CDMA underlay network with cognitive interference-minimizing code assignment and semi-blind interference suppression

In this paper, we propose a code-division multiple-access (CDMA) secondary network to operate cognitively as an underlay for a primary network. A novel interference-minimizing code assignment (IMCA) for secondary users is proposed such that the interference from existing primary and secondary users is minimized. This is accomplished by minimizing the mean square cross-correlations between the candidate codes and the received signal. Since the codes of the primary CDMA network are unknown to the secondary network the scheme operates blindly and the problem is formulated as a constrained optimization of a Boolean quadratic form. The solution is obtained exhaustively and we demonstrate significant performance gains compared to random code assignment (RCA). We then slightly relax the constraint on the user code space and obtain a near-optimal solution with complexity that is linear in the spreading factor. For the secondary network, we propose a slightly modified minimum-output energy receiver that operates semi-blindly as a first stage followed by parallel interference cancellation. For this receiver structure, we derive improved log-likelihood ratios for the single-user decoders based on partial knowledge of the user codes and channel coefficients. Significant error probability performance improvements of 2 dB or more are obtained due to IMCA in the coded case.