Receiver Sensitivity Improvement Research Articles

Adaptive Bayesian neural networks nonlinear equalizer in a 300-Gbit/s PAM8 transmission for IM/DD OAM mode division multiplexing.

The strong stochastic nonlinear impairment induced by random mode coupling appears to be a long-standing performance-limiting problem in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this Letter, we propose a Bayesian neural network (BNN) nonlinear equalizer for an OAM-MDM IM/DD transmission with three modes. Unlike conventional Volterra and convolutional neural network (CNN) equalizers with fixed weight coefficients, the weights and biases of the BNN nonlinear equalizer are regarded as probability distributions, which can accurately match the stochastic nonlinear model of the OAM-MDM. The BNN nonlinear equalizer is capable of adaptively updating its weights and biases sample-by-sample, according to the probability distribution. An experiment was conducted on a 300-Gbit/s PAM8 signal with three modes over a 2.6-km OAM-MDM RCF transmission. The experimental results demonstrate that the proposed BNN nonlinear equalizer exhibits promising solutions to effectively mitigate nonlinear distortions, which outperforms conventional Volterra and CNN equalizers with receiver sensitivity improvements of 1.0 dBm and 2.5 dBm, respectively, under hard-decision forward error correction (HD-FEC) thresholds. Moreover, compared with the Volterra and CNN equalizers, the complexity of the OAM-MDM is significantly improved through the BNN nonlinear equalizer. The proposed BNN nonlinear equalizer is a promising candidate for the high capacity inter-data center interconnects.

Performance enhanced dual-mode index modulation OFDM with OCT precoding in VLC systems

In this paper, dual-mode index modulation orthogonal frequency division multiplexing (DMIM-OFDM) enabled by orthogonal circulant matrix transform (OCT) precoding is proposed and experimentally demonstrated in visible light communication (VLC) systems. Since all available subcarriers are active to convey information bits, DMIM-OFDM can achieve higher spectral efficiency (SE) and energy efficiency (EE) than index modulation OFDM (IM-OFDM). In addition, to mitigate the uneven frequency-selective fading between different subblocks, channel-independent OCT precoding is applied to all subblocks to equalize the overall signal-to-noise ratio (SNR). The experimental results show that the DMIM-OFDM with OCT precoding outperforms the traditional IM-OFDM, DMIM-OFDM without precoding, and with DFT precoding at different SEs. For DMIM-OFDM VLC system with SE of 4.5 bit/s/Hz, by using OCT precoding, the receiver sensitivity improvement of 3 dB can be achieved at the bit error rate (BER) of 3.8e−3 after 3.5-m free-space transmission.

A hybrid encryption scheme of DNA and chaotic precoding for IMDD-FBMC systems

A new scheme that combines deoxyribonucleic acid (DNA) encryption algorithm and chaotic Walsh–Hadamard transform (WHT)/discrete Fourier transform (DFT) precoding to improve security performance is proposed for the first time in the filter bank multicarrier (FBMC) systems with intensity modulation and direct detection (IMDD). In our proposed scheme, we employed DNA and chaotic WHT/DFT precoding to achieve bit-level and symbol-level encryptions, respectively. With this hybrid encryption scheme, it is expected not only to enhance data transmission security but also to improve the bit error rate (BER) performance. We conducted experiments in the 16/32 quadrature amplitude modulation (QAM)-FBMC systems with IMDD. The relevant results show that our proposed scheme can improve receiver sensitivity by around 1 dB at the BER of 3.8e−3 compared with the scheme using only DNA encryption or without encryption. And a huge key space of 101806/101024 by DNA-chaotic WHT/DFT encryption is achieved. In addition, the net bit rate of transmission systems can be enhanced with a similar BER performance, compared to the optical orthogonal frequency division multiplexing (OFDM) systems.

