1,720,968 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
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
Dual stage carrier phase estimation for 16-QAM systems based on a modified QPSK-partitioning
Full text linkCoherent 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
Multi-Stage CPE Algorithms for 64-QAM Constellations
No full textWe propose and analyze a multi-stage architecture for carrier phase-estimation in 64-QAM systems, based on the cascade of several feed-forward elementary blocks. We outline the beneficial effect of increasing the number of elementary blocks
Multistage carrier phase estimation algorithms for phase noise mitigation in 64-quadrature amplitude modulation optical systems
Full text linkTwo novel low-complexity multi-stage digital feedforward carrier phase estimation (CPE) algorithms for 64-ary quadrature amplitude modulation (QAM) are proposed and analyzed by numerical simulations. The first stage is composed of a Viterbi&Viterbi block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the Viterbi&Viterbi algorithm after a 64-QAM-to-QPSK transformation, while the subsequent stages iterate a maximum likelihood estimation (MLE) algorithm for phase estimation. All proposed techniques are characterized by a high tolerance to laser phase noise: an OSNR penalty of 1 dB at bit error rate (BER) of 10−2, the proposed schemes can tolerate a linewidth times symbol duration product equal to 5.6e−5 and 7.1e−5, respectively. At 32 Gbaud, all of the above linewidth requirements can be met using commercial tunable lasers. The proposed schemes achieve a similar linewidth. tolerance with a reduced implementation complexity with respect to algorithms based on the blind phase search (BPS) metho
Blind modulation format identification for digital coherent receivers
Full text linkIn this paper, a simple novel digital modulation format identification (MFI) scheme for coherent optical systems is proposed. The scheme is based on the evaluation of the peak-to-average-power ratio (PAPR) of the incoming data samples after analog-to-digital conversion (ADC), chromatic dispersion (CD) and polarization mode demultiplexing (PMD) compensation at the receiver (Rx). Since at a particular optical signal-to-noise ratio (OSNR) value different modulation formats have distinct PAPR values, it is possible to identify them. The proposed scheme and the results are analyzed both experimentally and through numerical simulations. The results demonstrate successful identification among four modulation formats (MF) commonly used in digital coherent systems
Carrier Phase Estimation Through the Rotation Algorithm for 64-QAM Optical Systems
Full text linkA novel low-complexity two-stage digital feedforward carrier phase estimation algorithm based on the rotation of constellation points to remove phase modulation for a 64-ary quadrature amplitude modulation (QAM) system is proposed and analyzed both experimentally and through numerical simulations. The first stage is composed of a Viterbi and Viterbi (V&V) block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the V&V algorithm after the removal of phase modulation through rotation of constellation points. Comparison of the proposed scheme with constellation transformation, blind phase search (BPS) and BPS+MLE (maximum likelihood estimation) algorithm is also shown. For an OSNR penalty of 1 dB at bit error rate of 1e−2 , the proposed scheme can tolerate a linewidth times symbol duration product (Δν · Ts) equal to 3.7 × 1e−5 , making it possible to operate 32-GBd optical 64-QAM systems with current commercial tunable laser
Analytical and Experimental Results on System Maximum Reach Increase Through Symbol Rate Optimization
Full text linkWe investigated the reach increase obtained through nonlinearity mitigation by means of transmission symbol rate optimization (SRO). First, we did this theoretically and simulatively. We showed that the nonlinearity model that properly accounts for the phenomenon is the EGN model, in its version that specifically includes four-wave mixing. We then found that for PM-QPSK systems at full C-band, the reach increase may be substantial, on the order of 10-25%, with optimum symbol rates on the order of 2-6 GBd. We show that for C-band PM-QPSK systems over SMF, the potential mitigation due to SRO is greater than that ideally granted by digital backpropagation (the latter applied over a bandwidth of a 32-GBd channel). We then set up an experiment to obtain confirmation of the theoretical and simulative predictions. It consisted of 19 PM-QPSK channels, operating at 128 Gb/s per channel, over PSCF, with span length 108 km and EDFA-only amplification. We demonstrated a reach increase of about 13.5%, when going from single-carrier per channel transmission, at 32 GBd, to eight subcarrier per channel, at 4 GBd, in line with the EGN model predictions. We also extended the theoretical investigation of SRO to PM-16QAM, where we found a qualitatively similar effect to PM-QPSK, although the potential reach increase appears to be typically only about 50% to 60% that of PM-QPSK. Further investigation is, however, in order, specifically to explore the effect on PM-16QAM SRO of the removal of long-correlated phase and polarization nois
Linewidth-tolerant feed-forward dual-stage CPE algorithm based on 64-QAM constellation partitioning
No full textA detailed analysis of a novel low-complexity two-stage digital feed-forward carrier recovery algorithm for 64-QAM, based on the rotation of constellation points to remove phase modulation is presented. Its performance and complexity is compared with previously proposed algorithm
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