Hf

Satellite and terrestrial microwave 1 VHF | UHF | SHF , EHF ,

104 103 102 101 1 10-1 10-2 10-3 Wavelength (meters)

104 103 102 101 1 10-1 10-2 10-3 Wavelength (meters)

FIGURE 3.48 Radio spectra wavelength of 1 km to 10 km, whereas the extremely high frequency (EHF) band occupies the range from 30 to 300 GHz corresponding to wavelengths of 1 millimeter to 1 centimeter. Note that the progression of frequency bands in the logarithmic frequency scale have increasingly larger bandwidths, for example, the "band" from 1011 to 1012 Hz has a bandwidth of 0.9 x 1012 Hz, whereas the band from 105 to 106 Hz has a bandwidth of 0.9 x 106 Hz.

The propagation properties of radio waves vary with the frequency. Radio waves at the VLF, LF, and MF bands follow the surface of the earth in the form of ground waves. VLF waves can be detected at distances up to about 1000 km, and MF waves, for example, AM radio, at much shorter distances. Radio waves in the HF band are reflected by the ionosphere and can be used for long-distance communications. These waves are detectable only within certain specific distances from the transmitter. Finally, radio waves in the VHF band and higher are not reflected back by the ionosphere and are detectable only within line-of-sight.

In general, radio frequencies below 1 GHz are more suitable for omnidirectional applications, such as those shown in Table 3.6. For example, paging systems ("beepers") are an omnidirectional application that provides one-way communications. A high-power transmission system is used to reach simple, low-power pocket-size receivers in some geographic area. The purpose of the system is to alert the owner that someone wishes to communicate with him or her. The

System Description Distance

Paging Short message 10s of kilometers

Cordless telephone Analog/digital voice 10s of meters

Cellular telephone Analog/digital voice and data kilometers

Personal Communication Services Digital voice and data 100s of meters

Wireless LAN High-speed data 100 meters

TABLE 3.6 Examples of omnidirectional systems system may consist of a single high-powered antenna, or a network of interconnected antennas, or be a nationwide satellite-based transmission system. These systems deliver the calling party's telephone number and short text messages. Paging systems have operated in a number of frequency bands. Most systems currently use the 930 to 932 MHz band.

Cordless telephones are an example of an omnidirectional application that provides two-way communications. Here a simple base station connects to a telephone outlet and relays signaling and voice information to a cordless phone. This technology allows the user to move around in an area of a few tens of meters while talking on the phone. The first generation of cordless phones used analog radio technology and subsequent generations have used digital technology.

Application—Cellular Communications

Analog cellular telephone systems were introduced in 1979 in Japan. This system provided for 600 two-way channels in the 800 MHz band. In Europe the Nordic Mobile Telephone system was developed in 1981 in the 450 MHz band. The U.S. Advanced Mobile Phone System (AMPS) was deployed in 1983 in a frequency band of 50 MHz in the 800 MHz region. This band is divided into 30 kHz channels that can each carry a single FM-modulated analog voice signal.

Analog cellular phones quickly reached their capacity in large metropolitan areas because of the popularity of cellular phone service. Several digital cellular telephone systems based on digital transmission have been introduced. In 1991 Interim Standard IS-54 in the United States allowed for the replacement of a 30 kHz channel with a digital channel that can support three users. This digital channel uses differential QAM modulation in place of the analog GM modulation. A cellular standard based on code division multiple access (CDMA) was also standardized as IS-95. This system, based on direct sequence spread spectrum transmission, can handle more users than earlier systems could. In Europe the Global System for Mobile (GSM) standard was developed to provide for a pan-European digital cellular system in the 900 MHz band. These cellular systems are discussed further in Chapter 6.

