Intermodulation product suppression in OFDM system amplifiers

The need for linearity is one of the principal demands in the design of modern power amplifiers for OFDM systems. QPSK or QAM signals are typically generated by modulating both I and Q subcarriers so the resultant of these two components has simultaneous amplitude and phase modulation which changes with the contents of the transmitted information. Despite the fact that the number of subcarriers is constant the peak-to-average ratio (PAR) of the cluster varies and can be sometimes very high. In this case the amplifier goes to saturation. The consequence of this nonlinearity is the generation of interferences which fall in the same frequency range. These interferences can be solved by linearisation of amplifier characteristic at higher output level. In this paper the results of a relative simple algorithm for linearisation are presented. For simplicity in calculations 8 subcarriers are chosen to investigate the adequacy of the algorithm. Results of error phasors values by simulations are shown for both cases - with and without digital-pre-distortion (DPD). FFT and IFFT with 2048 samples are used to minimalize the computer errors because the interference supression results go to - 80 dB