Principles of Communication.pdf

Principles of

Communication

Course Outline 1. Introduction to Communications Systems

2. Noise

3. Amplitude Modulation

4. Single-Sideband Techniques

5. Frequency Modulation

6. Radio Receivers

7. Radiation and Propagation of Waves

8. Pulse Modulation

9. Digital Modulation

10. Broadband Communication System

Principles of Communication LAB

1. Passive, Active Filters, Tuned Circuits

2. AM Transmitter

3. Frequency Modulation

4. Pulse Amplitude Modulation

5. Diode Detection

6. Time Division Multiplexing

7. Frequency Division Multiplexing

8. Suggested Project : superheterodyne

receiver

Introduction: Communication System

Communication systems are designed to transmit information from one place to another.

Communication systems Design concerns: 1. Selection of the information–bearing

waveform;

2. Bandwidth and power of the waveform;

3. Effect of system noise on the received information;

4. Cost of the system.

RA 9292 Definition:

Communications

– refers to process of sending and/or

receiving information, data, signals and/or

messages between two or more points by

radio, cable, optical waveguide or other

devices and wired or wireless medium.

Evolution of Communication System:

Smoke Signal Tribal Drum Morse Code

Evolution of Communication System:

1837 – Samuel Morse invented the telegraph

system, the first to be commercially successful

communication system which uses electricity in

sending messages.

1866 – the use of telegraph cables that runs

under water.

1898 – twelve transatlantic cables in operation.

1876 – Alexander Graham Bell invented the

telephone, the first voice communication by

electrical means.

Importance of a radio in communication as a medium;

1865 – James Clerk Maxwell constructed the theoretical framework in radio communication.

1887 – Heinrich Rudolph Hertz verified the theories of Maxwell.

1901 – Guglielmo Marconi accomplished the first transatlantic communication via radio.

1906 – transmitters began to use specially designed high frequency alternators to transmit voice.

1920 – began the regular radio broadcasting.

The use of electronic system;

1904 – Sir John Ambrose Fleming invented

the diode tube.

1906 – Lee De Forest invented the triode for

amplification.

1947 – Brattain, Bardeen, and Shockley

invented the transistor.

1948 – transistor began to use for

amplification.

Two Broad Categories of

Communication System

1. Analog Communication System

2. Digital Communication System

Analog Communication System

An analog communication system transfers

information from an analog source to the sink.

Digital Communication System

A digital communication system transfers

information from a digital source to the sink.

Analog Information Source

An analog information source produces

messages that are defined on a continuous

form.

(e.g. microphone)

Digital Information Source

A digital information source produces a

finite set of possible messages. (e.g. typewriter)

Digital and Analog Communication Systems

A digital waveform is defined as a function of time that can have a discrete set of amplitude values.

An Analog waveform is a function that has a continuous range of values.

t

x(t)

t

x(t)

Analog Digital

Digital Communication Advantages

• Relatively inexpensive digital circuits may be used;

• Privacy is preserved by using data encryption;

• Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system;

• In long-distance systems, noise dose not accumulate from repeater to repeater. Data regeneration is possible

• Errors in detected data may be small, even when there is a large amount of noise on the received signal;

• Errors may often be corrected by the use of coding.

Disadvantages • Generally, more bandwidth is required than that for analog

systems;

• Synchronization is required.

Deterministic and Random Waveform

A Deterministic waveform can be modeled as a completely specified function of time.

A Random Waveform (or stochastic waveform) cannot be modeled as a completely specified function of time and must be modeled probabilistically.

Basic Properties of the em signal;

where:

𝑣 = 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟𝑥𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔𝑕𝑡 𝑐

𝑣𝑓 =1

𝑘 and k is dielectric constant of a certain medium

• The frequency is the number of cycles (i.e. from A to

B) in a one-second period. It may be the number of voltage polarity alternations or em field oscillations that

takes place in a span of time.

second

11cycle

Hz

• Wavelength is the distance required to complete one cycle at a

particular frequency. (i.e the

distance from point A to B represents

one wavelength)

A B

f

v

here, when phase is at 900

𝑎𝑖𝑛𝑠𝑡 = 𝐴𝑚𝑎𝑥 sin 𝜔𝑡 + 90

A B

Amplitude

Time

Amplitude

0o

90o

180o

270o

360o

• Phase is the location of the travelling wave at a fixed point

in time.

• Amplitude is the maximum

displacement of a continuous wave.

