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7/29/2019 TW Lecture1
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OFDM-MIMO implementation
in Line Of Sight
microwave/millimeter wave
link
Baruch Cyzs
baruch_cyzs@hotmail.com
7/29/2019 TW Lecture1
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Introduction
Implementation of OFDM-MIMO in line of
sight microwave link
Description of hardware prototype of mmwave PTP microwave that employs
OFDM-MIMO.
Important Implementation issues inmicrowave link that employs OFSM-MIMO
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The MIMO Spatial multiplexing
implementation The MIMO spatial implementation exploits
random independent and identical distributed(iid) channel.
The orthogonality of the channel is usuallyachieved by existing of reach scattering.
Spatial multiplexing suffer degradation in itsperformance if significant direct path (LOS)
exists in the Rician channel. LOS microwave link cannot implement MIMO
since it relies mainly on strong LOS component
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How can MIMO implemented in
LOS Microwave For 3 decades LOS microwave links use polar
multiplexing by transmitting via orthogonalpolarizations.
Witcom in 2001 has initiated new activity ofimplementing geometric spatial multiplexingproject.
Prior to project kickoff Witcom has initiated
extensive outdoor field test to evaluate MIMOperformance in 5.8GHz in Tel Aviv.
Test results has shown low rank (mostlysingular) channel even in near/non line of sight.
The results has driven Witcom to seek solutionin the geometric spatial multiplexing.
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MIMO SM field test in 5.8GHz
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The antenna array approach
As opposed to polar multiplexing in spatialmultiplexing the number of SM channels
can be greater than 2. The LOS microwave link multiplexing
employs antenna arrays at both sides (noneed to be equal number of elements)
The array antenna spacing is the keyfactor for achieving orthogonality.
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The Near Field multiplexing
The receiving array is located in the near
field of the transmitting array.
Since the wave front is not planar there isphase gradient upon the receiving array.
If the phase gradient is set to certain
predetermined value the link channelbecomes orthogonal.
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Geometry orthogonalization
R
R
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Linear antenna array requirement full
rank conditiond
R
R
dR
R
Phase difference between R and R:
360/(2*n) in optimal orthogonal condition
n antennas
1 4
2
opt
nRd
n
n
R
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The asymmetric case
dtR
R
R
R
n antennas
n
Rdd rt
dtdr
dr
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Optimal antenna spacing versus link
distance and frequency
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Singular values of dual array acts as
virtual channel gain
3 dB gain
optimal
Antenna spacing
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The optimal orthogonal case
characteristics Low sensitivity to antenna position.
No sensitivity to transversal shifts.
It is possible to work in suboptimal spacing
by employing adaptive modulation.
Antenna constellation can be linear or
regular polygon the same antenna spacing
rule holds.
( ) * ( )y t H x t( ) * ( )y t H x t * ( )y H x t
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h11
The channel Measurement
TX
TERMINAL
RX
TERMINAL
X1
X2
X3
y1
y2
y3
( ) ( )y t H x t( ) ( )y t H x t ( )y H x t
Measuring H matrix by a training/pilotsequence and calculating beam formers terms
for the channel separation
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Inherent diversity gain Apart of spatial multiplexing Beam formers exhibits
inherent diversity gain over SISO channel The gain depends on
Nt transmitters
Nr, receivers
Nc active sub-channels, for inherent systemgain:
10*[log( ) log( ) log( )]g Nt Nr Nc
NcNcNt
Nr
2 2
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Singular Value Decomposition
y
2
3
2
3
z
zUxVyU
zxVUyzHxy
domainf requncy
tztxthty
domaintime
HHH
H
)()(*)()(
X
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The de-multiplexing process
3
2
1
3
2
1
3
2
1
3
2
1
'
'
'
'
'
'
'
'
'
z
z
z
x
x
x
y
y
y
Noise statistics has not changed (unitary rotation)
Singular values represent virtual gain
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Graphical presentation of SVD
Encoding
&
Modulation
V
+
+
Z1
Z4
Decoding
&
DemodulationU
V U
xx yy
Diversity
gain
Carrier
separation
1
2
Precoding is needed for diversity gain
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Basic Block diagram - dual
antenna arrays
V21
V12
V22
V11 U11
U21
U12
U22
x1
x2
y1
y2
H11
H22
H21
H12
x1
x2y2
y1
Tx Beam former
Diversity Gain
Rx Beam former
SeparationChannel
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Capacity discussion - theoryTheoretical capacity
2 options:Transmitter knows channel state:
1log( )
n
i
W
C
total
0
i
P
N
Where satisfies
1( )
i
i
Water filling algorithm
1
log(1 )n
i
Cn
i
Transmitter does not know channel state:
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Capacity discussion in real life
Real modem has maximum throughput
so there exists maximum bound of
throughput for higher SNR values. It transmitter knows the channel it can
set the throughput accordingly in the
modulator.
This is in fact real-life water filling.
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The dual mode QR-SVD weight
computation In order to decouple beam former update instances inboth sides of the link coefficients was set by dualalgorithm.
