Abstract:
A method and apparatus for transmitting/receiving an ACKnowledgement/Negative ACKnowledgement (ACK/NACK) signal to support packet data retransmission in an Frequency Division Multiple Access (FDMA) wireless communication system are provided, in which a User Element (UE) generates an ACK/NACK signal for received packet data, determines whether the UE is set to support ACK/NACK repetition, transmits the ACK/NACK signal on a basic response channel mapped to one of a DCH on which the packet data was received and a Shared Control Channel (SCCH) carrying scheduling information about the packet data, if the UE is not set to support ACK/NACK repetition, and selects one of supplementary response channels for each ACK/NACK repetition, the supplementary response channels being allocated for ACK/NACK repetition and repeatedly transmits the ACK/NACK signal on the selected supplementary response channel according to a predetermined repetition factor, if the UE is set to support ACK/NACK repetition.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority under 35 U.S.C. §119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Aug. 18, 2006 and assigned Serial No. 2006-78413, and a Korean Patent Application filed in the Korean Intellectual Property Office on Jul. 27, 2007 and assigned Serial No. 2007-75638 the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for transmitting/receiving an ACKnowledgement/Negative ACKnowledgement (ACK/NACK) signal for received packet data in order to support Hybrid Automatic Repeat reQuest (HARQ) in a Frequency Division Multiple Access (FDMA) wireless communication system. 
     2. Description of the Related Art 
     With reference to  FIG. 1 , FDMA will first be described below.  FIG. 1  illustrates FDMA. 
     Referring to  FIG. 1 , FDMA is a technology for distinguishing physical channels in frequency. In general, all available resources are divided in time and frequency as indicated by reference numeral  101 . A minimum block is composed of one symbol in time and one subcarrier in frequency. This is called a Time-Frequency (TF) bin. A TF bin is an actual transmission unit carrying a modulation symbol on a physical channel. The total number of TF-bins depends on a total frequency bandwidth and the number of symbols transmittable in a Transmission Time interval (TTI). 
     In FDMA, different TF bins are allocated to different channels and different User Equipments (UEs). Basically, it is impossible to share one TF bin between different channels or different UEs. However, to achieve time diversity, frequency diversity, or spatial diversity for a low-rate channel, a TF bin can be shared by covering or spreading the TF bin with a code as with Code Division Multiplexing (CDM). For transmission of high-rate packet data, TF bins are purely allocated. Mapping between data symbols and TF bins is equivalent to subcarrier mapping  102 . The mapping relationship between a channel and TF bins is signaled beforehand or determined according to a predefined rule. 
     Since a channel carrying packet data or a channel carrying signaling data is allocated on a UE basis, TF bins are allocated to the channel and then the channel with TF bins is allocated to an intended UE. Although TF bins can be allocated on a UE basis, TF bins allocated to a specific channel form a logical channel as indicated by reference numeral  103 , which is preferable in terms of signaling. One logical channel (hereinafter, “channel”) is composed of a plurality of TF bins and the number of TF bins is determined, taking into account the characteristics of the channel. In determining the number of TF bins for a channel, the lowest data rate of packet data is considered if the channel is a packet data channel, and signaling overhead of information about a scheduled channel is considered if scheduling is carried out. For a control channel, the number of TF bins is determined according to the number of bits transmitted per TTI. 
     A channel to be allocated to a UE is scheduled every TTI or set by higher signaling. In the illustrated case of  FIG. 1 , if a Data CHannel (DCH)  103  is allocated to a UE, packet data symbols  105  for the UE are mapped to the DCH  103  by channel mapping  104  and then mapped to actual TF bins by subcarrier mapping  102 . During the subcarrier mapping  102 , the channel may be mapped to scattered TF bins (e.g. a DCH  106 ) or successive TF bins (e.g. the DCH  103 ), depending on whether frequency diversity is to be achieved or according to a TF bin 4 allocation algorithm. Since the channel is a logical channel, when only an allocated channel is transmitted as with a UE, there may not be a need for channel mapping because transmission symbols are simply mapped to predefined TF bins of the allocated channel. 
     HARQ is a technique for increasing a reception success rate by soft-combining initial transmission data with retransmission data without discarding the initial transmission data. A HARQ receiver determines whether a received packet has errors and transmits a HARQ ACK signal or a HARQ NACK signal to a HARQ transmitter according to the determination result. Accordingly, the HARQ transmitter retransmits the HARQ packet or transmits a new HARQ packet according to the received HARQ ACK/NACK signal. 
     HARQ is categorized into synchronous HARQ and asynchronous HARQ according to the timing of retransmission. In synchronous HARQ, a retransmission occurs a predetermined time after completion of a previous transmission, whereas a retransmission occurs irrespective of the time of a previous transmission in asynchronous HARQ. 
