Performance Analysis of Unipolar Code in Different Phase Modulations of Coherent Optical CDMA

Transcription

1 Engnrng Lttrs, 6:, EL_6 8 Prformanc Analyss of Unpolar Cod n Dffrnt Phas Modulatons of Cohrnt Optcal CDMA M. Massoud arbassan, Mmbr, IAENG and H. Ghafour-Shraz Abstract hs papr xamns th bt-rror rat () for an optcal drct-dtcton cod-dvson multpl-accss (OCDMA) systm mployng a novl prm cod famly, hrby rfrrd to as Doubl-Paddd Modfd Prm Cod (DPMPC) for th spradng and d-spradng opratons n addton to cohrnt homodyn dual-balancd dtcton. As a cohrnt modulaton, bnary phas-shft kyng (BPS) format s dployd whr th phas s modulatd by thr an xtrnal phas-modulator or njcton-lockng mthods. h phas lmtaton and th prformanc for both phas-modulatons ncludng multpl accss ntrfrncs (MAI) n a shot-nos lmtd rgm ar studd and modrat pnalty assocatd wth th lmtd phas xcurson s rvald. Indx rms Cohrnt balancd-dtcton, njcton-lockng, lmtd phas xcurson, multpl accss ntrfrnc (MAI), optcal cod-dvson multpl-accss (OCDMA). I. INRODUCION Cohrnt tm spradng optcal cod dvson multpl accss (OCDMA) as a multpl accss protocol that taks advantag of th xcss bandwdth n sngl-mod optcal fbr has attractd a lot of attnton bcaus of th supror prformanc ovr ncohrnt schms [-3]. It maps low-rat lctrcal or optcal sgnals nto hgh-rat optcal squncs to achv random multpl accss. In OCDMA, many nods transmt smultanously on a common frquncy (wavlngth) band. Accordngly, sgnals must b dsgnd to rduc mutual ntrfrncs. In ordr to achv ths rsult, ach data bt s subdvdd nto a numbr of bnary chps. h ffct of ths furthr modulaton s to sprad th sgnal's frquncy spctrum. h chp squnc consttuts a cod that prmts a bt stram broadcast on a ntwork to b slctd by mans of a corrlaton procss at th rcvr dstnaton. A larg numbr of chp cods can b assgnd to dffrnt usrs, n th procss of whch th st of optcal squncs ssntally bcoms a st of addrss cods or sgnatur squncs for th ntwork. Svral archtcturs hav bn consdrd for th us of CDMA wthn an optcal fbr, th most common systms us drct-dtcton M. M. arbassan and H. Ghafour-Shraz ar wth th Dpartmnt of Elctronc, Elctrcal and Computr, th Unvrsty of Brmngham, Brmngham, B5, U (-mals: mmk44@bham.ac.uk and h.ghafour-shraz@bham.ac.uk) wth bpolar cods such as Gold squncs; whras, unpolar cods lk prm cod famls, partcularly th proposd on (scton II), hav mor flxbl cod-lngth and ar almost orthogonal. o rtan th advantag of / cods as a powr savng opton, w consdr n ths study unpolar sgnalng along wth th mployng of bnary phas shft kyng (BPS) as a cohrnt modulaton. h capacty of th systm usng prm cods s lmtd by th maxmum achvabl bt-rat of th lctronc crcutry gnratng th psudo-nos (PN) squncs. W sttl on a maxmum attanabl chp-rat of Gchp/s and a dsrd bt-rat of hundrds of Mbps, ladng to a lmt n th lngth of th spradng squncs on th ordr of hundrds of chps pr bt. In ths study, th analyss as th sgnal-to-nos ratos (SNR) of OCDMA systms and corrspondng s usng a nw prm cod famly and two phas