TABLE OF CONTENTS A spread-spectrum system is used to spread a signal over a frequency band that is much wider than the minimum bandwidth required to transmit the information being sent. Typically, a multimegahertz bandwidth is used to transmit a voiceband signal of a few kilohertz. A spread-spectrum system can be used to improve spectrum efficiency in mobile-radio communications.
In the newly allocated 850-MHz mobile-telephone frequency spectrum, a band of 40 MHz (825 to 845 and 870 to 890 MHz) is assigned to mobile communications, as was mentioned in the introduction to this book. One-half of the frequency band is used for transmitting and the other half for receiving, with a one-way transmission bandwidth consisting of a 20-MHz continuous band. Communications channels can be evenly assigned a particular frequency for conventional use in a cellular planned system, or a coded waveform covering the total 20-MHz band can be assigned to a particular user. Codes that can be used include the following different kinds of modulation:
Direct-sequence-modulated system-Modulation of a carrier by a digital-code sequence whose bit rate is much higher than the information signal bandwidth.
Frequency-hopping system-Carrier-frequency shifting in discrete increments, in a pattern governed by a code sequence which determines the order of frequency usage.
Time-hopping system-The timing of transmission-e.g., low-duty-cycle or short-duration-is governed by a code sequence.
Time/frequency-hopping system [18]-The code sequence determines both the transmitted frequency and the transmission bit rate.
"Chirp" or pulse-FM modulation system-A carrier is swept over a wide...
