Nonlinear Amplifier Distortions in Ds-cdma Systems

A well-known feature of the DS-CDMA technique is its small sensitivity to nonlinear distortions with respect to multicarrier systems. Indeed, when phase-shift keying (PSK) modulations are used, the single-user DS-CDMA signal is characterized by an almost constant envelope, and therefore the transmitting high-power amplifier (HPA) can work efficiently, i.e., close to the saturation, without introducing significant distortions. In this paper, we evaluate the symbol-error rate (SER) performance of linear multiuser detectors in downlink channels, taking into account the presence of a nonlinear HPA at the transmitter. We underscore that our aim is to outline the performance degradation induced by the HPA when multiuser detectors designed for linear scenarios are employed. More sophisticated detectors specially designed for nonlinear environments [ReZh][MeGe] are not considered herein. Firstly, we present a statistical characterization of the nonlinear distortions. Successively, we extend to the linear de-correlating detector (LDD) and to the MMSE detector some of the results obtained in [CDT] for the MF in AWGN channels. Moreover, the degradation induced by the nonlinear amplifier is analyzed in fading Channels. For flat Rayleigh fading channels, a closed-form SER expression for the LDD is derived. For frequency-selective fading channels, a semi-analytic SER expression is obtained. We also evaluate the total degradation (TD), which is a parameter that allows to optimize the mean output power for a given HPA and a target SER. Simulation results, which validate the analytical approach, are presented for the RAKE receiver, the LDD, and the MMSE detector. INTRODUCTION The communication system has challenge of accommodating many users in a small area. The wireless domain is the current area of interest. The conventional systems used either frequency spectrum sharing or timesharing and hence there was the limitation on the capacity. With the advent of spread spectrum and hence CDMA, fixed bandwidth was used to accommodate many users by making use of certain coding properties over the bandwidth. But this system suffers from MAI (Multiple Access Interference) caused by direct sequence users. Multiuser Detection Technique is going to be the key to this problem. These detection schemes were introduced to detect the users‟ data in the presence of Multiple Access Interference (MAI), Inter Symbol Interference and noise. Spread spectrum CDMA systems (DS/CDMA) are becoming widely accepted and promise to play a key role in the future of wireless communications applications because of their efficient use of the channel and there allowness for nonscheduled user transmissions. Hence recent interests are in techniques, which can improve the capacity of CDMA systems. www.ijrtet.com International Journal of Research Trends in Engg and Tech 268 IJRTET VOL I-ISSUE V MAY 2016 The focus of most current research is on Wideband CDMA (W-CDMA) or NG (next generation) CDMA. In W-CDMA, the multimedia wireless network will become feasible. Not only voice, but also images, video and data can be transmitted by mobile phones or other portable devices. Achieving a higher data rate and higher capacity are two major goals for W-CDMA, which makes the multiuser interference problem more and more crucial. As Mobile communication systems based on CDMA are inherently subject to MultipleAccess Interference (MAI), since it is impossible to maintain orthogonal spreading codes in mobile environments. MAI (Multiple-Access Interference) limits the capacity of Conventional detectors and brings on strict power control requirements to alleviate the Near-Far problem. MOTIVATION and OBJECTIVE Direct sequence code division multiple access (DSCDMA) system is well known wireless technology. In DS-CDMA system, all of the users signals overlap in time and frequency cause mutual interference. This system suffers from Multiple Access Interference (MAI) caused by Direct Sequence users and near –far effect. The general structure of these detectors consists of a bank of matched filters. The detection is done on the basis of a filter matched to the pseudo-random sequence of the user. We refer to this detector as the conventional matched filter detector. Since the conventional matched filter was designed for orthogonal signature waveforms, it suffers from many drawbacks due to the MAI term which it does not take into account. Multi-user Detector (MUD) techniques exploit the character of the MAI by removal of the Multi-User Interference from each user‟s received signal before making data decision, and thus offer significant gains in capacity and Near-Far Resistance over the conventional receiver. Verdu's work shows that optimum Maximum-Likelihood Sequence Detector can completely eliminate MAI, thus greatly increase CDMA system capacity. However, the complexity of the Optimum detector is exponential in number of users, which is too complicate for practical implementation. There have been great ISSN (Online): Applied interests in finding sub optimum detectors with acceptable complexity and marginal performance degradation compared with the optimum detector. Sub optimum detectors can be classified into two linear multi-user detectors and subtractive interference canceller. Two of the most cited linear multi-user detectors are Decorrelating detector and MMSE detector. This work presents comparative study between linear multiuser detectors, and conventional single user matched filter in DSCDMA system. Analysis and simulations are conducted in synchronous AWGN channel, and Gold sequence and kasami sequence are used as the spreading codes. Multiuser detectors derivation is presented for synchronous DS-CDMA systems. The synchronous assumption considerably simplifies exposition and analysis and often permits the derivation of closed form expressions for desired performance measures. These are useful since similar trends are found in the analysis of the more complex asynchronous case. Further more, every asynchronous system can be viewed as equivalent synchronous system with larger effective user population. MEASURE OF PERFORMANCE The probability of error of bit error rate (BER), as a function of the signal–to–Noise Ratio (SNR), is a common and essential figure of merit for a communication system, indicating the feasibility of reliable data transfer across the channel. The BER can be used as a metric to compare different communication systems. Throughout this work we utilize the BER as a measure of performance for the multiuser detection schemes.