Patent Publication Number: US-2007121751-A1

Title: Methods and apparatus for beamforming training symbols in wireless multiple-input-multiple-output systems

Description:
TECHNICAL FIELD  
      The present disclosure relates generally to wireless communication systems, and more particularly, to methods and apparatus for beamforming training symbols in wireless multiple-input-multiple-output (MIMO) systems.  
     BACKGROUND  
      A data frame of a wireless MIMO system may include one or more training symbols such as preamble symbols, pilot symbols, and/or midamble symbols. In general, a preamble symbol may be a training symbol at the beginning of each data frame. Typically, the preamble symbol may be used for various synchronization tasks. A pilot symbol may be a training symbol to provide tracking information, which may be associated with a spatial channel. A midamble symbol may be a training symbol corresponding to a time slot (e.g., at the beginning of a user zone). To increase data throughput in wireless MIMO systems, some development efforts have been directed toward improving channel estimation for beamformed spatial channels and reducing pilot allocation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram representation of an example wireless communication system according to an embodiment of the methods and apparatus disclosed herein.  
       FIG. 2  is a block diagram representation of an example base station.  
       FIG. 3  is a block diagram representation of an example wireless MIMO system.  
       FIG. 4  is a flow diagram representation of one manner in which the example base station of  FIG. 2  may be configured to beamform training symbols.  
       FIG. 5  is a block diagram representation of an example processor system that may be used to implement the example base station of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION  
      In general, methods and apparatus for beamforming training symbols in wireless multiple-input-multiple output (MIMO) systems are described herein. The methods and apparatus described herein are not limited in this regard.  
      Referring to  FIG. 1 , an example wireless communication system  100  including a base station (BS)  110  and a subscriber station (SS)  120  is described herein. Although  FIG. 1  may depict one base station, the wireless communication system  100  may include additional base stations. In a similar manner, the wireless communication system  100  may include additional subscriber stations even though  FIG. 1  depicts one subscriber station.  
      The base station  110  may use a variety of 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, frequency-division multiplexing (FDM) modulation, orthogonal frequency-division multiplexing (OFDM) modulation, multi-carrier modulation (MDM), and/or other suitable modulation techniques to communicate via wireless links. For example, the base station  110  may implement OFDM modulation to transmit large amounts of digital data by splitting a radio frequency signal into multiple small sub-signals, which in turn, are transmitted simultaneously at different frequencies. In particular, the base station  110  may use OFDM modulation as described in the 802.xx family of standards developed by the Institute of Electrical and Electronic Engineers (IEEE) and/or variations and evolutions of these standards (e.g., 802.11, 802.15, 802.16, etc.) to communicate with the subscriber station  120 . In addition or alternatively, the base station  110  may operate in accordance with other suitable wireless communication protocols that require very low power such as Bluetooth, Ultra Wideband (UWB), and/or radio frequency identification (RFID) to communicate with the subscriber station  120 .  
      The base station  110  may also operate in accordance with other wireless communication protocols may be based on analog, digital, and/or dual-mode communication system standards. For example, the base station  110  may operate in accordance with wireless communication protocols such as orthogonal frequency division multiple access (OFDMA)-based standards, time division multiple access (TDMA)-based standards (e.g., Global System for Mobile Communications (GSM), General Packet Radio Services (GPRS), Enhanced Data GSM Environment (EDGE), Universal Mobile Telecommunications System (UMTS), etc.), code division multiple access (CDMA)-based standards, wideband CDMA (WCDMA)-based standards, variations and evolutions of these standards, and/or other suitable wireless communication standards.  
      The subscriber station  120  may be a laptop computer, a handheld computer, a tablet computer, a cellular telephone (e.g., a smart phone), a pager, an audio and/or video player (e.g., an MP3 player or a DVD player), a game device, a digital camera, a navigation device (e.g., a GPS device), and/or other suitable wireless electronic devices. The subscriber station  120  may communicate via wireless links as described in the 802.xx family of standards developed by the Institute of Electrical and Electronic Engineers (IEEE) and/or variations and evolutions of these standards (e.g., 802.11, 802.15, 802.16, etc.). In one example, the subscriber station  120  may operate in accordance with the 802.16 family of standards developed by IEEE to provide for fixed, portable, and/or mobile broadband wireless access (BWA) networks (e.g., the IEEE std. 802.16, published 2004). The subscriber station  120  may also use direct sequence spread spectrum (DSSS) modulation (e.g., the IEEE std. 802.11b) and/or frequency hopping spread spectrum (FHSS) modulation (e.g., the IEEE std. 802.11). Further, the subscriber station  120  may also operate in accordance with other suitable wireless communication protocols that require very low power such as Bluetooth, Ultra Wideband (UWB), and/or radio frequency identification (RFID) to communicate via wireless links. In addition or alternatively, the subscriber station  120  may communicate via wired links (not shown). For example, the subscriber stations  120  may use a serial interface, a parallel interface, a small computer system interface (SCSI), an Ethernet interface, a universal serial bus (USB) interface, a high performance serial bus interface (e.g., IEEE 1394 interface), and/or any other suitable type of wired interface to communicate. The methods and apparatus described herein are not limited in this regard.  
