Fronthaul based on pulse-width modulation in RSOA WDM PONS with broadband and coherent seeds

To avoid the bandwidth expansion of the optical digital fronthaul, recently it has been demonstrated that pulse-width modulation (PWM) is a viable solution for the analog optical fronthaul, as an alternative to standard radio over fiber. 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). The PWM optical signal can be transmitted over a wavelength-division multiplexing passive optical network (WDM PON). In this paper we compare the performance of two architectures exploiting reflective semiconductor optical amplifiers externally seeded either by broadband or by coherent lights. The various RRUto- BBU links are pooled through virtual point-to-point connections at different wavelengths generated by a sliced broadband source or by an array of distributed feedback Bragg (DFB) lasers. In both cases to reduce Rayleigh scattering impairments associated with external seeding, the proposed solution takes advantage of a polarization retracing circuit based on Faraday rotators. We compare the two architectures by evaluating the up-link transmission performance of 100 MHz RF signals over up to 20 km standard single mode fiber employing Long-Term Evolution (LTE) mobile signals with 64-QAM and 256-QAM modulations.