Electrical key components for high-bitrate optical OFDM systems
Final Report Abstract
Coherent optical orthogonal frequency multiplexing (CO-OFDM) has been shown to be the promising technique for high-capacity optical communications with bit rates towards 400 Gbit/s per wavelength. It enables scalability to the ever increasing demand of data rate and adaptability to reconfigurable optical networks. The Institute of Telecommunication (INÜ) in cooperation with the Institute of electrical and optical communications engineering (INT) proposed within this project to design and implement a real-time CO-OFDM system. The INÜ is responsible for the system parameter design part while the INT focus on the implementation of circuit technology. This report summarizes the work that are accomplished by INÜ. For a robust real-time data transmission, the INÜ research group has extensively investigated in [4] the digital to analog converter (DAC) quantization effect, the nonlinearity of Mach Zehnder modulator (MZM), channel estimation, time synchronization and general system parameter optimization. However, many other hardware imperfections, e.g., mismatch of transmit and receive local oscillator, imbalance of I and Q branch of the optical IQ-Modulator and phase noise of the laser diode (LD), shall also be surmounted. Furthermore, the electrical components such as DAC, MZM and photodiode (PD) exhibit in nature lowpass characteristics which degrade the system performance. Additionally, the ADC and DAC quantization effect have to be further addressed based on realistic hardware model and limited number of bits. The aforementioned problems are firstly incorporated based on realistic hardware models into the C++ simulation chain developed by INÜ. Through theoretical analysis and numerical simulation, the impact of the hardware imperfections on the system performance is studied and quantified by either increase of bit error rate (BER) or optical signal to noise ratio (OSNR) penalty. Then, algorithms at reasonable complexity are suggested/proposed to combat these problems. Since optical signal processing is expensive and usually more demanding, the suggested methods are mostly conducted in the electrical domain using digital signal processing (DSP) unit. For instance, a method of robust carrier frequency offset (CFO) estimation with reduced signaling overhead was proposed for CO-OFDM systems. It was shown that it works for a large range of frequency offsets (| fCFO | < 5 GHz) and exhibits superior robustness against noise and dispersive effects considering laser random phase walk. For the IQ imbalance, a thoroughly mathematical framework was established for CO-OFDM systems. An optimum joint IQ imbalance estimation/compensation and channel estimation/equalization method was proposed. Furthermore, a method for designing preamble or training sequences was presented, which is mathematically proved to be optimal under power constraint and shows significant performance improvement. Furthermore, companding transform technique combined with predistortion of nonlinear MZM characteristic function was studied along with an optimum parameter design. Based on a VHDL OFDM transmitter and receiver, the quantization effect with limited word lengths is examined in terms of BER and required OSNR for various ADC and DAC word length. In brief, the focus of this project is to tackle the main performance limiting impairments caused by both electrical and optical hardware components. All suggested methods for combating the impairments can be realized in the electrical domain without the need of optical modules.
Publications
- “Robust Carrier Frequency Offset Estimation with Reduced Overhead for Coherent Optical OFDM Systems.” In: Proceedings of ITG Symposium on Photonic Networks. May 2015, pp. 1–7
D. Rörich, I. Vaklinov, and S. ten Brink
- IQ Imbalance Compensation and Channel Estimation in Coherent Optical OFDM Systems.” In: 2016 10th IEEE International Conference on Signal Processing and Communication Systems (ICSPCS). Australia, Dec. 2016
Xiaojie Wang, Benedikt Leible, Wenhao Wang, David Rörich, and Stephan ten Brink
(See online at https://doi.org/10.1109/ICSPCS.2016.7843310) - “Verfahren zur Verringerung nichtlinearer Effekte in Sender und Empfänger optisch kohärenter Mehrträgersysteme.” PhD thesis. Institute of Telecommunications, University of Stuttgart, Sept. 2016
David Rörich
(See online at https://dx.doi.org/10.18419/opus-8923)
