# Real-Time Hybrid Multi-Sensor Fusion Framework for Perception in Autonomous Vehicles
## 3 Autonomous Vehicle Perception Sensors
### Radar  
Radar stands for Radio Detection and Ranging. Radar works on the principle of transmitting and detecting electromagnetic (EM) radio waves. Radars can measure information about the obstacles they detect like relative position based on the wave travel time and relative velocity based on the Doppler property of EM waves. The frequency of radars EM waves are not affected by different lighting or weather conditions; thus, they can detect at day or night, in foggy, cloudy, or snowy conditions. Automotive radars are usually mounted either behind a bumper or in the vehicle grille; the vehicles are designed to ensure the radar performance (the radar antenna ability to focus EM energy) is not affected no matter where they are mounted. For automotive applications, Frequency-Modulated Continuous Wave (FMCW) type radar is typically used [29]. FMCW radar transmits continuous waves or chirps in which the frequency increases (or decreases) over time usually in a sawtooth waveform.  
Figure 3: Camera sensor processing pipeline.  
These radars also typically work at frequencies of 24 GigaHertz (GHz), 77 GHz and 79 GHz. The GHz frequency corresponds to millimeter wavelengths; hence, they are also called Millimeter Wave (MMW) radars. There are three major classes of automotive radar systems depending the application: SRR (Short-Range Radar) mostly for parking assist and collision proximity warning, MRR (Medium-Range Radar) mostly for blind-spot detection, side/rear collision avoidance and LRR (Long-Range Radar) for adaptive cruise control and early collision detection mounted upfront. FMCW radar hardware consists of multiple components as shown in Figure 4.  
The Frequency Synthesizer generates the FMCW wave or chirp at the desired frequency. The wave is then amplified using a power amplifier depending on the radar class and the desired distance coverage for the radar. The further the desired range, the higher the power amplification needed for the signal. The transmit (TX) antenna converts the electrical energy of the signals to electromagnetic signals and transmits them through the air. The receive (RX) antenna receives the reflected signal. Field of View (FoV) of radar is determined by the antenna type, pattern, and gain value, discussing which is out of the scope of this paper. The antenna also increases the power of the signal by focusing the energy in a specific direction. The received signal is amplified through a Low Noise Amplifier (LNA) since some of the EM energy is attenuated by moving through air and the targets it hits during the signal travel. The amplified received signal is sent to a mixer. Mixer multiplies the reflected signal by the original signal generated by the synthesizer. Through this operation, mixer changes the frequency of the EM wave to the difference of the original signal and reflected signal frequency. The mixer will then output the delta frequency, also known as frequency-shift or intermediate frequency (IF). The output of the mixer is converted from analog to digital using an analog digital converter (ADC) so we can further process the signal for target detection and tracking.