Signal Transmission System

Abstract

To provide a transmitter with which it is possible to transmit a pulse signal and an RF signal with high quality in one transmission line.SOLUTION: A transmitter 100 comprises an RF signal generation unit 110, a pulse signal generation unit 120, a coupling unit 140, a linear driver 150, and a light-emitting element 160. The RF signal generation unit 110 generates an RF signal that is modulated by a carrier wave. The pulse signal generation unit 120 generates a pulse signal. The coupling unit 140 couples the RF and pulse signals and generates a superimposed signal. The linear driver 150 amplifies the superimposed signal and generates an amplified signal. The light-emitting element 160 converts the amplified signal to an optical signal and generates a superimposed optical signal. The pulse signal generation unit 120 generates a pulse signal so as to satisfy fr=n×fs, where fr represents the carrier frequency of the RF signal, fs represents a first frequency that is the reciprocal of modulation rate of the pulse signal, and n represents a natural number.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a transmitting device and a signal transmission system. [Background technology]

[0002] Conventionally, in optical communication using optical fiber as a transmission path, a method of superimposing a pulse signal and an RF (Radio Frequency) signal and transmitting the superimposed signal has been proposed. Patent Document 1 discloses a wireless communication system that wirelessly transmits a superimposed signal, in which a transmission signal and an interference signal are superimposed, from a transmitting device to a receiving device. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-34338 A Summary of the Invention [Problem to be solved by the invention]

[0004] The high-speed pulse signal applied to the wireless communication system disclosed in Patent Document 1 has frequency components over a wide band. Therefore, in a wireless communication system that transmits a high-speed pulse signal, this high-speed pulse signal may overlap with the frequency components of an RF signal transmitted by a carrier wave such as a radio signal, and each of them may become a noise component, making it difficult to transmit them simultaneously.

[0005] The present invention has been made in consideration of the problems inherent in the conventional techniques, and an object of the present invention is to provide a transmitting device capable of transmitting a pulse signal and an RF signal with high quality over a single transmission path. [Means for solving the problem]

[0006] A transmitting device according to an embodiment of the present invention includes an RF signal generating unit that generates an RF signal modulated by a carrier wave, a pulse signal generating unit that generates a pulse signal, a combining unit that combines the RF signal and the pulse signal to generate a superimposed signal, a linear driver that amplifies the superimposed signal and generates an amplified signal, and a light emitting element that converts the amplified signal into an optical signal and generates a superimposed optical signal, and the pulse signal generating unit generates a pulse signal to satisfy the following equation (1), where fr is the carrier frequency of the RF signal, fs is a first frequency that is the reciprocal of the modulation speed of the pulse signal, and n is a natural number. fr = n × fs (1)

[0007] A signal transmission system according to another aspect of the present invention includes the above-mentioned transmitting device, a transmission line for transmitting the superimposed optical signal output from the transmitting device, and a receiving device for receiving the superimposed optical signal from the transmission line, wherein the receiving device has a photodetector that receives the superimposed optical signal and converts it into a superimposed electrical signal, a branching section that branches the superimposed electrical signal, an RF signal demodulation section that amplifies one of the superimposed electrical signals branched at the branching section and demodulates it into an RF signal, and a pulse signal demodulation section that amplifies the other superimposed electrical signal branched at the branching section and demodulates it into a pulse signal. Effect of the Invention

[0008] According to the present invention, it is possible to provide a transmitting device capable of transmitting a pulse signal and an RF signal with high quality over a single transmission path. [Brief description of the drawings]

