Optimal sequences for CDMA with decision-feedback receivers

Abstract

We consider a symbol-synchronous code-division multiple-access (CDMA) system that is equipped with a multiuser decision-feedback receiver and for which power control is available. The users are each assigned a quality-of-service (QoS) threshold to be guaranteed by the system, and to cover scenarios for which there are multiple classes of users, these are not required to be equal to each other. For an ideal decision-feedback receiver, it is known that with enough power the system can always meet the users' QoS thresholds, so we instead minimize the sum of the users' received powers over system designs (i.e., signature sequences, power-control policy, and decision-feedback receiver) which guarantee the QoS requirements. It is found that the optimal design produces two classes of users, those whose sequences and powers satisfy with equality the generalized Welch bound inequality and those oversized users that are mutually orthogonal to each other and the rest of the users. In terms of power and bandwidth savings, the optimal sequences for the decision-feedback receiver are found to compare very favorably to optimal designs for linear receivers and to random sequences for the decision-feedback receiver. Index Terms-Code-division multiple access (CDMA), decision-feedback receiver, linear minimum mean-square-error (MMSE) receiver, power control, quality-of-service (QoS), signature sequences. I. INTRODUCTION T HE design of power-and bandwidth-efficient code-division multiple-access (CDMA) systems has of late received considerable attention. This has led naturally to the study of signal designs, by which we mean the design of the users' signature waveforms or transmit pulses so that certain performance metrics are optimized within the context of a variety of system attributes. A popular example is the maximization of sum capacity for the synchronous CDMA channel with additive Gaussian noise when the received powers of the users and the available bandwidth are fixed. As a function of processing gain less than or equal to the number of users, Rupf and Massey [1] derived signature sequences that optimize the sum capacity when received user powers are all equal to each other.

Successive interference cancellation for 3G downlink: Algorithm and VLSI architecture

“…The SIC-MUD [4] is an iterative algorithm based on the concept of interfering signal regeneration and subsequent cancellation. Our implementation of the algorithm operates on bursts corresponding to N transmit data symbols.…”

Codeword optimization for uplink CDMA dispersive channels

“…The following lemma, which uses results from majorization theory [15], is useful in proving this result. Majorization theory has been applied in relatively recent work in wireless systems to sum capacity problems [25]- [27] as well as to signal design and power control for CDMA systems [7]. We mention that the proof in [24] is based on stochastic ordering, which is a particular case of the more general majorization relation.…”

Interference avoidance and power control for uplink CDMA systems

“…We note that, for the downlink of a CDMA system an analysis can be found in [1], and for the uplink case algorithms for optimal allocation of powers and signatures can be found in [20], [21]. More recent work [7], [9] presents algorithms for signal design and power control subject to quality of service expressed in terms of signal-to-interference ratios or RMS bandwidth constraints. We note that these are centralized schemes in which calculations are performed at the receiver and results must be transmitted to users.…”