A Direct Sequence Spread-Spectrum Modem for Wideband HF Channels

The use of spread-spectrum techniques to reduce jamming vulnerability and probability of intercept is fundamental in many military communication systems. This paper describes the design of an experimental spread-spectrum modem (modulator-demodulator) built to interface with an existing HF test facility that offers an equalized 1 MHz bandwidth over skywave paths. The modem uses differential binary and quadrature phase shift keying data modulation and direct-sequence bi-phase spreading techniques. Programmable charge-coupled-device matched filters, which allow symbol-to-symbol code agility, and coherent pre-detection integration provide up to 30 dB of processing gain at a 500 bits/second data rate. Flexible data rate and range (time-of-arrival) tracking to maintain bit synchronization are incorporated into the system.