Previous | Table of Contents | Next |
Radio systems are designed for a certain area of coverage. Even early radio and TV broadcast systems used the concept of coverage areas to provide service to a defined service area. Therefore, the same frequencies could be reused to support service in metropolitan areas some distance away. For instance, 98.1 (MHz) on your FM dial might be WXYZ in New York and KFRC in San Francisco, California. Similarly, Channel 7 on your TV might be WFAA in Dallas, Texas and KGO in San Francisco. Broadcast TV stations in the United States can reuse frequencies if separated by at least 150 miles [12-6].
The formal concept of radio cells dates back to 1947 when Bell Telephone Engineers developed a radio system concept that included numerous, low-power transmit/receive antennae [12-1]. Scattered throughout a metropolitan area, such an architecture would increase the effective subscriber capacity of radio systems by breaking the area of coverage into small cells, or smaller areas of coverage. Each frequency could be reused in non-adjacent cells. Additionally, the cells can be split, or subdivided further as the traffic demands of the system increasethe costs of the network are highly scaleable.
Frequency reuse is sensitive to factors which should now be familiar as a result of the discussion of microwave and satellite systems in Chapter 3. Specifically, these factors include frequency, power level, antenna design, and topography. Higher frequency signals always attenuate to a greater extent over distance given the same power level. Antenna design is sensitive to wavelength and other factors. Topography is always an issue, as line of sight is always preferable.
Cells are generally defined in three categories, macrocells, microcells, and picocells (Figure 0112.1). As the cells shrink, the advantages of frequency reuse increase significantly. However, the costs of network deployment increase dramatically and the issues of switching traffic from moving transmitters from cell-to-cell increase considerably. Nonetheless, the increase in traffic-handling capacity can be quite remarkable, with associated increases in revenue potential. Assuming that 12 channels are available for use in a metropolitan area of a 60-mile radius, consider the following theoretical scenario shown in Figure 12.1.
Figure 12.1 Macrocells, microcells, and picocells.
Clearly, communications is going digital. Nonetheless, analog does have a place, if for no other reason than it is the incumbent technology. Transition from analog is expensive and disruptive, and will take some time to complete. This scenario holds true in the wireless, as well as the wired, world. Just as was the case in the wired world, digital wireless offers the advantages of more efficient use of bandwidth (spectrum), improved quality of transmission through enhanced error performance, increased throughput (a logical extension of diminished transmission errors), and improved security. There exist no less than 16 different means of modulating a radio signal, most of which relate to digital radio. Generally speaking, Amplitude Modulation (AM) alone is ineffective due to the phenomenon of fading. For the most part, a combination of Phase Shift Keying (PSK) and Amplitude Modulation (AM) is used to yield compression of up to 16:1.
Communications networks take great advantage of the concept of DAMA (Demand-Assigned Multiple Access). DAMA allows multiple devices to share access to the same network on a demand basis (first come, first served). There exist a number of ways in which multiple access can be provided in a wireless network; those techniques largely are mutually exclusive.
Previous | Table of Contents | Next |