Viterbi decoder

Abstract: An apparatus for providing and storing a state metric which is used for an add-compare-select (ACS) operation is disclosed in a Viterbi decoder using a number of ACS units in order to enhance decoding speed. A state metric memory in a Viterbi decoder uses a two-port memory, in which a memory bank for reading and writing a state metric of a first half among the N state metrics generated in a ACS unit and two memory banks for alternately reading and writing the state metric of the second half whenever a codeword is input, are incorporated into a single memory. As a result, a storage capacity for storing the state metrics can be greatly reduced compared with a conventional apparatus.

Description: The present invention relates to a Viterbi decoder, and more particularly, to a configuration of an apparatus for supplying and storing a state metric used during add-compare-select (ACS) operations in a Viterbi decoder using a number of ACS units in order to enhance decoding speed. The Viterbi decoder is an apparatus for decoding codewords encoded by a convolutional code, by using a maximum likelihood method. The Viterbi decoder compares an encoding state of a predetermined encoder with that of a received codeword, selects the closest encoding path, and decodes information transmitted from the selected path. Figure 1 is a block diagram showing a conventional Viterbi decoder. The Viterbi decoder shown in Figure 1 includes a branch metric calculator 10, an ACS unit 12, a state metric memory 14, a path memory 16, and a path tracing logic unit 18. When the apparatus shown in Figure 1 receives the received codeword, the branch metric calculator 10 calculates a similarity (Hamming distance) between the received codeword and a predetermined codeword which can be output from an encoder (not shown), and outputs a branch metric BM. The ACS unit 12 receives the branch metric BM provided from the branch state metric calculator 10 and a previous (old) state metric OSM stored in the state metric memory 14 as inputs, and performs an add-compare operation in order to select a survival path showing an encoding sequence closest to that of the received codeword, and calculate a current (new) state metric NSM of the survival path. The selected survival path is stored in the path memory 16, and the calculated current state metric NSM is stored in the state metric memory 14. The path tracing logic unit 18 refers to the survival path memory in the path memory 16 and outputs decoded data according to an encoding path of the received codeword. The number N of states which can exist per a single codeword among the received codewords in Viterbi decoding is defined as 2 K-1 , wherein K is defined as a constraint length. When the ACS unit 12 of Figure 1, which calculates the state metric of the survival path, has the number of states, that is, N, the apparatus shown in Figure 1 maintains the fastest processing speed. The reason is because N ACS units each simultaneously perform an ACS operation, with respect to the N states which can exist in correspondence to a single codeword. Therefore, in view of decoding speed, it is most preferable that N ACS units are used with respect to a codeword having N states. However, it becomes difficult to make an ACS unit having such a large capacity as the number of states (N) gets larger. Thus, as the state number N becomes larger, the number of ACS units must be appropriately selected. Since a convolutional code having a constraint length K of 9 is used in a general code division multiple access (CDMA) system, the state number becomes 2 K-1 (=256). If a Viterbi decoder is embodied using 256 ACS units, decoding speed is faster but more hardware is needed. Meanwhile, when a single ACS unit is used in a Viterbi decoder, less hardware is needed but decoding speed is slower. The paper by C.B. Shung, H.-D. Lin, P.H. Siegel and H.K. Thapar: "Area Efficient Architectures for the Viterbi Alogirthm" Globecom '90, vol 3, pages 1787-1793, December 1990, San Diego, USa desribes a systematic way of partitioning and scheduling a number of trellis states into a smaller number of ACSs which are connected by a fixed or multi-stage interconnection network to provide an architecture for implementation of the Viterbi algorithm, in order to explore are-time trade-offs. A Viterbi decoder using a single ACS unit is disclosed in the Korean patent application No. 90-20808 filed on December 17, 1990, by the same applicant, entitled "State metric storage device". This device can perform a fast input and output processing of a state metric in the case when a single ACS unit is used, but cannot be applied to the case when a number of ACS units are used. Meanwhile, a technology of enhancing decoding speed using a number of ACS units is disclosed in the Korean patent