This study presents a multiple-input multiple-output (MIMO) architecture utilising polarisation diversity to increase the capacity of high-frequency (HF) radio links through the ionospheric channel. A summary of physics focuses on the polarisation of radio waves propagated in the ionosphere. This preliminary step introduces the original idea of diversity in the transmitted polarisations to replace the classical space diversity for MIMO applications. This solution enables a reduction of the inter antenna spacing which is convenient in the context of decametric wavelengths. Simulations, referring to the theoretical expression of channel capacity and involving realistic models of ionospheric radio links, underline a significant increase in performances for this particular MIMO structure if compared to a single-input single-output system. Additionally, the trade-off between capacity gain and complexity appears balanced with the development of a 2 x N MIMO system transmitting two complementary circular polarisations. More specifically, the design of a 2 x 2 MIMO system is described in the context of a single carrier waveform and frequency-domain equalisation, resorting to classical solutions for the different signal processing modules. A global simulation, involving the entire transmission system, is carried out and gives an estimate of the maximal data transfer rate compatible with a requested quality of service.