Resource Allocation in OFDMA as an Optimization Problem

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.

Implementation of the Resource Allocation Task

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

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  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.

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  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).