Measurement and characterization of the temporal behavior of fixed wireless links

We analyze the methodology and present the interpretation of results obtained in the measurement of the temporal Ricean K-factor and the time-autocovariance function of the received power for fixed narrowband wireless links in urban residential environments. Our main observation is that the time-autocovariance function (and thus both the temporal K-factor and the coherence time) fundamentally depends on a combination of the characterization of the transmit-receive path and whether the street-level moving objects are in the field of view of the antennas. In most practical cases the latter refers only to the customer premises antenna, as the base station is typically placed at a much greater distance from moving scatterers. No significant basis was found for the popular conjecture that the K-factor is mainly influenced by the excess path loss. However, a strong correlation between coherence times and temporal K-factor was observed over a wide range of link conditions. We also found that the time-autocovariance function can be quite accurately modeled by the weighted sum of two exponentially decaying functions: one with a time constant on the order of 1 s and the other typically slower by one order of magnitude. The results will be helpful in establishing design criteria for radio systems such as multiple input-multiple output that need to estimate the channel response and its persistence in time and for media access control (MAC) layer transmission protocols whose efficiency is affected by the depth and duration of channel outages.