Abstract:
Disclosed is a method and an apparatus for subchannel assignment for suppressing inter-antenna interference in an Orthogonal Frequency Division Multiplexing Access (OFDMA) system based distributed wireless communication system equipped with antennas that are randomly distributed in a geographical manner and can simultaneously communicate with multiple Subscriber Stations (SSs). The method includes selecting and obtaining access to distributed antennas which satisfy the data transmission rate that an SS requires and with which the SS can communicate; re-queuing a distributed antenna having the maximum transmitted power within the same cell in high priority; and assigning subchannels to the relevant distributed antennas in an order from the relevant distributed antenna having the high priority.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of the earlier filing date, under 35 U.S.C. §119(a), to that patent application entitled “Method and Apparatus for Subchannel Assignment for Suppressing Interantenna Interference in OFDMA System,” filed in the Korean Intellectual Property Office on Mar. 2, 2007 and assigned Serial No. 2007-20985, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to Orthogonal Frequency Division Multiplexing Access (OFDMA) communication systems, and more particularly to a method and an apparatus for subchannel assignment for suppressing and minimizing inter-antenna interference in an OFDMA system. 
         [0004]    2. Description of the Related Art 
         [0005]    With an increase of the requirements of a user in relation to internet service, the need for a communication system that can efficiently offer internet service is increasing. The existing communication network has been developed for the main purpose of a voice service but has drawbacks in that the existing communication systems have a relatively narrow data transmission band width, and require an expensive charge for its usage. 
         [0006]    In order to resolve or correct such drawbacks, a study on a scheme of OFDM is being conducted as a representative example of a broadband wireless access scheme. 
         [0007]    The scheme of OFDM corresponds to a typical transmission scheme employing multi-carriers and to a scheme that converts a symbol queue input in series into parallel data, modulates a converted symbol queue through multiple subcarriers having mutual orthogonality, and then transmit a modulated symbol queue. The above-mentioned scheme of OFDM can be widely applied to digital transmission technology that needs high-speed data transmission, such as wireless internet, Digital Audio Broadcasting (DAB) and digital television, Wireless Local Area Network (WLAN), and the like. 
         [0008]    The scheme of OFDM (see for example, L. J. Cimini, “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Trans. Commn., vol. COM-33, no. 7, pp. 665-675, June 1985 and Richard Van Nee and Ramjee Prasad, “OFDM for Wireless Multimedia Communications,” Artech House, 2000.) corresponds to multiplexing technology that subordinately divides a bandwidth into multiple frequency subcarriers. 
         [0009]    In OFDM, an input data stream is divided into several parallel substreams having a reduced data rate (therefore, the symbol length increases). Then, each substream is modulated, and is transmitted, on a separate orthogonal subcarrier. An increase of the symbol length improves the robustness of the OFDM against delay diffusion. OFDM modulation can be realized by efficient Inverse Fast Fourier Transforms (IFFT), which in turn enables multiple subcarriers having low complexity. 
         [0010]    In the above OFDM system, channel resources employ an OFDM symbol in the time domain, and is enabled by using subcarriers in the frequency domain. Time and frequency resources consist of subchannels assigned to an individual user. 
         [0011]    Also, the scheme of OFDM corresponds to a scheme of multiaccess/multiplexing, provides a multiplexing operation relating to data streams from multiuser to Up Link (UL) multiaccess employing a Down Link (DL) subchannel and an UL subchannel. 
         [0012]    As previously described, the subcarrier is usually grouped into subsets called subchannels. For example, in a World interoperability for Microwave Access (WiMAX) system, the structure of OFDM symbol is made up of three kinds of subcarriers, including a data subcarrier for data transmission, a pilot subcarrier for an evaluation and synchronization, and a null subcarrier for a guard band and a DC carrier. An activated (data and pilot) subcarrier is grouped into subchannels. 
         [0013]    A WiMAX OFDM physical layer (see for example, IEEE 802. 16-2004 (Revision of IEEE Std 802. 16-2001), “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” October 2004 and IEEE 802. 16e-2005, “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems,” Feb. 2006) supports subchannelization both in a DL (downlink) and in an UL (uplink), and a unit of the minimum frequency/time resources of the subchannelization corresponds to one slot. 
         [0014]    Hence, research on an algorithm for assigning an adaptive subcarrier (subchannel) has been extensively carried out in a multi-user OFDM system. However, most of these algorithms are based on a Central Antenna based System (CAS). 
         [0015]    A Distributed Antenna System (DAS) that is based on the OFDMA can allow a subcarrier to be used by another antenna. 
