7. mmWave and Terahertz ISAC Perspective

Why High Frequency Helps Sensing

mmWave and THz systems provide large bandwidth and small wavelength. These properties are useful for sensing:

• Large bandwidth improves range resolution.
• Small wavelength improves sensitivity to small movements.
• Compact arrays provide high angular resolution.
• Narrow beams reduce some interference.

$$\Delta R = \frac{c}{2B}, \qquad f_D = \frac{2 v f_c}{c}$$

Wider bandwidth separates targets more finely in range, and a higher carrier turns the same motion into a larger Doppler shift.

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.

Why High Frequency is Difficult

High-frequency links are fragile:

• Blockage is severe.
• Penetration through walls is poor.
• Beam alignment is critical.
• Hardware impairments are more significant.
• Phase noise and synchronization matter.

ISAC must therefore combine high-resolution sensing with robust communication control.

Moving up in frequency: what improves and what gets worse

1. Formula and symbols

$$\Delta R=c/(2B),\quad\Delta v=\lambda/(2T_{\rm obs})$$

$$\theta_{\rm beam}\approx2/N,\quad L_{\rm FS}=20\log_{10}(4\pi d/\lambda)$$

• $B$: bandwidth
• $\lambda$: wavelength
• $T_{\rm obs}$: observation time
• $N$: elements across the aperture
• $d$: path range

Beamwidth is in radians and assumes the stated half-wavelength array model.

Observation time

10 ms

Target distance

20 m

Panel size

10 cm

2. Work with the current inputs

$$\Delta R = \frac{c}{2B}, \qquad \Delta v = \frac{\lambda}{2T_{\text{obs}}}, \qquad \theta_{\text{beam}} \approx \frac{2}{N} = \frac{2}{D/(\lambda/2)}, \qquad \mathrm{FSPL} = 20\log_{10}\frac{4\pi d}{\lambda}$$

3. Result

3.5 GHz1.50 m2 el./side, Doppler 23 Hz per m/s

28 GHz37.5 cm18 el./side, Doppler 187 Hz per m/s

140 GHz3.7 cm93 el./side, Doppler 934 Hz per m/s

300 GHz1.5 cm200 el./side, Doppler 2001 Hz per m/s

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.

mmWave ISAC

mmWave is a practical near-term ISAC band because 5G/6G systems already use mmWave-style beamforming. The same arrays used for directional communication can support radar-like sensing.

Use cases:

• Vehicle tracking.
• Indoor positioning.
• Gesture recognition.
• Beam prediction.
• Factory monitoring.

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.

THz ISAC

THz frequencies can offer even larger bandwidth and finer sensing resolution. They are promising for extremely high data rates and high-resolution imaging-like sensing.

Challenges:

• Very high path loss.
• Molecular absorption.
• Hardware limitations.
• Short-range deployment.
• Severe blockage.

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.

Role of RIS

RIS becomes especially important at mmWave/THz because blockage is the central weakness. A RIS can provide an alternate path and can help illuminate targets that are not visible through the direct path.

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.

Takeaway

mmWave and THz frequencies are natural for ISAC because they provide high-resolution sensing and high-rate communication. Their weakness is fragility, which makes beam management, RIS assistance, and robust tracking essential.

References for this section3

• F. Liu et al., "Integrated Sensing and Communications", IEEE JSAC, 2022.
• J. A. Zhang et al., "An Overview of Signal Processing Techniques for Joint Communication and Radar Sensing", IEEE JSTSP, 2021.
• X. Mu et al., "Reconfigurable Intelligent Surface-Aided Near-Field Communications for 6G", IEEE Vehicular Technology Magazine, 2024.