WO2006020568A1

WO2006020568A1 - Channel sounding for improved system performance

Info

Publication number: WO2006020568A1
Application number: PCT/US2005/028122
Authority: WO
Inventors: Joseph S. Levy
Original assignee: Interdigital Technology Corporation
Priority date: 2004-08-11
Filing date: 2005-08-09
Publication date: 2006-02-23
Also published as: AR050836A1; JP5254282B2; US9444531B2; EP1779702A1; CA2577980A1; US20060034165A1; US8917583B2; US20160087699A1; KR20130095239A; TWM289003U; KR20130021423A; ES2395941T3; KR20120085687A; KR20060050414A; MX2007001710A; EP1779702B1; US20160380712A1; CN101002498A; TWI385943B; US20150030095A1

Abstract

A transmitter generates and transmits a low rate signal to its intended receiver. Upon receiving the low rate signal, the intended receiver generates and transmits a channel sounding response (CSR), said CSR being a short burst having a predefined transmit format and carrying predetermined information. The transmitter then analyzes the CSR and determines uplink channel response, estimates downlink channel response, and determines appropriate transmit parameter settings based on the analysis and downlink response estimate. Adjustment of the transmit parameters can be made in either the MAC or PHY layer or in a combination of both. After adjusting its transmit parameters and modulating sub-carriers with user-data according to the determined transmit settings, the transmitter transmits the user-data to the receiver on a preferred portion of bandwidth. In a preferred embodiment, the transmitter also generates and transmits a transmit format control (TFC) signal containing the determined transmit parameter settings, including sub-carrier modulation information, to the receiver.

Description

[0001] CHANNEL SOUNDING FOR IMPROVED SYSTEM PERFORMANCE

[0002] FIELD OF INVENTION

[0003] The present invention relates to wireless communication systems.

More particularly, the present invention is a method and apparatus for improving channel and system performance in a wireless communication system.

[0004] BACKGROUND

[0005] Orthogonal Frequency Division Multiplexing (OFDM) refers to a data transmission scheme wherein user-data is split into smaller data streams and transmitted using sub-carriers that each has a smaller bandwidth than the total available transmission bandwidth. The efficiency of OFDM results from the orthogonality of the sub-carriers. That is to say, the sub-carriers are selected such that they do not interfere with each other during transmission, thus resulting in an efficient transmission scheme.

[0006] Multiple-Input Multiple-Output (MIMO) refers to a wireless transmission and reception scheme wherein both transmitter(s) and receiver(s) employ multiple antennas for transmission and reception. A MIMO system takes advantage of the spatial diversity or spatial multiplexing options created by the presence of the multiple antennas to increase throughput. [0007] A continuing challenge for OFDM-MIMO systems is system performance, i.e., capacity, reliability, etc. Towards this end, many techniques have been proposed for improving, for instance, channel capacity and/or reliability. An example of one such technique is referred to as "water-filling", another example is power control. Water-filling and power control describe processes whereby a transmitter estimates channel conditions using feedback signals from a receiver in the system. Based on these estimates, the transmitter attempts to transmit user data in a way that optimizes channel performance in view of the channel conditions. As with similar techniques, water-filling and power control rely upon knowledge of the transmission channel, via feedback signals, to optimize channel performance. The signaling overhead associated with these feedback signals, however, is significant and often limits any potential increase in system performance. In addition, generating and transmitting feedback signals causes delays which also limit potential increases in system performance. These drawbacks to feedback signaling are particularly evident in systems with rapidly changing channel conditions, systems transmitting large amounts of data, and/or systems utilizing a large number of sub-carriers. [0008] Accordingly, it is desirable to have a method and apparatus for efficiently estimating current channel conditions for use in improving overall system performance in OFDM-MIMO systems.

