Link Parameters

The following LoRa link parameters are proposed for amateur radio LoRa APRS 438:

Table 1 APRS 438 link parameters
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	`0x12`	`0x12`	`0x12`
header mode	explicit (20 bits)	explicit (20 bits)	explicit (20 bits)
CRC	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 the 0x34 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 \(R_c\):

\[R_s = \frac{R_c}{2^{SF}} = \frac{BW}{2^{SF}} = \frac{125\,000}{2^{11}} \approx 61\,\text{symbols/s}\]

The effective data rate \(DR\) or bit rate \(R_b\) can be obtained by taking into account the forward error correction:

(1)\[DR = R_b = R_s \cdot SF \cdot \frac{4}{4 + CR} = \frac{125\,000}{2^{11}} \cdot 11 \cdot \frac{4}{5} \approx 537\,\text{bits/s} \approx 67\,\text{byte/s}\]

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.