The orthogonal frequency division multiplexing (OFDM) modulation is one kind of multicarrier modulation. As the subcarriers are orthogonal, the system's inter symbol interface (ISI) is reduced significantly. The spectral efficiency is much higher compared to the single carrier modulation. Besides, the OFDM can resist the multi-path channel much better by importing the guard interval. Because of these advantages, the OFDM modulation is well used in underwater communication, since normal communication method met great difficulties when dealing with the band-limited and complex multi-path underwater acoustic channel.

However, some serious problems exist in OFDM communication system. One of the biggest troubles is the PAR problem which brings great challenges to the underwater acoustic-electric transducer. The signal with high PAR will be distorted badly and the system efficiency will be impaired severely. 

So researchers of Institute of Acoustics, Chinese Academy of Sciences carried out a series of studies and proposed a new companding technique, called "YL-Transform companding". With this method, the distribution of time domain samples is transformed into uniform distribution and the PAR is reduced from 12dB to 4.6dB for 256-channel QPSK OFDM. By changing the normal distribution into uniform distribution, the PAR of OFDM signal is reduced remarkably. While at the same time, the compressed signal is recovered at the receiver to keep the system's performance. The YL-Transform can be realized just by addition and multiplication operation and these are all simple calculation which can completely satisfy the real communication systems.

This research work is supported by the National High Technology Research and Development Program of China ('863' Program) and the National Natural Science Foundation of China.