Cellular IoT – The Dust Slowly Settles – Part 3 – Cat-M1 Devices

After an overview in part 1 and and introduction to rather simple enhancements for the Internet of Things (IoT) with LTE Cat-1 and Cat-0 devices in part 2, this 3rd part in my mini-series on the cellular Internet of Things (IoT) takes a look at some of the more profound changes in the LTE specifications for IoT, Cat-M1 and Cat-M2 devices and network support for them.

One driver of device complexity and power consumption of LTE are the very broad communication channels. LTE devices of all standard LTE device categories have to be able to monitor control channels and receive data in a channel that can be up to 20 MHz wide. For IoT devices where data rate is of secondary concern this was changed in two steps:

LTE Category M1 (Cat-M1) Devices

Cat-M1 devices, specified in 3GPP Release 13, only have to support a maximum channel bandwidth of 1.4 MHz and support a maximum data rate of 1 Mbit/s. This requires changes on the physical layer of the LTE air interface as the standard LTE control channels operate across the full standard LTE channel bandwidth (e.g. 20 MHz). As a consequence, additional control channels that are invisible to standard LTE devices have been introduced which are only spread across a 1.4 MHz bandwidth. Note that the overall LTE bearer can still be 20 MHz wide but CAT-M1 devices only see a 1.4 MHz broad part of it. To extend cell range or to offer better in-house coverage signaling information and user data can be repeated, i.e. there is additional redundancy.

Like CAT-0 devices discussed in the previous post, a software update on the network side is required. Without the upgrade, CAT-M1 devices won’t even see a network as the new signaling channels are not broadcast.

Many sources also mention CAT-M devices. So what is the difference between CAT-M and CAT-M1!? According to this post it’s the same thing. At some point CAT-M was renamed to CAT-M1, as a further 3GPP IoT work item in Release 13, now referred to as Narrow-Band Internet of Things (NB-IoT), introduces yet another device class, CAT-M2 (*) and even narrower channels. More about this in part 4 of this mini-series.

(*) Update 15.6.2018: The term is not used anymore in this context but has been reused for an evolution of CAT-M1

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