Post on 20-Nov-2015
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Methods for interference mitigation in wireless networks
Metody pro snen interferenc v bezdrtovch stch
Ing. Tom Dulk, FAI TBU in ZlinSupervisor: doc. RNDr. Vojtch Keslek, CSc.
2Problem: crosstalks between co-located 802.11 devices working on adjacent channels
Picture source: http://www.eliatel.cz/Design/obr/RIA_RIC/PICT0111.jpg
Crosstalks
Channel N Channel N+20 MHz
Channel N-20 MHz
3The reason of the problem
Transmitted signal @ channel f=2432 MHzmasks the weak remote signals in adjacentchannels.
CF 2.432 GHz
2.38200000000 GHz
Received signal @ channel f+20=2452 MHzReceived signal @ channel f-20=2412 MHz
4Why should we solve the problem ? Because it troubles all 802.11 networks, both indoor and outdoor. Because it causes drops of network throughput. When network usage
peaks, the througput decreases beyond tolerable values. The 2.4 GHz band is the most affected because it has very limited
frequency allocation. Unfortunatelly, the 2.4GHz band is also the most populated (notebooks, mobile phones, tablets, e-book readers, baby monitoring devices, ): In Czech republic, the VO-R/10/03.2007-4 regulation for 2.4GHz provides
85MHz, which is 4 non-overlapping 20 MHz channels... But in collocations, we can use only 2 channels (see Table 7 in thesis).
Current solutions? Do not use adjacent channels in collocations GPS synchronization at 802.11 frames level:
Only available with Ubiquity and Motorola Canopy devices, 802.11 does not support this. From its principles, the synchronization decreases the network througput. practical experiments with Ubiquity GPS do not show satisfactory results.
5Thesis objectives Build a specialized RF testbed for WLAN
measurements, which will allow measuring and testing the behaviour of the real radio devices.
Analyse cross-device and/or cross-technology interferences.
Propose new or analyze existing methods for interference mitigation usable within DSSS and OFDM transceivers
6Thesis resultsauthor's contribution to the state of the art
7The RF emulation testbed design
Testbed measurement modes: Measuring adjacent, alternate and non-adjacent
channel rejection Measuring CS/CCA (Carrier sense/channel clear
assessment) Testing 802.11n MIMO 2x2 including crosstalks
between the 2 streams The MAC layer experiments:
up to 24 wireless nodes can be connected emulating hidden nodes, media access algorithms etc.
8The RF emulation testbed design
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
DUT802.11n
Signal analyzer
9RF emulation testbed implementation
10
RF emulation testbed implementation
2x UPS
Virtualization servers (Fujitsu TX200S5,
Supermicro SuperServer)
Local console (keyboard, monitor,
KVM switch)
RF network
Ubiquity Bullet2 & M5
2x USRP2-N210GNU Radio
R&S FSV7
NETIO power control
2 LAN switches(hidden under the drawer)
Space for signal generator
11
Design and implementationof testbed measurements methodology For every kind of measurement, a method and
a SW tool had to be developed. The methodology strictly follows the 802.11
standard requirements SW tools are published at
http://sourceforge.net/projects/wificolab/
12
Testbed measurements methodology
Example: measuring adjacent channel rejection with 3 WLAN devices in standard modes
Virtualization serverRF
test-bedLAN switch
Control & management machine DUT (in AP mode)
Golden device (in AP mode)
Closed firmware
Closed firmware WLAN
WLAN
Signal Generator the interferer:
Broadcaster tool
The sender machineiperf -uc
(client UDP mode)
Golden device (in STA mode)
Closed firmware WLAN
iperf -us (server UDP mode)
VLAN4
VLAN3
VLAN2
VLA
N4
VLA
N3
VLA
N2
13
Testbed measurements methodology
Example: measuring CCAVirtualization server
RF test-bed
LAN switch
Control & management machine
DUT (in AP mode)
Golden device (in AP mode)
Closed firmware
Closed firmware WLAN
WLAN
Signal Generator the interferer:
Broadcaster tool
The receiver machineiperf -us
(server UDP mode)
Golden device (in STA mode)
Closed firmware WLAN
iperf -uc (client-sender
UDP mode)
VLAN4
VLAN3
VLAN2
VLA
N4
VLA
N3
VLA
N2
Management SW
14
Practical measurements examples Transmit chain parameters measurements:
The simplest measurement method (support built in the R&S FSV7 analyzer)
But extremely important, needed for finding the golden devices. Example:
AzureWave AR5BXB63 AW-GE780 miniPCI-e card versusUbiquity Bullet2
15
Practical measurements examples Receiver minimum sensitivity measurement
Pre-requisite for adjacent channel rejection and CCA performance measurements!
