Signling-Embedded Premble Design for Flexible Opticl Trnsport Networks Linglong Di nd Zhocheng Wng Tsinghu Ntionl Lbortory for Informtion Science nd Technology, Deprtment of Electronic Engineering, Tsinghu University, Beijing 84, Chin E-mil: dill@tsinghu.edu.cn Abstrct Coherent opticl orthogonl frequency division multiplexing (CO-OFDM) is promising technique for future elstic opticl trnsport networks. In CO-OFDM systems, the premble is usully used for timing nd frequency synchroniztion, nd dedicted pilots re dopted to crry signling for flexible system configurtions. In this pper, we propose judicious signlingembedded premble design to simultneously chieve the exct timing synchroniztion bsed on the idel Delt-like timing metric, the ccurte frequency synchroniztion with sufficient estimtion rnge, s well s relible signling trnsmission. This is chieved by designing the premble in the frequency, i.e., two identicl trining sequences with specific distnce occupy the even subcrriers of the premble, whereby the system signling is crried by the combintion of different trining sequences nd different distnces between two sequences. Such frequencydomin design would result in the time-domin premble hving conjugte symmetric property nd two repetitive prts. The former property is used to produce the idel Delt-like correltion function for exct timing synchroniztion, while the ltter one is used for ccurte frequency synchroniztion. Simultion results lso show tht the improved signling detection performnce cn be chieved. I. INTRODUCTION Enhncing the spectrum utiliztion is intensively demnded nd becomes more chllenging when next-genertion opticl trnsport networks is evolving from Gb/s to beyond Gb/s []. The pioneering work by Shieh [2] hs suggested coherent opticl orthogonl frequency division multiplexing (CO-OFDM) to provide spectrum-efficient nd elstic longhul high-speed opticl trnsmissions, whereby vrible signl bndwidth nd dt rte could be esily nd flexibly configured in gridless fshion [3] [6]. To further enhnce the system flexibility nd improve the spectrum efficiency for future opticl trnsport networks, it is highly expected tht CO-OFDM could support number of working modes, which re indicted by different inverse fst Fourier trnsform (IFFT) sizes, gurd intervl lengths, constelltion modultions, chnnel coding schemes, code rtes, etc [4]. However, extr dedicted resources (e.g., frequencydomin pilots) re usully required to relibly convey those importnt system signling, which suffers from loss in spectrum efficiency. Additionlly, in most CO-OFDM systems, the premble t the beginning of the trnsmission frme is used for timing nd frequency synchroniztion [7] []. The most widely used premble is the one proposed by Schmidl [7], [8], but it suffers from performnce degrdtion since the timing metric used for synchroniztion hs plteu even over single-pth chnnels. This problem cn be resolved by Minn s premble [9], [], wherein multiple repetitive prts with specificlly designed signs yield well-behved timing metric, but the timing metric of the Minn s premble is not shrp enough to cquire the exct timing of the trnsmission block, nd the frequency synchroniztion estimtion ccurcy is reduced. Motivted by the requirements of signling-bsed system flexibility nd premble-bsed synchroniztion, in this pper we propose novel signling-embedded premble design to simultneously chieve the exct timing synchroniztion bsed on the idel Delt-like timing metric, the ccurte frequency synchroniztion with sufficient estimtion rnge, s well s relible signling trnsmission. Those merits re chieved by the following design method in the frequency domin: two identicl Hdmrd sequences with specific distnce occupy the even subcrriers of the premble, whereby the system signling is crried by the combintion of different Hdmrd sequences nd different sequence distnces. Such frequencydomin design would result in the time-domin premble hving conjugte symmetric property nd two repetitive prts. The former property