OFDM for Underwater Acoustic Communications

Chapter 4PAPR Control

Due to the superposition of data symbols on a large number of subcarriers, an OFDM waveform has a large peak-to-average power ratio (PAPR). Since nonlinear amplification causes inter modulation among subcarriers and undesired out-of-band radiation, the power amplifier at the transmitter must operate with a large backoff to limit nonlinear distortion. The severity of the PAPR issue in practical systems depends on the system operating conditions.

• In short range underwater acoustic applications, the average power matters more than the peak power as the power amplifier is often not working in the full power mode. As long as the transmitter circuits can handle occasionally large peaks, the PAPR issue is not a major concern.
• In long range underwater acoustic applications, the power amplifier might work in the full power mode. With the peak power limited by the hardware, a signal with large PAPR leads to small average power output, reducing the transmission range. In such scenarios, the PAPR issue is a major concern.

From the implementation point of view, more bits are needed for quantization to synthesize a signal with large PAPR, which adds to the transmitter complexity.

Next, we explore the PAPR issue of OFDM modulation, and then present three simple yet effective PAPR reduction methods.

4.1 PAPR Comparison

Let denote one OFDM block in the passband, as in ...