As the number of IoT sensors and applications are growing, the unlicensed sub-GHz bands are starting to get crowded, demanding reliable, scalable technologies that can coexist, says András Gnandt, Wireless Technologies, Silicon Laboratories.
The relatively new mioty LPWAN (low power wide area network) wireless technology addresses these requirements with a new PHY (physical layer) and MAC (medium access control) approach called telegram splitting, defined in an open standard. Offering reliability and scalability, mioty is designed for massive commercial and industrial IoT deployments. Silicon Labs is a member of the mioty alliance, the industry body which provides an open, standardised ecosystem for mioty.
In this article, we look at the mioty technology and its target applications in more detail, including its low power performance which targets battery-powered and energy harvesting applications, and we calculate how this relates to battery lifetime.
Introduction
LPWAN technologies target applications where the end devices are deployed over a wide area with long-range transmission capabilities. They are typically battery-powered, transmitting relatively infrequently with low data rates to a base station or collector, to send from one to several hundred bytes of information to data centres. The spectrum used can be licensed or unlicensed.
The topology of an LPWAN network is typically a star, but other options are cellular like NB-IOT (narrow band IoT) LTE-M (long term evolution) or MESH such as for Wi-SUN. The LPWAN technology can be proprietary or standard-based and can support public or private networks for deployment.
The sub-GHz unlicensed ISM (industrial scientific medical) bands are fragmented region by region using different frequency ranges, such as the 169MHz, 433 MHz and 868 MHz band in Europe, 915 MHz band in US and the 434 MHz, 490 MHz, and 920 MHz bands in Asia. The ISM bands fall under different regional regulations regarding transmit output power, on-air time or duty cycling limitations per device for transmissions.
Mioty is an open ETSI standard-based LPWAN technology with a star topology using a disruptive transmission approach of telegram splitting, to enable reliable communication in the congested ISM bands, while still coexisting with the other currently available LPWAN technologies.
Since mioty is still an evolving technology, new device classes – like class B – and features will be introduced and added to the standard in the future to address new applications.
Mioty technology
Mioty is an open standard wireless LPWAN IoT technology based on the LTN (low throughput networks) standards. The LTN systems are covered in three documents: ETSI technical report TR 103 249, ETSI TS 103 358 and ETSI TS 103 357.
Mioty’s core innovation is telegram splitting multiple access (TSMA). In previous standards, continuous transmission in a channel is used by LPWANs to send the application data intended for a complete message. Protection against interference relies on FEC (forward error correction). Self-interference immunity is typically addressed by using different tweaks, such as different modulation modes and/or data rates.
Instead, the physical layer of mioty introduces a new method called telegram splitting. Here, a 100 kHz channel is divided into 24 or 25 instances of 3 kHz wide sub-channels and the application data is transmitted by radio bursts as sub-packets, using a pseudo-random pattern.
This frequency-hopping method gives better protection against interferers in time or frequency domain or both. In the case of self-interference, only sub-packets can collide, with reduced probability due to short bursts and long pauses between bursts. Combined with a 1/3 FEC the application data can be recovered even for the edge case where 50% of the sub-packets are lost. Thus, mioty gives wireless communication in harsh RF environments and helps the deployments of massive IoT.
To target a low-cost solution the selected modulation for TS-UNB PHYs is GMSK (gaussian minimum shift keying) with precoding and FEC, available for almost all IoT silicon, and enables the use of efficient non-linear RF power amplifiers. Besides the ISM bands, there is no limitation on using other bands, and all allowed bands are possible in the 133-966MHz range. Channel bandwidth of 25 kHz, 100 kHz and 725 kHz can be used, defined by the carrier spacing of the TSMA pattern.
Two modulation rates of 2380,371 Symbols/s and 396,729 Symbols/s can cover ULP (ultra low power) and ER (extended range) modes. The gross data rates are 512 bit/s and 85 bit/s for the two modes respectively.
Calculating low power performance
To meet the demands of the IoT, silicon vendors are creating wireless system on chip (SoC) devices for mioty, with extremely low power consumption. For example, Silicon Labs’ EFR32FG23 offers an optimised combination of ultra-low transmit and receive radio power and RF, making it possible for IoT end nodes to achieve wireless range of over a mile, while operating on a coin cell battery for 10 years or more.
Taking a 2200 mAh battery as power supply for the endpoint, and using the datasheet parameters of the EFR32FG23, we calculated the battery lifetime for three examples of different application data sizes: 10, 50 and 200 bytes. The charge used per message was calculated in µAh.
Table 1 shows the related transmission intervals that are possible if we plan to reach 15 years of battery lifetime. We have based these figures on sleep current of 1.2 µA when the SoC is in sleep mode at EM2 energy level with 16 kB RAM retention, and we have used the transmission output power level for the European region as +14 dBm where the SoC uses 25 mA when transmitting the radio bursts. With a typical 1% yearly self-discharge rate of the battery, a 20% self-discharge was calculated for a 15 years time period.
Application data sizeMessage on-air timeTotal message timeCharge per messageTransmission interval10 bytes363 ms3.7 s2.5 µAh5 msg/h50 bytes969 ms10.1 s6.7 µAh2 msg/h200 bytes3240 ms34.1 s22.5 µAh1 msg/2h
Table 1: mioty low power calculations – examples that can achieve 15 years of battery lifetime for a 2200 mAh battery capacity
Looking at these transmission intervals and application data sizes, mioty looks a good fit for smart metering for utilities where only several uplinks per day is enough, and for environmental monitoring where several messages per hour are needed, for example 10 bytes of application data every 12 minutes.
The charge required per message shows that mioty can be used even for energy harvesting applications, since the energy levels are in the usable range of such applications, using solar, heat, mechanical or induction harvesting.
Target market segments
Mioty technology provides reliable, low-power communication using the new TSMA method. Adding robustness to the mix is a good fit for smart metering, especially for cases where the metres are in a hard to reach or deep indoor environment such as water and gas metres.
Mioty can support up to 3.5 million uplink messages per gateway per day in a 2.5 km circle, and 110,000 devices can be deployed per gateway. With this level of scalability and with a 5 km range in urban areas and 15 km range for flat areas, infrastructure monitoring can also be supported for a wide area, including critical points such as airports, bridges and road maintenance.
As environmental concerns are rising today, this means that temperature, air quality and other types of sensors can be widely deployed. They can be powered by energy harvesting, thus lowering maintenance costs and reducing the waste of batteries.
Sustainability of smart buildings can be addressed to maintain the energy usage of buildings with environmental sensors and control the energy management systems. With multicast group messages, street and building lighting can also be covered by mioty.
Alerts sent by predictive maintenance applications combined with machine learning can help to optimise the maintenance of building machinery or detecting water and gas leakages. This reduces response times to start repairs, and cuts wastage of gas and water.
András Gnandt
Conclusions
Mioty is a wireless technology that uses a telegram splitting approach. It is ideal for industrial IoT and smart cities, providing low-power wireless links over long distances.
The author is András Gnandt, Wireless Technologies, Silicon Laboratories.
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