LoRaWAN airtime: why radio conditions matter

How spreading factor and duty cycle shape real smart metering installations

Key takeaways

• ‍A LoRaWAN connection is not simply good or bad. In the European EU868 band, meters on the same network can operate at spreading factors from SF7 to SF12.
• Each step up in spreading factor roughly doubles airtime. For a 10-byte payload, a message at SF12 spends about 24 times longer on air than at SF7.
• The default LoRaWAN uplink channels in EU868, like most additional channels, fall into sub-bands with a 1% duty-cycle limit, so longer airtime reduces how often a device may transmit.
• Maximum payload shrinks to 51 bytes at SF10 to SF12, which is often less than a full M-Bus telegram.
• Gateways are also subject to duty-cycle limits, which makes downlinks and confirmed messages a scarce resource in large installations.

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A 10-byte meter reading takes about 62 milliseconds to transmit at spreading factor 7. The same reading at spreading factor 12 takes almost 1.5 seconds, roughly 24 times longer. Both meters sit on the same LoRaWAN network, both deliver their data, and both count as covered. In terms of battery, channel capacity and how often they are allowed to transmit, they are in very different situations.

The idea works like a conversation in a forest. Two people standing close can talk quickly. Across a wide clearing, the same sentence has to be spoken slowly and clearly to be understood, and it takes longer. In radio terms, distance is not only metres: a concrete wall or a meter shaft two floors underground can put a meter further away from the gateway than hundreds of metres of open air. LoRaWAN adds one more rule: whoever talks for a long time must then stay silent for a long time.

Coverage is not binary

A typical scenario looks like this: one water meter is installed in a ground-floor utility room with a clear path to the gateway. Another sits two floors underground in a concrete meter shaft. Both are online. The first transmits at a high data rate. The second needs the slowest, most robust data rate to reach the gateway.

For remote meter reading, that difference decides how much data each device can send, how often, and at what energy cost. Coverage alone says little about whether a LoRaWAN installation will work well at scale.

What spreading factor does

LoRa modulation trades speed for robustness. In the European EU868 band, LoRaWAN uses spreading factors SF7 to SF12 on 125 kHz channels, mapped to data rates DR5 (SF7) down to DR0 (SF12). A higher spreading factor spreads each bit over a longer signal.

A higher spreading factor improves receiver sensitivity, allowing weaker signals to be decoded at the cost of a lower data rate and longer airtime. That is why SF12 reaches meters that SF7 cannot.

Spreading factor is not transmission power. A higher spreading factor does not mean the device shouts louder; it means it speaks more slowly. Transmission power is a separate parameter.

Airtime in numbers

Time on air (airtime) is how long one transmission occupies the radio channel. The values below are calculated for EU868 with 125 kHz bandwidth, coding rate 4/5 and the standard 13-byte LoRaWAN frame overhead.

Longer airtime has three consequences. The radio stays active longer, and radio transmission is one of the largest contributors to battery drain in a battery-powered meter. The channel is occupied longer, which raises the risk of collisions as the number of devices grows. And airtime is exactly what regulatory limits are measured in.

Duty cycle: transmission time is rationed

LoRaWAN operates in licence-exempt spectrum, governed in Europe by ETSI EN 300 220-2 and national implementations of ERC Recommendation 70-03. The 863 to 870 MHz band is divided into six sub-bands with different limits:

• 863 to 865 MHz: 0.1% duty cycle
• 865 to 868 MHz: 1% duty cycle, where networks commonly place additional uplink channels (867.1 to 867.9 MHz)
• 868.0 to 868.6 MHz: 1% duty cycle, containing the three default LoRaWAN uplink channels
• 868.7 to 869.2 MHz: 0.1% duty cycle
• 869.4 to 869.65 MHz: 10% duty cycle, commonly used for downlinks
• 869.7 to 870 MHz: 1% duty cycle

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A 1% duty cycle allows a transmitter 36 seconds of airtime per hour in that sub-band. Devices using listen-before-talk with adaptive frequency agility (LBT+AFA) can follow different rules, so "1% duty cycle" is not a universal LoRaWAN rule.

The limit is counted separately for each sub-band. After a transmission, the device must wait roughly 99 times its airtime before transmitting again on the same sub-band, but it may use another sub-band in the meantime.

