Spatial diversity and high data rate waveforms-is it worth doing?

In the early days of HF communications, CW and FSK waveforms were the prevalent means for data communications. These tone based signaling methods were very susceptible to frequency selective fading caused by multipath. To improve the reliability of these links, spatial diversity was commonly used. As newer, more robust waveforms were developed for HF, it was felt that spatial diversity was no longer needed. This paper investigates the possible performance improvements of using spatial diversity on modern HF waveforms such as the high data rate waveforms defined in US MIL-STD-188-110B. Several different channel conditions are studied to determine if spatial diversity should perhaps be revisited as a way of further improving the reliability of HF links