TABLE OF CONTENTS Two major advantages of frequency hopping are that it can be implemented over a much larger frequency band than it is possible to implement direct-sequence spreading, and that the band can be divided into noncontiguous segments. Another major advantage is that frequency hopping provides resistance to multiple-access interference, while not requiring power control to prevent the near-far problem. Since direct-sequence systems cannot escape the near-far problem by hopping, accurate power control is crucial but becomes much less effective as the fading rate increases. These advantages of frequency hopping will be decisive in many applications. For example, the Bluetooth system and combat net radios use frequency hopping to avoid the near-far problem.
Frequency-hopping systems are usually part of a frequency-hopping code-division multiple-access (FH/CDMA) network in which all systems share the same M frequency channels. In a synchronous FH/CDMA network, the systems coordinate their frequency transitions and hopping patterns. Consequently, as many as M frequency-hopping signals can be simultaneously accommodated by the network with insignificant multiple-access interference at any of the active receivers. Network coordination is much simpler to implement than for a DS/CDMA network because the timing alignments must be within a small fraction of a hop duration instead of a small fraction of a spreading-sequence chip. Multipath signals and errors in range estimates can be accommodated at some cost in the energy per information bit by increasing the switching time between frequency-hopping pulses. However, some type of centralized or cellular architecture is...
