In this chapter we propose a combined Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO), i.e. OFDM-MIMO system, in order to increase the capacity of High Altitude Platform (HAP) communication links. In the proposed system, the OFDM parameters and various compact MIMO antenna array configurations are investigated and their performance, in term of capacity, is analysed. In addition, since these special compact MIMO antenna array configurations depend on the array elements being positioned very closely together, the effect of mutual coupling and spatial correlation will also be analysed and taken into account when performing the simulation for this combined diversity system. Further, we will investigate the influence of the separation angle between the multiple relaying platforms on system performance, and determine the optimal separation angles that maximize the total capacity of the system.