels. The SNR estimate may be quantized to a value with a certain number of bits. In an embodiment, the SNR estimate is mapped to an SNR index, e.g., using a look-up table.

In another embodiment, the CSI comprises a signal power and an interference power plus a noise power. These two components may be derived separately and provided to each transmission channel for data transmission.

In another embodiment, the CSI includes signal power, interference power, and noise power. These three components may be derived and provided to each transmission channel used for data transmission.

In another embodiment, the CSI includes the signal-to-noise ratio plus a list of interference powers for each observable interference term. This information may be derived and provided to each transmission channel used for data transmission.

In another embodiment, the CSI comprises a matrix form (e.g., N for all transmit-receive antenna pairs)_T×N_RComplex term) and matrix form (N)_T×N_RComplex term) noise plus interference components. The base station may then suitably combine the signal components and the noise-plus-interference components for the appropriate transmit-receive antenna pairs to derive a quality for each transmission channel used for the data transmission (e.g., a post-processing SNR for each transmitted data stream, as received at the terminal).

In another embodiment, the CSI includes a data rate indicator for each transmitted data stream. The quality of the transmission channel used for data transmission may be initially determined (e.g., based on an estimated SNR for the transmission channel) and a data rate corresponding to the determined channel quality may be identified (e.g., based on a look-up table). The identified data rate indicates a maximum data rate that can be transmitted on the transport channel for the required level of performance. The data rate is then mapped to a merged Data Rate Indicator (DRI) representation, which can be efficiently encoded. For example, if the base station supports (up to) seven possible data rates for each transmit antenna, a 3-bit value may be used to represent DRI, where, for example, zero may indicate a zero data rate (i.e., no transmit antenna is used) and 1 to 7 may be used to indicate a different data rate of 7. In a general implementation, the quality measurements (e.g., SNR estimates) are mapped directly to the DRI according to, for example, a look-up table.

In another embodiment, the CSI includes power control information for each transmission channel. The power control information may include a single bit for each transmission channel to indicate a request for more or less power or it may include multiple bits to indicate the magnitude of the change in the requested power level. In this embodiment, the base station may use the power control information fed back from the terminal to adjust data processing and/or transmit power.

In another embodiment, the CSI includes an indication of the particular processing scheme used at the base station for each transmitted data stream. In this embodiment, the indicator may identify the particular coding scheme and the particular modulation scheme used to transmit the data stream to achieve a desired level of performance.

In another embodiment, the CSI comprises a differential indication of a particular quality measurement of the transmission channel. Initially, some other quality measurement of the transmission channel is determined and reported as a reference measurement. Thereafter, the quality of the transmission channel continues to be monitored and the difference between the last reported measurement and the current measurement is determined. The difference between the two is then scaled to one or more bits, and the scaled difference is mapped to and represented by the difference indicator and then reported. The differential indicator may indicate that the last reported measurement was increased or decreased by some particular value (or that the reported measurement was maintained). For example, the differential indicator may indicate that (1) the SNR observed for a particular transmission channel has increased or decreased by some particular value, or (2) the data rate should be adjusted by some particular amount or some other change. The reference measurements may be transmitted periodically to ensure that differential indicators and/or erroneous reception of these indicators do not accumulate.

Other forms of CSI may also be used and are within the scope of the invention. In general, the CSI includes any form of sufficient information that may be used to adjust the processing at the base station such that the transmitted data stream achieves a desired level of performance.

The CSI may be derived from signals transmitted from the base station and received at the terminal. In an embodiment, the CSI is derived from a pilot reference included in the transmitted signal. Alternatively or additionally, CSI may be derived from data included in the transmitted signal.

