cdma in opticsmanauss · • consequence: basic modulation: im (nearly always) • consequence of...

1

CDMA in Optics

Prof. István FigyesBudapest University of

Technology and Economics

ISSSTA Manaus Aug. 2006

CDMA/Optics - Frigyes 2

Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?• 2. Few words on MA – CDMA• 3. Concepts in optical communications• II. OCDMA: binary baseband transmission• 4. SS-CDMA concepts in optics• 5. Temporal coding OCDMA• 6. Spectral coding OCDMA; bipolar coding



Outline

• 7 2-3 dimension coding; application in long haul and elswhere

• 8 Performances – receiver structures• 9 Network studies

• III OCDMA in Radio over Fiber • 10 What is RoF?• 11 Basic tools in OCDMA-analog• 12 OCDMA design in RoF

2

I. Introduction



1 Apology• CDMA? • Definition• Optics? • Basic concepts



Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?

2. Few words on MA – CDMA• 3. Concepts in optical communications• II. OCDMA: binary baseband transmission• 4. SS-CDMA concepts in optics• 5. Temporal coding OCDMA• 6. Spectral coding OCDMA; bipolar coding

3



2 What is MA? - definition

• 1. One channel – multiple users• 2. These (either sources or sinks or both):

geographically dispersed• 3. Information: intended to (one or more)

dedicated user(s)• (Comment: thus

broadcast excluded by 3multiplex excluded by 2)



2 What is MA? - examples • MA channels –

examples (both RF):• 1st appearence:

satellite

• Best known: mobile

Communication network



2 What is MA? optical examples

• Optical examples:• 1. Star

• 2. DBDQ

Source

E/O

O/E

Sink

Star C

S/SS/SEO/OE

S/SS/SEO/OE

S/SS/SEO/OE

S/SS/SEO/OE

S/SS/SEO/OE

4



2 What is CDMA?

• Information signal spectrum is spread • Spreading code: individual to users (in O-

CDMA called sometimes: signature code)• Having appropriate correlation properties• Appropriate: -low (non-zero-shifted) auto-

correlation (needed for synchronization)• -low cross-correlation

(needed for low Multi User Interference)



2 What is CDMA? Why in optics?

• In optics: WDM – well evolved, mature technology, widespread application

• But: not very good for LAN: not flexible, not scalable; central control needed

• CDMA: all these; no central control; and in optics: required broad band available

• Broader application than LAN possible• (Several components developed for WDM

are applicable to CDMA)



Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?• 2. Few words on MA – CDMA

3. Concepts in optical communications• II. OCDMA: binary baseband transmission• 4. SS-CDMA concepts in optics• 5. Temporal coding OCDMA• 6. Spectral coding OCDMA; bipolar coding

5



3 Optical communications – a few basic concepts: light sources

• Natural access to the optical channel: via its power (intensity)

• As: in (nonthermic) light sources (LD, LED): electrons→photons, current→power

• Consequence: basic modulation: IM (nearly always)

• Consequence of that: signals are non-negative (significant difference to RF)



3 Optical communications – a few basic concepts; comment on mod.

• While power and so IM is the most plausible

• field-strength and PSK QAM etc. not impossible (so-called: coherent systems)

• However: much more complicated;• thus: not applied very often.



3 Optical communications – a few basic concepts: medium

• Transmission medium: optical fiber• Adverse characteristics:• 1.attenuation (minimal at 850, 1300, 1500

nm) • 2. dispersion:

( ) ( ) ( )

( ) distortionlinear constv/1 i.e.

