Selected Papers

Electromagnetic Interference to 5.2-GHzband Wireless LANs
Yoshiharu Hiroshima†, Yoshiharu Akiyama, andHiroshi Yamane
Abstract
Although the electromagnetic environment and interference characteristics for 2.4-GHz-band wireless
local area networks (LANs) have been studied, those for 5.2-GHz-band ones have not. This paper presents measurements for the electromagnetic environment in a city and describes the characteristics ofinterference from microwave medical treatment machines to 5.2-GHz-band wireless LANs, whichshould be taken into consideration.

1. Introduction
The rapid development of telecommunications systems, especially wireless communications systems,
has made it easy to introduce information technology(IT) into homes and access it in offices and cities.
Examples of wireless communications systems thathave spread widely now include some using the 2.4GHz ISM (industrial, science, and medical) band.
This band can be used in Japan without a radio
license if proof of conformity to technical standards
is provided. Therefore, since wireless systems andISM equipment are used, interference in this band isan unavoidable problem. Consequently, the 5.2-GHz
band, which has few sources of interference, isattracting attention, and regulations for outdoor use
are also advanced.
Although the electromagnetic environment and theinterference characteristics for the 2.4-GHz-band
have been studied [1], [2], those for 5.2-GHz-band
wireless LANs have not, so research on the electromagnetic environment and interference characteristics is needed to establish methods for designing basestations of such wireless systems when they are set ina city area.
We investigated the following to understand the† NTT Energy and Environment Systems Laboratories
Musashino-shi, 180-8585 Japan
E-mail: hiroshima.yoshiharu@lab.ntt.co.jp

18

actual electromagnetic compatibility (EMC) problems of 5.2-GHz-band wireless LANs that are close
to an interference source in a city area.
(1) Measurements of the electromagnetic environment of the 5.2-GHz band
We measured the electromagnetic field strength
in some city locations. The measurements wererepeated at various times of the day.
(2) Research on the characteristics of interference
from microwave medical treatment machines towireless LANs
We built an experimental communication system and noise-injecting system. We usedmicrowave medical treatment machines(MMTMs) as a source of interference in a cityarea and a commercial 5.2-GHz-band wireless
LAN card. The characteristics of communication
degradation caused by the interference from theMMTM to the wireless LAN were measured.
Several models of wireless LAN equipment andMMTMs were prepared. The communication
system and the injecting system consisted ofcable paths.
2. Investigation of electromagnetic environment
The electric field strength of the 5.2-GHz band
(4.9–5.3 GHz) at some points in the city was measured to understand the electromagnetic environmentof 5.2-GHz-band wireless LANs. The measurement
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70Amp.
Dipole antenna

DC power

Cable
3m

Spectrum
analyzer

Field strength (dBµV/m)

DC 15 V
60

50

4030
4.9

Measuring vehicle

Fig. 1. Setup of measuring electromagnetic environment.

Vol. 2 No. 11 Nov. 2004

55.1
Frequency (GHz)

5.2

5.3

Fig. 2. Outdoor electric field intensity(changes for different places).
70
Field strength (dBµV/m)

setup is shown in Fig. 1. We used a measuring vehicle with an extendable arm on its roof (3.3 m abovethe ground) and a dipole antenna and signal amplifier. The signal amplifier was installed as near theantenna as possible considering signal attenuation.
Electromagnetic field measurement was carried out
while moving at about 10 km/h using the measuring
equipment (spectrum analyzer) installed in the measuring vehicle. The maximum value of received signals for 30 s was taken as data. At places with muchnoise, the measuring vehicle stopped and measurements that changed over time were repeated at various
times to see if the environmental factors changed withtime. The measurement parameters were as follows.
• Resolution band width (RBW): 1 MHz,
• Video band width (VBW): 1 MHz.
Examples of measurement results are shown inFigs. 2 and 3. Measured places were roads facing
drugstores (two places), one fast-food store, and one
school as examples of comparatively quiet environments. Figure 2 shows that noise appeared over thewhole measured band near the store, while no noise
appeared at the school etc. in the comparatively quietenvironment. Figure 3 shows the results of measuring
every other hour in front of drugstore #2 in Fig. 2. We
think that the peaks were observed at random because
the noise was wideband and its appearance time andsweep time were asynchronous.
To clarify the cause of the broadband noise measured near the drugstore, we measured the time andfrequency characteristics using a realtime spectrum
analyzer. In this measurement, we used a horn antenna, which has high directivity, to pinpoint the sourceof interference.
The measurement results when turning the antenna