Example: Ubiquity Bullet M5 (MIMO 1x1) in 802.11n mode
-46-47
-48-49
-50-51
-52-53
-54-55
-56-57
-58-59
-60-61
-62-63
-64-65
-66-67
-68-69
-70-71
-72-73
-74-75
-76-77
-78-79
-80-81
-82-83
-84-85
-86-87
-88-89
-90-91
-92-93
-94-95
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
40,00
45,00
MCS7 (65 Mbit/s)MCS6 (58.5 Mbit/s)MCS5 (52 Mbit/s)MCS4 (39 Mbit/s)MCS3 (26 Mbit/s)MCS2 (19.5 Mbit/s)MCS1 (13 Mbit/s)MCS0 (6.5 Mbit/s)
Signal level [dBm]
Thro
ugpu
t [M
bit/s
]
16
Practical measurements examples Adjacent channel rejection the first important
measurement for co-location scenarios Example: Cisco AIR-AP1231G-E-K9 vs. Ubiquity Bullet2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 470%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
54 Mbits48 Mbits36 Mbits24 Mbits18 Mbits12 Mbits9 Mbits6 Mbits11 Mbits5.5 Mbits
Signal level difference between working and adjacent channels [dB]
Pac
ket l
oss
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Practical measurements examples CCA performance in presence of adjacent channel
interference the 2nd important measurement for co-location Manufacturers will probably never publish such data this
scenario is not required by 802.11 standard Example: Cisco AIR-AP1231G-E-K9 vs. Ubiquity Bullet2
-60 -59 -58 -57 -56 -55 -54 -53 -52 -51 -50 -49 -47 -46 -45 -44 -43 -42 -410%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
54 vs. 11 Mbit/s54 vs. 6 Mbit/s11 vs. 11 Mbit/s
Adjacent channel interfering signal [dBm]
Tran
smitt
er th
roug
put d
rop
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More about CCA performance Cisco Carrier Busy Test for 6Mbit/s interference @ 2342 MHz, -46dBm
Frequency [MHz] Carrier Busy
2412 19%2417 96%2422 95%2427 94%2432 94%2437 89%2442 99%2447 92%2452 86%2457 3%2462 1%2467 0%2472 0%
All these channels are unusable for this DUT (Cisco) device, because the interferer is too strong!
19
Spectral analysis of OFDM/DSSS adjacent channel properties
For 802.11 DSSS, the spectral components do not interfere in the baseband:
f [Hz]
|U| [V]
fc+11MHz +22MHz +33MHz
1/T 2/T 3/T 4/T 5/T 6/T 7/T 8/T 9/T
2 D .T
-11MHz
fc
+20MHz
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Spectral analysis of OFDM/DSSS adjacent channel properties
For OFDM, the spectral components collide even between subcarriers because of symbol changes:
f [kHz]
|U| [V]
Subcarrier -2 0 1 2 -1
-625 -312.5 0 312.5 625125 -125 250 375 500
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Adjacent channel interference mitigation methods
22
Know your enemy (and know yourself)
With standard 802.11 device, following conditions must be met for co-location deployment: |S
max_colocated| < CCA_threshold
where |S
max_colocated| is the strongest measured signal level in adjacent channels.
CCA_threshold is the maximum signal level in adjacent channel for which the tested device CCA still returns channel free result.
|Smax_colocated
| < |Smin_remote
| + Adj_channel_Rejection
CCA_threshold and Adj_channel_Rejection must be measured only once (by somebody who knows how)...
|Smax_colocated
| and |Smin_remote
| can be measured on-site by the device own signal level detector
23
Other mitigation methods Improving the ACPR
Filtering in baseband PAPR reduction RF signal filtering
Coordinated TPC and DFS Active noise cancellation Distributed OFDM symbol-level synchronization
24
Follow-up works and things to do The testbed was already used in these thesis:
Vgner, A.: Real performance of devices operating on 802.11n. MSc. thesis, Faculty of Electrical Engineering and Communication, Brno University of Technology, 2011.
linz, P.: Issues of highly stressed wireless networks based on 802.11b/g standards. Masters thesis, Masaryk's univesity in Brno, 2012.