[0009] SUMMARY

[0010] The present invention is a method and apparatus for improving system performance in Multiple-Input, Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) wireless communication systems. A transmitter generates and transmits a low rate signal to its intended receiver. Upon receiving the low rate signal, the intended receiver generates and transmits a channel sounding response (CSR), said CSR being a short burst having a predefined transmit format and carrying predetermined information. The transmitter then analyzes the CSR and determines uplink channel response, estimates downlink channel response, and determines appropriate transmit parameter settings based on the analysis and downlink response estimate. Adjustment of the transmit parameters can be made in either the MAC or PHY layer or in a combination of both. After adjusting its transmit parameters and modulating sub-carriers with user-data according to the determined transmit settings, the transmitter transmits the user-data to the receiver on a preferred portion of bandwidth. In a preferred embodiment, the transmitter also generates and transmits a transmit format control (TFC) signal containing the determined transmit parameter settings, including sub-carrier modulation information, to the receiver. [0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a flow diagram illustrating a channel sounding scheme for improving system performance in Multiple-Input, Multiple-Output (MIMO)

Orthogonal Frequency Division Multiplexing (OFDM) communication systems;

[0013] Figure 2 is a MIMO-OFDM transmitter-receiver pair configured to use channel sounding pulses to improve system performance; and

[0014] Figure 3 is MIMO-OFDM wireless communication system wherein a base station and a wireless transmit/receive unit (WTRU) each comprise a transmitter-receiver pair in accordance with the present invention.

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0016] Herein, a wireless transmit/receive unit (WTRU) includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to herein, a base station includes but is not limited to a Node-B, site controller, access point or any other type of interfacing device in a wireless environment.

[0017] In a preferred embodiment, channel sounding pulses are used for improving channel and system performance in Orthogonal Frequency Division Multiplexing (OFDM) systems utilizing Multiple-Input, Multiple-Output (MIMO) equipment. The sounding pulses enable MIMO-OFDM transmitters, for instance, to assess current channel conditions and hence, to format transmit data packets that optimize throughput in view of the channel conditions. [0018] In accordance with the present embodiment, a MIMO-OFDM transmitter generates and transmits a low rate signal, such as a request for a sounding pulse (CSRq) to an intended receiver. Upon receiving this request, the receiver generates a channel sounding response (CSR) and transmits it to the requesting transmitter. This CSR is preferably a short burst formatted with predetermined transmit parameters that assure its successful reception given the particular system configuration and environment. Included in the CSR is information known to the transmitter. The transmitter, upon receiving the CSR, processes the information and determines current channel conditions. Based on these determinations, the transmitter modulates user data to sub-carriers and adjusts its transmit parameters to maximize channel capacity, reliability, and/or any other channel performance characteristic as required by a user, using any of the various channel optimization techniques including water-filling and power control. Utilizing CSR pulses to assess channel conditions, rather than conventional feedback signals, enables the transmitter to format and transmit data packets that optimize channel performance without incurring all of the overhead and delay of conventional channel-improvement approaches. [0019] Referring now to Figure 1, a flow diagram 100 illustrating a channel sounding scheme for improving system performance in MIMO-OFDM wireless communication systems is shown. Prior to transmitting data, a transmitter generates a channel sounding response request in the form of a low rate signal and transmits it to a receiver (step 102). This request is preferably a low data rate signal, such as a data packet header, containing source (i.e., transmitter) and destination (i.e., intended receiver) information. Upon receiving and processing the low rate signal (step 104), the receiver generates and transmits a predefined channel sounding response (CSR) (step 106), preferably as a short burst or pulse, to the transmitter. The CSR is preferably predefined in terms of its size, number of symbols, amplitude, etc., to assure successful reception at the transmitter given the particular system configuration and/or the receiver's allocated resources. Included in the CSR is information the transmitter may use in assessing current channel conditions.

[0020] At the transmitter, the CSR is received and information transmitted as part of the CSR is processed (step 108) and utilized to characterize the current channel conditions (step 110). This characterization includes determining uplink channel response via measuring the amplitude, phase, and quality of each received sub-carrier at each antenna; and estimating downlink channel response. If a particular sub-carrier indicates a high error rate, for instance, the transmitter will not modulate that sub-carrier with large amounts of data. Conversely, if a particular sub-carrier arrives at the transmitter with a relatively low error rate, the transmitter will more heavily modulate that sub-carrier with user data.

