Cooperative Spectrum Sharing With Bidirectional Secondary Transmissions

Existing cooperative spectrum-sharing protocols either separate the primary and the secondary transmissions or allow the secondary user (SU) to transmit only in the time slot when it is relaying the primary user´s signal. In this paper, we propose a new cooperative spectrum-sharing protocol that multiplexes the secondary transmissions with both the primary and the relay transmissions. This way, a pair of SUs can bidirectionally communicate with each other while one of them is serving as a relay to assist in the transmission from the primary transmitter to the primary receiver. Specifically, the transmission is on a two-time-slot basis. In the first time slot, in addition to the primary transmitter´s transmission, the SU who does not act as the relay also transmits. In the second time slot, the SU who acts as the relay will split its power to relay the primary signal to the primary receiver while transmitting its own signal to the other SU. In particular, it is considered that the relay SU is equipped with multiple antennas, while all the other terminals have a single antenna each. Based on the proposed protocol, we consider two decode-and-forward (DF) schemes and one amplify-and-forward (AF) scheme. We evaluate the achievable rate regions of the rate of the primary link versus the sum rate of the secondary links of the proposed schemes. The Pareto boundary of the achievable rate region is found by optimizing the transmit power, the beamforming vectors, and the power splitting factor at different nodes to maximize the sum rate of the secondary links for a given rate of the primary link. It is demonstrated through numerical results that our proposed bidirectional protocol allows the SUs to utilize the spectrum in a more efficient way without sacrificing the primary link´s rate, as compared with the existing one-directional scheme in the literature.