Wednesday, 23 April 2014

Discovering Sigfox

Image taken from
A few weeks ago We were invited to a workshop hosted by Abertis Spain, introducing SIGFOX: a cellular/mobile provider offering no less than connectivity for the Internet of Things through its star-based cell infrastructure, acting as a transport channel, even for devices running on batteries.

In a telecommunication market vastly competing for higher bandwidth and high data rates, SIGFOX proposal is quite simple yet appealing: lower data rates and low power consumption equals lower rates/prices (€1-€14), which is good as you can plan ahead what billing plan fits your expected data traffic, giving you exact numbers when having to dimension your network and its possible growth.

Abertis Telecom is SIGFOX operator in Spain, allowing to use its telecommunications infrastructure (broadcast, satellite and mobile communications) to support applications built on top of SIGFOX.  This is a good thing to have in mind when planning a network and its possible growth over time, specially in Smart Citiy applications in which new devices have to be added to either expand the service coverage or add new features, to zones that may lack the proper infrastructure, thus avoiding extra civil work.

But of course, coverage can still be a problem for any RF-based solution :)

As most IoT/WSN applications at the end can be simplified to a basic origin/sink approach, the SIGFOX model can be embraced and ported to existing applications, keeping the power consumption as low as the transceiver allows, as taken from the SIGFOX page:

"Typical emission consumption of a SIGFOX Ready™ modem varies from 20mA to 70mA and they consume next to nothing when inactive"
Which is possible of course by restricting data traffic to one-direction only with low updating rates :)

Now the technical part:

What I found atractive about SIGFOX was the fact that they use sub 1-GHz RF transceivers, thus anyone can port its existing application and adapt to its protocol, not having to purchase a specific radio model or closed HW, as SIGFOX bussiness is focused only on being the carrier, however a certification process is required as expected.

SIGFOX operates in ISM bands (license-free frequency bands), it currently uses 868MHz on Europe (as defined by ETSI and CEPT) and the 902MHz in the USA (as regulated by the FCC), offering different profiles or classes, 0-3, being 3 the worst, mostly for well-covered zones.  I imagine the difference between classes is the transmission power levels, which is expected as to avoid self-interference the wisest thing to do is to confine devices into its cell, avoiding overhearing and interfering with each other and others.

As said earlier, SIGFOX focuses on low throughput devices, restricted to send up to 140 messages per day, each message of maximum 12 bytes (payload data).  Altough this may seem as too low compared to most byte-hungry messages going on the airwaves (up to 128 bytes for most RF transceivers), it actually fits most existing applications, leaving just enough space for data and sensor readings (up to six sensors each with 16-bits measures).  The slow data rate is expected (RSSI are close to -130dBm, so I'm guessing >1Kbps), and in fact is what SIGFOX is promoting, so you will need to think your application towards these constrains, fortunately there are quite a few possible scenarios in which this will work.

My main concern is of course self-interference, as it is right now with fewer radio stations installed all around, reaching to an existing one may require transmitting at a higher power, which in CEPT region places us below the 500mW limit of the 869.400-869.650MHz band, where you can have channels up to 25KHz or even use the whole bandwidth as a high-velocity channel.  The AFA/LBT/duty cycle requirements of the regulation mitigates to some point the self-interference, SIGFOX devices do not use a fixed frequency channel but a random one so there is lesser chances to being interfered twice, also as update rates are quite low, retransmissions attempts can be sky-rocketed as no close to real-time responses are expected, but not all applications are as relaxed.  As we already know, with proper network planning and infrastructure deployment, cells can be adjusted and dimensioned to restrict this effect and the class concept plays an important role in this, but this is a known and expected problem needed to be addressed eventually (surely it is already on their mind!).

The folks at Abertis were kind enough to give free SIGFOX demonstrators, here's ours:

SIGFOX demonstrator with short antenna and user button (back)

The next step will be testing the kit myself and try to sniff some traffic, adapt my sub-1GHz node (a CC1120-based device running on top of a TI MCU) to SIGFOX and develop a simple demo, some colleagues at the office have interesting ideas about what to connect to.

 Documentation available Here

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