A communication device that modulates light with radio frequency signals.

Abstract

To prevent the level of a RF signal obtained by photoelectrically converting modulated light from becoming excessively small.SOLUTION: A first communication device, which is located in a first site among a first and a second site that are connected by a first optical fiber, comprises: first addition means that adds up a first RF signal, of which a first frequency in a radiofrequency (RF) band is modulated, and a first sinusoidal signal of a second frequency in the RF band; and first modulation means that modulates double sidebands of an optical carrier with a signal that includes the first RF signal and first sinusoidal signal outputted by the first addition means, so as to generate a first modulated light. It is determined such that the sum of or the difference between the first and second frequencies is equal to a prescribed third frequency, and that the level of the second RF signal of the third frequency obtained by photoelectrically converting the first modulated light after being transmitted through the first optical fiber falls within a prescribed range.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to radio over fiber (RoF) communication technology.

Background Art

[0002] For example, in a mobile communication network, in order to simplify the configuration of an antenna site where an antenna is installed, a radio over fiber (RoF) system can be used for communication between the antenna site and a housing site that houses the antenna site. In RoF, a transmitting site generates modulated light by intensity-modulating continuous light with a radio frequency signal (RF signal) that is a signal in a radio frequency band, and transmits the generated modulated light to a receiving site. The receiving site restores the RF signal by photoelectrically converting the received modulated light with a photoelectric converter.

[0003] Intensity modulation is classified into double-sideband (DSB) modulation that generates DSB modulated light having both an upper sideband (USB) and a lower sideband (LSB), and single-sideband (SSB) modulation that generates SSB modulated light having only one of the upper sideband (USB) and the lower sideband (LSB). Patent Document 1 discloses an SSB modulator that generates SSB modulated light. Generally, an SSB modulator has a more complex configuration and is more expensive than a DSB modulator that generates DSB modulated light. Therefore, using DSB modulation in RoF is advantageous in terms of cost.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, when using DSB modulation, the level of the RF signal output by the photoelectric converter at the receiving site may be low due to the chromatic dispersion of the optical fiber. Specifically, in the case of DSB modulation, the photoelectric converter outputs an RF signal that is a combination of the first beat component of the optical carrier and USB and the second beat component of the optical carrier and LSB. Since the frequencies of USB and LSB are different, the phases of USB and LSB rotate differently from each other due to the chromatic dispersion of the optical fiber. Therefore, the phase relationship between the first beat component and the second beat component changes depending on the point of photoelectric conversion. For example, if the first beat component and the second beat component are in phase at the point of photoelectric conversion, that is, if the phase difference between the first beat component and the second beat component is an even multiple of π, the level of the RF signal output by the photoelectric converter will be maximum. On the other hand, if the phase difference between the first beat component and the second beat component is an odd multiple of π, the first beat component and the second beat component will cancel each other out, and the level of the RF signal output by the photoelectric converter will be minimum.

[0006] Figure 1 shows an example of the RF signal level output from a photoelectric converter when DSB-modulated light propagated through a 20km single-mode fiber is photoelectrically converted. In Figure 1, the RF signal level is shown as the attenuation relative to the maximum level of the RF signal output by the photoelectric converter. The horizontal axis in Figure 1 is the center frequency of the RF signal. As shown in Figure 1, the RF signal level (attenuation) is a function of the center frequency of the RF signal. The RF signal level is also a function of distance. For example, in Figure 1, when the center frequency of the RF signal is set to 30GHz, the RF signal level becomes very small. However, this is the case when DSB-modulated light is propagated through a 20km single-mode fiber, and the RF signal level output from the photoelectric converter changes depending on the length of the optical fiber through which the DSB-modulated light was propagated.