application No. 96-7208 filed on March 18, 1996, by the same applicant, entitled "ACS device of Viterbi decoder". This device uses four ACS units in order to enhance decoding speed. However, since the device includes a typical state metric memory, that is, a state metric memory comprised of a single memory device, the input and output speed of the state metric are somewhat slow. A technology for solving the above problems is disclosed in the Korean patent application No. 97-8603, filed on 0000, 00, 1997, by the same applicant, entitled "State metric storing method and apparatus for Viterbi decoder". This apparatus uses two state metric storing units, that is, a state metric storing unit comprised of two memory devices, to thereby efficiently supply and store the state metric used during add-compare-select operation. However, since a memory capacity of 2N is required, wherein N is the number of states, the memory capacity of the state metric storing unit gets larger as N becomes larger, which is burdensome. With a view to solve or reduce the above problems, it is an aim of preferred embodiments of the present invention to provide a Viterbi decoder using a number of ACS units in order to enhance decoding speed and which can supply and store a state metric with a small capacity. According to a first aspect of the invention, there is provided a Viterbi decoder having a branch metric calculator for calculating a similarity between an input codeword and a predetermined codeword which can be output from an encoder and outputting 2N branch metrics where N is the number of states, an add-compare-select (ACS) portion for performing an add-compare operation by receiving 2N branch metrics and N previous state metrics provided from the branch metric calculator, selecting a survival path showing an encoding sequence closest to that of the received codeword, and calculating N current state metrics, and a state metric memory for providing the N previous state metrics to the ACS unit or storing the N current state metrics generated by the ACS unit. wherein said ACS unit comprises M ACS units, where M=2 n , n∈ 1, 2, ... and M<N, which each perform N/M operations during N/M cycles whenever a code word is input, the Viterbi decoder characterised in that said state metric memory comprises: a two-port memory including a memory bank for simultaneously reading and writing a state metric of the first half among the N state metrics generated in said ACS unit, and two memory banks for alternately reading and writing the state metrics of the second half whenever a codeword is input; counter for counting a proceeding sequence of said cycles; a write address generator for generating a write address of said memory based on the count result of said counter; and a read address generator for generating a read address of said memory based on the count result of said counter. Preferably, said memory is a two-port memory. Said state metric memory may further comprise a write cross unit for realigning the state metrics which are written to the memory according to the structure of said ACS unit. Said state metric memory may further comprise a read cross unit for realigning the state metrics which are read from the memory according to the structure of said ACS unit. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of a conventional Viterbi decoder; Figure 2 is a block diagram of another conventional Viterbi decorder; Figure 3A and 3B are trellis diagrams for explaining the operation of the apparatus shown in Figures 1 and 2, in the case when a constraint length is 9; Figure 4 is for explaining the operation of the add-compare-select portion shown in Figure 2; Figure 5 is a detailed block diagram showing the add-compare-select portion shown in Figure 2; Figure 6 shows the operation of the add-compare-select portion shown in Figure 2; Figure 7 shows a configuration of the state metric storing portion shown in Figure 2; Figure 8 is a block diagram of a Viterbi decoder according to embodiments of the present invention; and Figure 9 is a detailed block diagram showing the state metric storing portion shown in Figure 8. The structure and effect of embodiments of the present invention will be described with reference to the attached drawings. Another conventional Viterbi decoder shown in Figure 2 and disclosed in the Korean patent application 97-8603, includes a branch metric calculator 20, an ACS unit 22 comprised of four ACS units 22a-22d, a state metric memory 28 having two memories 28a and 28b, a path memory 26, and a path tracing logic unit 24. The ACS unit 22 includes four ACS units 22a-22d. Each of the ACS units 22a-22d performs an ACS operation with respect to N/4 states. Since the number of states is 256 when a code ratio is 1/3 and a constraint length is 9, each of the ACS units 22a-22d performs an