In es iga ion o wi ed and wi eless se ices
based on OFDM DSB-RC ansmission in he
p esence o modula ion chi p o a DEMZM
Paulo Almeida1,* and Hen ique Sil a1,2
1Ins i u o de Telecomunicações, Depa men o Elec ical and Compu e Enginee ing, Uni e si y o Coimb a,
Coimb a 3030-290, Po ugal
2[email p o ec ed]
*[email p o ec ed]
Abs ac : In his pape , we in es iga e he pe o mance o in ensi y
modula ion wi h op ical ca ie educed by biasing a dual-elec ode Mach-
Zehnde modula o (DEMZM) below i s quad a u e poin , in he p esence
o con olled modula ion chi p. The e ec s o he modula ion chi p and o
he bias poin o a DEMZM on he ecei ed signal a e analy ically de i ed
o small-signal ope a ion. The in e ac ion o hese wo e ec s is assessed
in e ms o op ical signal o noise a io (OSNR) equi ed o a BER = 10−9,
h ough a compa ison be ween double sideband (DSB) modula ion and
double sideband - educed ca ie (DSB-RC) modula ion, by nume ical
simula ion. We ound ha he powe o he op ical ca ie has impac on he
op imum alue o he α chi p pa ame e and ha a posi i e alue o he α
chi p pa ame e can be he op imum alue o d i e a DEMZM, depending
on he cen al equency o he o hogonal equency di ision mul iplexing
(OFDM) signal and on he DEMZM bias poin .
©2013 Op ical Socie y o Ame ica
OCIS codes: (060.2330) Fibe op ics communica ions; (060.4080) Modula ion.
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(C) 2013 OSA
16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30764
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Con e ence,(2012), pape OW3B.2.
1. In oduc ion
Con e gence o wi ed and wi eless access ne wo ks in o a single hyb id op ical access
in as uc u e based on adio-o e - ibe (RoF) ne wo ks has gained conside able momen um
in ecen yea s, due o he possibili y o o e ing he inal use a single simpli ied access o
b oadband se ices. Fu he mo e, he me ging o wo di e en access ne wo ks allows
educing upg ade and managemen cos s o u u e b oadband access ne wo ks [1]. This
con e gence, applied on long- each passi e op ical ne wo ks (LR-PONs), allows educing he
ne wo k cos o ope a ion u he , since a la ge geog aphic a ea can be co e ed wi h a single
cen al o ice (CO) and wi hou op ical-elec ical-op ical con e sions a he middle o he
ne wo k.
In ensi y modula ion di ec -de ec ion (IMDD) sys ems a e la gely used in op ical access
ne wo ks due o hei simplici y and educed cos . Howe e , IMDD sys ems p esen some
d awbacks, such as powe ading induced by ch oma ic dispe sion, which occu s a e
pho ode ec ion due o he inhe en double sideband (DSB) spec um, and low modula ion
e iciency, since a leas 50% o he op ical powe is was ed in he op ical ca ie , which does
no ca y any in o ma ion.
To o e come he la e d awback, he modula ion e iciency has been imp o ed wi h
double sideband - educed ca ie (DSB-RC) modula ion [2], ob ained by educing he powe
o he op ical ca ie wi h an op ical no ch il e , such as a Fab y-Pé o il e in e lec ion
mode [3] o a na ow band ibe B agg g a ing [4]. Mo e ecen ly, a cos -e ec i e me hod
based on biasing he Li hium Nioba e (LiNbO3) Mach-Zehnde modula o (MZM) below i s
quad a u e poin (QP), o educe he powe o he op ical ca ie , has been demons a ed [5].