[0007] This invention provides a technique to prevent the level of the RF signal obtained by photoelectric conversion of modulated light from becoming too low. [Means for solving the problem]

[0008] According to one aspect of the present invention, a first communication device located at the first site of a first site and a second site connected by a first optical fiber comprises: a first adding means for adding a first RF signal modulated in the radio frequency (RF) band and a first sine wave signal in the RF band; and a first modulation means for generating a first modulated light by performing double-band modulation of an optical carrier with a signal including the first RF signal and the first sine wave signal output by the first adding means, wherein the first frequency is the frequency of the first RF signal and the second frequency is the frequency of the first sine wave signal. Both are configured to be changeable, and the first frequency and the second frequency are The sum or difference is determined to be a predetermined third frequency, and the level of the second RF signal of the third frequency obtained by photoelectric conversion of the first modulated light after transmission through the first optical fiber is determined to be within a predetermined range. [Effects of the Invention]

[0009] According to the present invention, it is possible to prevent the level of the RF signal obtained by photoelectric conversion of modulated light from becoming too low. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an example of the relationship between the level and frequency of an RF signal obtained by photoelectric conversion of modulated light that has propagated a predetermined distance. [Figure 2] A diagram illustrating the transmitter side configuration of a communication device according to several embodiments. [Figure 3] A diagram showing the frequency components of signals and modulated light generated within a communication device according to several embodiments. [Figure 4] Diagrams illustrating the receiving side configuration of a communication device according to several embodiments. [Figure 5] This figure shows an example of the relationship between the level of an RF signal obtained by photoelectric conversion of modulated light propagating a predetermined distance and the frequency of a sinusoidal signal. [Figure 6] Diagrams illustrating the receiving side configuration of a communication device according to several embodiments. [Figure 7] A diagram illustrating the transmitter side configuration of a communication device according to several embodiments. [Modes for carrying out the invention]

[0011] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features from the multiple features described in the embodiments may be combined arbitrarily. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0012] <First Embodiment> In the following description, it is assumed that a first site and a second site connected by an optical fiber communicate via RoF (Resonance of Flight). In this embodiment, the second site is an antenna site where an antenna is installed and which communicates wirelessly with a wireless device (WD) or user equipment (UE) of a mobile communication network, and the first site is a receiving site that accommodates one or more antenna sites. The first site transmits RF signals to the second site via an optical fiber. Similarly, the second site transmits RF signals to the first site via an optical fiber. In this embodiment, the transmitting side configuration of the first site and the transmitting side configuration of the second site are the same, and the receiving side configuration of the second site and the receiving side configuration of the first site are the same. Therefore, the transmitting side configuration of the first site and the receiving side configuration of the second site will be described below.

[0013] Furthermore, in the following description, the second site is assumed to transmit a wireless signal with a center frequency of f3 to the WD. In such a case, the first site typically modulates continuous light (optical carrier) with an RF signal with a center frequency of f3 and transmits it to the second site. However, as explained using Figure 1, depending on the length of the optical fiber connecting the first and second sites, the level of the RF signal restored at the second site may become too low. This embodiment aims to prevent the level of the restored RF signal from becoming too low. In the following description, the center frequency of a bandwidth-containing signal such as an RF signal or modulated light will simply be referred to as "frequency".

[0014] Figure 2 is a configuration diagram of the transmission side of a communication device installed at the first site, that is, a configuration diagram of a transmission device provided in the communication device. An RF signal with a frequency fx is input to the frequency converter 11. The RF signal is a signal that carries information, that is, a modulated signal. As described above, the frequency fx is the center frequency. The frequency converter 11 frequency-converts the RF signal with the frequency fx into an RF signal 81 (Figure 3(A)) with the frequency f1 based on the sine wave signal with the frequency fy from the variable oscillator 10 and outputs it to the adder 13. Note that the value of the frequency f1 is the sum or difference between the value of the frequency fy and the value of the frequency fx. The variable oscillator 12 outputs a sine wave signal 82 (Figure 3(A)) with the frequency f2 to the adder 13. The adder 13 adds (combines) the RF signal 81 and the sine wave signal 82 and outputs it to the DSB modulator 14. Figure 3(A) shows the frequency components of the signal output by the adder 13. In Figure 3(A), it is assumed that the frequency f2 is higher than the frequency f1, but the frequency f2 may be lower than the frequency f1.

[0015] The DSB modulator 14 generates DSB-modulated light by DSB-modulating an optical carrier wave with the signal from the adder 13 and transmits it to the second site. Hereafter, the DSB-modulated light is simply referred to as "modulated light". Figure 3(B) shows the frequency components of the modulated light. The reference numeral 90 is the optical carrier wave. The reference numeral 91 is the USB corresponding to the RF signal 81, and the reference numeral 93 is the LSB corresponding to the RF signal 81. Also, the reference numeral 92 is the USB corresponding to the sine wave signal 82, and the reference numeral 94 is the LSB corresponding to the sine wave signal 82.

