QAM Formats Research Articles

Nonlinearity-tolerant transmission using modified phase-conjugate twin wave for M-PSK signals

Abstract Using modified phase-conjugate twin wave (M-PCTW) scheme, in which additional information bits are modulated one of the conjugated wave, is helpful to increase the spectral efficiency (SE) of the conventional PCTW scheme. In this paper, we show that the phase modulated formats, but not the conventional square QAM formats are more suitable to M-PCTW scheme. Results of 1600 km nonlinear transmission show that for polarization multiplexed (PM) 8-PSK format, performance improvement from M-PCTW is about 3.3 dB, while it is about 2.2 dB for PM 8-QAM formats. For PM 16-PSK (QAM) modulation, performance improvement from M-PCTW is about 3.4 dB and 1.6 dB, respectively.

Effect of Line-of-sight Channels on DC-biased Optical Filter Bank Multicarrier Visible Light Communication with Multiple LED Arrays

This paper investigates the impact of propagation delay and channel loss due to the use of multiple LED arrays in visible light communication (VLC) systems based on filter bank multicarrier (FBMC) modulation. FBMC offers greater spectral efficiency, and asynchronous transmission and is a promising alternative scheme to orthogonal frequency division modulation (OFDM). The proposed FBMC model is based on 4-quadrature amplitude modulation (QAM) and 16-QAM formats and uses 100 symbols and 600 input bits per symbol. In this paper, the VLC-FBMC system is designed based on the line-of-sight (LOS) model under the additive white Gaussian noise (AWGN) channel. Comparison analyses between different bit rates in terms of bit error rate (BER), best sampling point, and signal-to-noise ratio (SNR) requirement have been carried out to show the delay and loss effect on communication quality and system performance. The results demonstrate that the proposed FBMC model achieves a bit rate of up to 29.296 Mbit/s with a low BER of 10-3 and less SNR penalty in high QAM formats, demonstrating its potential as a viable alternative to OFDM for future VLC systems.

Coherent Nyquist optical pulse transmission at nearly 1-Tb/s/λ over 1,600 km with a capacity of 21.5 Tb/s using PS-32 QAM signals

We demonstrate the 1,600-km transmission at nearly 1-Tb/s/λ signals with a capacity of 21.5 Tb/s. Probabilistic shaping was newly applied to high-speed coherent optical Nyquist pulse transmission systems to maximize the transmission capacity. Employing a 160-GBd PS-32 QAM format, WDM signals at nearly 1-Tb/s/λ were successfully transmitted over 1,600 km with a capacity of 21.5 Tb/s.

System impact and calibration method of power imbalances for a dual-polarization in-phase/quadrature optical transmitter.

For the latest 400-Gb/s or upcoming 1-Tb/s single-carrier optical fiber communications systems, dual-polarization quadrature amplitude modulation (DP-QAM) based on in-phase (I) and quadrature (Q) optical transmitter is the only possible solution. In a coherent DP-IQ transmitter, the power difference between the I and Q branches or orthogonally polarized (X and Y) channels are known as the IQ or XY power imbalance, respectively. Uncompensated IQ and XY power imbalances are always troublesome and can significantly limit the performance in long-haul transmission. In this work, we first experimentally investigate the impact of IQ and XY power imbalances on various QAM formats and baud rates in high-speed transmission. A DP-16-QAM up to 86 GBd is investigated, aiming for beyond 400 Gb/s applications. Then, a convenient pre-calibration method based on optical interference is proposed and experimentally demonstrated to detect and compensate both the IQ and XY power imbalances. This method can use the existing modulator structure and low-speed photodiode in the DP-IQ transmitter to obtain the optical interference power by injecting a specific coded electrical signal. After adjusting the time skew of each channel, the power imbalance between two channels can be calibrated by minimizing the optical interference power. The results fit well with the given theoretical analysis, and the proposed technique can be adopted to the DP-IQ transmitter with different QAM formats.