[0009] [Figure 1] 1 is a diagram showing a configuration of a signal transmission system according to a first embodiment. [Diagram 2] 1 is a block diagram showing a configuration of a transmission device according to a first embodiment. [Figure 3A] FIG. 4 is a diagram showing an example of a waveform of a pulse signal in the time domain. [Figure 3B] FIG. 4 is a diagram showing an example of a waveform of a pulse signal in the frequency domain. [Figure 4A] FIG. 2 is a diagram showing an example of a waveform of an RF signal in the time domain. [Figure 4B] FIG. 2 is a diagram showing an example of a waveform in the frequency domain of an RF signal. [Diagram 5] FIG. 4 is a diagram showing an example of a waveform obtained by combining a pulse signal and an RF signal in the first embodiment. [Figure 6A] 1 is a block diagram showing an example of a configuration of a receiving device according to a first embodiment. [Figure 6B] 1 is a block diagram showing an example of a configuration of a receiving device according to a first embodiment. [Figure 7A] 3 is a diagram showing an example of a waveform demodulated by a pulse signal demodulation unit of the receiving device according to the first embodiment. FIG. [Figure 7B] 3 is a diagram showing an example of a waveform demodulated by an RF signal demodulation unit of the receiving device according to the first embodiment. FIG. [Figure 7C] 4 is a diagram showing an example of a waveform when a signal demodulated by an RF signal demodulation unit of the receiving device according to the first embodiment is down-converted to baseband. FIG. [Figure 8A] FIG. 2 is a diagram for explaining an eye pattern with respect to a cutoff frequency according to the first embodiment. [Figure 8B] FIG. 2 is a diagram for explaining an eye pattern with respect to a cutoff frequency according to the first embodiment. [Figure 8C] FIG. 2 is a diagram for explaining an eye pattern with respect to a cutoff frequency according to the first embodiment. [Figure 9] FIG. 4 is a diagram for explaining a change in Q value with respect to a cutoff frequency according to the first embodiment. [Figure 10A] 4A to 4C are diagrams for explaining eye patterns with respect to the filter order of a low-pass filter of the receiving device according to the first embodiment. [Figure 10B] 4A to 4C are diagrams for explaining eye patterns with respect to the filter order of a low-pass filter of the receiving device according to the first embodiment. [Figure 11] FIG. 11 is a block diagram showing a configuration of a transmission device according to a second embodiment. [Figure 12A]1A and 1B are diagrams for explaining the relationship between a pulse waveform and a frequency spectrum. [Figure 12B] 1A and 1B are diagrams for explaining the relationship between a pulse waveform and a frequency spectrum. [Figure 13] FIG. 11 is a diagram illustrating a frequency spectrum of a superimposed signal obtained by superimposing a pulse signal and an RF signal in a transmission device according to a second embodiment. [Figure 14] FIG. 11 is a block diagram showing a configuration of a transmission device according to a third embodiment. [Figure 15] FIG. 11 is a block diagram showing a configuration of a receiving device according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The signal transmission system 10 according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In addition, in the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (Signal Transmission System 10) The signal transmission system 10 according to the present embodiment superimposes a pulse signal represented by a pulse waveform and an RF signal (Radio Frequency, high-frequency wireless signal) and transmits the superimposed signal as an optical signal. In a home or a car, a pulse signal of a LAN (Local Area Network) and an RF signal received from an antenna are wired using separate signal lines. This increases the number of wires, which increases costs and makes it difficult to reduce the number of wires. In addition, since the RF signal is made high-frequency by a carrier wave, the frequency may be lowered by IF (Intermediate Frequency) or the like when transmitting the signal through a coaxial cable or the like. In this embodiment, a signal obtained by superimposing a pulse signal and an RF signal at the electrical stage is converted to light and transmitted through a transmission path such as a single optical fiber.

[0012] (First embodiment) 1 is a diagram showing a configuration of a signal transmission system 10 according to the first embodiment. As shown in FIG. 1, the signal transmission system 10 includes a transmitting device 100, a receiving device 200, and a transmission path 300.

[0013] (Transmitting device 100) The transmitting device 100 is a device that superimposes a pulse signal and an RF signal and transmits the superimposed signal. Fig. 2 is a block diagram showing the configuration of the transmitting device 100 according to the first embodiment. As shown in Fig. 2, the transmitting device 100 includes an RF signal generating unit 110, a pulse signal generating unit 120, a coupling unit 140, a linear driver 150, and a light emitting element 160. The transmitting device 100 may be configured as a transmitting / receiving device including a receiving device.

[0014] The RF signal generating unit 110 generates and outputs an RF signal S1 that is modulated by a carrier wave or that has been modulated by a carrier wave. In this embodiment, the RF signal S1 generated by the RF signal generating unit 110 may be a signal input from an external source such as a terrestrial digital broadcasting or a mobile phone. In this case, the RF signal generating unit 110 outputs the RF signal S1 input from the outside to the combining unit 140 described below.

[0015] The pulse signal generating unit 120 generates a pulse signal S2 and outputs it to the combining unit 140 described below. The pulse signal generating unit 120 receives a digital signal (not shown) from the outside and generates a pulse signal based on the input digital signal. Note that in the first embodiment, the digital signal is not limited to being generated based on a signal input from the outside, and may be generated by the pulse signal generating unit 120 and output as a pulse signal S2.

[0016] Fig. 3A is a diagram showing an example of a waveform in the time domain of a pulse signal. Fig. 3B is a diagram showing an example of a waveform in the frequency domain shown by Fourier transforming the pulse signal. Fig. 4A is a diagram showing an example of a waveform in the time domain of an RF signal. Fig. 4B is a diagram showing an example of a waveform in the frequency domain shown by Fourier transforming the RF signal.

[0017] In the first embodiment, the pulse signal generating unit 120 generates a pulse signal by modulating it at a predetermined frequency. Specifically, the pulse signal generating unit 120 generates a pulse signal so as to satisfy the following formula (1), where fr is the carrier frequency of the RF signal, fs is the frequency that is the reciprocal of the modulation speed T of the pulse signal (hereinafter referred to as a first frequency fs), and n is a natural number. fr = n × fs (1)

[0018] In addition, the first frequency fs, which is the reciprocal of the modulation speed T of the pulse signal, is a frequency indicated by 1 / T in the example shown in FIG. 3B.

