Mitsubishi Electric Research Laboratories, Inc.
Andreas F. Molisch, Andrés Alayón-Glazunov, Peter Almers, Gunnar Eriksson,
Anders J. Johansson, Johan Karedal, Buon Kiong Lau, Neelesh B. Mehta, Fredrik Tufvesson, Shurjeel Wyne
MIMO antennas, propagation channels, and their impact on system design
page / 2MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Wireless propagation channels describe how electromagnetic signals get from transmitter to receiver
It is the propagation channel that distinguishes wireless communications from wired communications:
• Multipath propagation ( -> Fading, Time variations)• Interference
page / 3MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Antennas describe how electromagnetic waves are launched from TX and received at RX
Antennas are interface between RF electronics and channel• TX antennas determine how waves are sent off into space• RX antennas receive the waves
Complex antenna patterns determines how multipath components interact with antennas
Channels and antennas determine the possibilities of signal transmission schemes and signal processing
page / 4MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Understanding channels is vital for theory
Any breakthrough in communications theory is based on simplifiedchannel model! But at some point we must ask“Which effects are real, and which are artifacts of the channel model?”
(A. Lapidoth)
Example: Rayleigh fading results in finite probability that receive power is larger than transmit power
• Too unlikely to matter in most cases• Multiuser diversity always selects user with instantaneously
best SNR• What if number of users becomes very large ?
page / 5MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Understanding channels is vital for system testing
Comparison of different systems:Different systems may “win” in different channelsChannel model for standards need to be chosen carefully
Comparison of MIMO-OFDM-based proposals for 802.11n
page / 6MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Understanding channels is vital for system design
System parameters have to be chosen according to propagation channelSystem cannot cover all thinkable “worst cases” (too inefficient); has to be designed just right for the channels in which it should operateExample:
• Repetition frequency for training sequence of MIMO channel estimator
page / 7MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Understanding channels is an inspiration for theory and system design
The channel creates the problems for effective data transmissionUnderstanding where the problems are coming from gives ideas for how to circumvent themExample:
• RF preprocessing for antenna selection
page / 8MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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So……
“Know thy channel”(Solomon Golomb)
page / 9MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Definition of MIMO
What is a MIMO system?A MIMO system consists of several antenna elements, plus adaptive signal processing, at both transmitter and receiver, the combination of which exploits the spatial dimension of the mobile radio channel.
H1,1
H2,1Hn,1T
Transmitter Receiver
Antenna 1
Antenna 1
Antenna 2
Antenna 2
Antenna nT
Antenna nR
H1,nR
H2,nR
Hn,T nR
SignalprocessingData source
Signalprocessing Data sink
Channel
page / 10MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Goals of MIMO
• increase power• beamforming
Array Gain
• multiply data rates• spatially orthogonal
channels
SpatialMultiplexing • mitigate fading
• space-time coding
Diversity
page / 11MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Diversity vs. array gain
Diversity: reduce probability that signals at all antenna elements are in fading dip simultaneouslyBeamforming: increase mean SINR when receiving signals from certain direction
page / 12MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Spatial multiplexing
Each MPC can carry independent data streamBeamforming view:
• TX antenna “targets” energy onto one scatterer• RX antenna receives only from that direction
Channel capacity grows linearly with number of antenna• C~min(Nt, Nr, Nscatt)
page / 13MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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History
Diversity:• Receive diversity: since 1940s• Transmit diversity: early 1990s
Wittneben; Winters
• Space-time codes in late 1990sTarokh et al.; Alamouti
Spatial multiplexing:• Invented by Winters 1987• Theoretical treatment in mid-1990s
Paulraj; Telatar; Foschini&Gans; Raleigh and Cioffi, Tarokh et al.
