High Spectral Efficiency Of Cascade Overlap Code Division Multiplexing Research

Bandwidth efficiency is one of the most important objectives in the design of mobile communication system in the future. Overlapped Code Division Multiplexing (OVCDM) is a new technology that can increase the bandwidth efficiency and improve the BER performance together. There are bandwidth efficiency gain and constraint relationship produced by symbols'overlapping, which is the key point of OVCDM. Because OVCDM can be seen as a code with rate greater than 1, when it is used as a subcode in concatenation schemes, this concatenation can provide high bandwidth efficiency in terms of the product of the component code rate and also extremely large coding gain.In traditional concatenated codes, long code with excellent performance can be successfully constructed by short codes, but the cost of good performance is that total code rate declines. OVCDM can be seen as a code with code rate greater than 1, so if we use OVCDM with short constraint length as component code of a concatenated code, through appropriate match design, substantial coding gain will be obtained and the total code rate is increased as well, and system's spectral efficiency will be improved. Therefore, concatenated OVCDM is a very valuable research topic.In the concatenated OVCDM system, a nonlinear OVCDM code and a linear OVCDM code are concatenated by a symbol interleaver. In the receiver, iterative coding is taken between two Soft-Input-Soft-Output decoders, which are based on non-binary symbol-based Maximum A-Posteriori (MAP) decoding algorithm. Selection of concatenated structure is critical. This paper mainly studied two kinds of concatenations:serial concatenation and turbo product concatenation. The performance of these two systems is decided by both two code matrices and the symbol interleaver. For turbo product concatenation, the ratio between the row code and the column code is another important parameter. Symbol-based EXIT (Extrinsic Information Transfer) chart is an effective tool to search optimal concatenated structure. We redesigned the obtained system parameters based on theoretical performance results of the concatenated OVCDM system, with respect to performance and complexity and so on.Although concatenated OVCDM systems have a lower threshold of steep drop, and performance in the low SNR (Signal to Noise Ratio) is very good, but there is high error floor, which leads to the final performance in high SNR range worse than that of code and modulation scheme in LTE (Long Term Evolution). Through the error histogram analysis, we found:the probability that some uncorrectable errors exist is large, and the small errors are distributed in many frames in OVCDM system. For this situation, the idea of precoding is proposed, that is, a traditional error-correcting code is added before the original concatenated OVCDM system. Simulation results show that in AWGN channel the precoding scheme can indeed significantly reduce the small uncorrectable errors of each frame, effectively improve the performance of both bit error ratio and frame error ratio, and the ultimate performance is better than that of LTE. For the multi-path fading channel, combining precoding scheme and channel interleaving have a remarkable effect to improve the frame error ratio of the system. We focused on the system design and performance using two kinds of precodings-BCH codes and shortened BCH codes, and summarized the different characteristics of these two schemes. In order to facilitate project implementation, we preliminarily explore the quantization of the constellation points output of the concatenated OVCDM system and get some useful conclusions.In order to reduce the complexity of the system, concatenated OVCDM system based on constellation multiplexing is proposed. In this scheme the second stage OVCDM code matrix is simplified as a matrix with two rows and one column, while in the receiver, corresponding BCJR decoding process is simplified to the symbol-level soft demodulator, so system complexity can be significantly decreased. We studied different constellation multiplexing systems including serial concatenation and turbo product concatenation, and analyzed the system performance in AWGN channel and multi-path fading channel, including the first-stage OVCDM encoder structure, mapping method of the output constellation and search of the best second-stage multiplexing matrix. We mainly optimized the constellation multiplexing with spectral efficiency 4bit/s/Hz. Simulation results show that the performance almost equals to that of original system when bit error ratio is in 10-5 level, while the complexity is reduced to 1/64. It also has obvious error floor. Similarly, precoding scheme can be used to eliminate the error floor effectively. We also researched and simulated the precoding scheme of constellation multiplexing. Finally, combining precoding scheme and constellation multiplexing, we successfully designed and realized a low-complexity concatenated OVCDM system that has spectral efficiency 5.82bit/s/Hz. Its gap to the Shannon limit is only 1.9dB.