90° 1 LO 1 2 90° 2 LO LO 90° 1 2 3 4 1 2 90° 3 LO 4

Signal LO Single-Polarization Downconverter I Single-Polarization Downconverter II LO Signal Single-Polarization Downconverter I PBS

Down-converter LO Signal Phase Estimator Loop Filter + + + Σ Σ −

Fast Axis Slow Axis RX Signal PBS PC Im Re Im Re

−90° h v I

−90° Splitter DSP Output Bit encoding

O/E Down-Conversion + Sampling Transmitter Receiver    Channel   Channel OFDM TX OFDM RX IFFT Add Prefix P/S Size Nc Length Npre MZ O/E Down-Conversion + Sampling LO FFT Remove Prefix S/P

PBS LO  −90°  + − Sampler / DSP

Dual-Polarization Down-Converter LO p(t) t=mT Dual-Polarization Down-Converter Digital Equalizer Symbol Decision Σ + −

Digital Coherent Receiver TX Laser Receiver Channel MZ #1 PBS MZ #2 LO Laser Digital Coherent Receiver LO Signal FF Phase Estimator  Symbol Decision Soft Phase Estimator

… … ○ ● □ ■

 Dual-Polarization Down-Converter DD Avg. LO

+ −  MZ Dual-Polarization Down-Converter LO OFDM TX #1 OFDM TX #2 PBS OFDM TX #1 OFDM TX #2 OFDM RX #2 Dual-Polarization Down-Converter LO Soft Phase Est. Avg.   + − NLPN Comp. Symbol Decision TED Loop Filter

PBS −90° Pol. Rotate

+  −   Dual-Polarization Down-converter LO −90° Dual-Polarization Down-converter LO + −  Single-Polarization Down-converter LO Ts LPF  −90° Single-Polarization Down-converter LO

Soft Phase Estimator Symbol Decision Decision Decision

Dual-Pol. Down-Converter Digital Signal Processor PBS Data Modulation Pulse Carver Transmitter Channel Dual-Pol. Down-Converter LO LPF Digital Signal Processor Receiver TX Laser DCF + Digital Signal Processor Transmitter Channel Receiver