For some ten years or more, mobile phone (cellular) networks have been the only universal wireless communications technology available to makers and operators of Machine-to- Machine (M2M) communications equipment. For M2M applications, the venerable GPRS (2G) technology has been the entry-level choice of mobile phone network; the newer 3G and 4G technologies offer progressively higher data rates, at a higher connection cost.
All of these mobile phone technologies, however, have serious drawbacks for M2M users: the data rate is far higher than required by most M2M applications. In addition, the high charges that the mobile network operators levy to connect even the simplest of wireless devices reflect the high data rate that the network can support.
What is more, the technology tends to perform poorly when used in harsh or extreme environments. In short, for most M2M applications, using a mobile phone network for universal wireless coverage is expensive.
Soon, however, many users will have the choice of two new wide-area network technologies. This article compares these two newcomers to the M2M scene.
Low power and wide area coverage
Both the new networks fall into a new category of universal network called public Low-Power Wide-Area Network (LPWAN).
Interestingly, the topology of the two new network types is exactly the same as that of the cellular phone technologies: their star topology also has a Base Transceiver Station (BTS) at its centre. But unlike 2G, 3G or 4G systems, an LPWAN uses a modulation scheme that sacrifices data throughput in order to gain greater tolerance of interference and attenuation of the signal. At the same time, the technology calls for receivers with very high sensitivity.
In other words, unlike a mobile phone network, an LPWAN is optimised for the low-power, low data-rate requirements of M2M and IoT applications.
First steps towards public network roll-out
Both new public LPWAN technologies operate at frequencies in the ISM licence-free bands.
SIGFOX™ is one; LoRa™, an LPWAN technology developed by semiconductor manufacturer Semtech, is the other.
Operation of SIGFOX public networks
A SIGFOX public network covers France, Spain, the UK and the Netherlands; beginning in 2015, several field trials were taking place in cities around the world, and nationwide network deployment was starting in Portugal, Denmark, Belgium and the US, as shown in Figure 1.
SIGFOX plans to have national coverage in more than 60 countries by 2020.
An OEM which wishes to join the SIGFOX public network just needs a client module that runs the SIGFOX client stack, and an 868MHz radio transceiver that can perform Differential Binary Phase-Shift Keying (DBPSK) modulation for the uplink and Gaussian Frequency Shift Keying (GFSK) for the downlink.
The gateways and all the networking and application software for transporting the data are provided by SIGFOX to ensure the same quality experience whichever country the objects are communicating in. According to SIGFOX, open-area range for transmissions can be longer than 15km, allowing a network with universal coverage to be created with a relatively small number of cells.
SIGFOX does not use a proprietary modulation scheme, so independent semiconductor and module manufacturers can make transmitters and transceivers which conform to the SIGFOX specification. Atmel, for instance, already supplies the ATA8520 family of SIGFOX-compliant products.
Atmel has also announced the introduction of a fully integrated SIGFOX RF transceiver.
SIGFOX: performance, costs and limitations
In the SIGFOX system, the number of transmissions per day is limited to 140 uplink messages, each of up to 12 bytes, and only four downlink messages of up to 8 bytes. Latency is in the range 3-5ms.
SIGFOX users only pay an annual subscription fee for each node, for provision of network communication service.
Implementing a LoRa wide-area network
The route to the development of a universal Long Range (LoRa™) network has been different from that of SIGFOX.
Based on the Chirp Spread Spectrum (CSS) technique, LoRa is able to vary the length of the so-called ‘spreading factor’ (between 6 and 12 bits) and the bandwidth to match the bit rate required, in a range from 20bits/s up to 41kbits/s. LoRa is a completely asynchronous digital modulation scheme.
Unlike SIGFOX, LoRa technology is fully intended for use in private networks, as well as public networks. In addition, the high performance of the LoRa technology is proven by its ability to receive signals as much as -22dB below the noise floor, coupled with adjacent-channel rejection of at least 69dB with a 25kHz offset – some 30dB better than when using FSK modulation at 868MHz on the same transceivers.
At one time, ISM-band radios for industrial applications and operating at frequencies below 1GHz were typically limited to an open-field range of up to 2km. Semtech introduced LoRa transceiver ICs to provide industrial users, operating closed private networks, with a much longer range of up to 15km between a node and gateway.
The core of the RF implementation offering such performance is provided by the Semtech SX1272 transceiver, which supports a frequency range of 860-1,020MHz, and the SX1276 with a wider range of 137-1,020MHz. Sensitivity reaches a peak of -148dBm in the SX1276.
For applications with very high numbers of end nodes, Semtech has developed a solution for the concentrator: the highly efficient Semtech SX1301 baseband chipset and two Semtech SX1257 I/Q modulators. A concentrator built around these Semtech chips will handle as many as 10,000 nodes.
LoRa in public LPWANs
Recently, Semtech has worked with partners including IBM and Actility to develop a protocol stack for large-scale networks based on its technology, called LoRa_WAN. It is comprised of a client, a server and packet forwarder firmware, as shown in Figure 2.
The introduction of LoRa_WAN is expected to facilitate the introduction of many large-scale private and public LoRa networks in the coming months and years. The roll-out of LoRa networks is supported by the foundation of the LoRa Alliance in December 2014. The alliance includes:
- Various end-node module manufacturers, including Semtech, IMST, Microchip, MultiTech, Link Labs and Embit
- Concentrator manufacturers using the SX1301, including IMST, Kerlink, MultiTech and Embit
- Various network infrastructure operators
- IBM and Actility, providers of cloud servers running on the LoRa_WAN_Server software
The existence of concentrator modules from suppliers such as Kerlink, Embit, IMST and MultiTech means that the hardware for a LoRa BTS can be very rapidly developed. When supplied by Future Electronics, the concentrator may be shipped with pre-loaded IBM or Actility software. For users which intend to connect devices over a private network, rather than relying on the existence of a public LoRa network with the required coverage, this software makes network implementation far quicker and easier than it would otherwise be.
It is worth noting that this private network capability is not available to users of the SIGFOX technology.
A minimum of 15km open-field range between concentrator and node allows for the creation of large cells to quickly achieve wide-area coverage.
All communications over the LoRa_WAN are secured with AES 128- bit encryption, as shown in Figure 3.
In addition, the LoRa_WAN protocol stack manages both the adaptive data-rate and adaptive output-power capabilities of the LoRa technology, for optimisation of power consumption and signal strength.
Using the new options for LPWAN deployment
As Table 1 shows, the two new LPWAN technologies have some differences as well as many similarities.
LoRa and SIGFOX in different ways offer industrial users a clear means to save both power and cost in M2M and IoT applications. But new products and networks using the LoRa and SIGFOX technologies are constantly emerging, and so it will often be helpful for industrial equipment OEMs and others to take advice from experts in the field of M2M wireless networks, such as the specialists at the Future Connectivity Solutions division of Future Electronics, before deciding on the best way to implement LPWAN technology for their application.
|Data Rate||Maximum Sensitivity in Currently Available Hardware||Maximum Output Power||Public/ Private Network||Number of Messages per Day|
|LoRa™||20bits/s to 41kbits/s||-148dBm||Up to 20dBm||Private or public||Public network: dependent on contract with operator Private network: no limit|
|SIGFOX||Uplink: 100bits/s Downlink: 600bits/s||-132dBm @ 600bits/s -142 dBm @ 100bits/s||14dBm (EU) 23dBm (US)||Public-only||140 uplink, 4 downlink|
Table 1: Comparison of the features of LoRa and SIGFOX