Externally-seeded WDM PON for next generation mobile access based on pulse-width modulation

Recently it has been demonstrated that pulse width modulation (PWM) can represent a viable solution for the analog optical fronthaul alternative to standard radio over fibre, which still avoids the bandwidth expansion of the digital fronthaul. The PWM encodes the analog samples at the transmitter onto the duration of the on/off keyed optical signal, splitting the sampling and quantization of the radio signal between remote radio units (RRUs) and baseband units (BBUs). In particular in this contribution the PWM optical signal is transmitted over a wavelength division multiplexed passive optical network (WDM PON) architecture exploiting externally-seeded reflective semiconductor optical amplifiers. The various RRU-to-BBU links are pooled through virtual point-to-point connections at different wavelengths generated by an array of DFB lasers. To reduce Rayleigh scattering impairments associated with external seeding the proposed solution takes advantage of a polarization retracing circuit based on Faraday rotators. We demonstrate the up-link transmission of 100-MHz RF signals up to 20-km standard single mode fibre with error vector magnitude percentages compatible with 64 QAM and 256 QAM modulations.