Wireless systems can be classified according to whether they have a narrowband or wideband architecture. Narrowband systems support low-bit-rate transmission, whereas wideband systems support high-bit-rate transmission. A system is defined as narrowband or wideband depending on the bandwidth of the transmission physical channels with which it operates. The system channel bandwidth is assessed with respect to the coherence bandwidth. The coherence bandwidth is defined as the frequency band within which all frequency components are equally affected by fading due to multipath propagation phenomena. Systems operating with channels substantially narrower than the coherence bandwidth are known as narrowband systems. Wideband systems operate with channels substantially wider than the coherence bandwidth. In narrowband systems, all the components of the signal are equally influenced by multipath propagation. Accordingly, although with different amplitudes, the received narrowband signal is essentially the same as the transmitted narrowband signal. In wideband systems, the various frequency components of the signal may be differently affected by fading. Narrowband systems, therefore, are affected by nonselective fading, whereas wideband systems are affected by selective fading.

The coherence bandwidth, Bc , depends on the environment. It is approximately given by

Bc = (2πT)-1

in hertz, where T, in seconds, is the delay spread, as defined next. In a fading environment, a propagated signal arrives at the receiver through multiple paths. The time span between the arrival of the first and the last multipath signals that can be sensed by the receiver is known as delay spread. The delay spread varies from tenths of microseconds, in rural areas, to tens of microseconds, in urban areas. As an example, consider an urban area where the delay spread is T = 5µs. In such an environment, the coherence bandwidth is calculated as Bc = 32 kHz. Therefore, a system is considered to be narrowband if it operates with channels narrower than 32 kHz. It is considered to be wideband if it operates with channels several times wider than 32 kHz.

Another important definition within this context concerns coherence time. The coherence time, Tc, is defined as the time interval during which the fading characteristics of the channel remain approximately unchanged (slow change). This is approximately given as

Tc = (2 fm)-1

where fm is the maximum Doppler shift. The Doppler shift, in hertz, is given as v/λ, where v, in m/s, is the speed of the mobile terminal and λ, inm, is the wavelength of the signal.