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Cooperative primary-secondary transmission using superposition coding and successive interference cancellation

In a collaborative primary-secondary transmission system, a secondary transmitter (ST) working as a relay is allowed to transmit its own signal superimposed on the primary signal only if it does not intervene the primary transmission. The spectrum room for the secondary usage is thanks to the link gain achieved by the relaying. At the secondary receiver (SR), it also receives the primary signal first, removes it contained in the superposed signals and gets its own signal. In this case, how near the SR to the primary transmitter is important and hence the location of SR is somewhat restrictive. In this paper, we show successive interference cancellation (SIC) only with the superposed signal does not degrade the performance of the collaboration and moreover enlarges the area in which the spectrum sharing is successful. We provide two protocols: SIC-OT and SIC-ET. SIC-OT optimally allocates time slots as well as the transmit power but SIC-ET allocates only the power. Numerical examples are used to verify that the proposed protocols provide better performance than the previous collaboration schemes.