[0016] Figure 4 is a configuration diagram of the receiving side of the communication device installed at the second site, that is, the configuration of the receiving device provided in the communication device. The photoelectric converter 20 performs photoelectric conversion of the modulated light received from the first site and outputs an electrical signal. The electrical signal includes the beat components of any combination of the components in Fig. 3(A), that is, the optical carrier 90, USB91, USB92, LSB93, and LSB94. The frequency of the beat component is the difference between the frequencies of the two components included in the combination (the distance on the frequency axis). As an example, the frequencies of the beat components of USB92 and LSB93, and the beat components of USB91 and LSB94 are f1 + f2, which is the result of frequency-converting the RF signal 81 with frequency f1 to frequency (f1 + f2).

[0017] Also, the photoelectric converter 20 outputs, like the beat component of USB91 and USB92, the result of frequency-converting the RF signal 81 with frequency f1 to frequency (f2 - f1). Furthermore, the photoelectric converter 20 outputs, like the beat component of USB92 and the optical carrier 90, the sine wave signal 82 with frequency f2, and also outputs, like the beat component of USB91 and the optical carrier 90, the RF signal 81 with frequency f1.

[0018] As described above, the photoelectric converter 20 outputs the result of frequency-converting the RF signal 81 with frequency f1 to frequency (f1 + f2), the result of frequency-converting it to frequency (f2 - f1), etc. However, in the following description, the sum component, that is, the frequency (f1 + f2) will be used. However, the difference component may also be used. In this embodiment, the frequencies f1 and f2 are determined such that the frequency (f1 + f2) becomes the frequency f3 of the wireless signal transmitted by the second site to the WD. That is, f1 and f2 are determined such that f3 = f1 + f2. The filter 21 extracts the RF signal with frequency f3 included in the electrical signal output by the photoelectric converter 20. The RF signal with frequency f3 is converted into a wireless signal with frequency f3 and transmitted to the WD.

[0019] Thus, the transmitting device does not perform DSB modulation with the RF signal of frequency f3, which is the frequency of the wireless signal to be transmitted to the WD, but rather with the RF signal 81 of frequency f1 and the sine wave signal 82 of frequency f2. As explained using Figure 1, if DSB modulation is performed with the RF signal of frequency f3, the level of the RF signal of frequency f3 output by the photoelectric converter 20 may become too low. The level of the RF signal of frequency f3 output by the photoelectric converter 20 is determined by the length of the optical fiber connecting the first site and the second site, and the wavelength dispersion of the optical fiber, but these parameters cannot be changed after the optical fiber has been laid.

[0020] On the other hand, there are many (theoretically infinite) frequency pairs of frequencies f1 and f2 to obtain frequency f3, and generally, there exists a frequency pair that keeps the level of the RF signal at frequency f3 output by the photoelectric converter 20 within an acceptable range. Therefore, by appropriately setting the values ​​of frequency f1 and frequency f2, it is possible to prevent the level of the RF signal at frequency f3 output by the photoelectric converter 20 from becoming too low.

[0021] The following outlines the method for determining frequency pairs. Figure 5 is similar to Figure 1 and shows the relationship between the level of the RF signal with frequency f3 output by the photoelectric converter 20 and the frequency f2 of the sine wave signal 82. However, in Figure 1, the horizontal axis was the frequency of the RF signal when the sine wave signal 82 was absent, whereas in Figure 5, the horizontal axis is the frequency of the sine wave signal 82. Since frequency f3 is a predetermined value, and therefore frequency f1 is determined once frequency f2 is determined, the horizontal axis in Figure 5 can also be considered as the combination of frequencies f1 and f2, i.e., a frequency pair. The graph in Figure 5 can be calculated based on the frequency f0 of the optical carrier, the chirp parameter of the DSB modulator 14, the length L of the optical fiber connecting the first site and the second site, and the chromatic dispersion value D per unit length.

