Patent Publication Number: US-8971247-B2

Title: Methods, devices, and systems for efficient retransmission communications

Description:
BACKGROUND 
     1. Field 
     One feature generally relates to wireless communications, and more particularly, to methods and devices that retransmit information due to reception of erroneous information. 
     2. Background 
     The increased availability of Wireless Local Area Networks (WLANs) has allowed stations, such as desktop computers, laptop computers, hand held personal digital assistants (PDAs), and mobile phones, to wirelessly connect with one another through a variety of networks, such as Local Area Networks (LANs) and the Internet, to transfer data between them. For example, a user can take a laptop computer from a desk into a conference room to attend a meeting and still have access to a local network to retrieve data and have access to the Internet via one or more modems or gateways present on the local network without being tethered by a wired connection. 
     A WLAN may be comprised of four primary components. These components may include stations (STAs), one or more access points (APs), a wireless medium and a distribution system. The network is built to transfer data between stations that may include computing devices with wireless network interfaces. For example, laptop computers, desktop computers, mobile phones, and other electronic devices having wireless network interfaces are examples of stations within a network. Access points are devices that allow stations to connect to one another and transfer data. Examples of access points include routers, centralized controllers, base stations, and site controllers. 
     Data is transmitted over a WLAN via packets. Packets contain control information and payload information (e.g., the data); the form of packets vary depending on the communication protocol. A station may desire to transfer multimedia content, such as video, over the WLAN to another station via one or more access points. 
     In wireless (radio) communications, Multiple-Input and Multiple-Output (MIMO) offers significant increases in data throughput with little to no additional bandwidth or transmit power. This is achieved this by higher spectral efficiency (i.e., more bits per second per hertz of bandwidth) and link reliability or diversity (e.g., reduced fading). Multi-User (MU)-MIMO permits a network device (access node) to communicate with multiple client stations at each transmission window, by sending packets to multiple receivers over different channels during a particular transmission window. 
     The Institute of Electrical Engineers (IEEE) 802.11 standard denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters). In conventional IEEE 802.11 communications, a Media Access Control (MAC) Protocol Data Unit (MPDU) may be used to exchange messages/data between entities at the MAC layer of a protocol stack via a packet-switched network. One or more MPDUs may be encapsulated within a PHY Protocol Data Unit (PPDU) that is transmitted at the PHY layer of a protocol stack. For instance, the maximum number of MPDUs per PPDU may be eight (8). During transmission, some MPDU packets may be lost or corrupted. Consequently, a transmission scheme is implemented to resend lost MPDU packets. However, in MIMO communications, the retransmission scheme can lead to inefficiencies beyond just retransmitting an erroneous packet. 
     Therefore, there is a need for improved efficiency in MIMO communications that include retransmission protocols. 
     SUMMARY 
     Embodiments of the present disclosure include devices, methods, and computer readable medium for improved efficiency in MIMO communications that include retransmission protocols. 
     A first method operational at a transmitter device is provided for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. A plurality of concurrent data streams are transmitted within a first transmission window, each concurrent data stream including one or more MPDUs and associated with a different recipient, the first transmission window having a first length. The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). An indication of a retransmission subset of the MPDUs to be retransmitted for each concurrent data stream may then be obtained or received. Consequently, the retransmission subset is transmitted for each concurrent data stream within a second transmission window, wherein a length of the second transmission window is equal to a longest of the retransmission subsets of the concurrent data streams. In one example, the length of the second transmission window may be equal to or less than the first length of the first transmission window. In another example, each of the concurrent data streams in the first transmission window carries a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of a second length, where the second length is less than the first length. 
     In one example, an acknowledgment may be received for each MPDU in the first transmission window that is successfully received. The indication of the subset of the MPDUs to be retransmitted for each concurrent data stream may be obtained from a lack of acknowledgments received for the subset of MPDUs. 
     According to one feature, an error correcting code may be generated for the MPDUs in at least one data stream of the concurrent data streams of the first transmission window. The error correcting codes may be appended to the MPDUs in the at least one data stream of the concurrent data streams. 
     In one implementation, no new MPDUs are added to the concurrent data streams in the second transmission window thereby making the length of the second retransmission window less than the first length. 
     In another implementation, one or more new MPDUs may be added to the concurrent data streams in the second transmission window so that each concurrent data stream in the second transmission window have the same duration. The first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in a first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window carries a second PPDU in the first data stream, and any new MPDUs added to the second PPDU has a packet index exceeding the maximum packet index. 
     In accordance with one aspect, at least a first data stream in the concurrent data streams may have MPDUs of a first duration and a second data stream in the concurrent data streams has MPDUs of a second duration. In accordance with another aspect, at least two data streams in the concurrent data streams have different data rates. 
     Similarly, a transmitter device may be provided for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. The transmitter device may include a communication interface coupled to a processing circuit. The communication interface may serve to communicate with a plurality of receiver devices. The processing circuit may be configured assemble for transmission a plurality of concurrent data streams within a first transmission window, each concurrent data stream including one or more MPDUs and associated with a different receiver device, the first transmission window having a first length. The processing circuit may receive, from the plurality of receiver devices, an indication of a retransmission subset of the MPDUs to be retransmitted for each concurrent data stream. Consequently, the processing circuit may assemble for transmission the retransmission subset for each concurrent data stream within a second transmission window, wherein a length of the second transmission window is equal to a longest of the retransmission subsets of the concurrent data streams. The processing circuit may be further configured to encapsulate each concurrent data stream within a Physical (PHY) protocol data unit, each of the concurrent data streams in the first transmission window carries a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of a second length, where the second length is less than the first length. 
     A second method operational in a transmitter is provided for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. A plurality of concurrent data streams are transmitted within a first transmission window, each concurrent data stream including one or more MPDUs and is associated with a different recipient, the first transmission window having a first length. As a result, indication of a retransmission subset of the MPDUs to be retransmitted may be received or obtained for at least one of the concurrent data streams. The plurality of concurrent data streams are then transmitted within a second transmission window, the at least one of the concurrent data streams in the second transmission window including the retransmission subset of the MPDUs and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. An indication may then be obtained or received that at least one packet of at least one of the retransmission subsets includes an error from transmitting the second plurality of concurrent data streams within the second transmission window indicating a need to retransmit at least one MPDU again. Consequently, the plurality of concurrent data streams may be retransmitted within a third transmission window, at least one concurrent data stream in the third transmission window including the at least one MPDU and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the third transmission window is equal to the first length. At least a first data stream in the plurality of concurrent data streams may have MPDUs of a first duration and a second data stream in the plurality of concurrent data streams has MPDUs of a second duration, and the first length is a first time duration. The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). At least two data streams in the concurrent data streams may have different data rates. 
     In accordance with one implementation, one or more new MPDUs may be added to the concurrent data streams in the second transmission window so that each concurrent data stream in the second transmission window have the same total duration. The first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in a first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window carries a second PPDU in the first data stream, and any new MPDUs added to the second PPDU has a packet index exceeding the maximum packet index. 
