Promising Modulation Format Research Articles

OFDM-Based VLC Systems: A Systematic Review

Visible Light Communications (VLC) represents a new technology of wireless communications allowing high data rate, high-speed internet access, green and friendly communication system, especially for indoor users, where the use of Light Emitting Diodes (LEDs) is growing as a viable alternative to traditional illumination. Orthogonal frequency division multiplexing (OFDM) is a promising modulation format for optical wireless communication (OWC); however, precise channel estimation is required for synchronization and equalization. Moreover, one of the challenging issues for OFDM is its high Peak-to-Average Power Ratio (PAPR) due to the superposition of all subcarriers; therefore, the system requires a high linearity and dynamic range. This paper provides a systematic overview of OFDM-based VLC systems, highlighting the essential aspects of PAPR reduction and channel estimation techniques. Several technical challenges have been addressed to realize the full potential of OFDM-based VLC technology. Moreover, we provide new insights into the over-explored and under-explored areas, which lead us to identify open research problems of VLC based on OFDM. Concisely, this paper serves as a guide and a starting point for researchers willing to research VLC using OFDM.

Recent Advances in Equalization Technologies for Short-Reach Optical Links Based on PAM4 Modulation: A Review

In recent years, short-reach optical links have attracted much more attention and have come to constitute a key market segment due to the rapid development of data-center applications and cloud services. Four-level pulse amplitude modulation (PAM4) is a promising modulation format to provide both a high data rate and relatively low cost for short-reach optical links. However, the direct detector and low-cost components also pose immense challenges, which are unforeseen in coherent transmission. To compensate for the impairments and to truly meet data rate requirements in a cost-effective manner, various digital signal processing (DSP) technologies have been proposed and investigated for short-reach PAM4 optical links. In this paper, an overview of the latest progress on DSP equalization technologies is provided for short-reach optical links based on PAM4 modulation. We not only introduce the configuration and challenges of the transmission system, but also cover the principles and performance of different equalizers and some improved methods. Moreover, machine learning algorithms are discussed as well to mitigate the nonlinear distortion for next-generation short-reach PAM4 links. Finally, a summary of various equalization technologies is illustrated and our perspective for the future trend is given.

Joint clock recovery and feed-forward equalization for PAM4 transmission.

With the rapid development of cloud services, data-center applications and the Internet of Things, short-reach communications have attracted much more attention in recent years. 4-level pulse amplitude modulation (PAM4) is a promising modulation format to provide both high data rate and relatively low cost for short-reach optical interconnects. In this paper, a joint clock recovery and feed-forward equalization algorithm (CR-FFE) is proposed to simultaneously eliminate the inter-symbol interference (ISI) and track large sampling clock offset (SCO) in PAM4 transmission. The algorithm estimates timing error according to the difference between two tap coefficients of fractionally spaced equalizers, thus solving the problem of incompatible prerequisites between clock recovery and channel equalization. A 10GHz directly modulated laser (DML) based 50-Gbit/s PAM4 transmission experiment is implemented to investigate the performance of the proposed algorithm. Experimental results show that the proposed CR-FFE algorithm can resist SCO up to 1000 ppm after 40 km standard single-mode fiber (SSMF) transmission under the 2x10-2 SD-FEC BER threshold, which is dramatically improved comparing with that of 20 ppm in traditional CR cascaded by FFE algorithm.

Investigation of a novel structure for 6PolSK-QPSK modulation

Benefiting from the high spectrum efficiency and power efficiency, 6PolSK-QPSK (6-ary polarization-shift keying quadrature phase-shift keying) is a promising modulation format in coherent optical communication. We proposed a novel structure to generate the 6PolSK-QPSK with two dual-drive MZMs. Simulation results show that the proposed structure can generate 6PolSK-QPSK effectively and the performance is nearly the same to the traditional one.

Investigation of quadrature imbalance compensation algorithm for coherent 6PolSK-QPSK

6PolSK-QPSK is a promising modulation format in optical fiber communication. Because of the damage suffered during the transmission and reception, a series of algorithms are needed to be adopted to recover the original data. We proposed a novel quadrature imbalance compensation algorithm based on the data statistical properties. Simulation results show that the quadrature imbalance can be well compensated with the proposed algorithm.

