# A Systematic Review of Perception System and Simulators for Autonomous Vehicles Research
## 2 Environment Perception System
### LiDAR (Light Detection and Ranging)  
LiDAR systems were initially developed in the 70s to measure elements in sea or land from satellites or airplanes. It was specifically developed for the detection of submarines by the American Navy. LiDAR systems base their operation on the measurement of the time of flight of a pulsed light emitted from a laser diode until it is received by an emitter. The emission are in infrared ranges (905 nm or 1550 nm). Emissions at 905 nm require less energy than those emitted at 1550 nm because the water in the atmosphere begins to absorb energy from 1400 nm. This initial disadvantage of power increase at 1550 nm is used by the aqueous liquid of the eye to totally filter this wavelength, making them less harmful than LiDAR at 905 nm [10]. Lasers used for vehicles belong to Class 1 [11] and are safe under all conditions of normal use. LiDARs use the ToF principle to carry out the measurement of distance between emission and reception. These can be classified according to the type of information they obtain from their environment in 2D or 3D LiDARs or it can be classified according to their construction rotary or solid state LiDAR. 2D LiDAR obtains information from the environment by projecting a single laser beam on a rotating mirror perpendicular to the axis of rotation (see Figure 5a). The 3D LiDAR allows to obtain a 3D map of great accuracy to be obtained of the environment; for that purpose, they use a set of diodes lasers mounted on a pod that rotates at high speed (see Figure 5b). The number of lasers installed in the pod determines the accuracy of the point cloud obtained in each turn. Currently we can find 3D LiDARs that integrate from 4 to 128 lasers or channels with a horizontal FOV of 360 grades and vertical FOV that oscillates between 20-45 grades with accuracy of a few centimetres. Depending of the number of channels, 3D LiDAR are used in Adaptive Cruise Control (ACC), object avoidance, object identification or 3D mapping. LiDAR is affected by weather conditions such as rain, snow, fog or dusty environments due to the diffraction of light in these environments. Furthermore, they reduce their operating range detection depending on the reflectivity of the objects that are reached by the laser beams. The maximum detection capacity depending on the type of reflectivity of the material to be detected is presented in the datasheet provided by the manufacturer.  
Figure 5: Operating schemes: (**a**) Rotating 2D LiDAR, (**b**) rotating 3D LiDAR, (**c**) solid state 3D LiDAR.  
The last kind of device based on laser measurement that has arrived in the autonomous vehicles world is the solid state LiDAR. Solid state LiDAR allows a 3D representation of the scene to be obtained around the LiDAR without the use of mobile parts in the device. A micro mirror MEMS circuit carries out the synchronization with a beam laser to scan the horizontal FOV in multiples lines. For that, the micro-mirror reflects the beam over a diffuser lens, which creates a vertical line that touches the objects (Figure 5c). The light reflected is captured by a lens and is sent to a photodetector array to build the first line of a 3D matrix. The process is repeated until a point cloud of the scene is created. This feature notably increases its durability, reduces maintenance tasks, and decreases its price. Solid state has a smaller FOV than the rotary LiDAR. The trends in perception system are replacing the current rotating 3D LiDAR by a set of solid state LiDARs integrated around the vehicle.