Abstract:
A subcarrier system generates a phase comparison signal in a transmitter and transmits the phase comparison signal together with an optical subcarrier multiplex signal in the same transmission channel of an optical network. A receiver measures a phase distortion between a phase reference signal and the received phase comparison signal for each modulation section of the received subcarrier symbols, calculating at least one correction value and correcting time jitters or phase impairments of all parallel received or regained subcarrier symbols as a function of the phase distortion of the received phase comparison signal. Time jitter/phase jitter can be compensated without high hardware expenses.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention refers to a subcarrier multiplex system according to claim  1 . 
       BACKGROUND OF THE INVENTION 
       [0002]    Optical subcarrier multiplex systems like OFDM (Orthogonal Frequency Division Multiplex) systems have gained high attention in the recent years due to high intrinsic robustness against dispersion and its high spectral efficiency. The data signal is converted into parallel symbols and transmitted via multiple subcarriers. A great variety of modulation formats of the subcarriers is feasible like AM, FSK, PSK, QAM. Each subcarrier transmits at a relative low bit rate, the modulation period of a symbol is relatively long compared with the duration of a bit. Especially OFDM systems require orthogonal subcarriers and a phase stable signals. The electrical subcarrier signal can be used to modulate the optical power of an optical source (laser) directly or to modulate an external optical modulator. 
         [0003]    An OFDM system is described by Arthur James Lowery, Senior Member, IEEE, Liang Bangyuan Du, and Jean Armstrong in “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems” in JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 1, JANUARY 2007 131. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0004]    Although subcarrier systems like OFDM have high tolerances against chromatic dispersion they suffer from nonlinear effects like SPM (Self Phase Modulation) and XPM (Cross Phase Modulation) in the fiber as well as from laser chirp in interaction with dispersion. These effects result in time/phase jitter of the subcarrier signals. The patent application WO 94/03987 discloses a simple analogue circuit to compensate the laser chirp induced distortion at the transmitter or at the receiver. 
         [0005]    The object of this invention is to compensate the impairment of subcarrier signals. 
         [0006]    This problem is solved by a subcarrier multiplex system described in claim  1 . 
         [0007]    The transmitter of the subcarrier system is designed for generating a phase comparison signal and transmitting the phase comparison signal together with an optical subcarrier multiplex signal in the same transmission channel,
   the receiver is designed for measuring a phase distortion between a phase reference signal and the phase comparison signal for each modulation section of the parallel received subcarrier symbols,   calculating at least one correction value and correcting time jitters or phase impairments of all the parallel received or regained subcarrier symbols as a function of the phase distortion of the received phase comparison signal.   
 