Probabilistically-Shaped DMT for IM-DD Systems with Low-Complexity Fast WHT-Based PDSP

Transmission capacity and receiver sensitivity of an intensity-modulation direct detection (IM-DD) optical discrete multi-tone (DMT) system can be improved by using the probabilistically shaping (PS) technique. However, different probabilistic distributions will be required owing to the unbalanced signal-to-noise ratio (SNR) among data-carrying subcarriers (SCs) induced by the imperfect frequency response of optical/electrical devices, which can increase the implementation complexity of the PS-DMT transceiver. In this work, different signal pre-processing schemes including pre-equalization, Walsh–Hadamard transform (WHT)-based full data-carrying SCs precoding (FDSP) and fast WHT-based partial data-carrying SCs precoding (PDSP) are investigated for SNR equalization in a short-reach PS-DMT transmission system. After transmission over 50 km single-mode fiber, the experimental results indicated that three pre-processed signals have almost the same generalized mutual information (GMI) performance and receiver sensitivity improvements. The proposed fast WHT-based PDSP scheme may be a good option for the implementation of the PS-DMT transmission systems with a large SC SNR fluctuation regarding computational complexity.

Performance Enhancement of W-Band RoF System Using 4D Trellis Coded Modulation OFDM With Precoding

In the paper, a four-dimensional trellis-coded modulation 16-quadrature amplitude modulation-orthogonal frequency division multiplexing (4D TCM 16QAM-OFDM) combined with precoding scheme is proposed and experimentally demonstrated in W-band radio-over-fiber (RoF) system. In W-band RoF system, frequency selective fading can cause unbalanced signal-to-noise ratio (SNR) distribution on the sub-carriers of the 4D TCM 16QAM-OFDM signals. Orthogonal circulant matrix transform (OCT) precoding, discrete Fourier transform (DFT) precoding, and constant amplitude zero autocorrelation sequence (CAZAC) precoding can be used to balance the SNR over the data subcarriers of 4D TCM 16QAM-OFDM signals. Moreover, DFT precoding and CAZAC precoding can reduce the peak-to-average power ratio (PAPR) of 4D TCM 16QAM-OFDM signals. After 50km single-mode fiber (SMF) and 5m free space transmission, the experimental results show that, at the bit error rate (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> , compared with 4D TCM 16QAM-OFDM using OCT precoding and DFT precoding, 4D TCM 16QAM-OFDM using CAZAC precoding can achieve the receiver sensitivity improvement of 2.33 dB and 0.48 dB respectively. In addition, compared with 4D TCM 16QAM-OFDM without precoding, it can achieve the receiver sensitivity improvement of 3 dB by using CAZAC precoding. Furthermore, 4D TCM 16QAM-OFDM has higher spectral efficiency than 8QAM-OFDM at the same symbol rate. The receiver sensitivity for 4D TCM 16QAM-OFDM using CAZAC precoding is enhanced about 1.1 dB, compared with 8QAM-OFDM using CAZAC precoding.

Beaconless angle-of-arrival tracking with improved receiver sensitivity and tracking precision for free-space optical communications

One of the technically challenging issues of free-space optical communication systems is the mitigation of angle-of-arrival (AoA) fluctuations, mainly arising from the vibration of receiving end and atmospheric turbulence. A common approach is to employ an AoA tracking system at the receiver. Beacon-based AoA tracking requires the beacon light, thus the system could be complicated and bulky when implemented. Beaconless AoA tracking could help to minimize the optical components required. This tracking typically requires an optical beam splitter to direct the received optical signal both to the beam position sensor and data photo-detector (PD). However, the presence of beam splitter results in optical power loss and tracking imprecision. In this paper, we propose a design of AoA tracking system based on an integrated beam positioning sensor and data PD to obviate the need for beacon light and beam splitter. We carry out a proof-of-concept experimental demonstration of AoA tracking systems for a 10-Gb/s free-space optical link over 104 m, comparing its performance with that of the conventional system. The demonstration shows a loss reduction of >2.9 dB by using the proposed design. It also shows that the proposed design improves the tolerance to AoA fluctuation by more than a factor of 2 at a target bit-error ratio of 10−3. The tracking time is measured to be 171 ms at an AoA fluctuation of 250 μrad. We show that the loop controller and fast steering mirror bandwidths are the main factors limiting the tracking time.

