System and method for beamforming using rate-dependent feedback in a wireless network

Embodiments of multicarrier receiver and method for beamforming using rate-dependent feedback in a wireless network are generally described herein. Other embodiments may be described and claimed. In some embodiments, decimated beamforming matrices are generated for groups of two or more subcarriers and provided to a multicarrier transmitter. The number of subcarriers in a group may be based on a receive data rate. The multicarrier transmitter uses the decimated beamforming matrices to beamform signals for transmission to the multicarrier receiver.

TECHNICAL FIELD

Some embodiments of the present invention pertain to wireless communication systems. Some embodiments of the present invention pertain to wireless communication systems that communicate beamformed signals using multiple antennas.

BACKGROUND

Beamforming is used in many conventional wireless communication systems to help improve the signal-to-noise ratio of received signals and increase system throughput. Some types of beamforming use feedback from a receiver to compensate for the effects of the channel. The feedback required for these beamforming techniques may consume channel bandwidth which in turn, may reduce system throughput. This feedback also consumes energy.

Thus, there are general needs for systems and methods for beamforming that consume less channel bandwidth and reduce energy consumption. There are also general needs for systems and methods for beamforming that consume less channel bandwidth and reduce energy consumption while maintaining acceptable throughput levels.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustrate specific embodiments of the invention to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments of the invention set forth in the claims encompass all available equivalents of those claims. Embodiments of the invention may be referred to herein, individually or collectively, by the term “invention” merely for convenience and without intending to limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed.

FIG. 1is a block diagram of a multicarrier transmitter in accordance with some embodiments of the present invention. Multicarrier transmitter100generates multicarrier communication signals130from input bit stream101using two or more of transmit antennas114. In some embodiments, multicarrier transmitter100may be part of a multiple-input multiple-output (MIMO) multicarrier communication system in which two or more of transmit antennas114are used to convey data streams to a multicarrier receiving station having two or more receive antennas, although the scope of the invention is not limited in this respect.

In accordance with some embodiments of the present invention, multicarrier communication signals130are generated by applying decimated beamforming matrices (V)121to two or more subcarriers of symbol-modulated subcarriers107. Each of decimated beamforming matrices (V)121may be generated from channel estimates that may be generated by a receiver (e.g., receiver200shown inFIG. 2). The number of subcarriers that each decimated beamforming matrix121is applied to may depend on a receive data rate. These embodiments are described in more detail below.

Multicarrier transmitter100may comprise channel encoder102to encode input bit stream101and spatial-frequency interleaver104to generate one or more spatial streams105from encoded bit stream103. In some embodiments, channel encoder102may be a channel encoder which may add error-checking to the bit stream. In some embodiments, spatial-frequency interleaver104may generate one or more spatial streams105which may be transmitted to a multicarrier receiving station with two or more receive antennas. In some embodiments, spatial-frequency interleaver104may generate one or more spatial streams105by selecting bits of encoded bit stream103in some predetermined fashion.

Multicarrier transmitter100may also comprise one or more symbol mappers106associated with each of spatial streams105to map bits of spatial streams105to symbols to generate symbol streams. In some embodiments, symbol mappers106may also map bits of spatial streams105to subcarriers associated with a multicarrier communication channel to generate symbol-modulated subcarriers107for each of spatial streams105. In some of these embodiments in which a multicarrier communication signal uses fifty-two subcarriers, each symbol mapper106may generate fifty-two corresponding symbol-modulated subcarriers in the frequency domain. These may be referred to as frequency-domain symbol-modulated subcarriers107. In some embodiments, symbol mappers106may comprise quadrature-amplitude-modulation (QAM) mappers, although the scope of the invention is not limited in this respect.

In some embodiments, symbol mappers106may map bits in accordance with a modulation assignment that may range from zero bits per symbol to up to ten or more bits per symbol. In some embodiments, the subcarrier modulation level may comprise one or more of binary phase shift keying (BPSK), which communicates one bit per symbol, quadrature phase shift keying (QPSK), which communicates two bits per symbol, 8PSK, which communicates three bits per symbol, 16-quadrature amplitude modulation (16-QAM), which communicates four bits per symbol, 32-QAM, which communicates five bits per symbol, 64-QAM, which communicates six bits per symbol, 128-QAM, which communicates seven bits per symbol, and 256-QAM, which communicates eight bits per symbol. Modulation levels with even higher data communication rates per subcarrier may also be used.

