Please note: I acknowledge that this question is related to LoRa as opposed to The Things Network or LoRaWAN specifically, however I have spent a number of hours online researching this and cannot seem to find an answer. I have also asked this question on the Electrical Engineering Stack Exchange, but it is still not clear to me what the reason is. I am asking the question on this forum as I have not come across another “LoRa” specific forum.
I also acknowledge that some may accept that this is part of the LoRa standard and do not see a need to question it, however I consider it an important characteristic which I would like to understand.
Start of Post:
I’m trying to learn about how the LoRa transmission standard works.
I am familiar with the concept of bit, symbol and chip.
I understand that the chip rate (number of frequency jumps per second) is equal to the bandwidth. For example, a bandwidth of 125 kHz means there are 125,000 chips per second.
It is not clear to me why the bandwidth (the range of frequencies that the chips can be) are linked to how many chips there are per unit of time (how quickly the transmission frequency moves across the bandwidth).
Why are they linked? In other words, why is the transmit time of each chip fixed to the bandwidth? Is it convention or is there a scientific/mathematical reason for it? Why can’t the chip rate be 250,000 chips per second for a bandwidth of 125 kHz instead of 250 kHz?
Sources:
Understanding the relationship between LoRa chips, chirps, symbols and bits
https://www.mobilefish.com/download/lora/lora_part15.pdf (slide 3)
https://www.frugalprototype.com/wp-content/uploads/2016/08/an1200.22.pdf (page 10)
