Phase Estimation for Coherent Optical OFDM

Abstract

Abstract-Phase estimation is one of the enabling functionalities in coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) receivers. In this letter, we compare pilot-aided and data-aided phase estimation methods for a CO-OFDM transmission experiment at 8 Gb/s over 1000-km standard single-mode fiber without optical dispersion compensation. We also show that as few as five subcarriers are sufficient for pilot-aided phase estimation. IndexTerms-Coherent communications, orthogonal frequency-division multiplexing (OFDM), phase estimation.

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

“…4. At the same time, the number of subcarriers may be changed according to (6) and (7). Table II reports the values with 73.5 MSymbol/s for each subcarrier.…”

“…Consequently, phase noise effects can vary significantly for them. In this work, coherent detectionis realized through digital phase estimation and compensation by pilot-aided phase estimation described in [7]. Data-aided phase estimation is not considered here due to the implementation difficulties when QAM formats are used [20].…”

“…As a result, the sampling sequence before OFDM demodulation includes laser phase noise. The transmission model of CO-OFDM can be found in [7], which is a parallel multiple subcarrier transmission model. By assuming a perfect FFT window synchronization, the CO-OFDM transmission model is given by (1) (2) where and are the M-QAM modulated data of the th subcarrier in the th OFDM symbol before and after transmis-sion, is the transmission channel response at th subcarrier, is the laser phase noise sampling sequence with the time interval, is complex white Gaussian noise.…”

“…In (1), CPE is independent of subcarrier and can be estimated and compensated by using several pilot subcarriers described in [7]. The channel response in optical transmission is slowly varying and therefore is independent of OFDM symbol within a block of OFDM symbols.…”

“…• Some hardware imperfections of the front end can be addressed by software, such as the laser phase drift [7] and the nonlinear transfer function of the optical modulator [9]. • The transmission channel characteristics can be monitored by means of receiver signal processing [10].…”