...2

g

221

⇒≠=

+−+−+=

ωβ

ωωβωωββωβ

dd

ccc

6



3 Optical communications – a few basic concepts: medium• 3. nonlinearity:

• nonlinear scattering• 4. birefringence:

(n depends on polarization) ⇒random variation of polarization state

( )Pfn =β and



3 Optical communications – a few basic concepts: noise sources

• Shot noise (photons):• Intensity noise (light-source characteristic)• MUI (like in RF CDMA)• Beat noise, 2 sources (or PIIN):• : coherence time• Photodetector dark current (usually very

small)• Thermal noise (no, in optical band)

IeBi 2_2 =

BIi Cτ=_2

Cτ



3 Optical communications –; some basic components

• For „broadcasting”: star coupler• For spectral-to-spatial decomposing a

wideband signal: diffraction grating• For manipulating spatially decomposed

signals: SLM, Spatial Light Modulator. Pixels are individually controlled to change light (polarization, phase or amplitude/intensity); liquid crystal or MEMS

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II. OCDMA transmission ofbaseband digital signals



Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?• 2. Few words on MA – CDMA• 3. Concepts in optical communications• II. OCDMA: binary baseband transmission

4. SS-CDMA concepts in optics• 5. Temporal coding OCDMA• 6. Spectral coding OCDMA; bipolar coding



4. Spectrum spreading concepts in optical applications

• Two concepts: 1) temporal coding2) spectral coding

• Other cathegorization: 1) „1D”:spreading in time only

2) „2D”:time+wave-length

3) „3D”:2D+space• Further: 1) on-off

2) M-ary PPM (biphase, if M=2)

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• Temporal coding:• info signal: binary 0: 0 intensity• binary 1: given intensity• SS signal: binary 0: 0 intensity • binary 1: user specific

signature sequence of short pulses• Spectrum spreading ratio: T/Tc ;(Tc: chip-

time)




• Spectral coding:• (unmod) wide-band signal is generated• spectrally decomposed• amplitude or phase of the spectral

components varied (modulated) according to user’s code.

• Details later



Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?• 2. Few words on MA – CDMA• 3. Concepts in optical communications• II. OCDMA: binary baseband transmission• 4. SS-CDMA concepts in optics

5. Temporal coding OCDMA• 6. Spectral coding OCDMA; bipolar coding

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5. Temporal coding OCDMA; basic difference – RF/optics

Signal format: in RF bipolar (PSK, QAM, whatever)

Thus auto (or cross) corr.: low magnitude of ± sequences

In opt: IM – thus strictly positive

0 + 0 0 0

0 + 0 0 0 0 + 0 0 0

+ - - + -

+ - - + - + - - + -



5. Temporal coding OCDMA: Optical Orthogonal Codes

• For good corr: low density – „low weight”

• Requirement: • crosscorr: low (≤λc)– any 2 (user)

codes, any time shiftautocorr: low (≤λa); τ≠0



5. Temporal coding OCDMA Optical Orthogonal Codes

• A few equations:• Spreading sequence (or code): xk=0,1;

( ) ( ) ( ) ( ) ...2,1,0;.;.1±±=+=−= ∑

∞

−∞=

nTntxtxkTtPxT

txk

ckc

( ) ∑−

=+=

1

0ˆF

klkkx xxlR ( ) ∑

−

=+=

1

0, ˆ

F

klkkyx yxlR

( ) ax lR λ≤≠ 0 ( ) cyx lR λ≤,

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• Code designation:

• (w: weight, number of 1-s; F=T/Tc=SS factor)

• in strict OOC: λa=λc=1• Max number of OOCs:

.

( )cawF λλ ,,,

( )11−−

≤wwFN




• Spreader and de-spreader (correlator)• This version: so-called „passive”• Data dem: threshold device; • 1: ≥ threshold

0: < threshold




STARCOUPLER

DELAYτ2

DELAYτw

STARCOUPLER

DATASOURCE

DATAMOD

LIGHTPULSE G

„PASSIVE” CODER

„PASSIVE” DECODER

STARCOUPLER

DELAYT

DELAYT-τ2

DELAYT-τw

STARCOUPLER

PHOTODET

DATADEM∫

CT

dt0

11




OPTICALMULTIPL

SEQUE.GEN.

