HK1124446A - Pilots for mimo communication systems - Google Patents

Description

Pilot for MIMO communication systems

This application is PCT international application number: PCT/US2003/034520, PCT International filing date: 24/10/2003, chinese national application number: 200380104907.2, entitled "pilots for MIMO communication systems".

The present application claims priority from united states provisional applications No. 60/421,309, No. 60/438,462, No. 60/421,428 and No. 60/438,601, the first three application filing dates all being 10/25/2002 and the last application filing date being 1/7/2003, entitled "MIMO WLAN SYSTEM", "Channel Calibration for a Time division multiplexed Communication System", "Channel Calibration and spatial processing for TDD MIMO Systems" and "Pilots for MIMO Communication Systems", respectively, in that order, all assigned to the assignee of the present invention and incorporated herein by reference in their entirety.

Background

FIELD

The present invention relates generally to data communications, and more particularly to pilots suitable for use in multiple-input multiple-output (MIMO) communication systems.

Background

MIMO systems using multiple (N)_T) Transmitting antenna and a plurality of (N)_R) The receive antennas are used for data transmission. From N_TA transmitting antenna and N_RThe MIMO channel formed by the receiving antennas can be decomposed into N_SA separate channel of which N_S≤min{N_T，N_R}。N_SEach of the individual channels corresponds to a dimension. The MIMO channel may provide improved performance (e.g., increased transmission) if additional dimensionalities are established by the multiple transmit and receive antennasCapacity for transmission and/or greater reliability).

In a wireless communication system, data to be transmitted is first modulated onto a Radio Frequency (RF) carrier signal to generate an RF modulated signal, which is more suitable for transmission over a wireless channel. For MIMO systems, up to N may be generated_TA RF modulated signal, and can be simultaneously modulated from N_TThe transmit antennas are transmitted. The transmitted RF modulated signal may reach N through multiple propagation paths within the wireless channel_RA receiving antenna. The propagation path characteristics typically change over time due to a number of factors, such as fading, multipath, and external interference. Thus, the transmitted RF modulated signal may experience different channel conditions (e.g., different fading and multipath effects) and may be associated with different complex gains and signal-to-noise ratios (SNRs).

To achieve higher performance, it is often necessary to describe the wireless channel response. For example, the transmitter may require a channel response to implement spatial processing (described below) to transmit data to the receiver. The receiver may also need the channel response to perform spatial processing on the received signal to recover the transmitted data.

In many wireless communication systems, the pilot transmitted by the transmitter assists the receiver in performing multiple functions. The pilot is typically generated based on known symbols and implemented in a known manner. The pilot may be used by the receiver for channel estimation, timing and frequency acquisition, data demodulation, and so on.

There are several challenges in the design of the pilot structure for MIMO. As one consideration, the pilot structure needs to account for the additional dimensionalities created by the multiple transmit and multiple receive antennas. As another consideration, since pilot transmission represents overhead within a MIMO system, it is desirable to minimize pilot transmission as much as possible. Moreover, if the MIMO system is a multiple access system that supports communication with multiple users, the pilot structure design needs to be such that pilots supporting multiple users do not consume a large portion of the available system resources.

There is therefore a need in the art for a MIMO system pilot technique that addresses the above considerations.

SUMMARY

Pilots suitable for use in MIMO systems are provided herein. These pilots may support various functions such as timing and frequency acquisition, channel estimation, calibration, etc., as needed for proper system operation. Different types of pilots designed and used for different functions may also be considered.

The various types of pilots may include: beacon pilots, MIMO pilots, steered reference or steered pilots, and carrier pilots. The beacon pilot is transmitted from all transmit antennas and may be used for timing and frequency acquisition. The MIMO pilot is also transmitted from all transmit antennas but covered with different orthogonal codes assigned to the transmit antennas. The MIMO pilot may also be used for channel estimation. The steered reference is transmitted on a particular eigenmode of the MIMO channel and is user terminal specific. The steered reference may be used for channel estimation and possibly for rate control. The carrier pilot may also be transmitted on certain designated subbands/antennas and may be used for phase tracking of the carrier signal.

The respective pilot transmission schemes may be designed based on a combination of these different pilot types. For example, on the downlink, the access point may transmit a beacon pilot, a MIMO pilot, and a carrier pilot for all user terminals within its coverage area, and optionally a steering reference to any active user terminals receiving downlink transmissions from the access point. On the uplink, a user terminal may send a MIMO pilot for calibration and may send a steered reference and a carrier pilot when scheduled (e.g., for downlink and/or uplink data transmissions). The process of transmitting and receiving these types of pilots is described in detail below.

Various aspects and embodiments of the invention are 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 illustrates a multiple access MIMO system;

FIG. 2 illustrates an example frame structure for data transmission within a TDD MIMO-OFDM system;

FIG. 3 illustrates downlink and uplink pilot transmission for an example pilot transmission scheme;

fig. 4 shows a block diagram of an access point and a user terminal;

fig. 5 shows a block diagram of a TX spatial processor that may generate a beacon pilot;

FIG. 6A shows a block diagram of a TX spatial processor that can generate a MIMO pilot;

FIG. 6B shows a block diagram of an RX spatial processor that provides channel response estimates based on received MIMO pilots;

FIG. 7A shows a block diagram of a TX spatial processor that can generate a steering reference; and

fig. 7B shows a block diagram of an RF spatial processor that provides a channel response estimate based on a received steered reference.

Detailed Description

The word "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

Fig. 1 illustrates a multiple-access MIMO system 100 that supports multiple users and is capable of implementing the pilots described herein. MIMO system 100 includes a plurality of Access Points (APs) 110 that support communication for a plurality of User Terminals (UTs) 120. For simplicity, only two access points 110a and 110b are shown in fig. 1. An access point is generally a fixed station used for communicating with user terminals. An access point may also be referred to as a base station or using some other terminology.

The user terminals 120 may be dispersed throughout the system. Each user terminal may be a fixed or mobile terminal in communication with the access point. A user terminal may also be called an access terminal, a mobile station, a remote station, User Equipment (UE), a wireless device, or some other terminology. Each user terminal may communicate with one or possibly more access points on the downlink and/or uplink at any given moment. The downlink (i.e., forward link) refers to transmission from the access point to the user terminal, and the uplink (i.e., reverse link) refers to transmission from the user terminal to the access point. As used herein, an "active" user terminal is a terminal that receives downlink transmissions from an access point and/or sends uplink transmissions to the access point.

Within fig. 1, access point 110a communicates with user terminals 120 a-120 f, and access point 110b communicates with user terminals 120 f-120 k. The assignment of user terminals to access points is generally based on received signal strength rather than distance. At any given moment, a user terminal may receive downlink transmissions from one or more access points. System controller 130 is coupled to access points 110 and may be designed to perform a number of functions, such as (1) coordinating and controlling the access points coupled thereto, (2) routing data among the access points, and (3) accessing and controlling communication with user terminals served by the access points.

I. Pilot frequency

Pilots suitable for use in MIMO systems, such as that shown in fig. 1, are provided herein. These pilots may support various functions that may be required for proper system operation, such as timing and frequency acquisition, channel estimation, calibration, and so forth. The pilots may be considered to be of different types designed and used for different functions. Table 1 lists four types of pilots and a short description of an example pilot design. Fewer, different, and/or additional pilot types may also be defined and are within the scope of the present invention.

Table 1-pilot types

Pilot frequency type	Description of the invention
		Beacon pilot	Pilots transmitted from all transmit antennas and used for timing and frequency acquisition
MIMO pilot	Pilot frequency with different orthogonal codes sent from all transmitting antennas and used for channel estimation
		Steering reference or steering pilot	Pilot transmitted for a particular user terminal on a particular eigenmode on a MIMO channel and used for channel estimation and possibly rate control
Carrier pilot	Pilot frequency for carrier signal phase tracking

Steered reference and steered pilot are synonyms.

Various pilot transmission schemes may be designed based on combinations of these different pilot types. For example, on the downlink, the access point may transmit a beacon pilot, a MIMO pilot, and a carrier pilot for all user terminals within its coverage area, and may optionally transmit a steering reference to any active user terminal receiving downlink transmissions from the access point. On the uplink, a user terminal may send a MIMO pilot for calibration and may send a steered reference and a carrier pilot when scheduled (e.g., for downlink and/or uplink data transmissions). The process of transmitting and receiving these various types of pilots is described in detail below.

The pilots described herein may be used for various types of MIMO systems. For example, pilots may be used for (1) single carrier MIMO systems, (2) multicarrier MIMO systems, which may use Orthogonal Frequency Division Multiplexing (OFDM) or some other multicarrier modulation techniques, (3) MIMO systems implementing multiple access techniques, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and code division multiple access (cdma), (4) MIMO systems implementing Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), and/or Code Division Multiplexing (CDM) for data transmission, (5) MIMO systems implementing Time Division Duplexing (TDD), Frequency Division Duplexing (FDD), and/or Code Division Duplexing (CDD) for data transmission, and (6) other types of MIMO systems. For clarity, the pilots for a MIMO system implementing OFDM (i.e., a MIMO-OFDM system) are first described below, followed by the pilots for a TDD MIMO-OFDM system.

OFDM effectively partitions the overall system bandwidth into multiple (N)_F) Orthogonal sub-bands, which are also referred to as tones, frequency bins, or frequency sub-channels. For OFDM, each subband is associated with a respective subcarrier upon which data is modulated. For a MIMO-OFDM system, each subband may be associated with multiple eigenmodes, and each eigenmode of each subband may be viewed as an independent transmission channel.

For clarity, specific pilot structures are described below for an example MIMO-OFDM system. In the MIMO-OFDM system, the system bandwidth is divided into 64 orthogonal subbands (i.e., N)_F64) that are assigned indices of-32 to + 31. Of these 64 subbands, 48 subbands (e.g., with indices of ± {1.. 6, 8.. 20, 22.. 26}) may be used for data transmission, 4 subbands (e.g., with indices of ± {7, 21} may be used for carrier pilot and possibly signaling, the DC subband (with index of 0) is not used, and the remaining subbands are not used, as guard subbandsThe remaining 12 subbands are unused. This OFDM subband structure is described in further detail in the aforementioned provisional U.S. application No. 60/421, 309. Different numbers of subbands and other OFDM subband structures may also be implemented for MIMO-OFDM systems and are within the scope of the invention.

