The idea of Distributed Space-Time Codes (DSTCs) was proposed to achieve spatial and temporal diversity without having channel information at the relays in wireless networks. In that scheme, distributive relays cooperate with each other to generate a Space-Time Code (STC) at the receiver. However, DSTCs have poor error rate performances at low signal to noise ratios (SNRs) that may disqualify them from being used in next generation mobile communications systems. To overcome this issue, this paper proposes a novel scheme based on the joint use of DSTCs and turbo codes, which we call “Distributed Space-Time Turbo Codes (DST-TCs)”. We also propose a Fast Forced Symbol Method (FFSM) which significantly improves the coding gain of the proposed DST-TCs. The FFSM is based on repeated decoding operations and does not require any significant change in the design of encoder and decoder of DST-TC. The FFSM provides 0.9 dB of coding gain over simple DST-TC at frame error rate (FER) of 10-5.

Thanks to the probabilistic message passing performed between its component decoders, a turbo decoder is able to provide strong error correction close to the theoretical limit. However, the minimum Hamming distance (dmin) of a turbo code may not be sufficiently large to ensure large asymptotic gains at very low error rates (the so-called flattening effect). Increasing the dminof a turbo code may involve using component encoders with a large number of states, devising more sophisticated internal permutations, or increasing the number of component encoders. This paper addresses the latter option and proposes a modified turbo code in which a fraction of the parity bits are encoded by a rate-1, third encoder. The result is a noticeably increased dmin, which improves turbo decoder performance at low error rates. Performance comparisons with turbo codes and serially concatenated convolutional codes are given.

The paper presents a computationally efficient distance measurement method for double binary turbo codes, such as these used in the digital video broadcast with return channel via satellite (DVB-RCS) standard, based on Garello's method. Distance spectra for all standardized DVB-RCS packet sizes and all standardized code rates are presented. A new interleaver design for DVB-RCS based on the dithered relative prime (DRP) interleaving approach is also presented. A minimum distance (d/sub min/) of 36 has been achieved for an unpunctured ATM packet of 424 information bits with a DRP interleaver, whereas the d/sub min/ of the standardized DVB-RCS interleaver is 31. A d/sub min/ of 38 has been achieved for an unpunctured MPEG packet of size 1504 information bits with a DRP interleaver, whereas the d/sub min/ of the standardized DVB-RCS interleaver is 33. Simulation results for code rate 1/3 show an improvement at high signal to noise ratios of at least 0.15 dB and 0.25 dB for ATM and MPEG packets, respectively.