     With reference to  FIG. 2 , a synchronous HARQ operation will be described in more detail.  FIG. 2  illustrates a basic HARQ operation. 
     Referring to  FIG. 2 , a HARQ transmitter transmits an initial HARQ packet on a DCH  202  by a predetermined process in step  203 . A HARQ receiver decodes the initial HARQ packet and determines whether the initial HARQ packet has errors by a Cyclic Redundancy Check (CRC) check. In the presence of errors, the HARQ receiver stores the HARQ packet in a buffer and transmits an HARQ NACK to the HARQ transmitter on an ACK CHannel (ACKCH)  202  in step  205 . In step  206 , the HARQ transmitter retransmits the HARQ packet. The HARQ receiver soft-combines the stored HARQ packet with the retransmission HARQ packet and performs a CRC check in step  207 . If the combined HARQ packet still has errors, the HARQ receiver stores the HARQ packet in the buffer and transmits an HARQ NACK to the HARQ transmitter. However, if decoding of the combined HARQ packet is successful, the HARQ receiver transmits an HARQ ACK to the HARQ transmitter in step  208 . 
     The HARQ transmitter repeats the above operation until it receives an HARQ ACK from the HARQ receiver or the number of retransmissions for the HARQ packet reaches a predetermined retransmission number. 
     Now an ACK/NACK transmission method will be described. 
     Conventionally, a dedicated channel is allocated for a UE so that the UE can transmit an ACK/NACK signal. Under an environment where channels are non-orthogonal as with Code Division Multiple Access (CDMA), the total amount of available resources is limited by transmit power or reception interference level rather than it is directly related to the number of codes. Therefore, allocation of a code to each UE is not a significant problem in terms of resource utilization even if the UE does not use the dedicated channel. However, T-F resources are orthogonal and the amount of T-F resources directly affects that of available resources in FDM. Hence, when T-F resources allocated to an ACKCH are not used, it is a waste of resources. In other words, dedicated allocation of resources for ACK/NACK transmission on a UE-by-UE basis is inefficient in terms of resource utilization in an FDMA system. 
     In this context, one-to-one mapping between ACKCHs and DCHs or Shared Control CHannels (SCCHs) has been proposed and is under discussion in order to support HARQ efficiently in the FDMA system. 
       FIG. 3  illustrates one-to-one mapping between DCHs and ACKCHs. 
     Referring to  FIG. 3 , reference numerals  302  to  305  are DCHs and reference numerals  307  to  310  denote ACKCHs. The DCHs  302  to  305  are mapped to the ACKCHs  307  to  310  in a one-to-one correspondence and an ACK/NACK signal for a received DCH is transmitted on a predetermined ACKCH mapped to the DCH. If packet data is received on a first DCH  302  (DCH # 1 ), an ACK/NACK signal for the packet data is transmitted on a first ACKCH  307  (ACKCH # 1 ). If packet data is received on a second DCH  303  (DCH # 2 ), an ACK/NACK signal for the packet data is transmitted on a second ACKCH  308  (ACKCH # 2 ). The mapping between the ACKCHs and the DCHs enables ACK/NACK transmission without allocating dedicated frequent resources to UEs. 
       FIG. 18  illustrates one-to-one mapping between SCCHs and ACKCHs. 
     Referring to  FIG. 18 , reference numerals  1802  to  1805  are SCCHs and reference numerals  1807  to  1810  denote ACKCHs. The SCCHs  1802  to  1805  are mapped to the ACKCHs  1807  to  1810  in a one-to-one correspondence and an ACK/NACK signal for a received DCH is transmitted on a predetermined ACKCH mapped to an SCCH by which the DCH has been scheduled. If scheduling information about packet data is received on a first SCCH  1802  (SCCH # 1 ), an ACK/NACK signal for the packet data is transmitted on a first ACKCH  1807  (ACKCH # 1 ). If scheduling information about packet data is received on a second SCCH  1803  (SCCH # 2 ), an ACK/NACK signal for the packet data is transmitted on a second ACKCH  1808  (ACKCH # 2 ). 
     ACK/NACK repetition will be described below. 
     Typically, an ACK/NACK TTI is equal in length to a TTI of a general downlink frame or an uplink frame. When a Mobile Station (MS) at a cell boundary needs a transmit power exceeding a maximum allowed power, for ACK/NACK transmission, it transmits an ACK/NACK signal with the maximum allowed power. The resulting decreased received signal level renders the ACK/NACK transmission unreliable. To avert this problem, a High Speed Downlink Packet Access (HSDPA) system repeats the same ACK/NACK signal, so that instantaneous power level requirements are decreased as much as a repetition number and thus the ACK/NACK signal can be transmitted within a maximum allowed power level. Information about whether an ACK/NACK signal is repeated (hereinafter, referred to as ACK/NACK repetition setting information) is set in an upper-layer signaling message or a Medium Access Control (MAC) message by a network. 