modulaton mthods wth cohrnt dtcton ar nvstgatd. In ths papr th analyss starts wth ntroducng nw prm cod famly namd doubl-paddd modfd prm cod (DPMPC) n th followng scton. Cohrnt OCDMA archtctur s dscussd n scton III whr followd by systm analyss of xtrnal phas modulator wth dual-balancd dtctor. Scton V compars th rsults and xamnatons of xtrnal phas modulator wth njcton-lockng mthods and fnally rvals th prformanc pnalty mposd on th OCDMA systm as a rsult of th lmtd phas xcurson of ±.4π [4]. h rmarks com n th concludng scton. II. 'DOUBLE-PADDED MODIFIED PRIME CODE (DPMPC)' SRUCURE Accordng to th varous prm cod famls ntroducd n [5-8], th proposd optcal sgnatur squnc (DPMPC) s gnratd through rpatng (paddng) frstly, th fnal squnc-stram of th modfd prm cod (MPC) [5] squnc and scondly th fnal squnc-stram of th prvous modfd prm squnc (rotatng) n th sam group. In fact, th paddng ordr can also b appld vc vrsa, whras f th paddng ordr changs durng th cod gnraton, th cross-corrlaton valu also changs nto undsrabl valus (ncrass); thrfor, th paddng ordr has to b followd for th whol cod squncs. It s now clar whr th xprsson doubl-paddd coms from. Fnally, ths two squncs ar paddd nto ach MPC squnc and (Advanc onln publcaton: 9 Fbruary 8)

2 Engnrng Lttrs, 6:, EL_6 8 consquntly th cod nlargs by P as compard to MPC and by P as compard to both paddd modfd prm cod (PMPC) [6] and nw-modfd prm cod (nmpc) [7]. hs mpls an ncras n th chp-rat (procssng factor n spradng) whch maks th nw cod mor scur (.. lss or no ntrcpton) and also prmts th OCDMA systm to oprat at hghr bt-rat. It s ncssary to not that th paddd squncs cannot only b th fnal squnc-stram of MPC but also thy can b any stram of MPC squncs. hs s du to th unqunss of ach MPC squnc-stram that maks ach cod matchlss aganst ach othr. h nw cod-famly has P groups, ach of whch has P squnc cods. h lngth of ach cod s (P P) and th wght s (P). h total numbr of avalabl squncs s P. abl I shows an xampl of th DPMPC for P3. Rfrrng to abl I ach cod conssts of two parts, MPC and group squnc-stram (GSS) parts. For xampl, n squnc C th MPC part s and ts GSS part s whch comprss th last squnc-stram of C and C,.. C ( MPC) ( GSS). Smlarly, by addng th (th last part of C, MPC squnc-stram) to th last squnc-stram of C w can gnrat C of DPMPC. Snc synchronous OCDMA s consdrd, th auto- and cross-corrlaton functon C mn for any par of cods m and n at synchronous tm () s gvn by, P, f m n C, f m n, m and n shar th sam group () mn, f m n, m and n ar from dffrnt groups whr m,n {,,...P }. Fgurs to 3 llustrat th corrlaton functon C mn for cods of P3. In Fg. th auto-corrlaton valus of C ar dsplayd at ach chp synchronzaton postons. As clarly shown n Fgs. and 3, th cross-corrlaton valus of dffrnt cods at xactly synchronzd tm () s for th cods n a sam group and for thos ar n dffrnt groups. As an xampl, data stram s followd by th cod squncs to show how thy work. In Fg. at ach synchronous tm, th systm follows th data at ts maxmum