Abstract:
A broadband wireless communication system using a plurality of Frequency Allocations (FAs) is provided. A method for packet transmission of a packet of a transmitting end includes dividing one encoded packet into a plurality of parts, mapping a plurality the plurality of parts of the packet to the FAs through the plurality of different FAs transmission, when a re-transmission request is received, re-mapping the plurality of parts of the packet to the FAs such that at least one of the at least one of the plurality of parts is re-mapped to an FA that is different than an FA previously mapped thereto. Retransmitting the encoded packet by at least one of a number of subunits.

Description:
PRIORITY 
     The present application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Mar. 26, 2008 and assigned Serial No. 10-2008-0027781, the entire disclosure of which is hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a broadband wireless communication system and, more particularly, to an apparatus and method for supporting hybrid automatic repeat request in a broadband wireless communication system. 
     BACKGROUND OF THE INVENTION 
     Today, many wireless communication techniques are being proposed to achieve a high-speed mobile communication. Among them, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is accepted as one of the most promising techniques for a next generation wireless communication. The OFDM scheme is expected to be widely used in the future wireless communication techniques, and currently is used as a standard in the Institute of Electrical and Electronics Engineers (IEEE) 802.16-based Wireless Metropolitan Area Network (WMAN) referred to as the 3.5 generation technology. 
     Meanwhile, the wireless communication systems are evolving to provide a high-speed data service in comparison with a legacy system or to address an implementation issue. In such a system evolution process, various systems may coexist in the same area according to a degree of compatibility with the legacy system. For example, a new system may be installed in an area where an IEEE 802.16e system exists. In this case, the new system has to be able to provide services not only to a legacy Mobile Station (MS) but also to a new MS. 
     A currently used OFDM-based broadband wireless communication system has a structure wherein only an MS using a single bandwidth can be supported using one Frequency Allocation (FA). Therefore, to support a new MS, using a wider bandwidth to be developed in the future, an FA of the system has to be changed to a new FA having a bandwidth corresponding to the wider bandwidth used by the new MS. However, due to the change of the FA, the system cannot provide a service to a legacy MS using a narrow bandwidth. That is, there is a problem in that all legacy MSs have to be changed while changing the FA of the system. Accordingly, there is a need for a method of supporting both a legacy MS, using a narrow bandwidth, and a new MS, using a wide bandwidth, in an evolution process of a broadband wireless communication system. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention to solve at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and method for supporting both a mobile station using a narrow bandwidth and a mobile station using a wide bandwidth in a broadband wireless communication system. 
     Another aspect of the present invention is to provide an apparatus and method for using a plurality of frequency allocations according to a frequency overlay scheme in a broadband wireless communication system. 
     Another aspect of the present invention is to provide an apparatus and method for increasing a retransmission gain by using hybrid automatic repeat request when a plurality of frequency allocations are used according to a frequency overlay scheme in a broadband wireless communication system. 
     Another aspect of the present invention is to provide an apparatus and method for determining frequency allocations to be used for packet retransmission when a plurality of frequency allocations are used according to a frequency overlay scheme in a broadband wireless communication system. 
     In accordance with an aspect of the present invention, a method for packet transmission of a packet of a transmitting end using a plurality of Frequency Allocations (FAs) in a wireless communication system is provided. The method includes dividing one encoded packet into a plurality of parts, mapping the plurality of parts of the packet to the FAs through the plurality of different FAs transmission, and when a re-transmission request is received, re-mapping the plurality of parts of the packet to the FAs such that at least one of the plurality of parts is re-mapped to an FA that is different than an FA previously mapped thereto. Retransmitting the encoded packet by at least one of a number of sub-units. 
     In accordance with another aspect of the present invention, an apparatus for transmitting a packet using a plurality of Frequency Allocations (FAs) in a wireless communication system is provided. The apparatus includes a plurality of transmitters for transmitting the plurality of parts through the FAs, at least one mapper for mapping the parts of the packet to the FAs, a controller for changing a mapping relation between a plurality of parts of the packet and the FAs such that at least one of the plurality of parts is re-mapped to an FA that is different than an FA previously mapped thereto. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts 
         FIGS. 1A and 1B  illustrate examples of frequency band use for supporting all Mobile Stations (MSs) using different bandwidths in a broadband wireless communication system; 
         FIGS. 2A and 2B  illustrate examples of encoded packet mapping in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIGS. 3A and 3B  illustrate examples of Frequency Allocation (FA) change by using an address mapping scheme in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIGS. 4A to 4D  illustrate an example of FA change by using an interleaving column switching scheme in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 5  illustrates a flowchart for a packet transmission process of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 6  illustrates a flowchart for an FA change process using an address mapping scheme of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention; 
         FIG. 7  illustrates a flowchart for an FA change process using an interleaving column switching scheme of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention; and 
         FIG. 8  illustrates a block diagram of a transmitting end of a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A through 8 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system. 
