Patent Description:
In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks may be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks may be designated respectively as a wide area network (WAN), metropolitan area network (MAN), local area network (LAN), or personal area network (PAN). Networks also differ according to the switching/routing technique used to interconnect the various network nodes and devices (e.g., circuit switching vs. packet switching), the type of physical media employed for transmission (e.g., wired vs. wireless), and the set of communication protocols used (e.g., Internet protocol suite, SONET (Synchronous Optical Networking), Ethernet, etc.).

Wireless networks are often preferred when the network elements are mobile and thus have dynamic connectivity needs, or if the network architecture is formed in an ad hoc, rather than fixed, topology. Wireless networks employ intangible physical media in an unguided propagation mode using electromagnetic waves in the radio, microwave, infra-red, optical, etc. frequency bands. Wireless networks advantageously facilitate user mobility and rapid field deployment when compared to fixed wired networks.

In order to address the issue of increasing bandwidth requirements that are demanded for wireless communications systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. With limited communication resources, it is desirable to reduce the amount of traffic passing between the access point and the multiple terminals. For example, when multiple terminals send uplink communications to the access point, it is desirable to minimize the amount of traffic to complete the uplink of all transmissions. Thus, there is a need for an improved protocol for uplink transmissions from multiple terminals.

<CIT> discloses that simultaneous, multi-user uplink communication is scheduled in a wireless network (<NUM>) by transmitting a poll message (<NUM>) to a plurality of access terminals (<NUM>-<NUM>) in response to receiving a first request to transmit data (<NUM>) via uplink. The poll message (<NUM>) includes a solicitation for requests to transmit data from each of the plurality of access terminals (<NUM>-<NUM>). The poll message (<NUM>) also includes a medium reservation and schedule for transmission of the requests from the access terminals. Based on the requests received from the access terminals, a number of the access terminals (<NUM>, <NUM>) are selected for simultaneous transmission of data (<NUM>, <NUM>) via uplink. A transmit start message (<NUM>) is sent to each of the selected access terminals (<NUM>, <NUM>) indicating when and for how long the selected access terminals (<NUM>, <NUM>) may transmit data via uplink. After the data (<NUM>, <NUM>) is received, a block ACK message (<NUM>, <NUM>) is sent to each of the selected access terminals (<NUM>, <NUM>) indicating successful simultaneous communication.

Further prior art is known from <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

The underlying problem of the present invention is solved by the subject matter of the independent claims.

Various aspects of the novel systems, apparatuses, and methods are described more fully hereinafter with reference to the accompanying drawings. The teachings disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or function presented throughout this disclosure. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the novel systems, apparatuses, and methods disclosed herein, whether implemented independently of or combined with any other aspect of the invention. In addition, the scope of the invention is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the invention set forth herein. It should be understood that any aspect disclosed herein may be embodied by one or more elements of a claim.

Although particular aspects are described herein, many variations and permutations of these aspects fall within the scope of the disclosure. Although some benefits and advantages of the preferred aspects are mentioned, the scope of the disclosure is not intended to be limited to particular benefits, uses, or objectives. Rather, aspects of the disclosure are intended to be broadly applicable to different wireless technologies, system configurations, networks, and transmission protocols, some of which are illustrated by way of example in the figures and in the following description of the preferred aspects. The detailed description and drawings are merely illustrative of the disclosure rather than limiting, the scope of the disclosure being defined by the appended claims.

Wireless network technologies may include various types of wireless local area networks (WLANs). A WLAN may be used to interconnect nearby devices together, employing widely used networking protocols. The various aspects described herein may apply to any communication standard, such as Wi-Fi or, more generally, any member of the IEEE <NUM> family of wireless protocols.

In some aspects, wireless signals may be transmitted according to a high-efficiency <NUM> protocol using orthogonal frequency-division multiplexing (OFDM), direct-sequence spread spectrum (DSSS) communications, a combination of OFDM and DSSS communications, or other schemes. Implementations of the high-efficiency <NUM> protocol may be used for Internet access, sensors, metering, smart grid networks, or other wireless applications. Advantageously, aspects of certain devices implementing this particular wireless protocol may consume less power than devices implementing other wireless protocols, may be used to transmit wireless signals across short distances, and/or may be able to transmit signals less likely to be blocked by objects, such as humans.

In some implementations, a WLAN includes various devices which are the components that access the wireless network. For example, there may be two types of devices: access points ("APs") and clients (also referred to as stations, or "STAs"). In general, an AP serves as a hub or base station for the WLAN and an STA serves as a user of the WLAN. For example, a STA may be a laptop computer, a personal digital assistant (PDA), a mobile phone, etc. In an example, an STA connects to an AP via a Wi-Fi (e.g., IEEE <NUM> protocol such as <NUM>. 11ah) compliant wireless link to obtain general connectivity to the Internet or to other wide area networks. In some implementations an STA may also be used as an AP.

The techniques described herein may be used for various broadband wireless communication systems, including communication systems that are based on an orthogonal multiplexing scheme. Examples of such communication systems include Spatial Division Multiple Access (SDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA) systems, Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems, and so forth. An SDMA system may utilize sufficiently different directions to simultaneously transmit data belonging to multiple user terminals. A TDMA system may allow multiple user terminals to share the same frequency channel by dividing the transmission signal into different time slots, each time slot being assigned to different user terminal. A TDMA system may implement GSM or some other standards known in the art. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which is a modulation technique that partitions the overall system bandwidth into multiple orthogonal sub-carriers. These sub-carriers may also be called tones, bins, etc. With OFDM, each sub-carrier may be independently modulated with data. An OFDM system may implement IEEE <NUM> or some other standards known in the art. An SC-FDMA system may utilize interleaved FDMA (IFDMA) to transmit on sub-carriers that are distributed across the system bandwidth, localized FDMA (LFDMA) to transmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA) to transmit on multiple blocks of adjacent sub-carriers. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDMA. A SC-FDMA system may implement 3GPP-LTE (3rd Generation Partnership Project Long Term Evolution) or other standards.

