Frequency shift device and optical communication system

Abstract

A frequency shift device comprising: a frequency converter that shifts the frequencies of a plurality of signals so that waves of frequencies generated from a plurality of signals and different from the signals do not interfere with the plurality of signals; and a multiplexer that multiplexes the plurality of signals.

Description

Frequency shift device and optical communication system

[0001] The present invention relates to a frequency shifting device and an optical communication system.

[0002] There is an FM bulk transmission system that uses FM bulk conversion to convert multi-channel video signals into optical signals, transmits the signals through an optical transmission path, demodulates them at the receiving end, and outputs them to a video viewing device such as a TV. In an FM bulk transmission system, CNR characteristics and distortion characteristics (CSO (Composite Second Order) characteristics and CTB (Composite Triple Beat) characteristics) are used as indicators of transmission quality.

[0003] Toshiaki Shimoba et al., "Study on wideband RF signal transmission system using FM batch conversion method with all-channel phase modulation," IEICE General Conference, 2021. Ryo Miyatake et al., "Optical transmission experiment on FM conversion method with wideband phase modulation," IEICE Communications Express, 2021.

[0004] In FM broadcast transmission systems, the multi-channel video signals used as input signals are CATV or BS / CS signals output from facilities such as broadcasting stations. The frequencies of these signals may be multiples of each other. As a result, frequencies such as harmonics and second- and third-order distortion may overlap with the frequencies of other waves in the multi-channel video signal, causing deterioration of distortion characteristics on the receiving side.

[0005] The object of the present invention is to improve distortion characteristics.

[0006] One aspect of the present invention is a frequency shifting device including a frequency converter that shifts the frequencies of a plurality of signals so that the frequencies are not in a multiple relationship, and a multiplexer that multiplexes the plurality of signals.

[0007] The present invention can improve the distortion characteristics.

[0008] FIG. 1 is a diagram showing a configuration of an optical communication system according to a first embodiment; FIG. 2 is a diagram showing examples of harmonics, second-order distortion, and third-order distortion generated by two signals; FIG. 3 is a diagram showing a configuration of a frequency shift device according to a first embodiment; FIG. 4 is a flowchart showing the operation of the frequency shift device according to the first embodiment; FIG. 5 is a flowchart showing a method for setting the frequency shift amount of the frequency converter 11 by a computer; FIG. 6 is a diagram showing a configuration of an optical communication system according to a second embodiment; FIG. 7 is a graph showing CNR characteristics; FIG. 8 is a graph showing CSO characteristics; and FIG. 9 is a graph showing CTB characteristics.

[0009] Embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing the configuration of an optical communication system 1 according to a first embodiment. The optical communication system 1 is a system for transmitting video by optical signals. The optical communication system 1 includes a frequency shifter 10, an optical transmitter 20, an optical transmission line 30, and an optical receiver 40.

[0010] The frequency shifter 10 receives input of video signals of each channel from facilities such as a broadcasting station. The video signals of each channel are, for example, CATV signals or BS / CS signals. The frequency shifter 10 shifts the frequency of the input video signals of each channel. The multiple signals generated by shifting the frequencies are collectively called frequency-shifted signals. Signals obtained by dividing a multi-channel video signal into individual channels are, in other words, signals obtained by dividing signals of multiple frequency bands into specific frequency bands. The frequency shifter 10 will be described in detail later.

[0011] The optical transmitting device 20 converts the frequency-shifted signal into an optical signal by frequency modulation and intensity modulation. The optical transmitting device 20 transmits the optical signal to the optical receiving device 40 via an optical transmission path 30. The optical receiving device 40 receives the optical signal from the optical transmitting device 20, and demodulates the optical signal by intensity demodulating and FM demodulating it to demodulate the frequency-shifted signal. The optical receiving device 40 outputs the demodulated frequency-shifted signal to, for example, an external video playback device 90. The video playback device 90 is, for example, a television.

