Deterioration of a signal waveform due to ISI and Circuit techniques to prevent the
deterioration
• Inter-symbol interference (ISI) is a type of distortion resulting from dispersive
communication channels. ISI can cause problems during the reception of signals. ISI limits
the error-free reception of signals. To mitigate this, engineers can use Nyquist pulse shaping
techniques.
• Pulse shaping is the process of choosing the time-domain and spectral shape of symbols so
that pulses do not spread or overlap.
• Nyquist pulse shaping criterion introduced at the transmission end leads to improved signal
reception without ISI
• Nyquist pulse shaping criterion introduced in the transmission end leads to a more reliable
reception of signals without ISI distortions
The Frequency Response of Communication Channels:
In digital communication, most communication channels are band-limited, and band-
limited channels have finite bandwidth. They can be considered equivalent to band-limited
linear filters. Most of these channels are either baseband or passband.
The frequency response H(f) of communication channels can be given by equation 1
(below). The derivative of the phase response gives the group delay of the channel under
consideration, as shown in equation 2 (below)
• The frequency response of a baseband channel and passband channel is shown in the
figure below. The baseband channel response is similar to a low pass filter, whereas the
passband channel resembles that of a band-pass filter.
Baseband and passband channel frequency responses
• From the amplitude and phase responses of a channel, we can conclude whether
distortion is introduced into the signal transmitted.
• The communication channel can be considered distortionless, or a ‘non-distorting
channel’, if A(f) is constant with frequency and 𝜙 (f) is a linear function of frequency.
However, in most cases, the amplitude response A(f) is not a constant, which causes
amplitude distortions in the signal transmitted over the channel. If 𝜙(f) is not a linear
function of frequency (non-constant group delay), it introduces phase distortion or delay
distortions. The channels with delay distortions are called dispersive channels
Intersymbol Interference and Why It Matters:
In digital communication, pulses transmitted through dispersive channels reach the
output side of the channel at different time intervals due to non-constant group delay. This
results in intersymbol interference (ISI), which means interference between the adjacent
transmitted pulses. The term ‘symbol’ is a synonym for data bits or pulses in digital
communication. As the name suggests, the symbols overlap due to the dispersive
characteristics of the communication channels.
• ISI is prevalent in high data rate communication systems. When the communication channel
bandwidth is greater than the signal bandwidth, ISI is minimized. When the bandwidth of
the signal and the channel bandwidth is close to each other, then the overlapping of symbols
is greater and ISI increases.
• ISI is responsible for degrading the signal that is going to be transmitted. This makes the
reconstruction of the original information at the receiving end difficult. In such cases,
communication is hindered, which is why controlling ISI is so critical.
Nyquist Pulse Shaping:
• Pulse shaping is one technique used in communication to overcome the degradation of
signals due to ISI. Pulse shaping involves the process of choosing the time-domain and
spectral shape of the symbols so that the pulses do not spread or overlap.
• Nyquist pulse shaping criterion is followed to achieve zero ISI. According to Nyquist pulse
shaping criterion, to achieve zero ISI, digital pulses should satisfy the following conditions
given in equation (3), where Tb is the bit period:
• Pulses that satisfy the above conditions have zero ISI, as the values of the pulses at
sampling periods are either equal to 1 at the center of the pulse or equal to zero at points
where other pulses are centered. The input pulses shaped by following the Nyquist pulse
shaping criterion result in the efficient reconstruction of original information at the
receiver end.
• To achieve zero ISI, pulses can be shaped as sinc pulses, rectangular pulses, or raised
cosine pulses. Various pulse shaping filtering schemes can be employed at the
transmission end of communication systems to achieve a pulse shape that results in the
perfect reconstruction of original signals, including sinc filtering, raised-cosine filtering,
and square-root raised cosine filtering.