The teachings herein may be incorporated into (e.g., implemented within or performed by) a variety of wired or wireless apparatuses (e.g., nodes). In some aspects, a wireless node implemented in accordance with the teachings herein may comprise an access point or an access terminal.

An access point ("AP") may comprise, be implemented as, or known as a NodeB, Radio Network Controller ("RNC"), eNodeB, Base Station Controller ("BSC"), Base Transceiver Station ("BTS"), Base Station ("BS"), Transceiver Function ("TF"), Radio Router, Radio Transceiver, Basic Service Set ("BSS"), Extended Service Set ("ESS"), Radio Base Station ("RBS"), or some other terminology.

A station "STA" may also comprise, be implemented as, or known as a user terminal, an access terminal ("AT"), a subscriber station, a subscriber unit, a mobile station, a remote station, a remote terminal, a user agent, a user device, user equipment, or some other terminology. In some implementations an access terminal may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol ("SIP") phone, a wireless local loop ("WLL") station, a personal digital assistant ("PDA"), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or smartphone), a computer (e.g., a laptop), a portable communication device, a headset, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a gaming device or system, a global positioning system device, or any other suitable device that is configured to communicate via a wireless medium.

<FIG> is a diagram that illustrates a multiple-access multiple-input multiple-output (MIMO) system <NUM> with access points and user terminals. For simplicity, only one access point <NUM> is shown in <FIG>. 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 using some other terminology. A user terminal or STA may be fixed or mobile and may also be referred to as a mobile station or a wireless device, or using some other terminology. The access point <NUM> may communicate with one or more user terminals <NUM> at 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. A system controller <NUM> couples to and provides coordination and control for the access points.

While portions of the following disclosure will describe user terminals <NUM> capable of communicating via Spatial Division Multiple Access (SDMA), for certain aspects, the user terminals <NUM> may also include some user terminals that do not support SDMA. Thus, for such aspects, the AP <NUM> may be configured to communicate with both SDMA and non-SDMA user terminals. This approach may conveniently allow older versions of user terminals ("legacy" stations) that do not support SDMA to remain deployed in an enterprise, extending their useful lifetime, while allowing newer SDMA user terminals to be introduced as deemed appropriate.

The system <NUM> employs multiple transmit and multiple receive antennas for data transmission on the downlink and uplink. The access point <NUM> is equipped with Nap antennas and represents the multiple-input (MI) for downlink transmissions and the multiple-output (MO) for uplink transmissions. A set of K selected user terminals <NUM> collectively represents the multiple-output for downlink transmissions and the multiple-input for uplink transmissions. For pure SDMA, it is desired to have Nap ≤ K ≤ <NUM> if the data symbol streams for the K user terminals are not multiplexed in code, frequency or time by some means. K may be greater than Nap if the data symbol streams can be multiplexed using TDMA technique, different code channels with CDMA, disjoint sets of sub-bands with OFDM, and so on. Each selected user terminal may transmit user-specific data to and/or receive user-specific data from the access point. In general, each selected user terminal may be equipped with one or multiple antennas (i.e., Nut ≥<NUM>). The K selected user terminals can have the same number of antennas, or one or more user terminals may have a different number of antennas.

The SDMA system <NUM> may 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. The MIMO system <NUM> may 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). The system <NUM> may also be a TDMA system if the user terminals <NUM> share the same frequency channel by dividing transmission/reception into different time slots, where each time slot may be assigned to a different user terminal <NUM>.

<FIG> illustrates a block diagram of the access point <NUM> and two user terminals <NUM> and 120x in MIMO system <NUM>. The access point <NUM> is equipped with Nt antennas 224a through 224ap. The user terminal <NUM> is equipped with Nut,m antennas <NUM>ma through <NUM>mu, and the user terminal 120x is equipped with Nut,x antennas <NUM>xa through <NUM>xu. The access point <NUM> is a transmitting entity for the downlink and a receiving entity for the uplink. The user terminal <NUM> is 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, Nup user terminals are selected for simultaneous transmission on the uplink, and Ndn user terminals are selected for simultaneous transmission on the downlink. Nup may or may not be equal to Ndn, and Nup and Ndn may be static values or may change for each scheduling interval. Beam-steering or some other spatial processing technique may be used at the access point <NUM> and/or the user terminal <NUM>.

On the uplink, at each user terminal <NUM> selected for uplink transmission, a TX data processor <NUM> receives traffic data from a data source <NUM> and control data from a controller <NUM>. The TX data processor <NUM> processes (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 processor <NUM> performs spatial processing on the data symbol stream and provides Nut,m transmit symbol streams for the Nut,m antennas. Each transmitter unit (TMTR) <NUM> receives and processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) a respective transmit symbol stream to generate an uplink signal. Nut,m transmitter units <NUM> provide Nut,m uplink signals for transmission from Nut,m antennas <NUM>, for example to transmit to the access point <NUM>.

Nup user terminals may be scheduled for simultaneous transmission on the uplink. Each of these user terminals may perform spatial processing on its respective data symbol stream and transmit its respective set of transmit symbol streams on the uplink to the access point <NUM>.

At the access point <NUM>, Nup antennas 224a through <NUM>ap receive the uplink signals from all Nup user terminals transmitting on the uplink. Each antenna <NUM> provides a received signal to a respective receiver unit (RCVR) <NUM>. Each receiver unit <NUM> performs processing complementary to that performed by transmitter unit <NUM> and provides a received symbol stream. An RX spatial processor <NUM> performs receiver spatial processing on the Nup received symbol streams from Nup receiver units <NUM> and provides Nup recovered uplink data symbol streams. The receiver spatial processing may be 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 processor <NUM> processes (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 sink <NUM> for storage and/or a controller <NUM> for further processing.

