5G ATG Radio Frequency Standard for In-flight Internet Access

Today, connecting to in-flight Wi-Fi for work and video streaming while traveling has become commonplace, and one of the core technologies supporting this experience is the 5G ATG (Air to Ground) terminal. As a key device for achieving high-speed communication at high altitudes, ATG terminals must meet stringent radio frequency standards, with reference sensitivity being a crucial indicator determining communication quality. This article will delve into the technical principles, radio frequency specifications, and reference sensitivity requirements of 5G ATG terminals, helping you understand the technological support behind high-altitude internet access.

5G ATG

Bridges for high-altitude internet access

The core mission of ATG technology is to provide high-speed internet access for airplane passengers by establishing a connection between airborne equipment and the ground-based 5G base station network. Its communication link can be simply summarized as: ground network — 5G base station — ATG equipment — Wi-Fi — user terminal. Compared to satellite communication, ATG technology is not only cheaper but also leverages the high bandwidth of 5G to achieve faster network speeds. However, it faces two major challenges: the Doppler effect caused by the ultra-high speed of aircraft and signal coverage and stable access in ultra-high-altitude environments.

Besides terrestrial 5G base station connections, high-altitude internet access can also be achieved through a route of "terrestrial network — satellite earth station — satellite — airborne satellite antenna and terminal — Wi-Fi — user terminal." However, the high cost and speed limitations of satellite communication make ATG (Advanced Telecommunications Group) the more mainstream choice currently. To ensure compatibility and coverage, 3GPP has clearly defined seven 5G frequency bands that can be used for ATG, covering both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes, as follows:

frequency band	Uplink frequency range（MHz）	downlink frequency range（MHz）	duplex mode
n1	1920–1980	2110–2170	FDD
n3	1710–1785	1805–1880	FDD
n34	2010–2025	2010–2025	TDD
n39	1880–1920	1880–1920	TDD
n41	2496–2690	2496–2690	TDD
n78	3300–3800	3300–3800	TDD
n79	4400–5000	4400–5000	TDD

5G ATG

Key RF Features：Antenna and Output Power

| Antenna design specifications

The antenna configuration of an ATG terminal directly affects signal transmission and reception performance. 3GPP has established clear standards for different antenna types:

For ATG terminals equipped with one or more omnidirectional antennas, the conduction characteristics are based on the parameters at the antenna connector.
ATG terminals equipped with antenna arrays (similar to phased array antennas in base stations) use the characteristics at the transceiver array boundary (TAB) connector as a reference. This flexible antenna design is intended to adapt to the aircraft fuselage environment, ensuring stable signal acquisition even in high-altitude, high-speed scenarios.

| Maximum output power requirement

ATG terminals need to report their rated maximum output power via the UE capability parameter maxOutputPowerATG-r18. The reporting conditions are the maximum modulation order supported by the terminal and the full physical resource block (PRB) configuration within the NR carrier channel bandwidth. The measurement duration is at least one subframe (1ms). The value range of this parameter is 23 to 40 dBm, and the output power tolerance for different frequency bands is uniformly ±(2+TT) dB (TT is the test tolerance).

Furthermore, the power control requirements differ for terminals with different antenna configurations:

For terminals with multiple omnidirectional antennas that do not declare antennaArrayType-r18 capability, the measured maximum output power must be maintained within ±2 dB of the reported rated power;
For terminals equipped with antenna arrays and declaring this capability, the measured power must meet the rated power ±2 dB requirement at the transceiver array boundary (TAB). The test tolerance TT is adjusted according to the frequency band: 0.7 dB for frequencies ≤ 3.0 GHz and 1.0 dB for frequencies 3.0 GHz < f ≤ 6.0 GHz.

ATG UE

ATG UE Reference Sensitivity

Reference sensitivity (REFSENS) is a key indicator for evaluating the reception performance of ATG terminals. It refers to the minimum average power at which the terminal's throughput still meets or exceeds the requirements of a specified reference measurement channel at a specific power level. Since 3GPP has not yet finalized test requirements for terminals with more than two transmit ports or TAB array antennas, current mainstream test standards focus on traditional ATG terminals with 2Rx (2 receive ports) and 4Rx (4 receive ports), and the test requirements are consistent with those for single-carrier configurations.

| FDD band reference sensitivity requirements

The reference sensitivity of the FDD band needs to be determined by combining the frequency band, subcarrier spacing (SCS), and channel bandwidth. The following are the core test standards (all including test tolerance TT):

2Rx ports：Taking the n1 band as an example, when SCS=15kHz, the reference sensitivity of the 5MHz channel bandwidth is -96.8 dBm. As the bandwidth increases to 50MHz, the sensitivity increases to -89.6 dBm. The sensitivity of the n3 band is generally higher than that of n1, with -79.7 dBm at 15kHz SCS and 50MHz bandwidth.

4Rx ports：Compared to the 2Rx port configuration, the 4Rx configuration offers a significant improvement in sensitivity. For example, in the n1 band with a 15kHz SCS and a 50MHz bandwidth, the reference sensitivity reaches -92.3 dBm; in the n3 band with the same configuration, it reaches -82.4 dBm. The diversity gain of the multiple receiver ports effectively improves the ability to receive weak signals.

| TDD band reference sensitivity requirements

The reference sensitivity of the TDD band is calculated using a formula, and its core is related to the number of physical resource blocks (NRBs) corresponding to the channel bandwidth. The following are the calculation standards for typical bands (including TT):

n34/n39 frequency band：With 2Rx ports and SCS=15kHz, the sensitivity formula is -100+10log10 (NRB/25)+TT; with 4Rx ports, the formula is reduced by 2.7 dB to reflect the performance advantages of multi-port.

n41/n78/n79 frequency band：As a mainstream TDD frequency band for 5G, its sensitivity formula is adjusted according to the characteristics of the frequency band. Taking the n78 frequency band as an example, the formula for the 2Rx port at 15kHz SCS is -95.8+10log10 (NRB/52)+TT, while the 4Rx port is reduced by 2.2 dB to ensure stable reception even in high bandwidth scenarios.

| Test tolerance supplement

The test tolerance TT for reference sensitivity is consistent with the tolerance standard for the maximum output power of the terminal: 0.7 dB when frequency ≤ 3.0 GHz and 1.0 dB when 3.0 GHz < f ≤ 6.0 GHz. This tolerance needs to be included in the final sensitivity index calculation to ensure the rigor of the test results.

As the core hub of high-altitude communication, the RF standard for 5G ATG terminals revolves around three core principles: stability, efficiency, and compatibility. From frequency band planning to antenna design, from output power control to reference sensitivity requirements, each specification provides a technical basis for solving communication challenges in high-altitude, high-speed scenarios. Reference sensitivity, a key indicator of a terminal's ability to receive weak signals, directly determines the internet experience for aircraft in long-distance, weak-coverage areas. The configuration of multiple receiver ports and refined formulaic standards further enhance the practicality and reliability of ATG technology. With the continuous improvement of 3GPP standards, future ATG terminals will support more antenna configurations and frequency band combinations, bringing a faster and more stable experience to high-altitude communication, making "freedom of internet access in the air" a common occurrence.