A New Selective Fading Model: Application to Propagation Data

Channel transmission models for use in estimating the performance of radio systems on line-of-sight paths at 6 GHz are explored. The basis for this study is the simple three-ray multipath fade, which provides a channel transfer function of the form H(ω) = a1 − b exp −j(ω − ω 0 )τ, where a is the scale parameter, b is a shape parameter, τ is the delay difference in the channel, and ω 0 is the (radian) frequency of the fade minimum. This model is indistinguishable from an ideal channel model, within the accuracy of existing measurements. The propagation data that confirm the model were obtained in summer 1977 from a 26.4-mile hop near Atlanta, Georgia. The received power at 24 sample frequencies spaced at 1.1 MHz and centered on 6034.2 MHz was continuously monitored and recorded during periods of anomalous behavior. The model is applied to estimating the statistics of the channel delay difference, τ. The average delay difference giving rise to significant selectivity in the channel is between 5 and 9 ns. The distribution of delay difference is obtained for delay differences greater than 10 ns. The channel is found to have more than 3 dB of selectivity (difference between maximum and minimum attenuation in band) due to delay differences greater than 20 ns for more than 70 seconds in a heavy fading month. (This is comparable to the time the channel attenuation of a single frequency exceeds 40 dB.) The three-path model requires further simplification for narrowband channel application. For a channel with 30 MHz bandwidth, a model with fixed delay of 6.3 ns provides a sufficiently accurate representation of all observed channel conditions. The resulting nonphysical model is used to statistically characterize the condition of the fading channel. The statistics of the parameters of the fixed delay model are almost independent and of relatively simple form. The distribution of the shape parameter b is of the form (1 − b) 2.3 . The distribution of a is lognormal.