• Prototypes in early 2000s• Standardized systems for large-scale deployment: after 2005
- IEEE 802.11n, 3GPP Release 7, Wimax, 3GPP-LTE
page / 14MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversity Array design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 15MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Types of MIMO channel models: transfer function matrix
Transfer function from each transmit- to each receive antenna System-oriented description: signals at antenna connectorsEasy to measureNo connection to physics of propagationAssumes specific antenna array configuration
H 1,1H2,1HN ,1r
Antenna 1
Antenna 1
Antenna 2
Antenna 2
Antenna Nt
Antenna NrH1,Nt
HNr,Nt
page / 16MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Types of MIMO channel models: double-directional model
Parameters of the multipath componentsChannel-oriented descriptionIndependent of antenna properties
M. Steinbauer, A. F. Molisch, and E. Bonek, ”The double-directional mobile radio channel”, IEEE Antennas Prop. Mag., 43, No. 4, 51-63 (2001).
Transmitter Receiver
a1 ,τ
ττ
1 T,1
T,2T,3
,
,,
, R,1
R,2R,3 aa23 ,,
23 ,,
page / 17MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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The RUSK Lund Channel sounder for measuring A fast switched measurement system for radio propagation investigations at 300 MHz, 2 GHz and 5 GHz.Up to 240 MHz bandwidthMIMO capacity determined by the switches, currently 32 elements at each side.Multipath parameter extraction by SAGE/RIMAX algorithm
page / 18MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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algorithms•algorithmdevelopment
•space-timecoding
•transceivertechniques
•`bad urban´•`rural´• ...
canonicalchannels
analyticalframework
•signalprocessing
• informationtheory
• ...
physical wavepropagation
•COST259 •GSCM•ray tracing• ...
Spatial channel models – an overview
antennaconfiguration
canonicalconfigurations
•number•geometry•polarization
networklevel
• link capacity•cdf of BER•interference ...
canonicalchannels
typicalenvironmentscanonicalenvironments
page / 19MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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The Double-directional Propagation Channel
Propagation ChannelDouble-directional
DODs
MT
R R T Th(t, , , , )τ ϕ θ ϕ ,θ M
DOAs
"Single-directional" Channel for DOAs
h(t, )τh(t, )τ
TX-Site RX-Site
Radio Channel
scatterers
R Rh(t, , , )τ ϕ θ
h,x R ,x T i1N hi,R,i,T,i gRRgTT ej kR,ixR ej kT,ixT
double-dir. Impulse response antenna pattern element location factortransferfunction
page / 20MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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…used now in almost all standardized MIMO models
COST 259: macro-micro- and picocells
3GPP: cellular systems in urban and suburban area
802.11n: indoor WiFi systems
COST 273: macro-, micro-, picocells, peer-to-peer, fixed wireless
M. Steinbauer and A. F. Molisch (eds.), “Directional channel models”, Chapter 3.2 (pp. 132-193) of “Flexible Personalized Wireless Communications”, L. Correia (ed.), Wiley, 2001
Spatial Channel Modeling Ad-hoc group (A. Kogiantis, et al.: SCM text version 6.0, SCM AHG Doc. 134, Jan. 2003.
G. Calcev, D. Chizhik, B. Goeransson, S. Howard, H. Huang, A. Kogiantis, A. F. Molisch, A. L. Moustakas, D. Reed and H. Xu, “A Wideband Spatial Channel Model for System-Wide Simulations”, IEEE Trans. Vehicular Techn., 56, 389-403,2007.
V. Erceg, et al., “TGn channel models”, IEEE document 802.11-03/940r4, May 2004.
A. F. Molisch and H. Hofstetter, “The COST 273 MIMO channel model”, in L. Correia (ed.), “Mobile Broadband Multimedia Networks”, Academic Press, (2006).
