Multipath fading at microwave frequencies causes highspeed digital radio systems on line-of-sight paths to experience outage, where outage is defined here as the number of seconds in a month that the bit-error rate (BER) on an unprotected hop exceeds a threshold. The expected or predicted outage of a radio system may be calculated using a statistical model of fading in conjunction with characteristics of the radio determined by stressing it with a multipath fading simulator in the laboratory. To determine the effects of transmitted power levels (or nominal received carrier-to-noise ratio) on outage may require the simulation of 500-1000 channel conditions to characterize the radio completely. This paper describes a method of generating the complete characterization from a reduced set of simulation measurements; the method reduces the required number of measurements by a factor of about 20. Over a wide range of operating conditions, the outage calculated with this simplified method differs by less than 25 percent from that obtained with the complete characterization.

A statistical model of multipath fading and detailed radio characterization measurements made in the laboratory provide the basis for a calculation of the time, during a heavy fading month, that a given digital radio system will be unavailable because of high bit error rates induced by multipath fading, thermal noise, and interference. The author shows that a simple algebraic expression for outage agrees with the detailed calculation over a wide range of possible interference levels. The simple expression also agrees with field observations. Performance is shown to depend on five parameters: three of these (dispersive fade margin, flat fade margin, and carrier-to-noise ratio at critical BER) are characteristic of the radio system as installed on a given path, one describes the interference environment, and one specifies the incidence of fading on the path.

Selective fading due to muitipath on a typical narrowband line-of-sight microwave radio path is represented with a simplified three-ray model. This model provides a satisfactory description of the frequency transfer characteristics of almost all fades observed during three separate propagation experiments. Data were obtained for a 30 MHz bandwidth at 6 GHz using both a standard horn reflector and a parabolic dish, and for a 20 MHz bandwidth at 4 GHz using the horn. The statistics of the parameters of the model were found to be substantially the same for all three cases. The incidence of selective fading scales, within a factor of two, with the incidence of fading at a single frequency, for 1-month observation intervals.