[0022] For example, if the DSB modulator 14 can ignore the chirp parameter, like a Mach-Zehnder modulator, the graph in Figure 5 follows equation (1). |cos{πcDL(f2 2 -f1 2 ) / f0 2}| 2 (1) In equation (1), c is the speed of light.

[0023] The frequency pair is determined such that the level of the RF signal at frequency f3 output by the photoelectric converter 20 is within an acceptable range (a predetermined range). For example, if the acceptable range of the RF signal level at frequency f3 (attenuation relative to the maximum value) is 0 to A (where A is a negative value), then a frequency pair that is above the dashed line in Figure 5 is selected. As shown in Figure 5, there are many frequency pairs that keep the RF signal level within the acceptable range, and the frequency pair actually used (hereinafter referred to as the "used frequency pair") is selected from these many frequency pairs that keep the RF signal level within the acceptable range so as to satisfy the following conditions.

[0024] [Condition 1] The sinusoidal signal 82 with frequency f2 must be outside the bandwidth of the RF signal 81 with frequency f1. Therefore, if the bandwidth of the RF signal 81 is ΔRF, the frequency pair used must satisfy the condition that the difference between f2 and f1 is greater than ΔRF / 2.

[0025] [Condition 2] As described above, the photoelectric converter 20 also outputs an RF signal 81 with frequency f1 and a sine wave signal 82 with frequency f2. Interference occurs if the sine wave signal 82 with frequency f2 is within the bandwidth of the RF signal with frequency f3, or if there is an overlapping region between the bandwidths of the RF signal 81 with frequency f1 and the RF signal with frequency f3. Therefore, the frequency pair used must satisfy the condition that the difference between frequencies f1 and f3 is greater than ΔRF, and the difference between frequencies f2 and f3 is greater than ΔRF / 2.

[0026] [Condition 2'] The filter 21, located in the subsequent stage, extracts the RF signal of frequency f3 from the electrical signal output by the photoelectric converter 20. Condition 2 was a condition to prevent interference between the RF signal of frequency f3 and the RF signal 81 of frequency f1 and the sine wave signal 82 of frequency f2. However, considering the performance of the filter 21, Condition 2 can be replaced with Condition 2', which states that the difference between frequencies f1 and f3, and the difference between frequencies f2 and f3, are greater than a predetermined number ΔF. ΔF is set considering the passband characteristics of the filter 21, such that the levels of the RF signal 81 of frequency f1 and the sine wave signal 82 of frequency f2 in the signal output by the filter 21 are smaller than a first threshold.

[0027] Furthermore, it is possible to use both conditions 2 and 2', rather than using only one of them. In other words, the frequency pair can be selected from a frequency pair in which the RF signal 81 at frequency f1 and the sine wave signal 82 at frequency f2 do not interfere with the RF signal at frequency f3, and the levels of the RF signal 81 at frequency f1 and the sine wave signal 82 at frequency f2 included in the electrical signal output by the filter 21 are smaller than the first threshold.

[0028] If there are many frequency pairs that satisfy condition 1 and condition 2 and / or condition 2', one of these frequency pairs is selected as the usable frequency pair. For example, the configuration may be such that the frequency pair that has the highest RF signal level at frequency f3 among the frequency pairs that satisfy the conditions is selected as the usable frequency pair.

[0029] Furthermore, the frequency pairs can be further narrowed down by setting the following optional conditions, and then the frequency pair that has the highest RF signal level at frequency f3 can be selected as the usable frequency pair. For example, the upper and lower limits of frequencies f1 and f2 can be set as optional conditions. This takes into account the characteristics of the variable oscillators 10 and 12. In this case, the usable frequency pair will be selected from frequency pairs in which frequencies f1 and f2 are within the range from their lower limit to their upper limit. In addition, an optional condition can be set to make frequency f1 lower than frequency f2.

[0030] If no frequency pair satisfies condition 1 and condition 2 and / or condition 2', consider whether the acceptable range for the RF signal level can be further relaxed, that is, whether the value A in Figure 5 can be made lower, and if so, relax it. For condition 2', consider whether the predetermined number ΔF can be made smaller, and if so, make it smaller.

