Utilizing the sub-1GHz spectrum for the Internet of Things - Embedded.com

Utilizing the sub-1GHz spectrum for the Internet of Things

Editor’s Note: In this Product How-To, Atmel’s Magnus Pederson describes how to take advantage of the expanded number of sub-1 GHz channels available in the new IEEE 802.15.4 standard to build for ZigBee / 802.15.4, 6LoWPAN, and high-speed ISM networks using the company’s AT86RF212B transceiver.

Low power wireless data communications are used to control a multitude of applications ranging from wireless controlled toys and 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 from a single battery without any maintenance.

This area of radio spectrum, also termed the ISM band (industrial, scientific and medical), has become overcrowded because it is shared by everything from microwave ovens to Wi-Fi routers and Bluetooth-based headsets. More spectrum is needed to allow for better link reliability and greater data throughput.

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 new channels is 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 less-crowded spectrum for ISM applications, the use of the 769 – 935 MHz frequencies offers 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 20/40 kb/s to 100 kb/s/250 kb/s.

Leading the development of sub-GHz applications are the new wireless transceiver ICs such as Atmel’s AT86RF212B, a low power, low voltage 769 – 935 MHz transceiver specifically designed for ZigBee / 802.15.4, 6LoWPAN, and high-speed ISM applications. The only external components required are a crystal, bypass capacitors, and an antenna. All analog 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. 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.

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

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 enable a more robust approach.

The link budget is the dynamic area between receiver sensitivity and transmitter output power. For example, the 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 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 1000 kb/s. Selecting the right data rate for the design 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, 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 careful balancing of the many potential applications and use cases that might be encountered in actual use.

There may be additional control components required to integrate controlwith your selected transceiver. The AT86RF212, for example, providesthe necessary logic signals to facilitate the automatic control of anexternal RF front-end without the need for any firmware interaction.

Designingsmart meters that can be installed anywhere within the house requiresconsideration of the system’s antenna position. Wireless signals willtake multiple paths, and as we’ve all learned from using Wi-Fi, theindoor environment provides many challenges. Signal may travel alongmultiple paths before finally being received.

Each of thesebounces can introduce phase shifts, time delays, attenuations, and evendistortions that can destructively interfere with one another at theaperture of the receiving antenna.

Antenna diversity, where morethan one antenna is used to receive the signals, is especiallyeffective at mitigating these multipath situations. This is becausemultiple antennas offer a receiver several observations of the samesignal. Each antenna will experience a different interferenceenvironment.
Thus, if one antenna is experiencing a deep fade, it islikely that another has a sufficient signal. Collectively such a systemcan provide a robust link. The AT86RF212B device, for example, uses twoantennas to select the most reliable RF signal path. This is done bythe radio transceiver during preamble field search without anyinteraction from the application software.

Figure 3: Antenna diversity improves reliability

ManyInternet of Things (IoT) designs will be battery powered, and in mostcases from a single cell. Smart energy and building controls will relyon wall-mounted sensors, so having an ultra-low-power consumptionprofile will be essential if the product is to gain wide consumer andindustry acceptance.
Developers need to profile the overall powerbudget and take full advantage of sleep modes of the hostmicrocontroller and wireless transceiver. As a guide, the AT86RF212Bdevice has a sleep consumption of 0.2 uA, receiver on of 9.2 mA and whentransmitting at 5 dBm power a consumption of 18 mA.

Beforeembarking on a new IoT design, engineers need to review the use casesanticipated and select an appropriate wireless transceiver. While thereare many technical considerations, developers also need to be mindful ofany tools that might be available to aid a faster development cycle.Any tools that analyze power consumption and error testing together withlibrary code for the host MCU will greatly assist this aspect of thedesign.

Availability of low-level IEEE802.15.4 MAC drivers, andfor smart metering and other mesh-based applications, a mesh networkingstack is essential. A well-supported wireless transceiver will have areadily available development or evaluation board on which prototypedesigns can be quickly tested and debugged prior to completing thedesign.

Magnus Pedersen is Atmel’s Product MarketingDirector MCU Wireless. He holds a bachelor degree in Tele-/Radiocommunication from Trondheim College of Engineering (1994). He has heldpositions as Technical Officer in the Royal Norwegian Air Force, ProductManager, Sales and Marketing manager, R&D Manager and ChiefTechnology Officer (CTO) in Q-Free ASA. Based in Trondheim, Norway atthe AVR Microcontroller Design Centre he is responsible for Atmel’swireless product line worldwide, focusing on IEEE 802.15.4 compliantwireless products.

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