SigFox is a cellular style, long range, low power, low data rate form of wireless communications that has been developed to provide wireless connectivity for devices like remote sensors, actuators and other M2M and IoT devices.
The SigFox wireless interface has been developed to enable any communications that take place to consume a minimum amount of power. In this way the remote devices can run on battery power for very extended periods without the need for any battery changes or maintenance.
In addition to this, M2M and IoT communications will communications over extended distances and the SigFox system has been designed to enable the transmissions to cover long distances, enabling a limited number of base stations to be used.
Using a cellular style approach, the remote SigFox nodes are able to communicate with base stations which have Internet connectivity, thereby enabling remote control and data collection from anywhere with Internet connectivity.
In this way, SigFox is able to provide a low data rate connection to anywhere that is covered by a network at very low cost for many M2M and IoT applications.
SigFox M2M application areas
The SigFox network and technology is aimed at the low cost machine to machine application areas where wide area coverage is required. There are a number of applications that need this form of low cost wireless communications technology. Areas where the SigFox network may be used include:
- Home and consumer goods
- Energy related communications - in particular smart metering
- Healthcare - in particular the mHealth applications that are starting to be developed
- Transportation - this can include the automotive management
- Remote monitoring and control
- Retail including point of sale, shelf updating, etc
What is SigFox
SigFox provides a cellular style network operator that provides a tailor-made solution for low-throughput Internet of Things and M2M applications.
For a host of applications from smart meters to control nodes that need connectivity over long ranges the only option until recently has been to use a cellular connection. This option has several disadvantages because cellular phone systems are focussed on voice and high data rates. They are not suited to low data rate connections as the radio interface is complex and this adds cost and power consumption - too much for most M2M / IoT applications.
The SigFox network is aimed at providing connectivity for a variety of applications and users. It is not aimed at one area, but at being for general use by a variety of different types of users. The SIGFOX network performance is characterised by the following:
- Up to 140 messages per object per day
- Payload size for each message is 12 bytes
- Wireless throughput up to 100 bits per second
SigFox radio access network
In view of the low data rates used for IoT connections, the SIGFOX network employs Ultra-Narrow Band, UNB technology. This enables very low transmitter power levels to be used while still being able to maintain a robust data connection.
The SigFox radio link uses unlicensed ISM radio bands. The exact frequencies can vary according to national regulations, but in Europe the 868MHz band is used; in the US it is 915MHz; and 433MHz in Asia.
The density of the cells in the SigFox network is based on an average range of about 30-50km in rural areas and in urban areas where there are usually more obstructions and noise is greater the range may be reduced to between 3 and 10km. Distances can be much higher for outdoor nodes where SIGFOX states line of sight messages could travel over 1000km, although more usual figures will be much less than this.
The overall SigFox network topology has been designed to provide a scalable, high-capacity network, with very low energy consumption, while maintaining a simple and easy to rollout star-based cell infrastructure.
SigFox operation is based around the use of very narrow bandwidths. The uplink and downlink have different characteristics:
- Uplink: The uplink bandwidth is just 100 Hz in the European area, although 600 Hz is allowed in the USA, and the modulation scheme is DBPSK, differential binary phase shift keying.
In Europe the uplink frequency availability is limited to frequencies between 868.00 and 868.60 MHz and the maximum power is limited to 25 mW. Also the European Union has limited the maximum duty cycle to a maximum mean transmission time of 1% to fairly share the spectrum usage between all users of this and other similar communications systems.
- Downlink: For the downlink the channel bandwidth is 1.5 kHz and GFSK, Gaussian frequency shift keying is used as the modulation format. This provides a data rate of 600 bps.
The downlink frequency band is limited to frequencies between 869.40 and 869.65 MHz. In the case of the downlink, the power output is limited to a maximum of 500mW with 10% duty cycle. Again this duty cycle is limited by Eu regulations.
Although the duty cycle limitations affect all IoT / M2M type applications using the 868MHz band, it limits the Sigfox data transmission. It means that the maximum length of a SigFox packet is 24 bytes, and of this, the payload data may occupy a maximum of 12 bytes. Accordingly it can be seen that with the data rate of 100bps each packet transmission takes about 2 seconds. Also each transmission from a SigFox device consists of three packets transmitted each transmitted on three different frequencies chosen from a pseudorandom sequence. The multiple transmission of the data is sued to ensure the integrity of the data received.
Early versions of SigFox only supported uplink transmissions, but later the system evolved to support bi-directional communication, even though there is significant link asymmetry as seen from the details of the uplink and downlink.
In terms of the system protocol, the downlink communication, i.e., data from the base stations to the end devices can only occur following an uplink communication.
Also the number of messages sent each day is limited, making the system ideal for low data rate remote monitoring nodes, etc. In view of this acknowledgements are not set, but the uplink communication reliability is provided by the use of time and frequency diversity as well as the duplication of transmissions.
As SigFox is a proprietary standard different numbers of messages are allowed each day according to the plan which has been selected.
Sigfox system parameters summary
The key figures for the Sigfox radio interface are summarised in the table below:
|Parameters for Sigfox System
| Downlink: GFSK
|Downlink channel bandwidth
|100 / 600 Hz (Eu / USA)
|Uplink channel bandwidth
|Uplink data rate
| Eu: 100 bps
USA 600 bps
|Downlink data rate
SIGFOX is one of a number of systems that is being deployed to meet the growing demand for M2M and IoT applications. Each of the different systems has its own characteristics and the size of the market will mean that there is space for several different competing systems.
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