BASEBAND TRANSMISSION AND RECEPTION
Inter symbol Interference
Generally, digital data is represented by electrical pulse, communication
channel is always band limited. Such a channel disperses or spreads a pulse
carrying digitized samples passing through it. When the channel bandwidth is
greater than bandwidth of pulse, spreading of pulse is very less. But when
channel bandwidth is close to signal bandwidth, i.e. if we transmit digital data
which demands more bandwidth which exceeds channel bandwidth, spreading
will occur and cause signal pulses to overlap. This overlapping is
called InterSymbol Interference. In short it is called ISI. Similar to
interference caused by other sources, ISI causes degradations of signal if left
uncontrolled. This problem of ISI exists strongly in Telephone channels like
coaxial cables and optical fibers.
The main objective is to study the effect of ISI, when digital data is transmitted
through band limited channel and solution to overcome the degradation of
waveform by properly shaping pulse
(Source:Brainkart)
The effect of sequence of pulses transmitted through channel is shown in fig.
The Spreading of pulse is greater than symbol duration, as a result adjacent
pulses interfere. i.e. pulses get completely smeared, tail of smeared pulse enter
into adjacent symbol intervals making it difficult to decide actual transmitted
pulse. First let us have look at different formats of transmitting digital data.In
base band transmission best way is to map digits or symbols into pulse
waveform. This waveform is generally termed as Line codes.
EYE PATTERN
The quality of digital transmission systems are evaluated using the bit error rate.
Degradation of quality occurs in each process modulation, transmission, and
detection. The eye pattern is experimental method that contains all the
information concerning the degradation of quality. Therefore, careful analysis
of the eye pattern is important in analyzing the degradation mechanism.
• Eye patterns can be observed using an oscilloscope. The received wave is
applied to the vertical deflection plates of an oscilloscope and the saw tooth
wave at a rate equal to transmitted symbol rate is applied to the horizontal
deflection plates, resulting display is eye pattern as it resembles human eye.
• The interior region of eye pattern is called eye opening
(Source:Brainkart)
We get superposition of successive symbol intervals to produce eye pattern as
shown below.
Fig 3.1Eye pattern (Source:Brainkart)
• The width of the eye opening defines the time interval over which the received
wave can be sampled without error from ISI
• The optimum sampling time corresponds to the maximum eye opening
• The height of the eye opening at a specified sampling time is a measure of the
margin over channel noise.
The sensitivity of the system to timing error is determined by the rate of closure
of the eye as the sampling time is varied. Any non linear transmission distortion
would reveal itself in an asymmetric or squinted eye. When the effected of ISI
is excessive, traces from the upper portion of the eye pattern cross traces from
lower portion with the result that the eye is completely closed.
Example of eye pattern:
Binary-PAM Perfect channel (no noise and no ISI)
Fig 3.2 Example of eye pattern: Binary-PAM with noise no ISI (Source:Brainkart)
Pulse Shaping
It is the process of changing the waveform of transmitted pulses. Its
purpose is to make the transmitted signal better suited to its purpose or
the communication channel, typically by limiting the effective bandwidth of the
transmission. By filtering the transmitted pulses this way, the inter symbol
interference caused by the channel can be kept in control. In RF
communication, pulse shaping is essential for making the signal fit in its
frequency band.
Typically pulse shaping occurs after line coding and modulation.
Need for pulse shaping
Transmitting a signal at high modulation rate through a band-limited
channel can create inter symbol interference. As the modulation rate increases,
the signal's bandwidth increases. When the signal's bandwidth becomes larger
than the channel bandwidth, the channel starts to introduce distortion to the
signal. This distortion usually manifests itself as inter symbol interference.
The signal's spectrum is determined by the modulation scheme and data
rate used by the transmitter, but can be modified with a pulse shaping filter.
Usually the transmitted symbols are represented as a time sequence of dirac
delta pulses. This theoretical signal is then filtered with the pulse shaping filter,
producing the transmitted signal.
In many base band communication systems the pulse shaping filter is
implicitly a boxcar filter. Its Fourier transform is of the form sin(x)/x, and has
significant signal power at frequencies higher than symbol rate. This is not a big
problem when optical fibre or even twisted pair cable is used as the
communication channel. However, in RF communications this would waste
bandwidth, and only tightly specified frequency bands are used for single
transmissions. In other words, the channel for the signal is band-limited.
Therefore better filters have been developed, which attempt to minimize the
bandwidth needed for a certain symbol rate.
An example in other areas of electronics is the generation of pulses where
the rise time need to be short; one way to do this is to start with a slower-rising
pulse, and decrease the rise time, for example with a step recovery diode circuit.
