News and New Products

Wireless-Sensor Networks Find Fit in Unlicensed Band

By Margery Conner, EDN -- Electronic News, 3/21/2006

Low-power, short-range, low-data-rate wireless networks use the unlicensed RF band. In addition to their overriding need to operate on severely constrained energy sources, such as using one battery for their entire lifetimes or employing energy they scavenge from the environment, these networks must contend with interference from sources such as Bluetooth headsets and microwave ovens. These networks use network protocols that largely determine the networks' efficiency, robustness, and security. As such, network designers need to first determine which sub-band is best for their application: 900MHz or 2.4GHz. In addition to selecting the frequency band, they also need to decide on a hardware-transceiver scheme: a roll-your-own or an SOC (system-on-chip) approach. They must also decide which network protocol to use.

Both the 900MHz and 2.4GHz bands have advantages. Both reside in the ISM (industrial/scientific/medical) band, an unlicensed frequency band of 902MHz to 928MHz, 2.4GHz to 2.483GHz, and 5.725GHz to 5.875GHz. Almost all of the transceiver and SOC products targeting wireless-sensor-network applications use the 900MHz to 928MHz and 2.4GHz to 2.483GHz bands. The 900MHz band touts long broadcast range because of its relatively longer wavelength and its correspondingly longer battery life. However, lower frequency means the use of a larger antenna than a 2.4GHz system requires. And, if you plan to sell your system into a global market, you will quickly encounter a lack of standardization in the 900MHz range. For example, in Europe, you cannot use the 900MHz to 928MHz band because it is part of the GSM (Global System for Mobile communications) network for cell-phone communication and, thus, is unavailable.

For more of this story, written by Electronic News sister publication EDN, click here.