June 2011 | NTIA Technical Memo TM-11-477
Christopher Redding; Christopher J. Behm; Timothy J. Riley; Robert B. Stafford
Abstract: Wireless communications experience temporal variations in received signal level that are characteristic of the propagation channel. Propagation channel fading at VHF and above is predominantly caused by the complex scattering environment through which the radio waves travel. The techniques used in receiver design to mitigate the effects of fading are well established, but the constraints of limited spectral resources and the insidious characteristics of the propagation environment demand increasingly complex modulation techniques to accomplish this mitigation. To maintain system reliability and usability, receivers must be built to increasingly higher tolerances to cope with propagation-induced errors. This report describes a re-examination of the fading channel. Specifically, it presents a reassessment of the assumptions implicit in the understanding of fading channels. This effort involves the measurement, analysis, and assessment of propagation fading and describes the limitations of classical fading theories when applied to ever more sophisticated modulation techniques. The ultimate purpose of this report is to investigate the statistical fading behavior of the radio channel within time frames significant to the reception of digitally-modulated signals. Measurements of the mobile radio channel were conducted to analyze the fast fading characteristics of the public safety frequency band. The measurement system is described, as are improvements made to increase its capability to measure the channel characteristics. Data analysis was conducted without bias toward conventional fading assumptions to provide an independent theoretical understanding of RF fading propagation. Finally, an assessment of the efficacy of classical fading theory in the testing of existing public safety VHF transceivers is discussed with conjectures about future transceiver testing and design.
Keywords: public safety; propagation characteristics; fading channel; Rayleigh distribution; short-term measurements; quadrature channel measurements; 700 MHz
For technical information concerning this report, contact:
Institute for Telecommunication Sciences
Disclaimer: Certain commercial equipment, components, and software may be identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.