# Design of PAM-8 VLC Transceiver System Employing Neural Network-Based FFE and Post-Equalization
## 1 Introduction  
With the rapid growth in network traffic caused by the increasing number of communication devices, high-quality media transmission, and real-time contact requirements, radio spectrum congestion has become a severe problem. Radio frequency (RF) signals only cover the 20 kHz to 300 GHz bandwidth in the electromagnetic spectrum, while visible light covers the 400 THz to 800 THz spectrum bandwidth [1]. This wide, unlicensed spectrum bandwidth provides a solution to the radio spectrum congestion problem, and it has led to research focused on visible light communication (VLC). VLC technology utilizes visible light as a communication medium to transmit programmed data by changing the intensity of the light source. Then, the modulated light is received by a photodiode (PD) and transferred to a photoelectric current, which is processed in a receiver to recover the transmitted signal [2]. Beyond providing sufficient bandwidth to relieve radio spectrum congestion, other unique characteristics of visible light, such as its line-of-sight (LOS) property and lack of electromagnetic interference (EMI), guarantee the superiority of VLC compared with traditional RF-based technologies [3]. Therefore, VLC technology has been considered as a potential indoor communication method with high security and low cost. Meanwhile, visible light owns a much lower attenuation in water than RF signals. Thus, VLC technology is suitable for underwater communication. However, VLC technology suffers from intrinsic problems, such as limited communication bandwidth and limited signal-to-noise ratio (SNR) for high-order modulation. Thelimited communication bandwidth impedes the transmitted signal data rate in VLC systems since commercial white LEDs only provide from several MHz to 10 MHz of communication bandwidth [4; 5]. To increase the communication data rate, there are three solutions. The first one is changing the device structure to downscale the size of LEDs. To increase the communication data rate, the most straightforward method is increasing the symbol rate through extending the modulation bandwidth. The main limitation of LED bandwidth is caused by large intrinsic capacitance, which can be reduced by downscaling the size of LEDs. Therefore, micro-LEDs (\(\mu\)-LEDs), the size of which is a small percent of normal LEDs, have hundreds of MHz of bandwidth [6; 7]. Due to the limitation on bandwidth caused by the intrinsic characteristics of LEDs, some works have sought \(\mu\)-LEDs with larger bandwidths for implementing high-speed VLC systems [8; 9; 10]. The second solution is applying high-order modulation schemes, such as pulse-amplitude modulation (PAM), QAM, carrier-less amplitude and phase modulation (CAP), and discrete multi-tone modulation (DMT) [11; 12; 13; 14], to increase the efficiency of modulation bandwidth. This method increases the data rate without changing the symbol rate by improving the data number carried by each symbol. The third one is utilizing equalization schemes, such as passive and active equalizers, feed-forward equalization (FFE), decision feedback equalization (DFE), and neural network (NN)-based equalization [12; 13; 15; 16; 17; 18], to extend the modulation bandwidth. These solutions have been adopted synthetically to further extend the modulation bandwidth. In [8], by adopting a \(\mu\)-LED that owned the 906 MHz bandwidth, a 4 Gbps underwater VLC with a PAM-4 modulation scheme was achieved over a distance of 2 m. A 655 MHz \(\mu\)-LED was utilized for implementing a data transmission rate of 11.95 Gbps for a VLC with an orthogonal frequency-division multiplexing (OFDM) modulation scheme [9]. With a T-bridge pre-equalizer, the \(-3\) dB bandwidth of a VLC system based on HV-LED was extended to more than 700 MHz, which was used to achieve a 3 Gbps underwater VLC transmission over 1.2 m with a 16-quadrature amplitude modulation (QAM) [10]. In [12], PAM-4 and FFE were combined to increase the communication data rate to 2 Gbps.