Microwave Engineering Europe - May 2008 - (Page 22)

22 WIRELESS NETWORKING — ISM then jump synchronously between predetermined channels. The advantage of this scheme is that the network does not stay long on a single channel, so if there is heavy interference and all communication is lost on one channel, it can be retried on another channel in the next timeslot. The disadvantage is that the network has to be synchronous so that if the network moves into a channel where all communication is lost due to interference, the members of the network will all decide to move on to the next channel at the same time. This, in addition to the distribution of channel lists, adds overhead to software complexity and network trafÞc. The weakness of the algorithm is that if an interferer like a WiFi network causes interference in ¼ of the available channels Figure 5: Static frequency hopping. in the 2.4 GHz band, the throughput of the network will be reduced by ¼ and the latency will be high if several channels with heavy interference are chosen successively. Adaptive frequency hopping Adaptive frequency hopping ( Figure 6) is an enhancement on the static frequency hopping algorithm which enables the master in a system to remember which channels are free of interference and use those more frequently. The master keeps a table where a score is held for each channel. The values in this table are continuously updated based on the quality of communication in the channel. What channels to jump to next are chosen deterministically or using weighted random based on the value in the tables. The list of the channels to be used next is distributed to the devices in the network. For many applications with synchronous operation such as keyboard/mouse, remote control and audio, an adaptive frequency hopping algorithm can be very effective in mitigating interference. This is the system utilized by Bluetooth version 1.2 and TIÕs RadioDesk. Frequency agility Frequency agility (Figure 7) can be considered an extremely slow frequency hopping algorithm. The network will stay on the same channel until the packet error rate (PER) goes above a certain threshold level. In practice, it will Þnd a channel with little interference and stay there until a new interferer imposes itself. This scheme is used in the Wireless USB standard and is optional in the upcoming ZigBee 2007 standard. The advantage of frequency agility is that there is little overhead and good throughput when there is little change in the interference patterns. A challenge for frequency agile systems is that it is generally not desirable to change channels when disturbed by a frequency hopping interferer like Bluetooth. The system should be patient enough to wait for a frequency hopping interferer to move on to the next channel while agile enough to move on quickly if a new static interferer imposes itself. The disadvantage of frequency agility is that when unacceptable interference is detected and the frequency has to be changed, there is no reliable way for the master to inform the members of the network of the frequency change. Instead timeouts will have to be used, and the resulting latency in changing frequency may be unacceptable in for example user interface systems. Conclusion Due to the high number of interferers, coexistence issues have to be considered for any RF network designed to work in the 2.4 GHz ISM band. The key to achieving good coexistence is to start out with a radio with good selectivity so that your network is minimally disturbed by interference in neighboring frequencies. Depending on the requirements of the application, a frequency hopping or frequency agility algorithm should be implemented to ensure that your network does not get stuck on a frequency with an aggressive interferer. By taking these measures, it is possible to design RF networks for the 2.4 GHz band that meet the latency, throughput and power consumption requirements of most applications. Figure 6: Adaptive frequency hopping. Figure 7: Frequency agility. 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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