On the downlink, at the access point <NUM>, a TX data processor <NUM> receives traffic data from a data source <NUM> for Ndn user terminals scheduled for downlink transmission, control data from a controller <NUM>, and possibly other data from a scheduler <NUM>. The various types of data may be sent on different transport channels. TX data processor <NUM> processes (e.g., encodes, interleaves, and modulates) the traffic data for each user terminal based on the rate selected for that user terminal. The TX data processor <NUM> provides Ndn downlink data symbol streams for the Ndn user terminals. A TX spatial processor <NUM> performs spatial processing (such as a precoding or beamforming) on the Ndn downlink data symbol streams, and provides Nup transmit symbol streams for the Nup antennas. Each transmitter unit <NUM> receives and processes a respective transmit symbol stream to generate a downlink signal. Nup transmitter units <NUM> may provide Nup downlink signals for transmission from Nup antennas <NUM>, for example to transmit to the user terminals <NUM>.

At each user terminal <NUM>, Nut,m antennas <NUM> receive the Nup downlink signals from the access point <NUM>. Each receiver unit <NUM> processes a received signal from an associated antenna <NUM> and provides a received symbol stream. An RX spatial processor <NUM> performs receiver spatial processing on Nut,m received symbol streams from Nut,m receiver units <NUM> and provides a recovered downlink data symbol stream for the user terminal <NUM>. The receiver spatial processing may be performed in accordance with the CCMI, MMSE, or some other technique. An RX data processor <NUM> processes (e.g., demodulates, deinterleaves and decodes) the recovered downlink data symbol stream to obtain decoded data for the user terminal.

At each user terminal <NUM>, a channel estimator <NUM> estimates 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 estimator <NUM> estimates the uplink channel response and provides uplink channel estimates. Controller <NUM> for each user terminal typically derives the spatial filter matrix for the user terminal based on the downlink channel response matrix Hdn,m for that user terminal. Controller <NUM> derives the spatial filter matrix for the access point based on the effective uplink channel response matrix Hup,eff. The controller <NUM> for 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 <NUM>. The controllers <NUM> and <NUM> may also control the operation of various processing units at the access point <NUM> and user terminal <NUM>, respectively.

<FIG> illustrates various components that may be utilized in a wireless device <NUM> that may be employed within the wireless communication system <NUM>. The wireless device <NUM> is an example of a device that may be configured to implement the various methods described herein. The wireless device <NUM> may implement an access point <NUM> or a user terminal <NUM>.

The wireless device <NUM> may include a processor <NUM> which controls operation of the wireless device <NUM>. The processor <NUM> may also be referred to as a central processing unit (CPU). Memory <NUM>, which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor <NUM>. A portion of the memory <NUM> may also include non-volatile random access memory (NVRAM). The processor <NUM> may perform logical and arithmetic operations based on program instructions stored within the memory <NUM>. The instructions in the memory <NUM> may be executable to implement the methods described herein.

The processor <NUM> may comprise or be a component of a processing system implemented with one or more processors. The one or more processors may be implemented with any combination of general-purpose microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate array (FPGAs), programmable logic devices (PLDs), controllers, state machines, gated logic, discrete hardware components, dedicated hardware finite state machines, or any other suitable entities that can perform calculations or other manipulations of information.

The processing system may also include machine-readable media for storing software. Software shall be construed broadly to mean any type of instructions, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Instructions may include code (e.g., in source code format, binary code format, executable code format, or any other suitable format of code).

The wireless device <NUM> may also include a housing <NUM> that may include a transmitter <NUM> and a receiver <NUM> to allow transmission and reception of data between the wireless device <NUM> and a remote location. The transmitter <NUM> and receiver <NUM> may be combined into a transceiver <NUM>. A single or a plurality of transceiver antennas <NUM> may be attached to the housing <NUM> and electrically coupled to the transceiver <NUM>. The wireless device <NUM> may also include (not shown) multiple transmitters, multiple receivers, and multiple transceivers.

The wireless device <NUM> may also include a signal detector <NUM> that may be used in an effort to detect and quantify the level of signals received by the transceiver <NUM>. The signal detector <NUM> may detect such signals as total energy, energy per subcarrier per symbol, power spectral density and other signals. The wireless device <NUM> may also include a digital signal processor (DSP) <NUM> for use in processing signals.

The various components of the wireless device <NUM> may be coupled together by a bus system <NUM>, which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus.

Certain aspects of the present disclosure support transmitting an uplink (UL) signal from multiple STAs to an AP. In some embodiments, the UL signal may be transmitted in a multi-user MIMO (MU-MIMO) system. Alternatively, the UL signal may be transmitted in a multi-user FDMA (MU-FDMA) or similar FDMA system. Specifically, <FIG>, and <FIG> illustrate UL-MU-MIMO transmissions 410A, 410B, 1050A, and 1050B that would apply equally to UL-FDMA transmissions. In these embodiments, UL-MU-MIMO or UL-FDMA transmissions can be sent simultaneously from multiple STAs to an AP and may create efficiencies in wireless communication.

An increasing number of wireless and mobile devices put increasing stress on bandwidth requirements that are demanded for wireless communications systems. With limited communication resources, it is desirable to reduce the amount of traffic passing between the AP and the multiple STAs. For example, when multiple terminals send uplink communications to the access point, it is desirable to minimize the amount of traffic to complete the uplink of all transmissions. Thus, embodiments described herein support utilizing communication exchanges, scheduling and certain frames for increasing throughput of uplink transmissions to the AP.

<FIG> is a time sequence diagram illustrating an example of an UL-MU-MIMO protocol <NUM> that may be used for UL communications. As shown in <FIG> and in conjunction with <FIG>, the AP <NUM> may transmit a clear to transmit (CTX) message <NUM> to the user terminals <NUM> indicating which STAs may participate in the UL-MU-MIMO scheme, such that a particular STA knows to start an UL-MU-MIMO. In some embodiments, the CTX message may be transmitted in a payload portion of a physical layer convergence protocol (PLCP) protocol data units (PPDUs). An example of a CTX frame structure is described more fully below with reference to FIGs. <NUM>-<NUM>.

