5. 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.

range resolution is roughly proportional to 1 / bandwidth

So wider bandwidth generally means sharper distance separation between targets.

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.

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.

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.

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.

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.