     The present invention to be described below relates to a technique for simultaneously supporting Mobile Stations (MSs) using different-sized bandwidths. In particular, the present invention relates to a technique for applying Hybrid Automatic Repeat Request (HARQ) when multi-Frequency Allocation (FA) access is achieved according to a frequency overlay scheme. Although an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA)-based wireless communication system will be described as an example hereinafter, the present invention may also apply to other types of wireless communication systems. 
     First, a wireless communication system considered in the present invention will be described in brief. 
       FIGS. 1A and 1B  illustrate examples of a bandwidth switching process in a broadband wireless communication system. If a system operating an FA having a bandwidth of 10 Mega Hertz (MHz) intends to operate an FA having a bandwidth of 20 MHz, the following switching schemes are expected to be used. 
     In a first scheme, as shown in  FIG. 1A , while operating an FA having a bandwidth of 10 MHz  105 , an FA having a bandwidth of 20 MHz  110  is also operated simultaneously at a separate frequency band. In a second scheme (i.e., a frequency overlay scheme), as shown in  FIG. 1B , while operating an FA having a bandwidth of 10 MHz  115 , an FA having a bandwidth of 20 MHz  120  is also operated simultaneously by combining two FAs each having a bandwidth of 10 MHz  115  at the same frequency band. A wide bandwidth is required when operating a separate FA, as shown in  FIG. 1A . Therefore, in terms of spectral efficiency, it is preferable to use the frequency overlay scheme of  FIG. 1B . Therefore, a broadband wireless communication system conforming to the frequency overlay scheme of  FIG. 1B  is taken into account in the present invention. 
     In a broadband wireless communication system of the present invention, a resource allocated to an MS having multi-FA access capability has a format  200  of  FIG. 2A . As shown in  FIG. 2A , a resource for transmitting an encoded packet  205  is allocated throughout two FAs (i.e., FA 1   201  and FA 2   202 ). A MAP message  205  for reporting information regarding an address of the allocated resource, the number of slots in each FA, and the like, is located in any one of the FAs. For example, when k slots are allocated to the encoded packet  205 , the allocated slots are n slots included in the FA 1   201  and (k−n) slots included in the FA 2   202 . 
     If the encoded packet  205  needs to be retransmitted due to unsuccessful transmission of the packet, as shown in  FIG. 2B , a part of the encoded packet  205   a  transmitted using the FA 1   201  is retransmitted using the FA 2   202 , and a part of the encoded packet  205   b  transmitted using the FA 2   202  is retransmitted using the FA 1   201 . That is, an FA used at initial transmission differs from that used at retransmission. As a result, an additional frequency gain is produced. 
     As a specific example of changing an FA used at retransmission as described above, the present invention proposes an address mapping scheme and an interleaving column switching scheme. 
     First, the address mapping scheme will be described. In a wireless communication system of the present invention, to transmit an encoded packet  300  using an allocated resource, a transmitting end divides the encoded packet into a plurality of slot-sized parts, and maps the respective parts to actual physical slots. When the encoded packet is retransmitted, according to a predetermined rule, the transmitting end recalculates positions of the physical slots to be mapped with the respective parts. In this case, the rule is defined such that parts mapped to a k th  FA at n th  transmission are mapped to (k+1) th  FA at (n+1) th  transmission. The slot can be expressed in a Resource Unit (RU) or a subchannel. 
