Up to now we have discussed guided transmission. The signal is guided or conducted down some sort of a "pipe." The "pipes" we have covered included wire pair, coaxial cable, and fiber-optic cable. Radio transmission, on the other hand, is based on radiated emission.
The essential elements of any radio system are (1) a transmitter for generating and modulating a "high-frequency"6 carrier wave with an information baseband, (2) a transmitting antenna that will radiate the maximum amount of signal energy of the modulated carrier in the desired direction, (3) a receiving antenna that will intercept the maximum amount of the radiated energy after its transmission through space, and (4) a receiver to select the desired carrier wave, amplify the signal, detect it, or separate the signal from the carrier. Although the basic principles are the same in all cases, there are many different designs of radio systems. These differences depend upon the types of signals to be transmitted, type of modulation (AM, FM, or PM or a hybrid), where in the frequency spectrum (see Figure 2.6) in which transmission is to be affected, and licensing restrictions. Figure 2.14 is a generalized model of a radio link.
The information transport capacity of a radio link depends on many factors. The first factor is the application. The following is a brief list of applications with some relevant RF bandwidths:
• Line-of-sight microwave, depending on the frequency band: 2, 5, 10, 20, 30, 40, 60 MHz.
• SCADA (system control and data acquisition): up to 12 kHz in the 900-MHz band.
6In the context of this book, "high-frequency" takes on the connotation of any signal from 400 MHz to 100 GHz.
• Satellite communications, geostationary satellites: 500 MHz or 2.5-GHz bandwidths broken down into 36- and 72-MHz segments.
• Cellular radio: 25-MHz bandwidth in the 800/900-MHz band. The 25-MHz band is split into two 12.5-MHz segments for two competitive providers.
• Personal communication services (PCS): 200-MHz band just below 2.0 GHz, broken down into various segments such as licensed and unlicensed users.
• Cellular/PCS by satellite (e.g., Iridium, Globalstar), 10.5-MHz bandwidth in the 1600-MHz band.
• Local multipoint distribution system (LMDS) in 28/38-GHz bands, 1.2-GHz bandwidth for CATV, Internet, data, and telephony services (Ref. 5).
Bandwidth is also determined by the regulating authority (e.g., the FCC in the United States) for a particular service/application. Through bit packing techniques, described in Chapter 9, the information carrying capacity of a unit of bandwidth is considerably greater than 1 bit per Hz of bandwidth. On line-of-sight microwave systems, 5, 6, 7, and 8 bits per hertz of bandwidth are fairly common.
Chapter 9 provides a more detailed discussion of radio systems.
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