compander

In most recent researches [6][7][8][10][11][13][14], companders have been designed by modifying the amplitude distribution of the OFDM signal.

Since there will always be a minimum amount of noise present in the signal even at very high SNR, the OFDM systems employing companders have a noise floor.

This is in contrast with the companders in [6][8][9][10][13][14][15] (including the functions in Fig.

Performance comparison with the companders found in existing researches is also presented.

We found that without decompanding, all the six companders under consideration yield approximately equal BER, which is equal to the BER of the original OFDM signal.

Some instances of these companders are piecewise linear transforms in [3-4] and non-linear transforms in [5-9].

In general, companders are deterministic functions designed to modify OFDM signal amplitude after modulation operation, while assuming that the amplitude of the signal is a Rayleigh random process.

This means that the output of companders designed to preserve average power may show more significant aberrations from expected behavior when larger QAM constellations are used for modulation.

If D is a measure of companding distortion for a certain companding function and all of the J companders have similar mathematical form, design specifications and constraints, then, by virtue of the law of total expectation, the over-all mean companding distortion remains same as for the fixed compander.