valu (auto-corrlaton), whl n Fgs. and 3 at mnmum valus (cross-corrlaton). If thr corrlaton valu or data s th output s also at ach synchronzd tm of ; snc th cod and data ar multpld. hs cod was also appld to ncohrnt OCDMA systm [9] and htrodyn cohrnt OCDMA schm []. III. COHEREN OCDMA SYSEM ARCHIECURE In th optcal drct-dtcton CDMA systm, th outgong data s frst ncodd usng nw prm cod squncs. h ncodd data s thn modulatd usng an xtrnal phas modulator; ths corrsponds to multplcaton of th optcal carrr and nabls th systm to mploy cods basd on n-phas corrlatons. h us of an xtrnal Mach-Zhndr modulator for ths applcaton was vrfd xprmntally n []. At th ntndd rcvr, anothr phas modulator s usd to dmodulat th rcvd sgnal usng th sam prm cod ABLE I DOUBLE-PADDED MODIFIED PRIME CODES FOR P3 Cods MPC GSS Part C C C C C C C C C squnc usd for ncodng at th transmttr. Basd on th corrlaton proprts of th slctd famly of prm cods, th sgnal that has bn modulatd twc by th sam cod wll d-sprad, whl sgnals modulatd by dffrnt cods wll b furthr sprad n th frquncy doman and hnc rmovd. h optcal phas modulator could b followd by a Fabry-Prot optcal fltr that rjcts unwantd crosstalk orgnatng form th MAI outsd th fltr bandwdth. h optcal output of th fltr conssts of th dsrd sgnal and rsdual co-channl ntrfrnc. In cohrnt dtcton OCDMA systm, a hgh qualty rfrnc lasr s cntrally locatd and ndvdual usrs us ths rfrnc for njcton-lockng of nxpnsv dstrbutd fd-back (DFB) lasrs to act as a sourc for transmsson and also srv as th local oscllator for cohrnt homodyn dtcton. h phas s modulatd by controllng th njcton currnt to th DFB lasr. In ths cas, data s phas modulatd by a prm squnc and transmttd ovr th ntwork. At th rcvr a DFB lasr s slavd to th rfrnc lasr, phas modulatd by th orgnal squnc and thn combnd cohrntly wth th rcvd CDMA sgnal. Snc th sam sourc s usd for transmsson and rcpton w hav homodyn dtcton. h cohrntly mxd optcal sgnals ar ncdnt on a dual-balancd dtctor whos lctrcal output consrvs phas nformaton. h possbl bpolar lctrcal sgnal s ntgratd ovr a bt ntrval and th rsult compard to a thrshold to form th fnal bt stmat. Whn phas modulaton s achvd by an xtrnal modulator, chp-rat can rang to th Gchp/s; n contrast, th njcton currnt adjustd to achv phas modulaton has th ordr of Gchp/s chp-rat du to th lmtd phas xcurson assumd n [4] howvr, ths rangs hav nowadays changd nto much hghr valus du to componnts mprovmnts. hs rducton n chp-rat may b justfd to avod th xpns of an xtrnal modulator. On th othr hand, as mntond, f w pos th sam lmt of Gchp/s to th chp-rat and want to kp th bt-rat suffcntly hgh (hundrds of Mbps), w ar lmtd to spradng squncs havng lngths n th ordr of hundrds. As Gold squncs ar N n - chps long, wth n bng an odd ntgr, w hav lmtd th rsults to n 9,.. a lngth of 5, (Advanc onln publcaton: 9 Fbruary 8)

