Turbo-code Termination Schemes and a Novel Alternative for Short Frames

Recently a coding scheme, termed turbo-codes was proposed, which achieves results very close to the Shannon-limit. The encoding and decoding of turbo-codes is reviewed. In particular, a comparison of the current methods used for terminating the trellis of the turbo-code are described. A novel approach to termination of the code is introduced which removes the need to transmit tail bits across the channel. A reduction in transmitted symbols by 2 percent is achieved for PCS CDMA type applications. Simulations con-rm that the novel approach is better in terms of BER than other methods of termination. Perhaps the most exciting and potentially important development in coding theory, in recent years, has been the announcement of \turbo-codes" 1]. Turbo-codes were rst proposed by Berrou et al. 1] as a way of dramatically reducing the errors in a forward error correction system. The scheme combines the concepts of iterative decoding, soft-in/soft-out decoding, Recursive Systematic Convolutional (RSC) encoding, and non-uniform random interleaving. A turbo-code encoder consists of two RSC encoders, separated by an interleaver. One reason RSC encoders are chosen is that they give a lower BER, at low SNR, compared to a classical Non Systematic Convolutional (NSC) code 1]. The turbo-code decoder uses an iterative technique , using two soft decision input/output trellis decoders in each decoding stage. The maximum-a-posteriori (MAP) trellis decoder minimises the BER. Two of these MAP de-coders were used within each turbo-code decoder stage to produce the original results 1]. The information in a turbo-code decoder is processed as blocks of data, whereas most convolutional codes operate in a continuous stream which are then decoded with the Viterbi Algorithm. For a block turbo-code decoder to attain optimum performance the initial and nal code states of the data block transmitted need to be known. Currently, termination methods transmit additional bits, called the tail, so that the code can be terminated in the zero state. To avoid a further delay penalty, in a mobile environment , turbo-codes must be designed to take into account the speech block size. The considered CDMA mobile radio system of Blanz et al. 2] used a 192 bit block size. To terminate this 16 state code, four symbols are required. This adds a 2 percent overhead to the transmission rate R. This paper proposes a novel termination scheme which addresses the problem of how to eeciently terminate turbo-codes. This is achieved by not transmitting a tail. …