Start frame for distributed MIMO

Embodiments of the present disclosure generally relate to communication, and more specifically to distributed multiple-input multiple-output (MIMO) systems. For certain aspects, methods and apparatus are provided for transmitting a start frame containing information regarding parallel MIMO transmissions in a transmit opportunity and transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame. For certain aspects, methods and apparatus are provided for receiving a start frame containing information regarding parallel MIMO transmissions in a transmit opportunity and transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame. For certain aspects, methods and apparatus are provided for receiving a start frame containing information regarding parallel MIMO transmissions in a transmit opportunity and receiving one or more parallel MIMO streams in accordance with the information contained in the start frame.

TECHNICAL FIELD

The present disclosure generally relates to communication, and more specifically to distributed multiple-input multiple-output (MIMO) systems.

BACKGROUND

Some wireless technologies, such as Spatial Division Multiple Access (SDMA), allow for multiple transmissions to occur in parallel without causing a collision. A potential application of such technologies is the distribution of multiple video streams in parallel. For example, a video stream may be 2 Mbps for Standard Definition Television (SDTV) using Moving Picture Expert Group 1 (MPEG1), 8 to 25 Mbps for High Definition Television (HDTV) using MPEG2, and up to 54 Mbps for Blu-ray.

In certain applications, video streams may be transmitted using Multiple-input-multiple-output (MIMO) techniques. MIMO generally refers to simultaneous transmissions from multiple transmit antennas to multiple receive antennas.

SUMMARY

Certain embodiments provide a method for wireless communications. The method generally includes transmitting a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide a method for wireless communications a method for wireless communications. The method generally includes receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide a method for wireless communications. The method generally includes receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and receiving one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes logic for transmitting a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and logic for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes logic for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and logic for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes logic for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and logic for receiving one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes means for transmitting a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and means for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes means for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and means for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide an apparatus for wireless communications. The apparatus generally includes means for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and means for receiving one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide a computer-program product for wireless communications comprising a computer readable medium having a set of instructions stored thereon, the set of instructions being executable by one or more processors. The set of instructions generally includes instructions for transmitting a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and instructions for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide a computer-program product for wireless communications comprising a computer readable medium having a set of instructions stored thereon, the set of instructions being executable by one or more processors. The set of instructions generally includes instructions for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and instructions for transmitting one or more parallel MIMO streams in accordance with the information contained in the start frame.

Certain embodiments provide a computer-program product for wireless communications comprising a computer readable medium having a set of instructions stored thereon, the set of instructions being executable by one or more processors. The set of instructions generally includes instructions for receiving a start frame containing information regarding parallel multiple-input multiple-output (MIMO) transmissions in a transmit opportunity and instructions for receiving one or more parallel MIMO streams in accordance with the information contained in the start frame.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure provide techniques and apparatus that may be utilized in distributed MIMO systems.

An Example Wireless Communication System

The multi-antenna transmission techniques described herein may be used in combination with various wireless technologies such as Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM), Time Division Multiple Access (TDMA), and so on. Multiple user terminals can concurrently transmit/receive data via different (1) orthogonal code channels for CDMA, (2) time slots for TDMA, or (3) subbands for OFDM. A CDMA system may implement IS-2000, IS-95, IS-856, Wideband-CDMA (W-CDMA), or some other standards. An OFDM system may implement IEEE 802.11 or some other standards. A TDMA system may implement GSM or some other standards. These various standards are known in the art.

FIG. 1shows a multiple-access MIMO system100with access points and user terminals. For simplicity, only one access point110is shown inFIG. 1. An access point is generally a fixed station that communicates with the user terminals and may also be referred to as a base station or some other terminology. The access point may also be a mobile station, in which case it may be referred to as a group owner. A user terminal may be fixed or mobile and may also be referred to as a mobile station, a wireless device, a client, or some other terminology. Access point110may communicate with one or more user terminals120at any given moment on the downlink and uplink. The downlink (i.e., forward link) is the communication link from the access point to the user terminals, and the uplink (i.e., reverse link) is the communication link from the user terminals to the access point.