Multicarrier transmitter100may also comprise beamformer108to apply decimated beamforming matrices121to frequency-domain symbol-modulated subcarriers107of each spatial stream105and generate beamformed symbol-modulated subcarriers109for subsequent transmission by a corresponding one of transmit antennas114. In some embodiments, beamformer108may apply decimated beamforming matrices121as weights to weight symbol-modulated subcarriers107. These embodiments are described in more detail below.

In some embodiments, beamformed symbol-modulated subcarriers109may be associated with a particular antenna and antenna stream. In some embodiments, the number of antenna streams may be greater than or equal to the number of spatial streams105, although the scope of the invention is not limited in this respect. In some embodiments, the number of antenna streams may correspond to the number of transmit antennas114. Although multicarrier transmitter100is illustrated with three antenna streams (e.g., each associated with one of transmit antennas114) and two spatial streams105, the scope of the invention is not limited in these respects.

Multicarrier transmitter100may also comprise inverse Fourier transform (IFT) circuitry110for each antenna stream to perform an inverse Fourier transformation on beamformed symbol-modulated subcarriers109to generate time-domain samples111. In some embodiments, IFT circuitry110may add a guard interval (GI) including a cyclic prefix (CP), although the scope of the invention is not limited in this respect.

Multicarrier transmitter100may also comprise digital-to-analog conversion (DAC) and radio-frequency (RF) circuitry112associated with each antenna stream to respectively digitize and convert time-domain samples111to RF multicarrier time-domain signals for subsequent transmission by a corresponding one of transmit antennas114.

In some embodiments, decimated beamforming matrices may be received from a multicarrier receiver in compressed form. In these embodiments, multicarrier transmitter100may also comprise decompression unit122to decompress compressed decimated beamforming matrices (V)123to provide decimated beamforming matrices121to beamformer108. Decompression unit122may operate when a multicarrier receiver transmits compressed decimated beamforming matrices123as feedback to multicarrier transmitter100.

In some embodiments in which the multicarrier communication signals comprise a plurality of substantially orthogonal subcarriers, each of decimated beamforming matrices121may be applied by beamformer108to two or more subcarriers depending on the decimation factor. In some embodiments, beamformer108may interpolate decimated beamforming matrices121to generate a separate beamforming matrix to apply to each subcarrier. These embodiments are discussed in more detail below.

In some MIMO embodiments, beamformer108may comprise a plurality of individual subcarrier beamformers which may be associated with each subcarrier. Each subcarrier beamformer may apply the elements of one of decimated beamformer matrices121to frequency-domain symbol-modulated subcarriers107to generate beamformed symbol-modulated subcarriers109for each transmit antenna114. In these embodiments, beamformed symbol-modulated subcarriers109may be associated with a number of transmit antennas114which may be greater than or equal to a number of spatial streams105, although the scope of the invention is not limited in this respect.

FIG. 2is a block diagram of a multicarrier receiver in accordance with some embodiments of the present invention. Multicarrier receiver200may communicate multicarrier signals with a multicarrier transmitter, such as multicarrier transmitter100(FIG. 1). Receiver200receives radio-frequency (RF) signals through one or more of antennas201, processes the received signals, and generates decoded bit stream229. Receiver200is illustrated inFIG. 2as a multicarrier receiver which may receive and process multicarrier signals, such as orthogonal frequency division multiplexed (OFDM) signals and orthogonal frequency division multiple access (OFDMA) signals, however the scope of the invention is not limited in this respect.

Receiver200may include radio-frequency (RF) circuitry202to down-convert the received signals and analog-to-digital conversion (ADC) circuitry204to digitize the received signals and generate digital time-domain signals205. Receiver200may also include frequency correction circuitry206to correct any frequency offset present in the received signals. In some embodiments, receiver200may also include cyclic prefix removal circuitry208to remove a cyclic prefix from frequency-corrected time-domain signals207. Receiver200may also include Fourier-transform (FT) circuitry210to perform a Fourier transform on digital time-domain signals209to generate frequency-domain signals215. In some embodiments, Fourier-transform circuitry210may provide a frequency-domain signal for each subcarrier of a received multicarrier communication signal. In some embodiments, Fourier-transform circuitry210may perform a discrete Fourier transform (DFT), such as a fast Fourier transform (FFT), although the scope of the invention is not limited in this respect.

