# Sensitivity Analysis of Self-Interference Cancellation in Differentially-Fed Shared-Patch Antenna Arrays For In-Band Full-Duplex Applications

Hanan Ouled Angoura, Hadi Hijazi, François-Maël Robert, Meriem Nesrine Djama, Ulrich Simpore, Marc Le Roy, Raafat Lababidi, Roland Gautier,

André Pérennec

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## To cite this version:

Hanan Ouled Angoura, Hadi Hijazi, François-Maël Robert, Meriem Nesrine Djama, Ulrich Simpore, et al.. Sensitivity Analysis of Self-Interference Cancellation in Differentially-Fed Shared-Patch Antenna Arrays For In-Band Full-Duplex Applications. 2025 32nd IEEE International Conference on Electronics, Circuits and Systems (ICECS), Nov 2025, Marrakech, Morocco. pp.1-4, 10.1109/ICECS66544.2025.11270633. hal-05411534

HAL Id: hal-05411534

https://hal.univ-brest.fr/hal-05411534v1

Submitted on 11 Dec 2025

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# Sensitivity Analysis of Self-Interference Cancellation in Differentially-Fed Shared-Patch Antenna Arrays For In-Band Full-Duplex Applications

Hanan Ouled Angoura¹*, Hadi Hijazi², François-Maël Robert¹, Meriem Nesrine Djama¹, Ulrich B.Y. Simpore¹,

Marc Le Roy¹, Raafat Lababidi², Roland Gautier¹, André Pérennec¹

¹Univ Brest, ²ENSTA | Institut Polytechnique de Paris, UMR 6285, F-29200 Brest

* hanan.ouledangoura@univ-brest.fr

**Abstract**—This paper investigates the influence of practical balun imperfections on self-interference cancellation (SIC) in differentially-fed shared-patch antennas designed for 2.5 GHz In-Band Full-Duplex systems. A four-port square patch antenna is employed as a reference to conduct a sensitivity analysis, quantifying SIC degradation induced by such imperfections. The study is further extended to a 1×4 linear array, constructed from the reference element, to examine the effects of feeding imbalance and beam steering at the array level. To mitigate these sensitivities, an enhanced antenna design incorporating decoupling slots and metallic vias between adjacent ports is introduced, yielding improved impedance matching, port isolation, and SIC performance. The proposed design demonstrates superior robustness against balun imperfections.

**Keywords**—Antenna array, balun imbalance, beam steering, differentially-fed antenna, IBFD, ISAC, in-band full-duplex, JCAS, near-field cancellation, self-interference cancellation.

## I. INTRODUCTION

In Band Full-Duplex (IBFD) communication systems have attracted significant attention due to their potential to double the spectral efficiency by allowing simultaneous transmission and reception on the same frequency band [1]. Beyond spectral efficiency, IBFD also opens promising perspectives for Integrated Sensing and Communications (ISAC) and Joint Communication and Sensing (JCAS) systems, where the ability to simultaneously transmit and receive is fundamental to enable seamless fusion of communication and sensing functionalities. However, the primary challenge in IBFD systems lies in the management of self-interference (SI), which arises from the coupling between the transmit and receive ports of the antenna [2]. Effective self-interference cancellation (SIC) is essential to enable the practical deployment of IBFD technologies, especially in antenna arrays where mutual coupling and beam steering effects further complicate SI mitigation [3].

A significant portion of the SI originates at the antenna interface, where traditional techniques, such as physical separation or polarization diversity, are often insufficient for compact systems [2]-[4]. To address this, antenna-level SIC techniques have been proposed, with particular emphasis on near-field cancellation (NFC) [5]. In particular, this technique has proven to be highly effective both for separate Tx/Rx apertures and in shared ones. This method exploits an anti-phase feeding technique to achieve a near-field minimum along the receiving port axis, thereby minimizing the residual self-interference.

Several studies have explored geometric modifications to improve antenna isolation, such as a partially reflecting surface (PRS), capacitive slot loading. For instance, the work presented in [6] proposes a dual-polarized filtering IBFD antenna using a PRS with cross-shaped slots and shorting vias to enhance impedance matching, frequency selectivity, and inter-port isolation. Similarly, metallic vias have been employed to suppress surface waves and reduce mutual coupling between ports [7]. In [8], defected ground structures (DGS) combined with cross slots and shorting pins were integrated beneath a symmetrical differential feed, enhancing both inter-port isolation and impedance matching. Moreover, [9] introduced a differential-feed neutral coupling combined with optimized baluns, effectively mitigating Tx/Rx coupling through balanced excitation and structural symmetry.

