Abstract:
The invention relates to a bandpass filter (OFI) which is mounted downstream of an optical amplifier (OV) and allows noise to be largely reduced. In order for said bandpass filter to be able to optimally receive burst signals (BS 1,  BS 2,  . . . , BSN) transmitted by several user devices (ONT 1,  ONT 2,  . . . , ONTN) also in a central node (OLT), the bandpass filter is set to the respective received carrier frequencies (TF 1 to TFN). Because of time constraints, this is possible only if the carrier frequencies (TF 1,  . . . ) or associated filter setting values (FE 1,  . . . ) have already been stored and the bandpass filter is preset.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application is a national stage application of PCT/EP2007/057113, filed Jul. 11, 2007, which claims the benefit of priority to German Application No. 10 2006 035 097.9, filed Jul. 28, 2006, the contents of which hereby incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The invention relates to a method and to arrangement for improving the signal quality of received optical burst signals. 
       BACKGROUND OF THE INVENTION 
       [0003]    In point-to-multipoint networks—such as the passive optical network (PON)—optical burst, signals (databursts) are transmitted in time-division multiplex method from user devices (ONTs) to a central node (OLT). In this context, the carrier frequencies can be slightly different from burst signal to burst signal especially when inexpensive lasers are used. Between the user devices and the receiving device of the central node or, respectively, in the receiving device of the central node, at least one optical amplifier is arranged which adds optical noise, the so-called “amplified spontaneous emission”. To obtain from the demodulator a data signal which is as free of interference as possible, it is necessary to filter the received optical signal with as narrow a bandwidth as possible in order to reduce the optical noise as far as possible. 
         [0004]    In patent application GB 2315 939 A, a fiber amplifier is described which is followed by a tracking filter for reducing the noise. The amplifier described is suitable for use in point-to-point connections. In point-to-multipoint connections, however, this would assume the presence of precisely identical carrier frequencies for all transmitting devices of the user devices. This again necessitates frequency-stabilized or frequency-controlled lasers which, however, cost considerably more than normal lasers. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention, in point-to-multipoint systems, discloses a cost-effective method for improving the signal quality and arrangements suitable for carrying out the method. 
         [0006]    The method according to one embodiment of the invention uses a tunable bandpass filter and, in particular, has the advantages that neither a stabilization of the lasers of the user devices is required nor a service channel from the central node to the user devices (ONTs) is required for tuning the frequencies of their transmit lasers to the passband of the optical bandpass filter. The essential factor is a presetting of the bandpass filter in order to adapt it in time to the characteristics of the burst signals in the shortest time. 
         [0007]    A further advantage is given by the fact that any user devices can be connected since these do not have to have any additional devices for frequency control. 
         [0008]    To optimize also the demodulation and the subsequent decision for a binary value, the controller can control the optical amplifier or an electrical amplifier following the demodulator in order to equalize the different amplitudes of the burst signals before the decider stage. 
         [0009]    By connecting two (or more) bandpass filters in parallel, the problem of an adjusting time which is too long can be avoided. 
         [0010]    Whilst one bandpass filter is active, the other bandpass filter can be adjusted to the carrier frequency of the subsequent burst. 
         [0011]    A further embodiment uses a filter bank with several parallel-connected optical bandpass filters, the center frequencies of which deviate from one another. For each received burst signal, the optimum filter is selected by a controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The invention will now be described in greater detail with reference to exemplary embodiments, in which: 
           [0013]      FIG. 1  shows a basic circuit diagram of a passive optical network. 
           [0014]      FIG. 2  shows an upstream frame for burst signals. 
           [0015]      FIG. 3  shows a basic circuit diagram of an optical receiver. 
           [0016]      FIG. 4  shows a variant of the filter arrangement. 
           [0017]      FIG. 5  shows a central node (OLT). 
           [0018]      FIG. 6  shows a variant of the central node with filter banks. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  shows the basic circuit diagram of a central node OLT with transmitting device TX and receiving device RX, which node is connected to several user devices ONT 1 -ONTN via an optical fiber OF and a splitter SP. The user devices send burst signals BS 1 -BSN in time-division multiplex mode, here called burst time-division multiplex signal BNS, within an upstream pulse frame FRU shown in  FIG. 2 . A longer pause can occur after the last burst signal. 