PHOTODET: ∫

T

dt0

DATADEM

CW LIGHTSOURCE

ON-OFFMOD

DATAMOD

SEQUEGEN

DATASOURCE

„ACTIVE” CODER

„ACTIVE” DECODER



Network – once again

STARCOUPLER

E/OSOURCE

E/OSOURCE

E/OSOURCE

O/E SINK

O/E SINK

O/E SINK




• Discussion: • In order to λa&c=1:

w must be lowF highpeak/average high

• N is low• Typical values:

F=1000+; w<10; N≈10

0 1 1 0 1

c1

c2 c1+c2

MUI, info1: „0”

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5. Temporal coding OCDMA Improved-performance codes

• Significant research in improving – mainly increasing Nat the expense: higherλa and λc

– The results are mainly based on number theory

• Examples:– quadratic congruence codes:– (p2-1,p,2,4); N=p-1; p: any prime– perfect difference: (F,w,1,2) and: N>F;

BER very nearly as good as OOC



5. Temporal coding OCDMA; synchronous OCDMA, prime

codes• Until now: asynchronous

users – However: if synchronised no

requirement on λa

– And shifted version is also applicable as code

• Prime codes (p2-1,p,p-1,2)very good though λa=w(nearly): N=p(p-1)

• Synchr: Bus architecture –common chip-gen. – via the fiber



5. PPM in temporal coding OCDMA

• Rather than simple On-Off-Keying + spectrum spreding

• M-ary PPM + spectrum spreding• Example: binary: 011101 octonary: 35

PPM

PPM+OOC

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5. PPM in temporal coding OCDMA

• Digital M-ary PPM: orthogonal signal set• Well known: performance is improving with

increased M. (If received power constraint)• E.g.: it is shown: given BERmax and N (max

number of OOC-s, max number of users) there exists M0:BER< BERmax if M> M0 and N simultaneous users

• (Of course: M-times shorter TC is needed)



Outline

• I. Introduction• 1. Apology: CDMA? Optical comm.?• 2. Few words on MA – CDMA• 3. Concepts in optical communications• II. OCDMA: binary baseband transmission• 4. SS-CDMA concepts in optics• 5. Temporal coding OCDMA

6. Spectral coding OCDMA; bipolar coding



6 Spectral coding OCDMA

• Light of wideband source is spectrally decomposed

• Modulated in SLMAdjusting to destination’s code

LED

DATASOURCE

SLM

fGRATING FOR SPECTRALDE-;

STARCOUPLER

RECOMPOSITION

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• Broadband light source: coherent or non-coherent

• Coherent: narrow (~100 fsec) pulse • Non-coh: LED, SLD, • Modulation: Intensity or Phase

SLM: attenuator SLM: phase shifterδf

fProgrammable




• Breakthrough 1 (Kavehrad): for IM spectral coding cheap light sources can be applied;

• for everything else: expensive (mode-locked) lasers are needed



6 Spectral coding OCDMA: spectrum spreading code

• Breakthrough 2: bipolar coding with unipolar pulses

• Signature codes: rows of N×N Hadamardmatrix: cH(f) (orthogonal matrix; rows: N/2 1s-N/2 0s); or: M-sequence

• Coding(user k): ( ) ( )fcfc HkHk :1;:0

DATASOURCE

COMBINER

( )fcHkCODER

CODER( )fcHk

0

1

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6 Spectral coding OCDMA: decoder

Under ideal circumstances: no MUI;„ideal”: flat LED spectrum, no other noise

STARCOUPLER

3 DBCOUPLER

( )fcHkDECODER

DECODER( )fcHk

PHOTODET

PHOTODET

++

-

RX #k ⎟⎟⎠

⎞⎜⎜⎝

⎛∫T

dt0

INTE-GRATE



Outline

7 2-3 dimension coding; application in long haul and elswhere





7. 2-3 D coding; application in longhaul and elswhere („matrix codes”)• OCDMA: originally for LAN (short distance)• Less advantageous for long haul: • high F, low w: very short pulses, high SS –

distorted by dispersion, disp. compens. needed• A possible solution: 2D (matrix) coding:

encoding in t and f (remark: this is a mixture of DS and FH)

• Multiwavelength lasers – developed for WDM –can be used

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7. 2-3D coding; application in longhaul and elswhere

