Requirements for Radar System for automotives for ADAS applications

Why Radar?

The radar sensor detects objects and measures their relative velocity and position. For that purpose, the sensor has four antenna elements that simultaneously transmit radar waves in the frequency range between 76 and 77 GHz. These waves are reflected by objects. By comparing the amplitudes and phases of the signal echo received by the antenna elements, precise conclusions on the objects’ position can be drawn. The relative speed and distance of objects is measured using Doppler Effect (shift in frequency between the reflected and transmitted signals) and time lag.

The radar sensor can be applied to a series of functions which can also be accomplished parallell. By the identification of objects in the driveway the radar sensor is able to measure the distance and the relative speed of these objects. The subsequent object classification in relevant and non-relevant barriers enables the recognition of critical situations. The radar enables the realization of an intelligent, predictive and adaptable warning system for innovative safety functions. Thereby measures to avoid accidents or reduce the risk of injury are triggered in critical situations. In addition a precise determination of speed over ground and the integration of the radar into assistance systems for the optimization of the efficiency are possible. Via intelligent networking of the radar sensor with components and systems or by integrating information of other sensors such as a camera or additional radar sensors we enable new applications or enhancements of existing functions. Through the use of multiple sensors and components we further increase the safety and productivity without increasing cost.

Type and the Number of Radars needed (for a 360° view):

1.      Long Range Radar – 1

2.      Mid-Range Radar – 2

3.      Short Range Radar – 6

Requirement for Long Range Radar:

We need one LRR at the front of the vehicle to cover a range up to 250 m, especially for ACC (Adaptive Cruise Control), which will help the EV to maintain a specified distance and relative speed with respect to the vehicle moving ahead.

Requirement for Mid-Range Radar:

In total we need 2 Mid-Range Range Radars with a range of 100 m, one at the front of the EV and another at the back side. In front we need a radar for EBS (Emergency Breaking System) and for a more detailed and greater azimuth coverage of the front. It will also be used for effective detection of Pedestrian, road sign, manholes etc. It will also assist the Automatic Parking System. The Radar at the back will be needed to measure the relative speed and the distance of the vehicle coming from back in a range of 100 m, which give necessary warning and will also assist lane change if required.

Requirement for Short Range Radar:

In total we need 6 Short Range Radars with a range up to 15 m, two at the front sideways, two at the mid of the vehicle  on both the sides (the one between the front and back seat on four-door cars), and two at rear sideways. The front and rear sideways radar will help us in Blind Spot Detection, lane departure warning, Automatic Parking System etc. The two at sideways will give a more surround view, which will help us to detect the pedestrians and vehicles moving from the sideways at the places like a four-way crossing etc.

Principle to be used:

The FMCW (Frequency Modulated Continuous Waveform) Radio Detection Principle has been developed and used over many years to accurately measure targets at short distances. Frequency-Modulated Continuous-Wave (FMCW) Radar is a special type of radar radiates continuous transmission power like a simple continuous wave radar (CW-Radar). We can control FMCW-radar frequency, which can help us to evaluate the object’s range. CW-Radar has the disadvantage that we cannot use it to evaluate the object’s range because its frequency is stable .The received signal in FMCW is a time-delayed copy of the transmitted signal where the delay , is related to the range. Because the signal is always sweeping through a frequency band, at any moment during the sweep, the frequency difference , is a constant between the transmitted signal and the received signal.   is usually called the beat frequency. Because the sweep is linear, one can derive the time delay from the beat frequency and then translate the delay to the range. Refer the link mentioned below for more information. http://www.radartutorial.eu/02.basics/Frequency%20Modulated%20Continuous%20Wave%20Radar.en.html

Frequency Band for Operation:

The two bands available for automotive use are 77 and 24 GHz, but 24GHz frequency band has a few limitations. These include the potential for interference with radio astronomy and satellite services and also disadvantage of having bandwidth limitations of 200 MHz bandwidth due to other usage of the ISM spectrum, as a result, these radars will be phased out of new vehicles by 2022 in Europe. Although in India we have only 76-77 GHz band available but soon the entire band will be made available for the automotive in India also. The 77GHz band (from 76-81GHz), often called long-range radar (LRR), is the future for automotive radar. The technical advantages of the 77GHz band include that the higher frequency pairs effectively with a smaller antenna. The relationship between the antenna size and the frequency is linear, so 77GHz systems will need antenna sizes a third of the size of the current 24 GHz ones; in case you were wondering, the antenna is typically mounted in the vehicle bumper. The 77GHz devices also benefit from benefit from higher permitted transmit power. Perhaps most important, the wider bandwidth available in the 77 GHz band enables greater accuracy and, as a result, provide drivers with better object resolution. With some clever signal processing, this enhanced resolution will improve the detection and avoidance of big objects, like cars, and allow the avoidance of smaller ones, like pedestrians, too. This will complement the machine-vision systems that can already provide this accuracy but struggle in situations with poor visibility.  Due to the width of available frequencies, a move from 24GHz to 77GHz can achieve 20x better performance in range resolution and accuracy. The range resolution of a 77GHz system can be 4cm versus 75cm for 24GHz radar, allowing better detection of multiple objects that are close together.

Required Bandwidth:

Larger bandwidth gives greater accuracy, higher object resolution and better velocity resolution. For LRR and MRR the bandwidth needed will be 500 MHz, and for SRR the bandwidth needed will be 4GHz, as more bandwidth gives us a better dynamic range.

Range Resolution Requirements:

Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Pulse width is the primary factor in range resolution. A well-designed radar system, with all other factors at maximum efficiency, should be able to distinguish targets separated by one-half the pulse width time. Therefore, the theoretical range resolution of a radar system can be calculated from the following formula:

As the bandwidth increases, the achievable range resolution is decreasing and this means the monitored objects can be seen more accurately. The maximum detected range becomes smaller when the bandwidth increases. For LRR the Range Resolution should be at least 1m at a max. Range of 250 m. For the MRR it should be around 0.5m, and for SRR it should be less than 0.1m, so that we distinguish two closely spaced targets.

Velocity Resolution:

Though radars provide highly accurate information about the velocity and range, the LRR should have velocity resolution less than 2.5 kmph. The MRR with a velocity resolution of 1 kmph and SRR with a 0.5 kmph velocity resolution. The velocity resolution can be increased simply by increasing the frame time with no additional hardware cost.
Azimuth: The SRR should be able to sweep and angle of ±75°. The LRR upto and angular sweep of ±5°, whereas the MRR ±45° will serve the purpose.  
Elevation Angle:

The LRR with an elevation of ±3° can detect an object of height 5.24 above and below the ground at a distance of 100 m in front of EV.

Velocity Accuracy:

Radar are accurate enough to achieve a relative velocity with an accuracy of 0.1 m/s.

Angular Accuracy:

The LRR, MRR and SRR are desired to have and angular accuracy of 0.1°, 0.5° and 1° respectively.

Distance Accuracy:

The LRR, MRR and the SRR must measure distances with an accuracy of 1m, 0.1m and 0.02m respectively.