     For ACK/NACK repetition, a repetition factor should be set in order to indicate whether an ACK/NACK signal is repeated and how many times the repetition occurs. For example, if the repetition factor is a non-zero number, the ACK/NACK signal is repeated as many times as the repetition factor. If the repetition factor is 0, the ACK/NACK signal is transmitted only once. 
     When a system with ACKCHs mapped to DCHs or SCCHs supports ACK/NACK repetition, it faces some problems, which will be addressed with reference to  FIG. 4 . In the illustrated case of  FIG. 4 , one cell has two UEs, UE # 1  and UE # 2 . UE # 1  is located at a cell boundary and UE # 2  is near to a Node B at the center of the cell. 
     First, one-to-one mapping between DCHs and ACKCHs will be described. 
     Referring to  FIG. 4 , three ACK/NACK repetitions are set for UE # 1  so that it can transmit an ACK/NACK signal reliably. Since UE # 2  has sufficient transmit power, UE # 2  is supposed to transmit an ACK/NACK signal only once. 
     Upon receipt of packet data on a first DCH  402  (DCH # 1 ) in step  405 , UE # 1  transmits an ACK/NACK signal on a first ACKCH  402  (ACKCH # 1 ) at time k=4 in step  407  and then repeats the ACK/NACK signals on ACKCH # 1  at time k=5 and k=6 in steps  408  and  409 . Meanwhile, a Node B may transmit packet data to UE # 2  on DCH # 1  during the next TTI through scheduling in step  406 . Then UE # 2  transmits an ACK/NACK signal for the received packet data on ACKCH # 1  at time k=5 in step  410 . Thus, the ACK/NACK signals from UE # 1  and UE # 2  collide on ACKCH # 1  at time k=5. This data collision occurs because UEs share the ACKCHs and the DCHs are mapped to the ACKCHs in a one-to-one correspondence. 
     ACK/NACK repetition is viable on the premise that an ACKCH can be allocated to a UE for a plurality of TTIs. However, data is transmitted on a DCH only during one TTI and the one-to-one mapping between ACKCHs and DCHs does not allow for allocation of an ACKCH mapped to the DCH long enough for ACK/NACK transmission. As SCCHs are also transmitted on a TTI basis, the one-to-one mapping between SCCHs and ACKCHs illustrated in  FIG. 18  leads to the same problem. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and apparatus for supporting ACK/NACK repetition when ACKCHs are mapped to DCHs or SCCHs in an FDMA wireless communication system. 
     Another aspect of the present invention is to provide a method and apparatus for enabling ACK/NACK repetition by allocating more ACKCHs than SCCHs or SCCHs in an FDMA wireless communication system. 
     A further aspect of the present invention is to provide a method and apparatus for selecting a supplementary ACKCH for ACK/NACK repetition, when basic ACKCHs are allocated for as many DCHs or SCCHs and simultaneously, supplementary ACKCHs are allocated in an FDMA wireless communication system. 
     Still another aspect of the present invention is to provide a method and apparatus for supporting ACK/NACK repetition by allocating a supplementary ACKCH dedicatedly to a UE that will repeats an ACK/NACK signal in an FDMA wireless communication system. 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method for transmitting an ACK/NACK signal to support a retransmission of packet data received from a Node B in a UE in an FDMA wireless communication system, in which the UE generates an ACK/NACK signal for received packet data, determines whether the UE is set to support ACK/NACK repetition, transmits the ACK/NACK signal on a basic response channel mapped to one of a DCH on which the packet data was received and an SCCH carrying scheduling information about the packet data, if the UE is not set to support ACK/NACK repetition, and selects one of supplementary response channels for each ACK/NACK repetition, the supplementary response channels being allocated for ACK/NACK repetition and repeatedly transmits the ACK/NACK signal on the selected supplementary response channel according to a predetermined repetition factor, if the UE is set to support ACK/NACK repetition. 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method for receiving an ACK/NACK signal from a UE to support a retransmission of packet data in an FDMA wireless communication system, in which it is determined whether the UE is set to support ACK/NACK repetition, an ACK/NACK signal for transmitted packet data is received on a basic response channel mapped to one of a DCH on which the packet data was transmitted and an SCCH on which information about the packet data was transmitted, if the UE is not set to support ACK/NACK repetition, and one of supplementary response channels is selected for each ACK/NACK repetition, the supplementary response channels being allocated for ACK/NACK repetition, and the ACK/NACK signal is received repeatedly on the selected supplementary response channel according to a predetermined repetition factor, if the UE is set to support ACK/NACK repetition. 