# Design of PAM-8 VLC Transceiver System Employing Neural Network-Based FFE and Post-Equalization
## 1 Introduction
In [13], a 64-QAM was adopted for a higher data rate with an adaptive deep-learning equalizer. A CAP modulation scheme with a hybrid post-equalizer was utilized in [13], achieving a data rate of 8 Gbps. DMT modulation with an adaptive bit- and power-loading algorithm was adopted in [14] to achieve a data rate of 1 Gbps. Ref. [15] combined DCO-OFDM and long short-term memory (LSTM), while [16] presented 64-QAM signal transmission with a joint time-frequency neural network to realize an underwater VLC system with a data rate of 2.85 Gbps. In [17], PAM-4 and PAM-8 schemes were adopted with an LSTM equalizer to reach a communication distance of 1.2 m at 1.15 Gbps, and an underwater VLC system with a data rate of 1.22 Gbps was implemented in [18] with PAM-7 modulations and a density-based spatial clustering of applications with noise (DBSCAN) algorithm. VLC systems implemented with discrete components can lead to large parasitic components, which introduce extra noise to the transmitted signals, increase the signal loss, and limit the bandwidth. The aforementioned results cause stability and installation problems. High-bandwidth \(\mu\)-LEDs are expensive and not achievable in practical implementation. From the perspective of practical application, a highly integrated VLC system with a dedicated transmitter and receiver is superior to a discrete system. The typical architecture of an integrated VLC transceiver system is presented in Figure 1, which consists of a transmitter, a light source, a light signal detector, and a receiver. On the transmitter side, the source data are encoded and modulated in the baseband. Then, the modulated signal is sent to the LED driver. This signal controls the driving current of the light source to generate a corresponding optical signal. On the receiver side, the transmitted optical signal is converted into an electrical signal by a PD. A transimpedance amplifier (TIA) converts the photoelectronic current into voltage with amplification for further signal processing. A clock and data recovery (CDR) module then extracts the clock and retimes the data for subsequent demodulation and decoding processes in the baseband. There are several works related to the design of integrated VLC systems. In [19], a fully integrated VLC transmitter was designed and implemented in a 0.35 um process with a 266 kbps data rate. Ref. [20] proposed a micro-LED display driver with a VLC transmitter implemented in a 0.5 um process to achieve a data rate of 1.25 Mbps. Ref. [21] presented a 24 Mbps VLC receiver with an ambient light rejection function at a 1.6 m distance. In [22], a fully integrated receiver in a 0.13 um process for visible laser communication was presented with a maximum data rate of 500 Mbps. However, it can be noticed that these works have only focused on the design of either the transmitter or the receiver without systematically evaluating the integrated transceiver system. In addition, these works have performed with rather low data rates compared with discrete VLC transceiver systems [13; 14; 15; 16; 17; 18] due to a lack of suitable equalization schemes. In this paper, we propose a PAM-8 VLC transceiver system for implementing Gbps communication with a \(\upmu\)-LED with a 50 MHz bandwidth. It consists of an integrated analog front-end (AFE) and an offline digital baseband, which aims to overcome limited bandwidth by the combination of a high-order modulation scheme and a hybrid equalization method. The AFE is composed of a current-mode logic (CML) driver, passive equalizers, and a TIA. A 50 MHz \(\upmu\)-LED modelled in both circuit and Verilog-A code is used as the light source, which is fitted from the experimental data. To extend the bandwidth of this LED, passive equalizers applied after the driver and an NN-based FFE algorithm in the baseband are adopted on the transmitter side, while a radial basis function neural network (RBFNN)-enabled post-equalization is employed in the off-line baseband of the receiver. PAM-8 modulation is chosen for further increasing the data rate.