Connect the Dots: Rohm Combines Sigfox and IEEE 802.15.4k for Global, Long-Range IoT

Create: 09/12/2017 - 14:39
IoT global deployments

Long- range, wide-area networking for the IoT is showing promise, but customers need a quick, low-power wireless access (LPWA) solution to get moving. While developers of those solutions are eager to help, the answer as to which wireless interface is best continues to waddle in ambiguity. Rohm Semiconductor has offered yet another solution.

Last week I discussed how Sierra Wireless was helping developers and solution providers move forward despite the ambiguity by offering a chip that handles both NB-IoT and LTE-M (Cat-M1) in the licensed cellular bands. I had surmised that combinations of licensed and unlicensed would be the way forward. I even somewhat facetiously predicted that, with silicon being so cheap, we’d soon see chips supporting licensed and unlicensed options, adding unlicensed Sigfox and LoRaWAN LPWA to ICs that also support NB-IoT and LTE-M.

Well, a few days later, Rohm Semiconductor announced its ML7404, a chip that supports two unlicensed interfaces, Sigfox and IEEE 802.15.4k, adding yet another wrinkle to the LPWA landscape (Figure 1).

Figure 1: Rohm’s ML7404 IC combines Sigfox and IEEE 802.15.4k wireless interfaces in a single chip. (Image source: Rohm Semiconductor).

Some of the nuances of Sigfox and its big competitor LoRaWAN have been covered previously, but IEEE 802.15.4k is new to these pages. It’s an official standard that derives from the IEEE’s work on the 802.15.4 media access control (MAC) and physical layer (PHY) to make it more suited to low-energy, critical infrastructure monitoring (LECIM). Note that 802.15.4 is the standard upon which Zigbee is based, so it’s well known already, though interfaces can never consume too little power: less is always better, for a given data rate and range.

While the 802.14.4k standard supports direct-sequence spread-spectrum (DSSS) and frequency shift keying (FSK) modulation, the ML7404 supports only DSSS and it operates in sub-GHz bands. The baseline IEEE standard 802.15.4 operates in both the 2.45-GHz and sub-GHz bands, but lower-frequency, sub-GHz operation allows for greater signal propagation distances and lower absorption by obstacles.

Combo Reduces Risk, Accelerates Deployments

By combining the two sub-GHz interfaces, Sigfox and IEEE 802.15.4k, Rohm’s ML7404 gives solution providers the benefit of having the global compatibility of both interfaces, along with Sigfox’s large number of subscribers and IEEE 802.15.4k’s DSSS signal robustness in the face of interferers. It also reduces design risk. The protocol stack for 802.15.4k will be provided free as open-source software from Rohm’s partner stack vendors.

Like other combination chips, the goal is to simplify choices so designs can move forward. Meeting this goal is important as systems are increasingly designed with a global customer in mind. The ML7404’s dual-interface support gives developers the assurance that a single SKU will work globally, from Europe to the United States and Japan, to effect greater adoption of devices and services.

Customers need to be able to focus on acquiring, analyzing and acting upon the data, instead of worrying about which wireless interface will work in their particular region. It’s the job of solution providers to solve that particular problem, and the ML7404 helps with that.

On-chip BPSK for Sigfox Lowers Power

Sometimes how a provider solves a problem has enormous effects on the final outcome. True, higher integration and flexibility lead to greater power consumption and a generally larger IC footprint. It also requires more software support. That said, the designers of the ML7404 made some interesting design choices to offset the downsides.

For instance, instead of generating the Sigfox BPSK waveforms in an off-chip microcontroller (MCU), as is normally the case, the ML7404’s on-chip dedicated circuitry develops the BPSK-modulated signal (Figure 2). This eliminates the need for an external MCU, saving power.

Figure 2: The ML7404 implements Sigfox-required BPSK modulation in hardware to avoid the need for a separate MCU. (Image source: Rohm Semiconductor)

As it stands, the IC consumes 1 mW off a 1.8- to 3.6-V supply, with a transmit current of 34 mA @ 20 mW in either Sigfox or 15.4k mode. The receive current is 13.6 mA. Most critically, the current consumed in sleep mode, which is what the devices will be in for the majority of their working life, is 1.2 microamps (µA).

To help get providers moving on a design, the ML7404 is supported by an evaluation kit that bundles sample programs, such as a simple MAC, and various test scenarios, along with reference module designs. Manuals and tools can be downloaded from LAPIS’s site (Rohm acquired LAPIS semiconductor, the designers of the ML7404.)

As the Mobile World Congress unfolds this week in San Francisco, it’s likely there will be more IoT developments that involve integration and other enhancements. We’ll keep you up to date on those, and any other announcements that will help meet the need for fast, efficient, low-power, low-cost IoT solutions for the next generation of systems. Let us know what you’d like to hear more about that will help you develop and deploy your IoT solutions.

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Patrick Mannion
Patrick Mannion is an independent writer and content consultant who has been working in, studying, and writing about engineering and technology for over 25 years.

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