Designing for Low ISI in an OFDM Modem

One problem that arises in OFDM systems is when the currently received OFDM symbol is distorted by the adjacently transmitted OFDM symbols. This is known as intersymbol interference (ISI) and is a function of the combined impulse response of the wireless channel and the transmit and receive digital/analog filters. This paper illustrates the effect of the digital and analog filters on ISI in a typical 802.11a/g WLAN OFDM modem. It is shown that the filters, rather than the actual wireless channel, dominate the impulse response. Two techniques, increasing the digital interpolation ratio and the use of frequency domain interpolation, are then introduced with the effect of reducing the impulse response and ISI. The result of implementing these two techniques at the transmitter is to reduce the required cyclic prefix by 50% for an ISI requirement of 40 dB.