[0031] In the configuration shown in Figure 2, if frequency fx is equal to frequency f1, the variable oscillator 10 and frequency converter 11 can be omitted. Also, in the configuration shown in Figure 2, if frequency fx is equal to frequency f3, the frequency converter 11 can perform frequency conversion using the sine wave signal 82 of frequency f2 output by the variable oscillator 12. In this case, the variable oscillator 10 can be omitted.

[0032] <Transformation Form 1> As described above, the transmitter configuration of the first site and the transmitter configuration of the second site are the same, and the receiver configuration of the first site and the receiver configuration of the second site are the same. Therefore, Figure 2 is also a configuration diagram of the transmitter of the second site, and Figure 4 is also a configuration diagram of the receiver of the first site. In the case of the second site, the frequency fx of the RF signal input to the frequency conversion unit 11 corresponds to the frequency of the radio signal transmitted by the WD. If the radio section is frequency division duplexing (FDD), frequency fx is different from frequency f3, and if the radio section is time division duplexing (TDD), frequency fx is frequency f3. Frequencies f1 and f2 can be set individually for the direction from the first site to the second site (hereinafter referred to as the first direction) and the direction from the second site to the first site (hereinafter referred to as the second direction).

[0033] However, typically, the first optical fiber that carries modulated light in the first direction and the second optical fiber that carries modulated light in the second direction are housed in the same optical cable, and their lengths are approximately the same. Furthermore, the first and second optical fibers are usually of the same type, and their dispersion characteristics are also similar. Therefore, the frequency pair of frequencies f1 and f2 selected for the first direction can be used as is for the second direction.

[0034] As described above, the photoelectric converter 20 of the receiving device at the second site also outputs a sine wave signal 82 with frequency f2. Therefore, if the frequency pairs used in the first and second directions are the same, the variable oscillator 12 can be omitted in the transmitting device at the second site. Figure 6 shows the configuration of the receiving device at the second site in this case. In Figure 6, a separator 22 is provided instead of a filter 21. The separator 22 outputs an RF signal with frequency f3 and a sine wave signal 82 with frequency f2 separately. The sine wave signal 82 with frequency f2 output by the separator 22 is input to the adder 13 of the transmitting device at the second site.

[0035] <Transformation Form 2> Furthermore, if the frequency pairs used in the first and second directions are the same and the TDD method is used in the wireless section, an RF signal with frequency f3 is input to the transmitter at the second site. In this case, the frequency of the sine wave signal required to frequency convert the RF signal with frequency f3 to an RF signal with frequency f1 is frequency f2. Therefore, the sine wave signal 82 with frequency f2 output by the separation unit 2 in Figure 6 can be used for frequency conversion in the frequency conversion unit 11 of the transmitter at the second site, and the variable oscillator 10 can be omitted in the transmitter at the second site. In this case, the variable oscillator 12 of the transmitter at the first site generates a sine wave signal 82 during both the transmission period in the first direction and the transmission period in the second direction, and the DSB modulator transmits modulated light including the optical carrier wave 90, USB 92, and LSB 94 to the second site even during the transmission period in the second direction.

[0036] <Second Embodiment> Next, the second embodiment will be described, focusing on the differences from the first embodiment. When the wireless section is TDD, an RF signal with frequency f3 is input to the transmitter at the second site. Here, in the first direction, DSB modulation is performed using an RF signal with frequency f1 and a sine wave signal with frequency f2, and in the second direction, DSB modulation is performed using an RF signal with frequency f3 and a sine wave signal with frequency f2. Note that f1 + f2 = f3. As shown in Figure 6, the receiver at the second site can extract a sine wave signal with frequency f2. Therefore, in this case, as shown in Figure 7, the transmitter at the second site only needs to input the RF signal with frequency f3 based on the wireless signal from the WD and the sine wave signal with frequency f2 from the receiver to the adder 13, and the variable oscillator 10, frequency converter 11, and variable oscillator 12 can be omitted.

