LTE-M

LTE-M

The evolution of IoT infrastructure and widespread adoption of IoT devices creates a need for cellular communication technologies optimized for IoT. LTE-M (Long Term Evolution for Machines) — also known as LTE-CatNB — and NB-IoT (Narrow Band IoT) — also known as LTE-CatNB — have emerged as two leading IoT cellular technologies to help meet IoT cellular connectivity requirements. 

Both technologies are growing rapidly, with LTE-M available in countries in the Americas, Europe, Australia, and Asia. In fact, according to GSMA, a total of 60 LTE-M commercial networks have been launched worldwide.

Deployment status of LTE-M and NB-IoT (Source)

Two of the key characteristics any commercially viable IoT connectivity technologies must have are:

• Low power consumption- Many IoT devices are battery-operated, so power consumption is critical
• Low cost- Cost is an important factor in the adoption of IoT devices

Additionally, IoT applications tend to have lower bandwidth and speed requirements than traditional IT infrastructure. For most IoT devices, speeds in the Kbps are good enough.

LTE-M and NB-IoT were developed with these tradeoffs in mind. They offer lower power consumption and lower cost while operating at lower bandwidth and lower speed compared to 4G (LTE-Cat1 and above) networks.

2G (GSM) and 3G were viable low-cost cellular options for IoT networks in the past, but the shift of consumer cellular networks to 4G and later is phasing them out of modern applications. LTE-M and NB-IoT fill this gap because they can use 4G infrastructure.

This article will explore LTE-M in depth including its features, differences from other cellular connectivity technologies, applications, and best practices for selecting and using LTE-M solutions.

Executive summary

Below is a summary of the features and use cases of LTE-M.

Network Infrastructure for LTE-M

LTE-M works on existing LTE infrastructure. As an IoT developer, you only need to worry about the selection of the LTE-M module and hardware integration. The rest of the network infrastructure is taken care of by the cellular service provider whose SIM card you use in your module. The diagram below provides an overview of how LTE-M network infrastructure works. 

LTE Network Architecture. (Source)

As you can see, on the one end, there is the UE (User Equipment), i.e., your device, and on the other, there’s the PDN (Packet Data Network), that you’d like to connect to. The UE is connected to eNodeB which is the physical hardware that communicates wirelessly with the UE (similar to the base transceiver station with GSM). MME (Mobility Management Entity) authenticates the UE with the help of the HSS (Home Subscriber Server) database. 

The HSS database knows which user should be able to access which PDN, their Quality of Service, and so on. The PDN is connected to the PGW (PDN Gateway), which is connected to the SGW (Serving Gateway). The SGW ensures that the users don’t face any disruption while moving between different eNodeBs, and acts as the link between the eNodeB and the PGW.

For a deeper dive, you can read the TutorialsPoint article on LTE network infrastructure.

LTE-M vs. NB-IoT

So far, we’ve discussed how LTE-M and NB-IoT are both relatively low-cost and low-bandwidth cellular technologies popular for IoT applications. However, they do have several key differences.

From an application perspective, as a rule of thumb: use LTE-M for mobile IoT devices, and use NB-IoT if the device will is stationary and transmits at very low data rates.

The table below details the differences in data rates, bandwidth, and latency for LTE-M and NB-IoT.

LTE-M vs. NB-IoTMetricLTE-MNB-IoTMax Data Rate (Downlink)1 Mbit/s26 kbit/sMax Data Rate (Uplink)1 Mbit/s66 kbit/sBandwidth1.4 MHz180 kHzLatency10-15 ms1.6-10s

However, these aren’t the only differences. The sections below examine some of the other differences between LTE-M and NB-IoT.

LTE-M vs. NB-IoT: Power Consumption

Both LTE-M and NB-IoT use PSM (Power Saving Mode) and eDRX (extended Discontinuous Reception) to save power. Generally, people assume that LTE-M consumes more power than NB-IoT. However, that is not always true. It depends on the application. While LTE-M may consume more power during data transmission, short message durations may enable LTE-M to consume less power overall than NB-IoT.

LTE-M vs. NB-IoT: Mobility

LTE-M performs seamless cell handover (i.e., switching between cell towers or eNodeBs). NB-IoT does not.

Once NB-IoT moves out of the range of a tower, it increases the power to stay connected. If NB-IoT disconnects from a tower, it must register with the network again. Thus, if your application requires mobility, LTE-M is the way to go.

LTE-M vs. NB-IoT: Technology

NB-IoT uses a slightly different technology (DSSS modulation) than LTE-M. LTE-M works only on 4G architecture. NB-IoT works with 2G and 3G architecture as well. Going forward, both LTE-M and NB-IoT are projected to support 5G.

