Radar target generation and detection - Hardware

Github: https://github.com/williamhyin/SFND_Radar_Target_Detection

Why Radars?

The autopilot system is perceived by the input of a plurality of sensors, each sensor has its own advantages and disadvantages, we use to compensate for the disadvantages of other radar sensors. Biggest advantages is that it can directly measure the radar radial velocity, although laser , cameras, other sensors need to be calculated by derivation of the speed, it can be directly determined speed radar, which also helps to distinguish the moving object and a stationary object. another advantage is that the radar is rarely influenced by environmental factors, may be complicated illumination, rain work around the clock under snow conditions.

FMCW automotive radars

Radar early in the 1930s has been put into use by the military when they are mainly used to detect aircraft, but since then, radar technology has made considerable progress. Today, they are increasingly being used as an advanced driving the auxiliary systems (ADAS) automotive radar sensors in the image below shows the use of a set of radar sensors and other sensors in the vehicle:

Vehicle sensor configuration (source: cdn.rohde-schwarz.com)

RF technology and advances in digital signal processing technology makes the low cost, small size, high efficiency becomes possible radar.

Radar has the ability to accurately measure the impact of long-range target speed and accurate spatial information, at the same time without fear of bad light and weather, and therefore become an important autopilot sensor applications. Most cars are equipped with radar for adaptive cruise control of the ACC a sensor for detecting a vehicle is too close, keep their distance and speed to follow the vehicle in front. with the improvement of autopilot level, more and more radar sensors are used.

The development trend of radar sensors

As shown below using the radar transmission and detection of electromagnetic waves work:

Electromagnetic waves are reflected when the obstacle. If these reflected waves are received again in their origin, then this means that an obstacle in the propagation direction for the influence from dark radar frequency electromagnetic energy, can be penetrate clouds and fog. this makes it possible to determine the position of the radar system because the distance the like, or dark weather invisible target road modern radar can extract more information than a distance signal from the target echo signals.

Radar principle (Source: electricalelibrary )

Automotive radars used not so large volume of military radar, the sensor is small, it can be easily mounted below the front grille or bumper as shown below, a different radar component parts.

Bosch Automotive Radar

Radome: the radome is a structural, weatherproof housing, to protect the radar antenna radome to avoid excessive weakening of the material or an electromagnetic wave transmission antenna an electromagnetic wave received, for the actual radio wave. on is transparent.
The printed circuit board radar: It is analog hardware, including a radar transceiver and radio waves to produce the desired antenna.
A printed circuit board and the process: This includes digital signal processing (DSP) unit

Radar automotive applications in the millimeter-wave radar autopilot areas are mainly three bands, namely, 24GHz, 77GHz and 79GHz, generally divided into long-range radar and short-range radar, long-range radar can measure objects as far as 250m, but vision is very small, and short-range radar is designed to have a larger field of vision, but can not see far.

Radar vs Deal

Comparative radar and lidar [Source: cleantechnica ]

Radar and Lidar is a hot topic of dispute, but in fact, these sensors are complementary to the target using the laser radar can produce high-resolution imaging of the reflected laser beam. The shape and capable of providing accurate position information, but in a bad lidar weather conditions will not work because small wavelength does not allow work well in fog or rain. in addition, the laser radar sensor is an expensive, the cost ranges from $ 35,000 to $ 100,000, as of 2019. is lidar the technology of choice Waymo the radar does not have the ability to produce high-resolution images, but it has a high accuracy rate estimates based on the Doppler phenomenon. in addition, the radar wavelength allows it to target perceived in bad weather conditions. the most important thing is low the manufacturing cost of the radar. the radar unit may be a cost as low as a few hundred dollars, which may be deployed such that a plurality of car manufacturers radar sensor 360 sensed. Tesla as dependent on its primary radar sensors, lidar and not included in its sensor fusion system. that is money the more the better, no money carefully selected.

Radar Properties

Next, a general overview see you will see the signal characteristics, including the definition of the general form and Equation signal wavelength signals

Single wave parameters

** wavelength (the lambda) ** physical length from a point to the next point with a wave of a wave, calculated as follows:
$\lambda=\frac{\text { speed of light }}{\text { frequency }}$
** Frequency (Frequency) is the number of waves per second through in hertz (GHz) units higher the frequency, the shorter the wavelength w automotive radar generally operates in a band (76GHz - 81GHz).. This signal is called frequency. millimeter-wave, because the wavelength of millimeter. a signal of bandwidth (bandwidth) ** is the difference between the highest and lowest frequency component of the continuous band.

** Amplitude (Amplitute) ** is the strength of the signal. It is generally defined as the RF signal power / electromagnetic field (dB / dBm), associated larger configuration radar sensing output power and the amplitude of the received signal radar signal, the radar visibility the higher the car radar can operate at maximum 55dBm output power (316w).

Frequency signal

Amplitude of the signal

Phase of the signal

** Phase (Phase) ** is a specific time point of the waveform cycle, measured in radians. A complete cycle is 360 °. Frequency may also be defined as a phase with respect to time, frequency and phase of the first-order signal Derivative.
$\begin{aligned} &\text {frequency}=\frac{d \varphi}{d t}\\ &\varphi=\text {phase of the signal} \end{aligned}$
This Doppler characteristics of moving targets for measuring the frequency shift. Graph showing the phase.