     Similarly, a transmitter device may be provided for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. The transmitter device may include a communication circuit coupled to a processing circuit. The communication circuit adapted for communicating with a plurality of receiver devices. The processing circuit may be configured to assemble for transmission a plurality of concurrent data streams within a first transmission window, each concurrent data stream including one or more MPDUs and is associated with a different recipient, the first transmission window having a first length. As a result, the processing circuit may receive, from the plurality of receiver devices, an indication of a retransmission subset of the MPDUs to be retransmitted for at least one of the concurrent data streams. Consequently, the processing circuit may assemble for transmission the plurality of concurrent data streams within a second transmission window, the at least one of the concurrent data streams in the second transmission window including the retransmission subset of the MPDUs and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. The processing circuit may be further configured to obtain an indication that at least one packet of at least one of the retransmission subsets includes an error from transmitting the second plurality of concurrent data streams within the second transmission window indicating a need to retransmit at least one MPDU again. The processing circuit may consequently retransmit the plurality of concurrent data streams within a third transmission window, at least one concurrent data stream in the third transmission window including the at least one MPDU and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the third transmission window is equal to the first length. 
     A first method operational in a receiver device is provided to facilitate retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. A first data stream may be received within a first transmission window including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. 
     The receiver device may then determine which MPDUs in the first data stream are successfully received and which MPDUs are received with errors. An acknowledgment may then be sent (by the receiver device) indicating each MPDU in the first data stream that is received with errors. As a result, the receiver device may receive the first data stream within a second transmission window including a second number of retransmitted MPDUs, the second transmission window having a second length equal to or less than the first length and including one or more retransmitted MPDUs for at least one of the concurrent data streams, the second length is equal to a longest of the retransmitted MPDUs within the concurrent data streams. The receiver device may determine that the second number of MPDUs includes one or more retransmitted MPDUs that were previously determined to have been received with errors. Likewise, the receiver device may also determine that the second number of MPDUs includes one or more new MPDUs. The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). In one example, each of the concurrent data streams in the first transmission window may carry a first PPDU of the first length and each of the concurrent data streams in the second transmission window may carry a second PPDU of the second length, where the second length is less than the first length. In another example, the first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in a first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window may carry a second PPDU in the first data stream, and any new MPDUs added to the second PPDU has an index exceeding the maximum packet index. In some instances, the second length of the second transmission window may be equal to or less than the first length of the first transmission window. In some implementations, at least one data stream in the concurrent data streams has MPDUs of a first duration and another data stream in the concurrent data streams has MPDUs of a second duration. In some instances, at least two data streams in the concurrent data streams may have different data rates. 
     Similarly, a receiver device may be provided that facilitates retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. The receiver device may include a communication circuit coupled to a processing circuit. The communication circuit may be configured to communicate with a transmitter device. The processing circuit may be configured to receive a first data stream within a first transmission window including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. The processing circuit may determine which MPDUs in the first data stream are successfully received and which MPDUs are received with errors. An acknowledgment may then be transmitted by the receiver device indicating each MPDU in the first data stream that is received with errors. Consequently, the receiver device may receive the first data stream within a second transmission window including a second number of retransmitted MPDUs, the second transmission window having a second length equal to or less than the first length and including one or more retransmitted MPDUs for at least one of the concurrent data streams, the second length is equal to a longest of the retransmitted MPDUs within the concurrent data streams. 
     The processing circuit may be further adapted to: (a) determine that the second plurality of MPDUs includes one or more retransmitted MPDUs that were previously determined to have been received with errors; and/or (b) determine that the second number of MPDUs includes one or more new MPDUs. 
     A second method operational in a receiver device is provided to facilitate retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. A first data stream may be received within a first transmission window including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. The receiver device may determine which MPDUs in the first data stream are successfully received and which MPDUs are received with errors. As a result, an acknowledgment may be sent indicating each MPDU in the first data stream that is received with errors. Consequently, the first data stream may be received within a second transmission window, for each concurrent transmission stream the second transmission window including one or more retransmitted MPDUs and/or one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. The received device may then (a) determine that the first data stream within the second transmission window includes one or more retransmitted MPDUs that were previously determined to have been received with errors, and/or (b) determine that the first data stream within the second transmission window includes one or more new MPDUs. In one implementation, the first number of MPDUs may be buffered in a receive buffer that is longer than the first length. The first number of MPDUs may be removed (from the buffer) according to a sequence order. 
     Buffer space in the receive buffer may be released for the retransmitted MPDUs responsive to determining that the first data stream in the second transmission window includes one or more retransmitted MPDUs and the one or more retransmitted MPDUs are successfully received. As a result, the one or more new MPDUs may be buffered in the released buffer space. MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). Each of the concurrent data streams in the first transmission window may carry a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of the first length. 
     The first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in the first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window may carry a second PPDU in the first data stream, and any new MPDUs added to the second PPDU has an index exceeding the maximum packet index. 
     At least the first data stream in the concurrent data streams may have MPDUs of a first duration and a second data stream in the concurrent data streams may have MPDUs of a second duration. 
     Similarly, a receiver device may be provided to facilitate retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. The receiver device may include a communication interface coupled to a processing circuit. The communication interface may be adapted to communicate with a transmitter device. The processing circuit may be configured receive a first data stream within a first transmission window including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. The processing circuit may then determine which MPDUs in the first data stream are successfully received and which MPDUs are received with errors. As a result, the processing circuit may send an acknowledgment indicating each MPDU in the first data stream that is received with errors. Consequently, the processing circuit may receive the first data stream within a second transmission window, for each concurrent transmission stream the second transmission window including one or more retransmitted MPDUs and/or one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. A receive buffer may be adapted to buffer the first number of MPDUs, the receive buffer being longer than the first length. The processing circuit may be further adapted to remove the first number of MPDUs from the receive buffer according to a sequence order. 
     In one example, the processing circuit is further configured to release buffer space in the receive buffer for the retransmitted MPDUs responsive to determining that the first data stream in the second transmission window includes one or more retransmitted MPDUs and the one or more retransmitted MPDUs are successfully received. The receive buffer may be further adapted to buffer the one or more new MPDUs in the released buffer space. 
    
    
     
       DRAWINGS 
         FIG. 1  is a block diagram of one example of a Wireless Local Area Network (WLAN) environment where a source station transmits data to a destination station through one or more networks or a peer-to-peer link. 
         FIG. 2  illustrates an Open System Interconnection (OSI) model that may be implemented as part of a protocol stack used by a device (e.g., transmitter and/or receiver) for communications. 
         FIG. 3  illustrates a Media Access Control (MAC) Protocol Data Unit (MPDU) including a MAC header and a MAC trailer, with an Internet Protocol (IP) packet serving as the payload for the MPDU according to one embodiment. 
         FIG. 4  is a block diagram of an exemplary transmitter device according to an embodiment of the disclosure. 
         FIG. 5  is a block diagram of an exemplary receiver device according to an embodiment of the disclosure. 
         FIG. 6  illustrates retransmission in accordance with one embodiment. 
         FIG. 7  illustrates a second embodiment of retransmission using a reduced size retransmission window in which only retransmitted MPDUs are sent in a subsequent PPDU. 
