Allocation of electromagnetic spectrum λ= = f 1 In the figure, λ = c/f, where: λ is the wavelength in meters; c is the propagation speed of light (identical to that of a radio wave) in meters per second (approximately 300,000 km/s); f is the frequencyin Hz (definedas1 cycleper second). 125
Radio propagationmodes 2 Radio waves at different frequency bands propagate in different ways. Some of these propagation modes are as follows: Ground wave: The radio wave follows the surface of the Earth, and thus communication over the horizon is possible. Skywave: The radio wave is reflected from the ionosphere back to Earth. The wave is reflected back from the Earth s surface and back to the Earth again making long-distance communication possible. The communication quality is not stable because the characteristics of the ionosphere vary with time. Line of sight: The radio wave propagates along the straight line from the transmitter to the receiver. A general requirement for good performance is that the receiving antenna be visible from the transmitter. The radio frequencies above 100 MHz that propagate in line-of-sight (LOS) mode are used in most modern communication systems. 126
OpticalCommunications 3 At the infrared light frequencies just below visible light (wavelength 400 700 nm) a transmission medium, optical fiber, provides very low attenuation. Optical fiber is the most important media for high-capacity long-distance transmission. It is used in national long-distance networks as well as in international and intercontinental submarine cable systems. Commercial optical communication systems use binary light pulses. When the transmitted information is in binary form, the receiver either detects light or does not: this is also called intensity modulation (similar to BPSK: constellation symbols 0 and 1 ). Modern optical systems are also able to use transmitted light as a carrier wave in the same way that radio systems do. Radio systems are able to change phase and frequency of the carrier wave, not just intensity. Traditionally, one optical signal occupies the whole fiber although a small portion of its very wide frequency is usable. However, development of narrowband optical transmitters and optical filters has made it possible to increase the data transmission capacity by inserting multiple optical channels into the same fiber with the help of the dense wavelength division multiplexing (DWDM) system. 127
The free spaceattenuationformula 4 Attenuation, or loss, in free space: Same expression as above in decibel: The free-space loss formula may give results that are too optimistic in actual conditions (i.e. real terrestrial propagation). Supplementary attenuation (L s ) is introduced if there is a hill, a building, or a wall on or close to the straight line between the transmitting and receiving antennas. 128
Antennasand LOS propagation 5 Link loss was calculated assuming that antennas are isotropic, i.e. they transmit and receive equally to and from all directions. However, practical antennas have a focusing effect that can be expressed through an antenna gain, g T or g R depending if the antenna is the transmit antenna or the receive antenna, respectively. The maximum transmitting and receiving gain (along the direction of maximum radiation) of an antenna with effective aperture area Ae is: g T or g R : T R The value of Aefor a dish or horn antenna approximately equals its physicalarea. Received power and overall radio link loss when antenna gains are considered: (L Tot ) L s L Tot = L L s = g T g R Decibel expression: L tot, db Schematic of LOS radio link 129
TransmissionMedia 6 In modern communications,to interconnect far-end and near-end equipment, Transmission systems may use (mainly): copper cables optical cables radio channels 130
Coppercableasa transmissionmedium 7 131
CopperCables 8 Attenuation in copper cable increases with frequency approximately according to the following formula: AdBis attenuation in decibels, f is the frequency, and k is a constant specific for each cable Twisted Pair Twowiresare twisted together to reduce external electrical interference and interference from one pair to another in the same cable Usedin the telecommunications networks in subscriber lines, in 2-Mbps digital transmissions with distances up to 2 km between repeaters, in DSLs up to several megabits per second 132
OpticalFiberCables 9 Fiber optic links are used as the major media for long-distance transmission, now traditionally in the access/metro networks, and more and more in the access network. Main characteristics are: High transmission capacity: very large bandwidth, to carry very high data rates, up to several hundred Gbps(transport/metro networks) and to several hundred Mbps (access network). Low cost:cost of the fiber has decreased to the level of a twisted-pair cable Tolerance against external interference: e.m. disturbances have no influence on the light signal inside the fiber. Small size and low weight:fiber material weighs little and the fiber diameter is only of the order of a hundred micrometers instead of a millimeter or more for copper wire. Unlimited material resource: Quartz used in glass fibers is one of the most common materials on Earth. Low attenuation: Attenuation in modern fibers is less than half a decibel per 133 kilometer and it is independent of the data rate.
Opticalfiberattenuation 10 134
Radio transmission 11 Advantages Radio transmission does not require any physical medium. Radio systems are quick to install Because no digging of cable into the ground is required, the investment costs are much lower than cable transmission Disadvantages Spectrum is a limited resource. Interference between systems Bandwidth is generally much lower than that carried with cable systems The use of radio frequencies is regulated by the ITU-R at the global level and, for example, by ETSI at the European level and the FCC in the UnitedStates. 135