HK1111534A - A communication system using ofdm for one direction and dsss for another direction - Google Patents

Description

Communication system using OFDM in one direction and DSSS in another direction

Technical Field

The present invention relates to communication systems, and more particularly to multi-user systems that utilize the basic modulation format of Orthogonal Frequency Division Multiplexing (OFDM) and spread spectrum transmission.

Background

With the growing demand for high-speed wireless services, more throughput per bandwidth is required to accommodate more users with higher data rates, while retaining a guaranteed quality of service (QoS) across the entire packet network. While the definition of "wireless broadband" may change, it is generally accepted that next generation wireless broadband networks must be able to provide a wide range of services, from high quality voice to high definition video, over IP-centric, high speed (> 10Mbps downlink and > 512Kbps uplink) wireless channels. See Shingo Ohmori et al, "The Future Generation of Mobile Communications Based on Broadband Access Technologies," IEEE Communication Magazine, 12.2000.

Due to the asymmetric nature of packet traffic, the requirements of the wireless uplink (from the user to the base station or access point) and downlink (from the base station or access point to the user) are very different. High throughput/spectral efficiency is most important in the heavy traffic downlink even though this means that the base station comprises more hardware and higher cost power amplifiers. On the other hand, the amplifier efficiency modulation scheme is decisive for the user terminal in order to reduce costs and improve power efficiency. Obviously, separate design optimization methods must be employed in the design of the uplink and downlink modems. However, almost all current systems, such as the common GSM and IS-95 networks, utilize a consistent modem and multiple access architecture for both the uplink and downlink. The result is that the efficiency of the overall system is compromised.

Orthogonal Frequency Division Multiplexing (OFDM) and Direct Sequence Spread Spectrum (DSSS) are two modulation techniques used for wireless communications. In OFDM, as shown in fig. 1, a wide bandwidth is divided into a plurality of narrowband subcarriers, which are arranged to be orthogonal to each other. The signals modulated on the subcarriers are transmitted in parallel. In DSSS, modulation symbols are first spread with a spreading sequence over the available bandwidth and then transmitted. In Code Division Multiple Access (CDMA), multiple subscriber stations communicate with a base station using DSSS signaling with different spreading sequences.

OFDM is an efficient technique for multiple fading channels. In well-designed systems, the frequency response of each subcarrier may be flat or nearly flat. Thus, only a very simple channel equalization or even no channel equalization is required. Another significant advantage of OFDM is that it allows for optimal power and rate allocation to maximize channel capacity. This inherent advantage is even more significant in cellular systems with multiple users, where the channel response of each user is different. In this case, by judiciously allocating subcarriers to multiple users, the capacity throughput of the entire system can be maximized.

On the other hand, OFDM also has some disadvantages. One of the drawbacks is the peak-to-average power (PAP) ratio of the large OFDM signal. This is an important obstacle for implementing OFDM based systems. A large PAP ratio means a stricter linearity requirement or large back-off for the power amplifier, resulting in higher cost or lower transmission power. This is highly undesirable for implementing user terminals, which in turn dominate the system cost due to their large number. Furthermore, in order to achieve the maximum capacity of OFDM with adaptive user allocation, it is often necessary to feed back channel measurements at the user to the base station. This also increases overhead and complicates system control.

DSSS generally processes multiple channels by using a so-called rake receiver, which coherently adds together the signals received from multiple delay paths. However, when the data rate is high and the spreading factor is low, the performance of the rake receiver is degraded. Moreover, DSSS signals equally utilize the entire frequency spectrum, including high gain frequencies and low gain frequencies. Therefore, the potential capacity of DSSS is less than that obtained with OFDM with adaptive subcarrier allocation. On the other hand, DSSS signals typically have a lower PAP than OFDM signals. Furthermore, code division multiple access can be enabled with DSSS, which provides strong multiple access flexibility in many multiple access schemes. Therefore, DSSS is always an attractive technology, especially for user terminal transmission.

Both OFDM and DSSS are widely used for wireless communication, but in most systems a single technique is used for the downlink and uplink. For example, in a UMTS W-CDMA system, DSSS is used for both downlink and uplink, while in IEEE 802.11a, OFDM is used for both downlink and uplink. This means that both advantages and disadvantages are present in the system. For more information on W-CDMA, see "WCDMA for umts" by h.holma and a.toskala, John Wiley & Sons, inc., 2000. For more information on OFDM, see "OFDM for Wireless multimedia communications", Artech House publication, 2000 by r.van Nee and r.prasad.

Disclosure of Invention

A method and apparatus for communication is described. In one embodiment, a method for communicating with a user includes transmitting an Orthogonal Frequency Division Multiplexing (OFDM) signal to the user and receiving a Direct Sequence Spread Spectrum (DSSS) signal from the user.

Drawings

The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.

Fig. 1A shows an OFDM signal and subcarriers in the frequency domain.

Fig. 1B shows a plurality of subcarriers and a cluster.

Fig. 2 shows OFDM subcarrier clusters and pilot symbols in the time-frequency domain.

Fig. 3 illustrates a communication network that uses OFDM for transmission in one direction and CDMA for transmission in the other direction.

Fig. 4 is a block diagram of one embodiment of a base station transmitter that uses OFDM for downlink communications.

Fig. 5 is a block diagram of one embodiment of a user terminal receiver.

Fig. 6 is a block diagram of one embodiment of a user terminal transmitter using DSSS/CDMA for uplink communications.

Fig. 7 is a block diagram of one embodiment of a base station receiver and downlink subcarrier allocator.

Fig. 8 shows a data format of an uplink transmission signal.

Fig. 9 shows a data format of a downlink transmission signal.

Fig. 10 shows pilot subcarriers for frequency tracking.

Fig. 11 is a block diagram of one embodiment of a duplex system for bi-directional transmission with CDMA and improved data rate in one direction with an additional OFDM channel.

Fig. 12 shows channel responses related to different users.

Figure 13 is a flow diagram of one embodiment of a process for allocating subcarriers.

Fig. 14 shows a time and frequency grid of OFDM symbols, pilots, and clusters.

Fig. 15 illustrates user processing.

Fig. 16 illustrates an example of fig. 15.

FIG. 17 illustrates one embodiment of a format for arbitrary cluster feedback.