Tolerance of SCM Nyquist and OFDM signals for heterogeneous fiber-optic and millimeter-wave mobile backhaul links under the effect of power amplifier saturation induced clipping

Extremely high frequency carrier communications such as millimeter-wave (mmW) is considered a promising solution for the future mobile networks. To compensate for the large path loss of the millimeter-wave links, saturated power amplifiers are inevitably used during amplify-and-forward relaying, which potentially causes system degradation due to power saturation induced clipping. In this paper, the tolerance of OFDM and Nyquist subcarrier multiplexing (SCM) signals against power amplifier saturation induced clipping is studied in the context of fiber-optic and millimeter-wave integrated communications for heterogeneous mobile backhaul networks. In the system, 10 Gbaud OFDM and Nyquist SCM signals are converted to an intermediate frequency passband before converted to the optical domain for a radio-over-fiber transmission to a remote antenna unit (RAU). At RAU, the optical radio signals are converted back to the electrical domain at millimeter-wave range. Those are then amplified by an power amplifier prior to a millimeter-wave communication to a remote radio head (RRH) for detection. Since Nyquist SCM signal exhibits a level of lower peak-to-average power ratio (PAPR) than that of OFDM, Nyquist SCM signals are more tolerant against the clipping caused by the power amplifier saturation. For 16 QAM, at clipping ratio (CR) of 4.5 dB, one-band Nyquist SCM signal shows a signal-to-noise ratio (SNR) performance over 10 times higher than that of OFDM. While the increase in the number of Nyquist SCM bands reduces the tolerance, Nyquist SCM signal maintains a better tolerance than OFDM one with respect to 16 QAM, 32 QAM and 64 QAM formats, which suffer severer impact with larger SNR penalty for higher modulation levels.

10 Tbit/s QAM Quantum Noise Stream Cipher Coherent Transmission Over 160 Km

We describe in detail our recent demonstration of a 10 Tbit/s secure physical layer transmission that we achieved by using digital coherent QAM quantum noise stream cipher (QNSC) and injection-locked WDM techniques. We used an FPGA-based transmitter and receiver to demonstrate a 165 channel polarization-multiplexed WDM 5 Gbaud 128 QAM/QNSC (70 Gbit/s) on-line transmission over 160 km with a spectral efficiency of 6 bit/s/Hz. In the present system, the original 128 QAM data were encrypted in a 1024 × 1024 QAM format using basis information. The encrypted signal was then masked by a large ASE noise, which reduced the detection “success” probability for an eavesdropper to 0.13% for each symbol. Furthermore, the multiplicity of the original QAM data and the seed keys used to generate the basis information were arbitrarily changed with time, which makes the decryption much more difficult.

Low-Complexity Geometric Shaping

Approaching Shannon's capacity via geometric shaping has usually been regarded as challenging due to modulation and demodulation complexity, requiring look-up tables to store the constellation points and constellation bit labeling. To overcome these challenges, in this paper, we study lattice-based geometrically shaped modulation formats in multidimensional Euclidean space. We describe and evaluate fast and low complexity modulation and demodulation algorithms that make these modulation formats practical, even with extremely high constellation sizes with more than $10^{28}$ points. The uncoded bit error rate performance of these constellations is compared with the conventional QAM formats in the additive white Gaussian noise and nonlinear fiber channels. At a spectral efficiency of 2 bits/sym/polarization, compared with 4-QAM format, transmission reach improvement of more than 38% is shown at the hard-decision forward error correction threshold of $2.26\times 10^{-4}$.

CMOS DAC Supported 1.1 Tb/s/λ DWDM Transmission at 9.8 bit/s/Hz Over DCI Distances

We report on a 16-nm CMOS DAC based transmitter optimization enabling bitrates up to 1.1 Tb/s. We describe the transmitter optimization and derive the maximization of the net bitrate. Based on back-to-back experiments we optimize the entropy of the probabilistically shaped QAM format, which we further apply for subsequent transmission experiments over data center interconnect distances. Finally, we successfully demonstrate DWDM transmission of 17 channels in a 112.5 GHz grid operated at 105 GBaud over 80 km up to 118 km achieving a net bitrate of 1.1 Tb/s and spectral efficiencies of 9.8 bit/s/Hz.

