Sub-GHz Wireless MCUs and Components for IoT

Created: November 15, 2021
Updated: October 10, 2024

In recent years, the use of IoT devices has grown massively, with a lot of it happening in the background in areas like industrial production, infrastructure, home automation, smart meters, and wearable electronics. In the consumer space, IoT devices mostly connect to shorter-range indoor networks, normally over WiFi or Bluetooth. Today, more devices are integrating over long distances with low-frequency protocols or taking a hybrid approach with high frequency and low-frequency protocols on the same device. Bringing it all together involves the fusion of multiple wireless protocols alongside digital processing and an embedded application.

Why has there been a continued focus on sub-GHz wireless in these systems, especially when we already have many useful protocols like Bluetooth, WiFi, cellular, and other 2.4 GHz ISM band options? Sub-GHz wireless has its advantages, and there is much more support from IoT service providers for these products. This all means it’s much easier to both build a private network architecture and connect it to your cloud services through a base station, or to access cloud services through an existing wireless carrier. In the US, the major telecoms now offer IoT services over their networks, and you can set up your own cloud service platform that connects with your IoT hardware using the major cloud service providers.

At the end of the day, if you can’t get a sub-GHz protocol onto your board, then you can’t use it to take advantage of long-range, low-power wireless communication and the services these protocols enable. In this article, we’ll look at some of the major considerations in low-power, long-range wireless connectivity within the widely recognized sub-GHz band.

Selecting a Sub-GHz Wireless Option

Building IoT products with sub-GHz wireless connectivity require selecting a chipset that can support these frequencies and that implements the desired wireless protocol for your IoT network. Early MCUs used in IoT devices did not include these features, instead requiring a dedicated module or requiring emulation in the device application. Today, there are several chipsets and fully integrated MCUs that support multiple sub-GHz protocols. Some of these products will also support a higher frequency ISM band in the 2.4 GHz range, and possibly WiFi up to 5 GHz. You can read more about the basics of IoT protocol selection here.

The mix of various standards and protocols will determine which frequencies will be available in your design, which will be a primary driver of power consumption. When choosing a wired or wireless networking protocol, the data rate is usually the primary consideration. In sub-GHz wireless, the major advantages are the low power consumption of these protocols and the long-range available at these frequencies. Therefore, matching device lifetime and communication range requirements to the application are typically more important for end devices on the network.

Balancing Range and Power

High frequency and low-frequency protocols differ in two major aspects that determine their ideal application areas: attenuation and power consumption. Lower frequencies generally correspond to lower power consumption and longer range, so sub-GHz protocols are ideal for these IoT applications. The low-frequency transmission also has fewer problems with obstacles like hills, buildings, etc., so this long-range capability eliminates the need for repeater sites and base stations. Contrast this with the next wave of 5G rollouts, where mini base stations will need to be implemented for service delivery to end-users.

A simple way to get started estimating the power requirements at a transmitter for a given distance and transmission frequency (really the wavelength) is to use the Friis path loss formula. This formula illustrates the tradeoff between transmission frequency (or rather wavelength) and range:

Where:

Pr = Received power
Pt = Transmitted power
Dt = Directivity of the transmitter
Dr = Directivity of the receiver
d = Distance between the transmitter and receiver antennas
λ = Transmission wavelength

Effectively, if you know the receiver sensitivity (specified in dBm), then you can determine the required transmitter power for a given wavelength and line-of-sight transmission distance. In general, doubling the transmission range requires increasing the power budget for your wireless link by 6 dB. In addition, we can see that doubling the frequency reduces the received power by 6 dB. Note that these are all idealized factors depending on line-of-sight transmission between two antennas. A device deployed in a real scenario will experience losses from absorption, multipath propagation and reflections, and even the weather. Therefore, make sure to consider a realistic margin of safety for your system to account for the possibility of limited range.

Important Specifications for Sub-GHz Chipsets

While range and transmission frequency are the major considerations in designing sub-GHz IoT devices, there are a few other specifications that should be considered in these designs.

Power Consumption

Sub-GHz wireless products (and every other wireless product) won’t have a specific range specification, or if they do it will just be an estimate. They will have a power output value for a given current specified as an EIRP value (equivalent isotropically radiated power, in units of dBm). An antenna with directivity/gain greater than 1 can be used for directed transfer and could be used to reduce the power consumption required to transmit data. Total system power consumption can be reduced further by using a system with lower standby current, low-power modes, and wake-up timers. Given all of these factors, power consumption can be minimized, and devices can be designed to have a total useful lifetime of over 10 years on a coin cell battery.

Receiver Sensitivity

As was mentioned above, the receiver sensitivity and transmission frequency will determine the system range. Channels with larger bandwidths will require a more sensitive receiver, which might limit the range in your sub-GHz link. Compensating for this may require increasing transmission power, limiting range, using a lower data rate, or possibly moving to a different protocol for your application. Antenna gain/directivity also plays a role here and can compensate for lower sensitivity by providing directional transmission between devices on the network.

