Ken Bednasz, VP Application Engineering, Telit
IoT creates limitless remote monitoring and control opportunities for everything from tracking packages, livestock or humans through “wearables”, providing security alerts from remote locations or adding sensors to smart grids for entire cities. It has been forecast there will be an installed base of 31 billion IoT devices in 2020 rising to 75 billion by 2025.
New innovations in cellular narrowband technology have been developed to expand potential IoT applications by extending the reach of mobile networks, reducing costs, increasing ROI, and reducing energy consumption to make battery operation over many years a practical reality.
Narrowband IoT (NB-IoT) is a Low Power Wide Area Network (LPWAN) technology that can be deployed in unused spectrum blocks in existing LTE/4G and upcoming 5G mobile networks. The technology, which has been developed to deliver battery lives of ten years or more, is part of Release 13 of 3GPP’s globally recognized and adopted standards for cellular communications.
IoT devices based on NB-IoT connectivity can target a truly global market opportunity and are likely to make a major contribution to realising the stellar forecasts. NB-IoT is poised to revolutionize businesses that rely on access to data in hard-to-reach locations including cellars and basements, warehouses and rural areas.
NB-IoT modules are coming to market which offer improved cost-efficiency compared with earlier generation cellular technologies. These are opening the way for IoT applications that haven’t previously gained traction because of inadequate performance and/or prohibitive cost.
Potential applications for NB-IoT include:
Adding new remote control or status monitoring features can add real value to all sorts of products and significantly reduce lifetime cost of ownership for end-users. In many cases the marginal cost of differentiating a product by connecting it to the IoT will be low.
Scoping the power budget
The first stage in transforming something into an IoT device is to identify what data can be generated and captured and how that data can be acted upon to add-value assuming it can be analysed remotely.
The frequency with which it is desirable for the device to communicate – to either transmit data or to receive instructions or updates – and the quantity of data that needs to be transferred during each transmission – will determine the power requirements of the IoT connectivity and, consequently, the life expectancy of a battery-powered IoT connectivity solution.
To achieve its exceptional low power performance NB-IoT takes advantage of two key 3GPP functionalities: Power Saving Mode (PSM) and Extended Discontinuous Reception (eDRX).
In PSM, the device tells the network that it is going to go dormant. In response to some event trigger or at a pre-determined time, depending on the specific characteristics of the use case, the device wakes up and transmits to the network. It can then remain in receive mode for a defined period so that it can be reached if needed. Since the device is dormant for the entirety of the PSM window, power consumption is significantly reduced.
Extended Discontinuous Reception (eDRX) builds on 3GPP’s earlier Discontinuous Reception (DRX) LTE feature. In an LTE network, the network pages devices typically every 1.28 seconds. While DRX allowed devices to sleep for 10.24 seconds – eight normal paging cycles – eDRX allows the device to advise the network how many ‘hyper frames’ of 10.24 seconds it intends to sleep for before making itself available again.
Manipulation eDRX settings allows the designer to optimise a PSM-capable device’s wakefulness for the IoT application. These optimizations are ideally suited for small volumes of periodic data traffic.
Incorporating NB-IoT modules
As small and highly integrated computing devices, NB-IoT modules have been engineered to enable connectivity to be added to products of any complexity with ease.
The Telit xE910 family of modules can be used to illustrate typical design considerations. This platform delivers a comprehensive set of connectivity features in a compact 28.2 x 28.2mm LGA form factor. It has been deployed in utility metering, home and commercial security, and situations with limited mobility such as POS and logistics terminals.
With variants for deployment on 2G, 3G, and 4G networks, the family’s common form factor, electrical and programming interfaces allow developers to implement a “design once, use any-where” strategy: it provides a migration path for existing 2G and 3G devices, extending their product life-cycle into today’s increasingly 4G/LTE cellular environment.
The NE910C1 is the LTE NB-IoT evolution of the xE910 series delivering maximum uplink to 20 kbps (single-tone) and downlink to 250 kbps. It supports power saving mode (PSM) and extended discontinuous reception (eDRX). It has been designed to enable rapid IoT/M2M implementation where low cost and low power are more relevant than high speed.
In an IoT device the power supply circuitry and board layout are very important parts of the full product design and have a significant influence on overall performance. Power supply design needs to consider three different design areas: electrical design of the power source; thermal design and PCB layout.
To deliver the nominal required output of 3.8V, for battery-powered devices a single 3.7V Li-Ion cell is well suited to powering xE910 modules. The battery must be rated to supply peak currents up to 0.6 Amps for LTE/4G.
Click image to enlarge
Figure 2. Telit LE910 - compact cellular IoT module family measuring 28.2 x 28.2 x 2.2 mm with variants capable of delivering up to 150 Mbps download speeds
An appropriate low ESR bypass capacitor is required to limit current absorption peaks: a 100μF tantalum capacitor with a rating of at least 10V is appropriate for most applications.
The bypass capacitor should be placed as close as possible to the module’s power input and the PCB trace from the capacitor to the module should be wide enough to ensure a constant voltage level even during current peaks.
The battery connector needs to be designed to avoid polarity inversions when connecting the battery. Alternatively, a diode should be inserted close to the power input to protect the module from inversion.
The power supply, input cables and tracks should be placed on the board in such a way as to guarantee that the high current return paths in the ground plane do not overlap any noise-sensitive circuitry.
The antenna connection is also an important aspect of the full product design as it can have a significant impact on the overall performance. Where the antenna is placed close to the battery or supply lines an EMI filter, such as a ferrite bead, should be inserted on VBATT pins.
NB-IoT connectivity also has applications in externally-powered products and systems such as smart street lighting or parking meters. In such use-cases, battery-power can be replaced by a variety of well understood voltage-dropping techniques to ensure the nominal 3.8V supply and current requirement can be delivered while protecting the module from excessive peaks.
Through the availability of NB-IoT modules previously unimaginable asset tracking and remote status monitoring applications can now be realized to deliver real added value to individuals, enterprises and society at large.