M.Sc Thesis

M.Sc StudentShalom Aviv
SubjectPhase Recovery Systems for Coherent Optical Transmission
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Moshe Nazarathy


In recent times, direct detection has become the dominant optical detection method. Direct detection only senses the power of a received signal, losing phase information. The ability to compensate the channel impairments at this method is very limited. A more advanced detection method, also based on direct detection, is differential detection, where the phase difference between successive symbols is recovered using delay interferometers (DI).   

The most modern technology is coherent detection, which allows complete information from the electrical field, both amplitude (power) and phase, to be obtained. Equivalently, we can use the in-phase (I) and quadrature (Q) signals in the two polarizations - all the available degrees of freedom. After digitalization, digital signal processing (DSP) can be used for compensate the channel impairments. Using this method, we can achieve the highest spectral and power efficiency. Specifically, this thesis deals with carrier phase recovery by DSP.

We review state-of-the-art methods of carrier phase recovery for coherent detection. We present a simulation of comparative performance under the nonlinear link model, showing the performance-complexity trade-off superiority of a novel polar MSDD algorithm developed in our EE group.  

We also consider whether coherent detection can be improved by modifying transmission constellations. Use of high order modulation formats improves spectral efficiency. We explore  non-rectangular high order constellations, in particular those with circular symmetry. The motivation for testing such constellations as an alternative to conventional rectangular QAM is that the circular configuration might be intuitively more conducive for mitigating phase noise impairment. In addition, we explore a specific 1/5/5/5 16 order constellation, and propose a differential precoding scheme to overcome its origin point phase indefinite, as well as a sub-optimal heuristic bits-to-symbol mapping scheme. Finally, we demonstrate its performance superiority over the rectangular M-QAM constellation in systems where phase noise is dominant.    

The main innovation in this thesis is the presentation of a novel differential detection scheme for the DFT-S OFDM transmission method. DFT-Spread (DFT-S) OFDM is a variant of OFDM that has multiple advantages; however one disadvantage is its degraded phase noise tolerance due to sampling rate slow-down in each DFT-S sub-band. Here, we propose a phase recovery method for 16-QAM coherent transmission, based on the combination of non-redundant (de)interleaving, differential precoding, and multi-symbol delay detection (MSDD), applicable to either conventional DFT-S OFDM receivers or to filter-bank sub-banded DFT-S OFDM receivers. Laser and nonlinear phase noise tolerance is significantly improved.