Once a user terminal <NUM> receives a CTX message <NUM> from the AP <NUM> where the user terminal is listed, the user terminal may transmit the UL-MU-MIMO transmission <NUM>. In <FIG>, STA 120A and STA 120B transmit UL-MU-MIMO transmission 410A and 410B containing physical layer convergence protocol (PLCP) protocol data units (PPDUs). Upon receiving the UL-MU-MIMO transmission <NUM>, the AP <NUM> may transmit block acknowledgments (BAs) <NUM> to the user terminals <NUM>.

<FIG> is a time sequence diagram illustrating an example of an UL-MU-MIMO protocol that may be used for UL communications. In <FIG>, a CTX frame is aggregated in an A-MPDU message <NUM>. The aggregated A-MPDU message <NUM> may provide time to a user terminal <NUM> for processing before transmitting the UL signals or may allow the AP <NUM> to send data to the user terminals <NUM> before receiving uplink data.

Not all APs or user terminals <NUM> may support UL-MU-MIMO or UL-FDMA operation. A capability indication from a user terminal <NUM> may be indicated in a high efficiency wireless (HEW) capability element that is included in an association request or probe request and may include a bit indicating capability, the maximum number of spatial streams a user terminal <NUM> can use in a UL-MU- MIMO transmission, the frequencies a user terminal <NUM> can use in a UL-FDMA transmission, the minimum and maximum power and granularity in the power backoff, and the minimum and maximum time adjustment a user terminal <NUM> can perform.

A capability indication from an AP may be indicated in a HEW capability element that is included in an association response, beacon or probe response and may include a bit indicating capability, the maximum number of spatial streams a single user terminal <NUM> can use in a UL-MU- MIMO transmission, the frequencies a single user terminal <NUM> can use in a UL-FDMA transmission, the required power control granularity, and the required minimum and maximum time adjustment a user terminal <NUM> should be able to perform.

In one embodiment, capable user terminals <NUM> may request to a capable AP to be part of the UL-MU-MIMO (or UL-FDMA) protocol by sending a management frame to AP indicating request for enablement of the use of UL-MU-MIMO feature. In one aspect, an AP <NUM> may respond by granting the use of the UL-MU-MIMO feature or denying it. Once the use of the UL-MU-MIMO is granted, the user terminal <NUM> may expect a CTX message <NUM> at a variety of times. Additionally, once a user terminal <NUM> is enabled to operate the UL-MU-MIMO feature, the user terminal <NUM> may be subject to follow a certain operation mode. If multiple operation modes are possible, an AP may indicate to the user terminal <NUM> which mode to use in a HEW capability element, a management frame, or in an operation element. In one aspect the user terminals <NUM> can change the operation modes and parameters dynamically during operation by sending a different operating element to the AP <NUM>. In another aspect the AP <NUM> may switch operation modes dynamically during operation by sending an updated operating element or a management frame to a user terminal <NUM> or in a beacon. In another aspect, the operation modes may be indicated in the setup phase and may be setup per user terminal <NUM> or for a group of user terminals <NUM>. In another aspect the operation mode may be specified per traffic identifier (TID).

<FIG> is a time sequence diagram that, in conjunction with <FIG>, illustrates an example of an operation mode of a UL-MU-MIMO transmission. In this embodiment, which is according to the invention, a user terminal <NUM> receives a CTX message <NUM> from an AP <NUM> and sends an immediate response to the AP <NUM>. The response may be in the form of a clear to send (CTS) <NUM> or another similar signal. In one aspect, requirement to send a CTS may be indicated in the CTX message <NUM> or may be indicated in the setup phase of the communication. As shown in <FIG>, STA <NUM> A and STA 120B may transmit a CTS <NUM>408A and CTS <NUM>408B message in response to receiving the CTX message <NUM>. The modulation and coding scheme (MCS) of the CTS <NUM>408A and CTS <NUM>408B may be based on the MCS of the CTX message <NUM>. In this embodiment, CTS <NUM>408A and CTS <NUM>408B contain the same bits and the same scrambling sequence so that they may be transmitted to the AP <NUM> at the same time. The duration field of the CTS <NUM> signals may be based on the duration field in the CTX by removing the time for the CTX PPDU. The UL-MU-MIMO transmission 410A and 410B are then sent by the STAs 120A and 120B as listed in the CTX <NUM> signals. The AP <NUM> may then send acknowledgment (ACK) signals the STAs 120A and 120B. In some aspects, the ACK signals may be serial ACK signals to each station or BAs. In some aspects the ACKs may be polled. This embodiment creates efficiencies by simultaneously transmitting CTS <NUM> signals from multiple STAs to an AP <NUM> instead of sequentially, which saves time and reduces the possibility of interference.

<FIG> is a time sequence diagram that, in conjunction with <FIG>, illustrates another example of an operation mode of a UL-MU-MIMO transmission. In this embodiment, user terminals 120A and 120B receive a CTX message <NUM> from an AP <NUM> and are allowed to start and UL-MU-MIMO transmission a time (T) <NUM> after the end of the PPDU carrying the CTX message <NUM>. The T <NUM> may be a short interframe space (SIFS), point interframe space (PIFS), or another time potentially adjusted with additional offsets as indicated by an AP <NUM> in the CTX message <NUM> or via a management frame. The SIFS and PIFS time may be fixed in a standard or indicated by an AP <NUM> in the CTX message <NUM> or in a management frame. The benefit of T <NUM> may be to improve synchronization or to allow a user terminals 120A and 120B time to process the CTX message <NUM> or other messages before transmission.