Fig. 18 illustrates one embodiment of a base station.

Detailed Description

Methods and apparatus for integrating OFDM and CDMA technologies are described. In one embodiment, a method for communicating with a user includes transmitting an Orthogonal Frequency Division Multiplexing (OFDM) signal to the user and receiving a Direct Sequence Spread Spectrum (DSSS) signal from the user.

The techniques described herein increase and potentially maximize downlink throughput and, in turn, increase and potentially optimize uplink power efficiency while maintaining multiple access flexibility for the overall system. In one embodiment, OFDM is used for the downlink to increase spectral efficiency and bit rate, and possibly maximize it. This is an important feature for today's internet access because of the asymmetry of the technology. DSSS/CDMA for the uplink avoids the problem of large peak-to-average ratios for OFDM and increases and potentially maximizes multiple access flexibility. CDMA techniques other than DSSS may alternatively be used. For example, Frequency Hopping (FH) may be used.

In one embodiment, the subcarriers of the OFDM downlink are adaptively allocated to multiple users to increase and possibly maximize system capacity. Uplink CDMA signals from multiple users received at the base station may be used for adaptive allocation.

While at least one embodiment is described for wireless communications, the teachings of the present disclosure are equally applicable to wired communications, for example, such as but not limited to cable modems.

In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, without detail, in order to avoid obscuring the present invention.

Some portions of the detailed description that follows are presented in terms of algorithms and symbol expressions which operate on data bits within a computer memory. These algorithmic descriptions and expressions are the means used by those skilled in the data processing arts to more effectively convey the substance of their work to others skilled in the art. An algorithm, as the term is used here, and as it is used generally, is conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of the above terms and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing" or "computing" or "calculating" or "determining" or "displaying" or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

The invention also relates to an apparatus for performing such operations. The apparatus may be specially constructed for the required purposes, or it may be constructed of a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), Random Access Memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus.

The algorithms and expressions presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read-only memory ("ROM"), random-access memory ("RAM"), magnetic disk storage media, optical storage media, flash-memory devices, and electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others.

Overview

Fig. 1B shows a plurality of subcarriers, such as subcarrier 101, and cluster 102. A cluster, such as cluster 102, is defined as a logical unit that is contained in at least one physical carrier, as shown in fig. 1B. A cluster may contain contiguous or non-contiguous subcarriers. The mapping between a cluster and its subcarriers may be fixed or reconfigurable. In the latter case, the base station informs the user when the cluster is redefined. In one embodiment, the spectrum includes 512 subcarriers, and each cluster includes four consecutive subcarriers so that there are 128 clusters.

In one embodiment, each base station transmits pilot symbols simultaneously, and each pilot symbol occupies the entire OFDM frequency bandwidth, as shown in fig. 2. In one embodiment, each pilot symbol has a guard time length or duration of 128 microseconds, the combination of which is approximately 152 microseconds. After each pilot period, there is a predetermined number of data periods followed by another set of pilot symbols. In one embodiment, four data periods are used to transmit data after each pilot, and each period is 152 microseconds.

Fig. 3 is a block diagram of a communication network that transmits using OFDM in one direction and CDMA in the other direction. The processing modules in fig. 3 and other figures are executed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, etc.), software (such as is run on a general purpose computer system or a dedicated machine), or a combination of both.

Referring to fig. 3, a communication network 300 includes a plurality of communication systems (e.g., base stations, access points, head end devices, mobile units, subscribers, remote devices, terminal units, etc.). Although only two systems are shown, the communication network 300 may include more than two systems. At one site, system 350 includes a CDMA transmitter 301 and an OFDM receiver 302 that transmit information, where CDMA transmitter 301 uses CDMA modulated signals for wireless communication and OFDM receiver 302 processes wireless communications in the form of OFDM signals received from other locations in the network. The switch 303 then switches between the CDMA transmitter 301 and the OFDM receiver 302 so that only one of them is coupled to the antenna 310 at any one time.

For Time Division Duplex (TDD) systems that use time division multiplexing to support full duplex communication, or other systems that communicate bi-directionally on the same frequency, switch 303 comprises a time switch. In an alternative embodiment, the switch 303 is replaced by a frequency diplexer for Frequency Division Duplex (FDD), or other such system, which transmits and receives information using different frequency bands in each direction.

System 351 includes an OFDM transmitter 306 and a CDMA receiver 305, where OFDM transmitter 306 is configured to generate OFDM signals for communicating via wireless transmission to other stations in the system and CDMA receiver 305 is configured to process CDMA signals received from other stations in the system. The switch 304 (or duplexer) couples the CDMA receiver 305 or the OFDM transmitter 306 to the antenna 314 at a certain moment. Switches/duplexers 304 and 303 allow antennas 311 and 310 to be used for transmission and reception, respectively, simultaneously.

In one embodiment, system 350 comprises a subscriber in a mobile communication system, while system 351 comprises a base station. Therefore, as shown in fig. 3, OFDM is used for downlink. The use of OFDM for the downlink may maximize spectral efficiency and bit rate. CDMA for the uplink substantially avoids the large peak-to-average ratio problem of OFDM and provides multiple access flexibility.

In one embodiment, the subcarriers of the OFDM downlink are adaptively allocated to multiple users, thereby enabling multiplexing and increasing (and potentially maximizing) system capacity. Information extracted from uplink CDMA signals received at a base station from multiple users may be used for adaptive subcarrier allocation. As described in detail below.

In one embodiment, a 5MHz spectrum is used for each downlink OFDM channel. In the pulse-shaping case, the net bandwidth for data transmission is 4MHz, divided into 512 subcarriers that are transmitted in parallel. In one embodiment, each OFDM symbol has a duration length of 128 microseconds with a guard interval of 24 microseconds. Thus, the total symbol period is 152 microseconds. In one embodiment, all subcarriers in one OFDM symbol are used for one user. Traffic to multiple users is implemented by Time Division Multiplexing (TDM), e.g., different users use different OFDM symbols at different times. In another embodiment, the subcarriers in one OFDM symbol can be used by multiple users, each using a portion of the total subcarriers. In one embodiment, any subcarrier may be allocated to any user. In another embodiment, the granularity of subcarrier allocation is increased to a fixed number of subcarriers, referred to as a cluster, as shown in fig. 1A and 2. Any user may be assigned any cluster. User clustering reduces the overhead of sub-carrier indexing.

