Abstract:
Systems and methods for reducing macro-femto bas station interference are disclosed. In one aspect, macro-femto interference is reduced by configuring t macro bases station to avoid using resources allocat to the femto base station.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to wireless communication systems and method. 
       BACKGROUND 
       [0002]    Some mobile network operators are investigating the possibility of providing home and/or small area coverage for a limited number of users using a small base station, commonly called a “femto” base station (or femto NodeB for WCDMA or femto eNodeB (E-UTRAN NodeB) for long term evolution (LTE)). Other common names are Home NodeB (HNB) for WCDMA or Home eNodeB (HeNB) for LTE. 
         [0003]    A femto base station may provide normal LTE/WCDMA coverage for end users in a so called femto cell, and may be connected to the mobile operator&#39;s network using some kind of IP based transmission. One alternative is to use fixed broadband access (e.g. xDSL or Cable) to connect the femto base station to the network. 
         [0004]    In some systems (e.g., LTE systems) the coverage area (i.e., femto cell) of a femto base station may overlap with the cell of a large base station, commonly called a “macro” base station, and the femto base station may use the same frequency spectrum (or part of the same frequency spectrum) as the macro base station. Accordingly, at times, the users of the femto and/or macro base station may experience uplink and/or downlink interference. This interference is referred to as “macro-femto interference”. In this document, for purposes of brevity, the overlaying cellular network is always referred as “macro” layer, even though it may consist of both macro, micro and pico cells. 
         [0005]    Thus, there exists a need to reduce this macro-femto interference. 
       SUMMARY 
       [0006]    In one aspect, the patent provides a method performed by a first base station (e.g., a macro base station) for reducing interference in a communication system comprising the macro base station and a second base station (e.g., a femto base station) located within or near a cell serviced by the macro base station, wherein the macro base station is allocated a frequency spectrum and the femto base station is allocated a frequency spectrum that at least partially overlaps with the frequency spectrum allocated to the macro base station. In some embodiments, the method performed by the macro base station includes: (a) determining a performance metric; (b) comparing the determined performance metric to a predetermined threshold; and (c) determining whether to assign to a user terminal a resource block that is (i) within the frequency spectrum allocated to the femto base station or (ii) within a time slot allocated to the femto base station, characterized in that this determination is based, at least in part, on the result of the comparison of the performance metric with the predetermined threshold. 
         [0007]    The bandwidth of the frequency spectrum allocated to the femto base station may be smaller than the bandwidth of the frequency spectrum allocated to the macro base station and the frequency spectrum allocated to the femto base station may completely fall within the frequency spectrum allocated to the macro base station. 
         [0008]    In some embodiments, at least a portion of the frequency spectrum allocated to the macro base station does not overlap with the frequency spectrum allocated to the femto base station, the performance metric corresponds to an amount of the non-overlapping frequency spectrum that is currently available for use, and the step of determining whether to assign to the user terminal a resource block that is within the frequency spectrum allocated to the femto base station comprises determining whether the amount of the non-overlapping frequency spectrum that is currently available for use is (a) greater than the predetermined threshold or (b) greater than or equal to the predetermined threshold. The macro base station may assign to the user terminal a resource block that is within the frequency spectrum allocated to the femto base station if, and only if, the amount of the non-overlapping frequency spectrum that is currently available for use is (a) less than the threshold or (b) less than or equal to the threshold. 
         [0009]    In some embodiments, steps (a) through (c) are performed in response to receiving a request from a UE. Also, in some embodiments, the macro base station or another node in the network (e.g., a gateway node) may control an active frequency spectrum of the femto base station based, at least in part, on a resource need of the macro base station. 
         [0010]    In some embodiments, the above mentioned performance metric may correspond to the number of available resource blocks that are not within the frequency spectrum allocated to the femto base station, and the step of determining whether to assign to the user terminal a resource block that is within the frequency spectrum allocated to the femto base station comprises determining whether the number of available resource blocks is (a) less than the predetermined threshold or (b) less than or equal to the predetermined threshold. 
         [0011]    In another aspect, the invention provides a communication system in which interference (e.g., macro-femto interference) is reduced. In some embodiments, the communication system includes: a first base station (e.g., a macro base station) servicing a first coverage area, the macro base station being allocated a frequency spectrum for servicing the first coverage area; and a second base station (e.g., a femto base station) servicing a second coverage area, the femto base station being allocated a frequency spectrum for servicing the second coverage area, wherein the second coverage area at least partially overlaps with the first coverage area, the frequency spectrum allocated to the femto base station at least partially overlaps with the frequency spectrum allocated to the macro base station, and the macro base station is configured to avoid using (i) a portion of its allocated frequency spectrum that overlaps with frequency spectrum allocated to the femto base station and/or (ii) time slots allocated to the femto base station, unless performance reasons require the use of one or more of the time slots and/or frequencies allocated to the femto base station. 
