Microwave Communication System

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Wave Propagation through sky, ground, Tropospheric, Repeaters, duct propagation, Fading, Diversity,

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Microwave Communication System Subject: Microwave & Radar Engineering Class: 6 th Sem ECE Presented By Kulwinder Singh Lecturer ECE S. R. S. Govt Polytechnic College for Girls Ludhiana Email: kulwinderpannu@gmail.com Mobile: 97813-00151 03/04/2012 1 PUNJAB EDUSAT SOCIETY (PES)

Introduction :

Introduction The main function of a microwave communication system is to ensure the transmission of microwave signal from transmitter to receiver. The three main mechanisms used are Ground waves Sky waves Tropospheric waves. 03/04/2012 2 PUNJAB EDUSAT SOCIETY (PES)

Background:

Background Earlier system used FDM voice band circuits and used conventional non coherent frequency modulation techniques. Modern systems carry “Pulse Code Modulated TDM voice-band circuits use more modern digital modulated techniques like phase shift keying. 03/04/2012 3 PUNJAB EDUSAT SOCIETY (PES)

Capacity of Microwave Systems:

Capacity of Microwave Systems The capacity of a microwave communication system varies from less than 12 voice band channels to more than 22000 channels. Terminal Station Repeater Station Repeater Station Terminal Station 03/04/2012 4 PUNJAB EDUSAT SOCIETY (PES)

Working of W System:

Working of W System The information originates and terminates at the terminal stations. Repeaters simply relay the information to the next downlink microwave station. The physical location of station should be like that no physical obstruction should prevent transmission of signals. 03/04/2012 5 PUNJAB EDUSAT SOCIETY (PES)

Working of W System:

Working of W System Signal is transmitted through earth’s atmosphere. Hence these systems have obvious advantage of carrying thousands of information channels without the need for physical facilities such as co-axial cables or optical fibers. 03/04/2012 6 PUNJAB EDUSAT SOCIETY (PES)

Analog Link:

Analog Link Microwave Transmitter F1 FM Modulator FDM Terminal IF 70MHz 0-6 MHz Baseband Telephone Channels Microwave Receiver Microwave Transmitter F2 Microwave Link 4GHz Microwave Link 4GHz Microwave Receiver FM Demodulator Microwave Receiver Telephone Channels IF 70MHz 0-6 MHz Baseband 03/04/2012 7 PUNJAB EDUSAT SOCIETY (PES)

Analog Link Transmitter:

Analog Link Transmitter It consists of two terminal stations and one or more repeaters. At the sending end several input channels say telephone and television are frequency multiplexed within baseband of 0-6MHz. This signal is then used to modulate a 70MHz IF signal. 03/04/2012 8 PUNJAB EDUSAT SOCIETY (PES)

Analog Link Transmitter:

Analog Link Transmitter This IF is finally up-converted to 4GHz signal. This signal is amplified and fed through a directional antenna towards a repeater station which may at a distance of 50km. 03/04/2012 9 PUNJAB EDUSAT SOCIETY (PES)

Analog Link Repeater:

Analog Link Repeater At the repeater station, the signal is down-converted to the IF, because it is easier to amplify IF signal with narrowband amplifier. Then this IF signal is amplified with the help of a narrowband amplifier. This signal is then again up-converted to new higher frequency (F2) to avoid interference. Alternate repeaters use alternate frequencies. 03/04/2012 10 PUNJAB EDUSAT SOCIETY (PES)

Analog Link Receiver:

Analog Link Receiver At the receiver side signal is first down-converted to IF frequency. Amplified and again down-converted to baseband signal. Finally these signals are de-multiplexed and received by the users. 03/04/2012 11 PUNJAB EDUSAT SOCIETY (PES)

Repeaters:

Repeaters W TX RX TX W Repeater W RX 03/04/2012 12 PUNJAB EDUSAT SOCIETY (PES)

Type of Repeaters:

Type of Repeaters Depending upon the stage at which they amplify the signal, the repeaters can be classified into following three types IF repeaters. Baseband Repeaters RF repeaters. 03/04/2012 13 PUNJAB EDUSAT SOCIETY (PES)

