Evaluation of visible light communication channel delay profiles for automotive applications
© Lee et al.; licensee Springer. 2012
Received: 27 March 2012
Accepted: 6 November 2012
Published: 27 December 2012
In this article, we present channel delay profiles based on simulated data regarding the practical conditions for the use of visible light communication (VLC) in automotive applications such as Intelligent Transportation Systems (ITS). Practical vehicular LED headlamp and street lamp that consider the lighting regulation for transportation are used to design the ITS scenarios based on VLC. We modeled two usage scenarios, crossroad and metropolitan street, using the CATIA V5 tool. Measurements for the VLC channel delay profile evaluation were then gathered by using a ray-tracing scheme employing commercial LightTools software under the vehicle-to-vehicle and vehicle-to-infrastructure (V2I) communication links. From the obtained channel impulse responses from both scenarios, we derived the VLC channel delay profiles. From them, we found that the common property of the delay profile was composed of dominant multiple line of sight (LOS) links and a less number of non-LOS delay taps. However, the channel delay profile for the V2I link and metropolitan scenario show more dispersive channel characteristics due to the reflection and diffusion of the visible light.
Next-generation LED lighting has more advantages than existing fluorescent and incandescent lighting in terms of its long life expectancy, high tolerance to humidity, low-power consumption, and minimal heat generation. Recently, there have been many attempts to combine LED with IT technology. Among them, visible light communication (VLC), the convergence of illumination and communication, has been developed [1–6]. In general, VLC uses intensity modulation with direct detection scheme, which uses the intensity of the light to transmit data. Human eyes perceive only the average intensity when the light modulates faster than the maximum flickering time period, defined as 5 ms . Therefore, both acceptable lighting and communication can be implemented simultaneously. The corresponding VLC standardization has recently been published by the IEEE standard association . VLC has many applications in indoor lighting used for home and office environments and in outdoor lighting for vehicular environment. The VLC applications are summarized in the VLC standard group .
Among these applications, the adaptation of VLC for Intelligent Transportation Systems (ITS) is of high interest to various VLC research groups [5–12]. In order to obtain intelligence in transportation, ITSs have utilized the advantages of the Information and Communication Technologies (ICT), providing several different technological systems that help users. Among the possible ICTs, ITSs are now considering the use of VLC, focusing on two aspects. First of all, it is possible to repurpose the transportation lighting infrastructure for communication. Recently, the transportation lighting infrastructure such as street lamps, traffic lights, automotive lamps, etc., is changing to LEDs. Therefore, in the case of an ITS based on VLC, it will be possible to make use of the conventional automotive and traffic LEDs. Consequently, the cost incurred in constructing the ITS infrastructure will be reduced. Second, the electro magnetic compatibility problem, which is a very serious problem in ITSs based on RF signals, will be minimized since visible light and the conventional RF signals occupy different parts of the electromagnetic spectrum.
To adopt VLC in an ITS, the first requirement is to know the VLC channel characteristics in the ITS scenario. In the IEEE VLC standard , the technical consideration document outlines the “Types of Channels” for VLC in regards to automotive applications. However, it does not discuss any channel parameter details that describe the reference channel model. The related research papers use either a mathematical channel model that utilizes the simplified automotive environments [7–9], or use their own developed channel modeling simulators, which still contain many great simplifications and assumptions [10–12].
