Exclusion and Guard Zones in DS-CDMA Ad Hoc Networks

The central issue in direct-sequence code-division multiple-access (DS-CDMA) ad hoc networks is the prevention of a near-far problem. This paper considers two types of guard zones that may be used to control the near-far problem: a fundamental exclusion zone and an additional CSMA guard zone that may be established by the carrier-sense multiple-access (CSMA) protocol. In the exclusion zone, no mobiles are physically present, modeling the minimum physical separation among mobiles that is always present in actual networks. Potentially interfering mobiles beyond a transmitting mobile's exclusion zone, but within its CSMA guard zone, are deactivated by the protocol. This paper provides an analysis of DS-CSMA networks with either or both types of guard zones. A network of finite extent with a finite number of mobiles and uniform clustering as the spatial distribution is modeled. The analysis applies a closed-form expression for the outage probability in the presence of Nakagami fading, conditioned on the network geometry. The tradeoffs between exclusion zones and CSMA guard zones are explored for DS-CDMA and unspread networks. The spreading factor and the guard-zone radius provide design flexibility in achieving specified levels of average outage probability and transmission capacity. The advantage of an exclusion zone over a CSMA guard zone is that since the network is not thinned, the number of active mobiles remains constant, and higher transmission capacities can be achieved.

I. Introduction

Direct-Sequence code-division multiple-access (DS-CDMA) ad hoc networks are realized by using direct sequence spread-spectrum modulation while the mobiles or nodes of multiple users simultaneously transmit signals in the same frequency band. All signals use the entire allocated spectrum, but the spreading sequences differ. DS-CDMA is advantageous for ad hoc networks because it eliminates the need for any frequency or time-slot coordination, imposes no sharp upper bound on the number of mobiles, directly benefits from inactive terminals in the network, and is capable of efficiently implementing bursty data traffic, intermittent voice signals, multibeamed arrays, and reassignments to accommodate variable data rates. Furthermore, DS-CDMA systems are inherently resistant to interference, interception, and frequency-selective fading.