OFDM encoding with reduced crestfactor

Orthogonal Frequency Division Multiplexing (OFDM) has been proposed for future high speed wireless LANs and digital (television and audio) broadcasting as a promising multicarrier modulation technique. OFDM offers robustness against significant channel degradation by dispersion. For wireless LANs, the design of the transmitter power amplifier and the characteristics of the modulated signal are key factors deciding the coverage range performance and the product cost. The peak-envelope-power of the transmitter power amplifier is an important parameter in the design of small sized, low power wireless LAN devices. This paper describes an OFDM system with a data encoder/decoder that improves the peak-to-average power ratio of the transmitted signal. In this scheme, the data to be transmitted is converted into symbols for the individual subcarriers, such that the resultant of a pair of symmetrically positioned subcarriers is orthogonal to the resultant of another pair of subcarriers with the same center of symmetry. This conversion gives a reduced ratio between peak envelope-power and the mean (RMS) power, which reduces constraints on the power stage design. Further, the impact on the bit-error rate of clipping the signal in the transmitter before the output amplifier is discussed