RF standards for short-range wireless connectivity

RF standards for short-range wireless connectivity

New standards provide optimized solutions for the differing needs and priorities of different applications

BY MARY O’KEEFFE
Analog Devices, Norwood, MA
http://www.analog.com

The term “short-range device” (SRD) refers to a device capable of wireless communications over a relatively short distance—from just a few centimeters up to a few kilometers. Several wireless standards defining the communications processes for such devices exist, with new standards continuing to evolve.

Constraints

Governments impose restrictions on the use of the frequency spectrum. Figure 1 shows the UHF (300-MHz to 3-GHz) ISM unlicensed frequency bands available in different parts of the world.

Fig. 1. Several UHF (300-MHz to 3-GHz) ISM unlicensed frequency bands are available in different parts of the world.

As the figure shows, no common unlicensed ISM band is available below 2.4 GHz, although some RF transceivers will support operation across several of these sub-1-GHz bands. The Analog Devices ADF7020/-1, for example, supports operation from 135 to 950 MHz. Many designs use proprietary communications protocols in this frequency range.

Despite the inherent range advantage of the lower-frequency bands, the global nature of the 2.4-GHz band makes it attractive for many SRD communications protocols such as Bluetooth, WLAN and ZigBee. Irrespective of the communications protocol used, countries apply additional constraints driven by factors such as safety and quality of performance; these constraints are required to limit interference between different equipment. Some examples relevant for the 2.4-GHz band are shown in Table 1 .

Existing standards

The various communications protocols each offer advantages and disadvantages, with the optimum choice depending on the application. Bluetooth, for example, offers data rates up to 3 Mbits/s, whereas 802.11g enables data rates as high as 54 Mbits/s and ZigBee is limited to 250 kbits/s.

While 802.11g has the higher data rate, it also has higher cost and higher power consumption. ZigBee has the advantage of low power consumption. It can also support a high number of nodes. For example, Bluetooth’s maximum of 8 nodes in a net could be a limiting factor in an industrial application.

For sensor applications, where only a limited amount of data must be transferred and low power consumption is of significant value, ZigBee appears to have an advantage over Bluetooth or WLAN. For applications such as wireless headsets, the data rate offered by Bluetooth meets the requirements while maintaining a relatively low cost.

The Wibree standard currently in development can operate in a standalone mode or as a complement to Bluetooth, offering a lower-power solution than Bluetooth and a maximum data rate of 1 Mbit/s. This is a higher data rate than ZigBee, but its range would be shorter than a low-power ZigBee device. Other factors that differentiate the various standards include latency and resilience.

Developing standards

The industrial environment is one where resilience is of particular value. The recently ratified Wireless Hart standard is targeted at the industrial space. The SP100 group is also looking at a standard for industrial applications.

Both the Wireless Hart standard and the ISA-SP100.11a standard, which is in development, indicate the use of an 802.15.4-compatible radio. The 802.15.4 radio also constitutes the physical layer for the ZigBee standard.

While several short-range device standards are already in existence, new standards are continuing to evolve. These are driven by and targeted at providing optimized solutions for the differing needs and priorities of different applications. ■

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