METHOD AND DEVICE FOR SIGNAL REPETITION

Abstract: The invention provides a method and a device for, with a low bit error probability, repeating signals in an electro-optical network. The method comprises the following steps: receiving and converting of an optical signal to an electrical; inverting and/or application of a non-linearity on the electrical signal; converting of the electrical signal to an optical; and further transmitting of the optical signal.

Description: Method and device for signal repetition The present invention relates to a device and a method for telecommunications, especially for signal processing in optical or electro-optical networks. Technical Field of the invention In the field of telecommunications today there exist optical as well as electrical networks and combinations thereof. The purely optical networks have large transmission capacities for point to point transfers but with more complex network configurations the electrical networks can be superior in certain respects. The reason for this is that the complex networks require repeated operations, such as distribution of light, filtering, switching etc., and general operations where losses and thereby noise generating amplification are involved. A completely optical network is analogue which makes it difficult for it to cope with repeated operations such as distributing light, filtering, switching etc., and general operations where losses and thereby noise generating amplification are involved. Description of Related Art Because the completely optical networks have limitations with more complex network configurations, different types of electro optical network configurations exist, in which communications go over fibre optic connections but where the optical signals in certain places and for certain purposes are converted to electrical signals and treated as such, whereafter they are reconverted to optical signals. One such purpose is, for example, repeating of signals along the length of the connection. One factor which contributes to deciding a network's transmission capacity is the bit error probability. A number of different factors influence the probability for bit errors and among these can be named, for example, noise and dispersion. Dispersion means that different wavelengths propagate at different speeds through the fibres, and a pulse which is sent through the fibres therefore spreads out which gives rise to intersymbol interference. Intersymbol interference means that adjacent pulses go into each other and thereby give an increased risk for incorrect detecting of the pulses. The performance of a communication network can partly be shown in a so-called eye opening diagram. Such a diagram is obtained when a digital signal is connected to the vertical input of an oscilloscope and its time-base triggered in step with the symbols. Each symbol then sweeps over the earlier and they form an eye pattern. As a measure of the network's performance a so-called eye opening penalty, below called EOP, can be defined according to the following formula: EOP = 10 vl i O υ -log(A/B) [dB] where B is the eye's vertical opening, i.e. the distance from the zero-level to the one- level, for the ideal case where only the network's attenuation irifluences the signals, and A is the same distance for the case where the signal is also influenced by dispersion and the like. If the eye opening is reduced then the margin for permitted noise is reduced and thereby the margin for error decisions less. Consequently, the lower the EOP, the better performance the net has. It is further desirable that a communication network has a high transparency with respect to e.g. bit rate. This means that the communication functions for a large number of different bit rates, i.e. that the network's architecture does not place too narrow requirement for given bit rates. The same concerns transparency in another respect, e.g. with respect to the signal protocol used, i.e. which type of coding is used, for example. It is consequently also desirable that the network's architecture does not limit which type of signal protocol that can be used. Summary Accumulations of noise as well as intersymbol interference origmating from dispersion in the fibres increase the bit error probability which limits the network's transmission capacity. This gives problems with scaling in the more or less optical networks, i.e. the network cannot be built sufficiently large and complex and also problems with transparency with respect to, for example, bit rate. In non-linear circuits, noise can comprise a time jitter of the signal flanks (jitter) and signal level noise (amplitude noise). It is an object of the invention to provide a method and a device for reliable signal processing, especially repeating, and transmission of information within communication networks with the retention of a low bit error probability. It is also an object of the invention to solve the above problems concerning communication network's scalability, bit rate transparency, noise and intersymbol interference. A further object of the invention is to solve the above problems concerning scalability, transparency, noise and dispersion at bit rates up to and over 10 Gb/sec. The above mentioned objects are fulfilled with a method and a device which have the features stated in the independent claims. Further features and developments of the invention are given in the other claims. The invention achieves the above objects through using as repeaters simple analogue bit rate transparent OEO-circuits (opto-electric-optic) comprising an inverter, and