Some physical constraints on the use of "Carrier-free" waveforms in radio-wave transmission systems

Are there practical applications in radio-wave transmission of inherently broad-band "carrier-free" waveforms such as Walsh functions? We demonstrate in this paper that the band-limiting constraints of radiating systems limit mission of the low-frequency spectral components of such waveforms and severely restrict the radiation directivity that may be achieved in systems that employ them. We also find that dispersion in the propagating medium poses difficulties for the use of such waveforms, and we present an example that illustrates their impracticality in a case in which medium-related limits on coherence bandwidth are of concern. Considerations of capital cost, reliability, and maintenance militate against the use of these waveforms and in favor of conventional spread-spectrum modulation schemes where such service is required. Troublesome aspects of engineering analysis methods for systems employing such modulation schemes are also exposed, as are some practical and economic difficulties to be expected if they are to be introduced as co-users of the electromagnetic spectrum with conventional radio systems.