In this article, we present a VLC channel delay profiles based on simulated data regarding the practical ITS usage scenario. In order to describe the practical VLC channel more precisely, we considered the lighting regulation for transportation, such as the light distribution for vehicular LED headlamps and street lamps and other optical properties when designing the ITS scenario. For the simulations, two urban usage scenarios, crossroad and metropolitan street, were modeled using the CATIA V5 tool . Measurements for VLC channel delay profile evaluation were gathered using a Ray-Tracing scheme employing LightTools  software, a widely accepted commercial optical engineering tool, in vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) cases. These tools are efficient when determining optical channel models, since they provide visual understanding and enable the practical modeling of simulation environments. We used the Lambertian reflection property for the reflectance parameter of each object for interference effect and reflectance in order to provide a more realistic scene. It is known that the reflection pattern of most surfaces can correctly be approximated using the Lambert or the Phong models. If there are strong specular components in the reflection pattern of the surfaces, the Phong model approximates those patterns well unlike that of the Lambert model [15, 16]. However, the surface conditions in urban areas are irregular and reflect optical signals without favoring any particular direction. Therefore, because the reflection patterns are diffuse, they can correctly be approximated using the Lambertian reflection model. Finally, the channel delay profiles were evaluated from the measurements. From the derived channel delay profiles, we can observe that the common property of the delay profile was composed of dominant multiple line of sight (LOS) links and a less number of non-LOS (NLOS) delay taps. However, the delay profile for the V2I link and metropolitan scenario shows greater dispersive channel characteristics due to the reflection and diffusion of the visible lights.
2. The VLC channel environment modeling
2.1. The ITS VLC light source modeling
We model two practical LED light sources for VLC in the ITS, an automotive LED headlamp and an LED street lamp.
2.2. The ITS VLC usage scenario modeling
The first is a crossroad scenario composed of vehicles running in four directions, buildings, roads/fields, and traffic lights. Vehicles waiting for the traffic signals in crossroad can exchange information with the traffic lights and other vehicles, such as traffic information, vehicle conditions, etc. The second scenario is a metropolitan street, which has a longer street area, higher buildings, vehicles, and street lamps. The second scenario is the more important ITS service scenario, since many vehicles in urban streets will benefit from several types of traffic information in order to enhance the traffic flow efficiency. Other important service scenarios, i.e., a high-way scene, may easily be applied based on the metropolitan street scenario. Note that the two proposed scenarios consider both V2V and V2I communication links.
3. The VLC channel simulation and delay profile evaluation
3.1. The VLC channel simulation results
3.2. The VLC channel delay profile evaluation
The VLC channel delay profile of the crossroad scenario
Relative delay (ns)
Average intensity (dB)
Relative delay (ns)
Average intensity (dB)
The VLC channel delay profile of the metropolitan street scenario
Relative delay (ns)
Average intensity (dB)
Relative delay (ns)
Average intensity (dB)
From the tables, we can observe that the delay spreads from the metropolitan street scenario are more dispersive than those from the crossroad scenario because a great amount of diffused optical rays due to the interference objects produce NLOS delay taps with more dispersive delays. In addition, the V2I cases in both scenarios show more dispersive delay spreads than the V2V cases. This is because there are strong LOS optical ray components caused by the LED headlamp in the V2V cases.
Channel models are the essential baselines when we evaluate the basic performance of VLC systems through simulation. For the use of VLC in automotive ITS applications, we derived the VLC channel delay profiles based on the simulated data using LightTools and CATIA V5 software. These are efficient tools used to obtain optical channel models providing visual understanding and reflect practical modeling of simulation environments. For a more detailed modeling of the two urban usage scenarios (the crossroad and metropolitan scenarios), the shape and optical characteristics of the practical light sources (the LED headlamp and street light) and the infrastructure (buildings, roads/fields, vehicles) were used. From the derived channel delay profiles, we can observe that the common property of the delay profile was composed of dominant multiple LOS links and a less number of NLOS delay taps. However, the delay profile from the V2I link and metropolitan scenario had more dispersive channel characteristics due to the reflection and the diffusion of the visible light. Regarding future work, it will be required to measure a real VLC channel under the crossroad and metropolitan street scenarios and compare them to our simulated results. Because it requires a great deal of effort to measure a real channel, it will be meaningful to verify the validity of our channel evaluation schemes to confirm that it will provide a very useful tool for obtaining VLC channels in several operation scenarios.
This study was supported by the Yeungnam University Research Grant.
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