The effect of filtering on the performance of OFDM systems

Filtering is required in the practical implementation of orthogonal frequency division multiplex (OFDM) systems operating at radio frequencies. These filters smear the transmitted waveform, and cause inter-symbol interference (ISI) between the blocks, forcing a further extension to the cyclic prefix. The tighter the filtering, the smaller the frequency bandwidth (guard-band) needed between adjacent channel users and the longer the prefix extension. Equations are developed that relate the prefix extension to the guard-band given the required subchannel signal-to-noise ratio (SNR). Numerical evaluation is then used to show the trade off between time domain and frequency domain capacity loss. There is an optimum guard-band loss, that will minimize the total (time+frequency) capacity loss. The total loss reduces as the inverse fast Fourier transform (IFFT) size N increases and the required SNR reduces. The capacity loss exceeds 40% when N=32 and SNR is 33 dB (needed for 8DPSK with an symbol error rate (SER) of 3% and a margin of 10 dB). Some of the loss can be reduced by using better filters (Chebyshev II), but the general trend of the results imply that schemes with a low number of subchannels (N<16) will not work well with high order modulations.