Precoding V coefficient in transmitter updated in slowmanner (sigma-beam sterring effect) on diversity gainby SVD calculation exploiting slow return channel.
Receiver beam former is calculated by QRdecomposition in fast manner update locally at the
receiver (delta-null steering effect):
zUz
QRQRU
QRHV
HR
T
11)(
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The QR-SVD characteristics
If preceding V is set by SVD result R becomes
diagonal with singular values at its diagonal.
In the case of the the optimal orthogonal spacingR becomes diagonal and V is not needed.
Off diagonal elements energy of R (upper
triangle) proportional to the non unitary noise
enhancement.
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The spatial multiplexing
implementation Witcom has built in the first half of the decade a
prototype system that utilizes the LOS mm wave
MIMO technology. The project was calledTeraWave.
This system was tested with successful results
for 9 months in France Telecom site.
Unfortunately due to marketing reasons theprogram has discontinued.
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Terawave outline architecture
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TeraWave general specification Frequency: 23GHZ
Bandwidth 28MHz.
Capacity: STM-4 (622MBS).
4 parallel channels 155MBS each employpolar+spatial multiplexing.
Modulation: OFDM 46 subcarriers/symbol up to128QAM.
Full pilot symbol every 16 OFDM symbol. Coding: Turbo Product Code
Outline: Full digital IDU connected via fiber to dualODUS direct mounted to dish antenna.
DSP calculates SVD/QR coefficients in zero forcingfashion.
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TeraWave gallery
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Test site in France
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The Spatial/polar system
Beam forming for multiple spatial channels separation
OFDM optimized modulation for spatial system
Smart mux for payload delivery over multiple spatial
channels
encoder
encoder
encoder
IFFT
IFFT
IFFT
Modulator
beam
formaerModulator
Modulator
OFDM framing
Demodulator
Demodulator
Demodulator FFT
FFT
FFTbeam
formaer
decoder
decoder
decoder
OFDM synchronizer
Demux
Mux
Data inData out
Spatial architecture
Transmitter Reciver
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MACframer
M
u
x
FEC
fr
amer
ARQ
memory
management
return
channel
SDH
Ethernat
Payload
QoS
Beam
former
TPC IFFTUp
converterIF / RF
OFDM
framer
spatial
channels
TPC
TPC
adptive modulation control
code rate QAM
Up
converterIF / RF
clock LO reference
Up
converterIF / RF
Fiber
channels
IFFTOFDM
framer
IFFTOFDM
framer
from receiver
TeraWave transmission system
ODUsIDU
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MACframer
M
u
x
FEC
fr
amer
ARQ
memory
management
return
channel
SDH
Ethernat
Payload
Beam
former
TPC FFTDown
converterIF / RF
OFDM
framer
spatial
channels
TPC
TPC
adptive modulation control
code rate QAM
Down
converterIF / RF
clock LO reference
Down
converterIF / RF
Fiber
channels
channel
estimator
FFTOFDM
framer
FFTOFDM
framer
to transmit
TeraWave receiving system
ODUsIDU
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Resource AllocationConstant throughput mode in (TDM radio):
Assign modulation modes so as to maximize gain margin (dB
above minimum S/N required for reliable communication).
Channels with gain margin below a threshold are turned off if
throughput can be maintained with fewer channels.
Variable throughput mode (in packet radio):
When all channels have minimum gain margin, reduce
throughput in order to maintain gain margin.
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Beamformer ImplementationSVD loop
[V] [U']
SVD
/QRcalculator
Actual channel[H(t)]
[U']
[V] []
Insertpilots
Extractpilots
Reference
PilotGenerator
X
Virtual Channel
DATADATA
Pilot
Generator
Return
Channel
CPE PhaseNormalization
x
Random phase
(t)
Estimatedvirtual channel
transferfunction
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Implementation issues in mm wave
OFDM-MIMO Significant dynamic issues have been found due to thelarge aperture of the antenna array and interruptions closeto the antennas that caused too rapid changes indifferential channel.
Differential phase noise due to separate LOs in ODUs.
High common phase noise due to large PLL factor in23GHz.
Larger back off due of OFDM compared to SC.
128QAM require for low implementation degradation CINRof above 35dB.
PTP microwave require 99.999% availability that reflectBER=10-12 which permits less than 5 minutes outage peryear!.
Interpolation filters degrade symbol+CP periodity - a factthat increased noise in higher BB frequency. Remedy:
Over sample OFDM, better recover of symbol timingphase.
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To grant TeraWave signal processing
means to combat the known OFDM
drawbacks in order to eliminate most ofthe inferiorities compared to single carrier
system.
The challenge was met!
The TeraWave challenge
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MIMO-OFDM Phase noise error
discussion In SISO OFDM channel error :
Close to LO carrier common phase error.
Far from LO carrier Inter carrier Interference
Spatial OFDM error suffers from uncorrelated noise: Common phase error cause CPE error in each modem
that can be corrected by CPE compensation.
Differential Phase error cause uncorrected cross-talk
between sub channels that cannot be compensated byconventional CPE.
Channel Doppler causes mainly differential phase
error
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Correlated phase noise -
analysis Terawave has high OFDM symbol rate. Most of phasenoise is CPE type.