IF Repeaters:

IF Repeaters The IF repeater works at the level of intermediate frequency. The received signal is down-converted to IF level only and amplified. Finally this signal is again up-converted to RF and retransmitted. 03/04/2012 14 PUNJAB EDUSAT SOCIETY (PES)

IF Repeater:

IF Repeater BPF Mixer IF amp & filter IF amp & filter Mixer BPF RF Power Amplifier Microwave Generator RF RF IF IF RF RF RF IF signals 03/04/2012 15 PUNJAB EDUSAT SOCIETY (PES)

Baseband Repeater:

Baseband Repeater In this type of repeater signal is fully down-converted to the level of baseband signals. These signals are then amplified and converted back to IF and finally to RF signals. Then this amplified signal is retransmitted. 03/04/2012 16 PUNJAB EDUSAT SOCIETY (PES)

Baseband Repeater:

Baseband Repeater BPF Mixer IF amp & filter IF amp & filter Mixer BPF RF Power Amplifier Microwave Generator RF RF IF IF RF RF RF Baseband amplifier and equalizer 03/04/2012 17 PUNJAB EDUSAT SOCIETY (PES)

RF Repeaters:

RF Repeaters In radio frequency repeaters the incoming signal is not down-converted to IF or baseband levels. Instead it is converted to output radio frequency. Then it is amplified. And finally retransmitted. 03/04/2012 18 PUNJAB EDUSAT SOCIETY (PES)

RF Repeater:

RF Repeater BPF Mixer BPF RF Power Amplifier Microwave Generator RF RF in RF out = RF in  LO RF out RF out 03/04/2012 19 PUNJAB EDUSAT SOCIETY (PES)

Troposphere and its Properties:

Troposphere and its Properties Earth Troposphere 0-12km Stratosphere 12-50km Mesosphere 50-85km Thermosphere 50-85km 03/04/2012 20 PUNJAB EDUSAT SOCIETY (PES)

Troposphere:

Troposphere It is lowermost layer of earth’s atmosphere. All weather and human activities take place in it. Green house effect also occur in troposphere. This layer extends to an altitude of 16 km - 18 km over tropical region to less than 10km over poles. 03/04/2012 21 PUNJAB EDUSAT SOCIETY (PES)

Troposphere:

Troposphere About 80% of total air mass is concentrated in this region. Its name came from Greek word “ tropos ” meaning “mixing”, so this region is constantly in motion. Temperature in this layer decreases with increase in height and falls to -68 0 F at the top. 03/04/2012 22 PUNJAB EDUSAT SOCIETY (PES)

Tropospheric Duct Formation:

Tropospheric Duct Formation Under normal conditions temperature decreases with height in this layer. At times, however, an unusual situation develops in which layers of warm air are formed above layers of cool air. This condition is known as TEMPERATURE INVERSION. 03/04/2012 23 PUNJAB EDUSAT SOCIETY (PES)

Tropospheric Duct Formation:

Tropospheric Duct Formation These temperature inversions cause channels, or ducts, of cool air to be sandwiched between the surface of the Earth and a layer of warm air. If a transmitting antenna extends into such a duct of cool air and radio wave enters the duct at a very low angle of incidence, vhf and uhf transmissions may be propagated far beyond normal line-of-sight distances. 03/04/2012 24 PUNJAB EDUSAT SOCIETY (PES)

Tropospheric Duct Formation:

Tropospheric Duct Formation These long distances are possible because of the different densities and refractive qualities of warm and cool air. The sudden change in density when a radio wave enters the warm air above a duct causes the wave to be refracted back toward Earth. When the wave strikes the Earth or a warm layer below the duct, it is again reflected or refracted upward . 03/04/2012 25 PUNJAB EDUSAT SOCIETY (PES)

Duct Propagation:

Duct Propagation 03/04/2012 26 PUNJAB EDUSAT SOCIETY (PES)

Duct Propagation:

Duct Propagation 03/04/2012 27 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation Virtually all weather phenomena occur in troposphere. many changes in weather phenomena cause variations in humidity and an uneven heating of the Earth's surface. As a result, the air in the troposphere is in constant motion. This motion causes small turbulences, or eddies, to be formed, as shown by the bouncing of aircraft entering turbulent areas of the atmosphere. 03/04/2012 28 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation These turbulences are most intense near the Earth's surface and gradually diminish with height. They have a refractive quality that permits the refracting or scattering of radio waves with short wavelengths. This scattering provides enhanced communications at higher frequencies. 03/04/2012 29 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation Radio waves of frequencies below 30 megahertz normally have wavelengths longer than the size of weather turbulences. These radio waves are, therefore, affected very little by the turbulences. 03/04/2012 30 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation On the other hand, as the frequency increases into the VHF range and above, the wavelengths decrease in size, to the point that they become subject to tropospheric scattering. The usable frequency range for tropospheric scattering is from about 100 megahertz to 10 gigahertz. 03/04/2012 31 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation When a radio wave passing through the troposphere meets a turbulence, it makes an abrupt change in velocity. This causes a small amount of the energy to be scattered in a forward direction and returned to Earth at distances beyond the horizon 03/04/2012 32 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation This phenomenon is repeated as the radio wave meets other turbulences in its path. The total received signal is an accumulation of the energy received from each of the turbulences. 03/04/2012 33 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation This scattering mode of propagation enables VHF and UHF signals to be transmitted far beyond the normal line-of-sight. To better understand how these signals are transmitted over greater distances, first consider the propagation characteristics of the space wave used in VHF and UHF line-of-sight communications 03/04/2012 34 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation When the space wave is transmitted, it undergoes very little attenuation within the line-of-sight horizon. When it reaches the horizon, the wave is diffracted and follows the Earth's curvature. Beyond the horizon, the rate of attenuation increases very rapidly and signals soon become very weak and unusable. 03/04/2012 35 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation Tropospheric scattering, on the other hand, provides a usable signal at distances. This is because of the height at which scattering takes place. The turbulence that causes the scattering can be visualized as a relay station located above the horizon. 03/04/2012 36 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation It receives the transmitted energy and then reradiates it in a forward direction to some point beyond the line-of-sight distance. A high gain receiving antenna aimed toward this scattered energy can then capture it. The magnitude of the received signal depends on the number of turbulences causing scatter and the gain of the receiving antenna. 03/04/2012 37 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation The scatter area used for tropospheric scatter is known as the scatter volume . The angle at which the receiving antenna must be aimed to capture the scattered energy is called the scatter angle . 03/04/2012 38 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation 03/04/2012 39 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation The signal take-off angle (transmitting antenna's angle of radiation) determines the height of the scatter volume and the size of the scatter angle. A low signal take-off angle produces a low scatter volume, which in turn permits a receiving antenna that is aimed at a low angle to the scatter volume to capture the scattered energy. 03/04/2012 40 PUNJAB EDUSAT SOCIETY (PES)

Troposcatter Propagation:

Troposcatter Propagation As the signal take-off angle is increased, the height of the scatter volume is increased. When this occurs, the amount of received energy decreases. There are two reasons for this: (1) scatter angle increases as the height of the scatter volume is increased; (2) the amount of turbulence decreases with height. 03/04/2012 41 PUNJAB EDUSAT SOCIETY (PES)

Fading:

Fading Fading is deviation of the attenuation that a signal experiences over certain propagation media. The fading may vary with time, geographical position or radio frequency, and is often modelled as a random process. 03/04/2012 42 PUNJAB EDUSAT SOCIETY (PES)

Frequency Diversity:

Frequency Diversity 03/04/2012 43 PUNJAB EDUSAT SOCIETY (PES)

Space Diversity:

Space Diversity 03/04/2012 44 PUNJAB EDUSAT SOCIETY (PES)

Space Diversity:

Space Diversity 03/04/2012 45 PUNJAB EDUSAT SOCIETY (PES)

Microwave Protection Switching Arrangement:

Microwave Protection Switching Arrangement 03/04/2012 46 PUNJAB EDUSAT SOCIETY (PES)

PowerPoint Presentation:

THANKS 03/04/2012 47 PUNJAB EDUSAT SOCIETY (PES)

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