[0021] Once the channel conditions are known in the uplink and estimated for the downlink (step 110), the transmitter determines appropriate transmit parameter settings (step 112), (e.g., antenna selection, antenna power, bandwidth selection, earner power, carrier coding, carrier modulation, etc.), makes the appropriate parameter adjustments (step 114), and accordingly modulates its sub-carriers (step 116), preferably using a water-filling, power control, or similar technique. It should be noted that the parameter adjustments may occur in the MAC layer, in the PHY layer, or in a combination of the two. The formatted data packets are then transmitted on select portion of bandwidth to the receiver (step 118). Optionally, the transmitter tracks the channel performance estimates derived from current and previous CSR measurements (step 112a), enabling the transmitter to predict future channel conditions for use in optimizing the channel performance of future data transmissions.

[0022] It should be understood that the overall channel performance of a communication link remains relatively static even though the performance of a particular sub-carrier and/or antenna pair may change quite rapidly. This is particularly true if the communication link has sufficient bandwidth and spatial diversity. Accordingly, the size of the transmitted data packets may be fixed, leaving only the encoding parameters to be adjusted, which can occur in near real time based on received CSRs. Transmitting fixed-sized data packets greatly simplifies the MAC layer's complexity. There is, however, some added complexity required in the PHY layer, particularly if the PHY layer is configured to determine and implement the final encoding scheme

[0023] Prior to, after, or in parallel with transmitting the formatted data packets (step 118), the transmitter may optionally generate and send a transmit format control (TFC) signal to the receiver (step 120). This TFC signal includes information regarding the transmit parameter settings and identifies which sub- carriers have been modulated by which modulation schemes (e.g., QPSK, 16 QAM, 256 QAM, etc.), and/or which coding types and data rates have been used. Providing this type of information to the receiver as part of the TFC signal is an enhancement which simplifies overall receiver decoding complexity. Alternatively, if a TFC signal is not generated or not successfully received at the receiver, the receiver may determine TFC information on its own via a trial and error method, hereinafter referred to as "blind TFC detection". [0024] To further improve the overall system performance, the transmitter and/or receiver may monitor CSR signals emitted by other receiver(s) in the system, assess the communication link between themselves and the receiver(s) emitting the CSRs, and maintain a history of these channel conditions for use in future communications with that receiver.

[0025] Referring now to Figure 2, a MIMO-OFDM transmitter 202 and receiver 204 configured in accordance with the present invention are shown. Included in the transmitter 202 is a channel sounding signal processor 201 for generating low rate sounding request signals, for processing received channel sounding response signals, and preferably, for assessing channel conditions of a communication link between itself and receivers. In addition, the transmitter

202 includes a MAC layer processor 203 for setting data transmit parameters including data rates, coding schemes, packet formats, etc., a physical (PHY) layer processor 205 for spreading data bits across sub-carriers and across transmit antennas 207i, 2072, ...207n according to the MAC parameter setting processor

203 or optionally, according to the PHY layer processor's 205 own transmit parameter settings, an optional transmit format control (TFC) processor 206 for processing information from the MAC processor 203 and/or the PHY layer processor 205, an optional signal monitoring processor 208 for monitoring CSR signals transmitted between other receiver-transmitter pairs, an optional memory component 210 for maintaining a history of channel conditions and determined transmit parameters, and a plurality of transmit/receive antennas

[0026] Included in the receiver 204 is a plurality of transmit/receive antennas 209i, 2092, ...209_n, a channel sounding processor 211 for processing channel low rate sounding requests, for generating channel sounding response (CSR) signals, and preferably, for assessing channel conditions of a communication link between itself and other transmitters and/or receivers. In addition, the receiver 204 includes an optional TFC processor 213 for processing received TFC control information and for determining TFC information via blind detection, a data packet processor 215 for decoding and demodulating received data packets according to the information provided by the TFC processor 213, an optional signal monitoring processor 217 for monitoring CSR signals transmitted from other receivers, a memory component 219 for maintaining a history of channel conditions, and an optional adjustment processor 221 for adjusting transmit parameters based on the channel histoiy.