Receiver200may also include channel transfer function generator216to generate channel estimates231for each data subcarrier based on received frequency-domain signals215. In some embodiments, channel estimates231may comprise weights for each data subcarrier. Receiver200may also include channel equalizer212to weight the subcarriers of received frequency-domain signals215based on channel estimates231to generate channel-equalized frequency-domain signals213. In accordance with some embodiments, the application of the weights by channel equalizer212may help compensate for the effects of the communication channel through which the received signals may have propagated. In some embodiments, channel equalizer212may substantially cancel the effects of the communication channel. In some embodiments, channel-equalized frequency-domain signals213may comprise a symbol, such as a quadrature-amplitude modulated (QAM) symbol, for each data subcarrier, although the scope of the invention is not limited in this respect.

Receiver200may also include demodulator214to demodulate channel-equalized frequency-domain signals213and generate bit-metrics227for each data subcarrier. In some embodiments, symbol demodulator214may be a quadrature-amplitude modulation demodulator and the symbols may comprise QAM symbols. Receiver200may also include decoder228to perform an error-correction decoding operation on bit metrics227to generate decoded bit stream229. In these embodiments, bit metrics227may represent probabilities (e.g., soft bits rather than actual hard bits), which may be decoded using soft-decision decoding.

In some embodiments, decoder228may be a forward-error-correcting (FEC) decoder. In some embodiments, decoder228may be a low-density parity check (LPDC) decoder that performs layered decoding, although the scope of the invention is not limited in this respect. In some embodiments, decoder228may perform LDPC layered decoding operations based on a parity-check matrix for a predetermined LDPC code, although the scope of the invention is not limited in this respect.

In some embodiments, receiver200may also perform a deinterleaving operation prior to the operation of decoder228. In some of these embodiments, the deinterleaving operation may be a block deinterleaving operation on blocks of hard bits or on blocks of bit metrics227, although the scope of the invention is not limited in this respect.

In accordance with embodiments of the present invention, channel transfer function generator216may generate channel transfer function matrices (H)217from received frequency-domain signals215. In some embodiments, channel transfer function generator216may generate one channel transfer function matrix for each subcarrier of received frequency-domain signals215. In some embodiments, channel transfer function matrices217may be generated from channel estimates which may be estimated from preamble symbols and/or pilot subcarriers of received multicarrier communication signals.

Multicarrier receiver200may also comprise decimator218to decimate channel transfer function matrices217to generate decimated channel transfer function matrices (H)219based on receive data rate225. Receive data rate225may be provided by multicarrier transmitter100(FIG. 1). Decimator218may generate one of decimated channel transfer function matrices219for groups of two or more subcarriers. In some embodiments, decimator218may combine and/or average groups of channel transfer function matrices217to generate each decimated channel transfer function matrix219.

Multicarrier receiver200may also comprise decomposition unit220which may perform a decomposition on decimated channel transfer function matrices219to generate decimated beamforming matrices (V)221. Decimated beamforming matrices221may be provided to multicarrier transmitter100(FIG. 1) as feedback for use in beamforming and may correspond to decimated beamforming matrices121(FIG. 1).

Although decimator218is illustrated as coming before decomposition unit220in the processing path, the scope of the invention is not limited in this respect. In other embodiments, decomposition unit220may come before decimator218. In these embodiments, decomposition unit220may decompose channel transfer function matrices217into beamforming matrices, and decimator218may decimate the beamforming matrices to generate decimated beamforming matrices221.

In some embodiments, decomposition unit220may perform a singular value decomposition (SVD) on decimated channel function matrices219to generate decimated beamforming matrices221. In some embodiments, the SVD operations performed by decomposition unit220may decompose a wireless channel into three components based on the following equation:
{tilde over (H)}=U*S*VT.

In this equation, {tilde over (H)} represents one of decimated channel transfer function matrices219, U and V are unitary matrices, VTis the transpose of matrix V, and S is a rectangular matrix having real nonzero-valued diagonal elements and zero-valued non-diagonal elements. In this equation, matrix V corresponds to one of decimated beamforming matrices221. The SVD operation may effectively decompose the wireless channel into three independent channels in which the S matrix may be viewed as a ranking for the three independent channels.