Sample problems:

1. Calculate the wavelength in (a) free space,

(b) transmission line, with dielectric medium

constant of 1.5, corresponding to a frequency

of 27 MHz.

2. Express the positive cosine function

representation of the following signals ;

(a) 𝑣 𝑡 = 50 sin 𝜔𝑡 + 𝜋

(b) 𝑖 𝑡 = 10 1 − 𝑐𝑜𝑠2𝜔𝑡

Bandwidth

• The span of frequencies within the spectrum

occupied by a signal for conveying information

(e.g. music uses 0 to 20 KHz --> BW = 20KHz)

Water

(information)

Bandw

idth

• Sending information in a short amount of time

requires more bandwidth

Standard voice channel bandwidth;

f1 = 300Hz , f2 = 3400Hz

BW = 3400 Hz – 300 Hz

BW= 3.4 kHz

TV broadcasting channel bandwidth;

e.g. ABS-CBN operates at 54 MHz to 60 MHz

BW = 60 MHz – 54 MHz

BW = 6 MHz

Decibels - used in almost every part of electronic communication system to express the ratio of two power levels or voltage levels.

Definition:

- one tenth of a Bel

𝐵𝑒𝑙 = log (𝑃𝑜𝑢𝑡

𝑃𝑖𝑛)

𝐷𝑒𝑐𝑖𝐵𝑒𝑙 =𝐵𝑒𝑙

10= log (

𝑃𝑜𝑢𝑡

𝑃𝑖𝑛)

Example Problem;

1. Find the ratio between P2 and P1, in dB, if

(a) P1 = 2W and P2 = 3W; (b) P1 = 3W and P2 = 2W

dB Gain vs dB Loss

If Po is the output power of a device and Pi is the input power then the Gain in dB is;

𝐴𝑝 𝑑𝐵 = 10 log𝑃𝑜

𝑃𝑖

If Po is less than Pi then the negative gain results to a Loss in the system.

Example:

1. An attenuator has a loss of 26 dB. If the power of 2 W is applied to the attenuator, find the output power.

Other dB notation

dBm – based on the reference 1mW of power at the input.

𝐴𝑝 𝑑𝐵𝑚 = 10 log𝑃𝑜

1𝑚𝑊

dBW – use 1W as reference input power

dBk – use 1kW as reference

Note :

dB+dBk = dBK

dB+dBm = dBm

dB+dBW = dBW

Example Problems:

1. Convert a power level of 5W to (a)dBm and

(b)dBk

2. If a signal with a power level of -12dBm were

applied to the system as shown, What is the

output level of the system?

in Ap Ap out

20dB 15dB

3. Convert 10 dBW to dBm and dBk.

dB in Current and Voltage Gains:

From power formula;

𝑃 = 𝐼2𝑅 =𝑉2

𝑅

It follows that, voltage gain in dB is;

𝐴𝑝 𝑑𝐵 = 10 log

𝑉𝑜𝑢𝑡2

𝑅𝑉𝑖𝑛2

𝑅

𝐴𝑣 𝑑𝐵 = 20log (𝑉𝑜

𝑉𝑖)

Similarly the current gain in dB is given by;

𝐴𝑝 𝑑𝐵 = 10 log𝐼𝑜𝑢𝑡2𝑅

𝐼𝑖𝑛2𝑅

𝐴𝑖 𝑑𝐵 = 20 log𝐼𝑜

𝐼𝑖

Ex. A signal in cable tv system has an

amplitude of 3mV in 75 ohms. Calculate its

level in dBmV and dBm.

Parts of a Communication System

All communication systems contain three main sub systems: Transmitter

Channel

Receiver

Other parts of communication system:

Information Input Source

Output Destination/Sink

Noise

Transmitter Receiver

Block Diagram of A Communication System

TRANSMITTER:

The signal-processing block is used for more efficient transmission.

Examples:

In an analog system, the signal processor may be an analog low-pass filter to restrict the bandwidth of m(t).

In a hybrid system, the signal processor may be an analog-to-digital converter (ADC) to produce digital signals that represent samples of the analog input signal.


TRANSMITTER:

The transmitter carrier circuit converts the processed base band signal into a frequency band that is appropriate for the transmission medium of the channel.

Example:

An amplitude –modulated (AM) broadcasting station with an assigned frequency of 850 kHz has a carrier frequency fc=850kHz. The mapping of the base band input information waveform m(t) into the band pass signal s(t) is called modulation. It will be shown that any band pass signal has the form

𝑠 𝑡 = 𝑅 𝑡 𝑐𝑜𝑠ω𝑐𝑡 + 𝜃𝑡 ω𝑐 = 2𝜋𝑓𝑐𝑡

If R(t)=1 and θ(t) = 0, s(t) would be a pure sinusoid of frequency f=fc with zero bandwidth.