To mi iga e he o me d awback, he powe ading induced by ch oma ic dispe sion in
o hogonal equency di ision mul iplexing (OFDM) IMDD sys ems, he in oduc ion o
con olled nega i e modula ion chi p by a dual elec ode MZM (DEMZM) has been shown o
mi iga e he dispe sion penal y [6]. Up o now, he ansmission o op ical signals in he
p esence o modula ion chi p has been demons a ed only o in ensi y modula ed signals wi h
ull op ical ca ie , h ough DEMZMs [6], semiconduc o op ical ampli ie s [7], elec o-
abso p ion modula o s [8], o lase diodes [9, 10]. In [11], i has been analy ically
demons a ed ha , inside a bias ol age ange a ound he QP o a DEMZM, he exp essions
de i ed o he α chi p pa ame e componen s can be used o es ima e he modula ion chi p,
whe eas a he maximum ansmission poin (MATP) o minimum ansmission poin (MITP)
o he in ensi y ans e unc ion o he DEMZM he exp essions de i ed a e no alid due o
singula i ies in he exp ession o he α chi p pa ame e . Unlike he o he a o emen ioned
de ices, he exp ession o he α chi p pa ame e o he DEMZM does no depend on he
a e age op ical powe , i.e. powe o he op ical ca ie . This opens he possibili y o
imp o ing he modula ion e iciency o IMDD sys ems, by employing DSB-RC modula ion
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(C) 2013 OSA
16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30765
along wi h con olled modula ion chi p o mi iga e he powe ading induced by ch oma ic
dispe sion.
In his pape , we in es iga e analy ically and nume ically he pe o mance ob ained wi h
an op ical hyb id signal (OHS) composed by a cus om OFDM Gigabi E he ne (OFDM-GbE)
signal and h ee independen OFDM ul a-wideband (OFDM-UWB) signals, when
modula ion chi p is in oduced in he in ensi y modula ion wi h educed ca ie based on a
DEMZM. The op ical ca ie is educed by biasing he DEMZM below i s QP, in a bias poin
de e mined as he bes alue o all signals conside ed.
The emainde o his pape is o ganized as ollows. A ma hema ical desc ip ion o he
e ec o modula ion chi p on he ecei ed signal and he e ec o he DEMZM bias poin on
he modula ion e iciency is p esen ed in Sec ion 2. In Sec ion 3, he modeling o a
wa eleng h di ision mul iplexed passi e op ical ne wo k (WDM-PON) ope a ing wi h a
single wa eleng h is desc ibed, based on he equi alen block diag am o he sys em used o
e alua e he pe o mance ob ained wi h he ecei ed OHS, when ansmi ed in he p esence
o modula ion chi p. In he same sec ion, he main pa ame e s equi ed o gene a e he
baseband OFDM signals a e p esen ed. The nume ical simula ion esul s compa ing he e ec
o modula ion chi p on DSB and DSB-RC modula ions a e p esen ed in Sec ion 4, and he
main speci ica ions equi ed o d i e he DEMZM and limi a ions o he ansmission sys em
ound a e poin ed ou . Finally, in Sec ion 5 he main conclusions o his wo k a e d awn.