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided an apparatus of a UE for transmitting an ACK/NACK signal to support a retransmission of packet data received from a Node B in an FDMA wireless communication system, in which a response channel decider selects a response channel that will carry an ACK/NACK signal according to information about all response channels, one of information about a DCH on which packet data was received and information about an SCCH on which scheduling information about the packet data was received, and a repetition factor for the ACK/NACK signal, a response generator generates the ACK/NACK signal for the packet data, and a multiplexer maps the ACK/NACK signal to physical layer resources corresponding to the selected response channel and transmits the ACK/NACK signal repeatedly according to the repetition factor. 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided an apparatus for receiving an ACK/NACK signal from a UE to support a retransmission of packet data to the UE in an FDMA wireless communication system, in which a response channel decider selects a response channel on which an ACK/NACK signal will be received according to information about all response channels, one of information about a DCH on which packet data was transmitted and information about an SCCH on which scheduling information about the packet data was transmitted, and a repetition factor for the ACK/NACK signal, a demultiplexer extracts the ACK/NACK signal from signals received on physical channels according to the selected response channel, and a response decoder decodes the extracted the ACK/NACK signal and acquires ACKNACK information for the packet data transmitted to the UE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates FDMA; 
         FIG. 2  illustrates a basic HARQ operation; 
         FIG. 3  illustrates one-to-one mapping between DCHs and ACKCHs; 
         FIG. 4  illustrates a problem encountered with ACK/NACK repetition when DCHs are mapped to ACKCHs in a one-to-one correspondence; 
         FIG. 5  illustrates ACKCH allocation according to a first exemplary embodiment of the present invention; 
         FIG. 6  illustrates ACK/NACK transmission according to the first exemplary embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating ACK/NACK selection according to the first exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram of an ACK/NACK transmitter according to the first exemplary embodiment of the present invention; 
         FIG. 9  is a block diagram of an ACK/NACK receiver according to the first exemplary embodiment of the present invention; 
         FIG. 10  illustrates ACKCH allocation according to a second exemplary embodiment of the present invention; 
         FIG. 11  illustrates ACK/NACK transmission according to the second exemplary embodiment of the present invention; 
         FIG. 12  is a flowchart illustrating ACK/NACK selection according to the second exemplary embodiment of the present invention; 
         FIG. 13  illustrates ACKCH allocation according to a third exemplary embodiment of the present invention; 
         FIG. 14  illustrates ACK/NACK transmission according to the third exemplary embodiment of the present invention; 
         FIG. 15  illustrates ACKCH allocation according to a fourth exemplary embodiment of the present invention; 
         FIG. 16  illustrates ACK/NACK transmission according to the fourth exemplary embodiment of the present invention; 
         FIG. 17  is a flowchart illustrating ACK/NACK selection according to the fourth exemplary embodiment of the present invention; and 
         FIG. 18  illustrates one-to-one mapping between SCCHs and ACKCHs. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of exemplary embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     Exemplary embodiments of the present invention provide a method for solving a problem encountered with ACK/NACK repetition in the case where ACKCHs are mapped to DCHs or SCCHs and thus shared among UEs in an FDMA wireless communication system. 
     For this purpose, the present invention allocates more ACKCHs than DCHs or SCCHs to support ACK/NACK repetition. Therefore, the following description is made of a method for allocating supplementary ACKCHs, a method for selecting an ACKCH in a UE that will transmit an ACK/NACK signal repeatedly, a transmitter for transmitting an ACK/NACK signal, and a receiver for receiving an ACK/NACK signal. 
     While the present invention is applicable to any ACKCH allocation for downlink HARQ and uplink HARQ, it will be described in the context of an uplink ACK/NACK allocation to support downlink HARQ. 
     Embodiment 1 
     In accordance with a first exemplary embodiment of the present invention, supplementary ACKCHs are allocated, besides basic ACKCHs. If a UE is supposed to transmit an ACK/NACK signal repeatedly, the UE always transmits repeated ACK/NACK signals on a supplementary ACKCH. 
       FIG. 5  illustrates ACKCH allocation according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , three DCHs  501  (or SCCHS) are allocated. Compared to the conventional technology in which three ACKCHs are allocated for one-to-one mapping to the DCHs or SCCHs, six ACKCHs  502  and  503  are allocated to support ACK/NACK repetition in this exemplary embodiment of the present invention. Three of the ACKCHs  502  and  503  are basic ones  502  and the other three are supplemental ones  503 . 