Modified low-bandwidth sub-Nyquist sampling receiving scheme in an IM/DD OFDM system enabled by improved optical shaping.

In this paper, a modified low-bandwidth sub-Nyquist sampling receiving scheme enabled by optical shaping is investigated in an intensity modulation/direct detection (IM/DD) orthogonal frequency-division multiplexing (OFDM) system, which can reduce the sampling rate and analog bandwidth of an analog-to-digital converter (ADC) at the receiving end. By changing the phase matrix of preprocessing, the modified scheme can distinguish different groups of data only by controlling the delay of the shaping module. In addition, the proposed RF sharing architecture can further reduce the cost and increase the feasibility of the scheme. Based on arcsine digital pre-distortion (DPD) technology, a DPD optical pulse shaping scheme is proposed to achieve better spectrum aliasing in the optical domain. With the help of the DPD shaping, we successfully experimentally demonstrate the 12.5-GHz/44.45-Gbit/s IM/DD OFDM system with low-bandwidth (3.125 GHz) and sub-Nyquist sampling rate (6.25 GSa/s) ADC. The experiment results show that the proposed scheme can not only effectively achieve low-bandwidth reception, but also achieve about 0.4 dB receiver sensitivity improvement compared with the traditional high-bandwidth scheme at BER of 3.8×10-3 after 10.2 km standard single mode fiber transmission, which indicates that the proposed scheme is a promising low-cost candidate to provide large transmission capacity for the next-generation network.

Multi-band DFT-S 100 Gb/s 32 QAM-DMT transmission in intra-DCI using 10 G-class EML and low-resolution DAC.

In this paper, 100 Gb/s/λ 32 quadrature amplitude modulation discrete multi-tone (QAM-DMT) transmission using 10 G-class electro-absorption modulated laser (EML) and 4/5-bit digital-to-analog converters (DACs) are experimentally demonstrated to meet the requirement of intra-datacenter interconnection (intra-DCI). Unequal length multi-band (ULM) discrete Fourier transform spread (DFT-S) precoding is investigated to alleviate the distortion induced by the high peak-to-average power ratio (PAPR) of DMT. The results show that the required computational complexity of ULM DFT-S precoding with 2-bands (k1=256, k2=64) decreases sharply compared to the traditional DFT-S technique with only about 0.5 dB receiver sensitivity penalty. In addition, compared to the equal length multi-band (ELM) DFT-S precoding, the ULM DFT-S precoding can bring about 2.5 dB receiver sensitivity improvement with slight added computational complexity. With the assistance of ULM DFT-S precoding and noise shaping (NS) technique, the bit-error ratio (BER) of 100 Gb/s 32 QAM-DMT signal generated by 5-bit DAC over 2-km single-mode fiber (SMF) transmission can reach the hard-decision forward error correction (HD-FEC) threshold with received optical power (ROP) of -6.5 dBm, with only additional 39.9% multiplier and 33.7% adder.

Performance Improvement of the Free-Space Optical Communication Link Using Spatial Diversity Reception-Assisted OFDM Signals

Orthogonal frequency division multiplexing (OFDM) technology is presented for use in free-space optical (FSO) communications accompanied by the spatial diversity reception. Using quadrature phase shift keying (QPSK) modulation formats, the OFDM signals show robustness to support high spectral efficiency and compatibility with the spatial diversity reception to improve receiver sensitivity. Compared with the single-carrier QPSK signal, the OFDM-QPSK signal with 64 sub-carriers can reduce the BER from 2.87 × 10−3 to 2.98 × 10−4 at the SNR of 6 dB. Using a two-aperture spatial diversity reception with OFDM, the BER can be reduced from 2.45 × 10−3 of a single receiver to 6.10 × 10−4 under moderate turbulence conditions. Under strong turbulence, the BER of the single receiver is 2.14 × 10−2. It can be improved to 1.16 × 10−3 by using four-aperture receivers, and even 6.87 × 10−4 by using six-aperture receivers. The optimized aperture number should be selected according to channel conditions.