         [0010]    The inventive idea is to measure the distortion of a transmitted single phase comparison signal and to correct all impaired received subcarrier signals according to that phase distortion. 
         [0011]    The correction function of the subcarrier system can be designed easily if the time jitters of all parallel received subcarrier symbols are corrected by the same time shift as a function of the phase distortion of the received phase comparison signal. 
         [0012]    Therefore only a common jitter control value is derived to correct the time jitter of all subcarrier symbols. 
         [0013]    For improved compensation enhanced jitter correction values for individual phase correction of received subcarrier symbols are calculated considering different time jitter of the received subcarrier symbols. 
         [0014]    For digitally systems it is advantageously that phase jitters and/or amplitude of all parallel regained symbols (SD S ) are compensated according to the phase distortion of the received phase comparison signal. 
         [0015]    The correction is done by digital data processing. 
         [0016]    For improved phase compensation the phase impairments of each received subcarrier symbol or regained symbol are corrected individually by an enhanced calculated phase correction value, which implies a correction factor considering individual phase jitter of the received subcarrier symbols. 
         [0017]    The invention can be advantageously applied to an OFDM (Orthogonal Frequency Division Multiplex) subcarrier system. 
         [0018]    More details and improvements of the invention are described in further depending claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Examples of the invention including a presently preferred embodiment are described below with reference to accompanying drawings, where 
           [0020]      FIG. 1  shows a schematic diagram of subchannel system according to the invention, 
           [0021]      FIG. 2  shows adjacent channels of a subchannel/OFDM system, 
           [0022]      FIG. 3  shows an OFDM system according to the invention, 
           [0023]      FIG. 4  shows details of a OFDM system, and 
           [0024]      FIG. 5  shows a modified receiver according of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The diagram  FIG. 1  illustrates subcarrier system with a transmitter  1  and a receiver  3 . An optical subcarrier multiplex signal OSMT is generated and transmitted over a transmission network  2 , e.g. an optical fiber, to the receiver  3 . 
         [0026]    At the transmitter  1  a high speed digital serial signal DST is fed to a serial-parallel converter  10  and converted into parallel data bits. Constant number of parallel bits, e.g. 4 bits, are converted by a symbol generation unit  11  into a symbol sequence ST S , s=1, 2, . . . , N. A group of N of these symbols is then parallel modulated onto different subcarriers in a subcarrier modulation unit  12 , e.g. by quadrature phase shift keying. The parallel generated subcarrier symbols SST S  are then combined in a combiner  13  to a subcarrier multiplex signal SMST, which is modulated onto an optical carrier in an optical modulation unit  14  and transmitted as optical subcarrier multiplex signal OSMT in a transmission channel  2  to the receiver  3 . A modulation section or a symbol ST S  duration is much longer than a bit duration of the serial signal DST because of the conversion into symbols and the parallel transmission of the symbols. After the transmission of a group of symbols the next group of symbols is transmitted. 
         [0027]    Usually additional signals are transmitted in neighbour channels.  FIG. 2  shows three channels CH 1 , CH 2  and CH 3  of a WDM (wavelength division multiplex) system transmitting signals in these channels over the same optical fiber. The subcarrier symbols SST S , s=1, 2, . . . , N of the regarded N subchannel system are transmitted over the middle channel CH 2 . The neighbour channels CH 1 , CH 3  have a shorter and a longer wavelength λ. Both neighbour channels, as mentioned before (and also additional channels), as well as nonlinear effects impair the transmitted optical subcarrier multiplex signal OSMT. The phase of the subcarrier signals in the regarded channel CH 2  is disturbed. This causes a loss in signal quality, especially when the phase of the subcarriers is important for the demodulation. 
         [0028]    At the receiver  3  an impaired optical subcarrier multiplex signal OSMR is received and converted in an optical receiver unit  31  into an electrical subcarrier multiplex signal SMSR, which is fed via a jitter correction unit  32  to a separation unit  33 , e.g. an electrical comb filter, which separates the subcarriers (divides the channel into subchannels). In a subcarrier demodulation unit  34  the received subcarriers symbols SSR S  are demodulated and the symbols SR S , s=1, 2, . . . , N regained. Then the symbols are converted into parallel bits in a symbol conversion unit  35 , and a parallel-serial converter  36  converts the groups of parallel bits into a digital serial signal DSR. 
         [0029]    The time jitter of the received subcarrier multiplex signal SMSR is compensated by a controlled jitter compensating unit  32 , which is a controllable delay. 
         [0030]    Details of the compensation method will be explained regarding a special subcarrier system. As an example an OFDM (orthogonal frequency diversity multiplex) system is illustrated in  FIG. 3 . In a transmitter  1 OT the subcarrier modulation unit  12  is replaced by an OFDM modulation unit  12 F, and in a receiver  3 OT the separation unit  33  and the demodulation unit  34  is replaced by an OFDM demodulator  33 F_ 34 F. The elements for compensation are explained later. 
         [0031]    Essential parts of an OFDM system are illustrated in  FIG. 4 , which is only used for explanation of the OFDM part. The generation of symbols and the optical modulation/demodulation are not shown. An OFDM transmitter  12 F_ 13 F receives the bits of the signal DST already converted into symbols ST S  as described before. In the shown analogue system the symbols ST 1 -ST N  are modulated by modulators MO 1 -MO N  onto the subcarriers SC S =exp(j2πf s t), s=1, 2, . . . , N. The generated subcarrier symbols are combined to the subcarrier multiplex signal SMST and transmitted in the transmission channel CH 2 /transmission network  2 , where they are impaired by different disturbing linear and nonlinear effects, indicated by H(f, P), wherein H(f)=frequency depending effects and H(P)=power depending effects causing a time/phase jitter of the transmitted STS symbols ST s . 
         [0032]    In a digital system a group of symbols ST 1 , ST 2 , . . . , ST N  is converted parallel into subcarrier symbols SST S =SST 1 , SST 2 , . . . , SST N  by an Inverse Fourier Transformation FFT −1 . 
         [0033]    In the analogue OFDM system shown in  FIG. 4  a received subcarrier multiplex signal SMSR is fed to a OFDM demodulator  33 F_ 34 F, where it is split in a splitter  33 F and fed, together with associated subcarrier frequencies −SC S =exp(−j2πf s t), to correlators CO 1 -CO N . Because all the subcarrier signals with one exception are orthogonal to the subcarrier supplied to a correlator, which outputs only one sequence of symbols SR S  having the same carrier frequency. 
         [0034]    In a digital OFDM system the received subcarrier multiplex signal SMSR is demodulated by applying Fast Fourier Transformation to regain the symbols SR S =SR 1 , SR 2 , . . . , SR N . 
         [0035]    The time/phase corrections is applied by
       Correction of the time jitter of the complete OFDMR signal by applying a time correction signal TC to a common jitter correction unit  32  or   Applying individual time correction signals TC S  to jitter correction elements  32   S  inserted in the signal paths of the correlators CO S .       
 
