Can I let LoRa send with 27dBm power? [EU]

Modified on Mon, 4 Mar, 2019 at 3:17 PM

LoraWAN EU868 devices are capable of operating in the 863 to 870 MHz frequency band.
The specification requires devices to support a minimum set of common frequencies to guarantee communications.
These frequencies are 868.1, 868.3 and 868.5 MHz.

Five extra frequencies are added during the OTA procedure: 867.1, 867.3, 867.5, 867.7, 867.9 MHz.
Although these are not part of the specification, they are the most commonly used set of frequencies.

LoraWAN devices in the EU must follow the ETSI regulations for Short Range Devices.
ETSI regulation: https://www.etsi.org/deliver/etsi_en/300200_300299/30022002/03.01.01_60/en_30022002v030101p.pdf

The ETSI regulations define the maximum duty cycle and the maximum radiated power a device may transmit.

band	frequency start	frequency end	effective radiated power e.r.p.	maximum duty cycle
K	863	865	25 mW (14 dBm)	0.1 %
L	865	868	25 mW (14 dBm)	1.0 %
M	868	868.6	25 mW (14 dBm)	1.0 %
N	868.7	869.2	25 mW (14 dBm)	0.1 %
P	869.4	869.65	500 mW (27 dBm)	10.0 %
R	869.7	870	25 mW (14 dBm)	1.0 %

The duty cycle limits pose a limit on how much data a device may send during a certain time period on each band.
For each transmission a device must calculate how much time on air (TOA) the transmission will take an update the current used duty cycle for the corresponding band.

Example:

Band M poses a 1 % DC limit which corresponds to 36 seconds of TOA per hour.
A lorawan packet of 50 bytes sent on datarate 0 takes about 2.3 seconds.
This means you can send 15 packets in one hour on band M.

This example also shows that it is important to increase the datarate and decrease the packet size as much as possible to reduce the TOA.

When a device transmits on one of these eight frequencies, the server/gateway chooses to reply using one of two methods:

1. reply on the same frequency and datarate after 1 second (rx window 1)
2. reply on a fixed frequency and datarate after 2 seconds (rx window 2)

The fixed channel for RX 2 is 869.525 MHz using datarate 0 (band P).

The selection is made based on the remaining available duty cycle that the CloudGate is allowed to send.

The selection algorithm tries to spread the usage on all bands (L, M, P) evenly to keep the choice between using RX 1 and RX 2 available as long as possible.

The result of this algorithm is that at one time the gateway will send with an e.r.p of 14 dBm, and another time using 27 dBm.
Since the device is only allowed to send at 14 dBm, it is considered rare that an uplink sent at 14 dBm cannot be replied to using a downlink with the same output power.