A performance analysis of channel fragmentation in dynamic spectrum access systems

Ed Coffman,Gil Zussman,Florian Simatos,Philippe Robert,Shuzo Tarumi

doi:10.1007/s11134-012-9317-7

Abstract

Dynamic Spectrum Access systems offer temporarily available spectrum to opportunistic users capable of spreading transmissions over a number of non-contiguous subchannels. Such methods can be highly beneficial in terms of spectrum utilization, but excessive fragmentation degrades performance and hence off-sets the benefits. To get some insight into acceptable levels of fragmentation, we present experimental and analytical results derived from a mathematical model. According to the model, a system operates at capacity serving requests for bandwidth by assigning a collection of one or more gaps of unused bandwidth to each request as bandwidth becomes available. Our main result is a proof that, even if fragments can be arbitrarily small, the system remains stable in the sense that the average total number of fragments remains bounded. Within the class of dynamic fragmentation models, including models of dynamic storage allocation that have been around for many decades, this result appears to be the first of its kind. In addition, we provide extensive experimental results that describe behavior, at times unexpected, of fragmentation as parameter values are varied. Different scanning rules for searching gaps of available spectrum, all covered by the above stability result, are also studied. Our model applies to dynamic linked-list storage allocation, and provides a novel analysis in that domain. We prove that, interestingly, a version of the 50 % rule of the classical, non-fragmented allocation model holds for the new model as well. Overall, the paper provides insights into the behavior of practical fragmentation algorithms.

Highlights

This paper focuses on the analysis of dynamic resource allocation algorithms and is motivated by applications of these algorithms to Dynamic Spectrum Access Networks
It brings out characteristics of the fragmentation process that need to be borne in mind in implementations, where these characteristics show parameter values that must be avoided, if a system with fragmentation is to operate efficiently
The experiments were conducted with a discrete-event simulator written in C that includes a stochastic arrival process, a capability that we intend to explore in future research

Summary

Introduction

This paper focuses on the analysis of dynamic resource allocation algorithms and is motivated by applications of these algorithms to Dynamic Spectrum Access Networks ( known as Cognitive Radio Networks). Cognitive Radios can adapt their transmitter parameters to the environment in which they operate and are viewed as key enablers of efficient use of the underutilized wireless spectrum [1, 7, 8, 17]. Under the basic model of Cognitive Radio Networks [1], Secondary Users (SUs) are allowed to use white spaces ( known as spectrum holes) that are not used by the Primary Users but must avoid interfering with active Primary Users (for instance, see Fig. 1). The physical-layer aspects of OFDM-based Dynamic Spectrum Access have been extensively studied recently, allowing channel fragmentation introduces several new problems [12, 20, 23] that significantly differ from classical Medium Access Control (MAC).

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