Time-Frequency Packing for High Capacity Coherent Optical Links

We consider realistic long-haul optical links, with linear and nonlinear impairments, and investigate the application of time-frequency packing with low-order constellations as a possible solution to increase the spectral efficiency. A detailed comparison with available techniques from the literature will be also performed. We will see that this technique represents a feasible solution to overcome the relevant theoretical and technological issues related to this spectral efficiency increase and could be more effective than the simple adoption of high-order modulation formats.Comment: 10 pages, 9 figures. arXiv admin note: text overlap with arXiv:1406.5685 by other author

Optical Time-Frequency Packing: Principles, Design, Implementation, and Experimental Demonstration

Time-frequency packing (TFP) transmission provides the highest achievable spectral efficiency with a constrained symbol alphabet and detector complexity. In this work, the application of the TFP technique to fiber-optic systems is investigated and experimentally demonstrated. The main theoretical aspects, design guidelines, and implementation issues are discussed, focusing on those aspects which are peculiar to TFP systems. In particular, adaptive compensation of propagation impairments, matched filtering, and maximum a posteriori probability detection are obtained by a combination of a butterfly equalizer and four 8-state parallel Bahl-Cocke-Jelinek-Raviv (BCJR) detectors. A novel algorithm that ensures adaptive equalization, channel estimation, and a proper distribution of tasks between the equalizer and BCJR detectors is proposed. A set of irregular low-density parity-check codes with different rates is designed to operate at low error rates and approach the spectral efficiency limit achievable by TFP at different signal-to-noise ratios. An experimental demonstration of the designed system is finally provided with five dual-polarization QPSK-modulated optical carriers, densely packed in a 100 GHz bandwidth, employing a recirculating loop to test the performance of the system at different transmission distances.Comment: This paper has been accepted for publication in the IEEE/OSA Journal of Lightwave Technolog

Spectral Efficiency Optimization in Flexi-Grid Long-Haul Optical Systems

Flexible grid optical networks allow a better exploitation of fiber capacity, by enabling a denser frequency allocation. A tighter channel spacing, however, requires narrower filters, which increase linear intersymbol interference (ISI), and may dramatically reduce system reach. Commercial coherent receivers are based on symbol by symbol detectors, which are quite sensitive to ISI. In this context, Nyquist spacing is considered as the ultimate limit to wavelength-division multiplexing (WDM) packing. In this paper, we show that by introducing a limited-complexity trellis processing at the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a denser-than-Nyquist channel packing (i.e., a higher spectral efficiency (SE)) is possible at equal reach. By adopting well-known information-theoretic techniques, we design a limited-complexity trellis processing and quantify its SE gain in flexi-grid architectures where wavelength selective switches over a frequency grid of 12.5GHz are employed.Comment: 7 pages, 9 figure

Next-generation long-haul optical links: Higher spectral efficiency through time-frequency packing

We consider realistic long-haul optical links, where nonlinear effects represent the main impairment, and investigate the application of time-frequency packing with low-order constellations as a the most viable solution to increase the spectral efficiency. We will see that this technique allows to overcome the relevant theoretical and technological issues related to this spectral efficiency increase and is more effective than the simple adoption of high-order modulation formats which are more sensitive to nonlinear effects

Absolute Polar Duty Cycle Division Multiplexing for High-Speed Fiber Optic Communication System

