Advanced digital signal processing for coherent optical OFDM transmissions

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been actively considered as a potential candidate for long-haul transmission and 400 Gb/s to 1 Tb/s Ethernet transport because of its high spectral efficiency, efficient implementation, flexibility and robustness against linear impairments such as chromatic dispersion and polarization mode dispersion. However, due to the long symbol duration and narrow subcarrier spacing, CO-OFDM systems are sensitive to laser phase noise and fibre nonlinearity induced penalties. As a result, the development of CO-OFDM transmission technology crucially relies on efficient techniques to compensate for the laser phase noise and fibre nonlinearity impairments. In this thesis, high performance and low complexity digital signal processing techniques for laser phase noise and fibre nonlinearity compensation in CO-OFDM transmissions are demonstrated. For laser phase noise compensation, three novel techniques, namely quasipilot-aided, decision-directed-free blind and multiplier-free blind are introduced. For fibre nonlinear compensation, two novel techniques which are referred to as phase conjugated pilots and phase conjugated subcarrier coding, are proposed. All these abovementioned digital signal processing techniques offer high performances and flexibilities while requiring relatively low complexities in comparison with other existing phase noise and nonlinear compensation techniques. As a result of the developments of these digital signal processing techniques, CO-OFDM technology is expected to play a significant role in future ultra-high capacity optical network. In addition, this thesis also presents preliminary study on nonlinear Fourier transform based transmission schemes in which OFDM is a highly suitable modulation format. The obtained result paves the way towards a truly flexible nonlinear wave-division multiplexing system that allows the current nonlinear transmission limitations to be exceeded

Carrier Recovery in burst-mode 16-QAM

Wireless communication systems such as multipoint communication systems (MCS) are becoming attractive as cost-effective means for providing network access in sparsely populated, rugged, or developing areas of the world. Since the radio spectrum is limited, it is desirable to use spectrally efficient modulation methods such as quadrature amplitude modulation (QAM) for high data rate channels. Many MCS employ time division multiple access (TDMA) and/or time division duplexing (TDD) techniques, in which transmissions operate in bursts. In many cases, a preamble of known symbols is appended to the beginning of each burst for carrier and symbol timing recovery (symbol timing is assumed known in this thesis). Preamble symbols consume bandwidth and power and are not used to convey information. In order for burst-mode communications to provide efficient data throughput, the synchronization time must be short compared to the user data portion of the burst. Traditional methods of communication system synchronization such as phase-locked loops (PLLs) have demonstrated reduced performance when operated in burst-mode systems. In this thesis, a feedforward (FF) digital carrier recovery technique to achieve rapid carrier synchronization is proposed. The estimation algorithms for determining carrier offsets in carrier acquisition and tracking in a linear channel environment corrupted by additive white Gaussian noise (AWGN) are described. The estimation algorithms are derived based on the theory of maximum likelihood (ML) parameter estimation. The estimations include data-aided (DA) carrier frequency and phase estimations in acquisition and non-data-aided (NDA) carrier phase estimation in tracking. The DA carrier frequency and phase estimation algorithms are based on oversampling of a known preamble. The NDA carrier phase estimation makes use of symbol timing knowledge and estimates are extracted from the random data portion of the burst. The algorithms have been simulated and tested using Matlab® to verify their functionalities. The performance of these estimators is also evaluated in the burst-mode operations for 16-QAM and compared in the presence of non-ideal conditions (frequency offset, phase offset, and AWGN). The simulation results show that the carrier recovery techniques presented in this thesis proved to be applicable to the modulation schemes of 16-QAM. The simulations demonstrate that the techniques provide a fast carrier acquisition using a short preamble (about 111 symbols) and are suitable for burst-mode communication systems

Carrier Synchronization in High Bit-Rate Optical Transmission Systems

In this dissertation, design of optical transmission systems with differential detection and coherent detection is briefly described. More over, algorithms for carrier synchronization and phase estimation with their implementation in high bit-rate optical transmission systems are proposed

Optical Space Division Multiplexing in Short Reach Multi-Mode Fiber Systems

The application of space division multiplexing to fiber-optic communications is a promising approach to further increase the channel capacity of optical waveguides. In this work, short reach and low-cost optical space division multiplexing systems with intensity modulation and direct detection (IM/DD) are in the focus of interest. Herein, different modes are utilized to generate spatial diversity in a multi-mode fiber. In such IM/DD systems, the process of square-law detection is inherently non-linear. In order to obtain an understanding of the channel characteristics, a system model is developed, which is able to show under which conditions the system can be considered linear in baseband. It is shown that linearity applies in scenarios with low mode cross-talk. This enables the use of linear multiple-input multiple-output (MIMO) signal processing strategies for equalization purposes. In conditions with high mode cross-talk, significant interference occurs, and the transmitted information cannot be extracted at the receiver. Furthermore, a method to determine the power coupling coefficients between mode groups is presented that does not require the excitation of individual modes, and hence it can be realized with inexpensive components. In addition, different optical components are analyzed with respect for their suitability in MIMO setups with IM/DD. The conventional approach with single-mode fiber to multi-mode fiber offset launches and optical couplers as well as a configuration that utilizes multi-segment detection are feasible options for a (2x2) setup. It is further shown that conventional photonic lanterns are not suited for MIMO with IM/DD due to their low mode orthogonality during the multiplexing process. In order to enable higher order MIMO configurations, devices for mode multiplexing and demultiplexing need to be developed, which exhibit a high mode orthogonality on one hand and are low-cost on the other hand