2. Theo e ical analysis
2.1 E ec o modula ion chi p o a DEMZM on he ecei ed signal
A complex- alued baseband OFDM signal up-con e ed o he cen al equency, (ω =
2π ), can be exp essed by:
() ()
()
()
()
cos sin
i q
x
x x
ωω
=− (1)
whe e xi( ) and xq( ) a e he baseband in-phase and quad a u e componen s o he OFDM
signal wi h ze o mean and ene gy σ2
xi and σ2
xq, espec i ely, a he ou pu o he OFDM
modula o , be o e he RF up-con e e . The op ical ield a he ou pu o he LiNbO3
DEMZM wi h ini e ex inc ion a io (ER) can be exp essed by [12]:
()
()
()
()
()
11 2 2
2,
2
cbias bias
j jVx V jVx V
VV
MZM
Pe
E e e
ππ
ππ
ω
γ
−+




=+





(2)
whe e he ampli ude d i e ol ages o any alue o he α chi p pa ame e a e exp essed as
[13]:
() ()
()
2
12
cos sin
22cos 1
bias bias
bias
VV VV
VmV
VV
ππ
π
π
γπ γαγπ
γπ γ
+−
=++ (3)
() ()
()
22
cos 1 sin
22cos 1
bias bias
bias
VV VV
VmV
VV
ππ
π
π
γπ αγπ
γπ γ
++
=− ++ (4)
whe e Vbias = - (Vbias1 + Vbias2) is he bias ol age o he DEMZM, de e mined by he DC
ol ages Vbias1 and Vbias2, Vπ is he hal -wa e ol age o he DEMZM, m is he modula ion
index, P is he a e age powe o he con inuous wa e (CW) lase emi ing a he equency νc
(ωc = 2πνc) ed in o he op ical inpu o he DEMZM, and γ is he scaling ac o , be ween 0
and 1, used o accoun o he unbalance be ween he a ms and ela ed wi h he ER o he
DEMZM [12]. Fo balanced DEMZMs, his scaling ac o is 1. Unde small-signal
modula ion condi ions (m << 1), he highe o de sidebands a e small and can be igno ed, and
hus he op ical ield signal gi en by Eq. (2) can be simpli ied o:
#196157 - $15.00 USD
Recei ed 20 Aug 2013; e ised 4 No 2013; accep ed 21 No 2013; published 6 Dec 2013
(C) 2013 OSA
16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30766
()
() ()
{
}
0
2,
2
c c
c
j j
j
MZM
P
E Ee Ee Ee
ωω ωω
ω
−+
−+
≅++ (5)
wi h
[]
[]
**
11 2 2
01122
11 2 2
)
)
)
E
EAA
E
γ
γ
γ
−
+


=− ΓΛ + ΓΛ


=Γ +Γ
=ΓΛ+ΓΛ
(6)
wi h Γ1 = exp(-jπVbias1/Vπ), Γ2 = exp(jπVbias2/Vπ), Λk = J1(θkq( )) + jJ1(θki( )), Ak = J0(θki( )) +
J0(θkq( )), whe e Jn(·) is he Bessel unc ion o he i s kind and o de n (n = 0,1), and θki( ) =
πVkxi( )/Vπ, θkq( ) = -πVkxq( )/Vπ a e he modula ion phase shi s o he baseband in-phase and
quad a u e componen s, espec i ely, when applied o he elec ode k = 1,2. The supe sc ip *
deno es he complex conjuga e and 1j=−.When his op ical signal is ansmi ed o e a
dispe si e single mode ibe , a phase shi is induced in o each op ical sideband, ela i e o
he op ical ca ie . The Taylo ’s expansion o he dispe sion p opaga ion cons an o he
dispe si e ibe can be exp essed as [14]:
() ( ) ()
2
01 2
2,
cc
βω β β ω ω β ω ω
≈+ − + − (7)
whe e
()
|c
mm
mdd
ωω
ββωω
=
=. The e ec o highe o de ibe dispe sion a 1550 nm is
neglec ed. Fo he sideband cen e ed a he equency ωc ± ω , he dispe sion p opaga ion
cons an is gi en by:
()
2
01 2
2.
c
βω ω β β βω β ω
±
±==± + (8)
A e ansmission o e a s anda d single-mode ibe (SSMF) o leng h L, he op ical ield can
be w i en as:
()
()
{
()
}
0
0
2
,.