Rectangular Orthogonal Digital Filter Banks Based on Extended Gaussian Functions

Rectangular orthogonal digital filter banks (ODFBs) based on square-root-raised-cosine (SRRC) functions are widely utilised to realise flexible and elastic multi-channel aggregations for fixed and mobile networks. However, long digital filter lengths are required to minimize digital filtering-associated signal distortions. In this paper, based on the extended Gaussian function (EGF), a novel rectangular ODFB with excellent robustness against the short digital filter length-induced truncation effect is proposed. Optimum digital filter parameters of the EGF-based ODFBs are identified and verified in multi-channel hybrid OFDM-digital filter multiple access (DFMA) PONs based on intensity modulation and direct detection (IMDD). By making use of the identified optimum digital filter parameters, extensive comparisons of digital filter characteristics and corresponding multi-channel upstream PON performances are made between the EGF-based ODFBs and the SRRC-based ODFBs. It is shown that to achieve a similar aggregated upstream signal transmission capacity, the EGF-based ODFB reduces the digital filter DSP complexity by a factor of 4. For a digital filter length as short as 8, in comparison with the SRRC-based ODFB, the EGF-based ODFB introduces &gt;1.5 dB (&gt;0.8 dB) improvements in upstream receiver sensitivity for 5-bits (8-bits) DACs&#x002F;ADCs, increases the aggregated upstream signal transmission capacity by &gt;5.5&#x0025;, enlarges the ONU launch power dynamic range by &gt;2.5 dB and improves the frequency offset tolerance by a factor of &gt;1.5. In addition, the EGF-based ODFB also enhances upstream performance robustness against ONU symbol timing offsets.

Underwater optical wireless communication performance enhancement using 4D 8PAM trellis-coded modulation OFDM with DFT precoding.

A 4D 8-pulse amplitude modulation trellis-coded modulation-based orthogonal frequency division multiplexing (4D 8PAM TCM-OFDM) scheme combined with discrete Fourier transform (DFT) precoding is proposed and experimentally demonstrated in an underwater optical wireless communications system. It can resist power fading caused by the attenuation, scattering, and reflection in the water channel. The experimental results show that, after transmission over the water-air channel and at the bit error rate of 3.8×10-3, the improvement in the receiver sensitivity is 0.88 dB using the 4D 8PAM TCM-OFDM scheme, compared to the conventional 64QAM-OFDM scheme. In addition, the proposed 4D 8PAM TCM-OFDM scheme combined with DFT precoding can compensate for unbalanced impairments, and it can obtain 1.28 dB receiver sensitivity improvement, compared to the conventional 64QAM-OFDM scheme with DFT precoding.

Low-complexity absolute-term based nonlinear equalizer with weight sharing for C-band 85-GBaud OOK transmission over a 100-km SSMF.

A low-complexity absolute-term based nonlinear feed-forward equalizer (FFE) combined with a decision-feedback equalizer (DFE) with weight sharing (AT-NLE-WS) is proposed and experimentally performed in a C-band 85-GBaud on-off keying (OOK) transmission system over a 100-km standard single-mode fiber (SSMF). By applying the k-means clustering algorithm to reduce weight redundancy, the required number of real-valued multiplications per symbol (RNRM) of the proposed AT-NLE-WS is only 14 for a bit error ratio (BER) under a KP4-forward error correction (FEC) threshold of 2.4 × 10-4. Compared with FFE-DFE, polynomial based nonlinear FFE-DFE (P-NLE), and AT-NLE, the proposed AT-NLE-WS saves >93% real-valued multiplications under the KP4-FEC threshold. In addition, compared with FFE-DFE, the proposed AT-NLE-WS can simultaneously achieve an approximately 2-dB improvement of receiver sensitivity and reduce the complexity by >80%. All experimental results show that AT-NLE-WS is a very attractive approach for practical implementation of low-cost optical interconnections with a data rate beyond 50 Gb/s and transmission distance up to 100 km.