[0027] For clarity and solely for illustrative pui'poses, the transmitter 202 and the l'eceiver 204 shown in Figure 2 are hereinafter described as separate devices operating independently in a MIMO-OFDM system. It should be understood, however, that these devices 202, 204 are preferably configured to co¬ exist as inter-related components of a single MIMO-OFDM network device, such as a base station or a WTRU, as shown in Figure 3. The MIMO-OFDM wireless communication system 300 of Figure 3 comprises a base station 301 and WTRU 302 communicating over a wireless interface, and an RNC 250 for controlling the base station 301. As the Figure illustrates, both the base station 301 and WTRU comprise a transmitter 202 - receiver 204 pair configured in accordance with the present invention.

[0028] Referring back to Figure 2, in the transmitter 202, prior to processing a data stream Tx for transmission, a low rate channel sounding request signal is generated in the channel sounding signal processor 201. This sounding request is then passed to the transmit antennas 207i, 2072,...207_n for transmission to the receiver 204 via a wireless interface. Upon receiving the low rate request, the receiver 204 processes the request and generates a channel sounding response (CSR) in its channel sounding processor 211. As described above, the CSR is preferably a short burst formatted to assure reception at the transmitter 202 and includes information known to the transmitter 202 for use in assessing current channel conditions. Once generated, the CSR is sent to the receiver's antennas 209i, 2092,...209_n for transmission to the transmitter 202. [0029] The CSR is then received at the transmitter 202 and processed in the transmitter's channel sounding processor 201. The channel sounding processor 201 analyzes the information transmitted as part of the CSR and uses this information to characterize current channel conditions in the uplink, and to estimate downlink channel response. These channel characterizations are then sent to the MAC layer processor 203 and/or to the PHY layer processor 205 where they are used to set data transmit parameters including: sub-carrier allocation, transmit antenna allocation, sub-carrier transmit power, transmit antenna power, sub-carrier coding, bandwidth selection, etc. Optionally, with regard to selecting a desired portion of bandwidth on which to transmit, the transmitter 202 may comprise a separate processor (not shown) configured to operate as a bandwidth selection unit. The PHY layer processor 205 then formats the transmit data packets, modulates the various sub-carriers (not shown) with user data, and maps the modulated sub-carriers to the transmit/receive antennas 207i, 2072,...207_n, using a channel capacity optimization scheme, such as water- filling, a channel reliability optimization scheme, or any other channel performance optimizing scheme in accordance with the transmit parameter settings. The formatted data packets are sent to the transmit/receive antennas 207i, 207s, ...207_n for transmission to the receiver 204 using preferred portions of the bandwidth. Optionally, the transmitter 202 maintains a history of channel condition estimates for use in optimally transmitting future data packets. [0030] Prior to, after, or in parallel with transmitting the formatted data packets, the optional TFC processor 206 generates and transmits a TFC signal via the transmit/receive antennas 207i, 2072,...207_n over the wireless interface. This TFC signal indicates to the receiver 204 the transmit parameter settings of the transmitted data packets and identifies the location (i.e., on which sub- carriers the data bits are being transmitted), the coding schemes and the modulation schemes (e.g., QPSK, 16 QAM, etc.) used for the transmitted data packets. [0031] If a TFC signal is transmitted, the receiver 204, receives the TFC signal and processes it in its optional TFC processor 213. This TFC processor 213 extracts the formatting and modulation information from the TFC signal and sends it to the data packet processor 215 for use in decoding and demodulating received data packets. Otherwise, if a TFC signal is not successfully detected by the receiver 204, the TFC processor 213 gathers available TFC information using a blind detection-type process.

[0032] To further improve system capacity and efficiency, the transmitter

202 and receiver 204 can monitor the CSRs generated by other receivers (not shown) using their respective signal monitoring processors 208, 217 and thereafter, assess and estimate the channel conditions between themselves and the other receiver(s). In the transmitter 202 and the receiver 204, their respective channel sounding processors 201, 211 may be configured to perform these channel assessments and estimates. Alternatively, the transmitter 202 and receiver 204 may each comprise additional processors (not shown) configured to function as a signal analyzer that assesses uplink channel conditions and as an estimator for estimating downlink channel conditions based on the channel assessments, respectively. This channel condition information may be utilized by both the transmitter 202 and receiver 204 to maintain a history of the channel conditions for use in determining transmit parameters of future communications with the receiver(s). This history may be stored in their respective memory components 210, 219.

