Construction of equiangular signatures for synchronous CDMA systems

Abstract

Abstract-Welch bound equality (WBE) signature sequences maximize the uplink sum capacity in direct-spread synchronous code division multiple access (CDMA) systems. WBE sequences have a nice interference invariance property that typically holds only when the system is fully loaded and the signature set must be redesigned and reassigned as the number of active users changes to maintain this property. An additional equiangular constraint on the signature set, however, maintains interference invariance. Finding such signatures requires imposing equiangular side constraints on an inverse eigenvalue problem. This paper presents an alternating projection algorithm that can design WBE sequences that satisfy equiangular side constraints. The proposed algorithm can be used to find Grassmannian frames as well as equiangular tight frames. Though one projection is onto a closed but non convex set, it is shown that this algorithm converges to a fixed point, and these fixed points are partially characterized.

Interference avoidance with incremental power updates for uplink CDMA systems

“…We note that equicorrelated WBE sequence sets may not exist for all values of K and N and are in general hard to be generated [3]. Finally, we note also the fact that the proposed algorithm is capable of tracking variable target SINRs.…”

“…More precisely, instead of adjusting codeword and power to maintain current SINR above the target SINR, at any step of the proposed algorithm a given user k will perform the adaptation to maximize its spectral efficiency subject to constraints on SINR and codeword. We note that since the SINR of a given user depends on the user codeword and power, as seen in equation (3), the corresponding spectral efficiency η k depends implicitly also on the user codeword s k . By plugging in the expression of γ k from equation (3) we write user k's spectral efficiency as a function of its codeword and power…”

Dynamic Space-Code Multiple Access (DSCMA) System: A Double Interference Cancellation Multiple Access Scheme in Wireless Communications System

“…This lower bound is called as Welch bound (Li, 2003). Signature sequences that maximize the sum capacity in the uplink of CDMA systems in AWGN channel are known to satisfy Welch's bound on the total squared correlation with equality (Heath et al, 2004).…”

Adaptive Interference Avoidance for Dynamic Wireless Systems

“…We note that other related algorithms for codeword adaptation for uplink CDMA systems [2]- [7] have the same characteristic, and are not adaptable to changing numbers of active users in the system. In order to overcome this limitation, recent research [8] proposes using Grassmannian signatures for use in dynamic systems with variable number of users in the system. These are designed to support a maximum number of active users in the system subject to a given interference level, and have the nice property that interference among users does not change when less users are active in the system.…”

“…These are designed to support a maximum number of active users in the system subject to a given interference level, and have the nice property that interference among users does not change when less users are active in the system. As noted in [8] the disadvantage associated with equiangular Grassmannian signatures is that they may not exist for any desired system configuration specified by given number of users and processing gain value.…”