      Further, the wireless communication system  100  may include other wireless personal area network (WPAN) devices, wireless local area network (WLAN) devices, wireless metropolitan area network (WMAN) devices, and/or wireless wide area network (WWAN) devices such as network interface devices and peripherals (e.g., network interface cards (NICs)), access points (APs), gateways, bridges, hubs, etc. to implement a cellular telephone system, a satellite system, a personal communication system (PCS), a two-way radio system, a one-way pager system, a two-way pager system, a personal computer (PC) system, a personal data assistant (PDA) system, a personal computing accessory (PCA) system, and/or any other suitable communication system (not shown). Accordingly, the wireless mesh network  110  may be implemented to provide WPANs, WLANs, WMANs, WWANs, and/or other suitable wireless communication networks. Although certain examples have been described above, the scope of coverage of this disclosure is not limited thereto.  
      In the example of  FIG. 2 , a base station  200  may include an input source  210 , a channel identifier  220 , and a beamformer  230 . The beamformer  230  may be coupled to the input source  210  and the channel identifier  220 . The input source  210  may provide one or more data streams to the beamformer  230 . For example, a data stream may include a portion of a data frame. In another example, a data stream may include one or more data frames. Each frame may include one or more training symbols. In particular, a training symbol may be a preamble symbol, a pilot symbol, and/or a midamble symbol.  
      In general, a preamble symbol may be a training symbol located at the beginning of each frame and used for various synchronization tasks. A pilot symbol may be a training symbol to provide information for channel tracking and estimation, which may be associated with a transmit antenna and/or a spatial channel. A midamble symbol may be a training symbol corresponding to a time slot (e.g., at the beginning of a user zone). A user may carry and/or operate a subscriber station (e.g., SS  120  of  FIG. 1 ), and a user zone may be a set of subcarriers and a set of time slots within an OFDMA frame associated with the subscriber station. For example, the pilot and midamble symbols may be used to enhance channel estimation in broadband channels for a particular user.  
      The channel identifier  220  may identify a plurality of spatial channels available to the base station  200 . The plurality of spatial channels may be shared by two or more subscriber stations (e.g., one shown as  120  in  FIG. 1 ). For example, the plurality of spatial channels may be assigned to two or more subscriber stations with each subscriber station including one receiver with multiple active receive antennas (e.g., point-to-point MIMO), multiple receivers with each receiver having an active receive antenna (e.g., point-to-multiple-point MIMO), or a combination thereof.  
      As described in detail below, the beamformer  230  may compute beamforming weights associated with each of the plurality of spatial channels identified by the channel identifier  220  to beamform pilots associated with the data streams from the input source  210 . The base station  200  may also include a plurality of transmitters  240 , generally shown as  242 ,  244 , and  246 , and a plurality of antennas  250 , generally shown as  252 ,  254 , and  256 . The plurality of transmitters  240  may be coupled to the beamformer  230 . Each of the plurality of transmitters  240  may be coupled to one of the plurality of antennas  250 . For example, the transmitter  242  may be coupled to the antenna  252 , the transmitter  244  may be coupled to the antenna  254 , and the transmitter  246  may be coupled to the antenna  256 . Although  FIG. 2  depicts three transmitters, the base station  200  may include additional or fewer transmitters. In a similar manner, the base station  200  may include additional or fewer antennas even though  FIG. 2  depicts three antennas. The methods and apparatus described herein are not limited in this regard.  
      Referring to  FIG. 3 , an example wireless MIMO system  300  may include a base station  310  and one or more subscriber stations, generally shown as  320  and  325 . The wireless MIMO system  300  may include a point-to-point MIMO system and/or a point-to-multiple point MIMO system. For example, a point-to-point MIMO system may include the base station  310  and the subscriber station  320 . A point-to-multiple point MIMO system may include the base station  310  and the subscriber station  325 . The base station  310  may transmit data streams to the subscriber stations  320 ,  325  simultaneously. For example, the base station  310  may transmit two data streams to the subscriber station  320  and one data stream to the subscriber station  325 . Although  FIG. 1  may depict two subscriber stations, the wireless MIMO system  300  may include additional or fewer subscriber stations.  
