Microwave Engineering Europe - May 2008 - (Page 20)

20 WIRELESS NETWORKING — ISM a modulation format efﬁciently resists narrowband interference, it is subject to interference from any other directly overlaying DSSS signal. Conversely, a narrowband FHSS (Frequency Hopping Spread Spectrum) modulation signal resists DSSS interferers better than other narrowband interferes. FHSS is used in Bluetooth and many proprietary systems. What are the effects of poor coexistence When an RF link is subject to interference, the result is that the packets will contain errors when they are received, and packets are lost. Many two-way RF protocols like IEEE 802.15.4 ensure quality of service in face of occasional packet loss by requiring the receiver to acknowledge the receipt of a packet by transmitting a so called ACK packet. If the ACK packet is not received by the transmitter, the original packet is resent. This is repeated until the ACK is properly received or the communication link is deemed by the transmitter to be lost. One-way RF protocols usually address packet loss by transmitting the same packet Figure 2: Two 2.4 GHz wireless mice subjected to interference from a WiFi ﬁle transfer. Figure 3: An RF receiver is not only disturbed by unwanted signals within the frequencies it is trying to receive, but to a certain extent also by signals transmitting on close by frequencies. a number of times in order to increase the probability of it reaching the receiver intact. There are three main effects of packet loss due to interference; increased latency, increased power consumption and reduced throughput. The latency increase can be illustrated by a two-way system with ACK where the ﬁrst packet is not received properly due to interference. The transmitter will have to wait a timeout period while waiting for the ACK packet from the receiver before assuming that the original packet was lost. It must then resend the original packet. In an environment with heavy interferers and a system that is not robust to interference, such retransmissions can potentially have to be repeated many times before the communication is successful, thereby increasing the latency. A wireless mouse drawing circles is efﬁcient in illustrating an example of the user experience being affected by latency increase due to poor coexistence. In Figure 2, two 2.4 GHz wireless mice are being subjected to interference from a WiFi ﬁle transfer between the computer and a router 3 meters away from the receiving dongle. The solution on the left is clearly experiencing intermittent latency issues that are manifested in the computer assuming straight line movement between two coordinates. The solution on the right has good selectivity and an adaptive frequency hopping algorithm which leaves it unaffected by the interference. Retransmissions increase the amount of time both transmitter and receiver have to stay active to complete a transmission, and will therefore directly increase the power consumption of the network. For battery driven applications such as sensor networks, this could lead to battery lifetimes being signiﬁcantly shorter than anticipated in high-interference environments. Frequent retransmissions will also reduce the total throughput of highthroughput networks. When streaming audio over an RF-link, for example, the maximum data rate must be reduced if interference causes signiﬁcant packet loss in order to avoid clicks. How to achieve good coexistence While there will in some environments be signiﬁcant interference in some parts of the 2.4 GHz ISM band, there will always be parts of the band that are good for communicating. A frequency hopping or frequency agile algorithm will ensure that a network is not Figure 4: Interferers in two neighboring channels on the limit of causing enough interference at the receiver to cause packet errors. Microwave Engineering Europe ● May 2008 ● www.mwee.com http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - May 2008

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.