A user terminal may also communicate peer-to-peer with another user terminal For example, in the illustrated example, UTs120e,120f,120g,120h, and120iare shown with peer-to-peer links. These UTs may also have a link with the AP in addition to the illustrated peer-to-peer link with a UT.

While portions of the following disclosure will describe user terminals120capable of communicating via SDMA, for certain embodiments, the user terminals120may also include some user terminals that do not support SDMA. Thus, for such embodiments, an AP110may be configured to communicate with both SDMA and non-SDMA user terminals. This approach may conveniently allow older versions of user terminals (“legacy” stations) to remain deployed in an enterprise, extending their useful lifetime, while allowing newer SDMA user terminals to be introduced as deemed appropriate.

System100employs multiple transmit and multiple receive antennas for data transmission on the downlink and uplink. Access point110is equipped with Napantennas and represents the multiple-input (MI) for downlink transmissions and the multiple-output (MO) for uplink transmissions. A set of Nuselected user terminals120collectively represents the multiple-output for downlink transmissions and the multiple-input for uplink transmissions. For pure SDMA, it is desired to have Nap≧Nu≧1 if the data symbol streams for the Nuuser terminals are not multiplexed in code, frequency or time by some means. Numay be greater than Napif the data symbol streams can be multiplexed using different code channels with CDMA, disjoint sets of subbands with OFDM, and so on. Each selected user terminal transmits user-specific data to and/or receives user-specific data from the access point. In general, each selected user terminal may be equipped with one or multiple antennas (i.e., Nut≧1). The Nuselected user terminals can have the same or a different number of antennas.

The SDMA system100may be a time division duplex (TDD) system or a frequency division duplex (FDD) system. For a TDD system, the downlink and uplink share the same frequency band. For an FDD system, the downlink and uplink use different frequency bands. MIMO system100may also utilize a single carrier or multiple carriers for transmission. Each user terminal may be equipped with a single antenna (e.g., in order to keep costs down) or multiple antennas (e.g., where the additional cost can be supported).

FIG. 2shows a block diagram of access point110and two user terminals120mand120xin MIMO system100. Access point110is equipped with antennas224athrough224t. User terminal120mis equipped with antennas252mathrough252mu, and user terminal120xis equipped with antennas252xathrough252xu. Access point110is a transmitting entity for the downlink and a receiving entity for the uplink. Each user terminal120is a transmitting entity for the uplink and a receiving entity for the downlink. As used herein, a “transmitting entity” is an independently operated apparatus or device capable of transmitting data via a wireless channel, and a “receiving entity” is an independently operated apparatus or device capable of receiving data via a wireless channel. In the following description, the subscript “dn” denotes the downlink, the subscript “up” denotes the uplink, Nupuser terminals are selected for simultaneous transmission on the uplink, Ndnuser terminals are selected for simultaneous transmission on the downlink, Nupmay or may not be equal to Ndn, and Nupand Ndnmay be static values or can change for each scheduling interval. The beam-steering or some other spatial processing technique may be used at the access point and user terminal.

On the uplink, at each user terminal120selected for uplink transmission, a TX data processor288receives traffic data from a data source286and control data from a controller280. TX data processor288processes (e.g., encodes, interleaves, and modulates) the traffic data for the user terminal based on the coding and modulation schemes associated with the rate selected for the user terminal and provides a data symbol stream. A TX spatial processor290performs spatial processing on the data symbol stream and provides transmit symbol streams for the antennas. Each transmitter unit (TMTR)254receives and processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) a respective transmit symbol stream to generate an uplink signal. transmitter units254provide uplink signals for transmission from antennas252to the access point.

Nupuser terminals may be scheduled for simultaneous transmission on the uplink. Each of these user terminals performs spatial processing on its data symbol stream and transmits its set of transmit symbol streams on the uplink to the access point.

