Wireless Short-Range Devices: Designing A Global License-Free System For Frequencies <1 GHz

Introduction
The term short-range device (SRD) is intended to cover radio transmitters that provide either unidirectional or bidirectional communication and have little capability of causing interference to other radio equipment. One cannot list all the applications of SRDs, because they provide many different services. Among their more popular applications are:

• Telecontrol for home- or other building-automation systems
• Wireless sensor systems
• Alarms
• Automotive, including remote keyless entry and remote car-starting
• Wireless speech and video

Designers of SRD wireless systems need to use great care in choosing the radio's communication frequency. In most cases, the choice is limited to those portions of the spectrum that allow license-free operation given that certain specifications and conditions on usage are met. Table I lists the frequency bands available globally.

The 2.4-GHz band is widely used by designers who want to build systems that can operate worldwide. In fact, it has become the frequency band of choice for such standards as Bluetooth, WLAN, and ZigBee. The 5.8-GHz band has also attracted some attention—in cordless phones or the 802.11a version of WLAN, for example.

For systems that require both wider range and lower power, however, the sub-1-GHz bands remain compelling due to reduced co-existence issues and greater transmission range, as both of these affect power consumption -- an important consideration in battery-powered applications.