Broadband Impairment Compensation in Hybrid Fiber-Wireless OFDM Long-Reach PONs

This paper proposes and demonstrates experimentally the broadband transmission impairment compensation in optical and electrical domain, enabling the provision of a bundle of simultaneous services using orthogonal frequency division multiplexing (OFDM) modulation in hybrid wireless-optic networks. The combination of electrical pre-compensation and fixed optical dispersion compensation is assessed in long-reach passive optical networks (LR-PONs): i) An electrical pre-compensation technique using extra RF-pilots located in the free spectrum of a bundle of OFDM-based services is proposed to evaluate the response of the transmission channel. The impact of the number of extra RF-pilots and the digital signal processing algorithm used for channel estimation is analyzed experimentally for different configurations. Using the proposed electrical pre-compensation technique, an error vector magnitude (EVM) improvement of 5.2 dB in the higher frequency OFDM signal is achieved. The electrical compensation enables the long-reach transmission at 100 km of simultaneous long term evolution (LTE), worldwide interoperability for microwave access (WiMAX) and ultra wideband (UWB) signals and an I/Q OFDM signal providing GbE connectivity. ii) Fixed optical dispersion compensation is used to overcome partly the dispersion-induced impairments of the long-reach transmission. With the combination of electrical and optical impairment compensation approaches, we demonstrate that the electrical compensation technique is able to counterbalance the residual dispersion of the optical access network. An EVM fluctuation below 1 dB is obtained for residual dispersion values ranging from -425 to 425 ps/nm. The EVM of the highest frequency UWB signal centered at 4.49 GHz is improved by 8.8 dB when using the combination of electrical and optical compensation, enabling the simultaneous transmission of the bundle of OFDM-based signals in a 125 km LR-PON.