In another embodiment, the CSI includes one or more signals transmitted on the uplink from the terminal to the base station. In some systems, there may be a degree of correlation between the uplink and downlink (e.g., Time Division Duplex (TDD) systems, where the uplink and downlink share the same bandwidth in a time division multiplexed manner). In these systems, the quality of the downlink may be estimated (to the necessary degree of accuracy) based on the quality of the uplink, which may be estimated based on signals (e.g., pilot signals) transmitted from the terminals. The pilot signals represent the way the base station can estimate the CSI observed at the terminal.

The signal quality at the terminal may be estimated according to various techniques. Some such techniques are described in the following patents, which are assigned to the assignee of the present invention and incorporated herein by reference:

● U.S. Pat. No. 5799005 entitled "SYSTEM AND METHOD FOR detecting AND detecting received PILOT POWER AND PATH loss A CDMA COMMUNICATION SYSTEM", filed 25/8 of 1998,

● U.S. Pat. No. 5903554 entitled "METHOD AND APPATUS FOR MEASURING LINKQUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEM", filed on 11/5 of 1999,

● U.S. Pat. No. 5056109YIJI 5265119, both entitled "METHOD AND APPATUSFOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONESYSTEM", filed on 8/10 AND 23/11/1993 respectively, AND

● U.S. Pat. No. 5799005 entitled "SYSTEM AND METHOD FOR detecting AND detecting received PILOT POWER AND PATH loss A CDMA COMMUNICATION SYSTEM", filed 25/8/1998,

● U.S. Pat. No. 6097972 entitled "METHOD AND APPARATUS FOR PROCESSING PROCENDROVER CONTROL SIGNALSIN CDMA MOBILE TELEPHONE SYSTEM" was filed on 8/1/2000.

It is also possible to find ways of estimating a single data transmission from a pilot signal or data transmission in many papers in the technical field. One such channel estimation scheme is described in the paper by f.ling, entitled "Optimal Reception, performance bound, and current-Rate Analysis of referenced Coherent CDMA Communications with Applications," ieee transmission on Communication, 10 months 1999.

Various types of CSI information, as well as various CSI reporting mechanisms, are described in U.S. Pat. No. 6574211 issued to Padovani et al, 6/3/2003, entitled "METHOD AND APPARATUS FORHIGH RATE PACKET DATA TRANSMISSION", filed on 11/3/1997, assigned to the assignee of the present invention, AND described in "TIE/EIA/IS-856 cdma2000Hi gh Rate Packet Data air interface Specification", both of which are incorporated herein by reference.

The CSI may be reported back to the base station using various CSI transmission schemes. For example, the CSI may be transmitted fully or differentially or a combination of the above. In an embodiment, the CSI is reported periodically and a differential update is sent back based on previously transmitted CSI. In another embodiment, the CSI is only sent when there is a change (e.g., if the change exceeds a certain threshold), which may reduce the effective rate of the feedback channel. As an example, the SNRs may be returned only when they change (e.g., differentially). For OFDM systems (with or without MIMO), correlation in the frequency domain may be used to reduce the amount of CSI fed back. As an example of an OFDM system, if it corresponds to N_MThe SNR for a particular spatial subchannel of the frequency subchannels is the same, it is possible to report the SNR and the first and last frequency subchannels for which this condition holds. Other compression and feedback channel error recovery techniques that reduce the amount of data to be fed back for the CSI may also be used and are within the scope of the invention.

Elements of the base station and the terminals may be implemented with one or more Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), processors, microprocessors, controllers, microcontrollers, Field Programmable Gate Arrays (FPGAs), programmable logic devices, other electronic units, or any combinations of the foregoing. Some of the functions and processes described herein may also be implemented in software executing on a processor.

Some aspects of the present invention may be implemented in a combination of software and hardware. The process of scheduling, i.e., selecting terminals and assigning transmit antennas, for example, may be based on program code executing on a processor (e.g., scheduler 534 in fig. 5).

Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the concepts described and these concepts may have applicability in other sections throughout the entire description.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (31)

1. A method of scheduling downlink data transmissions sent to a plurality of terminals in a wireless communication system, comprising:

forming one or more sets of terminals for possible data transmission, wherein each set is selected from all active terminals to be considered for scheduling and comprises a unique combination of one or more terminals and corresponds to the set to be evaluated;

forming one or more transmit antenna and terminal pairs for each set, wherein each transmit antenna and terminal pair corresponds to a particular assignment of a plurality of transmit antennas to one or more terminals in the set;

evaluating the performance of each transmitting antenna and terminal pair;

selecting one of a plurality of evaluated transmit antenna and terminal pairs based on the evaluated performance of the transmit antenna and terminal pairs;

transmitting data from one or more transmit antennas assigned to the terminal to each scheduled terminal in the selected transmit antenna and terminal pair; and

terminals to be considered for scheduling are prioritized.

2. The method of claim 1, wherein forming one or more transmit antenna and terminal pairs for each set comprises:

identifying the transmitting antenna and the terminal pair with the best performance among all the unassigned transmitting antennas;

allocating the transmitting antennas of a pair to the terminals of the pair, an

The assigned transmit antenna and terminal are removed from consideration.

3. The method of claim 1, wherein each set is evaluated based in part on channel state information for each terminal in the set, wherein the channel state information indicates channel characteristics between the transmit antennas and the terminal.

4. The method of claim 3, wherein the channel state information for each terminal comprises an estimate of a signal-to-noise-and-interference ratio derived at the terminal from signals transmitted from the transmit antennas.

5. The method of claim 4, wherein each set of one or more terminals to be evaluated is associated with a respective signal-to-noise-and-interference ratio matrix obtained by one or more terminals in the set.

6. The method of claim 3, further comprising:

the coding and modulation scheme for each transmit antenna is determined based on channel state information associated with the transmit antenna.

7. The method of claim 1, wherein all active terminals to be considered for scheduling comprise one or more SIMO terminals, each terminal designated to receive a single data stream.

8. The method of claim 1, wherein all active terminals to be considered for scheduling comprise one or more MIMO terminals, each designated to receive multiple data streams from multiple transmit antennas.

9. The method of claim 8, wherein the selected set comprises a single MIMO terminal.

10. The method of claim 8, wherein each scheduled MIMO terminal performs successive cancellation receiver processing to recover data transmitted to the MIMO terminal.

11. The method of claim 4, wherein one or more antenna beam sets are evaluated by each terminal to be considered for scheduling to provide one or more vectors of signal-to-noise-and-interference ratios, one vector for each antenna beam set.

12. The method of claim 1, wherein each set includes a plurality of terminals with similar link margins.

13. The method of claim 1, wherein said evaluating comprises:

a performance metric is calculated for each transmit antenna and terminal pair.

14. The method of claim 13, wherein the performance metric is system throughput.

15. The method of claim 13, wherein the transmit antenna and terminal pair with the best performance metric is selected for scheduling.

16. The method of claim 1, wherein the multiple transmit antennas are assigned to one or more terminals in each set based on priorities of the terminals in the sets.

17. The method of claim 16, wherein the highest priority terminal in the set is assigned the transmit antenna associated with the highest throughput and the lowest priority terminal in the set is assigned the transmit antenna associated with the lowest throughput.

18. The method of claim 1, further comprising:

the terminals to be considered for scheduling are limited to a group of terminals with the N highest priorities, where N is greater than or equal to one.

19. The method of claim 1, further comprising:

maintaining one or more metrics for terminals to be considered for scheduling; and

wherein the priority for each terminal is determined based in part on one or more metrics maintained for the terminal.

20. The method of claim 19, wherein one metric maintained for each terminal is related to an average throughput obtained by the terminal.

21. The method of claim 1, wherein the priority for each terminal is further determined and related to a quality of service based on one or more factors maintained for the terminal.

22. The method of claim 1, wherein one or more terminals in the selected set are scheduled for data transmission on a channel comprising a plurality of spatial subchannels.

