Chapter 1: Telecommunication Fundamentals Block Diagram of a communication system Noise n(t) m(t) Information (base-band signal) Signal Processing Carrier Circuits s(t) Transmission Medium r(t) Signal text Processing mˆ ( t) Transmitter Receiver All communication systems involve three main sub systems: the transmitter the channel the receiver. m (t) : The information input waveform. The message delivered by the receiver is denoted by m ˆ ( t). The { } indicates that the message received may not be the same as that transmitted. m ˆ ( t) may be corrupted by noise in the channel. The message information may be in analogue or digital form, depending on the particular system. The message may represent audio, video or some other type of information. m(t) Signal Processing Carrier Circuits s(t) Channel r(t) Transmitter The signal processing block at the transmitter conditions the source for more efficient transmission. For example, in an analogue system the signal processor may be an analogue low-pass filter that is used to restrict the bandwidth of m (t). In a digital system, the signal processor may be an analogue to digital converter which produces digital word that represents samples of the analogue input signal. The transmitter carrier circuit converts the processed base band signal that is appropriate for the transmission medium of the channel. For example, if the channel consists of a fiber-optic cable, the carrier circuits convert the base band input (i.e. frequencies near f = 0 ) to light frequencies and the transmitted signals, s (t) is light. s (t) is said to be band pass signal, because it is designed to have frequencies located in a band about Carrier frequency ( f c ).
The mapping of the base band input information waveform m (t) into the band pass signal s(t) is called modulation. For example, an amplitude-modulated (AM) broadcasting station with an assigned frequency of 50 Hz has a carrier frequency of f c = 50Hz. Carrier Frequency Amplitude 1 Mapping is called Modulation S(f) - KHz 0 KHz { Low Pass f C =50 KHz 5 KHz {6 KHz Channels may be classified into two categories: Wire Wireless Band Pass Some examples of wire channels are: Twisted-pair telephone lines Coaxial cables Waveguides Fiber-optic cables Some typical wireless channels are: Air Vacuum Seawater In general, the channel medium attenuates the signal so that the noise of the channel or the noise introduced by an imperfect receiver causes the delivered information mˆ ( t) to be deteriorated from that of the input m (t). The channel noise may arise from natural electrical disturbances (e.g. lightning) or from artificial sources, such as high-voltage transmission lines, ignition systems of cars, or switching circuits of a nearby digital computer. The channel may provide undesirable multi paths between its input and output that have different time delays and attenuation characteristics.
The channel characteristics may vary with time, which makes the signal fade at the channel output r (t). You have probably observed this type of fading when listening to distant shortwave stations. The receiver takes the corrupted signal at the channel output and converts it to a base band signal. The signal processing circuits at the receiver clean up this corrupted signal and deliver the output m ˆ ( t). Frequency allocations: Wireless communication systems often use the atmosphere for the transmission channel. Here, interference and propagation conditions are strongly dependent on the transmission frequency. Theoretically, any type of modulation (e.g. amplitude modulation, frequency modulations, single-side band, phase-shift keying, frequency-shift keying etc) could be used at any transmission frequency. However, the government regulations specify the modulation type, bandwidth, power and type of information that a user can transmit over desired frequency bands. Frequency assignments and technical standards are set internationally by the International Telecommunication Union (ITU). Communication Frequency Band: Frequency Range Designation Typical uses 3-30 KHz Very Low Frequency (VLF) Submarine communication 30-300 KHz Low Frequency (LF) Long range navigation, Radio beacons 300-3000 KHz Medium Frequency (MF) AM broadcasting 3-30 MHz High Frequency (HF) Amateur radio, International broadcasting, Ship communication 30-300 MHz Very High Frequency (VHF) VHF Television, FM two-way radio, Aircraft navigation 300-3000 MHz (0.3-3 GHz) Ultra High Frequency (UHF) UHF Television, Cellular Phone, GPS etc. 3 GHz 30 GHz` Super High Frequency (SHF) Satellite communication, Radar microwave link Note: KHz = 3 Hz, MHz = 6 Hz, GHz = 9 Hz
Radio Wave Propagation: Radio waves are propagated, depending on the frequency, along the surface of the earth, through the atmosphere and by reflection (or scattering) from the ionosphere or troposphere. The ionosphere is that part of the atmosphere, at heights above about 50 Km, in which free ions and electrons exist in sufficient quantities to effect the propagation of radio waves. Four principal regions or layers affect the propagation of radio waves (see below). At frequencies below 30 MHz regular long distance transmission is possible by the way of ionospheric reflections. The D region is of low electron density mainly causing absorption of radio waves passing through, but sufficiently reflective to LF( 30 300KHz). (D-region ionization is mainly in daytime). The E region is ionized mostly by solar ultraviolet and x-rays in the daytime. E layer is an important reflecting medium for daytime HF( 3 30MHz) propagating and at night for MF( 0.3 3MHz) and LF( 30 300KHz) propagation. The F region is the most important reflecting layer for long distance high-frequency communication day and night.
