Modern wireless radio systems range from relatively simple cordless telephones to mobile cellular systems and the emerging Personal Communication Systems (PCS). It is useful to consider such diverse systems as cordless telephone and mobile cellular radio to illustrate some of the fundamental characteristics of wireless communication systems [Cox, 1992].
Table 1: Summary of relevant characteristics of cordless telephone and
cellular mobile radio.
A summary of the relevant parameters and characteristics for cordless telephone and cellular radio is given in Table 1. As evident from that table, the fundamental differences between the two systems are speech quality and the area covered by a base station. The high speech quality requirement in in the cordless application is the consequence of the availability of tethered access in the home and office and the resulting direct competition with wire-line telephone services. In the mobile cellular application the user has no alternative to the wireless access and may be satisfied with lower, but still acceptable, quality of service.
In cordless telephone applications the transmission range is short because the base station can simply be moved to a conveniently located wire-line access point (wall jack) to provide wireless network access where desired. In contrast, the mobile cellular base station must provide access for users throughout a large geographical area of up to approximately 30 km (20 miles) around the base station. This large coverage area is necessary to economically meet the promise of of uninterrupted service to roaming users.
The different range requirements directly affect the transmit power and antenna height for the two systems. High power transmitters used in mobile cellular user sets consume far more power than even complex signal processing hardware. Hence, sophisticated signal processing, including speech compression, voice activity detection, error correction and detection, and adaptive equalization, can be employed without substantial impact on the battery life in portable hand sets. Furthermore, such techniques are consistent with the goals of increased range and support of large numbers of users with a single, expensive base station. On the other hand, the high mobile cellular base station antennas introduce delay spreads which are one or two orders of magnitude larger than those commonly observed in cordless telephone applications.
Clearly, the two systems considered above are at extreme ends of the spectrum of wireless communications systems. Most notably, the emerging PCS systems fall somewhere between the two. However, the comparison above highlights some of the system characteristics which should be considered when discussing access methods for wireless communication systems.
