Abstract

Orthogonal frequency-division multiplexing (OFDM) is an attractive transmission technique for high-bit-rate communication systems. One major drawback of OFDM is the high peak-to-average power ratio (PAPR) of the transmitter’s output signal. The selected mapping (SLM) approach provides good performance for PAPR reduction, but it requires a bank of inverse fast Fourier transforms (IFFTs) to generate a set of candidate transmission signals, and this requirement usually results in high computational complexity. In selected mapping (SLM) method a whole set of candidate signals is generated representing the same information, and then the most favourable signal as regards to PAPR is chosen and transmitted. SLM scheme is one of the initial probabilistic approaches for reducing the PAPR problem, with a goal of making occurrence of the peaks less frequent, not to eliminate the peaks. The scheme can handle any number of subcarriers and drawback associated with the scheme is the overhead of side information that needs to be transmitted to the receiver. This paper studies the problem of peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) system. It introduces a new PAPR reduction method that performs signal processing in the OFDM time domain together with the SLM method. The simple non-linear amplitude pre-distortion process of the proposed method changes the statistical characteristics of the OFDM symbol from Rayleigh distribution into Gaussian-like distribution. This change in statistical distribution is accompanied by changes in the peak and average power values and consequently reduction in the PAPR value. For the QPSK OFDM system with 128 sub-carriers, 3dB reduction in PAPR value is achieved. The proposed method shows that SLM with square rooting have higher PAPR reduction compare to the original SLM method. The approach is very flexible and can work with arbitrary number of sub-carries without restriction on the type of modulation applied on them.

Key words: Orthogonal frequency division multiplexing (OFDM), peak –to-average power ratio (PAPR), slm, statistical distribution.