Patent Publication Number: US-2017373732-A1

Title: Feedback scheme for mu-mimo

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, claims the benefit of and priority to, U.S. patent application Ser. No. 14/794,184, filed on Jul. 8, 2015, which is a continuation of U.S. patent application Ser. No. 14/018,266, filed on Sep. 4, 2013, and issued as U.S. Pat. No. 9,112,669, which is a continuation of U.S. patent application Ser. No. 12/845,515, filed on Jul. 28, 2010, and issued as U.S. Pat. No. 8,547,955. The subject matter of the above are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Multiple-user multiple-input multiple-output (MU-MIMO) systems can transmit and receive signals to/from multiple users at a single antenna array at the same time. In a MU-MIMO system, multiple signals are sent in parallel and are kept separate from one another by transmitting (or receiving) each signal in a different (e.g., orthogonal) direction or in a different spatial channel. The process of transmitting (or receiving) a signal in a specific direction using an antenna array is known as beamforming. By selecting a beam for each signal that will produce limited interference with other parallel beams, multiple signals can be transmitted or received at the same time. Beams are often calculated to be orthogonal to one another in order to minimize the interference between the beams. 
     There are two different types of beamforming systems: implicit and explicit. In implicit beamforming the access point calculates the beamforming matrix based on reciprocal channels. That is, the access point receives uplink signals and based on the reception quality and direction of the uplink signals, the access point determines the direction and other parameters for the downlink beams. In explicit beamforming the access point sends out a sounding signal to the mobile stations. The mobile stations then send feedback to the access point regarding the quality of the received signal. The access point can then use the feedback to generate beams for future downlink signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a wireless communication system. 
         FIG. 2  illustrates an example of a mobile station for communicating in the wireless communication system of  FIG. 1 . 
         FIG. 3  illustrates an example of an access point for communicating in the wireless communication system of  FIG. 1 . 
         FIG. 4  illustrates an example chart for managing feedback for explicit beamforming in the system of  FIG. 1   
     
    
    
     DETAILED DESCRIPTION 
     The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims. 
     The present inventors have recognized, among other things, a feedback scheme for explicit beamforming enabling multiple mobile stations to provide feedback to an access point in a MU-MIMO system. In the feedback scheme the access point allocates uplink time slots to each mobile station for feedback information. In order to allocate the uplink time slots, the access point determines the length of the feedback from each mobile station. In some examples, the access point can control the length of the feedback from the mobile stations by assigning a number of downlink streams to each mobile station as well as limiting the feedback to a specific type, and a specific modulation and coding scheme. As used herein a stream corresponds to a stream of data to be transmitted as a beam using beamforming techniques and/or spatial multiplexing (SDMA) techniques. 
       FIG. 1  illustrates an example of a wireless communication system  100 . The wireless communication system  100  can include a plurality of mobile stations  102  in wireless communication with an access network  104 . The access network  104  forwards information between the mobile stations  102  and another communications network  106 . Communications network  106  can include the internet, a private intranet, or other network. 
     In an example, each mobile station  102  can include one or more antennas  114  for transmitting and receiving wireless signals to/from an access point  118  in the access network  104 . The access point  118  can implement the air interface to the mobile stations  102 , and can transmit and receive signals with an antenna array  116  coupled thereto. The access point  118  can be communicatively coupled to the communications network  106  for forwarding information to/from the mobile stations  106 . 
       FIG. 2  illustrates an example of a mobile station  102 . The mobile station  102  can include a memory  202  for storage of instructions  204  for execution on processing circuitry  206 . The instructions  204  can comprise software configured to cause the mobile station  102  to perform actions for wireless communication between the mobile station  102  and the access point  118 . The mobile station  102  can also include an RF transceiver  208  for transmission and reception of signals with the antenna  114 . 
     In some examples, the mobile station  102  can be a personal digital assistant (PDA), a laptop or desktop computer with wireless communication capability, a web tablet, a net-book, a wireless telephone, a wireless headset, a pager, an instant messaging device, a digital camera, an access point, a television, a medical device (e.g., a heart rate monitor, a blood pressure monitor, etc.), or other device that can receive and/or transmit information wirelessly. 