[0019] The pulse signal generating unit 120 generates a pulse signal to satisfy formula (1), thereby preventing the RF signal S1 and the pulse signal S2 from interfering with each other. Furthermore, the natural number n in formula (1) is a value of 2 or more. As a result, the carrier frequency fr of the RF signal becomes more than twice the first frequency fs, which is the reciprocal of the modulation speed of the pulse signal, and the power of the pulse signal in the frequency domain is located in a valley of the signal close to 0. Therefore, it is possible to prevent the RF signal S1 and the pulse signal from interfering with each other.

[0020] The coupling unit 140 has a function of generating a superimposed signal S3 in which the RF signal S1 and the pulse signal S2 are superimposed. An example of the coupling unit 140 is a directional coupler. FIG. 5 is a diagram showing a waveform in the frequency domain of the superimposed signal S3 in which the RF signal S1 and the pulse signal S2 are superimposed. As shown in FIG. 5, the superimposed signal S3 superimposed in the coupling unit 140 of the transmission device according to the first embodiment suppresses the RF signal S1 and the pulse signal S2 from interfering with each other. In addition, in the example shown in FIG. 5, an example in which the value of the natural number n in the formula (1) is 2 is shown. As shown in FIG. 5, the carrier frequency fr of the RF signal is twice the first frequency fs, which is the inverse number of the modulation speed of the pulse signal, and the power of the pulse signal in the frequency domain is located in a valley of the signal close to 0. Therefore, it is possible to suppress the RF signal S1 and the pulse signal from interfering with each other.

[0021] The linear driver 150 generates an amplified signal S3b by amplifying the superimposed signal S3a output from the combining unit 140, and outputs the amplified signal S3b to the light emitting element 160. In this embodiment, the linear driver 150 corresponds to an amplifier for driving the light emitting element 160.

[0022] The light-emitting element 160 is an element that converts an electrical signal into an optical signal. The light-emitting element 160 is connected to the linear driver 150 and the transmission path 300. The light-emitting element 160 converts the amplified signal S3b output from the linear driver 150 into a superimposed optical signal S4 (optical signal), and transmits the superimposed optical signal S4 to the transmission path 300. The light-emitting element 160 has an electrical-optical conversion function. The light-emitting element 160 may be an element such as a laser diode or an LED (Light Emitting Diode). Since the transmission device 100 can convert into an optical signal using one light-emitting element 160, an increase in the number of components of the system can be suppressed.

[0023] The transmission path 300 is a single communication line that connects the transmitting device 100 and the receiving device 200. An example of the transmission path 300 is an optical fiber. The optical fiber is made of, for example, glass or resin. The optical fiber is configured to include, for example, a core layer through which an optical signal propagates and a cladding layer that covers the periphery of the core layer. In the optical fiber, the optical signal propagates in the core layer by being totally reflected at a predetermined angle at the boundary surface between the core layer and the cladding layer. The optical fiber can have a smaller wire diameter than a metal wire and can be made lighter.

[0024] (Receiving device 200) The receiving device 200 is a device that receives a signal. As shown in Fig. 6A, the receiving device 200 includes a light receiving element 210, a coupler 220, an RF signal demodulation unit 230, and a pulse signal demodulation unit 240. The receiving device 200 may be configured as a transmitting / receiving device including a transmitting device.

[0025] The light receiving element 210 converts the superimposed optical signal S4 sent from the transmission path 300 into a superimposed electrical signal S5. The superimposed electrical signal S5 output from the light receiving element 210 is branched by the coupler 220. One of the superimposed electrical signals S5 branched by the coupler 220 is sent to the RF signal demodulation section 230. The other of the superimposed electrical signals S5 branched by the coupler 220 is sent to the pulse signal demodulation section 240. The coupler 220 corresponds to a branching section.

[0026] The RF signal demodulation unit 230 includes a low-noise amplifier 231 and a band-pass filter 232. The low-noise amplifier 231 is an amplifier having a gain at the carrier frequency fr of the RF signal, and outputs an amplified electrical signal S6. The band-pass filter 232 is a band-pass filter having a signal bandwidth equal to or greater than the first frequency fs of the pulse signal. The low-noise amplifier 231 and the band-pass filter 232 extract an RF signal from the superimposed optical signal S4. That is, the RF signal demodulation unit 230 has a function of amplifying the superimposed electrical signal S5 and demodulating it into an RF signal. FIG. 7B is a diagram showing an example of an RF signal demodulated by the RF signal demodulation unit 230. Also, FIG. 7C is a diagram showing an example of a waveform when the demodulated RF signal is down-converted to a baseband signal.

