Abstract:
A method of allocating communication resources for a communication between a transmitter and a receiver in a communication system, the communication resources being divided in time periods and frequency sub-bands. The transmitter receives a channel quality measurement sent by the receiver. The transmitter performs allocation of a first part of the communication resources to the receiver according to the channel quality measurement if allocation of the first part is selected, or allocation of a second part of the communication resources to the receiver if allocation of the first part is not selected. The transmitter informs the receiver of allocated communication resources, and the allocated communication resources being designated for frequency localized channels or for frequency distributed channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/955,895 filed on Dec. 13, 2007, which is a continuation of International Patent Application No. PCT/CN2005/000857 filed on Jun. 15, 2005. The afore-mentioned patent applications are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of radio communication systems, and in particular to a method and system for allocating communication resources, especially for packet-based, multi-user cellular communication systems. 
       BACKGROUND 
       [0003]    In packet-based, multi-user cellular communication systems, such as multi-user OFDM (Orthogonal Frequency Division Multiplexing) systems, a scheduler-device that makes decisions as to which user is assigned which radio resources and when is typically employed. From time to time, the users report the quality of their respective radio channels to the base station, whereupon the base station makes a scheduling decision. In uplink, the base station measures the channel quality, e.g., from pilot signals transmitted by the users. The scheduler may exploit the fact that the users&#39; channels change independently from each other, i.e., channels of one or more users may be fading, or, also, one or more channels allocated to a specific user may be fading, while others are not. Typically, a user is assigned radio resources when its channel conditions are good. Accordingly, the scheduler improves the performance of the system (in terms of cell throughput) as compared to systems that do not exploit the users&#39; channel quality through a scheduler. 
         [0004]    The extent to which the scheduler improves the system performance depends in downlink on the quality of the feedback information. Good, detailed and accurate feedback of the channel quality are necessary. There are situations, however, where such accurate and timely feedback is not possible. A user may, for instance, move at such a high speed that a channel quality measure is outdated and obsolete by the time it reaches the base station. Another example of unreliable feedback measures occurs when a cell-edge user has bad signal-to-noise ratio on the uplink feedback channel and the quality measure is simply detected erroneously at the base station. 
         [0005]    Scheduling may also be inefficient if the channel varies considerably in time during a transmission time interval, i.e. in case of high Doppler spread. For certain types of data scheduling may not be desirable, e.g. for data with low latency requirements and low data rates. Feedback data is a typical example of such kind of data. In this case, dedicated channels are more appropriate. 
         [0006]    For these situations, the system may provide a frequency-distributed channel in order to provide a high-diversity link-performance. Accordingly, users with reliable channel quality feedback and with data appropriate for scheduling are assigned radio resources when and where their respective channel conditions are known to be good, other users are assigned frequency-distributed channels. 
         [0007]    A problem, however, is how to provide both high link-diversity (frequency-distributed) channels and high multiuser-diversity (frequency-localized) channels at the same time in an efficient way. 
         [0008]    An attempt to solve this problem is disclosed in IEEE Std 802.16-2004, “Standard for Local and Metropolitan Area Networks”, Part 16: “Air Interface for Fixed Broadband Wireless Access Systems”, 2004, wherein the above problem has been solved through the use of so-called ‘zones’. A zone is a time period during which a certain type of channel is transmitted. Each radio frame contains two zones, one for the transmission of frequency-localized channels followed by one for frequency distributed channels in a pure time-multiplexing fashion. In the header of each radio frame information is conveyed as to when in time one zone changes into the next. 
         [0009]    A disadvantage with this solution, however, is that the link-diversity is limited. 
         [0010]    Accordingly, there is a need for a system and method with improved link-diversity. 
       SUMMARY 
       [0011]    Embodiments of the present invention provide a system and a method for allocating communication resources in a multi-user cellular communication system, which has an improved link-diversity as compared to the known prior art. 
         [0012]    In accordance with the embodiments of the present invention, communication resources are divided in periods of time and frequency sub-bands, wherein part of the communication resources are used for frequency-localized communication channels, and part of the communication resources are used for frequency distributed channels. The method includes the steps of:
       classifying part of said sub-bands as sub-bands carrying frequency-distributed channels, and   classifying part of said sub-bands as sub-bands carrying frequency-localized channels.       
 
         [0015]    This has an advantage that frequency-localized communication and frequency-distributed communication can be performed simultaneously and without interruption, since the potential delay associated with each of the two channel types, imposed by the ‘zone’-structure of the prior art, is eliminated. Further, since the present invention allows data to be transmitted continuously, the embodiments of the present invention have an advantage that link-diversity is improved because even if the signal quality is poor during part of a transmission frame, signal quality during the rest of the frame may be sufficient enough to ensure a correct transmission. Even further, the embodiments of the present invention have an advantage that there is no delay until the desired type of channel (localized or distributed) is available, since both types of channels always are available, which is a substantial advantage, in particular for packet data transmissions with demands for fast retransmissions. 