For OFDM, the data transmitted on each available subband is first modulated (i.e., symbol mapped) using a particular modulation scheme (e.g., BPSK, QPSK, or M-QAM) selected for that subband. One modulation symbol may be transmitted on each available subband in each symbol period. Each modulation symbol is a complex value corresponding to a particular point within the signal constellation for the selected modulation scheme. Signal values of zero may be transmitted on the unused subbands. For each OFDM symbol period, modulation symbols for the usable subbands and zero signal values for the unused subbands (i.e., for all N_FModulation symbols and zeros for each subband) are transformed using an Inverse Fast Fourier Transform (IFFT) to obtain a constellation comprising N_FA converted symbol adapted for sampling. To combat intersymbol interference (ISI), a portion of each converted symbol is often repeated (which is also referred to as adding a cyclic prefix) to form a corresponding OFDM symbol, which is then transmitted over the wireless channel. An OFDM symbol period, also referred to herein as a symbol period, corresponds to the duration of one OFDM symbol.

1. Beacon pilot

The beacon pilot comprises a pilot from N_TA particular set of pilot symbols for each transmission of the transmit antennas. The same set of pilot symbols at N designated for beacon pilot transmission_BTransmitted over one symbol period. General formula N_BAnd may be any integer value of one or more.

In the exemplary embodiment, the set of pilot symbols for the beacon pilot is a set of 12BPSK modulation symbols for 12 particular subbands, which is referred to as a "B" OFDM symbol. The 12BPSK modulation symbols for the B OFDM symbol are given in table 2. Zero signal values are transmitted on the remaining 52 unused subbands.

TABLE 2 Pilot symbols

Sub-band index	Beacon pilot b (k)	MIMO Pilot p (k)	Sub-band index	Beacon pilot b (k)	MIMO Pilot p (k)	Sub-band index	Beacon pilot b (k)	MIMO Pilot p (k)	Sub-band index	Beacon pilot b (k)	MIMO Pilot p (k)
													0	0	-13	0	1-j	1	0	1-j	15	0	1+j
-26	0	-1-j	-12	-1-j	1-j	2	0	-1-j	16	1+j	-1+j
												-25	0	-1+j	-11	0	-1-j	3	0	-1-j	17	0	-1+j
-24	1+j	-1+j	-10	0	-1-j	4	-1-j	-1-j	18	0	1-j
												-23	0	-1+j	-9	0	1-j	5	0	-1+j	19	0	1+j
-22	0	1-j	-8	-1-j	-1-j	6	0	1+j	20	1+j	-1+j
												-21	0	1-j	-7	0	1+j	7	0	-1-j	21	0	1+j
-20	-1-j	1+j	-6	0	-1+j	8	-1-j	-1+j	22	0	-1+j
												-19	0	-1-j	-5	0	-1-j	9	0	-1-j	23	0	1+j
-18	0	-1+j	-4	1+j	-1+j	10	0	1+j	24	1+j	-1+j
												-17	0	1+j	-3	0	-1+j	11	0	1-j	25	0	1-j
-16	1+j	-1+j	-2	0	1-j	12	1+j	-1+j	26	0	-1-j
												-15	0	1-j	-1	0	-1+j	13	0	-1-j		0	0
-14	0	1+j	0	0	0	14	0	0

For the example embodiment shown in Table 2, for the beacon pilot, BPSK modulation symbols (1+ j) are transmitted in subbands-24, -16, -4, 12, 16, 20, and 24, and BPSK modulation symbols- (1+ j) are transmitted on subbands-20, -12, -8, -4, and 8. A signal value of zero is transmitted on the remaining 52 subbands of the beacon pilot.

The B OFDM symbols are designed to facilitate user terminal system timing and frequency acquisition. For the B OFDM example embodiment described above, only 12 out of 64 total subbands are used, and these subbands are spaced apart by four subbands. These 4 subband spacings allow the user terminal to have an initial frequency error of up to two subbands. The beacon pilot allows the user terminal to correct its initial coarse frequency error and to correct its frequency so that the phase drift over the beacon pilot duration is small (e.g., less than 45 degrees over the beacon pilot duration at a sampling rate of 20 MHz). If the beacon pilot duration is 8 musec, the 45 degree (or less) phase drift over 8 musec is equal to 360 degrees over 64 musec, which is approximately 16 kHz.

The 16kHz frequency error is typically too large for operation. Additional frequency correction may be obtained using the MIMO pilot and the carrier pilot. These pilots cover a duration long enough so that the user terminal frequency can be corrected to within a desired target (e.g., 250 Hz). For example, if the TDD frame is 2 milliseconds (as described below) and if the user terminal frequency is accurate to within 250Hz, there will be less than a half cycle of phase change over one TDD frame. The phase difference between the TDD frames of the beacon pilot can be used to frequency lock the user terminal to the clock of the access point, effectively reducing the frequency error to zero.

In general, the set of pilot symbols for the beacon pilot may be derived using any modulation scheme. Thus, other OFDM symbols using BPSK or some other modulation scheme for the beacon pilot may also be used and are within the scope of the invention.

In the example design, four transmit antennas may be used for beacon pilot transmission. Table 4 lists the OFDM symbols to be transmitted from each of the four transmit antennas for a beacon pilot transmission covering two symbol periods.

Table 3-beacon pilots

Symbol period	Antenna 1	Antenna 2	Antenna 3	Antenna 4
					1	B	B	B	B
2	B	B	B	B

MIMO Pilot

MIMO pilots comprise from N_TA particular set of pilot symbols for each transmission of the transmit antennas. N assigned for MIMO pilot transmission for each transmit antenna_PThe same set of pilot symbols is transmitted for each symbol period. However, the set of pilot symbols for each transmit antenna is "covered" with a unique orthogonal sequence or code assigned to that antenna. Covering is the process by which a given pilot or data symbol (or set of L pilot/data symbols with the same value) to be transmitted is multiplied by all L chips of an L-chip orthogonal sequence to obtain L covered symbols, which are then transmitted. Decovering is the complementary process of covering, i.e. multiplying the received symbols by the same orthogonal sequence of L chipsThe L chips of the column to obtain L decovered symbols, which are then accumulated to obtain the transmitted pilot or data symbol estimates. Coverage acquisition from N_TN of transmitting antenna_TOrthogonality among the pilot transmissions and allow the receiver to distinguish between the individual transmit antennas, as described below. The duration of the MIMO pilot transmission depends on its use, as described below. General formula N_PAnd may be any integer value of one or more.

One set or different sets of pilot symbols may be used for N_TA transmitting antenna. In an example embodiment, one set of pilot symbols is used for all N as MIMO pilots_TOne transmit antenna and the set includes 52 QPSK modulation symbols for the 52 usable subbands, which is referred to as a "P" OFDM symbol. The 52 QPSK modulation symbols for the P OFDM symbol are given in table 2. Signal values of zero are transmitted on the remaining 12 unused subbands.

The 52 QPSK modulation symbols form a unique "word" that is used to facilitate channel estimation for the user terminal. The unique word is selected to have a minimum peak-to-average variation in the waveform generated based on the 52 modulation symbols.

It is well known that OFDM is generally associated with higher peak-to-average variations than transmit waveforms of other modulation techniques (e.g., CDMA). Thus, to avoid circuit clipping (e.g., power amplifiers) on the transmit chain, the OFDM symbols are typically transmitted at a reduced power level, i.e., backed off from the peak transmit power level. The backoff is used to account for waveform variations of the OFDM symbol. By minimizing the peak-to-average variation in the waveform of the P OFDM symbol, the MIMO pilot may be transmitted at a higher power level (i.e., with less back-off applied to the MIMO pilot). The higher transmit power of the MIMO pilot may then result in improved received signal quality of the MIMO pilot at the receiver. Smaller peak-to-average variations also reduce the amount of distortion and non-linearity generated by circuitry in the transmit and receive chains. These various factors may result in improved accuracy of the channel estimates obtained based on the MIMO pilots.

The OFDM symbol with the minimum peak-to-average variation can be obtained in various ways. For example, a random search may be conducted with a large number of pilot symbol sets randomly formed and evaluated to find the set with the smallest peak-to-average variation. The table of P OFDM symbols shown in table 2 may be used for example OFDM symbols for MIMO pilots. In general, the set of pilot symbols for the MIMO pilot may be derived using any modulation scheme. Thus, various OFDM symbols derived using QPSK or some other modulation scheme may also be used for MIMO pilot, and this is within the scope of the invention.

Various orthogonal codes may be used to cover in N_TP OFDM symbols transmitted on the transmit antennas. Examples of such orthogonal codes include Walsh codes and Orthogonal Variable Spreading Factor (OVSF) codes. Pseudo-random and quasi-orthogonal codes may also be used to cover the P OFDM symbols. An example of a pseudo-random orthogonal code is an M-sequence, well known in the art. An example of quasi-orthogonal codes IS the quasi-orthogonal function (QOF) defined by IS-2000. In general, various types of codes may be used for the covering, some of which are described above. For simplicity, the term "orthogonal code" is used herein to refer to any type of code suitable for covering pilot symbols. The orthogonal code length (L) is selected to be greater than or equal to the number of transmit antennas (e.g., L ≧ N)_T) And L orthogonal codes are available. Each transmit antenna is assigned a unique orthogonal code. At N from each transmit antenna_PN transmitted in one symbol period_PThe P OFDM symbols are covered with orthogonal codes assigned to the transmit antennas.