[0027] The pulse signal demodulation unit 240 includes a transimpedance amplifier 241 and a low-pass filter 242. The transimpedance amplifier 241 is an amplifier having a first frequency band, and outputs an amplified electrical signal S7. The low-pass filter 242 is used to remove an RF signal. The transimpedance amplifier 241 and the low-pass filter 242 extract a pulse signal from the superimposed optical signal S4. That is, the pulse signal demodulation unit 240 has a function of amplifying the superimposed electrical signal S5 and demodulating it into a pulse signal. FIG. 7A is a diagram showing an example of a demodulated pulse signal. As shown in FIG. 7A, the signal demodulated by the pulse signal demodulation unit 240 includes an RF signal, but the RF signal is suppressed to a degree that does not affect the pulse signal.

[0028] 6B, the receiving device 200 may include an optical splitter 221, and after splitting the superimposed optical signal S4, an RF signal and a pulse signal may be extracted by an RF signal demodulator 230 and a pulse signal demodulator 240, respectively. In this case, it becomes possible to apply the photodetector 210 to the superimposed optical signal S4 split by the optical splitter 221, and it becomes possible to suppress the frequency components to be removed at the stage of the photodetector 210.

[0029] Furthermore, the optical splitter 221 splits the superimposed optical signal S4 at a fixed ratio (for example, a 1:1 ratio). Note that the splitting ratio is not limited to 1:1, and the superimposed optical signal S4 may be split at other ratios.

[0030] In the first embodiment, the cutoff frequency fc of the low-pass filter 242 is determined and set in advance by calculation. The cutoff frequency fc is determined by the following formulas (2) to (4) based on the relationship between the carrier frequency fr of the RF signal S1 output from the RF signal generating unit 110 and a first frequency fs that is the reciprocal of the modulation speed T in the pulse signal generating unit 120.

[0031] First, when fr≧2×fs, that is, when the carrier frequency fr of the RF signal is equal to or greater than twice the first frequency fs, a cutoff frequency fc that satisfies the relationship in the following equation (2) is applied. Here, γ is the order of the low-pass filter 242. 0.6×fs≦fc(γ≧4)≦1×fs (2)

[0032] Furthermore, when 1.2×fs≦fr<2×fs, that is, when the carrier frequency fr of the RF signal is 1.2 times or more and less than twice the first frequency fs, a cutoff frequency fc that satisfies the relationship in the following formula (3) is applied, where γ is 4≦γ≦7. 0.6×fs≦fc(γ)≦0.75×fs (3)

[0033] Furthermore, when 1.2×fs=fr, that is, when the carrier frequency fr is 1.2 times the first frequency fs, the cutoff frequency fc that satisfies the relationship of the following equation (4) is applied. fc(γ≧7)≦0.75×fs (4)

[0034] In the first embodiment, the carrier frequency fr and the first frequency fs that satisfy the relationship fr<1.2×fs are outside the applicable range.

[0035] 8A to 8C are diagrams showing eye patterns when a fourth-order Butterworth LPF (Low Pass Filter) is used for low pass filter 242 and the cutoff frequency fc is changed. Fig. 8A shows an example in which the cutoff frequency fc is 0.5×the first frequency fs=600 MHz. Fig. 8B shows an example in which the cutoff frequency fc is 0.5×the first frequency fs=900 MHz. Fig. 8C shows an example in which the cutoff frequency fc is the first frequency fs=1200 MHz.

[0036] As shown in Fig. 8A, as the cutoff frequency fc becomes lower, the effect of inter-symbol interference becomes greater, and the Q value deteriorates (Q value: quality factor). For example, in the example of Fig. 8A, the high-frequency components of the pulse signal are suppressed by low-pass filter 242, so the amplitude of the pulse waveform with the shortest level 1 is attenuated. In contrast, the pulse waveform with continuous levels of 1 has more low-frequency components, so the amplitude attenuation is less. Therefore, in the eye pattern shown in Fig. 8A, the difference in amplitude causes a state called inter-symbol interference, and the Q value deteriorates.

[0037] Furthermore, as shown in Fig. 8C, when the cutoff frequency fc becomes higher, the Q value deteriorates due to the influence of the RF signal. This is because, in Fig. 8C, the power of the RF signal is large and the RF signal appears as noise compared to the pulse signal, causing the Q value to deteriorate.

[0038] Generally, when an RF signal is superimposed on a pulse signal, the RF signal is regarded as noise. Therefore, the Q value is determined by the power of the RF signal suppressed by the filter and the amplitude of the pulse signal. In this embodiment, the Q value is expressed as Q=(signal amplitude) / (rms noise at signal 0 level+rms noise at signal 1 level). (rms: root mean square).