         [0016]    After performing the classification of the frequency sub-bands, the classification may be changed from time period to time period, after a certain number of time intervals or at predetermined intervals. Which sub-bands are of which type can be transmitted on a broadcast channel in the beginning of, or prior to, a time period. This has the advantage that a distribution of frequency-localized and frequency-distributed communication resources which optimises system throughput always can be used. 
         [0017]    A code representing a particular arrangement of the communication resources may be transmitted to the receiver, wherein the code may be used by the receiver to retrieve the communication resource scheme to be used. This has the advantage that rather complicated communication resource schemes may be communicated to the receivers without substantial signalling. 
         [0018]    The classification of the frequency sub-bands may be kept from time period to time period. This has the advantage that the system may be standardized, i.e. the sub-bands may always be used for a specific kind of communication. 
         [0019]    The frequency-distributed channels may consist of frequency hopping channels, time or code multiplexed channels, or interleaved frequency multiplexed channels. This has the advantage that link-diversity of the frequency-distributed channels may be increased even further. 
         [0020]    The embodiments of the present invention also relate to a transmitter and a multi-user communication system. 
         [0021]    Further advantages and features of the embodiments of the present invention will be disclosed in the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  shows a prior art method of transmitting frequency-localised and frequency-distributed communication. 
           [0023]      FIG. 2  shows a communication resource scheme suitable for use with the embodiments of the present invention. 
           [0024]      FIG. 3  shows an exemplary embodiment of the present invention. 
           [0025]      FIG. 4  shows another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0026]    As described above, communication in a packet-based multi-user communication system can be performed using frequency-localized channels, i.e. channels which are assigned to users based on channel quality measurements. As also is stated above, in certain situations, such as when a channel quality measure is outdated and obsolete by the time it reaches the base station due to a fast moving user, or when communicating data with low latency requirements and low data rates, frequency-localized communication may be undesirable. In such cases, communication using frequency-distributed channels may be preferable. 
         [0027]    Accordingly, there is a need for a system utilizing both types of communication. 
         [0028]    When using frequency-localized and frequency-distributed channels, the channels must be complementary (i.e., they must use disjoint resources) and yet as many of the physical radio resources as possible must be assigned to users (resources must not be unused). 
         [0029]      FIG. 1  shows a prior art solution. As can be seen in the figure, the frequency band is divided into sub-bands 10a-10j, each consisting of a number of sub-carriers (not shown). Further, the allocation of the sub-bands are divided into transmission frames, of which one is shown, each consisting of a certain period of time, divided into time slots TS1-TS8, which in turn are divided into a number of symbols. When it is desired to have both frequency-localized and frequency-distributed communication, each frame is divided into two zones, of which the first zone Z1 is used for the frequency-localized channels followed by a second zone Z2 for frequency distributed channels in a pure time-multiplexing fashion. As stated above, information as to when one zone changes into the next is conveyed in the header of each radio frame. As stated above, a disadvantage with this solution is that frequency-localized communication and frequency-distributed communication cannot be performed simultaneously. Further, a delay associated with each of the two channel types is imposed by the ‘zone ’-structure. Even further, time diversity of the frequency distributed channels, is degraded since these channels are limited to only one of the zones Z1, Z2. 
         [0030]    In  FIG. 2  is shown a communication resource scheme suitable for use with the embodiments of the present invention. The disclosed system is a multiple-carrier OFDM system, having a two-dimensional structure (time and frequency). The frequency spectrum of the OFDM system is divided into ten sub-bands 20a-20j, preferably constituting equal portions of the frequency spectrum. Equal frequency sub-bands are preferred to facilitate resource management (for example, it is easier to allocate the available resources). However, division into non-equal frequency sub-bands is, of course, also possible. Each sub-band is divided into a number of sub-carriers, for example, each sub-band may consist of 20 sub-carriers (not shown), however, sub-bands consisting of any number of sub-carriers are possible, e.g., 1, 5, 100 or any other number. 
         [0031]    In the time domain, the frequency spectrum is divided into time-slots, which typically has the length of a number of OFDM symbols. The figure shows one time slot consisting of eight OFDM symbols S1-S8. The frequency/time spectrum thus constitutes a communication resource scheme, wherein, the smallest resource allocated to a user is one sub-band during one OFDM symbol (for frequency-distributed communication, as will be described below). 
         [0032]    Instead of, as in the prior art, divide a transmission frame in different time zones, which are used for frequency-localized and frequency-distributed communication, respectively, the communication resource scheme is divided in frequency. 
         [0033]    In this exemplary embodiment, the sub-bands 20a, 20c, 20e, 20g, 20i are used for frequency-localized communication, while the sub-bands 20b, 20d, 20f, 20h, 20j are used for frequency-distributed communication. Further, as can be seen in the figure, there are three users UE1-UE3 communicating on the frequency-localized channels (UE1 on sub-band 20e; UE2 on 20c and 20i; and UE3 on 20a and 20g), and two users UE4, UE5 communicating on the frequency-distributed channels. Accordingly, the embodiments of the present invention allow that all users may benefit from continuous data transmission, irrespective of which type of communication that is utilized. Further, the continuous transmission has the advantage that data transmission using frequency-distributed channels are spread all over time slot and thus also all over a frame, and not just part of it, which improves time diversity of the channel. Further, the embodiments of the present invention have an advantage that it increases the throughput in the system, since, for example, frequency-localised communication can always be performed, which, in turn, allows communication with users when they have a good channel quality, irrespective of when in, e.g., a transmission frame. 
