Future wireless systems will need to accommodate information sources with different data rates. DirectSequence Code-Division Multiple-Access (DS/CDMA) is a multiple access technique that is well suited to provide multi-rate access. Thus, in this paper, multi-rate communication systems are considered for the transmission of DS/CDMA wireless signals. Performance for maximum likelihood-based detection is studied in the context of two multi-rate access methodologies: multi-code access where high data rate users multiplex their information streams onto multiple codes; and variable spreading length access where signature sequences of different lengths are assigned to users with different data rates. Various maximum likelihood-based detection schemes for the variable spreading length system are considered as they can achieve nearoptimal performance and thus provide reference points for comparison with sub-optimal schemes. In addition, asymptotic multi-user performance measures are calculated ...

Recently, decorrelator-based receivers have been investigated for demodulating data in a dual-rate synchronous DS/CDMA system. The proposed receivers were a high-rate decorrelator (HRD) matched to the data rate of the high-rate users and a low-rate decorrelator (LRD) matched to the data rate of the low-rate users. In this paper, the probabilities of error attainable for high-rate and low-rate users with the use of the HRD and the LRD are analyzed. It is proven that the LRD offers superior performance to that of the HRD for all users. Asymptotic analysis provides bounds for the performance difference for high-rate users. The results are generalized to multi-rate systems. To increase fidelity, a Sliding-Window Decorrelator is proposed which demodulates a high-rate user's data by a soft decoding rule from the outputs of several decorrelators sliding along the received signal sequence. The results show that it performs better than the HRD while maintaining smaller demodulation delay and co...

Recently, decorrelator-based receivers have been investigated for demodulating data in a dual-rate synchronous DS/CDMA system. The proposed receivers were a high-rate decorrelator (HRD) matched to the data rate of the high-rate users and a low-rate decorrelator (LRD) matched to the data rate of the low-rate users. In this paper, the probabilities of error attainable for high-rate and low-rate users with the use of the HRD and the LRD are analyzed. It is proven that the LRD offers superior performance to that of the HRD for all users. Asymptotic analysis provides bounds for the performance difference for high-rate users. The results are generalized to multi-rate systems. To increase fidelity, a Sliding-Window Decorrelator is proposed which demodulates a high-rate user's data by a soft decoding rule from the outputs of several decorrelators sliding along the received signal sequence. The results show that it performs better than the HRD while maintaining smaller demodulation delay and co...

Abstract — A dual rate synchronous DS/CDMA system provides service to low bit rate and high bit rate users. In a fixed duration interval, a low rate user transmits one bit while a high rate user transmits M bits. The differences in the bit transmission rates result in different processing gains for each class of user. In this paper, we propose a decision feedback decorrelator for the dual rate synchronous DS/CDMA system which uses modified correlators and initiates the bit decision process at the end of the high rate bit interval. Every step of the decision process is executed by utilizing the decisions of all previously decoded users. This dual rate decision feedback receiver (evaluated by simulation) is found to outperform two types of decorrelators for dual rate CDMA systems. It is also observed that as the interferers grow stronger relative to the desired user, the performance of the decision feedback receiver for decoding the desired user approaches the single user bound. I.

This paper advocates the employment of burst-by-burst adaptive transceivers, which are capable of accommodating the time-variant channel quality fluctuation of wireless channels. This paper is concluded with a range of performance figures and system design guidelines for wireless systems. Keywords---Burst-by-burst adaptive video transmission, H.263 codec, joint detection CDMA, MPEG4 codec, power control, 3G systems, 2G systems, video, video over cordless telephones, video telephony, wireless video, wireless video communications. I. STATE-OF-THE-ART SYSTEM COMPONENT DEVELOPMENTS A. Introduction and Outline The subject of mobile radio communications has reached a state of maturity over the past two decades, as indicated by the excellent monographs by Jakes [1], Lee [2]--[4], Parsons and Gardiner [5], Parsons [6], Pahlavan and Levesque [7], Feher [8], Goodman [9], P

this paper we present results comparing maximum likelihood based joint detection for two of the more viable multi-rate access schemes. The objective is to determine if one multi-rate access scheme is inherently better than the other. Our consideration of maximum likelihood based detection schemes is motivated by the need to determine benchmarks for performance of other detection methods. As the methods under study are near-optimal, they can fulfill this benchmarking role. The two access schemes to be examined are multi-code access and variable spreading length access. The comparison will be based on the determination of asymptotic multi-user performance measures. To this end, bounds 1

Multi-rate communication systems are considered for the transmission of Direct-Sequence Code-Division MultipleAccess wireless signals. Performance for jointly optimal detection is studied in the context of multi-code access where high data rate users multiplex their information streams onto multiple codes; and variable spreading length access where signature sequences of different lengths are assigned to users with different data rates. Preliminary results are obtained for various methods of jointly optimal detection for the variable spreading length system. In addition, asymptotic multi-user performance measures are calculated and bounded to compare performance of the two systems. 1 Introduction As wireless systems proliferate, there is a desire to be able offer wireless transmission services to accommodate a variety of diverse sources. Possible sources such as voice, video, and data communications have inherently different data rates. We shall consider providing multi-rate communica...

This paper examines truncated window decorrelators for an asynchronous DS/CDMA system supporting users transmitting at different bit rates. We decode a user by extending the observation window over sufficient number of its bits. To characterize practical window sizes, simple upper and lower bounds for the asymptotic efficiency of both the truncated window and infinite window decorrelators are developed. Empirical results show that as the length of the observation window increases, the bounds converge rapidly to the asymptotic efficiency of the infinite window decorrelator. The complexity of the receiver depends strongly on the ratio of the maximum to minimum bit rates.

Optimum near-far resistance is studied for synchronous dual-rate DS/CDMA systems. Three multi-rate access schemes are considered: multi-code (MC) access where high-rate users multiplex their data bits onto multiple codes and form a single-rate system; variable spreading length (VSL) access where the spreading lengths of signature sequences are inversely proportional to users' data rates; and variable chipping rate (VCR) access where the chipping rates of the signature sequences are proportional to users' data rates. In order to remove the influence of signature sequences in the comparison of the three schemes, random signature sequences are assumed. Optimum near-far resistance is then averaged over all possible realizations. Two types of code sets are considered for the VSL system: general random codes and random repetition codes. Bounds and approximations are provided for the average optimum near-far resistance. Analytical results show that the performance depends on the access scheme...

Among multi-access communications techniques, CDMA (Code Division Multiple Access) is interference-limited. Conventional single-user receivers suffer from the nearfar problem in CDMA cellular communications systems. One method to suppress multi-access interference is digital beamforming by using base-station antenna arrays. However, using beamforming alone cannot solve the near-far problem. Verd'u demonstrates that multi-user signal detection can be used to eliminate multi-access interference by utilizing all active users' spreading codes at the base-station. This thesis addresses the incorporation of array signal processing with multi-user signal detection in the CDMA terminal to base-station uplink. In particular, this thesis proposes a method - jointly estimating the unknown channel array response vectors and detecting the bits from all users.