Fig. 4 is a block diagram of one embodiment of a base station transmitter that uses OFDM for downlink communications. Referring to fig. 4, the base station includes N processing paths or links, labeled 1 through N, each of which includes a Forward Error Correction (FEC) encoder 402 followed by an interleaver 403 followed by a modulator 404. There is one processing path for each of the n users to communicate with the base station. In one embodiment, a Medium Access Control (MAC) (not shown) or other multiplexing mechanism is used to route user data to the processing paths of the various clusters.

As shown in FIG. 4, user data 401_1-nIncluding data sent to the respective users. A Multiplexer (MUX)480, operating as part of the subcarrier allocator, that receives user data 401_1-nAnd outputs cluster data and user data modulated onto the subcarriers, wherein the cluster data is generated as a result of the allocator allocating the subcarrier groups to the respective users for transmission. In an alternative embodiment, MUX480 is not included and the user data is sent directly to Forward Error Correction (FEC) encoder 402.

The above-described cluster data is first encoded by a Forward Error Correction (FEC) encoder 402 in a manner well known in the art. The result of the encoding is to add embedded sufficient redundant information in the user data to allow the receiver to correct it. The user data is then interleaved by interleaver 403. interleaver 403 reorders the code symbols (e.g., bits) in the user data in such a way that consecutive code symbols are separated by a plurality of symbols in the sequence to be transmitted. As is well known in the art. Thereafter, the base station modulates the interleaved user data with a modulator 404 using a digital modulation method such as QPSK, 16QAM, or other methods described below. The modulated data is processed on all subcarriers (intended for multiple users) with an Inverse Fast Fourier Transform (IFFT)405 as is well known in the art. The output of the IFFT405 is input to a parallel-to-serial converter 406 which converts the output of the parallel IFFT405 into a serial output OFDM signal in a manner well known in the art. In one embodiment, an additional guard interval (cyclic prefix) is inserted there. The final OFDM signal is transmitted through an RF channel.

In one embodiment, the base station adaptively allocates subcarriers to users to increase (and potentially maximize) spectral efficiency. Fig. 12 shows the channel responses associated with different users. As shown in fig. 12, the channel responses corresponding to the two users are different. Multi-user adaptive loading increases overall system capacity by allocating subcarriers with relatively high signal-to-noise ratios to users. The frequency response is sent to a subcarrier allocator at the base station for adaptive user allocation, so that only subcarriers with relatively high signal-to-noise ratios are allocated to users for downlink transmission. Also, the FEC coding and modulation scheme may be adaptive depending on the signal-to-noise ratio of one or more subcarriers.

In one embodiment, the downlink SNR per subcarrier is measured by the user. This information is sent back to the base station subcarrier allocator, which collects SINR information from all users. The subcarrier allocator may then perform a best or sub-best allocation algorithm to allocate subcarriers having relatively high SNRs to users. In another embodiment, described below, SNR information is derived directly from the uplink signal transmitted by each user. Both techniques will be described in detail below. The two techniques for collecting SNR information can also be combined. The two techniques are combined, for example, by using a weighted average of the two. In addition, such combining may be time-based, where each technique operates at a different time and is discontinuous, and uses SNR information derived from both techniques.

Fig. 5 is a block diagram of a user terminal receiver having the function of processing a received OFDM signal in reverse order of the processing shown in fig. 4. The resulting data for the user is sent to its upper data link layer.

In one embodiment, the received signal is sampled in a time sequence, the samples being stored in a memory. Upon receiving a predetermined number of samples (e.g., 512 samples), the serial-to-parallel connector 506 converts the incoming OFDM signal (sample format) to a parallel format in a manner well known in the art. The output of the serial-to-parallel converter 506 is received by a Fast Fourier Transform (FFT)505, which implements the fast fourier transform in a manner well known in the art. The output of the FFT505 is sent to one of the different path numbers. That is, these outputs of the FFT505 are coupled to a plurality of processing paths labeled 1 through n.

Each processing path includes a demodulator 504 that demodulates the signal using a demodulation technique to reverse the modulation applied by the base station as described above. The receiver then deinterleaves the demodulated signal using a deinterleaver 503 in a manner well known in the art. The receiver takes the reordered demodulated data from the deinterleaver 503 using the FEC decoder 502 to produce user data 501 in a manner well known in the art. In one embodiment, the output of the FEC decoder 502 is clustered data.

The demultiplexer (Demux)507 may be part of the Media Access Control (MAC) layer that operates on the FEC decoder 502₁To 502_nGenerates user data from a plurality of subcarriers by demultiplexing 502₁To 502_nIn the above sub-carriers, the user data is carried on the cluster of sub-carriers.

Note that in a software implementation of the receiver in which the processing modules in fig. 5 are implemented in software, the signals received with the antenna are sampled and the samples are stored in memory for processing by the respective processing modules.

Fig. 6 is a block diagram of one embodiment of a user terminal transmitter using DSSS/CDMA for uplink communications. Referring to fig. 6, uplink data is first encoded with a forward error correction code in an FEC encoder 602 and then interleaved by an interleaver 603 in the same manner as described above. The receiver then modulates the interleaved data via a modulator 604. For the modulated interleaved data, the receiver provides the spreading code of the user after modulation via a spreading processing module 605. The spread spectrum signal is pulse-shaped and transmitted through a Radio Frequency (RF) channel.

Fig. 7 is a block diagram of one embodiment of a base station with a receiver and a downlink subcarrier allocator. Referring to fig. 7, there are n processing paths coupled to downlink OFDM subcarrier allocator 707. In one embodiment, each processing path is for a single user. Since all other paths perform the same way, only one of the paths will be described.

Note that in a software implementation of the receiver in which the processing modules in fig. 7 are implemented in software, the signals received using the antennas are sampled and the samples are stored in memory for processing by the respective processing modules.

