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2.1 Basic radar functions
Radar systems radiate electromagnetic energy into space through antennas or antenna arrays. The radiated electromagnetic energy "illuminates" surrounding targets. An "illuminated" target intercepts some of the radiated energy and reflects some back to the radar system. Radar systems utilize one or more receive channels to detect reflected energy to determine the range, speed, and relative angle of a target.
According to the different types of waveforms emitted by radar transmitters, radar systems can be divided into pulse radar and continuous wave radar. Pulse radar consists of a repetitive train of short duration pulses. The distance to the target is measured based on the time delay between the transmitted pulse and the received pulse. Unlike pulse radar, continuous wave radar usually emits electromagnetic waves continuously over a period of time. By comparing the received signal with the transmitted signal, the characteristics of the target are obtained. In automotive applications, continuous wave radar systems dominate due to their advantages in several aspects. Compared with pulse radar, continuous wave radar has the characteristics of low peak transmit power, simple structure, and high integration. It has a wide range of applications, especially in the automotive field. This chapter attempts to provide a comprehensive and consistent explanation of the basic principles of radar technology for automotive applications. Although many concepts are the same between pulse radar and continuous wave radar, in this book, more emphasis is placed on continuous wave radar.
The functions of automotive radar can be divided into detection, tracking and imaging. In this chapter, the focus is on detection, as well as the basic techniques of signal processing to perform the task. Tracking and imaging are discussed in the following chapters. For target detection, the fundamental problem is to determine whether the echo received by the receiver is a reflection from the object or just noise. For continuous wave radar, detection decisions are usually made by comparing the received echo signal amplitude to a threshold, which can be predefined or calculated in real time. For a robust radar system, thresholds need to be adaptively calculated from real-time radar data.
Continuous wave radar require some type of modulation in order to obtain the range of the target. Modulation is used to encode distance information into echo signals, and then the echo signals are extracted through signal processing. For example, chirped frequency modulated continuous wave (FMCW) radar encodes the range of a target into the frequency of a baseband signal. In phase modulated continuous wave radar, the range information is encoded according to the phase code sequence, and the phase code sequence is extracted by calculating the correlation between the echo and the original code sequence. Despite the various types of modulation, the range resolution (ΔR) of a radar is inversely proportional to the bandwidth (BW) of the transmitted signal:
（2.1）
In automotive radar applications, a larger BW usually facilitates better range resolution.
Radar can also use the Doppler effect to obtain the relative speed of the target. This is one of the main advantages of automotive radar over other automotive sensors such as cameras and LiDAR, which stands for "Light Detection and Ranging." The Doppler effect is the change in frequency or phase of electromagnetic waves relative to a target moving relative to the radar. It is named after Austrian physicist Christian Doppler, who described the phenomenon in 1842. The reason for the Doppler effect is that as a target moves toward a radar, each successive crest of an electromagnetic wave is reflected from a position closer to the radar than the previous crest of the electromagnetic wave. Therefore, each electromagnetic wave takes slightly less time to reach the radar than the previous electromagnetic wave. Therefore, the time interval between consecutive electromagnetic wave peaks reaching the radar is reduced, resulting in a reduction in the phase of the received electromagnetic wave. Conversely, if the target is far away from the radar, each wave is reflected farther from the radar than the previous wave, so the arrival time between successive electromagnetic waves increases and the phase increases. The radar can obtain the target's relative velocity by calculating the phase evolution between a series of emissions from the target.
For automotive applications, simply obtaining the relative distance and speed of a target is often not enough. In order to make correct decisions, such as emergency braking, the vehicle also needs to know the target's location in 3D space. As shown in Figure 2.1, the automotive radar measures the position of the target (P) in the spherical coordinate system. The +x axis is the axis view direction, usually perpendicular to the radar antenna plate. The angle φ on the x-y plane is the azimuth angle, and the angle θ is the elevation angle. In radar systems, there are several ways to obtain the azimuth and elevation angles of a target. One of the most well-known methods is the use of mechanical spinners [1]. In this method, a radar with a very narrow radiation beam is mounted on a rotator and mechanically scans the environment. The relative elevation and azimuth angles of the target are given by the position of the rotator. Mechanically scanned radar has a wide range of applications in the military; however, due to its bulk, it is not suitable for automotive applications, which require the radar to be compact and low-cost. Other angle measurement methods include phased arrays, digital beamforming, and multiple-input multiple-output technologies, which do not require any mechanical rotating structures and are highly integrated [1]. This chapter provides a brief introduction to phased array and digital beamforming technology. Multiple input and multiple output techniques are discussed in detail in the next chapter.
Figure 2.1 Spherical coordinate system for radar measurement