2
c c
c
j j
jLj
jL jL
P
E L Ee e Ee e Ee e
ωω ωω
βω
ββ
−−+
−
−−+
−+
≅++
(9)
Conside ing a limi ed bandwid h posi i e-in insic-nega i e (PIN) pho odiode wi h
esponsi i y ℜ, he pho ocu en is exp essed as:
() ()()
{
}
()
*
,, DC s
i E LE L i i =ℜ = + (10)
whe e iDC he DC pho ocu en and is( ) he signal pho ocu en . Conside ing he small-signal
app oxima ion, Jn(x)≈1/n!(x/2)n, we de i ed he ollowing exp ession o he signal
pho ocu en as a unc ion o he sys em pa ame e s:
() ( ) () ()
[]
()
()
22
2
21cos a c an cos
c
sbias iq
D L
i PmF x jx
c
πλ
π
γ
ααωτ
≈ℜ + + + −


 (11)
whe e Fbias = sin(π(Vbias1 + Vbias2)/Vπ), D is he dispe sion pa ame e , λc is he ca ie
wa eleng h, and τ = β1L is he ime delay su e ed by he op ical signal p opaga ing a dis ance
L h ough op ical ibe wi h a g oup delay pe uni leng h β1. In he de i a ion o Eq. (11), a
e y na ow-bandwid h OFDM signal was assumed, such ha he ampli ude ac o cos(·)
p esen in Eq. (11) o he cen al equency can be assumed cons an o e all he signal
bandwid h. I should be no ed ha , in he case o a eal- alued OFDM signal, he exp ession
o Eq. (11) con inues alid, wi h xq( ) = 0.
The a e age powe o he pho ode ec ed signal can be app oxima ed by:
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16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30767
() ( )
()()
()
2
22
22
22 2 2
21cosa c an
c
s L L bias xi xq
D L
Pi R RPmF
c
πλ
πγ σ σ α α
=≈ℜ + + +



(12)
whe e ⋅ deno es he a e aging ope a o in he ime domain and RL is he load esis o whe e
he RF powe is measu ed. The RF powe gi en by Eq. (12) is in acco dance wi h Eq. (1) in
[15], when no malized by he RF powe in op ical back- o-back (B2B) con igu a ion,
conside ing as modula ing signal a eal sinusoidal signal (xq( ) = 0). Thus, Eq. (12) is he
gene al exp ession o he RF powe a e pho ode ec ion, in he p esence o he modula ion
chi p. F om Eq. (12) we can conclude ha : (1) When he DEMZM is biased a i s null
in ensi y poin , (Vbias1 and Vbias2 equal o Vπ/2), he e is no signal pho ocu en loca ed a he
equency ω ; his is clea since, in his case, a he ou pu o he DEMZM only odd-o de
ha monics o ω a e p esen , and he e is no op ical ca ie o bea wi h he 1s o de
ha monics o gene a e he pho ocu en signal a ω . (2) The nega i e alues o he chi p
pa ame e ac o a c an(α), wi hin he a gumen o he cosine unc ion, ha e he e ec o
gene a ing a phase shi opposi e o ha caused by he cumula i e dispe sion along he ibe ;
hus, he i s null occu s o a g ea e ibe leng h in he p esence o nega i e chi p, allowing
mi iga ion o he powe ading induced by ch oma ic dispe sion o a gi en dis ance. (3) The
chi p pa ame e has also impac on he maximum ecei ed powe , h ough he ac o (1 + α2).
In o de o alida e Eq. (12), Fig. 1 depic s he RF powe o he ecei ed signal
no malized by i s maximum, ob ained analy ically om Eq. (12) and om nume ical
simula ion o an OFDM-UWB signal ollowing he ECMA-386 s anda d [16] cen e ed a he
equency = 3.960 GHz, which co esponds o he 1s o 14 channels. The DEMZM is
biased a he quad a u e poin . Mo e de ails abou he OFDM-UWB signal and he op ical
sys em pa ame e s conside ed a e p o ided in Sec ion 3.
F om Fig. 1 i is concluded ha he analy ical model de i ed o he signal pho ocu en
and i s RF powe shows excellen ag eemen wi h he nume ical simula ion esul s ob ained,
wi h a small disc epancy. This small disc epancy be ween he cu es can be due o he high
peak- o-a e age powe a io (PAPR) o he componen s xi( ) and xq( ). Hence, he
app oxima ion o he Bessel unc ions in Eq. (11) may no be accu a e since xi( ) and xq( )
ha e high-peaks a some poin s. Fu he mo e, i is obse ed ha he i s null o α = −1
occu s o a la ge ibe leng h han o α = 0.
