prof. z. ghassemlooy icee 2006, iran 1 dh-pim employing lmse equalisation for indoor optical...

Prof. Z. Ghassemlooy ICEE 2006, Iran

1

DH-PIM Employing LMSE Equalisation For Indoor Optical

Wireless Communications

Z. Ghassemlooy, W. O. Popoola, and

N. M. Aldibbiat Optical Communications Research Group,

School of Engineering and Technology, The University of Northumbria,

Newcastle, U.K.Web site: http://soe.unn.ac.uk/ocr


2

Contents

Overview of Optical Wireless Communications (OWC)

Modulation Techniques

ISI and Equalisation

Simulation Results

Concluding Remarks


3Optical Wireless –What Does It Offer ?

High data rate (in particular line of sight)

Immunity to electromagnetic interference

Abundant unregulated bandwidth

High security compared with RF

Absence of multipath fading (due to the use of IM/DD)

Complementary to RF Etc.


4OWC Links – Types

Diffuse Uses single or multiple source and detector

- No requirement for alignment between them

Robust to blocking and shadowing Allows roaming Multiple paths (reflections)

- Result in inter-symbol interference (ISI).

Limited bandwidth- Due to large capacitance of the large area detectors

Line of sight Uses single or multiple source and detector

- Requires alignment between them

High bandwidth and no multipath induced ISI Allows roaming Suffers from blocking and shadowing


5Diffuse Systems – How to Combat Noise and Dispersion

Noise Filtering: Optical or electrical

Match Filtering: Maximises SNR, the optimum detection method in the presence

of noise. (Time reversed copy of received pulse convolved with received data stream)

Coding: Block codes, convolution codes (MLSD), turbo codes. Increase

performance by adding redundant data!

Equalisation: Channel distortion compensating filters: - Zero Forcing Equaliser (ZFE)- Minimum Mean Square Equaliser (MMSE)- Decision Feedback Equaliser (DFE)


6Modulation Tree

DifPAM

Pulse Modulation

Analogue Digital

Pulse Time Pulse Shape Pulse Time

Isochronous Anisochronous Isochronous Anisochronous

PSMPAM

PIMPIWMPFMSWFM

PWMPPM

DPPMMPPMDPWMPCM

DPIMDPIWMdifPPMDH-PIM

RZRBAMIManchester

NRZNRZ(L)NRZ(I)Miller code


7Digital PTM Schemes

Symbol1

Symbol2

Symbol3

OOK

PPM

PIM

DH-PIM

Time

bT

sT2sT

A

0 0 0 0 0111 1 11 1

H2H1

Redundantspace

M = 4 bits

L = 24=16 slots

( = 2)

Info.

L1

Info.

Info.

(2) (10) (15)

OOK Simple to implement High average power requirement Suitable for Bit Rate greater tha 30Mb/s Performance detoreaites at higher bit rates

PPM Complex to implement Lower average power requirement Higher transmission bandwidth Requires symbol and slot synchronisation

DPIM Higher average power requirement compared with PPM Higher throughput Built in symbol synchronisation Performance midway between PPM and OOK

DH-PIM The highest symbol throughput Lower transmission bandwidth than PPM and DPIM Built in symbol synchronisation Higher average power requirement compared with PPM and DPIM

Digital PTM Schemes

DH-PIM-

Frame Structure

0

Nawras,2005


10DH-PIM – Characteristics

3 4 5 6 7 80

4

8

12

16

20

24

28

32

M [bit]

Nor

mal

ised

ban

dwid

th r

equi

rem

ents

DH-PIM1 DH-PIM

2

DH-PIM3

DPIM

PPM


11DH-PIM – Characteristics

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-120

-100

-80

-60

-40

-20

0

20

40

Normalised frequency

Pow

er S

pect

rum

Den

sity

(dB

)

Synchronisation

2 3 4 5 6 7 8 9 100.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

M [bit]

Nor

mal

ised

pac

ket

tran

smis

sion

rat

e

DH-PIM3

DH-PIM1

DH-PIM2

DPIM

PPM

Higher packet transmissionthroughput


12DH-PIM System

DH-PIMencoder

Transmitterfilterp(t)