[0012] The following describes waves of different frequencies generated by a video signal, which is an AC waveform. For example, harmonics whose frequencies are integer multiples of the video signal are generated using the video signal as a fundamental wave. Harmonics are generated, for example, when distortion occurs in the fundamental wave, resulting in the inclusion of multiple components of the fundamental wave. Signals caused by second-order distortion and third-order distortion are also generated from two signals. The frequency of a signal generated by second-order distortion is the sum or difference of the frequencies of the two signals that generated it. A signal generated by third-order distortion is a signal generated by the fundamental wave and a double wave. The frequency of a signal generated by third-order distortion is the difference between twice the frequency of one of the two signals that generated it and the frequency of the other signal.

[0013] FIG. 2 shows an example of harmonics, second-order distortion, and third-order distortion generated by two signals. The example shown in FIG. 2 shows a case where the frequency is f ａ A signal with frequency f ｂ This shows the signals generated by the harmonics generated by the signal A (hereinafter referred to as the B signal) and the signals generated by the second and third order distortions between the A and B signals. The frequencies of the harmonics are integer multiples of the frequencies of the A and B signals. ａ , 3f ａ , 2f ｂ The frequency of the signal generated by the second-order distortion between the A and B signals is f ａ +f ｂ and f ｂ -f ａ The frequency of the signal generated by the third-order distortion between the A signal and the B signal is 2f ａ -f ｂ and 2F ｂ -f ａ In the example shown in FIG. ｂ and 2F ａ Assume that f is approximately equal. ａ and f ｂ -f ａ are almost equal, so the frequency f ａ The frequency f of the signal generated by the second-order distortion in the vicinity ｂ -f ａ As a result, the A signal interferes with the signal generated by the second-order distortion, causing deterioration of the distortion characteristics of the A signal.ｂ and 2F ａ are almost equal, the frequency f of the B signal ｂ The frequency of the double wave generated by the A signal is 2f ａ As a result, the B signal interferes with the second harmonic generated by the A signal, causing deterioration of the distortion characteristics of the B signal.

[0014] Next, the frequency shifter 10 will be described in detail. Fig. 3 is a diagram showing the configuration of the frequency shifter 10 according to the first embodiment. The frequency shifter 10 includes a frequency converter 11 and a multiplexer 12. The frequency converter 11 receives the video signals c of each channel. １ , c ２ ,...c Ｎ are input to the frequency converter 11. The frequency converter 11 shifts the frequencies of the input signals.

[0015] signal c ｋ (k=1, 2, ... N) ｃｋ The frequency converter 11 converts the signal c ｋ Each has a frequency Δf ｋ By multiplying the signals, the frequency of each signal f ｃｋ The signal c after frequency shift is ｋ The respective frequencies of ｃｋ +Δf ｋ Signal c ｋ Each has a frequency Δf ｋ Multiplying the signal with frequency f ｃｋ -Δf ｋ A signal with frequency f ｃｋ -Δf ｋ The signal can be removed by a filter or the like.

[0016] The frequency converter 11 calculates the frequency shift amount Δf ｋ is determined under the constraints of not causing signal interference and the constraints of the receiving side.

[0017] The following describes the constraints to prevent signal interference. ｋ The signal c generated from ｋWaves with different frequencies (such as harmonics and waves generated by second and third order distortion) are generated by multiple signals c ｋ In order to avoid interference with ｋ If the frequencies of the signal and the wave are "sufficiently separated", there will be almost no degradation in the quality of the signal. Here, "sufficiently separated" is an amount determined by the requirements of the optical communication system 1, and it is difficult to determine uniquely. For example, there are cases where the wave is separated by 1 / 100 or more of the frequency of the signal. In other words, in this example, the frequency f of the signal １ and wave f ２ is "sufficiently far away" １ -f １ / 100 to f １ +f １ / 100 range ２ However, there may be cases where the waves are more than 1 / 10 of the signal frequency apart, or more than 1 / 1000 of the signal frequency apart. If the requirements of the optical communication system 1 stipulate that "a specified frequency separation is sufficient," then the frequencies of the signal and the waves can be said to be "sufficiently separated" if they are separated by the specified frequency.