2.2 E ec o DEMZM bias poin on he modula ion e iciency
Fo he small-signal analysis o he e ec o DEMZM bias ol age on he op ical powe s and
pho ode ec ed RF powe , he DEMZM is conside ed wi hou inse ion losses, balanced (γ =
1), and he modula ion is assumed chi pless (α = 0). Fo hese condi ions, om Eq. (5) and
Eq. (6), he powe o he op ical ca ie is gi en by:
()
2
2
012
241cos ,
2
OC bias bias
P
PEPVV
V
π
π

 
==++

 
 

 
(13)
and he a e age op ical powe o he signal sidebands is gi en by:
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Recei ed 20 Aug 2013; e ised 4 No 2013; accep ed 21 No 2013; published 6 Dec 2013
(C) 2013 OSA
16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30768

015 30 45 60 75 90 105 120 135 150
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fibe leng h (km)
No malized RF powe
= 0
=-1
Analy ical Nume ical
Fig. 1. No malized RF powe s as a unc ion o ibe leng h, o an OFDM signal cen e ed a
3.96 GHz.
()
()
()
()
22
22 2
222
12
22
2
22
=1cos.
2
OSB
xi xq bias bias
PP
PEE E
PmVV
V
π
π
πσσ
−+ +
 
=+=
 
 


+− +





(14)
The RF powe in op ical B2B con igu a ion, gi en by Eq. (11) o L = 0 and unde he
a o emen ioned condi ions, can be ew i en aking in o accoun he p oduc o Eq. (13) by
Eq. (14):
2.
L OC OSB
PRPP=ℜ (15)
Figu e 2 depic s he op ical powe o he ca ie , he op ical powe o he signal sidebands,
he o al op ical powe (POC + POSB) and he pho ode ec ed RF powe , no malized by hei
maximum alues, as unc ions o he bias ol age no malized by he hal -wa e ol age o he
DEMZM. The no malized powe o he op ical ca ie (POC) is equal o he in ensi y ans e
unc ion o an ideal DEMZM o nega i e bias ol ages.
In Fig. 2 i can be obse ed ha , when he bias ol age o he DEMZM dec eases om he
QP owa ds he MITP, he powe o he op ical ca ie dec eases whe eas he op ical powe o
he signal sidebands inc eases in he same p opo ion, and consequen ly he o al op ical
powe becomes domina ed by he powe o he signal sidebands.
O he wise, when he bias ol age is inc eased om he QP owa ds he MATP, he powe
o he op ical ca ie inc eases and he o al op ical powe is domina ed by he ca ie op ical
powe . The pe o mance and each o he op ical communica ion sys em a e de e mined by
POSB. By analogy wi h ampli ude modula ion in RF communica ion sys ems [17], he
modula ion e iciency is exp essed by η = POSB/(POC + POSB) × 100. The e o e, he e iciency
eaches 100% i he op ical ca ie is o ally supp essed, which equi es he addi ion o an
op ical local oscilla o a he ecei e side o pe o m cohe en de ec ion. In di ec de ec ion
sys ems, he mos used sys ems in op ical access ne wo ks, a ac ion o he op ical ca ie
mus be ansmi ed o enable di ec -de ec ion. The e o e, he e iciency eaches 100% i he
op ical ca ie is o ally supp essed, which equi es he addi ion o an op ical local oscilla o
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a he ecei e side o pe o m cohe en de ec ion. In di ec de ec ion sys ems, he mos used
sys ems
Fig. 2. Rela ionship be ween he op ical powe s and he RF powe as unc ions o he Vbias
ol age.
in op ical access ne wo ks, a ac ion o he op ical ca ie mus be ansmi ed o enable
di ec -de ec ion. Typically, he op imum pe o mance is eached when he powe o he
op ical ca ie is educed and equal o he op ical powe o he sidebands [4]. The e o e, wi h
POC = POSB, he op imum modula ion e iciency is 50% o di ec -de ec ion sys ems.
