This article will cover technical aspects of LoRa and SigFox technology. Although there are similarities like working in same frequency bands, using unlicensed spectrum and playing in the lower cost end of the market (which shouldn’t be covered by telcos), there are some major differences since LoRa can work both ways, it can support higher data rates and it’s not proprietary. On the other hand SigFox although proprietery network controlled by SigFox it can do global roaming that is still not supported by LoRa. In the end a technical comparison table and business oriented chart is given with the brief conclusion.
LoRa is a spread-spectrum radio technology that is developed by company Semtech. Because of its spread spectrum nature and forward error correction, LoRa signals can be read well below the noise floor which gives it an additional security from eavesdropping. Thanks to good tolerance to frequency offsets, cheaper oscillators and transceivers can be used. LoRa transceivers typically support various bandwidths to spread the signal over 125kHz, 250kHz or 500kHz channels. Several spreading factors are used from SF7 to SF12. Higher spreading factor like SF7 improves the system gain so the range can be higher but due to longer transmission time capacity is lower. On the other hand lower spreading factors like SF12 have lower system gain and smaller range from the gateway but the biggest capacity. With smallest 125 kHz channel and spreading factor SF7 the possible capacity is only 300 bit/s and by using 500 kHz channel and SF12 it’s possible to have up to 50 kbit/s of capacity.
On top of the physical LoRa layer the LoRa Alliance has defined upper layers (these would be OSI or TCP/IP layer 2 and 3) and a network architecture that is known as LoRaWAN. Since the LoRa alliance is open in nature the ecosystem of chip and modules providers, device and network infrastructure equipment manufacturers, and network management solution providers is currently growing at a fast pace to be able to produce better, interoperable, simpler and cheaper IoT devices and solutions.
Since the link budgets of LoRa are typically more than 150 dB, quite good ranges can be achieved. In rural environments the range can be up to 15 km, but in urban environment due to shadowing from buildings range vary from around 2 to 3 km.
LoRaWAN network architecture is star-of-stars topology which consists of end devices, gateways and in the core a central network server or LNS (Lora Network Server) from where all the management of network is done. The LoRa gateways are connected to the LNS via IP connection and they don’t do any processing on the packets i.e. they just add information that identifies the gateway. Because of this a message from an end node will be received by all the gateways within the range. The LNS then finds multiple messages and selects only the one from the most suitable gateway.
There are three different types of LoRaWAN classes, each with its own way of receiving and transmitting signals:
Class A end-devices allow bidirectional communications where each end device’s uplink transmission is followed by two short downlink receive windows. This type of operation is used when the end-device only require downlink communication shortly after the uplink packets are sent. At any other time, downlink communications from the server will have to wait for the next scheduled uplink.
Class B end devices open extra receive windows at scheduled times. To be able to do this, gateways must provide the synchronization by regularly broadcasting a beacon. Class B is recommended for lower latency use cases and currently is rarely used.
Class C end devices have nearly continuously open receive windows, which are only closed when transmitting. This is not applicable for battery power devices i.e. it is used only for mains powered devices.
LoRa can also provide geolocation services on top of the data communication by highly accurate time stamping the messages arriving to the gateways using the fine GPS clock which allows to synchronize the gateways within few nanoseconds accuracy. I have written about this in more details here.
SigFox is the name of the French company that provides and promotes proprietary SigFox technology and manages the operations of other national SigFox public network operators. Unlike LoRa which is more open and where there can be more operators in one country and even smaller private LoRa networks, SigFox is a network operator that deploys and manage the network. Actual worldwide coverage is quite good and is being quickly deployed in other continents. Being the only provider (one provider per country is assigned) allows SigFox to offer global roaming which is still not possible with LoRaWAN.
In wireless systems for a defined output power, the achievable range is partially determined by the bandwidth of the receiver. So the smaller the bandwidth is, the lower the sensitivity of the receiver (and the range extended. There are some practical tra trade-offs since very narrow bandwidth also means very low data rates and very long time on air, which will reduce the battery lifetime. Additional problem with very long on air time is increased possibility of interference/collisions with other wireless systems which is particularly a problem since more systems are using these unlicensed frequency bands.
In practice narrowband systems typically use a data rate of less than 1 kbps and a 10 kHz channel bandwidth which puts high requirements on the RF transceiver. SigFox uses a bandwidth which is even 100 times narrower so it is called Ultra Narrow Band (UNB) with a channel width in the uplink of 100 Hz in Europe and 600 Hz in the USA. Hence by using a DBPSK modulation scheme the uplink data rates are 100bps in Europe and 600 bps in the USA. In the downlink the channel bandwidth is 1.5kHz modulated with GFSK to provide a data rate of 600bps.
In Europe the uplink frequency band is from to 868,00 to 868,60 MHz, with a maximum output power of 25mW and a maximum mean transmission time of 1%. The downlink frequency band is from 869,40 to 869,65 MHz, with a maximum output power of 500mW with 10% duty cycle. The duty cycles are defined by European regulation to fairly share the spectrum in the unlicensed ISM bands (it affects LoRa as well).
The maximum length of a packet is 24 bytes, where the user data occupy up to 12 bytes. With 100bps data rate, each packet transmission takes about 2 seconds. While transmitting each device sned three packets that are transmitted on pseudorandom frequencies.
SigFox offer tiered option plans for how many uplink transmissions you are allocated per day, as well as how many downlink transmissions you get from the Network server to your device: Platinum (101 to 140 uplink messages + 4 downlink); Gold (51 to 100 uplink messages + 2 downlink); Silver (3 to 50 uplink messages + 1 downlink); One (1 to 2 uplink messages + no downlink).
My personal opinion is that in this comparison LoRa seems like a better solution due to the fact that it has a bigger ecosystem, much higher data rate, it can be bidirectional and that it supports geolocation. SigFox will probably still be used but for specific applications where small messages are sent rarely from the device to the gateway.
[Some of the text and tables are taken from the article “LPWAN as Enabler for Widespread Geolocation
Solutions” written by Juan Nogueira Nine, Stephane Boudaud and Fabien Ferrero/Leonardo Lizzi. Some pictures are taken from the presentation from company SagemCom which can be a bit biased since they sell LoRa solutions.]
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