is used to produce the idel Delt-like correltion function for exct timing synchroniztion, while the ltter one is used for ccurte frequency synchroniztion. The reminder of this pper is orgnized s follows. The proposed signling-embedded premble design is described in Section II. The corresponding timing/freqeuncy synchroniztion nd signling detection re presented in Section III. Numericl results re shown in Section IV. Finlly, conclusions re drwn in Section V. Nottion: We use the upper nd lower boldfce letters to denote mtrices nd column vectors, respectively; F N denotes the normlized N N fst Fourier trnsform (FFT) mtrix whose (n+,k+)th entry is exp( j2πnk/n)/ N; ( ) md ( ) T denote the complex conjugte nd trnspose, respectively; ngle{x} mens the ngle of the complex-vlued x; Finlly, x indictes tht x is frequency-domin signl. II. SIGNALING-EMBEDDED PREAMBLE DESIGN In this section, we propose novel signling-embedded premble design to simultneously chieve the following three merits: ) Idel Delt-like timing metric for very ccurte timing synchroniztion; 2) Accurte frequency synchroniztion 978--4799-4482-8/4/$3. 24 IEEE
Frequency Domin: Gurd Hdmrd Bnd Sequence g 2 n Time Domin: Sequence Distnce d b c N /4 d en /4 b u Hdmrd Sequence k N /2 k2 IFFT Hdmrd Sequence 2 n Zero Gurd Bnd g cn /4 d e N /4 u N Fig.. Frequency-domin nd time-domin structures of the signlingembedded premble. with sufficient estimtion rnge; 3) Relible system signling trnsmission. The signl structure of the proposed premble in the frequency nd time domins re illustrted by the top nd bottom prts of Figure, respectively. In the frequency domin, the premble p = [ p, p,, p N ] T of length N is composed of three distinct prts: ) Two gurd bnds filled with zero subcrriers re locted t high frequency bnd nd low frequency bnd (including the DC subcrrier), nd ech gurd bnd hs g subcrriers; 2) Two identicl Hdmrd sequences occupy two seprted regions with the distnce of Δd, nd ech region hs 2 n+ subcrriers; 3) In the middle, Δd zero subcrriers re used to seprte the two Hdmrd sequences. The Hdmrd sequence ũ of length 2 n is extrcted from ny column of the n-order rel-vlued Hdmrd mtrix H n recursively generted by [] H = [ + + + ], H n = 2 2 [ ] +Hn +H n +H n H n 2 n 2 n. () Then, the Hdmrd sequence ũ = {ũ k } 2n k= is mpped on the even subcrriers of two regions with the frequency-domin distnce Δd s below p k = ũ (k k)/2, mod(k, 2) =,k k<k +2 n+, ũ (k k2)/2, mod(k, 2) =,k 2 k<k 2 +2 n+,, others, (2) where k = g +Δd/2, nd k 2 = k +2 n+ +Δd. Unlike conventionl prembles minly used for timing nd frequency synchroniztion, we propose to embed the system signling in the premble so tht no extr dedicted overhed will be required to crry the system prmeters. This is chieved by the combintion of selected Hdmrd sequence nd the specific sequence distnce ccording to the configured signling. More specificlly, different signling corresponds to distinct Hdmrd sequences nd/or sequence distnces, both of which could be detected by the receiver to recover the trnsmitted signling. First, when one specific Hdmrd sequence ũ hs been selected, one specific sequence distnce Δd could convey certin signling. For exmple, in Figure we ssume the following vlues N = 248, 2 n+ = 52,g = 28, sothevlueofδd could be configured to vry within the rnge (256, 768] (the minimum sequence distnce Δd min should be lrger thn zero, e.g., 256 mentioned here, so virtul subcrriers locted in the middle of the signl bndwidth cn be used to llevite the crrier lekge effect [2]). Ber in mind tht only the even subcrriers re used in the premble, Δd should be n even number, so the distnce rnge (256, 768] could crry 8-bit signling (notice tht (768 256)/2 =2 8 ). Second, since Δd cn crry no more thn 9-bit signling even when g = nd Δd min =, more signling bits could be provided by selecting different Hdmrd sequences. For exmple, the signling bits could be incresed from 8 to if four possible Hdmrd sequence cndidtes cn be used. The receiver cn esily identify