LTE-M vs. GSM

Until GSM is completely phased out, IoT engineers may need to choose between LTE-M and GSM. If your device is expected to last for more than 2-3 years, then choosing LTE-M is a no-brainer, as most operators plan to shut down 2G services by 2025.

If you are developing a device for shorter lifespans, here are some essential factors to help you make the decision.

LTE-M vs. GSM: Cost

The cost of an LTE-M module can be 2x or more than a GSM module. This is perhaps the primary reason there is some inertia in moving from GSM to LTE-M. For instance, Quectal’s GSM Module (M95) costs ~5 USD, while the LTE-M module (BG95) costs ~12 USD.

LTE-M vs. GSM: Power consumption

LTE-M uses less power than GSM (23 dBm vs. 32 dBm). Thus, LTE-M is a better choice if power consumption is a significant consideration

LTE-M vs. GSM: Bandwidth and data rate

As you would expect, LTE-M wins hands down here. GSM has ~200 kHz bandwidth compared to 1.4 MHz for LTE-M. The data rate for GSM peaks at about 50 kbps, while it is 1 Mbps for LTE-M.

LTE-M vs. GSM: Range

LTE-M cell towers have nearly double the range (radius) of GSM cell towers. Therefore, a single cell tower can serve the LTE module for longer distances. This may not always be noticeable but it helps for mobile applications.

LTE-M vs. LTE-Cat1 and higher

In an earlier section, we saw that LTE-M is not yet available in all regions. In these regions, if one wishes to migrate from GSM to LTE, their best bet is LTE-Cat1.

While LTE-Cat1 has several features that are overkill for IoT, it’s the best option compared to the higher iterations of LTE (Cat4, Cat 6, and so on). Therefore, if LTE-M isn’t available, LTE-Cat1 is best for applications that require mobility or higher data rates than NB-IoT.

However, to give you a perspective, here is how LTE-M, LTE-Cat1, and LTE-Cat4 compare in terms of data rate and bandwidth.

LTE-M recommendations and best practices

Now that we’ve covered the characteristics of LTE-M and how it compares to other cellular connectivity options, let’s review some best practices and recommendations.

Determine the need for LTE-M

While LTE-M is well suited for many IoT use cases; it is not the best option for some applications. For example, if you have a very low-bandwidth stationary application — like a smart meter that just sends out one reading per hour — then NB-IoT is best. Similarly, LTE-M is less viable than Ethernet if you have a high-bandwidth latency-sensitve use case — like a smart CCTV camera video streaming application.

Choose the right LTE-M module

Module vendors (Simcomm, Quectal, ublox, etc.) offer multiple LTE-M modules. Here are a few to consider to ensure you select the right module(these apply for LTE-Cat1 and above too):

• Frequency Bands served- Different network providers use different frequency bands to provide LTE services. For example, Reliance Jio in India uses bands 3, 5, and 40. Thus, if someone is using a Jio sim card with the LTE module, then the module must work in at least one of the bands 3, 5, or 40. You can find the supported bands in your region here.
• 2G/3G fallback- If your application requires the module to be deployed in very remote regions without 4G support, you may need modules with 2G/3G fallback. Look for solutions that can use sim cards of providers that support 2G/3G along with 4G.
• Availability- It may so happen that an LTE-M module is available for purchase in your country, but no network provider supports LTE-M. Check the availability of LTE-M services in your country or region before purchasing a module.
• Presence of GNSS (Global Navigation Satellite System)- Since LTE-M is very popular for mobile applications, several LTE and LTE-M modules include built-in GNSS support. If your application requires GNSS, check for the supported constellations (GPS, Galileo, GLONASS, etc.) and choose the one that best matches your needs.

Minimize work when migrating from GSM to LTE-M

Many applications are upgrades instead of greenfield applications where you start from scratch. If you’re migrating from GSM to LTE-M and want to minimize engineering work and cost, try to reuse as much of your GSM code and infrastructure as practical.

For example, you’ll find a lot of AT Commands to be similar. Therefore, instead of rewriting application code from scratch, compare the AT commands manual for your GSM and LTE-M modules and make only the necessary changes. 

Similarly, on the hardware side, while the module footprints change (LTE modules are usually larger than GSM modules), vendors try to ensure pinout compatibility.

Conclusion

LTE-M is an ideal cellular communication technology for many IoT applications, especially mobile ones like asset tracking. It balances power, cost, and data requirements for IoT in a commercially viable way.

Many regions already have LTE-M support, and more are catching on. For those areas where LTE-M isn’t available, LTE-Cat1 is a viable alternative. There is also NB-IoT for low data-rate stationary applications. Understanding the features and tradeoffs of these different IoT connectivity technologies will help you make an informed decision.