     For example, when the encoded packet is transmitted using three (3) FAs, address mapping is changed by retransmission as shown in  FIGS. 3A and 3B .  FIGS. 3A and 3B  illustrate an example of FA mapping at initial transmission and retransmission when an encoded packet  300  occupying nine (9) slots is transmitted. Referring to  FIG. 3A , the encoded packet  300  is divided in a slot unit, and among the divided parts  301  to  309 , a part  0   301  and a part  1   302  are mapped to slots  311 ,  313 , within a FA 1   310 , a part  2   3023  to a part  4   305  are mapped to slots  321 ,  323 ,  324  within an FA 2   320 , and a part  5   306  to a part  8   309  are mapped to slots  331 - 334  within an FA 3   330 . Further, when the encoded packet  300 , mapped as shown in  FIG. 3A , is retransmitted, the retransmitted encoded packet  300 ′ is mapped as shown in  FIG. 3B . Referring to  FIG. 3B , a part  0   301  and a part  1   302  are mapped to slots  322 ,  324  within an FA 2   320 , a part  2   303  to a part  4   305  are mapped to slots  331 ,  333 ,  334  within an FA 3   330 , and a part  5   306  to a part  8   309  are mapped to slots  311 - 314  within an FA 1   310 . For this, according to a predetermined rule, the transmitting end calculates an FA to be mapped with each part of the retransmitted packet  300 ′. In  FIGS. 3A and 3B , an identical number of slots are allocated to each FA, and thus all parts are retransmitted using an FA different from that used at initial transmission. However, unlike  FIGS. 3A and 3B , if a different number of slots are allocated to each FA, only some of the parts can be transmitted using an FA different from that used at initial transmission. 
     Next, the interleaving column switching scheme will be described. In a broadband wireless communication system of the present invention, a transmitting end performs block interleaving on an encoded packet in a symbol unit, and thereafter, transmits the resultant packet. For example, when the encoded packet consists of thirty (30) symbols, interleaving is performed as shown in  FIG. 4A . That is, the transmitting end performs interleaving by writing the thirty (30) symbols in a horizontal-axis direction in an interleaving buffer configured with an 8×4 matrix form, and by reading out the thirty (30) symbols in a vertical-axis direction from the interleaving buffer. Upon reading out the thirty (30) symbols arranged in a matrix form, the transmitting end changes an FA used at retransmission by changing an order of reading out the symbols along the columns. 
     For example, when the encoded packet consisting of the thirty (30) symbols are transmitted using three (3) FAs, distribution of the symbols at initial transmission and retransmission is as shown in  FIG. 4B  through  FIG. 4D . If the symbols are read out in an order of a 1 st  column, a 2 nd  column, a 3 rd  column, and a 4 th  column at initial transmission  405 , distribution of the symbols transmitted using each FA is as shown in  FIG. 4B . Referring to  FIG. 4B , symbols  1 ,  5 ,  9 ,  13 ,  17 ,  21 ,  25 ,  29 ,  2 , and  6  are transmitted using an FA 1   421 , symbols  10 ,  14 ,  18 ,  22 ,  26 ,  30 ,  3 ,  7 ,  11 , and  15  are transmitted using an FA 2   422 , symbols  19 ,  23 ,  27 ,  4 ,  8 ,  12 ,  16 ,  20 ,  24 , and  28  are transmitted using an FA 3   423 . 
     If the symbols are read out in an order of the 2 nd  column, the 3 rd  column, the 4 th  column, and the 1 st  column at first retransmission  410 , distribution of the symbols transmitted using each FA is as shown in  FIG. 4C . Referring to  FIG. 4C , symbols  2 ,  6 ,  10 ,  14 ,  18 ,  22 ,  26 ,  30 ,  3 , and  7  are transmitted using the FA 1   421 , symbols  11 ,  15 ,  19 ,  23 ,  27 ,  4 ,  8 ,  12 ,  16 , and  20  are transmitted using the FA 2   422 , and symbols  24 ,  28 ,  1 ,  5 ,  9 ,  13 ,  17 ,  21 ,  25 , and  29  are transmitted using the FA 3   423 . 
     In addition, if the symbols are read out in an order of the 3 rd  column, the 4 th  column, the 1 st  column, and the 2 nd  column at second retransmission  415 , distribution of the symbols transmitted using each FA is as shown in  FIG. 4D . Referring to  FIG. 4D , symbols  3 ,  7 ,  11 ,  15 ,  19 ,  23 ,  27 ,  4 ,  8 , and  12  are transmitted using the FA 1   421 , symbols  16 ,  20 ,  24 ,  28 ,  1 ,  5 ,  9 ,  13 ,  17 , and  21  are transmitted using the FA 2   422 , and symbols  25 ,  29 ,  2 ,  6 ,  10 ,  14 ,  18 ,  22 ,  26 , and  30  are transmitted using the FA 3   423 . 
     That is, as shown in  FIG. 4B  to  FIG. 4D , most symbols constituting the encoded packet are transmitted using different FAs at each retransmission by using the interleaving column switching scheme. 