At access point110, antennas224athrough224apreceive the uplink signals from all Nupuser terminals transmitting on the uplink. Each antenna224provides a received signal to a respective receiver unit (RCVR)222. Each receiver unit222performs processing complementary to that performed by transmitter unit254and provides a received symbol stream. An RX spatial processor240performs receiver spatial processing on the received symbol streams from receiver units222and provides Nuprecovered uplink data symbol streams. The receiver spatial processing is performed in accordance with the channel correlation matrix inversion (CCMI), minimum mean square error (MMSE), soft interference cancellation (SIC), or some other technique. Each recovered uplink data symbol stream is an estimate of a data symbol stream transmitted by a respective user terminal An RX data processor242processes (e.g., demodulates, deinterleaves, and decodes) each recovered uplink data symbol stream in accordance with the rate used for that stream to obtain decoded data. The decoded data for each user terminal may be provided to a data sink244for storage and/or a controller230for further processing.

On the downlink, at access point110, a TX data processor210receives traffic data from a data source208for Ndnuser terminals scheduled for downlink transmission, control data from a controller230, and possibly other data from a scheduler234. The various types of data may be sent on different transport channels. TX data processor210processes (e.g., encodes, interleaves, and modulates) the traffic data for each user terminal based on the rate selected for that user terminal. TX data processor210provides Ndndownlink data symbol streams for the Ndnuser terminals. A TX spatial processor220performs spatial processing (such as a precoding or beamforming, as described in the present disclosure) on the Ndndownlink data symbol streams, and provides transmit symbol streams for the antennas. Each transmitter unit222receives and processes a respective transmit symbol stream to generate a downlink signal. transmitter units222providing downlink signals for transmission from antennas224to the user terminals.

At each user terminal120, antennas252receive the downlink signals from access point110. Each receiver unit254processes a received signal from an associated antenna252and provides a received symbol stream. An RX spatial processor260performs receiver spatial processing on received symbol streams from receiver units254and provides a recovered downlink data symbol stream for the user terminal The receiver spatial processing is performed in accordance with the CCMI, MMSE or some other technique. An RX data processor270processes (e.g., demodulates, deinterleaves and decodes) the recovered downlink data symbol stream to obtain decoded data for the user terminal.

At each user terminal120, a channel estimator278estimates the downlink channel response and provides downlink channel estimates, which may include channel gain estimates, SNR estimates, noise variance and so on. Similarly, a channel estimator228estimates the uplink channel response and provides uplink channel estimates. Controller280for each user terminal typically derives the spatial filter matrix for the user terminal based on the downlink channel response matrix Hdn,mfor that user terminal Controller230derives the spatial filter matrix for the access point based on the effective uplink channel response matrix Hup,effController280for each user terminal may send feedback information (e.g., the downlink and/or uplink eigenvectors, eigenvalues, SNR estimates, and so on) to the access point. Controllers230and280also control the operation of various processing units at access point110and user terminal120, respectively.

FIG. 3illustrates various components that may be utilized in a wireless device302that may be employed within the wireless communication system100. The wireless device302is an example of a device that may be configured to implement the various methods described herein. The wireless device302may be a base station104or a user terminal106.

The wireless device302may include a processor304which controls operation of the wireless device302. The processor304may also be referred to as a central processing unit (CPU). Memory306, which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor304. A portion of the memory306may also include non-volatile random access memory (NVRAM). The processor304typically performs logical and arithmetic operations based on program instructions stored within the memory306. The instructions in the memory306may be executable to implement the methods described herein.

The wireless device302may also include a housing308that may include a transmitter310and a receiver312to allow transmission and reception of data between the wireless device302and a remote location. The transmitter310and receiver312may be combined into a transceiver314. A single or a plurality of transmit antennas316may be attached to the housing308and electrically coupled to the transceiver314. The wireless device302may also include (not shown) multiple transmitters, multiple receivers, and multiple transceivers.

The wireless device302may also include a signal detector318that may be used in an effort to detect and quantify the level of signals received by the transceiver314. The signal detector318may detect such signals as total energy, energy per subcarrier per symbol, power spectral density and other signals. The wireless device302may also include a digital signal processor (DSP)320for use in processing signals.

The various components of the wireless device302may be coupled together by a bus system322, which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus.

The wireless system shown inFIGS. 1-3may refer to the SDMA system where antennas at the access point are located in sufficiently different directions, which insures no interference between simultaneously transmitted spatial streams dedicated to different user terminals. For certain embodiments of the present disclosure, the wireless system shown inFIGS. 1-3may refer to the multiuser system where a precoding (beamforming) of the transmission signal is applied providing orthogonality between spatial streams dedicated to different user terminals, while the access point antennas do not necessarily need to be located in sufficiently different directions.