[0018] frequency f ｃｋ At least two of the frequencies are in a multiple relationship. Here, "multiple" means, for example, ｂ and 2F ａ This includes two frequencies being approximately multiples of each other, such as when the frequencies f of two signals are approximately equal. ｃｋ When the frequencies f are multiples of each other, as mentioned above, the frequency of the wave from one signal is not "sufficiently far" from the frequency of the other signal, and interference between the signals may occur. ｃｋ +Δf ｋ The frequency shift amount Δf ｋ For example, each frequency shift amount Δf ｋ By not setting the same value, f ｃｋ +Δf ｋ In order to prevent only the influence of the second harmonic, the frequency f ｃｋAny two frequencies f ｃα +Δf α and f ｃβ +Δf β It is sufficient that the formula (1) is not satisfied for all combinations of the above.

[0019] To prevent the influence of only the third harmonic, use frequency f ｃｋ Any two frequencies f ｃα +Δf α and f ｃβ +Δf β It is sufficient that the formula (2) is not satisfied for all combinations of the above.

[0020] To prevent only the influence of the signal caused by the second-order distortion, ｃｋ +Δf ｋ It is sufficient that the formulas (3) and (4) are not satisfied for any combination of the frequencies.

[0021] In equation (4), α may be equal to β, or α may be equal to γ.

[0022] To prevent only the influence of third-order distortion on the signal, ｃｋ +Δf ｋ It is sufficient that the formula (5) does not hold for any combination of arbitrary frequencies. In equation (5), α=β may be satisfied.

[0023] The signals generated by the second harmonic, the third harmonic, the second-order distortion, and the third-order distortion are signal c ｋ In order to prevent interference with ｃｋ +Δf ｋ Δf so as not to satisfy equations (1) to (5) ｋ The value of the input signal c is determined. ｋ Shift the frequency of

[0024] The following describes the constraints on the receiving side. ｋ If is too large, the signal c ｋmay be processed as a signal of a different frequency by the video playback device 90 on the receiving side. ｋ The absolute value of the signal c is determined so as to be equal to or less than the upper limit of the frequency shift amount that is ignored by the video reproduction device 90 on the receiving side. ｋ Each frequency shift amount Δf ｋ is within a specified dither value range. The dither value is a value indicating the frequency of noise added before quantization to avoid patterns that may occur due to quantization. The range of the dither value is, for example, ±20 kHz in a video transmission system. The frequency shift amount Δf ｋ is within the range of the specified dither value, the frequency shift amount Δf is applied to the processing in the video reproducing device 90 on the receiving side in the same manner as the processing for the noise added by the dither. ｋ is ignored, so the process is normal. ｋ Each frequency shift amount Δf ｋ is within the range of frequency error allowed by the video reproduction device 90 on the receiving side. ｋ is within the range of the frequency error allowed by the video reproduction device 90 on the receiving side, the frequency shift amount Δf ｋ is ignored and processing continues normally.

[0025] The frequency shift amount Δf is determined based on the constraints to prevent signal interference and the constraints on the receiving side. ｋ A specific example of each frequency shift amount Δf ｋ is a value determined by a random function that outputs a value equal to or less than the upper limit of the constraint on the receiving side, for example, under the constraint that overlapping of values ​​is not permitted. ｋ Since these do not have the same value, it can be said that they are determined under the constraint of not causing interference between signals, and that they are below the upper limit of the receiver's constraint, so they are determined under the receiver's constraint.