[0033] In accordance with the present invention, the transmitter 202 may reuse the transmit parameter settings, preferably stored in the optional memory component 210, as set by the MAC layer processor 203 and/or the PHY layer processor 205 for subsequent data transmissions such time that a future CSR indicates a change in channel conditions. Alternatively, the transmitter 202 may use historical results from previously received CSR(s), also stored in the optional memory component 210 or in a secondary memory component (not shown), to predict when a change in channel conditions will occur and at that time, adjust the transmit parameters accordingly. Similarly, the receiver 204 may maintain a history of channel conditions in its optional memory component 219 for use in adjusting transmit parameters via its optional adjustment processor 221. [0034] Although not particularly specified, the frequency at which a transmitter requests channel sounding information from a receiver depends on a variety of factors. Examples of such factors include, but are not limited to: system configuration, number of sub-carriers, number of spatial channels, volatility of the communication link, communication environment, and the like. In general terms, a transmitter must request a CSR often enough to maintain accurate knowledge of the channel. As an example, a transmitter may begin by requesting CSRs at predetermined time intervals. As the transmitter begins to accumulate CSR data, the transmitter may use this data to estimate the rate at which channel conditions change and accordingly request CSRs according to the change frequency.

[0035] The present invention may be implemented in any type of wireless communication system, as desired. By way of example, the present invention may be implemented in any type of 802-type system, UMTS-FDD, UMTS-TDD, TDSCDMA, CDMA2000, OFDM-MIMO or any other type of wireless communication system. The present invention may also be implemented on an integrated circuit, such as an application specific integrated circuit (ASIC), multiple integrated circuits, logical programmable gate aiτay (LPGA), multiple LPGAs, discrete components, or a combination of integrated circuit(s), LPGA(s), and discrete component(s).

[0036] While the present invention has been described in terms of various embodiments, other variations, which are within the scope of the invention, as outlined in the claims below, will be apparent to those skilled in the art. Further, although the features and elements of the present invention are described in the various embodiments in paiticular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.

Claims

CLAIMS What is claimed is:

1. A method for improving system performance in a Multiple-Input, Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system comprising:

(a) transmitting a low rate signal;

(b) transmitting a channel sounding response (CSR) responsive to the low rate signal, said CSR being a short burst having a predefined transmit format and carrying predetermined information;

(c) analyzing the CSR;

(d) determining transmit parameter settings based on said analysis;

(e) adjusting transmit parameters and modulating sub-carriers with user- data according to the determined settings; and

(f) transmitting the user-data with the adjusted transmit parameters.

2. The method of claim 1, wherein the low rate signal is a data packet header including source and destination information.

3. The method of claim 1, wherein the user-data is modulated to the sub-camers using a channel capacity optimization technique.

4. The method of claim 3, wherein the channel capacity optimization technique is water-filling.

5. The method of claim 1, wherein the user-data is modulated to the sub-carriers using a channel reliability optimization technique.

6. The method of claim 5, wherein the channel reliability optimization technique is power control.

7. The method of claim 1, further comprising saving the determined transmit parameter settings and transmitting subsequent user-data according to the saved transmit parameter settings.

8. The method of claim 1, further comprising generating a transmit format control (TFC) signal, said TPC signal comprising the determined transmit parameter settings, including sub-carrier modulation information, and transmitting said TFC with the adjusted user-data to a receiver.

9. The method of claim 1, wherein adjusting the transmit parameters step occurs in a medium access control (MAC) layer of a transmitter.

10. The method of claim 1, wherein adjusting the transmit parameters step occurs in a physical (PHY) layer of a transmitter.

11. The method of claim 1, wherein adjusting the transmit parameters step occurs in a combination of a MAC layer and a PHY layer of a transmitter.

12. A method for improving system performance in a Multiple-Input, Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system comprising:

(a) monitoring CSR signals transmitted between a transmitter and a receiver;

(b) assessing channel conditions of a communication link between the transmitter and the receiver;

(c) maintaining a history of said channel conditions;

(d) adjusting transmit parameters based on the channel history; and

(e) communicating with the transmitter and receiver utilizing the adjusted transmit parameters.

