The core activities did not encounter any key challenges, although a number of computational complexities with user terminal were identified, mainly in relation to the chosen MMSE-SIC (Minimum Mean Squared Error-Successive Interference Cancellation) based detectors. MMSE-SIC based detectors have been chosen for the study for performing the best with our IM techniques. However, even the low complexity iterative MMSE-SIC, at the core of the CDM-based solution, would require a capability to perform a relatively large number of mathematical operations for implementing its real-time MMSE filter, which varies from bit to bit. With the Overlay technique, the optimality of detector’s operation depends on high accuracy of clock and phase recovery (synchronisation) as well as SNIR estimation. To meet the requirements for proper operation of our IM techniques the user terminals seem to require significant additional processing power and circuitry, whose costs might not be justifiable for a relatively modest capacity gain of ~25%, under ideal conditions.
Furthermore, it has been shown that aggressive frequency reuse schemes require superior resource sharing and scheduling strategies as well to improve the offered capacity fairness. That is, beyond our physical layer focus, optimal implementation of our IM techniques, each requires a modified/new radio resource management to optimize, respectively, scheduling in overlay coding and bandwidth-on-demand, in our spreading-CDMA technique, potentially using a combined random access and DAMA scheme for the latter.
Finally, it is of importance to be reminded of some key practical issues (outside the scope of this study) that regardless of the planned frequency reuse scheme could seriously hinder actual implementation of large multi-beam systems. These include insufficient Ka-band gateway spectrum, resulting in a large number of gateways and/or use of less explored higher frequencies, such as Q/V-bands, introducing new technical challenges.