Basic Principle of Communication

Introduction

1.         The word communication means exchange of ideas and views between two or more people. Basically communication needs a common language and a source of energy for carrying the transmission, a receiver and a media through which this energy is transferred.

2.         In electronics, the term communication means sending, receiving and processing of information by electronic means. Initially communication started with wired telegraphy in 1840s. Some decade’s later telephony came up as an evolved communication system.

3.         In twentieth century, during World War II (1939-45) the triode (the electronic tube) was invented and that gave rise to new development in Radio Communication. It was more refined through invention of transistor in 1950s, integrated circuits in 1965 and other semi-conducting devices in the following years. More recently the use of satellite, Fibre optics with participation of computer and data communication has given the communication a new shape even more wide spread.

4.         A modern communication system involves, sorting, processing and storing of information before transmission. Sorting means selecting of wanted information from a lot and rejecting unwanted ones. Processing involves converting the information into electrical waveforms. Then actual transmission follows after filtering of noise. Finally we have reception, which includes processing steps such as decoding (detection), storage (computer memory) and interpretation i.e., display in the form of analog or digital etc., In this reference form of communication includes, radio telephony and telegraphy, broadcasting, point to point mobile communication, computer communications, radar, radio telemetry and radio aids navigation. Most of these described briefly in following chapters. It is most evident that the world will be paralyzed if communication is detached from it.

5.         Establishment of Communication.        It is not possible to transmit the speech or low frequency signal directly into the atmosphere because of following reasons:

(a)          Attenuation is more.

(b)          Power required for transmission will be very high.

(c)          All radio channels will interfere with each other.

(d)          Length of antenna required is very high.

6.         Thus this speech is modulated over a high frequency signal called carrier, transmitted by a device called transmitter into air or any other medium. Modulation is the process of superimposing low frequency signal upon high frequency signal (carrier). Modulation is carried out in transmitter.

7.         This modulated signal is directed to distant end called receiver through medium. This medium may be solid, liquid or gas. (Fig 1).

Fig 1 Wireless Audio Communication

8.         At receiving end this signal is picked up by the antenna and detected. Detection is the process of separating intelligence from carrier. Likewise a telephone call, or a picture or other type of signal is also modulated with carrier and guided through medium. At receiving end these signals are reproduced after detection (Fig 5.2).

Fig 2 Wireless Video Communication

9.         Signal can be modulated in terms of:

(a)       Amplitude Modulation (AM).       When the amplitude of high frequency carrier wave is changed in accordance with the intensity of signal, it is called amplitude modulation. In amplitude modulation only the amplitude of the carrier wave is changed in accordance with intensity of signal However, the frequency of modulated wave is remains the same. i.e carrier frequency. Fig shows the principle of amplitude modulation. Fig. ‘b’ shows the audio electrical signal whereas fig ‘a’ shows carrier wave constant amplitude. fig 3‘c’ shows amplitude modulated wave form(AM).Note that the amplitude of both positive and negative half cycles of  carrier wave are changed in accordance with the signal. For instance, when the signal is increasing in the positive sense, the amplitude of carried wave also increases. On the other hand, during negative half cycle of the signal, the amplitude of carrier wave decreases. An electronic circuit called modulator does amplitude modulation.

Fig 3 Amplitude Modulation

(b)       Frequency Modulation (FM).       The type of modulation in which frequency (number of alternations per second) carrier is varied with amplitude of modulating signal (speech).  It is used in Television Broadcasting System, Satellite Communication System etc.

Fig 4 Frequency Modulation

(c)        Phase Modulation (PM).   The type of modulation in which a 90° phase shifted carrier is added in amplitude modulated wave. It is used in Satellite communication, Fibre optics transmission etc.

10.         Likewise detection can also be in terms of amplitude, frequency or phase modulation. Amplitude modulation has the advantage of simplicity of equipment where as frequency modulation and phase modulation has advantage of less noise, better clarity but range is comparatively low.

Types of Radiated Wave

11.       Continuous Wave (CW).     It consists of radiated waves during the time when key is pressed and no radiation during the time the key is not pressed.

12.       Modulated Continuous Wave (MCW).     It consists of changing the amplitude of radiated wave during the periods of information, i.e., when the key is pressed.

13.       Radio Telephony (RT).     It is the form of MCW, in which the variation of waves amplitude is due to human speech.