         FIG. 8  illustrates another embodiment of retransmission using a reduced size retransmission window in which some new MPDUs are sent with retransmitted MPDUs. 
         FIG. 9  illustrates yet another embodiment of retransmission using full retransmission windows. 
         FIG. 10  illustrates one embodiment of error correction coding for an Alternate MPDU (A-MPDU). 
         FIG. 11  is a flow diagram illustrating exemplary steps that may be carried out as part of a transmit/retransmit process. 
         FIG. 12  is a flow diagram illustrating another exemplary steps that may be carried out as part of a transmit/retransmit process. 
         FIG. 13  illustrates another example of how multiple data streams may be accommodated and retransmitted within a transmission window. 
         FIG. 14  illustrates a retransmission alternative for data streams of different rates in which a reduced size retransmission window is used. 
         FIG. 15  illustrates a retransmission alternative for data streams of different rates in which a reduced size retransmission window is used and new packets or units may be added to data streams to pack the retransmission window. 
         FIG. 16  is a flow diagram illustrating exemplary receive process according to one or more embodiment of the disclosure. 
         FIG. 17  is a flow diagram illustrating other exemplary steps that may be carried out as part of a receive process according to one or more embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings in which is shown, by way of illustration, specific embodiments in which the disclosure may be practiced. The embodiments are intended to describe aspects of the disclosure in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and changes may be made to the disclosed embodiments without departing from the scope of the disclosure. The following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Overview 
     Embodiments of the present disclosure include devices, methods, and computer readable medium for improved efficiency in MIMO communications that include retransmission protocols. 
     Techniques are presented herein that allow for multiple retransmission policies for different Media Access Control Protocol Data Units (MPDUs) that include a stream of data (e.g., general data as well as video, and audio streams) to be transmitted from a transmitter to a receiver. For example, one feature provides for a method operational to improve transmission efficiency by reducing a window size of a retransmission window or modifying how retransmission packets are processed to allow for more new packets to be transmitted along with retransmission packets. 
     Exemplary Network Environment 
       FIG. 1  illustrates an exemplary WLAN environment  100  where a source STA  102  transmits data to a destination STA  108 , which may occur through one or more networks  104  or a direct peer-to-peer link (not shown). In some embodiments, the source STA  102  and/or the destination STA  108  may wirelessly connect via the network  104  (e.g., via one or more access points). In other embodiments, many access points or no access points and/or one or more intermediate networks may exist between the source STA  102  and the destination STA  108 . 
     In general, each of the source STA  102  and/or the destination STA  108  may include a transmitter and a receiver to provide two-way communications. As a result, when referring to a transmitter device and a receiver device, a person of ordinary skill in the art would understand that both might be embodied in each of the source STA  102  and/or the destination STA  108 . 
     Most wireless communication networks, including WLANs, may be broken down into different sections in order to help conceptualize the inner workings and structure of the network. For example, the Open Systems Interconnection model (OSI model) is a way of sub-dividing a communications system into successively smaller parts often referred to as layers. A “layer” may be defined as a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it. Embodiments presented herein for providing retransmission policies for different MPDUs that comprise a stream of data transmitted from a source STA  102  to a destination STA  108  may be implemented and conceptualized within such a scheme. 
       FIG. 2  illustrates an Open System Interconnection (OSI) model that may be implemented as part of a protocol stack used by a device (e.g., transmitter and/or receiver) for communications. In general, content/data from the higher layers of a protocol stack (e.g., host layers) may be encapsulated within the lower layers (e.g., media layers). In one example, the Media Access Control (MAC) Protocol Data Units (MPDUs) may be generated at the Data Link Layer and may encapsulate Application Layer, Presentation Layer, Session Layer, Transport Layer, and/or Network Layer data. As a non-limiting example, embodiments of transmitter devices and receiver devices discussed herein may implement a protocol stack based on this OSI model  200 . 
     A Protocol Data Unit (PDU) describes or includes data and its overhead at a particular layer of a protocol stack. Each layer of a protocol stack may have a unique PDU. At the transmitter device, as data is sent down the protocol stack, it is encapsulated at each layer by adding a header and possibly a trailer. At the receiver device, data is decapsulated as it goes back up the protocol stack. For example, as illustrated in  FIG. 2 , a transport layer PDU may be referred to as a segment, a network layer PDU may be referred to as packet or datagram, a data link layer PDU may be referred to as a frame, and a physical layer PDU may be referred to as bits. 
       FIG. 3  illustrates a Media Access Control (MAC) Protocol Data Unit (MPDU)  300  including a MAC header  312  and a MAC trailer  318 , with an Internet Protocol (IP) packet serving as the payload for the MPDU  300  according to one embodiment. An MPDU  300  may be referred to as being within an MPDU frame  310 , which includes an MPDU payload  320  encapsulated by the MAC header  312  and the MAC trailer  318 . The MPDU payload  320  may include an IP payload  325  encapsulated with an IP header  322 . 
     A Physical (PHY) Protocol Data Unit (PPDU) may include a plurality of MPDUs (e.g., each like MPDU  300 ) that may be sequentially identified by an index (e.g., n, n+1, n+2, . . . n+i). That is, in order to correctly reconstruct content in the payload portion of a plurality of MPDUs, the sequence in which MPDUs are removed from a receiver buffer (queue) is important. In some receiver systems, MPDUs must be removed (e.g., sent up stream for further processing) in sequential order (e.g., according to their indices and/or only when the packets in a PPDU are complete). If a first MPDU is received with errors (e.g., or errors that do not permit recovery of the first MPDU with error correcting codes), then the subsequent MPDUs (e.g., MPDUs of higher indices within the same PPDU) cannot be removed from a receive buffer until such first MPDU is successfully retransmitted. Thus, retransmission schemes are needed that permit efficiently retransmitting MPDUs while maximizing MPDU throughput and/or minimizing dead air transmissions. 
     Exemplary Transmitter Device 
       FIG. 4  is a block diagram of an exemplary transmitter device  400  according to one embodiment of the disclosure. The transmitter device  400  may include a processing circuit (e.g., a processor, processing module, etc.)  410 , a memory  420 , and a communication interface  430 . The processing circuit  410  may be communicatively coupled to the memory  420  and communication interface  430  to transmit and/or receive data to/from a receiver device over a wireless communication link  438 . In one example, the communication interface  430  may comprise a transmitter circuit and/or receiver circuit (e.g., a transceiver or modem device) adapted or configured for wireless communications with a receiver device. For example, the communication interface  430  may comprise a transmitter circuit, including one or more components of a transmitter chain, to transmit data from the transmit queue  435 . 
     In one example, the processing circuit  410  may include, among other modules, an assembler  415  that may control assembly of MPDUs  300  into multiple data streams for transmission in a MIMO environment, as is explained more fully below. 
     The communication interface  430  may implement a transmission queue  435  for collecting data prior to transmission. According to one implementation, the transmitter device  400  may be configured to transmit the multiple data streams to multiple receiver devices over the communication interface  430  in a MIMO environment. 
     In one example, the processing circuit  410  may include an Error Correction Coding (ECC) encoder  418  for encoding error correction codes and appending error correction to packets that are transmitted as discussed more fully below. 