While significant progress has been made in developing decoupling techniques to enhance isolation and achieve high SIC in IBFD antennas, these advancements are mostly evaluated under ideal conditions. The effectiveness of these approaches relies heavily on maintaining precise symmetry in the antenna geometry and feeding network. In practical implementation, several imperfections can significantly degrade SIC performance, and little attention has been given to the sensitivity of SIC to real-world non-idealities. Particularly:

* Balun phase and magnitude imbalances: differential feeding requires precise symmetry between signal paths. Any mismatch in amplitude or phase between the balun outputs disrupts this symmetry, reducing NFC efficiency and consequently degrading SIC.

* Beam steering using phase control: in a phased array, steering the beam away from the main direction requires changing the phase shift between antenna elements, it is then necessary to analyze the NFC mechanisms at the array level.

These practical issues motivate a thorough sensitivity analysis to understand and quantify their impact to bring the IBFD antenna system closer to real-world conditions. To the best of our knowledge, there is no comparative study in the literature that systematically quantifies the degradation of SIC performance as a function of these combined parameters in a realistic IBFD antenna system.

This paper is organized as follows: section II presents the reference differentially fed Tx/Rx patch antenna, with an evaluation of its input matching and SIC performance under both ideal and imbalanced feeding conditions. A detailed sensitivity study highlights the impact of phase and amplitude errors. Section III extends the analysis to a 1×4 array based on the reference antenna, focusing on SIC degradation caused
<figure><img src="image_3.png" /><figcaption>Fig. 1: (a) IBFD antenna and its feeding network, (b) S-parameters performance under ideal and 5° phase imbalance, (c) SIC sensitivity to feeding phase and magnitude imbalance. {L = 60.94 mm, W = 30.47 mm, ΔP = 8 mm}</figcaption></figure>

by balun imperfections and beam steering effects. Section IV presents an improved antenna design that incorporates decoupling slots and impedance tuning vias and evaluates its robustness using the same sensitivity analysis. Finally, the conclusion and perspectives summarize the main findings and outline directions for future research.

## II. INFLUENCE OF BALUN IMBALANCE ON THE REFERENCE TX-RX PATCH

The baseline structure used in this study is a square patch antenna in Fig. 1(a) designed to resonate at 2.5 GHz. Four feeding ports are symmetrically positioned in pairs and are differentially fed to implement the NFC technique for IBFD application or to reduce cross-polarization. An external ideal balun is used to feed Tx₁ and Tx₂ with signals of equal amplitude and opposite phase, while another balun is employed at the receiving ports (Rx₂ and Rx₁) to combine the residual signals destructively, as depicted in Fig. 1 and detailed in [5]-[9]. Initially, in this differentially fed antenna, the optimal inter-port distance (ΔP) for achieving good impedance matching was identified as 6 mm.

This configuration was validated via simulation in CST, and its performance was presented in [10], which included a detailed analysis of the coupling matrix and SIC performance. However, the practical realization of this structure faced hardware limitations. Specifically, using commercially available connectors (e.g., SMA, SPMP), the structure was not feasible at such a narrow inter-port spacing. UFL connectors were then considered as an alternative due to their compact size and surface-mount compatibility on the antenna ground plane. However, their partially unshielded design introduced undesired inter-port coupling, degrading the isolation performance. Consequently, the feeding network was redesigned to increase the port spacing to 8 mm, enabling the integration of SPMP connectors. This modification resulted in a slight degradation of the differential input matching ($|S_{TxTx}|_{dB} = |S_{RxRx}|_{dB} = -13\text{dB}$).

To evaluate the structure sensitivity to realistic imperfections, a parametric analysis was conducted considering imbalance in differential feeding. Simulations were conducted in CST (Time Domain Solver) to evaluate the impact of a phase mismatch at the feeding ports. As a first investigation, a 5° phase imbalance was applied at one of the Tx ports. As shown in Fig. 1(b), the resulted isolation (SIC) dropped from 149 dB (ideal condition) to 117 dB under imbalance, highlighting the vulnerability of the structure to imperfections introduced by baluns and other practical elements or placements in the feeding network.

To quantify the effects of real-world imbalances due to balun imperfections, a Monte Carlo sensitivity analysis was conducted in MATLAB. The S-parameters obtained from CST were imported, and random deviations in both amplitude and phase were applied. The resulting SIC degradation trend is shown in Fig. 1(c). For instance, above the green area delimited approximately by a phase imbalance of $\Delta\phi=2.5°$ and magnitude imbalance of $\Delta M=0.4$ dB, the SIC degrades quickly below 110 dB, although this level remains relatively high. In practice, we plan to use a 2.5 GHz balun (e.g. BLJC4-252R+ from mini-circuits) where the maximum imbalance may range up to 2 dB in amplitude and 5° in phase (0.2 dB and 0.8° for typical values).

