⚡ electrical engineering

Multi-Band Patch Antenna Array for Out-of-Band Aided Millimeter Wave Communication

This paper investigates the impact of colocating millimeter-wave antenna structures in front of sub-6 GHz arrays for out-of-band aided communication, demonstrating through simulations and measurements that such configurations result in negligible degradation to the sub-6 GHz radiation performance.

Original authors: Faruk Pasic, Jure Soklič, Robert Langwieser, Stefan Schwarz, Christoph F. Mecklenbräuker

Published 2026-04-06

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CC BY 4.0

Original authors: Faruk Pasic, Jure Soklič, Robert Langwieser, Stefan Schwarz, Christoph F. Mecklenbräuker

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

Imagine you are trying to have a conversation with a friend in a noisy, crowded room.

The Problem: The "Fast Lane" vs. The "Slow Lane"
In the world of wireless internet, we have two main "lanes":

The Sub-6 GHz Lane: This is like a wide, old highway. It's a bit slower in terms of raw speed, but it's very reliable. It can go around obstacles (like walls) and doesn't get blocked easily.
The Millimeter Wave (mmWave) Lane: This is a brand-new, super-high-speed race track. It can carry massive amounts of data (like 4K video or huge downloads), but it's very fragile. If a single person walks in front of it, the signal breaks.

To get the best of both worlds, engineers want to use the reliable "highway" to help guide the fragile "race track." They want to use the slow lane to tell the fast lane exactly where to aim so it doesn't get blocked.

The Challenge: Stacking the Antennas
To make this teamwork work, the antennas for both lanes need to be right next to each other, looking in the exact same direction. It's like having a navigator (the sub-6 GHz antenna) standing right behind the driver (the mmWave antenna) to shout directions.

But here's the catch: If you put a big metal object (the mmWave antenna) right in front of the navigator, you might accidentally block the navigator's view or mess up their voice. In antenna terms, the mmWave structure might ruin the signal quality of the sub-6 GHz antenna.

The Experiment: The "Shadow" Test
The authors of this paper wanted to know: If we stack these antennas on top of each other, does the big mmWave antenna act like a bad shadow that ruins the sub-6 GHz signal?

They built a model where they placed a "mmWave antenna" (which is actually just a flat board with metal patterns on it) directly in front of a "sub-6 GHz antenna." They tested this in two ways:

Computer Simulations: Like a flight simulator, they modeled the physics to see what would happen.
Real-World Lab Tests: They built actual antennas in their lab and measured the signals with special equipment.

The Results: A Friendly Neighbor
The results were surprisingly good news!

The Navigator is Fine: When they put the mmWave antenna in front of the sub-6 GHz antenna, the sub-6 GHz signal barely noticed. It was like putting a thin, transparent sheet of glass in front of a person shouting; the voice came out almost exactly the same. The signal strength dropped by a tiny, almost unnoticeable amount (less than 0.25 dB).
The Driver Gets a Boost: Interestingly, when they looked at the mmWave antenna, the sub-6 GHz antenna behind it actually helped! It acted like a backstop or a mirror. Instead of the mmWave signal leaking out the back (wasting energy), the sub-6 GHz antenna reflected it forward. This made the mmWave signal even stronger and more focused.

The Big Takeaway
Think of it like two people standing in a line to pass a ball. You might worry that the person in front would block the person behind. This paper proves that if you design them correctly, the person in front doesn't block the person behind at all. In fact, the person behind helps the person in front throw the ball straighter.

Why This Matters
This discovery is a green light for future 6G and advanced 5G networks. It means engineers can safely stack these different types of antennas on the same device (like your phone or a cell tower) without worrying that they will ruin each other's performance. We can finally combine the speed of the "race track" with the reliability of the "highway" to give us faster, more stable internet.

1. Problem Statement

Future wireless communication systems aim to integrate sub-6 GHz and millimeter wave (mmWave) frequency bands to meet high data rate demands. A key strategy involves using reliable out-of-band information from the sub-6 GHz band (which has better propagation characteristics) to aid mmWave communication.

To ensure both systems experience similar multipath propagation effects, the antenna arrays for both bands must be co-located and precisely aligned (sharing the same geometric center). A common hardware implementation places the mmWave array directly in front of the sub-6 GHz array. However, this configuration raises a critical concern: does the physical presence of the mmWave antenna structure degrade the radiation characteristics (gain, beamwidth, pattern) of the underlying sub-6 GHz array? If the degradation is significant, the feasibility of such co-located designs is compromised.

