LoRa radio technology
Long-range radio in the 433 MHz band – how the nodes reach their gateway
LoRa (Long Range) is a radio technology for small amounts of data over long distances. It needs very little energy, works without a mobile contract and, in the licence-free frequency range, needs no radio licence. That is exactly what makes it ideal for readings: a node sends a short data packet, goes back to sleep and gets by on a battery for a long time.
The HC-50 uses LoRa for the link between nodes and gateway. The gateway then passes the values on via WiFi, see HC-50 series.
The HC-50 at a glance
| Radio chip | SX1262 |
| Frequency | 433 MHz |
| Bandwidth | 125 kHz |
| Spreading factor | SF9, a good compromise between range and transmission time |
| Network | Own, private LoRa network (not LoRaWAN). You need no network operator and no access to public networks |
| Transmit power | Regulated automatically by the node: as low as possible, as high as necessary |
| Range | Typically several kilometres, depending on location and surroundings |
Range
How far a signal carries can be estimated in advance. The illustrations above show a simulation with the online service Radio Mobile Online by radio amateur Roger Coudé. It assumes a transmit power of 10 mW (10 dBm) and takes losses and gains from antennas and cables into account only roughly.
A simulation remains a calculation. In practice the range depends strongly on terrain and buildings, plus interference from other transmitters on neighbouring frequencies and the weather. With clever placement of the gateways, inexpensive, wide-area coverage is nevertheless quite feasible.
What improves the range
- Clear line of sight: The fewer buildings, hills and forest between node and gateway, the better. An elevated position on a mast or on the roof helps most.
- Distance from metal: Large metal surfaces and reinforced concrete in the immediate vicinity weaken the signal.
- Suitable antenna: See next section.
- Short antenna cable: Every metre of cable costs signal.
Tips for choosing a location can be found under Gateway mounting and in the section "Location and mounting" of the node guide.
Antennas
Gateway and nodes have an internal antenna and are therefore protected all around. For most uses it is sufficient. An external antenna is worthwhile if the location is difficult, e.g. with a lot of metal around, or if the range is to be increased significantly. Please contact us about this.
There is a wide variety of antennas on the market. Inexpensive LoRa boards usually come with a small antenna, as seen in the picture. Under good conditions it already reaches 2 to 3 km.
For a base station an antenna with omnidirectional radiation and the highest possible gain is suitable. A long antenna cable, on the other hand, brings losses. So the aim is to find a compromise that works in everyday use. Depending on the mounting situation and desired coverage, you can experiment with different antennas.
Example: DeLock 433 MHz antenna with 3 dBi gain
Radio parameters
Frequency, spreading factor and bandwidth determine how far and how fast transmission takes place. On the HC-50 they are fixed, you do not have to configure anything. The background knowledge helps, however, to understand the system's behaviour.
Frequency
In the EU the 433 MHz and 868 MHz bands are permitted for LoRa. The HC-50 uses 433 MHz. At the same transmit power, lower frequencies propagate more favourably and penetrate obstacles better than higher ones.
All HC-50 devices of an installation use the same radio channel. If there are several gateways within mutual range, please get advice beforehand, see Firmware features.
Spreading factor
The spreading factor (SF) determines how many symbols are used to encode the payload data. For LoRa it is defined from SF6 to SF12. With SF7 there are 128 symbols, with SF11 already 2048 for the same data. A packet of 64 bytes takes about 120 ms with SF7, and well over a second with SF11.
| Small SF (e.g. SF7) | Large SF (e.g. SF11) | |
|---|---|---|
| Transmission time | short | long |
| Power consumption | low | about 10 times higher |
| Range | shorter | greater |
| Radio cell | small | large |
| Participants in the network | more possible | fewer possible |
The HC-50 works with SF9. A data packet thus takes around 0.2 seconds. That is long enough for good range and short enough to spare the battery and to observe the prescribed transmission time limit.
Bandwidth
The bandwidth of the LoRa modulation is adjustable; defined values include 31.25 kHz, 62.5 kHz, 125 kHz, 250 kHz and 500 kHz. A smaller bandwidth needs considerably more time for a message. Bandwidths below 125 kHz only work with special hardware (including a temperature-compensated crystal, TCXO). From 125 kHz upwards all common LoRa chipsets run stably. The HC-50 therefore uses 125 kHz.
Signal quality: RSSI and SNR
Two values describe how good a radio link is. The gateway records them for each packet received and supplies them as readings.
- RSSI (Received Signal Strength Indicator) is the strength of the received signal in dBm. The value is always negative: the closer to 0, the stronger the signal.
- SNR (Signal-to-Noise Ratio) is the distance between signal and background noise in dB. If it is positive, the signal is above the noise. If it is negative, it is buried in the noise.
The special thing about LoRa: the radio technology can still read signals below the noise level. The larger the spreading factor, the further below. What matters is therefore not only how strong the signal is but how much margin remains to the sensitivity limit (link margin).
On the left the signal (RSSI, green) is clearly above the background noise (blue), the distance between them is the leeway. On the right it is below and can still be read, as long as it stays above the sensitivity limit of the spreading factor.
Reference values for this limit at 125 kHz bandwidth:
| Spreading factor | Sensitivity |
|---|---|
| SF7 | −123 dBm |
| SF8 | −126 dBm |
| SF9 (HC-50) | −129 dBm |
| SF10 | −132 dBm |
| SF11 | −133 dBm |
| SF12 | −136 dBm |
Rule of thumb: with about 10 dB margin to the sensitivity limit, a connection is reliable even in rain, foliage or slightly changed conditions. If the RSSI is only just above it, a higher location or a better antenna helps.