The received signal samples are input to a correlator 701 which despreads the samples with the same spreading sequence used throughout the transmission and correlates the input signal with the spreading code of the user. In an alternative embodiment, the correlator 701 may be replaced with a matched filter. The receiver inputs the output of the correlator 701, which is a correlation result, to the rake receiver 702 and the channel estimator 703. Rake receiver 702 processes the correlation results by means of maximal ratio combining in a manner well known in the art, including performing demodulation and outputting the processed results to deinterleaver 705. The deinterleaver 705 performs deinterleaving and outputs the unscrambled data to the FEC decoder 706. FEC decoder 706 performs FEC decoding in a manner well known in the art. The output of FEC decoder 705 is user data. The decoded data is then sent to the upper data link layer.

The channel estimator 703 estimates the channel response and provides the estimate to the rake receiver 702 and the FFT 704. The rake receiver 702 uses the channel estimates to determine which fingers (fingers) to select for combing (combining). FFT 704 transforms the channel response to a frequency response in a manner well known in the art.

An allocator 707 receives the frequency response for the multiple users from FFT 704 and allocates subcarriers based on the received response.

In one embodiment, each user is assigned a unique spreading sequence. Furthermore, the uplink transmission signal may contain a unique training sequence, as described below in connection with fig. 8. The sequence is used at the base station to estimate the channel. Once the channel time response is estimated, its frequency response is derived with FFT 704. The frequency responses of all users are sent to subcarrier allocator 707 for adaptive subcarrier allocation, as shown in fig. 7.

In one embodiment, after channel estimation, the size of FFT 704 in number of processing points is the same as that used for downlink OFDM transmission. In another embodiment regarding subcarrier clustering, the size of the channel estimated FFT 704 is smaller than the FFT used for downlink OFDM transmission. For example, if the size of the FFT 704 for downlink OFDM is 512 and the number of consecutive subcarriers in the cluster is 16, only a 32-point FFT is required for the channel frequency response at the base station receiver.

In another embodiment, the channel frequency response of the associated user is estimated based on the uplink spread spectrum signal without using a training sequence or pilot signal. The frequency response is estimated to within the phase ambiguity and the amplitude response is utilized in the subcarrier allocation.

Fig. 8 illustrates one embodiment of a data format for a CDMA signal in a time frame. Referring to fig. 8, data symbols 801 and 803 flank an optional training symbol, referred to herein as a midamble 802. Optional training symbols (midambles) are preferably inserted in the middle of the frame, which can be used for channel estimation for CDMA signal correlation detection. The spreading code of the midamble may be different from the spreading code of the data symbols. For midambles, long spreading codes (e.g., twice as long) can improve the channel estimation of the receiver and thus improve overall performance.

The uplink CDMA signals from the subscriber units may be synchronous or asynchronous. For synchronous CDMA, each uplink signal arriving at the base station is aligned in time. This simplifies receiver operation at the base station. For example, with respect to FIG. 7, the correlation processing for all of the users may be combined, for example, using a multi-dimensional signal transform.

In one embodiment, all subscriber units are synchronized in time and frequency with their base stations. The base station broadcasts a "beacon signal" periodically, followed by a regular OFDM symbol. The beacon signal is used for synchronization by the subscriber unit and preferably occurs once in a time frame of, for example, 10 microseconds. In one embodiment, the beacon signal itself is an OFDM signal or a plurality of OFDM signals. In another embodiment, the beacon signal includes a spread spectrum Pseudonoise (PN) sequence, as shown in fig. 9. Referring to fig. 9, although only four PN sequences are shown, any number may be used. In one embodiment, the first PN sequence PN1, or some other predetermined number of PN sequences, can be used for time synchronization at the subscriber unit by sequence correlation in a manner well known in the art. The PN sequences (PN2 following PN1) are preferably identical and can be used at the subscriber unit by sequence correlation to frequency track and compare the phase difference between the correlation result pairs. In one embodiment, there must be multiple PN2 sequences and they are shorter than the PN1 sequence.

In one embodiment, a switch in a transmitter having a single output and a pair of inputs, one of which is coupled to receive a PN sequence from a PN sequence generator and the other of which is coupled to an FFT output, is coupled to output data having the format shown in fig. 9.

In one embodiment, pilot subcarriers are inserted into the OFDM symbol, as shown in fig. 10, so that the subscriber unit can further measure and correct carrier frequency error (frequency tracking).

In one embodiment, the uplink CDMA signals are power controlled to reduce and potentially minimize interference with each other. The power control may be performed in an open or closed loop manner, preferably by a combination of both. A power control unit at the user controls its transmission power. The power control unit receives power adjustment commands, which may be locally generated (open loop) or received from the base station (closed loop). For open loop power control, the subscriber unit monitors the downlink signal power to adjust its own transmit power. Since the CDMA signal is wideband and the multi-user OFDM downlink signal may not occupy the full bandwidth as shown in fig. 2, there may be a mismatch in the downlink and uplink power measurements. One way to solve this problem is to always transmit a full bandwidth pilot OFDM symbol in the downlink, as shown in fig. 2. The subscriber unit measures the downlink pilot symbol power to adjust its transmission power. One embodiment of a user is shown in U.S. patent application Ser. No. 09/38,086 entitled "OFDM with added negative sub-container-Cluster configuration and selected Loading," filed on 12/15/2000, assigned to the assignee of the present invention, and incorporated herein by reference. In closed loop power control, the power of the uplink CDMA signal is measured at the base station receiver. The power adjustment required by each subscriber unit is then done on the downlink transmission signal. For closed loop power control, the base station measures the uplink power and sends power control commands to the users instructing the users to adjust their power levels.

In one embodiment, the downlink and uplink transmissions are arranged by Frequency Division Duplexing (FDD). In this case, the transmission and reception are separated by an RF duplexer. In another embodiment, the downlink and uplink are arranged by Time Division Duplexing (TDD). In this case, the time switch controls transmission and reception.

In another embodiment, CDMA is used for downlink and uplink transmissions. To further enhance the data rate of the downlink, additional OFDM channels are used as shown in fig. 11. Referring to fig. 11, two communication systems (e.g., communication units, stations, etc.) are shown in the duplex system. Communication system 1150 includes a CDMA transmitter 1101, a CDMA receiver 1102, and an OFDM receiver 1103 coupled to an antenna 1105 via a switch or duplexer 1104. Similarly, communication system 1151 comprises a CDMA receiver 1108, a CDMA transmitter 1110, and an OFDM transmitter 1109 coupled to an antenna 1106 via a switch or duplexer 1107.

