Exhaustive gaussian approach for performance evaluation of direct-detection OFDM systems employing square and cross QAM

An exhaustive Gaussian approach (EGA) is proposed to evaluate, through numerical simulation, the bit error ratio (BER) of direct-detection orthogonal frequency-division multiplexing systems employing square and cross quadrature amplitude modulation. Excellent agreement between the BER estimates from the direct error counting (DEC) and the EGA is shown for different levels of optical signal-to-noise ratio and signal distortion. It is shown that the EGA requires about three orders of magnitude less in computation time than the DEC method for BER levels around 10-6, with the difference getting higher for lower BER levels.