P. Almers, et al.. Survey of Channel and Radio Propagation Models for Wireless MIMO Systems. EURASIP Journal on Wireless Communications and Networking, 2007, 2007.
page / 21MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversity Array design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 22MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Multipath propagation causes angular dispersion
Important propagation mechanisms:
Over-the-rooftop
Waveguiding in street canyons
Reflection at far scatterers
page / 23MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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3D-measurements at BS – RX3 (microcell): setup
S
WE
NC
athe
dral
177°
-50°
RX3
-140
°
TX30°
20°
130°
-170
°
53°
135°
100
200
m0
h=30
m
h=28
m
h=30
m
h=23
mh=
29 m
h≈60
m
h=28
m
h=29
m
h=24
m
h=25
m
BO
F
80°
Stre
et 2
Stre
et 1
Yard
M. Töltsch, J. Laurila, A. F. Molisch, K. Kalliola, P. Vainikainen, and E. Bonek, „Spatial characterization of urban mobile radio channels“, IEEE JSAC 20, 539-549 (2002).
page / 24MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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M. Töltsch, J. Laurila, A. F. Molisch, K. Kalliola, P. Vainikainen, and E. Bonek, „Spatial characterization of urban mobile radio channels“, IEEE JSAC 20, 539-549 (2002).
page / 25MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Measurement Campaign
Macro/Microcell:
• same measurement routes• same array orientation• BS antenna height
difference of 6.5m
Figure courtesy of K. Hugl and E. Bonek;Joint work with P. Vainikkainen
page / 26MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Deviation between geometrical MS position and DOA
Above rooftop:correlated!
Below rooftop:not correlated!
Figure courtesy of K. Hugl and E. Bonek;Joint work with P. Vainikkainen
page / 27MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Results – Propagation mechanisms
G. Eriksson, F. Tufvesson, and A. F. Molisch, “Investigation of the Radio Channel for Peer-to-Peer Multiple Antenna Systems at 300 MHz”, Proc. IEEE Globecom 2006, (2006).
Non-LOSLOS path blocked by a buildingOther buildings close to the LOS path
page / 28MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Results – Propagation mechanisms
Rx1-1
Rx3-1
µsµs
page / 29MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversityArray design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 30MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Why do we care about angular dispersion?
Angular dispersion determines correlation between signals at antenna elements
• For a fixed array structure: the bigger angular spread, the smaller the correlation
Correlation determines the capacity of MIMO
2
2
log det / /
log 1
R
Hn
T
iT
C bits s Hzn
n
γ
γ λ
= +
= +
∑
I HH
page / 31MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Capacity with correlation
page / 32MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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It all depends on the rms angular spread ?
Folk law: “MIMO and diversity properties determined by rmsangular spread” ⇒ can model far scatterers by increasing angular spread of local scatterersWhere does it come from?
• [Asztely and Ottersten 1996]: correlation coefficient can be approximated by function that depends only on rms angular spread
BUT• requires several assumptions (stated in the paper !)
- rms angular spread small- maximum angular spread small- Etc.
page / 33MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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MIMO capacity
Cdf of the capacity for: two specular sources (solid), single cluster (dashed), two clusters (dotted).
0 5 10 15 20 25 30 35 400
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1
AS=30 deg
capacity [bit/s/Hz]
cdf(c
apa
city
)
AS=5.8 deg
0 5 10 15 20 25 30 35 400
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1
AS=30 deg
AS=5.8 deg
capacity [bit/s/Hz]
cdf(c
apa
city
)
2*2 array 8*8 array
A. F. Molisch, "Effect of far scatterer clusters in MIMO outdoor channel models", Proc. 57th IEEE Vehicular Techn. Conf., 534-538 (2003).
page / 34MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Line-of-sight reduces capacity for constant receive power
A. F. Molisch, M. Steinbauer, M. Toeltsch, E. Bonek, and R. Thoma, „Capacity of MIMO systems based on measured wireless channels“, “, IEEE JSAC 20, 561-569 (2002).
page / 35MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Limited number of scatterers
A. F. Molisch, M. Steinbauer, M. Toeltsch, E. Bonek, and R. Thoma, „Capacity of MIMO systems based on measured wireless channels“, “, IEEE JSAC 20, 561-569 (2002).
page / 36MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Are TX and RX directional spectra independent?