In one embodiment, the CDMA transmitter and receiver pairs in each communication system are implemented as CDMA transceivers. In one embodiment, both systems include a CDMA transceiver and an OFDM transceiver, the OFDM transceiver including an OFDM transmitter and an OFDM receiver.

Although fig. 11 shows a point-to-point connection, the system may include other units (e.g., users) with CDMA transmitters and receivers and OFDM transmitters or receivers or both. Likewise, other units may be in a communication system and have a CDMA transmitter and a CDMA receiver without OFDM communication capability. On the other hand, the additional units may have OFDM communication capability (OFDM transmitter and/or receiver) but no CDMA communication capability.

In one embodiment, referred to herein as turbo mode, each channel (e.g., CDMA downlink, CDMA uplink, and OFDM downlink) occupies a different frequency spectrum. For example, the CDMA downlink may use 5MHz channels in the frequency range 2110-2170MHz while the CDMA uplink may use 5MHz channels in the 1920-1980MHz range, while the OFDM downlink may use 5 or 10MHz channels over the high frequency range. In turbo mode, the pilot signal used for subscriber unit synchronization can be carried on a downlink CDMA channel or a downlink OFDM channel. The power control signal can also be carried on a downlink CDMA channel or a downlink OFDM channel. When a pair of CDMA uplink and downlink channels has been established, such as IS-95 CDMA system or UMTS W-CDMA system, it IS preferable to use the downlink CDMA channel for uplink synchronization and power control and to use the beacon symbols, pilot subcarriers embedded in the OFDM channel for receiving the downlink OFDM signal. The overhead of these synchronization symbols can be further reduced if the synchronization signals of the CDMA downlink are effectively utilized.

An exemplary subcarrier/cluster allocation scheme

FIG. 13 is a flow diagram of one embodiment of a process for assigning clusters to users. The process is performed by processing logic that may comprise hardware (e.g., dedicated logic and circuitry), software (such as is run on a general purpose computer system or a dedicated machine), or a combination of both.

Referring to fig. 13, each base station periodically broadcasts pilot OFDM symbols to each user within its cell (or sector) (processing module 101). The pilot symbols, commonly referred to as sounding sequences or signals, are known to the base station and the user. In one embodiment, each pilot symbol covers the entire OFDM frequency bandwidth. The pilot symbols for different cells (or sectors) may be different. The pilot symbols may be used for a variety of purposes: time and frequency synchronization for cluster-assigned channel estimation and signal-to-interference/noise (SINR) ratio measurement.

Next, each user continuously monitors the reception of pilot symbols and measures SINR and/or other parameters, including inter-cell interference and intra-cell traffic for each cluster (processing block 1302). Based on this information, each user selects one or more clusters that have good performance relative to each other (e.g., high SINR and low traffic load) and feeds back information about these candidate clusters to the base station over a predetermined uplink access channel (processing module 1303). For example, SINR values above 10dB may indicate good performance. Similarly, a cluster utilization below 50% may also indicate good performance. Each user selects a cluster that has relatively better performance than the other clusters. This selection results in each user selecting the cluster that they are willing to use based on the measured parameters.

In one embodiment, each user measures the SINR of each user cluster and reports these SINR measurements to its base station over the access channel. The SINR value may comprise an average of SINR values for each subcarrier in a cluster. Alternatively, the SINR value of a cluster may be the worst SINR among the SINR values of subcarriers in the cluster. In another embodiment, the SINR value of a weighted average of subcarriers in a cluster is used to generate the SINR value for the cluster. This is particularly useful in clusters of diversity differences, where the weights applied to the subcarriers may be different.

The information feedback from each user to the base station contains the SINR value for each cluster and also indicates the coding/modulation rate the user desires to use. As long as the base station knows the order of the information in the feedback, no cluster index is needed to indicate which SINR in the feedback corresponds to which cluster. In an alternative embodiment, the information in the feedback is ordered according to which cluster has the best performance for the user relative to each other. In this case, an index is needed to indicate the cluster corresponding to the associated SINR value.

Based on the feedback received from the users, the base station further selects one or more clusters for the users among the candidate clusters (processing module 1304). The base station may utilize additional information available at the base station, such as: traffic load information on each subcarrier, the amount of traffic requests queued at the base station for each band, whether the band is over-utilized, and how long the user has been waiting to transmit information. Subcarrier load information of neighboring cells can also be exchanged between base stations. The base station may use this information in subcarrier allocation to reduce inter-cell interference.

After cluster selection, if a connection to the user has been established, the base station informs the user about the cluster assignment either through a downlink common control channel or through a dedicated downlink traffic channel (processing block 1305). In one embodiment, the base station also informs the user about the appropriate modulation/coding rate.

Once the basic communication link is established, each user can continue to send feedback to the base station using a dedicated traffic channel (e.g., one or more predetermined uplink access channels).

In one embodiment, the base station allocates all clusters to the user at once. In an alternative embodiment, the base station first allocates a plurality of clusters, referred to herein as base clusters, for establishing data links between the base station and the users. The base station then sequentially allocates a plurality of clusters, referred to herein as secondary clusters, to the users to increase the communication bandwidth. The assignment of the base cluster may be given a higher priority and the assignment of the secondary cluster may be given a lower priority. For example, the base station first ensures that the base cluster is allocated to the user and then tries to fulfill further requests from the user on the secondary cluster. Alternatively, the base station may first assign the secondary cluster to one or more users and then assign the primary cluster to other users. For example, the base station may assign the base and auxiliary clusters to one user before assigning any clusters to other users. In one embodiment, the base station assigns the base cluster to a new user and then determines whether any other users request the cluster. If not, the base station allocates a secondary cluster to the new user.

Processing logic retrains from time to time by repeating the above process (processing block 1306). This retraining may be performed periodically. This retraining compensates for variations in user movement and any interference. In one embodiment, each user reports to the base station an updated selection of its cluster and its associated SINR. The base station further reselects and informs the user about the new cluster allocation. Retraining may be initiated by the base station, and in this case, the base station requests a particular user to report its updated cluster selection. In addition, retraining may also be initiated by the user when observing deterioration of their channel.

Adaptive modulation and coding

In one embodiment, different modulation and coding rates are used to support reliable transmission on channels with different SINRs. Signal spreading over multiple subcarriers may also be employed to improve reliability under very low SINR conditions.