Referring to <FIG>, in conjunction with <FIG>, the UL-MU-MIMO transmission <NUM> may have a common duration. The duration of the UL-MU-MIMO transmission <NUM> for user terminals utilizing the UL-MU-MIMO feature may be indicated in the CTX message <NUM> or during the setup phase. To generate a PPDU of the required duration, a user terminal <NUM> may build a PLCP service data unit (PSDU) so that the length of the PPDU matches the length indicated in the CTX message <NUM>. In another aspect, a user terminal <NUM> may adjust the level of data aggregation in a media access control (MAC) protocol data unit (A-MPDU) or the level of data aggregation in a MAC service data units (A-MSDU) to approach the target length. In another aspect, a user terminal <NUM> may add end of file (EOF) padding delimiters to reach the target length. In another approach the padding or the EOF pad fields are added at the beginning of the A-MPDU. One of the benefits of having all the UL-MU-MIMO transmissions the same length is that the power level of the transmission will remain constant.

In some embodiments, a user terminal <NUM> may have data to upload to the AP but the user terminal <NUM> has not received a CTX message <NUM> or other signal indicating that the user terminal <NUM> may start a UL-MU-MIMO transmission.

In one operation mode, the user terminals <NUM> may not transmit outside an UL-MU-MIMO transmission opportunity (TXOP) (e.g., after CTX message <NUM>). In another operation mode user terminals <NUM> may transmit frames to initialize a UL-MU-MIMO transmission, and then may transmit during the UL-MU-MIMO TXOP, if for example, they are instructed to do so in a CTX message <NUM>. In one embodiment, the frame to initialize a UL-MU-MIMO transmission may be a request to transmit (RTX), a frame specifically designed for this purpose (an example of a RTX frame structure is described more fully below with reference to <FIG> and <FIG>). The RTX frames may be the only frames a user terminal <NUM> is allowed to use to initiate a UL MU MIMO TXOP. In one embodiment, the user terminal may not transmit outside an UL-MU-MIMO TXOP other than by sending an RTX. In another embodiment, a frame to initialize an UL MU MIMO transmission may be any frame which indicates to an AP <NUM> that a user terminal <NUM> has data to send. It may be pre-negotiated that these frames indicate a UL MU MIMO TXOP request. For example, the following may be used to indicate that a user terminal <NUM> has data to send and is requesting an UL MU MIMO TXOP: an RTS, a data frame or QoS Null frame with bits <NUM>-<NUM> of the QoS control frame set to indicate more data, or a PS poll. In one embodiment, the user terminal may not transmit outside an UL MU MIMO TXOP other than by sending frames to trigger this TXOP, where this frame may be an RTS, PS poll, or QOS null. In another embodiment, the user terminal may send single user uplink data as usual, and may indicate a request for a UL MU MIMO TXOP by setting bits in the QoS control frame of its data packet. <FIG> is a time sequence diagram illustrating, in conjunction with <FIG>, an example where the frame to initialize a UL-MU-MIMO is a RTX <NUM>. In this embodiment the user terminal <NUM> sends to the AP <NUM> a RTX <NUM> that includes information regarding the UL-MU-MIMO transmission. As shown in <FIG>, the AP <NUM> may respond to the RTX <NUM> with a CTX message <NUM> granting an UL-MU-MIMO TXOP to send the UL-MU-MIMO transmission <NUM> immediately following the CTX message <NUM>. In another aspect, the AP <NUM> may respond with a CTS that grants a single-user (SU) UL TXOP. In another aspect, the AP <NUM> may respond with a frame (e.g., ACK or CTX with a special indication) that acknowledges the reception of the RTX <NUM> but does not grant an immediate UL-MU-MIMO TXOP. In another aspect, the AP <NUM> may respond with a frame that acknowledges the reception of the RTX <NUM>, does not grant an immediate UL-MU-MIMO TXOP, but grants a delayed UL-MU-MIMO TXOP and may identify the time of the TXOP is granted. In this embodiment, the AP <NUM> may send a CTX message <NUM> to start the UL-MU-MIMO at the granted time.

In another aspect, the AP <NUM> may respond to the RTX <NUM> with an ACK or other response signal which does not grant the user terminal <NUM> an UL-MU-MIMO transmission but indicates that the user terminal <NUM> shall wait for a time (T) before attempting another transmission (e.g., sending another RTX). In this aspect the time (T) may be indicated by the AP <NUM> in the setup phase or in the response signal. In another aspect an AP <NUM> and a user terminal <NUM> may agree on a time which the user terminal <NUM> may transmit a RTX <NUM>, RTS, PS-poll, or any other request for a UL-MU-MIMO TXOP.

In another operation mode, user terminals <NUM> may transmit requests for UL-MU-MIMO transmissions <NUM> in accordance with regular contention protocol. In another aspect, the contention parameters for user terminals <NUM> using UL-MU-MIMO are set to a different value than for other user terminals that are not using the UL-MU-MIMO feature. In this embodiment, the AP <NUM> may indicate the value of the contention parameters in a beacon, association response or through a management frame. In another aspect, the AP <NUM> may provide a delay timer that prevents a user terminal <NUM> from transmitting for a certain amount of time after each successful UL-MU-MIMO TXOP or after each RTX, RTS, PS-poll, or QoS null frame. The timer may be restarted after each successful UL-MU-MIMO TXOP. In one aspect, the AP <NUM> may indicate the delay timer to user terminals <NUM> in the setup phase or the delay timer may be different for each user terminal <NUM>. In another aspect, the AP <NUM> may indicate the delay timer in the CTX message <NUM> or the delay timer may be dependent on the order of the user terminals <NUM> in the CTX message <NUM>, and may be different for each terminal.

In another operational mode, the AP <NUM> may indicate a time interval during which the user terminals <NUM> are allowed to transmit a UL-MU-MIMO transmission. In one aspect, the AP <NUM> indicates a time interval to the user terminals <NUM> during which the user terminals are allowed to send a RTX or RTS or other request to the AP <NUM> to ask for an UL-MU-MIMO transmission. In this aspect, the user terminals <NUM> may use regular contention protocol. In another aspect, the user terminals may not initiate a UL-MU-MIMO transmission during the time interval but the AP <NUM> may send a CTX or other message to the user terminals to initiate the UL-MU-MIMO transmission.

