Proposed Modulation Format Research Articles

Nonlinearity-Tolerant Probabilistically-Shaped Four-Dimensional Modulation at Spectral Efficiency of 9 Bits/Four-Dimensional Symbol

Network traffic between data centers has grown rapidly in recent years. Therefore, an optical modulation format with high spectral efficiency is desired in inter-data center links at data rates of more than 400 Gb/s in each wavelength. In order to further improve the transmission performance in single-span inter-data center links, a nonlinearity-tolerant probabilistically-shaped 4D modulation format with a spectral efficiency of 9 bits/4D symbol is proposed. The proposed modulation format fills the gap between polarization-multiplexed 16- and 32-ary quadrature-amplitude modulation (PM-16QAM and PM-32QAM). In numerical simulations, we compare our proposed modulation format with probability-amplitude-shaped 64QAM (PAS-64QAM) and 512-set-partitioned 32QAM (512SP-32QAM) at a spectral efficiency of 9 bits/4D symbol in a single-span transmission link. The simulation results indicate that the proposed modulation format shows better linear and nonlinear performances than a 512SP-32QAM format. Compared with PAS-64QAM format, the proposed modulation format can provide similar transmission performance with improved fiber nonlinear tolerance.

Photonics generation of microwave linearly chirped signal with amplitude and phase modulation capability

A photonics method is proposed to generate a microwave linearly chirped signal with improved central frequency, doubling bandwidth, amplitude and phase modulation capability. In the scheme, a dual-polarization integrated modulator combined with an optical filter are utilized to generate an orthogonally polarized optical signal. Then, the optical signal is injected into a polarization modulator (PolM) to perform data modulation. By controlling the polarization state of the orthogonally polarized optical signal, amplitude or phase of the generated microwave linearly chirped signal will be changed according to the driving data sequence. The proposed scheme is verified by simulation. Linearly chirped signals with central frequency tuned from 14 to 29 GHz, bandwidth tuned from 4 to 10 GHz are generated. ASK linearly chirped signals with bit-rate tuned from 156.25 to 1250 Mbps are demonstrated. Generation of a PSK linearly chirped signal and its coherent demodulation are verified. Besides, 4ASK, 4PSK and QPSK linearly chirped waveforms are achieved, and time-bandwidth product improvement is confirmed. The proposed method features parameter tunability and modulation format agility, which can be potentially employed in radar and wireless covert communication systems.

Probabilistically Coded Modulation Formats for 5G Mobile Fronthaul Networks

The centralized/cloud radio access network architecture is being considered as a promising solution for fifth-generation cellular networks. In particular, the stringent capacity and latency requirements of the fronthaul segment call for innovations in the design of new architectures and transceivers that support adaptive communication resource allocation between centralized baseband processing units and remote radio heads. In this paper, we propose probabilistically coded three-level pulse amplitude modulation and five-point quadrature amplitude modulation schemes in hybrid mobile fronthaul (MFH) networks that support both coherent detection and direct detection. We propose encoding and decoding algorithms and optimize the decoder configuration through simulations. Based on a distributed pseudorandom encoding technique developed in our earlier work, we adapt the modulation type and code rate per fronthaul connection in real-time. Using two separate software-defined networking enabled downlink and uplink transmission testbeds employing code division multiplexing technology, we demonstrate the benefits of our proposed modulation formats for multicast transmission in hybrid networking scenarios as well as the advantages of adaptive bidirectional transmission in MFH networks.

Polarization-Ring-Switching for Nonlinearity-Tolerant Geometrically Shaped Four-Dimensional Formats Maximizing Generalized Mutual Information

In this paper, a new four-dimensional 64-ary polarization ring switching (4D-64PRS) modulation format with a spectral efficiency of 6 bit/4D-sym is introduced. The format is designed by maximizing the generalized mutual information (GMI) and by imposing a constant-modulus on the 4D structure. The proposed format yields an improved performance with respect to state-of-the-art geometrically shaped modulation formats for bit-interleaved coded modulation systems at the same spectral efficiency. Unlike previously published results, the coordinates of the constellation points and the binary labeling of the constellation are jointly optimized. When compared with polarization-multiplexed 8-ary quadrature-amplitude modulation (PM-8QAM), gains of up to 0.7 dB in signal-to-noise ratio are observed in the additive white Gaussian noise (AWGN) channel. For a long-haul nonlinear optical fiber system of 8,000 km, gains of up to 0.27 bit/4D-sym (5.5% data capacity increase) are observed. These gains translate into a reach increase of approximately 16% (1,100 km). The proposed modulation format is also shown to be more tolerant to nonlinearities than PM-8QAM. Results with LDPC codes are also presented, which confirm the gains predicted by the GMI.

A Novel Four-Dimensional Modulation Format Based on Subset Selection

Coherent optical transmission systems have a four-dimensional (4-D) signal space (two quadratures in two polarizations), which can be used to create 4-D modulation formats that have better sensitivity than traditional two-dimensional modulation signal. In this paper, we propose an interesting four-dimensional modulation format based on subset selection for coherent optical fiber communications. We compare the proposed modulation format to the polarization multiplexed quadrature amplitude modulation(PM-QAM) and the set-partitioning M polarization-multiplexed QAM (M-SP-QAM). Simulation results show that for the same spectral efficiency, the proposed modulation format can get SNR gains of up to1dB compared with PM-MQAMs at 7% forward exchange control (FEC) threshold at a BER of 1e-3 in back-to-back system. After 800 km transmission distance, the signal-to-noise ratio (SNR) gain is 1.05dB. It is also shown that the mapping process of the proposed modulation format has lower latency and occupies less memory units compared with the commonly used look-up-tables mapping method.

Performance Analysis of IMC in FSO Systems Over Gamma–Gamma Channel

Free-space optical communication (FSO) offers high security and high bandwidth, which is why it is widely used in digital communication systems. However, atmospheric turbulence or scintillation induces a voluminous degradation to the signal. So, different modulation techniques are introduced to improve the performance of FSO systems. In this paper, a novel inverted Manchester code (IMC) and a 3 bits-per-symbol modulation format are proposed, referred to as IMC/BPSK. The BERs of the IMC/BPSK amplitude branch and phase branch are studied theoretically and numerically over the gamma–gamma channel model separately, and the capacity is also been studied. The simulation comparison with 8-PPM is also made. The results show that the BER performance of IMC/BPSK is better than that of 8-PPM with the same symbol rate. Therefore, the proposed modulation format is feasible, which can effectively improve the tolerance degree of the atmospheric turbulence effect.

Double sideband suppressed carrier modulation for stable fiber delivery of radio frequency standards

We theoretically and experimentally investigate the properties of a double sideband suppressed carrier modulation transmission scheme for long distance microwave frequency standard dissemination. The proposed method effectively doubles the transmitted frequency obtaining enhanced phase noise and stability performance. Suppressing the optical carrier also eliminates the dispersion induced signal fading. Important transmission parameters like optical modulation depth and linearity are also improved. Allan deviation and phase noise measurements prove the superior performance of the proposed modulation format.

Assessment of absolute added correlative coding in optical intensity modulation and direct detection channels

The performance of absolute added correlative coding (AACC) modulation format with direct detection has been numerically and analytically reported, targeting metro data center interconnects. Hereby, the focus lies on the performance of the bit error rate, noise contributions, spectral efficiency, and chromatic dispersion tolerance. The signal space model of AACC, where the average electrical and optical power expressions are derived for the first time, is also delineated. The proposed modulation format was also compared to other well-known signaling, such as on-off-keying (OOK) and four-level pulse–amplitude modulation, at the same bit rate in a directly modulated vertical-cavity surface-emitting laser-based transmission system. The comparison results show a clear advantage of AACC in achieving longer fiber delivery distance due to the higher dispersion tolerance.

Investigation on four wave mixing effect in various optical Fibers for different spectral efficient orthogonal modulation Formats

The paper analyzes the four wave mixing (FWM) effect in different spectral efficient orthognal modulation formats at equal channel spacing of 100GHz and 50GHz to design long haul wavelength division multiplexing (WDM) optical system. Further, the comparison of reduction of FWM for existing and proposed modulation format have been analyzed by varying the laser input power from −10dBm to 10dBm.

Stealth transmission of temporal phase en/decoded polarization-modulated-code-shift-keying optical-code-division multiple-access signal over synchronous digital hierarchy network with asynchronous detection

An innovative approach for security-enhanced optical stealth transmission in a synchronous digital hierarchy network is proposed and experimentally investigated. The security enhancement is achieved through a signal modulation format, so-called polarization-modulated-code-shift-keying, which is implemented with two superstructured fiber Bragg gratings-based optical-code-division multiple-access encoders and a polarization modulator. The proposed modulation format can provide a constant energy level for both bits 0’s and 1’s, which avoids secure vulnerability of single-stealth-user with on-off-keying modulation before coupling into the host channel and after the cascade of filters. Moreover, a self-made cost-effective gain-switched distributed feedback laser with relatively narrow spectrum is first employed as a stealth optical source, which greatly reduces the system cost and complexity. The stealth signal is recovered and detected asynchronously. The experimental results show high secure performance and robustness against eavesdropping, while keeping a bit error rate below forward error correction limit.

Performance investigation of Raman erbium-doped fiber amplifier hybrid optical amplifier in the scenario of high-speed orthogonal-modulated signals

We propose a new optical data format based on simultaneously modulating nonreturn-to-zero and polarization-shift-keying for high-speed applications by means of efficient utilization of bandwidth. To the best of our knowledge, this article reports on the first investigation of a Raman erbium-doped fiber amplifier hybrid optical amplifier using a hybrid/orthogonal modulation technique in the scenario of 8- and 20-channel dense wavelength division multiplexed systems with high-data rates at 0.2 and 0.8 nm channel spacing, respectively. The results show that the proposed modulation format offers significant advantages in offering high-spectral efficiency values with acceptable quality factors and bit error rates. © 2014 Society of Photo-Optical Instrumentation

A novel modulation format of 50% RZ polarization interleaved multiplexing DQPSK with high CD and polarization crosstalk tolerance in high-speed self-homodyne receiving system

We propose a novel modulation format of 50% RZ polarization interleaved multiplexing DQPSK for high-speed self-homodyne receiving systems. The proposed modulation format enables excellent tolerance of chromatic dispersion and polarization crosstalk. In self-homodyne receiving systems, the proposed format can reduce the cost of adaptive equalization module. It has potential applications in long-reach self-homodyne optical communication systems.

Next-Generation TDM-PON Based on Multilevel Differential Modulation

In this letter, a next-generation time-division multiplexed passive optical network (TDM-PON) based on multilevel differential modulation formats is proposed. Multilevel modulation enables the efficient scaling of bit rates without resorting to very high-speed electronics and photonics, while differential detection simplifies the detection process. The 40 Gb/s downstream signal is implemented with Star 16 quadrature amplitude modulation, while two upstream channels are evaluated, a simple, asymmetrical 10 Gb/s on-off keying signal and a 30-Gb/s differential eight phase-shift keying signal. Experimental results show very good receiver sensitivities, indicating the proposed modulation format's suitability for future very high-speed TDM-PONs.

Iterative Polar Quantization-Based Modulation to Achieve Channel Capacity in Ultrahigh-Speed Optical Communication Systems

In this paper, we propose a nonuniform coded-modulation format based on iterative polar quantization (IPQ) as a scheme to enable achieving channel capacity in ultrahigh-speed optical communication systems. The proposed modulation format is coded with structured low-density parity-check (LDPC) codes optimized for Gaussian channels and, in combination with polarization-multiplexing, can achieve 800 Gb/s per wavelength aggregate rate and beyond utilizing the currently available components operating at 50 GS/s. Using coded IPQ, we show that we can achieve capacity for signal-to-noise ratios (SNRs) of up to 25 dB and increase the total propagation distance over optical transmission systems by 275 km over coded star-quadrature amplitude modulation (QAM).

Novel optical orthogonally modulation scheme for superimposing DPSK signals on dark RZ signals

A configuration of transmitter for superimposing DPSK signals on dark RZ signals was proposed. The transmitter comprises two optical phase modulators and a delayed line Mach–Zehnder interferometer. A new orthogonally modulation format with tunable duty cycle and extinction ratio in the optical domain was generated and discussed. The results show that the proposed modulation format offers significant advantages in offering high spectral efficiency values, high extinction ratio, and requiring electronics with reduced bandwidth.

Progress in optical modulation formats for high-bit rate WDM transmissions

With the progress of optical communication systems, and especially the constraints brought by wavelength division multiplexing (WDM) transmissions and increased bit rates, new ways to convert the binary data signal on the optical carrier have been proposed. It appears clearly now that several of the methods proposed by research laboratories will be applied into commercial products soon due to the large improvements generated. This paper intends to summarize some of the most interesting proposed modulation formats for high bit rate (especially 40 Gb/s) WDM transmissions

Noise statistics of and BER estimation using demodulated signals for direct detection differential polarization-phase-shift keying

The noise statistics of optically demodulated signal of the recently proposed modulation format, namely differential polarization-phase-shift keying, are analyzed. It is found that, in the linear regime, the Gaussian approximation is reasonably accurate in estimating the BER. In the nonlinear regime, the noise of the demodulated signal is dominated by the effect of nonlinear phase noise and obeys chi-squared statistics with a degree of freedom of one. Gaussian approximation underestimates the BER in the nonlinear regime. A two-step BER estimation method accounting for both linear beat noise and nonlinear phase noise is proposed. The effectiveness of this two-step BER estimation method at all power levels has been established by comparison with direct error counting.

Optical Phase and Intensity Modulation Using Dark Pulses

A new optical modulation format is described, based on the simultaneous modulation of the amplitude and the phase of an optical signal. The proposed modulation format offers significant advantages in wavelength-division-multiplexing transmission, offering high spectral efficiency values, high extinction ratio, and requiring electronics with reduced bandwidth.

Comparison of Modulation Formats for 42.7-Gb/s Single-Channel Transmission Through 1980 km of SSMF

We experimentally compare nonreturn-to-zero and return-to-zero on-off keying with a number of recently proposed modulation formats for 1980-km 42.7-Gb/s single-channel transmission over standard single-mode fiber. Substantial performance improvements are obtained with the new formats.

CF-RZ-DPSK for suppression of XPM on dispersion-managed long-haul optical WDM transmission on standard single-mode fiber

We present a new optical modulation format chirp-free return-to-zero differential phase shift keying (CF-RZ-DPSK), which enables wavelength-division-multiplexing (WDM) transmission at 10 Gb/s/ch at a channel spacing of 100 GHz over 3000 km without significant impairments due to cross-phase modulation (XPM). A transmitter setup is presented, which allows a simple implementation of CF-RZ-DPSK with two Mach-Zehnder modulators in push-pull operation. The robustness toward XPM is shown theoretically with the help of a simple analytical model for the XPM-induced phase modulation. The superior performance of CF-RZ-DPSK over other modulation formats [RZ-ampfitude shift keying (ASK), nonreturn-to-zero (NRZ)-DPSK, and NRZ-ASK, respectively] is clarified. Finally, simulation results for CF-RZ-DPSK in comparison to RZ-ASK show the superior performance of the newly proposed modulation format in a dense WDM setup.