A Signal Space Diversity-Based Time Division Broadcast Protocol in Two-Way Relay Systems

This paper considers a two-way relay system with time division broadcast (TDBC) protocol. In such protocol two end-sources exchange information with each other through help of a half-duplex decode- and-forward relay. To enhance the spectral efficiency in this system, we propose a novel scheme which incorporates signal space diversity with TDBC protocol. In the proposed scheme, original data symbols are rotated by a certain angle before being transmitted, and then the end- sources and the relay cooperate for transmitting in-phase and quadrature components of two consecutive rotated symbols. Thereby, the number of transmitted symbols are doubled over three time slots, which results in an increase in spectral efficiency. In particular, for this system we first derive a closed-form expression for the end- to-end (E2E) error probability with an arbitrary constellation to acquire all the resulting non- uniform constellation cases. Then, using the derived expression, we optimize the rotation angle at the different values of the signal-to-noise ratio to further improve the E2E error probability performance. Numerical results corroborate the theoretical analysis and show that the scheme proposed herein provides not only higher spectral efficiency, but also higher reliability transmission.