2018-02-04 1 Wireless Communication Channels Lecture 7: Directional channel models Channel sounding EITN85, FREDRIK TUFVESSON ELECTRICAL AND INFORMATION TECHNOLOGY HT2018 Wireless Communication Channels 2 Directional channel models The spatial domain can be used to increase the spectral efficiency of the system – Smart antennas – MIMO systems Need to know directional properties – How many significant reflection points? – Which directions? – Model incoming angle (direction of arrival) and outgoing angle (direction of departure) to scatterers Model independent of specific antenna pattern
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2018-02-04
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Wireless Communication ChannelsLecture 7: Directional channel modelsChannel sounding
EITN85, FREDRIK TUFVESSON
ELECTRICAL AND INFORMATION TECHNOLOGY
HT2018 Wireless Communication Channels 2
Directional channel models
The spatial domain can be used to increase the spectralefficiency of the system
– Smart antennas– MIMO systems
Need to know directional properties– How many significant reflection points?– Which directions?– Model incoming angle (direction of arrival) and
outgoing angle (direction of departure) to scatterersModel independent of specific antenna pattern
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HT2018 Wireless Communication Channels 3
Double directional impulse response
hÝt, r TX , r RX,b,I,H Þ= >§=1
NÝrÞ
h§Ýt, r TX, r RX,b,I,H Þ
TX position RX position
delay direction-of-departure
direction-of-arrival
h§Ýt, r TX, r RX,b,I,H Þ= |a §|e jj§NÝb?b§ÞNÝI?I§ÞNÝH ?H §Þ
number of multipath componentsfor these positions
HT2018 Wireless Communication Channels 4
Double directional impulse responsewith slightly different notation:
Time and locationis omitted here!
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HT2018 Wireless Communication Channels 5
Physical interpretation
ltW
Y
HT 2018 Wireless Communication Channels 6
Directional models
• The double directional delay power spectrum is sometimes factorizedw.r.t. DoD, DoA and delay.
• Often in reality there are groups of scatterers with similar DoD and DoA –clusters
I§H §
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HT 2018 Wireless Communication Channels 7
Angular dispersion
• At the base station the angular spread is often modeledas Laplacian
Assign positions for scatterersaccording to given distributions
Derive impulse response giventhe scatterers and distributionsfor the signal properties.
Used in the COST 259model, COST 273,COST 2100, WINNER3GPP/3GPP2
Geometry-Based Stochastic Channel Model(GSCM)
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Geometry-Based Stochastic Channel Model(GSCM)
BS
MS 1
Cluster
Local cluster
Local cluster
Cluster
MS 2
Create an ”imaginary” map for radio wave scatterers(clusters)
HT2018 Wireless Communication Channels
Courtesey:K. Haneda, Aalto Uni.
HT2018 Wireless Communication Channels 12
MIMO channel
channel matrix
úúúúú
û
ù
êêêêê
ë
é
=
)()()(
)()()()()()(
)(
TxRxRxRx
Tx
Tx
21
22221
11211
ttt
tttttt
t
MMMM
M
M
hhh
hhhhhh
LMOMM
LL
H
signal model ( ) ( ) ( )å-
=
-×=1
0
D
ttt
tt xHy
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HT2018 Wireless Communication Channels 13
Deterministic modeling methodsSolve Maxwell’s equations with boundary conditionsProblems:• Data base for environment• Computation time“Exact” solutions• Method of moments• Finite element method• Finite-difference time domain (FDTD)High frequency approximation• All waves modeled as rays that behave as in geometrical optics• Refinements include approximation to diffraction, diffuse
scattering, etc.
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Ray launchingTX antenna sends out rays in different directionsWe follow each ray as it propagates, until it either
– Reaches the receiver, or
– Becomes too weak to be relevantPropagation processes
– Free-space attenuation– Reflection– Diffraction and diffuse scattering:
each interacting object is sourceof multiple new rays
Predicts channel in a whole area (for one TX location)
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HT2018 Wireless Communication Channels 15
Ray tracingDetermines rays that can go fromone TX position to one RXposition
– Uses imagining principle– Similar to techniques
known from computerscience
Then determine attenuation of allthose possible paths
Example: Ray tracing
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Required basestation power toconnect to aWCDMA cell phone.Example fromStuttgart.
Courtesey: Awe-communications
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Example: Ray tracing
HT2018 Wireless Communication Channels 17
Coverage for aWCDMA cell phone.Example fromStuttgart.
