Rain outage performance of tandem and path diversity 18-GHz short hop radio systems

The performance with respect to rain outage time of dual path diversity and non-path diversity (tandem) arrangements for 18-GHz short hop radio systems is computed and compared. The analysis is based on two extrapolations of R. A. Semplak's 1 three-year average of the measured probability distributions for rain attenuation at 18.5 GHz on a 6.4-km hop in New Jersey. The effects of merge hops and joint fading between hops in the diversity system, dependence of the rain attenuation distribution on hop length, and uncertainty in the tail of the distribution are included. The results show that (i) the performance of tandem systems relative to diversity systems increases as the system length increases, (ii) the difference in the number of repeaters per unit length required for short and long tandem systems is small, (iii) the performance of the diversity system is strongly dependent on the amount of joint fading between parallel paths, and (iv) the performance of the tandem system is strongly dependent on the tail of the attenuation distribution. Neither of the latter two factors is known from rain attenuation measurements, but if the joint attenuation probabilities are sufficiently high, then diversity shows no advantage over tandem for either of the assumed extrapolations. The uncertainty in the tail of the attenuation distribution and the sensitivity of the tandem system performance to it emphasize the need for reliable attenuation measurements out to a probability of about 10 −7 .