**2.1. Modulation scheme**

The automotive 24 GHz radar sensor allows a simultaneous and unambiguous measurement of target range *R* and radial velocity *v*r even in multiple target situations. This is achieved by combining the advantages of the Frequency Shift Keying (FSK) waveform and the Frequency Modulated Continuous Waveform (FMCW) in a so called Multi Frequency Shift Keying (MFSK) waveform [19], which is already used in commercial automotive radar sensors to enable Adaptive Cruise Control (ACC) or Blindspot Detection (BSD) [20], [21]. Applying an FSK waveform, the target range *R* and radial velocity *v*r can be measured. However, there is no range resolution. Multiple objects measured at the same spectral line in the Doppler spectrum result in an unusable range information, as the determination procedure assumes a single target. To mitigate this drawback, the FMCW waveform resolves targets in range *R* and velocity *v*r. Limitations will occur in this case in multi target situations due to ambiguous measurements. The specific MFSK waveform is applied in the 24 GHz Radar sensor for a range and Doppler frequency measurement even in multi target situations with a bandwidth of *f*sweep = 150 MHz and a resulting range resolution of Δ*R* = 1.0 m. It is a classical step-wise frequency modulated signal with a second linear frequency modulated signal in the same slope but with a certain frequency shift *f*step integrated into this waveform in an intertwined way. The chirp duration is denoted by *T*CPI = 39 ms which results in a velocity resolution of Δ*v* = 0.6 km/h.

It is important to notice that this waveform is not processed by a matched filter or analyzed by an ambiguity function. Instead it is processed in a non-matched filter form to get an unambiguous and simultaneous target range and Doppler frequency measurement with high resolution and accuracy. The echo signal of the stepwise and intertwined waveform is downconverted by the corresponding instantaneous transmit frequency into baseband and sampled at the end of each short frequency step. This time discrete signal is Fourier transformed separately for the two intertwined signals to measure the beat frequency *f*<sup>B</sup> which is simultaneously influenced by the target range *R* and radial velocity *v*r.

2 Will-be-set-by-IN-TECH

24 and 77 GHz band are also strong candidates for automotive safety systems. Compared with vision systems, they have some additional important advantages of robustness in all weather conditions, simultaneous target range and radial velocity measurement and a high update rate. These properties are especially important for pedestrian recognition, as the object

Object class? Object class?

**Figure 1.** Daily traffic situation in an urban area with an oncoming vehicle and pedestrians walking on

This chapter presents the modulation scheme of an automotive radar sensor and explains the features of pedestrians and vehicles by which a robust classification is possible in an urban

The automotive 24 GHz radar sensor allows a simultaneous and unambiguous measurement of target range *R* and radial velocity *v*r even in multiple target situations. This is achieved by combining the advantages of the Frequency Shift Keying (FSK) waveform and the Frequency Modulated Continuous Waveform (FMCW) in a so called Multi Frequency Shift Keying (MFSK) waveform [19], which is already used in commercial automotive radar sensors to enable Adaptive Cruise Control (ACC) or Blindspot Detection (BSD) [20], [21]. Applying an FSK waveform, the target range *R* and radial velocity *v*r can be measured. However, there is no range resolution. Multiple objects measured at the same spectral line in the Doppler spectrum result in an unusable range information, as the determination procedure assumes a single target. To mitigate this drawback, the FMCW waveform resolves targets in range *R* and velocity *v*r. Limitations will occur in this case in multi target situations due to ambiguous measurements. The specific MFSK waveform is applied in the 24 GHz Radar sensor for a range and Doppler frequency measurement even in multi target situations with a bandwidth of *f*sweep = 150 MHz and a resulting range resolution of Δ*R* = 1.0 m. It is a classical step-wise frequency modulated signal with a second linear frequency modulated signal in the same slope but with a certain frequency shift *f*step integrated into this waveform in an intertwined way. The chirp duration is denoted by *T*CPI = 39 ms which results in a velocity resolution of

It is important to notice that this waveform is not processed by a matched filter or analyzed by an ambiguity function. Instead it is processed in a non-matched filter form to get an unambiguous and simultaneous target range and Doppler frequency measurement with

area from a moving vehicle with a mounted 24 GHz radar sensor, see Figure 1.

classification should be available immediately and at any time.

the sidewalk.

Δ*v* = 0.6 km/h.

