Most of the optical networks-on-chip (ONOCs) architectures proposed so far in the literature rely for their operation on wavelength division multiplexing (WDM). A limitation to the WDM approach for on-chip communications stems from the difficulty of integrating light sources on a silicon CMOS compatible platform. Therefore, to optimise the use of the available lasers, a further dimension inherent to optical communications can be leveraged to implement on-chip networking, namely the spatial (modal) one. In mode-division multiplexing (MDM) techniques, distinct communication channels are allocated, on the same common wavelength, to the various modes of a multi-mode waveguide. While for on-chip WDM networks the awareness on the requirements of functional building blocks is well established, for on-chip MDM systems, on the contrary, the design constraints at both device and system levels are still an open issue of investigation. The aim of this contribution is to explore, by means of analytical (scattering matrix formalism) and numerical (FDTD) tools, the design space of different MDM building blocks.