Linear and nonlinear impairments severely limit the transmission performance of high-speed visible light communication systems. Neural network-based equalizers have been applied to optical communication systems, which enables significantly improved system performance, such as transmission data rate and distance. In this paper, a memory-controlled deep long short-term memory (LSTM) neural network post-equalizer is proposed to mitigate both linear and nonlinear impairments in pulse amplitude modulation (PAM) based visible light communication (VLC) systems. Both 1.15-Gbps PAM4 and 0.9Gbps PAM8 VLC systems are successfully demonstrated, based on a single red-LED with bit error ratio (BER) below the hard decision forward error correction (HD-FEC) limit of 3.8 x 10 −3 . Compared with the traditional finite impulse response (FIR) based equalizer, the Q factor performance is improved by 1.2dB and the transmission distance is increased by one-third in the same experimental hardware setups. Compared with traditional nonlinear hybrid Volterra equalizers, the significant complexity and system performance advantages of using a LSTMbased equalizer is demonstrated. To the best of our knowledge, this is the first demonstration of using deep LSTM in VLC systems. IntroductionVisible light communications (VLC) based on light emitting diodes (LEDs) has become an attractive and promising technology due to its cost effectiveness, immunity to electromagnetic interference, license-free and high security [1]. In recent years, transmission rate of visible light systems has been increasing with the use of` high-order modulation, such as orthogonal frequency division multiplexing (OFDM) [2], carrier-less amplitude and phase modulation (CAP) [3] and pulse amplitude modulation (PAM) [4].Equalizer is one of the most critical parts of the VLC systems. As a typical communication system, the transmitted signal of VLC systems is distorted in amplitude and delay causing the inevitable inter-symbol interference (ISI). On the other hand, nonlinear distortion has gradually become a new bottleneck in high-speed transmission systems due to nonlinear V-I response of LED source [5] and other origins that have not been well-modeled such as non-linear distortion arising from the transmitter driving circuits and the electrical amplifier (EA). Recently, Adaptive finite impulse response (FIR) filter based linear equalizers in VLC systems have been widely studied, such as scalar modified constant multi-modulus algorithm (S-MCMMA) blind equalization algorithm [6], data-aided recursive least square (RLS) [7] and least mean squares (LMS) [8]. In order to compensate for both linear and nonlinear effects, hybrid equalization schemes have been shown to be effective approaches to improve the performance of VLC systems, such as adaptive FIR linear equalizer with
Accelerator-based neutron source have been considered to be practical for boron neutron capture therapy (BNCT). Based on experience with a parameters of the Brookhaven National Laboratory BMRR reactor neutron source, which has been used in treatment experiments, the future accelerator-based neutron source for BNCT should have the properties of low energy distribution (< 100 keV) and high flux (about 10(9) neutrons per second per square centimeter) in the patient zone. Using protons to bombard thick 7Li targets, generating neutrons via the 7Li(p,n)7Be reaction, is one of the optimal choices for this kind of neutron source. Neutron yield data versus incident energy are necessary in order to select the proper incident energy and for estimating how high the incident proton current should be. The required proton beam current intensity is one of the key parameters for an accelerator useful for BNCT. In the present work, neutron yields of the 7Li(p,n)7Be reaction with a thick lithium target and incident energies of 1.885 and 1.9 MeV were measured at 0 degree with respect to the incident beam direction. The results are (3.08 +/- 0.17) x 10(12) and (5.71 +/- 0.32) x 10(12) neutrons/C sr, respectively. Neutron yield angular distribution measurements at 2 MeV incident energy were also performed. The proton beams were generated by the Peking University 4.5 MV electrostatic accelerator. The emitted neutrons from these reactions have the advantages of low energy distribution and forward angular distribution, which are requirements for a BNCT neutron source. The data obtained in this work can be used as a reference to study the accelerator-based neutron sources for BNCT.
Aiming to improve the performance of free space visible light communication (VLC) system, we combined the use of rotation coding together with geometric shaping (GS) technology in carrierless-amplitude-and-phase (CAP) modulation visible light communication system. In this paper, we designed a new GS-16QAM constellation named Triangular. Triangular GS-16QAM has a better noise resistance due to its larger value of the minimum Euclidean distance. Based on the excellent new 16QAM constellation, we further improve the performance of VLC system utilizing a proposed new coding technology named rotation coding after the quadrature-amplitude-modulation (QAM) mapping. The rotation coding can reduce the overlapping problem of the outer constellation points induced by non-linear impairment. Finally, the upper gain about 1 dB is achieved with rotation coding in 1.2 m free space VLC communication when the transmission speed is relatively low about 80 Mb/s. While the transmission speed is relatively high around 2 Gb/s in CAP-VLC system, Triangular constellation with rotation coding still has positive effect for transmission quality with about 1 dB enhancement than the square 16QAM. All of these results are verified by simulations and experiments.
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