Abstract:
The invention describes method and an arrangement for transmitting an orthogonal frequency diversity multiplex signal via an optical filter. OFDM channels located near an edge of an OFDM spectrum are copied and shifted to an opposite edge of the OFDM spectrum and transmitted via the optical filter. At the receiver symbols are derived from original and the copied OFDM channels. Then the symbols having a better signal quality are elected for further processing.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention refers to a method and an arrangement for transmitting an orthogonal frequency diversity multiplex signal via at least one filter. 
       BACKGROUND OF THE INVENTION 
       [0002]    Orthogonal frequency diversity multiplex (OFDM) is a promising modulation technique well known from wireless and wired communication systems. A large number of closely-spaced orthogonal subcarriers carry the data information. 
         [0003]    Since a few years OFDM has been proposed for fiber-optic communication systems and has found many potential applications varying from the access to long-haul networks. OFDM offers many advantages that make it interesting for the use of fiber-optic applications such as negligible linear crosstalk, scalability to higher order modulation formats, etc. Because of the small and well defined spectrum of the OFDM signal, it has a high tolerance with respect to narrowband optical filtering. However, one of the main disadvantages of OFDM is that an optical bandwidth filter must be centered precisely around the complete OFDM signal as the tolerance with respect to filter offset is very low. 
         [0004]    The problem of a frequency offset of optical filters is illustrated in  FIG. 1  showing the original OFDM spectrum (black) and the attenuated spectrum (white). In this figure it can be seen that an offset of an optical filter directly leads to attenuation of the subcarriers located near the edge of the OFDM spectrum. As a result, the signal noise ratio (SNR) of these subcarriers is deteriorated and the overall bit error rate (BER) is steeply increased. 
         [0005]    Today, optical OFDM has not been commercialized. However in the experimental investigations reported with optical OFDM so far, the center wavelength of the OFDM signal is tuned precisely to the filter shape of optical filters that are used in the transmission line. For proof-of-principle experiments this is a valid method, however, in commercial systems this would imply that expensive lasers are required with precise locking over their lifetime. In addition, the OFDM systems require stringent specifications with respect to their bandwidth and alignment to the ITU (International Telecommunication Union) grid. 
       PRIOR ART 
       [0006]    To improve the signal quality different kinds of diversity are suggested for OFDM communication systems. The US Patent Application 2006/0193268 A1 mentions in the “Abstract” the different possibilities of diversity transmission. 
         [0007]    In the German Patent Application 2314630 Erich Burger discloses a method for an optimized evaluation of two received diversity signals. According to the signal quality the signals are added with equal or different amplitudes, or the better signal is selected. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide a method and an arrangement to cope with the instability of optical bandwidth filters and to mitigate filtering penalties. 
         [0009]    The inventive idea to mitigate the influence of a spectral drift of an optical filter is
       copying optical channels of an OFDM signal located near an edge of an OFDM spectrum to obtain copied optical channels having shifted carrier frequencies adjacent to an opposite edge of an OFDM spectrum,   transmitting an optical OFDM signal comprising in addition these copied optical channels carrying duplicated symbols,   receiving and demodulating an OFDM transmission signal,   evaluating the quality of regained symbols of the copied optical channels and regained symbols of the allocated original optical channels, and   selecting those symbols having a better signal quality or combining allocated original an copied symbols to obtain optimized symbols.       
 
         [0015]    For a not predicable filter drift is advantageous
       copying optical channels near both edges of the OFDM spectrum to gain copied optical channels having shifted carrier frequencies adjacent to opposite edges of the OFDM spectrum.       
 
         [0017]    Copying of the optical channels is preferable executed by modulating the symbols onto shifted baseband carriers with frequencies adjacent to an opposite edge of a OFDM baseband spectrum. 
         [0018]    At the receiver the symbols with better signal quality are selected by
       evaluating training symbols of the copied subcarrier signals and of the allocated original subcarrier signals to determine the signal quality.       