       FIG. 3  illustrates an example of an access point  118 . The access point  118  can include a memory  302  for storage of instructions  304  for execution on processing circuitry  306 . The instructions  304  can comprise software configured to cause the access point  118  to perform actions for wireless communication between the mobile station  102  and the access point  118 . The access point  118  can also include an RF transceiver  308  for transmission and reception of signals using the antenna array  116 . The processing circuitry  306  can be configured to implement beamforming with the antenna array  116 . In an example, the processing circuitry  306  can be configured to use the antenna array  116  to implement adaptive beamforming in a MU-MIMO system. That is, multiple beams can be implemented at the same time to different mobile stations  102 . Moreover, the direction of each beam can change dynamically according to changes in the signal path to a given mobile station  102 . The access point  118  can include a network switch, router, or hub for sending and receiving information with the communications network  106 . 
     In an example, the mobile station  102  and access point  118  can be configured to operate in accordance with one or more frequency bands and/or standards profiles. For example, the mobile station  102  and access point  118  can be configured to communicate in accordance with specific communication standards, such as the Institute of Electrical and Electronics Engineers (IEEE) standards. In particular, the mobile station  102  can be configured to operate in accordance with one or more versions of the IEEE 802.11ac communication standard for MU-MIMO Wi-Fi. 
     The mobile station  102  and access point  118  can also be configured to operate in accordance with one or more versions of the IEEE 802.16 communication standard for wireless metropolitan area networks (WMANs) including variations and evolutions thereof. For example, the mobile station  102  and access point  118  can be configured to communicate using the IEEE 802.16-2004, the IEEE 802.16(e), and/or the 802.16(m) versions of the 802.16 standard. For more information with respect to the IEEE 802.16 standards, please refer to “IEEE Standards for Information Technology—Telecommunications and Information Exchange between Systems”—Metropolitan Area Networks—Specific Requirements—Part 16: “Air Interface for Fixed Broadband Wireless Access Systems,” May 2005 and related amendments/versions. 
     In some examples, the mobile station  102  and access point  118  can be configured to communicate in accordance with one or more versions of the Universal Terrestrial Radio Access Network (UTRAN) Long Term Evolution (LTE) communication standards, including LTE release 8, LTE release 9, and future releases. For more information with respect to UTRAN LTE standards, see the 3rd Generation Partnership Project (3GPP) standards for UTRAN-LTE, release 8, March 2008, including variations and later versions (releases) thereof. 
     In some examples, RF transceiver  208  and RF transceiver  308  can be configured to transmit and receive orthogonal frequency division multiplexed (OFDM) communication signals which comprise a plurality of orthogonal subcarriers. In broadband multicarrier examples, the mobile station  102  and access point  118  can be configured to communicate in accordance with an orthogonal frequency division multiple access (OFDMA) technique. 
     In other examples, the mobile station  102  and access point  118  can be configured to communicate using one or more other modulation techniques such as spread spectrum modulation (e.g., direct sequence code division multiple access (DS-CDMA) and/or frequency hopping code division multiple access (FH-CDMA)), time-division multiplexing (TDM) modulation, and/or frequency-division multiplexing (FDM) modulation. 
       FIG. 4  illustrates an example timing flowchart  400  for a feedback scheme for explicit beamforming in the MU-MIMO system  100 . To start the feedback process, the access point  118  sends out a sounding package  402  to the mobile stations  102 . The sounding package  402  can correspond to one or more downlink frames  404 . The sounding package  402  can announce the group ID for the downlink frames  404 , as well as the media access control (MAC) address for the mobile stations  102  to be associated with the group ID. 
     The sounding package  402  can also include a feedback schedule for the mobile stations  102 . The feedback schedule can identify time slots for each of the mobile stations  102  associated with the Group ID. In an example, in order to efficiently schedule the feedback time slots, the access point  118  determines the length of the feedback from each mobile station  102  and assigns the time slots accordingly. For example, the access point  118  can announce that a first mobile station  102  has a feedback time slot  406  scheduled at the beginning of the feedback period. After determining the length of the feedback from the first mobile station  102 , the access point  118  can assign a second mobile station  102  to a feedback time slot  408  immediately following the end of the feedback  406  from the first mobile station  102 . The access point  118  can continue assigning feedback time slots  410 ,  412  in this manner until all the mobile stations  102  associated with the group ID have a time slot in the feedback period. 
