Chapter 14. Wireless Communications

Most people consider mobility to be one of their most cherished abilities and rights. In fact, what do we take away from those who have been deemed injurious to their fellow humans? Mobility. The freedom afforded by mobility is very precious to humans, indeed.

One area in which we like to exercise our freedom of mobility is at work, and the mobile workforce is growing all the time. Mobile workers, often called road warriors, are people who are away from their primary workplace at least 20% of the time. Today, about one-third of the population fits this category.

Another area in which we want to use our freedom to be mobile is with the Internet. The mobile Internet is already a reality in some places, and it will become more prevalent elsewhere in the near future, as more and more Internet-ready cellular phones and other wireless devices make their way into people's pockets. Eventually, and maybe within just a few years, there will be more mobile Internet devices than PCs connected to the Internet; wireless is predicted to become the dominant form of Internet communication by 2009. These trends, as well as the trend toward portable computing in the form of wearables, contribute to our growing interest in wireless.

We tend to think of wireline facilities as the default and of wireless as something extra that needs to be justified. Wireless has often been justified in situations, such as when wire cannot be physically applied (for example, in a jungle, in a desert, on a mountaintop); when wire cannot be economically justified (for example, when the expense of burying cable is too great); when time is an issue; and when mobility is a factor. The future will see mixed-media approaches (including wireline and wireless solutions), depending on the given situation. For example, we may want to bring fiber as close as possible to an end node, but then to engage in a more mobile lifestyle, we might rely on a wireless link for the last few meters.

The building blocks for the wireless world include cellular and PCS (Personal Communication Services) service providers, cell phone makers, infrastructure companies, handheld computer makers, electronic parts manufacturers, wireless portals, e-commerce companies, movie and music studios, automobile companies, and satellite companies. The wireless world will really have a big impact on transportation. We are reengineering the way vehicles will operate, and things such as traffic management systems, smart cars, smart highways, and navigational systems are very much a part of the telecommunications sector.

Wireless networks will be a very important part of our future. Wireless networks fall into the same domains as wireline networks wide area networks (WANs), metropolitan area networks (MANs), local area networks (LANs), and personal area networks (PANs) and are applied in support of mobile (such as cellular and PCS systems), fixed (as in wireless local loop), and broadcast (television) applications.

Wireless has been with us a very long time, in the form of radio, which involves inducing an electrical current at a remote location via the propagation of electromagnetic waves, through space, with the intent of communicating information. This technology is more than 100 years old, so we could turn things around and say that wires can be thought of as private radio spectrum.

The key issues in wireless are technological (for example, standards, bandwidth, performance, spectrum reuse) and political (for example regulation, spectrum allocation). Regulation has a tremendous influence on what is made available, who can use it, and how it's allocated among the various competitive providers. Bandwidth is a huge consideration in anything to do with telecommunications, and in the realm of wireless, bandwidth refers to the amount of spectrum available to transport information. Narrowband wireless applies a baseband, or single-channel, approach; wideband wireless applies a broadband, or channelized, approach. Bandwidth is often limited by regulation, technology, and environmental conditions in that order.

There are a number of key areas to consider when it comes to the performance of wireless:

Path loss The path loss in decibels (dB) represents the ratio of the strength of the transmitted signal to the received strength.

Multipath Multipath is the artifact of reflections and echoes. For example, with antenna televisions, there are times when an image has a ghost figuration about it and that is an echo. Multipath can create secondary and tertiary signals that compete with the primary signal.

Fading There are a number of propagation characteristics, and they vary with the different frequencies. As a mobile station moves through a cell, the multipath signals abruptly and rapidly add to and subtract from each other. As a result, very good signals are interspersed with very poor signals. This effect is referred to as a Rayleigh fade (named after the physicist Lord Rayleigh). The multipath delays can be predicted on a statistical basis, and components can be designed to handle the problem.

Interference and noise Interference and noise are byproducts of precipitation in the air, metals in the environment, or a variety of other anomalies. Error-correction techniques are needed to fix these problems.

Antenna design, position, and orientation Antennas are constantly being developed to better deal with the problems of wireless transmissions. Antenna design is as critical as the prevailing radio conditions in which the antennas are operating.

The following sections cover various aspects of wireless communications, including spectrum reuse and the various types of wireless networks WANs, MANs, LANs, and PANs.