 
         [0020]    The realisation of the features above is done by corresponding means used in the shown embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Presently preferred examples of the invention are described below with reference to accompanying drawings, where 
           [0022]      FIG. 1  the characteristic of an optical bandwidth filter for OFDM signals, 
           [0023]      FIG. 2  shows an embodiment of an OFDM transmission system according to the invention, 
           [0024]      FIG. 3  and  FIG. 4  show diagrams illustrating the copying of subcarrier information, and 
           [0025]      FIG. 5  and  FIG. 6  show the extended OFDM spectra in relation with the optical bandwidth filter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 2  illustrates a simplified block diagram of an OFDM transmission system. Only the functional units relating to the invention are shown. The system may be adapted for polarisation multiplex signals as well as for different kinds of coding and modulation. 
         [0027]    First, the general operation of an OFDM system, even though known to those skilled in the art, may by explained shortly. A data signal DS is received at the transmitter input  1  and converted in a serial-parallel-converter  2  into a sequence of parallel data words, each comprising P 1 -Pm bits. Each data word P 1 -Pm is converted (coded) into a group of symbols S 1 -Sn (e.g. QAM quaternary amplitude modulation may be used). Orthogonal baseband subcarriers are then modulated by n sequences of these symbols. Today, this feature is carried out by a digital IFFT (Inverse Fast Fourier Transformation) processing unit  4 . The obtained subcarrier signals B 1 -Bn are then converted (added) in a parallel-serial-converter  5  into an OFDM baseband signal BMS, which in the shown embodiment comprises a real component MSI and an imaginary component MSQ, both modulating an optical carrier in a modulation unit  6 . The n subcarrier signals B 1 -Bn, also denoted as baseband channels, are converted into n optical signals referred to as optical channels CH 1 -CHn ( FIG. 3 ,  FIG. 4 ). The generated optical OFDM signal OTS is transmitted via an optical fiber  18  to a receiver. An optical filter  7 , this expression includes any band limiting element, is inserted between transmitter and receiver and/or a second filter  10  may be inserted at the transmitter/receiver. 
         [0028]    A band limited OFDM transmission signal ORS is received at input  9  of a receiver  11 . The transmission signal is coherent demodulated (converted into an electrical signal) and sampled. The regained OFDM baseband signal BMS is split into a plurality of equal parallel signals by a second serial-parallel-converter  12  and a FFT (Fast Fourier Transformation) is applied to these signals in the FFT-unit  13 , which outputs n sequences of symbols S 1 -Sn (the same reference signs are used for the signals in the OFDM transmitter and the OFDM receiver for reasons of clarity). Of course, the regained symbols S 1 -Sn may be impaired by different effects while being transmitted. The parallel symbols S 1 -Sn are estimated in a decoder (symbol estimation unit)  15  and converted into parallel data words P 1 -Pm, then multiplexed by the second parallel-serial-converter  16  into the data signal DS and output at the receiver output  17 . 
         [0029]    As stated above, the OFDM transmission signal OTS may be impaired. The invention refers to impairments by the optical filter  7  or other bandwidth limiting effects. According to  FIG. 3 , optical channels CG 1  and CGp (CG 1 , CGp—representing a group of e.g. 1—ca. 10 channels) located near the edges of the optical OFDM spectrum are “copied” to optical channels CC 1 , CCp adjacent to opposite edges of the optical OFDM spectrum. In other words, the copied channels are diversity channels, which shifted carrier frequencies are adjacent to the original OFDM bandwidth. 
         [0030]    If the filter pass-band varies to lower frequencies—solid line in FIG.  5 —the original channels with higher frequencies CHp and the channels CC 1  “copied” to higher frequencies are impaired. But the original channels CG 1  and the copied channels CCp at the other filter edge are not impaired. These “channels” are selected instead of the impaired channels CHp, CC 1 ; or more exact, the symbols transmitted via these undisturbed optical channels are selected by the OFDM receiver. If the filter pass-band drifts in the other direction the copied channels CHp, CC 1  are selected instead of the channels CCp, CH 1 . 
         [0031]    Usually certain filters drift in the same direction. If the filter drift is known, it is sufficient to copy optical channels CHq from the insecure filter edge to channels CCq located at the opposite edge of the OFDM spectrum as shown in  FIG. 4 . If the optical channels (subcarriers) are shifted (or the filter pass-band is shifted from “a” to “b”) pass-band drifts in both directions are also correctable. 
         [0032]      FIG. 6  shows that the optical channels CHq are seriously impaired by the filter drift while the copied channels CCq are undisturbed. 
         [0033]    The “copying” of the optical channels is preferable done in the OFDM baseband while generating subcarrier signals B 1 -Bn. 
