Wireless transmission systems need to cope with fluctuating RF channel conditions caused by all sorts of unpredictable variations in the environment. RF signals are hardly stable for more than a few seconds and the weaker the signal the larger the fluctuations. The graph reproduced above is typical of the RSSI (Received Signal Strength) of a weak signal reported by a LoRa receiver: over 12 dB variation in a matter of a few minutes and this is by no means a special case. Anyone having practiced RF transmission in urban environment has been confronted with this major problem of fading, unpredictable and very hard to circumvent.
It is of course beneficial for a communications system to use the highest bitrate (lowest Spreading Factor SF) whilst ensuring a reliable transmission over the RF channel.
With LoRaWAN, an end device has to rely on the RSSI of the last messages received from the gateway (downlink) to adjust its bitrate or SF to the channel conditions. The issue with LoRaWAN is that, by construction, downlink is subject to restrictions due to the impossibility of receiving uplink messages while downloading. In practice, an end device may have to rely on messages received hours before and therefore has to set up its SF with a margin in order to stand a reasonable chance for its message to go through, indeed as much as 12 dB in case of a weak signal. One SF grade corresponds to 3 dB, 12 dB corresponds to a margin of 4 SF. The device may therefore have to set SF12 where SF8 would have sufficed, resulting in a transmission time 16 times longer than necessary.
Before uploading a message, a PicoWAN end device first scans a dedicated beacon channel to detect the gateway around with the best RSSI. Every second, each PicoWAN gateway emits a signal burst in the form of a “ping” which is about the shortest LoRa signal possible as well as the most powerful compliant with regulations. The end device will scan the beacon channel for half a second and pick up the gateway with the strongest signal to upload its message using one of 32 channels. It will use the highest LoRa bitrate (or lowest Spreading Factor SF) and lowest power suitable for the instant radio channel conditions derived directly from the RSSI) of the signal just received.
This scheme ensures that the end device is able to assess the quality of the communications channel right before it uploads its message. It can then adjust precisely the transmission speed (or SF) in order to make best use of the transmission channel capacity and save on battery power consumption.