Channel Estimation Based on Complex-Valued Neural Networks in IM/DD FBMC/OQAM Transmission System

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) is a promising candidate for 5G mobile fronthaul system. Due to the inherent imaginary interference of FBMC/OQAM signals, an accurate channel estimation is particularly indispensable. In this paper, an efficient method is proposed based on complex-valued neural networks (CVNN) for an accurate channel estimation of FBMC/OQAM signals with low computational complexity and pilot overhead. Here, the channel frequency response (CFR) is first calculated for training the CVNN. Next, the CFR estimated from the extracted pilots is exploited as the input of the trained CVNN to yield the CFR of data symbols. We experimentally demonstrate a 12.5-GBd intensity modulation direct detection (IM/DD) FBMC/OQAM transmission system over 30-km and 50-km standard single mode fibers (SSMF). The experimental results show that the proposed method achieves 3-dB and 1-dB receiver sensitivity improvement at the bit error ratio (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> with only 5% pilot overhead respectively, compared to the conventional least square (LS) and linear minimum mean error (LMMSE) methods. When the pilot overhead decreases from 10% to 1%, its BER performance is always better than LS and LMMSE. For the computational complexity, its complexity is the same order of magnitude as LS and lower than LMMSE. On the other hand, the CVNN requires less hidden neurons to keep the similar BER performance as real-valued neural networks (RVNN). Meanwhile, it has excellent resilience to fiber chromatic dispersion over 30-km and 50-km SSMF.

81-GHz W-band 60-Gbps 64-QAM wireless transmission based on a dual-GRU equalizer.

We propose a dual gated recurrent unit neural network based on nonlinear equalizer (dual-GRU NLE) for radio-over-fiber (ROF) communication systems. The dual equalization scheme is mainly based upon GRU algorithm, which can be trained via two steps including I-GRU and Q-GRU. By using the dual-GRU equalizer, 60-Gbps 64-QAM signal generation and transmission over 10-km SMF and 1.2-m wireless link at 81-GHz can be achieved. For the digital signal processing (DSP) at receiver, comparison between CMMA equalizer, Volterra equalizer, and dual-GRU equalizer are demonstrated. The results indicate that the proposed dual-GRU NLE significantly mitigates the nonlinear distortions. The dual-GRU equalizer has a better BER performance in receiver sensitivity than the traditional CMMA and Volterra equalizer. At the expense of large complexity, an improvement of receiver sensitivity can be achieved as much as 1 dB compared with Volterra equalizer at the BER of 2×10-2. To the best of our knowledge, this is the first time to propose a novel dual-GRU equalizer, which is promising for the development in millimeter-wave photonics for B5G applications and beyond.

Retracted: Peak to average power ratio reduction of ZT DFT‐S‐OFDM signals using hybrid improved monarch butterfly optimization‐PTS scheme with TR‐clipping scheme