which one is selected due to the orthogonlity of different Hdmrd sequences. Since sequence discrimintion vi correltion hs higher complexity thn sequence distnce detection, using sequence distnce to crry signling is preferred if the sequence distnce could provide the required signlling bits. Note tht if no signling is embedded in the premble, the fixed Hdmrd sequence ũ nd sequence distnce Δd will be ssigned. In the time domin, the premble p =[p,p,,p N ] T is obtined by pplying IFFT to the frequency-domin premble p, yielding p = αf H N p, (3) where α is power scling fctor to mke the premble hve the sme verge power s dt symbol with N subcrriers, e.g., α = N/2 n+. The time-domin premble follows the form p =[b c N/4 d e N/4 b c N/4 d e N/4 ] T, (4) }{{}}{{} where =[b c N/4 d e N/4 ] of length N/2 is composed of four prts: both b nd d hve only one entry, while c N/4 nd e N/4 hve the length of N/4. In prcticl CO-OFDM systems, the premble p will be protected by its cyclic prefix to void the interference due to opticl impirments including chromtic dispersion (CD) [3]. Compred with Schmidl s nd Minn s prembles [7], [9], the proposed premble p simultneously enjoys the following two time-domin fetures: ) Similr to Schmidl s design, p is lso composed of two identicl time-domin prts, e.g., p =[ ] T, becuse only the even subcrriers re occupied by non-zero signls in the frequency domin; 2) Menwhile, within prt, c N/4 nd e N/4 re conjugte symmetric, i.e., c n = e N/4 n, n N/4. (5) The first feture is useful for ccurte frequency synchroniztion, nd the second one is essentil for ccurte timing synchroniztion, both of which will be ddressed in detil in the following Section.
III. PREAMBLE-BASED JOINT TIMING/FREQUENCY SYNCHRONIZATION AND SIGNALING DETECTION In this section, the specilly designed premble will be used to chieve ccurte timing/frequency synchroniztion s well s physicl lyer signling trnsmission. A. Joint Timing nd Frequency Synchroniztion Using the conjugte symmetric property of the premble, the exct strting point d of the CO-OFDM symbol is cquired by mximizing the timing metric M(d) s below { } { } d =rg mx {M(d)} (d) 2 =rg mx{ P d d R 2 (d) }, (6) where P (d) = N/2 n= q d+nq N+d+n, (7) denotes the correltion function of the received premble {q n } N n=, nd N/2 R(d) = q d+n 2 (8) n= represents the verge power of the received premble [3] used to normlize the correltion function P (d) in (6). After timing synchroniztion, crrier frequency offset (CFO) estimtion (or frequency synchroniztion) should be performed before further processing. In CO-OFDM systems with intrdyne receiver front end, the CFO Δf between the locl opticl lser oscilltor (OLO) nd the incoming signl is t most equl to the OFDM symbol rte /T (or one subcrrier spcing) [4], i.e., the normlized CFO denoted by Ω=ΔfT stisfies Ω (CFO in opticl OFDM systems is most likely to be bout %-2% of one subcrrier spcing [4], so Ω.2 is usully vlid). Thus, the receiver only needs to perform the fine (frctionl) CFO estimtion. Similr to Schmidl s method [7], the CFO estimte Ω cn be obtined by exploiting the time-domin structure of the proposed premble with two identicl hlves s below Ω = π ngle {P (d )}, (9) where P (d )= N/2 n= q d +nq N/2+d+n. () Due to the phse mbiguity issue, the estimtion rnge of (9) is (, +], which is sufficient for CO-OFDM systems since only the fine CFO estimtion is required s mentioned bove. On the other hnd, the proposed premble cn lso del with CFO much lrger thn one subcrrier spcing, e.g., by exploiting the property tht the integrl CFO results in the frequency-domin shifting of the ctive subcrriers but cn be esily determined by compring the received premble with the locl known premble in the frequency domin [5]. Note tht the correltion function P (d ) in () is used for both timing nd frequency synchroniztion in Schmidl s method [7], while P (d ) is only used for CFO estimtion but P (d ) in (6) is used for timing synchroniztion in our proposed scheme. B. Signling Detection