through introducing an intentional non-linearity in preferably the electronics. According to the invention a method is produced comprising that a signal which is to be repeated, is inverted and/or given a non-linearity, and also a device comprising inverters and/or non-linearity units. With the method and the device according to the invention the following advantages are obtained: The bit error rate (below called BER) accumulates much more slowly through the nonlinear pulse forming if the amplitude noise is the predominant bit error source. This is the case for instance if the system band width is significantly greater than the maximum allowable bit rate, since increased band width reduces the jitter. The intersymbol interference also accumulates more slowly through non-linear pulse foraiing. This reduction of intersymbol interference increases, i.e. the performance of the network is improved further, through mverting of the signals as performed according to the invention. These effects lower the bit error probability for the transfer on the communication network. Furthermore, no clocking of the signals is required. This implies further advantages as the solution becomes cheaper to implement without clock. Furthermore the implementation is easier without a clock with higher bit rates in the transfer, and the network's transparency with respect to bit rate becomes higher. Further, with a method and a device according to the invention one can use both RZ- pulses (Return to Zero) and NRZ-pulses (Ηon-Return to Zero). The transparency of the network with respect to the modulation method thereby is increased. In relation to the completely optical case the advantage is also obtained that an electronic supervising signal is available which facilitates the supervision and error- finding of the communication. Brief description of the drawings In order to make the present invention easy to understandable and carry out, it will be described by means of illustrating examples and with reference to the appended drawings, in which similar elements have the same reference numerals and in which: Figure 1 shows a device according to a first embodiment of the invention. Figure 2 shows an example of a non-linearity function in a non-linearity unit according to the invention. Figure 3 shows noise accumulation expressed in BER (Bit Error Rate) as a function of the non-linearity if the amplitude noise is the predominant. Figure 4 shows a device accordmg to a second embodiment of the invention. Figure 5 shows intersymbol interference expressed in eye-opening-penalty (below called EOP) in dB, as a function of the fibre connection length in kilometres for different non-linearities in comparison with the completely optical case. Figure 6 shows a device according to a third embodiment of the invention. Detailed description of embodiments Fig. 1 shows a device 1 according to a first embodiment of the invention. The device 1 comprises an optical input 2 connected to an input 3 on an opto-electrical converter 4. An output 5 on the opto-electrical converter 4 is connected to an input 6 on a filter 7. An output 8 on the filter 7 is connected to an input 9 on a non-linearity unit 10. An output 11 on the non-linearity unit 10 is connected to an input 12 on an amplifier 13. An output 14 on the amplifier 13 is connected to an input 15 on an electro-optical converter 16. An output 17 on the electro-optical converter 16 is connected to an output 18 on the device 1. According to this embodiment the non-linearity unit 10 is placed between the filter 7 and the amplifier 13, but other embodiments are conceivable where the non-linearity 10 is placed in some other position along the chain in the device 1, in a manner well known in the technical field. Other embodiments can also be conceived where the constituent units are comprised in each other in different combinations in a manner well known to the skilled person. For example, the amplifier 13 and filter 7 can be comprised in the same unit. The amplification can also be divided up into several amplifiers, and the amplifier 13 or amplifiers can comprise automatic regulation of e.g. their amplification factor. The different units can also be cascade-connected in different order in a manner also well known to the skilled person. According to this embodiment the non-linearity lies in the electrical domain. In line with the idea of the invention one can, however, also conceive that e.g. the non- linearity is placed in the optical domain. For example, the electro-optical converter 16 can comprise the functions of the non-linearity unit 10. A further possibility is to move all of the functions of the invention to the optical domain. The opto-electrical converter 4 and the electro-optical converter 16 in this case are removed and the other constituent parts are optical. The device 1 can be used as a repeater in an optical communication network. In this case the input 2 and output 18 of the device 1 are connected to an optical fibre connection (not shown) along which the signal needs to be repeated because of the length of the connection. The optical signal received on the input 2 are converted to an electrical signal by the opto-electrical converter 4. The electrical signal is filtered by the filter 7. The filter 7 can be a pure low-pass filter but also other types of filters well known witfiin the technical field can be used. It is also possible to vary the relative position between components constituent in the device in a manner well known to the skilled person. The output signal from the output 8 of the filter 7 is fed to