Connecting Wireless-Based Internet of Things Applications Using the Sub-1Ghz Spectrum

By Magnus Pedersen, Director, Atmel

The use of low power wireless data communications has permeated all aspects of our busy lives’. From wireless controlled toys, baby monitors to home automation applications, many of these designs use the 2.4 GHz radio frequency spectrum and come under the IEEE 802.15.4 standard. Designed to support the huge numbers of possible applications requiring short range and low data rates, and unlike Wi-Fi and Bluetooth, the standard is aimed at products that have extremely low power consumption and that can operate for several years without any maintenance from a single battery.

Also termed the ISM band, (industrial, scientific and medical), this particular area of radio spectrum has become over crowded. Already shared with everything from microwave ovens, Wi-Fi routers and Bluetooth-based headsets, there has been an increasing need to open up more spectrum to accommodate the increasing need for more link reliability and data throughput. While the link level protocols have been designed to be very resilient against interference by employing techniques such as channel switching and spread spectrum, the impact of these may affect data rates.

When the first IEEE 802.15.4 standard was issued in 2003, the specification provided 16 channels at 2.4 GHz, 1 channel at 868 MHz and 10 channels in the 928 MHz. Recent updates to the standard have expanded the number of sub-1 GHz channels available. Initially aimed at Europe and North America, the number of these new channels is now expanding in Europe (3 channels) and North America (30 channels).

The most recent version of the IEEE 802.15.4 standard also provides support for new Sub1GHz bands in China (779-787MHz) and Japan (915-930MHz) . Apart from offering more less-crowded spectrum for ISM applications, the use of the 769 – 935 MHz frequencies offer more reliable propagation characteristics inside buildings; ideally suiting applications such as smart metering, industrial lighting and environmental controls. Recent advances in the modulation techniques used for 802.15.4 have also increased potential data throughput rates from 20/40kb/s to 100 kb/s/250kb/s.

Leading the development of sub-GHz applications are new wireless transceiver ICs such as Atmel’s AT86RF212B device. This low power, low voltage 769 – 935 MHz transceiver is specially designed for ZigBee / 802.15.4, 6LoWPAN, and high-speed ISM applications. Providing a complete SPI-to-antenna solution, the only external components required are a crystal, bypass capacitors and an antenna. All analogue radio, digital modulation/demodulation, and data buffering takes place on the chip. The transceiver also incorporates an on-board 128-bit AES encryption engine that provides a 16-byte encryption within 24 us.

 

 

 

 

 

 

Figure 1 – Block diagram of Atmel AT86RF212B single-chip radio transceiver

In addition to supporting current IEEE 802.15.4 modulation schemes, the AT86RF212B also supports proprietary data rates up to 1,000 kb/s enabling high-speed ISM applications.

Like any wireless design, RF performance is critical both in terms of receiver sensitivity and transmitter power. Taking account of both parameters, the “Link Budget” defines the range and robustness of a wireless system. The higher the link budget is, the better range you can achieve, and the extra margins aid making a more robust approach. The link budget is the dynamic area between receiver sensitivity and transmitter output power. For example, the Atmel AT86RF212B radio transceiver device has a receiver sensitivity of -110 dBm and a transmitter output power of +10 dBm, so its link budget is 120 dB. Another aspect of the link budget metric is that receiver sensitivity will be influenced by the data rate and operating frequency. While not necessarily of importance for short range use, it may impact designs that are designed to meet the though requirements of systems in harsh environments, demanding years of maintenance free operation from a single battery cell. Examples are gas- and water-meters, industrial lighting control, environmental monitoring, and other proprietary systems up to 1000kb/s. Selecting the right data rate for the design also impacts the range and power consumption. For example, lowering a data rate from 1,000 kb/s to 20 kb/s can increase the range by a factor of 6x. However, the key point for this article is that by reducing the frequency from 2,400 MHz to 915 MHz will increase range by 2.6x.

 

 

 

 

Figure 2 – Free space range vs frequency

While adding an external front-end stage will increase range and link robustness, it will also increase power consumption. This will need careful consideration balancing the many potential applications and use cases that might be encountered in actual use. There may also be additional control components required to integrate control with your selected transceiver. The Atmel AT86RF212, for example, provides the necessary logic signals to facilitate the automatic control of an external RF front-end without the need for any firmware interaction.

Introducing smart meters that can be installed anywhere within the house requires consideration of the system’s antenna position. Wireless signals will take multiple paths, and as we’ve all learnt from using Wi-Fi, the indoor environment provides many challenges. Signal may travel along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and even distortions that can destructively interfere with one another at the aperture of the receiving antenna. Antenna diversity, where more than one antenna is used to receive the signals, is especially effective at mitigating these multipath situations. This is because multiple antennas offer a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link. The AT86RF212B device, for example, uses two antennas to select the most reliable RF signal path. This is done by the radio transceiver during preamble field search without any interaction from the application software.

 

 

 

 

 

Figure 3 – Antenna diversity improves reliability

 

Many of the Internet of Things (IoT) designs being considered will be battery powered, and in most cases from a single cell. Smart energy and building controls will rely on wall-mounted sensors so having an ultra low power consumption profile will be essential if the product is to gain wide consumer and industry acceptance. Developers will need to carefully profile the overall power budget and take full advantage of sleep modes of the host microcontroller and wireless transceiver. As a guide, the AT86RF212B device has a sleep consumption of 0.2 uA, receiver on of 9.2 mA and when transmitting at 5 dBm power a consumption of 18 mA.

Before embarking on a new IoT design, engineers need to carefully review the use cases anticipated and select an appropriate wireless transceiver. While there are many technical considerations developers also need to be mindful of any tools that might be available to aid a faster development cycle.

Specifically from the wireless perspective, any tools that might analyse power consumption and error testing together with library code for the host MCU will greatly assist this aspect of the design. Availability of low-level IEEE802.15.4 MAC drivers, and for smart metering and other mesh-based applications, a mesh networking stack are essential. A well-supported wireless transceiver would also have a readily available development or evaluation board on which prototype designs can be quickly tested and debugged prior to completing the design.