• Principle: -order code in a matrix-correspond rows to frequencies,

columns to chip-time-slots-add 0-s to the code if

F ≠ rows×columns• Example: original OOC: (1010011); matrix:

meaning: TS1: λ1 & λ3 TS2: λ3TS3: λ1

(λ here: wavelength, not correlation) ⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

−−

1100

101



7. 2-3D coding; application in long haul and elswhere

• Further properties: SS of one λ: number of columns (being <F)

• cardinality can be multiplied by down-shifting rows:

• Note: 3D also exists: t, λ, fiber, with 3D matrices

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

−

−

0010111

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛−−

1011100



7. Recently introduced 2D codes/1

• Search for codes does not decrease; new-ones introduced in 2005-2006

• W/T MPR (Multi Pulse per Row): equal performance to OOC (or its folded – i.e. W/T – version)

• better cardinality• better spectral efficiency• (by the way: not too much better)

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• 2D spectral/space codes (system) were introduced (2006)

• apply spectral amplitude coding• spectral: as seen before• space: several fibers – in certain sense

– also coded• Use 2 independent max. length pseudo-

random sequences (space & spectr) –shifted for individual users („M matrices”)




•Principle of generating an M-Matrix-Codefrom two M-sequences




The (broadcast) network

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Encoder

DecoderRoughly: 2× that

of 1D(AWG: Arraeyed Waveguide Grating)



Outline

• 7 2-3 dimension coding; application in long haul and elswhere8 Performances – receiver structures

• 9 Network studies• III OCDMA in Radio over Fiber

• 10 What is RoF?• 11 Basic tools in OCDMA-analog• 12 OCDMA design in RoF



8 Performances – receiver structures – OOC

• Application of CDMA, star-arrangment is assumed (repeated below)

• Assumptions: MUI is the only source of errors

• chip sync (yields upper bound for BER)

• OOC is dealt with for first

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8 Network – once again

STARCOUPLER

E/OSOURCE

E/OSOURCE

E/OSOURCE

O/E SINK

O/E SINK

O/E SINK




• Points: sensitivityelectrical speed/BW

• 1.Passive integr. (BW:1/Tc; note: Th≤w)

• 2. Active integr. (BW: 1/T - << 1/Tc)

StCT-τ1

T-τ2

T-τw

StC PhD Int Tc Thr

Opt. multiplier

Sequence generator

∫T

dt0

Data demPhoto detector




• In this case Th = w• And: Pr{BE|1}=0

Pr{BE|0}==Pr{Σ MUI chips ≥w}

• Some computed results: (note: (F,w,1,1) codes)1.(2000,5,1,1), BER vs. Th (perhaps lower for noise in reality)

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• BER vs w,Th andnumber ofsymultaneous users



h(t) PD Int+Th


• 3. Opt. Hard limiter (e.g.+passive integr.)

• (decreases MUI: only if interfeing power at each user pulse is individually higher than limiter threshold of 1)

h(t) PD Int+Th




• 4. Double-optical hard limiter

• (interfering pulse only if large individual rather than small other pulses)

h(t) PD Int+Th

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• 5. Chip-level detection – electrical (integration time: Tc)

• Same effect as double hard limiter – high electrical BW needed

• Note: there is no opt hard limiter (!!)

PD Int+Th Count ≥wPD Count ≥w



8 Performances – receiver structures – SAC

• Remember: SAC: Spectral Amplitude C• Receiver (once again): differential concept• Thus: unipolar version of bipolar code

sequences can be applied

3 DBCOUPLER

( )fcHkDECODER

DECODER( )fcHk

PHOTODET

PHOTODET

++

-

RX⎟⎟⎠

⎞⎜⎜⎝

⎛∫T

dt0

INTE-GRATE




• Results in ideally no MUI• It turns out: main source of degradation:

PIIN (Phase Induced Intensity Noise)• due to beating of light from different

sources• I.e. =noncoherent sources: power

(intensity) additive

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• However: if nonzero correlation (coherence) time of sources, within this time interval:

