Multi-user Interference Cancellation In Asynchronous WCDMA

In a code division multiple access(CDMA) system, the presence of the multiple access interference(MAI) limits the number of users that can simultaneously communicate over the channel. To overcome the detrimental effects of the MAI, the multi-user detection (MUD) is introduced. Due to its high computational complexity, the optimum multi-user detection algorithm can not be implemented in practical receiver, so various sub-optimum multi-user detection algorithm which have significant reduction in computation complexity but with negligible performance degradation were proposed. Multi-stage interference cancellation (MIC) is one of the sub-optimum multi-user detection algorithm which have been studied extensively.Most of the previous paper on multi-user detection focus on the receiver's performance such as computational complexity, Bit Error Rate (BER), multi-user efficiency and etc, but little attention was paid on the system's actual capacity gain by use of MUD. In this dissertation, we studied the potential capacity benefits by the use of Multi-stages interference cancellation algorithm in an asynchronous WCDMA uplink, which transmit data at rate of 384 kbps in 5 MHz bandwidth, given 10-5 Bit Error Rate(BER) performance requirement. A multi-user detection system based on CDMA reverse link was modeled in Matlab platform. Performances for different number of users were evaluated through simulations.Simulation results show :For a multi-user detection receiver which combined Forward Error Control code(FEC) and multi-stage interference cancellation algorithm , the key factor which determine whether the detector can convergence is the error probability of the outputs of match filters. It will affect the working status of Viterbi decoder and the reliability of interference estimation.The Viterbi decoder has error correcting threshold, when the error rate of its input signal is above the threshold, the error probability of its output will be much higher; it's also a factor which restrict the convergence of the MUD detector. Pre-decoder interference cancellation to the outputs of Match filter can alleviate its impact .If the BER of the signals which was used to estimate interference is higher than 0.1, partial interference cancellation will get better results. Simulation show that when the average BER of all interfere user is between 0.1~0.2, 0.4 is the optimized cancellation coefficient , there is an average of 0.05 BER improvements.The interfere power was determined by the cross correlation coefficient of each users' signature waveform; higher cross correlation coefficient will cause the errors propagate between users and stages, and it may cause the MUD detector divergence. Partial interference cancellation pertain to a particular cross correlation coefficient can effectively resolve this problem.For an AWGN channels, the conventional CDMA detector can only supports three high bit rate users in WCDMA at a bit-error rate of 10-5 for the constraint length 7, 1/2-rate convolutional coded system. The iterative algorithm proposed in this thesis can triple the number of users in the target application in the absence of inter-cell interference.