Convolutive superposition for multicarrier cognitive radio systems

Recently, we proposed a spectrum-sharing paradigm for single-carrier cognitive radio (CR) networks, where a secondary user (SU) can maintain or even improve the performance of a primary user (PU) transmission, while also obtaining a low-data rate channel for its own communication. According to such a scheme, when the SU is able to receive the signal transmitted from the PU, it can superimpose its transmission on the PU signal by simple multiplicative precoding, without requiring any cooperation between the primary and secondary systems. The scope of this paper is to extend such a paradigm to a multicarrier CR network, where the PU employs an orthogonal frequency-division multiplexing (OFDM) modulation scheme. To improve its achievable data rate, the SU is allowed to transmit multiple symbols in parallel over the OFDM subcarriers of the primary system. In particular, the SU convolves its block-precoded symbols with the received PU signal in the time domain, which gives rise to the term convolutive superposition. An information-theoretic analysis of the proposed scheme is developed, which considers different amounts of channel state information at the secondary transmitter, as well as different precoding strategies for the SU. Extensive simulations illustrate the merits of our analysis and designs, in comparison with other CR schemes, by using as a performance indicator the ergodic channel capacity.