Frequency hopping spread spectrum
For frequency hopping spread spectrum (FHSS) systems, the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel. The digital data is broken into uniform sized bursts which are transmitted on different carrier frequencies. Instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth. The pseudorandom change of the carrier frequencies of the user randomizes the occupancy of a specific channel at any given time, thereby allowing for multiple accesses over a wide range of frequencies. In the FR receiver, a locally generated PN code is used to synchronize the receiver’s instantaneous frequency with that of the transmitter. At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel since narrowband FM or FSK is used. The difference between FHMA and a traditional FDMA system is that the frequency hopped signal changes channels at rapid intervals.
Frequency hopping transmitter:
- The first step is the modulation of user data according to one of the digital-to-analog modulation schem
- In next step, frequency hopping is performed, based on a hopping sequence.
- The hopping sequence is fed into a frequency synthesizer generating the carrier frequencies fi. Second modulation uses the modulated narrowband signal andthe carrier frequency to generate a new spread signal with frequency of fi f0 fora 0 and fi f1 for a 1 respectively.
Frequency hopping receiver:receiver of an FHSS system has to know the hopping sequence and must stay synchronized. Performs the inverse operations of the modulation to reconstruct user data. Several filters are also needed
Slow hopping frequency hopping:If the channel changes at a rate less than or equal to the symbol rate, it is called slow frequency hopping the transmitter uses one frequency for several bit periods
- Figure1.37shows five user bits with a bit period tb.
- Performing slow hopping, the transmitter uses the frequency f2 for transmitting the first three bits during the dwell time td.
- Then, the transmitter hops to the next frequency f3
Fast hopping frequency hopping:If the rate of change of the carrier frequency is greater than the symbol rate then the system is referred to as a fast frequency hopping system.
- The transmitter changes the frequency several times during the transmission of a single bit.
- Figure below shows the transmitter hops three times during a bit period.