         [0034]    A preferable embodiment for “copying” the optical channels is shown in  FIG. 2 . The symbols S 1 , S 2  (allocated to subcarrier signals B 1  and B 2 ) are duplicated and the duplicated symbols SC 1 -SC 2  are modulated onto lower (or higher) subcarriers generating the copied subcarrier signals BC 1  and BC 2 . The “copied” subcarrier signals BC 1  and BC 2  are converted into “copied” optical signals referred to as “copied channels”. Regarding  FIG. 4  and  FIG. 6  the “original subcarrier signals” B 1  and B 2  correspond to the original CHq channel group and the “copied subcarrier signals” BC 1 , BC 2  correspond to the copied channels CCq. 
         [0035]    In a transmission system according to the invention, the bandwidth of each optical filter  7 ,  10  remains the same, the bandwidth of the transmission signal has the same amount, but the required bandwidth range is enhanced according to the possible filter drift. 
         [0036]    At the OFDM receiver the copied symbols SC 1 , SC 2  are derived from copied subcarrier signals (BC 2 , BC 2 ). The signal quality of the recovered original symbols S 1 , S 2  and allocated copied symbols SC 1 , SC 2  carrying the same information is evaluated by an evaluation unit  14 . The symbols with the better signal quality are selected, and these elected symbols SE 1 , SE 2  are fed to the decoder  15 . The amplitudes of the symbols or the subcarrier signals respectively are in most cases sufficient as quality criterions. More sophisticated criteria e.g. OSNR (optical signal noise ratio), error rate if FEC (forward error correction) is applied, or a quality factor may be used. Selected is in a first embodiment of the estimation unit  14  the subcarrier signal (baseband channel) with the better signal quality, but symbol by symbol selection may be also applied. 
         [0037]    In another embodiment, the values of the allocated symbol S 1 , SC 1  and S 2 , SC 2  may be averaged. This is advantageous when original and copied channels are impaired. In a more advanced embodiment, optimized selected symbol values SE 1 , SE 2  may be calculated considering quality (dependent) factors Q 1 , Q 2 , e.g. according to SE=(Q 1 ·S 1 +Q 2 ·SC 1 )/(Q 1 +Q 2 ) (S 1 , SC 2 —symbols with equal amplitudes, Q 1 , Q 2 = 0 - 1 ). The best function may be achieved by experiment. The selected or calculated symbols our output by the estimation unit  14  and converted into data bits. It is also advantageous to use time multiplexed trainings symbols to determine the signal quality of the symbol sequences (baseband channels). 
         [0038]    The invention may be used prophylactical even if impairments by a filter are not expected in the near future. The present invention is not limited to the details of the above described principles. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. 
       REFERENCE SIGNS 
       [0039]      1  transmitter input 
         [0040]      2  serial-parallel-converter 
         [0041]      3  coder 
         [0042]      4  IFFT unit 
         [0043]      5  parallel-serial-converter 
         [0044]      6  modulation unit 
         [0045]      7  transmitter output 
         [0046]      8  first optical filter 
         [0047]      9  receiver input 
         [0048]      10  optional optical filter 
         [0049]      11  optical receiver (demodulation/sample unit) 
         [0050]      12  second serial-parallel-converter 
         [0051]      13  FFT unit 
         [0052]      14  evaluation unit 
         [0053]      15  decoder 
         [0054]      16  second parallel-serial-converter 
         [0055]      17  receiver output 
         [0056]      18  optical fiber 
         [0057]    DS data signal 
         [0058]    P 1 -Pm data word(s) 
         [0059]    S 1 -Sn parallel symbols 
         [0060]    SE 1 , SE 2  selected symbols 
         [0061]    B 1 -Bn, subcarrier signals (baseband channels) 
         [0062]    BC 1 , BC 2  copied subcarrier signals 
         [0063]    BMS OFDM baseband signal 
         [0064]    OTS optical OFDM (transmission) signal 
         [0065]    ORS band-limited OFDM (transmission) signal 
         [0066]    CH 1  1st channel group 
         [0067]    CHp pth channel group 
         [0068]    CHq qth channel group 
         [0069]    CC 1  copied first channel group 
         [0070]    CCp copied pth channel group 
         [0071]    CCq copied qth channel group