LoRa Explained: How It Sends Miles Without Cell Towers

LoRa (Long Range) is a wireless modulation technique that sends small data packets over distances of several miles using very little power, without needing cell towers, Wi-Fi, or satellites. It works by chirping a signal across a wide frequency band, which makes it extremely resistant to interference and allows receivers to pick up signals far below the noise floor. This article explains the technology behind LoRa, how it achieves its range, and how you can use it for your own long-range projects.

Formula

The data rate (DR) in bits per second is approximately:

DR = SF × (4/5) × (BW / 2^SF)

Where:

• SF = Spreading factor (7 to 12, unitless)
• BW = Bandwidth in hertz (usually 125, 250, or 500 kHz)
• 4/5 = Coding rate factor (for 4/5 coding, common in LoRa)

For time on air (TOA) in seconds, a simplified formula is:

TOA ≈ (8 + payload_bytes × 8 + 16) / DR

This ignores preamble and header overhead but gives a close estimate for typical packets.

Worked Example

Using the default values from the calculator: SF=7, BW=125 kHz, payload=20 bytes.

1. Convert BW to Hz: 125 kHz = 125,000 Hz
2. Calculate symbol rate: BW / 2^SF = 125,000 / 128 = 976.5625 symbols/s
3. Data rate: SF × (4/5) × symbol rate = 7 × 0.8 × 976.5625 = 5,468.75 bps
4. Time on air: (8 + 20×8 + 16) / 5468.75 = 184 / 5468.75 = 0.0336 s (about 34 ms)

The calculator shows a data rate of 5.5 kbps and a time on air of 0.03 s, matching the manual calculation.

What Is LoRa and Where Did It Come From?

LoRa is a proprietary spread-spectrum modulation technique developed by Cycleo (acquired by Semtech in 2012). It uses a form of chirp spread spectrum (CSS), where the signal frequency sweeps up or down over time. Each chirp encodes data, and the receiver correlates the chirps to decode the message. Because the chirps are wideband, they are very robust against noise, Doppler shift, and multipath fading.

The LoRaWAN protocol sits on top of LoRa, defining how devices connect to gateways, how data is encrypted, and how devices join a network. While LoRa is the radio modulation, LoRaWAN is the network layer. You can use LoRa without LoRaWAN (e.g., point-to-point links), but most long-range IoT deployments use LoRaWAN.

LoRa operates in the sub-GHz ISM bands, which are license-free in most countries. Common frequencies: 868 MHz (Europe), 915 MHz (North America), 433 MHz (Asia). These bands allow low-power transmissions without a license, but with duty cycle limits (e.g., 1% in Europe).

How Does LoRa Achieve Miles of Range?

LoRa’s range comes from a combination of three factors:

• Spreading factor (SF): Higher SF (e.g., 12) means each bit is spread over more chirps, making it easier for the receiver to detect the signal even when it’s below the noise floor. This increases range but reduces data rate.
• Low data rate: Slower data rates mean more energy per bit, which improves signal-to-noise ratio. A typical LoRa link might send 0.3 kbps to 50 kbps.
• Receiver sensitivity: LoRa receivers can detect signals as low as -137 dBm (with SF12, BW125 kHz), which is far below what Wi-Fi or Bluetooth can handle.

In practice, a LoRa link with a small antenna can reach 2-5 miles in urban areas and 10-15 miles in open rural areas. With high-gain antennas and ideal conditions (e.g., over water), links of 30+ miles have been demonstrated. The record for a LoRa link is over 700 km, achieved with stratospheric balloons, but that’s not typical for ground use.

Range also depends on the environment: hills, buildings, and trees block signals. LoRa’s chirp modulation helps with multipath (signal reflections) but cannot penetrate solid obstacles like mountains.

How to Choose LoRa Settings for Your Project

Selecting the right spreading factor, bandwidth, and transmit power is a trade-off between range, data rate, and battery life.

• Spreading factor: Use SF7 for short-range, high-throughput links (e.g., indoor sensors). Use SF12 for maximum range, but accept a data rate of only ~0.3 kbps.
• Bandwidth: 125 kHz is the standard for LoRaWAN. 250 kHz and 500 kHz double or quadruple the data rate but reduce sensitivity (and range).
• Transmit power: Most LoRa devices allow +14 dBm to +20 dBm (25 mW to 100 mW). Higher power increases range but drains battery faster and may violate local regulations.
• Coding rate: A coding rate of 4/5 is common; higher redundancy (e.g., 4/8) improves error correction but lowers throughput.

For a typical sensor that sends a few bytes every hour, choose SF10, BW125 kHz, and +14 dBm. This gives a good balance of range (~5 miles) and battery life (years on a coin cell).

Common Problems and Misconceptions

“LoRa is like Wi-Fi but longer range.” Not exactly. LoRa sends tiny packets (up to 255 bytes) at very low data rates. It cannot stream video or audio. It’s designed for sensor data, not internet browsing.

