Praise for Fundamentals of WiMAX

6.3 Resource-Allocation Techniques for OFDMA 209• BLER and received SINR: In adaptive-modulation theory, the transmitter needs to knowonly the statistics and instantaneous channel SINR. From the channel SINR, the transmittercan determine the optimum coding/modulation strategy and transmit power [8]. In practice,however, the BLER should be carefully monitored as the final word on whether the datarate should be increased (if the BLER is low) or decreased to a more robust setting.• Automatic repeat request (ARQ): ARQ allows rapid retransmissions, and hybrid-ARQgenerally increases the ideal BLER operating point by about a factor of 10: for example,from 1 percent to 10 percent. For delay-tolerant applications, it may be possible to accepta BLER approaching even 70 percent, if Chase combining is used in conjunction with H-ARQ to make use of unsuccessful packets.• Power control versus waterfilling: In theory, the best power-control policy from a capacitystandpoint is the so-called waterfilling strategy, in which more power is allocated to strongchannels and less power allocated to weak channels [11, 12]. In practice, the opposite maybe true in some cases. For example, in Figure 6.8, almost nothing is gained with a 13dBSINR versus an 11dB SINR: In both cases, the throughput is 3bps/Hz. Therefore, as theSINR improved from 11dB to 13dB, the transmitter would be well advised to lower thetransmit power, in order to save power and generate less interference to neighboring cells [3].• Adaptive modulation in OFDMA: In an OFDMA system, each user is allocated a block ofsubcarriers, each having a different set of SINRs. Therefore, care needs to be paid to whichconstellation/coding set is chosen, based on the varying SINRs across the subcarriers.6.3 Resource-Allocation Techniques for OFDMAThere are a number of ways to take advantage of multiuser diversity and adaptive modulation inOFDMA systems. Algorithms that take advantage of these gains are not specified by theWiMAX standard, and all WiMAX developer are free to develop their own innovative procedures.The idea is to develop algorithms for determining which users to schedule, how to allocatesubcarriers to them, and how to determine the appropriate power levels for each user oneach subcarrier. In this section, we will consider some of the possible approaches to resourceallocation. We focus on the class of techniques that attempt to balance the desire for highthroughput with fairness among the users in the system. We generally assume that the outgoingqueues for each user are full, but in practice, the algorithms discussed here can be modified toadjust for queue length or delay constraints, which in many applications may be as, if not more,important than raw throughput. 2Referring to the downlink OFDMA system shown in Figure 6.3, users estimate and feedbackthe channel state information (CSI) to a centralized base station, where subcarrier andpower allocation are determined according to users’ CSI and the resource-allocation procedure.Once the subcarriers for each user have been determined, the base station must inform each user2. Queueing theory and delay-constrained scheduling is a rich topic in its own right, and doing it justicehere is outside the scope of this chapter.