         [0012]    In some embodiments, the bandwidth of an active frequency spectrum of the femto base station is controlled based, at least in part, on (1) a capacity need of the femto base station and/or (2) a capacity need of the macro base station. For example, the bandwidth of the active frequency spectrum of the femto base station may be increased upon a determination that (1) the capacity need of the femto base station meets or exceeds a threshold and/or (2) the capacity need of the macro base station is less than or equal to a threshold. The macro base station may be configured to avoid utilizing only the frequencies that fall within the active frequency spectrum of the femto base station. Accordingly, the macro base station may include a storage unit that stores information identifying the femto base station&#39;s active frequency spectrum. 
         [0013]    In some embodiments, the macro base station is further configured to: (a) determine a performance metric; (b) compare the determined performance metric to a predetermined threshold; and (c) determine whether to assign to a user terminal a resource block that falls within frequency spectrum allocated to the femto base station, characterized in that this determination is based, at least in part, on the result of the comparison of the performance metric with the predetermined threshold. The performance metric may correspond to the amount of that portion of the frequency spectrum allocated to the macro base station that does not overlap with the frequency spectrum allocated to the femto base station that is currently available for use 
         [0014]    In still another aspect, the invention provides a base station that is configured to reduce the interference (e.g., macro-femto interference). In some embodiments, the base station includes means for reducing interference, characterized in that the means for reducing the interference comprises resource avoiding means for avoiding using only certain resources unless performance reasons require the use of one or more of the certain resources, wherein the certain resources include: (i) frequencies within a frequency spectrum allocated to a second base station and/or (ii) time slots allocated to the second base station. 
         [0015]    The resource avoiding means may include means for: (a) determining a performance metric; (b) comparing the determined performance metric to a predetermined threshold; and (c) determining whether to assign to a user terminal a resource block that is (i) within the frequency spectrum allocated to the second base station or (ii) within a time slot allocated to the second base station, characterized in that this determination is based, at least in part, on the result of the comparison of the performance metric with the predetermined threshold. The determination may be further based on one more downlink CQI values received from the user terminal and/or one or more uplink CQI values estimated by the base station. The performance metric may correspond to a current amount of frequency spectrum that is available for the macro base station to assign to user terminals. 
         [0016]    The above and other aspects and embodiments are described below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. 
           [0018]      FIG. 1  illustrates a system according to an embodiment of the invention. 
           [0019]      FIG. 2  illustrates different resource blocks. 
           [0020]      FIG. 3  illustrates an example frequency spectrum allocation according to an embodiment. 
           [0021]      FIG. 4  illustrates an example frequency spectrum allocation according to another embodiment. 
           [0022]      FIG. 5  is a flow chart illustrating a process according to an embodiment of the invention. 
           [0023]      FIG. 6  is a flow chart illustrating a process according to an embodiment of the invention. 
           [0024]      FIG. 7  is a functional block diagram of a macro base station according to some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring to  FIG. 1 ,  FIG. 1  illustrates a system  100  according to some embodiments of the invention. As illustrated in  FIG. 1 , system  100  includes a macro base station  102  having a coverage area  103  and a femto base station  104  having a coverage area  105 . As further illustrated, at least a portion of coverage area  105  (or “cell  105 ”) is within cell  103 . 
         [0026]    Macro base station  102  may be configured to communicate with user equipment (UE)  110  (e.g., UE  110   b  and/or  110   c ) using a frequency spectrum having a certain bandwidth. For example, base station  102  may be configured to communicate with UEs  110  using a frequency spectrum having a bandwidth of 20 MHz (e.g., a frequency spectrum of 700 MHz to 720 MHz). Macro base station  102  may be configured to allocate resource blocks (RBs) to a UE (e.g., UE  110   c ) for downlink and uplink communications, as illustrated in  FIG. 2 . In the exemplary  FIG. 2 , the differently shaded blocks represent different UEs and their allocated RBs in the frequency and time domains. Accordingly, an RB may have a frequency component and a time component. RBs may be allocated to a UE based on signal quality. For example, a UE  110  may report to macro base station  102  a channel quality indicator (CQI) value that represents the quality of a downlink signal. Similarly, base station  102  may estimate a CQI value that represents the quality of an uplink signal received from UE  110 . Base station  102  can use this information to avoid scheduling the UE  110  on RBs that experience bad quality. 
         [0027]    The maximum number of RBs in the frequency domain is dependent on the bandwidth of the available frequency spectrum. For example, with a bandwidth of 20 MHz, the maximum number of RBs in the frequency domain is 100, and with a bandwidth of 10 MHz the maximum number of RBs in the frequency domain is 50. 