In an exemplary embodiment, four transmit antennas are available and a 4 chip Walsh sequence, W, is assigned to the MIMO pilot₁＝1111，W₂＝1010，W₃＝1100，W₄1001. For a given Walsh sequence, a value of "1" indicates that a P OFDM symbol is transmitted, and a value of "0" indicates that a P OFDM symbol is transmitted. For a P OFDM symbol, each of the 52 QPSK modulation symbols within the P OFDM symbol is inverted (i.e., multiplied by-1). The result of each transmit antenna coverage is a sequence of P OFDM symbols covered by that transmit antenna. The covering is actually done separately for each subband to generate a sequence of covered pilot symbols for that subband. The sequence of covered pilot symbols for all subbands forms a covered P OFDM symbol sequenceAnd (4) columns.

Table 4 lists the OFDM symbols to be transmitted from each of the four transmit antennas for MIMO pilot transmission spanning four symbol periods.

TABLE 4-MIMO Pilot

Symbol period	Antenna 1	Antenna 2	Antenna 3	Antenna 4
					1	+P	+P	+P	+P
2	+P	-P	+P	-P
					3	+P	+P	-P	-P
4	+P	-P	-P	+P

For this set of 4 chip Walsh sequences, MIMO pilot transmission can occur over an integer multiple of four symbol periods to ensure orthogonality among the four pilot transmissions from the four transmit antennas. The Walsh sequence is simply repeated for MIMO pilot transmissions longer than the Walsh sequence length.

For a subband index K ∈ K, where K ± {1.. 26} for the example subband structure described above, a channel response matrix may be used for a wireless channel of a MIMO-OFDM systemH(k) And (4) describing a set. Each subband matrixH(k) Comprising N_TN_RValue h_i，j(k) N, where i e {1.. N_RN and j e.g. {1.. N_TIn which h is_i，j(k) Indicating the channel gain between the jth transmit antenna and the ith receive antenna.

The MIMO pilot may be used by the receiver to estimate the response of the wireless channel. In particular, to recover the pilot transmitted from transmit antenna j and recovered by receive antenna i, the OFDM symbol received on antenna i is first multiplied by the Walsh sequence assigned to transmit antenna j. Then all N of the MIMO pilots are accumulated_P"decover" OFDM symbols for one symbol period, where accumulation can be achieved separately for each of the 52 available subbands. Accumulation may also be achieved in the time domain for the received OFDM symbols (after removing the cyclic prefix for each OFDM symbol). Accumulation is also performed on a per sample basis over multiple received OFDM symbols, where the samples for each OFDM symbol correspond to different subbands if accumulation is performed after FFT and different time indices if accumulation is performed before FFT. The result of the accumulation isWhere K e K are the channel response estimates from transmit antenna j to receive antenna i for the 52 usable subbands. The same process can be implemented to estimate the channel response from each transmit antenna to each receive antenna. Pilot processing provides N for each subband_TN_RA plurality of complex values, wherein a complex value is a channel response estimation matrix for the subbandOf (2) is used.

The pilot processing described above may be implemented by an access point to obtain an uplink channel response estimateAnd may also be implemented by a user terminal to obtain a channel response estimate for the downlink

3. Steering reference or steering pilot

For MIMO-OFDM systems, the channel response matrix for each sub-bandH(k) May be "diagonalized" to obtain N for that subband_SAn eigenmode, wherein N_S≤min{N_T，N_R}. This may be done by responding to the channel response matrixH(k) Implementing singular value decomposition or pairingH(k) Is implemented by performing eigenvalue decomposition on the correlation matrix ofR(k)＝H ^H(k)H(k) In that respect For clarity, singular value decomposition is used for the following description.

Channel response matrixH(k) The singular value decomposition of (a) may be expressed as:

H(k)＝U(k)∑(k)V ^H(k)，k∈K，(1)

whereinU(k) Is thatH(k) Of the left eigenvector of (N)_R×N_R) A unitary matrix;

∑(k) is thatH(k) Of singular values of (N)_R×N_T) A diagonal matrix;

V(k) is thatH(k) Of the right eigenvector of (N)_T×N_T) A unitary matrix; and

“^H"denotes conjugate transpose.

Unitary matrixMBy usingM ^H M＝IIn whichIIs a unit array.

Singular value decomposition is further described in detail in Gilbert Strang, second edition entitled "Linear Algebra and ItsApplications," Academic Press 1980. Eigenmodes generally refer to theoretical constructions. A MIMO channel may also be viewed as comprising N available for data/pilot transmission_SA spatial channel. Each spatial channel may or may not correspond to an eigenmode depending on whether the spatial processing at the transmitter successfully diagonalizes the MIMO channel. For example, if the transmitter does not know or has only an imperfect estimate of the MIMO channel, the data streams are transmitted on the spatial channels (and not the eigenmodes) of the MIMO channel. For simplicity, the term "eigenmode" is used herein to refer to the case where a diagonal MIMO channel is attempted, although it may not be completely successful due to imperfect channel estimation.

Diagonal matrix for each subband∑(k) Containing non-negative real values along the diagonal, with zeros remaining. These diagonal terms are calledH(k) And represents an independent channel (or eigenmode) of the MIMO channel for the k-th subband.

The eigen-decomposition may be a channel response matrix for each of the 52 usable subbandsH(k) Implemented independently to determine N for that subband_SAn eigenmode. Each diagonal matrix∑(k) May be ordered such thatWherein sigma₁(k) Is the largest singular value, σ₂(k) Is the second largest singular value, etc., and σ_NS(k) Is the smallest singular value of the k-th subband. When for each diagonal matrix∑(k) When ordering singular values of, the correlation matrixU(k) AndV(k) are ordered accordingly. After sorting, σ₁(k) The singular values representing the best eigenmode of subband k are also referred to as the "dominant" eigenmodes.

A "wideband" eigenmode may be defined as a set of eigenmodes of the same order for all subbands after ordering. Thus, the mth wideband eigenmode includes the mth eigenmode of all subbands. The eigenmodes of each wideband are associated with a respective set of eigenvectors for all subbands. The "dominant" eigenmodes are associated with each matrix of each subband after orderingThe eigenmodes associated with the largest singular value in the inner.

Matrix arrayV(k) Including N that may be used for spatial processing at the transmitter_TAn eigenvector, whereinAnd isv _m(k) Is thatV(k) Column m of (1), whereinV(k) Is the eigenvector of the mth eigenmode. For unitary matrices, the eigenvectors are orthogonal to each other. Eigenvectors are also referred to as "steering" vectors.

The steered reference (i.e., steered pilot) includes one or more slave N_TA set of pilot symbols transmitted by the transmit antennas. In one embodiment, a set of pilot symbols is transmitted on a set of subbands of a wideband eigenmode in a given symbol period by performing spatial processing with a set of steering vectors for the wideband eigenmode. In another embodiment, multiple sets of pilot symbols are transmitted on multiple disjoint sets of subbands for multiple wideband eigenmodes in a given symbol period by performing spatial processing with multiple sets of steering vectors for the wideband eigenmodes (using subband multiplexing, described below).For clarity, the following description assumes that one set of pilot symbols is transmitted on one wideband eigenmode in a given symbol period (i.e., no subband multiplexing).

In an embodiment, the set of pilot symbols that manipulate the reference is the same as the P OFDM symbols used for the MIMO pilots. However, various other OFDM symbols may also be used to manipulate the reference, and this is within the scope of the invention.

The steering reference for the transmission of the mth wideband eigenmode (using beamforming, which can be described below) can be expressed as:

x _m(k)＝v _m(k)·p(k)，k∈K，(2)

whereinx _m(k) Is the m-th eigenmode of the k-th subband (N)_TX 1) transmit vector;

v _m(k) a steering vector of an mth eigenmode of a kth subband; and

p (k) is the pilot symbol for the kth subband (e.g., as given in table 2).

(Vector)x _m(k) Including N from the k-th sub-band_TN transmitted by transmitting antenna_TAnd a transmit symbol.

The steering reference may be used by the receiver to estimate a vector that may be used for spatial processing of data reception and transmission, as described below. The process of manipulating the fiducials is described in further detail below.

4. Carrier pilot

The example OFDM subband structure described above includes four pilot subbands with indices of-21, -7, and 21. In one embodiment, the carrier pilots are transmitted on the four pilot subbands in all symbol periods not used for some other type of pilot. The carrier pilot can be used by the receiver to track RF carrier signal phase variations and drift in the oscillators at the transmitter and receiver. This may provide improved data demodulation performance.

In one embodiment, the carrier pilots comprise four pilot sequences P_c1(n)，P_c2(n)，P_c3(n) and P_c4(n) which are transmitted on the four pilot subbands. In one embodiment, the four pilot sequences are defined as follows:

P_c1(n)＝P_c2(n)＝P_c3(n)＝-P_c4(n)，(3)

where n is the index of the pilot period (or OFDM symbol).

The pilot sequence may be defined based on the respective data sequences. In one embodiment, the pilot sequence P_c1(n) is based on the polynomial g (x) x⁷+x⁴+ x generation. Where the initial state is set to all ones and the output bits are mapped to signal values 1 * -1 and 0 * 1 as follows. Pilot sequence P_c1(n), where n ═ {1, 2.. 127}, and can be expressed as:

P_c1(n)＝{1，1，1，1，-1，-1，-1，1，-1，-1，-1，-1，1，1，-1，1，-1，-1，1，1，-1，1，1，-1，1，1，1，1，1，1，-1，1，1，1，-1，1，1，-1，-1，1，1，1，-1，1，-1，-1，-1，1，-1，1，-1，-1，1，-1，-1，1，1，1，1，1，-1，-1，1，1，-1，-1，1，-1，1，-1，1，1，-1，-1，-1，1，1，-1，-1，-1，-1，1，-1，-1，1，-1，1，1，1，1，-1，1，-1，1，-1，1，

-1，-1，-1，-1，-1，1，-1，1，1，-1，1，-1，1，1，1，-1，-1，1，-1，-1，-1，1，1，1，-1，-1，-1，-1，-1，-1，1}

pilot sequence P_c1The values "1" and "-1" within (n) may be mapped to pilot symbols using a particular modulation scheme. For example, using BPSK, "1" may be mapped to 1+ j, and "-1" may be mapped to- (1+ j). If there are more than 127 OFDM symbols, the pilot sequence may be repeated so that P_c1(n)＝P_c1(n mod 127), where n > 127.

