Capacity Variation of Indoor Radio MIMO Systems Using a Deterministic

Model

A. Grennan DIT

C. Downing DIT

B. Foley TCD

Introduction

MIM0 = MULTIPLE INPUT MIM0 = MULTIPLE INPUT MULTIPLE OUTPUT MULTIPLE OUTPUT

Antenna Array Implementation Radio Channel Efficiency Improvement Multipath Reflections Utilized

Structure of the presentation

Ray-tracing room model Validation of the model MIMO systems theory Results of investigations Conclusions and direction of future work

Why use a Ray-tracing Model?

Modeling is an inexpensive alternative to site specific measurements

Traditional statistical models have focussed on power coverage requirements

Ray-tracing provides information on specific direction of arrival of rays

Antenna parameters and other physical aspects of room may be accurately modeled

A custom tool permits easy modification of all parameters

Typical Ray-trace 3-D

-20

0

20

-8-6-4-202468

-8

-6

-4

-2

0

2

4

6

8

Validation of Model

Developed laboratory experiment Employed established techniques for

measurements Directly compared simulated and

measured rays Determined delay spread from

measured data and compared to simulated prediction

Simulated and Measured Data Single

Reflective Surface

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 10-8

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5SINGLE REFLECTIVE SURFACE

TIME (SECS)

AM

PLIT

UD

E

Simulated and Measured Data (2

surfaces)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 10-8

0

1

2

3

4

5

6TWO REFLECTIVE SURFACES

TIME (SECS)

AM

PLIT

UD

E

Simulated and Measured Delay

Spread

0

1

2

3

4

0 1 2 3

DISTANCE (METRES)

DE

LAY

SP

RE

AD

(nS

)

simulation

model room

High Bit Rate Radio using Multi-element Antennas

(MIMO System)

System proposed by Foshini and Gans et al of Lucent Technologies

Standard radio channel capacity increases by 1 bit/sec/Hz for 3dB increase in SNR

Using multi-element antennas the capacity increases linearly with the number of elements in the array

This capacity increases without limit and is not restricted by multipath

Multipaths in 4 X 4 system and Channel matrix

For a single channel the efficiency is

C/B = log2 (1+ |hij |2 * )

where C is the bit rate, B is the channel bandwidth and is the signal to noise ratio and

k

kkkij

dj

daH

2

sin2

cos

where is the wavelength assuming frequency of 5.2 GHz (60 mm)

N

i

i

NBC

12 1log/

The gain for the array is determined by calculating the eigenvalues, i of HH*.

Thus, the overall system efficiency is given by

Virtual parallel channels in

4 X 4 system

RESULTS

Random matrix versus simulated measurements

RMS delay spread and Capacity Effect of increased element

spacing and signal correlation

0 10 20 305

10

15

20

25

30

35

Transmit- Receive Range Meters

4x4 arrays

Spectral Efficiency

BITS/SEC/HZ

random

4x4

2X2

Random versus Simulated 1

S/N 18 dB/2 antenna

element spacing

Random versus Simulated 2

0 2 4 6 8 10 12 14 1610

15

20

25

30

35

40SIMULATED TX. CENTERRANDOM SIMULATED TX. AT WALL

SPECTRAL EFFICIENCY

BITS/SEC/HZ

DISTANCE IN METERS

S/N 18 dB

5 antenna

element spacing

Random versus Simulatedcomment

Random matrix does not accurately model fluctuations due to movement of arrays relative to

reflective surfaces

And

Is only idicative of results when gain is at a maximun (center of room) or the element spacing in

the array is large, thus decorrelating signal components.

RMS delay spread

M

kk

M

kkk

P

P

1

1

M

kk

M

kkk

P

P

1

1

2

2

and

22 )( This is the power weighted impulse response of the channel

where the first moment and second moments of the power delay profile are defined

Simulated RMS delay spread

0 10 20 30 400

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

-8 RMS Delay Spread

Time

Secs

Transmit- Receive Range Meters

RMS delay spread with Spectral Efficiency

0 10 20 30 400

0.5

1

1.5

2

2.5

3

3.5x 10

-8 RMS Delay Spread and Spectral Efficiency

Time

Secs

Transmit- Receive Range Meters

4x4 arrays S/N 18 dB

/2 antenna

element spacing

RMS delay spreadcomment

In case of small number of elements and when the spacing of the elements is small the delay spread is a good indication of the efficency fluctuation with distance but not when larger arrays are used.

The traditional radio system designer would seek to position antennas so as to minimise delay spread but the opposite is required for mimo systems.

Comparison of array sizes

0 10 20 306

8

10

12

14

16

18

20

22

24

26

Transmit- Receive Range Meters

4x4 arrays

Spectral Efficiency

BITS/SEC/ HZ

8x8

2x2

S/N 18 dB

/2 antenna

element spacing

Effect of increasing element spacing

0 10 20 305

10

15

20

25

30

35

Transmit- Receive Range Meters

4x4 arrays

Spectral Efficiency

BITS/SEC/ HZ

8 wavelength

2 wavelength

S/N 18 dB

/2 and 8

Element spacing

Signal correlations and Capacity

element spacing4x4 system

Signal correl-ationPair 1

Signal correl-ationPair 2

Meanbits/sec/hz

/2 0.9718 0.9674 11.7

2 0.9738 0.9350 13.0

5 0.9510 0.8529 17.9

8 0.9326 0.7906 20.9

Blocked line-of-sight1

0 5 10 15 20 25 30 355

10

15

20

25

30

35

40

WITH LINE OF SIGHTBLOCKED LOS

SPECTRAL EFFICIENCY

BITS/SEC/HZ

DISTANCE IN METERS

S/N 18 dB

4x4 system

/2 antenna

element spacing

Blocked line-of-sight2

0 5 10 15 20 25 30 355

10

15

20

25

30

35

40

45

50

BLOCKED LOSWITH LOS

SPECTRAL EFFICIENCY

BITS/SEC/HZ

DISTANCE IN METERS

S/N 18 dB

8x8 system

/2 antenna

element spacing

Conclusions

Random matrices are limited in predicting mimo performance for indoor environment

Location of transmitter/receiver pair may have to be chosen carefully to avoid ‘nulls’

Antenna element spacing/signal correlation is the most critical factor limiting system efficiency indoors