Interference Avoidance for Wireless Systems

Abstract

By definition, users of a multiple access communications system share the same medium, and such shared use leads to mutual interference. The usual methods for combating interference include channel assignment, power control and receiver signal processing. Interference avoidance is a recently developed tool which presupposes the existence of agile transceivers which can adapt their modulation and reception methods to suit the ambient environment. Thus interference avoidance, as implied by the name, seeks to place each user in that portion of the signal space where there is least interference. Greedy iterative application of this procedure optimizes various measures of multiple access performance including sum capacity and user capacity. As such, interference avoidance should be especially useful in unlicensed environments where no central control is assumed.

Codeword optimization for uplink CDMA dispersive channels

“…This is a particular case of the more general result proved in [28], which states that in general, a simultaneous water-filling solution corresponds to maximum sum capacity for a multiaccess vector channel. A proof for this particular case, which has been established independently in [28], can be found in [17]. Thus, application of greedy interference avoidance yields an optimal ensemble of codewords that maximizes sum capacity.…”

“…Proof: Full proof of this result can be found in [17]. Now suppose user k applies greedy interference avoidance and replaces its current codeword s k with the minimum eigenvector of the corresponding autocorrelation matrix of the interference-plus-noise R k .…”

“…Interference avoidance was originally introduced in the context of MMSE receiver filters [23], [24], but we concentrate our attention on greedy interference avoidance that uses matched filter receivers and was introduced in [21], [22] and explored in more detail in [17]. In order to review the greedy interference avoidance procedure, we consider the uplink of a synchronous CDMA system in which each user is assigned a unit norm N -dimensional codeword s to convey its information symbol b .…”

“…Thus, the sequence of sum capacity values along any update trajectory must be strictly increasing. A detailed proof of convergence to the optimal point for this algorithm can be found in [17].…”

“…Interference avoidance is a class of adaptive modulation techniques by which users in a CDMA system adapt their codewords in response to changing patterns of interference in the environment. Interference avoidance was introduced in the context of "chip-based" direct sequence CDMA systems [23], [24] and minimum mean square error (MMSE) receivers, and it was developed in a more general signal space framework in [17], [21], and [22].…”