         [0016]    In general, a DAS (see for example, A. M. Adel, A. Saleh, A. J. Rustako, and R. S. Ramon, “Distributed Antennas for Indoor Radio Communications,” IEEE Trans. Commun., vol. 35, pp. 1245-1251, December 1987 and S. Zhou, M. Xhao, X. Xu, J. Wang, and Y. Yao, “Distributed Wireless Communications System: a New Architecture for Future Public Wireless Access,” IEEE Commun. Mag., vol. 17, no. 3, pp. 108-113, March 2003) can provide macro-diversity that controls a large-scale fading and reduces an access distance by distributing antennas geometrically. The DAS has been introduced so as to solve a coverage area problem in an indoor wireless system, and afterwards has been applied to the performance improvement of a Code Division Multiple Access (CDMA) system. 
         [0017]      FIG. 1  is a view illustrating a coverage area by each distributed antenna that is distributed around a Base Station (BS) centered in a DAS. With reference to  FIG. 1 , in the DAS, the antennas of the BS are uniformly distributed geometrically, and with each antenna of the BS as the center of a hexagon area, an overall area can be divided into six hexagonal subareas. If an average access distance decreases in the DAS, as transmission power is reduced, inter-antenna interference diminishes, and capacity can increase. Channel conditions of the antennas of the BS are measured and analyzed by a subscriber station (SS) in each frame, and then an antenna M having the maximum gain can be selected as a serving antenna in the next frame. The value of M is equal to or greater than ‘1.’ Herein, the value of M is confined to being ‘1’ to have a positive value. 
         [0018]    If the number of antennas equals ‘P’ within a cell of the DAS, the number of developed subcarriers becomes P times as many as a CAS. Thus, an assignment of resources developed more complicated in the DAS. 
         [0019]    At present, in the DAS based on the OFDMA, an algorithm for assigning subchannels can be classified into several kinds as in the following. 
         [0020]    1. Each antenna develops all subchannels. 
         [0021]    2. All subchannels are assigned to cells only once. This implies that if any subchannel is used by one antenna in a cell, the subchannel cannot be employed even by any other antenna within the cell. 
         [0022]    3. Each subchannel is assigned from a global viewpoint, and in order to obtain diversity gain, it is allowed for two adjacent antennas to use one SS through the same subchannel. 
         [0023]    However, if each antenna develops all subchannels as described above, this is the same as cell division from a standpoint of frequency reuse, and incurs inter-antenna interference similar to co-channel interference in the cell division. Also, if all subchannels are developed by one remote antenna and one SS, even though interference is excluded from another antenna, this is a waste of bandwidth, and problems arise, for example in hot-zones. 
         [0024]    Hence, at present, even though two antennas are sufficiently far away from each other in the OFDMA-based DAS, its subchannels cannot be reused. 
         [0025]    In this manner, if each antenna develops all subchannels, this is the same as cell division from a viewpoint of frequency reuse, and causes such problem that a serious inter-antenna interference similar to co-channel interference is incurred. 
       SUMMARY OF THE INVENTION 
       [0026]    Accordingly, the present invention provides a method and an apparatus for subchannel assignment for suppressing inter-antenna interference in an OFDMA system, by assigning subchannels different from one another to antennas adjacent to one another, which in turn minimizes inter-antenna interference and satisfies the transmission rate required by a user. 
         [0027]    In accordance with an aspect of the present invention, there is provided a method for subchannel assignment for suppressing inter-antenna interference in an Orthogonal Frequency Division Multiplexing Access (OFDMA) system-based distributed wireless communication system equipped with distributed antennas that are randomly distributed in a geographical manner and can simultaneously communicate multiple to Subscriber Stations (SSs), including the steps of selecting and obtaining access to distributed antennas which satisfy the data transmission rate that an SS requires and with which the SS can communicate, re-queuing a distributed antenna having the maximum transmitted power within the same cell a highest priority, and assigning subchannels to the relevant distributed antennas in order from the relevant distributed antenna having the highest priority. 
         [0028]    In accordance with another aspect of the present invention, there is provided an apparatus for subchannel assignment for suppressing inter-antenna interference in an Orthogonal Frequency Division Multiplexing Access (OFDMA) system-based distributed wireless communication system having a Base Station (BS) apparatus for subchannel assignment, including a first assigning unit for finding a subchannel used by a distributed antenna adjacent to a relevant distributed antenna assigned another subchannel; and a second assigning unit for specifying a high priority for a distributed antenna having a maximum transmitted power and assigning subchannels to the relevant distributed antennas in an order from the relevant distributed antenna having the high priority. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The above and other exemplary features, aspects, and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0030]      FIG. 1  is a view illustrating a coverage area by each of a plurality of distributed antenna of a Base Station (BS) centered in a general DAS; 
           [0031]      FIG. 2  is a view illustrating a method for assigning a subchannel which avoids adjacent antennas using the same subchannel in an OFDMA system according to an embodiment of the present invention; 
           [0032]      FIG. 3  is a configuration block diagram illustrating a part of an overall internal configuration of a BS in which the assignment of subchannels is performed during a DL of an OFDMA system according to an embodiment of the present invention; and 
