Empirical Study on Implicit Polarization Diversity and Space Diversity for Short Range Indoor-to-Outdoor Radio Links below 1 GHz

To assess the properties of short-range indoor-tooutdoor radio channels for applications like telemetry and smart metering we performed several field trials. They were operated in urban and commercial areas for several weeks each. Multiantenna receivers were used to record signal levels from two different antennas in parallel. Due to multipath propagation, in some of the channels low signal levels were observed. We therefore investigated if diversity techniques can be applied to improve reception, focussing on implicit polarization diversity and space diversity. From measurements over long time periods we derived the distributions of correlation coefficients and signal levels obtained from more than 80 radio links each using 3, 7 and 27 channels in the 169 MHz, 434 MHz and 868 MHz bands, respectively. For implicit polarization diversity and space diversity with horizontal spacing of antennas of one to six wavelengths we find the signals from the two antennas to be sufficiently decorrelated (correlation coefficents < 0.7) for most of the radio links and channels. In contrast, for space diversity with two antennas placed on the same mast at different heights, in many cases the signals were not sufficiently decorrelated. With respect to cross-polarization imbalance, we find values between -3 dB and 9 dB in the different trials and in the different frequency bands. The results show that generally both diversity schemes have the potential to significantly improve the performance of short range indoor-to-outdoor radio transmission systems. If space diversity with two vertically polarized antennas is not feasible due to lack of a second antenna mast, implicit polarization diversity will perform well, in particular if the imbalance of signal levels can be compensated e.g. by using a cross-polarized antenna with appropriately higher gain.