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Maximising sub-1GHz spectrum for IoT

Posted: 01 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:wireless data? ISM band? Internet of Things? IoT? sub-1GHz?

Low power wireless data communications are employed to control a multitude of applications ranging from wireless controlled toys and baby monitors to home automation applications. Many of these designs use the 2.4GHz 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 2.4GHz, 1 channel at 868MHz and 10 channels in the 928MHz. Recent updates to the standard have expanded the number of sub-1GHz 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 C 935MHz 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 C 935MHz 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 analogue radio, digital modulation/demodulation, and data buffering takes place on the chip. The transceiver also incorporates an on-board 128bit AES encryption engine that provides a 16B 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.

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.

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