14.       Frequency of radio system ranges in a wide spectrum. It is used in various fields. These are tabulated in the following page:

Frequency Band	Range	Use
VHF	03 – 30 K Hz	Long distance point to point communication.
Low Frequency	30 – 300 K Hz	Marine, Navigational Aids
Medium Frequency	300 – 3000 K Hz	Broadcasting, Marine
High Frequency	3000 – 30000 K Hz	Communication of all types
VHF	30 – 300 M Hz	TV, FM Broadcasting, Radar, Air Navigation and aviation communication
UHF	300 – 3000 M Hz	Radar, Microwave relays, short distance communication
Super High Frequency	3000 – 30000 M Hz	Radar, Radio relays, Navigation, Experiments
Extremely High Frequency	30000 – 3000000 MHz (30–300 GHz)	Experimental

15.       Communication means exchange of views. In order to establish Communication we need a transmitter, a receiver and a media. A low frequency signal may be audio, video or other type is modulated inside the transmitter and through a media directed towards the receiver where signal is recovered after detection. The purpose of communication is to convey information from one place to other.

16.       Typical Communication Systems.        Communication plays a vital role in flying station. With the help of signal and radio equipment.  ‘Communication links’ are established between ground to air, air to ground, ground to ground, for passing various information between one place to another. Fig 5 shows channelising networks of ground station.

Fig 5 Typical Channelising of Networks in an Airfield

17.       Air to Ground & Ground to Air Communication.        The basic purpose of this network is to aid the flying aircraft and to supply various information demanded by the aircraft, e.g., homing/bearing, weather conditions, landing aids or other mission control information. It is carried out with the help of signal equipments fitted both in aircraft and at ground. In terms of frequency, these equipments are divided into two categories, viz., HF equipment and VHF equipment.

18.       Generally a VHF trans-receiver is equipped in aircraft, which is of comparatively small size to reduce the weight and occupies less space. These are called airborne equipments and established for RT communication only.

19.       In case if the above fails, then pilot can be aided by HF trans-receiver which are not popularly used now a days.

20.       Besides the above, at ground, numbers of facilities are extended to the aircraft and ground stations. These are discussed in the following paragraphs.

21.       HF Range.    Inter tower RT (Open/Confidential) and stand by facilities to HF RT.

(a)         Met broadcast RT reception channel.

(b)         NOTAM reception channel.

(c)          Teleprinter

.

(d)         Telex.

22.       VHF Range. VHF transmitter and receiver to establish the communication between ground to air. The number of channel will vary from station to station as per demand.

23.       Navigation Aids.

(a)       Medium frequency beacon (for Airborne Radio Compass).

(a)          V/UHF direction finder and Standby set of DF.

(a)          Instrument Landing System.

(i)            Marker

(ii)          Localiser

(iii)         Glide Path

(b)         Eureka/Rebecca (Radar).

(c)          Ground control approach (GCA) it consists of procession approach radar and search radar equipment.

24.       In addition to the above, tape recorders are placed in air traffic control (ATC) to record the conversation of pilot with ground station. Such recordings are very helpful in aircraft accident/incident investigations. The same job is carried out in aircraft by equipments like

(a)       Cockpit voice recorder.

(b)       Digital Flight Data Recorder (Black box).

25.       Black Box is totally full proof from all external stimuli.  Likewise detection can also be in terms of amplitude, frequency or phase.  Amplitude modulation has advantage of simplicity of equipment where as FM/PM has advantage of less noise, better clarity but range is comparatively low.

26.       Fibre Optical Communication System. It is a long haul in communication which use light as a media for carrying various intelligences.  e.g. Audio, Video, analog or digital Signal.  This concept was firstly given by American scientist Charles Kao and George Hookem. It was practically shaped by M/S Corning Glass Works by making a optical Transmitter (Txr), optical Receiver (Rxr) and a propagating Media. Later it was further improved by Bell Labs of USA.  Today this system has emerged out as leader of communication.  Silica in its pure form possesses optical qualities; that’s why it is used for this purpose.

27.       Salient Features of Fibre Optics. The salient features are as follows:

            (a)       Wide Band Width.   It can accommodate number of channels through one media. AT&T’s cable can handle 6000 simultaneous telephone calls, through a pair of cable. Latest WT-4 cable which is made by Bell Labs can handle 2,30,000 channels through a single pair of line. It can handle 2MB/Sec (Million Bits/Sec). With this rate 30 volume of 20^th Century Chamber’s Dictionary (1700 pages) can be transmitted in less than a sec.

(b)       Attenuation.     In earlier days from 10 db/ km to 2 db/ km attenuation was brought and it gives satisfactory result. Today lowest attenuation is 0.19 db per km at 1.55 m wavelength of light. Lowest attenuation in co-axial system is 10db/km. 1 db/km is general attenuation without a repeater of 100 km.