[0039] FIG. 9 is a graph showing the change in the Q value with respect to the cutoff frequency fc when the filter orders of the low-pass filter 242 are 4 and 5. The horizontal axis of FIG. 9 uses fc / fs, which is the ratio of the cutoff frequency fc to the first frequency fs. The vertical axis of FIG. 9 shows the Q value with respect to fc / fs. As shown in FIG. 9, the larger the order of the low-pass filter 242, the larger the attenuation rate of the Q value. Therefore, the larger the order of the low-pass filter 242, the greater the effect of removing the RF signal. However, if the cutoff frequency fc is set too low, the frequency component of the pulse signal is also removed, and the effect of inter-symbol interference also increases. Therefore, the lower limit of the cutoff frequency fc can be lowered as the order of the low-pass filter 242 is lower. Also, the higher the order, the greater the upper limit of the cutoff frequency fc, but the upper limit of the cutoff frequency fc also depends on the first frequency fs and the carrier frequency fr.

[0040] 10A and 10B show eye patterns when the carrier frequency fr of the RF signal shown in FIGS. 8A to 8C is changed from 2.4 GHz to 1.8 GHz. FIG. 10A shows an example in which the order of the low-pass filter 242 is 4 and the Q value is 14. FIG. 10B shows an example in which the order of the low-pass filter 242 is 5 and the Q value is 20. As shown in FIGS. 10A and 10B, when the order of the low-pass filter 242 is 4, the Q value is 14, but by changing the order of the low-pass filter 242 to 5, the Q value is improved to 20. This is because the attenuation rate of the Butterworth low-pass filter is expressed as 6 dB × order, and therefore, as the order increases, the attenuation of the frequency of the carrier frequency fr of the RF signal increases. That is, the signal transmission system 10 includes the low-pass filter 242 that satisfies the formulas (2) to (4), and thereby it becomes possible to demodulate the RF signal and the pulse signal while suppressing the influence of inter-symbol interference.

[0041] As described above, the transmitting device 100 according to the first embodiment includes an RF signal generating unit 110, a pulse signal generating unit 120, a combining unit 140, a linear driver 150, and a light emitting element 160. The RF signal generating unit 110 generates an RF signal modulated by a carrier wave. The pulse signal generating unit 120 generates a pulse signal. The combining unit 140 combines the RF signal and the pulse signal to generate a superimposed signal. The linear driver 150 amplifies the superimposed signal to generate an amplified signal. The light emitting element 160 converts the amplified signal to an optical signal to generate a superimposed optical signal. The pulse signal generating unit 120 generates a pulse signal so as to satisfy fr=n×fs, where fr is the carrier frequency of the RF signal, fs is the first frequency that is the reciprocal of the modulation speed of the pulse signal, and n is a natural number.

[0042] As a result, the transmitting device 100 according to the first embodiment can suppress mutual interference between the RF signal S1 and the pulse signal. Also, compared to a conventional signal transmission method using wavelength multiplexing, the transmitting device 100 can eliminate the need for multiple light sources and wavelength filters, suppressing an increase in the number of components of the system and enabling a reduction in manufacturing costs by suppressing an increase in the size of the system.

[0043] Moreover, the natural number n in the above formula (1) is equal to or greater than 2. As a result, the carrier frequency fr of the RF signal becomes twice the first frequency fs, which is the reciprocal of the modulation speed of the pulse signal, and the power of the pulse signal in the frequency domain is located in a valley of the signal close to 0. Therefore, it is possible to suppress mutual interference between the RF signal S1 and the pulse signal.

[0044] Second embodiment As described above, one specific embodiment has been described, but the above-mentioned embodiment is merely an example and does not limit the embodiments. For example, in the above-mentioned embodiment, a form in which the RF signal and the pulse signal can be prevented from interfering with each other due to the relationship between the carrier frequency fr of the RF signal and the first frequency which is the inverse of the modulation speed of the pulse signal has been exemplified. Here, a signal transmission system 10 according to a second embodiment which can further prevent the RF signal and the pulse signal from interfering with each other in the signal transmission system 10 will be described with reference to a configuration different from that of the first embodiment.

[0045] FIG. 11 is a block diagram showing a configuration of a transmission device 100 of a signal transmission system 10 in the second embodiment.

[0046] In the second embodiment, the pulse signal generating unit 121 is capable of generating a pulse signal that better prevents interference with the carrier frequency fr of an RF signal, compared to the pulse signal generating unit 120 in the first embodiment.

[0047] 12A and 12B are diagrams for explaining the relationship between a pulse waveform and a frequency spectrum. For example, the relationship between the time axis and the frequency axis (frequency spectrum) of a pulse signal of a rectangular pulse generated by OOK (On-Off keying) is as shown in FIG. 12A. The waveform of the time axis shown in FIG. 12A is shown on the frequency axis (frequency spectrum) by performing a fast Fourier transform (FFT). In addition, since the relationship between the time axis and the frequency axis shown in FIG. 12A is reversible, it is possible to show the inverse relationship of the relationship shown in FIG. 12A as shown in FIG. 12B by making the pulse shape on the time axis equivalent to the frequency axis. That is, it is possible to generate a signal with a suppressed high frequency component by generating a signal with a pulse shape on the frequency axis showing the relationship shown in FIG. 12B.