           [0033]      FIG. 4  is a flowchart illustrating a method for subchannel assignment for suppressing inter-antenna interference in an OFDMA system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    Exemplary embodiments of the present invention are now described in detail with reference to the accompanying drawings. The description includes particulars, such as specific configuration elements, that are presented in support of more comprehensive understanding of the present invention, and it will be obvious to those of ordinary skill in the art that changes in form and modifications may be made to the particulars and are considered to be within the scope of the present invention. 
         [0035]      FIG. 2  is a view illustrating a method for assigning a subchannel to adjacent antennas while avoiding using the same subchannel in an OFDMA system according to an embodiment of the present invention. 
         [0036]    With reference to  FIG. 2 , if a subchannel A is used by an antenna  1  offering services to an SS within a coverage area of the antenna  1 , adjacent antennas  2 ,  6 , and  7  avoid using the assigned subchannel A. The antennas  2 ,  6 , and  7  may employ any of the other subchannels except for the subchannel A. Herein, since an antenna  4  is not adjacent to the antenna  1 , the antenna  4  can use subchannel A, and thus there exists a minimal amount of interference between signals transmitted by antenna  4  and antenna  1 . 
         [0037]    The present invention according to a configuration described above avoids inter-antenna interference due to an assignment of the same subchannel and can achieve optimal use of channel resources, while satisfying the needs in services of an SS of a user. 
         [0038]      FIG. 3  is a configuration block diagram illustrating a part of an overall internal configuration of a base station (BS) in which the assignment of subchannels is performed during a downlink (DL) of an OFDMA system according to an embodiment of the present invention. An apparatus for assigning a subchannel according to the present invention is embodied through a subchannel assigning apparatus  30  included in the BS. 
         [0039]    A signal transmitted from the BS illustrated in  FIG. 3  passes through a frequency selective fading channel peculiar to each SS corresponding to the total number (K) of subscribers, and is then received by the SS of each subscriber. Referring to  FIG. 3 , the transmission rates required by users transmitted from SS of all subscribers and subchannel assignment information previously assigned to the respective SSs are input to the subchannel assigning apparatus  30  at each preset assignment period. The subchannel assigning apparatus  30  sequentially applies an assignment method according to the present invention at each assignment period utilizing the transmission rate required by each subscriber, and enables a first, second, and third assigning units  31 ,  32 , and  33  to perform an adaptive assignment of subchannels and the assignment of bits. 
         [0040]    Although not shown in  FIG. 3 , it would be recognized that the BS can be equipped with a receiving unit for receiving the transmission rate and subchannel assignment information required by each user (subscriber) and for providing the received required transmission rate and subchannel assignment information to the first, second, and third assigning units  31 ,  32 , and  33 . Since this can be implemented by using the widely known art, a detailed description need not be provided herein. 
         [0041]    Furthermore, subscribers&#39; data is assumed transmitted within the same cell that the transmission rate required by a subscriber, where the subscribers&#39; data is classified non-real-time data when the transmission rate required by the subscriber has a value of ‘0’ or a non-positive value and real-time data when the required transmission rate has the positive value. 
         [0042]    An assignment method according to the present invention will be specifically described with reference to  FIG. 4 . 
         [0043]    First, a description is made of fundamental conditions premised in the present invention in order to facilitate the understanding of the present invention. Assuming that there exists P antennas, N subchannels, and K users in an overall cell, the minimum data rate required for an SS according to each subscriber is represented as R min , and the channel quality from an antenna p to an SS k in relation to a subchannel n is represented as c k,n,p  expressed in terms of bits. For example, Qaudrature Phase Shift Keying (QPSK) coding having R ⅔ is used, c k,n,p  equals 4/3. 
         [0044]    The small letter k is a parameter necessary to distinguish between users. N means the total number of subchannels. c k,n  is the number of bits per symbol assigned to the nth subchannel of the kth user, and the number of bits per symbol c k,n  is, for example, a positive real number. It is possible to assign a positive real number not a positive integer to the number of bits per symbol through a combination of modulation and an error control code. 
         [0045]    Also, a symbol d refers to an adjacent antenna. For instance, if the antenna p is adjacent to an antenna q, d p,q  is set to ‘1.’ Otherwise d p,q  is set to ‘0.’ 
         [0046]    The first stage, a second stage, a third stage and a fourth stage assignment algorithms may be expresses as mathematical expressions according to an embodiment of the present invention, as follows:. 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 1 ST  STAGE ASSIGNMENT ALGORITHM 
               
