ISI in Channel

Causes for Intersynbol Interference
1. Multipath propagation
2. Inherent non-linear Frequency response of the channel

When the channel bandwidth is smaller than the signal bandwidth, the channel is bandlimited. Severe bandwidth Limitation causes intersymbol interference.
Also transmitting a signal at high modulation rate through a bandlimited channel can create intersymbol interference. As the modulation Rate increases the signals bandwidth increases. When the signals bandwidth becomes larger than the channel bandwidth, the channel starts to introduce distortion to the signal. This distortion is usually seen as intersymbol interference.

Difference Between Narrowband and Wideband systems

These are very important fundamental concepts in wireless systems. 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 signals 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 non-selective fading whereas wideband systems are affected by selective fading.

The Coherence bandwidth (Bc) depends on the environment.Bc=1/(2*pi*T) , where T is the Delay spread. 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 signal component that can be sensed by the receiver is known as Delay spread.

Coherence Time (Tc) is defined as the interval during which the fading characteristics of the channel remain approximately unchanged. Tc=1/(2*fm), where fm is the maximum doppler shift.

Recommended books for Radio Communications

1.Wireless Communications (Wiley - IEEE) by Andreas.F.Molisch
2.Fundamentals of Wireless Communication by David Tse and Pramod Viswanath
David Tse (Author), Pramod Viswanath (Author)