A　photonic-assisted　frequency　downconverter　with　self-interference　cancellation　and　fiber　dispersion　elimination　is　proposed　for　in-band　full-duplex　(IBFD)　radio-over-fiber　(ROF)　systems　based　on　stimulated　Brillouin　scattering.　In　this　work,　a　dual-polarization　Mach-Zehnder　modulator　(DPol-MZM)　is　employed　to　cancel　the　self-interference　signal　in　the　optical　domain,　thus　the　proposed　system　has　a　large　operation　bandwidth　without　the　limitation　of　the　electrical　bottleneck.　Meanwhile,　a　widely　tunable　microwave　photonic　filter　(MPF)　based　on　stimulated　Brillouin　scattering　(SBS)　is　constructed　to　perform　carrier-suppressed　single-sideband　modulation　of　LO.　Therefore,　the　proposed　photonic　frequency　downconverter　is　naturally　free　from　fiber　dispersion.　Furthermore,　thanks　to　the　characteristics　of　the　SBS-based　MPF,　such　as　large　out-of-band　rejection　ratio,　high　resolution　and　wideband　tunability,　the　proposed　system　has　a　high　flexibility　and　downconversion　efficiency,　which　is　of　great　significance　for　IBFD　ROF　systems.　(C)　2022　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement