Study On Peak-to-average Ratio Reduction In MC-CDMA System

Multi-carrier code division multiple access (MC-CDMA) technology is a kind of combination of OFDM and CDMA technology. The technology uses OFDM into the CDMA system, it reduces the data rate of the CDMA system, and enhances high speed data processing service ability. It is inherited from the OFDM and the CDMA technology. It has the unique superiority at the high speed data transmission, and has the anti-frequency selectivity ability, it also can against inter symbol interrupt. It has high spectrum utilization and, its modulation and demodulation could use discrete Fourier transform (DFT), it also can reduce the system complexity.As the same of other multi-carrier technology, one of the major disadvantages of MC-CDMA is its high peak-to-average power ratio (PAPR), because the MC-CDMA signal is summed by many independent sub-signals and the signal envelope distribution is Gaussian distribution. So it requires a long linear range of the high power amplifier (HPA), otherwise a significant power efficiency penalty and the degradation of the bit error rate (BER) will be occured. To overcome this drawback, in this paper, we investigate the PAPR problem and propose some solutions about the problem.We first discuss the construction of MC-CDMA system and the development of PAPR reduction methods,which provides the foundation of following discussion. Then analyze some commonly used spreading sequences and PAPR characteristics of some spreading codes, so as to avoid using the spreading code with high PAPR.Selected mapping (SLM) is a method to reduce PAPR of MC-CDMA system .In order to study the impact of different phase sequences on the reduction of the PAPR in MC-CDMA system using SLM. Based on the introduction of the usually selected as the phase sequences, we point out that they can be used as the phase sequences of the MC-CDMA system, and then more deeply analyze the PAPR characteristics for a phase sequence when its length is different and the number of the phase sequence set is different. Simulation results show that, compared with other sequences, Gold sequence can be used for the phase sequence in the actual system.for the same phase sequence, the shorter phase sequence we used, the smaller PAPR value we will get; the number of the phase sequence set is bigger, the smaller PAPR value we can get.In order to reduce the PAPR of MC-CDMA system and the computational complexity of partial transmit sequence(PTS) efficiently, a new method, un-uniform partition—suboptimal algorithm, is proposed in this paper. The simulation results show that , as compared with tradition method, uniform partition—optimal algorithm, the PAPR value increases only about 0.4 dB on some certain simulation parameters, while the computational complexity is reduced from exponential order operation to linear order operation.In order to reduce the system PAPR and the computational complexity of PTS efficiently, a new method which can reduce half of the OBPS algorithm computational complexity is proposed in this paper, and then propose several improved methods of DLPS algorithm. The simulation results show that, the proposed improved methods not only reduce the system PAPR, but also reduce the computational complexities heavily as compared with the traditional algorithm.