Mc-cdma System, Multi-user Detection Technology

Multicarrier CDMA systems combining OFDM with CDMA is a promising candidate for the future mobile communication systems, and now lots of experts are developing it. The thesis mainly researches on single-user detection and multi-user detection techniques, and has gained some theoretical and practical results.There are many key technologies about the single-user detection, such as synchronization between receiver and transmitter, channel estimation and diversity. The thesis mainly focuses on equalization in frequency diversity, which includes ORC equalization and controlled equalization etc. The ORC equalization has amplificatory effect on the noise, and the controlled equalization needs a threshold determined by the channel fading, these are very difficult to be implemented. Based on the analysis of bit error rate lower bound for the ORC technique of the MC-CDMA systems downlink in the frequency nonselective Rayleigh fading channel, an enhanced ORC technique that omits the weakest subcarrier(the subcarrier with the smallest channel fading factor) is proposed.Multi-user detection involves optimal detection, linear detection and nonlinear detection etc. In nonlinear detection, there are an amount of literatures about interference cancellation method. Interference cancellation detection is an iterative procedure, and its first stage is the conventional single user detection, so the first stage may have high bit error probability. Unfortunately, detection error in the first stage will have bad effects on the detection of subsequent stages. Taking this into account, an improved scheme of multi-stage detection(MSD) based on parallel interference cancellation(PIC) is put forward. The measure adopts a new enhanced ORC (EORC) to get initial data estimation, and EGC is used in the successive stages. The simulation results show that the performance of the improved means could get that of the MMSE method for the MC-CDMA systems downlink after one PIC stage, three stages PIC is a better trade off between the performance and the computational complexity.The conventional MMSE detection technique could not ensure achieving minimum bit error rate(MBER) theoretically. A new plan of multi-user detection based on MBER criterion is discussed for the MC-CDMA systems downlink in the fading channels. The discussed project can be carried out by gradient algorithm. Based on the estimation of kernel density function, an algorithm, called adaptive least bit error rate(LBER) multi-user detection, can be obtained. The simulation results indicate that the LBER has a better performance than the conventional NLMS algorithm in convergence and bit error rate, though they have the same computational complexity.