Multipathchannel

h(t)

n(t)

n-slotDH-PIMsequence

Unit energyfilter r(t)

(matched top(t))

R3

4 aveavePL

EqualiserEqualiser

Compute Ps in(n-1)th slot

Optimumthresholddetector


13OWC - Channel

Channel(ceiling bounce

Model)t -2T –T 0 T 2T 3T 4T 5T 6T 7T 8T

y(t)

ISI constituent

Developed by Carruthers and Kahn - Channel impulse response is fixed for a given position of Tx, Rx and intervening reflectors

rmsDa

tuat

aHath

1311

7

6

12

)(6

)0(),(

nsDrms 151 rmsD RMS delay spread

H(0) = path lossa = 2H/c, H = height of ceiling above Tx and Rx, c is the speed of lightu(t) = unit step function


14OWC –Equalisation

Linear Lattice Transversal

- Zero Forcing

- LMS

- Fast RLS

- Square-root RLS

Non-Linear DFE ML Symbol detector MLS

Linear Equaliser: Traversal filter structure that has a computational complexity which is a linear function of the channel dispersion length.


15OWC - Equalisation – contd.

Tx FilterMultipathchannel

Rx FilterEqualiser

Cj

yk

Equaliser output (estimate)

Noise nk

Ik kI

Discrete equivalent of the convolution of the Tx filter, channel and Rx filter with the information sequence Ik

....3,2,1,00

knfIIy k

knn

nknkk

ISI Noise

k

kjjkjk ycI

Where {cj} are the 2K +1 complex-valued tap weight/coefficients of the filter.


16Linear Zero ForcingEqualiser

- With a frequency response = h(t)-1. - Able to reduce ISI term at sampling points

kn jjkjnknkk ncqIIqI 0

ˆ {qn} is simply the convolution

of {cn} and {fn}. Signal ISI Noise

Equaliser with infinite number of taps, tap weights could be selected such that the ISI component is reduced to zero.For practical case: j = k

elsewhere

nfcq

jjnjn 0

01

Simple to implement, but not effective with noise.Compensate for the channel distortion at the expense of noise due a

large gain in the frequency range where attenuation is high

Simple to implement, but not effective with noise.Compensate for the channel distortion at the expense of noise due a

large gain in the frequency range where attenuation is high


17Least Mean Square Error Equaliser

Relaxing the zero ISI by selecting Cj such that the combined power of the residual ISI and additive noise at the equaliser output is minimised . I.e. minimising the mean error square:

kk II ˆ

The MSE for the equaliser2K+1 taps is

22ˆ)(

k

kjjkjkkk ycIEIIEkJ

The LMSE solution is obtained by dJ(k)/d{cj}.

KkkRkjRc iy

k

kjyj

3,2,1,0),()(

Autocorrelation matrix cross-correlation vector IyR

yyRT

iy

Ty

.

.

yT is the transpose of matrix yk-j and I represents the training signal.


18LMSEE – contd.

In contrast to zero-forcing equaliser, the LMSEs solution depend on the statistical properties of the noise as well as the channel induced ISI

Autocorrelation matrix cross-correlation vector IyR

yyRT

iy

Ty

.

.

yT is the transpose of matrix yk-j and I represents the training signal.

Where


19Simulation Process

EnterRb and Drms

1<Drms<15ns

Last ?

Stop

Out=Cm*hk*I + No

Enter No_symbGen L-DH-PIM

EvaluateCm, hk; No

Error = Error + 1 Error = Error Outk = Ik?

Last slot ?

Last Drms ?

SER = Error No of slots

Yes

YesNo

No No

No

Yes

Yes

Gen. Rand. {OOK} Start

Enter; SNR

-70<<-30 dBm


20Simulation Parameters

P P

Parameter Value

Average optical power -70 (dBm) ≤ ≤-30 (dBm)

Photodetector responsivity R 1

Threshold factor 0.5

Normalised delay spread DT 0.001 to 1.5

Bit rate Rb 1, 10, 50, 100, &150 Mbps

Alpha α 1 and 2

Number of OOK bits/symbol M 3 and 4

Background light current Ib 200 µA

Number of equaliser filter taps 3

No of OOK symbols 300,000


21

Unequalised Equalised

Results –Eye Diagrams

-0.4 -0.2 0 0.2 0.4

0

2

4

6

8

10

12

14

16

18

x 10-20

Time

Am

plitu

de

-0.4 -0.2 0 0.2 0.4

-1

0

1

2

3

4

5

6

7x 10

-20

Time

Am

plit

ude

8-DH-PIM2 at Rb = 100 Mbps, Drms = 15 ns and

P

= -30 dBm.


