Yuliang Gao

Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s short reach optical transmission systems.

Advanced modulation formats combined with digital signal processing and direct detection is a promising way to realize high capacity, low cost and power efficient short reach optical transmission system. In this paper, we present a detailed investigation on the performance of three advanced modulation formats for 100 Gb/s short reach transmission system. They are PAM-4, CAP-16 and DMT. The detailed digital signal processing required for each modulation format is presented. Comprehensive simulations are carried out to evaluate the performance of each modulation format in terms of received optical power, transmitter bandwidth, relative intensity noise and thermal noise. The performance of each modulation format is also experimentally studied. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10km of SSMF employing single band CAP-16 with EML. Finally, a comparison of computational complexity of DSP for the three formats is presented.

Experimental demonstration of 10 Gb/s multi-level carrier-less amplitude and phase modulation for short range optical communication systems.

Carrier-less amplitude and phase (CAP) modulation can be a good candidate for short range optical communications for considerable computational complexity reduction and simple system structure. In this paper, a detailed investigation on the digital filters in CAP modulation system is presented. An adaptive equalizer based on cascaded multi-modulus algorithm (CMMA) is used for the demodulation at the receiver. The impact of digital filter taps on system performance is investigated through comprehensive simulations and a 10 Gb/s CAP16 modulation system is demonstrated experimentally. The BER performance for different length of fiber link is measured. Compared with back-to-back (BTB) transmissions, 2 dB and 3.5 dB receiver power penalty are observed at BER of 10(-3) for 20 km and 40 km fiber link respectively. It clearly demonstrates the feasibility of the CAP16 modulation for the short range transmission systems.

Low-complexity and phase noise tolerant carrier phase estimation for dual-polarization 16-QAM systems

A low-complexity feed-forward carrier phase estimation (CPE) technique is presented for dual-polarization (DP)-16-QAM transmission systems. By combining QPSK partitioning, maximum likelihood (ML) detection and phase offset estimation between signals in different polarizations, simulation and experimental results for a 200 Gb/s DP-16-QAM system demonstrate similar linewidth tolerance to the best feed-forward CPE reported to date while the computational complexity is at least three times lower compared with other simplified feed-forward CPE techniques.

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

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.

40 Gb/s CAP32 System With DD-LMS Equalizer for Short Reach Optical Transmissions

Carrierless amplitude and phase (CAP) modulation is a promising candidate for short reach optical communications. In this letter, we propose the use of decision-directed least mean square (DD-LMS) adaptive equalization algorithm for high-order CAP transmission systems. The optimal length of taps and step size of DD-LMS algorithm is investigated for 30 Gb/s and 40 Gb/s CAP32 systems. A hybrid equalization technique that consists of DD-LMS and its aided two-stage pre-convergence is proposed that allows experimental transmission of 40 Gb/s CAP32 40 km standard single mode fiber (SSMF).

140-Gb/s 20-km Transmission of PAM-4 Signal at 1.3

We experimentally demonstrated 140-Gb/s transmission over 20-km standard single-mode fiber employing pulse-amplitude modulation (PAM)-4 and direct detection (DD) at 1.3 μm. DD faster than Nyquist is employed to compensate for channel impairments. The optimal length of taps of decision directed least mean square and the optimal tap coefficient for digital postfilter are investigated. A receiver sensitivity of -5.5 dBm at bit error rate of 3.8× 10-3 is realized for 140-Gb/s PAM-4 signal after 20-km transmission. To the best of our knowledge, this is the highest reported baud rate (70 GBd) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and DD for short reach communications.

\mu \text{m}

In this article, we describe advances in digital signal processing (DSP) techniques that enable Tb/s transmission, and software-defined flexible transponders that support adaptive modulation formats and elastic optical networks (EONs).

for Short Reach Communications

A feedforward carrier phase estimation algorithm using a modified QPSK partition approach followed by maximum-likelihood (ML) detection is proposed. Results demonstrate complexity reduction by more than a factor of 2 compared with other techniques.

Advanced DSP Techniques Enabling High Spectral Efficiency and Flexible Transmissions: Toward elastic optical networks

A modulation-format-independent or universal carrier phase estimation (U-CPE) technique is proposed for square 4-, 16-, and 64-ary quadrature amplitude modulation transmission systems for future elastic optical networks. The proposed U-CPE is based on a feedback architecture and eliminates the need to know the signal modulation format (MF) by using a cost function in the phase estimation update that is common to all square-shaped constellations. Compared with standard decision directed phase lock loop (S-DD-PLL) that requires the knowledge of the MF, the U-CPE method maintains close OSNR performance and linewidth tolerance. When MF is agnostic, the proposed U-CPE outperforms a modulation format oblivious (MFO)-DD-PLL in which QPSK decision rules are always used irrespective of actual MF of the signal.

Low-complexity two-stage carrier phase estimation for 16-QAM systems using QPSK partitioning and maximum likelihood detection

Carrierless amplitude and phase (CAP) modulation is generating increasing interest for short reach optical communications. In this letter, we present the use of a decision-directed least mean square (DD-LMS) adaptive equalization algorithm for CAP128/256 transmission systems. A novel preconvergence scheme using aided threshold decision is proposed, called decision-assisted modified cascaded multimodulus algorithm. With the help of a DD-LMS-based equalizer, we demonstrate for the first time transmission of 10-Gb/s CAP128/256 signal over 40-km standard single mode fiber using low-cost directly modulated lasers.