The Performance Simulation of Downlink MC-CDMA System with Spreading Sequence Allocation Procedure

• Main Authors: Ying-Jay Lin, 林英傑
• Other Authors: 袁正績
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/37932241680413837854

碩士 === 中華大學 === 電機工程學系碩士班 === 90 === In wireless communication systems, all spread spectrum signals will undergo the frequency-selective fading channel. In order to reduce the effect of this type of fading on system performance, the multicarrier code division multiple access (MC-CDMA) system is developed. The MC-CDMA system utilizes orthogonal frequency division multiplexing (OFDM) to divide the channel into many narrowband subchannels, such that each subchannel only undergoes flat fading, this in turn can also reduce the receiver complexity. This thesis simulates the BER performance of MC-CDMA system with four different combining strategies, which include orthogonality restoring combining (ORC), equal gain combining (EGC), maximum ratio combining (MRC), and minimum mean square error combining (MMSEC). Our results show that MMSEC is the optimum combining method in the MC-CDMA system. In addition, we will introduce a modified type of the MC-CDMA system, which we will denote S/P MC-CDMA system. Since the S/P MC-CDMA has incorporated the serial to parallel process, it prevents the signals from undergoing the frequency selective fading. Also, the same symbol will not be put on adjacent subcarriers, such that it could avoid running through burst errors, which will result in consecutive symbol errors. Based on our simulation results, the S/P MC-CDMA is superior to the MC-CDMA system. We also consider the effect of intersymbol interference (ISI) on system performance. Our results show that as long as the guard period is longer than the delay spread, ISI has little influence. Additionally, a spreading sequence allocation procedure is considered, which can also help improve the system performance.