     Hereinafter, an operation and structure of a transmitting end for retransmitting packets according to the aforementioned schemes will be described in greater detail. 
       FIG. 5  illustrates a flowchart for a packet transmission process of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the transmitting end initially transmits an encoded packet in step  501 . In this step, the transmitting end distributively maps the encoded packet to a plurality of FAs. In other words, the transmitting end transmits the encoded packet through a plurality of FAs. 
     After initially transmitting the encoded packet, proceeding to step  503 , the transmitting end determines whether acknowledge (ACK) or Non-ACK (NACK) on the encoded packet is transmitted from a receiving end. That is, the transmitting end determines whether the encoded packet needs to be retransmitted. The procedure of  FIG. 5  ends when the ACK is received. 
     Upon receiving the NACK, proceeding to step  505 , the transmitting end determines an FA to be used at retransmission with respect to each part of the encoded packet distributively mapped to each FA at initially transmission. In this step, the transmitting end determines FAs to be used at retransmission so that different FAs are mapped at initial transmission and retransmission to at least one of the subunit entities constituting the parts of the encoded packet. In other words, the transmitting end determines FAs to be used at retransmission so that each subunit entity is transmitted using one of the at least one FA excluding the FA used at a previous transmission. The subunit entity is a part or a symbol. The determination of the FAs for retransmission may be performed using various schemes. For example, the address mapping scheme of  FIG. 3  or the interleaving column switching scheme of  FIG. 4  can be used. When the address mapping scheme is used, the transmitting end recalculates FAs to be mapped with slot-sized parts included in each part. When the interleaving column switching scheme is used, the transmitting end reads out symbols from a block interleaving buffer in an order different from that used at previous transmission, and determines FAs to be mapped with the symbols in an order of reading out the symbols. 
     After determining the FA to be used at retransmission with respect to each part of the encoded packet, proceeding to step  507 , the transmitting end retransmits the encoded packet through the determined FA. That is, the transmitting end maps each subunit entity constituting the parts of the encoded packet to its corresponding FA, and thereafter, retransmits the resultant packet to the receiving end. 
       FIG. 6  illustrates a flowchart for an FA change process using an address mapping scheme of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention. The process of  FIG. 6  is an exemplary operation of step  505  of  FIG. 5  when the address mapping scheme is used. 
     Referring to  FIG. 6 , in step  601 , the transmitting end initializes m to total_FA, which indicates a total number of FAs used to transmit an encoded packet, initializes slot_idx to First_slot_idx, which indicates an index of a first slot of the encoded packet, and initializes L to Last_slot_idx, which indicates an index of a last slot of the encoded packet. 
     After initializing these variables (i.e., m, slot_idx, and L), proceeding to step  603 , the transmitting end determines whether the slot_idx is equal to L+1. If the slot_idx is equal to L+1, the procedure of  FIG. 6  ends. 
     Otherwise, if the slot_idx is not equal to L+1, proceeding to step  605 , the transmitting end sets n to pre_FA(slot_idx), i.e., an index of an FA mapped with a (slot_idx) th  slot of the encoded packet at previous transmission. Further, the transmitting end sets assign_FA(slot_idx), i.e., a retransmission FA of the (slot_idx) th  slot of the encoded packet, to a sum of ‘1’ and a value obtained as a result of performing a modulo operation on the n and the m. 
     In step  607 , the transmitting end sets the pre_FA(slot_idx) to assign_FA(slot_idx) set in step  605 . That is, to calculate an FA to be retransmitted at next retransmission, the transmitting end updates the pre_FA(slot_idx). 
     In step  609 , the transmitting end increments the slot_idx by ‘1’, and the procedure returns to step  603 . 
     By performing the procedure of  FIG. 6 , a retransmission FA of each part of the encoded packet is determined as a next FA of the FA used at previous transmission. That is, a mapping relation between the parts and the FAs is cyclic shifted. 
       FIG. 7  illustrates a flowchart for an FA change process using an interleaving column switching scheme of a transmitting end in a broadband wireless communication system according to an exemplary embodiment of the present invention. The process of  FIG. 7  is an exemplary operation of step  505  of  FIG. 5  when the interleaving column switching scheme is used. It is assumed, in the following description, that M×N-sized block interleaving is used in the present invention. 
