Patent Publication Number: US-8526407-B2

Title: Extended coordinated multipoint cells to mitigate inter-comp-cell downlink interference

Description:
TECHNICAL FIELD 
     The present invention relates generally to wireless cellular communications, and in particular to a system and method of mitigating inter-cell interference by increasing the extent of Coordinated Multi-Point cells. 
     BACKGROUND 
     Wireless cellular communication networks are nearly ubiquitous, and provide mobile voice and data communications to millions of subscribers. In a cellular network, a fixed transceiver (base station, NodeB, etc.) provides two-way radio communications with a plurality of subscribers within a geographic area, or cell (as used herein, the term sector is synonymous with cell). Method of suppressing intra-cell radio interference, such as time-division, frequency-division, and code-division multiplexing, and combinations thereof, are known in the art. Additionally, methods of suppressing inter-cell interference, such as frequency reuse patterns, are known in the art. In modern wireless cellular communication networks, inter-cell interference remains the dominant source of performance impairment, restricting data rates, system capacity, and the quality of delivered communication services. 
     Coordinated Multi-Point (CoMP) transmission is a technology to minimize inter-cell interference. A plurality of geographically contiguous cells—referred to as sub-cells—are grouped together to form a CoMP cell. Each CoMP cell has a central controller that coordinates transmission within its constituent sub-cells so as to minimize inter-cell interference within the CoMP cell (referred to herein as inter-sub-cell interference, or intra-CoMP cell interference). The CoMP cell controller minimizes inter-sub-cell interference by coordinating scheduling of transmissions to user equipment (UE) within the cells, and/or actively suppressing interference using signal processing techniques. 
     Although the CoMP system can be effective in minimizing intra-CoMP cell interference, the inter-CoMP cell interference still exists along the joined areas between neighboring CoMP cells. 
     SUMMARY 
     According to one or more embodiments described and claimed herein, inter-CoMP cell interference is reduced by “extending” at least one CoMP cell to include one or more UEs served by a neighboring CoMP cell in the extended CoMP cell&#39;s transmission calculation, so as to minimize interference to the UEs served by other CoMP cells. Each UE in a border sub-cell identifies neighboring CoMP cells from which it receives interference in excess of a threshold value, and includes the interfering CoMP cells in a close-neighbor set. The close-neighbor set is transmitted to the UE&#39;s serving CoMP cell controller. When downlink transmissions are scheduled to the target UE, the controller notifies the neighboring CoMP cells in the close-neighbor set, identifying the target UE. The neighboring CoMP cells then use information about the channel conditions from their transmit antennas to the target UE receive antennas, and construct the transmissions to UEs they serve, with the constraint that interference to the target UE is below a predetermined level. This process occurs reciprocally among all CoMP cells in a network. 
     One embodiment relates to a method of mitigating inter-CoMP cell interference by a CoMP cell controller managing transmissions in a plurality of sub-cells comprising a first CoMP cell. Information is received, from a controller of a second, neighboring CoMP cell, identifying a UE in a border sub-cell of the second CoMP cell scheduled to receive transmissions from the second CoMP cell, which UE also receives interference from antennas of the first CoMP cell. Using the information about the downlink channel to the UE in the border sub-cell of the second CoMP cell, transmissions are constructed to UEs in the first CoMP cell to maximize data rates received by the UEs in the first CoMP cell, subject to the constraint that interference experienced by the UE in the border sub-cell of the second CoMP cell, due to the constructed transmissions in the first CoMP cell, is below a predetermined level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional diagram of a wireless communication network comprising two Coordinated Multipoint (CoMP) cells. 
         FIG. 2  is a functional diagram of the network of  FIG. 1  wherein one CoMP cell is functionally extended to include a User Equipment in another CoMP cell. 
         FIG. 3  is a flow diagram of a method of interference reduction. 
         FIG. 4  is a graph of simulated intra-CoMP cell interference. 
         FIG. 5  is a functional diagram of the network of  FIG. 1  where in the two CoMP cells are dynamically defined, and overlap in terms of UEs and base stations. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a network  10  divided into two neighboring CoMP cells  12 ,  24 , of which only portions of each are depicted. A first CoMP cell  12  includes a controller  14  and fixed transceivers located, e.g., at base stations  16 ,  18 . The base stations  16 ,  18  (each of which may comprise a sub-cell) transmit downlink signals to User Equipment  20 ,  22  located in the CoMP cell  12 . As known in the art, controller  14  applies signal processing techniques, such as frequency-dependent beamforming, to construct the signal transmitted to UE  22 , where this signal is transmitted by the transceivers located at base stations  16  and  18 . The signal transmitted to UE  22  is constructed to maximize the data rate to targeted UE  22  while simultaneously keeping the interference caused to other UEs in the CoMP cell  12  below a certain threshold, thus reducing intra-CoMP-cell interference. The transmission to UE  20  in CoMP cell  12  is constructed analogously to maximize the data rate received by UE  20  while simultaneously keeping the interference caused to other UEs in CoMP cell  12  below a certain threshold. 
     A second CoMP cell  24  includes a controller  26  and fixed transceivers located, e.g., at base stations  28 ,  30 . The base stations  28 ,  30  transmit weighted downlink signals to UEs  32 ,  34  while keeping the interference to UEs in the CoMP cell  24  below a certain threshold. Note that UE  34  is proximate to CoMP cell  12 , and in particular is proximate to the UE  22  in the CoMP cell  12 . If the controller  14  of the CoMP cell  12  schedules transmissions to the UE  22  at the same time the controller  26  schedules transmissions to the UE  34  in CoMP cell  24 , the UE  34  will experience significant interference from the CoMP cell  12  transmissions to the UE  22 , since the controller  14  is unaware of the UE  32 , and cannot construct the transmissions at the base stations  16 ,  18  to mitigate interference the interference experienced by UE  32 . Similarly, transmissions scheduled to the UE  34  by the controller  26  will cause significant interference to the UE  22 . 
     According to one embodiment of the present invention, the CoMP cell  12  is “extended” to include the UE  34 , as depicted in  FIG. 2 , at least for the purpose of interference suppression. By including the UE  34  in its calculation for determining the transmissions to UEs in CoMP cell  12 , controller  14  may utilize known signal processing techniques, such as frequency-dependent beamforming, to maximize the data rate to targeted UEs  20  and  22 , while simultaneously suppressing interference caused to UE  34 , as well as possibly other UEs (not shown) in the CoMP cell  12 . In this manner, inter-CoMP cell interference to UE  34 , from CoMP cell  12 , may be minimized. This idea may be expanded to each UE  22 ,  34  in a border sub-cell that may experience interference from transmissions in a neighboring CoMP cell  24 ,  12 , as further explained herein. 
     The first step is to identify the UEs for which inter-CoMP cell interference is significant enough to warrant active mitigation. The UEs may most efficiently perform this analysis, and transmit their results to their respective CoMP cell controllers. Each UE (or at least, each UE in a border sub-cell) maintains a list of neighboring CoMP cells, and performs measurements of the path loss from each of the antennas belonging to each of its neighboring CoMP cells. For a neighboring CoMP cell “k,” the UE determines the minimum path loss to the antennas belonging to CoMP cell “k.” This minimum path loss is denoted by g_{k}. By convention, g — {0} denotes the minimum path loss to the CoMP cell that is serving this UE. A threshold value T is defined (in units of dB), such that a UE will be considered for intra-CoMP cell interference mitigation if there is at least one “k” greater than zero where 
     
       
         
           
             
               
                 
                   
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     The set of CoMP cells, i.e., the values of the index k, for which Eq. (1) is true, is referred to as the “close-neighbor set” for this UE. As an example, T=−3 dB would indicate that a UE will be considered for intra-CoMP cell interference mitigation if it measures a path loss to at least one non-serving CoMP cell that is within 3 dB of the path loss to its serving CoMP cell. The UE periodically transmits its close-neighbor set to its serving CoMP cell. 
     Each CoMP cell independently determines the subset of its UEs to which transmissions are scheduled in an upcoming predefined transmission duration, such as a frame, sub-frame, Transmission Time Interval (TTI), TMDA time slot, or the like. Of these, each CoMP cell then identifies the subset of its scheduled UEs that have a non-empty close-neighbor set—that is, the scheduled UEs in border sub-cells likely to experience interference from transmissions that may be scheduled in neighboring CoMP cells. The CoMP cell controller then transmits an identification of these scheduled UEs with non-empty close-neighbor sets to the relevant neighbor UEs, and receives similar identifications of UEs from the controllers of all neighboring CoMP cells. 
     For example, with reference to  FIG. 2 , the controller  26  of CoMP cell  24  determines that downlink transmissions will be scheduled, in a upcoming frame, to UEs  32  and  34  (and likely other UEs not shown and omitted from this discussion). Of these, only UE  34  reports a non-empty close-neighbor set to the controller  26 . Accordingly, the controller  26  transmits to the controller  14  of CoMP cell  12 , information identifying UE  34  and indicating that it is scheduled to receive downlink transmissions from CoMP cell  24  in an upcoming frame. 
     A method  100  of mitigating inter-CoMP cell interference by CoMP cell  12  is described with reference to  FIG. 3 . As described above, the controller  14  receives from CoMP cell  24  information identifying a UE  34  in a border sub-cell of a neighboring CoMP cell  24  scheduled to receive transmissions from the second CoMP cell  24 , which UE  34  also receives interference from antennas of the CoMP cell  12  (block  102 ). 
     To minimize interference to UE  34 , the controller  14  of CoMP cell  12  must have some information regarding the downlink channel from its transmit antennas to the receive antenna(s) of UE  34 . In one embodiment, with Time Division Duplexing (TDD) and assuming uplink-downlink channel reciprocity, this information can be obtained by transceivers in CoMP cell  12  receiving and analyzing the reference signals, also known as pilot signals, transmitted by UE  34 . In another embodiment, the controller  26  may direct the UE  34  to analyze reference symbols from CoMP cell  12  antennas and report channel conditions, which the controller  26  includes in the information it transmits to the controller  14  of CoMP cell  12 . In yet another embodiment, the UE  34  may report channel quality metrics directly to the controller  14 , via base stations  16  and  18 . In general, the controller  14  of the CoMP cell  12  may ascertain or estimate channel conditions between its transmit antennas and receive antennas of the UE  34  (block  104 ) in any manner, and based on any available information. 
     The controller  14  of CoMP cell  12 —armed with knowledge that downlink transmissions are scheduled to UE  34 , that the UE  34  sees interference from antennas in CoMP cell  12 , and information regarding the channel conditions from its transmitters to the UE  34  receiver—may include UE  34  in its global optimization for calculating what is transmitted from its antennas  16  and  18  to keep interference to UE  34  below a certain level. In particular, the controller  14  may construct transmissions to UEs  20  such that data rate to UEs  20  is maximized, subject to the constraint that interference to other UEs in CoMP cell  12  (e.g. UE  22 ) and to UE  34 , due to these transmissions, is below a predetermined level (block  106 ). In this manner, the CoMP cell  12  has “expanded” its coverage, at least in the sense that it includes a UE  34  served by a different CoMP cell  24  in its transmission construction. The method  100  repeats for every UE in CoMP cell  12 , and for every predetermined transmission duration (e.g., frame). 
     One algorithm for forming the weighted transmissions is the epsilon-forcing algorithm described by Kambiz Zangi, Dennis Hui, and Leonid Krasny, in U.S. patent application Ser. No. 12/404,773, titled, “System and Method for Coordinated Multipoint Downlink Transmissions,” filed on Mar. 16, 2009, assigned to the assignee of the present application, and incorporated herein by reference in its entirety. Other suitable methods are also known to those of skill in the art. Note that the interference suppression to UE  34  by CoMP cell  12  does not require the CoMP cell  12  controller  14  to have any knowledge of the data transmitted to UE  34  by CoMP cell  24  (e.g., data rate, modulation, etc.). In fact, a significant advantage of embodiments of the present invention is the small quantum of data that needs to be transmitted between CoMP cell controllers  14 ,  26 —basically, only the ID of affected UEs to which transmissions are scheduled, and in some cases, some channel information. 
     Although the embodiment discussed above has been explained with reference to CoMP cell  12  minimizing interference to a UE  34  served by CoMP cell  24 , in reality all CoMP cells  12 ,  24  in a network  10  will simultaneously minimize interference to all neighboring CoMP cell UEs which experience significant inter-CoMP-cell interference. For example, when the controller  26  transmits information to the controller  14  identifying UE  34  as a scheduled downlink recipient that has reported significant signal strength from CoMP cell  12  antennas, the controller  14  contemporaneously transmits to controller  26  corresponding information regarding, e.g., UE  22 . The controller  26  will then ascertain or estimate channel conditions from its transmit antennas to the UE  22  receiver antenna, and during the next downlink frame, will transmit signals from CoMP cell  24  antennas so as to minimize interference caused to UE  22 . 
     In embodiments of the present invention, the “expansion” of CoMP cell borders—at least for the purpose of interference mitigation—is UE-centric, since the identification of potentially interfering close-neighbor sets is performed by the UEs.  FIG. 5  depicts an embodiment of the present invention in which each UE defines own CoMP cell. That is, each UE defines the set of base stations that transmit downlink signals to it. 
     In  FIG. 5 , a dynamic CoMP cell for UE  22  comprises the base stations  16 ,  18 , and  30 , with an appropriately programmed processor at the base station  18  acting as the controller for this dynamic CoMP cell. Similarly, a dynamic CoMP cell for UE  32  comprises the base stations  28 ,  29 , and  30 , with the base station  28  serving as the CoMP cell controller. Note that base station  30  belongs to both the CoMP cell for UE  22  and the CoMP cell for UE  32 . Base station  16  is the serving base station for UE  20 ; base station  18  is the serving base station for UE  22 ; base station  30  is the serving base station for UE  34 ; base station  28  is the serving base station for UE  32 ; and base station  29  is serving base station for UE  33 . The downlink transmissions to UE  22  are transmitted from base stations  16 ,  18 , and  30 , and these transmissions would normally cause significant interference to the UE  32 , which is served by base station  28 . Note that base station  28  does not belong to the CoMP cell of UE  22 . 
     According to this embodiment, the transmissions to UE  22 , emanating from base stations  16 ,  18 , and  30 , are constructed in such a way to maximize the data rate received by UE  22  while simultaneously keeping the interference caused to UE  32  below a certain level. As described above, the base station  28  (or other CoMP cell controller for the UE  32  dynamic CoMP cell) must transmit to the controller of the dynamic CoMP cell for UE  22  (e.g., base station  18 ), that the base stations  16 ,  18 , and  30  are in its close-neighbor set, and that downlink transmissions are scheduled to UE  32  during an upcoming transmission duration. Those of skill in the art will readily recognize that transmissions to UE  32 , emanating from base stations  28 ,  29 , and  30 , can be constructed analogously to maximize the data rate received by user  32  while simultaneously keeping the interference caused to UE  22  below a certain level. 
       FIG. 4  graphs the results of system-level simulations comparing a simple CoMP system to an extended CoMP system according to embodiments of the present invention. Each CoMP cell comprises of sub-cells, and the threshold T=−3 dB. The simulated system has four transmit antennas per base station and two receive antennas per UE. As  FIG. 4  depicts, the extended CoMP system increased the bit rate in border sub-cells about 50% compared to the simple CoMP system, for the same system throughput. 
     According to embodiments of the present invention, inter-CoMP cell interference is reduced, with very low-bandwidth communication required between neighboring CoMP cells. In some embodiments, only an identification of a CoMP cell&#39;s own UEs to which downlink transmissions are scheduled, and which identified a target CoMP cell in its close-neighbor set, need be transmitted from one CoMP controller to another. In other embodiments, some channel condition information may also be shared between CoMP cell controllers. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.