# Communication system

All modes of long distance communication have been super ceded by electrical, electronic and optical modes. A communication system acts as a messenger. It consists of transmitter, communication channel and a receiver. A message signal has to be transformed appropriately to make it capable to transmission. The message signals for communication can be analog signals or digital signals. We cannot transmit an electronic signal in the audio frequency range (20 Hz to 20 kHz) over a long distance because.
1) The antenna or aerial should have a size comparable o the wavelength of signal (≈ 15 km)- which is too large.
2) Power of radiated 1/λ2. For good transmission, we need high powers and hence smaller wavelength or higher frequency.
3) Low frequency signals transmitted as such will get mixed up and there is no way to distinguish between them. Hence to transmit audio frequency signals, we use a high frequency signal called the carrier wave. The message signal or information is mounted on the carrier wave suitably. This is called modulation.

### Modulation

Modulation is the phenomenon of superimposing the audio frequency signals (called modulating signals) on a radio frequency carrier wave. If a career wave is represented by the equation.
vc = Vc sin(ωct+ɸ)
where Vc is maximum voltage, vc is instantaneous voltage at time t, ωc is angular frequency and ɸ is phase angle with respect to reference. Any of the three characteristics/parameters of the carrier wave may be varied by the modulating signal giving rise to following three types of modulation:
1) Amplitude modulation (AM)
2) Frequency modulation (FM)
3) Phase modulation (PM)

### Demodulation

It is the phenomenon of recovering the original modulating signals from the modulated wave, after transmission. Tuned radio frequency receiver and super heterodyne receiver are two common types of receivers.

### Data transmission

It refers to the transmission of data and data retrieval refers to the reception of the data. The term data is applied to a representation of facts, concepts or instructions suitable for communication, interpretation or processing by human beings or by automatic means. In most cases, data consist of pulse type of signals.

### Modem

A modem is a modulating and demodulating device. It connects one computer to another across ordinary telephone lines.

### Space communication

1) Space communication refers to the sending, receiving and processing of information through space.
2) The information (in the form of sound waves) can be sent from one place to another by superimposing it on undamped electromagnetic waves of high frequency called carrier waves.
3) With space communication, the information can be passed on from one place to another on surface of earth in extremely short interval of time.

### Ground wave propagation

1) The radio waves which travel through atmosphere following the surface of earth are known as ground waves or surface waves and their propagation is called ground wave propagation or surface wave propagation.
2) Ground waves or surface waves progress along the surface of earth. The ground wave or surface wave is vertically polarised to prevent short circuiting of the electric component of electromagnetic waves. The wave induces currents in the ground over which it passes and loses some energy by absorption. Apart from it, as ground waves or surface waves propagates over the surface of earth, the wave front of the wave gradually tilts over the surface of the earth and tilt of the wave front of the wave increases as the wave propagates over the earth.
3) It is due to this tilt that, the propagation of ground wave is limited.
4) The ground wave transmission becomes weaker with increase in frequency because more absorption of ground waves takes place at higher frequency during propagation through atmosphere.
5) The ground wave propagation is suitable for low and medium frequency. i.e. 2 MHz only.
6) The ground wave propagation is generally used for local band broadcasting and is commonly called medium wave.
7) The maximum range of ground or surface wave propagation depends upon:
a) The frequency of radio waves and
b) Power of transmitter
c) The range of transmission of ground wave propagation can be increased by increasing the power of transmitter which is so only for very low frequency band.

### Sky wave propagation

1) The sky waves are the radio waves frequency between 2 MHz to 30 MHz.
2) These radio waves can propagate through atmosphere and are reflected back by the ionosphere of earth’s atmosphere.
3) The sky waves can go from transmitter antenna to receiver antenna while travelling through sky and after reflection from ionosphere. Hence their propagation is known as sky wave propagation or ionosphere propagation.
4) In a single reflection from the ionosphere, the sky waves cover a distance not more than 4000 km.
5) With the help of sky wave propagation, communication over a very long distance around the globe is possible.
6) The signals received due to sky wave propagation are subjected to fading in which the strength or signal varies with time.

### Space wave propagation

1) The space waves are the radio waves of very high frequency (i.e. between 30 MHz to 300MHz or more).
2) The space waves can travel through atmosphere from transmitter antenna to receiver antenna either directly or after reflection from ground in the earth’s troposphere region. That is why the space wave propagation is also called as tropospherical propagation.
3) The space wave propagation is also called as line of sight propagation.
4) The space wave propagation is limited due to the line of sight distance and by the curvature of the earth. The line of the sight distance is the distance between transmitting antenna and receiving antenna at which they can see each other which is also called range of communication.
5) The range of communication of space wave propagation can be increased by increasing the height of transmitting and receiving antenna.
6) The space wave communication is utilized in television communication, radar communication and micro wave communication.
7) Television signals have the frequency range 80 MHz to 200 MHz.
8) The reception of television signals is possible either
a) By using communication geostationary satellite which reflects the signals to earth
b) By using tall antenna which may directly intercept the signals coming from transmitter antenna.

