ssion has, until now, implicitly assumed that all OFDM subcarriers are transmitted from the same transmitter to a certain receiver, i.e.: According to Section 4.3.3.1, one of the drawbacks of multi-carrier transmission are the corresponding large downlink transmission of all subcarriers to a single variations in the instantaneous transmit power, implying mobile terminal; a reduced power-amplifier efficiency and higher mobile- uplink transmission of all subcarriers from a single terminal power consumption, alternatively; the power- mobile terminal. amplifier output power has to be reduced with a reduced However, OFDM can also be used as a user-multiplexing range as a consequence. Being a kind of multi-carrier or multiple-access scheme, allowing for simultaneous transmission scheme, OFDM is subject to the same frequency-separated transmissions to/from multiple drawback. mobile terminals (see Figure 4.4-17). However, a large number of different methods have In the downlink direction, OFDM as a user-multi- been proposed to reduce the large power peaks of an plexing scheme implies that, in each OFDM symbol in- OFDM signal: terval, different subsets of the overall set of available In case of tone reservation, a subset of the OFDM subcarriers are used for transmission to different mobile subcarriers are not used for data transmission. In- terminals (see Figure 4.4-17a). stead, these subcarriers are modulated in such a way Similarly, in the uplink direction, OFDM as a user- that the largest peaks of the overall OFDM signal are multiplexing or multiple-access scheme implies that, in suppressed, allowing for a reduced power-amplifier each OFDM symbol interval, different subsets of the back-off. One drawback of tone reservation is the overall set of subcarriers are used for data transmission bandwidth loss due to the fact that a number of from different mobile terminals (see Figure 4.4-17b). In subcarriers are not available for actual data trans- this case, the term Orthogonal Frequency Division mission. The calculation of 