• In reality: coherence time (wide-line-width LED) very short though nonzero

cttEEEEI τδω <++=+= ;cos2 2122

21

221 EE



8 Performances – receiver structures – SAC (here OOC)

• Simulation result (different situation but explicit on PIIN effect: temporal, OOC, with and without PIIN)

MUI only

MUI+PIIN τc = Tc/5000τc = Tc/50



8 Performances – receiver structures – M-matrix coding

• This figure shows performance of M-matrix code – and also (1D) SAC and „permuted” M-matrix

23



Outline


• 8 Performances – receiver structures9 Network studies




9 OCDMA network studies

• While research of the 90s dealt mainly with good codes, their properties

• in this decade work started on networking issues; these include technology demonstrations, comparisons, protocols.

• However: no „optimal” architectures, protocols are (yet) found; some recent studies will briefly be mentioned.



9 OCDMA network studies/1Basic routing in LAN

STARCOUPLER

E/OSOURCE

E/OSOURCE

E/OSOURCE

O/E SINK

O/E SINK

O/E SINK

As seen often:„Broadcast-and-Select”

For „Select”:either: FTTR

or: TTFR

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9 OCDMA network studies/2: WDMA-CDMA LAN comparison

(COM. LET. 2002 Sept)

• WDMA: 1 user/wavelength• CDMA: 2D code

ph×w simultaneous usersall opt. or chip-level processing

• Slotted ALOHA protocol (retransmission if error – MUI or collision)

• Poisson packet arrival rate• Note: in chip level: (electr.)BW expansion

Number of wavelengthsCodeweight/wavelength



9 OCDMA network studies/2: WDMA-CDMA LAN comparison

(COM. LET. 2002 Sept)

• ResultsUtilization vs. offered load

Retransmission No vs. utilization

w 3, 7, 13: No of λ-s;„utiliz”:thrp/electr BW exp



9 OCDMA network studies/3: multiservice support (JLT, Feb 2006)

• Users may require different QoS or different transmission rate

• Better QoS (BER): higher code weight (w)• Higher rate: shorter code (less spreading)• Proposal: extended OOCs: MWML-OOC• code family strictly OOC (F,w,1,1), of

different F & w• (High cardinality but not infinite: F, w what

needed)

25



9 Network studies/4: medium access (Infocom, 2004)

• Random access protocols were investigated. The equivalents of

• ALOHA: CDMA-ALOHA• CSMA: Interference-sensing MA• CSMA/CD: Interference-detection MA



9 Network studies/4: medium access (Infocom, 2004)

Simulation result: throughput vs. Offered load



9 Network studies/5 – long-haul application of OCDMA (JLT 2000)

• Its flexibility can be of advantage • But: must be competitive with DWDM• For that: 1) have at least as many codes

as wavelengths in DWDM (i.e.: ≥8)• 2) operate at high data rates (i.e.,

greater than 2.5 Gb/s); • 3) propagate with high fidelity over the

installed or installable fiber links.• 2D OOC and 2D SAC can be appropriate

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III. OCDMA in Radio overFiber



Outline



• III OCDMA in Radio over Fiber 10 What is RoF?

• 11 Basic tools in OCDMA-analog• 12 OCDMA design in RoF



10.CDMA in Radio over FiberWhat Is RoF?

• RoF is an analog fiber optic link• Transmitting RF (MW, MMW) signals• Modulated by inf. bearing signal (digital)

S O U R C E

μW G E N

μW M O D

L D P D

μW m o d

27



10 Radio over Fiber: why?• In next generation wireless• higher capacaty („broadband for all”)• higher carrier (lower frequency bands full)• Both trends: smaller cells• Thus: many cells ⇒BS must be cheap• One possibility: simple BSs – control, signal

processing, management operations in central stations (CS) – connection via optical fibers

• BS then: E/O, O/E, (amplifiers) antenna



10 Radio over Fiber:Link design – e.g. in mobile

SOURCE

E/O O/E

O/E

E/O

SINK

CS

BS

1…30 km



10 CDMA in RoF• RoF together with fixed or mobile wireless:

a double-MA system• MA in wireless: more or less irrelevant• So here: CDMA in the optical layer• (Note: results of Prof. Komaki & group)

28



10 Radio over Fiber networks

• Two types of networkarchitecture: star –i.e. SDMA (space)

• bus – i.e. SCMA(subcarrier), WDMA, TDMA or CDMA

BS TS CS

fiber

wireless layer

optical layer

CC

BS

TS

C

C CS



Outline



• III OCDMA in Radio over Fiber • 10 What is RoF?

