to be communicated per scheme such as 16QAM may lead to an overall gain in symbol interval. At the same time, the bandwidth of the power efficiency compared to the use of QPSK. 16QAM 64QAM Figure 4.3-2 Signal constellations for (a) QPSK, (b) 16QAM and (c) 64QAM. High data rates in mobile communications CHAPTER 4.3 As an example, if a bandwidth utilization of close to is above a certain value, for QPSK, 16QAM, and two information bits per modulation symbol is required, 64QAM, respectively. Clearly, the probability for large QPSK modulation would allow for very limited channel peaks in the instantaneous power is higher in case of coding (channel-coding rate close to one). On the other higher-order modulation. hand, the use of 16QAM modulation would allow for Larger peaks in the instantaneous signal power imply that a channel-coding rate in the order of one half. Similarly, if the transmitter power amplifier must be over-dimensioned a bandwidth efficiency close to 4 information bits per to avoid the power-amplifier non-linearities that, occur modulation symbol is required, the use of 64QAM may at high instantaneous power levels, and cause corrup- be more efficient than 16QAM modulation, taking into tion to the signal to be transmitted. As a consequence, account the possibility for lower-rate channel coding and the power-amplifier efficiency will be reduced, leading corresponding additional coding gain in case of 64QAM. to increased power consumption. In addition, there will It should be noted that this does not speak against the be a negative impact on the power-amplifier cost. general discussion in Section 4.3.1 where it was con- Alternatively, the average transmit power must be cluded that transmission with high-bandwidth utilization reduced, implying a reduced range for a given data rate. is inherently power inefficient. The use of rate 1/2 High power-amplifier efficiency is especially important channel coding for 16QAM obviously reduces the for the mobile terminal, i.e. in the uplink direction, information data 