     Exemplary Receiver Device 
       FIG. 5  is a block diagram of an exemplary receiver device  500  according to one embodiment of the disclosure. 
     The receiver device  500  may include a processing circuit (e.g., a processor, processing module, etc.)  510 , a memory  520 , and a communication interface  530 . The processing circuit  510  may be communicatively coupled to the memory  520  and communication interface  530  to transmit and/or receive data to/from a transmitter device  400  ( FIG. 4 ) over a wireless communication link  538 . In one example, the communication interface  530  may include a transmitter circuit and/or receiver circuit (e.g., a transceiver or modem device) adapted or configured for wireless communications with a transmitter device. For example, the communication interface  530  may include a receiver circuit, including one or more components of a receiver chain, to receive data and store it in the receive queue  535 . 
     The memory  520  may include a receive buffer  525  for collecting MPDUs  300  as they are received and holding them until all MPDUs  300  for the PPDU are successfully received prior to passing them to a higher level in the communication architecture. 
     In one example, the processing circuit  510  may include, among other modules, a buffer control module  515  that may control the receive buffer  525  and the collection of the MPDUs  300 . 
     The communication interface  530  may implement a receive queue  535  for collecting data as it is received prior to moving the data to the memory  520  or the processing circuit  510 . According to one implementation, the receiver devices  500  may be configured to recognize transmissions including multiple data streams in a MIMO environment and determine which data stream of the multiple data streams is intended for that specific receiver device  500 . 
     In one example, the processing circuit  510  may include an ECC decoder  518  for decoding error correction codes and applying error correction to packets that are received as discussed more fully below. 
     Exemplary Retransmission Environment 
     In some communication protocols, MPDUs may be packed within a window (may also be referred to as a “slot”) to form a PHY Protocol Data Unit (PPDU). As a non-limiting example for the purposes of discussion herein, the PPDU may include eight (8) MPDUs. An MPDU may be generically referred to as a data unit and/or transmission unit. 
     In many communication protocols a transmitter device  400  ( FIG. 4 ) communicates with a single receiver device  500  ( FIG. 5 ). These types of communications are often referred to as unicast communications. In other communication protocols, a transmitter device  400  may communicate with multiple receiver devices  500 , but transmit the same information to all of the receiver devices. These types of communications are often referred to as multicast communications. MIMO environments allow communications that are even more robust by enabling a transmitter device  400  to send different information to each of multiple receiver devices  500 . These types of communications may be referred to herein as simulcast communications. 
     As a non-limiting example to transmit multiple data channels, the MIMO environment may use spatial multiplexing wherein the signal is split into multiple data streams that may be transmitted at a different phase, different frequency, or combination thereof within the same carrier frequency. A receiver device may be configured to recognize these different spatial signatures and extract the data stream that is intended for it. 
       FIG. 6  illustrates retransmission in accordance with one embodiment. In this example, eight (8) MPDUs may be transmitted within a PPDU to three different stations (STA- 1 , STA- 2 , and STA- 3 ) or destinations within a transmission window using a MIMO approach. In one embodiment, a first transmission window  610  includes transmissions (e.g., data streams) for the three different stations (STA- 1 , STA- 2 , and STA- 3 ) as illustrated by packets  1 - 8  for each data stream. A single packet  602  (e.g., MPDU) may also be referred as a data unit or transmission unit. Each packet  602  or unit may have a time duration t u    604 . It should be noted that the numbers 1-8 are used to designate a packet order within the first transmission window  610 , not to designate information within the packet. In addition, each packet in each data stream may include different information. In other words, packet  1   606  in data stream A (for station STA- 1 ) is transmitted concurrently with a packet  1   608  in data stream B (for station STA- 2 ) and a packet  1   610  in data stream C (for station STA- 3 ), but each of the packets  1   606 ,  608 , and  610  may include different information and may be destined for different receiver devices. 
     At the receiver devices  500 , the receive buffers  525  of each receiver device  500  may collect its corresponding set of the MPDU packets. In some communication protocols, a restriction on processing MPDU packets (e.g., within the same PPDU) may be that the packets be removed in sequence (e.g., according to MPDU indices) from the receive buffer  525 . Thus, if a transmission error occurs for packet n, then subsequent packets n+1, n+2, etc., within the same transmission window (or PPDU) cannot be removed from the receive buffer  525  until packet n is successfully retransmitted. More generally, the receive buffer at a receiver device may not flush MPDUs n+1, n+2, . . . , (e.g., in the same PPDU) until MPDU n has been received without errors. The efficiency of a transmission system may be negatively impacted if the receive buffer length is short (e.g., buffer length is equal to transmission window length) since there may not be sufficient space to receive additional MPDUs without first releasing the pending MPDUs. 
     In  FIG. 6  the packets are illustrated with different shading to indicate status of the packets. A current packet  640  that is successfully received is represented with no shading. A current packet that includes a packet error  650  is represented with a dark stipple shading. A retransmitted packet  660  is represented with a medium stipple shading. A new packet  670  within a second transmission window  620  is represented with a light stipple shading. A dead air packet  680  (i.e., a possible location for a packet with no data transmitted therein) is represented with a diamond shading. 
     Thus, as shown in the first transmission window  610 , MPDU packet  2  destined for STA- 1  includes a packet error  650 . Similarly, MPDU packet  4  destined for STA- 2  includes a packet error  650 . Finally, MPDU packets  7  and  8  destined for STA- 3  include packet errors  650 . These indications of packet status are also used in  FIGS. 7-9 . 
     In some embodiments, a packet error  650  may be indicated by no acknowledgement (ACK) being sent from the receiver devices  500  to the transmitter device  400  within a defined time frame. In other embodiments, a packet error  650  may be indicated by a negative acknowledgement (NACK) indicating that a packet includes an error being sent from the receiver devices  500  to the transmitter device  400  within a defined time frame. 
     In some protocols, as a result of the receive errors in the first transmission window  610 , a second PPDU in a second transmission window  620  with the same size (i.e., same number of packets) is sent. As shown in the second transmission window  620 , erroneous packet  2  for STA- 1  is resent as a retransmitted packet  660 , erroneous packet  4  for STA- 2  is resent as a retransmitted packet  660 , and erroneous packets  7  and  8  for STA- 3  are resent as retransmitted packets  660 . The protocol may attempt to send new packets  670  with the retransmitted packets  660  in the second transmission window. However, due to the windowing environment, there may be limits on how many new packets may be sent. That is, transmission windows are often sized to fit within the buffer length of receivers. If such receive buffer is already partially occupied by packets (i.e., MPDUs) that were not flushed/released due to a lower-indexed packet having errors, then the number of new packets that may be added to the second transmission window is limited. Adding more packets to the second transmission window than there is space available to buffer them at the receive buffer is pointless. 
     In the case of STA- 1 , only one new packet (packet  9 ) may be sent with the retransmitted packet  2  since the receive buffer  525  still has packets  3 - 8 . That is, the receive buffer  525  was only able to remove packet  1  since packet  2  had errors. So, at most, the receiving buffer for STA- 1  has room for packet  2  and one new packet (packet  9 ). 
