# Ionospheric Polarization Techniques for Robust NVIS Remote Sensing Platforms
## 2 NVIS Characteristics
### Polarization Diversity and Diversity Combining  
Polarization diversity was firstly applied to HF, radar, and imaging systems. Its potential for improving the capacity of wireless communications systems was demonstrated, despite some disappointing premature predictions [18]. The improvement generated by this diversity method is typically granted by an additional de-correlated channel provided by a polarization state that is orthogonal to the existing one.  
NVIS communications can clearly benefit from polarization diversity techniques due to one of these aspects: the two characteristic wave components with circular polarization of the opposite sense, both of which follow a different path through the ionosphere when a radio wave of a certain frequency reaches the ionosphere. The other key issue with evaluating parameters in this kind of scenario is cross-polarization discrimination (XPD), with low correlation co-efficients being achieved in most situations. Nevertheless, considerably more attention has been paid to spatial diversity due to the significant difference in the mean received signal level between the co-polarized and cross-polarized branches when one polarization is transmitted. Yet, spatial diversity is not the best solution after all, since at least 20\(\lambda\) horizontal and 15\(\lambda\) vertical separation distances are required for efficient outdoor diversity, resulting in an attraction gain for polarization diversity [19].  
The implementation of a diversity system based on polarization needs some extra infrastructure compared to a rather simple scenario. As with any other diversity environments, multiple antennas are needed, specifically at the receiver. Both multiple input multiple output (MIMO) and SIMO are valid solutions, with the MIMO being the one that provides a better performance as it involves multiple antennas in the transmitter.  
The effectiveness and success of both HF MIMO systems and diversity reception increases with the decrease of the correlation between the received signals. If the de-coupling between both characteristic waves is large enough and our system is correctly designed to combine the diversity correctly, these features will drastically improve HF MIMO and diversity reception. In [18], the behavior and the correlation of the two ionospheric characteristic waves were studied, where both signals achieved de-correlations that can reach values like 13 dB easily. Taking into account the propagation mechanism studied and the clear isolation (more than 12 dB) between both characteristic waves, it is safe to state that a scenario that implements diversity reception can greatly benefit from the use of a pair of orthogonal circularly polarized antennas. Specifically, the use of right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) antennas for HF MIMO will create two very isolated channels that will maximize MIMO gain. In the study done in this article, though, the transmit antenna will be a linear polarized one while having two circular polarized antennas in the receiver.  
All diversity techniques need a process for combining the different signals that arrive at the receiver. There are a lot of them, each one presenting different characteristics and gains. This article studies two different techniques: the selection combining, a method that selects the strongest signal received and discards the other, and the equal-gain combining, a technique that sums all the signals received coherently.