DIGITAL COMMUNICATION SYSTEMS
MSC -COURSE
Prepared by :
Nisreen Bashar AL-Madanat
Under Supervision of :
Dr. Ibrahim AL-Qatawneh
OUTLINES :
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Objectives
Introduction
Modes of communications
Digital data communication
Transmission timing
Goals in communication systems
Communication channels
Channel capacity
Noise in communication systems
Digital transmission
Multiplexing techniques
Digital modulation and demodulation
Selection of Encoding /Modulation schemes
Matched filter
Bandwidth efficiency
MPSK Error rate Performance
QAM Error performance
Performance comparison of various Digital Modulation schemes
Trade offs between BER ,POWER and BW
Industry tends
Summary
References
OBJECTIVES :
After completing this lecture the student will be able to :
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Have an over all view about the communication system and its
simplified block diagram.
Discuss the main concepts of communication systems.
Discuss and distinguish the types of communication systems and its
channels and Modes.
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Identify the digital data transmission methods and timing .
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Consider the basic goals or tradeoffs in communication system design .
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Identify the channel capacity and its related concepts.
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Aware the effect of noise in communication systems.
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Discuss the usefulness of digital transmission and its disadvantages.
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Discuss Multiplexing techniques and its types and applications.
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Identify modulation and its different types .
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Compare between different digital modulation schemes.
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Design and select a specific simple communication system based on
the trade offs and the performance comparison .
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Discuss industry trends.
INTRODUCTION
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Main purpose of communication is to transfer information
from a source to a recipient via a channel or medium.
Communication systems are found whenever information is to
be transmitted from one point to another.
Basic block diagram of a communication system:
BRIEF DESCRIPTION
Source: analog or digital
 Transmitter: transducer, amplifier, modulator,
oscillator, power amp., antenna
 Channel: e.g. cable, optical fiber, free space
 Receiver: antenna, amplifier, demodulator,
oscillator, power amplifier, transducer
 Recipient: e.g. person, (loud) speaker, computer
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DATA COMMUNICATION TERMS
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Data - entities that convey meaning, or information
Signals - electric or electromagnetic representations of data
Transmission - communication of data by the propagation and
processing of signals
Types of information
Voice,text data, video, music, email, numerical data ,
graphic data etc.
Types of communication systems
- Public Switched Telephone Network (voice,fax,modem)
- Satellite systems
- Radio,TV broadcasting
- Cellular phones
- Computer networks (LANs, WANs, WLANs)
INFORMATION REPRESENTATION
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Communication system converts information into
electrical electromagnetic/optical signals appropriate
for the transmission medium.
Analog systems convert analog message into signals
that can propagate through the channel.
Digital systems convert bits(digits, symbols) into signals
Computers naturally generate information as
characters/bits
 Most information can be converted into bits
 Analog signals converted to bits by sampling and
quantizing (A/D conversion)
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MODES
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OF COMMUNICATION
Based on whether the systems communicate on only
one direction or otherwise , the communication
modes are as indicated in the following figure :
DIGITAL DATA COMMUNICATION
In a digital communications system, there are two methods for
data transfer:
Parallel and Serial.
Parallel connections have multiple wires running parallel to
each other (hence the name), and can transmit data on all the
wires simultaneously. Serial, on the other hand, uses a single
wire to transfer the data bits one at a time.
Parallel Data The parallel port on modern computer systems is
an example of a parallel communications connection. The
parallel port has 8 data wires, and a large series of ground
wires and control wires. IDE hard disk connectors are another
good example of parallel connections in a computer system.
Serial Data The serial port on modern computers is a good example
of serial communications. Serial ports have a single data wire,
and the remainder of the wires are either ground or control
signals. USB is a good example of other serial communications
standards.
SERIAL VS. PARALLEL COMMUNICATION
TRANSMISSION TIMING (SYNCHRONIZATION )
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Synchronous and Asynchronous transmissions
are two different methods of transmission
synchronization. Synchronous transmissions are
synchronized by an external clock, while
asynchronous transmissions are synchronized by
special signals along the transmission medium.
SYNCHRONOUS TRANSMISSION
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Data/Strobe Synchronous Transmission
In synchronous transmission, the stream of data to be transferred is
encoded as fluctuating voltages on one wire, and a periodic pulse of
voltage is put on another wire (often called the "clock" or "strobe")
that tells the receiver "here's where one bit/byte ends and the next
one begins".
Synchronous transmission is carried out under the control of the
common master clock.
No start and stop bits are used instead the bytes are transmitted as
a block in a continuous stream of bits.
The receiver operates at exactly the same clock frequency as that of
transmitter. The grouping of these bits is the responsibility of the
receiver.
SYNCHRONOUS TRANSMISSION
Advantages:
 Lower overhead and thus, greater throughput
Disadvantages:
 Slightly more complex .
 Hardware is more expensive
 High cost
ASYNCHRONOUS TRANSMISSION
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In asynchronous transmission, there is only one wire/signal
carrying the transmission.
The transmitter sends a stream of data and periodically
inserts a certain signal element into the stream which can
be "seen" and distinguished by the receiver as a sync
signal.
That sync signal might be a single pulse (a "start bit" in
asynchronous start/stop communication), or it may be a
more complicated sync word.
ASYNCHRONOUS TRANSMISSION
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Advantages:
Simple, doesn't require synchronization of both
communication sides
Cheap, timing is not as critical as for
synchronous transmission, therefore hardware
can be made cheaper.
Set-up is very fast, so well suited for
applications where messages are generated at
irregular intervals, for example data entry
from the keyboard.
Disadvantages:
Large relative overhead, a high proportion of
the transmitted bits are uniquely for control
purposes and thus carry no useful information.
GOALS IN COMMUNICATION SYSTEM DESIGN
(TRADE –OFFS)
To maximize transmission rate, R
 To maximize system utilization, U
 To minimize bit error rate, Pe
 To minimize required systems bandwidth, W
 To minimize system complexity, Cx
 To minimize required power, Eb/No
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COMMUNICATION CHANNELS
•
Wire line Channels (Copper)
•
Microwave Radio
•
Satellite communication
•
Optical Fiber
CHANNEL CAPACITY
• Channel Capacity (C)
– the maximum rate at which data can be transmitted over a
given communication path, or channel, under given
conditions
 • Data rate (bps)
– rate at which data can be communicated , impairments,
such as noise, limit data rate that can be achieved
 • Bandwidth (B)
– the bandwidth of the transmitted signal as constrained by
the transmitter and the nature of the transmission medium
(Hertz)
 • Noise (N)
– impairments on the communications path
 • Error rate - rate at which errors occur (BER)
– Error = transmit 1 and receive 0; transmit 0 and receive 1
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SHANNON’S CHANNEL CAPACITY
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Equation:
C  B log 2 1  SNR 
Channel capacity C (bits/sec) is the speed
at which information can travel over a
channel with an arbitrarily low error rate
i.e. when a system is transmitting bits at
or below C then for any BER e>0 there
exists a code with block length n which
will provide a BER < e
Represents theoretical maximum that can
be achieved
In practice, only much lower rates
achieved
Formula assumes white noise (thermal noise)
 Impulse noise is not accounted for
 Attenuation distortion or delay distortion not
accounted for