         [0020]    In the user devices, the optical carrier frequencies of the burst signals are generated by. inexpensive lasers so that the carrier frequencies can deviate from one another considerably. In the receiving device RX of the central node OLT, the incoming burst signals are first amplified in an optical amplifier, not shown in  FIG. 1 . However, the optical amplifier or a further (bidirectional) amplifier BOV (drawn dashed in  FIG. 1 ) can also be arranged between splitter and central node. However, each optical receiving amplifier adds further optical noise, called ASE (amplified spontaneous emission), which further degrades the existing signal/noise ratio. This effect is therefore reduced by an optical bandpass filter connected downstream of the amplifier. The bandwidth of this filter is selected to be as narrow as possible which is why the center frequency or the passband must be tuned to the carrier frequencies of the different burst signals. Due to the accurate tuning of the center frequency, one-sided distortions of the optical burst signals, and thus of the demodulated binary signals, are avoided, at the same time. Although each burst signal contains a short preamble PR at the beginning of the burst, a sufficiently accurate frequency measurement and tuning of the bandpass filter is not possible during the short period of the preamble. According to the invention, this problem is solved by a controlled presetting (pretuning) of the bandpass filter. 
         [0021]      FIG. 3  shows a basic circuit diagram of the receiving device RX. The burst time-division multiplex signal BMS received via the optical fiber OF is supplied to an optical amplifier OV which amplifies it and adds ASE. The amplified signal is then supplied via an adjustable optical bandpass filter OFI to a demodulation device DEE. This contains a demodulator DEM which can be implemented by a photodiode in the case of amplitude modulation. The demodulator is followed by the series circuit of an electrical amplifier EV and a decider stage DEC which converts the demodulated and amplified signal into a binary signal DS and delivers it at a data output DA. 
         [0022]    Firstly, it is required to measure the carrier frequencies or to determine the filter adjustment values, respectively, for each burst signal. From the received burst time-division multiplex signal BMS, a measurement signal MS is branched off by a splitter SPM—here arranged following the optical amplifier OV—and supplied to a measuring device ME which measures the carrier frequencies TF 1 -TFN of the burst signals BS 1 -BSN and forwards the results to a controller CON (which can be part of an OLT system controller) which allocates, and stores, the measurement results to the burst signals directly or converted into filter adjustment values. The allocation to the burst signals can be made with the aid of the preambles PRl to PRN of the burst signals. During the connection setup, called “ranging”, since the central node knows in any case which ONT has been addressed by it, it can evaluate the preambles as control function. If the carrier frequencies are used as filter adjustment values, it must still be converted by means of stored tables or functions into (electrical) adjustment values which directly determine the filter characteristics. These electrical filter adjustment values can also be stored instead of the carrier frequencies and then used directly for presetting the bandpass filter. Measuring the carrier frequencies or determining the filter adjustment values is advantageously carried out during a turn-on phase (ranging) since there is adequate time available here for measurements. During the operation, measurements can then be made additionally, and, as a rule, slight corrections of the stored filter adjustment values or carrier frequencies. 
         [0023]    Instead of using frequency measurement, the filter adjustment values can be determined directly by tuning the bandpass filter to optimum signal quality—to the highest receive level in the simplest case. This can be done, for example, by using the look-in principle. 
         [0024]    During the operation, the controller CON of the central node OLT receives from the output of the decider stage DEC the preamble PR 1 -PRN, preceding each burst signal BS 1 -BSN (or the receive timeslot allocated to each received burst signal is known to the system controller of the central node in any case), and, after the end of a burst, sets the center frequency of the optical bandpass filter OFI in accordance with the expected carrier frequency for the following burst signal as early as possible. This can be done as early as at the end of the preceding burst signal. The ASE is reduced by a maximum by the optimally adapted bandwidth and the precisely set center frequency provides for optimum demodulation. 
         [0025]    If signals having different bandwidths are sent, the bandwidth of the bandpass filter can also be adapted by presetting. 