After timing synchroniztion nd CFO compenstion, the received time-domin premble {q n } N n= is then converted to the frequency-domin premble { q k } N k= by using N-point FFT. Afterwrds, the cross-correltion between the received premble nd the locl known premble in the frequency domin is clculted by Q(m) = 2 n+ 2 q m+k/2ũk/2 k=, mod (k,2)= 2 n α 2, mod (m, 2) =,g m g + Δd 2, () where only the even subcrriers of the received frequencydomin premble re used, nd ũ = {ũ k } 2n k= denotes one possible Hdmrd sequence cndidte. Since there re two identicl Hdmrd sequences in the frequency-domin premble, two correltion peks re expected t the subcrrier loctions k nd k 2 when the locl Hdmrd sequence mtches the trnsmitted one, nd the distnce between the two correltion peks should be k 2 k =2 n+ +Δd. Finlly, the system signling is detected by the sequence selection combined with the observed distnce Δd, nd the correct signling cn be used by the receiver for further processing. Unlike the conventionl signling scheme bsed on the ctul detection of the trnsmitted known trining signl, the proposed signling detection minly relies on the distnce of correltion peks ssocited with two known trining sequences. Since distnce detection is usully more robust to chnnel impirments thn signl detection, more relible signling detection cn be expected by the proposed premble, which will be verified lter in Section IV. C. Computtionl Complexity For correltion-bsed timing synchroniztion (6) nd CFO estimtion (9), computtion of the correltion function is similr to tht used in conventionl CO-OFDM systems [8], [], lthough the proposed premble hs better synchroniztion performnce. Additionlly, the proposed premble cn lso be used for signling detection with low complexity, becuse the multipliction required to clculte the cross-correltion function () in the frequency domin cn be simplified to direct ddition/subtrction due to the rel-vlued Hdmrd sequence s indicted by () nd (2). In summry, the proposed scheme hs low computtionl complexity. D. Extension Discussion The proposed signling-embedded premble with Deltlike timing metric cn be directly used by coherent opticl single-crrier frequency domin equliztion (CO-SC-FDE) systems to improve the timing synchroniztion performnce. In ddition, the cpbility of conveying the system prmeters
.2.2 Schmidl s Premble [8] Minn s Premble [] Propsed Premble 2 5 5 5 5 2.2 2 2.2 CFO =, No Noise CFO =.3, OSNR = db 2 2.2 2 2.2 CFO=., OSNR = 5 db CFO=.3, OSNR = db, Multipth Chnnel 2 2 Fig. 2. Comprison of the timing metric behviors in the bsence of noise nd chnnel distortion. The timing offset indictes the exct timing point. Fig. 3. Timing metric of the proposed premble under different CFOs nd OSNRs. The first three subplots re obtined over n AWGN chnnel, while the lst subplot over multipth chnnel. cn be exploited to relibly deliver physicl lyer signling without extr dedicted overhed. IV. NUMERICAL RESULTS In this section, the performnce of the proposed signlingembedded premble is numericlly evluted through simultions. The simultion setup is similr to the CO-OFDM system in [6]: The IFFT length is 248, the gurd intervl length is 52; The trnsmission link is composed of 6 opticlly mplified 8-km fiber spns, nd the electronic dispersion compenstion (EDC) vi one-tp filter is performed prior to chnnel estimtion t the receiver. The polriztion-diversity opticl hybrid detector hs the opticl locl oscilltor with the line bndwidth of khz. The opticl signl-to-noise rtio (OSNR) t the receiver is defined with the. nm noise bndwidth. Refer to [6] for more detils of the simultion setup. Figure 2 compres the timing metric behvior of the proposed premble with those of the clssicl Schmidl s nd Minn s prembles in the bsence of noise nd chnnel distortion. A plteu for Schmidl s premble [8] is observed, nd much shrper timing metric is chieved by Minn s premble []. However, neither of them is s shrp s the proposed premble tht enjoys the idel Delt-like timing metric, indicting