EEE 352 Analog Communication Systems

ELECTRICAL ENGINEERING DEPT

COMSATS INSTITUTE OF INFORMATION TECHNOLOGY

Introduction to Communication Systems

Lesson-1

Communication System

• Communication is the process of exchanging information between source and destination

• Routing of information requires a communication link - Channel to transmit information between source and destination.

• In past ages communication is carried over by runners, torches, pigeons etc. Such mediums are now obsolete in modern communication systems.

• Communication engineering deals with transmitting information through electrical signals, i.e. information or message such as spoken words, photographs, live scenes and sounds are first converted to electrical signals before being transmitted on communication link to receiver or destination.

• Electrical communication is reliable, fast and economical at the same time. Modern communication has applications such as e-banking, e-shopping, teleconferencing etc a possible reality.

Block Schematic of Basic Communication System

Baseband signal Recovered Baseband signal

Figure: Above depiction shows subsystems of a basic communication system in which sending receiving and processing of information is in electrical form.

Constituents of communication systems

(i) Source (input message) Source originates a message in non-electrical form such as human voice,

live scene, sound, data etc. Input message can be:

• Analog – A signal whose amplitude can take on any value in continous range i.e. analog signal amplitude can have infinite number of values

• Digital – amplitude limited to finite set of values e.g. binary signals have only two values a digital signal with M symbols is called M-ary signal (M = 2 is a binary signal).

Analog Signals An analog signal is a smoothly and continuously varying voltage or current. Examples are:

Sine waveVoiceVideo (TV)

Figure : Analog signals (a) Sine wave “tone.” (b) Voice.

• Digital Signals Digital signals change in steps or in discrete increments. Most digital signals use binary or two-state codes. Examples

are: Telegraph (Morse code) Serial binary code (used in computers)

Figure: Digital signals (a) Telegraph (Morse code). (b) Serial binary code.

(ii) Input Transducer:

A device that converts energy from one form to another.Convert an input signal into an electrical waveform.

Example: microphone converts human voice into electrical signal referred to as the baseband signal or message signal.

inputtransducer

Input messageBaseband/message signal

Eg. voice microphone Electrical signal

Principle: sound moves the cone and the attached coil of wire moves in the field of a magnet. The generator effect produces a voltage which "images" the sound pressure variation - characterized as a pressure microphone.

(iii) Transmitter (Tx):

Modifies or converts the baseband signal into format appropriate for efficient channel of transmission.

Example: If the channel is fiber optic cable, the transmitter converts the baseband signal into light frequency and the transmitted signal is light.

Transmitter also use to reformat/reshape the signal so that the channel will not distort is as much - Signal Conditioning using a pre emphasizer which is a low pass filter which limits the signal bandwidth

Modulation takes place in the transmitter. It involves static variation of amplitude, phase or frequency of the carrier in accordance to a message signal - Modulation is based on the principle of translating low frequency baseband signal to a higher frequency spectrum using modulators eg AM,FM modulators if baseband signal is analog otherwise digital modulation schemes are employed.

Transmitters may also include other subsystems for example sampler, quantizer and coder for transmission of digitized signals.Tx

transmitted signalBaseband/message signal

Optical signal

Modulated Stage

Carrier Oscillator

Power Amplifier Stages

Voltage Amplifier Stages

Baseband signal

s(t)

c(t)

x(t)

Modulated signal To channel

Amplifier Stages

Fig: Block Schematic Representation of an Analog Transmitter

The power of the modulated signal is amplified enough to reach the receiver stage of communication system before transmitted on channel.

(iv) Channel:

A medium through which the transmitter output is sent.

Divided into 2 basic groups:•Guided Electromagnetic Wave Channel – eg. wire, coaxial cable, optical fiber•Electromagnetic Wave Propagation Channel – eg. Wireless broadcast channel, mobile radio channel, satellite etc.

Introduces distortion, noise and interference – in the channel, transmitted signal is attenuated and distorted. Signal attenuation increase along with the length of channel.

This results in corrupted transmitted signal received by receiver, Rx

channel

Distortion & Noise

Transmitted signal Received signal

Communication channel can be modeled as a filter that changes the characteristic spectrum of baseband signal. This change is termed as channel distortion which is a result of different amplitude attenuation and phase shift of each frequency component of baseband signal.

• The signal is not only distorted by channel but also contaminated by additive noise, which is random and unpredictable.

• Causes: a) External – man made nearby noises, automobile ignition

radiation, florescent light, natural noise from lightning. b)Internal - thermal motion of electrons in conductors, diffusion or

recombination of charged carriers in semiconductor electronics etc • Additive noise have the effect of reducing intelligibility of

recovered baseband signal and to reduce signal-to-noise ratio.• Proper care techniques may minimize noise level to a certain

extent but it can never eliminate its effect on channel completely.

(v) Receiver (Rx)

Receiver decodes the received signal back to message signal – i.e it attempts to translate the received signal back into the original message signal sent by the source.

Reprocess the signal received from the channel by undoing the signal modification made by transmitter and the channel.Extract the desired signal from the received signal and convert it to a form that suitable for the output transducer – demodulator.

Demodulation takes place in the receiver.

(vi) Output transducer

Convert electrical signals to its original waveform eg loudspeaker.

14

RxOutput signalReceived signal Output

transducer

Output message

voicespeaker

Modes of communication

Broadcasting

Involves the use of a single powerful transmitter transmit to many receivers. Demodulation takes place in the receiver.

Information-bearing signals flow in one direction

Eg. TV and radio (Simplex)

ii. Point to point Communication

Where a communication process takes place over a link between a single transmitter and a receiver.

Information-bearing signals flow in bidirectional, which requires the use of a transmitter and receiver at each end of the link

Eg. Telephone (Full Duplex) and walkie talkie (Half Duplex)

Noise Immunity of Digital Signals

• Digital messages are transmitted using finite set of electrical waveforms.

• For example a binary Morse Code with only Mark and Space can be represented only two pulses say mark is A/2 and space is –A/2

• As can be seen clearly the data can be recovered correctly as long noise and distortion is within limits.

• Regenerative repeater stations placed along the path of communication channel allow clean detection of transmitted pulses.

• Thus preventing accumulation of noise along the signal path by periodically cleaning the pulses at repeater stations.

Noise in Analog Signals

• In contrast of digital signals waveforms, analog messages are important and a slight interference in the message will cause error in received signal.

• Noise adds up along the channel and may rise against the signal power thus decreasing the SNR of signal transmission.

• Amplification is of little help since it will tend to increase the noise level also along the signal power, further deteriorating the quality of received signal.

• Thus distance become a limiting factor of analog communication systems.

A/D and D/A• Analog to Digital conversion; Digital to Analog conversion

– Gateway from the communication device to the channel– Can be achieved by Sampling and Quantization– (Read book: pg 5-7)

A/D and D/A (cont)

• Quantization

Modulation• The basic idea here is to superimpose the message signal in

analog form on a carrier which is a sinusoid of the form

ACos(wt + φ) • There are three quantities that can be varied in proportion

to the modulating signal: the amplitude, the phase, and frequency.

• The first scheme is called Amplitude Modulation and the second two are called Angle Modulation schemes

Why Modulate

• Antenna size is a major concern• The radiating antenna should be (one tenth=0.1) or more of

the wavelength• For a speech signal (100 to 3000 Hz) corresponding

wavelength will be 3000 to 100 km• For 1MHz signal you need antenna size of only 30 meter

λ = v / f• Where v = 3x108 m/s

Why Modulate

• Simultaneous Transmission of several Signals

• Frequency Division Multiplex (FDM)

• Time Division Multiplex (TDM)

SNR, Bandwidth and Rate of Communication

1. Signal to Noise Ratio (SNR):

SNR is defined as the ratio of signal power to noise power. Noise distorts the signal that is accumulated along the path.

The dB value is calculated by taking the log of the ratio of the measured or calculated power (PS) w.r.t a reference power (PN) level.Commonly referred to as the power ratio form for dB

It is normally measured in Decibel (dB), defined as 10 times the algorithm (to base 10) of the power ratio.

Eg.: SNR of 10, 100 and 1000 correspond to 10, 20, and 30dBs, respectively.

dBm is a dB level using a 1mW reference.

Signal power, S related to the quality of transmission.

dBRV

RV

P

P

outn

ins

n

s

/

/log10log10

2

2

SNRdB

Example – A receiver produces a noise power of 200mW with no signal. The output level increases to 5 W when a signal is applied. Calculate SNR as in dBs.

Ans: 13.97 dBs

2 Bandwidth

Bandwidth is that portion of the spectrum occupied by a signal.

Specifically, bandwidth is the difference between the upper and lower frequency limits of the signal or the equipment operation range.

Figure below, shows the bandwidth of the voice frequency range from 300 to 3000Hz. The upper frequency is f2 and the lower frequency is f1. The bandwidth, then is 2700Hz

BW = f2 – f1

Bandwidth is the frequency range over which equipment operates or that portion of the spectrum occupied by the signal. This is the voice frequency bandwidth.

Bandwidth (B) of a channel is the range of frequencies that it can transmit with reasonable fidelity.

Bandwidth of an information signal is the difference between the highest and lowest frequencies contained in the information.

Bandwidth of a communication channel is the difference between the highest and lowest frequencies that the channel will allow to pass through it (ie: its pass band).

Data rate proportional to bandwidth

Randomness

• Noise is a limiting factor in communication systems

• Randomness is because of Noise, which is the essence of Communication System

Rate of Communication

Rate of information transmission is directly proportional with its bandwidth

Shannon limit for information capacity, CC(bps) = B log2 (1 + SNR)

Where C = information capacity (bps)

B = bandwidth (Hz) SNR = signal to noise ratio (no unit)

This is the upper limit for channel capacity for given bandwidth B and signal-to-noise ration SNR for which channel can pass Information with probability of error approximately equal to zero

Example 1 - For a standard telephone circuit with a SNR of 30dB and a bandwidth of 2.7 kHz, determine the Shannon limit for information capacity. Ans: 26.919 kbps

Example 2 – The telephone channel has a bandwidth of about 3kHz. Calculate the capacity of a telephone channel that has an SNR of 1023.Ans: 30 kbps