[0048] In the second embodiment, pulse signal generating unit 121 converts (modulates) an input or generated digital signal into a waveform as shown on the left side (time axis) of FIG. 12B to generate a pulse signal S2 as shown on the right side (frequency axis) of FIG. 12B. Pulse signal generating unit 121 outputs the generated pulse signal S2 to combining unit 140. Regarding the generation of a pulse signal in pulse signal generating unit 121, for example, a raised cosine pulse may be applied as an example of a pulse capable of suppressing high-frequency components. Note that in the second embodiment, the pulse signal generated in pulse signal generating unit 121 is applicable as long as it has a pulse shape capable of suppressing high-frequency components, and is not limited to a raised cosine pulse.

[0049] FIG. 13 shows a frequency spectrum of a superimposed optical signal S4 obtained by superimposing a 1.2 Gbps pulse signal using a raised cosine pulse and a 64-QAM OFDM signal having a carrier frequency fr of 2.4 GHz and a bandwidth of 100 MHz of the RF signal S1. (QAM: Quadrature Amplitude Modulation, OFDM: Orthogonal Frequency Division Multiplexing). As shown in FIG. 13, in the superimposed optical signal S4 output by the transmitting device 100 in the second embodiment, the waveform of the pulse signal and the waveform of the RF signal are separated on the frequency axis, and interference between the pulse signal and the RF signal is suppressed. Also, as shown in FIG. 13, the carrier frequency fr of the RF signal is 2.4 GHz, and the first frequency, which is the reciprocal of the modulation speed of the pulse signal, is 1.2 GHz. That is, in the second embodiment, the relationship of the formula (1) shown in the first embodiment is also established, and further, FIG. 13 shows an example in which the natural number n is 2.

[0050] As described above, the pulse signal generating unit 121 according to the second embodiment may generate a pulse signal by applying a raised cosine pulse capable of suppressing high frequency components. This allows the transmitting device 100 to more reliably suppress the RF signal S1 and the pulse signal from interfering with each other.

[0051] In the first embodiment, the width of the valley where the power of the frequency of the pulse signal becomes 0 is narrow, so the bandwidth of the RF signal that can suppress the influence of the pulse signal is limited, as shown in Fig. 5. On the other hand, in the second embodiment, there is no signal power at frequencies equal to or higher than the first frequency fs, as shown in Fig. 13, so that it is possible to superimpose an RF signal with a wide bandwidth.

[0052] In addition, in the first embodiment where the natural number n in the above formula (1) is 2 or more, the above formula (2) can be applied.

[0053] (Third embodiment) Next, a third embodiment will be described. In the following description, when the same reference numerals as those in the first and / or second embodiment are used, they indicate the same configuration as those in the first and / or second embodiment, and the preceding description will be referred to unless otherwise specified. Here, a configuration different from that of the first and / or second embodiment will be described for the signal transmission system 10 according to the third embodiment in which the setting values ​​of the pulse signal generating units 120, 121 and the digital filters 233, 243 are dynamically set in the signal transmission system 10.

[0054] Fig. 14 shows a block diagram of a transmitting device 100 according to the third embodiment. Fig. 15 shows a block diagram of a receiving device 200 according to the third embodiment. The transmitting device 100 according to the third embodiment differs from the transmitting device 100 according to the first embodiment in that it includes a calculation processing unit 170. The receiving device 200 according to the third embodiment also differs from the receiving device 200 according to the first embodiment in that it includes a filter setting unit 250.

[0055] In the signal transmission system 10 according to the third embodiment, first, only an RF signal is transmitted from the transmitting device 100 to the receiving device 200. In this case, for example, the pulse signal generating unit 120 is set not to operate, thereby realizing transmission of only an RF signal.

[0056] A demodulation section (not shown) in the receiving device 200 performs FFT (Fast Fourier Transform) on the RF signal transmitted to the receiving device 200, and the carrier frequency fr of the RF signal is extracted.

[0057] The arithmetic processing unit 170 of the transmitting device 100 acquires the carrier frequency fr from the receiving device 200 by a predetermined means. In this embodiment, the predetermined means may be configured such that the arithmetic processing unit 170 acquires the carrier frequency fr directly from a demodulation unit (not shown) of the receiving device 200. Alternatively, the carrier frequency fr may be sent to the arithmetic processing unit 170 of the transmitting device 100 via an external control device (not shown) that receives an output signal from the demodulation unit (not shown) of the receiving device 200.

[0058] When there are a plurality of carrier frequencies fr, the calculation processing unit 170 calculates the order and cutoff frequency fc such that all of the carrier frequencies fr existing within a frequency range demodulatable by a pulse signal demodulator (not shown) are 20 dB or less for the transmission power of the pulse signal. In the second embodiment, the calculation of the order and frequency in the calculation processing unit 170 is performed based on the above-mentioned formulas (2) to (4). Information about the order and cutoff frequency fc calculated by this calculation processing unit 170 is transmitted from the transmission device 100 to the reception device 200 by a pulse signal, and is set in the digital filter 233 and the digital filter 243 of the reception device 200.