**2.1. Modulation scheme**

**Figure 2.** MFSK waveform principle with two intertwined transmit signals.

$$f\_B = -\frac{2v\_\mathrm{r}}{\lambda} - \frac{2R \cdot f\_{\mathrm{sweep}}}{\mathrm{c}} \cdot \frac{1}{T\_{\mathrm{CPI}}} \tag{1}$$

In any case, a single target will be measured and will be detected on the same spectral line at position *f*<sup>B</sup> for the two intertwined signals. Therefore, after the detection procedure the phase difference ΔΦ between the two complex-valued signals on the spectral line *f*<sup>B</sup> will be calculated. The step frequency *f*step between the intertwined transmit signals determines the unambiguous phase measurement ΔΦ in the interval [−*π*;*π*). This phase difference ΔΦ again is influenced by the target range *R* and radial velocity *v*r described in Equation (2).

$$
\Delta\Phi = -\frac{2\pi}{f\_{\text{sample}}} \cdot \frac{2v\_{\text{r}}}{\lambda} - \frac{4\pi R \cdot f\_{\text{step}}}{c} \tag{2}
$$

The target range *R* and radial velocity *v*r can be determined by solving the linear equation described in Equation (1) and (2) in an unambiguous way. In this case, ghost targets are completely avoided since this waveform and signal processing combines the benefits of linear FMCW and FSK technology. The system design and the sensor parameters can be determined like in a linear FMCW radar system. The range and velocity resolution Δ*R* and Δ*v* are determined by the bandwidth *f*sweep of the radar sensor and the chirp duration *T*CPI as described in Equation (3) and (4), respectively.

#### 4 Will-be-set-by-IN-TECH 244 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications Pedestrian Recognition Based on 24 GHz Radar Sensors <sup>5</sup>

$$
\Delta R = \frac{c}{2} \cdot \frac{1}{f\_{\text{sweep}}} \tag{3}
$$

pedestrian, the velocity profile is less extended due to the moving direction of the pedestrian. Furthermore, the extension depends mainly on the azimuth angle under which the pedestrian is measured. In contrast, the radar echo signal in case of a vehicle shows a very narrow (*point*

Additionally, a *point shaped* range profile will occur in the case of a longitudinally or laterally moving pedestrian as the physical expansion is small compared to the range resolution of Δ*R* = 1.0 m. In contrast, a vehicle shows an *extended* range profile, due to several reflection points spaced in several range cells. The measurement result of a single observation is shown

0 4 8 12

Rprofile,ped

v

profile,veh

Pedestrian Recognition Based on 24 GHz Radar Sensors 245

Rprofile,veh

Velocity [m/s]

Under the use of an MFSK modulation signal, a range profile and the velocity profile can be extracted from a series of received signals as shown in Figure 4. As an example, four consecutive range and velocity measurements are depicted in a range Doppler diagram. The red dots show a longitudinally walking pedestrian, the blue crosses an in front moving vehicle. The figures depicted are based on radar measurements taken in an urban area with an ego speed of 50 km/h. It can be observed that neither velocity profile nor a range profile can be seen in the first measurement, consequently, those feature values are zero. In the second measurement, however, several range and velocity measurements allow to calculate an extended range profile for the vehicle and an extended velocity profile for the pedestrian.

(a) Measurement 1 (b) Measurement 2 (c) Measurement 3 (d) Measurement 4

*shaped*) velocity profile due to a uniform motion.

12

**Figure 3.** Range profile and velocity profile of a single measurement.

**Figure 4.** Sequence of range and velocity measurements.

16

v

profile,ped

20

Range [m]

24

in the range Doppler diagram in Figure 4.

$$
\Delta v = -\frac{\lambda}{2} \cdot \frac{1}{T\_{\rm CPI}} \tag{4}
$$

The table below shows the system parameters of the automotive radar sensor in detail.


**Table 1.** 24 GHz Radar Sensor Parameters.

Classical UWB-Radar Sensors have a sweep bandwidth of *f*sweep = 2 GHz. Using such a bandwidth, a high range resolution is determined, which allows also pedestrian classification. The technical challenge in this chapter is to realize pedestrian recognition based on a 24 GHz radar sensor with a bandwidth of only 150 MHz. This sensor is used in automotive applications, therefore an extension of the signal processing in terms of pedestrian classification is desirable.
