Abstract:
Advantage is taken of adaptive allocation techniques by intentionally creating multi-user diversity in an otherwise flat fading environment in order to improve system capacity. In one embodiment, multi-path distortion can be resolved to determine subscriber station (SS) diversity gain. Overall network capacity can be increased by allocating channel assignments to SSs within the network based on determined SS diversity gains. In one embodiment, intentional multi-path distortion is produced by transmitting a signal and a time-delayed version of the signal from a base station (BS).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/501,211 filed on Aug. 8, 2006. 
    
    
     TECHNICAL FIELD 
     This invention relates to wireless network channel allocations and more particularly to systems and methods using intentional multi-path distortion to facilitate such allocations. 
     BACKGROUND OF THE INVENTION 
     In wireless communication systems having selectively fading channels, different users experience peaks in their channel quality at different times or at different frequencies. This effect is called multi-user diversity. Multi-user diversity can be exploited in a selectively fading environment by designating users to transmit during times or at frequencies providing favorable channel conditions. For example, in a multi-carrier communication systems, significant performance improvements can be achieved by adaptively allocating to each subscriber multiple subcarriers based on, for example, load traffic information on each subcarrier, signal quality at the subscriber station, whether frequency bands are overused, and level of requests queued at the base station. 
     Adaptive allocation techniques are generally ineffective in flat fading environments. For example, adaptive allocation may not be practicable in a multi-carrier system with substantially similar subcarrier gains at the subscriber station. 
     BRIEF SUMMARY OF THE INVENTION 
     Advantage is taken of adaptive allocation techniques by intentionally creating multi-user diversity in an otherwise flat fading environment in order to improve system capacity. In one embodiment, multi-path distortion can be resolved to determine subscriber station (SS) diversity gain. Overall network Capacity can be increased by allocating channel assignments to SSs within the network based on determined SS diversity gains. In one embodiment, intentional multi-path distortion is produced by transmitting a signal and a time-delayed version of the signal from a base station (BS). 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a network system adapted according to an embodiment of the present invention; 
         FIG. 2  shows one embodiment of a process for allocation clusters to SSs; 
         FIG. 3  shows one embodiment of a subscriber processing system; and 
         FIG. 4  illustrates selective frequency assignments of an OFDMA network. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows one embodiment  10  in which base station (BS)  12  is in wireless communication with subscriber station (SS)  14  and SS  16 . BS  12  broadcasts signals  17 ,  18 ,  19 , which can be broadcast from separate antennas or from a common antenna. In one embodiment, a method for subcarrier selection for a system employing orthogonal frequency division multiple access (OFDMA) comprises partitioning subcarriers into groups of at least one cluster of subcarriers, receiving an indication of a selection by the subscriber of one or more clusters in the groups, and allocating at least one cluster in the one or more groups of clusters selected by the subcarrier for use in communication with the subscriber. As described in further detail herein, subcarrier selection may utilize diversity gain information determined by resolving intentional (artificial) multi-path distortion caused by transmission of a signal (for example signal  17  from OFDM modulator  130 ) and a time-delayed version of that signal (for example signal  18  from cyclic delay  131  and signal  19  from cyclic delay  132 ). 
     The techniques disclosed herein are described using OFDMA (clusters) as an example, but the concepts taught are not limited to OFDMA-based systems. Rather, the techniques may apply to multi-carrier systems in general. As can be seen in the following description, one advantage of these techniques is that minimal changes are required to the BS and SS relative to a standard OFDM system. 
     In one embodiment, subcarrier allocation is performed in each cell separately. Within each cell, the allocation for individual SSs is also made progressively as each new subscriber is added to the system as opposed to joint allocation for subscribers within each cell in which allocation decisions are made taking into account all subscribers in a cell for each allocation. 
     For downlink channels, each SS measures the channel and interference information for multiple subcarriers and then selects multiple subcarriers with good performance (e.g., a high signal-to-interference plus noise ratio (SINR)) and feeds back the information on these candidate subcarriers to the base station. The feedback may comprise channel and interference information on all subcarriers or just a portion of the subcarriers. In the case of providing information on only a portion of the subcarriers, an SS may provide a list of ordered subcarriers starting with those subcarriers which the subscriber desires to use, usually because performance is good or better than that of other subcarriers. The subchannels which comprise this list are decoded by demodulators  140  and  160  and the ordered list is provided by processors  141  and  161 , respectively. 
     Upon receiving the information from the SS, BS  12  further selects, for example, under control of processor  140 , the subcarriers among the candidates, utilizing additional information available at BS  12 , e.g., the traffic load information on each subcarrier, amount of traffic requests queued at BS  12  for each frequency band, whether frequency bands are overused, and/or how long a SS has been waiting to send information. BS  12  can use this information in subcarrier allocation to reduce inter-cell interference. 
       FIG. 2  shows flow diagram  20  which is one embodiment of a process for allocation clusters to SSs. The process is performed by processing logic that may comprise hardware (e.g., dedicated logic, circuitry, etc.), software (such as that which runs on, for example, a general purpose computer system or a dedicated machine), or a combination of both. 
     Referring to  FIG. 2 , processor  201  periodically broadcasts pilot OFDM symbols (for example from BS  12 ) to SS  14  and SS  16  within its cell (or sector). The pilot symbols, often referred to as a sounding sequence or signal, are known to both BS  12  and SSs  14 ,  16 . The pilot symbols can serve multiple purposes: time and frequency synchronization, channel estimation and signal-to-interference/noise (SINR) ratio measurement for cluster allocation. BS  12  broadcasts the OFDM symbol as signal  17  and time-delayed versions as signals  18 ,  19 . 