## III. 1x4 ANTENNA ARRAY SENSITIVITY ANALYSIS

The third section of this study investigates the 1x4 linear array configuration derived from the square patch antenna, as shown in Fig. 2(a). Unlike the single-element design, the patch length of the array was slightly adjusted to preserve a resonance at 2.5 GHz. This modification compensates for the frequency shift induced by mutual coupling effects between adjacent elements in the array. The reflection coefficient on the Tx port of the squared-patch array reaches -9 dB, while the Rx port achieves a better impedance match with -18 dB. This divergence in matching performance arises from the asymmetrical electromagnetic environment seen by the Tx and Rx ports within the array configuration. The mutual coupling between Tx and Rx ports using ideal baluns is simulated at -93dB. To study the impact of phase imbalance, parametric simulations were performed in CST by introducing various phase offsets. For instance, applying a 5° phase imbalance at the Tx₁ port of the first antenna element (Fig. 2(c)) increases the SI to -77 dB.

Further sensitivity analysis was performed using Monte Carlo simulations in MATLAB, still based on S-parameters from CST. Fig 2(b) shows the statistical distribution of SIC following phase and amplitude imbalances, and the results logically indicate a higher sensitivity to imbalance compared to a single patch, due to a cumulative inter-element coupling contribution with imbalance.

Fig. 2(d) shows the realized gain while steering in elevation along x-axis the main beam independently for Tx
<figure><img src="image_4.png" /><figcaption>Fig. 2: (a) 1x4 IBFD array based on reference antenna {L = 60.94 mm, θ = 29.54 mm}, (b) SIC sensitivity to phase and amplitude imbalance, (c) S-parameters under ideal and 5° phase imbalance conditions, (d) Tx and Rx radiation patterns {Δφ₁ = 0°, Δφ₂ = 22.5°, Δφ₃ = 45°, Δφ₄ = 67.5°, Δφ₅ = 90°, Δφ₆ = 112.5°} and (e) SIC degradation vs beam steering.</figcaption></figure>

and Rx in a basic phase array configuration. To do so, different phase shift value (Δφ) ranging from 0° to 112.5° (corresponding to a phase control by 4-bit phase shifters, i.e., LSB=22.5°) are applied between antenna elements, and θ is the angle between the tilted beam and the vertical direction. This analysis of independent Tx and Rx elevation beam angle is complementary to Fig. 2(e), which depicts SIC degradation under Tx and Rx elevation angle beam steering conditions. The plot exhibits specific angular regions where high SIC can still be preserved.

This section demonstrates that although the reference 1x4 array still achieves high isolation under ideal conditions, it remains highly sensitive to phase and amplitude imbalances, as well as beam steering operations. If not properly compensated, these imperfections would lead to significant SIC degradation in practice.

## IV. ENHANCED DIFFERENTIAL PATCH WITH SLOT DECOUPLING AND VIAS

To overcome the limitations observed in the reference design, particularly the degradation of input matching and isolation caused by hardware constraints that required increasing the ports spacing to 8 mm, a new patch antenna structure was developed, as shown in Fig. 3(a). While maintaining the same port spacing (e.g., 8 mm), the new design introduces two key modifications to reduce inter-port mutual coupling and improve impedance matching:

* Decoupling slots are inserted between adjacent Tx and Rx feeding ports to reduce near-field coupling. These slots act as barriers that interrupt the flow of surface currents between ports.

* Four metallized vias strategically placed to re-tune the input impedance of the differential feeding points. This

compensation is necessary because the introduction of the slots degrades impedance matching. Thus, the vias adjust the antenna's reactive properties to restore optimal matching.

The inter-port distance was deliberately maintained to ensure a consistent and fair comparison with the reference structure, but the slot and vias could have been optimized to increase the inter-port distance. As shown in Fig. 3(b), the inclusion of slots and vias significantly improves the input matching to (|S<sub>Tx,Tx</sub>|<sub>dB</sub> = -29 dB) and achieves an enhanced mutual isolation of 161 dB under ideal feeding conditions. Even with a 5° phase imbalance on a single port, the antenna maintains a strong SIC level of 133 dB, demonstrating improved robustness to excitation non-idealities. This improvement is further confirmed by the Monte Carlo-based sensitivity analysis shown in Fig. 3(c). The SIC remains higher than 110 dB in a range up to about 5° of phase offset and 0.8 dB amplitude variation.