         [0038]    Of course, in modern technology the modulation and demodulation is processed by digital computers. Delay elements are controlled storage units and corrections are performed by changing digital values. Therefore the elements shown in the figures have to be understood as functional units. 
         [0039]    To compensate the time/phase jitter a control signal has to be generated at the receiver. Therefore a phase comparison signal CST is generated by a subcarrier generation unit  15  ( FIG. 3 ) at the transmitter and transmitted parallel with the symbols as part of the SMST to the receiver. 
         [0040]    A possibility is to generate an unmodulated subcarrier signal as phase comparison signal CST ( FIG. 2 , which can be processed like the other subcarrier signals). Referring to  FIG. 3  and  FIG. 4  an orthogonal phase comparison signal CST is generated with the subcarriers SC S  and transmitted together with the subcarrier symbols SST S  ( FIG. 3 ,  FIG. 4 ). All received signals, the parallel received subcarrier symbols SSR S  and the received phase comparison signal CSR forming the subcarrier multiplex signal SMSR have approximately the same impairments: The same time distortion. 
         [0041]    The receiver of  FIG. 3  shows a delay element  39  and a time jitter correction unit  32  connected in series between the carrier demodulation unit  31  (photo diode) and the OFDM demodulator  33 F_ 34 F. 
         [0042]    A received phase comparison signal CSR has to be regained for compensation the impairments. The CSR can be regained by FFT (Fast Fourier Transformation) in the FFT control processor  34 C shown in  FIG. 3 . Therefore in a first embodiment a FFT control processor  34 C is also connected to the carrier demodulation unit  31 . The regained phase comparison signal CSR is compared (correlated) with an internal stable phase reference signal PS REF . The output signal represents the average phase difference (Φ REF −Φ C ) between the two signals for each modulation section (symbol duration). A jitter control circuit  38 T connected in series with the FFT control processor  34 C calculates and outputs a time jitter control signal TC, which controls a time jitter correction unit  32 , which is inserted in the main signal path in series with a delay element  39 . The delay of the delay correction unit  32  is varied according to the value of the jitter control signal TC: 
         [0000]        TC= (Φ REF −Φ C )/ω REF    (1); wherein
 