Analog Coherent Detection for Energy Efficient Intra-Data Center Links at 200 Gbps Per Wavelength

As datacenters continue to scale in size, energy efficiency for short reach (<; 2 km) links is a major factor for networks that may connect hundreds of thousands of servers. We demonstrate that links based on analog coherent detection (ACD) offer a promising path to simultaneously achieving significantly larger link budgets and improved link energy efficiency. A complete analysis is presented that considers the power consumption of all the photonic and electronic components necessary to realize an ACD link architecture based on 50 Gbaud (GBd) quadrature phase-shift keying (QPSK) signaling combined with polarization multiplexing to achieve 200 Gb/s/λ. These links utilize receivers that incorporate an optical phase-locked loop (OPLL) to frequency- and phase-lock the local oscillator (LO) laser to the incoming signal. QPSK modulation offers compelling advantages both in achievable link budget and in energy efficiency. Indeed, low-complexity electronics based on limiting amplifiers can be used as opposed to the linear front-ends, A/D converters, and digital signal processing (DSP) required for higher-order QAM or PAM formats. Our analysis indicates that links with 13 dB of unallocated budget operating at error rates of <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> can be achieved and is compatible with higher error rates that require forward error correction (FEC). We present a comparison of silicon and InP platforms and evaluate both traveling-wave and segmented modulator designs, providing an illustration of the wide design space before converging on the most promising architectures that maximize energy efficiency and minimize laser power. We establish the theoretical potential to achieve picojoule-per-bit energy efficiency targets.

Two-Stage Coded Modulation for Hurwitz Constellations in Fiber-Optical Communications

Four-dimensional signal constellations based on the checkerboard lattice $\boldsymbol{D}_4$ offer a packing gain over conventional QAM constellations per polarization. Due to the increased number of nearest neighbors such power-efficient 4D formats cannot be Gray-labeled and bit-interleaved coded modulation results in a considerable performance loss. Instead, a suited coded-modulation scheme must be tailored to the properties of the underlying signal lattice. We apply a low-complexity two-stage coded-modulation scheme for constellations based on the set of Hurwitz integers, an isomorphic representation of the $\boldsymbol{D}_4$ lattice. The proposed signaling scheme is evaluated in numerical simulations and fiber-optical system experiments. The performance is compared to a reference implementation using square QAM formats as proposed in the current 400ZR standardization activities. We demonstrate a 0.8 dB gain in required OSNR of the 512-ary Hurwitz constellation over dual-polarization 16-QAM (with 256 signal points in 4D) at the same symbol and bit rate. Additional numerical and experimental results show the potential for applications in the long-haul as well as the short-reach regime.

Analysis and simulation of reducing nonlinear interaction by shaping envelopes of transmitted signals in mode-division multiplexing systems

Nonlinear interaction induced by Kerr effects is a significant problem in the efforts made to ensure the successful development of transmission systems, such as mode-division multiplexing (MDM)-based optical multimode communication systems. A technique for reducing the interaction of the modes in MDM systems based on shaping envelopes of propagated signals is proposed. The envelopes of modes that carry m-array quadrature amplitude modulation (mQAM) are optically formed with a sinusoidal envelope to lower fiber nonlinearities by reducing the effective intensity and interference time between modes. The transmission performance of the proposed system is analytically characterized by developing an analytical model to estimate induced nonlinear phase noise and numerically investigated by examining the signal-to-noise ratio versus mode power. The effect of sinusoidal enveloped (SE)-4QAM format on the transmission distance is also explored for different mode combinations. Single-, two-, and five-mode transmissions are carried out to investigate the proposed method’s efficiency on the reduction of nonlinear interaction. In our system, the modes carry SE-4QAM format at rate 20 Gsymbol / s. The results show a significant enhancement in the performance of the MDM system when modes are modulated by SE-4QAM format. For example, the transmission distances of LP01 and LP11 are lengthened by 56.5% and 150% at the symbol error rate of 10 − 5, respectively.

Performance Comparison of Probabilistically Shaped QAM Formats and Hybrid Shaped APSK Formats With Coded Modulation

We study and compare performance of probabilistically shaped (PS)-64QAM formats (with spectral efficiency ranging from 3.6 to 8.6 b/s/Hz) and coded modulation (CM)-64/16APSK formats with PS or geometric shaping (GS) or hybrid shaping (HS) with spectral efficiency ranging from 4.8 to 8.8 b/s/Hz. We show that a well-designed CM-64/16APSK format could exceed the performance of PS-64QAM formats by a small margin both in back-to-back and transmission. This was verified for transmission distances from 4,435 to 22,176 km using laboratory hardware and our implemented FEC. We show similar nonlinearity compensation (NLC) benefit for both PS and CM formats using 3-stage NLC schemes. The NLC benefit increases from 1.2 to 1.6 dB as the transmission distance increases from 4,435 to 9,504 km and then nearly saturates at ∼1.6 dB after ∼10,000 km. Furthermore, we experimentally demonstrate 71.6 Tb/s (7.36 b/s/Hz) using a CM-64/16APSK format and 70.3 Tb/s (7.22 b/s/Hz) with a PS-64QAM format over 6,970 km using C+L band EDFA testbed.

