Praise for Fundamentals of WiMAX

114 Chapter 4 • Orthogonal Frequency Division Multiplexing• Discusses implementation issues for WiMAX systems, such as the peak-to-average ratio,and provides illustrative examples related to WiMAX.4.1 Multicarrier ModulationThe basic idea of multicarrier modulation is quite simple and follows naturally from the competingdesires for high data rates and ISI-free channels. In order to have a channel that does nothave ISI, the symbol time T shas to be larger—often significantly larger—than the channel delayspread τ. Digital communication systems simply cannot function if ISI is presents; an errorfloor quickly develops, and as T sapproaches or falls below τ, the bit error rate becomes intolerable.As noted previously, for wideband channels that provide the high data rates needed bytoday’s applications, the desired symbol time is usually much smaller than the delay spread, sointersymbol interference is severe.In order to overcome this problem, multicarrier modulation divides the high-rate transmitbit stream into L lower-rate substreams, each of which has T / L >> τ and is hence effectivelysISI free. These individual substreams can then be sent over L parallel subchannels, maintainingthe total desired data rate. Typically, the subchannels are orthogonal under ideal propagationconditions, in which case multicarrier modulation is often referred to as orthogonal frequencydivision multiplexing (OFDM). The data rate on each of the subchannels is much less than thetotal data rate, so the corresponding subchannel bandwidth is much less than the total systembandwidth. The number of substreams is chosen to ensure that each subchannel has a bandwidthless than the coherence bandwidth of the channel, so the subchannels experience relatively flatfading. Thus, the ISI on each subchannel is small. Moreover, in the digital implementation ofOFDM, the ISI can be completely eliminated through the use of a cyclic prefix.Example 4.1 A certain wideband wireless channel has a delay spread of1µ sec. We assume that in order to overcome ISI, T s≥ 10τ.1. What is the maximum bandwidth allowable in this system?2. If multicarrier modulation is used and we desire a 5MHz bandwidth,what is the required number of subcarriers?For question 1, if it is assumed that T s =10τ in order to satisfy the ISIfreecondition, the maximum bandwidth would be 1/T s = .1/τ =100 KHz, farbelow the intended bandwidths for WiMAX systems.In question 2, if multicarrier modulation is used, the symbol time goesto T = LT s . The delay-spread criterion mandates that the new symbol time isstill bounded to 10 percent of the delay spread: (LT s ) -1 = 100 Khz. But the5MHz bandwidth requirement gives (T s ) -1 = 5 MHz Hence, L ≥50 allows thefull 5MHz bandwidth to be used with negligible ISI.In its simplest form, multicarrier modulation divides the wideband incoming data stream into Lnarrowband substreams, each of which is then transmitted over a different orthogonal-frequencysubchannel. As in Example 4.1, the number of substreams L is chosen to make the symbol time