Phase sine wave

Referred to as phase two periodic signals phase difference when the difference is zero, said two signals are in phase, or in phase with each other.

General wave equation

$y(t)=A \cos \left(2 \pi \cdot f_{c} \cdot t+\phi\right)$

A is the amplitude of the signal
signal frequency is f_c
φ is the phase of the signal

FMCW

FMCW radar FMCW (Frequency-Modulated Continuous Wave radar) radar sensor is a continuous transfer of energy emission. FMCW radar has the ability to measure very close to the target and the target range and the ability to simultaneously measure the relative speed, which makes it an automobile the preferred type of radar.

FMCW waveform

Frequency Modulated Continuous Wave (the FMCW) is a frequency decrease signal over time as shown above, they are also referred upramps and downramps . FMCW radar two most common ways are sawtooth waveform and a triangular wave. A sawtooth waveform Usually only up ramps, ramp and the triangular waveform using the uplink and downlink ramps.

We often need to discuss called chirp ( Chirp radar signal), which is the frequency increase or decrease over time, increasing frequency, called up a Chirp- frequency drop is called Down-a Chirp . Many radars use repetitive chirp signals to calculate distance and speed chirp slope has a frequency bandwidth B_sweep (y-axis) and chirp time T_s (x-axis) determined.
$\text {Slope}=\frac{B}{T_{s}}$
Radar range resolution depends on B, and the maximum speed capability depends T_s.

A chirp sequence includes a number of a Chirp , in order to accurately measure the Doppler velocity, for each chirp sampled multiple times at multiple distance measurements, transmitted by the radar.

FMCW Hardware Overview

FMCW radar hardware implementation

Frequency Synthesizer: In automotive radar, the frequency synthesizer is to produce a frequency that the frequency chirp has reached the 77ghz element.
Antenna: an antenna converts electrical energy into an electromagnetic wave, electromagnetic radiation in the air, hit the target and reflected back to the radar antenna by a receiving antenna in a desired direction focus the energy to increase the strength of the signal The antenna pattern determines the apparent radar. field.
Mixer: In FMCW radar, the mixer mixer the echo signal with the frequency sweep signal generated by a frequency synthesizer subtracted to obtain frequency Delta- also called frequency shift signal or an intermediate frequency (IF).
```
IF = Synthesizer Frequency - Return Signal Frequency
```
Processor: The processor is all digital signal processing, detection, tracking, processing unit and other clustering algorithms that occurred during the unit may be a microcontroller, or even a. The FPGA .

FIG antenna

FIG antenna

The antenna pattern is the relative field strength geometric pattern antenna. Beamwidth of the antenna determines the field of view of the radar sensor. If the requirements for the radar only detect targets on the own lane, then the beam width must be small enough to cover the entire lane to achieve the desired range. If the beam width is greater than the width of the lane, it can also be perceived target other lanes. the antenna radiation includes not only the main beam, also includes side-lobe antenna side lobe is critical, because they can produce false alarms, and selection of interference from the undesired direction. As shown, the antenna side lobes pointing in different directions, can sense target is not within the main beam. in order to avoid detection of side-lobe, side lobe level necessary to suppress the peak of the main beam more than 30dB.

Antenna Type

In the most common automobiles 77GHz radar antenna is a patch antenna type. Patch array antenna having a low cost, simple manufacturing advantages, like low profile, ideal for automotive radar applications.

Patch array antenna

Radar Cross Section Overview

A radar energy reflecting target size and capacity is defined by a single, called Sigma, its unit is m ^ 2 square meters, the radar cross section is referred to. The display unit is a radar cross section area. Radar cross-sectional area depending on:

Geometry and physical characteristics of the target's outer

Smooth edge or surface waves cause scattering in all directions, thereby reducing the radar cross section, however, it makes an acute angle with the direction of the echo signal focused to the source, resulting in higher the RCS.
Radar irradiation direction
Radar transmitter frequency
Used in cars, trucks, bicycles materials, sometimes even for pedestrians clothing materials

RCS target vehicle

RCS unit conversion

RCS may also be used (dB) defined value, the following conversion method.
$R C S_{d B}=10 \log \left(R C S_{m^{2}}\right)$
Different targets have different RCS, we can convert them.

Radar Range Equation

Using the radar range equation may be designed to meet the requirements from the action of a radar transmitter, the receiver and antenna, its power, gain and noise performance has desirable.
$R=\sqrt[4]{\frac{P_{S} \cdot G^{2} \cdot \lambda^{2} \cdot \sigma}{P_{E} \cdot(4 \pi)^{3}}}$

R - maximum distance radar target
P_s - radar transmit power (dBm)
G - gain of the transmitting / receiving antenna (dBi) of
[lambda] - wavelength of a signal (m) of
[sigma] - RCS (m ^ 2)
P_E - Radar can detect a minimum power

The following figure shows the overall change in radar, radar signal intensity level

Transmitter power
The power amplifier to further improve the strength of the signal - emission chain Gain
Further amplifying the signal using an antenna
Unidirectional path loss represents a loss when the signal moves to a target signal strength
When the object reflector, the signal is amplified based on the target RCS
When the RCS gain back to the radar signal, and a similar loss in strength progresses
Receiver antennas before transmitting the return signal to the processing unit amplifies