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  Compensation techniques for group delay effects in transmit beamforming radio communicationUSPTO Application #: 20080095260Title: Compensation techniques for group delay effects in transmit beamforming radio communication Abstract: In a wireless communication device, transmit weights are iteratively processed to compensate for any group delay caused by receive synchronization in the receive device. A transmit matrix of weights is computed from signals transmitted by a second communication device and received at a plurality of antennas of the first communication device. The transmit matrix processing includes normalizing a transmit weight vector with respect to a mode of the transmit weight associated with one of the plurality of antennas. (end of abstract) Agent: Volpe And Koenig, P.C. Dept. Icc - Philadelphia, PA, US Inventor: Chandra Vaidyanathan USPTO Applicaton #: 20080095260 - Class: 375267000 (USPTO) Related Patent Categories: Pulse Or Digital Communications, Systems Using Alternating Or Pulsating Current, Plural Channels For Transmission Of A Single Pulse Train, Diversity The Patent Description & Claims data below is from USPTO Patent Application 20080095260. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 11/487,242 filed Jul. 14, 2006, which is a continuation of U.S. patent application Ser. No. 10/779,269, filed Feb. 13, 2004, which in turn claims priority from U.S. Provisional Application No. 60/476,982, filed Jun. 9, 2003; and U.S. Provisional Application No. 60/511,530, filed Oct. 15, 2003, which are incorporated by reference as if fully set forth. FIELD OF THE INVENTION [0002] Transmit beamforming radio communication techniques are disclosed and more particularly, techniques for compensating for group delay effects associated with orthogonal frequency division multiplex (OFDM) communication signals. BACKGROUND [0003] Examples of transmit beamforming radio algorithms that compute and use transmit weights for transmitting signals to another device are disclosed in U.S. Pat. No. 6,687,492, issued Feb. 3, 2004 and entitled "System and Method for Antenna Diversity Using Joint Maximal Ratio Combining" and in U.S. patent application Ser. No. 10/174,689, filed Jun. 19, 2002. According to these algorithms, receive weights associated with signals transmitted by a second device and received at multiple antennas of a first device are used to compute transmit weights for transmitting signals to a second device. The receive weights will include a group delay term due to group delay ambiguities in the receive synchronization algorithm used in the device for synchronizing to OFDM signals. In order to maintain desired performance of the above-described transmit beamforming radio algorithms, these group delay effects need to be removed. SUMMARY [0004] A method and apparatus in a wireless communication between a first communication device and a second communication device compensates for a group delay effect introduced by a synchronization algorithm. A baseband signal processor of the second communication device computes a transmit matrix of antenna weights from signals received at the plurality of antennas of the first communication device representing a plurality of modes simultaneously transmitted by the second communication device, wherein the transmit matrix distributes a plurality of modes among a plurality of antenna paths associated with corresponding ones of the plurality of antennas of the first communication device. The baseband signal processor processes the transmit matrix to compensate for the group delay, said processing includes normalizing the transmit matrix with respect to a reference mode corresponding to one of the plurality of antennas of the first communication device. The baseband signal processor applies the transmit matrix to a plurality of modes to be simultaneously transmitted from corresponding ones of the plurality of antennas of the first communication device to the second communication device. This process is repeated by the second communication device, and then again by the first communication device in an iterative manner allowing the transmit matrix of weights to converge. BRIEF DESCRIPTION OF THE DRAWINGS [0005] FIG. 1 is a diagram showing two devices that transmit beamform signals between each other. [0006] FIG. 2 is a flow chart of an algorithm by a device computes transmit weights based on received signals from the other device. [0007] FIG. 3 shows timing for an OFDM symbol generated without delay spread. [0008] FIG. 4 shows timing for an OFDM symbol generated with L-sample delay spread. [0009] FIG. 5 is a flow chart of the algorithm of FIG. 2, modified to compensate for group delay effects. [0010] FIG. 6 is a block diagram of a vector beamforming system in which the group delay compensation techniques may be used. [0011] FIG. 7 is a flow chart of an adaptive algorithm used in a vector beamforming system that includes processing steps to compensate for group delay. [0012] FIG. 8 is a block diagram of a radio communication device in which the techniques described herein may be employed. DETAILED DESCRIPTION [0013] FIG. 1 shows a system 10 is shown in which a first communication device and a second communication device 200 communicate with each other using radio frequency (RF) communication