Empirical channel model for unattended near ground sensor networks at UHF

Recently, unattended near ground wireless sensor networks are attracting increasing attention. The number of sensors needed to cover a certain area is of prime importance and therefore an accurate channel model is needed, especially to evaluate the maximum effective distance between adjacent terminals. In this paper, results of a measurements campaign performed in two environments, flat open yard and sandy dunes with bushes, are presented. Narrowband (NB) and wideband (WB) measurements at 916 MHz were conducted. The measured received power was compared with two ground wave prediction models. Additionally, we found that change of ground type leads to significant changes in both path loss and antenna gain. Based on the NB measurements we found a good match between predicted and measured received power values, and the predicted received power reflects the physical path profile. Based on the WB measurements we found that over flat open yard the channel can be classified as flat fading channel, while over sandy dunes with bushes the channel is rather frequency-selective with a coherence bandwidth of ~20MHz. Moreover, the temporal variations of the transfer function are negligible even though there was moderate wind during the measurements.