ACS operation 64 times (=256/4). That is, the ACS unit 22 calculates four current state metrics with reference to four previous state metrics and eight branch metrics at each cycle of 64 cycles. Figure 3A and 3B are trellis diagrams for explaining the operation of the apparatus shown in Figures 1 and 2. As is well known, the number of states which can be shifted whenever an information bit is input is limited to two at the stable state of a convolutional encoder, and a state to be shifted is consistent. For example, in the case of the apparatus shown in Figures 1 and 2, the 0th state can be shifted only to the 0th state or the 128th state and the first state can be shifted only to the first state or the 129th state. In each row divided by dotted lines shown in Figures 3A and 3B, the left-hand numbers represent the previous states and the right-hand numbers represent the current states. The solid lines connecting the states represent the branch metrics between the previous and current states. For example, the solid line connecting the previous 0th state with the current 0th state represents the branch metric between both the states. In the upper-left portion of Figure 3A, a value obtained by adding a state metric with respect to the previous 0th state and a branch metric between the previous 0th state and the current 0th state is compared with a value obtained by adding a state metric with respect to the previous 1st state and a branch metric between the previous 1st state and the current 0th state, and the smaller of the two values is taken as a state metric with respect to the current 0th state. Also, a value obtained by adding a state metric with respect to the previous 0th state and a branch metric between the previous 0th state and the current 128th state is compared with a value obtained by adding a state metric with respect to the previous 1st state and a branch metric between the previous 1st state and the current 128th state, and the smaller of the two values is taken as a state metric with respect to the current 128th state. The ACS unit 22 calculates the state metrics with respect to the current 0th, 128th, 1st and 129th states by referring to the state metrics with respect to the previous 0th through 3rd states and eight branch metrics at the first cycle among the 64 cycles, if a single codeword is input. The ACS unit 22a calculates the state metric with respect to the current 0th state by referring to the state metric with respect to the previous 0th and 1st states and the two branch metrics. The ACS unit 22b calculates the state metric with respect to the current 128th state by referring to the state metric with respect to the previous 0th and 1st states and the two branch metrics. The ACS unit 22c calculates the state metric with respect to the current 1st state by referring to the state metric with respect to the previous 2nd and 3rd states and the two branch metrics. The ACS unit 22d calculates the state metric with respect to the current 129th state by referring to the state metric with respect to the previous 2nd and 3rd states and the two branch metrics. Then, the ACS unit 22 calculates the state metrics with respect to the current 2nd, 130th, 3rd and 131st states by referring to the state metrics with respect to the previous 4th, 5th, 6th and 7th states and the eight branch metrics at the second cycle. The ACS unit 22a calculates the state metric with respect to the current 2nd state by referring to the state metric with respect to the previous 4th and 5th states and the two branch metrics. The ACS unit 22b calculates the state metric with respect to the current 130th state by referring to the state metric with respect to the previous 4th and 5th states and the two branch metrics. The ACS unit 22c calculates the state metric with respect to the current 3rd state by referring to the state metric with respect to the previous 6th and 7th states and the two branch metrics. The ACS unit 22d calculates the state metric with respect to the current 131st state by referring to the state metric with respect to the previous 6th and 7th states and the two branch metrics. The ACS unit 22 repeats the above operations until it obtains the state metric with respect to the current 126th, 254th, 127th and 255th states by referring to the state metric with respect to the previous 252nd, 253rd, 254th and 255th states and the eight branch metrics. If such an operation is generally expressed, it can be said that the ACS unit 22 calculates the state metric with respect to the current ith, (N/2+i)th, (i+1)th and (N/2+i+1)th states by referring to the state metric with respect to the previous 2ith, (2i+1)th, 2(i+1)th and {2(i+1)+1}th states and the eight branch metrics at each cycle. Here, BM0 is a branch metric from the previous 2ith state to the current ith state. BM1 