On the Bit Error Rate of OFDMA Employing Short Cyclic Prefix and Maximal Ratio Combining

Orthogonal frequency division multiple access (OFDMA) has been chosen as the fourth generation (4G) standard and also in the first releases of fifth generation (5G) mobile systems. Unfortunately, OFDMA uses a cyclic prefix that requires an increase in bandwidth and transmission power. In practical systems, there is no need to maintain a bit error rate (BER) lower than that required by a service. Hence, it is possible to reduce the cyclic prefix duration and maintaining an adequate performance. In this sense, in this work, the BER of OFDMA systems that employ a cyclic prefix shorter than the channel delay spread is evaluated. In addition, an antenna array at reception is considered. Diversity signals are combined employing maximal-ratio-combining (MRC). Assuming that noise plus intersymbol interference plus intercarrier interference can be modeled as a zero-mean Gaussian random variable, a closed form expression to calculate the BER for M-QAM modulations is derived. Simulation results show that our theoretical expression behaves as a lower bound, which becomes more accurate as the cyclic prefix duration tends to the channel delay spread.