23. The method of claim 1, wherein one or more terminals in the selected set are scheduled for data transmission on a channel comprising a plurality of frequency subchannels.

24. The method of claim 1, wherein a set of terminals is formed, and wherein the terminals in the set are selected according to the priorities of all terminals to be considered for scheduling.

25. An apparatus for scheduling data transmissions to a plurality of terminals in a wireless communication system, comprising:

means for forming sets of one or more terminals for possible data transmission, wherein each set is selected from all active terminals to be considered for scheduling and comprises a unique combination of one or more terminals and corresponds to the set to be evaluated;

means for forming one or more transmit antenna and terminal pairs for each set, wherein each transmit antenna and terminal pair corresponds to a particular assignment of a plurality of transmit antennas to one or more terminals in the set;

means for evaluating performance of each transmit antenna and terminal pairing;

means for selecting one of a plurality of evaluated transmit antenna and terminal pairs based on the evaluated performance of the transmit antenna and terminal pairs;

means for transmitting data from one or more transmit antennas assigned to the terminal to each scheduled terminal in the selected transmit antenna and terminal pair; and

means for prioritizing terminals to be considered for scheduling.

26. A base station in a multiple-input multiple-output (memo) communication system, comprising:

a transmit data processor for receiving and processing data to provide a plurality of data streams for transmission to one or more terminals scheduled for data transmission, wherein the data is processed in accordance with channel state information indicative of channel estimates for the one or more scheduled terminals;

a plurality of modulators for processing a plurality of data streams to provide a plurality of modulated signals;

a plurality of transmit antennas configured to receive and transmit a plurality of modulated signals to one or more scheduled terminals; and

a scheduler to: forming one or more sets of terminals for possible data transmission, wherein each set is selected from all active terminals to be considered for scheduling and comprises a unique combination of one or more terminals and corresponds to the set to be evaluated; forming one or more transmit antenna and terminal pairs for each set, wherein each transmit antenna and terminal pair corresponds to a particular assignment of a plurality of transmit antennas to one or more terminals in the set; evaluating the performance of each transmitting antenna and terminal pair; selecting one of a plurality of evaluated transmit antenna and terminal pairs based on the evaluated performance of the transmit antenna and terminal pairs; transmitting data from one or more transmit antennas assigned to the terminal to each scheduled terminal in the selected transmit antenna and terminal pair; and prioritizing terminals to be considered for scheduling.

27. The base station of claim 26, wherein the data streams for each transmit antenna are processed according to coding and modulation schemes selected for the transmit antennas by channel state information associated with the transmit antennas.

28. The base station of claim 26, further comprising:

a plurality of demodulators for processing the plurality of signals received via the plurality of transmit antennas to provide a plurality of received signals, an

A receive data processor for further processing the plurality of received signals to derive channel state information for a plurality of terminals in the communication system.

29. A multiple-input multiple-output communication system, comprising:

the base station of claim 26; and

one or more terminals, each terminal comprising:

a plurality of receiving antennas, each receiving antenna for receiving a plurality of modulated signals transmitted from a base station;

a plurality of front end units, each front end unit for processing signals from an associated receive antenna to provide a corresponding received signal;

a receive processor for processing a plurality of received signals from a plurality of front end units to provide one or more decoded data streams and further to derive channel state information for the plurality of modulated signals, an

A transmit data processor for processing the channel state information to be transmitted back to the base station.

30. The multiple-input multiple-output communication system according to claim 29, wherein the terminals are one or more terminals included in sets scheduled to receive data transmissions from the base station within a particular time interval, and wherein the set of one or more terminals scheduled to receive data transmissions is selected based at least in part on channel state information received from the one or more terminals in the sets.

31. The multiple-input multiple-output communication system according to claim 30, wherein the terminal is scheduled to receive data transmissions from one or more transmit antennas allocated to the terminal at the base station.