Praise for Fundamentals of WiMAX

4.2 OFDM Basics 121Example 4.2 In this example, we will find the minimum and maximum daterate loss due to the cyclic prefix in WiMAX. We will consider a 10MHz channelbandwidth, where the maximum delay spread has been determined tobe τ =5µsec. From Table 8.3, it can be seen that the choices for guardband size in WiMAX are G = {1/4, 1/8, 1/16, 1/32} and the number of subcarriersmust be one of L = {128, 256, 512, 1024, 2048}.At a symbol rate of 10MHz, a delay spread of 5 µ sec affects 50 symbols, sowe require a CP length of at least v =50.The minimum overhead will be for the largest number of subcarriers, so thisyields L = 2048. In this case, vL / = 50/2048 = 1/40.96 so the minimum guardband of 1/32 will suffice. Hence, the data rate loss is only 1/32 in this case.The maximum overhead occurs when the number of subcarriers is small.If L = 128, then vL / = 50/128, so even an overhead of 1/4 won’t be sufficientto preserve subcarrier orthogonality. More subcarriers are required.For L = 256, v/L < 1/4, so in this case ISI-free operation is possible, butat a data rate loss of 1/4.Sidebar 4.1 An Alternative PrefixOne alternative to the cyclic prefix is to use a zero prefix, which constitutes anull guard band. One commercial system that proposes this is the MultibandOFDM system that has been standardized for ultrawideband (UWB) operationby the WiMedia Alliance. a As shown in Figure 4.6, the multiband OFDMtransmitter simply sends a prefix of null data so that there is no transmitterpowerpenalty. At the receiver, the “tail” can be added back in, which recreatesthe effect of a cyclic prefix, so the rest of the OFDM system can functionas usual.Why wouldn’t every OFDM system use a zero prefix, then, since itreduces the transmit power by 10log 10 ((L+v)/L) dB? There are two reasons.First, the zero prefix generally increases the receiver power by 10log 10 ((L+v)/L) dB, since the tail now needs to be received, whereas with a cyclic prefix, itcan be ignored. Second, additional noise from the received tail symbols isadded back into the signal, causing a higher noise power σ 2 →((L+v)/L)σ 2 .The designer must weigh these trade-offs to determine whether a zero or acyclic prefix is preferable. WiMAX systems use a cyclic prefix.a. This was originally under the context of the IEEE 802.15.3 subcommittee, which hassince disbanded.