1. Introduction
With the continual expansion of cities, commuting distances and times have notably increased. In the majority of key metropolitan areas, residents spend over an hour each day commuting to and from work via public transportation [
1,
2]. Moreover, the everyday usage of transportation modes such as trains, buses, and subways is on a steady annual rise. Consequently, an increasing number of people are investing a significant part of their day in transit-time that could be alternatively used for work, reading, or indulging in multimedia content.
With the substantial surge in streaming platforms in recent years, subscribers’ consumption of multimedia content, particularly on-demand movies and TV series, has shown a marked increase. This trend signifies a clear shift in consumption patterns towards more selective and demanding behaviors [
3,
4]. Viewers now prefer to have control over what content they watch, the manner in which they watch it, and, most crucially, when and where they watch it. In essence, contemporary viewers seek ultra-high-definition (UHD) content that they can enjoy anywhere, anytime, and on-demand.
Currently, no services offer a fully satisfying experience in transportation due to an amalgamation of business- and technology-related hindrances. The business constraints are mainly due to restrictive data consumption models dictated by telephone and Internet service providers, which is not the focal point of this research. Technologically, the main challenges present as low signal coverage, inadequate speed, and the limited bandwidth accessible to users. These issues are amplified by an increasingly crowded radio spectrum, causing interference between radio frequency waves from assorted services like telephones and other network-connected devices through wireless fidelity (WiFi). A prospective solution to these challenges could be operating within different sections of the electromagnetic spectrum, perhaps the optical spectrum.
In optical communications, a particular scenario involves the exchange of information between transmitter and receiver devices, harnessing visible and infrared (IR) light sources. This type of communication is recognized as optical wireless communication (OWC). Traditionally, due to the low cost of optoelectronic and electro-optical components, IR technology has been extensively employed within OWC. However, its usage is confined by the need to sustain low transmission powers and speeds to evade potential health hazards [
5,
6].
In recent years, visible light communication (VLC) has made considerable strides, attaining data rates of up to several Gbps. However, video transmission systems documented in the existing literature significantly trail behind these upper limits. For instance, a study proposed in [
7] illustrates the concurrent transmission of audio and video signals using white and red Light-Emitting Diodes (LEDs), achieving data rates of up to 2 Mbps for video and 15 Mbps for audio over a 50 cm link, without the requirement of lenses or other optical elements to extend the distance. Similarly, ref. [
8] reported the implementation of a Field-Programmable Gate Array (FPGA)-based VLC system for real-time video transmission, achieving a communication distance of up to 5 m and employing a pulse position decision algorithm to enhance transmission reliability.
Other noteworthy studies include [
9], reporting a system reaching a maximum bit rate of 0.986 Mbps over an optical distance of 6 m, and [
10], achieving a bandwidth of 8 MHz and an optical distance of 8 m using an avalanche photodiode (APD) as a photodetector. Furthermore, ref. [
11] exhibited a maximum rate of 0.415 Mbps with video qualities ranging from 480p to 720p. A system achieving a bit rate of 7.14 Mbps over an optical link of 6.7 cm was developed in [
12], whereas the system proposed in [
13] managed video transmission at a rate of 0.115 Mbps over an optical link of 1 m. The research shared in [
14] demonstrated a system transmitting video at a rate of up to 0.115 Mbps over a 5 m distance between the transmitter and receiver. Lastly, the proposal in [
15] was based on high-powered lasers, which are infeasible for applications involving people due to potential damage to the cornea of the eyes [
5] and skin [
6].
Conversely, the study conducted in [
16] illustrates how Orthogonal Frequency-Division Multiplexing (OFDM) modulation can be employed for boosting spectral efficiency in VLC systems, thereby achieving data rates of up to several Gbps.
A development closely related to VLC is Light Fidelity (LiFi) technology, which integrates bidirectional and multi-user communication. This technology forms wireless optical networks with seamless handover capabilities supporting user mobility. LiFi offers higher bandwidth and transmission rates compared to radio frequency (RF) technologies such as WiFi. Although LiFi cannot penetrate opaque objects, this limitation can be advantageous for applications requiring high security levels.
Our proposed optoelectronics interfaces can be utilized to provide an innovative solution for transmitting UHD multimedia content in public transportation settings. By utilizing existing reading lights in public transport vehicles, our system sets itself apart from previous studies. Moreover, to our knowledge, no work has reported a VLC system capable of transmitting UHD video using an LED as a transmitter. Additionally, our system offers the flexibility of selecting the modulation scheme—be it On–Off Keying (OOK), OFDM, or any other scheme implemented in the FPGA—by simply toggling a switch.