In some embodiments, multicarrier receiver200may also comprise compression circuitry222to compress decimated beamforming matrices221to generate compressed decimated beamforming matrices ({tilde over (V)})223prior to transmission to multicarrier transmitter100(FIG. 1). One of many compression techniques may be used by compression circuitry222. In these embodiments, compressed decimated beamforming matrices223may correspond to compressed decimated beamforming matrices123(FIG. 1).

In some embodiments, multicarrier receiver200generates decimated beamforming matrices221for groups of two or more subcarriers. The number of subcarriers in a group may be based on a receive data rate225. Multicarrier receiver200may also receive signals from multicarrier transmitter100(FIG. 1) transmitted with multiple antennas at receive data rate225. Decimated beamforming matrices221(FIG. 1) may be individually applied to the subcarriers of the signals prior to transmission.

In some embodiments, the number of subcarriers in a group is lesser for higher receive data rates and greater for lower receive data rates. In other words, less beamforming feedback may be provided to the transmitter for lower receive data rates, and more beamforming feedback may be provided to the transmitter for higher receive data rates. This is because throughput at lower data rates may be less sensitive to beamforming. In this way, bandwidth consumption for feedback may be reduced when it is less needed.

In some embodiments, decimated beamforming matrices221may be generated based on a decimation factor. For higher receive data rates, the decimation factor may be decreased to decrease the number of subcarriers in a group. For lower receive data rates, the decimation factor may be increased to increase the number of subcarriers in a group. In some embodiments, the decimation factor may refer to the number of subcarriers in a group that are used to generate each of decimated beamforming matrices221. In some embodiments, for a maximum receive data rate, the decimation factor may have the value of two meaning that each decimated beamforming matrix is generated from two subcarriers and is used to beamform two subcarriers. In example embodiments that use fifty-two subcarriers, twenty-six decimated beamforming matrices may be provided as feedback. When the decimation factor is four, each decimated beamforming matrix may be generated from four subcarriers and may be used to beamform four subcarriers at the transmitter. In these example embodiments that use fifty-two subcarriers, thirteen decimated beamforming matrices may be provided as feedback when the decimation factor is four. Decimator factors may range from one to up to eight or more.

In some embodiments, a decimation factor of two may result in less than a 0.5 dB reduction in signal-to-noise ratio (SNR) at maximum data rates (e.g., above 100 Mega-bits-per-second (Mbps)). Accordingly, a decimation factor of two may generally be used for some maximum receive data rates without significant performance degradation, although the scope of the invention is not limited in this respect. A decimation factor of eight may result in less than a 0.5 dB reduction in SNR for lower data rates (e.g., below 20 Mbps). In some embodiments, decimator218may increase or decrease the decimation factor based on receive data rate225.

In some embodiments, multicarrier receiver200may determine receive data rate225from a packet header received from multicarrier transmitter100(FIG. 1) prior to generating decimated beamforming matrices221.

In some embodiments, for higher receive data rates225, each of decimated beamforming matrices221may be used by multicarrier transmitter100(FIG. 1) for beamforming a lesser number of subcarriers. For lower receive data rates225, each of the decimated beamforming matrices221may be used by multicarrier transmitter100(FIG. 1) for beamforming a greater number of subcarriers.

In some embodiments, each of channel transfer function matrices217may have a number of columns corresponding with the number of spatial streams (Nss) and may have a number of rows associated with the number of transmit antennas (Ntx). The use of the terms rows and columns may be interchanged without limiting the scope of the invention. In the case of multicarrier transmitter100(FIG. 1), there are two spatial steams (i.e., Nss=2) illustrated as being generated by spatial frequency interleaver104(FIG. 1) and there are three transmit antennas114(FIG. 1) (i.e., Ntx=3). In these embodiments, each of channel transfer function matrices217may be a 3×2 matrix. In the example of a multicarrier communication channel that uses fifty-two total subcarrier frequencies (i.e., the number of subcarriers), there may be fifty-two separate channel transfer function matrices217(i.e., one for each active subcarrier), although the scope of the invention is not limited in this respect.

In some beamforming embodiments, multicarrier transmitter100(FIG. 1) may implement ideal beamforming. In these embodiments, receiver200may provide non-decimated beamforming matrices to multicarrier transmitter100(FIG. 1) based on receive data rate225. In these embodiments, decimator218may refrain from decimating channel transfer function matrices217, allowing decomposition unit220to generate a beamforming matrix for each subcarrier for transmission to multicarrier transmitter100(FIG. 1). In these embodiments, the decimator factor may equal one.