Addi ionally, in Fig. 2 i can be obse ed ha he maximum RF powe is ob ained when POC
= POSB, o he bias ol age equal o −0.5Vπ, because he op ical powe s a e no malized.
Typically, a he ou pu o he DEMZM biased a he QP, he powe a io be ween he op ical
ca ie and he sidebands is app oxima ely 20 dB [4].
3. Simula ion se up
Figu e 3 ep esen s schema ically a hyb id long- each WDM-PON (LR-WDM-PON) o a
single wa eleng h ansmi ed om he op ical line e minal (OLT) loca ed in he CO o he
hyb id op ical ne wo k uni /base s a ion (ONU/BS) loca ed a he cus ome ’s p emises. In he
CO, he op ical ansmi e based on he DEMZM has he modula ion chi p con olled h ough
he ampli udes o he d i e signals [6].
MZM
CW
LASER
Chi p Con olle
V1 Adjus men
V2 Adjus men
Noise
Loading
OF
PIN
EDFA
VOA
ONU/BS
EHS RX
EA
VEA SSMF
SSMF
RN
80 km 0-20 km
CO
EDFA
()
1
V
()
2
V
UWB #1 TX
UWB #2 TX
UWB #3 TX
GbE TX
528
MHz
528
MHz
#1 #2 #3
#1#2#3
0
BOFDM Bg
UWBs GbE
Fig. 3. Simula ion ansmission scheme o an OHS o e a hyb id long- each FTTH ne wo k
wi h con olled modula ion chi p.
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An elec ical hyb id signal (EHS) is c ea ed by subca ie mul iplexing ou independen
OFDM signals, a cus om quad a u e ampli ude modula ion (QAM) OFDM-Gigabi E he ne
(GbE) signal wi h 1 GHz bandwid h, and h ee independen quad a u e phase shi keying
(QPSK) OFDM-UWB signals ollowing he ECMA-368 s anda d [16].
The h ee OFDM-UWB signals, UWB#1, UWB#2 and UWB#3, employing QPSK
modula ion wi h 528 MHz bandwid h, a e up-con e ed o he equencies 3.432, 3.960 and
4.488 GHz, espec i ely. These h ee channel signals pe o m no equency hopping, wi h he
h ee channels a ime equency codes (TFC) TFC5, TFC6 and TFC7, espec i ely. The
subca ie s alloca ion in he OFDM-GbE signal is simila o he UWB signals. The OFDM-
GbE signal is he unique ha is no ye s anda dized, and hus can be eely ansla ed in
equency, in o de o maximize he gua d band (Bg) be ween he DC equency and he lowe
edge o he GbE signal spec um and minimize he agg ega e bandwid h o he OFDM signals
(BOFDM), as illus a ed in Fig. 3. Thus, by alloca ing a gua d band o 100 MHz be ween he
GbE signal and he lowe equency UWB signal, he GbE signal is up-con e ed o he
cen al equency equal o 2.568 GHz, esul ing in a BOFDM = 2.684 GHz and a Bg = 2.068
GHz, which p oduce he a io Bg/BOFDM = 0.7705. The e o e, Bg is insu icien o
accommoda e all in e modula ion dis o ion p oduc s gene a ed upon di ec -de ec ion, since i
has been shown ha Bg should be a leas equal o BOFDM [18]. The main pa ame e s used o
gene a e he baseband OFDM signals used in he simula ions a e ga he ed in Table 1.
Table 1. Main Pa ame e s Used o Gene a e he Baseband OFDM Signals
GbE UWB1,2,3
Nominal Bandwid h [MHz] 1000 528
Sampling equency [MHz] 1056 528
FFT size (To al o subca ie s) 256 128
Da a subca ie s 216 100
Pilo subca ie s 24 12
Gua d subca ie s 15 10
Cyclic p e ix [samples] 64 37*
Symbol ime[μs] 0.303 0.312
Cons ella ion mapping QAM QPS
K
Cen al RF equency[GHz] 2.568 3.432;3.960 and 4.488
Ne bi a e [Mbi /s] 1425.7 641
Re e ences ~ [16] [16]
*The cyclic p e ix in he OFDM-UWB signals a e null.
