⚡ electrical engineering

User Mobility Demands Near-Field Communications in Terahertz Band Wireless Networks Beyond 6G

This paper demonstrates through a proof-by-contradiction framework that while stationary terahertz links can operate exclusively in the far field, practical mobile THz systems necessitate near-field-aware designs because far-field-only operation imposes unrealistically high power requirements to support target bandwidths.

Original authors: Peng Zhang, Vitaly Petrov, Arjun Singh, Emil Björnson, Josep Miquel Jornet

Published 2026-04-21

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Peng Zhang, Vitaly Petrov, Arjun Singh, Emil Björnson, Josep Miquel Jornet

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Question: Can We Keep 6G Simple?

Imagine you are trying to build the ultimate high-speed internet for the future (6G). You want to use Terahertz (THz) waves—the super-fast, invisible radio waves that can carry terabytes of data per second.

The paper asks a very simple, yet profound question: Can we design these mobile 6G systems so that they are "simple" (operating in the "far field"), or are we forced to deal with "complexity" (operating in the "near field")?

Here is the breakdown using everyday analogies.

1. The Two Worlds: The "Flat Wave" vs. The "Curved Wave"

To understand the paper, you need to know the difference between two ways radio waves behave:

The Far Field (The "Flat Wave"): Imagine you are standing on a beach, far away from a lighthouse. The light hitting your eyes looks like a flat, straight beam. In radio terms, this is the "Far Field." It's simple. Engineers love it because the math is easy, and the signal behaves predictably.
The Near Field (The "Curved Wave"): Now, imagine you walk right up to the lighthouse lens. The light hitting your face is no longer a flat beam; it's curving, wrapping around the glass, and hitting different parts of your face at different angles. This is the "Near Field." It's complex. The math is a nightmare, and the signal is messy.

The Problem: Terahertz waves need huge antennas (arrays) to work well. Because these antennas are so big, the "Near Field" (the messy, curved zone) stretches out very far. Usually, you have to be very far away to get into the "Far Field" (the simple zone).

2. The Experiment: The "Proof by Contradiction"

The authors wanted to see if we could force mobile phones to stay in the "Far Field" (the simple zone) even while moving around.

They set up a thought experiment (a "proof by contradiction"):

Assume we can keep the phone in the "Far Field" (simple zone).
Check if the phone can still get a strong enough signal to download movies at 6G speeds.
Check if the phone needs to use a realistic amount of battery power to do this.

The Result: They found a contradiction. To stay in the "simple" zone while moving, the phone would need to scream so loud (use so much power) that it would melt the battery or burn the circuitry.

3. The Analogy: The Flashlight and the Wall

Imagine you are holding a flashlight (the phone) and trying to shine a bright, tight beam of light onto a wall (the tower) 20 meters away.

Stationary (The Tripod): If you put the flashlight on a tripod and don't move, you can easily aim it perfectly. You can use a huge lens (antenna) to make the beam super tight and bright. You stay in the "Far Field," and everything works great. Verdict: Easy.
Mobile (The Handheld): Now, imagine you are walking around while holding the flashlight. Your hand shakes, you turn corners, and the angle changes.
- To keep the beam "flat" (Far Field) and hitting the wall perfectly, you would need to make the flashlight lens massive (physically impossible for a phone) OR turn the flashlight up to 100,000 watts (impossible for a battery).
- If you try to keep the lens small and the power low, the beam naturally becomes "curved" and messy as you get closer. You are forced into the "Near Field."

4. The Key Findings

The paper tested this across different frequencies:

Sub-6 GHz (Current 4G/5G): Like a standard flashlight. You can move around, and it's easy to stay in the "Far Field." No big deal.
mmWave (Current 5G): Like a laser pointer. It's getting harder. If you want super-fast speeds, you have to be careful, but it's still possible to stay in the "Far Field" for short distances.
Terahertz (Future 6G): This is the big one. The authors found that for mobile phones to get the massive speeds promised by 6G, it is physically impossible to stay in the "Far Field."
- If you try to stay in the "Far Field," the phone would need to transmit with the power of a small power plant.
- Conclusion: We must embrace the "Near Field." We have to accept that the waves are curved and messy, and we need to build complex, smart systems to handle that curvature.

5. Why Does This Matter?

For the last decade, engineers have been trying to design 6G systems based on "Far Field" assumptions because the math is easier.

This paper says: "Stop trying to force it."

If you are building a mobile 6G network, you cannot pretend the waves are flat. You have to design your system to handle the "curved" reality of the Near Field. It's like realizing you can't drive a car on a straight line forever; sometimes you have to turn, and your steering system needs to be ready for that.

Summary in One Sentence

While stationary 6G towers can enjoy simple, flat radio waves, mobile phones moving around will inevitably get stuck in the "messy" curved-wave zone, meaning we must design complex new systems to handle it rather than trying to avoid it.

1. Problem Statement

The paper addresses a fundamental feasibility question for Beyond-6G (B6G) Terahertz (THz) communications: Can a mobile THz communication system operate exclusively in the radiative far-field (FF) while supporting the ultra-wide bandwidths required for Tbps-level data rates?