Transfer learning simplified multi-task deep neural network for PDM-64QAM optical performance monitoring.

We experimentally demonstrate a transfer learning (TL) simplified multi-task deep neural network (MT-DNN) for joint optical signal-to-noise ratio (OSNR) monitoring and modulation format identification (MFI) from directly detected PDM-64QAM signals. First, we investigate the quality of amplitude histogram (AH) generation on the performance of OSNR monitoring and experimentally clarify the importance of higher electronic sampling rate in order to realize precise OSNR monitoring for high-order QAM format. Next, by implementing TL from simulation to experiment, when both 10Gbaud PDM-16QAM and PDM-64QAM signals are considered, the accuracy of MFI reaches 100% and the root-mean-square error (RMSE) of OSNR monitoring is 1.09dB over a range of 14-24dB and 23-34dB for PDM-16QAM and PDM-64QAM, respectively. Meanwhile, the used training samples and epochs can be substantially reduced by 24.5% and 44.4%, respectively. Since single photodetector (PD) and one TL simplified MT-DNN are used, the proposed optical performance monitoring (OPM) scheme with high cost performance can be applied for advanced modulation formats.

Equalization Enhanced Phase Noise in Coherent Receivers: DSP-Aware Analysis and Shaped Constellations

We revisit the analysis of equalization-enhanced phase noise (EEPN) arising in coherent receivers from the interaction between the chromatic dispersion compensation by an electronic equalizer and the phase noise of the local oscillator. Through numerical simulations we highlight EEPN characteristics and investigate its impact on the behavior of the carrier phase recovery algorithm. We show that the blind phase search, which is usually used in practice to recover the carrier phase, partially mitigates the EEPN. We detail a numerical approach to predict the system performance including the phase recovery algorithm, and show that taking into account EEPN characteristics relaxes the constraint on the system laser phase noise given by previous pessimistic analytical models. We present experimental validations of our claims, and address future advanced transoceanic systems using 98 GBd probabilistically shaped QAM formats.

400G Probabilistic Shaped PDM-64QAM Synchronization in the Frequency Domain

A low complexity digital signal processing architecture for dual-polarization coherent detection is presented. It synchronizes impairments in the frequency domain and uses synergies between the filtering processing stages to minimize overall complexity. We show how chromatic dispersion, skew, carrier frequency offset, timing error, and the state of polarization can be mitigated with a single filtering stage in the frequency domain. The error estimation in the frequency domain is based on the modified Godard algorithm, which calculates the clock tone with less than two samples per symbol and small timing jitter. The synchronization in the frequency domain is independent of the modulation format and therefore a good candidate for higher order modulation formats and so the synchronization of probabilistic shaped (PS) signals based on a high cardinality rectangular QAM format. We show the synchronization of PS polarization-division multiplexed (PDM)-64QAM signals with a symbol rate of 42.7 GBd in the frequency domain. The simulative results for the influence of the FFT size and the number of FFT blocks used for the error estimation are shown. Finally, we show experimental results for four different probabilistic shapings in back-to-back and transmission over 300 km, which are synchronized with the introduced digital signal processing (DSP).

A Reinforcement Learning-Based QAM/PSK Symbol Synchronizer

Machine Learning (ML) based on supervised and unsupervised learning models has been recently applied in the telecommunication field. However, such techniques rely on application-specific large datasets and the performance deteriorates if the statistics of the inference data changes over time. Reinforcement Learning (RL) is a solution to these issues because it is able to adapt its behavior to the changing statistics of the input data. In this work, we propose the design of an RL Agent able to learn the behavior of a Timing Recovery Loop (TRL) through the Q-Learning algorithm. The Agent is compatible with popular PSK and QAM formats. We validated the RL synchronizer by comparing it to the Mueller and Muller TRL in terms of Modulation Error Ratio (MER) in a noisy channel scenario. The results show a good trade-off in terms of MER performance. The RL based synchronizer loses less than 1 dB of MER with respect to the conventional one but it is able to adapt its behavior to different modulation formats without the need of any tuning for the system parameters.

