Patent Publication Number: US-2009225728-A1

Title: Analogue Beamforming

Description:
RELATED APPLICATION 
     This patent application claims priority to Provisional Application 61/035,123, “Analogue Beamforming,” filed by Kuze et al. on Mar. 10, 2008, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to wireless networks, and more particularly to analog beam forming and beam switching in networks according to the IEEE 802.16m standard. 
     BACKGROUND OF THE INVENTION 
     One object of networks designed according to the Worldwide Interoperability for Microwave Access (WiMAX) standard is to improve a spectral efficiency of networks, while keeping the cost of deployments to a minimum. Fixed WiMAX is based on the IEEE 802.16d standard, and mobile WiMAX on the IEEE 802.16e standard. 
     One way to do this is to use analog beam forming (ABF). The BS can switch arbitrarily through the available beams on the up link and the down link and communicates with the MS located in the active beams. The range of the cell is extended by the beam forming, which is important especially in rural areas. By adopting appropriate beam switching patterns, the interference can also be reduced. 
     Analog beam forming (ABF) is not the theoretic optimal way of using multiple antenna elements. Heterodyning all the signals to and from the baseband, and digitally processing the signals can achieve a higher capacity; see U.S. Pat. No. 6,307,506, “Method and apparatus for enhancing the directional transmission and reception of information.” However, ABF presents an excellent tradeoff between performance and complexity. For example, ABF can be performed with only a single radio frequency (RF) chain. 
     As another advantage, ABF can be combined with spatial multiplexing and other MIMO techniques. The set of N available antenna elements can be partitioned into K groups of M antennas, i.e., M×K=N, so that K beams are formed. In each beam, M antenna elements are available for spatial multiplexing. When dual-polarized antennas are used. 
     The IEEE 802.16 standards define a frame for the down link (DL) and up link (UL). The various fields and zones are described in complete detail in IEEE 802.16 standard “Part 16: Air interface for Broadband Wireless Access Systems,” and U.S. Publication 2008-0165881, “Method for Accessing Channels in OFDMA Mobile Multihop Relay Networks,” Tao et al. 
       FIG. 1  shows the conventional frame, which includes a DL subframe, and a UL subframe. The first symbol transmitted by the base station in the DL subframe is a preamble. The preamble enables the mobile stations to perform synchronization and channel estimation. 
     The frame includes a sequence of OFDM symbols, denoted in the horizontal time direction, and is indexed with integer k, {k, k+1, k+2, . . . , K}. Each OFDM symbol also includes a number of sub-channels, denoted in the vertical direction, and indexed with the integer, s {s, s+1, s+2, . . . , S}. 
     The first subchannel in the first two OFDMA symbols in the down link is the frame control header (FCH). The FCH is transmitted using QPSK rate 1/2 with four repetitions. The FCH specifies a length of the immediately succeeding down link MAP (DL-MAP) message and the repetition coding used for DL-MAP. 
     The BS uses the down link MAP (DL-MAP) and an up link MAP (UL-MAP) message to notify MS of the resources allocated to data bursts in the down link and up link direction, respectively, within the current frame. The bursts are associated with connection identifiers (CID). 
     Based upon a schedule received from the BS, each MS can thus determine when (i.e., OFDMA symbols) and where (i.e., subchannels) the MS should transceive (transmit or receive) with the BS. The first subchannels  203  in the UL subframe are used for ranging. 
     The receive/transmit gap (RTG) separates the frames, and the transmit transition gap (TTG) separates the subframes within a frame. This enables the transceivers to switch between transmit and receive modes. 
     However, that frame does not have zones to support ABF. 
     SUMMARY OF THE INVENTION 
     An orthogonal frequency division multiple access (OFDMA) network including a base station (BS) associated with a set of mobile stations (MS) in a cell. 
     The set of MS are grouped into sets of active MS, wherein each set of active MS corresponds to a beam formed at the BS. 
     The BS transmits a down link (DL) subframe using analog beam forming (ABF), wherein the DL subframe has one ABF zone for each set of active MS and each corresponding beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional superframe structure; 
         FIG. 2  is a schematic of a wireless network according to embodiments of the invention; and 
         FIG. 3  is a block diagram of a superframe according to embodiments of the invention; 
         FIG. 4  is a schematic of beam switching according to embodiments of the invention; and 
         FIG. 5  is a graph comparing networks with and without beam forming. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  shows a wireless network with a base station (BS)  201 , and a set of mobile stations (MS)  202  according to embodiments of our invention. The BS can form beams  210 - 211  within the cell  203  using a linear antenna array concatenated with a Butler matrix, see U.S. Patent Application 20060104197, “Method and system for economical beam forming in a radio communication system.” 
     Analog beam forming (ABF) requires that we define new zones in the superframe structure for the down link (DL) and the up link (UL). A zone is a time division duplexing technique that allows multiple transmission formats in the same DL and UL. 
     By partitioning the DL into multiple zones, the MS in different zones can be handled sequentially. Each ABF zone corresponds to a transmission interval in the DL, where a particular beam is active at the BS. Thus, the MS within the geographic coverage area of the active beam are grouped into an active set, and served during the corresponding zones. 
     The embodiment of the invention enable an efficient grouping of the MS into the active sets for the corresponding beams, and then serving all MS within each active set during the same zone of the DL or UL sub-frame. 
       FIG. 3  shows the frame structure according to embodiments of the invention. The DL subframe is partitioned into K ABF zones  310 . There is one zone for each of the two ABF zones. There is a preamble  301  at the beginning of each AFB zone  310 . The MS detect which preamble has the highest signal strength. This enables the MS to associate with the beam that has the best signal-to-interference ratio (SINR). 
     When ABF is used with  FIG. 3  shows more details of the frame structure when BF is used with adaptive modulation and coding (AMC). When ABF is used with full usage of subchannels (FUSC) and partial usage of subchannels (PUSC), advanced audio coding conventional (AAS) zones can be used. 
     The conventional DL-MAP includes the information about the ABF zones and the location (in the time-frequency domain) of the preamble  301  of each zone  310 . 
     Additionally, each zone has a UL-DL compressed map  302 . The map indicates the location of the subsequent UL ranging region  320 . The ranging regions are located in the UL subframe. The MS use these regions to signal to the BS that the MS is to be served in the associated DL zone. 
     During the initial reception of the ABF preambles  301 , each MS tries to detect the zone preamble. If the MS detects a certain zone preamble, then the MS can decode the UL-DL compressed map. 
     During the next UL subframe, the MS starts a ranging process by using in the ranging region indicated in the UL-DL compressed map. The MS can perform ranging for each analog BF preamble during the DL subframe. The MS transmit the ranging data to the BS, and the BS selects the active MS sets using the ranging data. 
     Interference Reduction with ABF 
     As shown in  FIG. 4 , ABF can also be used to reduce interference. The MS in different beams  1 - 4  are served at different times. Therefore, if BS 1  and BS 2  in adjacent cells arrange the down link transmission in such a way that the transmissions do not transmit to the same part of a cell edge at the same time, and similarly for the up link, then the interference at the MS is greatly reduced. 
     If the BS can coordinate the beams, then interference from adjacent cells can be almost completely eliminated, and only second-tier interference remains. If the BS cannot coordinate their beam switching, then the sequence in which beams are served can be selected randomly, and independently at each BS. This still leads to a stochastic reduction of the interference, similar to the reduction of interference in random frequency hopping or time-hopping impulse radio. 
     The hopping sequence can be determined at each BS, based on a base station identification (BS_ID). For example, the BS_ID is used as an initial value, i.e., seed, to a feedback shift register that generates a random hopping sequence. 
     Training for ABF 
     As described above, each zone has its own preamble that enables the MS to determine which beam is best. This requires that the MS receives all ABF preambles. A suboptimum solution is that the MS only receive the beams that are adjacent to the beam used for current communications. Additionally, it is not necessary for the MS to receive adjacent preambles during each frame. The time that the MS stays within one beam is usually quite large, on the order of seconds, so that infrequent listening is sufficient. If the MS is fixed, the detecting needs to perform again only when the MS moves. 
     For the up link, the MS can, from time to time, transmit the ranging signals in the zones associated with beams adjacent to the current beam. The periodic ranging is arranged so that collisions of the MS signal with that of other ranging signals are minimized. This is arranged by the BS, which controls the UL ranging through the UL map. 
     Feedback for Beam Scheduling 
     It may be helpful to let MS feedback to the BS the index of the best beam receives. The BS can use this feedback information to perform beam scheduling. With AMC, the feedback from the MS includes a channel quality indication for different beams for all the subchannels. This can further enable frequency and beam scheduling. 
     Performance for Training Structure for Basic Case 
     To test the performance enhancement achievable with ABF, we have simulated a small WiMAX network. We consider the down link case, and the average signal to interference and noise ratio (SINR) cumulative distribution function (CDF) at the simulated MS. To generate the CDF, a MS is randomly placed in a sector of interest. We assume that the MS communicates on the best available beam from the base station. The base station may communicate on N B  beams, where N B  is assumed to be either four or eight. 
     Each of the adjacent sectors interferers directly, i.e., the active beam from the adjacent sector are directed at the MS, with probability 1/N B . In this case, the interference from the adjacent sector is large. In the case when no direct interference is present from the adjacent sector, then a random beam, not directed at the MS, is assumed to be active, and the interference contribution from this sector is computed assuming a transmission from this sector. 
     The receiver noise is assumed to be additive white Gaussian (AWG). The SINR is determined at a thousand random locations within the sector. At each location five channel realizations are averaged to determine the SINR at each location. Other simulation assumptions are in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
             
