Cost 286 HamburgTELICE lab./Univ-Lille1 MIMO techniques for Improving Capacity and Robustness of...

cost 286 Hamburg TELICE lab./Univ-Lille 1

MIMO techniques for Improving Capacity and Robustness of Wireless

Communication in Railway Tunnels

Martine Liénard and Pierre Degauque

Telecommunication, interferences and electromagnetic compatibility Lab.

University of LilleFrance


Objectives

• Improve performance of GSM-R radio interface in underground transportation system (IST project Escort)– Provide multimedia services– Increase robustness of control and command

communication• Constraint : Keep GSM-R bandwidth, F = 900 MHz• Solution: use of multiple antennas at both transmission and

reception (Multiple Input Multiple Output MIMO)

C = log2 [det (I + H Hh)]

:average signal to noise ratio per receiving antenna, I is the identity matrix and the upper script h means the hermitian conjugate of the matrix.


• Problems?• Usually low correlation is ensured by multiple reflections

on distributed obstacles (not the case in tunnels)• Tunnel behaves as an oversize waveguide. Uncorrelated

channels due to the superposition of numerous hybrid modes?

• Low correlation is not a guarantee of high capacity.• Propagation modelling? Interesting..but for simple

geometrical configurations• Subway tunnel: Complexity of the shape: Change in the

transverse cross section, branch lines, sleepers on the track..

• Measurement campaigns in the Paris subway


Two tracks tunnelFrom A to D

Configuration of the subway line

one track tunnelFrom D to haxo


Configuration of the experiments

• Preliminary results: Direction of arrival (DOA) and position of the antennas– Mean angular spread smaller than 40°– Compromise between the antenna gain and the diversity gain if

MIMO techniques are used– Fixed antennas placed on a platform : horn 10 dBi gain at 900

MHz• Either parallel to the track (// to the tunnel axis) or along a «

diagonal line » or perpendicular to the tunnel axis– On the train: impossible to install the antennas on the roof.

Patch antennas put just behind the windscreen• Either along a horizontal line or at the four corners


horn antennas on platform (along a diagonal line)Patch antennas behind the windscreen


Wide band analysis: example of impulse responses

0

10

20

30

40

50

60

70

80

0 21 42 63 84 105

126

147

168

189

210

231

252

273

294

315

336

357

378

399

420

441

462

483

504

525

546

Time (nS)

Am

pli

tud

e (

lin

ea

r sc

ale

)

Patch 1

Patch 2

Patch 3

Patch 4

No significant change of the transmitted pulse. Flat channel


experimental set-up

Stored the transfer H matrix of complex impulse responses every 2m

RX

TX

•Channel sounder: 35 MHz bandwidth F=900MHz

• Consider 4 * 4 MIMO channel


Correlation coefficient between the horn antennas on the platform for three positions of the antennas inside the array: parallel to the track (//),

or along a diagonal line (diag) or perpendicular to the track


• Qualitative explanation: Ex: Two modes. In the transverse plane: interferences…amplitude and phase variation ..low correlation

• But change of the field distribution from one transverse plane to another depends on the difference of the phase constant of each mode…..

• Large spatial pseudo period along the tunnel axis

• On the train, the correlation coefficient between patch antennas are greater than 0.9 if the antennas are aligned (Maximum spacing: 45 cm!)

• Patches placed at the four corners of the windscreen


Mean H singular values (s.v.) for three antenna configurations

and comparison with the case of i.i.d. Rayleigh channels


Experimental results: influence of a narrowing effectCapacity versus distance TX-RX

distance: from 80m to 600 m

2 track tunnel 1 track tunnel 4X4 antennas

# SIMO CHANNEL


Statistical analysis: average complex correlation coefficient between antennas on train

2 tracks 1 track

• In the 2 tracks tunnel for d<300m

0.7<<0.8

many mode are excited• After the narrowing, in the 1 track

#0.98

Few modes propagate

H matrix is ill-conditionned

Tx antennas in the 2 track tunnel train moves from the 2 track to one track tunnel


Summary of the experimental characteristics of the MIMO channel in tunnel

Snr=10dB Siso Simo MIMO uniform tunnel d<300m

MIMO narrowing effect

MIMO Rayleigh

Max Capacity

bit/s/Hz

SNR=10dB

3.5 5.5 8.5 5.5 11

sv(1)

sv(4)~30 ~ 100 ~ 10

X3


Ccdf of the condition number (sv1/sv4) in different channels

1 track or 2 track tunnel for a range <300m model

NARROWING300m<distance tx-rx<600m

Iid channel

Consequence on the BER in presence of an AWGN and not only on the maximum capacity?


Comparison of the robustness of two transmission schemes in presence of AWGN. Influence of the average value of the condition

number of the H matrix (In tunnel: range and 2 track 1 track)

• Alamouti scheme (Increase the robustness of the link – STC- Same information on the two Tx antennas)

h12

s1 s0

S-/

/

Space time

coding

-S1* S0

S0* S1

h11

h21

h22

n1,n3

n2,n4

Channel estimation

combiner

Maximum likelihooddecoding

Channel estimation

r1

r2

0~s

1~s

0s 1s


Vertical Layered Space Time Architecture (V-Blast)

• Detection process uses linear combinatorial nulling and symbol cancellation to successively compute Si

• Simplified system diagram for a 2x2 transmission


Performance in presence of i.i.d Rayleigh channelsV-BLAST(4,4) and ALAMOUTI (2,2)

• Condition number of H matrices <7

• Identical spectral efficiency (4bit/s/Hz): Alamouti (2x2) 16QAM modulation

and MIMO V Blast 4x4. Comparison with the SISO case (1x1,16 QAM)


Performance in presence of correlated channelsInfluence of the condition number of H tunnel configuration

V-BLAST(4,4) and ALAMOUTI (2,2). SNR=10 dB

Alamouti 16QAM

V Blast


Conclusion

• MIMO Techniques give interesting results in tunnels, despite the few number of distributed obstacles.

• Waveguide effect of the tunnel: Alignment of the antennas plays a critical role: Best configuration: Perpendicular to the track (or along a diagonal line). Short distance (<300m): low correlation owing to the number of modes

• Improvement of the maximum channel capacity? (S/N = 10 dB)

SISO: 3 bit/s/Hz MIMO 8.2 bits/s/Hz

or (and) increase the robustness of the link (Decrease the BER for a given SNR)

• At large distance or/and small tunnel width :attenuation of the high order modes. High degree of correlation.

• Sensitivity of STC algorithms (as V Blast) to the condition number of the channel matrix: Fast increase of the BER (2 track 1 track) in presence of AWGN

Cost 286 HamburgTELICE lab./Univ-Lille1 MIMO techniques for Improving Capacity and Robustness of...

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