Upcoming PhD. thesis: linz, P.: Alternative 802.11 media access algorithms
Papers waiting for review: Turek,L., Dulk,T.: Packet Scheduler for Access Points in 802.11
Wireless Networks, Brno University of Technology FEEC, Radioengineering, Brno
25
Questions from prof. Raida's review
26
Previous works on DSSS - OFDM interference
SHIMIZU, Y., SANADA, Y. OFDM Interference suppression for DS/SS systems using complex FIR filter. In Proceedings of the IEEE Pacific Rim Conference on Communications, Computers and Signal Processing, 2007, p. 514517.
Proposed solution:
27
OFDM Interference suppression for DS/SS systems using complex FIR filter Application field: DS-UWB
OFDM modulation: BPSK 2nd Mod. DS/SS Bit rate of DS/SS Rb 150 Mbps Chip rate of DS/SS Rc = 3.75 Gcps Chip code c(n) Filtered DS-UWB, Ternary code (M:25) Filter delay Td 0.27 ns Filter coefficients h0 = 1,h1 = i Spectral null of DS/SS signal 0 GHz Num. of OFDM subcarriers Nsc 52 Total OFDM bandwidth 414 MHz OFDM symbol duration 13 s OFDM subcarrier bandwidth 15.6 MHz Signal-to-Interference bandwidth ratio R = 18
DS/SS bandwidth =7.452 GHz
28
OFDM Interference suppression for DS/SS systems using complex FIR filter The paper targets different scenario. In 802.11:
DSSS bandwidth ~ OFDM bandwidth DSSS is not the victim of OFDM.
DSSS is the predator, OFDM is the prey. 802.11 DSSS coexists & cooperates with
802.11 OFDM in the same channel this is handled by MAC protocol.This thesis deals only with adjacent channels interferences
29
Search engines for scientific databases
www.ObjectSpot.org - one of the iCamp project results, provides distributed/aggregated search in many repositories of scientific papers wireless emulation testbed keywords query
provides 64 results IEEEXplore: 84 results for the same query Google:
352000 results for the same keywords 3840 results for the wireless emulation testbed
exact phrase...
30
Wireless network emulator or simulator testbed ?
My testbed is not able to simulate all aspects of wireless networks, e.g. the antennas and free space propagation effects like multipath. Therefore, the word emulator is probably more appropriate.
http://en.wikipedia.org/wiki/Emulator#Emulation_versus_simulation: The word "emulator" was coined in 1963 at IBM during development of
the NPL (IBM 360) product line, using a "new combination of software, microcode, and hardware".
... However, before 1980, "emulation" referred only to emulation with a
hardware or microcode assist, while "simulation" referred to pure software emulation.
31
Testbed frequency range 2-6 GHz The frequency range is limited by:
Power splitters ZN4PD1-63+: 2-6 GHz Fixed attenuators VAT-XX+: 0-6 GHz Programmable attenuators:
Aeroflex Weinschel 3406T-55, 3408T-103: 0-6GHz(they have a bit better SWR and attenuation than JFW)
JFW 50P-1853 (solid state): 200-6000 MHz I did not have network analyzer (or generator)
to measure the insertion loss and SWR parameters at higher frequencies this is to be done.
32
Skin depth of for the Alu foil Skin depth (for >> ) where
=2 f, pro 2.4 GHz =15,08.109 rad/s is magnetic permeability for Al, ~
0=4.10-7 = 12,57.106 H/m
is conductivity, for Al =34,8.106 S/m (Al @ 2.4 GHz) = 1,7 .10-6 m = sqrt(2/(2*%pi*2.4e9 * 4*%pi*1e-7 * 34.8e6)) Plane wave absorbtion loss is:
Absorbtion loss of 17m aluminium foil @ 2.4GHz = -86 dB 20*log10((exp(1))^-(17/1.7))
For 11m foil, A=-56 dB
= 2
AdB=20 log et
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Shielding effectiveness For quicker calculations:
http://www.cvel.clemson.edu/emc/calculators/SE_Calculator/index.html
These calculations neglect: connector opening & connected UTP cable radiation Aluminium oxide layer on the alu foil
Alu foil thickness Plane wave absorbtion loss
Plane wafe reflection loss
11m @ 2,4GHz 56 dB 72 dB11m @ 5,4GHz 84 dB 69 dB22m @ 2,4GHz 112 dB 72 dB22m @ 5,4GHz 168 dB 69 dB
34
How the descriptions of DSSS and OFDM modulations fit in the thesis target?
