This chapter is from the book
This chapter is from the book
This chapter is from the book
Unbounded Medium
The atmosphere, the ocean, and outer space are all examples of unbounded media, in which electromagnetic signals originated by the source radiate freely into the medium and spread throughout it. The unbounded media are used by various radio frequency transmitting schemes to carry messages. The main feature of unbounded media is that when the signal is radiated from the transmitter, it radiates equally in all directions (unless restricted) and continues forever onward. As it moves farther from the source, the energy is spread over a larger area, so the level continually gets weaker at greater distances. As the wave moves through the medium, it is affected by natural disturbances that can interfere with the signal.
High-Frequency Radiotelephone
By convention, radio transmission in the frequency band between 3MHz and 30MHz is called high-frequency (HF) radio. Frequency bands within the HF spectrum are allocated by international treaty for specific services, such as mobile (aeronautical, maritime, and land), broadcasting, radio navigation, amateur radio, space communications, and radio astronomy. HF radio has properties of propagation that make it less reliable than some other frequencies. HF radio does, however, allow communications over great distances with small amounts of radiated power.
HF radio waves transmitted from antennas on the Earth follow two paths when they leave the antenna. The groundwave follows the Earth's surface, and the skywave bounces back and forth between the Earth's surface and various layers of the Earth's ionosphere. The groundwave is useful for communications up to about 400 miles, and it works particularly well over water. The skywave propagates signals for up to 4000 miles with a path reliability of about 90 percent. Data signals are carried on HF radio systems as continuous wave (CW) radio telegraphy at about 15 bits per second (bps), and frequency shift-keyed (FSK) single sideband signals are carried on HF at 75bps. Higher data-bit rates (up to 19200bps) are converted to standard 3KHz voice channel analog signals by modems, and these analog signals are transmitted on voice frequency (VF) carrier systems using HF radio.
Microwave Radio
The tall towers with large horns or the dish antennas that you see while driving through the countryside are the repeater stations for line-of-sight (LOS) microwave radio systems (sometimes called radiolink systems). Such systems can carry large quantities of voice and data traffic for several reasons:
They require no right-of-way acquisition between towers.
They can carry very large quantities of information per radio system, due to their high operating frequency.
They require the purchase or lease of only a small area of ground for installation of each tower.
Because the wavelength of the transmitted signal is short, an antenna of reasonable size can focus the transmitted signal into a beam. This provides greater signal strength at the receiver without increasing transmitter power.
Radiolink systems are subject to transmission impairments that limit the distance between repeater points and cause other problems. The microwave signals are treated as described here:
They are attenuated by solid objects (including the Earth). In addition, the higher frequencies are attenuated by rain, snow, and fog.
They are reflected from flat conductive surfaces (such as water and metal structures).
They are diffracted (split) around solid objects.
They are refracted (bent) by the atmosphere so that the beam can travel beyond the line-of-sight distance and be picked up by an antenna that is not supposed to receive it.
In spite of these possible problems, radiolink systems are highly successful and, until the late 1980s, carried a substantial part of all telephone, data, and television traffic in the United States. Beginning in the early 1980s, most long-distance communications carriers installed tens of thousands of miles of optical fiber. Since the late 1980s, most long-distance transmission has been moved off microwave systems to fiber-optic transmission systems. The microwave range of radio frequencies is allocated for various purposes by international treaty. Some of the frequency assignments for the United States are shown in Table 3.6.
Most common carrier radiolink systems carry analog signals, principally frequency modulation (FM). A few systems, however, carry digital signals. Two examples in the United States are the AT&T 3A-RDS radio system, which operates in the 11GHz band, and the AT&T DR-18 radio system, which operates in the 18GHz band. The 3A-RDS system carries DS3 digital signals at 44.736Mbps, and the DR-18 system carries DS4 digital signals at 274.176Mbps. The DS3 and DS4 signals, which are made up of several lower-bit-rate signals, are discussed in more detail later in this chapter.
Table 3.6 Frequency Assignments for Microwave Radiolink Systems
|
Service |
Frequency, GHz |
|
Military |
1.710–1.850 |
|
Operational fixed |
1.850–1.990 |
|
Studio transmitter link |
1.990–2.110 |
|
Common carrier |
2.110–2.130 |
|
Operational fixed |
2.130–2.150 |
|
Common carrier |
2.160–2.180 |
|
Operational fixed |
2.180–2.200 |
|
Operational fixed (TV) |
2.500–2.690 |
|
Common carrier and satellite (downlink) |
3.700–4.200 |
|
Military |
4.400–4.990 |
|
Military |
5.250–5.350 |
|
Common carrier and satellite (uplink) |
5.925–6.425 |
|
Operational fixed |
6.575–6.875 |
|
Studio transmitter link |
6.875–7.125 |
|
Common carrier and satellite (downlink) |
7.250–7.750 |
|
Common carrier and satellite (uplink) |
7.900–8.400 |
|
Common carrier |
10.7–11.7 |
|
Operational fixed |
12.2–12.7 |
|
CATV studio links |
12.7–12.95 |
|
Studio transmitter link |
12.95–13.2 |
|
Military |
14.4–15.25 |
|
Common carrier |
17.7–19.3 |
Terrestrial radiolink systems are point-to-point; that is, the signal is transmitted in a beam from a source microwave antenna across the Earth's surface to the antenna at which it is aimed. The width of the beam transmitted by a microwave antenna varies between 1º and 5º as a function of the frequency of transmission and antenna size. As a result, the transmission is highly directional, which is desirable if the information is intended for only one destination (for example, a telephone conversation). For many applications, however, the information has multiple destinations (for example, TV broadcasts), which makes the satellite radiolink system more practical and desirable.