22Results –Slot Error Rate vs SNR

-20 -15 -10 -5 0 5 10

10-5

10-4

10-3

10-2

10-1

Electrical SNR (dB)

Pro

babi

lity

of s

lot e

rror

(SE

R)

8-DH-PIM2, Drms

=10ns

Rb=1Mbps

Rb=10Mbps

Rb=50Mbps

Rb=100Mbps

Rb=1Mbps

Rb=10Mbps

Rb=50Mbps

Rb=100Mbps

Unequalised

Equalised

10-4

Significant improvement -37.5 and -35.5 dBm

of average optical power

@ SER of

10-4


23

-46 -44 -42 -40 -38 -36 -3410

-4

10-3

10-2

10-1

Average optical power requirement (dBm)

Pro

bab

ility

of

slot

err

or (

SE

R)

8-DH-PIM2 Rb=100Mbps; 3-Tap LMSE

Drms=2nsDrms=4nsDrms=6nsDrms=10nsDrms=2nsDrms=4nsDrms=6nsDrms=10ns

Unequalised

Equalised

Results – Slot Error Rate vs Avg. Optical Power


24

-15 -10 -5 0 5 10 15

10-6

10-5

10-4

10-3

10-2

10-1

Electrical SNR (dB)

Pro

bab

ilit

y o

f S

lot

erro

r (S

ER

)R

b=100Mbps; 3-tap L-ZFE; DT=1

8-DH-PIM2

8-DH-PIM1

8-DPIMOOK8-PPM

Results –Slot Error Rate vs SNR


25

10-2

10-1

100

0

0.2

0.4

0.6

0.8

1

1.2

Normalised Delay Spread DT

Po

wer

pen

alty

to

ach

ieve

SE

R=

10-3

(d

B)

Power penalty to achieve SER= 10-3 ;Rb=100Mbps; 3-tap L-ZFE

8-DH-PIM2

OOK8-DPIM8-DH-PIM

18-PPM

DT = DrmsRb

Results –Power Penalty vs DT


26

10-2

10-1

100

0

0.2

0.4

0.6

0.8

1

1.2

Normalised delay spread DT

Pow

er p

ena

lty t

o ach

ieve

SE

R=

10-3

LMSE and L-ZFE equalisers Rb=100Mbps

10-2

10-1

100

0

0.2

0.4

0.6

0.8

1

1.2

Normalised delay spread DT

LMSE and L-ZFE equalisers Rb=100Mbps

8-DH-PIM2

OOK8-DH-PIM

1OOK8-DH-PIM

28-DH-PIM

1

L-ZFE

LMSE

Results –Power Penalty vs DT

similar performance in very dispersive

environment

LMSE is better in less dispersive channels (DT <

0.2). LMSE compensates for

both dispersion and noise (dominant)

DT = DrmsRb

3-taps

Employing equalisation in DH-PIM leads to:- Reduced optical power level requirement at high data

rate and highly dispersive channels- Improve error performance in a dispersive channel

LMSE offered similar performance to LZEF, at

highly dispersive channel, but better performance

in less dispersive channels (line of sight) DH-PIM with equalisation is an attractive

modulation scheme for OWC where there is a

need for high throughput

Concluding Comments


28

Thank you!


29OWC – Issues + Solutions

Shadowing in non-line of sight links - Diversity schemes

Limited power (safety reason) - Power efficient modulation techniques

Noise due to the ambient light sources- Optical/electrical filtering- Modulation scheme with no or very little frequency

components at the low frequency bands Dispersion (due to multipath)

- Equalisation SNR variation with the distance and ambient noise

prof. z. ghassemlooy icee 2006, iran 1 dh-pim employing lmse equalisation for indoor optical...

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