  To support interference-free quasi-synchronous code-division multiple-access
(QS-CDMA) communication with low spectral density profile in a cognitive radio
(CR) network, it is desirable to design a set of CDMA spreading sequences with
zero-correlation zone (ZCZ) property. However, traditional ZCZ sequences (which
assume the availability of the entire spectral band) cannot be used because
their orthogonality will be destroyed by the spectrum hole constraint in a CR
channel. To date, analytical construction of ZCZ CR sequences remains open.
Taking advantage of the Kronecker sequence property, a novel family of
sequences (called "quasi-ZCZ" CR sequences) which displays zero
cross-correlation and near-zero auto-correlation zone property under arbitrary
spectrum hole constraint is presented in this paper. Furthermore, a novel
algorithm is proposed to jointly optimize the peak-to-average power ratio
(PAPR) and the periodic auto-correlations of the proposed quasi-ZCZ CR
sequences. Simulations show that they give rise to single-user bit-error-rate
performance in CR-CDMA systems which outperform traditional non-contiguous
multicarrier CDMA and transform domain communication systems; they also lead to
CR-CDMA systems which are more resilient than non-contiguous OFDM systems to
spectrum sensing mismatch, due to the wideband spreading.