In one embodiment, four pilot sequences P_c1(n)，P_c2(n)，P_c3(n) and P_c4(n) are transmitted on four different subband/antenna pairs. Table 5 shows the allocation of four pilot sequences to four pilot subbands and four transmit antennas.

Table 5

Sub-band	Antenna 1	Antenna 2	Antenna 3	Antenna 4
					-21	Pc1(n)	-	-	-
-7	-	Pc2(n)	-	-
					7	-	-	Pc3(n)	-
21	-	-	-	Pc4(n)

As shown in Table 5, the pilot sequence P_c1(n) is transmitted on subband-21 for antenna 1, pilot sequence P_c2(n) is transmitted on subband-7 of antenna 2, pilot sequence P_c3(n) is transmitted on subband 7 of antenna 3 and pilot sequence P_c4(n) are transmitted on sub-band 21 of antenna 4. Each pilot sequence is thus transmitted on a unique subband and a unique antenna. This carrier-pilot transmission scheme avoids the interference that would result if the pilot sequence were transmitted on multiple transmit antennas on a given subband.

In another embodiment, four pilot sequences are transmitted on the primary eigenmodes of their assigned subbands. The spatial processing of the carrier pilot symbols is similar to that of the steered reference, which can be described above and shown in equation (2). Steering vectors to transmit carrier pilots on the primary eigenmodesv ₁(k) For spatial processing. Thus, the pilot sequence P_c1Steering vector for (n)v ₁(-26) spatially processed, P_c2Steering vector for (n)v ₁(-7) spatially processed, Pilot sequence P_c3Steering vector for (n)v ₁(7) After spatial processing, the pilot sequence P_c4Steering vector for (n)v ₁(26) And (4) carrying out spatial processing.

Pilot for Single Carrier MIMO System

The pilots described herein may also be used for single carrier MIMO systems that do not use OFDM. In this case, much of the description above is still available but the subband index k is not required. For beacon pilots, the particular pilot modulation symbol b may be from N_TEach of the transmit antennas transmits. For MIMO pilots, a particular pilot modulation symbol p may be N_TCovered by orthogonal sequences and from N_TAnd transmitting by the transmitting antenna. Pilot symbol b may be the same as or different from pilot symbol p. The steering reference may be sent as shown in equation (2). However, the vector is transmittedx _mSteering vectorv _mAnd pilot symbol p is not a function of subband index k. The carrier pilots may be transmitted in a time division multiplexed manner or may simply be omitted.

For MIMO-OFDM systems, a cyclic prefix is typically used to ensure orthogonality across subbands with delay spread in the system, and an orthogonal code may identify a single transmit antenna. For single carrier MIMO systems, orthogonal codes rely on orthogonality and antenna identification. Thus, the orthogonal codes used to cover pilot symbols in a single carrier MIMO system can be selected to have good cross-correlation and peak-to-sidelobe characteristics (i.e., the correlation between any two orthogonal sequences used for coverage is small when there is delay spread in the system). The orthogonal code with good cross-correlation and peak sidelobe characteristics is the M-sequence and its time-shifted version. However, other types of codes may also be used to cover the pilot symbols of a single carrier MIMO system.

For wideband single-carrier MIMO systems, steering references may be sent in various ways to account for frequency selective fading (i.e., a frequency response that is not flat over the operating band). Several schemes for transmitting a steered reference in a wideband single carrier MIMO system are detailed below. In general, the transmitter may transmit reference waveforms that are processed in the same or similar manner as the processing used to transmit data on a particular wideband eigenmode. The receiver may correlate the received waveform with a locally generated copy of the transmitted reference waveform and extract channel information that allows the receiver to estimate a channel matched filter.

In the first schemeIn which the transmitter initially obtains steering vectors for the eigenmodesv _m(k) In that respect Steering vectorv _m(k) May be obtained by periodically transmitting OFDM pilot symbols, by frequency domain analysis of received MIMO pilots that have not been transmitted through OFDM, or by some means. For each value of k, where 1 ≦ k ≦ N_F，v _m(k) Is provided with N_TN of transmitting antenna_TN of item_T-a vector. The transmitter then pairs the steering vectorsv _m(k) N of (A)_TAnd performing inverse fast Fourier transform on the vector positions, wherein k is a frequency variable in IFFT calculation to obtain corresponding time domain pulses of the relevant transmitting antenna. Vector quantityv _m(k) Each vector position of (1) comprises N_FN of frequency sub-bands_FValue and corresponding time domain pulse is N_FA sequence of time domain values. The terminal then appends a cyclic prefix to the time domain pulse to obtain a steered reference for the transmit antenna. Generating N for each eigenmode_TA set of steering references, and can be derived from all N_TThe transmit antennas are transmitted in the same time slot. Multiple sets of pulses may be generated for multiple eigenmodes and may be transmitted in a TDD manner.

For the first scheme, the receiver samples the received signal to obtain a received vectorτ _m(n) removing the cyclic prefix and aligning the received vectorτ _m(n) performing a fast Fourier transform at each vector position to obtainH(k)v _m(k) The corresponding term of (2) is estimated. Receiving a vectorτ _mEach vector position of (N) (after cyclic prefix removal) includes N_FA time domain sample. The receiver then usesH(k)v _m(k) To synthesize a time-domain matched filter that may be used to filter the received data transmission. The time domain matched filter includes matched filtered pulses for each receive antenna. Time-Domain matched filter synthesis is described in commonly assigned U.S. patent application Ser. No. 10/017308 entitled "Time-Domain Transmit and Receive Processing with Channel Eigen-mode demodulation for MIMO Systems", filed 12/7/2001.

For the first scheme, the transmitter processing of steered references in a single carrier MIMO system is similar to that in a MIMO-OFDM system. However, other transmissions after manipulating the reference are sent on a single carrier waveform, such as described in the aforementioned U.S. patent application serial No. 10/017308. Also, the receiver uses the steered reference to synchronize the time-domain matched filter, as described above.

In a second scheme, the transmitter isolates a single multipath component of the wideband channel. This may be accomplished, for example, by searching the received MIMO pilot with a sliding correlator in a manner similar to that often used to search for multipath components in CDMA systems. The transmitter then processes the multipath components as a narrowband channel and obtains a single steering vector for each eigenmode multipath componentv _m. Also, a plurality of steering vectors may be generated for a plurality of eigenmodes of the multipath component.

Pilot structure for TDD MIMO-OFDM systems

The pilots described herein may be used for various MIMO and MIMO-OFDM systems. These pilots may be used for systems using downlink and uplink common or separate frequency bands. For clarity, an example pilot structure for an example MIMO-OFDM system is described below. For this MIMO-OFDM system, the downlink and uplink are Time Division Duplex (TDD) on a single frequency band.

Fig. 2 illustrates an embodiment of a frame structure 200 that may be used for a TDD MIMO-OFDM system. Data transmission occurs within TDD frame units, each frame spanning a particular duration (e.g., 2 milliseconds). Each TDD frame is divided into a downlink phase and an uplink phase. The downlink phase is further divided into a plurality of segments of a plurality of downlink transport channels. In the embodiment shown in fig. 2, the downlink transport channels include a Broadcast Channel (BCH), a Forward Control Channel (FCCH), and a Forward Channel (FCH). Similarly, the uplink phase is divided into multiple segments of multiple uplink transport channels. In the embodiment shown in fig. 2, the uplink transport channels include a Reverse Channel (RCH) and a Random Access Channel (RACH).

On the downlink, the BCH segment 210 is used to transmit one BCH Protocol Data Unit (PDU)212, which includes a beacon pilot portion 214, a MIMO pilot portion 216, and a BCH message portion 218. The BCH message carries system parameters for the user terminals in the system. FCCH segment 220 is used to transmit one FCCH PDU, which carries the allocation for downlink and uplink resources and carries other signaling for the user. The FCH segment 230 is used to transmit one or more FCH PDUs 232. Different types of FCH PDUs may be defined. For example, FCH PDU232 a includes a pilot portion 234a and a data packet portion 236 a. The FCH PDU232 b includes a single portion 236b of a data packet. FCH PDU232 c includes a single portion 234c of pilot.

On the uplink, the RCH segment 240 is used to transmit one or more RCH PDUs 242 on the uplink. Different types of RCH PDUs may also be defined. For example, the RCH PDU 242a includes a single portion 246a of a data packet. The RCH PDU 242b includes a pilot portion 244b and a data packet portion 246 b. The RCH PDU 242c includes a single portion 244c of pilot. RACH segment 250 is used by user terminals to access the system and send short messages on the uplink. RACH PDU 252 may be transmitted within RACH segment 250 and includes a pilot portion 254 and a message portion 256.

For the embodiment shown in FIG. 2, the beacon and MIMO pilot are transmitted on the downlink in each TDD frame of the BCH segment. The pilot may or may not be transmitted within any given FCH/RCH PDU. If the pilot is transmitted, it may occupy all or a portion of the PDU, as shown in FIG. 2. Pilots are transmitted within the RACH PDU to allow the access point to estimate the correlation vector during access. The pilot portion is also referred to as a "preamble sequence". The pilot transmitted within any given FCH/RCH PDU may be a steered reference or a MIMO pilot, depending on the purpose for which the pilot is used. The pilot transmitted within the RACH PDU is typically the steering reference, although a MIMO pilot may be transmitted instead. The carrier pilots are transmitted in pilot subbands and in portions not used for other pilots. The carrier pilots are not shown in fig. 2 for simplicity. The durations of the various portions within fig. 2 are not drawn to scale.

The frame structure and the transmission channel shown in fig. 2 are described in the above-mentioned U.S. provisional patent application No. 60/421309.

