Correlated frequency hopping-an improved approach to HF spread spectrum communications

Abstract

Tactical military operations often drive competing telecommunications requirements. For example, terrain and distances may preclude VHF/UHF line-of-sight communications, yet the HF environment may prove unsatisfactory from too many users in a limited bandwidth or intentional jamming. HF propagation, while favorable for over-the-horizon operations, allows relatively easy communications interception. The Correlated Hopping Enhanced Spread Spectrum (CHESS) waveform is introduced as an improved approach to high-speed, HF digital communications. In general terms, frequency hopping (FH) avoids narrowband interference better than direct sequence (DS) spread spectrum. CHESS improves upon current FH systems with a much shorter frequency dwell, and it does not modulate each hopped pulse. As a result, users sharing the spectrum with CHESS are less likely to experience adjacent or co-channel interference. Moreover, CHESS uses a differential frequency hopping (DFH) technique to encode data, which permits reconstruction of hops missed in the detection process. This error correction is over and above other methods that may be applied. The result is an exceptionally robust, low error rate communications system. Tests have yielded excellent bit error rates at up to 19.2 kbps over short- and long-haul propagation paths. The system uses relatively simple hardware. Essentially, an unmodulated direct digital synthesizer serves as a transmitter, and a fast Fourier transform card detects the signal. Digital signal processing as well as ADC and DAC form the basis of system performance.