3 Engnrng Lttrs, 6:, EL_6 8 Corrlaton Valu Auto-Corrlaton of DPMPC C,P3,D tm, Fg.. Auto-corrlaton valus of th C DPMPC for data stram of. ( s synchronzaton tm) Corrlaton Valu Cross-Corrlaton of DPMPC C & C,P3,D tm, Fg.. Cross-corrlaton valus of C and C (Sam Group) DPMPC for data stram of.( s synchronzaton tm) Corrlaton Valu cross-corrlaton of DPMPC,C & C,P3,D tm, Fg. 3. Cross-corrlaton valus of C and C (Dffrnt Groups) DPMPC for data stram. ( s synchronzaton tm) whras thr ar two stps furthr wth DPMPC whr P quals 3 and 9 wth achvabl cod-lngths (NP P) of 575 and 899 rspctvly. IV. EXERNAL PHASE MODULAION WIH DUAL-BALANCED DEECION o consrv nformaton contand n th phas of th optcal carrr, cohrnt dtcton s usd whrby a local optcal sourc s cohrntly combnd wth th rcvd nformaton-barng sgnal. In homodyn dtcton th local oscllator s at th carrr wavlngth and th output lctrcal sgnal s at basband. o lmnat th drct currnt (DC) componnt n ths basband sgnal a dual-balancd dtctor s usd. Obvously, a local oscllator wth much gratr powr than th total rcvd sgnal rsults n a shot-nos lmtd rgm whr dark currnt and rcvr thrmal nos ar nglgbl. h frst stp s to xamn th quatons that govrn th lctrcal output of a dual-balancd dtctor n a CDMA systm. Lt b th numbr of actv usrs and S ( t τ ) b th sgnatur squnc of usr. Ltτ b th rlatv tm-dlay btwn usrs and th dsrd usr (.g. usr #). h us of a unpolar spradng squnc s assumd; so that th squnc taks valus from th st{,} and also all usrs hav roughly th sam avrag powr gvn by Ŝ (thy travl narly th sam dstanc n th fbr and suffr th sam losss du to dsprsons) plus all usrs ar rcvd wth th sam polarzaton. At any nstant of tm t, C ( dnots th pcws-constant functon whch s th product of th bt valu and th squnc valu of usr at tm t, thus also takng valus θ, a from th st {,}. h ntal phas offst of usr s random varabl unformly dstrbutd ovr th ntrval (,π ). h rcvd sgnal s gvn by: s Sˆ C cos( ω ct θ ) () or n a phasor format as: j c t C t S Sˆ ( ω ( ). π π θ ) (3) h local oscllator wll b modulatd by th spradng squnc of th dsrd usr, S ( t ). h phasor form of th local oscllator sgnal s thn gvn by: j S t L Lˆ ( ω c t ( ). π π θlo) (4) h output of th dual-balanc dtctor s hnc: η η R[ LS] LS ˆ ˆ cos[ ( S( C ). π θ LO θ ] (5) η LS ˆ ˆ S t C t LO h ( ). ( ).cos( θ θ ) ν whr η s th (common) quantum ffcncy of th two dtctors, h s Planck s constant, s th fundamntal charg of an lctron, and v s th mployd optcal frquncy (wavlngth). Not that w assum th local oscllator tracks th phas of th dsrd usr, so wthout loss of gnralty w havθ LO θ. As mntond arlr, whn a strong local (Advanc onln publcaton: 9 Fbruary 8)

4 Engnrng Lttrs, 6:, EL_6 8 oscllator s mployd, th nos at th output s shot-nos lmtd and has on-sdd powr spctral dnsty of: η ˆ N L (6) h output of th dtctor (5), s ntgratd ovr a bt ntrval, ladng to th followng statstc [4]: S whr out η LS ˆ ˆ S. C η LS ˆ ˆ. b η LS ˆ ˆ [ b. R ( τ ) b. Rˆ ( τ )]., dt η Lˆ. n, N n( t ) dt.cos θ (7) b rprsnts th nformaton bt bng dtctd and b s th ovrlappng prvous bt of usr, and b s th ovrlappng followng bt of usr. h contnuous-tm partal cross-corrlaton functons ar