Multiplexing is one of the fundamental necessities in today’s digital communications. It allows multiple users to share the bandwidth of the transmission medium. In this dissertation a new design of the Duty cycle Division Multiplexing (DCDM) family, namely Absolute Polar Duty Cycle Division Multiplexing (APDCDM) which is based on the polar signaling and different return to zero (RZ) duty cycles is reported for high speed optical fiber communication systems. Unlike all the other techniques, in AP-DCDM different users share the communication medium to transmit in the same time period and at the same carrier wavelength, but with different duty cycles. The unique duty cycle for each channel helps to regenerate data at the receiver. Two different AP-DCDM designs, namely AP-DCDM with guard band (GB) and AP-DCDM without GB have been successfully demonstrated. This thesis is presented based on the alternative format which has been approved by University Putra Malaysia’s Senate, which is the manuscript-based format. The major difference between this alternative format and the conventional ones is that, this format uses published papers in place of the regular chapters on results and discussion. The first paper contains a novel concept of decision circuit and Bit-error-rate (BER) estimation method for AP-DCDM which is published in International Review of Electrical Engineering. This journal in indexed by ISI Thomson Scientific. The concepts have significant differences to those used in conventional microwave communication receivers. This is due to the unique characteristics of the multilevel signal produced in AP-DCDM system. The BER estimation method is validated by simulation and compared against bit-to-bit comparison method. The second paper contains the first design of AP-DCDM (AP-DCDM with guard band) which is published in Optical Fiber Technology journal (OFT) by Elsevier. This journal is indexed by ISI Thomson Scientific with 2008 impact factor of 1.253. It is demonstrated that AP-DCDM system has a clear advantage over conventional RZOOK. Complexity and performance comparison against other modulation formats namely Duobinary, Non-Return-to-Zero (NRZ)-OOK and RZ-Differential Quadrature Phase-Shift Keying (RZ-DQPSK) at aggregate speed of 40 Gb/s (2 x 20 Gb/s) are made. It is shown that AP-DCDM has less complexity and the best receiver sensitivity (-32 dBm) and better CD tolerance (±200 ps/nm). In reference to duobinary, AP-DCDM is less complex and has better receiver sensitivity but worse dispersion tolerance The third paper contains the second design of AP-DCDM (AP-DCDM without guard band) which is published in IET Journal of Optoelectronics by Institution of Engineering and Technology (IET), previously IEE. This journal is indexed by ISI Thomson Scientific with impact factor of 0.704. The system tolerance to signal impairments is investigated and it shows that the spectral width of the AP-DCDM can be furthered reduced which leads to better dispersion tolerance compared to other modulation techniques. The fourth paper presents the effect of self-phase-modulation on AP-DCDM system which is accepted for publication in IET Journal of Optoelectronics (with impact factor of 0.704) considering different number of channels, launched power and precompensation ratio. It was shown that SPM is a major factor that introduce penalty to the system. Nonetheless, our results indicate that transmission using AP-DCDM should be possible at the launched power of up to tens of dBm, which is consistent with the requirement of high-quality, long distance transmissions. Finally the fifth paper discusses the performance evaluation of AP-DCDM over Wave length Division Multiplexing (WDM), which is accepted for publication in Optics Communications by Elsevier, which is indexed by ISI Thomson Scientific with 2008 impact factor of 1.552. The narrow optical spectrum on AP-DCDM reduces the inter-channel coherent crosstalk. The possibility of setting channel spacing as narrow as 62.5 GHz for 40 Gbit/s AP-DCDM signal was confirmed. A capacity of 1.28 Tbit/s (32 x 40 Gbit/s) was packed into a 15.5 nm EDFA gain-band with 0.64 bit/s/Hz spectral efficiency by using 10 Gbit/s transmitter and receiver

Doppler-corrected differential detection system

Doppler in a communication system operating with a multiple differential phase-shift-keyed format (MDPSK) creates an adverse phase shift in an incoming signal. An open loop frequency estimation is derived from a Doppler-contaminated incoming signal. Based upon the recognition that, whereas the change in phase of the received signal over a full symbol contains both the differentially encoded data and the Doppler induced phase shift, the same change in phase over half a symbol (within a given symbol interval) contains only the Doppler induced phase shift, and the Doppler effect can be estimated and removed from the incoming signal. Doppler correction occurs prior to the receiver's final output of decoded data. A multiphase system can operate with two samplings per symbol interval at no penalty in signal-to-noise ratio provided that an ideal low pass pre-detection filter is employed, and two samples, at 1/4 and 3/4 of the symbol interval T sub s, are taken and summed together prior to incoming signal data detection