Temporal evolution analysis of indoor-to-outdoor radio channels in the 868-MHz ISM/SRD frequency band

Reliable wireless data links, which use the license free ISM/SRD frequency bands below 1 GHz are a key component for machine-to-machine communications in the environment of smart metering and smart grid. In contrast to mobile communication devices, the mounting position of such sensor, actor or metering devices is static over a long period. To design such specific low power wide area networks (LPWAN), knowledge about propagation channel and its temporal evolution is essential. Here, path loss and the multipath propagation conditions are the main aspects. To analyse the temporal evolution of typical sub-GHz ISM/SRD transmission channels, we performed two measurement field trials in the commercial area of Erlangen, Germany, in 2014 and 2016. Based on the measurement results for the 868 MHz ISM/SRD band, in this study the temporal evolution of the channel characteristic is adressed. Therefore we propose a two path channel model which describes the multipath propagation conditions. With reference to this model, a classification of the measured radio channels is done. Model parameters, like the amplitude of the first received signal path, are extracted from the measurement data and afterwards analysed with respect to the correlation between the two measurement field trials. The results show the unchanged characteristic of the majority of the measured radio channels after two years. Therefor, channels which indicate multipath propagation in 2014 are also observable in 2016. Furthermore, our correlation analysis of the model parameter show that besides the channel characteristic the temporal evolution of consecutive days in 2014 and 2016 is approximately constant. This indicates that the multipath propagation conditions are static over several years, and so our previous assumption of multipath effects of reflections on static structures, e.g. adjacent buildings, is plausible.