Advanced modulation formats and coherent detection in optical communications
Our research group examines techniques for mitigation of impairments in optical communications links and for increasing the capacity of those links. We address both optical and digital signal processing (DSP) solutions from a systems perspective. Maximizing the capacity of fiber communications is the rally cry for research in optical communi-cations this decade, with focus on 1) high order quadrature amplitude modulation (QAM), and 2) higher baud rates. We examine via simulation distortions to QAM from nonlinear amplification and mitigation in DSP; an experimental validation was recently completed. We work with an industrial partner to quantify theoretically and via experimentation the phase tracking improvement in QAM when using optical signal processing to suppress phase noise. We study the efficiency and range of wavelength conversion for QAM.
A key element in all our research is the development of stable, accurate, and efficient algorithms in offline digital signal processing of impairments in the optical channel. A typical experimental measurement involves coherent detection with a 22-GHz 3-dB bandwidth integrated receiver, the signal is digitized using two channels of a commercial 80-GS/s real-time oscilloscope with 30 GHz bandwidth. Signal processing is performed offline on 2 million captured samples. In the digital signal processing (DSP), we apply a Gaussian low-pass filter and do dispersion compensation. We then perform resampling and timing recovery.
A coarse non-data-aided fast Fourier transform-based frequency offset compensation is performed in a block-wise fashion over 30k symbols. We apply Wiener-Hopf-based decision-directed equalizer with 31 taps. We apply a minimum mean square error filter to mitigate the effect of limited receiver bandwidth. We then employ a decision-aided maximum likelihood algorithm to estimate the carrier phase]. Finally, we choose the closest symbol to the received I/Q coordinates from the QAM constellation and carry out symbol to bit mapping. We synchronize to the transmitted pseudo random binary sequence, count errors and estimate bit error rate (BER).
The optimization and robustness of the various algorithms being used is essential to extracting the best possible results from our experimental measurements, as well as simulations.
