NINETEENTH NATIONAL RADIO SCIENCE CONFERENCE, ALEXANDRIA, March 19-21, 2002 CENTRALIZED ADAPTIVE MMSE DETECTORS FOR MULTIRATE DS-CDMA IN MULTIPATH FADING CHANNELS Mostafa El-Khamy, Said Elnoubi, and Onsy Abdel Alim Department of Electrical Engineering, Faculty of Engineering, Alexandria University. Fax: (203) 5921853 E-mail: m_elkhamy@ieee.org Abstract: Future mobile systems will support services with different data rates. In this paper, we propose centralized feed-forward feedback architectures for adaptive Minimum Mean Square Error (MMSE) detectors for asynchronous multirate Direct Sequence Code Division Multiple Access (DS-CDMA) systems. A multipath Rayleigh fading channel is assumed. The MMSE detector is implemented using the Recursive Least Squares (RLS) algorithm. We compare between the different proposed architectures and the results are verified using simulations. 1. Introduction: With the rapid growth of cellular networks, portable computing, and high speed fixed networks as the Internet, there has been a great demand to integrate fixed wired networks with wireless mobile networks. Future third generation Direct Sequence Code Division Multiple Access (DS-CDMA) systems are required to support a variety of multimedia applications including voice, data, facsimiles, and video. To communicate multiple data rates using the same modulation scheme, different access schemes have been developed for multirate DS- CDMA. The popular schemes are the Multi-Code (MC), the Variable Spreading Length (VSL), and the Variable Chip Rate (VCR) access schemes. In the MC scheme, a user is assigned a number of spreading codes proportional to its data rate. In the VSL scheme, the code spreading gains are inversely proportional to the data rates. In the VCR scheme, the chip rates are proportional to the data rates. An MC CDMA access scheme results in a single rate system with a signaling rate equal to that of the lowest data rate but this scheme has a poor performance and suffers from the delays due to multiplexing the high rate data and transmitting it in parallel [1][2]. We consider centralized adaptive Minimum Mean Square Error (MMSE) receivers for multirate DS-CDMA systems employing the VCR or the VSL access schemes over an asynchronous multipath Rayleigh fading channel. The detector is centralized in the sense that different users with different data rates are detected. We propose different Feed-Forward Feed-Back (FF-FB) architectures for the implementation of the adaptive MMSE receiver in a multirate system. 2. Multirate System Model: For simplicity, we consider a dual data rate, asynchronous DS-CDMA system. The ratio between the High Rate (HR) and the Low Rate (LR) is M such that ) ( ) ( h s l s MT T = , (.) s T is the symbol duration, and the superscripts (l) and (h) refer to the LR and HR users respectively. The received baseband signal model can be written as ) ( ) ( ) ( ) ( ) ( ) ( 1 ) ( 1 ) ( t z n r n r t r n K k h k K k l k h l + ∑         ∑ + ∑ = ∞ -∞ = = = , ( 1) where K (l) and K (h) are the number of the LR and HR users respectively and ) (t z is an Additive White Gaussian Noise (AWGN) with zero mean and variance 2 / 2 o z N = σ . The received signal due to user k sampled at a multiple U of the chip rate is ∑ ∞ -∞ = - = m k k k k U mN n h m d A n r ) ( ) ( ) ( (.) (.) (.) (.) (.) , where (.) k A is the received amplitude of