         [0028]    Like macro base station  102 , femto base station  104  may be configured to communicate with user equipment (UE)  110  (e.g., UE  110   a ) using a frequency spectrum having a certain bandwidth. For example, femto base station  104  may be configured to communicate with UEs  110  using a frequency spectrum having a bandwidth of 5 MHz (e.g., a frequency spectrum of 715 MHz to 720 MHz). Also, like macro base station  102 , femto base station  104  allocates resource blocks to UEs. 
         [0029]    In those cases where the frequency spectrum used by the femto base station at least partially overlaps with the frequency spectrum used by macro base station  102 , there is a relatively large probability of macro-femto interference. The present invention aims to reduce this interference. In one aspect, the interference is reduced by dynamically using the spectrum available. 
         [0030]    For the sake of illustration, we shall describe an embodiment of the invention in the case where macro base station  102  is allocated a downlink frequency spectrum of 700 MHz-720 MHz, and femto base station is allocated a downlink frequency spectrum of 715 MHz-720 MHz (see  FIG. 3 ). Accordingly, in this example, the bandwidth of the spectrum allocated to femto base station  104  is less than the bandwidth of the spectrum allocated to macro base station  102 , and the spectrum allocated to femto base station  104  completely falls within the spectrum allocated to base station  102 . 
         [0031]    In this scenario, macro base station  102  is configured to avoid using the overlapping part of the spectrum (i.e., 715 MHz to 720 MHz). By doing so, the macro base station  102  effectively limits the downlink interference towards underlying femto base stations  104 . 
         [0032]    Macro base station  102  may be configured to avoid using the overlapping part of the spectrum (i.e., the portion of its spectrum that is shared with femto) by configuring macro base station  102  to use such shared portion if, and only if, certain criteria are satisfied. For example, macro base station  102  may be configured to use the shared portion of its spectrum if, and only if, the amount of the non-shared frequency spectrum (e.g., the number of RBs available from the non-shared portion of its spectrum—700 MHz to 715 MHz, in the example shown) is less than or equal to a threshold. For instance, if there are no RBs available from the non-shared portion of the spectrum, then macro base station  102  may use an RB that falls within the shared portion of the spectrum. In such an embodiment, macro base station  102  must know what portion of its spectrum is shared with femto. Accordingly, information identifying the spectrum allocated to femto base station  104  may be stored in a storage unit accessible to macro base station  102 . 
         [0033]    Referring now to  FIG. 4 ,  FIG. 4  illustrates an alternative frequency spectrum allocation between macro base station  102  and femto base station  104 . As in the previous embodiment, macro base station  102  may be allocated a 20 MHz bandwidth frequency spectrum (e.g., 700 MHz to 720 MHz). But in this embodiment, femto base station  104  may be allocated a bandwidth of 10 MHz (e.g., 710 MHz to 720 MHz). Additionally, in this embodiment, femto base station  104  can be dynamically configured to use less than all of the bandwidth allocated to it. That is, femto base station  104  may be configured to not use a portion of its allocated spectrum. The portion of the total allocated spectrum that femto base station  104  is configured to use is referred to as the “active” spectrum. As a specific example, at time t=0, femto base station  104  may have been allocated a frequency spectrum of 710-720 MHz, but also may have been configured to use only the frequency range 712.5-720, which is referred to as the active spectrum. 
         [0034]    In this embodiment, to reduce interference, macro base station  102  is configured to avoid using the portion of the femto base station  104 &#39;s active spectrum that overlaps with macro base station&#39;s spectrum. In the specific example shown in  FIG. 4 , macro base station  102  is configured to avoid using RBs that fall within the 712.5-720 MHz spectrum. 
         [0035]    As before, macro base station  102  may be configured to avoid using the portion of the femto base station  104 &#39;s active spectrum that overlaps with the macro base station&#39;s spectrum (i.e., the portion of its spectrum that is actively shared with femto) by configuring macro base station  102  to use such actively shared portion if, and only if, certain criteria (e.g., performance criteria) are satisfied. For example, macro base station  102  may be configured to use the actively shared portion of its spectrum if, and only if, there are no RBs available from the non-shared portion of its spectrum (i.e., 700 MHz to 712.5 MHz, in the example shown). In such an embodiment, macro base station  102  must know what portion of its spectrum is actively shared with femto. Accordingly, information identifying the femto&#39;s active spectrum may be stored in a storage unit accessible to macro. 