     Process  202  monitors the reception of the pilot symbols from each SS and measures the SINR and/or other parameters, including inter-cell interference and intra-cell traffic, of each cluster. Each SS encounters multi-path distortion created by the time-delayed transmission of the pilot symbols. 
     Process  203  estimates the frequency domain channel response, for example, by using a minimum mean square error (MMSE) channel estimator, to yield diversity gain information. 
     Based on this information, each SS, via process  204 , selects one or more clusters with good performance (e.g., high SINR and low traffic loading) relative to each other and feeds back the information on these candidate clusters to BS  12  through predefined uplink access channels. For example, SINR values higher than 10 dB may indicate good performance. Likewise, a cluster utilization factor less than 50% may be indicative of good performance. Each subscriber selects the clusters with relatively better performance than others. The selection results in each subscriber selecting clusters they would prefer to use based on the measured parameters. 
     Upon receiving the feedback from a SS, process  205  at BS  12  selects one or more clusters for the subscriber among the candidates. BS  12  may utilize additional information available at the base station, e.g., the traffic load information on each subcarrier, amount of traffic requests queued at the base station for each frequency band, whether frequency bands are overused, and how long a subscriber has been waiting to send information. The subcarrier loading information of neighboring cells can also be exchanged between base stations. The base stations can use this information in subcarrier allocation to reduce inter-cell interference. 
     Process  206  causes BS  12  to notify each SS about the cluster allocation through a downlink common control channel or through a dedicated downlink traffic channel if the connection to the subscriber has already been established. In one embodiment, BS  12  also informs the SS about the appropriate modulation/coding rates. Once the basic communication link is established, each SS can continue to send feedback to BS  12  using a dedicated traffic channel (e.g., one or more predefined uplink access channels). 
     Process  207  determines if retraining is necessary and, as needed, causes the process  20  to repeat. The retraining may be performed periodically. This retraining compensates for SS movement and any changes in interference. In one embodiment, each SS reports to BS  12  its updated selection of clusters. Then BS  12  further performs the reselection and informs the SS about the new cluster allocation. Retraining can be initiated by BS  12 , and in which case, BS  12  requests a specific subscriber to report its updated cluster selection. Retraining can also be initiated by the SS when it observes channel deterioration. 
       FIG. 3  illustrates one embodiment  30  of subscriber processing. The processing is performed by processing logic that may comprise hardware (e.g., dedicated logic, circuitry, etc.), software (such as that which runs on, for example, a general purpose computer system or dedicated machine such as processor  141  of  FIG. 1 ), or a combination of both. 
     Referring to  FIG. 3 , channel processor  301  performs channel estimation in pilot periods in response to pilot symbols. Channel processor  301  resolves multi-path distortion of the pilot symbols created, at least in part, by transmission of time-delayed versions of the pilot symbols by the BS antennae. Channel processor  301  uses the multi-path distortion information to determine diversity gain. A standard OFDM channel estimator may be used, such as a common 2D MMSE. To exploit the diversity gain, the SS only needs to calculate the signal to noise ratio (SNR) of each subcarrier, and report the subcarriers with the highest SNR to the BS. 
     Cluster selector  303  is coupled to channel processor  301  to perform cluster selection, generally based on SNR. The output of cluster selector  303  is input to cluster request processor  305 , which requests clusters and modulation/coding rates. The number of clusters selected may vary from 1 to the total number available. Indications of these selections are sent to BS  12  ( FIG. 1 ). The information is used for cluster selection to avoid clusters with heavy intra-cell traffic loading and/or strong interference from other cells. That is, a new subscriber may not be allocated use of a particular cluster if heavy intra-cell traffic loading already exists with respect to that cluster. Also, clusters may not be allocated if the interference is so strong that the SINR only allows for low-rate transmission or no reliable transmission at all. 
     Delay diversity may be used to increase frequency diversity of an orthogonal frequency division multiplexing access (OFDMA) network. In such a network, the increased frequency diversity can be used with a Forward Error Correction (FEC) code to lower Bit Error Rate (BER) or Frame Error Rate (FER) or both. 
       FIG. 4  illustrates selective frequency assignments of a OFDMA network yielding improved network throughput. By exploiting the selective fading characteristics of User #1 and #2 channels, an OFDMA network can allocate channels to SSs with the highest SNR. In one embodiment, a maximum time delay for signals  18  or  19  can be determined so that inter-symbol interference in an OFDMA network can be minimized by selecting time delays which are less than the determined maximum time delay. 
     The intentionally introduced delay should not exceed the designed cyclic prefix (CP) of the system. To maintain protection against multi-path delay, a fraction of the CP duration, perhaps ⅛ of the CP might be choose as the intentional delay. For example, in typical WiMAX systems, the CP is designed to tolerate multi-path delay of up to 8 microseconds (μs). Thus, 1 could provide sufficient frequency selectivity while retaining most of the protection against multi-path delay. The effective CP power would be ⅞ of the power without a delay. Additionally, the introduced delay may be slowly changed over time, and can be different among different cells or sectors. By changing delay pattern, the resulted frequency selectivity is also changing, so that traffic allocation may be more equally spread among multiple subcarriers. This results in each SS being served more fairly. Also, by using different delay patterns among various BSs, the inter-cell interference can be reduced since. This is because interfering signals from neighboring cells may experience fading and the intended signal, from the serving BS, may have a peak response, due to delay pattern differences. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.