MODULE 5

Radio communication,Satellite communication

RADIO COMMUNICATION

Basic Communication System

Block diagram of basic communication system

1. Transmitter: Convert the message signal produced by the source of information into a form suitable for transmission through the channel
2. Channel: Medium connecting transmitter and receiver
3. Receiver: It reconstructs a recognizable form of the original message signal for delivering it to the user of information

Frequency band allocation

Radio

• AM Broadcast
  • Medium wave 535-1605kHZ
  • Short wave 3-30 MHz
• FM Broadcast 88-108 MHz

Television

• VHF (Lower) Band I 47-68 MHz
• VHF (Upper) Band II 174-230 MHz
• VHF (Lower) Band III 470-598 MHz
• VHF (Upper) Band IV 606-870 MHz

Satellite

Modulation

• Transmission of information signals over long distances has some practical difficulties.
• It is often necessary to modulate the source information into a higher frequency analog signal called carrier.
• The carrier signal carries the information through the channel.
• The information signal modulates the carrier by changing its amplitude, frequency or phase

“Modulation is the process of changing any of the characteristics of the carrier signal in accordance with the information signal”

Need for Modulation

1. To reduce antenna height:
   • The minimum height of an antenna required for transmission and reception is given as L=λ/4
   • 

• • Therefore at low frequency the height of the antenna is high
  • If frequency becomes too small then length of the antenna becomes a large value. Such an antenna is impractical

1. Avoid mixing of signals:
   • When several signals get radiated in same frequency range from different sources, they may get mixed up.
   • So we need to modulate the signal with different carrier frequency

3. Increases the range of communication:

The energy of any wave depends on its frequency. The larger the frequency of the wave, greater the energy associated with it.

4. Allows multiplexing of signals:

• • Modulation permits transmission of two or more signals simultaneously over the same channel.

Types of Modulation

1. Amplitude Modulation
2. Frequency Modulation
3. Phase Modulation

Amplitude Modulation

Amplitude modulation is the process of varying the amplitude of the carrier wave in accordance with the instantaneous values of the information signal

Amplitude Modulation

Expression for Amplitude Modulated Wave

„em‟ is the instantaneous amplitude of the modulating signal (information signal) and „ec‟ representing the instantaneous amplitude of the carrier signals.

e_c = E_c cosω_ct

e_m = E_m cosω_mt

Amplitude of the modulated signal is given by

e_am = (e_m + E_c) cosω_ct …………………..(1)

The shape of the modulated signal is called AM envelope and is given

E_env = E_c + e_m

e_am = (E_c + E_m cosω_mt ) cosω_ct

The ratio of the maximum amplitude of the modulating signal to the maximum amplitude of the carrier signal is defined as Modulation index (m)

The total power of the modulated wave is given by,

P_t = P_LSB + P_c + P_USB

• Hence Bandwidth of AM wave is
  • BW= (f_c+f_m ) – (f_c-f_m ) =2 f_m
  • Thus the bandwidth of AM wave is twice the frequency of modulating signal.

AM Power Distribution

The total power of the modulated wave,

Where, Pc = carrier power
Ec = Maximum carrier voltage
R = load resistance

AM Power Distribution

Frequency spectrum of AM

• Modulation index (m) is given by, m = E_m/E_c

Amplitude Modulated Wave

Frequency Modulation

Frequency Modulation is process of varying the frequency of the carrier signal in accordance with the instantaneous amplitude of the modulating signal

• In FM, amplitude of the carrier remains constant.
• The variation in the carrier frequency from the unmodulated carrier frequency is called as frequency deviation (δ)
• The information is contained in the frequency deviation of the FM wave and frequency deviation is proportional to the amplitude of the modulating signal.

Carrier Signal

Modulating Signal

Modulated Signal

Mathematical Representation of FM Wave

Let „em‟ be the instantaneous amplitude of the modulating signal (information signal) and „ec‟ representing the instantaneous amplitude of the carrier signals.Then ,

e_c = E_c cosω_ct

e_m = E_m cosω_mt

Instantaneous deviation in frequency from the carrier is proportional to the amplitude of modulating signal,δ=k.em (k=a known constant)

• Then the instantaneous carrier frequency,

f_i = f_c + k e_m

f_i = f_c + k E_m cosω_mt

Maximum frequency deviation, Δf = kE_m

Therefore f_i = f_c + Δf cosω_mt

Bandwidth Requirement of FM

Modulation index (m_f ):

Radio Receivers

• A radio receiver picks up the signals transmitted by the transmitter with in the receiving stage.
• It converts the received modulated signal in to the original signal.
• There are mainly 2 type of receivers
  • Tuned Radio Frequency Receivers-single stage demodulation
  • Super heterodyne Receiver- converts the incoming station frequency to a low frequency called Intermediate frequency and then demodulates.

AM Super Heterodyne Receiver

• It is one of the AM receiver
• Uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency
• Provides superior selectivity and sensitivity compared with simpler designs.

Block Diagram of super heterodyne receiver

Block Diagram of super heterodyne receiver

• • The important characteristics that determines the quality of receiver are
    • Selectivity
    • Sensitivity
    • Fidelity
  • Selectivity: is the ability to select desired signal frequency and to reject unwanted signals.
  • Sensitivity: is the ability to respond to weak signals.
  • Fidelity: is the ability of a receiver to reproduce all audio frequencies with which the carrier is modulated.

Antenna & RF Amplifier:

• The radio signal is received using a suitable antenna
• The output of the antenna may be very small(few microvolts) so, it is amplified using a radio frequency (RF) amplifier

Local oscillator and mixer:

• The inputs to the mixer are signal from the RF amplifier and a sine wave from a variable frequency oscillator known as the local oscillator (LO)
• The mixer generates both sum and difference beat frequency signals
• Mixer output includes the original RF signal at fRF, the local oscillator signal at fLO, and the two new heterodyne frequencies fRF + fLO and fLO – fRF.
• Intermediate frequency is the difference between the oscillator frequency and that of radio signal i.e.,

I.F = f_LO – f_RF

• The intermediate frequency is standardized as 455kHz for all AM broadcast receivers.
• The difference frequency signal i.e, 455 kHz is selected using tuned circuits and applied to I.F amplifier for further amplification

I.F Amplifier:

• I.F Amplifier amplify signal at I.F = 455 kHz
• The output of the IF amplifier is given to demodulator or detector.

Demodulator or Detector:

• The audio frequency components or the modulating signal are extracted and the RF components are discarded. This process is called demodulation or detection

Audio Frequency Amplifier:

• Output of the demodulator is amplified by the audio frequency amplifier. It consists of voltage amplifier and power amplifier stages.

FM Radio Receiver

FM Radio Receiver

• The antenna picks up modulated signal transmitted from various stations.
• RF amplifier is tuned to the required incoming signal frequency.
• RF amplifier also amplifies the incoming weak signal and provides matching between the antenna and the receiver stage.
• The output of the RF amplifier is mixed with a locally generated signal in a mixer circuit.
• Incoming signal is converted into an Intermediate frequency.

FM Radio Receiver

• This IF signal is then amplified by an IF amplifier.
• This signal is applied to an amplitude limiter.
• This removes any amplitude variations due to noise and improves the signal to noise ratio.
• Output signal of Amplitude limiter is applied to RF demodulator.
• It identifies the frequency variations and recover the audio signal from the modulated wave.

Phase Modulation

• Phase Modulation is the process of varying the phase of the carrier wave in accordance with the instantaneous amplitude of the modulating signal

Limitations of Amplitude Modulation

• Efficiency is low because the side band power is one third of the total power.
• Long distance communication is not possible
• Any noise produce change the amplitude of the AM wave. So, the received signal is noisy and lacks audio clarity.

Comparison between AM and FM

FM has several advantages over AM

1. No wastage of power & efficiency is high
2. FM transmission can cover wide area.
3. Highly immune to noise
4. Co-channel interference is less
5. Amplitude is constant, which is independent of modulation index

Disadvantages of FM

1. Bandwidth requirement is high
2. Complex and expensive transmitting and receiving equipment
3. Since FM uses high frequency range, area of reception is limited only to line of sight.

Satellite communication

Satellite communication

• Most recent and fastest growing communication method which can telecast programmes all over the world.
• A communication satellite is a special man made vehicle placed in orbit around the earth that carries receiver and transmitter equipment.
• Uplink– Transmission from an earth station to the satellite
• Downlink-Transmission from a satellite to the earth station
• Transponder– Electronic system that takes an uplink signal converts it to a downlink signal
• Satellite orbiting the earth stays in position because the centripetal force on the satellite balances the gravitational force of the earth.

Satellite communication system

Satellite communication system

• Earth station picks up signals and they are transmitted towards the satellite at an uplink frequency.
• Uplink frequency- 5.9-6.4 GHz (C- band)
• Signal from earth station is fed to the transponder

Transponder

• Combination of receiver, amplifier and transmitter
• Receiver amplifies the signal and converts into downlink (3.7- 4.2 GHz)

Satellite system Link Models

• • A satellite system consists of 3 basic sections
    1. Uplink Model
    2. Transponder
    3. Downlink Model

Satellite uplink model

• • The modulator converts the input baseband signals to any of the modulated intermediate frequency.
  • The up converter converts the IF in to appropriate RF carrier frequency.
  • The High Power Amplifier ( HPA ) provides an appropriate gain and output power to propagate the signal to the satellite transponder.

Transponder

• The input BPF limits the total noise applied to Low Noise Amplifier.
• The frequency translator converts the high band uplink frequency to low band downlink frequency.
• The low power amplifier amplifies the RF signal for transmission through the downlink antenna to earth station receivers.

Satellite downlink model

Different types of satellites based on orbit

1. Low Earth Orbit (LEO)
2. Medium Earth Orbit (MEO)
3. Geostationary (GEO)

Geostationary Satellite

• • Geostationary (GEO) – Earth-orbiting satellite, Placed at an altitude of approximately 35800 km directly over the equator
  • It revolves in the same direction the earth rotates
  • One orbit takes 24 hours, the same length of time as the earth requires rotating once on its axis
  • Geostationary- appears nearly stationary in the sky as seen by a ground based observer
  • Covers one third of the earth
  • Using three satellite, it is possible to have global coverage
  • It can be accessed using a directional antenna, usually a small dish, aimed at the spot in the sky where the satellite appears.

Three geostationary satellites cover whole globe

Geostationary Satellite..

Advantage

• An directional antenna can be aimed and then left in position without further adjustment.
• Since highly directional antennas can be used, interference from surface based sources, and from other satellites is minimized.

Limitations

• The number of satellites that can be maintained in geostationary orbits without mutual conflict is limited
• Long round trip propagation delay

{distance that an electromagnetic wave must travel to and from a geostationary satellite is 71,600km so it takes 270ms between 2 ground stations}

satellite communication

• • Advantages
    • Satellite to satellite communication is very precise
    • Higher coverage area than terrestrial transmission
    • High flexibility
  • Dis-advantage
    • Satellite launching is costly