The EHS in ol ing ou sepa a ely ampli ied OFDM signals modula es ex e nally a
con inuous wa e (CW) lase wi h a linewid h o 5 MHz (con en ional DFB lase ) emi ing a
1550 nm, h ough a DEMZM wi h 25 dB DC ex inc ion a io, hal -wa e ol age Vπ = 5 V and
6 dB o inse ion loss. The modula ion index m is adjus ed h ough a a iable elec ical
a enua o (VEA). The model o he DEMZM is based on Eq. (2). The esul ing OHS a he
ou pu o he DEMZM is ansmi ed h ough SSMF o he cos ume ’s p emises. The SSMF is
modeled as he low-pass equi alen o a linea bandpass sys em. The SSMF is cha ac e ized
by an a enua ion o 0.22 dB/km and a ch oma ic dispe sion pa ame e D = 17 ps/km/nm.
A e ansmission h ough 80 km o SSMF, a he emo e node (RN), he op ical signal is
ampli ied by a noiseless e bium doped ibe ampli ie (EDFA) o compensa e o he
ansmission losses. F om he RN, he OHS is ansmi ed h ough ibe segmen s o leng h 0
km, 10 km and 20 km, o he hyb id ONU/BS loca ed a he cos ume ’s p emises, in o de o
simula e he di e en loca ions o he cus ome s ela i ely o he RN.
A he inpu o he hyb id ONU/BS, a a iable op ical a enua o (VOA) along wi h an
EDFA a e used o adjus he op ical signal- o-noise a io (OSNR) o he sys em, de ined in a
e e ence op ical bandwid h o 0.1 nm. The OSNR is e alua ed conside ing op ical noise o e
he wo pe pendicula di ec ions o pola iza ion.
In he hyb id ONU/BS, a second-o de supe -Gaussian op ical il e wi h 12.5 GHz ull
wid h a hal maximum (FWHM) bandwid h, cen e ed a 1550 nm, is used o educe he
ampli ied spon aneous emission (ASE) noise powe , and a PIN pho ode ec o wi h
esponsi i y o 0.9 A/W is conside ed. The ASE noise is modeled as Gaussian noise. The
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he mal noise o he ecei e is no conside ed, since a he ou pu o he PIN pho ode ec o i s
e ec is negligible, compa ed wi h he signal–ASE bea ing noise [19]. The ecei ed EHS is
demodula ed in he EHS ecei e (EHS RX), whe e each OFDM signal is sen o he
espec i e ecei e o pe o m down-con e sion and OFDM demodula ion. The OFDM
demodula ion includes emo al o cyclic p e ix, se ial- o-pa allel con e sion, FFT,
subca ie s equaliza ion, symbol de-mapping and pa allel- o-se ial con e sion. The
pe o mance o each o he ecei ed OFDM signals is assessed h ough he equi ed OSNR
(ROSNR) o a bi e o a e (BER) equal o 10−9, es ima ed by he e o ec o magni ude
(EVM) me hod applied o he cons ella ion o he ecei ed signal a he equalize ou pu [20].
4. Nume ical simula ion esul s and discussion
The simula ion esul s p esen ed in his pape we e p oduced wi h Ma lab®, conside ing 528
OFDM-GbE symbols and 512 OFDM-UWB symbols. Since he OFDM signals ha e a
spec um simila o Gaussian noise and di e en bandwid hs, he elec ical powe o each
signal was adjus ed o a powe spec al densi y equal o −139.6 dB/Hz.
