In the context of a safety assistance communication-based solution, OWC is a strong candidate for delivering the V2V communication power to cars. It is realized that the LED lighting is quipped to car mandatorily, meanwhile, a camera is considered as one of the indispensable sensors for car vision. The existence of supportive infrastructure enables the great potential for accepting OWC technology into the vehicle industry.
Imagine the vehicular environment, there are ambient lights from buildings, stars, and even traditional cars without OCC.
Such in a challenging environment as V2X, several technical questions must be answered:
- How to track and differentiate interested light-sources among others (including the sun/stars and ambient lights, building lamps, etc.)
- How to set up the link for high-rate transfer under mobility (wherein camera lens is a barrier)?
- What camera is applicable? Implementation & Market acceptance issue?
Figure 1 – Optical Wireless Communication in the vehicular scenario: interested light sources among thousand of noisy sources.
More explicitly, numerical considerations and requirements are to note such as Communication range, mobility, simultaneous links, data rate, link setup delay, camera frame rate, and so on…
Technical Performance Desired for Optical V2X
- Initial Considerations (or at least, desired performance for being applicable)
- Data rate/link: 20 kbps
- RoI camera frame rate: 10kfps
- Low-frame rate camera (30fps)
2. Performance Considerations (negotiable)
- Simultaneous links (e.g. 30 Tx) tracking and objects detection
- Mobility support: 110km/h
3. Performance Requirements
- Link initialization and switching: < 200 millisecond
- Communication Range: >100m for V2X
Figure 2 – Summary of Performance Considerations for Optical V2X
Considerations of Optical V2X using Camera
Figure 3 – Summary of Technical Considerations for an Optical V2X Solution
Figure 3 summarizes technical considerations for an Optical V2X Solution utilizing cameras.
Modulation rate greater than 200Hz
To support low image frame-rate processing (such as 30fps) in dealing flicker-free waveform
3. Nearly-point light sources
This is to operate at a challenging condition including far distance (>100m) and imperfect focus condition.
4. Mobility support
The identification of light sources (among other ambient lights) should be < 100ms.
5. Performance under dimming
The integration of high-rate data stream is with dimming so that the acceptable performance under dimming is required.
6. Camera compatibility
Compatible with either global shutter camera or rolling shutter camera.
7. Lighting infrastructure compatibility
Compatible with different types of lighting infrastructure such as single LED, multi-array LED, and Textured LED signage.
Reasons to promote Optical RoI signaling waveform
Figure 4 – Example of Optical V2X Use Case
Given the short discussion on the potential of V2V/V2X using light spectrum, numerous critical technical challenges remained unmitigated/uncovered. Among these circumstantial disadvantages requiring great effort, several ones are highlighted including,
- The selection and initialization of interesting links among thousands of possible artificial or natural light sources those are objected to eliminating. Rx should be able to detect the active light sources and initialize the communication with an acceptable-short delay. The traditional solution for object detection (such as car and its light sources detection) relying on a conventional computer vision seems to be improper for OCC link setup because of its detection accuracy, its complexity with processing time delay. Luckily, a smart camera with a RoI mode that can control its frame rate and imaging-related parameters is an enabler to set up the communication link for better communication quality. Such an intention of RoI camera usage to unleash its potential benefits for communication is desired.
- In addition to (1), the barrier for technical feasibility of the light source detection and link setup might come from the camera itself. Because of the imaging lens, even the best quality camera cannot perfectly focus on all the light sources simultaneously at different distances away from its position. This also means that the feasibility of the concept depends on whether OCC can work on the very-blurred image (in which light sources are out-of-focus) or not.
- The asynchronous communication mode must be supported for a wider range of applicable usages with the least unidirectional communication. More explicitly, at the receiver car, a camera just makes sampling, and the sampled image sequence should be decodable. Also, while a car is communicating with its paired car in a private communication mode, the other cars should also be able to identify the two cars and asynchronously receive a minimum data set in a public communication mode for safety assistance. The dedicated assignment/combination of the private mode and the public mode of communication supporting asynchronous reception is a hunger; however, it remains an unmitigated issue.
- High mobility is the specific requirement for V2V/V2X; however, this is one of the most challenging tasks because all the implementation of the above technical considerations jointly in such a challenging mobility condition is exponential times of complexity.
- The communication distance requirement for different traffic conditions has been discussed above. A required inter-vehicular distance for communication is up to 200m for highway or rural areas. At this far distance, the image of light sources becomes as small as nearly-point; so that the communication must support a nearly-point communication protocol. Recall that the demanding mobility is still included.
- Illumination is still a primary purpose of any light source no matter traffic lights, signage lights, or vehicular lights. So that the quality of modulated light intensity must adhere to the industrial light regulars, including the flicker-free requirement. Typical VLC PHY modes in IEEE 802.15.7-2011 operates at higher than 10kHz optical clock rate to ensure its spectrum interference to the perceptible light spectrum of human is acceptable, being a flicker-free modulation. Regardless of whether VLC or OCC is designed, the sampling rate must follow the Nyquist sampling theory. Still, a typical camera which relies on its frame rate for sampling cannot satisfy the Nyquist rate of above 10 kHz, so that the undersampling concept is the only rescue solution.
- In addition to (6), the light quality may require more than just flicker-free property. High-resolution dimming is necessary for particular scenarios. Certainly, the support for dimming is one of the technical considerations in any VLC/LiFi specification, and OCC is not an exception.
- The last-but-not-least consideration is about whether the proposed technology is compatible with a variety of hardware available on the market or not. This consideration is not about the technical feasibility but its success potential and economic feasibility. A designed OCC waveform should support either a global shutter camera type or a rolling shutter camera type or both. Also, it should consider different imaging-related parameters such as frame rate, shutter speed, resolution, etc. Also, the designed OCC waveform should apply to a wide range of light infrastructures of different size, shape and illumination characteristics.
The considerations mentioned above do not cover the need for a perfect vehicular OCC system entirely. These challenges uncover the need for a new technology in the field of Optical Wireless Communication called Optical RoI Signaling. In a short explanation, the RoI signaling stream is sent in prior to the high-rate connection as a reliable link setup procedure.
There is no universal agreement on the definition of Optical-RoI-Signaling yet. An understandable sentence (my definition) is:
“Optical-RoI-Signaling is an OWC solution using the RoI-camera for boosting the data rate and providing the reliable link setup procedure via the base-knowledge (e.g., (1) the ID of the light source, (2) the modulation format used next) being transmitted.”
As addressed, Optical-RoI-Signaling is a novel solution for Optical Wireless Communication in dealing with a challenging environment such as vehicular scenario. We will discuss particular solutions and compare them in a next post (see one here).