Drugstore #1
Fast-food store
Drugstore #2
School60

Time 1Time 2Time 3

50

40

30
4.9

5

5.1
Frequency (GHz)

5.2

5.3Fig. 3. Outdoor electric field intensity (changes over time).

toward drugstore #2 are shown in Fig. 4. Figure 4(a)shows the frequency characteristics of the maximum
value of received field strength for 30 s. In Fig. 4(b),the horizontal axis expresses frequency and the vertical axis expresses time. The frequency span is 30
MHz and the time span is 9.25 ms.
Noise covering the whole band was observed once
during a period of 9.25 ms and the measured frequency characteristics show that the noise occurred
all over the measured band from 4 to 6 GHz. It is possible that the noise was radiated from the drugstore’s
security system, but this was not confirmed.
3. Characteristics of interference from MMTMs
Equipment that radiates in the 5.2-GHz band otherthan current wireless LANs is restricted in terms of
installation location such as weather radar and is con19

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sidered to have a low possibility of interfering withwireless LANs. However, ISM equipment that uses
the 2.4-GHz band may radiate harmonics near 5.2
GHz, and may cause interference. Therefore, in thisstudy, we measured the characteristics of interference
to 5.2-GHz-band wireless LANs. As a source ofinterference, we chose the MMTM (which is ISMequipment) and observed the radiation near 5.2 GHz.
The specifications of the wireless LAN equipmentused for measurement and MMTMs as interference
sources are given in Tables 1 and 2, respectively.

that can choose the communication channel. The
measurement parameters were as follows.
• RBW: 10 kHz• VBW: 10 kHz• Holding time of maximum value: 30 s
Measurement results are shown in Fig. 5. The radiation from some MMTMs is also shown. The measured values for both a wireless LAN and MMTM are
the receiving port voltage [dBµV] of the measurement equipment. Noise of almost the same level assignals came from MMTMs on some channels.

3.1 Communication characteristics of wirelessLAN
Before measuring the interference characteristics,
we used a spectrum analyzer to measure the signal
spectrum of a model-A 5.2-GHz-band wireless LAN

3.2 Interference with wireless LAN
communication
The setup for measuring the interference characteristics is shown in Fig. 6. To eliminate multi-path ordelay effects, we replaced the radio transmission path
71.2

Noise covering the whole band

504030
4.0

4.5

5.0

5.5

9.25 ms

60

Field strength (dBµV/m)

Field strength (dBµV/m)

70

6.0–28.8

Frequency (GHz)

5.085

5.115
Frequency (GHz)(a) Field strength vs frequency(b) Spectrogram

Fig. 4. Electromagnetic field strength measured at drugstore #2.

Table 1. Wireless LAN equipment.

Frequency
Modulation

Model A

Model B

5.1–5.2 GHz

5.1–5.2 GHz

OFDM

OFDM

Transmission distance

90 m (6 Mbit/s)

100 m

Note

channel tunablechannel tuned automatically

Table 2. MMTMs.
Model 1Model 2Model 3

Model 4Maker

ABB

C

DriveTransformer
typeTransformer
typeTransformer
typeInverter type

150 W

100 W

100 W × 2 arms

200 W

Max. power

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80
Ch34 Ch38 Ch42 Ch4670
Voltage (dBµV)

60
50
40
30
20
Model 2Model 4

100
5.1

5.15

5.2

5.25

5.3

Frequency (GHz)

Fig. 5. Signal power of 5.2-GHz-band wireless LAN.

Step attenuator

Antenna

Powerdividers

3-dB
attenuator

PC
Wireless
Spectrum LANanalyzer card

MMTM

Fully anechoic room
Step attenuator
Wireless LAN card

PC
3-dB attenuators

Fig. 6. Setup for measuring interference characteristics.

by a cable transmission path (coaxial cable). In a fully
anechoic room (FAR) measurement, the coaxial cable
was connected to two antenna ports of a wireless
LAN card, and the output was compounded withpower dividers. The compounded output was connected to one antenna port of the wireless LAN cardoutside the FAR through the coaxial cable. The noisefrom an MMTM was inserted through power
dividers.
Throughput was chosen as an evaluation factor, andthe average throughput when changing the output ofthe MMTMs and the wireless LANs by two step
attenuators was measured. Commercial software wasused for throughput measurement. Measurements
were carried out ten times and the average, maximum, and minimum values were recorded. The
throughput characteristics without noise were also
recorded.
Vol. 2 No. 11 Nov. 2004