is a branch metric from the previous 2ith state to the current (N/2+i)th state. BM2 is a branch metric from the previous (2i+1)th state to the current ith state. BM3 is a branch metric from the previous (2i+1)th state to the current (N/2+i)th state. BM4 is a branch metric from the previous 2(i+1)th state to the current (i+1)th state. BM5 is a branch metric from the previous 2(i+1)th state to the current (N/2+i+1)th state. BM6 is a branch metric from the previous {2(i+1)+1}th state to the current (i+1)th state. BM7 is a branch metric from the previous {2(i+1)+1}th state to the current (N/2+i+1)th state. Figure 4 is a conceptual view for explaining the operation of the add-compare-select portion shown in Figure 2. Referring to Figure 4, the ACS unit 22 calculates the state metrics with respect to the current ith, (N/2+i)th, (i+1)th and (N/2+i+1)th states by referring to the state metrics with respect to the previous 2ith, (2i+1)th, 2(i+1)th and {(2(i+1)+1}th states and the branch metrics BMO-BM7 at each cycle among 64 cycles, if a single codeword is input. The ACS unit 22a calculates the state metric with respect to the current ith state by referring to the state metric with respect to the previous 2ith and (2i+1)th states and the two branch metrics BM 0 and BM 2 . The ACS unit 22b calculates the state metric with respect to the current (N/2+i)th state by referring to the state metric with respect to the previous 2ith and (2i+1)th states and the two branch metrics BM 1 and BM 3 . The ACS unit 22c calculates the state metric with respect to the current (i+1)th state by referring to the state metric with respect to the previous 2(i+1) and {2(i+1)+1}th states and the two branch metrics BM 4 and BM 6 . The ACS unit 22d calculates the state metric with respect to the current (N/2+i+1)th state by referring to the state metric with respect to the previous 2(i+1)th and {2(i+1)+1}th states and the two branch metrics BM 5 and BM 7 . Figure 5 is a block diagram showing in detail the add-compare-select portion shown in Figure 2. The apparatus shown in Figure 5 includes two adders 50 and 52, a comparator 54 and a selector 56. The adder 50 adds a branch metric applied to a BMu port and a previous state metric applied to a SMu port, and outputs a signal "A". Meanwhile, the adder 52 adds a branch metric applied to a BM1 port and a previous state metric applied to a SM1 port, and outputs a signal" "B". The comparator 54 compares the signal "A" from the adder 50 with the signal "B" from the adder 52, and outputs a low-level path select signal PS if A≤B and outputs a high-level path select signal PS if A>B. The selector 56 selects either the signal "A" from the adder 50 or the signal "B" from the adder 52, in response to the path select signal PS output from the comparator 54, and outputs a current state metric NSM. Each of the ACS units 22a-22d performs the operation of an input codeword 64 times. The first ACS unit 22a and the second ACS unit 22b perform the operation with respect to the even-numbered current state metrics, and the third ACS unit 22c and the fourth ACS unit 22d perform the operation with respect to the odd-numbered current state metrics. In more detail, the first ACS unit 22a calculates the state metric with respect to the uppermost 0th, 2nd, 4th, ..., 124th and 126th states among the even-numbered current state metrics. The second ACS unit 22b calculates the state metric with respect to the lowermost 128th, 130th, 132nd, ..., 252nd and 254th states among the even-numbered current state metrics. The third ACS unit 22c calculates the state metric with respect to the uppermost 1st, 3rd, 5th, ..., 125th and 127th states among the odd-numbered current state metrics. The fourth ACS unit 22d calculates the state metric with respect to the uppermost 129th, 131st, 133rd, ..., 253rd and 255th states among the odd-numbered current state metrics. Figure 6 shows the relationship between the input previous state metric OSM and branch metric BM input to the ACS unit 22 and the current state metric NSM output from the ACS unit 22. The previous state metric and branch metric input to the each of the ACS units 22a-22d and the output therefrom are as follows. As can be seen from Figure 6, the previous state metric with respect to the previous 0th, 4th, 8th, ..., 120th, 124th, 128th, 132nd, 136th, ..., 248th and 252nd states is applied to the Smu port of the ACS unit 22a according to the proceeding sequence of the operation cycle. The previous state metric with respect to the previous 1st, 5th, 9th, ..., 121st, 125th, 129th, 133rd, 137th, ..., 249th and 253rd states is applied to the Sml port of the ACS unit 22a. Also, the branch metric is applied to the Bmu port when the 0th, 4th, 8th, ..., 248th and 252nd states are shifted to the 0th, 2nd, 4th, ..., 124th and 126th states, respectively. The