On the Probabilistic Shaping and Geometric Shaping in Optical Communication Systems

We introduce and compare typical shaping schemes suitable for optical communications. The geometrically shaped-quadrature amplitude modulation (GS-QAM) formats are characterized by the non-equidistant spacing of constellation points, transmitted uniformly, and applied to improve system capacity. On the other hand, the well-known constant composition distribution matcher (CCDM) is applied for the generation of probabilistically shaped QAM (PS-QAM) formats. Mutual information (MI) is used as a metric to analyze the performances of regular/GS/PS-MQAM formats. In a linear amplified spontaneous emission noise limited region, it can be proved by the numerical simulation that MI performances of the GS-8/16QAM are always better than regular 8/16QAM and PS-8/16QAM; the largest shaping gains can be separately reached by PS-32QAM and GS-32QAM. We continue with the experimental demonstration on the 16QAM-based transmission system, and find that GS-16QAM generally has the best MI performance. We also find that the modulation-dependent nonlinear noises of the GS-8/16/32QAM are comparable to that of the regular 8/16/32QAM and generally lower than PS-8/16/32QAM. By using the enhanced Gaussian noise model, we observe that the GS-8/16QAM formats have better performances than regular 8/16QAM and PS-8/16QAM over multi-span transmission. Meanwhile, PS-32QAM formats provide superior performance over a relatively long transmission distance.

Optimal Pilot-Tone-Aided Multi-Core Fiber Transmission Using a Wideband Comb Transmitter

We systematically measure the noise properties of wide-band optical comb lightwave out of a dispersion-engineered nonlinear fiber mixer, which has restricted the use of such comb technology in the pilot-tone-aided coherent transmission over multi-core fibers. We compare the performance of three transmission technologies using different carrier sources, a 11.7-km multi-core fiber link and 10-GBaud 64 QAM signal. Superiority of shared carrier reception methods against self-homodyne detection is experimentally observed when the noisy optical comb is used as the carrier source. Modified shared carrier reception potentially provides further advantage when we employ higher order QAM formats.

64 × 0.5 Gbaud Nonlinear Frequency Division Multiplexed Transmissions With High Order Modulation Formats

Nonlinear frequency division multiplexing (NFDM) with the modulation of the nonlinear Fourier spectrum (both discrete and continuous parts) has been recently considered as a promising transmission method to combat fiber nonlinearity impairments. In this paper, we experimentally demonstrate the generation, detection, and transmission performance of NFDM systems where the transmitted information is encoded directly onto the continuous part of the nonlinear spectrum using 64 overlapped 0.5 Gbaud channels with high order QAM formats (e.g., 16 QAM, 32 QAM, and 64 QAM). In comparison with the conventional orthogonal frequency division multiplexing transmission, which is the linear counterpart of NFDM transmission, clear performance advantage of NFDM systems, up to 1.3 dB, has been observed. This result indicates for the first time the performance benefit of NFDM transmission over its linear counterpart, showing the high potential of this emerging technology.

RF-pilot aided modulation format identification for hitless coherent transceiver.

We propose a RF-pilot aided modulation format identification (MFI) technique to enable a hitless flexible coherent transceiver with fast format switching. For the MFI, modulation format information is encoded to the amplitude of the RF-pilot, which can be simultaneously used for the compensation of both laser phase noise and fiber nonlinearity. The proposed MFI technique is able to identify arbitrary modulation formats including multi-dimensional formats and hybrid QAM formats. The high accuracy of the proposed MFI scheme is experimentally demonstrated without sacrificing the tolerance of both laser phase noise and fiber nonlinearity for various modulation formats up to dual-polarization (DP) 64QAM. Finally, over 2240 km standard single mode fiber (SSMF) link, we experimentally demonstrate a hitless coherent transceiver with a fast block-by-block modulation format switching enabled by the proposed MFI.