13. A transmitter for use in a MIMO-OFDM wireless communication system comprising:

(a) a channel sounding processor for generating low rate signals and for analyzing received CSRs;

(b) a first processor for determining transmit parameter settings based on said analysis;

(c) a second processor for adjusting transmit parameters according to the determined settings;

(d) a PHY layer piOcessor for modulating sub-carriers with user-data according to the determined settings; and

(e) a plurality of transmit/receive antennas for receiving the CSR signals and for transmitting the user-data according to the adjusted transmit parameters.

14. The transmitter of claim 13, wherein the first and second processors are MAC layer processors.

15. The transmitter of claim 13, wherein the first processor is a MAC layer processor and the second processor is a PHY layer processor.

16. The transmitter of claim 13, wherein the first and second processors are PHY layer processors.

17. The transmitter of claim 13, wherein the PHY layer processor is configured to modulate user-data to the sub-carriers using a channel capacity optimization technique.

18. The transmitter of claim 17, wherein the channel capacity optimization technique is water-filling.

19. The transmitter of claim 13, wherein the PHY layer processor is configured to modulate user-data to the sub-carriers using a channel reliability optimization technique.

20. The transmitter of claim 19, wherein the channel reliability optimization technique is power control.

21. The transmitter of claim 13, further comprising a memory component for storing determined transmit parameter settings for use in transmitting subsequent user-data according to the stored transmit parameter settings.

22. The transmitter of claim 13, further comprising a TFC processor for generating TFC signals, said TFC signals comprising determined transmit parameter settings, including sub-carrier modulation information.

23. The transmitter of claim 13, further comprising:

(a) a signal monitoring processor for monitoring CSR signals transmitted between a second transmitter and a receiver;

(b) a signal analyzer configured to assess channel conditions of a communication link between itself and the receiver;

(c) a second memory component for maintaining a history of said channel conditions; and

(d) an adjustment processor for adjusting transmit parameters based on the channel history wherein communications with the receiver and second transmitter is performed using the adjusted transmit parameters.

24. The transmitter of claim 13, wherein said transmitter is a wireless transmit/receive unit (WTRU).

25. The transmitter of claim 13, wherein said transmitter is a base station.

26. A receiver for use in a MIMO-OFDM wireless communication system comprising:

(a) a channel sounding processor for processing received low rate signals and for generating responsive CSR signals, said CSR signals being short bursts each having a predefined transmit format and carrying predetermined information;

(b) a plurality of transmit/receive antennas for receiving the low rate signals and for transmitting the responsive CSR signals; and

(c) a data packet processor for processing user-data encoded sub-carriers.

27. The receiver of claim 26, further comprising:

(a) a TFC processor for processing received TFC signals, said TFC signals comprising transmit parameters settings of received data packets, including sub- carrier modulation information; and

(b) a data packet processor configured for decoding and demodulating user- encoded sub-carriers based on information provided by the TFC signals.

28. The receiver of claim 27, further comprising:

(a) a signal monitoring processor for monitoring CSR signals transmitted between a transmitter and a second receiver;

(b) a signal analyzer configured to assess channel conditions of a communication link between the transmitter and a second receiver;

(c) a memory component for maintaining a histoiy of said channel conditions; and

(d) an adjustment processor for adjusting transmit parameters based on the channel histoiy wherein communications with the transmitter and second receiver is performed using the adjusted transmit parameters.

29. The receiver of claim 28, wherein said receiver is a WTRU.

30. The receiver of claim 28, wherein said receiver is a base station.

31. A WTRU for use in a MIMO-OFDM wireless communication system comprising:

(a) a transmitter comprising:

(al) a first channel sounding processor for generating local low rate signals and for analyzing foreign CSR signals;

(a2) a first processor for determining transmit parameter settings based on the foreign CSR signal analysis;

(a3) a second processor for adjusting transmit parameters according to the determined settings; and

(a4) a PHY layer processor for modulating sub-carriers with user- data according to the determined settings;

(b) a receiver comprising:

(bl) a second channel sounding processor for processing foreign low rate signals and for generating local CSR signals responsive to the foreign low rate signals, said local CSRs being short bursts each having a predefined transmit format and carrying predetermined information; and

(b2) a data packet pi'ocessor for processing user-data encoded sub- carriers; and

(c) a plurality of transmit/receive antennas for receiving and transmitting communication signals.

