Peak-to-average-power ratio (PAPR) is a performance measurement that is indicative of the power efficiency of the transmitter. In the case of an ideal linear power amplifier where we achieve linear amplification up to the saturation point, we reach the maximum power efficiency when the amplifier is operating at the saturation point. A positive PAPR in dB means that we need a power backoff to operate in the linear region of the power amplifier. We can express the theoretical relationship between PAPR [dB] and transmit power efficiency as follows :
where η is the power efficiency and ηmax is the maximum power efficiency. For class A power amplifier, ηmax is 50% and for class B, 78.5%. Figure 7.1 shows the relationship graphically and it is evident from the figure that high PAPR degrades the transmit power efficiency performance.
A salient advantage of SC-FDMA over OFDMA is the lower PAPR because of its inherent single carrier structure. The lower PAPR is greatly beneficial in the uplink communications where the mobile terminal is the transmitter. As we showed in Section 3.4, time domain samples of the SC-FDMA modulated signals are different depending on the subcarrier mapping scheme and we can expect different PAPR characteristics for different subcarrier mapping schemes.