     Similarly, for STA- 2 , packet  4  had errors in the first PPDU transmission and MPDU packet  4  is therefore retransmitted in a subsequent second PPDU along with three new MPDU packets (packets  9 - 11 ). Three new packets  670  are possible because the original MPDU packets  1 ,  2 , and  3  were successfully received and can be removed from the receiving buffer  525  of STA- 2 , freeing space for the new MPDU packets  9 ,  10 , and  11 . 
     Likewise, for STA- 3 , packets  7  and  8  had errors in the first transmission window  610  and are retransmitted in the second transmission window  620 . Because original MPDU packets  1 - 6  were successfully/correctly received, new packets  9 - 14  can be appended to the retransmitted packets  7  and  8 . 
     The groups of packets to be retransmitted may be referred to herein as a retransmission subset. Thus for station STA- 1  the retransmission subset includes packet  2 , for station STA- 2  the retransmission subset includes packet  4 , and for station STA- 3  the retransmission subset includes packets  7  and  8 . 
     It should be noted that the second transmission window  620  illustrated in  FIG. 6  reflects the restriction imposed by certain protocols that received packets should be removed in sequence from the receive buffer. Thus, this retransmission scheme retransmits just enough packets per data stream as will fit in the receive buffer. 
       FIG. 13  illustrates another example of how multiple data streams may be accommodated and retransmitted within a transmission window. In this example, a first transmission window  1302  may include a plurality of data streams, where two or more of the data streams may have different transmission rates (e.g., packets or units in different streams may have different durations t u1 , t u2 , and t u3 ). For example, a first data stream A may include packets  1304  of a first duration t u1  a second data stream B may include packets  1306  of a second duration t u2 , and a third data stream C may include packets  1308  of a third duration t u3 . A second transmission window  1310  may serve to retransmit packets which had transmission errors in the first transmission window  1302 . For example, packet  1  of the first data stream A is retransmitted in the second transmission window  1310 , packet  4  of the second data stream B is retransmitted in the second transmission window  1310 , and packets  7  and  8  of the third data stream C are retransmitted in the second transmission window  1310 . This figure illustrates that data streams of different rates and/or packet duration may be transmitted within each transmission and/or retransmission window. 
     With the transmission protocol illustrated in  FIGS. 6 and 13 , depending on which MPDU packet in the sequence is retransmitted, a significant amount of padding (i.e., dead air) may be added to the PPDU carrying the retransmitted MPDU packets. As can be seen from  FIGS. 6 and 13 , a significant number of dead air packets  680  in the second transmission window  620 , which may be considered wasted air time because the receive buffer  525  cannot accept additional MPDUs in the second PPDU. This leads to lower network throughput efficiency. Therefore, a more efficient retransmission scheme is desirable that can make more efficient use the available space in a PPDU carrying retransmitted MPDUs. 
     Exemplary Retransmission Scheme  1   
       FIG. 7  illustrates a retransmission scheme with a reduced size retransmission window  720  in which only retransmitted MPDUs are sent in a subsequent PPDU. When one or more MPDUs in a first PPDU in the first transmission window  710  have packet errors  650 , only those one or more MPDUs are retransmitted in a second PPDU in the retransmission window  720  (also referred to herein as a second transmission window  720 ) as retransmitted packets  660 . New MPDUs are only sent after all previously transmitted MPDUs are successfully received (e.g., acknowledged). Because the second transmission window  720  is truncated to accommodate only the largest of the retransmission subsets (in this example the subset of packets  7  and  8  for station STA- 3 ), overall bandwidth for retransmission is reduced and there are significantly fewer dead air packets  680 . For example, in  FIG. 7 , only two dead air packets  680  are included as opposed to  FIG. 6 , where ten dead air packets  680  are included. 
     However, this approach of  FIG. 7  may still result in an inefficient retransmission if one of the packet streams has a large number of retransmitted MPDU packets because the retransmission window  720  size may approach the full size of a normal transmission window (e.g. the first transmission window  710 ). 
       FIG. 14  illustrates a retransmission scheme for data streams of different rates in which a reduced size retransmission window  1404  is used. A plurality of data streams A, B, and/or C of two or more transmission rates are send in the same first transmission window  1402 . However, packet  2  in a first data stream A, packet  4  in a second data stream B, and packets  7  and  8  in a third data stream C have errors during transmission. Here, only the packets or units that had errors in a first transmission window  1402  are retransmitted in a second transmission window  1404 , thereby allowing truncation of the second transmission window  1404  to the longest of the retransmitted packets in the plurality of data streams. 
     Exemplary Retransmission Scheme  2   
       FIG. 8  illustrates another embodiment of retransmission using a reduced size retransmission window  820  in which some new MPDUs are sent with retransmitted MPDUs. As with the embodiment of  FIG. 7 , when one or more MPDUs in a first PPDU in the first transmission window  810  have packet errors  650 , only those one or more MPDUs are retransmitted in a second PPDU in the retransmission window  820  as retransmitted packets  660 . Then, extra MPDUs are added as new packets  670  to each stream until no more new packets  670  can be added to any one stream. As with the embodiment of  FIG. 7 , the second transmission window  820  is still truncated to accommodate only the largest of the retransmission subsets (in this example the subset of packets  7  and  8  for station STA- 3 ). However, data streams with smaller retransmission subsets may include new packets  670  to pad that data stream such that there are no dead air packets  680 . Thus, the data stream for station STA- 1  can include new packet  9  and the data stream for station STA- 2  can include new packet  9 . In this scheme, the dead air space in the second transmission window  720  ( FIG. 7 ) is filled with new packets while limiting the second transmission window length  820  to the longest of the retransmission packets. The packet indices for the new packets  670  (e.g., new packet  9 ) added to the second PPDU in the retransmission window  820  may exceed the indices for the packets in the first PPDU in the first transmission window  810 . 
       FIG. 15  illustrates a retransmission alternative for data streams of different rates in which a reduced size retransmission window  1504  is used and new packets or units are added to data streams to pack to the retransmission window. However, packet  2  in a first data stream A, packet  4  in a second data stream B, and packets  7  and  8  in a third data stream C have errors during transmission. The packets or units that had errors in a first transmission window  1502  are retransmitted in a second transmission window  1504 . The length or duration of the second transmission window  1504  may be dictated by the data stream with the most (or longest duration) packet retransmissions. Unused space/slots in the retransmission window  1504  are filled with new packets or units. 
     Exemplary Retransmission Scheme  3   
       FIG. 9  illustrates yet another retransmission scheme using full retransmission windows. As with  FIGS. 6-8 , a first transmission window  910  (e.g., a first PPDU) includes packet errors  650  at packet  2  for station STA- 1  (data stream A), at packet  4  for station STA- 2  (data stream B), and at packets  7  and  8  for station STA- 3  (data stream C). The packets with errors are retransmitted as retransmitted packets  660  in a second transmission window  920  (e.g., a second PPDU). However, the transmitter device  400  includes new packets  670  for each data stream to fill out a full second transmission window  920  in the assumption that the receiver devices  500  can handle the new packets  670 . The new packets  670  may have packet indices that exceed the packet indices in the first PPDU. Thus, for stations STA- 1  and STA- 2  new packets  9 - 15  (in data streams A and B) and for station STA- 3  new packets  9 - 14  (in data stream C) are added to the second PPDU in the second transmission window  920 . 