               
                   
                 K p  = φ, ∀p 
               
             
          
           
               
                   
                 For (k = 1 : K) 
               
               
                   
                   
               
             
          
           
               
                   
                 
                   
                     
                       
                         { 
                         
                           
                             
                               Calculate 
                                
                               
                                   
                               
                                
                               
                                 c 
                                 
                                   k 
                                   , 
                                   p 
                                 
                               
                             
                             = 
                             
                               
                                 1 
                                 N 
                               
                                
                               
                                 
                                   ∑ 
                                   
                                     n 
                                     = 
                                     1 
                                   
                                   N 
                                 
                                  
                                 
                                   c 
                                   
                                     k 
                                     , 
                                     n 
                                     , 
                                     p 
                                   
                                 
                               
                             
                           
                           , 
                           
                             ∀ 
                             
                               p 
                               → 
                               
                                 
                                   MATH 
                                   . 
                                   
                                       
                                   
                                    
                                   EXPRESSION 
                                 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
               
                   
                 
                   
                     
                       
                         
                           Select 
                            
                           
                               
                           
                            
                           
                             p 
                             ′ 
                           
                         
                         = 
                         
                           
                             
                               
                                 arg 
                                  
                                 
                                     
                                 
                                  
                                 max 
                               
                               p 
                             
                              
                             
                                 
                             
                              
                             
                               ( 
                               
                                 c 
                                 
                                   k 
                                   , 
                                   p 
                                 
                               
                               ) 
                             
                           
                           → 
                           
                             
                               MATH 
                               . 
                               
