Link Parameters
The following LoRa link parameters are proposed for amateur radio LoRa APRS 438:
LoRa parameter |
uplink |
downlink |
alternative downlink |
---|---|---|---|
frequency |
438.050 MHz |
439.550 MHz |
434.425 MHz |
upchirp bandwith BW |
125 kHz |
125 kHz |
125 kHz |
spectrum |
438.050–438.175 MHz |
439.550–439.675 MHz |
434.425–434.550 MHz |
spreading factor SF |
11 |
11 |
11 |
code rate CR |
1 (5/4) |
1 (5/4) |
1 (5/4) |
preamble sync length |
8 symbols |
8 symbols |
8 symbols |
preamble sync word |
|
|
|
header mode |
explicit (20 bits) |
explicit (20 bits) |
explicit (20 bits) |
on (16 bits) |
on (16 bits) |
on (16 bits) |
|
IQ polarisation |
normal |
inversed |
inversed |
Note
Above preferred frequencies are outside the interfering 433—435 MHz ISM band and mostly respect the IARU Region 1 70 cm band plan.
Attention
The alternative downlink frequency is only intended for those countries where amateur radio is not allowed to transmit (e.g. Austria) or has secondary status (e.g. The Netherlands) on the preferred downlink frequency. Terminal-type end devices will expose an option to select the alternative downlink.
I‑gates using the alternative downlink frequency are fervently advised to emit at higher power levels (e.g. 7.5 W) to overcome the interference caused by ISM systems.
Note
In order to achieve a maximum range, Semtech — the company that developed LoRa — recommends selecting the maximum spreading factor \(SF = 12\). However, SF12 is extremely slow, offering only a mere 36.6 byte/s.
Likewise, the bandwidth is set to the smallest commonly available bandwidth among all LoRa ICs, namely \(BW = 125\,\text{kHz}\). This is by design also the chip rate \(R_c = BW\).
To avoid any further overhead to an already slow mode, the forward error correction (FEC) code rate is kept at \(CR = 1\), which corresponds to \(\frac{data}{data + FEC} = \frac{4}{5}\).
It was observed that amateur radio predominantly employs the LoRa sync word
0x12
; which is manufacturer recommended for private networks, and differs from the0x34
for a LoRaWAN.
With spread‑spectrum modulation, a symbol (or chirp with LoRa) consists out of many chips. The spreading factor \(SF\) is defined as the number of raw bits per symbol. Hence, each symbol or chirp holds \(2^{SF} = 2^{11} = 2048\,\text{chips}\).
This allows one to calculate the symbol rate \(R_s\) from the chip rate \(R_c\):
The effective data rate \(DR\) or bit rate \(R_b\) can be obtained by taking into account the forward error correction:
Above parameters are adequate for sending LoRa frames with short, compressed payloads over the almost longest possible distance when the number of participant nodes is relatively low.
See also
For an in depth tutorial slide series about LoRa (and LoRaWAN), please refer to Mobilefish.com, also available in video format on YouTube.