Microwave Oven Effects On Wireless Lans

Microwave Oven Effects On Wireless Lans Microwave Oven Interference on Wireless LANs Operating in the 2.4 GHz ISM Band Abstract – Commercial microwave ovens as applied in restaurants have two magnetron tubes and compared to domestic kitchen counterparts they spread the higher RF power and radiated heating energy more evenly. The domestic kitchen or residential microwave ovens have only one magnetron tube. The interference from the commercial type of microwave ovens is more difficult to characterise than the interference from the residential ones. The commercial type of microwave ovens radiate a CW-like interference that sweeps over tens of MHz during the two bursts per mains power cycle. The residential ones give a CW-like interference that has a more or less stable frequency near 2.45 GHz occurring once per mains power cycle.

The impact of the interference from the commercial type of microwave ovens on wireless LANs conforming the IEEE 802.11 standard for both DSSS (direct sequence spread spectrum) and FHSS (frequency hopping spread spectrum) has been evaluated. I. INTRODUCTION The release of the 2.4 GHz unlicensed band (2400 -2483.5 MHz) for ISM (industrial, scientific, medical applications) prompted a significant interest in the design of wireless LAN products. Interference from extraneous sources (unintentional radiators) impacts the reliability of communication in this 2.4 GHz ISM band. Sources of such interference are the millions of residential microwave ovens radiating at frequencies close to 2.45 GHz, and they have been described largely in the literature.

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Commercial microwave ovens, based on two magnetron tubes as used in restaurants, have been hardly described in the literature. Since commercial ovens are expected more often in the vicinity of office buildings with a high population density of office equipment and PCs, this type has been evaluated with respect to the nature of the interference signal and the impact on wireless LANs operating in the 2.4 GHz ISM band. At first, published material on residential microwave ovens, the reports from the NTIA (National Telecommunications and Information Administration, in the US) – [1] and [2] – are discussed. Next, the commercial microwave ovens and the nature of their interference is considered. The characterization of the interference from such ovens requires a dedicated measurement set up.

Then the robustness of wireless LANs based on DSSS and FHSS conformant to IEEE 802.11 against CW interference is discussed. At last, the interference from the commercial microwave ovens on wireless LANs based on DSSS and FHSS is measured with a dedicated set up and the impact of the interference nature is considered. II. NTIA REPORTS The NTIA makes [1] and [2] some pessimistic conclusions about the possibility of sustaining highly-reliable communication links in this band. The ubiquitousness of these ovens and the wideband interference picture that emerges from peak-power measurements using, for example, conventional spectrum analysers in max-hold mode and multiple sweeps, has led to these pessimistic conclusions. The NTIA describes measurement results for residential microwave ovens with a maximum EIRP for these radiators that lies lay between +16 and +33 dBm. Some shortcomings in the NTIA measurement methods are presented in [3].

The NTIA reports give results of frequency- and time-domain measurements. Spectrum analysers in max-hold mode were used to measure in the frequency domain, which resulted in traces that capture the peak emission, at each frequency sampling point, occurring during the time interval of observation. Spectrum analysers in zero-span trace mode were used to find how the signal power around the selected frequency varies over time. [3] mentions that the NTIA peak spectrum measurements and frequency-domain characterisation with time-domain plots show a pulsed III. RESIDENTIAL MICROWAVE OVENS Microwave ovens have become more popular over the last fifteen years and can be found in over two hundred million home kitchens. The heating source of these residential microwave ovens is based on a single magnetron tube mostly positioned in an upper corner. Without further provisions, such an oven would produce an uneven heating effect, because of static stable standing wave patterns inside the cavity of the oven.

Therefore, the usage of a rotating disk results in such a heating process at which the different sides of the rotated food or drink are illuminated more evenly. The power consumption is mostly in the 600 – 800 Watt range. 2445 MHz 2452 MHz 2459 MHz 2466 MHz # RES BW 10 kHz # VBW 10 kHz # SWP 15.0 sec Peak 10 dB/div 2.41 GHz 2.48 GHz Frequency Fig. 1. Max-hold spectrum for residential microwave oven.

# RES BW 3.0 MHz #VBW 1 MHz # SWP 30.0 msec Peak 10 dB/div fcentre 2.456 MHz Time 0 30 msec Fig. 2. Zero-span spectrum for residential microwave oven. NTIA measurement approach. With a high speed digital oscilloscope it can be shown, that during the active period the emitted signal is a CW with a frequency that moves over a few MHz.

The beginning of the burst looks like a pulsed CW of which the frequency can vary more, and the radiated signal strength is lower. The end of the burst looks like the pulsed beginning and also has a lower level. Although there are many differences between the emissions from ovens of different manufacturers, the centre burst frequency is mostly somewhere around 2450 – 2460 MHz, and the sweep goes over 2 – 6 MHz. Likewise, the total active period is about 8 msec (out of the 20 msec mains power cycle at 50 Hz, or 16 msec at 60 Hz) of which the first and last 1 msec of the burst considered the beginning and end, have a pulse nature. IV. COMMERCIAL MICROWAVE OVENS Microwave ovens which are used for commercial applications, are based on two magnetron tubes which are alternately active during one half of the mains power cycle of 20 msec. As illustrated in Fig.

3, the radiated electro-magnetic waves from the l/2 waveguides that are mounted on the two magnetron tubes are reflected by the rotating disk with metal mirror plates. This type of oven has a power consumption in the 1200 – 2500 Watt range and the cabinet is a more solid one of stainless steel. In an attempt to measure the max-hold spectrum for a commercial microwave oven, we found a characteristic as shown in Fig. 4, which occupies a much wider spectrum than the one found for a residential oven as illustrated in Fig. 1.

To characterise signals from a commercial oven we used an approach based on down mixing with a 2450 MHz carrier and filtering with a steep low-pass filter 1 to provide a baseband signal that is offered to a digital oscilloscope (a 125 MHz dual digital oscilloscope). In order to capture the oven activity over a full mains power cycle, we have selected a lower sampling resolution for the digital oscilloscope. Fig. 5, which is obtained in this way, illustrates the variation in the envelope of the CW-like microwave oven signal. A pulsed behaviour similar to the one found for a residential oven, is observed during the beginning and the end of the burst.

The commercial oven shows a random variation in frequency over tens of MHz, meaning that it covers a considerably wider frequency band than the domestic brother. The commercial oven gives a large power variation, as illustrated in Fig. 5. V. WIRELESS LANS BASED ON DSSS AND FHSS Currently available wireless LAN products employ either Bibliography REFERENCES [1] Gawthrop, F.

H. Sanders, K. B. Nebbia, J. J.

Sell, Radio spectrum measurements of individual microwave ovens, NTIA Report 94-303-1. [2] P. E. Gawthrop, F. H. Sanders, K.

B. Nebbia, J. J. Sell, Radio spectrum measurements of individual microwave ovens, NTIA Report 94-303-2. [3] S. Vasudevan, J.

Horne, M. K. Varanasi, Reliable wireless telephony using 2.4 GHz ISM band: Issues and solutions, in Proc. IEEE Fourth International Symposium on Spread Spectrum Techniques & Applications, Sept. 1996, (ISSSTA96), Mainz (Germany), pp.

790-794. [4] IEEE P802.11, Draft Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer Specification, P802.11 D5.1a, Jan. 1997. [5] A. Kamerman, Spread spectrum schemes for microwave-frequency WLANs, Microwave Journal, vol.

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