                                   
                               
                                
                               EXPRESSION 
                             
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
             
          
           
               
                   
                 K p′ = K p′ ∪ {k} (K p  corresponds to a set of all users that the antenna 
               
             
          
           
               
                 p serves) end} 
               
             
          
           
               
                   
                 2 ND  STAGE ASSIGNMENT ALGORITHM 
               
               
                   
                 For ( p = 1 : P ) 
               
             
          
           
               
                   
                 { K p = {k 1 , k 2 , . . . } 
               
               
                   
                   
               
               
                   
                 
                   
                     
                       
                         
                           Calculate 
                            
                           
                               
                           
                            
                           
                             c 
                             p 
                           
                         
                         = 
                         
                           
                             
                               1 
                               N 
                             
                              
                             
                               
                                 ∑ 
                                 
                                   k 
                                   = 
                                   
                                     k 
                                     1 
                                   
                                 
                                 
                                   
                                     
                                         
                                     
                                     k 
                                   
                                    
                                   
                                      
                                     
                                       K 
                                       p 
                                     
                                      
                                   
                                 
                               
                                
                               
                                 c 
                                 
                                   k 
                                   , 
                                   p 
                                 
                               
                             
                           
                           → 
                           
                             
                               MATH 
                               . 
                               
                                   
                               
                                
                               EXPRESSION 
                             
                              
                             
                                 
                             
                              
                             3 
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
             
          
           
               
                   
                 end} 
               
               
                   
                   
               
             
          
           
               
                   
                 if requeue is performed on P according to c p , c 1  is greater than any other c p . 
               
             
          
           
               
                   
                 3 RD  STAGE ASSIGNMENT ALGORITHM 
               
               
                   
                 U = {1, 2, 3, . . . , N} 
               
               
                   
                 ∀p, U p  = φ, V p  = φ 
               
               
                   
                 (At the very beginning, all subchannels can be available. 
               
             
          
           
               
                   
                 U p  is a set of subchannels assigned to the antenna p, and 
               
               
                   
                 V p  is a set of subchannels used by antennas adjacent to the antenna p) 
               
               
                   
                   
               
             
          
           
               
                   
                 For ( p = 1 : P ) 
               
             
          
           
               
                   
                 { K p  = {k 1 , k 2 , . . . } 
               
               
                   
                 for q = 1 : p − 1 
               
             
          
           
               
                   
                 if d p,q  = 1 then V p  = V p  ∪ U q   
               
             
          
           
               
                   
                 end} 
               
             
          
           
               
                   
                 (searching for all subchannels that have already been used by adjacent antennas) 
               
               
                   
                 For k = 1 : |K p | 
               
             
          
           
               
                   
                 R = 0 
               
             
          
           
               
                   
                 while R &lt; R min   
               
               
                   
                   
               
             
          
           
               
                   
                 
                   
                     
                       
                         
                           Select 
                            
                           
                               
                           
                            
                           
                             n 
                             ′ 
                           
                         
                         = 
                         
                           
                             
                               
                                 arg 
                                  
                                 
                                     
                                 
                                  
                                 max 
                               
                               
                                 n 
                                 ∈ 
                                 
                                   ( 
                                   
                                     U 
                                     - 
                                     
                                       V 
                                       p 
                                     
                                     - 
                                     
                                       U 
                                       p 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                             
                                 
                             
                              
                             
                               ( 
                               
                                 c 
                                 
                                   k 
                                   , 
                                   n 
                                   , 
                                   p 
                                 
                               
                               ) 
                             
                           
                           → 
                           
                             
                               MATH 
                               . 
                               
