COMMUNICATION SYSTEMS

COMMUNICATION SYSTEMS : OPTICAL COMMUNICATION, ELECTRICAL COMMUNICATION, RADIO COMMUNICATION,SATELLITE COMMUNICATION.


Introduction:-

The primary purpose of communication systems is information transportation, i.e, to transfer information from one place to another over a distance . Information in its original form can rarely be transported over a communication system. It needs to be represented in the form of signals that can be carried over communication systems. In today’s technology ,communication systems are capable of carrying signals in three energy forms;electrical,optical or electromagnetic. Communication technology deals with representation of information in the form of signals, transportaion of these signals efficiently and reliably to the specified destination and convert the signals back to the original form of information at the destination.
In earlier times, communication systems may have involved theuse of smoke signals, drums, semaphore flags or heliographs. In modern times,it involves the use of electronic devices such as telephone, television, radio and computers.

Basic elements:-
A basic communication systems consists of three elements:
a transistor that takes information and converts it to a signal.
a transmission medium that carries the signal and
a receiver that receives the signal and converts it back into the usable information.

Depending on the energy form and the medium used, four types of modern communication systems are prevalent. The type of communication system, the form of energy and the corresponding communication media are;

Communication type Energy Media
Electrical Electrical Metallic conductor
Optical Light Optical fibre or free space
Radio Electromagnetic Free space
Satellite Electromagnetic Satellite and free space together.

OPTICAL COMMUNICATION:-

Optical communication is any form of telecommunication that uses light as transmission medium. An optical communication system consists of a transmitter, which encodes a message in to an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal.



Forms of Optical Communication:-

There are many forms of non-technological optical communication including body language and sign language.
Techniques such as semaphore lines, shipflags, smoke signals and beacon fires were the earliest forms of technological optical communications.
The heligraph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer see flashes of light that can be used to sent a prearranged signaling code. Navy ships often use a signal lamp to signal in Morse code in a similar way.
Distress flares are used by mariners in emergencies, while light houses and navigation lights are used to communicate navigation hazards.
Aircraft use the landing lights at airports to land safely, especially at night. Aircraft landing on and aircraft carries use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relating to the best landing glide slope.
Optical fibre is the most common medium for modern digital optical communication. The transmitters in optical fibre links are generally light emitting diodes or laser diodes. Optical fibres are made of glass or plastic through which light can travel. The optical fibre acts like a tunnel. When light is launched at one end of the optical fibre it reaches the other end. Light energy has a much higher capacity than electrical energy to carry information. The merits of optical fibres stem from the fact that the basic material used in their construction is non metallic and electrically non conducting. The problems are mostly due to their delicate structure.
In optical cables the information is transmitted by packets of photons that have no charge and hence do not lead to electrostatic discharges. There is no possibility of spark or short circuit when a fibre is cut. Signal transmission in fibres is contained totally within the fibre. Therefore, no electromagnetic radiation takes place from the optical fibre.
Two principle light sources are used in fibre optic communication systems: Light Emitting Diodes (LEDs) and Injection Laser Diodes (ILDs) or simply laser diodes.LEDs are low cost devices but the light beam of LEDs is divergent and has many frequencies of emission. LASER stands for light amplification by stimulated emission of radiation. Light radiation is in the form of photons. By stimulating the radiation of photons, the light output can be amplified. This is the principle used in lasers.
Physically, an optical fibre is composed of the layers such as a core, cladding and protective coating. The core may be made of glass or plastic. Glass is used when fibre attenuation is to be minimized. When the core material is glass, the cladding is either glass or plastic. Fibres which use plastic for core have higher attenuation, but are cheap. Being economical the fibre with plastic core and cladding are used in many applications. Cladding serves two purposes: (1) since cladding is manufactured in a controlled environment, the interface between the core and cladding has a uniform refractive index throughout the cable, (2) cladding protects the core from absorbing surface contaminants. The protective coating gives mechanical strength to the fibre.
The propagation of light along a optical fibre may be described in terms of a set of guided electromagnetic waves called the modes of optical fibres. Only a certain discrete number of modes are capable of propagating along the fibre, depending upon the geometry of the fibre. The physical dimension and the configuration of the fibre may be chosen such that only one mode propagates through the fibre or many modes propagate. Accordingly fibres are classified as single mode fibres or multimode fibres.
As compared to co-axial and a waveguide communication fibre optical communication is considerably cheaper, particularly over long distances, because it requires limited repeater station. Fibre optic communication system is suitable for local area network, long distance trunk routes and installation in hazardous environment and power plants.
The other mode of optical communication is to use freespace as medium. Freespace optics systems are generally employed for ‘last mile’ communications and can function over distances of several kilometers as long as there is a clear time of sight between the source and the destination, and optical receiver can reliably decode the transmitted information.
When light is beamed in freespace like a pencil ray, there should be no obstruction in its path before reaching the receiving point. It is necessary that the communicating parties are in sight of each other with no obstruction between them. Such a form of communication is known as line-of-sight(LOS) communication.

