LTE-M, LTE Machine to Machine, M2M

- with the Internet of Things, IoT, and Machine to Machine, M2M communications becoming more widespread, there has been a growing need for a version of LTE that meets the needs of low power, low data rate and long battery life.

The Internet of Things, IoT and machine to machine, M2M communications are growing rapidly.

LTE, the Long Term Evolution cellular system is well placed to carry a lot of the traffic for machine to machine communications.

The issue is that LTE is a complex system capable of carrying high data rates.

To overcome this issue a "variant" of LTE, often referred to as LTE-M has been developed for LTE M2M communications.

Associated with it are new categories being launched in Rel 13 of the 3GPP standards known as LTE Cat 1.4MHz and LTE Cat 200kHz.

LTE-M key issues

There are several requirements for LTE M2M applications if the cellular system is to be viable in these scenarios:

  • Wide spectrum of devices:   Any LTE machine to machine system must be able to support a wide variety of different types of devices. These may range from smart meters to vending machines and automotive fleet management to security and medical devices. These different devices have many differing requirements, so any LTE-M system needs to be able to be flexible.
  • Low cost of devices:   Most M2M devices need to be small and fit into equipment that is very cost sensitive. With many low cost M2M systems already available, LTE-M needs to provide the benefits of a cellular system, but at low cost.
  • Long battery life :   Many M2M devices will need to be left unattended for long periods of time in areas where there may be no power supply. Maintaining batteries is a costly business and therefore any devices should be able to have a time between battery changes of up to ten years. This means that the LTE-M system must be capable of draining very little battery power.
  • Enhanced coverage :   LTE-M applications will need to operate within a variety of locations - not just where reception is good. They will need to operate within buildings, often in positions where there is little access and where reception may be poor. Accordingly LTE-M must be able to operate under all conditions.
  • Large volumes - low data rates:   As it is anticipated that volumes of remote devices will be enormous, the LTE-M must be structured so that the networks are be able to accommodate vast numbers of connected devices that may only require small amounts of data to be carried, often in short peaks but with low data rates.

Rel 12 updates for LTE-M

A number of updates were introduced in 3GPP Rel 12 to accommodate LTE-M requirements.

These updates mean that the cost of a low cost M2M modem could be 40 to 50% that of a regular LTE devices, making them comparable with EGPRS ones.

To accommodate these requirements a new a new UE category has been implemented LTE Category 0. These categories define the broad capabilities of the device so that the base station is able to communicate properly. Read more about LTE UE categories.

These low cost LTE-M, M2M modems have limited capability and are:

  • Antennas:   There is the capability for only one receive antenna compared to two receive antennas for other device categories.
  • Transport Block Size:   There is a restriction on the transport block size These low cost LTE-M devices are allowed to send or receive up to 1000 bits of unicast data per sub-frame. This reduces the maximum data rate to 1 Mbps in both the uplink and the downlink.
  • Duplex:   Half duplex FDD devices are supported as an optional feature - this provides cost savings because it enables RF switches and duplexers that are needed for the full performance modems to be removed. It also means there is no need for a second phase locked loop for the frequency conversion, although having only one PLL means that switching times between receive and transmit are longer.

LTE-M features planned for Rel 13

There are several features that are being proposed and prepared for the next release of the 3GPP standards in terms of LTE M2M capabilities. These include some of the following capabilities:

  • Reduce bandwidth to 1.4 MHz for uplink and downlink
  • Reduce transmit power to 20dBm
  • Reduce support for downlink transmission modes
  • Relax the requirements that require high levels of processing, e.g. downlink modulation scheme, reduce downlink HARQ timeline

The UE category for this is being termed 'Cat 1.4MHz'.

There is an additional enhancement for LTE-M under Rel 13 with a reduced bandwidth option of 200kHz in the uplink and downlink - often termed Narrowband or NB lte_m. By reducing the bandwidth and also the data rate, further simplification of the modem can be achieved. The UE category of this is being termed 'Cat 200kHz.'

LTE-Category Summaries

It is possible to summarise the various UE categories and options for LTE-M modems and systems.

Comparison of Capabilities and Modem Complexities for LTE-M
Capability Rel 8
Cat 4
Rel 12
Cat 0
Rel 13
'Cat 1.4 MHz'
Rel 13
'Cat 200kHz'
Downlink peak rate (Mbps) 150 1 1 0.2
Uplink Peak Rate (Mbps) 50 1 1 0.144
Number of antennas 2 1 1 1
Duplex mode Full Half Half Half
UE receive bandwidth (MHz) 20 20 1.4 0.2
UE transmit power (dBm) 23 23 20 23
Relative Modem Complexity 100% 40% 20% <15%

One of the key measurements for any machine node is the modem complexity. As this falls, so does the battery consumption. It can be seen that LTE Cat 1.4MHz and LTE Cat 200kHz have very lower complexity modems when compared to a Cat 4 modem.

Accordingly LTE Cat 1.4MHz and LTE Cat 200kHz will be widely used for LTE-M and M2M applications.

With a number of cellular style M2M wireless communication systems like LoRa and SIGFOX being deployed, LTE needs its own M2M capability to ensure that it is able to compete with these growing standards. Otherwise LTE may not be suitable for carrying this form of low data rate date from devices that require long battery life, etc. LTE-M is the cellular operators' answer to this.

By Ian Poole

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