When an IoT device is about to transmit, it should listen if some other IoT device is transmitting before itself transmits. This is just like humans, we wait until the other finishes before talking. This is called Polite Spectrum Access. If you transmit without listening first, it is referred to as Aloha.
Listen-before-talk (LBT) can increase the spectrum efficiency compared to Aloha by reducing the number of collisions and retransmissions.
In Aloha, each node transmits its packets without checking if the channel is busy, which can lead to collisions and retransmissions. This can result in a significant waste of bandwidth and reduce the overall spectrum efficiency.
LBT, on the other hand, requires each node to listen to the channel before transmitting, which can help to reduce the number of collisions and retransmissions. By listening to the channel, a node can determine if the channel is busy and wait until it is clear before transmitting.
Studies have shown that LBT can increase the spectrum efficiency compared to Aloha by a factor of 2-5, depending on the specific scenario and parameters. For example, a study by the European Telecommunications Standards Institute (ETSI) found that LBT can increase the spectrum efficiency by a factor of 2.5 compared to Aloha in a scenario with a large number of nodes and a high traffic load.
Here is a rough estimate of the spectrum efficiency gain of LBT compared to Aloha:
- Aloha: 18% (this is a typical value for Aloha in a scenario with a large number of nodes and a high traffic load)
- LBT: 45-60% (this is a rough estimate of the spectrum efficiency gain of LBT compared to Aloha, depending on the specific scenario and parameters)
LoRa is based on a modulation scheme called Chirp Spread Spectrum (CSS). It means that during transmission it increases/decreases the frequency at a rate depending on the speed. This makes it almost impossible for CSS to use Polite Spectrum Access.
The EU/US regulation states that an IoT device using the sub-GHz band (868MHz/915MHz) must only transmit 1% if the time, unless it is using Polite Spectrum Access (ETSI EN 300 220-2). Being polite allows for a much more efficient use of the frequency spectrum, which is very important as the frequency spectrum is a limited finite natrual resource. Going forward, we will have many more IoT devices using the spectrum, so using it efficiently is very important.
Almost all commercial off-the-shelf FSK transmitters is able to be polite and listen before talk. Surely we must access the frequency spectrum most efficiently and not use inpolite transmitters.