         [0036]    An advantage of this embodiment is that the femto&#39;s active spectrum may change over time to accommodate changes in load. For example, the femto&#39;s active spectrum may change based on capacity needs of femto base station  104  and/or macro base station  102 . For instance, if the capacity need of macro base station  102  increases greatly, macro base station  102  (or another node) may instruct femto base station  104  to reduce its active spectrum. In order for macro base station  102  to avoid using femto&#39;s active spectrum, macro base station  102  should be informed of changes to the femto&#39;s active spectrum (e.g., via an interface between base stations, in LTE this could be via the X2 interface) so it can store and use this information to avoid using RBs that fall within the femto&#39;s active spectrum. 
         [0037]    When macro base station  102  uses the portion of its allocated spectrum that is shared with femto base station  104 , macro base station  102  may select the UEs to which RBs from the overlapping part of the spectrum are allocated based on downlink CQI values received from the UEs and uplink CQI values estimated by macro base station  102 . For example, when macro base station  102  receives from a UE a bad downlink CQI value for an RB that is within the portion of macro&#39;s spectrum that is shared with femto base station  104 , macro base station  102  will not allocate to the UE RBs from the overlapping part of the spectrum because, based on the CQI, it is likely the UE is close to a femto base station  104 . 
         [0038]    Referring now to  FIG. 5 ,  FIG. 5  is a flow chart illustrating a process  500  according to an embodiment of the invention. Process  500  may begin in step  502 , where a frequency spectrum is allocated to macro base station  102  (e.g., 700 MHz-720 MHz). In step  504 , a frequency spectrum is allocated to femto base station  104  (e.g., 715 MHz-720 MHz). Additionally or alternatively, time slots may be allocated to femto base station  104 . In step  506 , macro base station  102  stores in a storage unit (e.g., a database or file system) information identifying the frequency spectrum and/or time slots allocated to femto base station  104 . In step  508 , macro base station  102  is configured to avoid using (a) the frequency spectrum and/or (b) time slots allocated to femto base station  104 . For example, as discussed above, macro base station  102  may be configured to avoid using the overlapping part of the spectrum (i.e., the portion of its spectrum that is shared with femto) by configuring macro base station  102  to use such shared portion if, and only if, certain criteria (e.g., performance criteria) are satisfied. As a specific example, macro base station  102  may be configured to use the shared portion of its spectrum if, and only if, the number of RBs available from the non-shared portion of its spectrum is less than or equal to a threshold. For instance, if there are no RBs available from the non-shared portion of the spectrum, then macro base station  102  may use an RB that falls within the shared portion of the spectrum. 
         [0039]    Referring now to  FIG. 6 ,  FIG. 6  is a flow chart illustrating a process  600  according to an embodiment of the invention. Process  600  may begin in step  602 , where macro base station  102  receives a request from a UE for an RB. In response, macro base station  102  (a) determines a performance metric (e.g., determines the number RBs from the portion of its frequency spectrum not shared with femto) (step  604 ), (b) compares the performance metric to a predetermined threshold (e.g., compares the number of RBs determined in step  604  to the threshold value) (step  606 ); and then decides whether to use a resource (e.g., frequency or time slot) allocated to femto base station  104  based on a result of the comparison (step  608 ). Accordingly, based on the result of the comparison, macro base station  102  performs either step  610  or  612 . In step  610 , macro base station  102  assigns to the UE a resource block that is not within the frequency spectrum and/or time slot assigned to femto, and in step  612 , assigns to the UE a resource block that is within the frequency spectrum and/or time slot assigned to femto base station  104 . 
         [0040]    Although many of the examples used the downlink spectrum allocation to illustrate the invention, the same principles are valid for the uplink direction of communication. 
         [0041]    In an alternative embodiment, rather than configuring macro base station  102  to avoid using resources (e.g., time slots and/or frequency spectrum) that are used by femto, macro base station  102  may be configured so that certain RBs (e.g., RBs from the shared portion of the spectrum) are transmitted with a reduced power. 
         [0042]    Referring now to  FIG. 7 ,  FIG. 7  is a functional block diagram illustrating a macro base station  102  according to some embodiments. As shown in  FIG. 7 , macro base station  102  may include a transmitting and receiving circuit  711  for transmitting data to and receiving data from a UE; a storage unit  704  (e.g., a non-volatile data storage) that stores software  708  for implementing the functions and features described above; and a processor  706  (e.g., a microprocessor) for executing software  708 . Storage unit  704  may also store information  709  identifying a resource (e.g., a frequency spectrum and/or time slots) allocated to each femto base station  104  that is located within the coverage area of macro base station  102 . 
         [0043]    While the processes described herein have been illustrated as a series or sequence of steps, the steps need not necessarily be performed in the order described, unless explicitly indicated otherwise. 
         [0044]    Further, while various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments.