Patent Publication Number: US-9433013-B2

Title: Centralized-scheduling method and apparatus for inter-cell interference coordination in heterogeneous network

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119(a) to Korean Patent Application Serial No. 10-2012-0011970, which was filed on Feb. 6, 2012 in the Korean Intellectual Property Office, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a centralized scheduling method and apparatus for minimizing inter-cell interference in a heterogeneous network including a macro Base Station (BS) and lower power Local evolved Node Bs (LeNBs), and more particularly, to a centralized scheduling method and apparatus for minimizing inter-cell interference in the wireless communication system adopting femto-eNB-based service coverage expansion (range expansion). 
     2. Description of the Related Art 
     In order to increase the coverage and capacity of a macro BS-only network, extensive research has been conducted to deploy LeNBs, particularly in association with the 3 rd  Generation Partnership Project Long Term Evolution-Advanced (3GPP LTE-A). For example, the coverage expansion can be achieved with a Heterogeneous Network (HetNet) implemented by deploying Remote Radio Heads (RRHs), pico or femto cells, or relays within the coverage of a macro cellular network. 
     A HetNet is capable of achieving a more gainful spatial reuse effect through cell splitting. However, the significant difference in transmission power between the macro and micro eNBs increases inter-cell interference. 
     Recently, a method for expanding the coverage of the micro eNB by offloading the macro eNB to the micro eNB has been disclosed to improve the capacity and fairness of the HetNet. 
     One approach for coverage expansion of the micro eNB is to increase the Transmission (Tx) power of the micro eNB and have each terminal attach to the eNB transmitting the single with the strongest Downlink (DL) Received Signal Strength (RSS) in the cell selection procedure. In the case, however, both the macro and LeNB terminals are likely to experience significant interference. 
     Another approach for coverage expansion of the micro eNB is to give the DL RSS of the micro eNB a positive offset (range expansion bias). In this case, the terminal selects the micro eNB to which it is nearer than the macro eNB, although the DL RSS of the macro eNB is stronger than that of the micro eNB. Accordingly, it is possible to expand the coverage of the micro eNB without increasing its transmission power. 
     However, the terminal attached to the micro eNB within its expanded area is likely to be influenced by significant inter-cell interference from the macro eNB having the stronger DL RSS than that of the micro eNB. 
     The coverage expansion of the micro eNB is capable of improving the capacity of the macro eNB by offloading the macro User Equipment (UEs) to the micro eNB. However, the low link performance of the UE located in the range expansion area of the micro eNB causes degradation of the sum capacity of the network. 
     To this end, there has been disclosed a multi-cell cooperative transmission technology to improve cell edge performance by mitigating the inter-cell interference, otherwise known as Coordinated Multi-Point transmission and reception technology (CoMP) under discussion in the 3GPP LTE Rel. 11 standard. The aforementioned problems, however, also may occur in the CoMP scenarios 3 and 4, i.e. CoMP scenarios in the HetNet environment. 
     In CoMP scenario 3, the multiple TPs (Transmission Points or low power RRHs) participating in coordination within the macro cell coverage have a different PCID from that of the macro cell. In CoMP scenario 4, the multiple TPs participating in coordination within the macro cell coverage have the same PCID as the macro cell. 
       FIG. 1A  illustrates a conventional HetNet which causes significant inter-cell interference. As shown in  FIG. 1A , the conventional HetNet includes macro eNBs  110 ,  112 , and  114 , micro eNBs  116 ,  118 ,  120 ,  122 ,  124 ,  126 , and  128 , and UEs  130 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146 , and  148 . Reference numerals  102 ,  104 , and  106  denote the coverage of macro eNBs  110 ,  112 , and  114 , respectively. 
     In the HetNet of  FIG. 1A , the coverage of each of the micro eNBs  116 ,  118 ,  120 ,  122 ,  124 ,  126 , and  128  has been expanded. Reference numeral  108  denotes the range expansion area of the micro eNB. 
     Reference numeral  150  denotes a desired signal between the UE and the eNB. In  FIG. 1A , the UEs  130  to  134  are scheduled by the macro eNBs, while the UEs  136  to  148  are scheduled by the micro eNBs. 
     In  FIG. 1A , reference numeral  152  denotes inter-cell interference. As shown in  FIG. 1A , it is noted that all of the UEs are experiencing significant inter-cell interferences in the conventional HetNet. 
       FIG. 1B  illustrates the conventional HetNet with blanked macro eNBs. Compared to  FIG. 1A , some of the inter-cell interferences in  FIG. 1B  are removed by blanking the macro eNBs. 
     The macro eNBs are blocked in the HetNet of  FIG. 1B  as compared to  FIG. 1A , which indicates that the macro eNBs have stopped data transmission. Accordingly, the inter-cell interferences from the micro eNBs and neighbor macro eNBs to the macro UEs  130 ,  132 , and  134  have been canceled. 
