Effects of FFT Size on PAPR of MC-CDMA System M. F. L. Abdullah Faculty of Electrical and Electronic Engineering University of Tun Hussein Onn Malaysia Johor, Malaysia faiz@uthm.edu.my M. F. Ghanim Computer Engineering Department College of Engineering University of Mosul Mosul, Iraq mayada_faris@yahoo.com A. Z. Yonis Communication Eng. Dept. College of Electronic Engineering University of Mosul Mosul, Iraq aws_zuher@yahoo.com Abstract—Due to the rapid development in technology, researchers are looking towards high speed wireless technologies which are suitable for high quality multimedia services. One of the most attractive and effective technologies which is used in fourth generation is Multi Carrier Code Division Multiple Access (MC-CDMA); this technology has many advantages such as robustness against frequency selective fading and high scalability in possible data transmission rate. From other point of view, MC- CDMA has drawback in transmission of signals because they have high Peak to Average Power Ratio (PAPR), the main trend of this paper is to evaluate PAPR of MC-CDMA system with different FFT size to obtain system with minimum PAPR. Keywords—MC-CDMA; SC-FDMA; PAPR; multicarrier. I.INTRODUCTION The conventional code-division multiple-access (CDMA) technique used in third generation system faced serious limitations by channel dispersion, causing inter symbol interference (ISI), and it requires advanced signal processing algorithms to contain it. Therefore, MC-CDMA employing multiple stream of data channel can combat channel dispersion, hence ISI, hereby increasing system capability to accommodate a higher number of users [1]. MC-CDMA is a combination of orthogonal frequency division multiplexing (OFDM) and CDMA, whereby the OFDM system generated by fast Fourier transform (FFT) is coded by a CDMA code to provide the multiple access [2]. There are number of approaches how to improve MC- CDMA performance (spreading code selection, PAPR reduction methods, multi-user receiver and nonlinear distortion compensation methods). So, MC CDMA systems have the inherent problem of a high PAPR, which causes serious performance degradation in the transmitted signal. When the signal passes through the High Power Amplifier (HPA), the high PAPR causes the peaks to enter into saturation region resulting in in-band radiation (IBR) and out-of- band radiation (OBR). The IBR degrades the performance by increasing BER, but the OBR results in Adjacent Channel Interference (ACI). The PAPR brings disadvantages like decreasing the SQNR (Signal-to-Quantization Noise Ratio) of ADC (Analog-to- Digital Converter) and DAC (Digital-to-Analog Converter) while degrading the efficiency of the power amplifier in the transmitter .It also increases BER and consumption of power. Use of HPAs result in increased cost, reduced battery life, increased co-channel interference and Inter Symbol Interference (ISI).Due to non-linear distortion in HPA; it further results in a loss of subcarrier orthogonality and spectral regrowth. Thus, if PAPR is reduced, less complexity of ADC and DAC, improved Signal to Noise Ratio (SNR) and BER will be obtained, enhancement of bandwidth and battery life with low power consumption will be gotten. The PAPR problem is more important in the uplink since the efficiency of power amplifier is critical due to the limited battery power in a mobile terminal [3] [4]. MC-CDMA systems are seen as special OFDM system when analyzing PAPR performance [5]. A wealth of methods for mitigating the PAR problem has been suggested in the literature. Some of the promising approaches are as follows [6]: Amplitude Clipping and Filtering, Coding, Selective Mapping, The adaptive pre distortion technique, and DFT Spreading. II.MULTI-CARRIER CDMA SYSTEM In recent years, many aspects have been investigated for the conventional MC-CDMA systems, including the maintenance of quality of service, elimination of interference, as well as multiple access interference (MAI), management of spreading code resources, and the design of advanced detection schemes [7]. So it is important to derive all the related equations of transmitter and receiver of MC-CDMA system. A. MC-CDMA Transmitter The OFDM scheme is insensitive to frequency selective fading but it has severe disadvantages such as difficulty in subcarrier synchronization and sensitivity to frequency offset and nonlinear amplification; on the other hand, the CDMA scheme has robustness against frequency selective fading [8]. 2013 IEEE 9th International Colloquium on Signal Processing and its Applications, 8 - 10 Mac. 2013, Kuala Lumpur, Malaysia 978-1-4673-5609-1/13/$31.00 ©2013 IEEE 182