Vehicular optical ranging and communication system

2.1 Using LED as the transmitter

In most conventional ranging systems, lasers or laser diodes (LDs) are usually used as the transmitter. Lasers and LDs can output a higher energy beam, compared to LEDs, which means they can be operated at a longer range. However, lasers and LDs are costly to acquire and maintain. While LEDs can offer an acceptable performance with a small fraction of cost.

Compact size is another advantage of LEDs. And lasers and LDs require more drive current to produce higher peak power, which makes the peripheral circuits for modulation usually complicated.

where I _target is the beam intensity on the target, τ _a is the atmospheric transmission loss, P _t is the LED transmitted power, θ _t is the LED beam angle, and R is the target distance. Collimating the LED beam is one possible way to mitigate this problem. But the collimation procedure is very difficult because of the LED core sharp. It requires a large diameter lens, which is not feasible for the LED array.

Another way is to address this problem at the receiver end. We will give details in the next section.

2.2 Using the PD array as the receiver

By employing a narrow beam laser or LD as the light source, the conventional ranging systems feature good lateral resolution. In order to produce a full view of the scene ranging data, a precise scanning mechanism is essential. On the other hand, for communication purposes, a tracking algorithm should be proposed to track the laser head and maintain the link. This is usually difficult in a moving situation.

Instead of constraining the beam width, limiting the receiver FOV is another approach to increase the ranging lateral resolution. The imaging lens makes it possible by providing different narrow FOVs to the elements of the PD array as the receiver sensor [15]. It can capture the entire scene while making the receiving process highly directional. The incoming light from different directions will be imaged to corresponding PDs as long as they can be separated wide enough in the real space. This will also help reduce the ambient noise since we are only interested in several certain PDs sensing high signal power. The imaging-based receiver allows distance measurement without scanning, while offering high lateral resolution and frame rate.

For the communication application, these PDs not only give our system great potential to increase the data rate but also enable our system to distinguish one user from another since they will be imaged to different PDs. This also provides a very effective media access control (MAC) for the future vehicular optical network since different users can transmit simultaneously without disturbing each other.

Increasing the number of PD will absolutely increase the resolution, but it will also increase the requirement of the data bus since it needs higher readout speed. Because only a few PDs are receiving optical signal, we need a tracking algorithm to find these PDs [16]. More PDs mean that a more complicated algorithm is needed. This is a trade-off when designing the system, so we need to choose the number based on the application requirement.

2.3 Spread spectrum waveform

Even though the imaging-based method can greatly reduce the interference between users in the LOS communication link, there still exists the possibility that the backscattered signal could interrupt another user. Also, the signal could be corrupted by the noise. The spread spectrum (SS) code is robust against multiuser and noise. Employing the pulsed SS waveform as the ranging signal can reach a high peak power, which will bring a higher signal-to-noise ratio (SNR) and provide a longer operation distance, while maintaining a relative low average power. Low average power is crucial to eye safety which is usually the limit of the optical ranging system. And the optical communication system will also benefit from the SS code on rejecting the multiuser interference. There are several SS codes we could use in our system, and the performance may vary depending on different applications [17].

3.1 Principle of the system

The original idea of ranging and communication using the SS waveform was proposed by Mizui et al. [9]. However, they just used a laser at the transmitter end and a single PD at the receiver end, which will bring the problems as discussed before: costly, bulky, requiring a fine scanning mechanism, and complicated tracking algorithm. Furthermore, it can only measure the distance instead of the location and only support point-to-point operation rather than between multiple vehicles. We will compare the modified system with the originally proposed one.

Figure 1 demonstrates the block diagram of the proposed ranging and communication system. First, vehicle no. 1 transmits a pulsed SS signature waveform s ₁(t) to vehicle no. 2. Then, vehicle no. 2 captures the signal filtered by an image lens and renews its neighbor list table (Table 1). After this, vehicle no. 2 multiplies information bits (SS modulation) to the signature waveform and transmits back to vehicle no. 1. Vehicle no. 1 acquires the distance measurement by sensing the time of flight (TOF) between the emitted light and received light and then extracts the information bits.

Neighbor	Position	Distance
s 1(t)	(x 1,y 1)	d 1
…	…	…
s k (t)	(x k ,y k )	d k
…	…	…

This system can work under two different modes. Even though vehicle no. 2 is not equipped with this system, vehicle no. 1 can still acquire the distance via the reflecting signal.

In this process, vehicle no. 1 can determine not only the distance but also the location of vehicle no. 2, because the imaging method can provide angle information of the incoming light. And if two or more users receive the ranging waveform, then they respond to it simultaneously, and their signal will be imaged to different areas of the PD array. The interference between different users can be mitigated significantly. The reflecting signal power falling into the corresponding PD, which mainly senses the active responding signal, is also limited in the case that only one single user responds.

3.2 Optical wireless communication network

where N! means the factorial of N.

Figure 2 illustrates the probability with different users n and different SS code lengths N. We can see that the probability of no two or more users are using the same SS code decreases very quickly as the number of users in the network increases, even with a large code length N. Conflict of SS code will degrade ranging accuracy and communication performance. A mechanism is needed to reduce the conflict.

Also, several certain SS codes should be reserved for special applications. For example, some SS codes should be assigned to broadcast emergency information. And these codes are pre-known to all users so they can demodulate the information correctly.

Another problem we need to take into consideration is the interference between the ranging process and the communication process. Although the spatial diversity and the code diversity have been applied to distinguish different users, the communication signal still can corrupt the ranging signal for the same user because these two processes use the same SS code. Thanks to the imaging-based receiving method and the 2-D PD array, only several certain PDs are corrupted. The reflecting ranging signal can still reach on most of PDs and the distance information can be acquired. Also, the computer vision technique can be applied to compensate the corrupted ranging information.

The optical communication links mainly depend on the LOS path and can be regarded as highly directional transmission. These characters make it beneficial in dealing with the broadcast problem. The broadcast information flows in a certain direction and will not flow back, which will help mitigate the broadcast problem occuring in the RF method.