     If the receiver devices  500  are configured to release space in the receive buffer  525  as soon as a previous PPDU is complete due to the retransmitted packet  660 , or include additional space for receiving the new packets  670 , then the receive buffer  525  should be able to handle the new packet  670  (which packet indices exceed the indices for the previous PPDU). For example, for STA- 1  (data stream A), as soon as retransmitted packet  2  is received, a full PPDU is complete including packets  1  and  3 - 8  from the first transmission window  910  and retransmitted packet  2  from the second transmission window  920 . Thus, as soon as retransmitted packet  2  is successfully received the receive buffer  525  may be released and the PPDU may be sent to the next highest level in the protocol stack. With the receive buffer  525  free, new packets  9 - 15  may be successfully placed in the receive buffer  525  as they are received. 
     However, if a retransmitted packet  660  is received again in error, the receiver buffer may not have sufficient space to receive some of the new packets (e.g., packets whose packet indices exceed the packet indices in the previous PPDU). For example, as illustrated by the received second transmission  930 , packet for station STA- 2  (data stream B), again has errors in the received second PPDU so the receive buffer  525  for station STA- 2  cannot release the remaining packets  5 - 8  (for the first PPDU). As illustrated by window  940 , the receive buffer of first length (e.g., 8 packets long) for station STA- 2  can only receive new packets  9 - 11  since packets  1 - 3  in the first transmission window (e.g., first PPDU) have been previously flushed. That is, packets  12 - 15  are dropped because the receive buffer  525  for station STA- 2  cannot release the space that includes packets  5 - 7  (in the first transmission window  910  for the first PPDU) until packet  4  is successfully received. The receive buffer keeps the space for the packet  4  available until such packet is received without error in a subsequent retransmission. 
     To obtain the maximum efficiency from the approach in  FIG. 9 , the receive buffer  525  may be increased to a second length so that it is larger than a single transmission window of a first length. In this case, all packets received in the second transmission window  930  are buffered as illustrated in window  950 . For example, the receive buffer  525  may be twice as long as a single transmission window. For instance, if the transmission window is eight (8) packets long, then the receive buffer may be 12, 16, 20 or 24 packets long. Consequently, rather than dropping packets  12 - 15 , these packets can be queued in the receive buffer  525 . 
     In  FIG. 9 , it illustrates that an extended receiver buffer is used by a receiver to receive data stream B. The receiver buffer  960  may be sixteen (16) packets long rather than eight (8) packets long. Receiver buffer  960  illustrates the reception and queuing of packets in the first transmission window  910 . Meanwhile, receiver buffer  960 ′ illustrates the receipt and queuing of packets in the second transmission window  920 . The extended receiver buffer  960 / 960 ′ permits queuing new packets received in a retransmission window thereby increasing throughput. 
     Also, as illustrated in  FIG. 13-15 , the data streams in  FIG. 9  may carry packets of different duration or size. 
     Exemplary Error Correcting Codes at MAC Level 
     In addition to implementing a modified retransmission approach, the MPDUs may include error correction coding in an attempt to recover packets with errors therein. As a non-limiting example, Raptor codes (i.e., rapid tornado codes) are a class of fountain codes that may be used for error corrections. Raptor codes may encode a given message having a number of symbols k into a potentially limitless sequence of encoding symbols such that knowledge of any k or more encoding symbols allows the message to be recovered with some non-zero probability. The probability that the message can be recovered increases with the number of symbols received above k becoming very close to 1, once the number of received symbols is only very slightly larger than k. A symbol can be any size, from a single byte to hundreds or thousands of bytes. 
       FIG. 10  illustrates one embodiment of error correction coding for an Alternate MPDU (A-MPDU)  1000 . A legacy section  1010  includes a transmission as discussed above with a PHY header  1012  and MPDUs  1  through K  1014 . A parity section  1020  includes Parity-MPDUs  1022  one through M. Thus there are a total of N=K+M packets in the transmission. If any K packets are received successfully (whether they are legacy packets or parity packets), the original legacy packets can be recovered using the error correction. As a non-limiting example, if a transmission includes 50 legacy packets and 5 parity packets, if the 50 legacy packets are all received correctly, no error correction is needed. However, if 5 or less of all the 55 packets (legacy packets and parity packets) include errors, the original 50 legacy packet can be recovered by applying error correction. 
     Exemplary Transmit Process 
       FIG. 11  is a flow diagram illustrating exemplary steps that may be carried out as part of a transmit/retransmit process  1100 . This method may be operational at a transmitter device for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. Reference also may be made to  FIGS. 4-10  and  13 - 14  when describing the transmit process  1100  of  FIG. 11 . 
     In operation block  1102 , a plurality of concurrent data streams may be transmitted within a first transmission window, each concurrent data stream including one or more MPDUs and associated with a different recipient, the first transmission window having a first length. For example, transmission windows  710 ,  810 , and  910  may include three data streams sent from the transmitter device  400  to multiple receiver devices  500  as stations STA- 1 , STA- 2 , and STA- 3 . The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). 
     Optionally, operation block  1104  indicates that an error correcting code may be generated for the MPDUs in at least one data stream of the concurrent data streams of the first transmission window and appended to the to the MPDUs in the at least one data stream of the concurrent data streams. Appending the error correcting code may reduce the number of packet errors in the first transmission window and reduce or eliminate the need for retransmission in the second transmission window. Such error correcting codes may permit reconstruction of MPDUs that were otherwise received with errors. 
     In operation block  1108 , an indication of a retransmission subset of the MPDUs to be retransmitted for each concurrent data stream may be obtained by the transmitter device  400 . Each of the receiver devices  500  may indicate to the transmitter device  400 , which, if any, of the packets within the first transmission window had errors. For example, in operation block  1106 , an acknowledgment may be received for each MPDU in the first transmission window that is successfully received. The retransmission subset of the MPDUs to be retransmitted may identify or indicate zero MPDUs (in the case that all packets of a data stream are successfully received) or one or more MPDUs (in the case where one or more MPDU are received with errors or with unrecoverable errors) for each data stream. 
     Alternatively, each of the receiver devices  500  may indicate to the transmitter device  400 , which of the packets within the first transmission window had were received successfully. Therefore, the absence of an acknowledgement (ACK) for a particular MPDU indicates packet errors. 
     Optionally, operation block  1110  indicates that one or more new MPDUs may be added to the concurrent data streams in the second transmission window so that each concurrent data stream in the second transmission window has the same total duration. Such new MPDUs may be added to each data stream according to the transmission schemes illustrated in  FIGS. 6 ,  7 , and/or  8 , for example. In one example, the first transmission window carries a first Physical (PHY) protocol data unit (PPDU) in a first data stream, the first PPDU having MPDUs with a maximum packet index, a second transmission window carries a second PPDU in the first data stream, and any new MPDUs added to the second PPDU has an index exceeding the maximum packet index. 
