Analysis of BER Performance in Presence of Nonlinear Distortion Due to PD-HPA in Downlink DS-CDMA Signals Abstract A predistorter-high power ampli fier (PD-HPA) pair has become a common practice in wireless communication to compensate for nonlinear distortion due to HPA. However, the PD-HPA pair still produces severe signal distortion when the input signal exceeds the PD-HPA’s saturation level. The effects of such distortion on bit error rate (BER) degradation in downlink direct sequence-code division multiple access signals (DS-CDMA) are analyzed. We establish which signal characteristics at the HPA input are the factors contributing to BER. Assuming that the baseband CDMA signal is characterized as a complex Gaussian process, we develop analytic expressions for the BER and the contributing factors to BER. Index Terms—CDMA, nonlinear distortion, pre-distortion. 1. Introduction Downlink direct sequence-code division multiple ac-cess (DS-CDMA) signals typically exhibit large dynamic range since they represent the sum of signals of many users. Unfortunately, when passed through a high power amplifier (HPA), this large dynamic range results in distortion for components falling in the highly nonlinear regions of the HPA, which degrades the system bit error rate (BER). A common parameter for characterizing the dynamic range of a signal is the signal peak-toaverage power ratio (PAR). PAR is often used as an indicator to how much harm the signal will suffer due to HPA nonlinearity, and also allows the system designer to determine the required amount of input back-off (IBO) to reduce nonlinear distortion effects. While PAR has its use, a signal’s dynamic range in relation to an HPA only captures only one feature of the signal’s interplay with the HPA. 2. Objective In this paper, an analytical description of the effects of HPA nonlinearities on the performance of a CDMA systems has been presented, for both synchronous (orthogonal) and asynchronous CDMA. It has been shown that, in a broad range of cases, the nonlinear distortion in the decision variables at the receiver can be modeled by means of a complex scale factor and an additive uncorrelated Gaussian noise: its variance and the scale factor have been evaluated analytically. Nonlinear distortion noise is usually smaller than MAI in asynchronous systems. This model, whose accuracy has been checked by means of simulation, has allowed an easy analytical evaluation of the TD for some classes of HPA models. 3. Proposed scheme In order to evaluate the system performance, in this section a statistical model for the decision variable , given by , is developed by taking into account the effects of the nonlinear channel. Having in mind the similarities between the multichannel CDMA signal and the multicarrier OFDM signal, both obtained with the superposition of many elementary signals (multiplied by the spreading sequence in the first case and by the sequence of samples of a sine wave in the latter case), we try to extend the theoretical model proposed in [8] for OFDM systems to CDMA systems. More precisely, we want to apply the extended Bussgang’s theorem to a CDMA signal provided that it can be modeled as a Gaussian signal. This approach is justified by considering that the signal at the input of the nonlinear block includes the sum of contributions which are uncorrelated and have the same statistics. If is sufficiently large, should become a complex Gaussian (nonstationary) random process as a consequence of the Central Limit Theorem. Of course, we should be careful when we want to extend this approach to a CDMA signal having its components with different power levels. 4. Software and hardware requirements Operating system : Windows XP/7. Coding Language : MATLAB Tool : MATLAB R 2012 System requirements: Hardware requirements: System : Pentium IV 2.4 GHz. Hard Disk : 40 GB. Floppy Drive : 1.44 Mb. Monitor : 15 VGA Colour. Mouse : Logitech. Ram : 512 Mb. 5. Conclusion In this letter, we studied how nonlinear distortion due to PDHPA affects BER degradation for transmitted downlink DSCDMA signals. We established analytically how the threshold exceeding rate RE and the variance of the clipped portion of the signal σ2c contribute to BER degradation. The motivation of this work is to provide system designers/operators with efficient tools that provide potential insight into the interactions between CDMA signals and the nonlinear PD-HPA, leading to better understanding of the impact of the PD-HPA on system BER. References [1] A. Conti, D. Dardari, and V. Tralli, “An analytical framework for CDMA systems with a nonlinear amplifier and AWGN,” IEEE Trans. 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