Long Term Evolution for Machines: LTE-M
LTE-M is the simplified industry term for the LTE-MTC LPWA technology standard published by 3GPP in the Release 13 specification. It specifically refers to LTE CatM1, suitable for the IoT. LTE-M is a low power wide area technology which supports IoT through lower device complexity and provides extended coverage, while allowing the reuse of the LTE installed base. This allows battery lifetime as long as 10 years or more for a wide range of use cases, with the modem costs reduced to 20-25% of the current EGPRS modems.
Supported by all major mobile equipment, chip set and module manufacturers, LTE-M networks will co-exist with 2G, 3G, and 4G mobile networks and benefit from all the security and privacy mobile network features, such as support for user identity confidentiality, entity authentication, confidentiality, data integrity, and mobile equipment identification. Commercial launches of LTE-M networks will take place globally in 2017.
Business Case & How it Works
The LTE channel is made up of resource blocks of about 230 kHz of spectrum, and LTE-M is part of the 1.4 MHz block, comprised of six resource blocks. LTE-M is more energy efficient because of its extended discontinuous repetition cycle (eRDX), which means the endpoint can communicate with the tower or the network on how often it will wake up to listen for the downlink. This can be anywhere from 10s to 40 minutes or more. While promising, as of April 2017, eDRX had not been deployed on any LTE-M1 networks.
LTE-PSM from Rel 12 had a similar feature, but extended sDRX was created specifically for LTE-M in Rel 13. PSM is an important part of LTE-M, since it allows devices to remain completely ideal, without having to re-join the network when it wakes up.
The advantage of LTE-MTC for M2M communications is that it works within the normal construct of LTE networks. In other words, a cellular carrier like Verizon, AT&T or Bell Canada only have to upload new software onto its base stations to turn on LTE-M and won’t have to spend any money on new antennas. It’s also five times simpler than a category 4 receiver—like that found in user equipment like a cell phone—because it needs only to understand and digitize 1.4 MHz of the channel instead of 20 MHz.
Sequans is a leading provider of chips specifically for LTE-M. Certified modules for LTE-M cost around $15 or less.
LTE-M has a little higher data rate than NB-LTE-M and NB-IoT, but it is able to transmit fairly large chunks of data. Thus, it can be used for applications such as tracking objects, wearables, energy management, utility metering, and city infrastructure. The data rates are theoretically around 100kb/s up and down, but generally average at 64kb/s on today’s network.
LTE eDRX (Extended Discontinuous Reception) and LTE PSM (Power Saving Mode)
We will briefly describe how eDRX and PSM work, and why it greatly improves the battery life for LTE-M1 connected devices. For applications like water meters, agricultural monitors, and cold chain monitors, this is a game changing technology.
LTE eDRX
A normal LTE paging cycle is 1.28s, during which the device can be contacted by the network if traffic is queued for that UE (User Equipment – LTE lingo for the “thing” connected to the network).
Power profile of an LTE-M1 UE in eDRX Mode
The not-yet-implemented LTE DRX improvement to LTE allowed UE devices to sleep for 10.24s between paging cycle which improved sleep current somewhat, but still not enough for multi-year battery life. LTE-M1 also extends the normal idle to 5.12s from 1.28s.
The eDRX innovation allows the UE to tell the network how many “hyper frames” (HF) of 10.24s it would like to sleep before checking back in. The maximum number of HFs a UE can sleep is settable by the mobile network operator, but it should be at least 40 minutes.
For a LTE-M1 device that transmits data once per day, and wakes up every 60 hyper frames to check for commands (this would be about every 10 minutes), a life of 4.7 years is achievable on 2 AA batteries.
LTE PSM
Power Saving Mode is a mode that the UE tells the network it is going to go dormant indefinitely. When the UE host device decides, based on some logic or timer, that it is time to transmit, it wakes up and transmits to the network, and remains in RX mode for 4 idle frames so that it can be reachable if needed.
Power Profile for LTE-M1 Power Saving Mode.
Since the device is dormant during the the entire LTE PSM window, it’s power consumption is extremely low. In fact, it can be even lower than a Sigfox or LoRaWAN module, since the TX rate for LTE-M1 is much higher (do to the lack of interference in the licensed spectrum).
An LTE-M1 devices that transmits once per day in full PSM mode could last well over 10 years on 2 AA batteries.
