A new approach to improve BER performance of a high Peak-to-Average ratio (PAR) OFDM signal over FM based Land Mobile Radios (LMR)

The performance for a FM system depends upon the RMS bandwidth (f_r) of the modulated FM signal and the bandwidth (W) of the modulating signal. Increasing the RMS value of the modulating signal (sigma_x) increases the RMS bandwidth of the FM signal. A typical FM radio clips the input modulating signal above a predefined threshold (A_c) to limit the maximum deviation (f_d) of the carrier frequency so as to avoid interference on adjacent channels. Thus a high peak-to-average (PAR) OFDM modulating signal will undergo non-linear distortion due to clipping by the FM radio. In this paper the effects of distortion due to clipping on the OFDM/FM system the will be studied. The performance limitations will be investigated for M-ary QAM modulation (M=16, 64, 256) and different IDFT sizes (N=64,256,512). A simple scaling method is proposed to improve the signal-to-noise ratio (SNR) at the FM receiver which would improve BER performance of the OFDM signal.