Wireless Transmission
Most of the technical aspects of wireless transmission will be addressed in Chapter 6. However, we make a few points to compare basic wireless transmission with metallic or fiber media.
Broadcast TV and cellular telephony are familiar forms of wireless communication. Whereas broadcast TV is a high-speed, one-way service and cellular telephony is a low-speed, two-way service, innovations are under consideration to provide high-speed, two-way communication using wireless.
Consumers have shown a willingness to pay for the convenience of being untethered. Cellular telephony has lower-quality, normally fewer service options and definitely higher pricing than wired telephone service. Nonetheless, it has experienced explosive growth worldwide. More than 500 million cellular handsets are in the hands of consumers today, as opposed to perhaps 150 million Internet users. And the lead of cellular telephony seems to be lengthening over Internet users annually. In 1998, more than 125 million cellular handsets were sold worldwide.
The movement to residential wireless high-speed two-way service is spurred on by these factors:
The observation that consumers will pay a premium for wireless service
The availability of technology such as digital signal processing (DSP) advances, which make new coding and noise-mitigation possible
The availability of spectrum as a result of recent government actions in the United States and abroad
The bit-carrying capability of a wireless channel is governed by these factors:
The amount of spectrum available (more spectrum means more bits)
The frequency of the channel (lower frequencies means more distance)
The modulation technique, which governs spectral efficiency
Local Multipoint Distribution Service (LMDS) has the largest amount of contiguous bandwidth available, at 1.1 GHz. This certainly compares favorably with, say, broadcast TV, which has 6 MHz.
Secondly, just as with metal, the higher the frequency, the less distance the signal can travel. LMDS is offered at 28 GHz, whereas broadcast TV is offered starting at 54 MHz. Thus, LMDS can travel only a few miles, and broadcast TV signals can be received for dozens of miles.
Finally, more aggressive modulation can yield more bits per symbol. Satellite services use a robust modulation, such as QPSK, which provides two bits per symbol; because it uses lower frequencies, broadcast digital TV can use more aggressive modulation, such as VSB or OFDM (in Europe).
Impairments
Open-air transmission is subject to serious impairments, and cellular phone subscribers can attest to this. Most impairments are conceptually the same problems as those confronted in wired media, but some variations exist. Among the key impairments are line of sight, multipath, absorption, and interference problems.
Line of Sight
If the frequency used is low enough, then radio signals can penetrate walls and roofs, like TV or cell phones, both of which operate at below 1 GHz. Beyond that, however, line of sight will be required between the transmitter and the receiver. Interference from natural elements such as trees and manmade structures must be taken into effect when deploying antennas and telling customers where they can receive service.
Multipath
When an open-air signal is transmitted, the signal is not radiated on a straight line to the receiver but is subject to dispersion. Attempts are made to guide the radio energy to minimize dispersion, but dispersion happens nonetheless.
If a structure is adjacent to the line of sight, then dispersed signals will bounce off the structure before terminating at the receiver. Because the dispersed signal took a different and longer path than a signal that took line of sight, two signals will arrive at the destination by different paths, hence the term multipath. Moreover, because the reflected signal took a longer path, it will arrive at the destination later. The result is delay spread, similar to the case of multimode fiber. Delay spread causes confusion at the receiver, and certain mitigation techniques are required. Excessive delay spreads and an excessive number of paths can render the intended signal indecipherable.
Absorption
For signals of sufficiently high frequency, rain can absorb enough energy to make the intended signal too weak. The only antidote to this is to use lower frequencies (usually very hard) or to use shorter paths by deploying more transmitters (also very hard and expensive). Because of the effects of rain and foliage, certain wireless services are infeasible in tropical geographies.
Interference
Other transmitters that inadvertently use the same spectrum in the same geographical area can adversely affect transmission. This can happen due to faulty equipment. For example, frequency reuse is a useful tool to maximize spectrum use over a wide geographical area. With frequency reuse, two transmitters may be granted use of the same frequency, provided that they are widely separated so as not to interfere with each other. A familiar case is AM radio: A station in Los Angeles may use the same frequency as a station in New York, but they won't interfere.
However, if two transmitters are closely spaced and one of them has a transmitter that is using excessive power, then they may interfere. This is why the Federal Communications Commission has regulations on power.
Spectrum allocation and various wireless services are discussed in Chapter 6.
Transmission Media Summary
Table 2-7 summarizes the maximum bit rate and distance characteristics of various media. These are compared according to generally available engineering practice as of this writing. Innovations are happening that will increase the maximum bit rate and distance.
Many other wireless options depend on the amount of bandwidth and where it is located. Some options are discussed in Chapter 6.