32. The WTRU of claim 31 , further comprising a memory component for storing determined transmit parameter settings for use in transmitting subsequent user-data according to the stored transmit parameter settings.

33. The WTRU of claim 32, further comprising a TFC processor for generating local TFC signals and for processing received foreign TFC signals, wherein said local TFC signals comprise the determined transmit parameter settings, including sub-carrier modulation information.

34. The WTRU of claim 33 , further comprising:

(a) a signal monitoring processor for monitoring CSR signals transmitted between network devices in the MIMO-OFDM wireless communication system;

(b) a signal analyzer configured to assess channel conditions of a communication link between a pair of the other network devices; and

(c) a second memory component for maintaining a history of said channel conditions.

35. A base station for use in a MIMO-OFDM wireless communication system comprising:

(a) a transmitter comprising:

(al) a first channel sounding processor for generating local low rate signals and for analyzing foreign CSRs;

(a2) a first processor for determining transmit parameter settings based on the CSR signal analysis;

(b) a receiver comprising:

(bl) a second channel sounding processor for processing foreign low rate signals and for generating responsive local CSR signals, said local CSR signals being short bursts each having a predefined transmit format and carrying predetermined information; and

(b2) a data packet processor for processing user-data encoded sub- carriers; and

36. The base station of claim 35, further comprising a memory component for stoiϊng determined transmit parameter settings for use in transmitting subsequent user-data according to the stored transmit parameter settings.

37. The base station of claim 36, further comprising a TFC processor for generating local TFC signals and for processing received foreign TFC signals, wherein said local TFC signals comprise the determined transmit parameter settings, including sub-carrier modulation information.

38. The base station of claim 37, further comprising:

(a) a signal monitoring processor for monitoring CSR signals transmitted between other netwox'k devices in the MIMO-OFDM wireless communication system;

(c) a second memoiy component for maintaining a history of said channel conditions.

39. A signaling method for improving communication-link performance in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system comprising at least one base station and at least one WTRU, the method comprising:

(a) transmitting a low rate signal comprising sounding instructions from a base station to a WTRU; (b) transmitting a sounding signal responsive to the low rate signal from the WTRU to the base station, said sounding signal being a short burst having a predefined transmit format and carrying predetermined information;

(c) analyzing the sounding signal and determining uplink channel response based on said analysis;

(d) estimating downlink channel response based on said determined uplink channel response;

(e) selecting a portion of a bandwidth on which to transmit user-data based on the estimated downlink channel response;

(f) adjusting transmit parameters according to the estimated downlink channel response; and

(g) transmitting user-data with the adjusted transmit parameters on the selected portion of the bandwidth.

40. The method of claim 39, wherein the base station instructs the WTRU to transmit sounding signals periodically.

41. A base station for improving communication-link performance in an OFDM wireless communication system, the base station comprising: a transmitter for generating and transmitting low rate signals comprising sounding instructions to a WTRU; a receiver for receiving sounding signals transmitted by the WTRU, said sounding signals being short bursts having a predefined formats and carrying predetermined information; a signal processor for determining an uplink channel response based on said sounding signals; an estimator for estimating a downlink channel response based on the determined uplink channel response; a transmit parameter adjusting processor for adjusting transmit parameters based on the estimated downlink channel response; and a bandwidth selection unit for selecting a portion of a bandwidth on which to transmit data based on the estimated downlink channel response; wherein the base station transmits user data with the adjusted transmit parameters to the WTRU using the selected portion of the bandwidth.

42. The base station of claim 41, wherein the transmitter is configured to generate and transmit low rate signals instructing the WTRU to transmit sounding signals periodically.

43. A WTRU for improving communication-link perfoπnance in an OFDM wireless communication system, the WTRU comprising: a receiver for receiving channel sounding instructions; and a ti'ansmitter for generating and transmitting sounding signals according to the channel sounding instructions.

44. The WTRU of claim 43, wherein the transmitter is configured to periodically transmit sounding signals in accordance with the sounding instructions.

45. An integrated circuit for use in a MIMO-OFDM wireless communication system comprising:

(c) a data packet processor for processing user-data encoded sub-carriers.