     In an example, in order to determine the length of the feedback to be received from the mobile stations  102 , the access point  118  specifies certain parameters for the feedback. For example, the length of the feedback for a mobile station  102  can vary based on the number of downlink streams for the mobile station  102  and the dimension of the feedback matrix received from the mobile station  102 . 
     Accordingly, in an example, the access point  118  specifies the dimension of the feedback matrix for the mobile station  102 . The dimension of the feedback matrix assigned by the access point  118  can be set to greater than or equal to the number of downlink streams on which data will be sent to the mobile station  102  in the downlink frame associated with the group ID. In an example, the number of streams allocated to a mobile station  102  is dependent upon the number of different mobile stations  102  to receive transmissions within a given downlink frame. When a large number of mobile stations  102  are to receive transmissions, there may be fewer streams available for use by each mobile station  102 . Accordingly, the number of streams allocated to the mobile station  102  may be less than if there were fewer mobile stations  102  to receive transmissions within the downlink frame. In an example, the access point  118  has a maximum of eight streams and a maximum of four different mobile stations  102  per downlink frame. Additionally, in an example, a maximum of four streams can be sent to a single mobile station  102 . In an example, the dimension of the feedback matrix assigned to a mobile station  102  can depend on the precoding type as well as the number of downlink streams. For example, when the access point  118  implements a simple MMSE precoding, the dimension of the feedback matrix can be set to equal the number of downlink streams (Ns×Nt). When the access point  118  implements a block diagonal precoding, then the dimension of the feedback matrix can be set to the number of receive antennas (Nr×Nt). The number of streams allocated to a mobile station  102  can also be based on the quality of previous transmissions from the mobile station  102 . For example, if the previous transmission experienced bad quality due to high interference between multiple streams, then the access point  118  can reduce the number of streams allocated to the mobile station  102  in future transmissions. 
     The access point  118  can also specify the feedback type for the mobile station  102  to use. In an example, the feedback types include un-compressed channel state information (CSI) feedback, un-compressed precoding matrix feedback, and compressed precoding matrix feedback. The un-compressed CSI feedback can comprise one or more columns of the Nr×Nt channel matrix, depending on the feedback matrix dimension specified by the access point  118 . As used herein, Nr corresponds to the number of receive antennas at the mobile station  102  and Nt corresponds to the number of transmit antennas in the antenna array  116  on the access point  118 . Additionally, Ns corresponds to the number of streams allocated to the mobile station  102  in the sounding package  402 . The un-compressed precoding matrix has the same dimensions as the un-compressed CSI feedback, but the content of the matrix is the right singular vector of the channel matrix, instead of CSI itself. The compressed precoding matrix comprises a compressed information representing the right singular matrix of the channel matrix. For example, if the access point  118  has four transmit antennas in the antenna array  116 , the mobile station  102  has two receive antennas, and the access point  118  indicates in the sounding package  402  that Ns=2, then the un-compressed CSI and precoding matrix would comprise a 4×2 matrix. The compressed precoding matrix would comprise a 4×2 right singular matrix. More compressed feedback types can also be used. In an example, the mobile station  102  notifies the access point  118  of the feedback types that the mobile station  102  supports. In an example, this notification of feedback type support can be provided by the mobile station  102  during association with the access point  118 . 
     In an example, the access point  118  can set the feedback type based on the precoding algorithm used by the access point  118 . For example, when the access point  118  is using a simple precoding algorithm, the access point  118  can specify a feedback type providing less information from the mobile stations  102 . When the access point  118  is using a complex precoding algorithm, the access point  118  can specify a feedback type providing more information from the mobile stations  102 . For example, if the access point  118  is using the simple minimum mean square error (MMSE) precoding algorithm, then the access point  118  can specify that the mobile stations  102  send feedback using the compressed precoding matrix. If the access point implements a block diagonal precoding algorithm then AP can specify full channel matrix (e.g., un-compressed CSI) from the mobile stations  102 . In an example, when the access point  118  specifies that the mobile station  102  is to receive fewer streams than the number of receive antennas at the mobile station  102 . In an example, the mobile station  102  can provide feedback based on reception with certain receive antennas and can receive the corresponding downlink frame with the same receive antennas. 