Modulation and Coexistence Considerations

Just as certain portions of ISM band protocols can experience coexistence challenges, sub-GHz bands can experience interference between channels. Sub-GHz protocols typically use keying modulation schemes (FSK, ASK, OOK, etc.). In some cases, spread spectrum mechanisms are used to increase the channel bandwidth, either by encoding data into a higher bit rate or with a scheme like frequency-hopping spread spectrum (FHSS). An example showing data rate increase being used to increase bandwidth for a given average transmission power is shown below.

Spread spectrum transmission concept. By spreading the transmitted data (blue) into higher bitrate encoding (red), the receiver can withstand potential sources of interference.

(Alt text: Spread spectrum transmission)

Spread spectrum signals are less prone to interference, but the transmitting and receiving circuits on the end devices need to have higher bandwidth to accommodate this spreading of power across the channel’s bandwidth. FHSS implementation will require additional testing to ensure EMC compliance and it will require compatible devices with sufficient receiver sensitivity on each end. In some devices, a dedicated transceiver module may be the best choice to provide sufficient sensitivity to receiver spread spectrum signals.

Sub GHz Radio and Transceiver Options

In short, there are two basic ways you can integrate sub-GHz radios into a new product and bring it onto a long-range IoT network:

Use a processor that includes sub-GHz wireless capabilities integrated onto the chip
Use an external sub-GHz transceiver that is compatible with your system’s host controller
Add a wireless module that contains all the required peripherals and

Depending on what your system needs to do, either option is viable as there are many components falling into both categories. The first two options will require a bit more effort if you’ve never designed things like filters, feedlines, antennas, or RF devices in general. However, there are highly integrated product lines from multiple vendors that support multiple sub-GHz bands; some excellent options are shown below.

Microchip, ATSAMR30M18A-I

The ATSAMR30M18A-I sub-GHz wireless module from Microchip functions as an MCU that includes an IEEE 802.15.4 compliant radio with an integrated antenna. This castellated SMD module includes an ARM Cortex-M0+ MCU with integrated 256 KB Flash memory, as well as an integrated transceiver for the 700/800/900MHz ISM band. As an easy-to-use SiP, it also includes some of the standard features users expect in MCUs, such as a 12-bit 350 ksps ADC, I2C operating up to 3.4 MHz, a USB 2.0 interface, and 16 GPIOs. It requires an external antenna; the table below includes a list of approved antennas, although other antennas could be used if they have similar specs and pass testing.

NXP Semiconductor, OL2385AHN

The OL2385AHN from NXP Semiconductor is a multiband wireless RF transceiver with an embedded MCU core that supports multiple sub-1 GHz bands (160 to 960 MHz). This device is a highly integrated transceiver with four selectable frequency ranges that supports multiple modulation schemes (400 kbps/200 kbps FSK, ASK, and OOK). On the board, a host controller can interface with this device via SPI, UART, or LIN protocol compatible UART. Some of the major application areas targeted with this component include LPWAN for smart infrastructure products, smart home technologies, M2M communication, and sensor networks.

NXP OL2385AHN radio transmitter block diagram. [Source: (Alt text: Sub-GHz design)

Texas Instruments, SimpleLink Wireless MCUs (CC13xx and CC430F51xx)

The SimpleLink line of wireless MCUs from Texas Instruments is one of my personal favorites for developing new IoT products that operate in sub-1 GHz bands. Some of the components in this product line also support multiple ISM bands, WiFi, Bluetooth, and others between 1 and 2 GHz. This product line includes some MCUs that are qualified for automotive products. The various products in the SimpleLink support these sub-1 GHz protocols:

IEEE 802.15.4
Wireless M-Bus (T, S, C, N mode)
6LoWPAN
Wi-SUN NWP
Amazon Sidewalk
MIOTY
ZigBee

If you’re using other products in the TI portfolio, you’ll find it easy to develop an application with TI’s SDK support for these products and peripheral devices for your IoT platform. These MCUs also interface with any other peripheral ASICs over standard digital interfaces, giving designers plenty of flexibility to build new IoT platforms.

The Future of Sub GHz IoT

Everyone continues to focus on WiFi, Bluetooth, and 5G just because they are so ubiquitous in the consumer space, but sub-1 GHz isn’t going anywhere and will continue to be the low-power backbone for IoT networks. The long range capabilities, low power consumption, and ease of implementation is too good to pass up, and it doesn’t make sense to contribute to further ISM or cellular congestion in persistent low data rate applications. Some of the components systems designers need in many sub-1 GHz applications fall into the following categories:

If you’re developing a custom solution that can support a range of possible frequencies or protocols, such as software-defined radio, you’ll need some additional components to build up your RF front-end:

When you need to find components for your next sub-1 GHz wireless system design, use the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to up-to-date distributor pricing data, parts inventory, and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

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