     Accordingly, there is a need in the art of enhanced cell association and Inter-Cell Interference Coordination (ICIC) technologies for efficient operation of the HetNet. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above-mentioned problems, and it is an object of the present invention to provide a centralized-scheduling method and apparatus that is capable of minimizing the inter-cell interference and thus maximizing the throughput in a HetNet adopting the micro eNB coverage expansion technique. 
     In accordance with an aspect of the present invention, an inter-cell interference control method of a server in a heterogeneous network includes establishing a connection with at least one base station, collecting channel information between the at least one base station connected to the server and terminals, determining interfering base stations causing interference to each terminal based on the channel information, and selecting the terminals to be scheduled in consideration of the interfering base stations. 
     In accordance with another aspect of the present invention, an inter-cell interference canceling apparatus of a heterogeneous network includes a communication unit which establishes a connection with at least one base station, and a control unit which controls collecting channel information between the at least one base station connected to the server and terminals, determining interfering base stations causing interference to each terminal based on the channel information, and selecting the terminals to be scheduled in consideration of the interfering base stations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1A  illustrates a conventional HetNet which causes significant inter-cell interference; 
         FIG. 1B  illustrates the conventional HetNet with blanked macro eNBs; 
         FIG. 2A  illustrates a HetNet operating with the centralized scheduling algorithm at a normal subframe according to an embodiment of the present invention; 
         FIG. 2B  illustrates a HetNet operating with the centralized scheduling algorithm at a macro blank subframe according to an embodiment of the present invention; 
         FIG. 3A  illustrates a macro blank subframe pattern in which the rate of macro blank frame to macro on frame is ½ (50%) according to an embodiment of the present invention; 
         FIG. 3B  illustrates a macro blank subframe pattern in which the rate of macro blank frame to macro on frame is ¼ (25%) according to an embodiment of the present invention; 
         FIG. 4  illustrates the inter-cell interference control procedure of the inter-cell interference cancellation apparatus according to an embodiment of the present invention; 
         FIG. 5  illustrates the details of the interference cell set determination step of  FIG. 4 ; 
         FIG. 6  illustrates the details of the inter-cell interference coordination step of  FIG. 4 ; 
         FIG. 7A  illustrates an scheduling candidate UEs sorting process of  FIG. 6 ; 
         FIG. 7B  illustrates an UE exclusion process of  FIG. 6 ; 
         FIG. 8A  illustrates an example of sharing the feedback channel information with the centralized scheduler  254 ; and 
         FIG. 8B  illustrates an example of sharing the channel information related to the macro and micro eNBs with the centralized scheduler  254 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Hereinafter, various embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted for the sake of clarity and conciseness. 
     The same reference numerals are used throughout the drawings to refer to the same or like parts. In the drawings, certain elements may be exaggerated or omitted or schematically depicted for clarity of the invention, and the actual sizes of the elements are not reflected. 
       FIG. 2A  illustrates a HetNet operating with the centralized scheduling algorithm at a normal subframe according to an embodiment of the present invention. 
     As shown in  FIG. 2A , the HetNet includes macro eNBs  210 ,  212 , and  214 , micro eNBs  216 ,  218 ,  220 ,  222 ,  224 ,  226 , and  228 , and UEs  230 ,  232 ,  234 ,  236 ,  238 ,  240 ,  242 ,  244 , and  248 . Unlike the HetNets of  FIGS. 1A and 1B , the HetNet of  FIG. 2A  further includes a centralized scheduler server  254 . Reference numeral  256  denotes optic backhaul or Ethernet backhauls connecting the centralized scheduler server  254  and the macro and micro eNBs. The eNBs exchange channel information through an X2 interface. 
     The HetNet of  FIG. 2A  includes the micro eNBs  216  to  228  having the coverage or range expansion areas. Reference numeral  208  denotes the range expansion area of each micro eNB. The UEs  230  to  234  are scheduled by the macro eNBs while the UEs  236  to  248  are scheduled by micro eNBs. 
     In  FIG. 2A , reference numeral  252  denotes inter-cell interference. Unlike  FIG. 1A , the inter-cell interferences from neighbor macro eNBs to a macro UE, the inter-cell interferences from the micro UEs to the macro eNBs, and the inter-cell interferences from neighbor micro eNBs to a micro UE are efficiently canceled in the HetNet of  FIG. 2A . 
       FIG. 2B  illustrates a HetNet operating with the centralized scheduling algorithm at a macro blank subframe according to an embodiment of the present invention. 
     Unlike  FIG. 2A , the macro eNBs are blocked at the macro blank subframe in the HetNet of  FIG. 2B . As compared to  FIG. 1B , the inter-cell interferences from neighbor micro eNBs to the micro UEs as well as the inter-cell interferences from neighbor macro eNBs to the macro UEs, from the macro eNBs to the micro UEs, and from the macro UEs to the micro UEs have been efficiently canceled in the HetNet of  FIG. 2B . 