1. Calibration (calibration)

For a TDD MIMO-OFDM system with a shared frequency band, the downlink and uplink channel responses may be assumed to be reciprocal to each other. I.e. ifH(k) Representing the channel response matrix from antenna array a to antenna array B for subband k, then a reciprocal channel means that the coupling from array B to array a is byH ^T(k) Is given inH ^TTo representHThe transposing of (1). For TDD MIMO-OFDM systems, reciprocal channel characteristics can be exploited to simplify channel estimation and spatial processing at the transmitter and receiver.

However, the frequency response of the transmit and receive chains at the access point is generally different from the frequency response of the transmit and receive chains at the user terminal. "effective" downlink channel response including applicable transmit and receive chain responsesH _dn(k) And "effective" uplink channel responseH _up(k) Can be expressed as:

H _dn(k)＝R _ut(k)H(k)T _ap(k)， k∈K，(4)

H _up(k)＝R _ap(k)H ^T(k)T _ut(k)，k∈K，

whereinT _ap(k) AndR _ap(k) frequency response N, which is the response of the transmit and receive chains at the access point for subband k_ap×N_apA diagonal matrix;

T _ut(k) andR _ut(k) is the sub-band k at the user terminal, the transmit chain and the receive chain correspond to N_ut×N_utA diagonal matrix;

N_apis the number of antennas at the access point; and

N_utis the number of antennas at the user terminal.

Combining the equations within the set of equations (4) yields the following results:

H _up(k)K _ut(k)＝(H _dn(k)K _ap(k))^T，k∈K，(5)

whereinK _ut(k)＝T _ut ^-1(k)R _ut(k) And isK _ap(k)＝T _ap ^-1(k)R _ap(k) In that respect Because of the fact thatT _ut(k)，R _ut(k)，T _ap(k) AndR _ap(k) is a diagonal matrix of the angles,K _ut(k) andK _ap(k) also a diagonal matrix.

Calibration can be implemented to obtain the actual diagonal matrixK _ap(k) AndK _ut(k) is estimated byAndwhere K ∈ K. Matrix arrayAndincluding a correction factor that takes into account the difference between the frequency responses of the transmit/receive chains at the access point and the user terminal. "calibrated" downlink channel response observed by a user terminalH _cdn(k) And the "calibrated" uplink channel response observed by the access pointH _cup(k) Can be expressed as:

k∈K，(6a)

k∈K，(6b)

k∈K.(6c)

the accuracy of the relationship in equation (6c) depends on the correlation matrixAndin turn, depends on the effective downlink and uplink channel responses used to derive the correction matricesAndthe estimated quality of (2). Correction vectorMay be defined to include onlyN of (A)_utA diagonal element, and correct the vectorMay be defined to include onlyN of (A)_apA diagonal element. Calibration is described in detail in the aforementioned U.S. patent application serial No. 60/421462.

The pilots described herein may also be used for MIMO and MIMO-OFDM systems that do not implement calibration. For clarity, the following description assumes that the calibration and correction matrix is implementedAndfor use in the transmit path at the access point and user terminal, respectively.

2. Beacon and MIMO pilot

As shown in fig. 2, the beacon pilot and the MIMO pilot are transmitted on the downlink within the BCH of each TDD frame. The beacon pilot may be used by the user terminal for timing and frequency acquisition. The MIMO pilot may be used by the user terminal to (1) obtain downlink MIMO channel estimates, (2) derive steering vectors for uplink transmissions, and (3) derive matched filters for downlink transmissions, as described below.

In an example pilot transmission scheme, the beacon pilot is transmitted in two symbol periods, and the MIMO pilot is transmitted in eight symbol periods after the start of the BCH segment. Table 6 shows the beacons and MIMO pilots for this example scheme.

Beacon and MIMO pilots of Table 6-BCH

Pilot frequency type	Symbol period	Antenna 1	Antenna 2	Antenna 3	Antenna 4
						Beacon pilot	1	B	B	B	B
MIMO pilot	2	B	B	B	B
						3	+P	+P	+P	+P
	4	+P	-P	+P	-P
						5	+P	+P	-P	-P
	6	+P	-P	-P	+P
						7	+P	+P	+P	+P

	8	+P	-P	+P	-P
						9	+P	+P	-P	-P
	10	+P	-P	-P	+P

The beacon pilot transmitted on the downlink may be represented as:

k∈K，(7)

whereinx _dn，bp(k) Is the transmit vector for subband k of the beacon pilot; and

b (k) is the pilot symbol sent by the beacon pilot on subband k, which is given in table 2. The beacon pilot is corrected by the correction vector as shown in equation (7)Scaled but not subjected to any other spatial processing.

The MIMO pilot transmitted on the downlink may be represented as:

k∈K，(8)

whereinx _dn，mp，n(k) Is the subband k of the downlink MIMO pilot within a symbol period N (N)_apX 1) transmit vector;

w _dn，nis the N of the downlink MIMO pilot at the access point within a symbol period N_apWith N transmitting antennas_apOf Walsh chips (N)_apX 1) a vector; and

p (k) is the pilot symbol for the MIMO pilot sent on subband k, which is given in Table 2.

As shown in equation (8), the MIMO pilot consists of vectorsw _dn，nIs covered and further corrected by a correction matrixScaled but not subjected to any other spatial processing. Identical Walsh vectorsw _dn，nFor all sub-bands, and hencew _dn，nNot a function of the subband index k. However, since each Walsh sequence is a unique sequence of 4 Walsh chips for a 4 symbol period,w _dn，nis a function of the symbol period n. Vector quantityw _dn，nThus including N for the access point symbol period N_apN of transmitting antenna_apA number of Walsh chips. For in table 6The scheme shown, four vectors for the first four symbol periods of MIMO pilot transmission on BCHw _dn，nWherein n ═ {3, 4, 5, 6} isw ₃＝[1111]，w ₄＝[1-11-1]，w ₅＝[11-1-1]Andw ₆＝[1-1-11]and four vectors repeated for the following four symbol periodsw _dn，n(where n is {7, 8, 9, 10}) is such thatw ₇＝w ₃，w ₈＝w ₄，w ₉＝w ₅Andw ₁₀＝w ₆。

the MIMO pilot transmitted on the uplink may be represented as:

k∈K，(9)

whereinx _up，mp，n(k) Is the subband k of the uplink MIMO pilot within a chip period N (N)_utX 1) transmit vector. Walsh vectors for uplink MIMO pilotsw _up，nWalsh vectors that can be used with uplink MIMO pilotsw _dn，nThe same or different. Example (b)E.g. if the user terminal is equipped with only two transmit antennas, thenw _up，nIncluding two Walsh sequences of length 2 or greater.

3. Spatial processing

As described above, the channel response matrix for each subband may be diagonalized to obtain N for that subband_SAn eigenmode. Calibrated uplink channel response matrixH _cup(k) The singular value decomposition of (a) may be expressed as:

k∈K，(10)

whereinU _ap(k) Is thatH _cup(k) Of the left eigenvector of (N)_ut×N_ut) A unitary matrix;

∑(k) is thatH _cup(k) Of singular values of (N)_ut×N_ap) A diagonal matrix; and

V _ut(k) is thatH _cup(k) Of the right eigenvector (N)_ap×N_ap) A unitary matrix.

Similarly, the calibrated downlink channel response matrixH _cdn(k) The singular value decomposition of (a) may be expressed as:

k∈K，(11)

wherein the matrixV _ut ^*(k) AndU _ap ^*(k) are respectivelyH _cdn(k) A unitary matrix of left and right eigenvectors.

As shown in equations (10) and (11), and based on the description above, the left and right eigenvector matrices for one link are the complex conjugates of the right and left eigenvector matrices, respectively, for the other links. For brevity, the matrix within the following description is addressedU _ap(k) AndV _ut(k) reference may be made to various other forms thereof (e.g., theV _ut(k) Can meanV _ut(k)，V _ut ^*(k)，V _ut ^T(k) AndV _ut ^H(k) ). Matrix arrayU _ap(k) AndV _ut(k) can be used for spatial processing by the access point and the user terminal accordingly, and as indicated by their subscripts.

In an embodiment, the user terminal may estimate the calibrated downlink channel response based on the MIMO pilot sent by the access point. The user terminal may then perform a calibrated downlink channel response estimationIs determined, wherein K ∈ K, to obtain a value for each subbandDiagonal matrix of left eigenvectors ofSum matrixThe singular value decomposition can be given asWhere the hat symbol "^" of each matrix indicates that it is an estimate of the actual matrix. Similarly, the access point may be based on the user terminalThe MIMO pilot estimates sent by the terminals calibrate the uplink channel response. The access point can implement the calibrated channel response estimationIs determined, wherein K ∈ K, to obtain a value for each subbandDiagonal matrix of left eigenvectors ofSum matrixThe singular value decomposition can be given asThe access point and the user terminal may also obtain the required eigenvectors based on the steered reference, as described below.

Data transmission may occur on one or more wideband eigenmodes per uplink. The particular number of wideband eigenmodes used for data transmission generally depends on the channel conditions and may be selected in various ways. For example, the wideband eigenmodes may be selected by using a water-filling process that attempts to maximize the overall throughput by (1) selecting the best set of one or more wideband eigenmodes to use, (2) allocating the total transmit power among the selected wideband eigenmodes.

MIMO-OFDM systems may therefore be designed to support multiple modes of operation, including:

● spatial multiplexing mode-for transmitting data on multiple wideband eigenmodes, an

● Beam steering mode-is used to transmit data on the primary (best) wideband eigenmode.

Data transmission over multiple wideband eigenmodes may be achieved by using a matrixU _ap(k) OrV _ut(k) A plurality of sets of eigenvectors within, where K e K (i.e., one set of eigenvectors per wideband eigenmode) are obtained by performing spatial processing. Table 7 summarizes the spatial processing at the access point and the user terminal for data transmission and spatial multiplexing modes.

