Microwave Engineering Europe - May 2008 - (Page 21)

WIRELESS NETWORKING — ISM 21 stuck on a channel with heavy interference, and good selectivity ensures that it is minimally disturbed by interferers in other parts of the band. Selectivity Selectivity is a hardware parameter of the receiver. 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. This is illustrated in Figure 3 where while the desired signal is in a channel that is not overlapping with the WiFi channel, the receiver will still receive interference from the WiFi transfer. Selectivity is a measure of how strong an interferer transmitting in a neighboring channel the receiver can tolerate without getting packet errors. Selectivity or jamming resistance is often referred to as adjacent and alternate channel rejection in data sheets and is measured in dB. Figure 4 shows interferers in two neighboring channels on the limit of causing enough interference at the receiver to cause packet errors. It can also be seen as a measure of how close a receiver can be to a jammer without “loosing” sensitivity. As a rule of thumb, for every 6 dB of selectivity increase, a receiver can be twice as close to a jammer without “loosing” sensitivity. For an example assuming free space and +10 dBm peak power from a jammer operating at +10 MHz from the wanted signal, the minimum distance before the receivers starts to get jammed is: Selectivity of 55 dB: 0.69 meters (jammer signal is -82 + 55 = -27 dBm) Selectivity of 28 dB: 15.4 meters (jammer signal is -82 + 28 = -54 dBm) Wireless standards like IEEE 802.15.4 set minimum requirements for receiver selectivity, but in order to achieve a robust system, these requirements should be surpassed. The ZigBee Alliance recently published a whitepaper on ZigBee coexistence with WiFi, highlighting the importance of the IEEE 802.15.4 requirements for selectivity. A common misconception with respect to interference is that increased link budget could compensate for selectivity. The analogy of a cocktail party is often used: It does not help to shout loud (high TX power) or to hear perfectly (good sensitivity) if one is not able to select (good selectivity) what to listen to. Shouting too loud (high TX power) can result in no one understanding each other anymore due to the resulting noise (interference). Static frequency hopping Frequency hopping (Figure 5) is a method where you continuously change channels for the network. An example of a frequency hopping algorithm is the implementation in Bluetooth versions 1.0 and 1.1. The master produces a pseudo-random sequence of channels to use in the succeeding timeslots and distributes it to the members of the network. The members of the network Building On The Excellence Of Our 100 Series… • Optimized Form Factor • High RF Power Applications • Lowest ESR @ Wireless • Superior Thermal Frequencies Performance • Highest Self • Ideal for Large and Resonance Small Signal • Rugged, Reliable NPO Applications Dielectric 800 SERIES Case A Capacitance Range 0.1 pF to 100 pF Capacitance Range 0.1 pF to 1000 pF Electrical Specifications Voltage Rating: Up to 250 WVDC IR: 105 M @ 25°C TCC: 0 ±30 PPM/°C 800 SERIES Case B Electrical Specifications Voltage Rating: Up to 500 WVDC IR: 105 M @ 25°C TCC: 0 ±30 PPM/°C .055" x .055" (1.4 mm x 1.4 mm) .110" x .110" (2.79 mm x 2.79 mm) AMERICAN ATC North America 631-622-4700 sales@atceramics.com TECHNICAL ATC Europe +46 8 6800410 sales@atceramics-europe.com CERAMICS ATC Asia +86-755-8366-4318 sales@atceramics-asia.com ISO 9001 REGISTERED w w w . a t c e r a m i c s . c o m Microwave Engineering ● May 2008 ● www.mwee.com http://www.atceramics.com http://www.atceramics.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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