In certain embodiments, a user terminal <NUM> enabled for UL-MU-MIMO may indicate to an AP <NUM> that it requests an UL-MU-MIMO TXOP because it has data pending for UL. In one aspect, the user terminal <NUM> may send a RTS or a PS-poll to request a UL-MU-MIMO TXOP. In another embodiment, the user terminal <NUM> may send any data frame, including a quality of service (QoS) null data frame, where the bits <NUM>-<NUM> of the QoS control field indicate a non-empty queue. In this embodiment the user terminal <NUM> may determine during the setup phase which data frames (e.g., RTS, PS-poll, QoS null, etc.) will trigger a UL-MU-MIMO transmission when the bits <NUM>-<NUM> of the QoS control field indicate a non-empty queue. In one embodiment, the RTS, PS-poll, or QoS null frames may include a <NUM> bit indication allowing or disallowing the AP <NUM> to respond with a CTX message <NUM>. In another embodiment, the QoS null frame may include TX power information and a per TID queue information. The TX power information and per TID queue information may be inserted in the two bytes of the sequence control and QoS controls fields in a QoS null frame and the modified QoS null frame may be sent to the AP <NUM> to request a UL-MU-MIMO TXOP. In another embodiment, referring to <FIG> and <FIG>, the user terminal <NUM> may send a RTX <NUM> to request a UL-MU-MIMO TXOP.

In response to receiving an RTS, RTX, PS-poll or QoS null frame, or other trigger frame as described above, an AP <NUM> may send a CTX message <NUM>. In one embodiment, referring to <FIG>, after the transmission of the CTX message <NUM> and the completion of the UL-MU-MIMO transmissions 410A and 410B, TXOP returns to the STAs 120A and 120B which can decide on how to use the remaining TXOP. In another embodiment, referring to <FIG>, after the transmission of the CTX message <NUM> and the completion of the UL-MU-MIMO transmissions 410A and 410B, TXOP remains with the AP <NUM> and the AP110 may use the remaining TXOP for additional UL-MU-MIMO transmissions by sending another CTX message <NUM> to either STAs 120A and 120B or to other STAs.

<FIG> is a message timing diagram of one embodiment of multi-user uplink communication. Message exchange <NUM> shows communication of wireless messages between an AP <NUM> and three stations 120a-c. Message exchange <NUM> indicates that each of STAs 120a-c transmits a request-to-transmit (RTX) message 802a-c to the AP <NUM>. Each of RTX messages 802a-c indicate that the transmitting station 120a-c has data available to be transmitted to the AP <NUM>.

After receiving each of RTX messages 802a-c, the AP <NUM> may respond with a message indicating that the AP <NUM> has received the RTX. As shown in <FIG>, the AP <NUM> transmits ACK messages 803a-c in response to each RTX messages 802a-c. In some embodiments, the AP <NUM> may transmit a message (e.g., a CTX message) indicating that each of the RTX messages 802a-c has been received but that the AP <NUM> has not granted a transmission opportunity for the stations 120a-c to uplink data. In <FIG>, after sending ACK message 803c, the AP <NUM> transmits a CTX message <NUM>. In some aspects, the CTX message <NUM> is transmitted to at least the stations STA 120a-c. In some aspects, the CTX message <NUM> is broadcast. In some aspects, the CTX message <NUM> indicates which stations are granted permission to transmit data to the AP <NUM> during a transmission opportunity. The starting time of the transmission opportunity and its duration may be indicated in the CTX message <NUM> in some aspects. For example, the CTX message <NUM> may indicate that the stations STA 120a-c should set their network allocation vectors to be consistent with NAV <NUM>.

At a time indicated by the CTX message <NUM>, the three stations 120a-c transmit data 806a-c to the AP <NUM>. The data 806a-c are transmitted at least partially concurrently during the transmission opportunity. The transmissions of data 806a-c may utilize uplink multi-user multiple input, multiple output transmissions (UL-MU-MIMO) or uplink frequency division multiple access (UL-FDMA).

In some aspects, stations STAa-c may transmit pad data such the transmissions of each station transmitting during a transmission opportunity are of approximately equal duration. Message exchange <NUM> shows STA 120a transmitting pad data 808a while STA 120c transmits pad data 808c. The transmission of pad data ensure that the transmissions from each of the STAs 120a-c complete at approximately the same time. This may provide for a more equalized transmission power over the entire duration of the transmission, optimizing AP <NUM> receiver efficiencies.

After the AP <NUM> receives the data transmissions 806a-c, the AP <NUM> transmits acknowledgments 810a-c to each of the stations 120a-c. In some aspects, the acknowledgments 810a-c may be transmitted at least partially concurrently using either DL-MU-MIMO or DL-FDMA.

<FIG> is a diagram of one embodiment of a RTX frame <NUM>. The RTX frame <NUM> includes a frame control (FC) field <NUM>, a duration field <NUM> (optional), a transmitter address (TA)/allocation identifier (AID) field <NUM>, a receiver address (RA)/basic service set identifier (BSSID) field <NUM>, a TID field <NUM>, an estimated transmission (TX) time field <NUM>, and a TX power field <NUM>. The FC field <NUM> indicates a control subtype or an extension subtype. The duration field <NUM> indicates to any receiver of the RTX frame <NUM> to set the network allocation vector (NAV). In one aspect, the RTX frame <NUM> may not have a duration field <NUM>. The TA/AID field <NUM> indicates the source address which can be an AID or a full MAC address. The RA/BSSID field <NUM> indicates the RA or BSSID of the STAs to concurrently transmit uplink data. In one aspect the RTX frame may not contain a RA/BSSID field <NUM>. The TID field <NUM> indicates the access category (AC) for which the user has data. The Estimated TX time field <NUM> indicates the time requested for the UL-TXOP and may be the time required for a user terminal <NUM> to send all the data in its buffer at the current planned MCS. The TX power field <NUM> indicates the power at which the frame is being transmitted and can be used by the AP to estimate the link quality and adapt the power backoff indication in a CTX frame.

