2,144 research outputs found
Design of Ad-Hoc Algorithms for Performance Optimization of High-Order Modulation Formats in Coherent Optical Transmission Systems
No full textCoherent optical detection has drawn a considerable interest in the past few years. This is mainly due to its high tolerance towards linear and non-linear fiber impairments and improved spectral efficiency, enabled by the use of polarization multiplexing (PM) and multilevel modulation formats. Using digital signal processing, coherent optical detection has made it possible to increase the spectral efficiency (SE) of optical systems well beyond the 1 b/s/Hz achievable in intensity modulated direct detection (IMDD) systems. Combined with polarization multiplexing (PM) and multilevel M-ary quadrature amplitude modulation (QAM) formats, coherent optical detection is considered to be the best candidate for future high capacity 100 and 400 Gbps wavelength-division multiplexing (WDM) systems. A critical part of coherent optical communication systems is the phase sensitive coherent receiver whose performance is limited by the phase noise that exists on the recovered data samples. Major source of phase noise is the finite linewidth of both transmitter (Tx) laser and receiver (Rx) local oscillator. Amplified Spontaneous Emission (ASE) creating nonlinear phase noise that interacts with the nonlinear Kerr effect, can also add in the phase noise of recovered data signal. This phase noise causes distortion and hence random rotation of the received constellation points. As a consequence, design of efficient carrier phase estimation (CPE) algorithms has become very important, especially while implementing high order modulation formats. This thesis presents several novel CPE algorithms for phase noise detection and compensation of high order QAM formats. Some algorithms make a phase estimation by increasing the number of symbols that took part in CPE while others use novel multistage architectures. The algorithms are based on a classic feed forward Viterbi&Viterbi (V&V) scheme. Performance of the algorithms in terms of complexity and linewidth times symbol duration product with other standard CPE algorithms is also given. It was observed that some of the proposed schemes give even better performance than the best available algorithms present in the literature. Also in optical communication systems frequency offset compensation (FOC) and carrier phase estimation (CPE) techniques require a prior knowledge of the modulation format but owing to the flexible transceivers, it is no longer guaranteed that the signals arriving at the receiver side would have the same, known in advance, modulation format (MF). The receiver thus needs to have some 'blind' or 'flexible' algorithms to adapt to these changes. Modulation format identification (MFI) is of high interest for the next generation fiber-optic networks as it could grant more autonomy and flexibility to the network. Elastic optical networks (EON) and cognitive optical networks (CON), with rate-adaptive transceivers supporting multiple modulation formats, have recently drawn a considerable interest as future optical networks. Although MFI for wireless systems has been thoroughly investigated, not much work has been done for the recognition/identification of modulation formats in fiber-optic networks. For MFI, a digital coherent receiver should be able to identify the MF of the incoming signals to ensure proper demodulation. Since FOC and CPE techniques require a prior knowledge of the modulation format, this makes blind MFI on-the-fly more difficult. As a consequence, we either need to develop FOC or CPE algorithms that are MF oblivious or develop some schemes that do MFI before entering the FOC or CPE blocks in the DSP receiver. This thesis also gives a simple and novel MFI scheme based on the evaluation of the peak-to-average-power ratio (PAPR) of the received data samples. To the best of our knowledge the proposed scheme is the simplest among all the schemes present in the literature. At a particular optical signal to noise ratio (OSNR) value, different modulation formats have distinct PAPR which can be used as defining parameter for their correct identification. Simulation and experimental results demonstrate successful identification of four commonly used modulation formats. Furthermore, the propose technique can also be extended to other lower or higher order formats. In order to practically generate these QPSK and QAM formats, the most commonly used modulator is the Dual Parallel Mach-Zender (DPMZ) modulator. DPMZ modulators are also used in a wide variety of radio-over-fiber (ROF) links. A stable biasing condition of these modulators is very important for high gain and low noise figure (NF) of these links. Being