Mobile Reception of Digital AM Audio and Data Services Using Long-Range Skywave Propagation

Digitization of AM radio systems promises a revival of the currently underutilized broadcasting bands below 30 MHz. Its prospects are near-FM sound quality, automatic tuning, and interference-free reception. Using skywave propagation in some of the bands, a potentially very large area can be covered by a single transmitter if the appropriate frequency and power are chosen. In this paper, we compare mobile and stationary reception at various frequencies, investigate the correlation between solar sunspot activity and reception quality, and analyze how non-audio data service can efficiently be broadcast with digital radio systems in the AM bands. Restrictions regarding the antenna size together with varying ionospheric conditions and and man-made noise make the reception of radio transmissions below 30 MHz in a car a challenging problem. Using the Digital Radio Mondiale (DRM) system, a total of 58 measurements were conducted both in rural and urban environments in fall 2005, most of them in the 49 m, 41 m and 17 m short wave bands, but also in long wave and medium wave bands. A stationary reference receiver recorded data of the same transmission in parallel with the mobile measurements. Comparison of both data sets allowed for a detailed analysis of the impact of mobility on the reception quality. The results show that particularly in rural areas, mobile reception is indeed reliable and perceived as high quality. Only when ionospheric disturbances push signals to just above the required Signal-to-Noise Ratio (SNR), in-car reception fades more frequently than at a stationary receiver. In urban and industrial areas, shadowing effects (tunnels, large buildings) and man-made noise (power lines, etc.) cause even more frequent dropouts. Thus most of the problems known from analog AM reception in cars will remain the same for digital broadcasting. However, where it works, the digital audio quality is clearly superior to that of analog AM. Therefore, we believe that digital ra- dio in the AM bands is a valuable complement to other digital broadcast services, e.g. by augmenting their coverage in rural areas, where network deployments for higher frequencies may economically not be feasible. Further, the data features-although limited-make digital broadcasting in the AM bands an interesting candidate for telematics and traffic information services covering very large areas.