Adaptive Single-user Receivers In Direct-sequence Cdma Systems

There is every indication that Direct-Sequence Code Division Multiple Access (DS-CDMA) will be one of the multiple access schemes for the third generation wireless mobile systems. CDMA is a flexible multiple access method suitable for supporting many new services (speech,data,video,etc.),which are becoming increasingly important for mobile communications. Besides supporting multiple data rates,future systems also have to meet enhanced performance and capacity demands. A CDMA system employing conventional receiver techniques is limited in capacity,since its performance can be significantly degraded by multiple access interference and the near-far problem. These problems can,however,be combated with multi-user receivers. Nevertheless,they require the knowledge of multiple access interference,such as the users' spread sequences,powers and time slots,etc.,and are computationally complex. So if a user knows the spread sequences of others,the security is not existed. While complex computing means the large consuming power in transmitters and receivers. The need of the control of power also increases the load of the systems. Therefore,these disadvantages prevented the usage of multi-user receivers. Single user detection techniques,however,require only the knowledge of desired user's spreading sequence and timing,and need few computations.This paper reviews a spectrum of adaptive single-user receivers found in literatures and analyzes their performance under a unified perspective to provide a basis of comparison. These structures of adaptive single-user receivers are N-tap CHRT LAR,CSFB-LAR,DSO-LAR,CW-FS-LAR,RW-FS-LAR and FS-DFAR. These adaptive receivers use NLMS algorithm to adapt those tap weights. Through simulation,it is clear that adaptive receivers,compared to the conventional receivers,provide large gains in system capacity and are near-far resistant. Especially,those fractionally spaced adaptive receivers,such as CW-FS-LAR,RW-FS-LAR and FS-DFAR,which exploit spectral correlation due to the cyclostationary nature of the DSSSsignal,are shown to greatly outperform those adaptive receivers that can notexploit this correlation. Multi-path results presented for two-ray and urbanchannels indicate that fractionally spaced adaptive receivers act as RAKEreceivers.