ACQUISITION OF SYNCHRONIZATION IN FAST FREQUENCY HOP RADIO

Nader  F. ZEIN

Fast‑Frequency‑Hopping is an effective technique to reduce the susceptibility of a communication system to interception and jamming. The problem considered here is the acquisition of synchronization between the transmitter and the receiver.

A system for acquiring synchronization in a fast‑frequency‑hop radio is presented.

The hopping rate is 16000 hops/sec and the information rate is one bit per hop, encoded onto one of 7000 channels spaced at 25 kHz. A serial‑search technique is used for acquisition. The novel aspect of the system is the degradation in selectivity, and hence in the capacity in rejecting interference, caused by the hop‑rate being comparable with the channel spacing.

Three different types of base‑band filter are considered, two of which are transversal filters and one is a bank of matched filters, matched to various pulse widths.

Two tasks have been accomplished:

     I) The response of these filters is simulated in the presence of worst case jamming signal

                   ii) One of the filters is implemented by using hardware.

In the first case, two fading models of the genuine signal were considered. In one model the genuine signal was assigned a constant magnitude for typically 90% of the time and was completely faded for the rest. The other model assumed two‑path fading. The jamming signal is modeled as pulses, which occur at random times, and at random frequencies. The pulses have Rayleigh amplitude‑phase distribution, with the same mean as the genuine pulses.

At the hop rate of 16000 hops/sec, the time scale of making decision is rather short. This had to be taken into account in the filter design.

The simulation results revealed that the matched filter was not as efficient as the transversal filter. However the matched filter has the advantage that it can readily be adapted to act as a demodulator once synchronization has been achieved. A typical value of the mean acquisition time, which corresponded to a total probability of missing the genuine signal of the order of 10, was found in the vicinity of 9 times the uncertainty period.