basics of communication systems

Lecture 1

BASICS OF COMMUNICATION SYSTEMS

Introduction

• Electronic Communication

– The transmission, reception, and processing of information with the use of electronic circuits

• Information

– Knowledge or intelligence that is communicated (i.e., transmitted or received) between two or more points

Introduction

• Digital Modulation

– The transmittal of digitally modulated analog signals (carriers) between two or more points in a communications systems

– Sometimes referred to as digital radio because digitally modulated signals can be propagated through Earth’s atmosphere and used in wireless communications systems

Introduction

• Digital Communications

– Include systems where relatively high-frequency analog carriers are modulated by relatively low-frequency digital signals (digital radio) and systems involving the transmission of digital pulses (digital transmission)

Introduction

ASK FSK PSK

QAM

Applications

1 • Relatively low-speed voice-band data communications

modems such as those found in most personal computers

2 • High-speed data transmission systems, such as broadband

digital subscriber lines (DSL)

3 • Digital microwave and satellite communications systems

4 • Cellular telephone Personal Communications Systems (PCS)

Basic Telecommunication System

Source Transducer Transducer Sink

Transmission Medium

Attenuation

In an electrical communication system, at the transmitting side, a transducer converts the real –life information into an electrical signal. At the receiving side, a transducer converts the electrical signal back into real-life information

Basic Telecommunication System

Source Transducer Transducer Sink

NOISE!!!

Transmission Medium

Note: As the electrical signal passes through the transmission medium, the signal gets attenuated. In addition, the transmission medium introduces noise and, as a result, the signal gets distorted.

Basic Telecommunication System

The objective of designing a communication system is to reproduce the electrical signal at the receiving end with minimal distortion.

Basic Telecommunication System

Channel

RS 232 Port RS 232 Port

Note: The serial ports of two computers can be connected directly using a copper cable. However, due to the signal attenuation, the distance cannot be more than 100 meters.

Basic Telecommunication System

Two computers can communicate with each other through the telephone network, using a modem at each end. The modem converts the digital signals generated by the computer into analog form for transmission over the medium at the transmitting end and the reverse at the receiving end.

Basic Telecommunication System

Source Baseband

Signal Processing

Medium Access

Processing Transmitter

Receiver Decoding of

Data

Baseband Signal

Processing Sink

Medium

(a) Transmitting Side

(a) Receiving Side

Basic Telecommunication System

In the case of a radio communication system for broadcasting audio programs, the electrical signal is transformed into a high-frequency signal and sent through the air (free space). A radio transmitter is used to do this. A reverse of this transformation – converting the high-frequency signal into an audio signal – is performed at the receiving station. Since it is a broadcasting system, many receivers receive the information.

Basic Telecommunication System

• In a mobile communication system, a radio channel has to be shared by a number of users. Each user has to use the radio channel for a short time during which he has to transmit his data and then wait for his next turn. This mechanism of sharing the channel is known as multiple access.

Basic Telecommunication System

1 • Multiplexer

2 • Multiple access

3 • Error detection and correction

4 • Source coding

5 • Signaling

Depending on the type of communication, the distance to be covered, etc., a communication system will consist of a number of elements, each element carrying out a specific function. Some important elements are:

Basic Telecommunication System Note:

Two voice signals cannot be mixed directly because it will not be possible to separate them at the receiving end. The two voice signals can be transformed into different frequencies to combine them and send over the medium.

Types of Communication 1 • Point-to-point communication

2 • Point-to-multipoint communication

3 • Broadcasting

4 • Simplex communication

5 • Half-duplex communication

6 • Full-duplex communication

Transmission Impairments

1

• Attenuation

• The amplitude of the signal wave decreases as the signal travels through the medium.

2

• Delay distortion

• Occurs as a result of different frequency components arriving at different times in the guided media such as copper wire or coaxial cable

3 • Noise

• Thermal noise, intermodulation, crosstalk, impulse noise

Transmission Impairments • Thermal Noise – occurs due to the thermal

agitation of electrons in a conductor. (white noise), N = kTB

• Intermodulation Noise – When two signals of different frequencies are sent through the medium, due to nonlinearity of the transmitters, frequency components such as f1 + f2 and f1 – f2 are produced, which are unwanted components and need to be filtered out.

Transmission Impairments

• Crosstalk – Unwanted coupling between signal paths

• Impulse Noise – occurs due to external electromagnetic disturbances such as lightning. This also causes burst of errors.

Analog Versus Digital Transmission

Analog Communication

The signal, whose amplitude varies continuously, is transmitted over

the medium.

Reproducing the analog signal at the receiving end is very difficult due to transmission impairments

Digital Communication

1s and 0s are transmitted as voltage pulses. So, even if the pulse s distorted due to noise, it is not very difficult to

detect the pulses at the receiving end.

Much more immune to noise

Advantages of Digital Transmission

More reliable transmission

• Because only discrimination between ones and zeros is required

Less costly implementation

• Because of the advances in digital logic chips

Ease of combining various types of signals (voice, video, etc.,)

Ease of developing secure communication systems

Questions:

1. What are the advantages of digital communication over analog communication?

2. Explain the different types of communication systems.

3. What are the different types of transmission impairments?

4. What is multiplexing?

5. What is signaling?

Lecture 2

INFORMATION THEORY

Claude Shannon

-Laid the foundation of information theory in 1948. His paper “A Mathematical Theory of Communication” published in Bell System Technical Journal is the basis for the entire telecommunications developments that have taken place during the last five decades. A good understanding of the concepts proposed by Shannon is a must for every budding telecommunication professional.