Referring toFIGS. 1 and 2together, in some embodiments, multicarrier receiver200may receive multicarrier communication signals130from multicarrier transmitter100through a communication channel having characteristics of channel transfer function matrices217. These channel transfer function matrices217may be used by a channel equalizer to equalize the received signals to generate a received bit stream. In some embodiments, multicarrier receiver200may apply non-linear MIMO equalization techniques to separate the spatial streams. In some embodiments, multicarrier transmitter100may receive decimated beamforming matrices121from multicarrier receiver200as part of a closed-loop MIMO feedback process. In some embodiments, when decomposition unit220performs an SVD on decimated channel transfer function matrix219, each of decimated beamforming matrices221may be unitary matrix having a number of rows equaling the number of the transmit antennas (Ntx), and a number of columns equaling the number of the spatial streams (Nss).

Although multicarrier transmitter100and multicarrier receiver200are illustrated as having several separate functional elements, one or more of the functional elements may be combined and may be implemented by combinations of software-configured elements, such as processing elements including digital signal processors (DSPs), and/or other hardware elements. For example, some elements may comprise one or more microprocessors, DSPs, application specific integrated circuits (ASICs), and combinations of various hardware and logic circuitry for performing at least the functions described herein. In some embodiments, the functional elements of multicarrier transmitter100and multicarrier receiver200may refer to one or more processes operating on one or more processing elements. Some embodiments of the present invention pertain to wireless communication devices that may include the elements of both multicarrier transmitter100and multicarrier receiver200.

In some embodiments, multicarrier transmitter100and multicarrier receiver200may communicate orthogonal frequency division multiplexed (OFDM) communication signals over a multicarrier communication channel. The multicarrier communication channel may be within a predetermined frequency spectrum and may comprise a plurality of orthogonal subcarriers. In some embodiments, the multicarrier signals may be defined by closely spaced OFDM subcarriers. Each subcarrier may have a null at substantially a center frequency of the other subcarriers and/or each subcarrier may have an integer number of cycles within a symbol period, although the scope of the invention is not limited in this respect. In some embodiments, multicarrier transmitter100and multicarrier receiver200may communicate in accordance with a multiple access technique, such as orthogonal frequency division multiple access (OFDMA), although the scope of the invention is not limited in this respect. In some embodiments, multicarrier transmitter100and multicarrier receiver200may communicate using spread-spectrum signals, although the scope of the invention is not limited in this respect.

In some embodiments, multicarrier transmitter100and/or multicarrier receiver200may be part of a communication station, such as a wireless local area network (WLAN) communication station including a Wireless Fidelity (WiFi) communication station, an access point (AP) or mobile station (MS). In some other embodiments, multicarrier transmitter100and/or multicarrier receiver200may be part of a broadband wireless access (BWA) network communication station, such as a Worldwide Interoperability for Microwave Access (WiMax) communication station, although the scope of the invention is not limited in this respect as multicarrier transmitter100and multicarrier receiver200may be part of almost any wireless communication device.

In some embodiments, multicarrier transmitter100and/or multicarrier receiver200may be part of a portable wireless communication device, such as a personal digital assistant (PDA), a laptop or portable computer with wireless communication capability, a web tablet, a wireless telephone, a wireless headset, a pager, an instant messaging device, a digital camera, an access point, a television, a medical device (e.g., a heart rate monitor, a blood pressure monitor, etc.), or other device that may receive and/or transmit information wirelessly.

In some embodiments, the frequency spectrums for the communication signals used by multicarrier transmitter100and multicarrier receiver200may comprise either a 5 gigahertz (GHz) frequency spectrum or a 2.4 GHz frequency spectrum. In these embodiments, the 5 GHz frequency spectrum may include frequencies ranging from approximately 4.9 to 5.9 GHz, and the 2.4 GHz spectrum may include frequencies ranging from approximately 2.3 to 2.5 GHz, although the scope of the invention is not limited in this respect, as other frequency spectrums are also equally suitable. In some BWA network embodiments, the frequency spectrum for the communication signals may comprise frequencies between 2 and 11 GHz, although the scope of the invention is not limited in this respect.