technologically mature and due to the fact that it has a linear Pockels effect, LiNBO3 is commonly used as a manufacturing material for MZ modulators. However, a change in temperature, radio-frequency (RF) heating or aging, can result in a drift of operating bias of LiNBO3 modulators. To cope with it, a close-loop control technique has to be developed that will follow this drift of the operating bias and will keep the DPMZ modulators under their optimal condition. This thesis also presents a close loop control technique for the automatic bias control of both the inner and outer MZ's of the DPMZ structure. This technique requires the generation of three separate low amplitude pilot tones at three different frequencies. These pilot tones are then clamped with the dc biasing voltage, at biasing ports of the MZ's. The pilot tones should have low amplitude so that they will not interfere with the RF signal data of the modulators. By carefully monitoring the beating between these tones, a close-loop control technique can be developed that will automatically monitor and follow the drift of the dc biasing voltage from its optimum and will enable the modulators to exhibit long term stability. In summary, this thesis makes important contributions by designing ad-hoc algorithms for performance optimization of high-order modulation formats in coherent optical transmission system
Pilot tones based polarization rotation, frequency offset and phase estimation for polarization multiplexed Offset-QAM Multi-Subcarrier coherent optical systems
No full textMulti-Subcarrier Offset-Quadrature Amplitude Modulation (OQAM) scheme has recently been proposed to increase the spectral efficiency (SE) of optical coherent communication systems. Thanks to the half symbol delay between the in-phase and quadrature components, OQAM signals can achieve channel spacing values as low as the symbol rate Rs, without suffering from inter channel interference (ICI) even in the condition of severe spectral overlapping. On the other hand, phase noise due to this half symbol delay not only results in the random rotation of the received constellation points but also introduces additional cross talk after equalization, when using standard equalization algorithms. Therefore, OQAM signals have very poor tolerance towards laser linewidth. In addition, for a polarization multiplexed (PM) system, polarization rotation between the x and y polarizations interacting with the phase noise can severely affect the performance.In this paper we propose a pilot tones (PTs) based approach for multi-subcarrier OQAM signals. The PTs are used to estimate frequency offset (FO),carrier phase (CP)and the polarization rotation matrix. An equalizer based on a modified constant modulus algorithm (CMA) is finally used to recover the signal
Dual stage carrier phase estimation for 16-QAM systems based on a modified QPSK-partitioning
Get PDFCoherent optical communications based on higher order modulation formats are severely affected by the phase noise of transmitter and receiver lasers. In this work, a novel yet simple scheme is presented for carrier phase estimation (CPE) of 16-ary Quadrature Amplitude Modulation (16- QAM) formats, based on a modified QPSK-partitioning algorithm. The proposed algorithm can tolerate a linewidth times symbol duration product of the order of 1e-4, with 1 dB penalty at a bit error rate (BER) of 1e-3. Tolerance can be further improved by introducing a maximum likelihood estimation (MLE) stage. Comparison of the scheme with other proposed algorithms is shown. The obtained results indicate that the presented approach can be used with the commercially available state of the art lasers for 16-ary Quadrature Amplitude Modulation (16- QAM) transmission at 100 Gbps
Automatic bias control of Mach-Zehnder modulators for QPSK and QAM systems
No full textA close-loop control technique for automatic bias control of inner and outer Mach-Zehnder modulators in a dual parallel configuration is presented. The Mach-Zehnders are configured as dual parallel optical modulators. The technique is based on applying low amplitude pilot tones at three different direct current biasing ports of the modulators. By carefully monitoring the amplitude of the beating between these pilot tones, a close-loop control technique can be developed that will automatically monitor and follow the drift of the direct current biasing voltage from its optimum. Hence the technique will enable the modulators to exhibit long term stability for the quadrature phase shift keying or M-quadrature amplitude modulation system
Performance analysis of different standard single stage CPE algorithms using MSE
