On designing quasi-ZCZ sequences under spectral constraints and its applications for cognitive radio networks

In cognitive radio (CR) systems, the spectrum opportunity is defined as spectrum holes which are not occupied by primary users. When existing sequence sets are spread over the available spectrum bands, their corresponding frequency-domain sequences have to be set to zero in order to satisfy the spectrum hole constraint, leading to damaged correlation properties. In this paper, a novel sequence design approach motivated by time/frequency synthesis is proposed for CR systems. When a zero correlation zone (ZCZ) sequence set (L, M, T) with sequence length L, family size M, and ZCZ width of T is chosen for the `modulation´ sequence in time domain, a quasi-zero-correlation zone (QZCZ) sequence set (LN, M, TN - 1) is obtained which features zero cross-correlation zones of width TN - 1, where N denotes the number of subcarriers in frequency domain, regardless of any CR spectrum hole constraint. As a result, the designed QZCZ sequences can be used as spreading sequences for quasi-synchronous CDMA-based CR networks.