techniques. This system is described in greater detail in U.S. application Ser. No. 10/174,689 referred to above. The devices use composite beamforming techniques when communicating with each other. In particular, communication device 100 has N plurality of antennas 110 and communication device 200 has M plurality of antennas 210. According to the composite beamforming (CBF) technique also described in the aforementioned co-pending application, when communication device 100 transmits a signal to communication device 200, it applies to (i.e., multiplies or scales) a baseband signal s to be transmitted a transmit weight vector associated with a particular destination device, e.g., communication device 200, denoted w.sub.tx,1. Similarly, when communication device 200 transmits a baseband signal s to communication device 100, it multiplies the baseband signal s by a transmit weight vector w.sub.tx,2 associated with destination communication device 100. The (M.times.N) frequency dependent channel matrix from the N plurality of antennas of the first communication device 100 to M plurality of antennas of the second communication device 200 is H(k), where k is a frequency index or variable, and the frequency dependent communication channel (N.times.M) matrix between the M plurality of antennas of the second communication device and the N plurality of antennas of the first communication device is H.sup.T(k). [0014] The transmit weight vectors w.sub.tx,1 and w.sub.tx,2 each comprises a plurality of transmit weights corresponding to each of the N and M antennas, respectively. Each transmit weight is a complex quantity. Moreover, each transmit weight vector is frequency dependent; it may vary across the bandwidth of the baseband signal s to be transmitted. For example, if the baseband signal s is a multi-carrier signal of K sub-carriers, each transmit weight for a corresponding antenna varies across the K sub-carriers. Similarly, if the baseband signal s is a single-carrier signal (that can be divided or synthesized into K frequency sub-bands), each transmit weight for a corresponding antenna varies across the bandwidth of the baseband signal. Therefore, the transmit weight vector is dependent on frequency, or varies with frequency sub-band/sub-carrier k, such that w.sub.tx becomes w.sub.tx(f), or more commonly referred to as w.sub.tx(k), where k is the frequency sub-band/sub-carrier index. [0015] While the terms frequency sub-band/sub-carrier are used herein in connection with beamforming in a frequency dependent channel, it should be understood that the term "sub-band" is meant to include a narrow bandwidth of spectrum forming a part of a baseband signal. The sub-band may be a single discrete frequency (within a suitable frequency resolution that a device can process) or a narrow bandwidth of several frequencies. [0016] The receiving communication device also weights the signals received at its antennas with a receive antenna weight vector w.sub.rx(k). Communication device 100 uses a receive antenna weight vector w.sub.rx,1(k) when receiving a transmission from communication device 200, and communication device 200 uses a receive antenna weight vector w.sub.rx,2(k) when receiving a transmission from communication device 100. The receive antenna weights of each vector are matched to the received signals by the receiving communication device. The receive weight vector may also be frequency dependent. [0017] Generally, transmit weight vector w.sub.tx,1 comprises a plurality of transmit antenna weights w.sub.tx,1,i=.beta..sub.1,i(k)exp(j.phi..sub.1,i(k)), where .beta..sub.1,i(k) is the magnitude of the antenna weight, .phi..sub.1,i(k) is the phase of the antenna weight, i is the antenna index, and k is the frequency sub-band or sub-carrier index (up to K frequency sub-bands/sub-carriers). The subscripts tx,1 denote that it is a vector that communication device 100 uses to transmit to communication device 200. Similarly, the subscripts tx,2 denote that it is a vector that communication device 200 uses to transmit to communication device 100. [0018] FIG. 2 shows a procedure 400 for determining near optimum transmit antenna weight vectors for first and second communication devices also described in detail in U.S. application Ser. No. 10/174,689 referred to above. The antenna weight parameters in FIG. 2 are written with subscripts to reflect communication between a WLAN access point (AP) and a station (STA) as examples of first and second communication devices, respectively. However, without loss of generality, it should be understood that this process is not limited to WLAN application. The AP has Nap antennas and the STA has Nsta antennas. Assuming the AP begins with a transmission to the STA, the initial AP transmit weight vector w.sub.T,AP,0(k) is [1,1, . . . 1], equal power normalized by 1/(Nap).sup.1/2 for all antennas and all frequency sub-bands/sub-carriers k. Phase for the transmit antenna weights is also initially set to zero. The subscript T indicates it is a transmit weight vector, subscript AP indicates it is an AP vector, subscript 0 is the iteration of the vector, and (k) indicates that it is frequency sub-band/sub-carrier dependent. The transmit weight vectors identified in FIG. 5 form an N.times.K matrix explained above. Continue reading... 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