branch metric is applied to the Bml port when the 1st, 5th, 9th, ..., 249th and 253rd states are shifted to the 0th, 2nd, 4th, ..., 124th and 126th states, respectively. The previous state metric with respect to the previous 0th, 4th, 8th, ..., 120th, 124th, 128th, 132nd, 136th, ..., 248th and 252nd states is applied to the Smu port of the ACS unit 22b according to the proceeding sequence of the operation cycle. The previous state metric with respect to the previous 1st, 5th, 9th, ..., 121st, 125th, 129th, 133rd, 137th, ..., 249th and 253rd states is applied to the Sml port of the ACS unit 22b. Also, the branch metric is applied to the Bmu port when the 0th, 4th, 8th, ..., 248th and 252nd states are shifted to the 128th, 130th, 132nd, ..., 252nd and 254th states, respectively. The branch metric is applied to the Bml port when the 1st, 5th, 9th, ..., 249th and 253rd states are shifted to the 128th, 130th, 132nd, ..., 252nd and 254th states, respectively. The previous state metric with respect to the previous 2nd, 6th, 10th, ..., 122nd, 126th, 130th, 134th, 138th, ..., 250th and 254th states is applied to the Smu port of the ACS unit 22c according to the proceeding sequence of the operation cycle. The previous state metric with respect to the previous 3rd, 7th, 11th, ..., 251st, and 255th states is applied to the Sml port of the ACS unit 22c. Also, the branch metric is applied to the Bmu port when the 2nd, 6th, 10th, ..., 250th and 254th states are shifted to the 1st, 3rd, 5th, ..., 125th and 127th states, respectively. The branch metric is applied to the Bm1 port when the 3rd, 7th, 11th, ..., 251st and 255th states are shifted to the 1st, 3rd, 5th, ..., 125th and 127th states, respectively. The previous state metric with respect to the previous 2nd, 6th, 10th, ..., 122nd, 126th, 130th, 134th, 138th, ..., 250th and 254th states is applied to the Smu port of the ACS unit 22d according to the proceeding sequence of the operation cycle. The previous state metric with respect to the previous 3rd, 7th, 11th, ..., 123rd, 127th, 131st, 135th, 139th, ..., 251st and 255th states is applied to the Sml port of the ACS unit 22d. Also, the branch metric is applied to the Bmu port when the 2nd, 6th, 10th, ..., 250th and 254th states are shifted to the 129th, 131st, 133rd, ..., 253rd and 255th states, respectively. The branch metric is applied to the Bml port when the 3rd, 7th, 11th, ..., 251st and 255th states are shifted to the 129th, 131st, 133rd, ..., 253rd and 255th states, respectively. The ACS uint 22a outputs the current state metrics with respect to the 0th, 2nd, 4th, 6th, ..., 124th and 126th states, respectively according to the proceeding sequnece of the operation cycle. The ACS unit 22b outputs the current state metrics with respect to the 128th, 130th, 132nd, 134th, ..., 252nd and 254th states, respectively. The ACS unit 22c outputs the current state metrics with respect to the 1st, 3rd, 5th, 7th, ..., 125th and 127th states, respectively. The ACS unit 22d outputs the current state metrics with respect to the 129th, 131st, 133rd, 135th, ..., 253rd and 255th states, respectively. A designated previous state metric must be provided at each cycle, in order that the ACS unit 22 normally operates. This is performed by the state metric memory 28 shown in Figure 2. The state metric memory 28 shown in Figure 2. includes two memories 28a and 28b, a multiplexer 28c and a control signal and address generator 28d. The two memories 28a and 28b repeat a read/write operation alternately whenever a single codeword is input. In more detail, if a single codeword is input, the state metric memory in the memory 28a is read and the read result is applied to the ACS unit 22 as a previous state metric. A current state metric newly generated by the ACS unit 22 is stored in the other memory 28b. When the next codeword is input, the state metric memory in the memory 28b is read and the read result is applied to the ACS unit 22 as a previous state metric. A current state metric newly generated by the ACS unit 22 is stored in the other memory 28a. As described above, since the two memories perform a read/write operation alternately whenever a codeword is input, the read and write operation can be simultaneously performed. Thus, a decoding operation can be quickly and efficiently performed. First, the reading operations of the memories 28a and 28b will be described. As shown in Figure 7, the memories 28a and 28b include eight memory banks SMM_A-SMM_H. The memory bank SMM_A stores the state metric with respect to the 0th, 4th, 8th, ..., 120th and 124th states. The memory bank SMM_B stores the state metric with respect to the 1st, 5th, 9th, ..., 121st and 125th states. The memory bank SMM_C stores the state metric with respect to the 128th, 132nd, 136th, ..., 248th and 252nd states. The