     With the allocated ACKCHs, UEs transmit ACK/NACK signals in the manner illustrated in  FIG. 6 . 
       FIG. 6  illustrates ACK/NACK transmission according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , UE # 1  is set to support ACK/NACK repetition, while UE # 2  is not set to support ACK/NACK repetition. A repetition factor R for UE # 1  is 2. When receiving packet data on DCH # 1  or scheduling information about the packet data on SCCH # 1  in a first frame (k=1) in step  605 , UE # 1  transmits an ACK/NACK signal on ACKCH # 1  in a fourth frame (k=4) in step  606 . As UE # 1  supports ACK/NACK repetition, it retransmits the same ACK/NACK signal on ACKCH # 4  mapped to DCH # 1  or SCCH # 1 , for ACK/NACK repetition in fifth and sixth frames (k=5 and 6) in steps  607  and  608 . In this manner, an ACK/NACK collision is avoided between UE # 1  and UE # 2  in the fifth frame (k=5). 
     In asynchronous HARQ, because retransmission time points are not fixed, three TTIs of ACKCH transmission do not affect retransmission time points. However, the retransmission time points of UE # 1  and UE # 2  may be changed in synchronous HARQ. Thus, a UE or a Node B uses a parameter that determines a repetition number in calculating a retransmission time point, to thereby secure an appropriate processing time. 
       FIG. 7  is a flowchart illustrating ACK/NACK selection in a UE according to the exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , the UE demodulates received packet data and generates an ACK/NACK signal depending on whether the packet data has errors in step  701 . In step  702 , the UE checks whether it is set to support ACK/NACK repetition to select an ACKCH that will carry the ACK/NACK signal. If ACK/NACK repetition setting information is set to repetition, or a repetition factor R is larger than 0, the UE is supposed to repeat the ACK/NACK signal. 
     If the UE is not supposed to repeat the ACK/NACK signal, it transmits the ACK/NACK signal on a basic ACKCH in steps  703  and  704 . The basic ACKCH is determined according to a DCH on which the packet data has been received or an SCCH that delivers scheduling information about the packet data. 
     If the UE is supposed to repeat the ACK/NACK signal, it performs (R+1) loops because as many ACK/NACK repetitions as the repetition factor R have to occur in step  705 . To be more specific, the UE determines whether a current ACK/NACK transmission is an initial transmission in step  710 . In the case of the initial ACK/NACK transmission (i=0, i is a variable indicating the number of repetitions), the UE selects the basic ACKCH mapped to the DCH or the SCCH in step  706  and transmits the ACK/NACK signal on the basic ACKCH. If i is greater than or equal to 1, the UE selects a supplementary ACKCH mapped to the DCH or the SCCH in step  707 . In step  708 , the UE retransmits the ACK/NACK signal on the supplementary ACKCH. 
     A Node B operates in a similar manner to the UE. In the method illustrated in  FIG. 7 , the Node B selects an ACKCH, receives the ACK/NACK signal from the UE on the selected ACKCH, and decodes it. 
       FIG. 8  is a block diagram of an ACK/NACK transmitter according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , an ACKCH decider  801  receives information about all ACKCHs  806 , information about a received DCH (or information about an SCCH carrying scheduling information about the DCH)  802 , and a repetition factor R  802  and selects an ACKCH  807  on which to transmit an ACK/NACK signal at a current transmission time point. The total ACKCH information  806  may be preset or notified by upper-layer signaling. The DCH information or the SCCH information is received from a receiver that has received packet data and the repetition factor R is received by upper-layer signaling. 
     An ACKCH generator  803  encodes or modulates an actual ACK/NACK bit  811  generated according to the reception result of the packet data in a predetermined format. 
     A Multiplexer (MUX)  804  maps the ACK/NACK signal received from the ACKCH generator  803  to predetermined physical layer resources according to ACKCH information  807  indicating the selected ACKCH received from the ACKCH decider  801 . 
     An Inverse Fast Fourier Transform (IFFT) processor  805  IFFT-processes the mapped ACK/NACK signal. 
       FIG. 9  is a block diagram of an ACK/NACK receiver according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , an ACKCH decider  906  receives total ACKCH information  905 , and information about a transmitted DCH or SCCH and a repetition factor R  901 , and determines an ACKCH to receive at a current time point. The total ACKCH information may be preset or informed by upper-layer signaling. The DCH or SCCH information is received from a transmitter that has transmitted packet data and the repetition factor R is notified by upper-layer signaling. 
     A Demultiplexer (DEMUX)  903  is aware of physical layer resources corresponding to the determined ACKCH based on ACKCH information  907  indicating the determined ACKCH. That is, the DEMUX  903  extracts the ACKCH from all physical channel resources received from a Fast Fourier Transform (FFT) processor  902 . A ACKCH decoder  904  acquires an actual ACK/NACK signal  908  by decoding and demodulating the ACKCH. 