       FIGS. 3A and 3B  illustrate patterns of macro blank subframes for use in the HetNet according to an embodiment of the present invention. 
       FIG. 3A  shows a macro blank subframe pattern in which the rate of macro blank frame to macro on frame is ½ (50%). As shown in  FIG. 3A , when the macro eNBs (or Macro BSs) are on, inter-cell interference occurs from the macro eNBs to the micro eNBs. 
       FIG. 3B  shows a macro blank subframe pattern in which the rate of macro blank frame to macro on frame is ¼ (25%). Compared to that of  FIG. 3A , the macro blank subframe pattern of  FIG. 3B  increases the probability of inter-cell interference from the macro eNBs to the micro eNB. 
       FIG. 4  illustrates the inter-cell interference control procedure of the inter-cell interference cancellation apparatus, i.e. centralized scheduler server  254 , according to an embodiment of the present invention. 
     The server  254  and the macro and micro eNBs are connected through the optic backhaul or Ethernet backhaul. The centralized scheduling can be implemented with the coordinated cells including the micro eNBs with the exception of the macro eNBs. In this case, the optic backhaul or Ethernet backhauls are established only between the server  254  and the micro eNBs. 
     Referring to  FIG. 4 , the centralized scheduler server  254  configures the range expansion bias to the micro eNB at step  402 . The centralized scheduler server  254  determines the range expansion bias in consideration of the macro eNB coverage, number of deployed micro eNBs, and transmission powers of the macro and micro eNBs. 
     The centralized scheduler server  254  controls the range expansion of the micro eNB in such a manner to assign positive offset (range expansion bias) to the DL RSS of the micro eNB. 
     In this manner, the centralized scheduler server  254  is capable of connecting the UE located near a micro eNB to the micro eNB, rather than to the macro eNB having the higher transmit power than that of the micro eNB without increasing the transmission power of the micro eNB. This expands the service coverage of the micro eNB. 
     The centralized scheduler server  254  determines the blank pattern of each macro eNB at step  404 . That is, the centralized scheduler server  254  determines the transmission power-off period of the macro eNB. 
     For example, the centralized scheduler server  254  may set the blank pattern of the macro eNB to the rate of macro blank subframe to normal subframe, such as ½, ¼, or ⅛. If the appearance frequency of the macro blank subframe to the normal subframe is set to a high value, the scheduling probability of the macro UE decreases. If the appearance frequency of the macro blank subframe to the normal subframe is set to a low value, the probability of the inter-cell interference from the macro eNB increases. 
     Accordingly, the centralized scheduler server  254  determines the blank pattern in consideration of the range expansion bias of the micro eNB, which has been set at step  402 . According to the operation mechanism of the centralized scheduler server  254 , it is possible to transmit data at a low power level rather than turning off the transmission power of the macro eNB at the macro blank subframe. 
     The centralized scheduler server  254  collects channel information between the eNBs and UEs that are collected and measured by the macro and micro eNBs within the coordinated cluster to share in common with the macro and micro eNBs at step  406 . 
     The channel information fed back by the UE to the macro and micro eNBs and shared with the centralized scheduler  254  may include a Channel Quality Indicator (CQI) and a Rank Indicator (RI). 
       FIG. 8A  illustrates an example of sharing the feedback channel information with the centralized scheduler  254 . Referring to  FIG. 8A , the UE  240  measures channel information for neighbor macro eNBs  210  to  214  as denoted by reference numerals  802  to  808  and micro eNB  220  and feeds back the channel information to the serving eNB  220  as denoted by reference numeral  810 , and the serving eNB  220  delivers the information fed back by the UE  240  to the centralized scheduler  254  as denoted by reference numeral  810 . 
     The channel information of the macro and micro eNBs that is shared with the centralized scheduler  254  may include RSRP and UpLink (UL) Sounding Reference Signal (SRS) power based on Cell Specific Reference Signal (CRS) or Channel State Information Reference Signal (CSI-RS). 
       FIG. 8B  illustrates an example of sharing the channel information related to the macro and micro eNBs with the centralized scheduler  254 . Referring to  FIG. 8B , the macro eNBs  210  to  214  and the micro eNB  220  receive the channel information measured by the UEs  232 ,  240 , and  246  as denoted by reference numerals  802  to  810  and send the received information with the centralized scheduler  254  as denoted by reference numerals  812  to  818 . 
     Returning to  FIG. 4 , all UEs select respective serving cells (serving eNB) at step  408 , by applying the range expansion bias of the preset micro eNB which has been set at step  402 . The micro UE fulfilling the following condition expressed below in Equation (1) with the range expansion bias is located in the expanded area of the micro eNB.
 