dfnd as: R Rˆ, j. j τ ( τ ) s ( t o ( τ ) s ( t τ τ ). s τ ). s j j ( t ) dt ( t ) dt whr τ τ τ j s th argumnt, wth τtakn to b zro. h nos n( n (7) s assumd a Gaussan random varabl wth zro man and unt varanc; all data bts ar ndpndnt, quprobabl and th dlays ar also ndpndnt and unformly dstrbutd ovr a bt ntrval. h frst trm n (7) s du to th dsrd usr, whl th scond trm s addtv wht Gaussan nos and th thrd s th multpl accss ntrfrnc. For furthr analyss, MAI has bn assumd a Gaussan dstrbutd; a vald approxmaton for a rasonabl numbr of actv usrs. h varanc of ach trm n th multpl accss ntrfrnc sums has bn shown n [4] to b approxmatly qual to 3N (8), whr N s th lngth of th sgnatur squnc ( 3N corrsponds to th corrlaton and / s du to th phas offs. h SNR s thn gvn by: η Lˆ. Sˆ. SNR η Lˆ Sˆ η.... Lˆ 3 N 3 N 4η Sˆ Not that 4 ηsˆ s th sngl-usr SNR ( E b N ), that shows SNR quals E b N f th MAI was zro (). Snc w hav BPS sgnalng n Gaussan nos, th can b calculatd drctly from th accomplshd SNR. Fgur 4 shows th comparson of vrsus th sngl-usr SNR for DPMPC squncs whn P9 and Gold-squnc of (9) -5 - N5, r5% P9, r5% N5, r5% P9, r5% N5, r5% P9, r5% Extrnal Phas Modulaton Sngl Usr SNR (db) Fg. 4. Prformanc of BPS-OCDMA systm usng an xtrnal modulator vrsus sngl-usr SNR ( E bn ) Extrnal Phas Modulaton EbN3dB, N5 EbN3dB, P9 EbN6dB, N5 EbN6dB, P9 EbN9dB, N5 EbN9dB, P No. of Smultanous Usrs, Fg. 5. Prformanc of BPS-OCDMA systm usng an xtrnal modulator vrsus numbr of smultanous actv usrs () 5 cod-lngth as usd n [-4]. As aformntond, f w pos th lmt of Gchp/s to th chp-rat and want to kp th bt-rat suffcntly hgh (hundrds of Mbps), w ar lmtd to spradng squncs havng lngths n th ordr of hundrds. As Gold squncs ar N n - chp long, wth n bng an odd ntgr, w ar lmtd to n9,.. a lngth of 5; whl, wth DPMPC thr ar two furthr stps, whr P quals 3 and 9 wth cod-lngths (NP P) of 575 and 899, rspctvly. Also for bttr undrstandng, th dsrd thrshold of -9 s ndcatd n Fg. 4. h paramtr r shown n th nst of Fg. 4 whch rangs from 5% to 5% dnots th prcntag of th total numbr of smultanous actv usrs. As th rsults show for a gvn valu of r whn th sgnal powr ncrass th systm dcrass; howvr, for hghr valus of r th systm dgrads. Also t can b sn that th prformanc usng th DPMPC surpasss th prformanc of Gold-squncs n dffrnt condtons. Fgur 5 xplans th systm bhavor vrsus th numbr of smultanous actv usrs, for dffrnt sngl-usr SNRs ( E ) and compars DPMPC wth P9 and Gold-squncs bn wth cod-lngth of 5. h fgur clarly shows that () as th (Advanc onln publcaton: 9 Fbruary 8)