memory bank SMM_D stores the state metric with respect to the 129th, 133rd, 137th, ..., 249th and 253rd states. The memory bank SMM_E stores the state metric with respect to the 2nd, 6th, 10th ..., 122nd and 126th states. The memory bank SMM_F stores the state metric with respect to the 3rd, 7th, 11th, ..., 123rd and 127th states. The memory bank SMM_G stores the state metric with respect to the 130th, 134th, 138th, ..., 250th and 254th states. The memory bank SMM_H stores the state metric with respect to the 131st, 135th, 139th, ..., 251st and 255th states. During the first 32 cycles of the 64 cycles, the state metrics stored in the memory banks SMM_A and SMM_B are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_E and SMM_F are applied to the ACS units 22c and 22d. During the next 32 cycles, the state metrics stored in the memory banks SMM_C and SMM_D are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_G and SMM_H are applied to the ACS units 22c and 22d. The control signal and address generator 28d applies a read address to a respective ACS unit so that during the first 32 cycles, the state metrics stored in the memory banks SMM_A and SMM_B are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_E and SMM_F are applied to the ACS units 22c and 22d. Then, the control signal and address generator 28d applies a read address to a respective ACS unit so that during the next 32 cycles, the state metrics stored in the memory banks SMM_C and SMM_D are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_G and SMM_H are applied to the ACS units 22c and 22d. The control signal and address generator 28d applies a control signal to the multiplexer 28c so that a state metric read from one of the memories is applied to the ACS unit 22 whenever a codeword is input. Next, the writing operation of the memories will be described. As can be seen from Figure 6, the ACS unit 22a performs the output operation in sequence of 0th, 2nd, 4th, 6th, ..., 124th and 126th states, and repeats the operation in sequence of SMM_A, SMM_E, SMM_A, SMM_E, .... The ACS unit 22b performs the output operation in sequence of 128th, 130th, 132nd, 134th, ..., 252nd and 254th states, and repeats the operation in sequence of SMM_C, SMM_G, SMM_C, SMM_G .... The ACS unit 22c performs the output operation in sequence of 1st, 3rd, 5th, 7th, ..., 125th and 127th states, and repeats the operation in sequence of SMM_B, SMM_F, SMM_B, SMM_F, .... The ACS unit 22d performs the output operation in sequence of 129th, 131st, 133rd, 135th, ..., 253rd and 255th states, and repeats the operation in sequence of SMM_D, SMM_H, SMM_D, SMM_H .... The control signal and address generator 28d generates an address so that during the odd-numbered cycles, a new state metric is stored in the memory banks of SMM_A, SMM_C, SMM_B and SMM_D, and during the even-numbered cycles, a new state metric is stored in the memory banks of SMM_E, SMM_G, SMM_F and SMM_H. In the Figure 2 apparatus, decoding speed can be enhanced by storing the previous state metric and the newly generated current state metric in each memory and alternately performing a read and write operation whenever a codeword is input. However, excessing memory capacity can be consumed for storing the state metrics. Furthermore, since the memory capacity of the state metric memory reaches two times the number of states N, such a problem gets larger as the number of the states gets greater. Figure 8 is a block diagram showing a Viterbi decoder including a state metric memory according to the present invention. In the Figure 8 apparatus, the same reference numerals are assigned to the same elements as those of Figure 2, whose detailed description will be omitted. The most distinctive characteristic of the state metric memory 38 shown in Figure 9 is the use of a two-port memory to reduce a storage capacity. As is well known, the two-port memory can perform reading and writing operations with respect to the same storage location at a single clock pulse, which differs from a conventional RAM. That is, data of the designated storage location is read at the rising edge of the clock and output via a read data bus, and data loaded in the write data bus is written to a designated storage location at the falling edge. As described above, since a reading and writing operation can be performed in the same storage location, the memory capacity of the state metric memory can be reduced. However, if the memory bank structure shown in Figure 7 is used without modification, a normal read and write operation cannot be performed. This is based on the following reason. The state metric with respect to the 128th-191st states is generated for the first 32 cycles of the 64 cycles as shown in Figures 3A and 3B. When the generated state metric is recorded in the storage location in which the