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For overlay cognitive radio networks (CRNs), transform domain communicationsystem (TDCS) has been proposed to support multiuser communications throughspectrum bin nulling and frequency domain spreading. In TDCS-based CRNs, eachuser is assigned a specific pseudorandom spreading sequence. However, theexistence of multiuser interference (MUI) is one of main concerns, due to thenon-zero cross-correlations between any pair of TDCS signals. In this paper, anovel framework of TDCS-based CRNs with the joint design of sequences andmodulation schemes is presented to realize MUI avoidance. With the uncertaintyof spectrum sensing results in CRNs, we first introduce a unique sequencedesign through two-dimensional time-frequency synthesis and obtain a class ofalmost perfect sequences. That is, periodic auto-correlation andcross-correlations are identically zero for most circular shifts. Thesecorrelation properties are further exploited in conjunction with aspecially-designed cyclic code shift keying in order to achieve the advantageof MUI avoidance. Numerical results demonstrate that the proposed TDCS-basedCRNs are considered as preferable candidates for decentralized networks againstthe near-far problem.

TDCS-based Cognitive Radio Networks with Multiuser Interference Avoidance