DATA COMMUNICATION PRINCIPLES Frequency Hopping Spread Spectrum FHSS

DATA COMMUNICATION PRINCIPLES : Frequency Hopping Spread Spectrum (FH-SS) for 2. 4 GHz Specification

Frequency Hopping Spread Spectrum (FH-SS) for 2. 4 GHz Specification Spread spectrum (SS) is a mechanism of sharing channel bandwidth for broadcast type access mechanisms (called multiple access). However, for the IEEE WLAN, it is not used as a multiple access mechanism. Instead, it is used as an interference combating mechanism. This is necessary because the network bandwidth is unlicensed and could be used by any device in an environment. Let be the signal bandwidth and be the channel bandwidth. Then, in order to use spread spectrum techniques, we must have In SS techniques, the signal bandwidth is spread to close to while keeping the total signal energy constant. By doing so, the signal energy carried per bandwidth is reduced a great deal. Consequently, a signal with very low power density (power per Hz) is used for transmission. This is shown in Figure 4 -13. The spreading is done in such a way that only intended receiver should be able to detect the signal.

In frequency hopping, this is done by dividing the wide channel bandwidth into many narrow channels, each with a bandwidth close to The signal is transmitted using one of the narrowband channels for a short duration after which this channel is replaced by another narrowband channel, and so on. Thus, by changing the narrowband channel in a pattern known only to the intended transceiver (transmitter/receiver) pair, a frequency-hopping pattern is implemented. This hopping is called fast frequency hopping if the channel hopping speed is faster than the bit rate. Contrarily, if more than one bit are transmitted per narrowband channel, then we call it as slow frequency hopping spread spectrum (SS-SFH). The IEEE WLAN specifies a slow frequency hopping mechanism.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press