The Research On The Optimization Of Turbo Coding For Broadband Wireless Communication Systems

Turbo codes, which are among the most widely used channel coding techniques, have significantly improved the spectrum efficiency of many wireless communication systems. Extensive studies have shown that the interleaving performance gain makes the error correction capability of turbo codes be relevant to the depth of the turbo codes inner interleaver. E.g., assuming the code rate, modulation type, and block error rate to be R=1/3, QPSK, and 10-4, respectively, the LTE-A turbo codes using the interleavers with large depth (i.e., K≥1000, where K is depth of the turbo codes inner interleaver) would outperform the ones using the interleavers with short depth (i.e., K≤200,) by1.5-3dB. For the sake of simplicity, we name the turbo codes using the interleavers with short depth by short turbo codes in this thesis, otherwise large turbo codes for short. To enhance the error correction ability of short turbo codes, we have studied the optimization of trellis termination for turbo codes, design of interleavers for short turbo codes, multi-user cooperative turbo coding, and multi-user ARQ. The main contributions are:1) Dual terminated turbo codes are the most popular turbo coding techniques. However, the trellis termination of dual terminated turbo codes would incur edge effect and extra tail bits, which will negatively impact the performance of turbo codes, especially the performance of short and high rate turbo codes. Hence, the continuous coding between both component encoders is used to mitigate the edge effect and decrease the length of tail bits, and two decoding algorithms are applied to decode the new coding scheme. Analyses show that the continuous coding between the both component encoders of dual terminated turbo codes can optimize the trellis termination of turbo codes, and finally improve their performance, especially the performance of short and high rate turbo codes.2) The low weight parity sequences of both component encoders of turbo codes are produced by the input sequence with self-terminated patterns of length less than-(?)k/2. Analyses of input sequences with self-terminated pattern show that maximum spread factor relative prime interleaver (MSF-RP) can guarantee to break down all input sequences with the self-terminated pattern of length less than ()k/2. Hence, MSF-RP interleaver can enhance the error correction abilities of short turbo codes.3) Compared to the techniques based on the orthogonal temporal-spectral-spatial resource assigned for every single user, more gain can be achieved by multi-user cooperation, which enables every one in the cooperation to explore more resource in a cooperative manner. Herein, we have investigated the multi-user cooperative turbo coding. Analyses show that, given the quality of service (QoS) in downlink, the log-signal-to-noise-ratio (SNR) is inversely proportional to the product of N and K, where N and K denote the number of users in cooperation and the message length of every user, respectively. That is, the larger the product of N and K, the lower the required log-SNR. Beside, the larger the product of N and K, the less the gain of the improvement. Hence, multi-user cooperative turbo coding is suitable for the scenario where the length of the message is relatively short.4) Based on the broadcasting nature and network coding, we have studied multi-user ARQ in downlink, where the base station broadcasts the linear combination of the retransmission packets from multi-user, and the retransmission packet is detected by belief propagation (BP) algorithm and softly combined with the previously received packet at the intended users before decoding. Analyses show that compared to single user ARQ, the spectrum efficiency can be improved by 8% by multi-user ARQ; meanwhile, the complexity increasing is negligible. Using the similar principle, we have extended the multi-user ARQ on the broadcasting link to the unicast link, where the sender transmits the linear combination of the retransmission packets from multi-stream by network coding, and the retransmission packet is detected by BP algorithm and softly combined with the previously received packet of the intended stream at the receiver before decoding. Furthermore, the retransmission packets of correctly recovered streams are canceled from the composite packet to improve the further decoding performance of the residual retransmission streams. Analyses show that, compared to single stream ARQ on the unicast link, the spectrum efficiency can be improved by 10% by multi-stream ARQ, meanwhile, the complexity increasing is less than 20%.In summary, aimed at improving the spectrum efficiency of turbo coded wireless communication systems, we have optimized the trellis termination for dual terminated turbo codes, designed the inner interleavers for short turbo codes, investigated the multi-user cooperative turbo coding, and studied multi-user ARQ. The techniques proposed in this thesis can optimize the performance of turbo coded wireless communication systems need to transmit traffic packages of various size, and have important practical and academic significance. Particularly, the performance of short turbo codes can be obviously improved by the contributions in this thesis, e.g., short turbo codes with MSF-RP interleavers and multi-user cooperative turbo coding. Hence, our work can significantly promote the abilities of turbo coded wireless communication systems to transmit short traffic packages.