Communication Systems_B.P. Lathi and Zhi Ding (Lecture No 40-42)

30820-Communication Systems
Week 15 – Lecture 40-42
(Ref: Chapter 7 of text book)
PRINCIPLES OF DIGITAL DATA
TRANSMISSION

Contents
• Digital Communication Systems
• Line Coding
• Digital Receivers and Regenerative Repeaters
308201- Communication Systems 2

Digital Communication Systems
• Digital Communication is a mode of communication where
the information is encoded digitally as discrete signals and
electronically transferred to the recipient.
• Digital Communication covers a broad area of
communications techniques including
– Digital Transmission
– Digital Radio
• Digital transmission is the transmission of digital pulses
between two or more points in a communication system.
• Digital Radio is the transmission of digital modulated analog
carriers between two or more points in a communication
system.

• A digital communication system consist of several
components
• The source of information can be analog or digital.
– Analog: Audio, Video Signals etc.
– Digital: Computer, Fax etc.
• The signal produced by the source is converted into digital
signals consisting of 1s and 0s.
– Source Encoding

(Channel Encoder)
• After source coding, the information sequence is passed
through the channel encoder.
– It adds redundancy in the binary information that can be
used at the receiver to overcome the effects of noise and
interference encountered during the transmission through
the channel.
– 𝑘 bits of information sequence is mapped into unique 𝑛
bits sequence called the code word.
– The amount of redundancy introduced is measured by
ratio 𝑛/𝑘. Its reciprocal i.e., 𝑘/𝑛 is known as the code rate.

(Digital Modulator & Demodulator)
• The binary sequence is then passed to digital modulator
which in turns convert the sequence into electrical signals to
be transmitted on the channel.
• The digital modulator maps the binary sequences into signal
waveform.
– e.g., represent 1 by sin 𝑥 and 0 by cos 𝑥
• The signal waveform is the passed though the channel.
– The communication channel is the physical medium that is used for
transmitting signals from transmitter to receiver.
• The digital demodulator processes the channel corrupted
transmitted waveform and reduces the waveform to the
sequence of numbers that represents estimates of the
transmitted data symbols.

(Channel)
• The modulation and coding used in a digital communication
system depends on the characteristics of the channel.
• Characteristics are whether the channel is linear or nonlinear,
and how free the channel is free from the external
interference.
• Five channels are considered in the digital communication i.e.,
– Telephone channels
– Coaxial cable
– Optical fiber
– Microwave radio and satellite channels.

(Channel Decoder & Source Decoder)
• The sequence of numbers then passed through the channel
decoder.
– The channel decoder attempts to reconstruct the original information
sequence form the knowledge of the code used by the channel
encoder and the redundancy contained in the received data.
– The average probability of a bit error at the output of the decoder is a
measure of the performance of the demodulator-decoder
combination.
• Source decoder tries to reduce the sequence from the
knowledge of the encoding algorithm.
– The approximate replica of the input signal at the transmitter side.
• Finally we get the desired signal in the desired format i.e.,
analog or digital using an output transducer.

(Signal Degradation)
• In digital communication, the main factors of
degradation of signal are
– Loss in signal to noise ratio (S/N)
• Decrease of desired signal power
• Increase of noise power
– Signal distortion caused by ISI
• The received pulses overlap one another; the tail of one pulse smears
into the adjacent symbol interval causing the loss of data in digital
communication.
– Distance
• As the distance is increased, there is a greater chance of signal
distortion.
• When the distance is large we use repeaters to amplify the signal
– Problem?
– The repeaters also amplify the noise.

Digital Communication System Analog Communication System
Advantages:
• Inexpensive digital circuits
• Privacy preserved (data encryption)
• Can merge different data and transmit
over a common digital transmission
system
• Error correction by coding
Disadvantages:
• Expensive analog components
• No privacy
• Cannot merge data from different
sources
• No error correction capability
Disadvantages:
• Large bandwidth
• Synchronization problem is relatively
difficult
Advantages:
• Smaller bandwidth
• Synchronization problem is relatively
easier

Line Coding
• The output of the source encoder is converted into electrical pulses
(waveforms) for the purpose of transmission over the channel.
– Line Coding or Transmission Coding
• The simplest line code is on/off or unipolar,
– binary 1 is transmitted by a pulse 𝑝(𝑡) and 0 is transmitted by no pulse.
• Another commonly used code is polar,
– 1 is transmitted by pulse 𝑝(𝑡) and 0 is transmitted by pulse – 𝑝(𝑡)
• Another popular code is bipolar or alternate mark inversion
– 0 is encoded by no pulse and 1 is encoded by 𝑝(𝑡) or −𝑝(𝑡) depending on
whether the previous 1 is encoded by – 𝑝(𝑡) or 𝑝(𝑡).

