frequency estimation for optical coherent m-qam system without removing modulated data phase

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  • nt

    W

    ive

    y q

    by a

    da

    4, t

    16-

    igital signal processing (DSP)irment compensation of thever, bed froment DSdth tolpproacfrequenO) laseon ofent. T

    symbols is introduced in this M-QAM scheme. In addition, it is to

    to counteract the noise. Simulations of 10-Gbaud 16-QAM and

    d Df

    fd,n yd,n2pDfnTs2pm

    Contents lists available at

    els

    Optics Comm

    Optics Communications 285 (2012) 36923696/fd,nS and the residual y0d,n.E-mail address: yusong@bupt.edu.cn (S. Yu).256-QAM coherent systems demonstrate that the range of the where m is an integer to ensure fd,n to fall into the range ofp,p. For simplicity, laser phase noise and ASE noise areneglected. It is worth to mention that in Eq. (2) the randomvariable of fd,n has been decomposed into its average value

    0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved.

    http://dx.doi.org/10.1016/j.optcom.2012.05.027

    n Corresponding author.derive that specic constellation points should be rotated by p=4, /fd,nSy0d,n, 2extended to M-ary quadrature amplitude modulation (M-QAM)format, which is more likely to be employed in the next genera-tion optical coherent communications system. Different from ourformer M-PSK scheme [8], partition according to the amplitude of

    l,n

    (Df l), Ts is the symbol duration, Nn represents ASE noise, andenotes the frequency offset.

    The argument of the signal is of the formestimator is necessary to ensure reliable performance of PE.We have presented a frequency estimation algorithm for

    M-ary phase-shift keying (M-PSK) optical coherent system [8].Compared with previous algorithm, the modulated data phase isnot required to be removed. In this paper, the algorithm is

    The nth received M-QAM symbol can be presented as

    Sn An expjyd,nyl,n2pDfnTsNn, 1

    where An, yd,n represent modulated amplitude and phase ofM-QAM format, y is the phase noise induced by laser linewidthIn optical coherent detection, dshows its competence for the impalong-haul transmission [13]. Moreoof phase recovery can be transferresoftware domain [1]. Among differtions (PEs), considering laser linewitation complexity, feed-forward aHowever, this approach needs thetransmit and the local oscillator (LMeanwhile, the frequency resoluticannot satisfy the above requiremcause of DSP, the stagehardware domain to

    P-based phase estima-erance and implemen-h [47] is preferred.cy offset between ther to be small enough.a typical tunable laserherefore, a frequency

    2. Principle

    For M-QAM coherent system, the modulated data phase israndomly chosen from the constellation states. Thus, the corre-sponding phase can be treated as a discrete noise. On the otherhand, the frequency offset is comparatively stable and its result-ing argument change would be periodical. Therefore, after Fouriertransform, the frequency offset can be extracted directly [8].1. Introduction frequency offset estimation can cover [1.2 GHz, 1.2 GHz],while the maximum estimation error is below 1 MHz.Frequency estimation for optical coheremodulated data phase

    Song Yu n, Yinwen Cao, Haijun Leng, Guohua Wu,

    State Key Laboratory of Information Photonics and Optical Communications, Beijing Un

    a r t i c l e i n f o

    Article history:

    Received 17 January 2012

    Received in revised form

    15 May 2012

    Accepted 16 May 2012Available online 31 May 2012

    Keywords:

    Coherent detection

    Fast Fourier Transform (FFT)

    M-ary quadrature amplitude modulation

    (M-QAM)

    a b s t r a c t

    For optical coherent M-ar

    can be extracted directly

    removing the modulated

    constellation points by p=Numerical simulations of

    algorithm.

    journal homepage: www.M-QAM system without removing

    anyi Gu

    rsity of Posts and Telecommunications, Beijing 100876, PR China

    uadrature amplitude modulation (M-QAM) system, the frequency offset

    pplying Fast Fourier Transform (FFT) to the signals argument, without

    ta phase. By categorizing the constellation points and rotating some

    his algorithm is robust to extract the frequency offset against the noise.

    QAM and 256-QAM coherent systems are presented to demonstrate this

    & 2012 Elsevier B.V. All rights reserved.

    SciVerse ScienceDirect

    evier.com/locate/optcom

    unications

  • pDfn

    S. Yu et al. / Optics Communications 285 (2012) 36923696 3693From Eq. (2), the average term is governed by

    Fig. 1. The division of constellation for 16-QAM when 2

    Table 1The derivation of /mS for range I.

    i m Possibility /mS

    4k k 13=16 i=43=16k1 3/16

    4k1 k 9=16 i=43=16k1 7/16

    4k2 k 5=16 i=43=16k1 11/16

    4k3 k 1=16 i=43=16k1 15/16/fd,nS/yd,nS/2pDfnTsS/2pmS 2pDfnTs2p/mS: 3Without loss of generality, 16-QAM is demonstrated to illus-

    trate the algorithm. In order to ensure that m is only dependenton yd,n, 2pDfnTs is divided into three ranges, range I:p=4 ip=2,p=2a ip=2; range II: p=2a ip=2,p=2a ip=2;range III: p=2a ip=2;3p=4 ip=2, where a arctan1=3 and iis an integer which is denoted as 4kr r 0;1,2;3.

