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