[0059] Furthermore, the pulse signal generating section 120 determines the modulation speed T of the pulse signal based on the first frequency fs obtained by the calculation in the calculation processing section 170, and generates the pulse signal S2 based on the modulation speed.

[0060] As described above, the transmitting device 100 of the signal transmission system 10 in the third embodiment further includes the arithmetic processing unit 170. The receiving device 200 further includes the digital filters 233, 243 and the filter setting unit 250. The RF signal demodulation unit 230 extracts the frequency of the RF signal based on the demodulated RF signal. The arithmetic processing unit 170 calculates the modulation speed of the pulse signal based on the frequency of the RF signal extracted by the RF signal demodulation unit 230. The filter setting unit 250 calculates the order and the cutoff frequency of the digital filters 233, 243 based on the frequency of the RF signal and the first frequency.

[0061] As a result, the signal transmission system 10 according to the third embodiment can dynamically set the modulation speed of the pulse signal in the pulse signal generating unit and the setting values ​​of the digital filters 233 and 243 according to the carrier frequency fr. Therefore, the signal transmission system 10 can more flexibly suppress mutual interference between the RF signal S1 and the pulse signal according to the system configuration.

[0062] (Other embodiments) Although the embodiments have been described in detail with reference to the drawings, the present embodiments are not limited to the contents described in the above embodiments. The components described above include those that a person skilled in the art can easily imagine and those that are substantially the same. Furthermore, the configurations described above can be appropriately combined. Various omissions, substitutions, or modifications of the configurations can be made without departing from the spirit of the embodiments.

[0063] The features of the transmitting device 100 and the signal transmission system 10 will be described below.

[0064] The transmitting device 100 according to the first embodiment includes an RF signal generating unit 110 that generates an RF signal S1 modulated by a carrier wave. The transmitting device 100 also includes a pulse signal generating unit 120 that generates a pulse signal S2. The transmitting device 100 also includes a combining unit 140 that combines the RF signal S1 and the pulse signal S2 to generate a superimposed signal. The transmitting device 100 also includes a linear driver 150 that amplifies the superimposed signal and generates an amplified signal. The transmitting device 100 also includes a light emitting element 160 that converts the amplified signal into an optical signal and generates a superimposed optical signal. The pulse signal generating unit 120 generates a pulse signal to satisfy the following formula (1), where fr is the frequency of the RF signal, fs is a first frequency that is the reciprocal of the modulation speed of the pulse signal, and n is a natural number. fr = n × fs (1)

[0065] According to the above configuration, the transmitting device 100 can suppress mutual interference between the RF signal S1 and the pulse signal. Also, compared to a conventional signal transmission method using wavelength multiplexing, the transmitting device 100 can eliminate the need for multiple light sources and wavelength filters, suppressing an increase in the number of components of the system and enabling a reduction in manufacturing costs by suppressing an increase in the size of the system.

[0066] In the transmission device 100 according to the second embodiment, the natural number n in the above formula (1) is 2 or more.

[0067] According to the above configuration, in the transmitting device 100, the carrier frequency fr of the RF signal is more than twice the first frequency fs, which is the reciprocal of the modulation speed of the pulse signal, and the power of the pulse signal in the frequency domain is located in a valley of the signal close to 0. Therefore, it is possible to suppress mutual interference between the RF signal S1 and the pulse signal.

[0068] The pulse signal generating section 120 of the transmitting device 100 according to the third embodiment may generate a pulse signal by applying a raised cosine pulse capable of suppressing high frequency components.

[0069] According to the above configuration, the transmitting device 100 can more reliably prevent the RF signal S1 and the pulse signal from interfering with each other.

[0070] The signal transmission system 10 according to the fourth embodiment includes the above-mentioned transmitting device 100, a transmission path 300 that transmits the superimposed optical signal output from the transmitting device 100, and a receiving device 200 that receives the superimposed optical signal from the transmission path 300. The receiving device 200 has a light receiving element 210 that receives the superimposed optical signal and converts it into a superimposed electrical signal. The receiving device 200 also has a branching unit that branches the superimposed electrical signal. The receiving device 200 also has an RF signal demodulation unit 230 that amplifies one of the superimposed electrical signals branched at the branching unit and demodulates it into an RF signal. The receiving device 200 also has a pulse signal demodulation unit 240 that amplifies the other of the superimposed electrical signals branched at the branching unit and demodulates it into a pulse signal.