The measured characteristics of interference to themodel-A wireless LAN from MMTMs are shown in
Figs. 7 and 8. Measurements were carried out forchannels 34 and 46 for which the noise fromMMTMs was obtained by measurement as shown in
Fig. 6. The horizontal axis is the sum of the attenuation caused by the two step attenuators.
For the measured value in channels 34 and 46,
when the attenuation exceeded 30 dB, throughput
decreased rapidly. This tendency was independent of
the existence of noise injected from MMTMs. We
think that the change in this throughput depended on
the attenuation of the transmitted signal and not on
the in-band interference from MMTMs. However,
only at the time of injection from model 4, throughput degradation was seen when the amount of attenuation was small. This is probably because the amplifier saturated when strong noise was radiated near the21Selected Papers

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Without noise
Model 1Model 2Model 3Model 4Throughput (Mbit/s)

201510500102030405060

Attenuation (dB)

Fig. 7. Interference from MMTM (Model A, Ch34).

25

Without noise
Model 2Model 3Model 4Throughput (Mbit/s)

20

1510500102030405060

Attenuation (dB)

Fig. 8. Interference from MMTM (Model A, Ch46).

25

Without noise
Model 1Model 2Model 3Model 4Throughput (Mbit/s)

201510500102030405060Attenuation (dB)

Fig. 9. Interference from MMTM (Model B, Ch34).

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band and was received by wireless LAN equipment.
If this degradation had been caused by in-band interference from MMTMs, the degradation would havebeen seen when the attenuation was big.
The measured characteristics for a model-B wireless LAN of interference from MMTMs are shown inFig. 9. Since one communication channel was set up
arbitrarily, model-B was measured for the single
channel. Like model-A, when the attenuation exceeded 30 dB, throughput decreased rapidly independentof the existence of injection from MMTMs. Moreover, only at the time of injection from model 4,throughput degradation was seen when the attenuation was small, this tendency was also the same.
Since the same tendency was seen even though models A and B use different operating systems, thisdegradation may not originate in the personal computer used for control and may influence wireless
LAN equipment.

Yoshiharu Hiroshima
Research Engineer, Energy Systems Project,
NTT Energy and Environment Systems Laboratories.
He received the B.E. and M.E. degrees in electrical and communication engineering fromTohoku University, Sendai, Miyagi in 1986 and1988, respectively. In 1988, he joined NTT Electrical Communication Laboratories, Musashino.
He has been researching EMC measurement for
telecommunication systems. He is a member of
the Institute of Electronics, Information andCommunication Engineers (IEICE).

Yoshiharu Akiyama
Senior Manager, Energy Systems Project, NTT
Energy and Environment Systems Laboratories.
He received the B.E. degree in electronic engineering from the University of Electro-Communications, Chofu, Tokyo in 1990. In 1990, hejoined NTT Electrical Communication Laboratories, Musashino. He has been researching EMCon wireless communications and home networking system. He is a member of IEICE.

4. Conclusion
To establish a base station design method for wireless LAN access systems, we investigated the electromagnetic environment of the 5.2-GHz band andthe characteristics of interference to 5.2-GHz-band
wireless LANs when MMTMs were chosen as
sources of interference.
Impulse noise appeared in outdoor environments.
Moreover, the maximum electric field strength wasestimated to be 60 dBµV/m. For the interference measurement of 5.2-GHz-band wireless LANs fromMMTMs, although there was no interference in theband, one model of MMTM showed that throughput
degradation might be caused by noise near the band.
In this paper, we considered only the relative level
difference between noise and signal. Since the accessspeed is set up according to the state of the transmission path in a 5.2-GHz-band wireless LAN, in orderto perform interference measurement in each transmission state, it is necessary to change the signal andnoise levels independently.

Hiroshi Yamane
Senior Research Engineer, Supervisor, Energy
Systems Project, NTT Energy and Environment
Systems Laboratories.
He received the B.E., M.E., and D.E. degrees
in electronic engineering from Ibaraki University, Hitachi, Ibaraki in 1980, 1982, and 1997,
respectively. In 1982, he joined the ElectricalCommunication Laboratories, Nippon Telegraphand Telephone Public Corporation (now NTT),
Ibaraki. He has been researching lightning surge
and overvoltage protection for telecommunications systems. He is a member of IEICE.

References
[1]

[2]

Y. Akiyama, Y. Hiroshima, H. Yamane, and F. Amemiya, “A Studyon EMC Issues of Wireless Home Networking,” ICEMC2002, pp.
173-178, 2002.
K. Takaya, Y. Maeda, and N. Kuwabara, “Experimental Investigationof BER and Throughput Characteristics for the Interference Between
2.4 GHz Band Middle Speed Wireless LAN Systems,” Trans. IEICE,
Vol. J82-B, No. 5, p. 1082.

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