         [0034]    In use, a scheduler is used to multiplex the frequency-localized channels onto sub-bands determined to be used for frequency-localized channels, and frequency-distributed channels are multiplexed onto sub-bands to be used for frequency-distributed channels. 
         [0035]    As is obvious to a person skilled in the art, any arrangement of the frequency-localized and frequency distributed channels may be used. In the example shown in  FIG. 1 , each type of communication is allocated half the resources. As is obvious to a person skilled in the art, however, the distribution of frequency-localized and frequency distributed channels is arbitrary, as long as at least one sub-band is used for either of the two types of communication. For example, if traffic in the cell varies in time, the channel distribution may vary in time as well. Preferably, however, the channel distribution is not changed too rapidly, i.e. a number of frames in a row utilizes the same distribution, as this substantially reduces signalling in the system. The base station may transmit, on a broadcast channel, which sub-bands are of which type, e.g. each time slot or each time there is a change in channel distribution. As is common in a system utilizing frequency localized channels, a scheduler makes decisions as to which user is assigned which frequency localized channels, whereupon this information is fed forward over a control channel to the users. Also, data regarding which user is assigned which frequency-distributed channels is communicated. 
         [0036]    Data transmission on the frequency distributed channels may use various techniques for increasing diversity further. For example, as is shown in  FIG. 2  for users UE4 and UE5, frequency hopping may be utilized. In this case, the base station and the users employ an algorithm to obtain the particular frequency hopping sequence for a frequency hopping channel. Further, a user may be allocated two or more sub-bands for frequency-distributed communication. 
         [0037]    Even further, in one embodiment of the present invention, there are one or more predefined channel resource schemes programmed in the base station and the receivers. In this way, the base station can transmit a code representing which scheme to use, e.g. on a broadcast channel, whereupon the receiver can use a look-up table to obtain the channel arrangement of the particular scheme. Each time the channel resource scheme is changed, the base station transmits the code representing the new scheme. This has an advantage that this kind of signalling is kept to a minimum. In an alternative embodiment, the base station may signal which frequency sub-bands that are to be used for which kind of signalling. This may be effected, e.g., at predetermined intervals, and/or each time the category (frequency localized or frequency distributed) of a sub-band is changed. As an even further alternative, the communication system standard may comprise only one configuration, which, accordingly always is used and thus has the advantage that no signalling regarding the communication resource scheme arrangement is necessary. 
         [0038]    The use of frequency hopping algorithms may, as disclosed above, be limited to those sub-bands that are used for frequency-distributed channels. However, in a system consisting of a plurality of base stations, it is often preferred to utilize frequency hopping patterns that in some way are optimised regarding to inter base station interference. If the employed frequency hopping patterns are generated based on the particular channel distribution of the base station, the frequency hopping patterns of neighbouring base stations, which utilizes different channel distributions, may disturb each other. Therefore, as is shown in the exemplary embodiment in  FIG. 3 , the frequency hopping algorithm may be generic in the sense that it applies to any classification of the sub-bands, however with the restriction that sub-bands for frequency localized channels may not be part of the generated hopping sequence, hence these bands are eliminated from the hopping patterns in case the generating algorithm makes them appear in the hopping sequence. 
         [0039]    Generic frequency-hopping sequences have the following advantages. It prioritizes the allocation of non-hopping (localized) channels, which heavily depends on the channel quality, allowing arbitrary allocation of the localized channels based on channel quality. Moreover, hopping patterns that are designed for limited mutual interference can be used without any modification, and with actual improvement of the inter-cell interference between the hopping patterns. This is illustrated in  FIG. 3 , wherein the hopping pattern for UE4 indicates that in TS2, sub-band 30a should be used, and in TS6, sub-band 30e should be used. In these time slots, however, according to the embodiments of the present invention, no data for UE4 is transmitted, but instead the frequency-localized data for UE3 in TS2, and UE1 in TS 6, is prioritised. 
         [0040]    This solution has the advantage that it reduces the downlink signalling, since the same frequency hopping patterns may be utilized irrespective of which channel distribution is utilized. Further, the balance between the needs for frequency-distributed channels and frequency-localized channels can vary between cells and, in a certain cell, over time. It is therefore desirable that different cells can employ different multiplexing configurations in order to efficiently serve the present users. For the same efficiency reason, it is desirable that the multiplexing configuration in a cell can change over time. 
         [0041]      FIG. 4  shows a further exemplary embodiment of the present invention. Here, the upper three sub-bands 40a-c are used for frequency-localized channels, while the lower three sub-bands 40d-f are used for frequency-distributed channels. In this embodiment, however, no frequency hopping is used, instead channel diversity is accomplished using interleaved frequency multiplexing, i.e., users UE4-UE6 each are allocated channels in each of the sub-bands 40d-f, i.e. each of the uses are allocated one or more sub-carriers in a sub-band. Further ways (not shown) to accomplish channel diversity includes time multiplexing and code multiplexing.