In some embodiments, multicarrier transmitter100and multicarrier receiver200may communicate in accordance with specific communication standards, such as the Institute of Electrical and Electronics Engineers (IEEE) standards including IEEE 802.11(a), 802.11(b), 802.11(g), 802.11(h) and/or 802.11(n) standards and/or proposed specifications for wireless local area networks, although the scope of the invention is not limited in this respect as they may also be suitable to transmit and/or receive communications in accordance with other techniques and standards. Some embodiments of the present invention may apply to the proposed Enhanced Wireless Consortium (EWC) specification of the IEEE 802.11 standards. In some broadband wireless access network embodiments, multicarrier transmitter100and multicarrier receiver200may communicate in accordance with the IEEE 802.16-2004 and the IEEE 802.16(e) standards for wireless metropolitan area networks (WMANs) including variations and evolutions thereof, although the scope of the invention is not limited in this respect as they may also be suitable to transmit and/or receive communications in accordance with other techniques and standards. For more information with respect to the IEEE 802.11 and IEEE 802.16 standards, please refer to “IEEE Standards for Information Technology—Telecommunications and Information Exchange between Systems”—Local Area Networks—Specific Requirements—Part 11 “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), ISO/IEC 8802-11: 1999”, and Metropolitan Area Networks—Specific Requirements—Part 16: “Air Interface for Fixed Broadband Wireless Access Systems,” May 2005 and related amendments/versions. Some embodiments may relate to the IEEE 802.11e proposed enhancement to the IEEE 802.11 WLAN specification that will include QoS (quality of service) features, including the prioritization of data, voice, and video transmissions.

Transmit antennas114used by multicarrier transmitter100and antennas201used by multicarrier receiver200may comprise two or more directional or omnidirectional antennas, including, for example, dipole antennas, monopole antennas, patch antennas, loop antennas, microstrip antennas or other types of antennas suitable for transmission of RF signals. In some MIMO embodiments that use two or more transmit antennas and two or more receive antennas, instead of two or more antennas, a single antenna with multiple apertures may be used. In these embodiments, each aperture may be considered a separate antenna. In some embodiments, each antenna may be effectively separated to take advantage of spatial diversity and the different channel characteristics that may result between each of transmit antennas114and each of the antennas of multicarrier receiver200.

FIG. 3is a functional diagram of a subcarrier beamformer in accordance with some embodiments of the present invention. Subcarrier beamformer300may correspond with one of the subcarrier beamformers that may be part of beamformer108(FIG. 1). Subcarrier beamformer300may be associated with one of the subcarriers of the multicarrier communication signals. In an example of a multicarrier communication signal having fifty-two total subcarriers, beamformer108(FIG. 1) of multicarrier transmitter100(FIG. 1) may functionally comprise fifty-two of subcarrier beamformers300. In some MIMO embodiments, subcarrier beamformer300weights frequency-domain symbol-modulated subcarriers307A and307B of each spatial stream325(for an associated subcarrier frequency) to generate two or more antenna streams329of symbol-modulated subcarriers309A,309B and309C. Three antenna streams329are illustrated, although the scope of the invention is not limited in this respect.

In some embodiments, subcarrier beamformer300may comprise one of weighting elements302A,302B and302C associated with each transmit antenna114(FIG. 1). Weighting elements302A-302C may weight and combine symbol-modulated subcarriers307A and307B associated with a corresponding subcarrier by applying elements of one row of decimated beamformer matrix121associated with a particular antenna and a corresponding subcarrier to generate symbol-modulated subcarriers309A,309B and309C. For example, weighting element302A may apply a corresponding first row of elements of decimated beamformer matrix121associated with the first antenna of a first antenna stream to combined symbol-modulated subcarriers307A and307B associated with a particular subcarrier frequency.

Depending on the decimation factor, each decimated beamformer matrix121may be applied to more than one subcarrier. For example, when the decimation factor is two, each of decimated beamformer matrices121may be applied to two subcarriers. In some embodiments, when beamformer108(FIG. 1) performs an interpolation on decimated beamformer matrices121to generate an interpolated beamforming matrix for each subcarrier, one interpolated beamforming matrix may be applied by each subcarrier beamformer300.