      The base station  310  may include an input source  330  and a beamformer  340 . The base station  310  may transmit two data streams from the input source  330  through two beamformed spatial channels over four transmit antennas  350 , generally shown as  352 ,  354 ,  356 , and  358 . Although  FIG. 3  depicts four transmit antennas, the base station  310  may include additional or fewer transmit antennas.  
      The input source  310  may provide a data/pilot symbol vector u. The data/pilot symbol vector u may be represented as  
         u   =     [           u   1               u   2           ]       ,       
 
 where u 1  may be transmitted through a first spatial channel and u 2  may be transmitted through a second spatial channel. The first and second spatial channels may be assigned to the subscriber station  320  including one receiver with multiple receive antennas (e.g., point-to-point MIMO). 
 
      To beamform the pilots of the two data streams, the beamformer  330  may determine a beamforming weight for each transmit antenna/spatial channel pair. In one example, the base station  300  may include eight transmit antenna/spatial channel pairs to correspond to the four transmit antennas and the two spatial channels. In particular, the beamformer  330  may use a beamforming matrix V based on the number of transmit antennas and the number of spatial channels of the base station  300 . Accordingly, the beamforming matrix V may be represented by a 4×2 matrix as  
         V   =     [           v   11           v   12               v   21           v   22               v   31           v   32               v   41           v   42           ]       ,       
 
 where v ij  is the complex weight for the j-th channel on i-th antenna. For example, v 11  may represent the beamforming weight for the first spatial channel on the antenna  352  while v 42  may represent the beamforming weight for the second spatial channel on the antenna  358 . In a similar manner, v 41  may represent the beamforming weight for the first spatial channel on the antenna  358  while v 12  may represent the beamforming weight for the second spatial channel on the antenna  352 . 
 
      The base station  310  may transmit a signal vector x to the subscriber station  320  via the plurality of transmit antennas  350 . The signal vector x may be represented as x=Vu, where V is the beamforming matrix and u is the data/pilot symbol vector. In one example, the antenna  352  may transmit x 1  and the antenna  354  may transmit x 2  of the signal vector x.  
      As indicated above, the base station  310  may transmit pilots via four spatial channels because the four transmit antennas may form the four spatial channels. Although the base station  310  may employ up to four spatial channels, the base station  310  may transmit pilots over two spatial channels formed by the four transmit antennas because the subscriber station  320  may receive at most two spatial streams with only two receive antennas. Accordingly, the beamforming matrix V may be represented by an M=N matrix to avoid redundancy, where M is the number of transmit antennas employed and N is the number of active spatial channels. In particular, active spatial channels may be spatial channels carrying data and used by the subscriber station  320  to receive data streams from the base station  310  via receive antennas. The base station  310  may release bandwidth allocated for pilots of inactive spatial channels. Thus, the base station  310  may increase data throughput by transmitting a greater amount of data with the released bandwidth.  
      By transmitting training symbols (e.g., pilot or midamble) via one antenna in a non-beamformed manner, for example, other antennas may not transmit data symbols because the data symbols may interfere with the training symbols. As a result, dedicated time slots may need to be reserved to transmit training symbols of a non-beamformed environment. In contrast, training symbols and data symbols of a beamformed environment may be transmitted simultaneously (e.g., during the same time slot) or concurrently (e.g., overlapping the same time slot) via distinct beamformed spatial channels without interference.  
      To avoid using additional bandwidth for transmitting information associated with the spatial channels, the base station  300  may beamform training symbols (e.g., pilots and/or midambles) so that the subscriber station  320  may identify the beamformed spatial channels to retrieve data from the base station  310  (e.g., coherent detection). In particular, the beamformed spatial channels may be the product of two matrices. The beamformed spatial channels may be represented as  
         Y   =       [           y   1               y   2           ]     =   HV       ,       
 
 where H is the channel matrix and V is the beamforming matrix. Accordingly, the subscriber station  320  may determine the beamformed channels Y to retrieve beamformed data transmitted from the base station  310 . Otherwise without beamformed training symbols, the subscriber station  320  may only estimate the channel matrix H using training symbols transmitted by the antennas without beamforming, and the base station  310  may be required to use additional bandwidth to transmit the beamforming matrix V to the subscriber station  320 . The methods and apparatus described herein are not limited in this regard. 