            
               
                   
                 Fast Fourier Transform size 
                 1024 
               
               
                   
                 Bandwidth 
                 10 MHz 
               
               
                   
                 Channel model 
                 Urban Macro 
               
               
                   
                 Center Frequency 
                 2.5 GHz 
               
               
                   
                 Inter site distance 
                 1000 m 
               
               
                   
                 Thermal noise 
                 −203 dBW/Hz 
               
               
                   
                 Receiver Noise Figure 
                 7 dB 
               
               
                   
                   
               
            
           
         
       
     
     The results are shown in  FIG. 5 . There is a significant improvement in the SINR with ABF. For a four beam network, the gain is approximately 22 dB at the median of the CDF. An additional 3 dB gain can be achieved with eight beams. 
     EFFECT OF THE INVENTION 
     The invention provides a simple but extremely efficient method for increasing SINR, and thus throughput in WiMAX networks. The method is not the optimum way of exploiting multiple antenna elements. A four-beam switching network cannot perform as well as a full four-antenna MIMO networks. However, the complexity of a four-beam switching network is much lower than a four-antenna MIMO network. 
     The complexity is identical to that of a single-antenna network with a single FRF chain, and just an additional switch and four antennas. Beam switching provides a low-cost and efficient solution both for range extension and for interference reduction. 
     Although the invention has been described with reference to certain preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the append claims to cover all such variations and modifications as come within the true spirit and scope of the invention.