Multipath Characteristics of Indoor-to-Outdoor Radio Channels in the 868-MHz Band

Reliable data links in license free ISM/SRD bands are a key requirement in machine-to-machine (M2M) communication scenarios. Therefore, a basic knowledge of the propagation channel is essential for the design of low power wide area (LPWA) networks, which operate in applications such as smart metering or infrastructure monitoring. With respect to findings from our previous work, the emphasis of this investigation is on the frequency selectivity of propagation channels. Here we present the first results of field trials in urban and commercial area environments for the 868MHz ISM/SRD band. To make multipath effects plausible for the frequency selectivity, we take the topographic conditions into account. Our findings show that a large number of propagation channels in the 868MHz ISM/SRD band are affected by strong multipath fading and can be classified in different categories. The multipath characteristics depend in a plausible way on the topographic conditions of the environment. With respect to the temporal evolution of these characteristics, a strong correlation between successive days is observed for propagation channels with predominant outdoor parts. In contrast, primarily indoor propagation channels show significant fluctuations over weekdays.