Spectrally efficient transform domain communication system with quadrature cyclic code shift keying

Transform domain communication system (TDCS), as an overlay cognitive radio communication system, has been proposed to obtain a low probability of interception by using spectrum bin nulling to synthesise an adaptive waveform corresponding to the spectrum sensing output. However, its low spectral efficiency limits potential practical applications. In this study, an efficient modulation scheme, namely quadrature cyclic code shift keying (Q-CCSK), is proposed for TDCSs by generating another fundamental modulation waveform as the data-bearing quadrature branch. Although using Q-CCSK doubles the spectral efficiency for TDCSs, the instinct inter-branch interference arises from the added quadrature branch. Through the orthogonality analysis, it is proven that the two branches (in-phase and quadrature- branches) are still satisfied the property of quasi-orthogonality. Moreover, the performances of TDCSs employing Q-CCSK in additive white Gaussian noise (AWGN) and multipath fading channels are discussed, respectively. Analytical and simulation results demonstrate that, compared with conventional schemes, the TDCS employing Q-CCSK can double the spectral efficiency with comparable systematic performance.