Frequency-selective channels are caused by delay spread in the channel. When delay spread is introduced into the channel model, intersymbol interference is observed at the receiver. Intersymbol interference denotes the effect of the channel introducing contamination to the current sample from previous samples. If the communication system does not compensate for this effect, the performance of the link can be degraded significantly. The adverse effects of delay spread can be even more dramatic if a strong interferer that is observed by a multiple-antenna receiver has a channel that is frequency selective. For example, consider a single-antenna interferer. Without delay spread, a capable multiple-antenna receiver can mitigate the effects of the interference. In channels with significant delay spread, the rank of the interference spatial receive covariance matrix can grow from rank-1 to full rank, because each receive symbol can contain contributions from multiple transmit symbols at various relative delays propagation through channels that cause independent spatial responses. Without changing the processing approach, this full-rank interference covariance matrix can overwhelm the communications link.

The frequency-selective channel can be represented in the frequency domain by employing a channel representation with coefficients at various frequencies, or in the time domain by employing a channel representation with coefficients at various delays (delay taps). To complicate the channel problem, if the channel is not static because of the motion of the transmitter, receiver, or scatterers, then compensating for delay spread can be more difficult.