On the Utility of Chirp Modulation for Digital Signaling

The issue of signal selection in binary data transmission is presented. The question of the relative utility of linear frequency sweeping (LFS or chirp), compared to PSK and FSK, in terms of error probability and spectrum usage, is discussed. The transmission media considered are the coherent, partially coherent, Rayleigh, and Rician channel models. Theoretically, LFS has unconditionally superior characteristics in the partially coherent and fading cases, for certain ranges of channel conditions. This is due to the more negative values of cross-coherence parameters possible with the LFS signal set over the FSK signal set. For the fading channel, theoretical supremacy of LFS over FSK depends upon the specular-to-Rayleigh signal power ratio and the adjustability of in-phase cross coherence, with a constraint upon quadrature phase cross coherence. From a practical standpoint, coherent reception of the LFS signal set has severe limitations. These are manifested primarily in two aspects: the need for phase synchronization of a chirp signal set, and the fact that the optimum value of cross coherence is highly sensitive to synchronization channel signal-to-noise ratio (SNR), and/or spectral-to-Rayleigh signal power ratio. The latter would require that modulation characteristics track the channel conditions in order to achieve the supremacy in performance theoretically predicted by optimization of the cross-coherence parameter in LFS.