     In operation block  1112 , the retransmission subset for each concurrent data stream is transmitted within the second transmission window, wherein a length (e.g., time duration) of the second transmission window is equal to a longest of the retransmission subsets of the concurrent data streams. Note that the “length” of the retransmission subsets may refer to a time/duration of a particular retransmission subset and/or transmission window. As non-limiting examples,  FIGS. 7 ,  8 ,  14 , and  15  illustrate that the length of second transmission window is limited by the length of the longest retransmission subset. For instance, in  FIG. 7 , where the transmission units/packets for all data streams are of the same length, the length of the second transmission window  720  (e.g., retransmission window) is data stream C (i.e., the longest retransmission subset) where two units/packets are being retransmitted. In another example, in  FIG. 14 , where the transmission units/packets for two or more data streams are of different length, the length of the second transmission window  1404  (e.g., retransmission window) is data stream A (i.e., the longest retransmission subset). Even though a single packet is being retransmitted in data stream A, its duration is longer than the packets for other data streams, therefore the length of the second transmission window  1404  is as long as the retransmission subset for data stream A. In one instance, each of the concurrent data streams in the first transmission window carries a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of a second length, where the second length is less than the first length. 
       FIG. 12  is a flow diagram illustrating another exemplary steps that may be carried out as part of a transmit/retransmit process  1200 . This method may be operational at a transmitter device for retransmitting Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. Reference also may be made to  FIGS. 4-10  and  13 - 14  when describing the transmit process  1100  of  FIG. 11 . The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). 
     In operation block  1202 , a plurality of concurrent data streams may be transmitted within a first transmission window, each concurrent data stream including one or more MPDUs and associated with a different recipient, the first transmission window having a first length. For example, transmission windows  910  may include three data streams sent from the transmitter device  400  to multiple receiver devices  500  as stations STA- 1 , STA- 2 , and STA- 3 . 
     Optionally, operation block  1204  indicates that an error correcting code may be generated for the MPDUs in at least one data stream of the concurrent data streams of the first transmission window and appended to the to the MPDUs in the at least one data stream of the concurrent data streams. Appending the error correcting code may reduce the number of packet errors in the first transmission window and reduce or eliminate the need for retransmission in the second transmission window. Such error correcting codes may permit to reconstruction of MPDUs that were otherwise received with errors. 
     In operation block  1206 , an indication of a retransmission subset of the MPDUs to be retransmitted for each concurrent data stream may be obtained by the transmitter device  400 . The retransmission subset of the MPDUs to be retransmitted may identify or indicate zero MPDUs (in the case that all packets of a data stream are successfully received) or one or more MPDUs (in the case where one or more MPDU are received with errors or with unrecoverable errors) for each data stream. Each of the receiver devices  500  may indicate to the transmitter device  400 , which, if any, of the packets within the first transmission window had errors. For example, in operation block  1106 , an acknowledgment may be received for each MPDU in the first transmission window that is successfully received. 
     Alternatively, each of the receiver devices  500  may indicate to the transmitter device  400 , which of the packets within the first transmission window had were received successfully. Therefore, the absence of an acknowledgement (ACK) for a particular MPDU indicates that packet errors. 
     In operation block  1208 , the plurality of concurrent data streams are transmitted within a second transmission window, the at least one of the concurrent data streams in the second transmission window including the retransmission subset of the MPDUs and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. Note that the “length” of the retransmission subsets may refer to a time/duration of a particular retransmission subset and/or transmission window. As non-limiting examples,  FIG. 9  illustrate that the length of second transmission window  920  is the same as the first transmission window  910 . Each of the concurrent data streams in the first transmission window may carry a first PPDU of the first length and each of the concurrent data streams in the second transmission window may carry a second PPDU of the first length. In one example, the first PPDU may have MPDUs with a maximum packet index, and the second PPDU may carry one or more new MPDUs having an index exceeding the maximum packet index. 
     In operation block  1210 , one or more new MPDUs are added to the concurrent data streams in the second transmission window so that each concurrent data stream in the second transmission window have the same total duration. Such new MPDUs may be added to each data stream according to the transmission scheme illustrated in  FIG. 9 , for example. However, in one example, at least a first data stream in the concurrent data streams has MPDUs of a first duration and a second data stream in the concurrent data streams has MPDUs of a different second duration. 
     In operation block  1212 , an indication is obtained that at least one packet of at least one of the retransmission subsets includes an error from transmitting the second plurality of concurrent data streams within the second transmission window indicating a need to retransmit at least one MPDU again. 
     In operation block  1214 , the plurality of concurrent data streams are transmitted within a third transmission window, at least one concurrent data stream in the third transmission window including the at least one MPDU and one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the third transmission window is equal to the first length. 
     Exemplary Receive Process 
       FIG. 16  is a flow diagram illustrating exemplary acts that may be carried out as part of a receive process  1600  according to one or more embodiment of the disclosure. This method may be operational in a receiver to facilitate retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. Reference also may be made to  FIGS. 4-10  and  13 - 14  when describing the transmit process  1600  of  FIG. 16 . 
     Operation block  1602  indicates that a first data stream within a first transmission window is received including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. These concurrent data streams may be transmitted in a MIMO environment and each of the receiver devices  500  would receive the data stream intended for it. The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). 
     Operation block  1604  indicates that a determination is made as to which MPDUs in the first data stream are successfully received and which MPDUs are received with errors (e.g., unrecoverable errors). 
     Operation block  1606  indicates that an acknowledgment may be sent (to the transmitter device  400 ) indicating each MPDU in the first data stream that is received with errors. 
     Operation block  1608  indicates that the first data stream may be received within a second transmission window including a second number of retransmitted MPDUs, the second transmission window having a second length equal to or less than the first length and including one or more retransmitted MPDUs for at least one of the concurrent data streams, the second length is equal to a longest of the retransmitted MPDUs within the concurrent data streams. This operation correlates with exemplary retransmission schemes  1  and  2  discussed above with reference to  FIGS. 7 ,  8 ,  13  and  14 , for example. 
     Operation block  1610  indicates that a determination may be made as to whether the second number of MPDUs includes one or more retransmitted MPDUs that were previously determined to have been received with errors. 
     Operation block  1612  indicates that a determination may be made that the second number of MPDUs includes one or more new MPDUs. As a result, this operation may correlate with exemplary retransmission schemes  2  discussed above with reference to  FIG. 8 . The first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in a first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window may carry a second PPDU in the first data stream, and any new MPDUs added to the second PPDU may have an index exceeding the maximum packet index. 
     In one example, each of the concurrent data streams in the first transmission window carries a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of the second length, where the second length is less than the first length. 
     In one example, the first number of MPDUs may be buffered in a receive buffer  525  and removal of the first number of MPDUs occurs according to a sequence order. The second length of the second transmission window may be equal to or less than a first length of the first transmission window. At least one data stream in the concurrent data streams may have MPDUs of a first duration and another data stream in the concurrent data streams may have MPDUs of a second duration. For instance, at least two data streams in the concurrent data streams have different data rates. 
