A Filter-Free, Image-Reject, Sub-Harmonic Downconverted RoF Link Without Fiber-Dispersion-Induced Power Fading

A filter-free, image-reject, sub-harmonic downconverted RoF link is proposed based on a dual-polarization quadrature phase-shift keying (DP–QPSK) modulator. At the remote antenna unit, the receiving radio frequency signal is applied to the upper QPSK modulator to achieve carrier-suppressed single-sideband (CS–SSB) modulation. The local oscillator (LO) is applied to the lower QPSK modulator, achieving sub-harmonic single-sideband (SH–SSB) modulation. The I/Q mixing is realized by exploiting a two-channel photonic microwave phase shifter, which mainly consists of a modulator, two polarization controllers, and two polarizers. The image interference signal can be rejected when combing the I and Q IF signals through a 90° electrical hybrid. Because the scheme is simple and filter-free, it has a good image-reject capability over a large frequency tunable range. Moreover, due to the special SH-SSB modulation, the modulated signals are immune to the chromatic dispersion-introduced power fading effect. Last, the sub-harmonic downconverter can decrease the frequency requirement of the LO signal. Experimental results show that an image rejection ratio (IRR) greater than 50 dB can be achieved when transmitted through a 25 km single-mode fiber (SMF). Simultaneously, under different RF signals and IF signals, the IRR has no periodic power fading, only small fluctuations. Image rejection capability of the scheme for the 50-MBaud 16-QAM wideband vector signal is also verified and the demodulation of the desired IF signal with a good EVM of less than 5% is realized.