Microwave Engineering Europe - May 2008 - (Page 18)

18 WIRELESS NETWORKING — ISM Achieving good coexistence in the 2.4 GHz ISM band As the worldwide open 2.4 GHz ISM band becomes ever more crowded, good coexistence becomes vital to the successful deployment of an RF solution. Poor coexistence leads to packet loss which translates to increased latency and power consumption due to retransmissions. The two most important factors to achieve good coexistence are selectivity and frequency hopping/agility algorithm. In addition to explaining the importance of coexistence, this presentation shows the effects of selectivity in practice through real world examples and discusses software implementations of frequency agility. By Peder Rand, Texas Instruments he worldwide availability of the 2.4 GHz ISM band has ensured its popularity with manufacturers of a wide variety of devices. Not only the number of devices deployed grows rapidly, but the number of radio technologies and protocols that share band is also on the rise. It is of course important that all these devices play nice and operate within the limits of regulations, but even if this is the case there is ample opportunity for radio interference between the devices in the band. Interference will cause packets to be lost, power consumption to increase and network performance to be reduced. The key to coexisting well with other devices in this band is robustness with respect to interference. Good selectivity with a frequency agility scheme suited for the application is the way to ensure this. Who are the interferers in the 2.4 GHz band The types of interferers in the 2.4 GHz ISM band have grown to include, amongst others, WiFi (IEEE 802.11b/g/n), Bluetooth®, cordless phones, analogue video and microwave ovens. Their footprint in terms of transmission energies and how they are distributed in the band over time varies. A typical scenario in a domestic environment is shown in Figure 1. Interferers can be loosely categorized into two types; stationary and frequency hopping. Stationary interferers stay on the same frequency for long periods of time, sometimes effectively blocking other communication around that frequency. Examples of this are WiFi, Microwave ovens and DSSS (Direct Sequence Spread Spectrum) cordless phones. Frequency hopping interferers on the other hand T continuously change which channel they operate on, and leave a footprint of interference across the whole band without blocking any single frequencies for more than a short while. A Direct Sequence Spread Spectrum (DSSS) modulation format sends redundant information spread out in the spectrum to improve the chances of being able to reconstruct the data sent at the receiver in face of an imperfect communication environment. DSSS is used in WiFi, ZigBee®, and other IEEE 802.15.4 networks. IEEE 802.15.4 sends 2 Mega chips spread out in the channel for a data rate of 250 Kbps. Although such Figure 1: A typical interferer scenario in a domestic environment. Microwave Engineering Europe ● May 2008 ● www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - May 2008 News Contents Comment Cover Feature: How to Succeed as a GaAs Foundry Wireless Networking: Wireless Coverage Where Everybody WINS Wireless Networking: Achieving Good Coexistence in the 2.4 GHz ISM Band GPS and Satellite: GPS developments: Galileo Moves Forward with Successful Giove-B Satellite Launch — Broadcom Targets AGPS in Mobile Phones and Devices Raising the Bar for the Radio: Making 802.11n Work Reducing Power Consumption in Ultrawideband Chips WiMax Catches Second Test Wave Products Calendar

Microwave Engineering Europe - May 2008

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