[0026] Each frequency shift amount Δf ｋis determined by, for example, a sequence of numbers whose values ​​monotonically increase and whose maximum value is equal to or less than the upper limit of the constraints on the receiving side. A sequence of numbers whose values ​​monotonically increase is, for example, a sequence of positive integers arranged in ascending order, such as 1, 2, and 3, and in this case, the frequency shift amount Δf ｋ is Δf ｋ = k [Hz]. Thus, the frequency shift amount Δf ｋ Since these do not have the same value, it can be said that they are determined under the constraint of not causing interference between signals, and that they are below the upper limit of the receiver's constraint, so they are determined under the receiver's constraint.

[0027] Each frequency shift amount Δf ｋ is determined by, for example, a sequence of monotonically decreasing values, the maximum value of which is equal to or less than the upper limit of the constraints on the receiving side. The sequence of monotonically decreasing values ​​is, for example, a sequence of M, M-1, M-2, and other values ​​obtained by subtracting positive integers from a predetermined number in ascending order, where M is a value equal to or less than the upper limit of the constraints on the receiving side. In this case, the frequency shift amount Δf ｋ is Δf ｋ = M-k [Hz]. This gives the frequency shift Δf ｋ Since these do not have the same value, it can be said that they are determined under the constraint of not causing interference between signals, and that they are below the upper limit of the receiver's constraint, so they are determined under the receiver's constraint.

[0028] Each frequency shift amount Δf ｋ As long as it is determined under the constraints of not causing signal interference and the constraints of the receiving side, it may be determined by associating it with a pre-set table or by solving an optimization problem.

[0029] The combiner 12 combines the frequency-shifted signal c １ , c ２ ,...c Ｎ The multiplexer 12 multiplexes the signals. A frequency-multiplexed signal is generated by the signal multiplexing. The multiplexer 12 outputs the frequency-multiplexed signal to the optical transmitter 20.

[0030] 4 is a flowchart showing the operation of the frequency shifter 10 according to the first embodiment. ｋ Frequency f ｃｋ Based on this, the frequency shift amount Δfｋ (Step S11). The frequency converter 11 determines the signal c ｋ The frequency shift amount Δf ｋ By multiplying by ｃｋ (Step S12). ｋ The frequency of ｃｋ +Δf ｋ After that, the multiplexer 12 multiplexes the frequency-shifted signals (step S13), and outputs the multiplexed signal to the optical transmitter 20 (step S14).

[0031] The plurality of signals c generated by the frequency shifter 10 ｋ Frequency f ｃｋ +Δf ｋ are not multiples of each other. Therefore, it is possible to prevent harmonics generated by one signal input to the optical transmitter 20 and signals generated by second-order and third-order distortion of two signals from occurring near the frequency of another signal. This allows the frequency shifter 10 to prevent deterioration of distortion characteristics on the receiving side.

[0032] The frequency-shifted signal is converted into an optical signal by the optical transmitting device 20, and the optical signal is demodulated by the optical receiving device 40. The amount of frequency shift added to the frequency-shifted signal is within the range of a specified dither value and is also within the range of frequency error allowed by the video reproducing device 90 on the receiving side, so that the amount of frequency shift is very small. Therefore, the amount of frequency shift is absorbed by the video reproducing device 90 on the receiving side, and a signal of the correct frequency can be transmitted.

[0033] In the above description, signals obtained by dividing a multi-channel video signal into individual channels are input to the frequency shifter 10. However, an undivided multi-channel video signal may also be input. In this case, the frequency shifter 10 may be provided with a demultiplexer to demultiplex the frequency-multiplexed multi-channel video signal, generate video signals for each channel, and input them to the frequency converter 11.

[0034] In the above description, the frequency converter 11 is configured to shift the frequency by the amount of frequency shift Δf ｋ is determined in advance,ｃｋ If is known, the frequency f ｃｋ Based on this, the frequency shift amount Δf ｋ and inputs the frequency shift amount Δf ｋ In this case, the frequency shift amount Δf ｋ may be calculated by a computer or by a human.

