title: Ultrafast and low overhead training symbol based channel estimation in coherent M-QAM single-carrier transmission systems.
abstract: We propose a training symbol based channel estimation (TS-EST) algorithm that estimates the 2 × 2 Jones channel matrix. The estimated matrix entries are then used as the initial center taps of the 2 × 2 butterfly equalizer. Employing very few training symbols for TS-EST, ultrafast polarization tracking is achieved and tap update can be initially pursued using the decision-directed least mean squares (DD-LMS) algorithm to mitigate residual intersymbol interference (ISI). We experimentally verify the proposed TS-EST algorithm for 112 Gbps PDM-QPSK and 224 Gbps PDM-16QAM systems using 10 and 40 training symbols for TS-EST, respectively. Steady-state and transient bit error rates (BERs) achieved using the TS-EST algorithm are compared to those obtained using the constant modulus algorithm (CMA) and the training symbol least mean squares (TS-LMS) algorithm and results show that the proposed TS-EST algorithm provides the same steady-state BER with a superior convergence speed. Also, the tolerance of the proposed TS-EST algorithm to laser phase noise and fiber nonlinearity is experimentally verified. Finally, we show by simulation that the superior tracking speed of the TS-EST algorithm allows not only for initial polarization tracking but also for tracking fast polarization transients if four training symbols are periodically sent during steady-state operation with an overhead as low as 0.57%.
authors.affiliation: Department of Electrical and Computer Engineering, McGill University, Montreal, Quebec H3A 2A7, Canada. mohamed.osman2@mail.mcgill.ca
authors.forename: Mohamed
authors.initials: M
authors.lastname: Morsy-Osman
authors.forename: Mathieu
authors.initials: M
authors.lastname: Chagnon
authors.forename: Qunbi
authors.initials: Q
authors.lastname: Zhuge
authors.forename: Xian
authors.initials: X
authors.lastname: Xu
authors.forename: Mohammad E
authors.initials: ME
authors.lastname: Mousa-Pasandi
authors.forename: Ziad A
authors.initials: ZA
authors.lastname: El-Sahn
authors.forename: David V
authors.initials: DV
authors.lastname: Plant