11 Basic tools in OCDMA-analog• 12 OCDMA design in RoF



11 Basic tools: OCDMA in analog (RF)

• Bandpass sampling + optical sampling• These resulting in PIM optical pulses

(1/sampling time)• And: spreading of spectrum of these – via

appropriate (user specific) SS sequences

29



11 Basic tools: OCDMA inanalog (RF)

• Basic tool 1: bandpass sampling. Note: sampling frequ > 2BRF (and not: > 2fmax)

• Basic tool 2: photonic sampling; • 1 source (a BS) below:

OPTICAL PIM PULSE TRAIN

LD MODext or D

OPTICALSWITCH

SEQUENCEGEN

COUPLER

DELAY



11 Basic tools: OCDMA in analog (RF)/3

• Resulting PIM pulses: DS spectrum spread (called sometimes: DOS), like in baseband

• Like there: F = Ts (i.e. sampling time)• Basic differences: -RoF is only one part of

the link, followed by wireless – not allowing low-weight (i.e. low power) codes

• -PIM rather than digital signal: sensitive to nonlinear distortion



Outline



• III OCDMA in Radio over Fiber • 10 What is RoF?• 11 Basic tools in OCDMA-analog

12 OCDMA design in RoF

30



12 RoF-OCDMA with prime codes

• „Strict” OOCs low weight• (Modified) prime codes can be applied• User sync must be insured• Further points: sensitive to near-far (here

very pronunced): therefore AGC• previous users (coinciding)

pulse can be switched off: decrease MUI, increase(somewhat) noise



12 RoF-OCDMA with primecodes/3: design example, uplink

AGC

LD

OSWc1(t)

AGC

LD

OSWck(t)

ST-C

OptA OSW

c1(t)

PD BPF

OptA OSW

cN(t)

PD BPF

Connecting fiber+ delay



12 RoF-OCDMA with bipolar (PN etc) codes

• Mainly due to high peak-to-average • requireing extremely linear amplifiers even

for prime codes (low w)• bipolar codes would be more appropriate• Of course this is possible with bias:

• But then: users’ codes are not orthogonal: total interference is present

( ) ( )[ ] ( )2

11 tctxPtP ++=

31



13 RoF-OCDMA with bipolar(PN etc) codes/2

• MUI cancellation is needed („multiuserdetection” not appropriate)

• One solution: Polarity Reversal Correlator:

COUPSW 2

PD 1

PD 2+

SW 1 +

–

BPF

( )2

1 tc+

( )2

1 tc−



13 RoF-OCDMA with bipolar(PN etc) codes/3: one design

• DOS network with PRC

AGC

LD

OSW[c1(t)+1]/2

AGC

LD

OSW(ck(t)+1)/2

ST-COptA PRC

[c1(t)+1]/2

PD BPF

OptA PRC

[cM(t)+1]/2

PD BPF



14 Short conclusions• CDMA, having fundamental role in wireless can

be advantageous in optical communications aswell

• However, definite positive character of IM opticalsignals requires different coding/modulationmethods

• Strictly Optical Orthogonal codes possible andimproved versions were introduced

• Differential detection makes the application ofbipolar codes possible

32



14 Short conclusions

• „Strict” OOCs and some improved werepresented

• Various receiver structures were shown• Based on Komaki’s results: application

in RoF was presented• Being a unique application of analog

spectrum spreading and code division• Unipolar (prime) and bipolar codes can

be applied



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cdma in opticsmanauss · • consequence: basic modulation: im (nearly always) • consequence of...

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