the non-linearity 10, where the signal is given an intentional non-linearity. The non-linearity unit 10 has a transfer function f (x) which gives an output signal which is a non-linear response to its input signal, which response can be, but does not need to be, non-binary. The signal is then amplified by the amplifier 13 and converted to an optical signal again by the electro- optical converter 16. The so obtained optical signal is retransmitted via the output 18 of the device 1 to the optical fibre connection (not shown). By means of the method according to this embodiment of the invention, a reducing effect of the bit error probability is obtained both through the noise suppression and the suppression of the intersymbol interference which the non-linear pulse forrning gives rise to on condition that the amplitude noise is the predominant noise. Fig. 2 shows the function in the non-linearity unit 10. x corresponds here to the signal in on the input 9 of the non-linearity 10, and f (x) corresponds to the signal on the output 11. A straight line in the diagram with a constant slope should correspond to the completely linear case, while a pure step function would illustrate the completely nonlinear case. Thus, the figure shows an example of a partially non-linear case. A non-linearity factor γ can be defined out of this according to: f f (x) = a tanh (b (x - 1/2)) 1/2 \ f (0) = 0 l f '(l/2) = l/γ γ = 0 corresponds to the completely non-linear case, and γ = 1,0 corresponds to the completely linear case. These equations define γ according to an embodiment of the non-linearity unit 10 according to the invention. A number of alternative definitions are obvious for the skilled person, where all of them to some extent are based on that γ consequently can be set to correspond to the part of space between logical "0" and logical " 1" which is occupied by the transition there between. Fig. 3 shows BER (Bit Error Rate) caused by amplitude noise accumulation for a link with approximately ten repeaters, as a function of the non-linearity which is applied to the signal through the non-linearity unit 10. The non-linearity which is applied to the signal according to the invention, gives a reducing effect on the bit error probability through noise suppression if the amplitude noise is the predominant source of bit error. The result is best with a complete non-linearity, i.e. γ = 0, but even an incomplete non- linearity gives a good result. This implies a large advantage in that a complete non- linearity can be difficult to realise at high bit rates. Through the invention it is possible therefore to use the noise-suppressing effect of the non-linearity without directly putting limits on the bit rate. The bit rate transparency of the network is consequently not limited either. With a method and a device according to the invention the bit error probability for the transmission on the communication network is lowered so that no clocking of the signals is required. This implies further advantages as the solution will be cheaper to implement without clocks. Furthermore, the implementation is more simple without clocks at higher bit rates in the transmission, and the transparency of the network with respect to bit rate becomes higher. Fig. 4 shows a device 41 according to a second embodiment of the invention. The device 41 comprises the same parts as the device 1, according to the first embodiment of the invention, connected in a similar way but with the difference that it also comprises an inverter 21. The inverter 21 is according to this embodiment connected between the filter 7 and the non-linearity unit 10, but other positions are also conceivable, in a manner well known to the skilled person. An input 20 on the inverter 21 is connected here to the output 8 of the filter 7, and an output 22 on the inverter 21 is connected to the input 9 of the non-linearity unit 10. For this second embodiment, variations can also be conceived, as mentioned in connection with the first embodiment. The function of the device 41 is similar to that of the device 1. In addition to the earlier mentioned noise suppression's favourable effect on the bit error probability, this is now further reduced through the inversion also suppressing the effect of the disperson of the fibre. This reduces in turn the intersymbol interference and thereby thus lowers the bit error probability further. Fig. 5 shows the intersymbol interference expressed in eye-opening-penalty (below called EOP) in dB, as a function of the fibre connection's length in kilometres, for different non-linearities of the invention according to the second embodiment. The best result is obtained at complete non-linearity but this can, as mentioned earlier, be difficult to achieve at high bit rates (over 10 Gb/sec). The diagram shows, however, that if the non-linearity factor γ<=0.5 then the EOP<=1.5 dB for communication connections with fibre lengths less than 1500 km with a distance between the repeaters of 30 km. If the non-linearity factor γ<=0.3 then the EOP<=1.0 dB for communication connections with fibre lengths less than 1500 km. Fig. 6 shows a device 61 according to a third embodiment of the invention. The device 61 comprises the same parts as the device 41 according to the second embodiment of the invention, combined in the similar way but with the difference that the non-linearity unit 10 is removed, and the output 22 of the inverter 21 is directly connected to the input 12 of the amplifier 13. For this third embodiment, variations can also be conceived, as mentioned in connection with the first embodiment.