(c)        Noise Immunity.      Fibre cable are inherently immune from any radiation, i.e., electro-magnetic or electrostatic interference, e.g. lighting motors, EHT (Extra High Tension) power lines, RF, cross talk and various pick up, etc, irrespective of data rate.

(d)       Size and Weight.     It offers greater signal handling capacity accompanied by smaller size and lighter weight. A copper cable, which can handle 36000 channels has a dia of 7.5 cm and weighs 11 kg/m.  But a fibre cable carrying 50,000 channels is only 1.25 cm in dia and weighs just 1.2 kg/m. Thus it saves cost, shipping and storage.

(e)       Compatibility with Conventional Electronics.    It perfectly matches with modern electronic communication system utilizing CMOS/TTL, IC and VLSI circuits.

(f)        Modular Design.     It can be easily designed into small modules, which can easily be replaced.  It eases the servicing and fault finding.

(g)       Security.       It is an almost perfectly secured system. Because it is extremely difficult to tap the information from cable without detection. If in case the cable is broken, light will escape and will be absorbed into space without any effective use.

(h)       Safety.   It is totally safe, because as it does not handle large voltages, no sparking or short circuit will be caused. It can be used in hazardous environments.

(j)         No scarcity of Raw Material.        It is made of silica, which is available on earth in abundance.

(k)        Insulating Medium.    Silica is a good insulating medium, which is utilized in several applications.

28.       Fibre optical transmission system can be divided into three main functional units:

(a)         The transmitter or Source.

(b)         The receiver or Detector. 

(c)          The propagating Media.

29.       Propagating Medium.        Plastic and glass fibre are used as propagating medium of light signal from transmitter to receiver. The principle of transmission of energy along with an optical fibre is similar to the concept of total internal reflection, which occurs when light in a glass core strikes the boundary of the glass sheath of lower refractive index at greater than critical angle. It is directly depending on the ratio of the two refractive indices. Plastic fibre cable is used in optical transmission because of their low cost, high source fibre, coupling efficiency and ease of handling. Their attenuation is high.

30.       Critical Angle.          It is that incident angle at which refracted light travels along with axis (surface of outer boundary). Striking rate is always greater than the critical angle. Optical fibre cable consists of glass clad with a sheath of different glass; core has a higher refractive index than sheath. Core has got very low optical scattering and absorption for low losses. There are three types of fibre:

(a)       Step Index Fibre (SI).

(b)          Graded Index Fibre.

(c)          Single Mode or Mono Index Fibre.

31.       Thus we can recall that fibre optics communication system is the one in which light is used as carrier to carry various types of signals. It needs an optical transmitter, an optical receiver and a physical optical cable as medium. It is so emerging out as the leader of communication. In India, P&T, Railways, ONGC, Defence Services and various public sectors are extensively using it.

32.       Laser Communication.     No other invention in the history of scientific development has made such a far-reaching impact in various areas of science and technology as LASER. It stands for Light Amplification by Stimulated Emission of Radiation.

33.       In 1954 the first low noise microwaves amplifier produced by Professor Towens and his colleagues, called MASER (Microwave Amplification by Stimulated Emission of Radiation). It was extremely low noise amplification of microwave signals by a Quantum-Mechanical Process. The LASER or optical MASER (L stands for light) is a development of this idea, which permits the generation or amplification of coherent light. Coherent means single frequency, in phase, directional and polarized. The Laser is a source of coherent electromagnetic wave at infra red and light frequencies. This ranges from 430 to 750 Tera Hertz (T Hz) (i.e., 430,000 G Hz to 750,000 G Hz) (1 T Hz = 1000 G Hz).

34.       The first Laser, using RUBY was proposed in 1958 and a scientist named Theodore Miaman in 1960 developed practical Laser. The first continuously operating Laser was followed in 1961 and used a mixer of helium and neon gases. Laser has a wide range of applications as in saving life in ophthalmic and other types of surgery and for military purpose.

35.       Principle.      Laser is based on the principle of spontaneous and stimulated emission of photon. The atom can without external force, spontaneously emit unwanted energy as photon. The released photon drains off enough energy from the atom to return to the ground state.  The process of spontaneous emission takes place very rapidly in a tiny fraction of a second. It can also happen that a photon leaving excited atom strikes another atom stimulating it to give up its photon sooner than it would have been released spontaneously. The collision process between photon and excited atoms starts a chain reaction, which causes more and more atoms to give up, thus releasing vast amount of energy. This energy starts building up a massive wave front, which grows with each collision between a photon and an excited atom. The length of time spent in excited state is called ‘life time’ and varies with atoms of different materials and energy levels.