Channel: Channels represents the path in which signals travel from transmitter to receiver. Very general classification of channels are:

Wire: Twisted-pair telephone line, coaxial

cable, waveguide, and fiber-optic cables.

Wireless: Air vacuum, and seawater.

In general, the channel medium attenuates the signal so that the delivered information deteriorated from that of the source. The channel noise may arise

from natural electrical disturbances or from artificial sources.


Receiver: The receiver takes the corrupted signal at the

channel output and converts it to be a base band signal that can be handled by the receiver’s base band processor.

The base band processor cleans up this signal and delivers an estimate of the source information to the communication system output.

In digital systems, the measure of signal deterioration is usually taken to be the probability of bit error P(e) – also called Bit Error Rate (BER) of the delivered data m(t).

In analog systems, the performance measure is

usually taken to be the Signal-to-noise Ratio (SNR) at the receiver output.

How to measure the effectiveness of a

communication system? We can measure the “GOODNESS” of a communication system in many ways:

How close is the estimate to the original signal m(t)? Better estimate = higher quality transmission Signal to Noise Ratio (SNR) for analog m(t) Bit Error Rate (BER) for digital m(t)

How much power is required to transmit s(t)?

Lower power = longer battery life, less interference

How much bandwidth B is required to transmit s(t)? Less B means more users can share the channel Exception: Spread Spectrum -- users use same B.

How much information is transmitted?

In analog systems information is related to B of m(t). In digital systems information is expressed in bits/sec.

Frequency Bands Regulations specify, modulation type, bandwidth,

power, type of information and etc. that a user can transmit over designed frequency bands.

Frequency assignments and technical standards are set internationally by International Telecommunication Union (ITU). Locally, these are set by NTC.

Each nation of ITU retains sovereignty over spectral usage and standards adopted in its territory.

Each nation is expected to abide by the overall frequency plan adopted by ITU.

Assignment:

1. Write the complete list of the VHF and UHF

television channel and its corresponding

operating frequencies.

2. Name the two basic type of spectrum

analyzer, and briefly describe how each

works.

3. What is the difference between the white and

pink noise?

4. Why is the noise power bandwidth greater

than the half power bandwidth of a system?

Noise

• Noise is any unwanted signal (random) that corrupts

and distort the desired signal

• Effects of Noise:

• Noise can cause the user to misunderstand the original signal

• Noise can cause the receiving system to malfunction

• Noise can result in a less efficient system

V

t

Noise Added

General Types of Noise 1. External Noise – noise created and defined

within the property of the channel or medium.

a. Atmospheric Noise – also called “static”

because lightning is the principal source of this

noise.

b. Industrial noise – a man-made noise thru the

intervention of man and/or man’s machine

and equipment.

c. Space Noise – or extra-terrestrial noise comes

from the sun, stars, and other space bodies

which radiates energy.

2. Internal Noise – originates within the communication equipment.

a. Thermal Noise – produced by random motion of electrons in a conductor due to heat.

𝑃𝑛 ∝ 𝐵𝑊 ∝ 𝑇

𝑃𝑛 = 𝑘𝑇𝐵𝑊

Where:

Pn - noise power, in Watts

T – temperature in Kelvin

k – Boltzmann’s constant, 1.38x10-23 J/K

Noise calculation

Noise Voltage;

𝑣𝑛 = 4𝑘𝑇𝐵𝑊𝑅

Due to several sources;

𝑣𝑛= 4𝑘𝑇𝐵𝑊 𝑅1 + 𝑅2 + ⋯ + 𝑅𝑥

𝑣𝑛 = √(𝑣𝑛1 + 𝑣𝑛2 + ⋯ + 𝑣𝑛𝑥)

b. Shot Noise – due to random arrival of

electrons or random variations in current

flow in an active device.

𝐼𝑛 = 2𝑞𝐼𝑜𝐵𝑊

Where:

In – rms noise current, in Ampere

Io – dc bias current , in Ampere

q – charge of an electron, 1.6x10-19 Coulomb

Exercises:

1. A diode noise generator is required to

produce 10 uV of noise in a receiver with an

input impedance of 75 ohms, and a noise

power bandwidth of 200kHz. What is the

current through the diode?