Claims: Claims 1. Method for signal repetition in a network, comprising: - mverting and/or applying a non-linearity on a signal which is to be repeated. 2. Method according to claim 1, characterized in that the application of a non- linearity is performed with the help of a non-linear, non-binary transfer function which has a non-linearity factor <=0.5 (γ<=0.5). 3. Method according to claim 1, characterized in that the application of a non- linearity is performed with the help of a non-linear, non-binary transfer function which has a non-linearity factor <=0.3 (γ<=0.3). 4. Method according to any of the above claims, characterized in that the signals which are to be repeated are data bit streams with a bit rate >= 10 Gb/sec. 5. Method according to any of the above claims, characterized in that the system band width is significantly greater than the maximum allowable bit rate. 6. Method according to any of the above claims, characterized in that the repeated signals retain an eye opening penalty <=1.5 dB (EOP<=1.5 dB). 7. Method according to any of the above claims, characterized in that the repeated signals retain an eye opening penalty <=1.0 dB (EOP<=1.0 dB). 8. Method for reduction of the bit error probability in signal transmission comprising: - applying an incomplete non-linearity on a signal with the help of a non-linear, non- binary transfer function. 9. Method according to Claim 8, characterized in that the non-linearity function has a non-linearity factor which is less than or equal to 0.5 (γ<=0.5). 10. Method according to Claim 8, characterized in that the non-linearity function has a non-linearity factor which is less than or equal to 0.3 (γ<=0.3). 11. Method for signal repetition in an electro-electrical network comprising the following steps: - receiving and converting an optical signal to an electrical signal; - inverting and/or applying a non-linearity on the electrical signal; - converting the electrical signal to an optical signal; and - txansntitting further the optical signal, where the steps are not necessarily performed in the given order. 12. Method for signal repetition in an electro-optical network comprising the following steps: a - receiving and converting of an optical signal to an electrical signal; b - filtering of the electrical signal; c - inverting of the electrical signal; d - applying an intentional non-linearity on the signal with the help of a nonlinear transfer function; e - electrical amplification of the signal; f - converting of the electrical signal to an optical signal; and g - transπtitting further the optical signal, where the steps b - e are not necessarily performed in the given order. 13. Method for signal repetition in an electro-optical network comprising the following steps: a - receiving and converting of an optical signal to an electrical signal; b - filtering of the electrical signal; d - applying a non-linearity on the signal with the help of a non-linear transfer function; e - electrical amplification of the signal; f - converting of the electrical signal to an optical signal; and g - transntitting further the optical signal, where the steps b, d and e are not necessarily performed in the given order. 14. Repeater in a communication network, comprising: - at least one inverter (21) and/or at least one non-linearity unit (10). 15. Repeater according to Claim 14, characterized in that the non-linear function of the non-linearity unit (10) is non-binary and has a non-linearity factor which is less than or equal to 0.5 (γ<=0.5). 16. Repeater according to Claim 14, characterized in that the non-linear function of the non-linearity unit (10) is non-binary and has a non-linearity factor which is less than or equal to 0.3 (γ<=0.3). 17. Repeater according to any of Claims 14-16, characterized in that the repeated signals retain an eye opening penalty <=1.5 dB (EOP<=1.5 dB). 18. Repeater according to any of Claims 14-16, characterized in that the repeated signals retain an eye opening penalty <=1.0 dB (EOP<=1.0 dB). 19. Repeater in an opto-electrical communication net, comprising: - opto-electrical converter (4); - filter (7); - inverter (21); - non-linearity unit (10) with a non-linear transfer function; - amplifier (13); and - electro-optical converter (16); not necessarily in the given order. 20. Repeater in an opto-electrical communication network, comprising: - opto-electrical converter (4); - filter (7); - non-linearity unit (10) with a non-linear transfer function; - amplifier (13); and - electro-optical converter (16); not necessarily in the given order. 21. Device for reduction of the bit error probability during signal transmissions, characterized by a transfer function which has a non-linearity factor which is different from 1.0 (γ≠l.O). 22. Device according to Claim 21, characterized in that the non-linearity factor is less than or equal to 0.5 (γ<=0.5). 23. Device according to Claim 21, characterized in that the non-linearity factor is less than or equal to 0.3 (γ<=0.3).

Inventors: BERGLIND EILERT, OEHLEN PETER, THYLEN LARS, BERGLIND, EILERT, OEHLEN, PETER, THYLEN, LARS
Applicants: BERGLIND EILERT, ERICSSON TELEFON AB L M, OEHLEN PETER, THYLEN LARS, BERGLIND, EILERT, OEHLEN, PETER, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL), THYLEN, LARS
Application year: 1997
Main classification: H04L12
IPC codes: H04B3, H04B10, H04L12