Joint APS is theproduct of marginalRx- and Tx-APS.
measurement
W. Weichselberger, M. Herdin, H. Özcelik, E. Bonek, “A Stochastic MIMO Channel Model With Joint Correlation of Both Link Ends,” IEEE Transactions on Wireless Communications, 5(1) , pages 90 - 99, 2006.
page / 37MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Are TX and RX directional spectra independent?
Joint APS is theproduct of marginalRx- and Tx-APS.
Kroneckerapproximation
W. Weichselberger, M. Herdin, H. Özcelik, E. Bonek, “A Stochastic MIMO Channel Model With Joint Correlation of Both Link Ends,” IEEE Transactions on Wireless Communications, 5(1) , pages 90 - 99, 2006.
page / 38MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversity Array design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 39MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Trend towards compact mobile terminals, limited space for antenna system.
Closely-spaced antennas have mutual coupling
Limits on antenna spacing
0.3λ @ 900 MHz
K510a
0.25λ
Figure courtesy of Sony-Ericsson
page / 40MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Matching for mutual coupling
page / 41MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Minimum admissible antenna spacing
MIMO capacity with mutual coupling with different matching strategies
0 0.5 10
1
2
3
4
5
6
7(a) WF Narrowband scenario=1
Antenna separation d/λ
Mea
n C
apac
ity (
bits
/s/H
z)
Reference (nc)Z
0 Match
Self MatchInput MatchMC Match
0 0.5 10
1
2
3
4
5
6
7(b) 120MHz BW
Mea
n C
apac
ity (
bits
/s/H
z)
Antenna separation d/λ
Reference (nc)Z
0 Match
Self MatchInput MatchMC Match
B. K. Lau, J. B. Andersen, G. Kristenson, and A. F. Molisch, “Impact of Matching Network on the Capacity of Compact MIMO systems”, Proc. Antennas 06 Conference.
page / 42MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversity Array design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 43MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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When are uniform linear arrays possible?
Base stations of cellular systems• Limit to number of antennas: wind load• Typical size of array: 4 elements
Access points for wireless LAN• Limit to number of antennas: size of access point• Typical size of array: 4 elements
Laptop• Limit to number of antennas: size of laptop; mounting on
backplane of screen or on edges?Not possible on handsets
page / 44MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Consequences of non-ULA structure
Different mean powers: e.g., antennas with maximum gain in LOS direction get more mean power than antennas pointing away
• Consequences for system design: modulation alphabet size, waterfilling, based on mean power is possible
Different fading statistics for different antenna elements
page / 45MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Small-Scale Amplitude Statistics
Some Tx-Rx combinations exhibit Rayleigh statisticsSome Tx-Rx combinations exhibit Rice statisticsSome Tx-Rx combinations exhibit ”other” statisticsMeasurement is ”LOS”!
Rows – Tx elements
Columns – Rx elements
t2/r2
t4/r4 (top)
t3/r3 (side)
t1/r1
A. Johanson, J. Karedal, F. Tufvesson, and A.F. Molisch, "MIMO channel measurements for Personal Area Networks", Proc. 61st IEEE Vehicular Techn. Conf., 171-176 (2005).
page / 46MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Polarization
Polarization offers more degrees of freedom without requiring more spaceOne antenna element can have two ports for two orthogonal polarizationsFading of orthogonal polarizations is independentMean power in co-polarized components is higher than in cross-polarized
TX RX
High power
Low power
Equal power
page / 47MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Contents
Double-directional channels versus transfer functionsAngular dispersion: how is it caused?Angular dispersion: impact on capacity and diversity Array design: how close can antennas be?Array design: beyond uniform linear arraysCase study: antenna selection
page / 48MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Antenna selection reduces complexity of MIMO
RF chains are major cost factorAntenna selection reduces number of chains, and thus costs and complexity Hybrid selection: use L out of N antennas
MIMO Channel
(H)
mod
mod
mod
demod
demod
demod
r receiving antennas
selected
demodA. F. Molisch and M. Z. Win, “MIMO systems with antenna selection”, IEEE Microwave Magazine March 2004, 46-56 (2004).
