RF communication is not always the best communication solution, so many researchers are looking for alternatives. One of the most popular alternatives is visible light communication (VLC), because of the modern optical communication breakthrough. We will show the pros and cons of both RF and VLC car-to-car communication that motivates us to build a hybrid system to harvest the benefits from both of them and overcome the limitation of each of them.
The Popularity of RF communication leads to overbooking in all its bands. New applications are trying to coexist with the current ones. However, there are no reserved bands for these applications. Then, one of the possible solutions is to remove a working application from its band and replace it with a new application in the same band (e.g., replacing the 3G mobile network with 5G) . That means all mobiles that support only 3G will stop working and their owners must buy new 5G phones to get the same service again . Moreover, the same problem will affect all other wireless devices (i.e., TV, radio, Wi-Fi, etc.).
On the other hand, the possible solution for RF spectrum scarcity is frequency reuse by both legacy and new applications through introducing possible separators (e.g., time, code, power, location, etc.) [3, 4]. However, it is only a temporary solution and limits new applications’ horizons while introducing more complexity to the system. For example, to decrease the interference in the system we must decrease the power of each tower which leads to dead zones and calls drops increase . In addition, to refill these zones more towers are added which increases the overall system cost and so on. This cycle appears in every aspect of the RF communication, enhancing any parameter affecting all other parameters with a global limitation to the achieved data rate .
The appearance of smart new applications (e.g., calls and video with hologram, auto-drive, etc.), which need a higher data rate than the RF can cover, opens the way to other communication techniques such as visible light communication . The VLC communication is popular of its higher data rate which can cover modern applications’ needs.
Also, the spread of industrial light sources technology and devices (e.g., LED, light detector) enables cost-effective VLC communications . VLC has an easy and small constellation to set up a communication link (i.e., LED, camera) at home where we can use the house light sources to communicate with our mobile phones’ cameras. At the street, we can use the street lights with our mobile cameras or the car headlight with another car parking camera. In a short distance, the VLC communications show a smaller bit error rate (BER) compared to RF at the same distance and power level because it has a smaller noise level. However, faraway links cannot be achieved through VLC and need another means of communication [9, 10].
Consequently, this paper combines the advantages of both VLC communication and RF communication for car-to-car applications. By merging VLC with RF, the hybrid communication system achieves higher range coverage and smaller BER.
The authors in  proposed selective relaying based on the transmitted data type. They categorized the data types into two major types. On the one hand, the important control short packets that need to reach the receiver even if the delay time is longer than expected. On the other hand, the large amount of downloading and streaming packets that require large bandwidth, small delay, and able to accept packet drop percentage. In their algorithm, the authors use coalitional game (CG) and analytic hierarchy process (AHP) to separate the packets’ flow for each type based on the car-to-car communication path properties. However, that algorithm recomputes each path property when a car enters or exits the neighboring zone which drains the system resources (e.g., batteries, processing). Moreover, the proposed system uses only RF communication which has a limited bandwidth and suffers from a high level of interference.
The authors in  proposed an air channel for the VLC communication with variable levels of foggy weather. The fog level is expressed through the meteorological visibility which varies from 10 m up to 10 km. However, what affects the VLC communication is meteorological visibility between 10 and 500 m. The authors proposed that the transmitter is LED and the receiver is a camera. For a constant distance between two cars and variable fog meteorological visibility, the BER is small for meteorological visibility up to 20 m at different LED light modulations. However, for meteorological visibility of 10 m or less and at the same LED light modulations the BER shows a dramatic increase that leads to consider the communication link as down. In contrast, no solution is proposed such as using the RF communication under this condition. Moreover, no study is presented at the multi-hop VLC communication under the fog weather condition.
The authors in  proposed a practical VLC communication system where the proposed system uses 4G telecom infrastructure as a backup. The VLC communication uses a single laser source as a transmitter and a light detector as a receiver. However, the throughput of the presented system is only 50 kbit/s at 10−7 BER and within small communication distances less than 50 m. The authors did not present detailed results to the VLC or RF communication in a car to car. Moreover, no MIMO is proposed to increase the VLC data rate.
The authors in  built a full VLC channel model from the car headlight as the source to the camera as the receiver that includes the distance between cars, the difference in azimuth, and elevation angles due to different road lanes and road irregular surface, respectively. The suspension system of each car that moves on irregular roads tries to compensate for the generated vibrations. However, these vibrations cause differentiation in the elevation angles that degrade the received signal by at most 4 dB.
