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

Magnus Pedersen, Atmel

May 25, 2013

Magnus Pedersen, AtmelMay 25, 2013

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.

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