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, guaranteeing close to 100% coverage of almost every inhabited region of the planet. 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, and as a result the cellular modules for embedding inside IoT equipment are over-specified, and too costly, for many 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, both of which offer dramatic cost savings compared to the mobile phone networks. 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). The name appears paradoxical: conventional approaches to wireless connectivity would suggest that a network device ought not to be able to operate at low power levels while at the same time transmitting over a wide area.
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. This enables the Transmit (output) power to be extremely low. At the same time, the technology calls for receivers with very high sensitivity, in order to maintain a connection with relatively weak incoming signals.
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. The result is that an LPWAN cell can offer broad coverage, potentially even bigger than a mobile phone cell, while using less power.
First steps towards public network roll-out
Both new public LPWAN technologies operate at frequencies in the ISM licence-free bands. Unlike mobile phone network operators, therefore, LPWAN operators do not need to buy expensive licensed radio spectrum.
Nevertheless, there is a considerable cost to creating a public wireless infrastructure, and inevitably it has taken time to reach a level of coverage at which new public LPWANs could satisfy more than a niche set of users. Now, however, the two LPWAN technologies have become viable options for OEMs and users.
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.
In France, SIGFOX owns and operates the network, develops the local ecosystem and sells communication subscriptions to the market. In other countries these activities are the responsibility of SIGFOX Network Operator (SNO) partners.
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. Some OEMs will develop their own design or module, but many will buy a ready-made SIGFOX Ready™ certified module.
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. Multiple base stations can receive and relay the same message; this native reception diversity combined with the ultra-narrowband signals’ rejection of interference help to make transmissions highly robust.
Close channel spacing in the uplink requires high selectivity at the base station. This is provided by cognitive software-defined radio receivers. The effect is to limit the complexity required in the end device’s modem, helping to keep OEMs’ implementations costs low.
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 ATA8520x family of SIGFOX-compliant products.
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. This makes SIGFOX suitable for applications that transmit small packets relatively infrequently, with long periods in a power-down state to preserve battery life. Utility meters are a good example of the kind of application that the SIGFOX network supports well.
SIGFOX users only pay an annual subscription fee for each node, for provision of network communication service. The annual fee is determined by the network operators in each country, but SIGFOX claims that in all cases, users will benefit from ‘groundbreaking prices [and] extremely low power consumption’.
Implementing a LoRa wide-area network
The route to the development of a universal LoRa (short for ‘Long Range’) 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. The LoRa technology is extremely good, providing a breakthrough in both range and power consumption. 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.
Whether public or private, a LoRa network calls for a concentrator at the centre of a star topology. Communication is bi-directional (half-duplex) natively.
The number of nodes connected to a concentrator depends on the application, and specifically the number of packets that are to be transmitted in a given period of time. 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 combination of the alliance and the LoRa_WAN protocol stack provide an ecosystem that network operators, including existing mobile network operators, can draw on to accelerate and lower the cost of LoRa public network deployments.
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.
This means that a public network implemented with the ready-made LoRa_WAN stack can offer users all the benefits of low power consumption, low cost and high security provided by the LoRa technology. There have already been important examples of service providers adopting LoRa. Orange, one of the largest mobile network operators in the world, selected the LoRa RF technology for an LPWAN scheduled for deployment in metropolitan France. Orange will use the network for ‘smart city’ applications in cities across France.
Using the new options for LPWAN deployment
As Table 1 shows, the two new LPWAN technologies have some differences as well as many similarities. Users who require low-cost, wide-area wireless coverage for end nodes running for years on a small battery were badly hampered by the high network charges and high Transmit power levels of cellular (mobile phone) modem chipsets.
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.
Table 1: comparison of the features of LoRa and SIGFOX
|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|