In some embodiments, each of transmit antennas114(FIG. 1) may transmit an OFDM symbol using the same subcarrier frequencies as the other transmit antennas. The OFDM symbols may be generated from symbol-modulated subcarriers309A,309B or309C associated with a particular antenna. The OFDM symbols transmitted together by transmit antennas114(FIG. 1) may represent one or more of spatial streams325. In some embodiments, at least two of transmit antennas114(FIG. 1) may be used to transmit at least one of spatial streams325, although the scope of the invention is not limited in this respect.

FIG. 4illustrates throughput performance curves for various decimation factors in accordance with some embodiments of the present invention. Throughput403(illustrated in Mbps) is shown on the y-axis of graph400, and receiver SNR405is illustrated on the x-axis of graph400. SNR curve401illustrates SNR performance for a decimation factor of one which was referred to previously as ideal beamforming. SNR curve402illustrates SNR performance for a decimation factor of two, SNR curve404illustrates SNR performance for a decimation factor of four, SNR curve408illustrates SNR performance for a decimation factor of eight, SNR curve416illustrates SNR performance for a decimation factor of sixteen, and SNR curve448illustrates SNR performance for a decimation factor of forty-eight. SNR curve450illustrates SNR performance when no beamforming is applied. In some embodiments, throughput403may be referred to good throughput.

Throughput403may correspond to receive data rate225(FIG. 2) that may be provided by multicarrier transmitter100(FIG. 1). As illustrated inFIG. 4, for lower data rates, there is less significance for sending a complete set of beamforming matrices as feedback (i.e., one matrix per subcarrier). For example, a decimation factor of eight may be suitable for lower data rates allowing feedback to be reduced. For lower data rates, feedback packets may, for example, be shortened by a factor of four relative to feedback packet lengths of higher data rates. Shorter feedback packets consume less bandwidth in the feedback path and may provide more time for the transmission of data by multicarrier transmitter100(FIG. 1) resulting in a higher throughput. Furthermore, multicarrier receiver200(FIG. 2) may consume less energy by transmitting less feedback helping to increase battery life for portable applications.

FIG. 5is a flow chart of a procedure for receiving beamformed signals in accordance with some embodiments of the present invention. Procedure500may be performed by a multicarrier receiver, such as multicarrier receiver200(FIG. 2), for receiving beamformed transmissions from a multicarrier transmitter, such as multicarrier transmitter100(FIG. 1).

Operation502comprises generating a channel transfer function matrix (H) for each subcarrier from received signals. Operation502may be performed by channel transfer function generator216(FIG. 2), although the scope of the invention is not limited in this respect.

Operation504comprises determining the decimation factor based on a receive data rate provided by the transmitter. In some embodiments, the receive data rate may be provided in a packet header, although the scope of the invention is not limited in this respect.

Operation506comprises decimating the channel transfer function matrices generated in operation502based on the decimation factor generated in operation504. Operation506generates decimated channel transfer functions for groups of two or more subcarriers. Operation506may be performed by decimator218(FIG. 2), although the scope of the invention is not limited in this respect.

Operation508comprises decomposing the decimated channel transfer function matrices to generate decimated beamforming matrices (V) for groups of two or more subcarriers. Operation508may be performed by decomposition unit220(FIG. 2), although the scope of the invention is not limited in this respect.

Operation510comprises transmitting the decimated beamforming matrices to the multicarrier transmitter. In some embodiments, prior to transmission, operation510may include compressing the decimated beamforming matrices, although the scope of the invention is not limited in this respect.

Operation512comprises receiving beamformed signals from the multicarrier transmitter. The receiving beamformed signals may have been transmitted at the receive data rate used in operation504to determine the decimation factor.

In some embodiments, operations502-512may be repeated on a symbol-by-symbol basis so that updated beamforming coefficients may be used for each transmitted OFDM symbol, although the scope of the invention is not limited in this respect.

Although the individual operations of procedure500are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently, and nothing requires that the operations be performed in the order illustrated. For example, operations502and504may be performed concurrently or in reverse order.

Unless specifically stated otherwise, terms such as processing, computing, calculating, determining, displaying, or the like, may refer to an action and/or process of one or more processing or computing systems or similar devices that may manipulate and transform data represented as physical (e.g., electronic) quantities within a processing system's registers and memory into other data similarly represented as physical quantities within the processing system's registers or memories, or other such information storage, transmission or display devices. Furthermore, as used herein, a computing device includes one or more processing elements coupled with computer-readable memory that may be volatile or non-volatile memory or a combination thereof.