what modulation to apply Multiple Access or OFDMA is also often used. 4 The term OFDMA is sometimes also used to denote the use of OFDM to multiplex multiple users in the downlink as illustrated in Figure 4.4-17a. OFDM transmission CHAPTER 4.4 Figure 4.4-17 OFDM as a user-multiplexing/multiple-access scheme: (a) downlink and (b) uplink. Figure 4.4-17 assumes that consecutive subcarriers are terminal (see Figure 4.4-19). Such transmit-timing con- used for transmission to/from the same mobile terminal. trol should adjust the transmit timing of each mobile However, distributing the subcarriers to/from a mobile terminal to ensure that uplink transmissions arrive terminal in the frequency domain is also possible as il- approximately time aligned at the base station. Since the lustrated in Figure 4.4-18. The benefit of such distrib- propagation time changes as the mobile terminal is uted user multiplexing or distributed multiple access is moving within the cell, the transmit-timing control a possibility for additional frequency diversity as each should be an active process, continuously adjusting the transmission is spread over a wider bandwidth. exact transmit timing of each mobile terminal. In the case when OFDMA is used as an uplink mul- Furthermore, even in case of perfect transmit-timing tiple-access scheme, i.e. in case of frequency multi- control, there will always be some interference between plexing of OFDM signals from multiple mobile subcarriers e.g. due to frequency errors. Typically this terminals, it is critical that the transmissions from the interference is relatively low in case of reasonable fre- different mobile terminals arrive approximately time quency errors, Doppler spread, etc. (see Section 4.4.8). aligned at the base station. More specifically, the trans- However, this assumes that the different subcarriers are missions from the different mobile terminals should received with at least approximately the same power. In arrive at the base station with a timing misalignment less the uplink, the pro