Antenna Sensitivity Enhancement through Noise Suppression in an LTE/GPS Communication Module

Article information

J. Electromagn. Eng. Sci. 2025;25(3):304-306

Publication date (electronic) : 2025 May 31

doi : https://doi.org/10.26866/jees.2025.3.l.26

Jeonghwan Kim ¹^,^a

, Seokju Wi ¹^,^b

, Hoseung Lee ¹^,^c

, Rui Li ¹^,^d

, Hyunwoong Shin ¹^,^e

, Hyung-Hoon Kim ²^,^f

, Hyeongdong Kim ¹^,^g

¹Department of Electronic Engineering, Hanyang University, Seoul, Republic of Korea

²Department of Cosmetic Science, Kwangju Women’s University, Gwangju, Republic of Korea

^*Corresponding Author: Hyeongdong Kim (e-mail: hdkim@hanyang.ac.kr)

ahttps://orcid.org/0009-0002-9957-2721

bhttps://orcid.org/0009-0003-8546-0435

chttps://orcid.org/0000-0002-4570-5720

dhttps://orcid.org/0009-0009-1641-309X

ehttps://orcid.org/0000-0003-4018-9648

fhttps://orcid.org/0009-0005-7879-6584

ghttps://orcid.org/0000-0003-4540-9451

Received 2024 June 3; Revised 2024 July 10; Accepted 2024 September 1.

Abstract

This letter discusses antenna sensitivity enhancement in an LTE/GPS communication module for vehicle-to-everything applications by addressing onboard noise suppression. The performance of antennas in smart vehicles is increasingly complex due to the concentration of electronic devices within small structures, which impacts signal reception. Onboard noise generated by other unknown electronic components on the PCB can couple through the resonance structure of the other antenna, significantly degrading performance. The proposed solution includes an equivalent noise circuit and two double-branch planar inverted-F antennas targeting the B1 and B5 LTE bands and the L1 and L5 GPS bands. The noise circuit can couple with the GPS antenna’s feed loop and requires a noise suppression method to prevent sensitivity degradation. The analysis shows that this approach can increase total isotropic sensitivity from −81.79 dB to −94.61 dB, representing a significant enhancement in antenna sensitivity performance.

Keywords: Antenna Sensitivity Enhancement; Onboard Noise Suppression; V2X Antenna

I. Introduction

As the development of vehicle-to-everything (V2X) technology accelerates, antenna performance requirements for smart vehicles are becoming increasingly complex [1, 2]. Antenna sensitivity is affected by the concentration of various electronic devices in small structures. Studies have explored how to improve antenna sensitivity through isolation from external factors because sensitivity directly influences the ability to receive signals [3–5]. However, electronic devices with internal antennas suffer greatly from onboard noise sources that are unintentionally generated by other electric components inside the printed circuit board (PCB). Moreover, onboard noise can be coupled with other resonances on the board, further reducing antenna performance.

This letter discusses the enhancement of antenna sensitivity through onboard noise suppression in an LTE/GPS communication module for V2X communication. The proposed module contains an equivalent noise circuit, which represents unexpected onboard noise, and two double-branch planar inverted-F antennas (PIFAs) targeting the B1 (2,100 MHz) and B5 (850 MHz) LTE communication bands and the L1 (1,575 MHz) and L5 (1,175 MHz) GPS communication bands. Each PIFA contains two radiation branches with different resonance frequencies sharing an identical feed loop, which exists for impedance matching [6]. In this application, a noise circuit with a resonance frequency of 850 MHz can couple with the feed loop of the GPS antenna, and the magnitude of the coupling will be stronger as the resonance frequency of the feed loop approaches that of the operating band. For this reason, a noise suppression method must be implemented to avoid antenna sensitivity degradation due to the coupling of the noise sources.

II. Module Design and Noise Suppression

Fig. 1 illustrates the overall communication module and the simulated model with a specific antenna structure and an equivalent noise circuit. The module was set to be 100 mm × 50 mm with a 1 mm-thick FR4 substrate (ɛ_r = 4.4), which is a commonly used vehicle tracker size. The simulation was conducted using the Ansys High-Frequency Structure Simulator (HFSS) tool in the driven modal solution type. The double-branch PIFA was designed in the upper left corner to operate in the B1 and B5 LTE communication bands, where L_{_L₁} = 18 nH, L_{_L₂} = 5 nH, and C_L = 2.2 pF. The branch containing L_{_L₁} was set to adjust the resonance frequency to 850 MHz to operate in the B5 band. The magnitude of C_L can control the input impedance as the resonance frequency of the feed loop gets closer to the radiation branch, creating greater coupling.

Fig. 1

Proposed antenna design with (a) a fabricated board, (b) a simulated model, (c) an LTE antenna, (d) a GPS antenna, and (e) an equivalent noise circuit.

