10 GHz channel spacing ultra-dense WDM networks transparently extended by mm-wave coherent RoF links

The extension of ultra-dense WDM passive optical networks (PONs) by millimetre-wave coherent radio-over-fiber (RoF) links is investigated. For a seamless and transparent integration of the RoF links coherent heterodyne detection is used for the generation of the wireless RF signals out of the optical baseband data signals. Simulations of the optical SIR in the PON are carried out in dependence on the WDM channel spacing. The projected impact on the transmission quality is derived and WDM CRoF experiments are performed. In the experiments three data modulated optical channels are transmitted over 10 km SMF to simulate the PON, before arriving at the E-band radio access unit (RAU). The RAU utilizes heterodyne detection with an optical LO for converting the 1 Gb/s optical baseband signal to a 75 GHz RF signal. After 40 m wireless transmission the signal is received by an antenna and downconverted to baseband using a low-cost SBD based receiver for envelope detection. The filtering of WDM channels with an optical channel spacing of down to 10 GHz showed very little penalty versus single channel transmission, relying only on the filter characteristic of the used LNAs and antennas and using no optical filters. Still the receiver´s sensitivity is only −57 dBm for a bit error rate (BER) of 2·10⁻³, thus it is shown that an ultra-dense WDM PON can be extended wirelessly using coherent RoF techniques without the need for optical filtering.