Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

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 study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems

Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.

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Multiband Carrierless Amplitude Phase Modulation for High Capacity Optical Data Links

We design staircase codes with overheads between 6.25% and 33.3% for high-speed optical transport networks. Using a reduced-complexity simulation of staircase coded transmission over the BSC, we select code candidates from within a limited parameter space. Software simulations of coded BSC transmission are performed with algebraic component code decoders. The net coding gain of the best code designs are competitive with the best known hard-decision decodable codes over the entire range of overheads. At 20% overhead, staircase codes are within 0.92 dB of BSC capacity at a bit error-rate of $10^{-15}$ . Decoding complexity and latency of the new staircase codes are also significantly reduced from existing hard-decision decodable schemes.

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Staircase Codes With 6% to 33% Overhead

Due to the rapid increase in network traffic in the last few years, many telecommunication operators have started transitions to 100-Gb/s optical networks and beyond. However, high-speed optical networks need more efficient forward error correction (FEC) codes to deal with optical impairments, such as uncompensated chromatic dispersion, polarization mode dispersion, and nonlinear effects, and keep the bit error rate (BER) at long distances sufficiently low. To address these issues, new FEC codes, called third-generation codes, have been proposed. A majority of these codes are based on soft-decision decoders and can provide higher coding gain as compared with their predecessors. This paper presents a thorough survey of third-generation FEC codes, suitable for 100 G and beyond optical networks. Furthermore, this paper discusses the main advantages and drawbacks of each scheme and provides a qualitative categorization and comparison of the proposed schemes based on their main features, such as net coding gain and BER. Information about the complexity of each scheme is given as well.

A Survey on FEC Codes for 100 G and Beyond Optical Networks

Current short-range optical interconnects capacity is moving from 100 to 400 Gb/s and beyond. Direct modulation of several laser sources is used to minimize bandwidth limitations of current optical and electrical components. This total capacity is provided either by wavelength division multiplexing or parallel optics; it is important to investigate on the ultimate transmission capabilities of each laser source to facilitate current capacity standards and allow for future demands. High-speed four-level pulse amplitude modulation at 25 GBd of a 1.5 μ m vertical-cavity surface-emitting laser (VCSEL) is presented in this paper. The 20 GHz 3 dB-bandwidth laser is, at the time of submission, the largest bandwidth of a 1.5 μ m VCSEL ever reported. Forward error correction (FEC) is implemented to achieve transmission over 100 m virtually error free after FEC decoding. Line rate of 100 Gb/s is achieved by emulation polarization multiplexing using 50 Gb/s signal obtained from a single VCSEL.

High-Speed 1550 nm VCSEL Data Transmission Link Employing 25 GBd 4-PAM Modulation and Hard Decision Forward Error Correction

100 Gb/s optical fiber transmission link with a single 1.5 um VCSEL has been experimentally demonstrated using 4-level pulse amplitude modulation

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100 Gb/s single VCSEL data transmission link

We propose a product code with shortened BCH component codes for 100G optical communication systems. Simulation result shows that 10 dB net coding gain is promising at post-FEC BER of 1E-15.

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Over 10 dB net coding gain based on 20% overhead hard decision forward error correction in 100G optical communication systems

This paper presents a revitalization effort towards exploiting multilevel polybinary signals for spectral efficient data links. Specifically, we present five level polybinary signaling for 10 Gbps signals. By proper coding to avoid error propagation and degeneracy of the bit error rate performance, a 10Gbps polybinary signal is successfully generated employing a 1.8 GHz Bessel filter with an electrical spectral efficiency of 5.5 bit/s/Hz. The experimental results show bit error rate performances below FEC level for transmission in singlemode and dispersion shifted fibers up to 20 km length.

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Five-level polybinary signaling for 10 Gbps data transmission systems.

This paper studies the application of beyond bound decoding method for high speed optical communications. This hard-decision decoding method outperforms traditional minimum distance decoding method, with a total net coding gain of 10.36 dB.

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Bomin Li

Papers

High-Speed 1550 nm VCSEL Data Transmission Link Employing 25 GBd 4-PAM Modulation and Hard Decision Forward Error Correction

100 Gb/s single VCSEL data transmission link

Over 10 dB net coding gain based on 20% overhead hard decision forward error correction in 100G optical communication systems

Five-level polybinary signaling for 10 Gbps data transmission systems.

Application of Beyond Bound Decoding for High Speed Optical Communications