     According to another feature, the buffer space in the receive buffer  525  may be released for the retransmitted MPDUs responsive to determining that the second number of MPDUs includes one or more retransmitted MPDUs and the one or more retransmitted MPDUs are successfully received. With the release, one or more of the new MPDUs may be placed in the receive buffer  525  in the released buffer space. 
       FIG. 17  is a flow diagram illustrating other exemplary acts that may be carried out as part of a receive process  1700  according to one or more embodiment of the disclosure. This method may be operational in a receiver to facilitate retransmission of Media Access Control (MAC) protocol data units (MPDUs) in a multi-user multiple-input and multiple-output (MU-MIMO) communication system. Reference also may be made to  FIGS. 4-10  and  13 - 14  when describing the transmit process  1700  of  FIG. 17 . In particular, this method may be implemented as part of the transmission scheme of  FIG. 9 . 
     Operation block  1702  indicates that a first data stream may be received within a first transmission window including a first number of MPDUs, the first transmission window having a first length and a plurality of concurrent data streams associated with different recipients. The MPDUs for each concurrent data stream may be encapsulated within a Physical (PHY) protocol data unit (PPDU). 
     Operation block  1704  indicates that a determination is made as to which MPDUs in the first data stream are successfully received and which MPDUs are received with errors. 
     Operation block  1706  indicates that an acknowledgment may be sent indicating each MPDU in the first data stream that is received with errors. 
     Operation block  1708  indicates that the first data stream may be received within a second transmission window, for each concurrent transmission stream the second transmission window including one or more retransmitted MPDUs and/or one or more new MPDUs so that a total length of each of the plurality of concurrent data streams in the second transmission window is equal to the first length. In one example, each of the concurrent data streams in the first transmission window carries a first PPDU of the first length and each of the concurrent data streams in the second transmission window carries a second PPDU of the first length. In another example, the first transmission window may carry a first Physical (PHY) protocol data unit (PPDU) in the first data stream, the first PPDU having MPDUs with a maximum packet index, the second transmission window may carry a second PPDU in the first data stream, and any new MPDUs added to the second PPDU may have an index exceeding the maximum packet index. 
     Operation block  1710  indicates that a determination may be made that the first data stream within the second transmission window includes one or more retransmitted MPDUs that were previously determined to have been received with errors. 
     Operation block  1712  indicates that a determination may be made that the first data stream within the second transmission window includes one or more new MPDUs. 
     According to one aspect, the first number of MPDUs may be buffered in a receive buffer that is longer than the first length and/or the first number of MPDUs may be removed (from the receive buffer) according to a sequence order. Buffer space in the receive buffer may be released for the retransmitted MPDUs responsive to determining that the first data stream in the second transmission window includes one or more retransmitted MPDUs and the one or more retransmitted MPDUs are successfully received. Consequently, the one or more new MPDUs may be buffered in the released buffer space. 
     According to another aspect, the receive buffer may be longer than the first length of the transmission windows. For example, the receive buffer may be twice or three times as long as the transmission windows, thereby allowing new MPDUs to be buffered even if previous MPDUs are received with errors (e.g., unrecoverable errors). 
     Specific implementations shown and described are only examples and should not be construed as the only way to implement the present disclosure unless specified otherwise herein. It is readily apparent to one of ordinary skill in the art that the various examples in the present disclosure may be practiced by numerous other partitioning solutions. 
     One or more of the components, acts, features and/or functions described herein and illustrated in the drawings may be rearranged and/or combined into a single component, act, feature, or function or embodied in several components, acts, features, or functions. Additional elements, components, acts, and/or functions may also be added without departing from the invention. The algorithms described herein may also be efficiently implemented in software and/or embedded in hardware. 
     In the description, elements, circuits, and functions may be shown in block diagram form in order not to obscure the present disclosure in unnecessary detail. Conversely, specific implementations shown and described are exemplary only and should not be construed as the only way to implement the present disclosure unless specified otherwise herein. Additionally, block definitions and partitioning of logic between various blocks is exemplary of a specific implementation. It is readily apparent to one of ordinary skill in the art that the present disclosure may be practiced by numerous other partitioning solutions. For the most part, details concerning timing considerations and the like have been omitted where such details are not necessary to obtain a complete understanding of the present disclosure and are within the abilities of persons of ordinary skill in the relevant art. 
     Also, it is noted that the embodiments may be described as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
     Those of ordinary skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout this description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. Some drawings may illustrate signals as a single signal for clarity of presentation and description. It will be understood by a person of ordinary skill in the art that the signal may represent a bus of signals, wherein the bus may have a variety of bit widths and the present disclosure may be implemented on any number of data signals, including a single data signal. 
     Elements described herein may include multiple instances of the same element. These elements may be generically indicated by a numerical designator (e.g.  110 ) and specifically indicated by the numerical indicator followed by an alphabetic designator (e.g.,  110 A) or a numeric indicator preceded by a “dash” (e.g.,  110 - 1 ). For ease of following the description, for the most part element number indicators begin with the number of the drawing on which the elements are introduced or most fully discussed. 
     It should be understood that any reference to an element herein using a designation such as “first,” “second,” and so forth does not limit the quantity or order of those elements, unless such limitation is explicitly stated. Rather, these designations may be used herein as a convenient method of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements may be employed there or that the first element must precede the second element in some manner. In addition, unless stated otherwise, a set of elements may comprise one or more elements. 
     Moreover, a storage medium may represent one or more devices for storing data, including read-only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine-readable mediums and, processor-readable mediums, and/or computer-readable mediums for storing information. The terms “machine-readable medium,” “computer-readable medium,” and/or “processor-readable medium” may include, but are not limited to non-transitory mediums such as portable or fixed storage devices, optical storage devices, and various other mediums capable of storing, containing or carrying instruction(s) and/or data. Thus, the various methods described herein may be fully or partially implemented by instructions and/or data that may be stored in a “machine-readable medium,” “computer-readable medium,” and/or “processor-readable medium” and executed by one or more processors, machines and/or devices. 
     Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine-readable medium such as a storage medium or other storage(s). A processor may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     The various illustrative logical blocks, modules, circuits, elements, and/or components described in connection with the examples disclosed herein 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 (FPGA) or other programmable logic component, 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 conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a number of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. A general-purpose processor, configured for executing embodiments described herein, is considered a special purpose processor for carrying out such embodiments. Similarly, a general-purpose computer is considered a special purpose computer when configured for carrying out embodiments described herein. 
     The methods or algorithms described in connection with the examples disclosed herein may be embodied directly in hardware, in a software module executable by a processor, or in a combination of both, in the form of processing unit, programming instructions, or other directions, and may be contained in a single device or distributed across multiple devices. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. A storage medium may be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. 
     Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware, software, or a combination thereof depends upon the particular application and design selections imposed on the overall system. 
     The various features of the invention described herein can be implemented in different systems without departing from the invention. It should be noted that the foregoing embodiments are merely examples and are not to be construed as limiting the invention. The description of the embodiments is intended to be illustrative, and not to limit the scope of the claims. As such, the present teachings can be readily applied to other types of apparatuses and many alternatives, modifications, and variations will be apparent to those skilled in the art.