rate, and thus also the bandwidth due to the importance of low mobile-terminal power utilization, to the same level as uncoded QPSK. consumption and cost. For the base station, high From the discussion above it can be concluded that, for power-amplifier efficiency, although far from irrelevant, a given signal-to-noise/interference ratio, a certain com- is still somewhat less important. Thus, large peaks in bination of modulation scheme and channel-coding rate is the instantaneous signal power is less of an issue for the optimal in the sense that it can deliver the highest-band- downlink compared to the uplink and, consequently, width utilization (the highest data rate within a given higher-order modulation is more suitable for the bandwidth) for that signal-to-noise/interference ratio. downlink compared to the uplink. 4.3.2.2 Variations in instantaneous 4.3.3 Wider bandwidth including transmit power multi-carrier transmission A general drawback of higher-order modulation schemes such as 16QAM and 64QAM, where information is also As was shown in Section 4.3.1, transmission with encoded in the instantaneous amplitude of the modu- a high-bandwidth utilization is fundamentally power lated signal, is that the modulated signal will have larger inefficient in the sense that it will require unproportionally variations, and thus also larger peaks, in its instantaneous high signal-to-noise and signal-to-interference ratios for power. This can be seen from Figure 4.3-3 which illus- a given data rate. Providing very high data rates within trates the distribution of the instantaneous power, more a limited bandwidth, for example by means of higher-order specifically the probability that the instantaneous power modulation, is thus only possible in situations where 1.0E + 00 1.0E - 01 1.0E - 02 1.0E - 03 1.0E - 04 16QAM 64QAM 1.0E - 05 Eb/No [dB] Figure 4.3-3 Distribution of instantaneous power for different modulation schemes. Same average power in all cases. CHAPTER 4.3 High data rates in mobile communications relatively high 