page / 49MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Switch implements adaptive antenna selection
MIMO Channel
(H)
mod
mod
mod
demod
r receiving antennas
Switch
demod
page / 50MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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RF-preprocessing recovers beamforming gain
Can be based on no channel state information (CSI), average CSI, or instantaneous CSI
A. F. Molisch and X. Zhang, „FFT-based Hybrid Antenna Selection Schemes for spatially correlated MIMO channels“, IEEE Comm. Lett., 8, 36-38 (2004).
X. Zhang, A. F. Molisch, and S. Y. Kung, “Variable-phase-shift-based RF-baseband codesign for MIMO antenna selection”, IEEE Trans. Signal Proc., 53, 4091-4103 (2005).
P. Sudarshan, N. B. Mehta, A. F. Molisch, and J. Zhang, „Channel Statistics-Based Joint RF-Baseband Design for Antenna Selection for Spatial Multiplexing”, IEEE Trans. Wireless Comm. 5, 3501-3511, (2006)
MIMO Channel
(H)
mod
mod
mod
demodSwitch
demod
RF-preprocessing
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Interpretation of preprocessing: Converting antenna selection to beam selection
0.2
0.4
0.6
0.8
1
30
210
60
240
90
270
120
300
150
330
180 0
No CSI available
page / 52MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Performance simulations
7
8
9
10
(normalized angular spread)
full-complexity system
3/8 antenna selection
3/8 preprocessingselection; no CSI
A. F. Molisch, M. Z. Win, Y. S. Choi, and J. H. Winters, “Capacity of MIMO systems with antenna selection”, IEEE Trans. Wireless Comm., 4, 1759-1772 (2005).
page / 53MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Handheld device
t2/r2
t4/r4 (top)
t3/r3 (side)
t1/r1
Device held at chestheight, in right hand of standing person (data mode)
page / 54MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Impact of antenna configurations
d
Configurations
lamda/2
lamda/2
d
PolarizationLine
Saw
Rectangular
Horizontal (H)
Vertical (V)
Alternate H & V (Alt HV)
Dual polarized (DP)
d
d
Configurations
lamda/2
lamda/2
d
PolarizationLine
Saw
Rectangular
Horizontal (H)
Vertical (V)
Alternate H & V (Alt HV)
Dual polarized (DP)
d
Configuration comparison for the AP - PC scenario HS-B at PC only. LOS. 4:2×2:2.
P. Almers, T. Santos, F. Tufvesson, A. F. Molisch, J. Karedal, and A. Johansson, “Antenna selection in measured indoor channels”, Proc. IEE Part H., in press.
page / 55MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Diversity gain
FC PSS opt PSS sopt FFTS HS-B PBS HS-R5
6
7
8
9
10
11
12
13
Aver
age
norm
aliz
edSN
R[d
B]
Algorithm
FullComplexity
.
RF-Preprocc.With instant.Channelstate info
FFT-basedselection
NormalAntennaSelection(no RFpreproc.)
No selection
uniform linear array
irregular array
oversimplifiedchannel model(i.i.d.)
measuredchannels
P. Almers, T. Santos, F. Tufvesson, A. F. Molisch, J. Karedal, and A. Johansson, “Antenna selection in measured indoor channels”, Proc. IEE Part H., in press.
page / 56MIMO antennas, propagation channels, and their impact on system designISWCS, October 2007
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Summary
MIMO signal processing and information theory deal with “effective” channel from TX antenna connectors to RX antenna connectors“Effective” channel is composed of (double-directional) propagation channel and antenna arrays; they interactAntenna array elements can be spaced closely, but only for narrowband case and only with appropriate matchingFor ULA, angular dispersion and antenna spacing determine capacity of MIMO systemFor non-ULA, signal statistics change; this can influence efficacy of signal processing schemes like antenna selection