ELECTRICAL COMMUNICATION:-

Electrical communication is the transfer of information using electrical energy. In earlier times, copper cables are used for electrical communication. Basically, there are three types of copper cables:
· Unshielded Twisted Pair(UTP)
· Shielded Twisted Pair(STP)
· Co-axial cables.
Twisting and shielding of pairs of wires serve to improve the reliability of information transfer. Twisting reduces electromagnetic interference at low frequencies. Shielding the wire pair with metallic braid reduces the effect of noise and electromagnetic interference at high frequencies. Twisted pair cables ideally suited for low capacity applications over short distances whereas co-axial cables are used for high capacity applications and long distance transmission. UTP cables are used in applications like data transfer whereas co-axial cables are used in applications like video transfer. For example, local are networks use UTPs whereas cable TV operators use co-axial cables.
In an analogue telephone network the caller is connected to a person, he wants to talk to by switches at various telephone exchanges. The switches from an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number. Once the connection is made, the caller’s voice is transformed to an electrical signal using a small microphone in the caller’s handset. This electrical signal is then sent through the network to the user at the other end where it its transformed back into sound by a small speaker in that person’s handset. There is a separate electrical connections that works in reverse, allowing the users to converse.
Despite the growth of the internet the characteristics of a local are networks remain distinct. It is at the data link layer through that most modern local area networks diverge from the internet. Whereas Asynchronous Transfer Mode(ATM) or Multiprotocol Label Switching (MPLS) are typical datalink protocol for larger networks, Ethernet and Token Ring are typical datalink protocols for local area networks.

RADIO COMMUNICATION:-

Radio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vaccum of space. Information is carried by systematically,changing some property of radiated waves, such as amplitude, frequency or phase. When radio waves pass an electrical conductor, the oscillating fields induce and alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.
Electromagnetic energy radiates through atmosphere in the form of waves. This energy is more commonly referred to as radio waves which are invisible to the naked eye and inaudible to humanears. Radio broadcasting and mobile, telephones are examples of radio communication.
Two parameters transmitter power and receiver sensitivity, are important in radio communication system. Larger the transmitter power, farther the distance at which the signals can be detected. Receiver sensitivity is an indication of the smallest signal power that can be recognized by a receiver. Higher receiver sensitivity means that the receiver can detect even very low signals. For example, if a mobile station transmits larger power it can cover a longer range. Similarly, if a mobile handset has higher receiver sensitivity it can catch the signals better and at longer distances.

Processes:-

Radio systems used for communications will have the following elements. Each systems contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate some property of energy produced to impress a signal on it. The transmitter sends the modulated electrical energy to tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic way that can move through free space.
Electromagnetic waves travel through space either directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion; some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible, this is the fundamental limit to the range of radio communications.
The electromagnetic wave is intercepted by a tuned receiving antenna, this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector subsystem. The receiver is tuned to respond preferentially to the desired signals, and reject undesired signals.
Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows and many other forms of entertainment. Radio was unique among methods of dramatic presentation in that it used only sound.