Propagation at frequencies below 300 KHz (LF) (Long Wave): Transmission in the frequency range below 150 KHz is in the region bounded by the earth and lower ionosphere. LF waves are of interest because of their depth of penetration below the earth s surface and are also regarded as a vital means of communication which deeply submerged submarines. Note: Reducing the ground conductivity increases the attenuation of the Radio Waves. Sea- water has high conductivity. Propagation at 300 KHz 3000 KHz (MF) (Medium Wave): During the day, at distances less than 00 Km, the Ground Wave is dominant because of intense
daylight D-Layer absorption of the MF radio wave. At night ionospheric reflection coefficient can be high, which causes the Radio wave to reflect and travel long distances. Propagation at high Frequencies about 3-30 MHz (HF) (Short Wave): This frequency range is usually reflected by the ionosphere day and night. The range of usable frequencies is limited on the upper end by the height and maximum electron density of the controlling layer and on the low-frequency end by absorption in the D region. > 30 MHz F Layer E Layer Tx D Layer Tx EARTH E layer reflecting causes the waves to travel up to 2000 Km Note : Ionospheric reflections permit radio communication to greater distances
The height of the transmitting antenna is that above the average level of ground and the distance between the transmitter and the receiver is about 3-15 Km.The receiving antenna height is taken m, considered representative of home-television. For mobile services the height of antenna will be less,around 3 m. Propagating above 1 GHz (Microwave Frequencies) Point-to-Point Microwave links at frequencies above 1 GHz are designed to achieve essentially free-space attenuation (no attenuation). Note: At high frequencies, losses along the ground become so great that the usefulness of the ground wave is limited to short distances. Propagation about 30 MHz (VHF & UHF) At frequencies much above 30 MHz, ionospheric reflections are not dependable and most communications depend upon line-of-sight propagation. Tx line of - sight Communication Rx Hand - Held Transmitter/Receiver In the VHF and UHF regions, propagation for point-to-point (Broadcasting and Mobile) services usually includes substantial terrain effects (e.g. the height of mountains, trees, buildings etc).
Example: Line of - sight length d Km The above figure shows the antenna heights required to provide smooth-terrain clearance on a 1 GHz path (microwave link) of length d (km). Fading: S R Tx Variations appear in the signal received due to time delayed multi-paths. Signal received ( S ) = signal direct ( S ) + signal reflected ( S ) If S d & S R are in phase signal S will increase in strength and if S d & SR are out of phase, signal S will decrease in strength. d R
Calculation of Antenna Length: LF: 30-300 KHz Antenna Length = = MW: 300-3000 KHz Antenna Length = = SW: 3-30 MHz Antenna Length = = 300 3 00 = m = 250m 3 3000 0 = m = 25m 6 30 = m = 2. 5m VHF: 30-300 MHz Antenna Length = = 300 6 1 = m = 0. 25m UHF: 300-3000MHz (Mobile telephone operation frequency: 900 MHz) 6 1 Antenna Length = = 900 = m = 0. 03m 12