###### Abstract
In a radio-over-fiber (RoF) link, an SMF length is placed between the remote antenna unit (RAU) and the central office (CO) to achieve antenna remoting, But, fiber-dispersion-induced power fading is a harmful effect in the RoF link. For the RoF receiver, the image-reject can be designed based on two PMs inside a Sagnac loop [26]. But an OBPF is needed to filter out the undesired sidebands. In addition, image rejection can be achieved by tuning the bias voltages of the modulators [27; 28]. However, the drift in the bias voltage is a problem for the long-term stable operation of the link and it will affect the image reject ratio. In Ref. [29], a single DP-QPSK modulator is used to realize the CS-SSB modulation of LO and RF signals, a technique that is immune to fiber-dispersion-induced power fading. And a 90\({}^{\circ}\) optical hybrid is used to reduce the imbalance between the two paths. In Ref. [30], a scheme that can realize both image suppression mixing and self-interference cancellation for the RoF link is proposed. But, bias control is a problem for the long-term stable operation of the link. In Ref. [31], a polarization-division multiplexing DPMZM is applied to modulate the RF and LO signals with CS-SSB modes. The scheme achieves high IRR and is not affected by fiber-dispersion-induced power fading. In Refs. [32; 33], image rejection mixers utilizing chip-based stimulated Brillouin scattering and interferometry are proposed, achieving ultra-high IRR. However, the RF pump signal must be changed according to the received RF signal. The frequency of the received RF signal has progressively shifted from the microwave band to the millimeter-wave band. In order to reduce the frequency requirement of the LO signal in the high-frequency mixer, higher-order LO sidebands should be considered. Thus, lower-frequency LO signals can also be optimized to take advantage of the great flexibility of microwave photonics to implement high-frequency mixers. But none of the above schemes take into account this problem. In our previous research, an all-optical, image-reject, fundamental/sub-harmonic downconverted RoF link using a PDM-MZM with a fiber-dispersion effect was proposed [34]. But due to the use of the fiber-dispersion effect to achieve image rejection, the system gain will be affected at certain frequency points. In this paper, we propose an image suppression sub-harmonic downconverted RoF link based on a DP-QPSK modulator. The RF signal is modulated in the upper QPSK with a CS-SSB modulation mode, while the LO signal is modulated in the lower QPSK with a special SH-SSB modulation mode. The modulated signals in the upper and lower arms are orthogonally polarization-multiplexed at the output of the DP-QPSK modulator. Since the DP-QPSK, PC, and PBS can form a photonic microwave shifter, two IF signals with a 90-degree phase difference can be obtained by building two separate paths. The two quadrature IF signals are then combined using a low-frequency quadrature hybrid, which introduces an additional \(\pi/2\) phase difference to ensure that the downconverted image is out of phase while the desired IF signals remain in phase. The feasibility of the proposed image-free microwave photonic sub-harmonic downconverter is theoretically analyzed and experimentally verified. The key contributions of our proposed approach are as follows: (1) Thanks to the SH-SSB modulation of the LO signal, the system's working frequency range relative to the LO signal has doubled. (2) The system has a large frequency tunable range, due to the absence of an optical filter. (3) Due to the special SSB modulation mode of RF and LO signals, the modulated signal can tolerate the dispersion-introduced power fading effect when transmitted over a length of optical fiber, making it suitable for antenna remoting applications.