What is the transmit power for the cellular radio?
Can the transmit power be changed? E.g., if I am using a high gain antenna, I may need to reduce transmit power to stay within EIRP limits.
According to the provisions of the 3gpp protocol, the terminal transmission power is dynamically adjusted according to the frequency band, and the maximum is generally 23dbm. However, in the network link, the base station will dynamically control the transmission power of the device according to the current device network environment. This process cannot be interfered by manual, so there is no way to adjust the power size. However, the maximum power value is in line with human safety and will not cause harm to the human body. Can this explanation answer your doubts?
Thanks @Javon_ma. That's useful information. I am not so worried about harm to the body but just trying to understand local regulations.
The FCC (reference - pdf) says,
Mobile Power. We adopt a 1 Watt (30 dBm) EIRP power limit for mobile devices, ... as adopted for the 3.7 GHz Service.
With the TX power maximum at 23 dBm this limits the antenna gain to 30 dBm - 23 dBm = 7 dBi.
However, I see some products like Waveform's antenna kit which advertise 11 dBi.
So if I use this antenna the base station will adjust my transmit power down to 19 dBm? Is there any reason to use an antenna with gain higher than 7 dBi if the transmit power will be lowered by the base station?
If I'm not connected to a base station, then will the TX power be unregulated, and hence too high?
One note - AT+QENG="servingcell" will output a comma-separated list and the second-to-last value is TX power in units of 0.1 dBm.
As far as I know, based on the following explanations, for specific regulations, you can refer to the 3GPP official agreement or the regulations of the regulatory authorities:
Question 1: Is it necessary to reduce the transmission power when using an 11 dBi antenna?
Yes. According to FCC regulations (for mobile devices in the 3.7 GHz band):
EIRP limit = Transmission power (TX) + Antenna gain - Line loss. Assuming line loss is negligible, then: 30 dBm (FCC limit) ≥ TX + Antenna gain. If the antenna gain is 11 dBi, the TX needs to be reduced to:
30 dBm - 11 dBi = 19 dBm
This configuration complies with the FCC requirement (19 dBm + 11 dBi = 30 dBm).
Question 2: Why use an antenna with a gain greater than 7 dBi? Even if the TX power needs to be reduced, the advantages of high-gain antennas include:
- Directional gain: High-gain antennas (such as 11 dBi) are usually directional and can focus energy in a specific direction, improving the quality of long-distance communication.
- Improved receive sensitivity: High-gain antennas can enhance signal reception capabilities, especially in weak signal environments.
- Optimized link budget: Reducing TX power may reduce interference, and directional coverage can increase the effective signal strength in a specific area.
Example: If the base station and user equipment are directionally fixed (such as in fixed wireless access), an 11 dBi antenna can provide better coverage in the target direction, compensating for the impact of reduced TX power.
Question 3: Is the TX power uncontrolled when not connected to a base station?
No. FCC compliance requirements are independent of the base station connection status:
• Mobile devices must be pre-configured with a TX power limit at the factory to ensure that the EIRP of any antenna combination is ≤ 30 dBm.
• Users cannot remove this limit on their own, otherwise the device will not be able to pass the FCC certification.