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The potential of time reversal in multiple antenna
systems
Persefoni Kyritsi, George Papanicolaou (Stanford University) Hung Tuan Nguyen, Jørgen Bach Andersen, Gert Frølund
Pedersen, and Patrick Eggers (Aalborg University)
• TR can achieve delay spread reduction and spatial focusing.– Delay spread* study: TR for fixed wireless applications
(FWA)– Spatial focusing study: TR in a multi-user context
• Discussion– Normalizations and objectives– What kinds of measurements do we need?
*Delay spread:
dhh
dhh
hhh
2
2
22
22 1
,1
MISO TR to a single user
tg jNT
tg j1
tg j2
User j
TX1
TX2
TXN
T
sj
• TR with antenna weighting:
• Weight selection algorithms
mm
TRpurem
hA
2
1
Pure TR Equal powerOptimal weighting
m
EPm hMA
11
opt
optoptm
mA
a
a )(
tsRAtr
tsththAtsthAthtr
thAtg
j
N
khhkjj
j
N
kkjkjkj
N
kjkjkjkjj
kjkjkj
TX
kjkj
TXTX
1
1
*
1
*
*
FWA: Measurement equipment
• Carrier frequency: 5 GHz
• Transmitted power: PT = 100mW
• 3dB bandwidth = 25MHz • 8 element uniform linear arrays (ULA) • Spacing: s = /2• Vertical polarization • Vertical (V) or Horizontal (H) orientation
FWA: Measument locations
LOCATION A
LOCATION B
LOCATION C
1.2km
2.6km
2km
29 floors (5, 19, 28)
Balconies SW & NE 28 floors (19, 29)
7 floors (roof)
Classification of MISO situations
Clear to cluttered Cluttered to clear
Delay spread reduction: heq/h
Spatial focusing in MISO TR to multiple users
• Each receiving antenna represents one of the Nr users
Interference calculation
• Signal on target user
• Interference from other users
• The SIR
tsRAtr
tsththAtsthAthtr
j
N
khhkjj
j
N
kkjkjkj
N
kjkjkjkjj
TX
kjkj
TXTX
1
1
*
1
*
r TX
klkj
N
jlll
N
khhklj tsRAtI
,1 1
2
2
max
max
tI
trSIR
j
j
j
Reminder: The near-far problem
BS
U1
U2
100
1
110
11S
100110
1010S
1,10
2
2
2
2*21
*122
2
2
1
2*21
*111
21
IR
txthtxththty
IR
txthtxththty
tthtth
Power control scenarios
• The scaling factors normalize so that the total transmitted power is kept constant
• Additional constraints– No power control across users
– Simple power control across users
||||
1*
measuredkjkj thA
rN
j
M
kmeasuredkj
rkj
th
MNconstA
1 1
2*
Multi-user operation– Nu=2 – Antennas of 2 different terminals at locations
along the route separated by distance d
Measurement route
BS
d d
Measurements• fc=2.14GHz,
• BW > 7MHz• 2 measurement routes of
l=1km• Transmitter
– 8 TX antennas
– htx=20m (Balcony on 5th floor)
• Receiver
_ On a trolley pulled by a van– Velocity 20-40km/hr– 4 RX antennas: A1, A2, B1,
B2 at the four corners
Results for NR=2, multi-user
0 50 100 150 200 250 3000
2
4
6
8
10
12
14
16
18
20
Distance in meters
SIR
in d
B
A1A2B1B2
Mean
Min
Route 1
0 50 100 150 200 250 3000
2
4
6
8
10
12
14
16
18
20
Distance in meters
SIR
in d
BA1A2B1B2
Route 2
Mean
Min
0 50 100 150 200 250 300-25
-20
-15
-10
-5
0
5
10
15
20
Distance in meters
SIR
in d
B A1A2B1B2Mean
Min
Route 1
0 50 100 150 200 250 300-25
-20
-15
-10
-5
0
5
10
15
20
Distance in meters
SIR
in d
B A1A2B1B2
Mean
Min
Route 2
With
out
pow
er
cont
rol
With
pow
er
cont
rol
Conclusions
• Spatial focusing achieved to different users with the mean SIR of up to 18dB for 8 transmitting antennas and 2 users.
• Multi-stream operation to a single user is not always possible