Multi Channel gateway Compliance

ข้อเขียนเกี่ยวกับการตั้งค่ากำลังส่งของ Multi Channel gateway ให้ Compliance ตามกฎหมายของ EU ซึ่งจำกัดกำลังส่ง 25mW erp โดยการแก้ไขค่า antenna_gain ใน ไฟล์ /opt/ttn-gateway/bin/global_conf.json  รายละเอียดมีดังนี้

The allowed tx power for EU868 is 14 dBm e.r.p., so you are allowed antenna's up to 2.15 dBi, or a bit higher if you take some loss into account. Above that you indeed meed to reduce the tx power.

The latest packet forwarder (not yet, but soon also on the TTN repository) has an option for this in the config json:

"antenna_gain": 0, /* antenna gain, in dBi */

For EU usage, you can enter your antenna gain minus 2.15 in this field. It will then reduce the tx power accordingly.

In the older versions you could reduce the power by modifying the tx_lut table. For every entry, the rf_powervalue defines the tx power these settings apply to. A bit messy, but it should work.

You could shift these values by the gain you have over 2.15. For example in your case,

"rf_power": 2,

becomes

"rf_power": 14,

The board will then output 2dBm if requested 14dBm. Et cetera.

Ideally, you'd use the tx_lut values from the iC88A datasheet, instead of the default of the packet forwarder. Also notice that these go up to 21dBm only, because this board is not licensed to transmit at 27dBm.

Keep in mind when using high gain antennas that you create an asymmetric situation. You have to reduce the tx power, but still have the gain on receive. Because of this you will receive messages from nodes than you can't reach anymore for downlinks. Ideally, we'd make the back-end aware of the antenna gain, such that it can know about this situation, and make downlink decisions accordingly.

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