Praise for Fundamentals of WiMAX

6.1 Multiple-Access Strategies for OFDM 203PowerTimePowerTimeUser 3User 6User 9User 2User 5User 8User 1 User 2 User 3User 1User 1User 1User 1User 1User 4User 1User 1User 7FrequencyFrequencyBlock ofSubcarriers(a) (b)Block ofSubcarriersFigure 6.1 (a) FDMA and (b) a combination of FDMA with TDMAPowerTimeUser 1User 2User 3User 4User 5FrequencyFigure 6.2 CDMA's users share time and frequency slots but use codes that allow the users to beseparated by the receiversecutive OFDM symbols; that is, the data symbol is multiplied by a length N code sequence andthen sent on a specific subcarrier for the next N OFDM symbols. Frequency-domain spreading,which generally has slightly better performance than time-domain spreading [13], entails eachdata symbol being sent simultaneously on N different subcarriers. MC-CDMA is not part of theWiMAX standards but could be deemed appropriate in the future, especially for the uplink.6.1.5 Advantages of OFDMAOFDMA is essentially a hybrid of FDMA and TDMA: Users are dynamically assigned subcarriers(FDMA) in different time slots (TDMA) as shown in Figure 6.3. The advantages of OFDMA startwith the advantages of single-user OFDM in terms of robust multipath suppression and frequencydiversity. In addition, OFDMA is a flexible multiple-access technique that can accommodate many