  For overlay cognitive radio networks (CRNs), transform domain communication
system (TDCS) has been proposed to support multiuser communications through
spectrum bin nulling and frequency domain spreading. In TDCS-based CRNs, each
user is assigned a specific pseudorandom spreading sequence. However, the
existence of multiuser interference (MUI) is one of main concerns, due to the
non-zero cross-correlations between any pair of TDCS signals. In this paper, a
novel framework of TDCS-based CRNs with the joint design of sequences and
modulation schemes is presented to realize MUI avoidance. With the uncertainty
of spectrum sensing results in CRNs, we first introduce a unique sequence
design through two-dimensional time-frequency synthesis and obtain a class of
almost perfect sequences. That is, periodic auto-correlation and
cross-correlations are identically zero for most circular shifts. These
correlation properties are further exploited in conjunction with a
specially-designed cyclic code shift keying in order to achieve the advantage
of MUI avoidance. Numerical results demonstrate that the proposed TDCS-based
CRNs are considered as preferable candidates for decentralized networks against
the near-far problem.