By employing OFDM modulation, our system can achieve data rates comparable to those reported in prior studies. Further, by leveraging high-efficiency LEDs and advanced signal processing techniques, our scheme enhances the transmission rate and improves energy efficiency and connection stability.
In this study, we conducted experimental tests in a passenger van, a specific environment that aptly represents the broader public transport context. Passenger vans, commonly used for passenger transport in many metropolitan areas, share key operational characteristics with other public transportation modes such as buses and trains. These features include constant movement, the presence of multiple users, and the need for efficient and secure communication systems.
Testing in a passenger van allowed us to evaluate the VLC technology under controlled yet realistic conditions, facilitating an accurate observation and measurement of the system’s performance. Since there is a similarity in lighting infrastructure and usage conditions, the results obtained can be extrapolated to other public transportation vehicles. Therefore, this experimental approach provides valuable and applicable information for implementing VLC systems in public transportation.
The integration of VLC in public transport offers several advantages over traditional wireless technologies. Unlike WiFi and RF, VLC is immune to electromagnetic interference, a crucial element in densely populated environments like trains and buses. Recent studies suggest that VLC can achieve higher spectral efficiency and transmission rates, reaching up to several Gbps in controlled scenarios [
16]. Additionally, the ability to utilize existing lighting infrastructure significantly reduces deployment and maintenance costs. These advantages make VLC an ideal solution for enhancing user experiences in public transportation by providing a secure, fast, and reliable connection for streaming UHD multimedia content. The studies conducted in [
7,
8,
12] also attest to the viability of VLC for real-time applications, a critical aspect of its implementation in transportation systems.
Hence, the driving force behind this study is to explore the practicality of the hardware implementation of optical transceivers and optoelectronic interfaces. The goal is to develop a VLC system that can be utilized for various use cases, including the transmission of UHD video in a passenger van.
With this in mind, a conceivable use case of this VLC system intends to leverage the reading lights pre-installed on the interior roofs of vehicles. This aims to equip public transportation with the ability to provide passengers access to UHD audio-visual content on handheld devices such as laptops, tablets, and smartphones. In order to play these audio-visual contents, users are required to connect a purpose-built transceiver or dongle to their devices’ USB port. The audio-visual content must be stored on a server that is accessible within the vehicle.
The proposed system encompasses two principal components: (i) a LiFi router and (ii) a USB dongle, as graphically represented in
Figure 1.
In the ensuing sections, the components and methods utilized in the development of the optoelectronic interfaces will be outlined. Initially, the proposed optoelectronic interface system will be detailed, covering its application in both the downlink and uplink. This will be followed by a more thorough elaboration on the design and implementation of the system components, including the LED and photodiode (PD) driver. Subsequently, we will describe the experimental set-up, testing procedure, and the system’s deployment in a passenger van. Finally, the most representative measurements characterizing the system will be presented, leading to the final conclusions.
4. Discussion and Conclusions
The implemented system comprises two parts. The first part is the ‘LiFi router’, which is made up of a stack of three boards, a visible LED, and an infrared receiver. It operates on a supply voltage of 12 V with a current draw of 920 mA. There are two power sources it can utilize: (i) a 12 V AC/DC adapter with a jack power connector or (ii) a 12 V battery. The ultra-white LED, driven by the implemented driver on one of the boards, achieves a bandwidth of 14.45 MHz, while the IR PD driver attains a bandwidth exceeding 12.27 MHz.
The second part is the USB dongle, which also consists of a stack of three boards, an IR LED, and a visible-light receiver. Operating with a 5 V supply voltage and drawing 1.12 A, this part can be powered in two ways: (i) via a 5 V AC/DC adapter with a jack power connector or (ii) via a USB Type-C connector. The infrared LED, accompanied by the implemented driver, achieves a bandwidth exceeding 20 MHz, whereas the visible photodiode (PD) driver, powered by the interface board at +5 V and −5 V, attains a bandwidth surpassing 11.66 MHz.
Additionally, the LED driver is designed for high-power white LEDs with a power consumption of approximately 1.5 W, replicating the reading lamps commonly found in mass transit. The first two stages of the driver consist of equalization amplification circuits designed to enhance the bandwidth of the LED. Each of these stages incorporates phase-advanced equalizers, resulting in a bandwidth of 14.45 MHz, marking a significant improvement (almost a decade) from the initial bandwidth of 1.8 MHz. The final stage encompasses a current source, comprising a voltage-biased N-MOSFET transistor that superimposes the modulated signal on the DC level necessary for the LED to achieve a luminous intensity of 75,000 lx.