SATELLITE COMMUNICATION:-

A communication satellite is an artificial satellite stationed in space for the purpose of telecommunications. Modern communication satellites use a variety of orbits including geostationary orbits, Moliniya orbits, other elliptical orbits and low earth orbits.
For fixed services, communication satellites provides a microwave radio relay technology complimentary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and handheld terminals, and for TV and radio broadcasting, for which application of other technologies such as cable is impossible.
Satellites communication also uses microwaves as carrier for source signals. Signals are beamed to the satellite from a groundstation using a microwave carrier, called uplink carrier. Satellite receives the signals and retransmits them to the ground over a different carrier, called downlink carrier. Satellite communication is used for broadcast applications like TV or Radio and also for two-way communication like telephone conversation. A TV studio uplinks information to the satellite and the downlink transmission is picked up by cable operators and distriuted to TV sets. A satellite as like a big microwave repeated in the sky. Itreceives the uplinksignal , filters out from the signal , amplifies the clean signal and retransmits the same to the ground at downlink frequency. The functions of noise filtering, signal amplification and frequency translation are performed by a device called transponder inside the satellite.Usually there are many transponders in a satellite each having a certain capacity to handle signals. Three frequency bands in the microwave region are used in satellite communication. Within the band, higher frequencies are assigned touplink and lower to down link.

Nomenclature
Uplink range
Downlink range
C- Band
5.9 – 6.4 GHz
3.7-4.2 GHz
Ku-Band
14.0 – 14.5 GHz
10.9-11.7 GHz
Ka-Band
27.5 – 31.0 GHz
17.7 – 21.2GHz

Principles of Satellite Communication:-

A communication satellite is essentially a microwave link repeater. It receives the energy beamed up at it by an earth station and amplifies and returns it to the earth at a frequency of about 2 GHz away. This prevents the interference between the uplink and downlink.
A stationary satellite or a succession of satellites in low elliptical orbits can be used for global communication. Satellites in close elliptical orbits requires relatively low transmitting powers and receiver sensitivities. But must tracked by the antennas of the ground stations. Stationary satellite present no tracking problems but are so far away that large antennas, high powers and receiver sensitivities are essential.
A geostationary satellite acts as a line-of-sight (LOS) device cum a sky tower to widen the visual horizon. It covers about one-third of earth’s surface. If three such satellites are judicially placed 120degree apart around the equatorial belt, atleast one of them will always be visible from any point on earth, except from the two poles. With three such global satellites in synchronous orbits around the equator , it is possible to cover a very large portion of the inhabited area of the world.
A satellite in a geostationary orbit appears to be in a fixed position to an earth based observer . A geostationary satellite revolves around the earth at constant speed once per day over the equator.
The geostationary orbit is useful for communication’s applications because ground based antennas , which must be directed towards the satellite , can operate effectively without the need for expensive equipment to track the satellite’s motion. Especially for applications that require a large number of ground antenna’s, the savings in ground equipment can more than justify the extra cost and onboard complexity of lifting a satellite in to the relatively high geostationary orbits.
An interesting aspect of satellite communication is that the cost of communication is independent of the distance between the source and the destination.It is immaterial whether the destination is 10km or 5000km away from the source. Satellite communication is economical for long distance communication. However , satellite communication involves much higher delay because the signals wil lhave to travel a distance equal to twice the altitude at which the satellite is placed.

Conclusion:-
In this context we discussed about communication systems. Communication systems perform a variety of functions to improve the efficiency and reliability of information transfer. The basic functions performed by a communication system at the transmitting end and the corresponding inverse functions performed at the receiving end. Also we discussed optical ,electrical , radio and satellite communication.