Study On Performance Of DS-CDMA With Frequency Domain Equalization

A very high-speed wireless access of 100Mb/s to 1Gb/s is required for fourth-generation mobile communication system. However, for such high-speed data transmissions, the channel is severely frequency-selective due to the presence of many interfering paths with different time delays. A promising wireless access technique that can overcome the channel frequency-selectivity and even take advantage of this selectivity to improve the transmission performance is DS-CDMA (direct sequence code division multiple access).In a broadband wireless communication system, frequency selective multipath fading severely degrades the bit error rate (BER) performance of DS-CDMA. An effective technique to improve the BER performance is a RAKE receiver. However, as the frequency selectivity of the multipath channel becomes more severe, the intercode interference (ICI) becomes stronger and the accurate channel estimation necessary for RAKE combining becomes more difficult. And the BER performance of DS-CDMA with RAKE receiver acutely degrades.Recently, for the reception of DS-CDMA signals, the MMSE frequency domain equalization is applied instead of RAKE combining. And the DS-CDMA with MMSE FDE has a better performance than DS-CDMA with RAKE in the frequency selective channel. Moreover, FDE has been adopted as a key technique for both DS-CDMA and multi-carrier (MC-) CDMA to overcome the channel frequency-selectivity. Especially, DS-CDMA transmission will probably be the most suitable technique due to lower peak-to-average power ratio (PAPR).This paper makes a more systematic research in the performance of DS-CDMA with FDE. Main research work is as follows:(1) A description of the development of DS-CDMA is given. We discuss DS-CDMA with RAKE receiver and describe some related issues of DS-CDMA with frequency-domain linear equalization. The performance of DS-CDMA with FDE based on MMSE is researched emphatically.(2) According to the shortage of conventional MMSE detection technique, a novel MMSE detection technique -PU-MMSE is provided and detection algorithm of this technique is derived.(3) Through simulation experiments, the performance of DS-CDMA with RAKE receiver and DS-CDMA with FDE is compared. Especially, we compare the performance difference of DS-CDMA with FDE based on two detection techniques (PU-MMSE and PC-MMSE).(4) Multi-carrier communication system and MC-CDMA are presented. By simulation experiments, the comparison of the BER performance between MC-CDMA and DS-CDMA with RAKE receiver is given and the BER performance of MC-CDMA and DS-CDMA with FDE is compared as well. Moreover, compared with MC-CDMA, the competitive advantages in the communication system of DS-CDMA with FDE are discussed.