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

17

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

18

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

20

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

21

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

33

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

38

Active noise cancellation Proposed & verified method:

39

Phase shifter

90 phase shift

40

http://www.amitecltd.com/

Another phase shifter...

Napkla

d

41

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

43

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.

44

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

Snmek 1Snmek 2Snmek 3Snmek 4Snmek 5Snmek 6Snmek 7Snmek 8Snmek 9Snmek 10Snmek 11Snmek 12Snmek 13Snmek 14Snmek 15Snmek 16Snmek 17Snmek 18Snmek 19Snmek 20Snmek 21Snmek 22Snmek 23Snmek 24Snmek 25Snmek 26Snmek 27Snmek 28Snmek 29Snmek 30Snmek 31Snmek 32Snmek 33Snmek 34Snmek 35Snmek 36Snmek 37Snmek 38Snmek 39Snmek 40Snmek 41Snmek 42Snmek 43Snmek 44