state metric with respect to the 128th-191st states is stored, they cannot be referred to during the next 32 cycles. In more detail, since the state metric generated during the first 32 cycles is the state metric with respect to the new state, and the state metric required by the ACS unit 22 is the state metric with respect to the previous state, the state metric with respect to the previous 128th-191st states cannot be used during the next 32 cycles when the state metric with respect to the 128th-191st states generated during the first 32 cycles is recorded in the same location. This causes a problem. To solve such a problem in embodiments of the present invention, a two-port memory is used, in which a memory bank with respect to the first half state and two memory banks with respect to the second half state are installed. The memory bank with respect to the first half state can perform a read and write operation simultaneously, and the two memory banks with respect to the second half state can perform a read and write operation alternately. That is, during the time when one memory bank performs a read operation, the other memory bank performs a write operation, which is alternately performed whenever a codeword is input. Figure 9 is a detailed block diagram showing the state metric memory shown in Figure 8. In the Figure 9 apparatus, a reference numeral 38a denotes an ACS counter, a reference numeral 38b denotes a write address generator, a reference numeral 38c denotes a read address generator, a reference numeral 38d denotes a two-port memory, a reference numeral 38e denotes a write cross unit, and a reference numeral 38f denotes a read cross unit. The ACS counter 38a performs a counting operation sequentially in sequence of 64 cycles. The write address generator 38b generates a write address of the memory 38d on the basis of the count result of the ACS counter 38a. The read address generator 38c generates a read address of the memory 38d on the basis of the count result of the ACS counter 38a. The write cross unit 38e changes a sequence of the state metric which is written in the memory 38d according to the structure of the ACS unit 22. The read cross unit 38f changes a sequence of the state metric which is read from the memory 38d according to the structure of the ACS unit 22. First, a read operation of the memory 38d will be described below. The memory 38d is comprised of 12 memory banks SMM_A through SMM_L as shown in Figure 9. The memory bank SMM_A stores the state metric with respect to the 0th, 4th, 8th, ..., 120th and 124th states. The memory bank SMM_B stores the state metric with respect to the 1st, 5th, 9th, ..., 121st and 125th states. The memory bank SMM_E stores the state metric with respect to the 2nd, 6th, 10th, ..., 122nd and 126th states. The memory bank SMM_F stores the state metric with respect to the 3rd, 7th, 11th, ..., 123rd and 127th states. The memory banks SMM_C and SMM_I store the state metric with respect to the 128th, 132nd, 136th; ..., 248th and 252nd states. The memory banks SMM_D and SMM_J store the state metric with respect to the 129th, 133rd, 137th, ..., 249th and 253th states. The memory banks SMM_G and SMM_K store the state metric with respect to the 130th, 134th, 138th, ..., 250th and 254th states. The memory banks SMM_H and SMM_L store the state metric with respect to the 131st, 135th, 139th, ..., 251st and 255th states. During the first 32 cycles of the 64 cycles, the state metrics stored in the memory banks SMM_A and SMM_B are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_E and SMM_F are applied to the ACS units 22c and 22d. During the next 32 cycles, the state metrics stored in the memory banks SMM_C and SMM_D are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_G and SMM_H are applied to the ACS units 22c and 22d. During the first 32 cycles of the 64 cycles, the read address generator 38c applies a read address to a respective ACS unit so that the state metrics stored in the memory banks SMM_A and SMM_B are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_E and SMM_F are applied to the ACS units 22c and 22d. Then, during the next 32 cycles, the read address generator 38c applies a read address to a respective ACS unit so that the state metrics stored in the memory banks SMM_C and SMM_D are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_G and SMM_H are applied to the ACS units 22c and 22d. When the next codeword is input, during the first 32 cycles of the 64 cycles, the read address generator 38c applies a read address to a respective ACS unit so that the state metrics stored in the memory banks SMM_A and SMM_B are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_E and SMM_F are applied to the ACS units 22c and 22d. Then, during the next 32 cycles, the read address generator 38c applies a read