TABLE 7 spatial processing of spatial multiplexing modes

In the context of table 7, the data is,s(k) is at N of subband k_SUp to N of modulation symbols transmitted on multiple eigenmodes_SA "data" vector of non-zero terms,x(k) is the transmit vector for sub-band k,r(k) is the received vector for subband k, and*(k) is a transmitted data vectors(k) Is estimated. The subscripts "dn" and "up" of these vectors indicate downlink and uplink transmissions, respectively.

On a broadband eigenmodeData transmission may be achieved by either "beamforming" or "beam steering". For beamforming, the eigenvectors for the modulation symbols for the primary wideband eigenmodesOrThe set is spatially processed, where K ∈ K. For beam steering, modulating symbolsOrIs spatially processed with K e K, a set of "normalized" (or "saturated") eigen vectors of (a). Normalized eigenvectorAndmay be derived as described below.

Spatial processing of spatial multiplexing and beam steering modes is described in the aforementioned provisional U.S. patent application serial nos. 60/421309 and 60/421428. The steering reference for the spatial multiplexing and beam steering modes is described below.

4. Operating reference

For reciprocal channels (reciprocals) (e.g., after calibration has been performed to account for differences between transmit/receive chains at the access point and the user terminal), steering references may be sent by the user terminal and used by the access point to obtainAndwithout estimating the MIMO channel or performing a singular value decomposition. Similarly, the steering reference may be sent by the access point and used by the user terminal to obtainAndwhere K ∈ K.

In one embodiment, the steering reference comprises a set of pilot symbols (e.g., P OFDM symbols) transmitted on one wideband eigenmode in a given symbol period by spatial processing with a non-normalized or normalized set of eigenvectors for the wideband eigenmode. In another embodiment, the steering reference comprises multiple sets of pilot symbols transmitted on multiple wideband eigenmodes in the same symbol period by spatial processing with multiple sets of unnormalized or normalized eigenvectors for the wideband eigenmodes. In either case, the steering reference is from all N's at the access point_apOne antenna transmission (for downlink) and from all N at the user terminal_utOne antenna transmit (for uplink). For clarity, the following description assumes that the steering reference is transmitted for one wideband eigenmode in a given symbol period.

A. Downlink steered reference-spatial multiplexing mode

For the spatial multiplexing mode, the downlink steering reference transmitted by the access point on the mth wideband eigenmode can be expressed as:

k∈K，(12)

whereinx _dn，sr，m(k) A transmit vector for the kth subband which is the mth wideband eigenmode;

is the eigenvector of the kth subband of the mth wideband eigenmode; and

p (k) is the pilot symbol to be sent on subband k for the steered reference (e.g., as given in table 2).

Steering vectorIs a matrixColumn m of (1), wherein

The received downlink steering reference at the user terminal in spatial multiplexing mode can be expressed as:

k∈K，(13)

wherein sigma_m(k) Is the singular value of the k subband of the mth wideband eigenmode.

B. Downlink steered reference-beam steered mode

For the beam steering mode, spatial processing at the transmitter is achieved using a "normalized" set of eigenvectors for the main wideband eigenmodes. With normalized eigenvectorsWith an overall transfer function different from that with unnormalized eigenvectorsTotal transfer function (i.e. of). A steering reference generated using the set of normalized eigen-vectors for the dominant wideband eigenmodes may then be transmitted by the transmitter and used by the receiver to derive a matched filter for the beam steering mode.

For the beam steering mode, the downlink steering reference transmitted by the access point on the primary wideband eigenmode can be expressed as:

k∈K，(14)

whereinIs the normalized eigenvector for the k-th subband of the dominant wideband eigenmode, which can be expressed as:

wherein a is a constant (e.g., a ═ 1); and is

θ_ui(k) Is the phase of the kth subband of the ith transmit antenna, which can be given as:

vector as shown in equation (15)N of (A)_apThe individual elements have the same amplitude but may differ in phase. Vector as shown in equation (16)The phase of each element in the interior is a slave vectorObtained from the corresponding element of (i.e. theta)_ui(k) Is fromIs obtained in which)。

The reception of the downlink steering reference at the user terminal for the beam steering mode may be expressed as:

k∈K.(17)

C. uplink steered reference-spatial multiplexing mode

For spatial multiplexing mode, the uplink steering reference sent by the user terminal on the mth wideband eigenmode can be expressed as:

k∈K.(18)

(Vector)is a matrixColumn m of (1), wherein

The uplink steering reference received at the access point in spatial multiplexing mode can be expressed as:

k∈K.(19)

D. uplink steered reference-beam steered mode

For the beam steering mode, the uplink steering reference transmitted by the user terminal on the primary wideband eigenmode can be expressed as:

k∈K (20)

normalized eigenvector of kth subband of eigenmode of main broadbandCan be expressed as:

wherein

Vector as shown in equation (22)Is derived from the eigenvectorThe corresponding element of (2) is obtained.

The reception of the uplink steering reference at the access point of the beam steering pattern may be represented as:

k∈K (23)

table 8 summarizes the spatial processing of the steered reference for spatial multiplexing and beam steering modes at the access point and user terminal.

TABLE 8 spatial processing of steering references

E. Steering reference transmission

For the example frame structure shown in fig. 2, the steering reference may be sent within a preamble sequence or pilot portion of an FCH PDU (for downlink) or an RCH PDU (for uplink). The steering reference may be sent in various ways.

In an embodiment, for spatial multiplexing mode, the steered reference is transmitted for one or more wideband eigenmodes of each TDD frame. The particular number of wideband eigenmodes transmitted within each TDD frame may depend on the duration of the steered reference. For the example design with four transmit antennas, table 9 lists the wideband eigenmodes of the steering reference within the preamble sequence for FCH/RCH PDUs of different preamble sequence sizes.

Table 9

Leader sequence	Using broadband eigenmodes
		0 OFDM code element	Without leader sequence
1 OFDM code element	Wideband eigenmode m, where m is frame count mod 4
		4 OFDM code elements	Cycling through all 4 wideband eigenmodes within the preamble sequence
8 OFDM code elements	Cycling through all 4 wideband eigenmodes twice within the preamble sequence

As shown in table 9, the steered reference is transmitted for all four wideband eigenmodes within the same TDD frame when the preamble sequence size is four or eight symbol periods. The steering reference transmitted by the access point within the preamble sequence of the FCH PDU during the nth symbol period may be expressed as:

k∈K，n∈{1...L}，(24)

where L is the preamble sequence size (e.g., L ═ 0, 1,4, or 8 for the example designs shown in table 9).

The steering reference transmitted by the user terminal in the preamble sequence of the RCH PDU during the nth symbol period can be expressed as:

k∈K，n∈{1...L}(25)

within equations (24) and (25), the four wideband eigenmodes are cycled through each 4 symbol period by manipulating the "mod" operation of the vector. This scheme may be used if the channel changes more quickly and/or during the early part of the communication session when a better channel estimate needs to be obtained quickly for proper system operation.

In another embodiment, the steered reference is transmitted for one wideband eigenmode within each TDD frame. The steered reference for the four wideband eigenmodes may cycle through four TDD frames. For example, steering vectors may be used by a user terminal for four consecutive TDD framesAndthe particular steering vector used to steer the reference within each TDD frame may be specified by a frame counter,the count may be sent within a BCH message. This scheme allows the use of shorter preamble sequences for the FCH and RCH PDUs. However, a longer time period may be required to obtain a better channel estimate.

For the beam steering mode, the normalized steering vector of the dominant broadband eigenmode is used to steer the reference, as shown in equations (14) and (20). The duration of the steering reference may be selected based on, for example, channel conditions.

When operating in a beam steering mode, a user terminal may transmit multiple steered reference symbols, e.g., one or more using normalized eigenvectorsOne or more eigenvectors using principal eigenvectorsAnd possibly one or more symbols using eigenvectors of other eigenmodes. By usingThe generated steered reference symbols may be used by the access point to derive an uplink matched filter vector. The vector is used by the access point to achieve matched filtering of uplink data transmissions sent by the user terminal using beam steering. By usingThe generated steering reference symbol may be used to obtainIt can be used to derive normalized eigenvectors for beam steering on the downlinkFor other eigenmodes, eigenvectors are usedToThe generated steering reference symbol may be used by an access point to obtainToAnd singular value estimates of these other eigenmodes. The information may be used by the access point to determine whether to use a spatial multiplexing mode or a beam steering mode for downlink data transmission.

For the downlink, the user terminal may estimate based on the calibrated downlink channel responseA downlink matched filter vector is derived for the beam steering mode. In particular, the user terminal is fromOf singular value decompositionAnd normalized eigenvectors can be derivedThe user terminal can exchangeMultiplication byTo obtainAlternatively, the steering vector may be used by the access point using normalized eigenvectorsThe steering reference is transmitted and may be processed by the user terminal in the manner described above to obtain a downlink matched filter vector for the beam steering pattern.

F. Subband multiplexing of steered references

For spatial multiplexing and beam steering modes, the steering reference may also be transmitted for multiple wideband eigenmodes in a given symbol period using subband multiplexing. The subbands used may be divided into a number of disjoint sets of subbands, one set for each wideband eigenmode selected for steering reference transmission. Each subband set may then be used to transmit a steered reference for the associated wideband eigenmode. For simplicity, the term "wideband eigenmode" is used herein even though the steered reference is only transmitted on a subset of all the usable subbands.

For example, the steered reference may be transmitted on all four wideband eigenmodes in one symbol period. In this case, the 52 usable subbands may be divided into four disjoint sets (labeled sets 1, 2, 3, and 4, for example), each set including 13 subbands. The 13 subbands in each set may be evenly distributed across the 52 available subbands. The steered reference for the primary wideband eigenmode may then be transmitted on 13 subbands in set 1, the steered reference for the second wideband eigenmode may be transmitted on 13 subbands in set 2, the steered reference for the third wideband eigenmode may be transmitted on 13 subbands in set 3, and the steered reference for the fourth wideband eigenmode may be transmitted on 13 subbands in set 4.