SummaryCurrent research on zero‐tail discrete Fourier transform spread OFDM, an add on to conventional discrete Fourier transform DFT‐OFDM modulation without cyclic prefix (CP). Its low power tail is configured for coping up with channel's delay spread. Advantageous feature of ZT DFT‐S‐OFDM has drawn researchers for deploying it as a potential waveform candidate to fifth generation (5G). However, the intrinsic shortcoming of high peak to average power ratio (PAPR) should be alleviated. In order to improve that work, we propose a hybrid PAPR scheme that implements optimized partial transmit sequence (PTS) with tone reservation (TR) with clipping technique. Its earlier work stresses for reducing PTS scheme's computational complexity with enhanced version of monarch butterfly optimization (MBO) for PAPR reduction in ZT DFT‐S‐OFDM system. The proposed model reduced the correlation among the candidate signals. In this paper, we further reduce PAPR by deploying TR clipping pair, which reduces the influence of clipping noise and decreases ZT DFT‐S‐OFDM signal's PAPR at same time. The proposed hybrid technique produces excellent PAPR reduction while improving receiver sensitivity and effective throughout. The performance comparison is done using the parameters like PAPR and BER to confirm that this introduced design exceeds current waveforms designs with more economical design structure.

Precoded OVSB-OFDM transmission system using DML with Kramers-Kronig receiver

Different from the use of IQ modulator or a dual-drive Mach-Zehnder modulator, we investigate an optical vestigial sideband orthogonal frequency division multiplexing (OVSB-OFDM) transmission system using a low-cost directly modulated laser. Precoding techniques and Kramers-Kronig (KK) algorithm are employed to enhance the bit error rate (BER) performance. The KK receiver is applied to eliminate the signal-signal beat interference caused by the square-law detection of the photodetector. Discrete Fourier transform (DFT) and orthogonal circulant matrix transform (OCT) precoding techniques can effectively average the signal-to-noise ratio over the data-carrying subcarriers and improve the BER performance. Through numerical simulation, we discuss the relationship between the order and center frequency of the optical filter and the carrier signal power ratio (CSPR). Furthermore, we study the influence of the KK algorithm and precoding techniques on receiver sensitivity under different transmission distances. The results show that both KK algorithm and precoding technologies can improve receiver sensitivity and extend the transmission distance. The performance of DFT precoding is better than that of OCT precoding. When there is no KK algorithm, the transmission distance of OCT precoding or no precoding is only 20 km at the BER of 3.8e-3; while DFT precoding can extend the transmission distance up to 60 km. With the help of the KK algorithm, OCT precoding and DFT precoding can further extend the transmission distance by up to 40 and 60 km, respectively, compared to the DFT precoding and without the KK algorithm case. The DFT precoding with the KK algorithm can improve the receiver sensitivity of more than 7 dB in terms of received optical power, compared to the no precoding and without the KK algorithm case.

8 × 10 Gb/s Downstream PAM-4 Transmission for Cost-Effective Coherent WDM-PON Application

We experimentally demonstrate digital signal processing (DSP)-free 8 × 10 Gb/s 4-level pulse-amplitude modulation (PAM-4) transmission for cost-effective coherent wavelength division multiplexing passive optical network (WDM-PON) with 20 GHz channel spacing over the C-band. The polarization-independent coherent receiver at optical network unit (ONU) including a 3 × 3 fiber coupler and a polarization beam splitter (PBS) followed by three single-ended photodiodes (PDs), is simple and cost-effective, because neither DSP nor phase/frequency locking is necessary for the coherent detection of PAM-4 signal. We numerically compare the performance between the transmitted signal diversity and the local oscillator (LO) signal diversity schemes, and optimize the parameters of heterodyne detection, in order to enhance the power budget. Based on the LO signal diversity scheme, we can achieve an average receiver sensitivity of -26 dBm for eight wavelength channels at a bit error ratio (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ), leading to a power budget of 29 dB after 25 km standard single-mode fiber (SSMF) transmission. Moreover, we verify an unequally spaced PAM-4 signaling, leading to contribution of 1 dB receiver sensitivity improvement for 10 Gb/s/λ PON.