Ini ially, he op imum bias ol age o he DEMZM equi ed o maximize he modula ion
e iciency o he sys em was in es iga ed o an op ical B2B con igu a ion. The DEMZM was
d i en in balanced push-pull mode. In Fig. 4, con ou plo s o he ROSNR o a BER = 10−9
a e shown as a unc ion o he modula ion index m o di e en alues o he bias ol age
no malized by he hal -wa e ol age o he DEMZM. No e ha a BER = 10−9 is enough o
ensu e ha he EVM limi o −14.5 dB a e ansmission, de ined in he s anda d o QPSK
OFDM-UWB signals, is espec ed.
Biasing he DEMZM below he QP, i ope a es in he non-linea egion o i s in ensi y
ans e unc ion, which deg ades he ansmi ed signals due o he gene a ion o ha monics
and in e modula ion dis o ion p oduc s. F om Fig. 4, i can be concluded ha he ROSNR
can be kep cons an when he DEMZM is biased below he QP, by dec easing m. The
ROSNR o he UWB #2 signal shown in Fig. 4(c)) p esen s a deg ada ion o nea ly 2 dB
ela i e o he UWB #1 and UWB #3 signals, due o linea c oss alk su e ed om he
adjacen channels. I can also be concluded ha he lowes bias ol age o he DEMZM is
limi ed by he GbE signal and is equal o −0.8 Vπ. The e o e, he op imum bias ol age equal
o −0.8Vπ was selec ed o de ine he DSB-RC modula ion, as he bes alue be ween all
signals conside ed. In Fig. 5, he ROSNR is p esen ed o all signals as a unc ion o m when
he DEMZM is biased a he QP, wi h Vbias = −0.5Vπ, and below he QP, a Vbias = −0.8Vπ.
Biasing he DEMZM a he QP, all signals p esen app oxima ely he same ROSNR as m
inc eases, whe eas when he DEMZM is biased a −0.8Vπ he GbE and UWB #3 signals
exhibi deg ada ion ela i ely o he o he signals, due o spu ious spec al componen s
esul ing om he nonlinea i y o he DEMZM ans e unc ion and he squa e-law o he
di ec -de ec ion. The GbE signal is deg aded because he a io Bg/BOFDM is lowe han 1, and
hus a ac ion o he GbE signal is dis o ed by in e modula ion spec al componen s nea
DC, a e pho ode ec ion. When he DEMZM is ope a ed below he QP, he 2nd ha monic
powe o he GbE signal a 5.136 GHz inc eases, a ec ing he UWB#3 signal cen e ed a
4.488 GHz. Fu he mo e, om Fig. 5 i is obse ed ha he op imum alues o m o d i e he
DEMZM a e 0.35 and 0.45, o he DEMZM biased a −0.8 Vπ and −0.5 Vπ, espec i ely. The
a e age OSNR imp o emen o DSB-RC (Vbias = −0.8Vπ) ela i e o DSB wi h ull ca ie
(Vbias = −0.5Vπ) o he op imum alues o m, among all signals, is nea ly 6 dB.
The d i e ol age ampli udes exp essed by Eq. (3) and Eq. (4) we e de i ed om he
small-signal app oxima ion chi p pa ame e componen s p esen ed in [11], and depend on he
bias poin o he DEMZM. The e o e, i is impo an o e alua e he accu acy o Eq. (3) and
Eq. (4) o he op imum alues o m ob ained o di e en bias poin s o he DEMZM
selec ed. The α chi p pa ame e alue a he DEMZM ou pu was measu ed as desc ibed in
[11,21]. Figu e 6 p esen s he absolu e e o be ween he desi ed α chi p pa ame e a he
ou pu o he DEMZM and he alue ob ained as a unc ion o m, o he wi ed and wi eless
signals modula ed in DSB-RC and DSB.
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Recei ed 20 Aug 2013; e ised 4 No 2013; accep ed 21 No 2013; published 6 Dec 2013
(C) 2013 OSA
16 Decembe 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030764 | OPTICS EXPRESS 30772