     Embodiment 2 
     A shortcoming of the first exemplary embodiment of the present invention is that successive allocation of the same DCH to UEs that support ACK/NACK repetition may cause an ACK/NACK collision between the UEs because the same supplementary ACKCH is used to deliver repeated ACK/NACK signals. To overcome this problem, a Node B scheduler should allocate data channels such that UEs for which ACK/NACK repetition is set do not receive the same data channel in a successive manner. For instance, in  FIG. 6 , if UE # 1  supports ACK/NACK repetition, scheduling is performed such that a DCH allocated to UE # 1  in a current TTI is allocated to UE-# 2  in the next TTI. Thus both UE # 1  and UE # 2  can transmit ACK/NACK signals without collision at time k=6. 
     In this context, a second exemplary embodiment of the present invention provides a method for supporting ACK/NACK repetition by allocating, to each UE, a supplementary ACKCHs for each ACK/NACK repetition of the UE. 
       FIG. 10  illustrates ACKCH allocation according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , three DCHs  1001  (or SCCHs) are allocated. Compared to the conventional technology in which three ACKCHs are allocated for one-to-one mapping to the DCHs or SCCHs, nine ACKCHs  1002  and  1003  are allocated to support ACK/NACK repetition in this exemplary embodiment of the present invention. Three of the ACKCHs  1002  and  1003  are basic ACKCHs  1002  and the other six are supplemental ACKCHs  1003 . In this case, a repetition factor R that can support ACK/NACK repetition without collision in a cell is 2. 
     To implement the second exemplary embodiment of the present invention, an algorithm is proposed to select an ACKCH to deliver an ACK/NACK signal at a given time point from among a plurality of ACKCHs. An ACKCH can be selected according to Equation (1):
 
ACKCH for  i   th  ACK/NACK transmission= i *(total number of DCHs or SCCHs)+(number of received DCH or SCCH( i= 0, . . . ,  R ))  (1)
 
     According to Equation (1), a different ACKCH is selected for each ACK/NACK repetition according to the current number of ACK/NACK repetitions and the number of a DCH or SCCH.
 
ACKCH for  i   th  ACK/NACK transmission=((frame number of  i   th ( i= 0)ACK/NACK transmission)modular( R+ 1))*(total number of DCHs or SCCHs)+(number of received DCH or SCCH)  (2)
 
     According to Equation (2), a UE selects a different ACKCH with respect to a different packet data reception time, using a frame number instead of the current number of ACK/NACK repetitions. The frame number is an absolute count of frames. For example, the frame number is a system frame number or a connection frame number in a WCDMA system. In the above equation, the number of a frame in which packet data has been received may be substituted for the frame number of the i th  (i=0) ACK/NACK transmission. 
     How a UE transmits an ACK/NACK signal by selecting an ACKCH using an ACKCH selection algorithm will be described below with reference to  FIG. 11 . 
       FIG. 11  illustrates ACK/NACK transmission according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 11 , UE # 1  and UE # 2  are set to support ACK/NACK repetition. A repetition factor R for UE # 1  and UE# 2  is 2. When receiving packet data on DCH # 1  in a first frame (k=1) in step  1105 , UE # 1  transmits an ACK/NACK signal on ACKCH # 4  in a fourth frame (k=4) in step  1106 . ACKCH # 4  is selected by the algorithm described in Equation (1) or Equation (2). 
     When the ACKCH is selected by Equation (1), UE # 1  selects ACKCH # 1  for DCH # 1  when i=0, ACKCH # 4  when i=1, and ACKCH # 7  when i=2, as indicated by reference numeral  1109 . In the same manner, UE # 2  transmits ACK/NACK signals on the same ACKCHs as those of UE # 1 , as indicated by reference numeral  1115 . However, since UE # 1  and UE # 2  transmit ACK/NACK signals at different times, there is no collision between them. 
     When the ACKCH is selected by Equation (2), UE # 1  selects ACKCH # 4  irrespective of i since it performs an i th (i=0) ACK/NACK transmission at k=4 as indicated by reference numeral  1100 . UE # 2  selects ACKCH# 7  because it receives data at k=2 and transmits an initial ACK/NACK signal at k=5, as indicated by reference numeral  1116 . 
       FIG. 12  is a flowchart illustrating ACK/NACK selection according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 12 , a UE demodulates received packet data and generates an ACK/NACK signal depending on whether the packet data has errors in step  1201 . In step  1202 , the UE checks whether the UE is set to support ACK/NACK repetition in order to select an ACKCH that will carry the ACK/NACK signal. If ACK/NACK repetition setting information is set to repetition or a repetition factor R is larger than 0, the UE will repeat the ACK/NACK signal. 