     In an example, the access point  118  can also specify the modulation and coding scheme (MCS) to be used by the mobile station  102  when encoding the feedback for sending to the access point  118 . In any case, once the access point  118  has specified these parameters, the access point  118  can determine that the length of the feedback from each mobile station  102  and assign time slots for the feedback according to the length of the feedback. 
     When a mobile station  102  receives a sounding package  402 , the mobile station  102  can measure the preamble of the sounding package  402  to determine the reception quality and direction. The mobile station  102  then calculates a feedback matrix of the feedback type (e.g., un-compressed CSI, un-compressed precoding, or compressed precoding) specified in the sounding package  402  based on the reception quality and direction. 
     In an example, the mobile station  102  can calculate the feedback matrix of the reception quality based on no interference from streams for other mobile stations  102 . Advantageously, this method of calculating the feedback matrix can be the same as for single user MIMO which can simplify the processes at the mobile station  102  since the mobile station  102  does not use different processes for MU-MIMO and SU-MIMO. In another example, the mobile station  102  can calculate the feedback matrix based on random and uniform interference from streams for other mobile stations  102 . For example, the mobile station  102  can calculate the number of streams for other mobile stations  102  based on the stream allocations provided by the access point  118  in the sounding package  402 . The mobile station  102  can then calculate the feedback matrix by assuming an interference signal from all the other streams randomly and uniformly arriving from all directions, where each stream is transmitted with the same transmit power. 
     Once the mobile station  102  has calculated the feedback matrix, the mobile station  102  sends the feedback matrix to the access point  118  within the time slot allocated to the mobile station  102  and using the MCS announced in the sounding package  402 . In an example, in addition to the feedback matrix, the mobile station  102  calculates a preferred MCS for use by the access point  118  in the downlink streams to the mobile station  102 . The preferred MCS can be calculated based on no interference from streams to other mobile stations  102 , or based on random and uniform interference from streams to other mobile stations  102 . 
     All the mobile stations  102  associated with the group ID can perform the feedback matrix calculation according to the announcements in the sounding package  402  and provide the feedback matrix within the time slot assigned by the sounding package  402 . 
     Once the access point  118  receives the feedback matrixes from the mobile stations  102 , the access point  118  can adjust the parameters of the precoding algorithm based on the feedback matrixes. In an example, for mobile stations  102  that calculated the feedback matrix based on no interference from streams for other mobile stations  102 , the access point  118  can adjust the feedback matrix to account for interference from other streams. In another example, the access point  118  can implement the feedback matrix without adjustment in the precoding algorithm. Once the access point  118  has adjusted the parameters based on the feedback matrixes received, the access point  118  can transmit the downlink frame  414  that corresponds to the group ID in the sounding package  402 . Accordingly, the downlink frame  414  can be generated using precoding adjustments based on the feedback from the mobile stations  102 . In an example, the access point  118  applies the adjusted precoding based on the feedback to the training field as well as to data fields (e.g., MAC protocol data units (MPDUs)) within the downlink frame  414 . 
     A mobile station  102  associated with the group ID can then receive the downlink frame  414  and decode the information therein. The mobile station  102  can then send an acknowledge (ACK) or negative acknowledgement (NACK) information  416  on an appropriate channel (e.g., an ACK channel) to the access point  118  evidencing whether the downlink frame  414  was accurately received by the mobile station  102 . In addition, in some examples, the mobile station  102  can perform an additional calculation to refine the preferred MCS to the used by the access point  118  based on the reception quality of the downlink frame  414 . This refined MCS can be piggybacked with ACK/NACK information  416  and sent to the access point  118  on, for example, an ACK channel. The access point  118  can then use the refined MCS for future transmissions to the mobile station  102 . Advantageously, this refined MCS takes into account actual interference from streams to other mobile stations  102 , since these other streams were transmitted in the downlink frame  414  along with the stream(s) to the mobile station  102  performing the calculation. 
     Embodiments may be implemented in one or a combination of hardware, firmware and software. Embodiments may also be implemented as instructions stored on a computer-readable medium, which may be read and executed by at least one processing circuitry to perform the operations described herein. A computer-readable medium may include any mechanism for storing in a form readable by a machine (e.g., a computer). For example, a computer-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. 
     The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.