Satellite Radiolink Systems
Figure 3.10 is a simple model of a satellite radiolink system. The satellite contains several receiver/amplifier/transmitter sections, called transponders, each of which operates at a slightly different frequency. Each of the 12 transponders on a satellite (12 on many of those placed in orbit during the 1980s, more on later orbited communications satellites) has a bandwidth of 36MHz. Individual transmitter sites, called uplink Earth stations, send narrow beams of microwave signals to the satellite. The satellite acts as a relay station. A transponder receives the signal from a single transmitter, amplifies it, and then retransmits it toward Earth on a different frequency. Note that the transmitting Earth station sends to only one transponder on a single satellite. The satellite, however, sends to all downlink receiving Earth stations in its area of coverage, called its footprint.
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Figure 3.10 Satellite radiolink system.
Several types of signals are carried by satellite systems. For example, 6MHz bandwidth standard TV programs, multiplexed 64 kilobits per second (Kbps) telephone channels, and high-speed data all can be carried simultaneously. One privately operated system, the Satellite Business System, which was merged with Hughes Network Systems, is all digital, and each of the 10 transponders per satellite is capable of carrying 43Mbps of digital data.
Commercial Satellites
Commercial communications satellites are launched into geostationary orbit at an altitude of 35,900 kilometers (22,300 miles) above the equator. This means that the geostationary satellite is orbiting the Earth at a constant speed and in the same direction as the Earth's rotation about its axis. The orbiting speed is such that it causes the satellite to have a fixed location with respect to the Earth. The Earth station antennas, therefore, can be fixed in position and do not have to track a moving target in the sky. The angle of view for a geostationary satellite is almost 120º wide. In principle, three such satellites equally spaced around the equator could cover the Earth from 60º north latitude to 60º south latitude. In practice, the coverage angle is restricted to less than 110º because the Earth station's antenna must be elevated above the local horizon by more than 5º.
LEOS Satellites
A relatively new type of satellite system referred to as a low Earth orbit satellite (LEOS) began service during the mid-1990s and can be expected to gain in popularity over the next few years. Placing a satellite into a low Earth orbit substantially reduces the amount of power required by a transmitter on Earth to bounce a signal off the satellite. Similarly, the satellite does not need to produce as strong a signal as required by a geostationary satellite to reach a receiver. Alternately, with the same amount of signal power as that used by a geostationary satellite, a LEOS signal can be received by a substantially smaller receiver. This makes a LEOS system ideal for supporting two-way paging, international cellular communications, and similar applications.
Because a low Earth orbit satellite's footprint may encompass an area on Earth for a few hours or less instead of permanently as by a geostationary satellite, the system operator must launch a large number of satellites to provide constant coverage to a fixed geographical area.
One example of an LEOS system is the Iridium system of 66 planned satellites that supports cellular communications on a worldwide basis. Iridium employs a system of relaying a call from one satellite to another until the call reaches a satellite that serves an Earth station that covers the destination of the call. This design strategy resulted in a considerable level of complexity that drove up the cost of call routing. In addition, Iridium phones are rather bulky and require a line-of-sight to a satellite. Although the basic intention of the service was laudable, it never obtained more than a small fraction of potential subscribers and was saved from extinction only by a contract with the U.S. Department of Defense.
Cellular Radio Systems
Americans have demonstrated an insatiable desire to communicate with each other anywhere, at any time. It seems that no location is too private, too noisy, or too busy to exclude the installation of a telephone or a data terminal. Because Americans spend a lot of time in their cars, mobile telephones and data terminals are in great demand. Each telephone conversation requires a separate radio channel, and because only a limited number of such channels were available in the past, the demand for mobile telephone channels far outstripped the radio frequencies available to provide them. However, in 1982, a system allowing the reuse of channels within a metropolitan area, called the cellular radio system, began trial operation in Chicago. This system provided many more mobile telephone channels. It rapidly gained acceptance throughout the United States in the late 1980s.
Figure 3.11 shows a diagram of a simple cellular system. A metropolitan area is divided into several cells, each of which is served by a low-powered transmitter and an associated receiver. The radio channels are suitable for data transmission up to 14.4Kbps as well as voice transmission. The number of radio channels assigned to each cell is sufficient for the predicted number of users in that cell at any one time. When a caller makes a call, his mobile unit automatically seizes a free channel in his current cell. When the caller moves out of the cell, the cell controller automatically switches control of the call from the cell being left to the one being entered. Even a different radio channel can be used, but the caller doesn't have to do anything and is never aware of anything happening. The call is linked from the cell controller to a central switching system. The central switching system can link the caller via radio to another mobile user or can access the public telephone network for connection to any fixed telephone.
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In Chapter 15, "Wireless Transmission," the operation of different types of cellular radio systems to include analog mobile phone service (AMPS), time-division multiple access (TDMA), and code-division multiple access (CDMA) are explained in detail.
Figure 3.11 Each cell contains a cell controller and transmitter-receivers for several