COM 5 – D 8 – E
INTERNATIONAL TELECOMMUNICATION UNION
TELECOMMUNICATION
STANDARDIZATION SECTOR
English only
STUDY PERIOD 2005-2008
Original: English
Questions:
8 and 16 /5
Geneva, 8-12 November 2004
STUDY GROUP 5 – DELAYED CONTRIBUTION 8
Source:
NTT
Title:
Example of trouble cases of Wireless LAN system
1
Introduction
A Wireless LAN system (W-LAN) is very useful system in the information society which makes
communication possible always anywhere. However, the electromagnetic environments in public
areas, such as a airport, railway station, underground shopping mall, and so on, are complicated.
Therefore, it is important for installing W-LAN system to maintain compatibility with
electromagnetic environment at installations.
This document introduces several examples of trouble cases in W-LAN system. Considering these
trouble cases, issues to be considered in relation to EMC are proposed in this document.
2
Example of Troubles
2.1
Interference between APs
When the wireless LAN system is installed in a hospital, it was sometimes impossible for wireless
terminals in specific area to communicate with Access Points (APs). Plan of hospital and the place
of APs are shown in Fig.1.
Spectrum of the W-LAN frequency band was measured in the troubled area and shown in Fig. 2.
From this figure, all frequency bands were used and it is difficult to distinguish W-LAN channels.
The reasons of the trouble are as follows.
(1) Too many APs are installed
(2) Inappropriate W-LAN channel setup (the channel of close frequency is set as the adjacent access
point).
For fixing this problem, some APs are removed and channel setup for access points were optimized
(avoid the overlap of frequency band among closer APs. As shown in Fig. 3, frequency spectrum
was separated and W-LAN channel becomes easy to distinguishable. After the countermeasure,
communication between wireless terminal and AP was established without mutual interference.
Contact:
Ryuichi Kobayashi
NTT East
Japan
Tel:+81-3-5739-3241
Fax:+81-3-6408-2914
Email:kobayasi@tasc.mod.east.ntt.co.jp
Attention: This is not a publication made available to the public, but an internal ITU-T Document intended only for use by the
Member States of the ITU, by ITU-T Sector Members and Associates, and their respective staff and collaborators in their ITU related
work. It shall not be made available to, and used by, any other persons or entities without the prior written consent of the ITU-T.
-2COM 5 – D 8 – E
Floor view
Troubled
Area
10m
Access points
(No. of AP was 15)
Fig. 1 AP set-up in the floor
Received level[dBuV]
100
80
60
40
2400
2420
2440
2460
2480
2500
Frequency[MHz]
Received level[dBV]
Fig.2 Measured spectrum before countermeasure
100
Ch1
Ch9
Ch5
80
Ch14
60
40
2400
2420
2440
2460
2480
2500
Frequency[MHz]
Fig.3 Measured spectrum after countermeasure
-3COM 5 – D 8 – E
Newly installed Wi-Fi service
Malfunction
AP
- Not able to connect
- Low through-put, etc
Power Injector
ONU
ch11
CAT5 cable
20m
ISP
AC100V
5m
Displayed system in shop
AP
Client
ch1
Fig.4 Configuration of W-LAN system
Received level[dBV]
Turn on Wi-Fi system(Peak value)
Turn off Wi-Fi system(Average value)
80
Displayed AP
(ch1)
70
Wi-Fi service’s AP
(ch11)
Wireless
speaker
system
60
50
40
Wireless TV
(No1)
30
Wireless TV
(No.2)
20
10
0
2390
2410
2430
2450
2470
2490
2510
Frequency[MHz]
Fig. 5 Measured spectrums
2.2
Interference between AP and other Wireless system
When WiFi service’s W-LAN system was installed in a consumer electronic shop, throughput was
very low (about 300kbit/sec) and sometimes connection between AP and W-terminal was lost.
Configuration of installation is shown in Fig. 4.We measured frequency spectrum in the shop and
the result is shown in Fig.5. Black line shows the spectrum when the AP is turned on, and Gray line
shows the result when the AP is turned off. From this figure, spectrum was overlapping between WLAN system and the other system that was used in the shop. From the investigation in the shop, we
found out that this frequency was used by wireless TV system and wireless speaker system.
Separating use of frequencies among WiFi-LAN system and the other systems is the only way to
solve this trouble. Therefore, coordination between WiFi LAN provider and shop owner is
necessary.
-4COM 5 – D 8 – E
Received level[dBV]
80
W-LAN
(ch1)
60
Unintentional emission
from MW oven
40
20
0
2423～2458MHz
2390
2450
2510
Frequency[MHz]
Fig.6 Measured spectrum when malfunction occurred
A C K p a ck et f ro m A P
D a ta p a ck et f ro m
term in a l eq u ip m en t
Time (7ms)
CH 9
U n in ten tio n a l em issio n
f ro m M W O v en
Frequency[MHz]
Fig.7 Measured spectrogram
2.3
Disturbance from unintended emission
When the wireless LAN system was installed in an office, connection of W-LAN system is
sometimes lost. The measured spectrum in office is shown in Fig. 6. From this figure, we found out
that disturbance appears in frequency range from 2423 to 2458MHz. When we searched for source
of disturbance, we found out that it was a leaked electromagnetic wave from the microwave oven
used in an adjacent building.
Since the frequency of a disturbance overlaps the frequency range of the channels 5 to 11 of WLAN, it is necessary to set the channel of W-LAN to1 to 4, or 12 to 14.
Figure 7 shows example of interference between W-LAN signal (CH.9) and electromagnetic wave
come from microwave oven.
-5COM 5 – D 8 – E
3
Summary
Following points should be considered as main causes of W-LAN trouble.
(1) Mutual interference in the same system
(2) Interference by other systems
(3) Disturbance from ISM equipment
It is required to take the above-mentioned issues into consideration not only in the case of a trouble,
but also during installation. Moreover, methodology for finding out causes and measurement tools
are different from that for conventional EMC problems. Therefore, it is important to take into
consideration not only characteristics of W-LAN or EM-environment, but also methodology for
solving trouble, measurement tools, and countermeasures in establishing K.emc or K.tnt.
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