In some embodiments, before an UL-MU-MIMO communication can take place, an AP <NUM> may collect information from the user terminals <NUM> that may participate in the UL-MU-MIMO communication. An AP <NUM> may optimize the collection of information from the user terminals <NUM> by scheduling the transmissions from the user terminals <NUM>.

As discussed above, the CTX message <NUM> may be used in a variety of communications. <FIG> is a diagram of an example of a CTX frame <NUM> structure. In this embodiment, the CTX frame <NUM> is a control frame that includes a frame control (FC) field <NUM>, a duration field <NUM>, a transmitter address (TA) field <NUM>, a control (CTRL) field <NUM>, a PPDU duration field <NUM> (as claimed), a STA information (info) field <NUM>, and a frame check sequence (FCS) field <NUM>. The FC field <NUM> indicates a control subtype or an extension subtype. The duration field <NUM> indicates to any receiver of the CTX frame <NUM> to set the network allocation vector (NAV). The TA field <NUM> indicates the transmitter address or a BSSID. The CTRL field <NUM> is a generic field that may include information regarding the format of the remaining portion of the frame (e.g., the number of STA info fields and the presence or absence of any subfields within a STA info field), indications for rate adaptation for the user terminals <NUM>, indication of allowed TID, and indication that a CTS must be sent immediately following the CTX frame <NUM>. The indications for rate adaptation may include data rate information, such as a number indicating how much the STA should lower their MCSs, compared to the MCS the STA would have used in a single user transmission. The CTRL field <NUM> may also indicate if the CTX frame <NUM> is being used for UL MU MIMO or for UL FDMA or both, indicating whether a Nss or Tone allocation field is present in the STA Info field <NUM>.

Alternatively, the indication of whether the CTX is for UL MU MIMO or for UL FDMA can be based on the value of the subtype. Note that UL MU MIMO and UL FDMA operations can be jointly performed by specifying to a STA both the spatial streams to be used and the channel to be used, in which case both fields are present in the CTX; in this case, the Nss indication is referred to a specific tone allocation. The PPDU duration <NUM> field indicates the duration of the following UL-MU-MIMO PPDU that the user terminals <NUM> are allowed to send. The STA Info <NUM> field contains information regarding a particular STA and may include a per-STA (per user terminal <NUM>) set of information (see STA Info <NUM><NUM> and STA Info N <NUM>). The STA Info <NUM> field may include an AID or MAC address field <NUM> which identifies a STA, a number of spatial streams field (Nss) <NUM> field which indicates the number of spatial streams a STA may use (in an UL-MU-MIMO system), a Time Adjustment <NUM> field which indicates a time that a STA should adjust its transmission compared to the reception of a trigger frame (the CTX in this case), a Power Adjustment <NUM> field which indicates a power backoff a STA should take from a declared transmit power, a Tone Allocation <NUM> field which indicates the tones or frequencies a STA may use (in a UL-FDMA system), an Allowed TID <NUM> field which indicates the allowable TID, an Allowed TX Mode <NUM> field which indicates the allowed TX modes, a MCS <NUM> field which indicates the MCS the STA should use, and a TX start time field <NUM> which indicates a start time for the STA to transmit uplink data. In some embodiments, the allowed TX modes may include a short/long guard interval (GI) or cyclic prefix mode, a binary convolutional code (BCC)/low density parity check (LDPC) mode (generally, a coding mode), or a space-time block coding (STBC) mode.

In some embodiments, the STA info fields <NUM>-<NUM> may be excluded from the CTX frame <NUM>. In these embodiments, the CTX frame <NUM> with the missing STA info fields may indicate to the user terminals <NUM> receiving the CTX frame <NUM> that a request message to uplink data (e.g., RTS, RTX or QoS Null) has been received but a transmission opportunity has not been granted. In some embodiments, the control field <NUM> may include information regarding the requested uplink. For example, the control field <NUM> may include a waiting time before sending data or another request, a reason code for why the request was not granted, or other parameters for controlling medium access from the user terminal <NUM>. A CTX frame with missing STA info fields may also apply to CTX frames <NUM>, <NUM> and <NUM> described below.

In some embodiments, a user terminal <NUM> receiving a CTX with a Allowed TID <NUM> indication may be allowed to transmit data only of that TID, data of the same or higher TID, data of the same or lower TID, any data, or only data of that TID first, then if no data is available, data of other TIDs. The FCS <NUM> field indicates the carries an FCS value used for error detection of the CTX frame <NUM>.

<FIG> is a diagram of another example of a CTX frame <NUM> structure. In this embodiment and in conjunction with <FIG>, the STA Info <NUM> field does not contain the AID or MAC Address <NUM> field and instead the CTX frame <NUM> includes a group identifier (GID) <NUM> field which identifies the STAs to concurrently transmit uplink data by a group identifier rather than an individual identifier. <FIG> is a diagram of another example of a CTX frame <NUM> structure. In this embodiment and in conjunction with <FIG>, the GID <NUM> field is replaced with a RA <NUM> field which identifies a group of STAs through a multicast MAC address.