www-gap.dcs.st-and.ac.uk/~history/
Mathematicians/Shannon.html
SIGNAL ENCODING CRITERIA
• What determines how successful a receiver will be in
interpreting an incoming signal?
Signal-to-noise ratio (SNR)
 Data rate
 Bandwidth (B)
 Inter-related quantities
• Increase in SNR decreases bit error rate
• Increase in data rate increases bit error rate
• Increase in bandwidth allows an increase in data rate
 Shannon Bound for AWGN non fading
channel
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CONCEPTS RELATED TO CHANNEL CAPACITY
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Shannon Bound for AWGN non fading channel
Nyquist Bandwidth
– For binary signals (two voltage levels)
• C = 2B
– With multilevel signaling (M-ary signalling)
• C = 2B log 2 M
• M = number of discrete signal or voltage levels
• N= number of bits
• M = 2^N
EXAMPLE OF NYQUIST AND SHANNON FORMULATIONS
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• Spectrum of a channel between 3 MHz and 4 MHz ;
SNRdB =24 dB
B  4 M Hz  3 M Hz  1 M Hz
SNR dB  24 dB  10 log10 SNR 
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SNR  251
Using Shannon’s formula
C  106  log 2 1  251  106  8  8Mbps
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How many signaling levels are required?
C  2 B log 2 M
 