    As for range I, all four cases are discussed in Fig. 1 and Table 1.Fig. 1 shows that the constellation is divided into two classes(open symbols and lled symbols). If the modulated data belongsto open symbols, m k1. If not, mk. The explanation andanalysis will be given in detail later.

    As for range I, if r1, which is corresponding to Fig. 1(a), lledsymbols exhibit argument less than 3p=4 and 2pDfnTsAp=42kp,p=2a2kp. Therefore, for lled symbols, mk isrequired to keep fd,nA p,p and for the rest symbols (opensymbols), one additional 2p is required. The requirement fordifferent m for different modulate data accounts for the two setsof symbols in Fig. 1. The possibility of open symbols is 3=16, whilethe possibility of lled symbols is 13=16. Therefore, /mSi=43=16. Considering Eq. (3) and Table 1 together, if 2pDfnTsis in range I, Eq. (3) can be rewritten as

    /fd,nS 2pDfnTs3p=8ip=2: 4Similarly, for range II and range III, we have

    /fd,nS 2pDfnTsp=2ip=2; 5or

    /fd,nS 2pDfnTs5p=8ip=2: 6In order to express Eqs. (4)(6) in a more general form, Gx isintroduced

    Gx 1, xry1,0, y1oxry2,1, x4y2:

    8>: 7

    Therefore

    /fd,nS 2pDfnTspG2pDfnTs=8y3ip=2, 8

    where y3 p=2.The trajectory of /fd,nS for 16-QAM (black solid line) is different

    from QPSK (black dashed line), as shown in Fig. 2. The argumentchange induced by frequency offset (argexpj2pDfnTs) is alsoplotted (gray solid line). It can be seen that the period of /fd,nSfor 16-QAM is the same as that of QPSK, a quarter ofargexpj2pDfnTs. Therefore, after applying FFT to /fd,nS, the rstharmonic frequency represents the absolute value of 4Df , as shownin Fig. 3.

    However, compared with the spectrum of /fd,nS for QPSK and16-QAM in Fig. 3(a) and (b), the amplitude of the rst harmonicfor 16-QAM is much lower, making it difcult to be extracted.

    In order to increase the amplitude of rst harmonic, rstly, wepartition different rings into two classes, CI and CII. In CI, the ringsconsist of constellation points that the shorter distance from axis Iand II is less than that from axis X and Y in Fig. 4(a), while CII

    includes the rest rings. Specically, CI is the middle ring and CII

    includes the inner and outer rings. We rotate CI by y, as shown inFig. 4(a). Considering the symmetry of the constellation, rotationof 0,p=2 is sufcient. The derivation of /mS is similar, and its

    Ts is in range I. (a)(d) denote r 1;2,3;0, respectively.expression is the same as Eq. (8). The only differences are y1, y2and b, which is shown in Table 2.

    To facilitate the expression of the rst harmonic F1, we usecontinuous variable t to substitute nTs, yielding

    F1 1

    T

    Z t0 Tt0

    /fd,nSejo1t dt, 9

    where T 1=4Df ando1 2p 4Df . Here, we assume Df40. Theintegral range is p=4;3p=4 and Eq. (9) can be further simplied

    F1 j

    16ej4y1 ej4y22: 10

    The amplitude of F1 is

    9F19 18cos2y1y2fcos2y1y22 cos2y1y2g1

    p: 11

    It is worth to mention that all the constellation points best lineup when the CI points are rotated by y p=4. And this can befurther conrmed by Figs. 3 and 4(b). It can be mathematically

  • S. Yu et al. / Optics Communications 285 (2012) 369236963694

    3/4calculated from Eq. (10) that the increase in 9F19 is 0.777, which isconsistent with Fig. 3.

    Fig. 5(a) is presented to illustrate the modied frequencyestimator. Firstly, partition the rings including CI by the amplitude;

    T/4 T/2

    Pha

    se (r

    ad)

    /4

    0

    /4

    /2

    /2

    3/4

    period of

    Fig. 2. /fd,nS and argexpj2pDfnTs, T 1=Df : The pe

    1 3 5 70

    0.05

    0.10

    0.15

    0.20

    0.25

    f/(4f)

    |FFT

    |

    QPSK

    1 30

    0.05

    0.10

    0.15

    0.20

    0.25

    f

    |FFT

    |

    16QAM w

    Fig. 3. Spectrum of /fd,nS. (a) /fd,nS for QPSK; (b) /fd,nS for 16-QA

    0.09

    0.10

    0.11

    0.12

    0.13

    0.14

    0.15

    0.16

    |F1|

    Y

    X

    III

    Fig. 4. (a) Rotation of CI by y. Black symbols: CI before rotation. Gray s

    then, rotate these rings by p=4; next, obtain the argument ofall the symbols; after that, use FFT to get the correspondingfrequency of the rst harmonic frequency; nally, divide it by 4 toget 9Df 9.

    3T/4 Tt

    Arg[exp(j2fnTs)]

    riod of /fd,nS is T/4 for both 16-QAM and QPSK.

    5 7/(4f)

    /o Rotation

    1 3 5 70

    0.05

    0.10

    0.15

    0.20

    0.25

    f/(4f)

    |FFT

    |

    16QAM w. Rotation

    M before p=4 rotation; (c) /fd,nS for 16-QAM after p=4 rotation.

    0 /8 /4 3/8 /2

    0.09

    0.16

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