OFDM CDMA versus DS CDMA Performance Evaluation for Fading Channels

In code division multiple access CDMA systems direct sequence DS applying maximum ratio combining MRC at the receiver has achieved high interest for mobile communications A second and rather novel technique is orthogonal frequency division multi plexing OFDM applying maximum likelihood detection MLD at the receiver In this paper the performance of both tech niques is compared considering the mobile radio environment The analytical results for both DS CDMA and OFDM CDMA are con rmed by Monte Carlo simulations The analysis enables a performance comparison between the DS CDMA system and the OFDM CDMA system with respect to the demands of low complexity receivers which is important for the system design The results show that OFDM CDMA outperforms DS CDMA in terms of spectral e ciency Introduction Multiple access systems based on DS CDMA have achieved increasing signi cance for mobile communications applica tions A promising concept is based on DS CDMA applying MRC at the receiver MRC takes advantage of the channel diversity to combat the multi path fading How ever the capacity of a DS CDMA system is limited by both multi user interference and inter symbol interference ISI in high data rate applications OFDM CDMA systems have been proposed lately OFDM is applied to combat the frequency selectivity of the channel using a simple one tap equalizer Further more OFDM prevents the ISI and inter carrier interference ICI by inserting a guard interval between adjacent OFDM symbols OFDM is typically used for audio TV and HDTV transmission over terrestrial channels and achieves high spectral e ciency The aim of this study is to carry out a performance com parison between DS CDMA and OFDM CDMA Both tech niques are considered for the down link base to mobile since for this application power e cient mobile receivers with low complexity are required Whereas for the up link mo bile to base a high complexity multi user detector is gener ally accepted to cope with the presence of several users in independent fading channels The paper is organized as follows In Section the DS CDMA transmission scheme applying MRC is presented The OFDM CDMA transmission scheme applying MLD is described in Section An analytical performance evalu ation for DS CDMA and OFDM CDMA is shown in Sec tion The validity of the analytical results is con rmed by simulation results in Section In Section a performance comparison between both techniques in terms of spectral ef ciency is carried out Finally Section summarizes the results DS CDMA with MRC The DS CDMA transmission scheme applyingMRC is shown in Figure for the down link The base station has a maxi mum user capacity of Nu users Every user i i Nu is assigned a spreading code vector Ci of period L Since good correlation properties of the spreading codes are re quired for MRC preferentially phased Gold codes are chosen The data bits di di f g of each user i are