In　this　paper,　we　propose　a　reference-modulated　permutation　index　differential　chaos　shift　keying　(RM-PI-DCSK)　over　multipath　fading　channels.　Due　to　quasi-orthogonal　property　between　different　permutated　chaotic　signals,　the　proposed　scheme　can　remarkably　suppress　the　inter-sequence　interference　to　recover　the　desired　information.　Moreover,　the　proposed　RM-PI-DCSK　has　one　more　modulation　bit　in　the　reference　segment　than　the　conventional　permutation　index　differential　chaos　shift　keying　(PI-DCSK).　Both　the　reference　signal　in　the　first　time-slot　and　a　permuted　replica　of　the　reference　in　the　second　time-slot　are　modulated　for　efficient　transmission.　The　theoretical　bit-error-rate　(BER)　performance　and　the　highly　consistent　simulation　results　demonstrate　that　the　proposed　RM-PI-DCSK　can　achieve　a　performance　gain　3　dB　over　the　conventional　PI-DCSK　at　a　BER　of　$10^{-4}$,　and　this　gain　becomes　larger　in　a　high-delay-spread　channel.　In　particular,　the　performance　superiority　of　the　RM-PI-DCSK　is　further　demonstrated　in　a　typical　ultra-wideband　(UWB)　channel　with　narrow-band　interference.　Therefore,　this　proposed　scheme　can　be　deemed　as　an　efficient　transmission　alternative　for　chaotic-based　wireless　communications,　such　as　in　wireless　local　area　network　(WLAN)　applications　and　indoor　applications　of　the　transmitted-reference　UWB.　©　1967-2012　IEEE.