“Higher spreading factor always means better range.” Yes, but it also means longer time on air, which increases the chance of packet collisions and drains battery. Also, some gateways limit the number of SF12 packets to keep the network fair.

“LoRa works anywhere.” No, it needs a gateway (for LoRaWAN) or a second LoRa device for point-to-point. The signal is line-of-sight friendly, but terrain and foliage matter.

Common issue: Antenna mismatch. Using an antenna tuned for 868 MHz on a 915 MHz device will severely reduce range. Always match antenna frequency to your region.

Another issue: Duty cycle limits. In Europe, a device can transmit only 1% of the time on a given channel. If you send too frequently, the device may be blocked or fined.

LoRa vs. Other Wireless Technologies

Here’s how LoRa compares to other common options:

• LoRa vs. Wi-Fi: Wi-Fi offers high data rates (Mbps) but only 50-100 m range and high power consumption. LoRa offers kbps but miles of range.
• LoRa vs. Bluetooth: BLE has similar data rates but a range of ~10 m. LoRa can cover a city.
• LoRa vs. Cellular (LTE-M/NB-IoT): Cellular has better coverage and higher data rates but requires a SIM card and subscription. LoRa is free (no monthly fee) but you must deploy your own gateways.
• LoRa vs. Sigfox: Sigfox is a proprietary ultra-narrowband network with similar range but lower data rate (100 bps) and a subscription fee. LoRa allows you to run your own network.

For battery-powered sensors that send small updates occasionally, LoRa is often the best choice because of its combination of range, low power, and open standard (LoRaWAN).

Costs and Hardware Needed

To start with LoRa, you need:

• LoRa modules: Semtech SX1276/SX1278 chips are common. Development boards like the Heltec LoRa-32 or TTGO LoRa32 cost $15-25.
• Gateway (for LoRaWAN): A single-channel gateway can be built for under $20 using a Raspberry Pi and an SX1276 module. A full 8-channel gateway like the Dragino LG01 costs $100-150.
• Antenna: A quarter-wave whip for 868/915 MHz costs $5-10. A high-gain directional antenna can cost $50+.
• Battery: A 18650 lithium cell ($5) can power a sensor for months or years, depending on transmit frequency.

There are no monthly fees if you run your own gateway. If you use a public LoRaWAN network (e.g., The Things Network), you can connect to community gateways for free, but coverage depends on volunteer infrastructure.

Safety and Regulations

LoRa operates in license-free ISM bands, but you must follow local rules:

• Europe (ETSI): Max transmit power 14 dBm (25 mW) for 868 MHz, duty cycle 1% per channel.
• North America (FCC): Max transmit power 30 dBm (1 W) for 915 MHz, but with frequency hopping and time limits.
• Asia: Varies by country; 433 MHz is common in China and Japan.

Always check your local spectrum regulator before deploying. Also, ensure your antenna is properly matched and grounded to avoid interference. LoRa is low power, so it’s generally safe, but don’t touch a transmitting antenna while it’s active.

Frequently Asked Questions

How far can LoRa actually reach? In an open field, a 100 mW LoRa module with a good antenna can reach 10-15 km. In a city, expect 2-5 km. With a high-gain antenna and perfect line of sight, 30+ km is possible, but not typical.

Can LoRa work without a gateway? Yes. You can set up two LoRa modules to communicate directly (point-to-point). This is useful for remote control or sensor links. For longer range and multiple devices, you need a gateway.

Is LoRa free? The radio spectrum is free to use (within regulations), but you need hardware. If you use a public network like The Things Network, there’s no subscription fee, but you may need to install your own gateway for reliable coverage.

What is the maximum data rate of LoRa? With SF7 and BW500 kHz, LoRa can reach about 50 kbps. In typical LoRaWAN settings (SF12, BW125 kHz), it’s only 0.3 kbps. That’s enough for a few bytes per message.

Can LoRa penetrate walls? Sub-GHz signals penetrate walls better than 2.4 GHz Wi-Fi, but not as well as lower frequencies like 433 MHz. In a building, expect 100-500 m range, depending on wall material. Concrete and metal reduce range significantly.

How long does a LoRa battery last? If you send a 20-byte message every hour at SF10, a 1000 mAh battery can last over a year. If you send every minute, it might last only a few weeks. Lower spreading factors and longer intervals extend battery life.

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in t | Time on air | s | 1 | 0
in p | Payload size | bytes | 20 | 0
in sf | Spreading factor | - | 7 | 0
in bw | Bandwidth | kHz | 125 | 0
out dr | Data rate | bps | (sf * (4/5)) / (2^sf / bw * 1000) * 1000 | 1
out toa | Time on air (calc) | s | (8 + p * 8 + 16) / ((sf * (4/5)) / (2^sf / bw * 1000)) / 1000 | 2