An additional radiation branch containing L_{_L₂} was inserted to operate in the B1 band. Another double-branch PIFA was designed in the upper right side to operate the L1 and L5 GPS communication bands, where L_{_G₁} = 9 nH, L_{_G₂} = 23 nH, and C_G = 1.0 pF. This was designed in mirror symmetry to the LTE antenna and with an identical operating mechanism. A noise equivalent circuit was inserted on the right side of the ground plane, where the capacitor C_N = 1.3 pF was used to set the resonance frequency of the B1 band. By defining the LTE antenna as port 1, GPS antenna as port 2, and noise circuit as port 3, Z13′ can be written as follows [7]:

(1) Z13′=Z13-Z12Z23Z22+ZL,

where Z13′ and Z₁₃ are the mutual impedances with and without the GPS antenna, Z₁₂ and Z₂₃ are the mutual impedances between the GPS antenna and the LTE antenna and the noise circuit, respectively, and Z₂₂ and Z_L are the self-impedance and terminated load impedance at the GPS antenna. Z₁₃ is a negligible value because the LTE antenna and the noise source are isolated. By comparing Z13′ and Z₁₃, the effect of the noise source coupling with the LTE antenna through the reference GPS antenna and the modified GPS can be estimated. If the resonance frequency of the feed loop of the GPS antenna approaches that of the noise circuit, Z₂₂ + Z_L approaches zero, and Z13′ is strongly impacted. In other words, antenna sensitivity degradation is exacerbated by the coupling of the noise source and the GPS antenna. To suppress the sensitivity degradation effect, it is necessary to isolate the coupling between the GPS feed loop and the noise circuit by tuning the resonance frequency.

Fig. 2 shows the simulated current distribution of the GPS antenna with different capacitance values. When C_G = 1 pF, the resonance frequency of the feed loop approached 850 MHz, and the magnitude of the loop resonance current was strengthened. This resulted in the maximization of Z₁₂Z₂₃/(Z₂₂ + Z_L), where Z₂₂ + Z_L approached zero. When C_G = 0.05 pF, the magnitude of Z₂₂ + Z_L significantly increased, making Z₁₂Z₂₃/(Z₂₂ + Z_L) negligible value.

Fig. 2

Current distribution of the GPS antenna at 850 MHz when (a) C_g = 1.0 pF and (b) C_g = 0.05 pF.

Fig. 3 represents the simulated reflection coefficient comparison of the LTE antenna for the cases of C_g = 1.0 pF and C_g = 0.05 pF in the GPS antenna and without an equivalent noise circuit. A resonance of the feed loop of the GPS antenna that could be coupled with the equivalent noise circuit antenna was observed when C_g = 1.0 pF. The reflection coefficient was fairly close to the case without a noise circuit when C_g = 0.05 pF. By tuning the resonance frequency of the feed loop, noise coupling could be avoided.

Fig. 3

Simulated reflection coefficient (S₁₁) of the LTE antenna.

Fig. 4 shows the simulated reflection coefficient and the measured total efficiency comparison of the GPS antenna between the cases of C_g = 1.0 pF and C_g = 0.05 pF. Changing the capacitance value led to an impedance mismatch, which resulted in a degradation of the total efficiency. However, the difference in the peak total efficiency between the two cases was less than 8%, which is not crucial for GPS antenna performance.

Fig. 4

Simulated reflection coefficient (S₁₁) and the measured total efficiency of the GPS antenna.

Table 1 shows the active test results, including the measured total radiated power (TRP) and total isotropic sensitivity (TIS) values in dBm for the proposed LTE antenna. TRP is the sum of all power radiated by an antenna, and TIS refers to the lowest detectable received signal strength. These values were calculated as follows [8]:

Table 1

Measured TRP and TIS of the LTE antenna for the B1 and B5 bands (unit: dBm)

(2) TRP=Pcond+Efficiency,

(3) TIS=Scond-Sinterference-Efficiency,

where generally used units of TRP, TIS, P_cond, S_cond, S_interference are in dBm, and efficiency is in dB. P_cond and S_cond, the conducted power and sensitivity from the commercial IC chip, were 22 dBm and −99 dBm, respectively, for the proposed application. These results imply that the IC chip generates a signal level of 22 dBm to the antenna input in transmitting mode and recognizes the signal, which is stronger than −99 dBm, in receiving mode. It is noted that lower TIS leads to better antenna sensitivity. S_interference is determined by noise coupling within the onboard noise source. For the TIS data comparison at the B5 band, the coupled noise source was suppressed by tuning the resonance frequency of the feed loop of the GPS antenna. In other words, S_interference was reduced, resulting in a TIS improvement of more than 12 dB.

III. Conclusion

In this letter, an onboard noise suppression method in an LTE/GPS communication module is proposed. This letter focused on antenna sensitivity degradation caused by unintentional noise source amplification and a method to avoid it. The analysis results showed that TIS increased from −81.79 dB to −94.61 dB, demonstrating the effectiveness of the noise suppression method in enhancing antenna performance.

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