An example of an encoding/modulation table is given in table 1 below.

TABLE 1

Scheme(s)	Modulation	Code rate
			0	QPSK 1/8 spreading	1/2
1	QPSK 1/4 spreading	1/2
			2	QPSK 1/2 spreading	1/2
3	QPSK	1/2
			4	8QPSK	2/3
5	16QAM	3/4
			6	64QAM	5/6

In the above example, 1/8 spreading means that one QPSK modulation symbol is repeated on eight subcarriers. The repetition/spreading may also be extended to the time domain. For example, one QPSK symbol may be repeated on four subcarriers of two OFDM symbols, again resulting in 1/8 spreading.

After initial cluster allocation and ratio selection, the coding/modulation rate can be adaptively changed according to the channel conditions observed at the receiver.

Pilot symbol and SINR measurement

In one embodiment, each base station transmits pilot symbols simultaneously, and each pilot symbol occupies the entire OFDM band, as shown in FIGS. 14A-C. Referring to fig. 14A-C, pilot symbols 1401 are shown traversing the entire OFDM frequency band for cells A, B and C, respectively. In one embodiment, each pilot symbol has a length or duration of 128 microseconds with a guard time, the combination of which is approximately 152 microseconds. After each pilot period, there is a predetermined number of data periods followed by another set of pilot symbols. In one embodiment, there are four data periods for transmitting data after each pilot, and each data period is 152 microseconds.

The user estimates the SINR of each cluster from the pilot symbols. In one embodiment, the user first estimates the channel response, including amplitude and phase, as if there were no interference or noise. When the channel is estimated, the user calculates interference/noise from the received signal.

The estimated SINRs may be sorted from maximum to minimum SINR and the cluster with the largest SINR value is selected. In one embodiment, the selected cluster has a SINR value greater than the minimum SINR that still allows for reliable (low-rate) transmissions supported by the system. The number of clusters selected may depend on the feedback bandwidth and the requested transmission rate. In one embodiment, the user always tries to send the most information about the cluster possible, depending on the choice of base station.

As described above, the estimated SINR values are also used to select the appropriate coding/modulation rate for each cluster. The SINR index may also indicate the particular coding and modulation rate that the user desires to use by using an appropriate SINR indexing scheme. Note that different clusters may have different modulation/coding rates, even for the same user.

The pilot symbols serve additional purposes in determining inter-cell interference. Since the pilots of multiple cells are broadcast simultaneously, they may interfere with each other (since they occupy the entire frequency band). Such collision of pilot symbols may be used to determine the amount of interference as a worst case. Thus, in one embodiment, the above estimation using this method is conservative, where the measured interference level is the worst case, assuming that all interferers are working. Therefore, the structure of the pilot symbol is such that it occupies the entire frequency band and causes collision between different cells, for detecting the worst-case SINR in a packet transmission system.

The user may again determine the interference level throughout the data traffic period. The data traffic period is used to estimate intra-cell traffic as well as inter-cell interference levels. In particular, the power difference in the pilot and traffic periods can be used to know the (intra-cell) traffic load and the inter-cell interference in order to select the desired cluster.

The interference level on a particular cluster may be lower because these clusters may not be used in neighboring cells. For example, in fig. 14, within cell a, there is less interference to cluster a because cluster a is not used in cell B (although it is used in cell C). Also within cell a, cluster B will experience lower interference from cell B because cluster B is used in cell B rather than cell C.

The modulation/coding rate based on the above estimation is robust against frequent interference variations caused by bursty packet transmissions. This is because the rate prediction is based on the worst case scenario, where all interferers are transmitting.

In one embodiment, the user utilizes available information from both the pilot symbol period and the data traffic period in order to analyze the presence of intra-cell traffic load and inter-cell interference. The goal of the user is to provide the base station with an indication of those clusters that the user desires to use. Ideally, the result of the user selection is a cluster with high channel gain, low interference from other cells, and high reliability. The user provides feedback information including the results, listing the desired clusters in order, or content not described herein.

FIG. 15 illustrates one embodiment of user processing. The process is performed by processing logic that may comprise hardware (e.g., dedicated logic, circuitry, etc.), software (such as is run on a general purpose computer system or a dedicated machine), or a combination of both.

Referring to fig. 15, a channel/interference estimation processing module 1501 performs channel and interference estimation in a pilot period in response to pilot symbols. Traffic/interference analysis processing module 1502 performs traffic and interference analysis in data cycles in response to signal information and information from channel/interference estimation module 1501.

The outputs of the channel/interference estimation processing module 1501 and the traffic/interference analysis processing module 1502 are connected to a cluster ordering and rate prediction processing module 1503 for performing cluster ordering and selection and rate prediction.

The output of the cluster ordering processing module 1503 is an input to the cluster request processing module 1504, which cluster request processing module 1504 requests clusters and modulation/coding rates. Indications of these selections are sent to the base station. In one embodiment, the SINR of each cluster is reported to the base station over the access channel. This information is used for cluster selection to avoid clusters with severe intra-cell traffic load and/or strong interference from other cells. That is, a new user may not be assigned to use a particular cluster if the cluster already has a severe intra-cell traffic load. Also, the cluster will not be allocated if the interference is so strong that the SINR only allows low rate transmission or completely unreliable transmission.

The channel/interference estimation by processing module 1501 is well known in the art by monitoring the interference due to the simultaneous broadcast of full-band pilot symbols in multiple cells. The interference information is passed to a processing module 1502, which uses the information to solve the following equation:

H_iS_i+I_i+n_i＝y_i

wherein S_iSignals representing sub-carriers (bands) I, I_iIs the interference of subcarrier i, n_iIs the noise associated with subcarrier i, and y_iIs the observed value of subcarrier i. In the case of 512 subcarriers, i may range from 0 to 511. I is_iAnd n_iAre not separated and can be considered as a quantity. Interference/noise and channel gain H_iIs unknown. In the whole pilot period, signal S_iRepresents a pilot symbol and observes a value y_iIs known so as to allow the determination of the channel gain H in the absence of interference or noise_i. Once the value is known, it can be substituted back into the formula to determine the interference/noise in the data cycle, since H is now_i、S_iAnd y_iAre all known.