Distributed MIMO

Certain embodiments of the present disclosure provide techniques and apparatus that may be utilized in distributed MIMO systems. As described above, MIMO generally refers to simultaneous transmissions from multiple transmit antennas to multiple receive antennas. The IEEE 802.11n standard, for example, specifies MIMO transmissions where the multiple transmit antennas belong to one user. MIMO may be used in a wide variety of wireless applications.

For example, in a home environment, audio-video (AV) traffic may be exchanged between several sources and destinations. As illustrated inFIG. 4, an access point (AP)402, illustratively shown with a wired connection to the Internet, may stream media to various devices, such as a Blu-ray player412, monitor (screen)416, and digital video recorder (DVR)418in a first room410, a screen426and audio device428located in a second room420, and a device436with integrated screen and speakers in a third room430. The AP may be, for example, a cable modem, set-top box, router, or the like.

As illustrated, devices connected with the AP may stream to various other devices. For example, the player412may stream to the screen416and a speakers414, the DVR may stream to screen426and audio device428. Certain devices may be both sources and receivers of streams.

As illustrated inFIG. 4, data may be sent from multiple sources to multiple destinations in parallel on the same channel, during distributed MIMO transmit opportunities (TXOPs). In the following disclosure, distributed MIMO TXOPs may also be referred to as MIMO TXOPs, or simply as TXOPs.

According to certain embodiments of the present disclosure, the start of a MIMO TXOP may be indicated by a start frame, which may be transmitted by the AP or another device in the network (that assumes a master role for the purpose of scheduling transmissions in the MIMO TXOP).

The start frame may specify various parameters of MIMO transmissions between devices during the pending TXOP. For example, the start frame may specify the transmitters and receivers during the pending TXOP, the spatial streams they may use, the maximum TXOP duration, and possibly the timing of response frames, such as block acknowledgements (BAs). The start frame may also synchronize the start of the transmission amongst the multiple transmitters.

As will be described below with reference to particular examples, during the TXOP, each transmitter may transmit data using beamforming to its intended receiver, while attempting to minimize the power received by the other receivers. As an alternative, a transmitter may send data on a predetermined set of spatial streams, which may be filtered by the intended receiver.

The number of transmit (Tx) and receive (Rx) antennas may be different in each of these cases, as will be described in greater detail below. In addition, the channel conditions between a transmitter and each of the destinations may be known at the transmitters, for example, either implicitly by receiving a suitable transmission from each of the receivers or explicitly by receiving a channel matrix or similar from each of the receivers.

In the following description, reference may be made to an AP as initiating a TXOP with a start frame. However, it should be understood that the master role of scheduling MIMO transmissions in a TXOP may reside at either an AP and/or at any station that is capable of scheduling. Further, a network may contain multiple such master devices.

FIG. 5illustrates an example MIMO system, in which an AP504transmits parallel MIMO streams to three stations502(STA5-STA7) and one station (STA1) transmit parallel MIMO streams to another station (STA8). The illustrated AP and stations may correspond, for example, to the components illustrated in the system ofFIG. 4.

According to certain embodiments, the stations allowed to participate in parallel MIMO transmissions during a TXOP may be selected and their transmission parameters may be controlled, for example, by an AP or non-AP station serving as a master device.

For example, to prevent near-far problems at the receivers during a distributed TXOP, the secondary signals should not be stronger than the primary signal by some threshold. As used herein, the term “primary signal” generally refers to the signal from the source transmitter, while the term “secondary signal” generally refers to signals from the other transmitters during the distributed TXOP. A measure of signal strength may be referred to herein as Received Signal Strength Indicator (RSSI).

The master device may take the signal strength requirement into account when scheduling transmitters and receivers for a distributed TXOP. For this purpose, the master device may periodically request a Received Signal Strength Indication (RSSI)-list from all receiving devices. The RSSI-list may contain the RSSI at the receiver for the devices listed in the RSSI-list request. If the receiving device has multiple receive antennas, it may report the maximum RSSI across all receive antennas, or the average RSSI over all antennas, where the averaging may be done on linear power values (e.g., rather than on logarithmic values).