(RSS i, LeNB ) dB &lt;(RSS i, Macro ) dB   , S={i| (RSS i, LeNB ) dB &lt;(RSS i, Macro ) dB , (RSS i, LeNB ) dB +Offset RE   _   Bias &lt;(RSS i, Macro ) dB }  (1)
 
     The centralized scheduler server  254  performs a handover of the UE which does not fulfill the outage condition among the UEs determined as range expansion LeNB UEs with the range expansion bias to a macro eNB at step  410 . The handover determination is performed with following formula.
 
(SINR i, LeNB ) dB  in Macroblank subframe&lt;Outage_Threshold  (2)
 
     The centralized scheduler server  254  determines whether the corresponding subframe is the macro blank subframe, at step  412 , by applying the macro blank pattern that has been configured at step  404 . 
     If the macro blank subframe is determined, the centralized scheduler server  254  stops data transmission to the UEs served by all macro eNBs connected to the centralized scheduler server  254  at the corresponding subframe at step  414 . In this manner, the centralized scheduler server  254  is capable of canceling the inter-cell interference from the high transmission power macro eNBs to the micro eNBs (LeNBs) at the macro blank subframes. 
     If the macro blank subframe is not determined, the centralized scheduler server  254  determines an interference cell set of the cells generating interference to each UE at step  416 . The interference cell set may include both the macro and micro eNBs (cells). If the corresponding subframe is a macro blank subframe, all macro eNBs are blanked and thus the macro eNB cells are excluded from the interference cell set. 
       FIG. 5  illustrates the details of the interference cell set determination step  416  of  FIG. 4 . 
     The centralized scheduler server  254  searches for a dominant interference to each UE at step  502 . There may be more than one dominant interference source per UE. The centralized scheduler server  254  searches all connected eNBs for the dominant interference without limitation to the coverage of a macro eNB. 
     The centralized scheduler server  254  sorts the dominant interferences of the individual UEs found at step  502  in a descending order of signal strength at step  504 . 
     The centralized scheduler server  254  determines a compensation weight value to be applied to the formula for determining a number of interference cells to be requested for blanking at step  506 . 
     The compensation weight value can be determined as follows: 
     Macro to Macro Compensation Weight=α 
     Macro to LeNB Compensation Weight=β 
     LeNB to Macro Compensation Weight=γ 
     LeNB to LeNB Compensation Weight=λ 
     Here, α is the compensation weight value to be applied when the serving cell of the UE i is a macro eNB and the eNB to be requested for blanking is another macro eNB. β is the compensation weight value applied when the serving cell of the UE i is a macro eNB and the eNB to be requested for blanking is a micro eNB. γ is the compensation weight value to be applied when the serving cell of the UE i is a micro eNB and the eNB to be requested for blanking is a macro eNB. λ is the compensation weight value to be applied when the serving cell of the UE i is a micro eNB and the eNB to be requested for blanking is another micro eNB. 
     The centralized scheduler server  254  adjusts the compensation weights α, β, γ, and λ to efficiently control the inter-cell interference in the coordinated cells. 
     The centralized scheduler server  254  determines a number of interference cells to be blanked per UE at step  508 . The numeral of interference cells to be blanked per UE is determined according to the following Equation (3). 
     
       
         
           
             
               
                 
                   
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     In Equation (3), INT 1  denotes the number of dominant interferences. R i (k) denotes the rate when the number of interference cells to be requested for blanking is k. W k  denotes the compensation weight factor determined according to the number of interference cells to be requested for blanking and is calculated in Equation (4) as follows.
 