The user uses the interference information from processing modules 1501 and 1502 to select the desired cluster. In one embodiment, the user uses the processing module 1503 to sort the clusters and also predict the available data rate when using the clusters. The predicted data rate information may be obtained from a look-up table having pre-calculated data rate values. Such a look-up table may store pairs of SINRs and their associated expected transmission rates. Based on this information, the user selects the cluster desired to be used according to predetermined performance criteria. The user uses the ordered list of clusters along with the coding and modulation rates known to the user to request the desired cluster to achieve the desired data rate.

FIG. 16 shows one embodiment of an apparatus that selects a cluster based on power difference. The scheme performs energy detection using available information in both pilot symbol periods and data traffic periods. The process of fig. 16 may be implemented in hardware (e.g., dedicated logic, circuitry, etc.), software (e.g., running on a general purpose computer system or a dedicated machine), or a combination of both.

Referring to fig. 16, the user includes an SINR estimation processing module 1601 to perform SINR estimation for each cluster in the pilot period, a power calculation processing module 1602 to perform power calculation for each cluster in the pilot period, and a power calculation processing module 1603 to perform power calculation for each cluster in the data period. Subtractor 1604 subtracts the power calculations in the data period from processing block 1603 from those in the pilot period from processing block 1602. The output of the subtractor 1604 is input to a power difference ranking (and group selection) processing module 1605, and the processing module 1605 performs cluster ranking and selection based on SINR and power differences between pilot and data periods. Once a cluster is selected, the user requests the selected cluster and coding/modulation rate by means of the processing module 1606.

More particularly, in one embodiment, the signal power of each cluster in the pilot period is compared to the traffic period according to the following equation:

P_P＝P_S+P_I+P_N，

wherein P is_PIs the measured power, P, for each cluster in the pilot period_DIs the measured power, P, in the traffic period_SIs the signal power, P_IIs the interference power, and P_NIs the noise power.

In one embodiment, the user selects to have a relatively high P where possible_D/(P_P-P_D) Is (e.g. greater than a threshold such as 10 dB) and avoids having a low P_P/(P_P-P_D) (e.g., such as below a threshold of 10 dB).

Alternatively, for each subcarrier in the cluster, the difference may be based on the energy difference between samples observed in the pilot period and in the data traffic period, which may be, for example:

img id="idf0003" file="A20071013913600261.GIF" wi="104" he="20" img-content="drawing" img-format="GIF"/

therefore, the user sums up the difference values of all subcarriers.

Depending on the actual implementation, the user may use the following metrics, which are SINR and P_P-P_DA combination function of the two to select a cluster:

β＝f(SINR，P_P/(P_P-P_D)

where f is a function of two input quantities. An example of f is a weighted average (e.g., isobaric weighting). Alternatively, the user may select a cluster based on their SINR and use only the power difference P_P-P_DClusters with the same SINR are distinguished. The difference may be less than a certain threshold (e.g., 1 dB).

SINR and P_P-P_DMay be averaged over time to reduce variation and improve accuracy. In one embodiment a moving average window is used that is long enough to neutralize statistical irregularities but short enough to capture the time varying characteristics of the channel and interference, e.g., 1 millisecond.

Feedback format for downlink cluster allocation

In one embodiment, for the downlink, the feedback contains an indication of the selected cluster and its SINR. An example format of arbitrary cluster feedback is shown in fig. 17. Referring to fig. 17, the user provides a cluster Index (ID) to indicate the cluster and its associated SINR value. For example, in the feedback, the user provides the cluster ID1(1701) and SINR of the cluster 1(1702), the cluster ID2(1703) and SINR of the cluster 2(1704), and the cluster ID3(1705) and SINR of the cluster 3(1706), etc. The SINR of the cluster may be generated using an average of the SINRs of the subcarriers. Therefore, a plurality of arbitrary clusters can be selected as candidates. As described above, the selected clusters may also be sorted in feedback to indicate priority. In one embodiment, the users may form a priority list of the cluster and send back the SINR information in order of degradation of priority.

Typically, an index to the SINR level, rather than the SINR itself, is sufficient to indicate the proper coding/modulation of the cluster. For example, the SINR indexing may be done with a 3-bit field to indicate 8 different ratios of adaptive coding/modulation.

Exemplary base station

The base station assigns the desired cluster to the requesting user. In one embodiment, the availability of a cluster assigned to a user depends on the overall traffic load on the cluster. Therefore, the base station selects a cluster having not only a high SINR but also a low traffic load.

Fig. 18 is a block diagram of one embodiment of a base station. Referring to fig. 18, the cluster allocation and load scheduling controller 1801 (cluster allocator) collects all necessary information including SINR (e.g., SINR/ratio index signal 1813 received by OFDM transceiver 1805) and user data, queuing fullness/traffic load (e.g., via user data buffering information 1811 from multi-user data buffer 1802) specifying the downlink/uplink of the cluster for each user. The controller 1801 uses this information to make a decision on cluster allocation and load scheduling for each user, and stores the decision information in a memory (not shown). The controller 1801 informs the user of the decision via a control signal channel (e.g., via control signal/cluster assignment 1812 of the OFDM transceiver 1805). The controller 1801 updates the decision during retraining.

In one embodiment, the controller 1801 also exercises admission control over user access because it is aware of the traffic load of the system. This may be accomplished by controlling user data buffer 1802 using admission control signal 1810.

Packet data of users 1 to N is stored in the user data buffer 1802. For the downlink, the multiplexer 1803 loads user data waiting to be transmitted into a cluster data buffer (cluster 1-M) by means of control of the controller 1801. For the uplink, the multiplexer 1803 transmits the data in the cluster buffer to the corresponding user buffer. The cluster buffer 1804 stores signals to be transmitted through the OFDM transceiver 1805 (for downlink) and signals received from the transceiver 1805. In one embodiment, each user may occupy multiple clusters and each cluster may be shared by multiple users (in a time-multiplexed manner).

It will of course be apparent to those skilled in the art that many alternatives and modifications may be made to the invention, after reading the foregoing description, and it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, the descriptions and details of the various embodiments are not intended to limit the scope of the claims, which in themselves recite only those features regarded as essential to the invention.