Prior to sending the RSSI-list request, the master device may request the transmitters to transmit a frame, allowing the receivers to measure RSSI under current channel conditions and assemble the RSSI-list. According to certain embodiments, obtaining the RSSI-list may be combined with obtaining the channel matrices (i.e. it may be combined with sounding).

Thus, in addition to specifying which devices may participate in MIMO transmissions during a TXOP, a start frame may also specify a TX power requirement for each transmitter. In this manner, a master device may avoid a near-far problem by reducing the Tx power for some transmitters in the distributed TXOP.

FIG. 6illustrates example operations600that may be performed by receiver and transmitter devices in a distributed MIMO system, in accordance with certain aspects of the present disclosure. The operations602and604may be performed by a transmitting device, while the operations606-610may be performed by a receiving device.

At602, a start frame containing information about a MIMO transmit opportunity (TXOP) is received (or transmitted if the device is acting as a master device). At604, data may be transmitted in one or more parallel MIMO stream(s) during the TXOP, with the data transmitted according to the information contained in the start frame.

At606, the (same) start frame is received by a receiving device and, at608, data is received from one or more transmitters, with the data received according to the information contained in the start frame. Optionally, at610, the received data may be acknowledged, for example, with block acknowledgement (BA) frames transmitted in accordance with timing information from the start frame.

FIG. 7illustrates an example exchange of MIMO transmissions, in accordance with the operations ofFIG. 6. As illustrated, a master device may transmit a start frame702containing information about transmissions in a distributed TXOP. During the TXOP, transmitting devices may send parallel MIMO transmissions704to receiving devices. The receiving devices may acknowledge data received in the TXOP with BA frames706. As illustrated, the start frame702may specify the distributed MIMO TXOP, may also synchronize the start of the transmissions704, and may also specify the timing of the BA frames706.

As noted above, according to certain embodiments, during the TXOP, each transmitter may utilize beamforming when transmitting to its intended receiver, while attempting to minimizing the power received by the other receivers. In such embodiments, each of the transmitters active during the MIMO TXOP may utilize at least as many TX antennas as the total number of spatial streams (SSs) that are used during the distributed MIMO TXOP.

Each receiver may utilize as many Rx antennas as spatial streams used to transmit to that station, as illustrated inFIG. 8. In the illustrated example, STA1transmits on two spatial streams to STA8, thus STA8may receive with two Rx antennas. AP may transmit to STA6and STA7using 1 stream, and to STA5using two streams. Thus, STA6and STA7may utilize one Rx antenna, while STA5may utilize two Rx antennas.

The channel state information between the transmitters and each of the receivers may be determined through sounding. Sounding may be based on implicit or explicit feedback. For implicit feedback, each receiver periodically transmits a frame which contains enough Long Training Fields (LTFs) to sound the channel. The receivers may do so autonomously or coordinated by the AP.

For explicit feedback, each transmitter may periodically transmit a sounding frame, to which each receiver may respond with a broadcast frame which contains a channel matrix between the transmitter and the station. The transmitters may transmit sounding frames912autonomously as illustrated inFIG. 9. As illustrated, the source (AP or STA1) may transmit a sounding frame910, while each receiving station may reply with its own channel matrix (e.g., in a sequential order scheduled in the sounding frame910).

As an alternative, transmitting devices (e.g., AP and/or STA1in this example) may coordinate sounding, in which case the channel matrices may be combined into a single frame1012as illustrated inFIG. 10, in response to sounding frames1010. As illustrated, each frame1012may include the channel matrices between the AP (determined based on the sounding frame1010A), as well as channel matrices between the STA1and responding station (determined based on the sounding frame1010S sent by STA1). The sounding frame1010S sent by STA1and the reply frames1012may be sent in a sequential order scheduled in the sounding frame1010A sent by the AP.

According to certain aspects, feedback provided via sounding frames may comprise channel matrices. As an alternative, or in addition, the feedback may also comprise (compressed) beamforming matrices (e.g., when the sounding approach described below with reference toFIG. 11is used).

FIGS. 11-13illustrate other example sounding exchanges, in which channel matrices may be transmitted (individually between stations or broadcast) in various frames in accordance with certain embodiments of the present disclosure. In addition to channel matrices, the RSSI for each of the transmitters may be included in the response frame(s).

