802.11-TX: Reduce transmit power, add notes about legal requirements

Readme.md

@@ -32,6 +32,25 @@ Just prints a line for each received CAM.
 Generates and transmits a CAM when a GNSS update was received.
 Depends on the `gnss` feature.
 
+Note: In germany, using the 5875 to 5915 MHz frequency band is only permitted for C-ITS use cases which implement transmit power reduction measures with at least 30 dB reduction range (or equivalent measures) and have a maximum EIRP of 33 dBm.
+This is based on EU regulation 2014/53/EU, so it applies to other EU countries in similar ways.  
+The relevant harmonized standard is ETSI EN 302 571 which mandates (excerpt of most relevant points):
+
+- a maximum power spectral density of 23 dBm/Mhz EIRP
+- transmit power control (TCP) in a range from 3 dBm up to the maximum output power of the equipment
+  * used for CEN DSRC and HDR DSRC interference mitigation by implementing measures from ETSI TS 102 792
+    + Mode A or B: Output power less than 10 dBm EIRP and less than -65 or -45 dBm/MHz EIRP in CEN DSRC band -> **needs to be measured**
+    + Mode C: Maximum transmission time of less than 1 ms and at least a few 100 ms between transmission (realistically) -> **can't be ensured with big messages**
+    + Mode D: "any" realistically possible transmission time, but at least 500 ms between transmissions for ~5 ITS stations/ 1000ms for 10 ITS stations -> **safe to assume when we only send CAMs at 1 Hz**  
+    + But modes C and D also need to:
+      - use protected zone information from CAM messages
+      - use a protected zone database, e.g. from www.asecap-pzdb.com
+  * used for dynamic congestion control (DCC)
+    + mandatory mechanism described in ETSI EN 302 571, chapter 4.2.10
+
+This is **not** implemented in this application!
+**So legally, you're not allowed to send with this.**
+
 ### UART
 Enable UART I/O protocol according to [docs/uart-protocol.md](./docs/uart-protocol.md).