36.       Application.      The application of laser is wide ranged. Few of them are discussed in the following paragraphs:-

(a)          Spectroscopy.        Spectroscope is an instrument used to study intensity and wave length of a ray of light. Powerful impact of laser beams can be applied in spectroscopy on the principle of Raman scattering to generate high intensity laser lights at many different frequencies.

(b)          Power Transmission.        A laser beam could be focused to heat a pot of coffee from a distance of 1000 miles. With the improved technology scientists are trying to achieve laser power lines from satellite to operate low power equipment.

(c)          Satellite Nudging.        Nudging means shifting slightly.  Light exerts a tangible pressure. A laser beam exerts a pressure of several pounds/sq inch over a tiny area. When satellite begin to slow down and thus be drawn towards the earth, a laser beam projected from earth can be used like a giant finger gently to push it back into higher orbit.

(d)          Radar.            Powerful laser pulses are capable of producing measurable reflection over greater distances than microwaves. At the 1.10-inch wavelength (X) of light, radar can detect much smaller object than can be done with inches long wavelength microwaves.  Even high velocity measurement can also be made.

(e)          Communication.     A laser beam is theoretically capable of simultaneously transporting voice of 10 channels. Thus more number of channels can be accommodated than any other communication. Laser beams have become a means of communication between earth and moon or other satellites.

37.       Due to the tremendous advantages and uses of laser devices, there are hundreds of institutes in advance countries, which are engaged in developing different types of laser. It is reliably estimated that new lasers are appearing at the rate of one per month. Days are not far away when laser devices will be used as house hold appliances like electrical appliances, one used now a days.

38.       Satellite Communication.    Satellite is a body, orbiting a planet. Satellites are natural as well as artificial.

39.       Classification.         The classification of the satellites is as below:

40.       Natural & Artificial Satellites.       Natural satellites are those heavenly bodies which move around a planet e.g., moon, whereas artificial satellite are those man made bodies, which are deliberately made to move around planet e.g., all satellites.

41.       Active & Passive Satellites.             Active satellites are those, which have active components like receiver, transmitter and frequency changer etc., e.g., INSAT 1B etc., whereas passive satellites are those metallic bodies which are used to reflect the signal. These do not receive or amplify the signal.

42.       Synchronous & Non-Synchronous Satellites.     Synchronous satellites are those, which are synchronised with the earth. These are placed into geo stationary orbit and need not to be tracked. e.g., INSAT 1B, INTELSAT etc., Non-synchronous orbit are those, which are not synchronised with earth.  They either move with faster or slower than the speed of earth. They are to be tracked e.g., Spy satellite etc.

43.       Salient Features.    The salient features of satellites are as below:

(a)          It works on a wide band Communication System.

(b)          It has long distance communication range.

(c)          Satellite connects one point with many other stations simultaneously unlike co-axial cable or microwave links, which go one point to another.

(d)          It is a flexible system, e.g. capacity of channels can be shifted from one geographical area to another on demand.

(e)          Operation is almost independent of atmospheric condition.

(f)           Signals are almost noise immune and of better quality.

(g)          Delay of approximately 600 ms occurs to receive a signal via satellite. This creates echo from ground station to satellite. It is 300 ms roughly.

(h)          Signal is subjected to cosmic noise/interference.

44.       Types of Satellite.  The following are types of satellites:

• Intelsat.         Expanded as International Telecommunication Satellite. It facilitates thecommunication between space and ground or region. These are internationally obtained. Mainly belong to America and generally channels are hired on loan, e.g., INTELSAT-I, II, III, IV, etc.
• Inmarsat.       International Maritime Satellite also known as International Maritime Telecommunication Satellite.  It provides communication from satellite to ship, mainly suitable for distress calls.
• Regional Satellite.      This belongs to a particular region, e.g., ARABSAT.  It is used for a particular region only.
• Domestic Satellite.     These are launched by a particular country for their domestic purpose, to provide communication for Radio, TV, Networks, Computers, etc. e.g., INSAT-1B, 1C etc.
• Experimental Satellite.      These are launched for various experiments e.g., for Meteorological, earth surface experiments, distress in sea, etc. e.g., Aryabhatta, Bhaskara, IRS, etc.
• Spy Satellite.     These do not revolve in synchronous orbit thus needs tracking.  These are utilised for spying purpose (example 1987 movement of Jaguar Sqn was traced next hour by Pakistan).
• Junk Eating Satellite.        It has two scorplans arms used to collect unwanted garbage existing in geo-stationary orbit.