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.
However, the wide beam angle is the main drawback of LEDs. In contrast to the beam angle of lasers or LDs, which is usually several milliradians, that of LEDs with a narrow beam is about 0.1 radians. This will bring in two problems: (1) the power density (power/area) decreases quickly as the measuring distance increases, and (2) lateral resolution is very low. We can see from the propagation function (Eq. 1) that the power intensity diminishes quickly over distance [14].
$$ I_{\text{target}}=\frac{4\tau_{a}P_{t}}{\pi{\theta_{t}R}^{2}} $$
((1))
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.
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.
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].