IoT Blog

IoT Blog

New Research Reveals LTE-M/CAT-M1 is Ideal for Deep Coverage IoT Applications

by Gus Vos, Chief Engineer, Technology Standards
How much coverage can LTE-M or CAT-M1 provide? It’s a difficult and technical question. In fact, I just finished editing a white paper to answer it, with lots of help and support from 15 other companies (AT&T, Sony, Ericsson, Nokia, Orange, Verizon, KT Corp, SoftBank, NTT DOCOMO, Virtuosys, TelcomSel, KDDI, Sequans, Altair, SK Telecom).  The paper is a bit long and on the technical side, so I thought I would summarize it here in a blog.

As we know, LTE-M is a Low-Power Wide-Area (LPWA) technology that enables low-cost devices for the Internet of Things (IoT), but the actual amount of coverage enhancement it provides has never been properly determined. This created uncertainty about whether LTE-M could suitably address IoT applications that need deep coverage performance, such as water metering. The 3GPP has published coverage targets, but targets are sometimes met, sometimes not met, and sometimes exceeded, thus Sierra Wireless, and the companies who worked with us on the paper, recognized that it’s very important to understand exactly how much coverage LTE-M can actually provide.

The other problem with using the published 3GPP targets is that the assumptions behind the targets are different, so you can’t simply grab those targets and compare them fairly. Thus the paper also goes into how these targets are calculated and the assumptions behind them, so a fair apples-to-apples comparison can be made.  The 3GPP specified coverage targets in terms of Maximum Coupling Loss (MCL).  MCL is a very common measure to describe the amount of coupling loss (in dBs) at which a service can be delivered. For example, without coverage enhancement, legacy LTE systems (before Release 13) can operate up to approximately 142 dB MCL.

The MCL calculation is fairly straightforward and is based on four main inputs: transmit power, receiver noise figure (NF), occupied channel bandwidth, and required SNR. The occupied channel bandwidth and required SNR are functions of the technology, but the transmit power and NF are based on the implement assumptions.  The 3GPP-published 155.7 dB MCL target assumes a 20 dBm device transmit power and a very conservative noise figures from 3GPP TR 36.888. However, this 155.7 dB MCL target translates to 160.7 dB MCL by assuming a 23 dBm device transmit power (common for LTE-M devices) and more realistic noise figures from 3GPP TR 45.820, which were also used in the cellular IoT study for EC-GSM-IoT and NB-IoT.  The white paper goes into the gory details of the MCL calculation—if you want to know more look at section 4 of the paper—but the net result is that there is a HUGE change from 155.7 to 160.7 dB MCL by using typical LTE-M assumptions for noise figure and transmit power.

Now that we understand the more appropriate 3GPP target for LTE-M is 160.7 dB MCL (not 155.7 dB MCL), this still leaves the question of what coverage LTE-M can actually support. To this end, Sierra Wireless and the group of supporting companies conducted an extensive link layer simulation (LLS) analysis of every LTE-M channel, which was the bulk of the work done for the paper.

For consistency, the simulation assumptions across the different channels are common and based on the simulation assumptions used in TR 45.820. The key finding is that LTE-M can provide 164 dB MCL or 21 dB of gain relative to legacy LTE devices. At 164dB MCL (or 21 dB of gain), data rates of 1400 bps in downlink and 250 bps in uplink are possible. For IoT applications that can tolerate lower data speeds and longer acquisition times, a gain of beyond 21 dB can also be supported. It’s important to note that these results are achieved without using base station (eNB) power spectral density (PSD) boosting, which can extend the coverage even more.

The white paper shows that LTE-M supports a very similar if not better coverage enhancement compared to other LPWA technologies (i.e. 164 dB MCL), which clears up any uncertainty that LTE-M can properly support IoT applications requiring deep coverage where latency, mobility and data speed requirements are less stringent. However, LTE-M is also a great choice for IoT applications requiring higher data rates, low latency, full mobility, and voice in typical coverage situations. Overall, this versatility allows LTE-M to support an extremely wide array of IoT applications, which helps to increase volume and drive economies of scale.