Orthogonal Multiband CAP Modulation Based on Offset-QAM and Advanced Filter Design in Spectral Efficient MMW RoF Systems

In this study, to the first time, we demonstrate an offset-QAM carrier-less amplitude and phase modulation (OQAM-CAP) technique for spectral efficient multi-user radio-over-fiber systems. In comparison with traditional QAM-based CAP modulation schemes, the digital filter for each channel in OQAM-CAP can be adaptively redesigned to obtain either higher spectral efficiency or lower computational complexity. Two kinds of digital filters, namely, square-root raised-cosine (SRRC) filters and isotropic orthogonal transform algorithm (IOTA)-based filters, are studied and applied to shape the frequency window of each channel in CAP systems. We compared the performances among SRRC-OQAM CAP, IOTA-OQAM CAP, and traditional SRRC-QAM CAP. It is experimentally demonstrated that SRRC-OQAM-CAP can achieve higher spectral efficiency, IOTA-OQAM-CAP can obtain higher computational efficiency, and SRRC-QAM-CAP can be used in uplink transmission with lower interchannel interference. Bidirectional experiments are conducted with around 2-GHz nine aggregated channels on 60-GHz optical millimeter wave. Less than 7% error vector magnitudes are realized over 15-km standard single-mode fiber and 1.5-m wireless channel.