As illustrated inFIG. 11, following a Sounding Start frame1102, multiple sources (STA1-STA4) may each transmit a sounding frame1110. Each receiver (STA5-STA8) may reply with their channel matrices sent in a reply frame in a sequential order scheduled in the start sounding frame1102.

As illustrated inFIG. 12, following a Sounding Start frame1202, multiple sources (STA1-STA4) may each transmit a sounding frame1210. A single receiver (e.g., STA6in this example) may reply with a frame1212containing the channel matrix for each source individually, in a sequential order scheduled in the start sounding frame1202.

As an alternative, illustrated inFIG. 13, following a Sounding Start frame1302, the multiple sources (STA1-STA4) may each transmit a sounding frame1310, and the single receiver (e.g., STA6in this example) may reply with a single broadcast frame1312containing the channel matrices for all of the sources.

During the TXOP, the transmitters may transmit on spatial streams that are known to the receiver beforehand, for instance through the start frame or through prior configuration, after which each receiver filters only the spatial streams it is required to receive. According to such embodiments, each transmitter uses at least as many Tx antennas as the number of spatial streams it is transmitting on, and each receiver may use at least as many Rx antennas as the total number of spatial streams used during the MIMO TXOP.

Such an embodiment is illustrated inFIG. 14. In the illustrated example, STA1transmits on two spatial streams to STA8, while the AP transmits to STA6and STA7using 1 stream, and to STA5using two streams, resulting in a total of six streams. Thus, the receiving stations (STA5-STA8) may all receive using six Rx antennas.

As noted above, the start frame may synchronize the start of the transmission amongst the transmitters. Ranging may be required in order to make the timing precise enough. A separate sounding sequence may not be required, because the transmissions may start with enough LTFs for the stations to determine how to filter the appropriate spatial streams.

Thus,FIGS. 15 and 16illustrate example embodiments, where channel matrices are sent in frames without separate sounding frames. As illustrated inFIG. 15, following a start frame1502and multi-user MIMO transmissions1504, stations may respond with Block Acknowledgements1506-1510. As illustrated, some Channel Matrix information may be included in a Block Acknowledgement (BA) frame (as shown with BA1508), while other Channel Matrix Information may be provided in separate frames1510(e.g., after the BAs).

In some cases, BAs may not contain Channel Matrix Information. For example, as illustrated inFIG. 16, following a start frame1602and multi-user MIMO transmissions1604, stations may respond with separate Block Acknowledgements1606, followed by separate reply frames1610containing Channel Matrix information.

According to certain embodiments, prior to being able to participate in distributed MIMO TXOPs, non-AP transmitters set up a direct link with their receivers. The non-AP transmitter (i.e., STA1) may inform the AP of this fact, after which the AP may include STA1in distributed MIMO TXOPs. Similarly, a non-AP transmitter may include the AP in distributed MIMO TXOPs when the non-AP transmitter gains a TXOP and not all available spatial streams are used. The AP and the non-AP transmitter may also still use TXOPs for themselves, without including other transmitters. However, when a TXOP is gained which uses less spatial streams than the smallest number of transmitters in some subset of stations, then including other transmitters will improve the overall efficiency.

According to certain embodiments, for a non-AP transmitter to include the AP in distributed MIMO TXOPs, the non-AP transmitter may need to be made aware of the active destinations for the AP. Similarly, an AP may need to be made aware of active destination(s) for the non-AP transmitter in order for the AP to include the non-AP transmitter. According to certain embodiments, this information may be conveyed through TSPECs or through new to be defined Action frames, or simply by announcing the direct link to the AP.

The various operations of the method described above may be performed by various hardware and/or software component(s) and/or module(s) corresponding to means-plus-function blocks illustrated in the Figures. Generally, where there are methods illustrated in Figures having corresponding counterpart means-plus-function Figures, the operation blocks correspond to means-plus-function blocks with similar numbering. For example, operations602-610illustrated inFIG. 6corresponds to means-plus-function blocks602A-610A illustrated inFIG. 6A.

Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals and the like that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles or any combination thereof.