Adaptive scheme for maintaining the performance of the in-home white-LED visible light wireless communications using OFDM

Spectral-efficient orthogonal frequency division multiplexing (OFDM) is a promising modulation format for the light-emitting-diode (LED) optical wireless (OW) visible light communication (VLC). VLC is a directional and line-of-sight communication; hence the offset of the optical receiver (Rx) and the LED light source will result in a large drop of received optical power. In order to keep the same luminance of the LED light source, we propose and demonstrate an adaptive control of the OFDM modulation-order to maintain the VLC transmission performance. Experimental results confirm the feasibility of the proposed scheme.

Analysis of RF-Pilot-Based Phase Noise Compensation for Coherent Optical OFDM Systems

In coherent optical long-haul transmission systems, orthogonal frequency-division multiplexing represents a promising modulation format. However, due to long symbol length, laser phase noise can be a major impairment. In this manuscript, the RF-pilot-based phase noise compensation scheme is analyzed and compared to conventional common-phase error compensation. It has been shown that the RF-pilot-based phase noise compensation scheme allows for a considerable increase in tolerable laser linewidth as compared to conventional common-phase error compensation at the cost of an increase in system complexity. For a 112-Gb/s transmission scheme, the tolerable linewidth is increased by a factor of ten as compared to common-phase error compensation.

Bit-Error Rate Performance of Coherent Optical M-ary PSK/QAM using Decision-Aided Maximum Likelihood Phase Estimation

The bit-error rate (BER) expressions of 16- phase-shift keying (PSK) and 16- quadrature amplitude modulation (QAM) are analytically obtained in the presence of a phase error. By averaging over the statistics of the phase error, the performance penalty can be analytically examined as a function of the phase error variance. The phase error variances leading to a 1-dB signal-to-noise ratio per bit penalty at BER=10(-4) have been found to be 8.7 x 10(-2) rad(2), 1.2 x 10(-2) rad(2), 2.4 x 10(-3) rad(2), 6.0 x 10(-4) rad(2) and 2.3 x 10(-3) rad(2) for binary, quadrature, 8-, and 16-PSK and 16QAM, respectively. With the knowledge of the allowable phase error variance, the corresponding laser linewidth tolerance can be predicted. We extend the phase error variance analysis of decision-aided maximum likelihood carrier phase estimation in M-ary PSK to 16QAM, and successfully predict the laser linewidth tolerance in different modulation formats, which agrees well with the Monte Carlo simulations. Finally, approximate BER expressions for different modulation formats are introduced to allow a quick estimation of the BER performance as a function of the phase error variance. Further, the BER approximations give a lower bound on the laser linewidth requirements in M-ary PSK and 16QAM. It is shown that as far as laser linewidth tolerance is concerned, 16QAM outperforms 16PSK which has the same spectral efficiency (SE), and has nearly the same performance as 8PSK which has lower SE. Thus, 16-QAM is a promising modulation format for high SE coherent optical communications.

40 Gb/s DBPSK and DQPSK formats for transparent 50 GHz DWDM transmission

Today's dense wavelength division multiplexed (DWDM) long haul and metropolitan networks are being upgraded from 10 Gb/s to 40 Gb/s, to support the growing traffic demands. Reconfigurable optical add/drop multiplexers (ROADMs) allow for transparent networking at 50 GHz channel spacing. This paper addresses the challenges to support 40 Gb/s by examining two promising modulation formats, namely, differential binary and quadrature phase shift keying (DBPSK and DQPSK). We investigated several variants for reception of the two formats and present performances under optical filtering, crosstalk, and polarization mode dispersion. We also discuss ROADM node architectures that have bearing on overall system performance.

Improving the performance of a crosspoint-switch-based optical buffer by using a differential phase-shift keying payload

We experimentally demonstrate a novel decimal optical buffer scheme based on a multiloop configuration and a single switch element—an optical crosspoint switch (OXS) matrix. Its variable-delay range is of 1 to 999 times the basic delay unit. The buffer dynamic reconfiguration can be achieved at nanosecond switching speed. Our results show that by using a differential phase-shift keying (DPSK) payload in the buffer it can outperform an on-off keying payload with 4-dB sensitivity improvement owing to its alleviation of patterning-induced degradation, clearly validating DPSK as a promising modulation format to overcome nonlinear impairments and to extend number of hops in all-optical packet-switching network.