Re: wireless-regdb: Wrong ETSI tranmit power without TPC

Chen-Yu Tsai
2025-05-16T08:15:54Z

On Thu, May 8, 2025 at 1:25 AM Alexander Wilhelm
<alexander.wilhelm@westermo.com> wrote:
>
> Hello devs,
>
> please correct me if I'm wrong. Since TPC is not used, the EU Rules dictates to
> decrease the transmit power by 3 dBm on some sub-bands. For the most of the EU
> countries the mW instead of dBm is used. Therefore they are halved (e.g for
> Germany (DE) in commit 81d251dda3985e1088bd89f5d9f565e63ba5a30f). The problem is
> that the values are then converted to dBm and rounded down and result in a wrong
> supported value. Take a look at example (Germany again):
>
> country DE: DFS-ETSI
> (2400 - 2483.5 @ 40), (100 mW)
> (5150 - 5250 @ 80), (200 mW), NO-OUTDOOR, AUTO-BW, wmmrule=ETSI
> (5250 - 5350 @ 80), (100 mW), NO-OUTDOOR, DFS, AUTO-BW, wmmrule=ETSI
> (5470 - 5725 @ 160), (500 mW), DFS, wmmrule=ETSI
> # short range devices (ETSI EN 300 440-1)
> (5725 - 5875 @ 80), (25 mW)
> # WiFi 6E
> (5945 - 6425 @ 320), (23), NO-OUTDOOR, wmmrule=ETSI
> # 60 GHz band channels 1-4 (ETSI EN 302 567)
> (57000 - 66000 @ 2160), (40)
>
> For the frequencies 5250 - 5350 the power of 100 mW results it 20 dBm. Correct.
> The frequencies 5470 - 5725 have a power of 500 mW. It results in 26 dBm. But I
> believe 27 dBm are allowed. For the frequncies 5725 - 5875 the value of 25 mW is
> also dictated. I'm not sure if I should expect 14 dBm, but I get 13 dBm.

Are you reading the converted numbers from the system, not the database
text file?

> Is there inconsistencies between dBm/mW conversions? Please give me a feedback.

The conversions don't always produce round numbers, and since we can't let
the system exceed the _actual_ limit, the values can only be rounded down.
For example, 500 mW converts to roughly 26.9897 dBm. We can't use 27 dBm,
since if the system outputs at 26.999 dBm, it would be in violation of
the rules.

Or, if we understand 3 dBm reduction to be halving the power, we could
fix up any rules that "have their base limit in mW and were converted to
dBm to apply the reduction" be rewritten in mW with the numbers halved.
That would require someone to go through the entries though. But if
the rules are already written in mW, and what you observe is the kernel
rounding down the numbers, then perhaps the kernel may need to support
both units.

Thanks
ChenYu