5 Engnrng Lttrs, 6:, EL_6 8 sngl-usr SNR ncrass th dcrass, () th systm prformanc s lmtd by MAI and () th numbr of smultanous actv usrs that DPMPC can accommodat s twc that of Gold-squnc. For xampl, whn E bn 6 db th DPMPC systm (wth P9) can tolrat maxmum of 8 smultanous usrs havng -9 whl that for th systm mployng Gold-squncs s 4 usrs. It also shows a rmarkabl capacty nhancmnt n ths systm by usng DPMPC du to flxbl cod-lngth Extrnal vs. Injcton-Lockng Modulaton r5%, P9 - Ext. Mod. r5%, P9 - Inj.-Lockng Mod. V. INJECION-LOCING PHASE MODULAION WIH PHASE LIMIAION Now xamnng th ffct of a phas lmtaton on th modulaton procss s studd. In ths cas, th njcton currnt of DFB lasr dod s modulatd to accomplsh PS sgnalng at th transmttr, wth phas xcurson lmtd to ±.4π and modulaton spd no gratr than Gchp/s. At th rcvr w dmodulat va njcton-lockng of th local oscllator, as t s no longr possbl to track th dsrd usr s ntal phas offst by njcton-lockng, anothr trackng mthod s assumd n that θ θ LO stll quals zro. By wrtng ±.4π ±π/.8π, th nformaton and local oscllator sgnals ar thrfor gvn as th followng form: j( ωct C.( π.8π ) π θ ) S Sˆ Sˆ L Lˆ j( ωct π. C π θ ) j ( ωct S( t ). π π ).. j.8πc j.8π. S( t ) () () h output of th dual-balancd dtctor undr ths crcumstancs s: η R[ L. S] η LS ˆ ˆ cos[ ( S( C ).( π.8π ) θ LO θ ] η LS ˆ () ˆ.sn (.8π ) cosθ η LS ˆ ˆ.cos (.8π ) S. C cosθ η LS ˆ ˆ. sn(.6π ) ( S( C ) sn θ Du to th phas lmtaton, two nw trms n th output of th dual-balancd dtctor ar appard. h frst trm n () s constant n tm and accountd as DC componnt whch can b stmatd and rmovd through th dual-balancd dtctor. h last trm n () nvolvs th wghtd-sum of th dffrnc of two cods (ssntally a nw psudo-cod). h wghtd-sum s boundd by th sum ovr th psudo-cods, whch n turn s nglgbl compard to that of th scond trm. hs s bcaus; th cod-lngth bcoms arbtrarly largr than th numbr of actv usrs (.. ). Hnc, th output can b N >> Sngl Usr SNR (db) Fg. 6. Prformanc comparsons of xtrnally and njcton-lockng modulatd BPS-OCDMA systm vrsus sngl-usr SNR No. of Smultanous Usrs, approxmatd by th scond trm alon plus nos. By ntgratng ovr a bt ntrval, th output sgnal can b obtand as: η Sout LS ˆ ˆ cos (.8π ) S(. C.cos θdt N n( dt Extrnal vs. Injcton-Lockng Modulaton EbN3dB, P9 - Ext. Mod. EbN3dB, P9 - Inj-Lockng Mod. Fg. 7. Prformanc comparsons of xtrnally and njcton-lockng modulatd BPS-OCDMA systm vrsus numbr of smultanous actv usrs () (3) η LS ˆ η ˆ. b..cos.8π Lˆ. n η LS ˆ ˆ cos.8π. [ b. R,( τ ) b. Rˆ,( τ )].cos θ Usng th sam Gaussan approxmaton for th mult-usr ntrfrnc, th SNR s achvd as follows: (Advanc onln publcaton: 9 Fbruary 8)

6 Engnrng Lttrs, 6:, EL_6 8 η cos.8π SNR η cos.8π ˆ. ˆ L S. 3N 4η ˆ 4 S cos.8π ˆ. ˆ L S. η.. Lˆ 3N (4) By comparng quatons (9) and (4), t s notcd that th only dffrnc s th cosn trm whch causs.6db dgradaton, whl ths s analogous to th sngl-usr whr t s xpctd a powr-loss of twc snc th lmtaton appls to both modulator and dmodulator, thn th dgradaton s dually qual to.db ( logθ, θ s th phas lmtaton). As shown n Fg. 6, th prformancs of both modulaton mthods hav bn valuatd vrsus sngl-usr SNR (.. E bn E b / N ) whn r 5% and P9. h xtrnal modulaton cas shows lowr ; howvr t may comproms th systm mplmntaton costs. Addtonally t can b obsrvd from Fg. 6, th dffrnc btwn two graphs s.db at any pont of th sam valu,.g. by rachng to th valu of -9, E bn of xtrnal modulaton s ndd to b 8.8dB but db