2. The circuit shows two resistors in series at

two different temperatures. Find the total

noise voltage and noise power produced

at the load, over a bandwidth of 100 kHz.

Signal-to-Noise Ratio

dBdBdB NSN

SSNR

log10

Where: S = signal power (in watts)

N = noise power (in watts)

• The SNR is important in determining how well the

system will operate or how successfully the system

can recover a weak signal

f (MHz)

dB

-10 dBm

-50 dBm

Noise floor

Variations of SNR

Ratio of signal plus noise power to noise

power alone. 𝑆 + 𝑁

𝑁

S/N for FM receivers that included distortion

(SINAD). 𝑆 + 𝑁 + 𝐷

𝑁 + 𝐷

Sample Problem:

1. A receiver produces a noise power of

100 mW with no signal. The output level

increases to 10 W when a signal is

applied. Calculate the (S+N)/N in dB.

2. Determine the signal power of a receiver

with SNR = 28 dB. This receives an

equivalent noise power of 500 mW.

Noise Figure Definition:

-a figure of merit which indicates how much a component or series of stages degrades the signal to noise ratio of a system.

𝑁𝐹 =

𝑆𝑁 𝑖𝑛

𝑆𝑁 𝑜𝑢𝑡

Where:

S/Nin - input signal to noise ratio

S/Nout – output signal to noise ratio

S/N in dB; NF(dB) = S/Nin(dB) - S/Nout(dB)

Example:

1. The signal power at the input of the

degenerating circuit in a receiving section is

1500 uW and the noise power received is 5

uW. At the output these values are 100 mW

and 3700 uW, respectively. What is the NF

of the sytem, in dB?

Equivalent Noise Temperature:

𝑁𝐹 =𝑇𝑒𝑞

290+ 1

Where:

NF - noise figure

Teq – equivalent noise temperature

Ex. An amplifier has an equivalent noise

temp. of (a)45K, (b)89oF. Calculate the dB

noise figure.

Information Capacity

• Information capacity is the rate at which data can be

transferred by a communications system

• Shannon’s Theory

N

SBC 1log2

where C = Channel or info capacity (in bits per sec)

B= channel bandwidth (in Hertz)

S = signal power (in watts)

N = noise power (in watts)

Information Measurement Definition: Information Measure (Ij)

The information sent from a digital source (Ij) when the jth massage is transmitted is given by:

where Pj is the probability of transmitting the jth message.

• Messages that are less likely to occur (smaller value for Pj)

provide more information (large value of Ij).

• The information measure depends on only the likelihood of sending the message and does not depend on possible interpretation of the content.

• For units of bits, the base 2 logarithm is used; if natural logarithm is used, the units are “nats”;

if the base 10 logarithm is used, the units are “hartley”.

Information Measurement Definition: Average Information (H)

The average information measure of a digital source is, where m is the number of possible different source messages. The average information is also called Entropy.

Definition: Source Rate (R) The source rate is defined as, where H is the average information T is the time required to send a message.

Information Measurement Ex. Find the information content of message that consists of

a digital word 12 digits long in which each digit may take on

one of four possible levels. The probability of sending any of

the four levels is assumed to be equal, and the level in any

digit does not depend on the values taken on by pervious

digits.

Answer: Possible combinations of 12 digits ( # of possible messages) = 412

Because each level is equally likely,

all different words are equally likely.

Channel Capacity & Ideal Comm. Systems

For digital communication systems, the “Optimum System” may defined as the system that minimize the probability of bit error at the system output subject to constraints on the energy and channel bandwidth.

Q: Is it possible to invent a system with no error at the output even when we have noise introduced into the channel?

Yes under certain assumptions !

According Shannon the probability of error would approach zero, if R<C

Where

R - Rate of information (bits/s) ; C - Channel capacity (bits/s)

B - Channel bandwidth in Hz ; S/N - the signal-to-noise power ratio

Note: Capacity is the maximum amount of information that a particular channel can transmit. It is a theoretical upper limit. The limit can be approached by using Error Correction

Channel Capacity & Ideal Comm. Systems

In analog systems, the OPTIMUM SYSTEM might be defined as the one that achieves the Largest signal-to-noise ratio at the receiver output, subject to design constraints such as channel bandwidth and transmitted power.

Question:

Is it possible to design a system with infinite signal-to-noise ratio at the output when noise is introduced by the channel?

Answer: No!

