Research On Complementary Coded Multi-user Wireless Communication Techniques And Code Design

Multiple access technique, which determines the pattern of sharing wireless resource among multiple users and channels, is one of the key physical layer techniques in wireless communications. Therefore, the development of multiple access technique marks the generational change and evolution of mobile communication systems. Compared with other multiple access schemes, code division multiple access (CDMA) possesses unique selective addressing capability, high security, resistance of narrow-band and multiple-path interferences (MPI), etc. However, due to the interference-limited problem, near-far effect and complex power-control mechanism resulting from bad correlation properties of traditional spreading codes, traditional CDMA failed to be selected as the mainstream multiple access technique in the fourth generation of mobile communications.In order to speed up the innovation of code division techniques, a new solution of next generation CDMA has been proposed based on a novel kind of spreading codes-complementary codes (CCs). Since CCs are able to realize both ideal auto-and cross-correlation properties with a new code design principle integrating real communication environment, it enable CDMA technique to get rid of the interference-limited problem. It opens up a new way for the innovation and application of the CDMA technique in the future wireless communications. In the last decade, the research on complementary coded CDMA (CC-CDMA) is mainly around refinement of the theory of CCs, code de-sign and system design over ideal communication environment. In fact, the research on communication systems based on CCs theory is practically still in its nascent phase. The special structure of CCs puts stress on the design of practical communication systems. Over complex communication environment, whether single spreading technique based on CCs is still able to resist interference or not has yet to be verified.Based on the above background and analysis, the multi-user wireless communica-tions based on CCs theory are analyzed and researched thoroughly in the thesis. Starting with the basic definition and properties of CCs and correlation functions, the new prob-lems caused by the special structure of CCs to the system design are analyzed in the thesis and we get the basic four constraints of CC-CDMA system design:1) separation of the element sequences;2) order match of the element sequences;3) synchronization of ele- ment sequences;4) equal gain combination. Based on these, the thesis compares the ca-pability of two classic CC-CDMA system architectures:serial time-division-multiplexing and parallel frequency-division-multiplexing, on the aspects of multiple access interfer-ence (MAI) and multi-path interference resistance. Then a problem is proposed that "the parallel frequency-division-multiplexed CC-CDMA will lose the interfere-free feature in frequency selective fading channels, since it won’t satisfy the4th constraint"Considering the differences between downlink and uplink communications, this the-sis launches the research on the above question in downlink and uplink CC-CDMA sys-tems respectively. For the downlink system, this thesis proposes a weighted combination scheme. Based on the analysis on the influence of classic single-user combination algo-rithms on bit error rate (BER) and anti-interference ability of the CC-CDMA system, a multi-user combination scheme based on minimum mean square error (MMSE) criterion is proposed. Then considering the computation cost and complexity of minimum mean square error combination (MMSEC), a corresponding simplified scheme is also proposed. Through theoretical analysis and simulation, we conclude:employing "downlink optimal CCs" with maximal-ratio combining is the most direct and low-cost way to improve the BER performance of downlink CC-CDMA systems over frequency selective fading chan-nels. With perfect channel state information in the receiver, this scheme is able to achieve MAI-free with full frequency diversity gain over independent frequency selective fading channels.For uplink CC-CDMA systems, this thesis proves that the uplink system is unable to resist MAI over frequency selective fading channels with the above proposed weighted combination scheme and we deduce the MMSEC coefficients for the uplink system. Through analysis and simulation, we prove that uplink MMSEC is able to provide good BER performance for the CC-CDMA system through a favorable trade-off between anti-MAI and frequency diversity gains. Then in order to resist the MAI and to reduce the complexity of MMSEC, this thesis proposes a joint pre-equalization and adaptive com-bining detection scheme. The pre-equalization part of the proposed scheme is able to reduce the times of iterations of the adaptive algorithm, while the adaptive combining part resolves the difficulty of combination resulting from the pre-equalization. Therefore, it achieves close to or even better BER performance than MMSEC.Then, facing multiple input and multiple output (MIMO) technique, the evolution of CCs is researched in this thesis. Based on the stating of principle of chip-level space-time coding based on three-dimensional complementary codes (3DCCs), this thesis focuses on the design of3DCCs and the corresponding multi-user MIMO systems. We derivate the theoretical bound of3DCCs with ideal correlation properties, and then propose a general-ized code design method for3DCCs with flexible trade-off between system overhead and correlation properties. In the end, the basic system architecture of MIMO-CC-CDMA based on chip-level space-time coding is proposed. The de-spreading and de-coding prin-ciples of both time-division-multiplexed and frequency-division-multiplexed MIMO-CC-CDMA are presented, and we also analyze their BER performance, anti-interference and multi-antenna diversity/multiplexing features. Both the theoretical analysis and simulated results prove the anti-interference features brought by3DCCs to multi-user MIMO sys-tems. Meanwhile both space diversity and multiplexing is supported by the proposed MIMO-CC-CDMA system, and it is also able to offer a flexible tradeoff between diver-sity gain and multiplexing capability to fit varying channel conditions and application requirements.