PAPR Reduction Methods for Noncoherent OFDM-MFSK

MFSK (M-ary frequency shift keying) can be combined with OFDM (orthogonal frequency division multiplexing) by grouping the subcarriers into groups of M and applying MFSK modulation to each of these groups. The detection of such an OFDM-MFSK modulation scheme can be noncoherent and it is very robust against Doppler shift and Doppler spread. The noncoherent detection of OFDMMFSK allows an arbitrary phase choice for all subcarriers in the transmitter. This degree of freedom can be exploited to reduce the PAPR (peak-to-average power ratio), which is a well known problem for multicarrier transmission schemes. In this paper we compare several PAPR reduction schemes and apply them to OFDM-MFSK. A new PAPR reduction algorithm is introduced, which adjusts the subcarrier phases of an OFDM symbol to obtain a low PAPR. The effects on the spectrum of the OFDM-MFSK signal in the presence of a nonlinear transmitter are considered and the improvements achievable by the various PAPR reduction schemes are shown. Finally the complexity of the presented schemes is compared.