<FIG> is a diagram of an example of a CTX frame <NUM> structure. In this embodiment, the CTX frame <NUM> is a management frame that includes a Management MAC Header <NUM> field, a Body <NUM> field, and a FCS <NUM> field. The Body <NUM> field includes an IE ID <NUM> field which identifies an information element (IE), a LEN <NUM> field which indicates the length of the CTX frame <NUM>, a CTRL <NUM> field which includes the same information as the CTRL <NUM> field, a PPDU Duration <NUM> field which indicates the duration of the following UL-MU-MIMO PPDU that the user terminals <NUM> are allowed to send, a STA Info <NUM><NUM> field and a MCS <NUM> field which can indicate the MCS for all the STAs to use in the following UL-MU-MIMO transmission, or an MCS backoff for all the STAs to use in the following UL-MU-MIMO transmission. The STA Info <NUM><NUM> (along with STA Info N <NUM>) field represent a per STA field that includes AID <NUM> field which identifies a STA, a number of spatial streams field (Nss) <NUM> field which indicates the number of spatial streams a STA may use (in an UL-MU-MIMO system), a Time Adjustment <NUM> field which indicates a time that a STA should adjust its transmission time compared to the reception of a trigger frame (the CTX in this case), a Power Adjustment <NUM> field which indicates a power backoff a STA should take from a declared transmit power, a Tone Allocation <NUM> field which indicates the tones or frequencies a STA may use (in a UL-FDMA system), an Allowed TID <NUM> field which indicates the allowable TID, and a TX start time field <NUM> which indicates a start time for the STA to transmit uplink data.

In one embodiment, the CTX frame <NUM> or the CTX frame <NUM> may be aggregated in an A-MPDU to provide time to a user terminal <NUM> for processing before transmitting the UL signals. In this embodiment, padding or data may be added after the CTX to allow a user terminal <NUM> additional time to process the forthcoming packet. One benefit to padding a CTX frame may be to avoid possible contention issues for the UL signals from other user terminals <NUM>, as compared to increasing the interframe space (IFS) as described above. In one aspect, if the CTX is a management frame, additional padding information elements (IEs) may be sent. In one aspect, if the CTX is aggregated in a A-MPDU, additional A-MPDU padding delimiters may be included. Padding delimiters may EoF delimiters (4Bytes) or other padding delimiters. In another aspect, the padding may be achieved by adding data, control or Management MPDPUs, as long as they do not require to be processed within the IFS response time. The MPDUs may include an indication indicating to the receiver that no immediate response is required and will not be required by any of the following MPDUs. In another aspect, the user terminals <NUM> may request to an AP <NUM> a minimum duration or padding for the CTX frame. In another embodiment, the padding may be achieved by adding PHY OFDMA symbols, which may include undefined bits not carrying information, or may include bit sequences that carry information, as long as they do not need to be processed within the IFS time.

In some embodiments, an AP <NUM> may initiate a CTX transmission. In one embodiment, an AP <NUM> may send a CTX message <NUM> in accordance with regular enhanced distribution channel access (EDCA) contention protocol. In another embodiment, an AP <NUM> may send a CTX message <NUM> at scheduled times. In this embodiment, the scheduled times may be indicated by the AP <NUM> to the user terminals <NUM> by using a restricted access window (RAW) indication in a beacon which indicates a time reserved for a group of user terminals <NUM> to access the medium, a target wake time (TWT) agreement with each user terminal <NUM> which indicates to multiple user terminals <NUM> to be awake at the same time to take part in a UL-MU-MIMO transmission, or information in other fields. Outside the RAW and TWT a user terminal <NUM> may be allowed to transmit any frame, or only a subset of frames (e.g., non-data frames). It may also be forbidden to transmit certain frames (e.g., it may be forbidden to transmit data frames). The user terminal <NUM> may also indicate that it is in sleep state. One advantage to scheduling a CTX is that multiple user terminals <NUM> may be indicated a same TWT or RAW time and may receive a transmission from an AP <NUM>.

<FIG> is a flow chart of an exemplary method <NUM> for wireless communication in accordance with certain embodiments described herein. A person having ordinary skill in the art will appreciate that the method <NUM> may be implemented by any suitable device and system.

In operation block <NUM>, the method <NUM> includes transmitting a clear to transmit (CTX) message to two or more stations, the CTX message indicating an uplink transmission opportunity, the CTX message further comprising a request that the two or more stations concurrently transmit uplink data at a specific time. In operational block <NUM>, the method <NUM> further includes receiving a plurality of uplink data from at least two stations at the specific time.

In some embodiments, an apparatus for wireless communication may perform some of the functions of method <NUM>. The apparatus comprises means for transmitting a clear to transmit (CTX) message to two or more stations, the CTX message indicating an uplink transmission opportunity, the CTX message further comprising a request that the two or more stations concurrently transmit uplink data at a specific time. The apparatus may further comprise means for receiving a plurality of uplink data from at least two stations at the specific time.

A person/one having ordinary skill in the art would understand that information and signals can be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that can be referenced throughout the above description can be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Various modifications to the implementations described in this disclosure can be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other implementations without departing from the scope of this disclosure. Thus, the disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the claims, the principles and the novel features disclosed herein. The word "exemplary" is used exclusively herein to mean "serving as an example, instance, or illustration.

Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a sub-combination or variation of a sub-combination.

The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). Generally, any operations illustrated in the Figures may be performed by corresponding functional means capable of performing the operations.

The various illustrative logical blocks, modules and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array signal (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller or state machine.

In one or more aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Thus, in some aspects computer readable medium may comprise non-transitory computer readable medium (e.g., tangible media). In addition, in some aspects computer readable medium may comprise transitory computer readable medium (e.g., a signal).

Claim 1:
An apparatus (120a) for wireless communication comprising:
means for receiving (<NUM>) a clear to transmit, CTX, message (<NUM>) from an access point (<NUM>), the CTX message being transmitted to two or more stations (120a, 120b) and indicating an uplink transmission opportunity, the CTX message comprising a physical layer convergence protocol, PLCP, protocol data unit, PPDU, duration field (<NUM>) and a request for the two or more stations to concurrently transmit uplink data at a specific time, wherein the PPDU duration field indicates a duration for the transmission of the uplink data; and
means for transmitting (<NUM>) uplink data at the specific time to the access point,
characterized in that
the means for transmitting is further configured to transmit a clear to send, CTS, message having a scrambling sequence at a time after receiving the CTX message and before the transmitting of the uplink data, the scrambling sequence being the same as a scrambling sequence of a clear to send, CTS, message transmitted by the another of the two or more stations.