Unipolar Signalling
Non-Return to Zero (NRZ)
• Duration of the MARK pulse (𝜏) is equal to the duration (𝑇0) of the
symbol slot
• Advantages:
– Simplicity in implementation
– Doesn’t require a lot of bandwidth for transmission
• Disadvantages:
– Presence of DC level (indicated by spectral line at 0Hz)
– Contains low frequency components
– Does not have error correction capability
– Long strings of zeros can cause loss of synchronization i.e., not transparent

Unipolar Signalling
Return to Zero (RZ)
• MARK pulse (𝜏) is less than the duration (𝑇0) of the symbol slot.
• Fills only the first half of the time slot, returning to zero for the
second half.
• Advantages:
– Presence of a spectral line at symbol rate which can be used as symbol
timing clock signal
• Disadvantages:
– Same as Unipolar NRZ case discussed earlier
– Occupies twice as much bandwidth as Unipolar NRZ
– Not transparent

Polar Signalling
Non-Return to Zero (NRZ)
• A binary 1 is represented by a pulse 𝑝(𝑡) and binary 0 is
represented by opposite pulse i.e., −𝑝(𝑡)
• Advantages:
– No DC component
• Disadvantages:
– Does not have error correction capability
– Does not posses any clocking component for ease of synchronization
• Not transparent

Polar Signalling
Return to Zero (RZ)
• A binary 1 is represented by pulse 𝑝(𝑡) and a binary 0 is represented by an
opposite pulse – 𝑝(𝑡)
• Fills only the first half of the time slot, returning to zero for the second
half.
• Advantages:
– Same as polar NRZ
– Presence of a spectral line at symbol rate which can be used as symbol timing clock
signal
• Disadvantages:
– Same as polar NRZ
– Occupies twice as much bandwidth as polar NRZ

Bipolar Signalling
• 0 is represented by absence of pulse while 1 is represented by alternating
voltage levels of +V and –V
• Bipolar NRZ
• Bipolar RZ
• Advantages:
– No DC component
– Occupies less bandwidth than unipolar and polar NRZ schemes
– Posses single error detection capability
• Disadvantages:
– Does not posses any clocking component for ease of synchronization
• Not transparent

• The duration of the bit is divided into two halves
– 1 is +ve in 1st half and –ve in the 2nd half
– 0 is –ve in 1st half and +ve in the 2nd half
• Advantages:
– No DC component
– Easy to synchronize
• Transparent
• Disadvantages:
– Because of greater number of transitions it occupies a significantly large bandwidth
– Does not have error detection capability
Manchester Signalling

Digital Receivers and
Regenerative Repeaters
• Regenerative repeaters are used at regular intervals along a digital
transmission line to detect the incoming digital signal and regenerate new
“clean” pulse for further transmission along the line.
• A receiver or a regenerative repeater performs three functions
– Reshaping incoming pulse using an equalizer
– Extract the timing information required to sample incoming pulses at
optimum instances
– Making symbol detection decisions based on the pulse samples

Equalizer
• A pulse train is attenuated and distorted by the transmission
medium.
– The attenuation is compensated by pre-amplifier
– The distortion is compensated by the equalizer
• Channel distortion is a form of dispersion, caused by an
attenuation of certain critical frequency components of the
data pulse train.
• An equalizer should have a frequency characteristic that is
inverse of that of the transmission medium.
– This will restore the critical frequency components and eliminate the
pulse dispersion
– Unfortunately it will also boost the critical frequency components of
the channel noise as well, known as noise amplification

Timing Extraction
• The received digital signals need to be sampled at the precise
instants.
– This requires a clock signal at the receiver in synchronism with the
clock signal at the transmitter (symbol or bit synchronization),
delayed by the channel response.
• There are three methods of synchronization
1) Derivation from a primary or a secondary standard (e.g., transmitter
and receivers slaved to a master timing source)
2) Transmitting a separate synchronizing signal (pilot clock)
3) Self synchronization, where the timing information is extracted from
the received signal itself.

Timing Extraction
• Because of its high cost, the first method is suitable for large
volumes of data and high speed communication systems.
• The second method in which part of channel capacity is used
to transmit the timing information, is suitable when the
available capacity is large in comparison to the data rate
• The third method is very efficient method of timing extraction
or clock recovery because the timing is derived from the
received message signal itself.

Timing Extraction
(Self-Synchronization)
• If the pulses are transmitted at a rate of 𝑅𝑏 pulses per second,
we require the periodic timing information i.e., the clock at
𝑅𝑏 Hz to sample the incoming pulses at a repeater.
• The timing information can be extracted from the received
signal itself if the line code is chosen properly.
• For example, if a RZ polar signal is rectified, it results in a
periodic signal which contains the desired periodic timing
signal of frequency 𝑅𝑏 Hz .
• When this signal is applied to a resonant circuit tuned to
frequency 𝑅𝑏 Hz, the output, which is a sinusoid of frequency
𝑅𝑏 Hz can be used for timing.

Timing Extraction
(Self-Synchronization)
• A digital signal, such as on/off signal (a) can be expressed as a
sum of a random polar signal (b) and a clock frequency
periodic signal (c)
• Because of the presence of the periodic component, we can
extract the timing information from this signal using a
resonant circuit tuned to clock frequency.

Timing Extraction
(Self-Synchronization
• The timing signal (resonant circuit output) is sensitive to the
incoming bit pattern.
– In on/off or bipolar case, a 0 is transmitted by ‘no pulse’
– If there are too many 0s in a sequence, there is no signal at the input
of the resonant circuit and the sinusoidal output of the circuit starts
decaying causing error in timing information.
• A line code in which the bit pattern does not affect the
accuracy of the timing information is said to be a transparent
line code.
– The RZ polar scheme (each bit is transmitted by some pulse) is
transparent.
– The on/off and bipolar are non-transparent.

Communication Systems_B.P. Lathi and Zhi Ding (Lecture No 40-42)

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