[0035] 5 is a flowchart showing a method for setting the frequency shift amount of the frequency converter 11 by a computer. First, the computer calculates the frequency shift amount of the signal c ｋ Frequency f ｃｋ (Step S21). Then, the computer acquires the frequency f ｃｋ Based on this, the frequency shift amount Δf ｋ (Step S22). ｋ The calculation method is the method described above, and is based on the frequency shift amount Δf under the constraints of not causing signal interference and the constraints on the receiving side. ｋ Then, the computer calculates the calculated frequency shift amount Δf ｋ is set in the frequency converter 11 (step S23). ｋ The calculated frequency shift amount Δf ｋ may be manually set in the frequency converter 11.

[0036] The signal input to the frequency shifter 10 is not limited to a signal obtained by dividing a multi-channel video signal into individual channels or a multi-channel video signal, but is not particularly limited to a signal obtained by frequency-multiplexing signals of different frequencies.

[0037] 6 is a diagram showing the configuration of an optical communication system 1 according to the second embodiment. The optical communication system 1 according to the second embodiment includes a frequency inverse shift device 50 in addition to the components of the optical communication system 1 according to the first embodiment. The frequency inverse shift device 50 applies a shift (hereinafter referred to as an "inverse shift") in the opposite direction to the frequency shift applied by the frequency shift device 10 to the frequency of the signal input from the optical receiving device 40. The frequency inverse shift device 50 outputs the signal to which the inverse shift has been applied to the video playback device 90.

[0038] The frequency reverse shift device 50 includes a demultiplexer 51, a reverse shift amount output unit 52, and a multiplexer 53. The demultiplexer 51 divides the signal input from the optical receiving device 40 into signals c of each channel. １ , c ２ ,...c Ｎ Split into

[0039] The reverse shift amount output unit 52 outputs the signal c ｋ Each has a frequency ΔF ｋ By multiplying the signals, the frequency of each signal f ｃｋ +Δf ｋ The signal c after frequency shift is ｋ The respective frequencies of ｃｋ +Δf ｋ -ΔF ｋ Signal c ｋ Each has a frequency ΔF ｋ Multiplying the signal with frequency f ｃｋ +Δf ｋ +ΔF ｋ A signal with frequency f ｃｋ +Δf ｋ +ΔF ｋ The signal ΔF ｋ is Δf ｋ -ΔF ｋ The inverse shift amount output unit 52 is configured to be able to communicate with the frequency shift device 10, for example, and outputs the frequency shift amount Δf applied by the frequency shift device 10. ｋ The information of frequency ΔF ｋCalculate the frequency ΔF ｋ The calculation of the inverse frequency shift amount ΔF may be performed by the frequency shifter 10. ｋ is the frequency shift amount Δf ｋ and input it manually to the reverse shift amount output unit 52.

[0040] The combiner 53 combines the frequency-shifted signal c １ , c ２ ,...c Ｎ are combined.

[0041] In the optical communication system 1 according to the second embodiment, the frequency inverse shifter 50 applies a frequency inverse shift to the signal. This allows the frequency shifter 10 to offset the frequency shift applied to the input signal by the inverse frequency shift even if the frequency shift applied exceeds the allowable frequency shift absorbed by the receiving video playback device 90. As a result, it is possible to prevent signal distortion caused by the frequency shift applied by the frequency shifter 10.

[0042] (Experimental Results) An experiment was conducted using the optical communication system 1 of the first embodiment. The video signal of each channel was frequency-shifted by the frequency shifter 10, the frequency-shifted signal was modulated into an optical signal by the optical transmitter 20, and the optical signal was transmitted to the optical receiver 40. The optical signal was demodulated by the optical receiver 40. The CNR characteristic, CSO characteristic, and CTB characteristic of the signal demodulated by the optical receiver 40 were measured using a spectrum analyzer.