         [0000]    ω REF —angular frequency of phase reference signal. 
         [0043]    The correction can be executed during the duration of a modulation section changing the delay continuously or in the middle of the modulation section. Special processing like direct comparing the phase comparison signal CSR and the phase reference signal PS REF  leads to a more actual control signal. A jitter compensated OFDM signal SMSC comprising all compensated subcarrier symbols SSR S  is then demodulated in the OFDM-demodulator  33 F_ 34 F. The delay element  39  with a time delay of about a modulation section (symbol) is in most embodiments necessary, because the jitter correction value TC has to be determined first. 
         [0044]    Because the impairment is slightly different to all subcarrier signals enhanced jitter correction values can be calculated introducing a jitter correction factor KT S : 
         [0000]        TCE   S   =KT   S (Φ REF −Φ C )/ω REF   +Δt   s    (2)
 
         [0045]    A constant delay time Δt S  can also be added to compensate the different transit time of the subcarrier signals. For the individual jitter compensation is executing by varying the delay time of the elements  32   S  in  FIG. 4 . 
         [0046]    An equivalent correction method is a corresponding time shift of all subcarrier signals SC S =exp(−j2πf s t) in the receiver. 
         [0047]      FIG. 5  shows a further embodiment of a receiver  3 OP. In a digital embodiment the actual phase values of the regained symbols SD S  are corrected. The correction of the phase or the time jitter are equivalent methods, but one skilled in the art may chose one method according to the chosen design or expected improvement. 
         [0048]    The phase distortion of the phase comparison signal and the symbols in this receiver are output by the same OFDM demodulator  33 F_ 34 F. Then phase correction values ΦC S =ΦC 1 −ΦC S  for all demodulated symbols SD S  are calculated in a phase control unit  38 P according to 
         [0000]      Φ C   S =(Φ REF −Φ C )ω S /ω REF )   (3)
 
         [0000]    wherein 
         [0049]    ΦC S =phase correction value of the carrier/symbols of subcarrier signal s=1, 2, . . . N, 
         [0050]    Φ REF =phase of a reference signal, 
         [0051]    Φ C =phase of the phase monitoring signal, 
         [0052]    ω S =2πf S  angular frequency of a subcarrier, 
         [0053]    ω REF =2πf REF  angular frequency of the phase reference signal. 
         [0054]    The phase correction is executed in a symbol correction unit  40  individually for all regained symbols SD S  (these are the uncorrected demodulated symbols) by correcting their phase values. This is easier than a correction of real and imaginary values of the symbols. If necessary according to the modulation mode, also the amplitude values of the symbols can be corrected according the amplitude variation of the received phase monitoring signal. 
         [0055]    In this embodiment the phase control unit  38 P is receiving further delay phase correction value ΔΦ S  e.g. from a forward error correction unit  37  to minimize the error rate. These delay correction values are added to the calculated phase correction values 
         [0000]      Φ CE   S   =KP   S (Φ REF −Φ C )(ω S /ω REF )+ΔΦ S    (4)
 
         [0056]    Delay differences of the subcarriers can be compensated by the delay correction. 
         [0057]    Because the impairment is slightly different to all subcarrier signals/symbols a correction factor K S  can be added to derive the enhanced correction values ΦCE S . The phase correction factors KP S  can be stored in an address table, derived considering the output of the FEC decoder, or calculated from a few measured values using a polynomial 
         [0000]        KP   S   =A+B (ω S /ω REF )+ C (ω S /ω REF ) 2    (5)
 
         [0000]    which leads to 
         [0000]      Φ CE   S =(( A+B  (ω s /ω REF )+ C (ω S /ω REF ) 2 )(Φ REF −Φ C )+ΔΦ S    (6),
 
         [0000]    wherein A, B, C=constant factors. 
         [0058]    The amplitudes of the received symbols can be corrected in a similar way. A long term amplitude average value A LT  of the received phase comparison signal (or of each subcarrier signal) is compared with the amplitude of the actual modulation section A AC . The amplitudes of all regained symbols are corrected according an amplitude correction value 
         [0000]        AC=A   LT   /A   AC    (7).
 