In Terawave there are no pilots in OFDM symbol. Fulltraining symbol is transmitted every 12-36 OFDM
symbols.
Channel model acquired after fading average of the pilotreference.
Channel model update rate 300Hz.
Safe acquisition and tracking for 128QAM requiresintegrated RMS phase error of less than 3.
Stringent requirement for MM wave receiver withsynthesizer with integrated RMS phase noise from300Hz of less than 3 in conventional phase recovery .
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Solution decision directed
CPE Correcting common phase error after equalizing
among all the sub-carrier in the OFDM symbol after
slicing each sub-carrier.
Can be done in both forward (correcting actual data advantage over SC modem) and backward feed.
Prone to slice error due to AWGN, channel cross-
talk ICI phase noise, channel behavior and non
linearityprocessing gain depends on number ofcarriers.
In Terawave simulation showes 30KHz dual order
loop bandwidth. Practical allowed integrated RMS
phase noise from 3KHz - 3
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The spatial phase error noise -
calculation
1
1
1
1
'
, 1
' ( ) ( )
' ( )
' ( )
: ( )
'
tr jj
r t
r t
r t
r t
r t
H U e U V e V
if
H U I j U V I j V
H I j U U V V
H I j U U V V
Define E error matrix
H I jE
U U V V
1
1
1
1
'
, 1
' ( ) ( )
' ( )
' ( )
: ( )'
tr jj
r t
r t
r t
r t
r t
H U e U V e V
if
H U I j U V I j V
H I j U U V V
H I j U U V V
Define E error matrixH I jE
U U V V
1
1
1
1
'
, 1
' ( ) ( )
' ( )
' ( )
: ( )
'
tr jj
r t
r t
r t
r t
r t
H U e U V e V
if
H U I j U V I j V
H I j U U V V
H I j U U V V
Define E error matrix
H I jE
U U V V
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The spatial phase error noise
2X2 case2 2
11 2 2 11 1 2 11 1 2
2 222 2 2 12 1 2 12 1 2
* *212 11 21 1 2 11 21 1 2
1
* *121 21 11 1 2 21 11 1 2
2
( ) ( ) (1 )( )
( ) ( ) (1 )( )
( ) ( )
( ) ( )
t r t t r r
t r t t r r
t t r r
t t r r
e v v
e v v
e v v u u
e v v u u
Common
CPE error
Diff. CPE error
Diff, x-talk
error
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Spatial phase error -
requirement Differential spatial phase noise causes CPE error andleakage from other spatial channel (noise like).
Without treatment algorithm the error budget force RMS
integrated phase error requirement of less than 0.7
degree tough solution in MM wave.
Alternatives:
To use common RF Lo (main contributor) for all ODUs.
Implication on deployment.
To use ultra quite separate RF LO, with basic high
frequency (low phase error multiplication).
To add to decision directed algorithm that mitigate
differential phase error mitigation the CPE leakage
compensation.
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Spatial phase noise
(CPE+leakage) mitigation
xd1 CPE
calc.
Antenna 1FFT
Antenna 2
FFT
decision
ycpe1
ycpe2
y1
y2cpe2
xd2
+
+u*11
u*12
u*21
u*22 CPE
decisioncpe1
2 112 12
1 2
* *212 11 21 1 2 11 21 1 2 1
1
* *121 21 11 1 2 21 11 1 2 2
2
1 211 1 *
2 11 21
22 2
1
( )
( )
( ) ( )
( ) ( )
( )
(
CPE
CPE
CPE
t t r r
xd
t t r r
xd
xd new old xd
xd new old xd
y CPE y
x decision y
y x
e ex x
e v v u u
e v v u u
eu u
*
12
*
11 21
)e
u u
ycpe2
1x
2x
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Differential Phase and amplitude
change due to wind
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Solution to dynamics
Phase tracking loop in receivers according
to master transmitter to avoid differential
phase error. Differential amplitude correction between
DSP calculation.
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Phase loop for high dynamics
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The amplitude correction
for high dynamics
u12
h21
h12
h22
12
u22
u21
h11
+
+
+
+x1
x2
u11
x2
x1X
X
XX
X
X
XXX
X
X
11
XX
22
X
X
21
2
X
X
1
211112121212212121121111111 )()( xuhuhxuhuhx
212122122222211212112222122 )()( xuhuhxuhuhx
leakage
Leakage path
Leakage path
Gain path
Gain path
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Phase noise and hit resilience
Phase
rotator
Phase error
measureFeedback
loop
control
dela
y
Feed
forward
loop
control
Phase
rotator
Phase
rotatordela
y
Phase
rotator
H
V
H error
V errorinput
-2
filter
Mediu
mRing
filter
M
UX
comparator
To phase
rotator
in ut
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The constellation before and
after
After Correction Before Correction
7/29/2019 TW Lecture1
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PAPR reduction in MIMO
Each FEC Block is interleaved among 4
channels.
Novel approach of multiplying output withunitary matrix .
Rotation is selected according to minimum
peak to average. 2-3 dB gain in this approach.
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