If the steered reference is transmitted on only a subset of all available subbands for a given wideband eigenmode, interpolation or some other technique may be used to obtain subband estimates for the steered reference transmission that are not used for that wideband eigenmode.

In general, the multiple sets of subbands may include the same or different numbers of subbands. For example, the number of subbands included in each set may depend on the SNR of the wideband eigenmodes associated with the set (e.g., more subbands may be assigned to sets associated with poorer quality wideband eigenmodes). Moreover, the subbands in each set may be uniformly or non-uniformly distributed across the available subbands. Multiple sets of subbands may also be associated with the same or different sets of pilot symbols.

Subband multiplexing may be used to reduce the amount of overhead required to transmit the steered reference, which may improve the efficiency of the system.

G. Channel estimation with steered reference

As equation (13) shows, at the user terminal, the received downlink steered reference (in the presence of noise) for the spatial multiplexing mode is roughlySimilarly, as shown in equation (19), at the access point, the received uplink steering reference (in the presence of noise) for the spatial multiplexing mode is approximatelyThe access point thus obtains based on the steering reference sent by the user terminalAnd σ_m(k) And vice versa.

Various techniques are used to process the steering reference. For clarity, the following description is directed to uplink steering reference processing. The vector received at the access point is given in equation (19) as

In one embodiment, to obtainEstimate of (2), received vector of steering reference sent on mth wideband eigenmoder _up，sr，m(k) First multiplied by the complex conjugate p of the pilot symbol^*(k) It is used to manipulate the reference. The result is then very integrated over multiple received steered reference symbols for each wideband eigenmode to obtainIs the mth broadband eigenmodeScaled left eigenvector. Vector quantityN of (A)_apEach of the terms is based on a vectorr _up，m(k) N of (A)_apOne of the items is obtainedObtained whereinr _up，m(k) N of (A)_apItem is from access point N_apSymbols received by each antenna. Since the eigenvectors have unit power, the singular value σ_m(k) Can be estimated based on the received power of the steered reference, which can be measured for each subband of each wideband eigenmode. Singular value estimationThen equals the amplitude of the pilot symbols p (k) divided by the square root of the received power.

In another embodiment, a Minimum Mean Square Error (MMSE) technique is used for steering reference based received vectorsr _up，sr，m(k) Obtaining a vectorIs estimated. Since the pilot symbols p (k) are known, the access point can deriveSuch that the received pilot symbols (in pairs of received vectors)r _up，sr，m(k) Achieved after matched filtering) and minimization of the mean square error between the transmitted pilot symbols. The use of MMSE techniques for spatial processing at a receiver is commonly described in U.S. patent application serial No. 09/993087, entitled "Multiple-access Multiple-Input Multiple-output (mimo) Communication System", filed on 11/6/2001.

The steered reference is transmitted for one wideband eigenmode (without subband multiplexing) in any given symbol period and can then be used to obtain one eigenvector estimate for each subband of the wideband eigenmode. Thus, the receiver can obtain an estimate of only one eigenvector in the unitary matrix in any given symbol period. Since multiple eigenvector estimates of the unitary matrix are obtained over multiple symbol periods, and due to noise and other sources of degradation within the wireless channel, the estimated eigenvectors of the unitary matrix (which are derived separately) cannot be orthogonal to each other. The estimated eigenvectors may thereafter be used for matched filtering of data transmissions received on the same link and/or spatial processing of data transmissions sent on other links. In this case, any error in orthogonality between these estimated eigenvectors may result in crosstalk between the data streams transmitted on the eigenmodes of the corresponding eigenvector. Crosstalk may deteriorate performance.

In one embodiment, the estimated eigenvectors of each unitary matrix are forced to be orthogonal to each other. The orthogonality of the eigenvectors may be achieved using Gram-Schmidt techniques, which are described in detail in the above-mentioned reference to Gilbert Strang, or other techniques.

Other techniques for processing the manipulation reference may also be used and are within the scope of the present invention.

The access point can thus estimate based on the steering reference sent by the user terminalAndwithout the need to estimate the channel response or to implementSingular value decomposition of (c).

Estimating a matrix at a user terminal based on a downlink steered referenceAndmay be implemented in a manner similar to that described above for the uplink steering reference.

For the beam steering mode, the received vector of the reference is steered on the uplinkCan be processed in a similar manner by the access point to obtainIs estimated. The conjugate transpose of the estimate is then the uplink transmission matched filter within the beam steering pattern. Manipulating the received vector of the reference on the downlinkCan be processed in a similar manner by the user terminal to obtainIs estimated. The conjugate transpose of the estimate is the matched filter of the downlink transmission in the beam steering mode.

5. Carrier pilot

The carrier pilots may be transmitted on the pilot subbands in various manners within the TDD frame structure shown in fig. 2. In one embodiment, four pilot sequences are reset for each transport channel. Thus, on the downlink, the pilot sequence is reset for the first OFDM symbol of the BCH message, again for the first OFDM symbol of the FCCH message, and for the first OFDM symbol sent on the FCH. In another embodiment, the pilot sequence is reset at the beginning of each TDD frame and repeated as needed. For this embodiment, the pilot sequence may be stopped (stopped) on the preamble portion of the BCH and FCH. The carrier pilots may also be transmitted in other manners and this is within the scope of the invention.

6. Pilot transmission scheme

Four types of pilots have been described above and may be used for MIMO and OFDM systems. These four different pilot types may be transmitted in various ways.

Fig. 3 illustrates downlink and uplink pilot transmission for an example pilot transmission scheme. Generally, block 310 corresponds to a system access phase, block 320 corresponds to a calibration phase, and block 330 corresponds to a normal operation phase.

The beacon pilot and MIMO pilot are transmitted by the access point on the downlink in each TDD frame (block 312) to allow all user terminals in the system to acquire system frequency and timing and estimate the downlink channel (block 314). Block 314 may be implemented as desired to access the system.

Calibration may be performed prior to normal operation to calibrate for differences between transmit/receive chains at the access point and the user terminal. For calibration, MIMO pilots may be implemented by the access point and the user terminal together (blocks 322 and 326). The uplink MIMO pilot may be used by the access point to derive an uplink channel estimate (block 324), and the downlink MIMO pilot may be used by the user terminal to derive or update an estimate of the downlink channel (block 328). The downlink and uplink channel estimates are then used to derive correction factors for the access point and the user terminal.

During normal operation, the steering reference may be transmitted by the user terminal on the uplink, either: (1) if and when the user terminal desires data transmission, or (2) if the user terminal is scheduled for data transmission (block 332). The uplink steered reference may be used by the access point to estimate a correlated unitary matrix and a diagonal matrix for the user terminal (block 334). The manipulation reference may optionally be transmitted by the access point to the user terminal (as shown by dashed box 336). The user terminal may continuously update its downlink channel estimate based on the downlink MIMO pilot and update the associated unitary and diagonal matrices (if transmitted) based on the downlink steered reference (block 338). The carrier pilots are transmitted on the pilot subbands by the access point (block 340) and the user terminals (block 334) during portions not used for other pilots. The downlink carrier pilot is used by the user terminal to track the downlink carrier signal phase (block 342) and the uplink carrier pilot is used by the access point to track the uplink carrier signal phase (block 346).

For the pilot transmission scheme shown in fig. 3, the user terminal estimates the downlink channel response based on the downlink MIMO pilot and transmits on the uplinkA longitudinal reference, which is then used by the access point to estimate the relevant unitary and diagonal matrices for the user terminal. In certain cases, the user terminal may obtain a bad estimate of the downlink channel response, in which case the downlink steering reference may be equally poor or may be worse. In the worst case, the steering vector used by the user terminal will result in a beam pointing towards the access point being zero. If this happens, the access point cannot detect the uplink steering reference. To avoid this situation, in the event that the user terminal detects that the access point is not receiving the steering reference correctly, the user terminal may perturb N of the steering vector he uses to steer the reference_utThe phase of each element. For example, if a user terminal specifies to send an uplink steering reference as part of a system access procedure, and if system access is not obtained after a certain number of access attempts, the user terminal may begin perturbing the phase of the steering vector element.

Various other pilot transmission schemes may also be implemented for MIMO and MIMO-OFDM systems and are within the scope of the present invention. For example, the beacon and carrier pilots may be combined into a single pilot that may be used for frequency and timing acquisition as well as carrier phase tracking. As another example, an active user terminal may send a MIMO pilot on the uplink instead of manipulating the reference.

MIMO-OFDM system

Fig. 4 shows a block diagram of an embodiment of an access point 110x and a user terminal 120x within MIMO-OFDM system 100. For clarity, in this embodiment, access point 110x is equipped with four antennas that may be used for data transmission and reception, and user terminal 120x is also equipped with four antennas for data transmission/reception. In general, the access point and the user terminal may each be equipped with any number of transmit antennas and any number of receive antennas.

On the uplink, at access point 110x, a Transmit (TX) data processor 414 receives traffic data from a data source 412 and signaling and other data from a controller 430. TX data processor 414 formats, codes, interleaves, and modulates (i.e., symbol maps) the data to provide modulation symbols. A TX spatial processor 420 multiplexes the modulation symbols from TX data processor 414 with pilot symbols, performs the required spatial processing, and provides four streams of transmit symbols to four transmit antennas.

Each Modulator (MOD)422 receives and processes a respective transmit symbol stream to provide a corresponding downlink modulated signal. The four downlink modulated signals from modulators 422a through 422d are then transmitted from antennas 424a through 424d, respectively.

At user terminal 120x, four antennas 452a through 452d receive the transmitted downlink modulated signals and each antenna provides a received signal to a respective demodulator (DEMOD) 454. Each demodulator 454 performs processing complementary to that performed at modulator 422 and provides received symbols. A Receive (RX) spatial processor 460 then performs spatial processing on the received symbols from all demodulators 454a through 454d to provide recovered symbols, which are estimates of the modulation symbols transmitted by the access point. An RX data processor 470 further processes (e.g., symbol demaps, deinterleaves, and decodes) the recovered symbols to provide decoded data, which is then provided to a data sink 472 for storage and/or a controller 480 for further processing.