[0037] Therefore, for a given frequency f3, a graph like Figure 5 is created showing the relationship between frequency f2 and the level of the RF signal output by the photoelectric converter 20 of the receiving device at the first site. Under the condition that the level of the RF signal output by the photoelectric converter 20 of the receiving device at the first site is within an acceptable range (predetermined range), candidate frequencies f2 that can be used are determined. Note that the acceptable range of the RF signal level output by the photoelectric converter 20 may be the same or different for the first and second directions. Then, the value of the frequency f2 to be actually used is determined from the candidate frequencies f2 that can be used. This is based on conditions similar to the first, second, and second' conditions in the first embodiment. For example, the RF signal with frequency f1 is used from the RF signal with frequency (f3+f2) and the RF signal with frequency (f3-f2)=f1 output by the photoelectric converter 20 of the receiving device at the first site. In this case, the third condition corresponding to the first condition, and the fourth and fourth' conditions corresponding to the second and second' conditions are as follows.

[0038] [Condition 3] The sinusoidal signal 82 with frequency f2 must be outside the bandwidth of the RF signal with frequency f3. Therefore, the condition that the difference between f2 and f3 is greater than ΔRF / 2 must be satisfied. This condition is satisfied by condition 2.

[0039] [Condition 4] The photoelectric converter 20 also outputs an RF signal with frequency f3 and a sine wave signal 82 with frequency f2. Interference occurs if these signals are within the bandwidth of the RF signal 81 with frequency f1. Therefore, the frequency pair used must satisfy the condition that the difference between frequencies f1 and f3 is greater than ΔRF, and the difference between frequencies f2 and f1 is greater than ΔRF / 2. This condition is satisfied by conditions 1 and 2.

[0040] [Condition 4'] When considering the performance of filter 21, condition 4 can be replaced with condition 4', which states that the difference between frequency f1 and frequency f3, and the difference between frequency f2 and frequency f1, are greater than a predetermined number ΔF. Note that the former condition is satisfied by condition 2'. The latter condition becomes a new condition.

[0041] In this way, a frequency pair that includes frequency f2, which is also usable in the second direction, is determined from among the frequency pairs available in the first direction. Then, by selecting the frequency pair to be used from the determined frequency pairs, the configuration of the transmitter at the second site can be simplified as shown in Figure 7.

[0042] <Third Embodiment> In the first embodiment, the relationship between the frequency f2 shown in Figure 5 and the level of the RF signal of frequency f3 output by the photoelectric converter 20 was determined by calculation, and a frequency pair was selected to satisfy predetermined conditions. However, from the formula for determining the relationship in Figure 5, if the chirp parameter of the DSB modulator 14 is a predetermined value, for example, 0.3 or less, setting f1 and f2 to values ​​near (f3) / 2 results in the level of the RF signal output by the photoelectric converter 20 being near the maximum value (0 dB).

[0043] Therefore, in this embodiment, both frequencies f1 and f2 are set to values ​​near (f3) / 2. Frequencies f1 and f2 are approximately half of frequency f3, and thus, generally, conditions 2 / 2' of the first embodiment are satisfied. Consequently, when frequencies f1 and f2 are set in this way, the only condition that needs to be satisfied is condition 1 of the first embodiment.

[0044] In the first embodiment, condition 1 did not consider the phase noise of the sine wave signal at frequency f2. When considering the phase noise of the sine wave signal at frequency f2, condition 1 becomes one where the difference between f2 and f1 is larger by ΔRF / 2+γ. Here, γ is determined such that the signal-to-noise ratio of the RF signal at frequency f1 after the addition of phase noise is greater than the second threshold that the transmitting side must satisfy. In other words, it is determined such that the degradation of the signal-to-noise ratio of the RF signal at frequency f1 due to phase noise is less than the third threshold. This modified first condition is applicable to each embodiment.

[0045] With the above configuration, it is possible to prevent the level of the RF signal obtained by photoelectric conversion of modulated light from becoming too low. Therefore, it becomes possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0046] The invention is not limited to the embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention. [Explanation of Symbols]

[0047] 13: Adder, 14: DSB modulator

Claims

1. A first communication device located at the first site of a second site connected by a first optical fiber, A first adding means for adding a modulated first RF signal in the radio frequency (RF) band and a first sine wave signal in the RF band, A first modulation means generates a first modulated light by performing double-band modulation of an optical carrier wave with a signal including the first RF signal and the first sine wave signal output by the first adding means, Equipped with, A first communication device in which a first frequency, which is the frequency of the first RF signal, and a second frequency, which is the frequency of the first sine wave signal, are both configured to be changeable, and the sum or difference of the first frequency and the second frequency is determined to be a predetermined third frequency, and the level of the second RF signal of the third frequency obtained by photoelectric conversion of the first modulated light after transmission through the first optical fiber is within a predetermined range.