On the other hand, different road lanes cause a change in the azimuth angles that degrades the received signal by at most 3 dB within the light beam field of view (i.e., half-power beamwidth) and are considered as blocked otherwise. Consequently, the change in azimuth and elevation angles leads the VLC communication to degrade in longer distances which is the main required parameter. However, the authors did not propose a solution to lower VLC car-to-car communication distance such as multi-hop or using RF as a backup means of communication.
The authors in  made a comparison between the communication through VLC and the communication through infrared. The authors built a practical model using real cars and used the engine control unit (ECU) of the car as a processing resource, the headlights as the transmitter, and a photodetector as the receiver. However, the presented results are somehow below the expectation because of the use of a photodetector as the receiver and not a camera. They study the weather effect on both the VLC and infrared and find out that VLC is less effective than infrared at foggy conditions. In contrast, VLC is better than infrared in all other weather conditions because the background ambient temperature of the cars works as interference in infrared communication that lowers its SNR.
In [16,17,18], a MIMO VLC performance analysis is introduced for indoor applications. In these applications, the MIMO idea means dividing the light lamp into sectors; each of them transmits independently toward the same camera. The resolution of the camera and the distance from the light lamp are the main factors that affect the BER, while the data rate is affected by the camera’s number of frames per second. Therefore, the trade-off in indoor applications is the maximum number of sectors in the light lamp, the camera’s resolution, and the number of frames. Increasing the MIMO VLC system dimensions will increase the system cost. Consequently, the authors’ target is to optimize the VLC system requirement for the different scenarios in indoor applications.
In [19,20,21], the authors proposed using variable intensity LED lights so that the modulation is not as simple as OOK. Despite higher-order QAMs increasing the data rate, it decreases the system BER and required high-resolution cameras to successfully detect the transmitted message. On the other hand, LED lights suffer from decreased intensity during their lifetime. Moreover, weather conditions affect the light intensity which requires a variable threshold between QAM levels. Therefore, the authors’ target is to optimize the variable threshold with respect to the desired QAM level while maintaining the BER within the acceptable level.
In [22, 23], the authors proposed using complex methods to overcome the VLC channel challenges. The proposed channel estimation is based on compressive sensing, and the block coding using bit-shuffle helps decrease the BER in the VLC system, especially in outdoor applications. Such techniques are required and adopted in much VLC research within academia.
In [24,25,26,27], the authors proposed using VLC in the vehicular car-to-car network. Adopting the VLC in car-to-car communication is driven by the increasing demand in car-to-car communication and the availability of LEDs, cameras, and processing units within the car structure. The authors investigate the BER performance in car-to-car communications and the most suitable methods to optimize the VLC communication for outdoor applications. Moreover, the author proposed not only using the VLC for car-to-car communication but also IoT sensors installed on each car.
In , the authors proposed using multi-hop to extend the VLC communication limited distance by using neighboring cars as relays. In the meantime, the authors proposed using terrestrial communication such as 5G or 6G as a backup. Such technique can extend the maximum VLC distance to multiple cars far away from the sender car while using the backup 6G if the chain of relays is broken.
In this paper, we aim to benefit from the modern technology of automotive manufacturing. Firstly, we use the car’s LED light and parking cameras to form VLC communication links between cars to transmit road data. Moreover, we benefit from VLC’s high data rate and small error rate for short coverage. Then, we extend the VLC communication coverage in the distance out of VLC single-hop range using the multi-hop technique by making other cars within the road work as repeaters between the source and destination while maintaining the high data rate and small error rate. Finally, when VLC reaches its time out of hopes, more coverage is realized by using RF communication which is known for its larger coverage even if its data rate is smaller. Therefore, by merging VLC with RF, the hybrid communication system assumes high range coverage and small BER. Our study is devoted to determining the performance parameter of the hybrid system including bit error rate BER, throughput and delay under clear weather, heavy fog, and heavy rain conditions. Our novelty is not only studying the addition of the multi-hop VLC relaying with the high data rate MIMO VLC and keeping the RF as backup, but also extending to study the effect on that system under different environmental conditions.
The rest of this paper is organized as follows: Sect. 2 describes the methods/experiments of the study. Section 3 describes the system model. Section 4 presents the proposed VLC multi-hop communication system. Section 5 depicts the simulation results and their analysis, and then, Sect. 6 concludes the paper.