• A 10-byte message at SF7 means about 6 seconds of silence on that sub-band.
• The same message at SF12 means almost 2.5 minutes.
• A 51-byte message at SF12 means more than 4.5 minutes.
• A device with channels in two 1% sub-bands, typical for networks using the additional 867 MHz channels, can therefore transmit roughly twice as often.

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For daily, hourly or even 15-minute readings, a single uplink stays within this limit. A 51-byte message at SF12 every 15 minutes uses about 11 seconds of airtime per hour, less than a third of a single sub-band's allowance. The limit becomes a real constraint at intervals of around 5 minutes at SF12 within one sub-band, or earlier when retransmissions and additional messages add up. At 15-minute intervals, the more significant cost of a high spreading factor is battery consumption and channel occupancy.

Payload limits and M-Bus telegrams

The maximum application payload also depends on the data rate. In EU868, DR0 to DR2 (SF12 to SF10) carry at most 51 bytes, DR3 (SF9) carries 115 bytes and DR4 to DR5 (SF8 and SF7) carry 222 bytes.

This matters for anyone connecting existing meters through an M-Bus converter. A full M-Bus telegram often exceeds 51 bytes, and the meters most likely to fall back to SF12 are exactly those in basements and shafts. Processing data in the converter before transmission addresses this at the source. VIF/DIF filtering sends only the values the receiving system needs, and the fragmentation strategies Truncate, Split and Multiframe cover cases where the full telegram has to arrive.

Downlinks and the gateway's own limits

Duty-cycle limits also apply to gateways. Every downlink, whether a configuration change, an ADR command or an acknowledgement of a confirmed uplink, consumes gateway airtime. One gateway serves all devices in its range, so hundreds or thousands of meters share its downlink capacity.

In a large installation, requesting confirmation for every reading quickly consumes this shared capacity. Confirmed uplinks are therefore best reserved for messages where guaranteed delivery matters, with routine readings sent unconfirmed.

Adaptive Data Rate

Adaptive Data Rate (ADR) lets the network server adjust a device's data rate and transmission power based on the link quality of recent uplinks. For stationary meters, the typical case in smart metering, ADR keeps devices with a good signal on fast data rates and short airtime instead of defaulting to conservative settings.

ADR does not fix poor network design. A meter in a shaft with consistently weak signal will stay at a high spreading factor. Gateway placement, antenna design and installation conditions remain part of the system design.

What to check before deployment

• Installation conditions: how many meters are likely to operate at SF10 to SF12 because of basements, shafts or thick walls
• Reading interval: a daily reading and a 15-minute interval create very different airtime budgets
• Payload size: whether the full M-Bus telegram is needed or whether filtered values are enough, especially below 51 bytes
• Downlink needs: how often devices need configuration changes or confirmed messages
• Network type: private network or public operator, since operators can apply their own policies on top of regulatory limits
• Scale: whether a configuration that works for ten test devices still fits the airtime budget for thousands

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Long range and low energy consumption are the reasons LoRaWAN is widely used for wireless meter reading and LoRaWAN energy monitoring. Neither comes automatically with the technology. A meter that is covered is not necessarily a meter that communicates efficiently. The difference comes from matching spreading factor, payload and reading interval to the conditions in which each meter actually operates.

The values in this article (airtime, duty-cycle limits and payload sizes) are indicative for the EU868 band and standard LoRaWAN parameters. Actual limits depend on national spectrum regulation, the LoRaWAN regional parameters version, the network operator and device configuration.

Glossary of terms used

• ADR (Adaptive Data Rate): a LoRaWAN mechanism that lets the network server adjust a device's data rate and transmission power to the radio conditions
• Airtime (time on air): the time one radio transmission occupies the channel
• Data rate (DR): a LoRaWAN index combining spreading factor and bandwidth, from DR0 (SF12) to DR5 (SF7) in EU868
• Downlink: a message sent from the network through a gateway to an end device
• Duty cycle: the maximum share of time a transmitter may transmit in a given sub-band within an observation period
• Gateway: a network element that receives LoRa transmissions from end devices and forwards them to the network server
• Payload: the application data carried in a message
• Spreading factor (SF): a LoRa modulation parameter that sets the trade-off between data rate, receiver sensitivity and airtime
• Uplink: a message sent from an end device to the network