[0071] According to the above configuration, the signal transmission system 10 can suppress mutual interference between the RF signal and the pulse signal. Compared to the conventional signal transmission method using wavelength multiplexing, the signal transmission system 10 can eliminate the need for multiple light sources and wavelength filters, suppressing an increase in the number of system components, and suppressing an increase in the size of the system, thereby enabling a reduction in manufacturing costs.

[0072] The pulse signal demodulation section 240 of the signal transmission system 10 according to the fifth aspect may have a low-pass filter 242 with a cutoff frequency fc that satisfies the relationship of the following equations (2) to (4), where fs is a first frequency that is the reciprocal of the modulation speed of the pulse signal and γ is an order. 0.6×fs≦fc(γ≧4)≦1×fs (2) 0.6×fs≦fc(γ)≦0.75×fs (3) fc(γ≧7)≦0.75×fs (4)

[0073] According to the above configuration, the signal transmission system 10 includes a low-pass filter that satisfies the expressions (2) to (4), and thus it becomes possible to demodulate an RF signal and a pulse signal with the influence of inter-symbol interference suppressed.

[0074] The transmitting device 100 of the signal transmission system 10 according to the sixth embodiment may further include an arithmetic processing unit 170. The receiving device 200 may further include digital filters 233, 243 and a filter setting unit 250. The RF signal demodulation unit 230 may extract the frequency of the RF signal based on the demodulated RF signal. The arithmetic processing unit 170 may calculate the modulation speed of the pulse signal based on the frequency of the RF signal extracted by the RF signal demodulation unit 230. The filter setting unit 250 may calculate the order and cutoff frequency of the digital filter based on the frequency of the RF signal and the first frequency.

[0075] According to the above configuration, the signal transmission system 10 can dynamically set the modulation speed of the pulse signal in the pulse signal generating unit and the setting values ​​of the digital filters 233 and 243 according to the carrier frequency fr. Therefore, the signal transmission system 10 can more flexibly suppress mutual interference between the RF signal S1 and the pulse signal according to the system configuration. [Explanation of symbols]

[0076] 10 Signal Transmission System 100 Transmitting device 110 RF signal generation section 120, 121 Pulse signal generating unit 140 Joint 150 Linear Driver 160 Light emitting element 200 Receiving device 242 Low-pass filter 300 Transmission Line

Claims

1. A signal transmission system comprising: a transmitting device that generates and outputs a superimposed optical signal; a transmission line that transmits the superimposed optical signal output from the transmitting device; and a receiving device that receives the superimposed optical signal from the transmission line, The transmitting device an RF signal generating unit that generates an RF signal to be modulated by a carrier wave; A pulse signal generating unit that generates a pulse signal; a combiner that combines the RF signal and the pulse signal to generate a superimposed signal; a linear driver that amplifies the superimposed signal and generates an amplified signal; a light-emitting element that converts the amplified signal into an optical signal and generates the superimposed optical signal; A calculation processing unit, The receiving device includes: a light receiving element that receives the superimposed optical signal and converts it into a superimposed electrical signal; A branching unit that branches the superimposed electrical signal; an RF signal demodulation unit that amplifies one of the superimposed electrical signals branched at the branch unit and demodulates the one of the superimposed electrical signals into the RF signal; a pulse signal demodulation unit that amplifies the other of the superimposed electrical signals branched at the branch unit and demodulates the other of the superimposed electrical signals into the pulse signal; A digital filter; A filter setting unit, the pulse signal generating unit generates the pulse signal so as to satisfy the following formula (1), where fr is a frequency of the RF signal, fs is a first frequency that is the reciprocal of a modulation speed of the pulse signal, and n is a natural number: The RF signal demodulation unit extracts a frequency of the RF signal based on the demodulated RF signal, The arithmetic processing unit calculates the modulation speed of the pulse signal based on the frequency of the RF signal extracted by the RF signal demodulation unit, The filter setting unit calculates an order and a cutoff frequency of the digital filter based on a frequency of the RF signal and the first frequency. fr=n×fs...(1)

2. 2. The signal transmission system according to claim 1, wherein the natural number n is 2 or more.

3. The signal transmission system according to claim 1 , wherein the pulse signal generating section generates the pulse signal by applying a raised cosine pulse capable of suppressing high frequency components.

4. The signal transmission system according to any one of claims 1 to 3, wherein the pulse signal demodulation unit has a low-pass filter with a cutoff frequency fc that satisfies the relationship of the following formula (2) when the frequency of the RF signal is equal to or more than twice the first frequency, satisfies the relationship of the following formula (3) when the frequency of the RF signal is equal to or more than 1.2 times but less than twice the first frequency, and satisfies the relationship of the following formula (4) when the frequency of the RF signal is 1.2 times the first frequency, where fs is a first frequency that is the reciprocal of a modulation speed of the pulse signal and γ is an order. 0.6×fs≦fc(γ≧4)≦1×fs (2) 0.6×fs≦fc(γ)≦0.75×fs (3) fc (γ≧7)≦0.75×fs (4)

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