Resource Allocation in OFDMA as an Optimization Problem

Resource Allocation in OFDMA as an Optimization Problem

Álvaro Pachón (Universidad Icesi, Colombia), Ubaldo García-Palomares (Universidad de Vigo, Spain) and Juan Manuel Madrid (Universidad Icesi, Colombia)
Copyright: © 2015 |Pages: 11
DOI: 10.4018/978-1-4666-5888-2.ch566
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Background

Problem Definition

The resource allocation problem can be stated as assigning resources (frequency and power blocks) with efficacy and efficiency, to a number of users demanding a variable set of services within a geographical area, accomplishing a balance between full satisfaction of user service requirements, and service provider financial / operational sustainability.

Problem Importance

The problem has two perspectives: a) The engineering perspective, since it is necessary to understand the challenges network operators face to offer services; and b) the service perspective, because success of the service provider depends on the variety of services offered to users, and the capacity to fully satisfy them.

Application Context

The context for problem formulation and solution is the downlink of a LTE (Long-Term Evolution) WMCS. LTE uses OFDMA (Orthogonal Frequency Division Multiple Access) as its multiple access technique at physical level.

OFDMA is a multiple access technique adopted by next-generation WMCS for their radio access interface, combining frequency and time domain access. Hence, the radio resource is represented in a time-frequency space, known as a resource grid. Channel bandwidth is divided into an orthogonal set of narrow-band carriers, allowing conversion of high serial data rates of broadband channels into parallel, orthogonal sets of smaller speed narrow-band channels, which experience flat frequency fading.

LTE, introduced by 3GPP, constitutes the evolution of cellular mobile communications towards a wideband end-to-end IP network. Several of its features are related to this chapter’s proposal: a) Use of OFDMA as the downlink multiple access technique; b) use of adaptive modulation and coding (AMC) schemes, selected according to channel condition and services demanded by users; and c) a high spectral efficiency requirement, leading to adoption of 1/1 frequency reuse schemes. In LTE, subcarriers in a channel are grouped in subsets known as Resource Blocks (RB). The average value of the SINR (Signal to Interference plus Noise Ratio) fully describes the subcarrier set, and determines the quality of received signal, the modulation/coding scheme to be used, and the achievable data rate.

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Approaches To Solution Of The Problem

The resource allocation problem in OFDMA offers multiple degrees of freedom, including implementation of the resource allocation task, time perspective for task execution, services offered by the service provider, WMCS architecture, optimization criterion, and resource scheduler architecture.

Implementation of the Resource Allocation Task

González et al. (2009) and Cicalo et al. (2011) propose two approaches for resource allocation:

  • Distributed approach. It enables each node to autonomously allocate resources, making the task simpler, lighter and less prone to failure in the computational sense. However, a global view of the system is not possible with this approach.

  • Centralized approach, featuring a central coordinator node. This node receives information from all other nodes, and performs resource allocation. Thus, it is possible to determine the global system status, in order to mitigate generated interference. Disadvantages of this approach include coordinator overload and/or failure (single choke point).

Key Terms in this Chapter

Signal to Interference plus Noise Ratio (SINR): Ratio between useful signal (signal with user information) and undesirable components disturbing it (noise and interference).

Optimization: Area of applied math, which specializes in selecting the best element within a given set, with respect to established criteria, using known principles and methods.

Wireless Mobile Communication Systems (WMCS): Communication systems using wireless channels to offer information transfer services to a population of mobile users within a geographical area. The need for efficient reuse of the electromagnetic spectrum prompts the division of the system in cells. Each cell has a base station, responsible for managing and allocating resources, allowing to satisfy the user service requirements.

Resource Allocation Problem: It can be stated as allocating the resources (frequency and power blocks) with efficacy and efficiency, to a number of users demanding a variable set of services within a geographical area.

Long-Term Evolution (LTE): Standard for wireless data communications introduced by the 3GPP ( 3rd Generation Partnership Project ) in Release 8. Its deployment is an important step on WMCS evolution towards a broadband, multiservice, end-to-end IP network.

Orthogonal Frequency Division Multiple Access (OFDMA): A multiple access technique adopted by next generation of WMCS for their radio access interface. It combines frequency and time access. By this reason, the radio resource is represented by a time-frequency space, known as a resource grid.

Interference: Anything altering or disrupting an information signal, as it travels along a channel from transmitter to a receiver.

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