DIMENSIONALITY THEOREM for Digital Signalling:

Nyquist showed that if a pulse represents one bit of data, noninterfering pulses can be sent over a channel no faster than 2B pulses/s, where B is the channel bandwidth.

Modes of Transmission:

Half-duplex

e.g. Two-way radio

e.g. broadcast radio/TV

Full-duplex e.g. Public-Switched

Telephone Network (PSTN)

sending of information

in one direction only Simplex


in either direction,

but in only one direction at a time


in both directions, simultaneously

• Modulation is the process by which the

communications signal that contains the information

(e.g. voice, data) is combined with another signal (I.e.

carrier)

• The result is a signal at frequencies more

compatible with the application and in a desired part

of the spectrum.

FM Basics (Analog Modulation)

Info Signal = sin 2fm t

t

1

t

A

Carrier = A sin 2fc t

Note: fm << f c

t

A

FM signal = A sin (2fc t + m sin2 f

m t )

freq> fC

freq= fC

freq< fC

Note: m = modulation index

= maximum carrier frequency

shift divided by the frequency of

the modulating signal, fm

t

Vc

Carrier = Vcsin 2f

c t

Note: fm << f

c

Info Signal = Vmsin 2fm t

t

Vm

Note: Vm < Vc

t

Vc

Vc

- Vm

Vc + Vm

envelope

carrier freq, fc

AM Signal

= (Vc+ Vmsin 2 fmt) sin 2fc t

AM Basics (Analog Modulation)

Digital

Modulation

• FSK – Frequency Shift Keying

Binary States: Logic 0 (high), Logic 1(low)

• ASK – Amplitude Shift Keying

Binary States: Logic 0 (absence of the carrier)

• PSK – Phase Shift Keying

Binary States: Logic 0 (0º, -90º), Logic 1(180º, +90º)

Multiplexing and Transmission

Media

In this section:

Multiplexing:

combining several sessions on to one medium

Properties of transmission media:

Characteristics of various transmission media

that impact their suitability for applicaitons.

64

Multiplexing Multiplexing: combining several signals onto

one line. Demultiplexing: taking a multiplexed signal and

recovering its original components

Frequency division multiplexing (FDM): using different frequency ranges for different signals Wave division multiplexing (WDM): same as

FDM, but with optical signals.

Time division multiplexing (TDM): each signal is allocated to a periodic time slot.

65

Frequency Division

Multiplexing Suppose that we have three phone signals that we want to

combine onto one line with higher bandwidth.

Allocate 4 KHz of bandwidth to each signal, which includes a “guard band” of unused frequency range to ensure signals don’t overlap.

Each signal originally uses the range 0.3 – 3.3 KHz.

Transform each signal to a different frequency range:

Signal 1: 20 – 24 KHz channel Use 20.5 KHz to 23.5 KHz, with 0.5 KHz of guard band on

each end.

Signal 2: 24 – 28 KHz

Signal 3: 28 – 32 KHz

At receiver, filters are used to isolate each channel, and then the frequency is transformed back to its original range.

66

FDM

67

FDM applications

High capacity phone lines

AM radio: 530 KHz to 1700 KHz, 10 KHz

bandwidth per station

FM radio: 88 MHz to 108 MHz, 200 KHz

bandwidth per station

TV broadcasts: 6 MHz bandwidth per TV

channel

First generation cell phones: each user gets

two 30 KHz channels (sending, receiving).

68

Wave Division Multiplexing

Essentially the same as FDM, except the

signals are optical and prisms are used to

combine/split signals instead of electrical

components.

Used to combine signals of different

frequencies (i.e. colours) onto one fibre-

optic cable.

69

Time Division Multiplexing

(TDM)

TDM is a digital method, as opposed to FDM which is analog.

Each signal is split into fixed-size units of time, and units from

each signal are sent alternately.

If signal represents bits, use a fixed size block of bits as the

unit.

Suppose that we have 3 signals to combine, and the time unit is

1 ms. Cycle through the signals as follows:

Send signal 1 for 1 ms.



Each signal has its own time slot; if it has nothing to send, the

slot is left empty to preserve synchronization.

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TDM

71

TDM applications Digital Service lines: DS-n

Implemented as telephone lines: T-n

Service Phone line Data rate # of voice channels

(DS-0) standard phone line

64 Kb/s 1

DS-1 T-1 1.544 Mb/s 24

DS-2 T-2 6.312 Mb/s 96

DS-3 T-3 44.736 Mb/s 672

DS-4 T-4 274.176 Mb/s 4032

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