Technology Requirements for an Alamouti-Coded 100 Gb/s Digital Coherent Receiver Using 3 × 3 Couplers for Passive Optical Networks

Digital coherent receivers employing Alamouti-coding are a promising solution for passive optical networks (PON) with downstream rates of 100 Gb/s/λ. To reduce the cost by utilizing high-volume photodiodes designed for 25 Gb/s data-center applications, we consider 28-Gbaud 16-QAM modulation format detected with a single-polarization coherent receiver based on a 3 × 3 coupler with single-ended detection. We investigate two alternative configurations: first, a receiver with a symmetric 3 × 3 coupler and second, an IQ coupler (asymmetric 3 × 3 coupler) with Alamouti-coding based polarization diversity. In this paper, we present a detailed investigation as to the feasibility of both the receivers for a wide range of parameters including quantization noise, relative intensity noise, I/Q mismatch, etc. It is found that such receivers can be realized with commercially available components. We also quantify the penalty associated with nonideal components to allow for lower cost designs with reduced specifications suitable for access applications. In addition, we present the adaptive DSP algorithms required to recover data for a single-carrier Alamouti-coded signal. Considering both linear and nonlinear impairments, a power budget of 36.4 dB and 33.1 dB is found at BER of 3.8 × 10
^-3
for a receiver with symmetric 3 × 3 coupler and IQ coupler, respectively, and thereby proving their applicability as receivers for 100 Gb/s/λ PON.