The processing for the uplink may be the same as or different from the processing for the downlink. Data and signaling are processed (e.g., coded, interleaved, and modulated) by a TX data processor 488, multiplexed with pilot symbols, and further spatially processed by a TX spatial processor 490. The transmit symbols from TX spatial processor 490 are further processed by modulators 454a through 454d to generate four uplink modulated signals, which are then transmitted via antennas 452a through 452 d.

At access point 410, the uplink modulated signals are received by antennas 424a through 424d, demodulated by demodulators 422a through 422d, and processed by an RX spatial processor 440 and an RX data processor 442 in a manner complementary to that implemented at the user terminals. The decoded data for the uplink may be provided to a data sink 444 for storage and/or to the controller 430 for further processing.

Controllers 430 and 480 control the operation of various processing units at the access point and user terminal, respectively. Memory units 432 and 482 store data and program codes used by controllers 430 and 480, respectively.

Fig. 5 illustrates a TX spatial processor 420a that may generate beacon pilots, and this may be implemented within TX spatial processor 420 in fig. 4. Processor 420a includes multiple beacon pilot subband processors 510a through 510k, one for each subband transmitting a beacon pilot. Each subband processor 510 receives pilot symbols b (k) for a beacon pilot and a correlation matrix for the associated subband

Within each subband processor 510, pilot symbols b (k) are used from the matrix by four multipliers 514a through 514dFour correction factors ofToScaled accordingly. Each multiplier 514 performs complex multiplication of the complex pilot symbols with a corresponding complex correction factor. The scaled pilot symbols from multiplier 514a are then provided to four buffer/multipliers 520a through 520d, respectively, which also receive the scaled pilot symbols from other subband processors 510. Each buffer/multiplexer 520 multiplexes the scaled pilot symbols for all subbands used for beacon pilot transmission with signal values of zero for the unused subbands and provides a stream of transmit symbols for the associated transmit antenna.

Fig. 6A shows a block diagram of TX spatial processor 420b, which may generate a MIMO pilot. Processor 420b may be TX spatial processor 420 in fig. 4 or490, but for clarity the implementation within TX spatial processor 420 is described below. Processor 420b may include multiple MIMO pilot subband processors 610a through 610k, one for each subband transmitting MIMO pilots. Each subband processor 610 receives pilot symbols p (k) for the MIMO pilot and a correction matrix for the associated subbandEach subband processor 610 also receives four Walsh sequences w₁To w₄They are assigned to four transmit antennas at the access point.

Within each subband processor 610, complex pilot symbols p (k) are passed through four Walsh sequences w by four complex multipliers 612a through 612d, respectively₁To w₄And (6) covering. The covered pilot symbols are further used by four complex multipliers 614a through 614d from the matrixFour complex correction factors ofToScaled accordingly. The scaled pilot symbols from multipliers 614a through 614d are then provided to four buffer/multipliers 620a through 620d, respectively. The sequential processing is as described above for fig. 5.

For processor 420b implementation within TX spatial processor 490, the number of Walsh sequences used depends upon the number of transmit antennas available at the user terminal. Furthermore, the scaling is applied to the matrix from the user terminalThe correction factor of (2).

FIG. 6B illustrates RX spatial processing that may provide channel response estimates based on received MIMO pilotsBlock diagram of device 460 b. Processor 460b may be implemented within RX spatial processor 440 or 460 of fig. 4, although an implementation of RX spatial processor 460 is described below for clarity. Processor 460b includes multiple MIMO pilot subband processors 650a through 650k, one for each subband used for MIMO pilot transmission. Each MIMO pilot subband processor 650 receives a vectorr(k) And the conjugated pilot symbol p of the associated subband^*(k) In that respect Each subband processor 650 also receives four Walsh sequences w assigned to the four transmit antennas at the access point₁To w₄。

Each MIMO pilot subband processor 650 includes four MIMO pilot subband/antenna processors 660a through 660d for the four receive antennas at the user terminal. Each processor 660 receives vectorsr(k) An term of (r)_i(k) In that respect Within each processor 660, a received symbol r_i(k) The conjugate pilot symbol p is first multiplied by a complex multiplier 662^*(k) In that respect The output of multiplier 662 is further multiplied by four complex multipliers 664a through 664d, respectively, by four Walsh sequences w₁To w₄. The outputs from multipliers 664a through 664d are then accumulated by accumulators 666a through 666d accordingly for the MIMO pilot transmission duration. Each multiplier 664 and accumulator 666 decovers one transmit antenna at the access point. The output from each accumulator 666 represents the channel gain estimate for subband k from transmit antenna j to receive antenna iChannel response estimation(where i ═ {1, 2, 3, 4} and j ═ 1, 2, 3, 4}) may be further averaged over multiple MIMO pilot transmissions (not shown in fig. 6B) to provide a more accurate channel response estimate.

As shown in FIG. 6B, each MIMO pilot subband/antenna processor 660 provides row vectors for associated receive antennas iWhereinIs calibrated channel response estimation of downlinkRow i (assuming the access point applies its correction matrix)). Processors 660a through 660d together provide a calibrated channel response matrixFour rows of.

Fig. 7A shows a block diagram of TX spatial processor 420c, which may generate a steering reference. Processor 420c may also be implemented within TX spatial processor 420 or 490 shown in fig. 4, although implementation of TX spatial processor 420 is described below for clarity. Processor 420c includes a plurality of steered reference subband processors 710a through 710k, one for each subband used to transmit a steered reference. To generate a steered reference for the spatial multiplexing modes, each subband processor 710 receives pilot symbols p (k), a steered vector, for each wideband eigenmode over which the steered reference is transmittedAnd receiving correction matrices for associated subbands

Within each subband processor 710, pilot symbols p (k) are multiplied by the steering vector for the mth wideband eigenmode by four complex multipliers 712a through 712d, respectivelyFour elements of (2)ToThe outputs from the multipliers 712a through 712d are further used by four complex multipliers 714a through 714d from the matrixFour correction factors ofToScaled. The scaled symbols from multipliers 714a through 714d are then provided to four buffer/multipliers 720a through 720d, respectively. The sequential processing is as described above.

To generate steering references on the downlink for the beam steering pattern, each subband processor 710 receives normalized steering vectorsRather than an unnormalized steering vectorFor implementation by processor 420c within TX spatial processor 490, each subband processor 710 may receive (1) a steering vector for each wideband eigenmode used to steer the reference, for a pair of spatial multiplexing modesOr (2) steering vectors for beam steering patternsIf subband multiplexing is used for the steering reference, steering vectors for multiple wideband eigenmodes can be used for disjoint sets of multiple subbands, as described above.

Fig. 7B illustrates an RX spatial processor 460c that may provide steering vectors and singular value estimates based on received steering references. Processor 460c may be implemented within RX spatial processor 440 or 460 in fig. 4, but for clarity, implementation within RX spatial processor 460 is described below. Processor 460c includes multiple steered reference subband processors 750a through 750k, one for each subband used to steer reference transmissions. Each subband processor 750 receives a vectorr(k) And receiving the conjugated pilot symbol p for the associated subband^*(k)。

Within each subband processor 750, a vector is receivedr(k) The four symbols in the symbol block are multiplied by the conjugate pilot symbol p by complex multipliers 762a to 762d, respectively^*(k) In that respect The outputs of multipliers 762a through 762d are then accumulated by accumulators 764a through 764d for the duration of the steered reference transmission for each wideband eigenmode, respectively. As shown in table 9, the steered reference may be transmitted for multiple wideband eigenmodes within the same steered reference transmission, in which case the accumulation is implemented separately for each of these wideband eigenmodes. However, the multiple steered reference symbols for any given wideband eigenmode (which may be sent within one or more steered reference transmissions) may be accumulated to obtain a higher quality estimate. Accumulators 764a through 764d provideAs shown in equation (13).

Since the eigenvectors have unity power, the singular values of each wide-band eigenmodeMay be estimated based on the received power of the steering reference. A power calculation unit 766 receives the outputs of multipliers 762a through 762d and calculates the received power P of the eigenmode steering reference for each subband k_m(k) In that respect Singular valueThen the square of the received power equal to the steered reference calculation is divided by the pilot symbol amplitude (i.e., pilot symbol amplitude))，

WhereinAnd r is_i(k) Is the received symbol on subband k for receive antenna i.

The outputs of accumulators 766a through 766d are then estimated from the inverse of the singular valuesAnd multipliers 768a through 768d are scaled accordingly to provide an estimate of the steering vector for each eigenmode,

steering reference processing for beam steering may be implemented in a similar manner. The processing of the steering reference on the uplink may also be implemented in a similar manner to obtain an estimate of the steering vector for each eigenmode,

the pilots described herein may be implemented by various means. For example, various types of pilot processing at the access point and at the user terminal may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the elements for processing pilots for transmission and/or reception may be implemented within: one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.

For a software implementation, some processing of various types of pilots (e.g., spatial processing of pilot transmissions and/or channel estimation based on received pilots) may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit 432 or 482 in fig. 4) and executed by a processor (e.g., controller 430 or 480). The memory unit may be implemented within the processor or external to the processor, in which case it can be coupled to the processor in a variety of ways known in the art.

Headings are included herein for reference and to aid in locating particular sections. These headings do not limit the scope of the concepts described therein under, which concepts may be applied to other sections throughout the specification.

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 (1)

1. A method for generating pilots in a wireless multiple-input multiple-output (MIMO) communication system, comprising:

obtaining a pilot symbol for each antenna within a plurality of antennas;

obtaining an orthogonal sequence for each antenna within a plurality of antennas, wherein the plurality of antennas are assigned different orthogonal sequences; and

the pilot symbols for each antenna are covered with an orthogonal sequence for the antenna to obtain a covered pilot symbol sequence for the antenna, where multiple covered pilot symbol sequences for multiple orthogonal pilots are obtained for multiple antennas.