 W   k+1   =W   k   +δ, W   0 =1  (4)
 
     When the corresponding subframe is a normal subframe, δ in Equation (4) is set to a value of one of α, β, γ, and λ. That is, if the serving cell of the UE i is a macro cell and the eNB to be requested for blanking is another macro eNB, δ is set to the value of α. If the serving cell of the UE i is a macro eNB and the eNB to be requested for blanking is a micro eNB (LeNB), δ is set to the value of β. 
     If the serving cell of the UE i is a micro eNB and the eNB to be requested for blanking is a macro eNB, δ is set to the value of γ, and if the serving cell of the UE i is a micro eNB and the eNB to be requested for blanking is another micro eNB, δ is set to the value of λ. 
     If the corresponding subframe is a macro blanking subframe, all macro eNBs are blanked and thus δ is set to the value of λ. 
     The centralized scheduler server  254  determines the interference cell set to be requested for blanking per UE at step  510 . In detail, the centralized scheduler server  254  determines the interference cell set to be requested for blanking per UE in such a manner of selecting as many eNBs as the number of blanking request interference cells per UE, which has been determined at step  508 , in a descending signal strength order. 
     Returning to  FIG. 4 , the centralized scheduler server  254  selects scheduling candidate UEs per eNB at step  418 . For example, the centralized scheduler server  254  is capable of selecting the UEs having maximum Proportional Fair (PF) metric value as scheduling candidate UEs. The PF metric formula can be modified by reflecting UE Quality of Service (QoS) and resource allocation ratio. 
     However, if the corresponding subframe is a normal subframe, the UEs served by the low power micro eNBs located at the range expansion area are disqualified from being the scheduling candidate. By disqualifying the LeNB UEs in the range expansion area that are exposed to the significant inter-cell interference from the high power macro eNBs from being the scheduling candidates, the outage probability of the LeNB UEs is reduced. 
     The centralized scheduler server  254  performs inter-cell interference coordination on the scheduling candidate UEs of all eNBs at step  420 . 
       FIG. 6  illustrates the details of the inter-cell interference coordination step  420  of  FIG. 4 . 
     The centralized scheduler  254  sorts the scheduling candidate UEs selected at step  418  in a descending order of PF metric value at step  602 , based on Equation (5) as follows.
 
PF Metric[ k]=R ( k, n )/ T avg( k, n )  (5)
 
     In Equation (5), k denotes the candidate UE index, n denotes time, R denotes channel quality as transmission data rate when the current UE k is allocated the target resource, and Tavg denotes the average data rate allocated to the UE k. That is, the better the channel quality of the UE and the lower the allocated average data rate, the higher the PF metric value. Through this scheduling mechanism, the average throughput of the system is improved and scheduling fairness to the UEs is guaranteed. Step  602  is described in detail with reference to  FIG. 7A . 
     As shown in  FIG. 7A , the centralized scheduler server  254  sorts the UEs selected as scheduling candidates in a descending order of PF metric value and configures the serving cell and interference cell set of each UE. 
     Returning to  FIG. 6 , the centralized scheduler server  254  excludes the UEs having the blanking request interference cell set of the corresponding UE as service cell from the scheduling candidates in the maximum PF metric first order at step  604 . Step  604  is described in detail with reference to  FIG. 7B . 
     As shown in  FIG. 7B , the centralized scheduler  254  excludes the UEs having any of the cells included in the interference cell set of the corresponding UE as the serving cell from the scheduling candidates in the maximum PF metric first order. 
     Returning to  FIG. 6 , when step  604  has been performed for all scheduling candidate UEs, step  606  is performed, in which the centralized scheduler  254  determines the UEs to be scheduled and the eNBs to be blanked in the corresponding subframe. 
     As described above, the centralized-scheduling method and apparatus of the present invention is capable of blanking (powering off) at least one macro or micro eNBs determined as dominant interference sources at every subframe, thereby stopping data transmission to the served UEs, and resulting in canceling the inter-cell interference. 
     The centralized-scheduling method and apparatus of the present invention is capable of minimizing the inter-cell interferences from the high power macro eNBs to LeNB UEs, from neighbor low power local eNBs to the LeNB UEs, from LeNBs to the macro UEs, and neighbor high power macro eNBs to the macro UEs, resulting in maximization of sum capacity of the HetNet. 
     The centralized-scheduling method and apparatus of the present invention introduces a centralized node to the legacy X2 interface-based CoMP so as to improve the inter-site (inter-eNB) CoMP performance, enabling the adoption of CoMP between eNB clusters, resulting in performance improvement in CoMP scenarios 3 and 4 under the HetNet environment. 
     Furthermore, the centralized-scheduling method and apparatus of the present invention is capable of connecting the centralized scheduler node and each eNB through a non-ideal backhaul such as Ethernet backhaul as well as optic backhaul, thereby improving the HetNet CoMP performance even when there is some delay on the backhaul between the centralized scheduler and each eNB. 
     Although embodiments of the present invention have been described in detail hereinabove with specific terminology, and the invention is limited thereto. While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention.