In this work we experimentally investigate the improved intra-channel fiber nonlinearity tolerance of digital subcarrier multiplexed (SCM) signals in a single-channel coherent optical transmission system. The digital signal processing (DSP) for the generation and reception of the SCM signals is described. We show experimentally that the SCM signal with a nearly-optimum number of subcarriers can extend the maximum reach by 23% in a 24 GBaud DP-QPSK transmission with a BER threshold of 3.8 × 10−3 and by 8% in a 24 GBaud DP-16-QAM transmission with a BER threshold of 2 × 10−2. Moreover, we show by simulations that the improved performance of SCM signals is observed over a wide range of baud rates, further indicating the merits of SCM signals in baud-rate flexible agile transmissions and future high-speed optical transport systems.

Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems.

Digital signal processing (DSP) combined with coherent detection has played a central role in the recent capacity expansion of optical networks. Optical superchannels aim to increase per-channel interface rates as well as per-fiber capacities of wavelength-division multiplexed (WDM) systems in a cost-effective manner. Superchannels circumvent the electronic bottleneck via optical parallelism and provide high-per-channel data rates and better spectral utilization, especially in transparent optical mesh networks. This article reviews recent advances in the generation, detection, and transmission of optical superchannels with channel data rates on the order of terabits per second (Tb/s). Enabling DSP techniques such as Nyquist-WDM, orthogonal frequency-division multiplexing (OFDM), software-defined modulation and detection, advanced coding, and joint DSP among the superchannel constituents are presented. Future prospects of DSP techniques for high-capacity superchannel transmission are also discussed.

Digital Signal Processing Techniques Enabling Multi-Tb\/s Superchannel Transmission: An overview of recent advances in DSP-enabled superchannels

We report the transmission of time domain hybrid QAM (TDHQ) signals for agile optical networks. A continuous tradeoff between spectral efficiency and achievable distance by mixing modulation formats including QPSK, 8QAM, and 16QAM is demonstrated in two scenarios: 1) 28 Gbaud non-return-to-zero (NRZ) signal for fixed 50 GHz grid systems; 2) superchannel transmission at date rates of up to 1.15 Tb/s and spectral efficiencies of up to 7.68 b/s/Hz. The TDHQ signal is generated using high-speed digital-to-analog converters (DACs) at the transmitter, and low-complexity digital signal processing (DSP) is proposed for processing the TDHQ signals at the receiver. Moreover, the nonlinearity tolerance of hybrid QAM signals with different configurations is investigated.

Spectral Efficiency-Adaptive Optical Transmission Using Time Domain Hybrid QAM for Agile Optical Networks

Optical orthogonal division multiplexing for long haul optical communications: A review of the first five years

In this paper we experimentally demonstrate transmission performance of optical DFT-spread OFDM systems in comparison with conventional OFDM systems. A 440.8-Gb/s superchannel consisting of 8 x 55.1-Gb/s densely-spaced DFT-S OFDM signal is successfully received after 1120-km transmission with a spectral efficiency of 3.5 b/s/Hz. It is shown that DFT-S OFDM can achieve an improvement of 1 dB in Q factor and 1 dB in launch power over conventional OFDM. Additionally, unique word aided phase estimation algorithm is proposed and demonstrated enabling extremely long OFDM symbol transmission.

Experimental demonstration of improved fiber nonlinearity tolerance for unique-word DFT-spread OFDM systems

We report and investigate the feasibility of zero-overhead laser phase noise compensation (PNC) for long-haul coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission systems, using the decision-directed phase equalizer (DDPE). DDPE updates the equalization parameters on a symbol-by-symbol basis after an initial decision making stage and retrieves an estimation of the phase noise value by extracting and averaging the phase drift of all OFDM sub-channels. Subsequently, a second equalization is performed by using the estimated phase noise value which is followed by a final decision making stage. We numerically compare the performance of DDPE and the CO-OFDM conventional equalizer (CE) for different laser linewidth values after transmission over 2000 km of uncompensated single-mode fiber (SMF) at 40 Gb/s and investigate the effect of fiber nonlinearity and amplified spontaneous emission (ASE) noise on the received signal quality. Furthermore, we analytically analyze the complexity of DDPE versus CE in terms of the number of required complex multiplications per bit.

Zero-overhead phase noise compensation via decision-directed phase equalizer for coherent optical OFDM.

In this paper, we present a carrier phase recovery (CPR) algorithm using a modified superscalar parallelization based phase locked loop (M-SSP-PLL) combined with a maximum-likelihood (ML) phase estimation. Compared to the original SSP-PLL, M-SSP-PLL + ML reduces the required buffer size using a novel superscalar structure. In addition, by removing the differential coding/decoding and employing ML phase recovery it also improves the performance. In simulation, we show that the laser linewidth tolerance of M-SSP-PLL + ML is comparable to blind phase search (BPS) algorithm, which is known to be one of the best CPR algorithms in terms of performance for arbitrary QAM formats. In 28 Gbaud QPSK (112 Gb/s) and 16-QAM (224 Gb/s), and 7 Gbaud 64-QAM (84 Gb/s) experiments, it is also demonstrated that M-SSP-PLL + ML can increase the transmission distance by at least 12% compared to BPS for each of them. Finally, the computational complexity is discussed and a significant reduction is shown for our algorithm with respect to BPS.

Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL.

We introduce and investigate the feasibility of a noniterative phase noise induced intercarrier interference (ICI) compensation scheme based on linear interpolation for coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) transport systems. The study of bit-error-rate (BER) performance for a 40-Gb/s 16-quadrature amplitude modulation (QAM) CO-OFDM signal demonstrates a significant improvement versus the conventional equalizer (CE) in the optical signal-to-noise ratio (OSNR) requirement. Moreover, the capability of this equalizer in conjunction with the back-propagation (BP) nonlinearity compensation scheme is investigated and a brief analysis of the computational complexity, in terms of the number of required complex multiplications, is provided. This practical approach does not suffer from error propagation while enjoying low computational complexity.

Noniterative Interpolation-Based Partial Phase Noise ICI Mitigation for CO-OFDM Transport Systems

We experimentally demonstrate the impact of equalization-enhanced phase noise (EEPN) on the performance of 56 Gbaud dual-polarization (DP) QPSK long haul transmission systems. Although EEPN adds additional noise to the received symbols, we show that this reduces the phase variance introduced by the LO laser, and therefore should be considered when designing the carrier phase recovery (CPR) algorithms and estimating system performance. Further, we experimentally demonstrate the performance degradation caused by EEPN when a LO laser with a large linewidth is used at the receiver. When using a 2.6 MHz linewidth distributed feedback (DFB) laser instead of a ~100 kHz linewidth external-cavity laser (ECL) as a LO, the transmission distance is reduced from 4160 km to 2640 km due to EEPN. We also confirm the reduction of the phase variance of the received symbols for longer transmission distances showing its impact on the CPR algorithm optimization when a DFB laser is used at the receiver. Finally, the relationship between the EEPN-induced penalty versus the signal baud rate and the LO laser linewidth is experimentally evaluated, and numerically validated by simulations.

Experimental investigation of the equalization-enhanced phase noise in long haul 56 Gbaud DP-QPSK systems.

We report an adaptive weighted channel equalizer (AWCE) for orthogonal frequency division multiplexing (OFDM) and study its performance for long-haul coherent optical OFDM (CO-OFDM) transmission systems. This equalizer updates the equalization parameters on a symbol-by-symbol basis thus can track slight drifts of the optical channel. This is suitable to combat polarization mode dispersion (PMD) degradation while increasing the periodicity of pilot symbols which can be translated into a significant overhead reduction. Furthermore, AWCE can increase the precision of RF-pilot enabled phase noise estimation in the presence of noise, using data-aided phase noise estimation. Simulation results corroborate the capability of AWCE in both overhead reduction and improving the quality of the phase noise compensation (PNC).

Mohammad E. Mousa-Pasandi

Papers

Zero-overhead phase noise compensation via decision-directed phase equalizer for coherent optical OFDM.

Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL.

Noniterative Interpolation-Based Partial Phase Noise ICI Mitigation for CO-OFDM Transport Systems

Experimental investigation of the equalization-enhanced phase noise in long haul 56 Gbaud DP-QPSK systems.

Data-aided adaptive weighted channel equalizer for coherent optical OFDM