Requirements of a Communication System

The requirement of a communication system is to transmit the information from the source to the sink without errors, in spite of the fact that noise is always introduced in the communication medium.

The Communication System

Information Source

Transmitter Receiver Information

Sink

Noise Source

Channel

Generic Communication System

Symbols Produced A B B A A A B A B A

Bit stream produced 1 0 0 1 1 1 0 1 0 1

Bit stream received 1 0 0 1 1 1 1 1 0 1

In a digital communication system, due to the effect of noise, errors are introduced. As a result, 1 may become a 0 and 0 may become a 1.

Generic Communication System as proposed by Shannon

Information Source

Source Encoder

Modulator Channel Encoder

Information Sink

Source Decoder

Demodulator Channel Decoder

Modulating Signal

Demodulating Signal

Modulated Signal

Explanation of Each Block

Information Source: Produces the symbols

Source encoder: converts the signal produced by the information source into a data stream

Channel Encoder: add bits in the source-encoded data

Modulation: process of transforming the signal

Demodulator: performs the inverse operation of the modulator

Explanation of Each Block

Channel Decoder: analyzes the received bit stream and detects and corrects the error

Source Decoder: converts the bit stream into the actual information

Information Sink: absorbs the information

Types of Source Encoding

• Source encoding is done to reduce the redundancy in the signal.

1. Lossless coding

2. Lossy coding

The compression utilities we use to compress data files use lossless encoding techniques. JPEG image compression is a lossy technique because some information is lost.

Channel Encoding

• Redundancy is introduced so that at the receiving end, the redundant bits can be used for error detection or error correction

Entropy of an Information Source

What is information?

How do we measure

information

??? ???

Entropy of an Information Source

H = log2 N bits/symbol

Where: N = symbols

Entropy of an Information Source

Example:

Assume that a source produces the English letters (from A to Z, including space), and all these symbols will be produced with equal probability. Determine the entropy.

Ans. H = 4.75 bits/symbol

Entropy of an Information Source

If a source produces (i)th symbol with a probability of P(i)

Entropy of an Information Source

• Example:

Consider a source that produces four symbols with probabilities of ½, ¼, 1/8, and 1/8, and all symbols are independent of each other. Determine the entropy.

Ans. 7/4 bits/symbol

Channel Capacity

• The limit at which data can be transmitted through a medium

Where: C = channel capacity (bps) W = bandwidth of the channel (Hz) S/N = signal-to-noise ratio (SNR) (dB)

SNR

Channel Capacity

• Example:

Consider a voice-grade line for which W = 3100 Hz, SNR = 30 dB. Determine the channel capacity.

Ans: 30.898 kbps

Question

1. To increase C, can we increase W?

2. To increase C, can we increase SNR?

Ans. 1. No, because increasing W increases noise as well, and SNR will be

reduced. 2. No, that results in more noise, called intermodulation noise

Shannon’s Theorems

• In digital communication system, the aim of the designer is to convert any information into a digital signal, pass it through the transmission medium and, at the receiving end, reproduce the digital signal exactly.

Shannon’s Theorems

• Requirements:

To code any type of information into digital format

To ensure that the data sent over the channel is not corrupted.

Source Coding Theorem

• States that “the number of bits required to uniquely describe an information source can be approximated to the information content as closely as desired.”

Example: Consider a source that produces two symbols A and B with equal probability.

Symbol Probability Code Word

A 0.5 1

B 0.5 0

Now, consider a source that produces these same two symbols. But instead of coding A and B directly, we can code AA, AB, BA, BB.

Symbol Probability Code Word

AA 0.45 0

AB 0.45 10

BA 0.05 110

BB 0.05 111

• NOTE:

Assigning short code words to high-probability symbols and long code words to low-probability symbols results in efficient coding.

0 110 110 10 111

A A B A B A A B B B

Channel Coding Theorem

• States that “the error rate of data transmitted over a bandwidth limited noisy channel can be reduced to an arbitrary small amount if the information rate is lower than the channel capacity.”

Example: Consider the example of a source producing the symbols A and B. A is coded as 1 and B as 0.

Symbols Produced

A B B A B

Bit Stream 1 0 0 1 0

Transmitting……………111000000111000

101000010111000 …………Received

NOTE

• Source coding is used mainly to reduce the redundancy in the signal, whereas channel coding is used to introduce redundancy to overcome the effect of noise.

Questions:

1. Draw the block diagram of a communication system and explain the function of each block.

2. What is entropy of an information source? Illustrate with examples.

3. What is source coding? What is the difference between lossless coding and lossy coding?

4. Explain the concept of channel capacity with an example.

5. What is channel coding? Explain the concept of error correcting codes.

Exercises

1. A source produces 42 symbols with equal probability. Calculate the entropy of the source.

2. A source produces two symbols A and B with probabilities of 0.6 and 0.4, respectively. Calculate the entropy of the source.

3. The ASCII code is used to represent characters in the computer. Is it an efficient coding techniques from Shannon’s point of view? If not, why?

Answers

1. 5.39 bits/symbol 2. 0.970 bits/symbol 3. In ASCII, each character is represented by seven bits. The

frequency of occurrence of the English letters is not taken into consideration at all. If the frequency of occurrence is taken into consideration, then the most frequently occurring letters have to be represented by small code words (such as 2 bits) and less frequently occurring letters have to be represented by long code words. According to Shannon’s theory, ASCII is not an efficient coding technique.

However, note that if an efficient coding technique is followed,

then a lot of additional processing is involved, which causes delay in decoding the text.