Context: THz systems require large antenna apertures to achieve sufficient array gain. Large apertures extend the radiative near-field (NF) region (defined by the Fraunhofer distance).
The Conflict: Operating in the near-field introduces significant complexity (spherical wave propagation, range-dependent beamforming, complex channel estimation). System designers would prefer to avoid this by staying in the far-field.
The Core Question: Is it physically possible to design a mobile THz link that remains in the far-field under realistic constraints (user mobility, device rotation, and limited transmit power), or is near-field operation unavoidable?

2. Methodology

The authors develop a proof-by-contradiction feasibility framework to determine the limits of far-field-only operation.

System Model:
- Point-to-point link between a stationary Access Point (AP) and a mobile User Equipment (UE).
- Both use square planar arrays with half-wavelength spacing.
- Mobility Modeling: Includes translational mobility (link distance $d$ varies between $d_{min}$ and $d_{max}$ ) and rotational mobility (UE rotation and relative angular variations).
- Misalignment: Accounts for practical device rotation (UE self-rotation) and motion-induced relative angle changes.
The Contradiction Framework:
The framework assumes the system operates only in the far-field and checks if two conditions can be satisfied simultaneously:
1. Geometric Far-Field Condition: The minimum link distance ( $d_{min}$ ) must be greater than or equal to the Fraunhofer distance ( $d_F$ ), which depends on the array apertures ( $D_1, D_2$ ) and wavelength ( $\lambda$ ).
2. Reliability (Link-Budget) Condition: The Signal-to-Noise Ratio (SNR) at the maximum link distance ( $d_{max}$ ) must meet a target threshold ( $SNR_{th}$ ). This depends on transmit power ( $P_t$ ), bandwidth ( $B$ ), and array gains (which degrade with rotation/misalignment).
Analytical Approach:
- The authors derive closed-form upper bounds on the maximum feasible bandwidth ( $B_{max}$ ) for a given set of parameters.
- If the required bandwidth for a specific application exceeds this bound, the assumption of "far-field-only" operation leads to a contradiction (either the aperture size becomes physically impossible, or the required transmit power becomes unrealistically high).
- They analyze four scenarios:
  1. Stationary, aligned.
  2. Mobile, UE rotated (1-angle and 2-angle).
  3. Mobile, relative direction variation (AP side rotation effects).

3. Key Contributions

Feasibility Framework: A novel analytical framework that jointly enforces geometric far-field constraints and link-budget reliability to determine the limits of far-field-only operation.
Closed-Form Bounds: Derivation of explicit mathematical limits for the maximum achievable bandwidth and minimum required transmit power for mobile THz links under various misalignment scenarios (UE rotation and relative motion).
Quantification of Mobility Impact: Demonstration that mobility introduces multiplicative penalties (mobility coefficient $M$ and aperture asymmetry $L$ ) that drastically reduce the feasible bandwidth compared to stationary links.
Cross-Band Comparison: A comparative analysis across sub-6 GHz, mmWave, and THz bands to show how the severity of near-field constraints scales with frequency.

4. Key Results

The numerical evaluation reveals a stark contrast between stationary and mobile scenarios, and between different frequency bands:

Stationary Links:
- Stationary THz links can operate exclusively in the far-field while supporting extremely large bandwidths (tens of GHz to THz).
- The required antenna apertures are physically realizable (e.g., ~10cm x 10cm at 300 GHz).
- Far-field constraints do not create an intrinsic bandwidth bottleneck for static backhaul or fixed links.
Mobile Links (The Critical Finding):
- For mobile THz access, far-field-only operation is essentially infeasible for target bandwidths in the tens-of-GHz range.
- Power Requirements: To maintain far-field operation with realistic mobility (e.g., distance variation from 0.5m to 25m) and device rotation, the required UE transmit power becomes prohibitively high (e.g., >40 dBm or 10 Watts for 100 GHz bandwidth). This far exceeds the capabilities of battery-powered mobile devices (typically limited to ~23 dBm).
- Impact of Misalignment: UE rotation and relative direction variations further tighten these limits, reducing the feasible bandwidth by orders of magnitude compared to the aligned case.
Cross-Band Comparison:
- Sub-6 GHz: Far-field-only operation is easily attainable even with mobility and rotation.
- mmWave: Far-field operation is feasible for moderate bandwidths (e.g., <2 GHz) but becomes difficult for multi-GHz targets, especially with rotation.
- THz: Far-field-only operation is unavoidably infeasible for high-bandwidth mobile access. The combination of large apertures (needed for gain) and mobility forces the system into the near-field.

5. Significance and Conclusion

Paradigm Shift: The paper challenges the assumption that future mobile THz systems can rely on traditional far-field plane-wave models. It proves that for mobile access, near-field-aware designs are not just an optimization option but a physical necessity.
Design Implications:
- System architects must abandon "far-field-only" simplifications for mobile THz.
- Future 6G/THz systems must incorporate spherical wave propagation models, range-dependent beamforming, and robust near-field channel estimation/tracking.
- Ignoring near-field effects in mobile THz design will lead to severe performance degradation or system failure due to unattainable power requirements.

In summary, while stationary THz links can avoid near-field complexities, user mobility fundamentally forces THz communications into the near-field regime, making near-field-aware signal processing and design essential for the realization of Beyond-6G mobile networks.