In 1995 personal communication services (PCS) licenses were auctioned in the U.S. for spectrum in the 1800/1900 MHz region. PCS is intended to extend digital cellular technology to a broader community of users by using low-power transmitters that cover small areas, "microcells." PCS thus combines aspects of conventional cellular telephone service with aspects of cordless telephones. The first large deployment of PCS is in the Japanese Personal Handiphone system that operates in the 1800/1900 band. This system is now very popular. In Europe the GSM standard has been adapted to the 1800/1900 band.

Application—Wireless LANs

Wireless LANs are another application of omnidirectional wireless communications. The objective here is to provide high-speed communications among a number of computers located in relatively close proximity. Most standardization efforts in the United States have focused in the Industrial/Scientific/Medical (ISM) bands, which span 902 to 928 MHz, 2400 to 2483.5 MHz, and 5725 to 5850 MHz, respectively. Unlike other frequency bands, the ISM band is designated for unlicensed operation so each user must cope with the interference from other users. In Europe, the high-performance radio LAN (HIPERPLAN) standard was developed to provide high-speed (20 Mbps) operation in the 5.15 to 5.30 GHz band. In 1996 the Federal Communications Commission (FCC) in the United States announced its intention to make 350 MHz of spectrum in the 5.15 to 5.35 GHz and 5.725 to 5.825 GHz bands available for unlicensed use in LAN applications. These developments are significant because these systems will provide high-speed communications to the increasing base of portable computers. This new spectrum allocation will also enable the development of ad hoc digital radio networks in residential and other environments.

Application—Point-to-Point and Point-to-Multipoint Radio Systems

Highly directional antennas can be built for microwave frequencies that cover the range from 2 to 40 GHz. For this reason point-to-point wireless systems use microwave frequencies and were a major component of the telecommunication infrastructure introduced several years ago. Digital microwave transmission systems have been deployed to provide long-distance communications. These systems typically use QAM modulation with fairly large signal constellations and can provide transmission rates in excess of 100 Mbps. The logarithmic, rather than linear, attenuation gave microwave radio systems an advantage over coaxial cable systems by requiring repeater spacings in the tens of kilometers. In addition, microwave systems did not have to deal with right-of-way issues. Microwave transmission systems can also be used to provide inexpensive digital links between buildings.

Microwave frequencies in the 28 GHz band have also been licensed for point-to-multipoint "wireless cable" systems. In these systems microwave radio beams from a telephone central office would send 50 Mbps directional signals to subscribers within a 5 km range. Reflectors would be used to direct these beams so that all subscribers can be reached. These signals could contain digital video and telephone as well as high-speed data. Subscribers would also be provided with transmitters that would allow them to send information upstream into the network. The providers of this service have about 1 GHz in total bandwidth available.

Application—Satellite Communications

Early satellite communications systems can be viewed as microwave systems with a single repeater in the sky. A (geostationary) satellite is placed at an altitude of about 36,000 km above the equator where its orbit is stationary relative to the rotation of the earth. A modulated microwave radio signal is beamed to the satellite on an uplink carrier frequency. A transponder in the satellite receives the uplink signal, regenerates it, and beams it down back to earth on a downlink carrier frequency. A satellite typically contains 12 to 20 transponders so it can handle a number of simultaneous transmissions. Each transponder typically handles about 50 Mbps. Satellites operate in the 4/6, 11/14, and 20/30 GHz bands, where the first number indicates the downlink frequency and the second number the uplink frequency.

Geostationary satellite systems have been used to provide point-to-point digital communications to carry telephone traffic between two points. Satellite systems have an advantage over fiber systems in situations where communications needs to be established quickly or where deploying the infrastructure is too costly. Satellite systems are inherently broadcast in nature, so they are also used to simultaneously beam television, and other signals, to a large number of users. Satellite systems are also used to reach mobile users who roam wide geographical areas.

Constellations of low-earth orbit satellites (LEOS) are being planned for deployment. These include the Iridium and Teledesic systems. The satellites are not stationary with respect to the earth, but they rotate in such a way that there is continuous coverage of the earth. The component satellites are interconnected by high-speed links forming a network in the sky.

Was this article helpful?

0 0

Post a comment