Praise for Fundamentals of WiMAX

210 Chapter 6 • Orthogonal Frequency Division Multiple Accesswhich subcarriers have been allocated to it. This subcarrier mapping must be broadcast to allusers whenever the resource allocation changes: The format of these messages is discussed inChapter 8. Typically, the resource allocation must be performed on the order of the channelcoherence time, although it may be performed more frequently if a lot of users are competing forresources.The resource allocation is usually formulated as a constrained optimization problem, toeither (1) minimize the total transmit power with a constraint on the user data rate [21, 39] or (2)maximize the total data rate with a constraint on total transmit power [18, 24, 25, 43]. The firstobjective is appropriate for fixed-rate applications, such as voice, whereas the second is moreappropriate for bursty applications, such as data and other IP applications. Therefore, in this section,we focus on the rate-adaptive algorithms (category 2), which are more relevant to WiMAXsystems. We also note that considerable related work on resource allocation has been done formulticarrier DSL systems [2, 6, 7, 41]; the coverage and references in this section are by nomeans comprehensive. Unless otherwise stated, we assume in this section that the base stationhas obtained perfect instantaneous channe-station information for all users. Table 6.1 summarizesthe notation that will be used throughout this section.6.3.1 Maximum Sum Rate AlgorithmAs the name indicates, the objective of the maximum sum rate (MSR) algorithm, is to maximizethe sum rate of all users, given a total transmit power constraint [43]. This algorithm is optimal ifthe goal is to get as much data as possible through the system. The drawback of the MSR algorithmis that it is likely that a few users close to the base station, and hence having excellentchannels, will be allocated all the system resources. We now briefly characterize the SINR, datarate, and power and subcarrier allocation that the MSR algorithm achieves.Table 6.1 NotationsNotationMeaningKLh kl ,P kl ,σ 2P totBNumber of usersNumber of subcarriersEnvelope of channel gain for user k in subcarrier lTransmit power allocated for user k in subcarrier lAWGN power spectrum densityTotal transmit power available at the base stationTotal transmission bandwidth