Wireless services need compatible networks. These networks bring the end-users within the realm of established wireless links so as to offer the service they need. Wireless networking refers to interconnecting the end-users via radio means.
Typically wireless communication involves modulating a high-frequency carrier by the baseband signal (or by a group of subcarriers, each modulated by a baseband voice or data signal) and radiating the modulated carrier as an electromagnetic wave using a suitable antenna. At the receiving end, an antenna system tuned to the central frequency, such as that of the carrier frequency, receives the EM wave.
The baseband signal(s) are recovered from the passband of the tuned-in radio frequency spectrum using appropriate demodulation techniques. Further, as needed, regenerative repeaters are interposed between the transmitting and receiving end.
The range of frequencies in the electromagnetic spectrum compatible for radio transmissions with the available technology, in general, spans widely, stretching from almost 100 kHz (termed as long-wave transmissions) up to about 60 GHz (called millimeter (mm) waves). This wide range of the EM spectrum is divided into specific bands and these bands are designated for specific applications.
Exclusively for modern wireless communication purposes, the UHF and/or microwave bands are used. (Classical radio-telegraphy and telephony adopted the short-wave band for long-distance communications.) There are unique signal impairment situations (as will be discussed in detail later in this chapter) associated with wireless telecommunications.
Wireless communication, in essence, is a point-to-point communication system, but there could be multiple transmission paths resulting from reflections and scattering of EM waves by physical structures such as buildings etc. or due to refractory effects caused by the atmosphere. The received signal is a vector sum of these multipathtraversed constituents, namely, the primary ray and the delayed secondary rays.
The extents of attenuation suffered by these rays would be different and may change with time. Such changes are significant in mobile and cellular phone applications. The attenuation is controlled by atmospheric conditions as well as by shadowing and other scattering of the EM waves involved.
In effect, wireless telecommunication signals face what is known as time-dependent “signal fading”. To counter the effects of fading, a fade-margin is facilitated. Increasing the transmitted power and/or incorporating frequency- and space-diversity receptions are envisaged in practice to facilitate the fade-margin. (In frequency diversity systems, the same intelligence is transmitted over more than one carrier.
It is expected that, even if one channel fades, the other channels are unlikely to fade. Hence, the information can be extracted from the unfaded channels. The space diversity system uses a single carrier but the reception is done at multiple, spatialdispersed receiving antenna/receiver systems. Again, if a faded reception is perceived at one receiving locale, the other locales may probably receive unfaded signals.
Therefore, the information can be recovered from these unfaded receptions.) Facilitating reliable communication, despite the inevitable fading conditions, poses, however, a host of challenges in the operational scenario of modern wireless telecommunication services.
Nevertheless, diversity-based system technology, different coding techniques, and spread-spectrum based strategies are adopted to minimize the effects of impairments in such wireless transmissions; and the networks implemented use such strategies consistent with the standards and application profiles. Apropos the variety in the existing wireless standards, a number of associated networks have emerged and have been adopted.