                                   
                               
                                
                               EXPRESSION 
                             
                              
                             
                                 
                             
                              
                             4 
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
             
          
           
               
                   
                 R = R + c k,n   ′ ,p 
               
               
                   
                   
               
             
          
           
               
                   
                 (subchannel n′ is assigned to the antenna p and a user k) 
               
             
          
           
               
                   
                 U p  = U p  + {n ′ } 
               
               
                   
                   
               
             
          
           
               
                   
                 4 TH  STAGE ASSIGNMENT ALGORITHM 
               
               
                   
                 For ( p = 1 : P ) 
               
             
          
           
               
                   
                 K p  = {k 1 , k 2 , . . . } 
               
             
          
           
               
                   
                 if ∃k ∈ K p  and to this k, R &lt; R min  then 
               
             
          
           
               
                   
                 while R &lt; R min   
               
               
                   
                   
               
             
          
           
               
                   
                 
                   
                     
                       
                         
                           
                             s 
                              
                             elect 
                           
                            
                           
                               
                           
                            
                           
                             n 
                             ′ 
                           
                         
                         = 
                         
                           
                             
                               arg 
                                
                               
                                   
                               
                                
                               max 
                             
                             
                               n 
                               ∈ 
                               
                                 ( 
                                 
                                   
                                     V 
                                     p 
                                   
                                   - 
                                   
                                     U 
                                     p 
                                   
                                 
                                 ) 
                               
                             
                           
                            
                           
                               
                           
                            
                           
                             ( 
                             
                               c 
                               
                                 k 
                                 , 
                                 n 
                                 , 
                                 p 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
                 R = R + c k,n,p   
               
             
          
           
               
                   
                 (subchannel n ′  is assigned to the antenna p and the user k) 
               
             
          
           
               
                   
                 U p  = U p  + {n′} 
               
               
                   
                   
               
             
          
         
       
     
         [0047]      FIG. 4  is a flowchart illustrating a method for subchannel assignment for suppressing inter-antenna interference in an OFDMA system according to an embodiment of the present invention. With reference to  FIG. 4 , the BS receives the transmission rates requested by users from the corresponding SSs at each predetermined assignment period during an initialization operation in step  410 . The data transmission rate c k,p  required by an SS served by an arbitrary antenna p is calculated by using the above MATH. EXPRESSION 1. 
         [0048]    In step  412 , the best p, i.e., having the highest c k,p , for an antenna is selected by using the above MATH. EXPRESSION 2. 
         [0049]    To give high priority to an antenna p selected in step  412 , re-queue is performed on multiple antennas by using the above MATH. EXPRESSION 3 in step  414 . 
         [0050]    In step  416 , a subchannel to be assigned to a distributed antenna is selected in order from the relevant distributed antenna with top priority. At this time, subchannels employed by other distributed antennas adjacent to a neighboring area of the relevant distributed antenna are found in step  418 . Herein, the number of other adjacent distributed antennas is at least one. 
         [0051]    Then, respective subchannels used by adjacent distributed antennas are compared with the selected subchannel by using the above MATH. EXPRESSION 4 in step  420 . 
         [0052]    In a case where a subchannel is the same as a subchannel to be assigned to an adjacent antenna is found in step  422 , i.e., if the selected subchannel has already been used by an adjacent antenna, the procedure returns back to step  416 , and repeatedly performs the described steps  416 ,  418 ,  420 , and  422  using the remaining subchannels. If the same subchannel is not found in step  422 , an assignment of the subchannel is performed to the antenna in step  424 . 
         [0053]    The above-described methods according to the present invention can be realized in hardware or as software or computer code that can be stored in a recording medium such as a CD ROM, an RAM, a floppy disk, a hard disk, or a magneto-optical disk or downloaded over a network, so that the methods described herein can be rendered in such software using a general purpose computer, or a special processor or in programmable or dedicated hardware, such as an ASIC or FPGA. As would be understood in the art, the computer, the processor or the programmable hardware include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code that when accessed and executed by the computer, processor or hardware implement the processing methods described herein. 
         [0054]    The merits and effects of exemplary embodiments, as disclosed in the present invention, and as so configured to operate above are described as follows. 
         [0055]    According to the present invention, by assigning subchannels different from one another to antennas adjacent to one another, inter-antenna interference is minimized while satisfying the transmission rate required by a user. 
         [0056]    While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. Therefore, the spirit and scope of the present invention must be defined not by described embodiments thereof but by the appended claims and equivalents of the appended claims.