### Microwaves propagation

1) Microwaves are electromagnetic waves of frequency 100 to 300 GHz; greater than those of T.V. signals. The wavelength of these waves is of the order of a few millimeters.
2) Microwaves can be transmitted as beam signals in a particular direction, much better than radio waves.
3) There is no spreading or bending of microwaves around the corners of any obstacle coming in their way.
4) Microwave transmission on the surface of earth is possible if the transmitting and receiving antenna is in the line of sight.
5) In order to increase the range of microwave transmission, on the surface of earth, we erect number of antennas at suitable distance called repeaters, in between transmitting and receiving antennas.
6) The Microwave communication cannot be used to cover the entire surface of earth due to the presence of various oceans and difficult mountains in-between.

### Satellite communication

1) The satellite communication is a mode of communication of signal between transmitter and receiver through satellite.
2) The satellite communication is like the line-of-sight- microwave communication.
3) The satellite communication is through space craft, provided with microwave receiver and transmitter. It is placed in an orbit around the earth.
4) In the satellite communication, a beam of modulated microwave from the transmitter is sent directly towards the communication satellite, which receives the coming signal, amplifies it and returns it to earth at the different frequency to avoid interference between the uplink to downlink.
5) The satellite communication can be achieved by two types of satellites.
a) By passive satellites- It is nothing but a metal coated plastic balloon or metallic sphere which works as a passive reflector. Moon, the natural satellite of earth is a passive satellite.
b) By active satellites- The active satellite carries antenna system, transmitter, receiver and power supply. It works as an active microwave repeater in the sky called transponder. Sputnik I launched by Russia in 1957 is the first active satellite. The present Indian communication satellites are INSAT 2B, INSAT 2C and INSAT-2D, which were launched in 1993, 1995 and 199 respectively.
6) A satellite communication is possible through geo-stationary satellite.
7) A single geostationary satellite cannot cover the whole part of the earth for microwave communication. It is so because, the large part of the earth is out of sight due to curvature of the earth.
8) In order to have the microwave communication link over the entire globe of earth, at least three geostationary satellites are required, which are 120˚ from each other.
9) INSAT 2C and 2D provide facilities like teleconferencing, computer to computer and voice- communication.
10) Satellite communications are categorized as either bus or pay load. The bus includes control mechanism that supports the pay load operation. The pay load is the actual user information that is conveyed through the satellite system.
11) Long distance communication beyond 30 MHz is not possible through sky wave, due to penetration of ionosphere by high frequency beyond critical frequency.

### Remote sensing

1) Remote sensing is a technique which is used to observe and measure the characteristic of the object at a distance.
2) Remote sensing done through a satellite. The remote sensing satellite should move on a fixed orbit round the earth in such a way so that it passes over a given location on the earth at the same local time. The orbit of such a satellite is known as sun-synchronous orbit. These satellites move in near polar orbits at an altitude of 918 km. The instrument on board is a multispectral scanner.
3) The Remote sensing satellite takes the repeated photographs of the particular location of the earth during its repeated journey over that location, from which comparative study can be made.
4) Satellite remote sensing systems provide us with data critical to weather prediction, agriculture forecasting, resource exploration and environmental monitoring.
5) The Indian remote sensing satellites are IRS-IA, IRS-IB and IRS-IC.

### Optical communication

1) It is a system of communication by which we transfer the information over any distance from one location to the other optical range of frequencies, using optical fibers.
2) The optical communication system is more economical than other system of communications and it has larger information carrying capacity and provides high quality service.
3) The information carrying capacity of communication system is directly proportional to its band width i.e. the wider, the band width; the greater is its information carrying capacity.
4) The band width of a system is the measure of percentage of its carrier frequency, which is about 10% of the carrier frequency.
5) Optical fibers are used in optical communications.
6) Light frequencies used in optical fiber system lie between 1014 Hz to 4 ×1014 Hz (i.e. 106to 405 GHz). The band width, corresponding to frequency 104 GHz is 103 GHz, which is around 104 times higher than the highest frequency microwave transmission.

### Optical fibers

An optical fiber is a long thread consisting of
(a) a central core of glass or plastic of uniform refractive index, which is surrounded by
(b) a cladding of material of refractive index less than of the core and
(c) a protective jacket of insulating material.