     Referring to  FIG. 7 , in step  701 , the transmitting end initializes L to total_tx_sym that indicates a total number of symbols of an encoded packet, initializes m to total_FA that indicates a total number of FAs used to transmit the encoded packet, and initializes F(fa_idx) to No_sym_FA(fa_idx) that indicates the number of symbols allocated to an (fa_idx) th  FA. The fa_idx is an integer greater than or equal to ‘1’ and less than or equal to the m. 
     In step  703 , the transmitting end sets col_idx to pre_srt_col_idx, i.e., a sum of ‘1’ and a value obtained as a result of performing a modulo operation on N and an index of column at which the symbols start to be read out at previous transmission, where N denotes the number of columns of block interleaving. For example, in a case where four columns  1 ,  2 ,  3 , and  4  exist, if the index of the column at which the symbols start to be read out at previous transmission is ‘1’, the col_idx is set to ‘2’ in step  703 . Further, the transmitting end sets buf_idx to 1. 
     In step  705 , the transmitting end determines whether the symbols are completely read out, that is, whether all symbols of the encoded packet are read out. If all symbols of the encoded packet are read out, the procedure proceeds to step  719 . 
     Otherwise, if all symbols of the encoded packet are not read out, proceeding to step  707 , the transmitting end sets row_idx to 0. 
     After setting the rox_idx to 0, proceeding to step  709 , the transmitting end determines whether the row_idx is greater than M−1, where M denotes the number of rows of block interleaving. In other words, the transmitting end determines whether the row_idx is greater than an index of a last row of block interleaving. If the row_idx is greater than M−1, proceeding to step  711 , the transmitting end increments the col_idx by ‘1’, and the procedure returns to step  705 . 
     Otherwise, if the row_idx is less than or equal to M−1, proceeding to step  713 , the transmitting end determines whether a sum of the col_idx and a value obtained as a result of performing multiplication on N and the row_idx is greater than the L. That is, the transmitting end determines whether all symbols of the encoded packet are read out. If the sum of the col_idx and the value obtained as a result of performing multiplication on N and the row_idx is greater than the L, proceeding to step  711 , the transmitting end increments the col_idx by 1, and the procedure returns to step  705 . 
     Otherwise, if the sum of the col_idx and the value obtained as a result of performing multiplication on N and the row_idx is less than or equal to the L, proceeding to step  715 , the transmitting end reads out an I_buffer((N×row_idx),col_idx) th  symbol, i.e., a symbol stored at an (N×row_idx) th  row and a (col_idx) th  column of a block interleaving buffer. Then the transmitting end stores the read-out symbols sequentially in a separate transmission buffer T_buffer( ). 
     Thereafter, the transmitting end increments the row_idx by ‘1’, and the procedure returns to step  709 . 
     If all symbols of the encoded packet are read out in step  705 , proceeding to step  719 , the transmitting end sets pre_srt_col_idx to a sum of ‘1’ and a value obtained as a result of performing a modulo operation on the pre_srt_col_idx and N. That is, to calculate an FA to be retransmitted at next retransmission, the transmitting end updates the pre_srt_col_idx. 
     In step  721 , the transmitting end initializes fa_idx to ‘1’, and initializes s to ‘1’. 
     After initializing these variables (i.e., fa_idx and s), proceeding to step  723 , the transmitting end determines whether the fa_idx is greater than the m. If the fa_idx is greater than the m, the procedure of  FIG. 7  ends. 
     Otherwise, if the fa_idx is less than or equal to the m, proceeding to step  725 , the transmitting end sets alloc_idx to ‘1’. 
     After setting the alloc_idx to ‘1’, proceeding to step  727 , the transmitting end determines whether the alloc_idx is greater than F(fa_dx), i.e., the number of slots allocated to an (fa_idx) th  FA. If the alloc_idx is greater than the F(fa_idx), proceeding to step  729 , the transmitting end increments the alloc_idx by ‘1’, and the procedure returns to step  723 . 
     Otherwise, if the alloc_idx is less than or equal to the F(fa_idx), proceeding to step  731 , the transmitting end sets assign_FA(T_buffer(s)), i.e., a retransmission FA of a symbol positioned at an s th  address of T_buffer ( ), to the fa_idx. 
     In step  733 , the transmitting end increments the alloc_idx by ‘1’, and increments the s by ‘1’°. Then, the procedure returns to step  727 . 
     Thereafter, steps  723  to  733  are repeated until the fa_idx is greater than m, so as to determine a retransmission FA of each symbol. 