8  106  2  10 6  log 2 M
4  log 2 M
M  16
NOISE IN COMMUNICATION SYSTEMS
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Without noise/distortion/sync. problem, we will never make
bit errors
The term noise refers to unwanted electrical signals that
are always present in electrical systems; e.g spark-plug
ignition noise, switching transients, and other radiating
electromagnetic signals.
Can describe thermal noise as a zero-mean Gaussian
random process.
A Gaussian process n(t) is a random function whose
amplitude at any arbitrary time t is statistically
characterized by the Gaussian probability density function
 1  n 2 
1
p ( n) 
exp     
 2
 2    
WHITE NOISE
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The primary spectral characteristic of thermal noise is that its
power spectral density is the same for all frequencies of interest
in most communication systems.
N
Gn ( f )  0 watts / hertz
Power spectral density Gn(f )
2
Autocorrelation function of white noise is
Rn ( )  1{Gn ( f )} 
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The average power Pn of white noise is infinite

p ( n) 


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N0
 ( )
2
N0
df  
2
The effect on the detection process of a channel with additive
white Gaussian noise (AWGN) is that the noise affects each
transmitted symbol independently.
Such a channel is called a memoryless channel.
The term “additive” means that the noise is simply superimposed
or added to the signal
SYSTEM NOISE
A constellation displaying significant noise.
 Dots are spread out indicating high noise and
most likely significant errors.
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Dots are
spread out
causing
errors to
occur
DIGITAL TRANSMISSION
ADVANTAGES
Why Digital ?
 – Increase System Capacity
• compression, more efficient modulation
 – Error control coding, equalizers,etc. possible to
combat noise and interference => lower power
needed
 – Reduce cost and simplify designs
 – Improve Security (encryption possible)
• Digital Modulation
 – Analog signal carrying digital data
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Digital techniques need to distinguish between discrete
symbols allowing regeneration versus amplification
Good processing techniques are available for digital
signals, such as medium.
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Data compression (or source coding)
Error Correction (or channel coding)(A/D conversion)
Equalization
Security
Easy to mix signals and data using digital techniques
DISADVANTAGES
Requires reliable “synchronization”
 Requires A/D conversions at high rate
 Requires larger bandwidth
 Non graceful degradation
 Performance Criteria
 Probability of error or Bit Error Rate
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MULTIPLEXING TECHNIQUES
Multiplexing
 Capacity of transmission medium usually exceeds
capacity required for transmission of a single signal
 Multiplexing - carrying multiple signals on a single
medium
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More efficient use of transmission medium
REASONS FOR WIDESPREAD USE OF
MULTIPLEXING
Cost per kbps of transmission facility declines with an
increase in the data rate
 Cost of transmission and receiving equipment declines
with increased data rate
 Most individual data communicating devices require
relatively modest data rate support
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MULTIPLEXING TECHNIQUES
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Frequency-division
multiplexing (FDM)
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Takes advantage of the fact that
the useful bandwidth of the
medium exceeds the required
bandwidth of a given signal
Time-division multiplexing
(TDM)
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Takes advantage of the fact that
the achievable bit rate of the
medium exceeds the required
data rate of a digital signal
** Statistical multiplexing
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is a type of communication link
sharing.
FDM VS TDM
STATISTICAL MULTIPLEXING
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In statistical multiplexing , a communication channel is divided into
an arbitrary number of variable bit-rate digital channels or data
streams
The link sharing is adapted to the instantaneous traffic demands of
the data streams that are transferred over each channel
When performed correctly, statistical multiplexing can provide a link
utilization improvement, called the statistical multiplexing gain.
Statistical multiplexing is facilitated through packet
mode or packet-oriented communication, which among others is
utilized in packet switched computer networks
In statistical multiplexing, each packet or frame contains a
channel/data stream identification number, or (in the case of
datagram communication) complete destination address information.
The packets to be transmitted are stored in a common buffer and are
then transmitted according to some specified scheduling rules.(the
adv. Of this is that the transmitter is never Idle as long as there are
packets to transmit.
COMPARISON WITH STATIC ,TDM AND FDM
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Time domain statistical multiplexing (packet mode communication) is similar
to (TDM), except that, rather than assigning a data stream to the same recurrent
time slot in every TDM frame , each data stream is assigned time slots (of fixed
length) or data frames (of variable lengths) that often appear to be scheduled in a
randomized order, and experience varying delay (while the delay is fixed in TDM).
Statistical multiplexing allows the bandwidth to be divided arbitrarily among a
variable number of channels (while the number of channels and the channel data
rate are fixed in TDM and FDM).
Statistical multiplexing ensures that slots will not be wasted (whereas TDM can
waste slots). The transmission capacity of the link will be shared by only those
users who have packets.
Statistical most costly .
TDM and FDM divide a communication link into channels with fixed transmission
rates , while in statistical it only uses the channel when it has packets to send.
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TDM and FDM less efficient for irregular flow of packets .
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The average delay for FDM and TDM is larger .
DIGITAL MODULATION AND DEMODULATION
Modulation
•All channels consist of some continuous parameter
•Must map discrete states onto continuous property
•Must have a decision circuit to map the state of the
modulated channel into a discrete state
•As number of levels or states M the behavior of the
digital system does not approach that of an analog system,
due to the decision circuit
MODULATION REVIEW
Modulation
- Converting digital or analog information to a waveform
suitable for transmission over a given medium
- Involves varying some parameter of a carrier wave
(sinusoidal waveform) at a given frequency as a function of
the message signal
- General sinusoid
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- If the information is digital changing parameters is called
“keying” (e.g. ASK, PSK, FSK)
MODULATION – DEFINITION
• Motivation
-Smaller antennas (e.g., l /4 typical antenna size)
• l = wavelength = c/f , where c = speed of light, f= frequency.
• 3000Hz baseband signal => 15 mile antenna, 900 MHz => 8 cm
- Frequency Division Multiplexing – provides separation of signals
- medium characteristics
- Interference rejection
- Simplifying circuitry
 • Modulation
– shifts center frequency of baseband signal up to the radio carrier
 • Basic schemes
- Amplitude Modulation (AM) Amplitude Shift Keying (ASK)
- Frequency Modulation (FM) Frequency Shift Keying (FSK)
- Phase Modulation (PM)
Phase Shift Keying (PSK)
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NUMBER OF LEVELS
Digital communications relies on a finite number
of discrete levels
 Minimum number of levels is two (binary code)
 Shannon Capacity helps determine optimum
number of levels for a given bandwidth, SNR,
and BER
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
Eb 
r  log1  r 
N0 