 
       FIG. 4  depicts one manner in which the example base stations of  FIGS. 2 and 3  may be configured to beamform training symbols in wireless MIMO systems. The example process  400  of  FIG. 4  may be implemented as machine-accessible instructions utilizing any of many different programming codes stored on any combination of machine-accessible media such as a volatile or nonvolatile memory or other mass storage device (e.g., a floppy disk, a CD, and a DVD). For example, the machine-accessible instructions may be embodied in a machine-accessible medium such as a programmable gate array, an application specific integrated circuit (ASIC), an erasable programmable read only memory (EPROM), a read only memory (ROM), a random access memory (RAM), a magnetic media, an optical media, and/or any other suitable type of medium.  
      Further, although a particular order of actions is illustrated in  FIG. 4 , these actions can be performed in other temporal sequences. Again, the example process  400  is merely provided and described in conjunction with the apparatus of  FIGS. 2 and 3  as an example of one way to configure a base station to beamform training symbols in wireless MIMO systems.  
      In the example of  FIG. 4 , the process  400  may begin with the base station  310  ( FIG. 3 ) identifying a plurality of subscriber stations that share a plurality of spatial channels (e.g., the subscriber station  320  of  FIG. 3 ) (block  410 ). The base station  310  may select the subscriber station based on separation of spatial channels and/or traffic schedules. In one example, the spatial signatures of two adjacent subscriber stations may be identical or relatively similar. To differentiate between subscriber stations, the base station  310  may select subscriber stations with different spatial signatures.  
      In another example, a change in beamforming weights during a burst of transmission to two subscriber stations may compensate for one of the subscriber stations receiving the transmission from the base station  310  earlier than the other subscriber station. However, the change in beamforming weights may cause an interruption of the channel status, which is an undesirable effect on the subscriber stations. Thus, the base station  310  may select subscriber stations with identical or relatively similar duration of transmission. Alternatively, the base station may continue to use the beamformed weights for spatial channels of any remaining subscriber stations to avoid the interruption. Further, the base station may change the beamformed weights used by the finished subscriber stations so that data may be transmitted to new subscriber stations through the new beamformed spatial channels.  
      The base station  310  (e.g., via the channel identifier  220  of  FIG. 2 ) may identify the plurality of spatial channels used by the subscriber station  320  (block  420 ). In particular, the base station  310  may identify a number of spatial channels based on a number of transmit antennas associated with the base station  310  and a number of receive antennas associated with the subscriber station  320 . In a point-to-point MIMO wireless system, for example, the subscriber station  320  ( FIG. 3 ) may include a single receiver with a plurality of antennas. In another example, a point-to-multiple-point MIMO wireless system may include a plurality of the subscriber stations with each subscriber station having a receiver with one active antenna (e.g., the subscriber station  325  of  FIG. 3 ).  
      In one example, the base station  310  may include two transmit antennas and the subscriber station  320  may include two receive antennas (e.g., the channel matrix H is a 2×2 matrix). Accordingly, two beamformed spatial channels may be formed. The base station  310  may use one or both beamformed spatial channels based on signal quality. For example, if the signal strength of one of the beamformed spatial channels is below a quality threshold for data transmission, the base station  310  may use the other beamformed spatial channel only.  
      Based on the number of spatial channels, the base station  310  (e.g., via the beamformer  230 ) may determine a beamforming weight associated with each of the plurality of spatial channels (block  430 ). Each beamforming weight may correspond to a transmit antenna/spatial channel pair. Based on the plurality of beamforming weights, the base station  310  may transmit one or more data streams and pilots associated with the data streams to the subscriber station  320  (block  440 ). In one example, the base station  310  may transmit data on a first spatial channel over a particular frequency and the pilot on a second spatial channel over the same frequency to the subscriber station  320  without causing interference.  
      Alternatively, the base station  310  may transmit one or more data streams and midambles associated with the data streams to the subscriber station  320 . The subscriber station  320  may retrieve the data from the base station  310  without explicitly knowing the beamforming matrix V because the data and training symbols may be transmitted via the same beamformed spatial channels. In particular, the base station  310  may not need additional bandwidth to transmit the beamforming matrix V. If the base station  310  includes more transmit antennas than the total number of active spatial channels, the base station  310  may select the spatial channels with the strongest signal strengths to transmit data to the subscriber station  320 . Further, the beamformed transmission may provide an “array gain” on the received signals if the base station  310  includes more transmit antennas than the total number of active spatial channels. Thus, channel estimation for beamformed spatial channels may be improved. The methods and apparatus described herein are not limited in this regard.  