       FIG. 8  illustrates a block diagram of a transmitting end of a broadband wireless communication system according to an exemplary embodiment of the present invention. The transmitting end having a structure of  FIG. 8  can use three (3) FAs according to a frequency overlay scheme. When the transmitting end can use two (2), four (4) or more FAs, a similar structure as that of  FIG. 8  is used. 
     Referring to  FIG. 8 , the transmitting end includes an encoder  802 , a symbol modulator  804 , a packet divider  806 , a plurality of subcarrier mappers  808 - 1  to  808 - 3 , a plurality of OFDM modulators  810 - 1  to  810 - 3 , a plurality of Radio Frequency (RF) transmitters  812 - 1  to  812 - 3 , a retransmission buffer  814 , and a retransmission controller  816 . 
     The encoder  802  performs channel coding on an input data bit-stream to generate an encoded packet. The symbol modulator  804  modulates the encoded packet to convert the packet into complex symbols. 
     To transmit a single encoded packet through a plurality of FAs, the packet divider  806  divides the single encoded packet into a plurality of parts to be transmitted using each FA. Further, the packet divider  806  provides each part to the subcarrier mappers  808 - 1  to  808 - 3  for managing each FA. 
     Each of the subcarrier mappers  808 - 1  to  808 - 3  maps the complex symbols to be transmitted using its corresponding FA to a frequency domain. Each of the OFDM modulators  810 - 1  to  810 - 3  converts the complex symbols mapped to the frequency domain into time-domain signals by performing an Inverse Fast Fourier Transform (IFFT) operation, and configures an OFDM symbol to be transmitted using its corresponding FA by inserting a Cyclic Prefix (CP). Each of the RF transmitters  812 - 1  to  812 - 3  up-converts a baseband signal into its corresponding RF signal, and transmits the RF signal through an antenna. 
     The retransmission buffer  814  stores the encoded packet that needs to be retransmitted. If an ACK is received for the stored encoded packet, the retransmission buffer  814  deletes the encoded packet corresponding to the ACK. If a NACK is received for the stored encoded packet, the retransmission buffer  814  provides the encoded packet corresponding to the NACK to the packet divider  806  under the control of the retransmission controller  816 . 
     According to the ACK or NACK fed back from a receiving end, the retransmission controller  816  determines whether the packet needs to be retransmitted. If the NACK is received, the retransmission controller  816  provides control such that the encoded packet stored in the retransmission buffer  814  is retransmitted. In particular, if the encoded packet is retransmitted, the retransmission controller  816  determines an FA to be used at retransmission so that a different FA is mapped at initial transmission and retransmission to at least one of the subunit entities constituting the parts of the encoded packet. Further, the retransmission controller  816  provides determined retransmission FA information to the packet divider  806 . The subunit entity is a part or a symbol. 
     The determination of the FAs for retransmission may be performed using various schemes. For example, the address mapping scheme of  FIG. 3  or the interleaving column switching scheme of  FIG. 4  can be used. When the address mapping scheme is used, the retransmission controller  816  determines a retransmission FA by performing the process of  FIG. 6 . That is, the retransmission controller  816  performs cyclic shifting on a mapping relation between the parts and the FAs. For this, the retransmission controller  816  recalculates FAs to be mapped with slot-sized parts included in each part. More specifically, the retransmission controller  816  evaluates an index of an FA mapped with one part at previous transmission, and calculates an index of an FA to be mapped with the part at retransmission by calculating a sum of ‘1’ and a value obtained as a result of performing a modulo operation on the index of the FA with a total number of FAs. 
     When the interleaving column switching scheme is used, the retransmission controller  816  determines a retransmission FA by performing the process of  FIG. 7 . Further, the retransmission controller  816  controls packet division of the packet divider  806  and an output of divided parts according to the determined retransmission FA. When the interleaving column switching scheme is used, the transmitting end further includes an interleaver (not shown) for performing block interleaving in a symbol unit. Accordingly, the retransmission controller  816  controls the interleaver (not shown) to read out symbols from a block interleaving buffer in an order different from that used at previous transmission, and determines FAs to be mapped with the symbols in an order of reading out the symbols. 
     According to exemplary embodiments of the present invention, in a broadband wireless communication system supporting a frequency overlay scheme, a transmitting end transmits packets through different FAs at initial transmission and retransmission, and thus, a receiving end can obtain an additional frequency gain when Chase Combining (CC) is performed. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.