2. Methodology

The authors employed a combined approach of full-wave electromagnetic simulations and experimental measurements to evaluate the mutual coupling and interference between the two bands.

Antenna Design:
- Structure: Uniform Linear Arrays (ULAs) of patch antenna elements.
- Configuration: The mmWave array is positioned in front of the sub-6 GHz array with an inter-array spacing ( $h_a$ ) of 10 mm. Both arrays share the same geometric center.
- Parameters:
  - Sub-6 GHz: 2.55 GHz carrier, RT-Duroid 5880 substrate.
  - mmWave: 25.5 GHz carrier, RO4350B substrate.
- Validation: The single sub-6 GHz patch design was validated via ANSYS HFSS simulations and physical measurements using a Vector Network Analyzer (VNA), showing a resonance deviation of only 0.5% from the target frequency.
Experimental Setup:
- Single Element Analysis: A single sub-6 GHz patch was tested with mmWave arrays of varying sizes (8, 16, and 32 elements) placed in front of it.
- Measurement Technique: Field-pattern measurements were conducted in an anechoic chamber. To simulate the mmWave array without active transmission, unetched Printed Circuit Boards (PCBs) with copper cladding (matching the mmWave array dimensions) were used as passive blockers.
- Array Analysis: Simulations were extended to full arrays (8-element sub-6 GHz ULA vs. varying mmWave ULAs) and the reverse scenario (mmWave array affected by the sub-6 GHz array).

3. Key Contributions

Feasibility Verification: The paper provides empirical evidence that co-locating mmWave and sub-6 GHz patch arrays is feasible with negligible performance degradation.
Comprehensive Evaluation: It analyzes the impact from two perspectives:
1. The effect of the mmWave structure on the sub-6 GHz radiation pattern.
2. The effect of the sub-6 GHz structure on the mmWave radiation pattern.
Hardware Realism: The study moves beyond theoretical models by including manufacturing tolerances, substrate variations, and physical measurement data alongside simulations.

4. Key Results

A. Impact of mmWave on Sub-6 GHz (Primary Focus)

Single Element:
- Gain Degradation: The presence of the mmWave array caused a maximum realized gain reduction of only 0.12 dB (simulation) and 0.25 dB (measurement).
- Beamwidth: The Half-Power Beamwidth (HPBW) remained largely stable. In the E-plane, it stayed at 68° (sim) or dropped slightly from 75° to 71° (meas, ~5% reduction). In the H-plane, changes were minimal (64° to 62°).
- Conclusion: Electromagnetic interaction at sub-6 GHz frequencies is minimal.
Full Array (8-element Sub-6 GHz ULA):
- Gain Degradation: The maximum reduction in main-lobe gain was approximately 0.15 dB.
- Beamwidth: E-plane HPBW decreased slightly from 68° to 66° (3%); H-plane remained unchanged at 12°.

B. Impact of Sub-6 GHz on mmWave (Reverse Scenario)

Gain: The presence of the sub-6 GHz array caused a marginal gain reduction in the mmWave array (from 15.16 dBi to 15.08 dBi).
Back-Lobe Suppression: Crucially, the sub-6 GHz array acted as an additional ground plane for the mmWave array. This suppressed the back-lobe radiation significantly, reducing the level from -3.4 dBi to -7.5 dBi, thereby improving the overall radiation performance of the mmWave system.

5. Significance and Conclusion

The study concludes that co-located sub-6 GHz and mmWave ULA configurations are highly feasible for out-of-band aided communication systems.

Performance: The mutual interference is negligible, with gain losses well below 0.3 dB, which is acceptable for practical deployment.
Benefit: The sub-6 GHz array not only supports the mmWave system via out-of-band information but also physically improves the mmWave array's directivity by suppressing back-lobe radiation.
Future Work: The authors suggest extending this analysis to Uniform Planar Arrays (UPAs) to assess radiation effects in two-dimensional antenna structures, which are common in modern Massive MIMO systems.

This research validates the hardware architecture required for next-generation multi-band 5G/6G systems, ensuring that the physical integration of diverse frequency bands does not compromise system reliability.