Internet of Things

Low Power Wide Area Networks
– Sigfox
Machine Type Communication
– Narrow Band IoT

Development of Internet of Things

The purpose of the Internet was originally to exchange data over a network. This usually involves access to external data for the use of people. The Internet can also be used, for example, to read the current meteorological data from a measuring station. In this case, data from sensors is read via the Internet. These scenarios are referred to as the Internet of Things (IoT). Over the course of the noughties, more and more sensors were connected to the Internet, so that there are now more connections to things than to people or servers. This was also one of the reasons to significantly increase the number of IP addresses with the introduction of IPV6.

A common way to connect sensors to the Internet was through so-called IoT hubs. This involves connecting sensors with a PAN or LAN network to a router that is connected to the Internet. Technologies such as WLAN, BT or ZigBee are used here. However, for many applications it would be easier to connect a sensor directly to the Internet. E.g. an electricity meter in the basement of a house.

It would be ideal for a utility company to be able to read the meter directly via radio at any time, or for the meter to regularly report its status via a network. It would be ideal if this could be done directly via a mobile network. However, there would be some problems with this. The sensors (with a mobile phone connection) would be too expensive and consume too much power. In addition, a mobile network could become overloaded if it “has to look after” too many sensors. On the other hand, common mobile devices, especially LTE devices, were far too powerful for the sensors. These do not need high transmission rates, but typically only have to transmit a few bytes.

Low Power Wide Area Networks

Driven by IoT, the idea of a network with large cells (wide area networks) emerged, but determined by end devices that only transmit little information and consume extremely little energy. This is called LPWAN.

SIGFOX

A company that recognized the opportunities of an LPWAN early on was called Sigfox. This was a French start-up founded in 2010. They developed their own LPWAN standard. This is based on narrow-band transmission in an unlicensed frequency band (860 MHz). The bandwidth is just 200 kHz. However, individual signals are extreme narrow up to 100 Hz. Data is transmitted at either 100 or 600 bits/s. This makes it possible to achieve very long ranges and penetrate deep into buildings even in the unlicensed band. Sigfox transmits with just 40 mW.

With investments of several hundred million dollars, Sigfox managed to build a global IoT network. Sigfox is either the network operator itself or cooperates with network operators. There is also close cooperation with a number of chip manufacturers who build inexpensive chips for IoT, which are then built into modules. A major application of Sigfox is, for example, tracking deliveries.

The disadvantage of Sigfox, however, is that it is not a “standard”. Whoever builds a system on Sigfox is dependent on Sigfox. Sigfox ran into financial problems in 2021 and had to file for bankruptcy in early 2022. Sigfox has now been sold to a Taiwanese IoT company.

Machine Type Communication

3GPP recognized the need for IoT communication, probably also through the success of Sigfox. The following performance features were seen for so-called Machine Type Communication (MTC).

  • Cost for a module less than 5 dollars
  • Thousands of possible devices in one cell
  • Extremely energy-saving operation with battery life of up to 10 years

To get closer to these goals, 3GPP specified three new categories for the LTE network.

  • LTE CAT-1, (up to 10 Mbit/s) no MIMO operation
  • LTE CAT-2, (up to 1 Mbit/s) half duplex operation and special Power Save mode (PSM)
  • LTE CAT-M1, 1.4 MHz bandwidth (sub-range of a normal LTE carrier)

Although these categories were easy to introduce, they did not result in the required performance characteristics being achieved. A new air interface had to be defined.

Narrow Band IoT

An important step to reduce power consumption is to reduce bandwidth. Power consumption is proportional to the bandwidth, especially when sending. In addition, it makes no sense to use a 20 MHz wide channel if you only want to transmit a few bytes. With 3GPP Release 13, a new air interface for MTC was created which is extremely narrowband. It only uses 180 kHz bandwidth. With a subcarrier spacing of 15 kHz, this corresponds to only 12 subcarriers. This means that only one resource block fits into the narrow band channel. The frame structure of the LTE system and its essential architecture are retained. However, many features of a “mobile system” are omitted because it is assumed from the outset that NB-IoT devices will be installed in a fixed location. This simplifies the protocol significantly.

There are three ways to place an NB-IoT channel:

  • Within an existing LTE channel
  • In the guard band (free protection area between LTE channels)
  • In a GSM channel (a GSM channel is 200 kHz wide)

In the downlink, only one end device is assigned every 12 subcarriers. However, it is possible to use only individual subcarriers in the uplink. If only 1 subcarrier is used, you can send with one twelfth of the transmission power. There is even the possibility of quartering a subcarrier again with intervals of 3.75 kHz and thus getting by with even lower performance. By bundling on just one extremely narrow channel, it is then possible to receive a signal even in extremely poor conditions (e.g. from the basement). With uplink, a device can be assigned 1, 3, 6 or all 12 subcarriers. The modulation here is only QPSK or BPSK.

NB-IoT has the same frame structure as “normal” LTE, i.e. 7 time slots per 0.5 ms. As with LTE, there are synchronization slots and very simple control channels.

NB-IoT was introduced in 2016 with Release 13 and was very soon tested by some mobile operators such as Deutsche Telekom.