         [0026]    In order to achieve optimum sampling of the demodulated electrical burst signals by the decider stage DEC in the case of amplitude modulation (on/off), either its input level can be kept constant or its threshold can be controlled. The amplitude can be controlled by either controlling the levels of the optical burst signals or the levels of the electrical signals supplied to the decider stage. A fast level control of the received optical signal can be carried out by a Raman or semiconductor amplifier. The measured level values or corresponding level adjustment values for the amplification are also stored in the controller CON and the optical amplifier is correspondingly preset. The electrical amplifier EV can be preset in the same manner so that the amplitude values of the input signal of the decider stage DEC remain constant. As an alternative, the threshold of the decider stage can be preset differently from burst signal to burst signal in order to always optimally sample the input signal. 
         [0027]    Since it is not yet possible at present to reset optical filters in the required very short time in the case of high PON data rates, the time problem can be solved by replacing the optical bandpass filter with two bandpass filters OFI 1  and OFI 2 , connected in parallel at the input end, as shown in  FIG. 4 . Whilst the burst signal BS 1  is output by the first optical bandpass filter OFI 1 , the second bandpass filter OFI 2  is already tuned etc. to the carrier frequency TS 2  of the second burst signal BS 2  so that more than the entire period of the burst signal BS 1  is available for the tuning. Naturally, this method can be expanded to more than two bandpass filters. 
         [0028]      FIG. 5  shows the basic configuration of a central node OLT with the receiving device RX according to the invention. In the downstream direction, a downstream time-division multiplex signal DMS is transmitted by the transmitting device TX by a multiplexer MUX and a bidirectional optical amplifier BOV via the optical fiber OF to the user devices ONT 1 -ONTN. In the opposite direction, the burst time-division multiplex signal BMS is received and supplied via the multiplexer MUX to the receiving device RX known from  FIG. 3 . The optical receiving amplifier thereof is now part of the bidirectional optical amplifier BOV. 
         [0029]      FIG. 6  shows a further central node OLT 2  with a variant of the receiving device RXV. The received burst time-division multiplex signal BMS is divided by a splitter SP 2  to different signal branches into which optical filters OF 1  to OFN are inserted with mutually differing center frequencies and overlapping passbands. Equivalent thereto is a wavelength-division demultiplexer in which the passbands of the channels also overlap. The outputs of all optical bandpass filters are in each case connected to a demodulation device, in this case photodiodes PD 1  to PDN, which deliver binary electrical signals of different quality in dependence on the correspondence of passband and carrier frequency. The quality ((pseudo) error rate, OSNR, eye opening) of the binary signals is measured in the measuring device MQ, the measurement of the level is adequate, as a rule, and is evaluated by the controller CON. The optimum binary signals are in each case stored, allocated to a burst signal, as filter selection values AF 1 , . . . , AFN. In the operating case, the qualitatively best binary signal is preselected for each burst signal and switched through via a switch SW (multiplexer) for further processing to a demodulation device DEE which delivers the optimum binary signal at the data output DA. 
       LIST OF REFERENCE DESIGNATIONS 
       [0000]    
       
         OLT Central node (Optical Line Terminal) 
         OF Optical fiber 
         OV Optical amplifier 
         SP Splitter 
         ONT 1 , User device (Optical Network Terminal) 
         SP 1  Splitter 
         OFI Adjustable optical filter 
         OFI 1 , OFI 2  Adjustable first/second optical filter 
         DEM Demodulator/optoelectrical transducer 
         EV Electrical amplifier 
         DEC Decider stage 
         CON Controller 
         ME Measuring device 
         DMS Downstream time-division multiplex signal 
         BMS (Upstream) burst time-division multiplex signal 
         TF 1  Carrier frequency 
         DEE Demodulation unit 
         FRU Upstream pulse frame 
         FRA Frame start 
         PR 1 , PR 2  Preamble 
         RX Receiving device 
         RA Frame start identifier 
         ME Measuring device 
         MS Measurement signal 
         DS Binary signal 
         DA Data output 
         BS Burst signal 
         TX Transmitting device 
         BOV Bidirectional optical amplifier