In this series:
Recently, while working on some improvements to Nutaq’s orthogonal frequency-division multiplexing (OFDM) reference design, I faced a very frustrating but instructive problem: inter-symbol interference (ISI). ISI is usually generated when transmitting in a multipath fading channel. In this kind of channel, multiple copies of the transmitted signal is received at different time intervals, which causes interference.
For simplicity, let’s assume that we have an OFDM transmission of two OFDM symbols with two loaded subcarriers. In a multipath fading channel, the received signal can be illustrated as follows:
Because we have a delayed copy of the original signal coming through our receive antenna, we can see that a portion of the first OFDM symbol creates interference with the second one, thus changing its amplitude and phase. On the low frequency subcarrier, this effect is less dramatic but on the second subcarrier there is no way to recuperate the information. The immediate fix to solve our problem, the cyclic prefix, was first presented by Peled and Ruiz in 1980