I was trying to analyze the 802.11 OFDM and DSSS in spectral domain to better understand their behavior when they coexist/compete in adjacent channels.
I think the spectral pictures I made helped my understanding a lot, but I am aware that the analysis is far from good. It does not contain: Mathematic/analytic formulas Probabilistic interference model nor BER analysis The analysis of baseband filtering and RF chain
influences
35
Questions from doc. Syrovtka
p. 22 needs rephrasing Eq. (29) should be:
Where (, ) are amplitude and phase unbalances of signal Q
p. 96 Figures 10-14 (broken references):corrected in current version: http://zamestnanci.fai.utb.cz/~dulik/dissertation/
Fig. 84 has mismatch in lables for MCS7 and MCS6,indentation of Fig. 21, 27, 37, 43, 44, 45, .83, 87 will be corrected before final submission
yunwantedy wanted =122 cos 122cos
36
Questions from prof. Vlek
37
Active noise cancelation Previous works:
S. W. Kim, Y. J. Chun, and S. Kim. Co-channel interference cancellation using single radio frequency and baseband chain. Communications, IEEE Transactions on, 58(7):2169 2175, 2010.
Jain, M. et al. Practical, real-time, full duplex wireless. In MobiCom 11: Proceedings of the 17th annual international conference on Mobile computing and networking, ACM, New York, NY, USA, 2011, pp. 301312, ISBN: 978-1-4503-0492-4
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Active noise cancellation Proposed & verified method:
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Phase shifter
90 phase shift
40
http://www.amitecltd.com/
Another phase shifter...
Napkla
d
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Pulse shaping filters in chapter 8.4.2 Root raised cosine filter Rectangular filter for 802.11 DSSS/OFDM
preambles:
or the simpler form for rest of the packet:
wT [t ]={sin220.5t /T TR T TR /2tT TR /2
1 T TR /2tT T TR /2
sin22 0.5tT /T TR TT TR /2tTT TR /2}wT [n]=wT [n T s]={ 1 1n790.5 n=0800 otherwise}
42
Pulse shaping filters in chapter 8.4.2 Results see Figures 104-110 and Table 8:
Rectangular filter gives same or better results, but requires less computing power (simple time-domain multiplication instead of convolution for root raised cosine filter)
RF power [dBm] ACPR
Rectangular filter Root cosine filter
0 -59 dB -58 dB
10 -59 dB -59 dB
20 -46 dB -40 dB
30 -24 dB -24 dB
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Ferroelectric and MEMS technologies for RF signal filtering
Amoss, J.W. et al. A Ferroelectric Microwave Switch. IEEE Transactions on Microwave Theory and Techniques. 13, 6 (Nov. 1965), pp. 789- 793.
Klimov, V.V. et al. Ferroelectric variable capacitors. Ferroelectrics. 7, 1 (1974), pp. 337-339
Yuliang Zheng et al. Compact Substrate Integrated Waveguide Tunable Filter Based on Ferroelectric Ceramics. IEEE Microwave and Wireless Components Letters. 21, 9 (Sep. 2011), pp. 477-479
Reinke, J.R. CMOS-MEMS Variable Capacitors for Reconfigurable RF Circuits. PhD thesis, Carnegie Mellon University, 2011.
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Work progress report Bc. a MSc. thesis, where we experienced the problem:
2005: Hale, B.: A wireless network implementation. Jaro, J.: Building WiFi network with internet access in a remote rural area. Sporek, J.: Server, router and WiFi AP providing access to university network for students and guests.
2007-2008: Frytk, V.: System for localization of users in a wireless network. Svitk, J.: Modeling, simulation and throughput analysis for the 802.11 protocols family. Bula, J.: Measuring the RF parameters of WiFi devices. Mat, J.: A system for monitoring large area networks.
For being able to measure the problem, I was lucky to get a grant: CESNET 351/2009 Wireless testbed with secure remote access for research and development of WLAN 802.11 applications and protocols, started April 2010, finished March 2011.
04-28-2011 state doctoral exam 11-11-2011 1st thesis version submitted 11-12-2011 the proposed interference mitigation method OFDM symbol-
level synchronization proven wrong by practical experiment 2-22-2012 2nd corrected version submitted
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