# Car Bumper Effects in ADAS Sensors at Automotive Radar Frequencies
## 2 Automotive Radar and Autonomous Vehicle  
Automotive radar has experienced a major evolution over the last decades as an essential part of the improvement of road safety systems [15], in the trend of increasing the importance of automotive electronics, in general, and ADAS, in particular. From the first road tests in the early seventies, radar-based safety systems have moved from luxury status to manufacturing in series for family vehicles due to the mass production of sensors. The first frequencies used for automotive radar were around 10 and 16 GHz [16]. However, the antennas had to be large to fulfill the requirement of narrow beam widths. After that, many other different frequencies were tested as follows [17; 18]: 35 GHz (selected due to the low atmospheric attenuation), 47 GHz, 60 GHz (especially in Japan) and 94 GHz (as there was military technology previously developed for this band, which reduces the cost of new designs or applications). Finally, the main emphasis was placed on the 24 GHz band, due to the low cost of sensors, and on the 77 GHz band, because of the reduced antenna size and larger bandwidth [16]. This last parameter becomes relevant when the number of sensors increases as more complex modulation schemes can be used to avoid mutual interference and ensure the coexistence of the different systems and vehicles. The earlier designs were based on Gunn oscillators, Schottky diodes, waveguides and horn antennas. Due to the arrival of GaAs Monolithic Microwave Integrated Circuits (MMICs) that can be integrated with microstrip antennas, more compact and lighter systems were possible [19]. SiGe MMICs allow multiple channels to be integrated into the same circuit, resulting in the incorporation of MIMO capabilities. Specifically, automotive radars have special requirements compared to other mm-wave transceivers, like supporting full-duplex operations in the same frequency band or fast chirping in a broad bandwidth [20]. With the improvements in automotive radar technology, the market of ADAS has continuously grown during the last ten years [16], automating an increasing number of driving tasks. The first application of automotive radar was the detection of vulnerable road users in blind spots. After that, the collision warning systems were followed by distance control and lane change assistance (in combination with image processing systems). In the currently used frequency band, the lowest frequencies (76-77 GHz) are intended for the so-called Long Range Radar (LRR), and the higher frequencies (77-81 GHz) for Short Range Radar (SRR) systems [21]. The LRR band is typically used to develop sensors for adaptive cruise control and collision detection systems, with typical operation ranges of up to 250 m. SRR sensors, on the other hand, are most often used for applications requiring high resolution, such as pedestrian or blind spot detection and lane change assistance. The requirements established for the automotive radar in the early research projects were 100 m range and 2.5\({}^{\circ}\) and 3.5\({}^{\circ}\) for azimuth and elevation beam widths, respectively [17]. The currently recommended characteristics for automotive radar in the 77 GHz band include:  
* Measurement ranges up to 50 m, 100 m or 250 m (depending on the application, as stated in the introduction);
* Bandwidths up to 1 or 4 GHz (for high-resolution applications);
* Maximum effective isotropic radiated power between 33 and 55 dBm;
* Antenna 3 dB-beamwidths (3 dB-BW) defined in Table 1[7; 22]. As observed in Table 1, the industry establishes differences between front and corner applications. Front applications comprise functionalities such as adaptive cruise control, and corner applications consist of those like blind spot detection. For example, current commercial frontal radars manufactured by _Bosch_ work in the range from 76 to 77 GHz, with detection ranges up to 302 m (depending on the model) and MIMO capabilities [23]. Bosch corner radars also work in the same frequency range with detection distances up to 160 m [24].