Fair comparison of complexity between a multi-band CAP and DMT for data center interconnects

Toeplitz concatenated matrix aided ICA algorithm for super-Nyquist multiband CAP VLC systems

In this paper, we demonstrate a Toeplitz concatenated matrix aided independent component analysis (TCM-ICA) equalizer that can skillfully compensate for the inter-channel interference (ICI) of super-Nyquist multiband carrierless amplitude and phase modulation (m-CAP) system. In the point-to-point super-Nyquist m-CAP visible light communication (VLC) scenario, we experimentally demonstrated a system-level average spectral efficiency (SE) enhancement of 0.50 b/s/Hz in comparison to the conventional scheme. In the multiple-input single-output (MISO) super-Nyquist 5-CAP scenario, the subcarrier-level Q factor enhancements of 4.4 dB, 5.2 dB, and 6.5 dB are achieved in comparison with the least-mean-square (LMS) post-equalizer for the 2^nd, 3^rd and 4^th subcarrier, respectively. As far as we know, the TCM-ICA equalizer is the most effective ICI compensation scheme to enhance SE in the super-Nyquist m-CAP systems.

Experimental multi-user VLC system using non-orthogonal multi-band CAP modulation

This paper provides experimental results for a multi-user visible light communications system using multi-band carrier-less amplitude and phase (m-CAP) modulation scheme. We optimize the system performance by adapting pulse shaping filter parameters, subcarrier spacing and allocating different baud rates to individual sub-bands called allocated m-CAP (Am-CAP). We show that a maximal system data rate of ∼468 Mb/s for four users can be supported while gaining higher flexibility for optimization and the same or lower computational complexity compared with the conventional m-CAP scheme.

Carrierless amplitude and phase modulation in wireless visible light communication systems

Visible light communications (VLCs) have attracted considerable interest in recent years owing to the potential to simultaneously achieve data transmission and illumination using low-cost light-emitting diodes (LEDs). However, the high-speed capability of such links is typically limited by the low bandwidth of LEDs. As a result, spectrally efficient advanced modulation formats have been considered for use in VLC links in order to mitigate this issue and enable higher data rates. Carrierless amplitude and phase (CAP) modulation is one such spectrally efficient scheme that has attracted significant interest in recent years owing to its good potential and practical implementation. In this paper, we introduce the basic features of CAP modulation and review its use in the context of indoor VLC systems. We describe some of its attributes and inherent limitations, present related advances aiming to improve its performance and potential and report on recent experimental demonstrations of LED-based VLC links employing CAP modulation. This article is part of the theme issue 'Optical wireless communication'.

Computational complexity comparison of single-carrier DMT and conventional DMT in data center interconnect

High peak-to-average power ratio (PAPR) is an inherent defect in intensity modulation and direct detection (IM/DD) discrete-multitone (DMT) system, which will cause serious signal nonlinear distortion over fiber transmission. Single carrier-DMT (SC-DMT), which also refers to the discrete-Fourier-transform spread DMT (DFT-spread DMT), is a promising technology for DMT signal PAPR reduction, but higher computational complexity is required due to the additional DFT/IDFT operations in transceiver. In this paper, we experimentally compare the performance of SC-DMT and conventional DMT (CDMT) signal when the computational complexity of SC-DMT transceiver is lower than CDMT by reducing the FFT size in SC-DMT. The results show that the receiver sensitivity of 20 GHz 1024-point FFT based SC-DMT improves by 0.7 dB than 8192-point FFT based CDMT for both 120 Gb/s 64QAM-DMT and 140 Gb/s 128QAM-DMT signal transmission over 2-km single mode fiber (SMF) at the BER of 3.8 × 10^-3 and 2.0 × 10^-2, respectively. It is the first time to find that the SC-DMT with lower transceiver computational complexity outperforms CDMT. In addition, fast-Hartley-transform (FHT) technique is employed to replace FFT for further transceiver computational complexity reduction. The results give out that FHT-based SC-DMT shows the same BER performance with FFT-based SC-DMT, while the computational complexity of the transceiver can be reduced by half.

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.

DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks

To address the continuous growth in high-speed ubiquitous access required by residential users and enterprises, Telecommunication operators must upgrade their networks to higher data rates. For optical fiber access networks that directly connect end users to metro/regional network, capacity upgrade must be done in a cost- and energy-efficient manner. 40 Gb/s is the possible lane rate for the next generation passive optical networks (NG-PONs). Ideally, existing 10 G PON components could be reused to support 40 Gb/s lane-rate NG-PON transceiver, which requires efficient modulation format and digital signal processing (DSP) to alleviate the bandwidth limitation and fiber dispersion. The major contribution of this work is to offer insight performance comparisons of 40 Gb/s lane rate electrical three level Duobinary, optical Duobinary, and four-level pulse amplitude modulation (PAM-4) for incorporating low complex DSPs, including linear and nonlinear Volterra equalization, as well as maximum likelihood sequence estimation. Detailed analysis and comparison of the complexity of various DSP algorithms are performed. Transceiver bandwidth optimization is also undertaken. The results show that the choices of proper modulation format and DSP configuration depend on the transmission distances of interest.