By John Walker

Deals updated info on advancements in cellular details platforms. The textual content additionally provides an summary of the sphere, together with GSM, mobile radio, cellular facts and ITS. extra sections disguise satellite tv for pc navigation structures, digital tolling and street pricing.

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Extra info for Advances in Mobile Information Systems (Artech House Mobile Communications Library)

Sample text

For such waves, in free space, the product of the wave frequency in Hertz and the wavelength in meters is about 3 × 108 m/s. Radio waves are electromagnetic waves within a convenient range of frequencies, say, between 3 kHz (3 × 103 Hz) and 3 THz (3 × 1012 Hz), although significant use is within the range of 9 kHz to 400 GHz, the range covered by the International Telecommunication Union (ITU) radio regulations. 1. Radio spectrum usage for communications and navigation purposes began at the lower frequencies, limited by the technology available at the time, and moved to higher frequencies as the demand for radiocommunication grew and as the necessary technologies were developed.

The bandwidth occupied by a modulated signal usually is within the range of a few times greater or smaller than the information rate (for digital streams, that is the bit rate). Much effort has been expended in the design of modulation methods that reduce the radio frequency (RF) bandwidth to the minimum necessary to improve spectrum utilization. Radio services using such narrow bandwidth have frequency assignments side by side in the spectrum, spaced by somewhat more than the necessary bandwidth.

Spread spectrum systems are technically complex, and they cannot effectively share the same frequency band with narrowband systems if the objective is to maximize spectrum utilization, but they may provide an effective solution for some communication needs. Rarely is an information stream continuous, thus requiring the full and exclusive use of a radio channel. Where the information flows are intermittent (a clear example is for telephony, where one party in a conversation usually speaks during less than 50% of a call, and the calls themselves are occasional), it is desirable that a number of users share a channel.

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