tht very precise timing synchroniztion cn be chieved by the proposed premble both in the singlepth nd multipth chnnels. The robustness of the timing metric to CFO is usully desirble. It is exmined by Figure 3 tht shows the impct of the normlized CFOs on the timing metric under different conditions. The timing metric is still Delt-like function which indictes tht successful timing synchroniztion cn be chieved even with CFO distortion. RMSE 2 3 Schmidl s Premble [8] Minn s Premble [] Proposed Premble 4 5 5 2 25 3 Opticl SNR (per informtion bit), OSNR [db/. nm] Fig. 4. Comprison of the RMSE performnce of the normlized CFO estimtion. Figure 4 compres the performnce of the normlized CFO estimtor (9) with those of the conventionl schemes in terms of root men squred error (RMSE). We cn observe tht the proposed CFO estimtor performs similrly s Schmidl s method [8] but much better thn Minn s solution []. The CFO cn be estimted t very smll error, for exmple, the RMSE is.3 when OSNR is db. Figure 5 shows the flse signling detection probbility of the proposed premble. For comprison, the performnce of
Flse Detection Probbility 2 3 Frequency Domin Pilots [8] Proposed Premble 4 7 6.5 6 5.5 5 4.5 4 3.5 3 Opticl SNR (per informtion bit), OSNR [db/. nm] Fig. 5. Comprison of flse signling detection probbility. the conventionl signling scheme bsed on frequency-domin pilots [8] hs lso been provided. As hs been ddressed in Section III, since the conventionl signling schemes rely on the ctul detection of the trnsmitted signl in the frequency domin, while the proposed signling detection relies on the distnce of correltion peks ssocited with two known trining sequence, better signling detection performnce cn be expected by the proposed scheme due to distnce detection is usully more robust to chnnel impirments. For exmple, when the trget flse detection probbility of 3 is considered, the proposed premble outperforms the conventionl scheme by bout.3 db, which demonstrtes the relible trnsmission of the importnt system signling. V. CONCLUSIONS Motivted by the requirements of signling-bsed system flexibility nd premble-bsed synchroniztion for CO-OFDM opticl trnsport networks, we propose novel signlingembedded premble design to simultneously chieve ccurte timing/frequency synchroniztion s well s relible signling detection. In the frequency domin, two identicl Hdmrd sequences with specific distnce occupy the even subcrriers of the premble, nd the system signling is crried by the combintion of different Hdmrd sequences nd different sequence distnces. The corresponding premble in the time domin hs conjugte symmetric property nd two repetitive prts, which produces the idel Delt-like correltion function for exct timing synchroniztion nd ccurte frequency synchroniztion. Additionlly, the distnce-bsed signling detection is robust to chnnel impirments, nd outperforms the conventionl scheme by bout.3 db for the flse detection probbility of 3. The proposed signlingembedded premble cn be lso directly used by coherent opticl single-crrier frequency domin equliztion (CO-SC- FDE) systems to improve the systme performnce. ACKNOWLEDGEMENT This work ws supported by Ntionl Key Bsic Reserch Progrm of Chin (Grnt No. 23CB3292), Ntionl Nturl Science Foundtion of Chin (Grnt Nos. 627266 nd 6285), nd ZTE fund project CON3725. REFERENCES [] A. R. Dhini, P.-H. Ho, nd G. Shen, Towrd green next-genertion pssive opticl networks (NG-PONs), IEEE Commun. Mg., vol. 49, no., pp. 94, Nov. 2. [2] W. Shieh nd C. Athudge, Coherent opticl orthogonl frequency division multiplexing, Electron. Lett., vol. 42, no., pp. 587 589, My 26. [3] W. Shieh, H. Bo, nd Y. Tng, Coherent opticl OFDM: Theory nd design, Opt. Express, vol. 6, no. 2, pp. 84 859, Jn. 28. [4] L. Di, C. Zhng, Z. Xu, nd Z. Wng, Spectrum-efficient coherent opticl OFDM for trnsport networks, IEEE J. Sel. Ares Commun., vol. 3, no., pp. 62 74, Jn. 23. [5] G. Shen, S. You, Q. Yng, Z. He, N. Yng, Z. 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