[0043] Fig. 7 is a graph showing CNR characteristics. Fig. 8 is a graph showing CSO characteristics. Fig. 9 is a graph showing CTB characteristics. In the graphs of Fig. 7 to Fig. 9, the vertical axis represents the CNR, CSO, and CTB values, respectively, and the horizontal axis represents the frequency of the video signal. Frequency shift amount Δf ｋ was determined using uniform random numbers. As shown in the legend, six values ​​of 15 kHz, 20 kHz, 30 kHz, 50 kHz, 80 kHz, and 100 kHz were used for the variance of the uniform random numbers. In the graphs of Figures 7 to 9, the values ​​of the Ministerial Ordinance on Video Signal Transmission established by the Ministry of Internal Affairs and Communications are listed as "Specs."

[0044] Frequency shift amount Δf ｋ In the range of 15 kHz to 100 kHz of the variance of the uniform random numbers that determine the CNR, CSO, and CTB values ​​were greater than the specifications. This shows that the optical communication system 1 of this embodiment can improve the CSO and CTB characteristics and distortion characteristics while maintaining the quality of the CNR characteristics. ｋ It can be seen that the larger the variance of the uniform random numbers that determine the coefficients, the larger the values ​​of CSO and CTB, and the more improved the distortion characteristics.

[0045] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments. The above-described embodiments are merely examples, and the present invention can be embodied in various forms with various modifications and improvements based on the knowledge of those skilled in the art, and designs within the scope of the present invention are also included.

[0046] For the sake of explanation, "frequency shift" and "frequency inverse shift" have been described as different processes, but "frequency inverse shift" is essentially the same process as "frequency shift." Therefore, "frequency inverse shift" may be read as "frequency shift," and the frequency inverse shift device 50 and the frequency shift device 10 may be collectively referred to as "frequency shift device."

[0047] REFERENCE SIGNS LIST 1 Optical communication system, 10 Frequency shift device, 11 Frequency converter, 12 Multiplexer, 20 Optical transmitter, 30 Optical transmission path, 40 Optical receiver, 50 Frequency inverse shift device, 51 Demultiplexer, 52 Inverse shift amount output unit, 53 Multiplexer, 90 Video playback device

Claims

1. A frequency shifting device comprising: a frequency converter that shifts the frequencies of multiple signals so that waves of different frequencies generated from the multiple signals do not interfere with the multiple signals; and a combiner that combines the multiple signals.

2. The frequency shifter according to claim 1, wherein the frequency converter determines the amount of frequency shift so that the absolute value of the amount of frequency shift is equal to or less than an upper limit value of the amount of frequency shift that is ignored by the receiving side.

3. The frequency shift device according to claim 1 or 2, wherein the frequency converter determines the amount of frequency shift so that the amount of frequency shift applied to each signal is not the same value.

4. The frequency shifter according to claim 1 or 2, wherein the frequency converter shifts the frequencies of the multiple signals so that second harmonics, third harmonics, signals caused by second-order distortion, and signals caused by third-order distortion generated from the multiple signals do not interfere with the multiple signals.

5. The frequency shifter according to claim 2, wherein the absolute value of the frequency shift amount is 20 kHz.

6. The frequency shifter according to claim 3, wherein the amount of frequency shift is determined by a random function that outputs a value equal to or less than an upper limit value of a receiving-side constraint, under the constraint that overlapping of values is not permitted.

7. The frequency shifter according to claim 3, wherein the amount of frequency shift is determined by a sequence of numbers whose values increase monotonically and whose maximum value is equal to or less than an upper limit value imposed by a constraint on the receiving side.

8. The frequency shifter according to claim 3, wherein the amount of frequency shift is determined by a sequence of numbers whose values monotonically decrease and whose maximum value is equal to or less than an upper limit value of a constraint on the receiving side.

9. An optical communication system comprising: a frequency shifter according to claim 1 or 2, which shifts the frequency of a signal to be transmitted and multiplexes said signal to be transmitted; and a frequency inverse shifter, on the receiving side, which inversely shifts the frequency of the signal.

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