r 
R
W
LIMITS ON COMMUNICATION CHANNELS

Eb 
r  log 
1  r N 

0 
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Two types of communication channels
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r<<1 – Power Limited
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High dimensionality signaling
schemes
Binary
r>>1 – Bandwidth Limited
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Low dimensionality
Multilevel
Proakis and Salehi, pp. 738
DIGITAL MODULATION TYPES
• Amplitude Shift Keying (ASK):
- change amplitude with each symbol
- frequency constant
- low bandwidth requirements
- very susceptible to interference
 • Frequency Shift Keying (FSK):
- change frequency with each symbol
needs larger bandwidth
 • Phase Shift Keying (PSK):
- Change phase with each symbol
- More complex
- robust against interference
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BASIC ENCODING TECHNIQUES
AMPLITUDE-SHIFT KEYING
o
o
One binary digit represented by presence
of carrier, at constant amplitude
Other binary digit represented by absence
of Carrier
• where the carrier signal is Acos(2pfct)
o
o
o
o
Very Susceptible to noise
Used to transmit digital data over optical
fiber
Amplitude of carrier wave is modulated
Equivalent BER vs SNR to baseband PAM
Proakis and Salehi, pp. 306
ANGLE MODULATION (PSK AND FSK)
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Frequency is time derivative of phase, PSK and
FSK are somewhat equivalent.
Proakis and Salehi, pp. 332
PSK: DIGITAL ANGLE MODULATION
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Usually in digital communications PSK is chosen over FSK
Easier to create multilevel codes
Possibility of using differential phase shift keying (DPSK)
Uses phase shifts relative to previous bit
 Eliminates need for local oscillator at receiver
 Use Gray Code to minimize effect of errors
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Proakis and Salehi, pp. 631
BINARY FREQUENCY-SHIFT KEYING
(BFSK)
Two binary digits represented by two different
frequencies near the carrier frequency
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– where f1 and f2 are offset from carrier frequency
fc by equal but opposite amounts
– B = 2([f2 – f1]/2 + fb)
• Where fb = input bit rate
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BER for BFSK
BI-PHASE SHIFT KEYING (BPSK)
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BPSK is the simplest method of digital transmission.
Data is transmitted by reversing the phase of the carrier.
The amplitude of the carrier remains constant.
Is a very robust transmission method but consumes
significant bandwidth
Data
Amplitude
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1
1
In Phase
0
180° Out
of Phase
0
-1
QUADRATURE AMPLITUDE MODULATION
• QAM is a combination of ASK and PSK
– Two different signals sent simultaneously on
the same carrier frequency –
– Change phase and amplitude as function of
input data
– Simple case 8 QAM (two amplitudes – 4
phases)
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Used in ADSL, Digital video broadcasting,
Microwave,…
 The amplitude of two carriers will be changed
according to the data signal (Quadrature)
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QAM MODULATOR AND DEMODULATOR
THE I/Q DIAGRAM
BIT-ERROR RATE
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QAM is a modulation type, where the
amplitude of two carrier signals will
be modulated according to the bits
sequence to be transmitted.
There are many kinds of QAM,
depending on the number of bits in
each sequence that will be
transmitted as one symbol.
Higher order QAM is more bandwidth
efficient than the lower order.
Higher order QAM is less power
efficient than the lower orders
because of the high bit-error rate.
Rectangular QAM is easier to
modulate and demodulate than nonrectangular QAM, but bit-error rate
is higher in rectangular QAM.
D PHASE-SHIFT KEYING (PSK)
Differential PSK (DPSK)
– Phase shift with reference to previous bit
• Binary 0 – signal burst of same phase as previous
signal burst
• Binary 1 – signal burst of opposite phase to
previous signal burst
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PERFORMANCE IN AWGN CHANNELS
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Similar to BPSK analysis have Pe for FSK, and
DPSK
M-ARY SIGNALING/MODULATION
What is M-ary signaling?
– The transmitter considers ‘k’ bits at a times. It
produces one of M signals where M = 2^k.
 Example: QPSK (k = 2)