       FIG. 5  is a block diagram of an example processor system  2000  adapted to implement the methods and apparatus disclosed herein. The processor system  2000  may be a desktop computer, a laptop computer, a handheld computer, a tablet computer, a PDA, a server, an Internet appliance, and/or any other type of computing device.  
      The processor system  2000  illustrated in  FIG. 5  includes a chipset  2010 , which includes a memory controller  2012  and an input/output (I/O) controller  2014 . The chipset  2010  may provide memory and I/O management functions as well as a plurality of general purpose and/or special purpose registers, timers, etc. that are accessible or used by a processor  2020 . The processor  2020  may be implemented using one or more processors, WLAN components, WMAN components, WWAN components, and/or other suitable processing components. For example, the processor  2020  may be implemented using one or more of the Intel® Pentium® technology, the Intele Itanium® technology, the Intel® Centrino™ technology, the Intel® Xeon™ technology, and/or the Intel® XScale® technology. In the alternative, other processing technology may be used to implement the processor  2020 . The processor  2020  may include a cache  2022 , which may be implemented using a first-level unified cache (L 1 ), a second-level unified cache (L 2 ), a third-level unified cache (L 3 ), and/or any other suitable structures to store data.  
      The memory controller  2012  may perform functions that enable the processor  2020  to access and communicate with a main memory  2030  including a volatile memory  2032  and a non-volatile memory  2034  via a bus  2040 . The volatile memory  2032  may be implemented by Synchronous Dynamic Random Access Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM), and/or any other type of random access memory device. The non-volatile memory  2034  may be implemented using flash memory, Read Only Memory (ROM), Electrically Erasable Programmable Read Only Memory (EEPROM), and/or any other desired type of memory device.  
      The processor system  2000  may also include an interface circuit  2050  that is coupled to the bus  2040 . The interface circuit  2050  may be implemented using any type of interface standard such as an Ethernet interface, a universal serial bus (USB), a third generation input/output interface ( 3 GIO) interface, and/or any other suitable type of interface.  
      One or more input devices  2060  may be connected to the interface circuit  2050 . The input device(s)  2060  permit an individual to enter data and commands into the processor  2020 . For example, the input device(s)  2060  may be implemented by a keyboard, a mouse, a touch-sensitive display, a track pad, a track ball, an isopoint, and/or a voice recognition system.  
      One or more output devices  2070  may also be connected to the interface circuit  2050 . For example, the output device(s)  2070  may be implemented by display devices (e.g., a light emitting display (LED), a liquid crystal display (LCD), a cathode ray tube (CRT) display, a printer and/or speakers). The interface circuit  2050  may include, among other things, a graphics driver card.  
      The processor system  2000  may also include one or more mass storage devices  2080  to store software and data. Examples of such mass storage device(s)  2080  include floppy disks and drives, hard disk drives, compact disks and drives, and digital versatile disks (DVD) and drives.  
      The interface circuit  2050  may also include a communication device such as a modem or a network interface card to facilitate exchange of data with external computers via a network. The communication link between the processor system  2000  and the network may be any type of network connection such as an Ethernet connection, a digital subscriber line (DSL), a telephone line, a cellular telephone system, a coaxial cable, etc.  
      Access to the input device(s)  2060 , the output device(s)  2070 , the mass storage device(s)  2080  and/or the network may be controlled by the I/O controller  2014 . In particular, the I/O controller  2014  may perform functions that enable the processor  2020  to communicate with the input device(s)  2060 , the output device(s)  2070 , the mass storage device(s)  2080  and/or the network via the bus  2040  and the interface circuit  2050 .  
      While the components shown in  FIG. 5  are depicted as separate blocks within the processor system  2000 , the functions performed by some of these blocks may be integrated within a single semiconductor circuit or may be implemented using two or more separate integrated circuits. For example, although the memory controller  2012  and the I/O controller  2014  are depicted as separate blocks within the chipset  2010 , the memory controller  2012  and the I/O controller  2014  may be integrated within a single semiconductor circuit.  
      Although certain example methods, apparatus, and articles of manufacture have been described herein, the scope of coverage of this disclosure is not limited thereto. On the contrary, this disclosure covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents. For example, although the above discloses example systems including, among other components, software or firmware executed on hardware, it should be noted that such systems are merely illustrative and should not be considered as limiting. In particular, it is contemplated that any or all of the disclosed hardware, software, and/or firmware components could be embodied exclusively in hardware, exclusively in software, exclusively in firmware or in some combination of hardware, software, and/or firmware.