Conversely, the developed PD driver, comprising three gain stages, attains a bandwidth of 11.66 MHz for the visible PD driver and 12.27 MHz for the IR PD driver. While these are modest values, they are sufficient to transmit UHD content as required.
In summary, the bandwidth of the visible downlink is 11.66 MHz, while the IR uplink achieves a bandwidth of 12.27 MHz. In [
10], a bandwidth of 8 MHz and a link distance of 8 m were reported. However, that system employed an APD as the photodetector. Considering that APDs require high bias voltages such as 65 V, and given that the receiver needs to be powered by a 5 V supply, using an APD is not viable.
To avoid potential spurious data, the visible optical link was configured to operate at a speed of 10 MHz, a submultiple of the 40 MHz sampling frequency determined by the ADC and DAC. Moreover, the maximum optical length achieved is 110 cm, accomplished by using a Fresnel lens at the receiver side with a focal length of 10 mm and a diameter of 13 mm. The system’s noise level amplitude is approximately 45 mVpp.
Likewise, the infrared optical link operates at 10 MHz, in harmony with the 40 MHz sampling frequency. It also achieves an operating distance of 110 cm, utilizing the same Fresnel lens. The system’s noise level amplitude is approximately 45 mVpp.
To ensure the system’s correct performance, the minimum signal-to-noise ratio (SNR) for both optical links is set at 10.74 dB.
The visible optical channel was modulated using a multicarrier modulation scheme, with each carrier modulated using Quadrature Phase Shift Keying (QPSK). This set-up achieved a BER of 3.325910−5 at a distance of 72.5 cm between the LED and the PD, both situated in the passenger van, reaching a bit rate of 11.25 Mbps.
The infrared link employs OOK with 8b10b encoding. As the data integrity requirements are less stringent than in the downlink, any packet lost due to interference is simply retransmitted, making this a suitable option. This uplink set-up reached a maximum data rate of 15.25 Mbps.
In conclusion, a system capable of streaming UHD video was successfully implemented using the hardware design of the optical transceivers and optoelectronic interfaces executed in this work. Specifically, this system achieved a bit rate of 15.25 Mbps for OOK and 11.25 Mbps for QPSK within the OFDM scheme. For context, the system reported in [
7] achieved up to 2 Mbps for video transmission using a red LED and reached an optical link of 50 cm, which is insufficient for UHD video transmission. In the work documented in [
8], a data transmission speed of 0.986 Mbps was achieved over a distance of 5 m. However, a VLC system with this bit rate cannot support the UHD transmission rate targeted in this study.
In [
11], a maximum bit rate of 0.415 Mbps was reported, with video qualities ranging from 480p to 720p -far from the UHD focus of this work -. In [
12], a system was developed with a bit rate of 7.14 Mbps over an optical link of 6.7 cm, while the optical link achieved in this work is 110 cm. Although [
13] displayed an optical link of 100 cm, the bit rate reached was only 0.115 Mbps, inadequate for UHD video transmission. An optical link of 5 m with a bit rate of 0.115 Mbps was exhibited in [
14], and [
9] presented an optical link distance of 6 m with a bit rate of 0.986 Mbps. Both of these instances fall short of the requirements for UHD video transmission.
Although [
15] reported a UHD video transmission with a bit rate of 100 Gbps, the system utilized high-power lasers for Free Space Optical (FSO) Communication links. It is well known that high-power lasers are prohibited in environments where people are present due to the risk of inducing cornea injuries [
5] and skin burns [
6]. Therefore, the system outlined in [
15] is unsuitable for user applications. Consequently, our proposed VLC system, capable of transmitting UHD video, remains the foremost option in the current state of the art for user applications. It is also compliant with the pre-existing lighting infrastructure in public transportation settings. A proof of concept was carried out by deploying the system in a passenger van, where it was successfully tested. Finally, a solar panel situated on the van’s roof was employed to power the user’s side, namely, the USB dongle and the user’s laptop, through a power bank battery. With this set-up, the user’s side was completely self-powered, charging the battery in 13.4 h under a luminous flux of 80,000 lx, while the battery life lasted 22.3 h.
The terms abbreviation list can be found in the following
Table 7.