Claims (42)

1. A method, comprising:

receiving an Orthogonal Frequency Division Multiplexing (OFDM) signal; and

a direct sequence spread spectrum, DSSS, signal is transmitted to a base station in response to the received OFDM signal.

2. The method of claim 1, wherein the receiving a DSSS signal comprises transmitting a Code Division Multiple Access (CDMA) signal.

3. The method of claim 2, wherein said transmitting a Code Division Multiple Access (CDMA) signal comprises transmitting a W-CDMA signal.

4. The method of claim 1, wherein the transmitted DSSS signal comprises a transmitted signal comprising a signal quality metric of the received OFDM signal.

5. A method according to claim 1, comprising determining a quality measure of the received OFDM signal.

6. The method of claim 5, wherein said determining a quality metric comprises determining a signal-to-noise ratio of the received OFDM signal.

7. The method of claim 1, wherein the receiving an OFDM signal comprises receiving one or more pilot signals.

8. The method defined in claim 1 wherein receiving the OFDM signals comprises receiving OFDM signals at a first frequency range and wherein transmitting DSSS signals comprises transmitting DSSS signals at a second frequency range.

9. The method of claim 8, wherein the first frequency range and the second frequency range do not overlap.

10. A subscriber unit, comprising:

a direct sequence spread spectrum DSSS transmitter; and

an orthogonal frequency division multiplexing, OFDM, receiver, wherein the subscriber unit is configured to transmit a DSSS signal from the DSSS transmitter in response to an OFDM signal received by the OFDM receiver.

11. The subscriber unit of claim 10, wherein the DSSS transmitter comprises a code division multiple access transmitter.

12. The subscriber unit of claim 10, wherein the DSSS transmitter comprises a DSSS transceiver.

13. The subscriber unit of claim 10, comprising a switch coupled to the DSSS transmitter and the OFDM receiver.

14. The subscriber unit of claim 13, comprising an antenna coupled to the switch.

15. A subscriber unit as claimed in claim 10, wherein the OFDM receiver is configured to determine a quality measure of the OFDM signal.

16. The subscriber unit of claim 10, wherein the subscriber unit comprises a mobile unit.

17. The subscriber unit of claim 10, wherein the OFDM receiver comprises: a Fast Fourier Transform (FFT) unit for performing FFT transform on the OFDM signal; and

a plurality of processing channels coupled to respective outputs of the FFT unit, each of the plurality of processing channels having a demodulator coupled to one of the respective outputs of the FFT unit.

18. The subscriber unit of claim 17, wherein the OFDM receiver comprises:

a deinterleaver coupled to the output of one or more of the demodulators; and

a forward error correction decoder coupled to an output of the deinterleaver.

19. The subscriber unit of claim 10, wherein the OFDM receiver and the DSSS receiver are configured to operate on non-overlapping frequency bands.

20. The subscriber unit of claim 10, wherein the subscriber unit is configured to demultiplex the OFDM signal based on time.

21. One method comprises the following steps:

receiving a direct sequence spread spectrum, DSSS, signal from a subscriber unit, wherein the DSSS signal is generated by the subscriber unit in response to the received orthogonal frequency division multiplexed, OFDM, signal.

22. The method of claim 21, wherein the receiving a DSSS signal comprises receiving a quality metric associated with the OFDM signal.

23. A method as claimed in claim 22, comprising transmitting the quality measure to an OFDM transmitter.

24. The method of claim 23, wherein the transmission quality metric comprises a transmission quality metric to an OFDM transmitter located at a base station, wherein the base station is configured as a DSSS signal from the subscriber unit.

25. A method of communicating with a user, the method comprising:

transmitting an orthogonal frequency division multiplexed, OFDM, signal to the user, wherein receiving the OFDM signal by the user results in a direct sequence spread spectrum, DSSS, signal generated by the user.

26. The method of claim 25, wherein the transmitting comprises:

allocating a first portion of the OFDM subcarriers to a first user; and

transmitting an OFDM signal to the first user according to the allocation.

27. The method of claim 26, wherein the transmitting further comprises:

allocating a second portion of the OFDM subcarriers to a second user; and

and transmitting the OFDM signal to the second user according to the allocation.

28. The method of claim 26, wherein the assigning is responsive to a quality metric received from the first user.

29. The method of claim 28, wherein the quality metric comprises a signal-to-noise ratio.

30. The method of claim 26, wherein the allocation is adaptive.

31. The method of claim 26, wherein the allocating comprises allocating only subcarriers determined to have at least one minimum quality metric.

32. A method according to claim 25, comprising receiving a signal representing a quality measure associated with the OFDM signal.

33. The method of claim 25, wherein said receiving a signal comprises receiving a signal representative of a signal-to-noise ratio of the OFDM signal.

34. A method according to claim 25, comprising selecting a code rate based at least in part on the quality measure.

35. The method of claim 25, comprising selecting a modulation scheme based at least in part on the quality metric.

36. The method of claim 25, wherein the transmitting comprises:

transmitting an OFDM signal of a first frequency range; and wherein generating the DSSS signal comprises:

a DSSS signal of a second frequency range is generated.

37. The method of claim 36, wherein the first frequency range and the second frequency range do not overlap.

38. A method of communication, the method comprising:

receiving an Orthogonal Frequency Division Multiplexing (OFDM) signal;

demultiplexing a plurality of subcarriers of the received OFDM signal;

identifying fewer than all of the separated subcarriers associated with the user;

outputting user data associated with the identified subcarriers; and

transmitting a Direct Sequence Spread Spectrum (DSSS) signal to a base station in response to the received OFDM signal.

39. A method of communication, the method comprising:

transmitting an orthogonal frequency division multiple access, OFDMA, signal to at least one user; and

receiving a direct sequence spread spectrum, DSSS, signal from the at least one user at a base station;

wherein the base station and the at least one user comprise link segments of a bidirectional communication link.

40. The method of claim 39, wherein the bidirectional communication link is asymmetric.

41. A method of communication, the method comprising:

receiving an orthogonal frequency division multiple access, OFDMA, signal at a subscriber station; and

transmitting a direct sequence spread spectrum, DSSS, signal from the subscriber station to a base station;

wherein the base station and the user comprise link segments of a bidirectional communication link.

42. The method of claim 41, wherein the bidirectional communication link is asymmetric.