4-Level Alternate-Mark-Inversion for Reach Extension in the O-Band Spectral Region

Alternate-mark-inversion (AMI) has been used in the past to improve the transmission performance of binary intensity modulation formats. In this work, we improved the tolerance of O-band 4-level pulse amplitude modulation (PAM4) transmission to channel impairments through the adoption of an AMI scheme. Improvements in receiver sensitivity were shown in both semiconductor optical amplifier (SOA) and bismuth-doped fibre amplifier (BDFA) equipped links. We confirmed the capability of 4-level AMI to extend the reach in the O-band, which was achieved through the mitigation of nonlinear and dispersive effects. We experimentally demonstrated a 4.28-dB Q-factor improvement for transmission over 60 km of SMF-28e. To offer an insight to the mechanisms leading to the improved tolerance to transmission impairments, we presented numerical simulations that illustrate the underlying method of action of AMI4.

A Comprehensive Performance Comparison of DFT-S DMT and QAM-DMT in UOWC System in Different Water Environments

In this paper, we experimentally demonstrate a 450-nm laser underwater optical wireless transmission system, in which a comprehensive investigation is made to show the significant performance improvement of the discrete Fourier transform spread discrete multi-tone (DFT-S DMT) as compared to the conventional quadrature amplitude modulation discrete multi-tone (QAM-DMT) modulation. DFT-S DMT outperforms QAM-DMT in terms of bit error rate (BER) performance due to low peak-to-average-power ratio (PAPR) and capability of counteracting high frequency fading in a band-limited underwater optical wireless communication (UOWC) system. In order to avoid signal-to-noise-ratio (SNR) degradation at fringe subcarriers, several zeros are padded at the edge of each block before DFT-S operation. The experimental results show the superiority of DFT-S DMT compared with QAM-DMT in different water environments, including turbidities, bubbles, and water flow. Data rates of ~16.16 Gbps and ~13.96 Gbps at a BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> are achieved by 16-QAM DFT-S DMT and 16-QAM-DMT over a 5-m water channel, respectively, which indicates that capacity enhancement of ~2.2 Gbps is obtained by the DFT-S DMT. Meanwhile, over 3-dB receiver sensitivity improvement can always be achieved by the DFT-S DMT at the tested underwater transmission distances. Combined with adaptive bit-power loading, 20.04 Gbps over a 5-m “clear ocean” channel transmission with a single laser diode (LD) is demonstrated. For a 35-m water link, the distance-data rate product reaches 498.4 Gbps*m. To the best of our knowledge, both the data rate and distance-data rate product are the largest among all the results reported for a single visible LD. Aimed at high-speed deep ocean applications, the studies are promising for future UOWC research.

Beyond 1.6 Tb/s Net Rate PAM Signal Transmission for Rack-Rack Optical Interconnects With Mode and Wavelength Division Multiplexing

In this article, a beyond 1.6 Tb/s net rate pulse amplitude modulation (PAM) communication system is experimentally demonstrated for 20 m rack-rack optical interconnects with low cost intensity modulation direct detection (IM-DD) architecture, which is enabled by mode division multiplexing (MDM) and wavelength division multiplexing (WDM). The MDM with three linearly polarized modes is realized by a multiplane light conversion (MPLC) mode multiplexer with the maximum modal crosstalk less than - 20 dB. The experimental results show that 2.01 Tb/s (1.84 Tb/s net rate) PAM-6 signal carried on three modes and four wavelengths is successfully transmitted over 20 m standard OM2 multimode fiber (MMF) with the BERs of received signals from all channels below hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. Look-up table (LUT) pre-distortion and Volterra nonlinear equalizer (VNLE) are employed for nonlinear impairment mitigation to improve the system performance. Compared to linear feedforward equalizer (FFE), 0.3 dB and 1 dB receiver sensitivity improvements are obtained by LUT and VNLE, respectively. To the best of our knowledge, our proposed scheme achieves the highest bit rate of 167.5 Gb/s per wavelength in MDM system with IM-DD structure. The experimental results show that our proposed PAM MDM communication scheme is a promising candidate for future 1.6 Tb/s short reach rack-rack optical interconnects.