Coherence bandwidth

Lore & Background

Coherence bandwidth varies over cellular or PCS communications paths because the multipath spread D varies from path to path. If the coherence bandwidth is greater than the data signal bandwidth, the channel can reasonably be assumed to be flat. Frequencies within a coherence bandwidth of one another tend to all fade in a similar or correlated fashion. The CDMA IS-95 waveform was designed with a bandwidth of approximately 1.25 MHz because in many urban signaling environments the coherence bandwidth is significantly less than 1.25 MHz. When fading occurs, it occurs only over a relatively small fraction of the total CDMA signal bandwidth; the portion of the signal bandwidth over which fading does not occur typically contains enough signal power to sustain reliable communications. This is the bandwidth over which the channel transfer function remains virtually constant.

Reader's Guide

Coherence bandwidth is a fundamental parameter in wireless system design, as it determines whether a channel will exhibit flat or frequency-selective fading. In the example of an urban cellular path with a delay spread of 1.9 μs, the coherence bandwidth is approximately 0.53 MHz, which results in frequency selective fading over the IS-95 bandwidth of 1.25 MHz. This selective fading characteristic was deliberately exploited in the design of the IS-95 CDMA waveform: because the coherence bandwidth in many urban environments is significantly less than 1.25 MHz, only a fraction of the signal bandwidth experiences correlated fading at any given time. The remaining portion of the bandwidth retains enough signal power to maintain reliable communications. The concept thus directly influences the choice of signal bandwidth and modulation schemes in cellular and PCS systems, ensuring that even in multipath-rich environments, communication links can be sustained.

Did You Know?

• Coherence bandwidth is approximately the reciprocal of the multipath time delay spread in rad/s.
• The IS-95 CDMA waveform was designed with a 1.25 MHz bandwidth because urban coherence bandwidth is often less than that.
• Frequencies within a coherence bandwidth fade in a correlated fashion.