| A technique of spread-spectrum transmission, interpolated frequency-hopping (IFH), is presented which employs a carrier that moves smoothly and continuously in frequency. This helps to alleviate problems such as spectral splatter and transient mismatch, which are a concern in conventional phase-locked loop based frequency-hopping spread-spectrum systems. In interpolated frequency-hopping, the pseudo-random hopping code is passed through a digital interpolation filter prior to controlling the synthesizer instantaneous frequency output. Laboratory measurements indicate that for certain cases, interpolated frequency-hopping shows a 12.5 dB improvement in average received IF power, has a much sharper roll-off of inband phase noise when compared to conventional hopping, and provides a phase-coherent IF after despreading. An interpolated frequency-hopping transceiver system consisting of a $Delta$-$Sigma$ frequency synthesizer, a $Delta$-$Sigma$ frequency discriminator, and $Delta$-$Sigma$ filter tap coefficients, is proposed. The system would be suitable for integrated mobile radio applications in slow fading environments. |
