[Paper Review] Guard Zones and the Near-Far Problem in DS-CDMA Ad Hoc Networks

Don Torrieri, Matthew C. Valenti|arXiv (Cornell University)|Jul 12, 2012

Wireless Communication Networks Research9 references3 citations

TL;DR

This paper analyzes guard zones—exclusion zones and CSMA guard zones—in DS-CDMA ad hoc networks to mitigate the near-far problem. Using a finite, uniformly clustered network model with Nakagami fading and Monte Carlo simulation, it shows that exclusion zones outperform CSMA guard zones in transmission capacity, especially when processing gain is high, as CSMA zones reduce capacity with only marginal outage improvement.

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.

Motivation & Objective

• To address the near-far problem in DS-CDMA ad hoc networks without centralized power control.
• To model a realistic finite network with finite mobiles and uniform clustering, avoiding stochastic geometry assumptions.
• To evaluate the tradeoffs between exclusion zones (physical separation) and CSMA guard zones (protocol-based deactivation) on network performance.
• To quantify transmission capacity and outage probability under Nakagami fading and shadowing using Monte Carlo methods.
• To compare DS-CDMA and unspread systems under varying guard zone configurations and network parameters.

Proposed method

• Models a finite ad hoc network with M mobiles placed uniformly in a bounded region using a uniform clustering process.
• Introduces two guard zones: an exclusion zone (r_ex) where no mobiles are physically present, and a CSMA guard zone (r_g) where interferers are deactivated via RTS/CTS handshake.
• Uses a closed-form expression for outage probability conditioned on network geometry and Nakagami-m fading, with shadowing factored in.
• Employs Monte Carlo simulation to compute spatially averaged outage probability and transmission capacity (TC), avoiding direct fading coefficient generation.
• Evaluates TC as a function of transmitter distance, guard zone radii, number of interferers, and processing gain (G_e).
• Compares DS-CDMA (G_e = 48) and unspread (G_e = 1) systems under varying guard zone and interference conditions.

Experimental results

Research questions

• RQ1 How do exclusion zones and CSMA guard zones affect transmission capacity in DS-CDMA ad hoc networks?
• RQ2 What is the tradeoff between maintaining higher active mobile counts (via exclusion zones) versus interference suppression (via CSMA zones)?
• RQ3 How does increasing the transmitter-receiver distance impact transmission capacity and outage probability?
• RQ4 How does the number of interferers influence the performance gain of DS-CDMA over unspread systems with and without guard zones?
• RQ5 Under what conditions does a CSMA guard zone provide significant outage improvement relative to its capacity cost?

Key findings

• Exclusion zones yield higher transmission capacity than CSMA guard zones because they do not reduce the number of active interferers.
• Transmission capacity degrades with increasing transmitter-receiver distance due to more nearby interferers, but the degradation is more gradual in DS-CDMA than in unspread systems.
• For high processing gain (G_e = 48), CSMA guard zones provide only marginal outage improvement at the cost of significant transmission capacity reduction.
• Transmission capacity increases roughly linearly with the number of interferers (M) in DS-CDMA without guard zones, but sublinearly with guard zones or in unspread systems.
• At M = 60, the transmission capacity of the unspread network is nearly identical with and without a CSMA guard zone, indicating diminishing returns.
• The performance gain of DS-CDMA over unspread systems is most pronounced when guard zones are absent or minimal, especially with large exclusion zones.