Thursday, December 4, 2014

LoRa in constructed environment

Suddenly LoRa has became one of the hottest technologies in the Internet of Things domain. The number of new customer cases we see in our company is increasing on a daily basis. But where's the beef?

There are two main use cases where LoRa is strong:
- Wide area wireless sensor network (W-WSN)
- Factory (or any facility) area netwok (FAN)

In W-WSN applications, individual diameter of individual LoRa cells may vary from 2 to 20 kilometers. This makes it possible to collect data from many sensors into single gateway for upstream via wired or wireless connections (3G/4G data mode). This is the modem killer scenario explained in previous posting.

Factory area network with star-topology.

FAN is perhaps more common use case for LoRa. Facility can be anything including factory, refinery, warehouse, seaport, shopping center, or office block, and all other cases where mainstream technologies like WiFi, Bluetooth, Zigbee, and other 2.4 GHz RF can't do the job without unreasonable number of access points or complex multi-hop routing algorithms and routing node.

LoRa modulation has certain characteristics which make it suitable for difficult constructed environments like a factory.

Aa a rule of thumb: the lower the frequence the longer the distance, or better peneration. Reduction from 2.4 GHz to Sub-GHz frequencies alone gives significant improvement. The CSS modulation of LoRa makes it possible to detect signals 20dB below the noise floor, yet giving improvement over traditional narrow-band Sub-GHz radios.

Multipath propagation

Multipath fading is a common problem in constructed environment. Signal may travel via multiple different routes due to reflections from different surfaces. It's like echo on a railway station making it very difficult to understand announcements given by loudspeakers. In radio technology, multipath propagation may cause signal amplification or attenuation with short displacement of the receiver.

Up-chirp, increasing frequency sweep in time domain.

Different frequencies have different reflection signature. Chirp Spread Spectrum (CSS) uses frequency sweeps to carry information. During the sweep, some frequency may attenuate, whereas some other frequency may be amplified. All in all, detection of signal in receiver side is very reliable. Consider the railway station example. If audio frequency sweeps - chirps - are broadcasted from loudspeakers, human can easily recognize whether the individual sweep is going from low to high (up-chirp) or from high to low (down-chirp), even if significant echo takes place.

Frequency also affects signal penetration. Due to various reasons, certain frequencies may penetrate better than others. By spreading the signal to wider bandwidth, we increase the probability that some signal gets through. According to our experiments, LoRa has been the only commonly available IoT RF technology, which can deliver data from inside a closed metallic storage container to outside world.

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