Spatial correlation and diversity of low-latitude multipath multimode HF skywave radio channels

To support the reliability and coverage of long-range skywave HF radio communication systems, it is necessary to consider technologies that involve multiple links, such as site diversity, network relay, or cooperative communication. This paper reports a study on the spatial correlation of signals between two HF skywave links with a common end and the benefit of site diversity systems using such links. For this study, two links in a low-latitude region are considered, each with a groundrange of 3044 km and 1895 km, both having a common transmitter and different receivers separated 1900 km apart. The correlation is evaluated from the results of ray-tracing simulations, verified with measurements and simulations for different dates. This study confirms that the correlation between two HF links is low, which can be explained as being the result of sum of multiple paths with random, uncorrelated phase-shifts. The study also proves that site diversity using two convergent links with selective combining can provide diversity gain and reduce the probability of outage significantly. To enhance the benefit of site diversity, a joint space-frequency diversity involving two different frequencies with opposite diurnal characteristics on the primary and alternative link is recommendable.