     If the UE does not repeat the ACK/NACK signal, the UE selects a basic ACKCH in a general ACKCH selection method and transmits the ACK/NACK signal on the basic ACKCH in steps  1203  and  1204 . The basic ACKCH is determined according to a DCH on which the packet data has been received or an SCCH that delivers scheduling information about the packet data. 
     If the UE repeats the ACK/NACK signal, the UE performs (R+1) loops because as many ACK/NACK repetitions as the repetition factor R must occur in step  1205 . More specifically, the UE selects an ACKCH by Equation (1) or Equation (2) in step  1206  and transmits the ACK/NACK signal on the selected ACKCH in step  1207 . 
     A Node B operates in a similar manner to the operation of the UE. In the method illustrated in  FIG. 12 , the Node B selects an ACKCH, receives the ACK/NACK signal from the UE on the selected ACKCH, and decodes it. 
     To implement the secondary exemplary embodiment of the present invention, an ACK/NACK transmitter and a ACK/NACK receiver are configured as in the first exemplary embodiment of the present invention, except that when Equation (2) is used, an ACKCH decider further receives a frame number as an input. 
     Embodiment 3 
     Despite the benefit of ACK/NACK repetition without collision between UEs that use the same DCH or SCCH, the second exemplary embodiment of the present invention requires as many additional ACKCHs as a repetition factor. Since ACK/NACK repetition will most likely occur for UEs at a cell boundary in real implementation, other UEs will not frequently use supplementary ACKCHs for ACK/NACK repetition. Thus, a third exemplary embodiment of the present invention proposes a method for a limited number of ACKCHs for each repetition to increase resource use efficiency. 
       FIG. 13  illustrates ACKCH allocation according to a third exemplary embodiment of the present invention. 
     Referring to  FIG. 13 , M 0  basic ACKCHs  1302  are allocated for as many DCHs or SCCHs. Thus, M 0  is the number of the DCHs or SCCHs. M 1  ACKCHs  1303  are allocated for a first ACK/NACK repetition and M 2  ACKCHs  1304  are allocated for a second ACK/NACK repetition. If more ACK/NACK repetitions are allowed, M 3 , M 4  . . . ACKCHs can be additionally allocated. M 0 , M 1  and M 2  are preset, or notified by upper-layer signaling. 
     From among the plurality of ACKCHs, an ACKCH to be transmitted at a given time point is selected according to Equation (3)
 
ACKCH for  i   th  ACK/NACK transmission=( i*M ( i− 1)+(number of received DCH or SCCH)) modular( Mi )( i= 0, . . . ,  R )  (3)
 
     Equation (3) is an algorithm for selecting a different ACKCH for a different ACK/NACK repetition using the current number of ACK/NACK transmissions, information about a DCH or an SCCH, and information about ACKCHs allocated for each repetition number. 
     As noted from Equation (3), the number of ACKCHs set for a repetition is less than the total number of DCHs, a collision may occur during ACK/NACK transmission. This collision can be avoided by scheduling DCHs such that UEs support ACK/NACK repetition do not select the same ACKCH in a scheduler. For example, if DCH # 1  (or SCCH # 1 ) and DCH # 3  (or SCCH # 3 ) are simultaneously allocated to two UEs supporting ACK/NACK repetition, a collision occurs between them. In this case, DCH # 1  (or SCCH # 1 ) and DCH # 2  (or SCCH # 2 ) are allocated to the two UEs, while DCH # 3  (or SCCH # 3 ) is allocated to another UE that does not support ACK/NACK repetition. Then the collision does not occur. 
       FIG. 14  illustrates ACK/NACK transmission according to the third exemplary embodiment of the present invention. 
     Referring to  FIG. 14 , UE # 1  and UE # 2  are set to support ACK/NACK repetition. A repetition factor R for UE# 1  and UE# 2  is 2, M 0 =3, M 1 =2, and M 2 =1. When receiving packet data on DCH # 1  in a first frame (k=1) in step  1405 , UE # 1  transmits an ACK/NACK signal on ACKCH # 4  in a fourth frame (k=4) in step  1406 . UE # 1  selects ACKCHs by Equation (3). Thus, UE # 1  selects ACKCH # 1  for DCH # 1  when i=0 in step  1406 , ACKCH # 4  when i=1 in step  1407 , and ACKCH # 6  when i=2 in step  1408 . Meanwhile, UE # 2 , which receives data on DCH # 2  at k=2 in step  1409 , selects ACKCH # 2  corresponding to DCH # 2  when i=0 in step  1410 , ACKCH # 5  when i=1 in step  1411 , and ACKCH # 6  when i=2 in step  1412 . 
