Guard Zones and the Near-Far Problem in DS-CDMA Ad Hoc Networks

Abstract: 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 is modeled as a uniform clustering process. The analysis uses a closed-form expression for the outage probability in the presence of Nakagami fading, conditioned on the network geometry. By using the analysis developed in this paper, the tradeoffs between exclusion zones and CSMA guard zones are explored for DS-CDMA and unspread networks.