NB-IoT

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NB-IoT

By the year 2017, the 3GPP has specified NB-IoT (Narrowband IoT) as the new LTE-compatible technology that will enable IoT connectivity over cellular networks.

As 2G technologies started being outdated and overshadowed by LTE, service providers started re-farming their 2G technologies and making them available for LTE use. Therefore, the fact that NB-IoT is actually narrowband, with a system bandwidth of 180 KHz, was useful as it allowed much spectrum flexibility, as now service providers can choose to deploy it either as a standalone solution in re-farmed 2G frequencies or as an in-band solution within the LTE carriers themselves.

NB IoT Design Principles

NB IoT was designed to solve the major issues encountered with already-existing cellular technologies when entering the IoT market. Keep in mind that the following comparisons are drawn against proprietary LPWANs. These issues, for 2G technologies, included relatively lower coverage and relative inability to support massive deployments. As for standard LTE, major issues when supporting IoT revolved around the the high device complexity needed to support LTE, which is reflected in the devices’ high cost and power consumption. Good news is that the goal was not create a generally “better” technology than LTE, the goal was rather to develop a technology more suitable for MTC. This was made easier due to the nature of MTC, which does not require very high throughput as humans need in LTE, nor does it need to encode huge blocks of voice data packets. Therefore, the mission was to tailor an already existing solution to better fit certain use cases, rather than developing a new more advanced generation in mobile communications.

Low Device Cost, Complexity and Power Consumption

For IoT applications, networks usually consist of a high number of devices, there is even a dedicated IoT vertical for such massive deployments called mMTC (massive machine-type communication). Therefore, keeping device costs at a minimal level is essential to ensure network feasibility. On the other hand, low device battery consumption is crucial for a realistic network. This is due to the fact that IoT deployments are usually in open fields, basements, and hard to reach places. This makes the process of periodic and frequent battery replacements very costly and unrealistic. For simplification purposes, device cost, complexity and power consumption will be addressed together as they are related and often dependent on each other.

To reduce device complexity, cost and power consumption, some complex LTE features had to be replaced with ones requiring less processing. For example, certain aspects of cell selection, synchronization and connection initialization had to be made simpler. For example, NB-IoT allows initial acquisition even when frequency drift is as high as 20 parts per million (ppm), as it allows the device to keep track of its frequency drift. In addition, reductions in data throughput were made, as in NB-IoT the maximum downlink transport block size (TBS) is limited to 680 bits, as opposed to several Mbs in standard LTE.

Furthermore, since channel coding and decoding consumes relatively high power, changes had to be made to the coding schemes NB-IoT uses. Standard LTE uses LTE Turbo codes, while NB-IoT uses a more simple scheme called LTE Tail-biting Convolution Code (TBCC).

Added to that, NB-IoT traded off some of standard LTE’s complex modulation techniques and MIMO (multiple input multiple output) transmission schemes, for more simple methods which require less processing capabilities, resulting in cheaper and less-power hungry devices. As a result, NB-IoT does not use MIMO, antenna diversity, high-order modulation techniques, nor fully duplex transmissions.

As for factors directly affecting power consumption, it was found that it is highly affected by the device’s behavior when inactive, otherwise knows as Sleep or Idle mode. Due tot he nature of MTC use cases, devices might spend much time in this idle state, where the device is not reporting any uplink sensor measurements for example, nor is it receiving updates or configurations through downlink. Though, the device still needs to perform some operations and send some radio packets, to keep its status in the network as available, these could be referred to as “Keep Alive Messages”. For NB-IoT,

Coverage Enhancement

For some IoT networks, such as smart metering applications for example, devices are placed behind concrete walls and in building basements. This adds to the importance of high coverage. As previously explained in the LTE-M section, coverage can be enhanced by reducing data rate. Moreover, NB-IoT utilizes repetitions to ensure that data packets reach their destination.

Support for Massive Deployments

NB-IoT aims at supporting the largest number of devices possible, this is achieved by utilizing the uplink transmission schemes which provide the highest spectral efficiency.

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