Fig. 4 illustrates the surface current distribution at 2.5 GHz for both the reference antenna and the slotted one under differential Tx excitation (i.e., Tx₁ and Tx₂). For the reference design, the surface current mainly concentrates along the upper and lower edges of the patch, forming a symmetrical pattern that reflects the balanced feeding. In contrast, the slotted antenna exhibits clearly a modified current distribution: the decoupling slots and central vias disturb the surface current flow and act as a complementary wavetrap decoupling technique that improves the Tx to Rx port isolation, as illustrated by the dark blue area around the Rx ports in Fig. 4(b). This perturbation leads to weaker near-field interactions between transmit and receive paths, which correlate with the improved SIC performance observed in Fig. 3C. These results validate the improved immunity versus
<figure><img src="image_5.png" /><figcaption>Fig. 3: (a) SIC sensitivity to phase and amplitude imbalance, (b) S-parameters under ideal and 5° phase imbalance, (c) SIC sensitivity to phase and amplitude imbalance. {L = 60.94 mm, Wp = 33.2 mm, ΔP = 8 mm, s₁ = s₂ = 0.4 mm, L1 = 4.1 mm, L2 = 4.6 mm}</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4: Surface current distribution under differential excitation of (a) the reference and (b) slotted patches.</figcaption></figure>

the imbalance of the modified radiating element. Further investigations are being considered to consolidate this immunity in a phased-array configuration by introducing decoupling or shielding structures between elements.

## V. CONCLUSION

This paper presented a comprehensive sensitivity analysis of SIC performance in differentially fed patch antennas designed for IBFD systems. Starting from a reference antenna with four symmetrically arranged feed ports, we investigated the impact of balun imbalances both in amplitude and phase on SIC performance, at both the single-element and array levels. The results reveal a strong sensitivity of the structures to these imperfections. The SIC degradation versus independent beam steering angle is also investigated.

To address these limitations, an enhanced antenna structure was proposed, integrating decoupling slots and impedance-tuning vias. This design achieved better isolation levels under ideal conditions and demonstrated improved robustness against feeding imperfections, making it a promising candidate for practical IBFD systems.

Future work will focus on integrating this improved antenna into larger arrays with independent beamforming capabilities and on investigating the impact of fabrication tolerances through prototype measurements.

## ACKNOWLEDGEMENTS

This work was supported in part by FullSSAT project, funded by the French Defense Innovation Agency (DGA/AID).

## REFERENCES

[1] T.-L. Hsu, M.-D. Wei, R. Negra, « Electrical Balance Duplexer as In-Band Full-Duplex Antenna Interface with Fast Orthogonal Searching Methodology », *EuMC*, April 2022, p. 71-74.

[2] K. E. Kolodziej, B. T. Perry, J. S. Herd, « In-Band Full-Duplex Technology: Techniques and Systems Survey », *IEEE Trans. MTT*, vol. 67, n° 7, July 2019.

[3] K. E. Kolodziej, J. G. McMichael, B. T. Perry, « Multitap RF Canceller for In-Band Full-Duplex Wireless Communications », *IEEE Trans. on Wireless Communications*, vol. 15, n° 6, June 2016.

[4] D. Bharadia, E. McMilin, S. Katti, « Full duplex radios », in *Proceedings of the ACM SIGCOMM 2013 conference on SIGCOMM*, Hong Kong, ACM, p.375-386, Aug. 2013.

[5] H. Hijazi, M. Le Roy, R. Lababidi, D. Le Jeune, A. Pérennec, « Ultra-wideband antenna system for in-band full-duplex applications », *IET Microwaves, Antennas & Propagation*, Dec. 2021.

[6] N. T. Le, T. Le-Huu, N. A. Nguyen, S. X. Ta, K. K. Nguyen, N. Nguyen-Trong, « Gain and Frequency-Selectivity Enhancement of Dual- Polarized Filtering IBFD Antenna Using PRS », *J. Electromagn. Eng. Sci*, vol. 23, n° 4, p. 318-325, July 2023.

[7] Y. He, Y. Li, « Dual-Polarized Microstrip Antennas With Capacitive via Fence for Wide Beamwidth and High Isolation », *IEEE Transactions on Antennas and Propagation*, vol. 68, n° 7, p. 5095-5103, July 2020.

[8] W. Yang, M. Xun, W. Che, W. Feng, Y. Zhang, Q. Xue, « Novel Compact High-Gain Differential-Fed Dual-Polarized Filtering Patch Antenna », *IEEE Trans. on Antennas and Propagation*, vol. 67, n° 12, p. 7261-7271, Dec. 2019.

[9] D. Prasad, S. Tripathi, P. Paltani, « An in-band full-duplex antenna for 2.4 GHz WLAN using differential-feed based neutral coupling for isolation enhancement », *MOTL*, vol. 66, Sept. 2024.

[10] H. O. Angoura, H. Hijazi, U. B. Y. Simpore, M. Le Roy, J. -T. Kubwimana, R. Lababidi, R. Gautier, S. Mallegol, G. Tanne, A. Pérennec, « Toward Using Monostatic Antennas With Near-Field Cancellation Technique in IBFD Phased Arrays », *IEEE ICM*, Dec. 2024.