A Low-complexity of VLC System using BPSK

— The design, implementation, and demonstration of visible light communication (VLC) system using Binary Phase Shift Keying (BPSK) modulation has been presented in this short paper. Our system is applied for indoor environment purpose. The test result shows that our VLC system able to work properly as expected, the BPSK constellation can be formed wirelessly through a visible light link. We obtained 13.4 kbps of maximum data rate transfer.

Associated with the modulation for VLC, in our previous work, we have been performed a VLC system with certain modulations, specifically a digital modulation: Pulse Width Modulation [3][4][5][6], Pulse Position Modulation [7], and the simplest modulation, OOK [8][9][10]. According to the comparison results among them, the mentioned modulations are wasteful in utilizing the available bandwidth, thus the communication speed is very limited [11]. One of the solutions is employed the multiplexing technique such as OFDM scheme to thrift the optical system's bandwidth [12].
In this work, we implement the simple OFDM scheme with Binary Phase Shift Keying (BPSK) mapper. The BPSK was chosen because of the most powerful of all types of PSK. It takes the highest distortion level to make the demodulator perform the wrong decision. The rest of this paper has been organized as follows, second section discusses the methodology. Third section looks to the successful demonstration. The last part is closed by fourth Section, that is conclusions and references. However, it must with a specific requirement that the two points are separated as far as 180 o . Fig. 1(a) shows the two points are located on the real axis (at 0 o and 180 o ). In BPSK modulation, the signal is reflected in two possibilities: signal '1' is represented by giving a phase-shift of a carrier signal with 0 o . Whereas the signal '0' is represented by shifting the signal phase carrier with 180 o . Fig. 1(b) depicts the simulation of BPSK in MATLAB. The BPSK is only able to modulate '1 bit' per symbol so it is not suitable for high-speed data rate applications.

Analog Front-End Design
Fig. 2 depicts the whole system of our VLC system. The USB is employed to connect the computer to the VLC module. The microcontroller STM32F4 was chosen as the main controller. We used components with a low-cost factor in order to meet the low-complexity requirement of VLC system. The antenna is utilized a commercially white LED while the PIN photodiode was employed as a photo-sensor.
As shown in Fig. 2, the physical layer of our VLC system contains a low-cost Bias-T and analog front-end receiver that has been realized by Op-Amps configuration. The Bias-T blocks consist of gain buffer, voltage reference circuit, DC-offset adder, and current amplifier. We discuss the design and implementation of Bias-T clearly in   Fig. 3(a) visualizes the demonstration of our VLC system using BPSK modulation in which the parameters for testing are referred to Table 1. It contains of seven variables (i.e. LED, Photodiode, channels setting, topology of link used, the environment condition of the performance test, initial optical angle, and distance of channel). There are 2 computers used for displaying the transmitted and received BPSK signal.

Results
Based on the performance test as shown in Fig. 3(b), we can conclude that the BPSK constellations can be formed through visible light link. However, those constellations that viewed from Fig. 3(b) are too small, thus we zoomed-out the achieved data as depicted in Fig. 4 for various distances of the optical channel. It can be concluded that the longer optical distance the higher error at receiver. The optical distance is about 50 cm for the best option.
The user interface was developed by the Delphi 7.0 software which is installed in PC transmitter as well as PC receiver. In summary, we obtained 13.4 kbps of data rate, this is the maximum speed transfer using BPSK modulation. The measurement methodology of available bandwidth and its SNR are discussed in separate papers (Please see the following references [28][29]). While the obtained bit rate and bit-error rate (BER) are discussed in [30-31].

Conclusions
In line with performance test, it can be summarized that the designed BPSK can met the qualification of with the related theories. Our VLC system can perform properly with optical distance up to 40 cm. We carefully designed our system to great deal with low-complexity target. Using BPSK modulation, we obtained the maximum bitrate of ~13.4 kbps. This work is part of Master thesis that can be found in [11].