for E bn of njcton-lockng mthod. h bhavor of ths two systms s ndcatd aganst ach othr and llustratd n Fg. 7 vrsus th numbr of smultanous actv usrs () whn E bn 3dB and P9. h xtrnal modulaton ndcats rmarkably nhancd prformanc, partcularly at th lowr numbr of smultanous actv usrs. REFERENCES [] X. Wang, N. Wada,. Hamanaka, A. Nshk, and. tayama, -usr, truly-asynchronous OCDMA xprmnt wth 5-chp SSFBG n/dcodr and SC-basd optcal thrsholdr, OFC 5, Anahm, CA, Postdadln papr PDP33, 5 [] X. Wang, N. Wada, G. Cncott,. Myazak, and. tayama, Dmonstraton of -usr,.7 Gbps truly asynchronous OCDMA usng FEC and a par of mult-port optcal-ncodr/ncodrs, n ECOC 5, Glasgow, U.., Postdadln papr hu 4.5.3, 5 [3] Z. Jang, D. S. So, S. D.Yang, D. E. Lard, R.V. Roussv, C. Langrock, A. M. Fjr, and A. M. Wnr, Four-Usr,.5-Gb/s, spctrally codd OCDMA systm dmonstraton usng low-powr nonlnar procssng, J. Lghtw. ch., vol. 3, no., pp , Jan. 5 [4] F. Ayad and L.A. Rusch, Cohrnt optcal CDMA wth lmtd phas xcurson, IEEE Comm. Lttrs, vol., no., Jan. 997 [5] W.C. wong, P.A. Prrr and P.R. Prucnal, "Prformanc comparson of asynchronous and synchronous cod-dvson multpl-accss tchnqus for fbr-optcal local ara ntworks", IEEE rans Commun., vol. 39, no., pp , 99 [6] M.Y Lu and H.W sao, "Cochannl ntrfrnc cancllaton va mployng a rfrnc corrlator for synchronous optcal CDMA systms", Mcrowav Opt. ch. Ltt., vol. 5, no. 6, pp.39-39, [7] F. Lu, M. M. arbassan and H. Ghafour-Shraz, Novl Famly of Prm Cods for Synchronous Optcal CDMA, J. Opt. Quant. Elctron., vol. 39, no., pp. 79-9, Jan. 7 [8] M. M. arbassan and H. Ghafour-Shraz, Phas-Modulatons Analyss n Cohrnt Homodyn Optcal CDMA Ntwork Usng a Novl Prm Cod Famly, Proc. WCE 7 (ICEEE), pp , London, U [9] M. M. arbassan and H. Ghafour-Shraz, "Frsh Prm Cods Evaluaton for Synchronous PPM and OPPM Sgnalng for Optcal CDMA Ntworks", J. Lghtw. ch., vol. 5, no. 6, pp. 4-43, Jun 7 [] M. M. arbassan and H. Ghafour-Shraz, "Prformanc Analyss of Htrodyn Dtctd Cohrnt Optcal CDMA Usng a Novl Prm Cod Famly", J. Lghtw. chnol., vol. 5, no., pp , Oct. 7 [] A. H. Gnauck, 4-Gb/s RZ-dffrntal phas shft kyd transmsson, n Proc. Optcal Fbr Communcatons (OFC 3), 3, Papr he VI. CONCLUSION h quatons wr ntroducd for th of optcal CDMA systm mployng a nw prm cod famly (DPMPC) and cohrnt dtcton wth both xtrnal phas modulaton and njcton-lockng undr th assumpton of a Gaussan dstrbuton for th multpl accss ntrfrncs. Consquntly, DPMPC outprforms th convntonal bpolar squncs n trms of flxbl cod-lngths and accordngly nhancng systm capacty. h lmtd phas xcurson causs svral complcatons.g. frstly sparat phas trackng s rqurd as t can no longr b accomplshd smultanously wth phas modulaton; scondly from a practcal pont of vw, thr s a DC-bas lvl n th dual-balancd dtctor output rqurng stmaton and rmoval; fnally thr s th dgradaton n quvalnt to a sgnal loss of.db; howvr ths loss s small n comparson to th MAI pnalty for multpl smultanous usrs. (Advanc onln publcaton: 9 Fbruary 8)