No full textThe performance of different standard single-stage carrier phase estimation (CPE) algorithms for multi-level modulation formats is analysed. The analysis is performed by calculating the mean square error (MSE) between the actual and the estimated phase noise. It was observed that the better the performance of an algorithm, the lower will be its MSE value. Moreover, MSE can also be used as an efficient benchmark for finding the optimum averaging window length at a particular value of signal to noise ratio (SNR) and laser linewidt
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Manahil Bilal Interview
No full textThis interview is with Manahil Bilal, a young Muslim woman and software developer in Spring, TX. Manahil describes life in her close-knit extended family and tells the story of her immediate family’s immigration and their adjustment to life in the US. She discusses her relationship with her Pakistani and Muslim identities over time and learning to embrace them despite misunderstandings by those around her. Manahil also talks about how 9/11 was taught in her schools, including misinformation by teachers and under-emphasis on the long term impact the attacks had on Muslims and those perceived to be Muslims in the US.Henry Luce FoundationReligious StudiesTranscript created with Otter.ai and edited by project assistant
Hydraulic simulations to evaluate and predict design and operation of the Chashma Right Bank Canal
Get PDFIrrigation systems / Irrigation canals / Flow control / Velocity / Canal regulation techniques / Hydraulics / Simulation models / Design / Operations / Crop-based irrigation / Distributary canals / Water delivery / Policy / Protective irrigation / Water allocation / Water requirements / Sedimentation / Water distribution / Equity / Water conveyance / Pakistan / Chashma Right Bank Canal
Data for: Effect of work function and cohesive energy of the constituent phases of Ti-50 at.% Al cathode during arc deposition of Ti-Al-N coatings
No full textThis data covers XRD of coatings and cathodes by both grades, deposition rates of the coatings and the hardness of the coatings
Average Contiguous Duration (ACD)-Based Quantization for Secret Key Generation in Generalized Gamma Fading Channels
Get PDFThe wireless channel-based Secret Key Generation (SKG) algorithms aim at securing the wireless link against unauthorized eavesdropping by exploiting the channel’s randomness for generating matching secret keys at the legitimate nodes for message encryption/decryption. To counter differences in hardware and noise conditions at the legitimate nodes, which can lead to key mismatch, the SKG algorithms typically include the intermediate steps of sampling, quantization, information reconciliation, and privacy amplification. These steps collectively aim to improve the performance trade-offs between Key Generation Rate (KGR), Key Agreement Probability (KAP), and Secret Key Randomness (SKR) properties. This paper derives a closed-form expression for the Average Contiguous Duration (ACD) of Generalized Gamma (GG) fading wireless channels. The ACD is a recently introduced novel quantifier for characterizing the second-order statistics of fading channels, which includes Average Fade Duration (AFD) as its special case. The proposed GG fading ACD expression is shown to include, as its special cases, the ACD for commonly observed fading distributions such as Gamma, Nakagami- , and Rayleigh. By exploiting the derived GG ACD expression, a multi-level quantization scheme for SKG is proposed that determines suitable quantization intervals for identical likelihood of an equal number of consecutive channel samples falling in each quantization interval. A comprehensive comparative analysis of the proposed ACD-based quantization for SKG is conducted in relation to conventional Uniform Quantization (UQ) and Cumulative Distribution Function (CDF)-based Non-Uniform Quantization (NUQ) schemes. The presented numerical results confirm the superior performance trade-off between KGR and KAP offered by the proposed ACD-based quantization in relation to that offered by UQ and CDF-based NUQ
Dual Stage CPE for 64-QAM Optical Systems Based on a Modified QPSK-Partitioning Algorithm
No full textIn this letter, a novel two-stage digital feed forward carrier recovery algorithm for 64-ary quadrature amplitude modulation (QAM) is proposed and analyzed. Due to the absence of any feedback loop, the approach shows a high tolerance toward laser phase noise. Different steps involving partition, selection, and rotation of symbols are also discussed. For an OSNR penalty of 1 dB at bit error rate of 10-3, the proposed scheme can tolerate linewidth times symbol duration product equal to 3.3×10-5 and hence can be used with the commercially available state-of-the-art tunable lasers for 64-QAM transmission at 400 Gb/
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