pagation distance and thus the path than the length of the cyclic prefix to preserve orthogo- loss of the different mobile-terminal transmissions may nality between subcarriers received from different differ significantly. If two terminals are transmitting with mobile terminals and thus avoid inter-user interference. the same power, the received-signal strengths may thus Due to the differences in distance to the base station differ significantly, implying a potentially significant in- for different mobile terminals and the corresponding terference from the stronger signal to the weaker signal differences in the propagation time (which may far unless the subcarrier orthogonality is perfectly retained. exceed the length of the cyclic prefix), it is necessary to To avoid this, at least some degree of uplink transmit- control the uplink transmission timing of each mobile power control may need to be applied in case of uplink Figure 4.4-18 Distributed user multiplexing. CHAPTER 4.4 OFDM transmission Without time alignment With time alignment UE #1 UE transmission UE #2 UE #1 UE #2 UE #1 T1 BS reception UE #2 T2-T1 Figure 4.4-19 Uplink transmission-timing control. OFDMA, reducing the transmit power of user terminals As a broadcast transmission according to Figure 4.4- close to the base station and ensuring that all received 21a has to be dimensioned to reach also the worst-case signals will be of approximately the same power. mobile terminals, including mobile terminals at the cell border, it will be relatively costly in terms of the resourses (base-station transmit power) needed to pro- 4.4.11 Multi-cell broadcast/ vide a certain broadcast-service data rate. Alternatively, taking into account the limited signal-to-noise ratio that multicast transmission and OFDM can be achieved at, for example, the cell edge, the achievable broadcast data rates may be relatively limited, The provisioning of broadcast/multicast services in especially in case of large cells. One way to increase the a mobile-communication system implies 