     The UE and a Node B operate in the same manner to as in the second exemplary embodiment of the present invention, except that Equation (3) is used instead of Equation (1) or Equation (2) in selecting an ACKCH in step  1206  of  FIG. 12 . 
     To implement the third exemplary embodiment of the present invention, an ACK/NACK transmitter and a ACK/NACK receiver are configured as in the first exemplary embodiment of the present invention and thus their description is not provided herein. 
     Embodiment 4 
     If a very small number of UEs support ACK/NACK repetition, allocation of shared ACKCHs results in ACKCH dissipation in the first, second and third exemplary embodiments of the present invention. Therefore, a fourth exemplary embodiment of the present invention is proposed in which a dedicated ACKCH is allocated to a UE supporting ACK/NACK repetition. 
       FIG. 15  illustrates ACKCH allocation according to the fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 15 , basic ACKCHs  1502  are allocated in a one-to-one correspondence with DCHs or SCCHs. ACKCHs  1503  and  1504  are allocated for UEs supporting ACK/NACK repetition. UE # 1  and UE # 3  support ACK/NACK repetition. Since the basic ACKCHs  1502  are shared in a cell, information about the basic ACKCHs  1502  is provided using fixed resources defined by a specification or in system information. ACK/NACK information is provided to UEs supporting ACK/NACK repetition such as UE # 1  and UE # 3 , along with channel information about the UEs by upper-layer signaling. In the illustrated case of  FIG. 15 , different ACKCHs are allocated to UE # 1  and UE # 3 , but a Node B scheduler may allocate the same ACKCH to UE # 1  and UE # 3  and schedule it in the manner that prevents ACK/NACK collision between them. 
       FIG. 16  illustrates ACK/NACK transmission according to the fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 16 , UE # 1  is set to support ACK/NACK repetition, while UE # 2  is set to not support ACK/NACK repetition. A repetition factor R for UE # 1  is 2. When receiving packet data on DCH # 1  or scheduling information about the packet data on SCCH # 1  in a first frame (k=1) in step  1605 , UE # 1  transmits/retransmits an ACK/NACK signal on ACKCH # 4  in fourth, fifth and sixth frames (k=4, 5 and 6) in steps  1606 ,  1607  and  1608  because ACKCH # 4  is allocated dedicatedly to UE # 1  supporting ACK/NACK repetition. 
     However, when receiving packet data on DCH # 1  or scheduling information about the packet data on SCCH # 1  in a second frame (k=2) in step  1609 , UE # 2  transmits an ACK/NACK signal on ACKCH # 1  corresponding to DCH # 1  or SCCH # 1  in the fifth frame (k=5) in step  1610  because UE # 2  does not support ACK/NACK repetition. 
       FIG. 17  is a flowchart illustrating ACK/NACK selection in a UE according to the fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 17 , the UE demodulates received packet data and generates an ACK/NACK signal depending on whether the packet data has errors in step  1701 . In step  1702 , the UE checks whether it is set to support ACK/NACK repetition to select an ACKCH that will carry the ACK/NACK signal. If ACK/NACK repetition setting information is set to repetition, or a repetition factor R is larger than 0, the UE will repeat the ACK/NACK signal. 
     If the UE does not repeat the ACK/NACK signal, it selects a basic ACKCH in the general ACKCH selection method and transmits the ACK/NACK signal on the selected basic ACKCH in steps  1703  and  1704 . The basic ACKCH is determined according to a DCH on which the packet data has been received or an SCCH that delivers scheduling information about the packet data. 
     If the UE repeats the ACK/NACK signal, it performs (R+1) loops because as many ACK/NACK repetitions as the repetition factor R have to occur in step  1705 . Without selecting a basic ACKCH for each ACK/NACK transmission, the UE transmits/retransmits the ACK/NACK signal on a dedicated ACKCH allocated to the UE in step  1706 . 
     A Node B operates in a similar manner to the operation of the UE. In the method illustrated in  FIG. 17 , the Node B selects an ACKCH, receives the ACK/NACK signal from the UE on the selected ACKCH, and decodes it. 
     As is apparent from the above description, the present invention supports ACK/NACK repetition when ACKCHs are mapped to DCHs or SCCHs in an FDMA wireless communication system. As ACK/NACK signals can be retransmitted without collision between UEs that support ACK/NACK repetition, even a UE remote from a Node B can transmit an ACK/NACK signal reliably, thereby expanding cell coverage. 
     While the invention has been shown and described with reference to certain exemplary embodiments of the present invention 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 present invention as defined by the appended claims and their equivalents.