M-ARY ERROR PERFORMANCE
• MPSK, as M increases
– the bandwidth remains constant,
– the minimum distance between signals reduces
=> increase in symbol error rate
 • MFSK, as M increases
– the bandwidth increases
– the performance improves but the minimum
distance between signals remains the same
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SELECTION OF ENCODING/MODULATION
SCHEMES
• Performance in an AWGN channel
– How does the bit error rate vary with the energy per bit
available in the system when white noise present
 • Performance in fading multipath channels
– Same as above, but add multipath and fading
 • Bandwidth requirement for a given data rate
– Also termed spectrum efficiency or bandwidth efficiency
– How many bits/sec can you squeeze in one Hz of bandwidth
for a given error rate
 • Cost
– The modulation scheme needs to be cost efficient
_ Circuitry should be simple to implement and inexpensive
(e.g. detection, amplifiers)

MATCHED FILTER
In order to detect a signal at the receiver, a linear
filter that is designed to provide the maximum
output SNR in AWGN for a given symbol
waveform is used.

This filter is called a “matched filter”
 • If the transmitted signal is s(t), the impulse
response of the matched filter can be shown to
be

This assumes that s(t) exists only for a duration
of T seconds. Let us look at the output for k = 1.
Compare with cross-correlation:
 The output of the matched filter is the crosscorrelation of the received signal and the time
shifted transmitted signal.
CORRELATION IMPLEMENTATION OF MATCHED
FILTER
BANDWIDTH EFFICIENCY
M-PSK ERROR RATE PERFORMANCE

Increasing M increases error rate and data rate
QAM ERROR PERFORMANCE
PERFORMANCE COMPARISON OF VARIOUS DIGITAL
MODULATION SCHEMES (BER = 10^-6)
TRADEOFFS BETWEEN BER, POWER AND
BANDWIDTH
Trade BER
performance for
power – fixed data
Rate

Trade data rate for
power – fixed BER

Trade BER for
data rate – fixed
power

INDUSTRY TRENDS
SUMMARY
REFERENCES
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
John G. Proakis, Masoud Salehi, Communications Systems
Engineering, Prentice Hall 1994
David Tipper/Associate Professor
Department of Information Science and
Telecommunications/University of Pittsburgh
http://www.tele.pitt.edu/tipper.html
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Communication systems (4th Edition)/Simon haykin
Constellations Demystified
Presented by Sunrise Telecom Broadband
Data communication and Network (4th Edition) by
Behrouz.A.Forouzan with Sophia Chung Fegan .
Fundamentals of DIGITAL COMMUNICATION by (Upamanyu
madhow)
Digital Communications, Fourth Edition, J.G. Proakis, McGraw Hill,
2000.
Agilent Digital Modulation in Communications Systems An
Introduction Application Note 1298
Digital Communications: Fundamentals and Applications, By
“Bernard Sklar”, Prentice Hall, 2nd ed, 2001
THANK YOU FOR YOUR TIME
AND ATTENTION