For a while I have been using this wireless technology to make my remote environmental sensors communicate to a central connected gateway. With great success I have to say, as I wasn’t expecting the quality of service that LoRa modulation provides.

In order to achieve 10km distance, the speed can be in the order of ~500-1kbps, although the sensors have low bandwidth anyway so even if they transmit every hour, it adds up to around 1-2kB per day. Most of the time only 10’s of bytes are transmitted only as the sensor sends an “i’m alive” packet once per day.

 

The developer of the LoRa modulation was Cycleo (Grenoble, France), later acquired by Semtech and at the time of writting, Semtech is the sole supplier of the RF chips to use this technology.

The following texts are taken from the Semtech “AN1200.22 LoRa™ Modulation Basics”  (14/09/2016) application note:

“LoRa is a proprietary spread spectrum modulation scheme that is derivative of Chirp Spread Spectrum modulation (CSS) and which trades data rate for sensitivity within a fixed channel bandwidth. It implements a variable data rate, utilizing orthogonal spreading factors, which allows the system designer to trade data rate for range or power, so as to optimize network performance in a constant bandwidth. LoRa is a PHY layer implementation and is agnostic with to higher-layer implementations. This allows LoRa to coexist and interoperate with existing network architectures”

” Multipath / fading Resistant – The chirp pulse is relatively broadband and thus LoRa offers immunity to multipath and fading, making it ideal for use in urban and suburban environments, where both mechanisms dominate.”

” Doppler Resistant – Doppler shift causes a small frequency shift in the LoRa pulse which introduces a relatively negligible shift in the time axis of the baseband signal. This frequency offset tolerance mitigates the requirement for tight tolerance reference clock sources. LoRa is ideal for mobile data communications links such as wireless tire-pressure monitoring systems, drive-by applications such as toll booth and mobile tag readers, and trackside communications for railroad infrastructure. ”

” FSK vs. LoRa Sensitivity Comparison – The principle of increasing the wanted signal bandwidth to transmit error free data over longer distance (i.e. in the presence of an increasing noise-to-signal ratio) is a fundamental principal of spread-spectrum communications and can be visualized by comparing the sensitivity of LoRa vs. a competitive landscape of FSK transceivers available as illustrated in Figure below: ”

lora-fsk_sensitivity

 

From the Link Labs website on 14/09/2016

” When processing a LoRa message, additional processing gain is achieved due to the modem’s ability to filter on the constant ramp chirp signal. This is how high sensitivity is achieved. In order to achieve “lock” to the LoRa signal, a long “constant chirp” preamble is transmitted. See Figure 1. This is really the power of LoRa, that an inexpensive chip with a cheap crystal, can achieve very high sensitivity.

LoRa preamble – This preamble can be set as a variable number of “symbols,” which are just the number of chirps. As you might imagine, there is no selectivity between a preamble from one LoRa transmitter vs. another. If there is a constant chirp at the right frequency and at the right chirp rate, a LoRa demodulator will listen to it, whether its from the intended system or not.

 

 

Once a LoRa modem has “locked” on to the preamble signal, the end of the preamble is signaled by the “reverse chirp” that is seen in Figure 1. Then the data transmission begins, which has a series of “symbols” that function much like M-ARY FSK symbols, but instead happen on a chirp.

 

 

Another powerful feature of LoRa is the ability to demodulate several “orthogonal” or simultaneous signals at the same frequency, assuming they have different chirp rates. In the datasheet, LoRa chirp rates are called “spreading factors,” with higher spreading factors denoting slower chirps. “