Event Details

A Variable Radix Backoff Strategy for Random Access Systems

Presenter: Yousry Salaheldin Abdel-Hamid
Supervisor: Dr. Fayez Gebali and Dr. T. Aaron Gulliver

Date: Mon, April 23, 2012
Time: 13:00:00 - 14:00:00
Place: EOW 430

ABSTRACT

Abstract:

The random access process and the contention resolution algorithm are key factors in the performance of most applications, particularly wireless networks. An efficient random access mechanism to improve connection setup, resource requests and best effort data transmission is an essential part of the design of a reliable communication system. The Binary Exponential Backoff (BEB) is the most widely used contention resolution protocol to reduce the probability of collisions.

However, it is shown that the BEB, which exponentially doubles the backoff window at every retransmission attempt may result in a significant amount unused resources, especially in a low to intermediate contention conditions. For this reason, a variable radix parameter (r) is therefore introduced to the backoff procedure to provide a reliable adaptive strategy to minimize wasted resources.

A general analytical multichannel Markov model of a contending node to investigate the variable radix strategy is first presented. Results using this model prove that adaptively varying the radix according to the number of contending stations can significantly improve performance compared to the binary exponential backoff algorithm.

The variable radix backoff strategy is further investigated in a practical application, namely the contention based BW request (BW-REQ) procedure in the IEEE 802.16 (WiMAX) standard. An analytical model of the BW-REQ procedure is presented which includes a variable radix in the backoff process. Results show that the BW-REQ throughput can be significantly improved by adaptively varying the radix according to the number of contending subscriber stations (SSs) and the available minislots in the random access channel (RACH).

Extensive simulation of the introduced models are presented which confirm the accuracy of the analytic results.