Research On Iterative Multiuser Detection Technology

Because multiple users share the wide frequency bandwidth, MAI (Multiple Access Interference) which is caused by the non-orthogonality of signals among each user constitutes a major impediment to the capacity and receiving quality of the DS-CDMA systems. MUD (Multiuser Detection) is an efficient and key technique to mitigate the effect of MAI by making the most information of spread serials, delay time, amplitude and phase about each user. Actually, it adopts channel coding and interleaving technique in most DS-CDMA systems. The researches of MUD are all in the coded systems in recent years. Be subjected to the idea of Turbo decoding, MUD can be processed combining with channel coding and decoding. So a kind of efficient MUD is developed which is called IMUD (Iterative Multiuser Detection). In this paper, the research is concentrated on IMUD. And the main works and contributions are as follows:1. By systematically studying lots of references about MUD and IMUD, the development of MUD technique and the current research status of IMUD are summarized in this paper. And the idea of IMUD is explained also.2. The idea of iterative process in IMUD comes of Turbo code. So the principle of Turbo coding and decoding is researched. SISO (Soft Input Soft Output) channel decoding algorithm—MAP (Maximum A-posterior Probability) algorithm used in iterative process is researched specially. The usage of the communication system simulation software SystemView combining with Matlab is studied. Then, the Turbo coding and decoding process based on MAP algorithm is simulated by SystemView united Matlab intuitively.3. Two typical received models of multiuser signals in DS-CDMA systems called synchronous AWGN (Additive White Gaussian Noise) channel and asynchronous multiple access reducing channel are researched. Several linear and non-linear traditional MUD algorithms in uncoded systems and their each advantages and disadvantages are analyzed. And then, these algorithms are simulated in synchronous AWGN channel and asynchronous multiple access reducing channel.4. The two existing IMUD algorithms of low complexity based on the traditional MUD discussed earlier are researched and simulated. Then, an IMUD algorithm based on PPIC (Partial Parallel Interference Cancellation) structure is proposed and the iterative structure of this IMUD is improved also. By simulation, MMSE (Minimum Mean Square Error), PIC (Parallel Interference Cancellation), PPIC IMUD algorithms and MMSE, PIC with improved structure, PPIC with improved structure IMUD algorithms are compared on BER (Bit Error Rate) performance respectively in the two channel models. In the asynchronous multiple access reducing channel, MMSE, PIC, PPIC IMUD algorithms nearly get the same performance and PIC, PPIC IMUD algorithms with improved structure get the little better performance than with former structure. And this result is analyzed from the aspect of resisting near-far effect. Finally, by analyzing the computing complexity, BER performance and existing advantages and disadvantages, the three IMUD algorithms are compared synthetically.