Study Of Parallel Interference Cancellation For Wideband CDMA

Besides "ping-pong" effect, constant-weight parallel interference cancellation (CW-PIC) is unreasonable because of the time variation of wireless channels and the randomicity of system loads in realistic systems. Thus, dynamic weight PIC is necessary for wideband CDMA. However, fewer theoretical results exist for the weight study of interference cancellation(IC) in multi-path fading channel, and some existing dynamic weighting schemes have little theoretical foundation. Therefore, it is stringent that in-depth research on parallel interference cancellation (PIC )over multipath fading channels should be implemented.In this thesis, taking example for WCDMA uplink, we investigate Rake receiver, partial parallel interference cancellator and smart antenna for CDMA over multipath fading channels and derive their closed-form expressions of bit error rate (BER). Numerical analysis and computer simulation is accomplished to validate our analysis. The main results are as follows:First, the variance of multiple access interference for complex CDMA is derived by Gaussian approximation method in chapter 2. This work is fundamental to performance analysis for WCDMA. Characteristic function method is adopted to derive the BER expression for coherent CDMA over fading channels in detail. This model can be employed to computer BER for many WCDMA receivers.The variance of residual interference for one path of a user is also educed and a close-form BER expression for PPIC is obtained. This work is fundamental to performance analysis of PIC in multipath channels.In chapter 3, we investigate optimizing weight for PPIC with perfect channel estimation, and argue its convergence and performance limit. This is the performance criterion of PIC study and design in the case of imperfect channel estimation.In the case of imperfect channel estimation, we first bring forward the idea of path-level optimizing and derive the expression determining the optimal interference cancellation weight (OICW) at path level for PPIC. Then we contrast path-level optimizing scheme with user-level optimizing one. The oscillatory behavior of user-level-optimized PIC (ULO-PIC) and the convergence of perfect channel estimation PIC (PCE-PIC) and path-level-optimized PIC (PLO-PIC) are proved. It shows us the method of design good PIC for realistic systems.Analysis indicates that the lower BER limit of multistage PPIC depends on power ratio, observation length of channel estimation, signal-noise-ratio and number of active users. This shows us some feasible techniques for improving PLO-PIC.In chapter 4, PLO-PIC is adapted to low complexity by using channel estimation, accompanied with simulation. The weight from this optimizing scheme can be updated with channel condition of the interesting signal and system loading, and the scheme is applicable to realistic systems, and even multirate CDMA systems.A receiver structure including smart antenae, Rake and PLO-PIC is presented and it's BER expression is given. The receiver has many advantages such as simplicity, practicality and anti-interference by space and time. The impact of antena number and cancellation stage on BER performance is investigated to lay the foundation of designing a receiver of low cost.