2. A conversion means that converts the frequency of the RF band signal and outputs the first RF signal of the first frequency, The first generation means for generating the first sinusoidal signal, A second generation means for generating a third sinusoidal wave signal, Equipped with, The first generation means is configured to change the value of the second frequency, which is the frequency of the first sinusoidal signal. The second generation means is configured to change the frequency value of the third sinusoidal signal, The first communication device according to claim 1, wherein the conversion means converts the frequency of the RF band signal using the third sinusoidal signal.

3. The first communication device according to claim 1, wherein the first frequency and the second frequency are determined such that the first sinusoidal signal is outside the bandwidth of the first RF signal.

4. The first communication device according to claim 3, wherein the first frequency and the second frequency are further determined such that the degradation of the signal-to-noise ratio of the first RF signal due to the phase noise of the first sinusoidal wave signal is less than a first threshold.

5. The chirp parameter of the first modulation means is less than or equal to a predetermined value. The first communication device according to claim 4, wherein the first frequency and the second frequency are the combination of the first frequency and the second frequency that satisfies the conditions that their sum or difference equals a predetermined third frequency, the level of the second RF signal is within the predetermined range, the first sine wave signal is outside the bandwidth of the first RF signal, and the degradation of the signal-to-noise ratio of the first RF signal due to the phase noise of the first sine wave signal is less than a first threshold, and the frequency difference between the first frequency and the second frequency is the smallest among these combinations of first and second frequencies.

6. The first communication device according to claim 3, wherein the first frequency and the second frequency are determined such that the first sinusoidal signal is outside the bandwidth of the second RF signal, and the bandwidths of the first RF signal and the second RF signal do not overlap.

7. The first communication device according to claim 6, wherein the frequency difference between the second frequency and the third frequency, and the frequency difference between the first frequency and the third frequency are determined such that the levels of the components of the first RF signal and the components of the first sine wave signal included in the signal including the second RF signal separated from the signal obtained by photoelectric conversion of the first modulated light are smaller than a second threshold.

8. A second communication device located at the second site, which receives the first modulated light via the first optical fiber from the first communication device according to any one of claims 1 to 7, A photoelectric conversion means that outputs a first electrical signal by photoelectric conversion of the first modulated light, A separation means for separating the second RF signal from the first electrical signal, Equipped with, The separation means separates the second sinusoidal signal of the second frequency from the first electrical signal. The second communication device further, A second adding means for adding the third RF signal of the first frequency and the second sinusoidal signal, A second modulation means generates a second modulated light by performing double-band modulation of an optical carrier wave with a signal including the third RF signal and the second sine wave signal output by the second adding means, Equipped with, The second modulated light is transmitted to the first communication device via a second optical fiber connecting the first site and the second site, and the second communication device.

9. The system further comprises frequency conversion means for frequency-converting the received fourth RF signal of the third frequency to the third RF signal, The second communication device according to claim 8, wherein the frequency conversion means performs the frequency conversion using the second sinusoidal signal.

10. The second communication device according to claim 8, wherein the first optical fiber and the second optical fiber are of the same type.

11. A second communication device located at the second site, which receives the first modulated light via the first optical fiber from the first communication device according to any one of claims 1 to 7, A photoelectric conversion means that outputs a first electrical signal by photoelectric conversion of the first modulated light, A separation means for separating the second RF signal from the first electrical signal, Equipped with, The separation means separates the second sinusoidal signal of the second frequency from the first electrical signal. The second communication device further, A second adding means for adding the third RF signal of the third frequency and the second sinusoidal signal, A second modulation means generates a second modulated light by performing double-band modulation of an optical carrier wave with a signal including the third RF signal and the second sine wave signal output by the second adding means, Equipped with, The second modulated light is transmitted to the first communication device via a second optical fiber connecting the first site and the second site. The second communication device is determined such that the level of a fourth RF signal, which is the sum or difference between the third frequency and the second frequency obtained by photoelectric conversion of the second modulated light after transmission through the second optical fiber, falls within a predetermined range.

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