Design For Narrowband IoT

Most low-power chips are designed with the assumption that batteries can be recharged or replaced, but there is a whole set of IoT devices under development that are expected to be always-on, communicate over a cellular infrastructure, and remain functional on a coin-sized lithium-ion battery for a decade or more.

Welcome to the world of Narrowband IoT (NB-IoT), a 3GPP standard (also known as Cat-NB1 and Cat-NB2) with reduced bandwidth and tighter power consumption requirements. The primary goal is to facilitate integration of the communications IP onto an SoC or microcontroller, including baseband or RF transceivers. This is different than traditional cellular implementations such as 3G, 4G and 5G, which typically consume more power and target different silicon process technology nodes.

The 3GPP standard defines several use cases based on a communications frequency of every 2 hours/once daily, and data bandwidth of 50 bytes/200 bytes. The expectation, with a 5Wh battery, is 10 years of battery lifetime for all use-case combinations. The standard also defines new power modes for extended standby, power-saving mode (PSM), as well as extended time intervals for network synchronization, extended discontinuous reception mode (eDRX).

“For Narrowband IoT, as well as the competitive technologies, it’s all about low power — as low as you possibly get it, especially being extremely low power when you’re asleep,” said Gerard Andrews, product marketing director for audio/voice IP at Cadence. “Many of these modems are going to be inactive for long periods of time, and some event will trigger them, fire them up, then send off data back over the cellular network to the data recording or data reporting station. So a big part of it is how little energy you can consume when you’re asleep. That ultimately will determine the total cost of ownership, along with a number of other issues, including how often you have to service it to change the batteries in the modem. It really comes down to low power from the design standpoint.”

Hakim Jaafar, head of marketing for wireless microcontrollers at STMicroelectronics, agreed. “NB-IoT, with its emphasis on low power/long battery life, low cost/high connection density, provides a powerful technology for many Industrial applications and ST is working closely with partners to assure appropriate products are available to meet customer needs and demand.”

To meet these stringent power consumption goals, designers need to rethink traditional communications architectures.

“Traditional LTE modem hardware and software architectures have incorporated multiple DSPs, several hardware accelerators, control cores, multiple OSs, stacks, etc., to manage the compute-intensive protocols and high-data-rate requirements of wideband LTE modems,” said Rich Collins, product marketing manager for IP subsystems at Synopsys. “Attempting to simply scale down this type of implementation to support NB-IoT does not yield a power- and area-efficient solution. A more flexible NB-IoT modem based on small, low-power CPU/DSP processors, a few targeted hardware accelerators, dedicated power management hardware, and tight integration between the baseband and RF transceiver can help achieve the required power and area goals.”

Other design considerations
The low-power processor must offer excellent code density and an efficient implementation of the software stack so that memory sizes can be kept small. If the memory footprint is small enough, off-chip DRAM can be avoided, keeping system costs down. Small code size is also important for achieving low power consumption, specifically by reducing the number of accesses to instruction memory, Collins said.

One of the most computationally intensive parts of the NB-IoT modem is the decoding of downlink data, which leverages the Viterbi algorithm. That is often a bottleneck in modem design. A software implementation of the Viterbi algorithm on a generic CPU/DSP processor provides limited performance improvement. In contrast, adding small, low-power hardware acceleration can significantly reduce the MHz requirements of Viterbi decoding over software-only implementations, he said.

Aside from hardware IP decisions, it is important to architect flexible power domains into the baseband/RF solution in order to minimize the number of elements within the NB-IoT modem that are required to be always-on. That includes such things as data retention, clocking and power management. The ideal solution involves multiple programmable power domains for both the baseband and the RF transceiver.

These modems typically operate on cellular networks. Getting them certified and tested so they can be deployed on carrier networks is a non-trivial task.

“A huge barrier there is that it’s a big investment to enter the market,” said Cadence’s Andrews. “If you’re successful and you get certified on AT&T, T-Mobile, and China Mobile, etc., then you fight it out with the other guys who’ve made it over this huge hurdle on first of all, the low power end of things. After that, you might be able to have some value added things on your modem such as the way you interact with certain sensors, or the way you leave some processing capability open for the end user to develop their application.”

In addition, these devices are expected to have a very long lifespan, which requires them to functional reliably for far longer periods than typical chip designs, said Paul Hill, director of marketing at Adesto Technologies.

“At the same time, with the Narrowband, you’re getting greater access to legacy technologies like 2G and 3G, which are becoming a very low-cost wireless medium, especially for low-data-rate devices,” Hill said. “Applications include luggage trackers and luggage security tags, which are connected anywhere in the world through a Narrowband interface rather than having to rely on a cell phone to connect through Bluetooth to another cell phone, and replicate through the cloud. Narrowband is an emerging technology that’s going to absolutely explode once the connectivity is more freely available.”

At the end of the day, in connected technologies as a whole, including Narrowband, whether it’s Bluetooth, WiFi, ZigBee or otherwise, in every domain, people are becoming extremely power conscious.

“Almost all of these devices — especially the Narrowband — are battery-powered and rely on small button cells,” Hill said. “No longer are electronics allowed to be built like Legos where you just connect building blocks together. Everybody has to know something about how long this can run on a battery, the limits of the battery, how much startup current can be drawn, how much programming that’s going to raise current can be used, how much standby time it’s possible the device will be in versus how much active time, among other considerations. All of these factors add up and influence the amount of energy consumed.”

The future
Beyond that, there is a concern for how much lithium is needed for these batteries, and what to do with that lithium once the batteries die. This becomes a green issue, which has broad ramifications for chip and system design.

“With COVID, we’ve seen in the last six to eight weeks now where environmentalists have said, ‘Look at how clear the skies are. Look how clear the oceans are becoming,’ Hill said. “This is all going to fuel that need to be more eco-friendly, and that affects how much power we consume. If we do have to use a battery, how long can we make that battery last? And can we use a more eco-friendly battery in this? All of this power consumption is going to filter through into the next generation products.”

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