Communication Systems
1. Some Major Milestones in the History of Communication
- In 1835 Invention of telegraph by Samuel F.B. Morse and Sir Charles Wheatstone. It resulted in tremendous growth of messages through post offices and reduced physical travel of messengers considerably.
- In 1876 Telephone is invented by Alexander Graham Bell and Antonio Meucci Perhaps the most widely used means of communication in the history of mankind.
- In 1895 Jagadis Chandra Bose and Guglieimo Marconi demonstrated wireless telegraphy. It meant a giant leap - from an era of communication using wires to communicating without using wires, (wireless)
- In 1936 Television broadcast (John Logi Baird) First television broadcast by BBC.
- In 1955 First radio FAX transmitted across continent. (Alexander Bain) The idea of FAX transmission was patented by Alexander Bain in 1843.
- In 1968 ARPANET - the first internet came into existence (J.C.R. Licklider) ARPANET was a project undertaken by the U.S. defence department. It allowed file transfer from one computer to another connected to the network.
- In 1975 Fiber optics developed at Bell Laboratories Fiber optical systems are superior and more economical compared to traditional communication systems.
- In 1989-91 Tim Bemers-Lee invented the World Wide Web. WWW may be regarded as the mammoth encyclopedia of knowledge accessible to everyone round the clock throughout the year.
2. Basic Terminology used in Electronic Communication Systems
- Any device that converts one form of energy into another can be termed as a transducer.
- Information converted in electrical form and suitable for transmission is called a signal.
- Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. Sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature.
- Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. \['0'\] Corresponds to a low level and \['1'\] corresponds to a high level of voltage/current.
- There are several coding schemes useful for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD). American Standard Code for Information Interchange (ASCII) is a universally popular digital code to represent numbers, letters and certain characters.
- In BCD, a digit is usually represented by four binary (0 or 1) bits. For example the numbers 0, 1, 2, 3, 4 in the decimal system are written as 0000, 0001, 0010, 0011 and 0100, 1000 would represent eight. It is a character encoding in terms of numbers based on English alphabet since the computer can only understand numbers.
- Range is the largest distance between a source and a destination up to which the signal is received with sufficient strength.
- Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal.
- The original low frequency message/information signal cannot be transmitted to long distances. Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the information. This process is known as modulation. As will be explained later, there are several types of modulation, abbreviated as AM, FM and PM.
- The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation.
- A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system. A communication satellite is essentially a repeater station in space.
3. Bandwidth of Signals
- In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.
- For speech signals, frequency range 300 Hz to 3100 Hz is considered adequate. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz-300 Hz) for commercial telephonic communication. To transmit music, an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kHz.
- Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.
4. Bandwidth of Transmission Medium
- Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space and fiber optic cable.
- Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz.
- Communication through free space using radio waves takes place over a very wide range of frequencies : from a few hundreds of kHz to a few GHz.
- Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.
- Spectmm allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.
5. Some Important Wireless Communication Frequency Bands
- Standard AM broadcast - \[540-1600\]kHz
- FM broadcast - \[88-108\]MHz
- Television - \[54-72\]MHz, \[76-88\]MHz, \[174-216\]MHz, \[420-890\]MHz
- Cellular Mobile Radio - \[896-901\]MHz, \[840-935\]MHz
- Satellite Communication - \[5.925-6.425\]GHz, \[3.7-4.2\]GHz
6. Jagadis Chandra Bose (1858-1937)
- He developed an apparatus for generating ultra short electro-magnetic waves and studied their quasioptical properties.
- He was said to be the first to employ a semiconductor like galena as a selfrecovering detector of electromagnetic waves. Bose published three papers in the British magazine, The Electrician' of 27 December, 1895.
- His invention was published in the 'Proceedings of The Royal Society' on 27 April, 1899 over two years before Marconi's first wireless communication on 13 December, 1901.
- Bose also invented highly sensitive instruments for the detection of minute responses by living organisms to external stimulii and established parallelism between animal and plant tissues.
7. Propagation of Electromagnetic Waves
- In communication using radio waves, an antenna at the transmitter radiates the Electromagnetic waves (em waves), which travel through the space and reach the receiving antenna at the other end.
- As the em wave travels away from the transmitter, the strength of the wave keeps on decreasing. Several factors influence the propagation of em waves and the path they follow. At this point, it is also important to understand the composition of the earth’s atmosphere as it plays a vital role in the propagation of em waves.
- Ground Wave
- To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength of the signal. At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground.
- In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth.
- A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz).
- Sky Waves
- In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.
- The ionosphere is so called because of the presence of a large number of ions or charged particles. It extends from a height of \[\sim 65\] Km to about 400 Km above the earth's surface. lonisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules.
- The ionosphere is further subdivided into several layers. The degree of ionization varies with the height. The density of atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionized.
- Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionisation is again low. However, at some intermediate heights, there occurs a peak of ionisation density. The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz).
- Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to total internal reflection in optics.
- Space Wave
- Another mode of radio wave propagation is by space waves. A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as satellite communication.
- At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths above the ground.
- Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth. If the signal is to be received beyond the horizon - then the receiving antenna must be high enough to intercept the line-of-sight waves.
8. Modulation and its Necessity
- As already mentioned, the purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.
- No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.
- For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least \[\lambda /4\] in dimension) so that the antenna properly senses the time variation of the signal.
- For an electromagnetic wave of frequency 20 kHz, the wavelength X, is 15 km. Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such baseband signals is not practical.
- We can obtain transmission with reasonable antenna lengths if transmission frequency is high (for example, if \[v\] is 1 MHz, then \[\lambda \] is 300 m). Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.
- Radiation from a linear antenna (length \[I\]) shows that the power radiated is proportional to \[{{(\iota /\lambda )}^{2}}\]. This implies that for the same antenna length, the power radiated increases with decreasing \[\lambda \], i.e., increasing frequency.
- Hence, the effective power radiated by a long wavelength aseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission.
- Another important argument against transmitting baseband signals directly is more practical in nature. Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed up and there is no simple way to distinguish between them. This points out towards a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.
- The above arguments suggest that there is a need for translating the original low frequency baseband message or information signal into high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.
- In doing so we take the help of a high frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses.
- As long as the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.
9. Mobile Telephony
- The concept of mobile telephony was developed first in 1970's and it was fully implemented in the following decade.
- The central concept of this system is to divide the service area into a suitable number of cells centred on an office called MTSO (Mobile Telephone Switching Office).
- Each cell contains a low-power transmitter called a base station and caters to a large number of mobile receivers (popularly called cell phones). Each cell could have a service area of a few square kilometers or even less depending upon the number of customers. When a mobile receiver crosses the coverage area of one base station, it is necessary for the mobile user to be transferred to another base station.
- This procedure is called handover or handoff. This process is carried out very rapidly, to the extent that the consumer does not even notice it. Mobile telephones operate typically in the UHF range of frequencies (about 800-950 MHz).