address to a respective ACS unit so that the state metrics stored in the memory banks SMM_I and SMM_J are applied to the ACS units 22a and 22b, and the state metrics stored in the memory banks SMM_K and SMM_L are applied to the ACS units 22c and 22d. Next, a write operation of the memory will be described below. If a certain codeword is input, the write address generator 38b generates a write address of the memory banks SMM_A, SMM_B, SMM_E, SMM_F, SMM_I, SMM_J, SMM_K, and SMM_I. When the next codeword is input, the write address generator 38b generates a write address of the memory banks SMM_A, SMM_B, SMM_E, SMM_F, SMM_C, SMM_D, SMM_G, and SMM_H. Accordingly, the memory banks SMM_A, SMM_B, SMM_E and SMM_F are used whenever each codeword is input. However, the memory banks SMM_C, SMM_D, SMM_G and SMM_H, and the memory banks SMM_I, SMM_J, SMM_K and SMM_L perform a read and write operation alternately. As described above, the embodiment of the present invention has been described with respect to the case when four ACS units are used. It is noted that the present invention can be applied without any modification even when 2 n (wherein n is 1, 2, ...) ACS units are used. In this case, each memory is divided into memory banks as many as the number of the ACS units. Therefore, it should be understood that the present invention retains to the scope of the appended claims. As described above, the apparatus according to the present invention provides an effect of reducing the memory capacity for storing the state metric by using a two-port memory. Also, the apparatus according to teachings of the present invention divides the memory -structure into one memory bank for the first half state and two memory banks for the second half state, incorporated by a single memory unit. A misoperation can be prevented by enabling one memory bank for the first half state to simultaneously perform a read and write operation, and enable the two memory banks to alternately perform a read and write operation by a codeword. The number of memories is equal to the number of the ACS units. In the conventional prior art, a memory capacity of two times the state number information amount is required, and the memory is divided into four times as many memories as the number of the ACS units, for the purpose of storing the state metrics. When the apparatus according to embodiments of the present invention is used, the memory capacity is only 1.5 times the state metrics, and the memory is divided into the number of the ACS units. Accordingly, the manufacturing cost of the Viterbi decoder can be greatly reduced.

Claims: A Viterbi decoder having a branch metric calculator (20) for calculating a similarity between an input codeword and a predetermined codeword which can be output from an encoder and outputting 2N branch metrics where N is the number of states, an add-compare-select (ACS) portion (22) for performing an add-compare operation by receiving 2N branch metrics and N previous state metrics provided from the branch metric calculator, selecting a survival path showing an encoding sequence closest to that of the received codeword, and calculating N current state metrics, and a state metric memory (38) for providing the N previous state metrics to the ACS unit (12) or storing the N current state metrics generated by the ACS unit (12), wherein said ACS unit (22) comprises M ACS units (22a-d), where M=2 n , n∈ 1, 2, ... and M<N, which each perform N/M operations during N/M cycles whenever a code word is input, the Viterbi decoder characterised in that said state metric memory (38) comprises: a two-port memory (38d) including a memory bank for simultaneously reading and writing a state metric of the first half among the N state metrics generated in said ACS unit, and two memory banks for alternately reading and writing the state metrics of the second half whenever a codeword is input; a counter (38a) for counting a proceeding sequence of said cycles; a write address generator (38b) for generating a write address of said memory based on the count result of said counter (38a); and a read address generator (38c) for generating a read address of said memory based on the count result of said counter (38a). The Viterbi decoder of claim 1, wherein said state metric memory further comprises a write cross unit (38e) for realigning the state metrics which are written to the memory according to the structure of said ACS unit. The Viterbi decoder of claim 1, wherein said state metric memory further comprises a read cross unit (38f) for realigning the state metrics which are read from the memory according to the structure of said ACS unit.

Inventors: CHOI SUNG-HAN, KONG JUN-JIN, CHOI, SUNG-HAN, KONG, JUN-JIN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1998
Main classification: H03M13
IPC codes: H03M13, G06F11