Courtesey: Awe-communicationsPropagation Models
HT 2018 Wireless Communication Channels 18
Channel measurementsIn order to model the channel behavior we need to measureits properties
– Time domain measurements» impulse sounder
» correlative sounder
– Frequency domain measurements» Vector network analyzer
Frequency domain measurementsUse a vector network analyzer or similar to determine thetransfer function of the channel
• Time domain properties via FFT• Using a large frequency band it is possible to get good
time resolution• As for time domain measurements, we need to know the
influence of the measurement system
( ) ( )* ( )* ( )meas TXantenna channel RXantennaH f H f H f H f=
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Channel sounding – directional antenna• Measure one impulse
response for eachantenna orientation
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HT 2018 Wireless Communication Channels 23
Channel sounding – antenna array
• Measure one impulseresponse for each antennaelement
• Ambiguity with linear array
d d d
h( )t h( )t h( )t
spatially resolved impulse response
Signal processing
linear array
x=0 x=d x=2d x=(M-1)d
h( )tt
HT 2018 Wireless Communication Channels 24
Real, multiplexed, and virtual arrays
• Real array: simultaneousmeasurement at all antennaelements
• Multiplexed array: short timeintervals betweenmeasurements at differentelements
• Virtual array: long delayno problem with mutualcoupling
RX RX
RX
RX
RX
Digital Signal Processing
Digital Signal Processing
Digital Signal Processing
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HT 2018 Wireless Communication Channels 25
Directional analysis
The DoA can, e.g., be estimated bycorrelating the received signals withsteering vectors.
An element spacing of d=5.8 cm andan angle of arrival of f =20 degreesgives a time delay of 6.6·10-11 sbetween neighboring elements
d
f
d sin f
HT 2018 Wireless Communication Channels 26
High resolution algorithms
• In order to get better angular resolution, other techniquesfor estimating the angles are used, e.g.:
– MUSIC, subspace method using spectral search– ESPRIT, subspace method– MVM (Capon’s beamformer), rather easy spectral
search method– SAGE, iterative maximum likelihood method
• Based on models for the propagation• Rather complex, one measurement point may take 15
minutes on a decent computer
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HT 2018 Wireless Communication Channels 27
RUSK LUND, our broadband MIMO channelsounder
• A fast switched measurementsystem for radio propagationinvestigations at 300 MHz, 2GHz and 5 GHz.
• Financed by Knut and AliceWallenbergs stiftelse, FOIand LTH
• MIMO capacity limited by theswitches, currently 32elements at each side.
HT 2018 Wireless Communication Channels 28
It’s all about measuring some delays...
In MIMO systems we use the fact thatthere are several paths between thetransmitter and receiver
These paths are characterized by a– time delay,
– phase shift,
– attenuation,
– angle of departure and
– angle of arrival
The angle of departure and angle ofarrival result in a slight difference intime delay for each of the antennaelements
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HT 2018 Wireless Communication Channels 29
It’s all about measuring some delays...
• In practice we measure the transfer functions betweeneach of the antenna elements, and we calculate theparameters of interest
HT 2018 Wireless Communication Channels 30
Working principle
Courtesy
MEDAV
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HT 2018 Wireless Communication Channels 31
Timing diagram
HT 2018 Wireless Communication Channels 32
Test signal – Multicarrier spread spectrum
0 0.5 1 1.5
-0.5
0
0.5
1
time [µs]
norm
.mag
nitu
de
Tx signalin time
0 0.5 1 1.5
-0.5
0
0.5
1
time [µs]
norm
.mag
nitu
de
Tx signalin time
5.1 5.15 5.2 5.25 5.30
0.2
0.4
0.6
0.8
1
frequency[GHz]
norm
.mag
nitu
de
Tx signalinfrequency
5.1 5.15 5.2 5.25 5.30
0.2
0.4
0.6
0.8
1
frequency
norm
.mag
nitu
de
Tx signalinfrequency
MSSS - Test Squence• periodic broadband Signal• high Correlation Gain• low Crest Factor• inherently band limited• flexible in generation• multiband possibility
(Up- /Downlink)
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HT 2018 Wireless Communication Channels 33
The measurement system
• 200 kg of batteries to allow for 6 hours of mobile measurements• 640 MHz sampling frequency, to allow high Doppler frequencies• 2 separate PCs to manage the data flow from the A/D converters• Oven controlled rubidium clocks to maintain synchronization
during wireless measurements• GPS and wheel sensors to position the system• Broadband patch antennas with 128 antenna ports at 2.6 GHz• Circular 300 MHz antennas with a diameter of 1.5 m
HT 2018 Wireless Communication Channels 34
RUSK LUND transmitter
• Baseband (Arbitrary Wave Form)Signal Generator
• Frequency Synthesizer• Rubidium Reference• Modulator• Power Amplifier• MIMO Control Unit• GPS
• bandwidths: up to 240 MHz• frequency grid 10 MHz• max. power 500 mW, with possibility for