         [0059]    FEC (forward error correction) is used to output an error free corrected signal DSRC. The correction can be applied to the parallel or serial bits. The number of corrections EC is used to optimize the time correction value/factor in the embodiment  FIG. 4  or the phase correction value/factor in the embodiment  FIG. 5  individual for each subcarrier symbol stream. An optimum is reached when an associated number of executed corrections is a minimum. 
       REFERENCE SIGNS 
       [0060]    DST input serial data signal 
         [0061]    DSR output serial data signal 
         [0062]    DSRC DSR FEC corrected serial data signal 
         [0063]    SMST (electrical) subcarrier multiplex signal at the transmitter (TX) 
         [0064]    OSMT transmitted optical subcarrier multiplex signal at TX 
         [0065]    OSMR received optical subcarrier multiplex signal 
         [0066]    SMSR (electrical) SMS at the receiver 
         [0067]    ST S  symbols at TX 
         [0068]    SST S  subcarrier symbol at TX 
         [0069]    SSR S  received subcarrier symbol 
         [0070]    SR S  transmitted/output/corrected symbol 
         [0071]    SD 1-N  detected symbol 
         [0072]    CST transmitted phase comparison signal 
         [0073]    CSR received phase comparison signal 
         [0074]    PS REF  phase reference signal 
         [0075]      1  transmitter 
         [0076]      1 OF OFDM transmitter 
         [0077]      2  transmission network (fiber) 
         [0078]      3  receiver 
         [0079]      3 OT OFDM receiver with jitter correction 
         [0080]      3 OP OFDM receiver with phase correction 
         [0081]      10  serial-parallel converter 
         [0082]      11  symbol generation unit 
         [0083]      12  subcarrier modulation unit 
         [0084]    FFT Fast Fourier Transformation 
         [0085]    OFDM Orthogonal Frequency Division Multiplexing 
         [0086]    OFDMS OFDM signal 
         [0087]      12 F FFT −1  processor unit 
         [0088]      12 F_ 13 F OFDM modulator 
         [0089]      13  combiner 
         [0090]      14  optical modulation unit 
         [0091]      15  subcarrier generation unit 
         [0092]    CH 1  first channel 
         [0093]      3 OT OFDM receiver for time jitter correction 
         [0094]      3 OP OFDM receiver for phase jitter correction 
         [0095]      31  carrier demodulation unit 
         [0096]      32  jitter correction unit 
         [0097]      32   S  jitter correction element 
         [0098]      33  separation unit 
         [0099]      33 F splitter 
         [0100]      33 F_ 34 F OFDM demodulator 
         [0101]      34  subcarrier demodulation unit 
         [0102]      34 C FFT control processor 
         [0103]      34 F FFT processor 
         [0104]      35  correction unit 
         [0105]      35   S  s th  correction circuit 
         [0106]      35  symbol conversion unit 
         [0107]      36  parallel-serial converter 
         [0108]      37  FEC decoder unit 
         [0109]      38 T jitter control unit 
         [0110]      38 P phase control unit 
         [0111]      39  delay element 
         [0112]      40  symbol correction unit 
         [0113]      41  FEC decoder 
         [0114]    SC S  subcarrier signal 
         [0115]    CS phase comparison signal 
         [0116]    TC jitter correction signal/value 
         [0117]    TCE S  enhanced individual jitter correction value 
         [0118]    ΦC S  phase correction value 
         [0119]    ΦCE S  enhanced phase correction value 
         [0120]    Φ REF  phase of the reference signal, 
         [0121]    Φ C  phase of the phase comparison signal, 
         [0122]    ΔΦ S  phase delay correction value, 
         [0123]    Δt S  time delay correction value 
         [0124]    K S  correction factor 
         [0125]    ω S  2πf S  angular frequency of a subcarrier, 
         [0126]    A, B, C factor, 
         [0127]    ω REF  2πf REF  angular frequency of the phase reference signal, 
         [0128]    A AC  actual amplitude, 
         [0129]    A LT  long time average amplitude, 
         [0130]    AC=A LT /A AC  amplitude correction factor, 
         [0131]    DSRC FEC corrected data signal, 
         [0132]    EC error correction.