Cellular connectivity for IoT: the case for NarrowBand-IoT

Author : Steve Bovingdon, Field Application Engineer at Anritsu

06 February 2018

Figure 1: NB-IoT in relation to existing LTE cellular technologies
Figure 1: NB-IoT in relation to existing LTE cellular technologies

As the economies of scale provided by the 3GPP ecosystem help drive down prices, NB-IoT will apply well to long-range connectivity of devices with low data rate, low mobility requirements. This piece reviews why testing of low cost IoT devices is still critical to ensure optimum battery performance and minimal interference.

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NarrowBand-IoT (NB-IoT) has a strong case for playing a key role in wirelessly connecting the billions of devices that experts predict will connect our smart cities, homes and offices – and providing a new revenue stream for mobile network operators. As a consequence of rival low power wide area network technologies (such as LoRAWan and Sigfox) growing market share, NB-IoT has rapidly emerged from 3GPP, taking just 9 months following the study phase.

NB-IoT as defined by 3GPP Release 13 is a new radio access technology (RAT). The connection of massive numbers of devices, extended coverage, long battery life, low cost and support for low data rates were key requirements shaping this new technology. Not wanting to be constrained by existing technologies in achieving these goals, 3GPP developed a non-backward compatible variant of LTE that will co-exist with current cellular standards.

However, it still shares many key concepts, including the use of OFDMA for the downlink. This re-use will help to speed up modem development. To reduce complexity and cost, many LTE features that are not as applicable for connected devices, and that are only sending periodic small packets of data, have been stripped out including higher modulation schemes and more complex transmission modes for MIMO). Also, features such as IMS, emergency calls and public warning systems are not in the Release 13 version of the specification. As with other 3GPP cellular technologies, new features will be added in future releases: connected mode mobility, multicast transmissions and positioning features are all under consideration for Release 14.

Deployment

Figure 2: NB-IoT deployment possibilities
Figure 2: NB-IoT deployment possibilities

Utilising a 180MHz channel, NB-IoT can be deployed as a standalone carrier (possibly re-using GSM channels), guard band of an LTE carrier or in-band, occupying a resource block of an existing LTE carrier. As such, for some operators, the capital expenditure of rolling out NB-IoT is minimised by re-using existing cell sites. NB-IoT in Release 13 will be deployed within existing operating bands.

Air access scheme

To reduce RF front end costs, half duplex operation has been chosen in Release 13, so devices can only transmit or receive at any one time, unlike LTE where full duplex is supported. Akin to LTE, NB-IoT uses OFDMA access technology in the downlink and the option of single-tone transmission (3.75 KHz or 15 KHz), or lternatively, SC-FDMA multi-tone transmission in the uplink – using the same 15 kHz sub carrier spacing as LTE. UE (user equipment) uplink transmission capabilities are signalled in its initial RRC Connection Request.

Enhanced coverage

Extended coverage, with deeper penetration into buildings to cater for basements or subterranean installations, is a key requirement for NB-IoT, requiring the technology to cope with additional losses of around 15 to 20dB.

Figure 3: eDRX reducing power consumption
Figure 3: eDRX reducing power consumption

This additional coverage is achieved by using more modest modulations schemes, such as QPSK and even BPSK, along with reduced transport block sizes and repeated transmissions.

For instance, the Master Information Block or MIB (the first message decoded by the UE when accessing a cell) is transmitted with a periodicity of 640ms (compared to 40ms for LTE), during which 8 independently-coded chunks are each transmitted 8 times. This repetition aids reception by the UE.

Downlink data can also be repeated, signalled to the UE in the DCI of the NPDCCH (a new control channel for B-IoT); up to 2048 repetitions can be configured to significantly aid reception at the cell edge.

Similarly, in the uplink, changes have been made to accommodate the narrow bandwidth and to enhance the ange. The new narrow band physical random access channel (NPRACH) operates with single-tone 3.75 KHz transmission, but uses frequency hopping to reduce the likelihood of selective fading impacting reception. The NPRACH transmission can be repeated up to 128 times to extend coverage.

Extended battery life

Figure 4: Anritsu ME7800L Simple Conformance Test System
Figure 4: Anritsu ME7800L Simple Conformance Test System

One of the aims for NB-IoT was to achieve device battery life of greater than 10 years, as many use cases are envisaged where mains power is not readily available. Take for example a remote water meter, or devices used for tracking of goods, animals or vulnerable people – battery charging would not be practical. A simpler RF front end, as an outcome of supporting reduced bandwidth and more modest modulation schemes, reduces power consumption. But in addition, two other key 3GPP concepts, ‘Power Save Mode’ (PSM), introduced in 3GPP Release 12, and ‘Enhanced Discontinuous Reception’ (eDRX) in Release 13, will result in devices being able to power off the radio or move into an extended suspended state. Aimed at devices only sending occasional packets of data, being able to switch off the radio, and not listen for paging, results in significant power savings.

With PSM, UEs remain in the connected state, but essentially the radio is powered down, and as such, uses significantly less power. After waking from the sleep state, UEs are not required to re-attach to the network; and this reduced signalling further improves power consumption, as well as decreases the signalling burden on the network.

DRX itself is not a new concept for cellular devices and provides a mechanism for devices in idle mode not to be required to monitor paging or control channels. Prior to Release 13, the DRX timer allowed for devices not to monitor paging channels for 2.76 seconds. For NB-IoT, in Release 13, the maximum value for eDRX is 2.91 hours – a considerable increase.

Testing

Testing is a crucial part of the development process for any cellular device.

Functional testing, such as how the device performs at the furthest reaches from the cell or deep within a basement, and how the battery life is impacted by operation at the cell edge, are just a few aspects to consider – and difficult to achieve without the right network simulator. For IoT devices, the performance of the modem could be even more critical, due to the costs of replacing poorly functioning devices. Rather than a consumer walking into a high-street store, it could require a maintenance engineer to be dispatched to a remote location – a costly exercise if repeated on a mass scale.


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