Amplifying device, transmission module, and transmission system

Abstract

To provide an amplifier or the like that can transmit a signal in the high-frequency band without degrading the quality for a transmission path that includes a device with insufficient frequency response characteristics in a high-frequency band, and suppress noise in the transmission path when the SNR of the signal is low.SOLUTION: An amplification device is placed in front of a transmission path having a frequency domain in which the gain is attenuated as the frequency increases, receives a signal containing a frequency component in the frequency domain, amplifies the signal with an amplification degree that reduces attenuation of the frequency component in the transmission path, and outputs the amplified signal to the transmission path, and reduces the deterioration of the SNR of the signal in the transmission path.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an amplification device, a transmission module, and a transmission system.

Background Art

[0002] Conventionally, development of RF transmission technology that suppresses noise increase and distortion such as third-order intermodulation distortion has been promoted using low-noise, high-output, and high-linearity devices.

[0003] For example, in the TV optical transmission system configuration of Patent Document 1, in an electro-optical conversion device in a TV optical transmission system for transmitting a TV-RF signal through an optical fiber, a current regulator for changing the current of a semiconductor laser diode light source is provided. Further, in an electro-optical conversion device in a TV optical transmission system for transmitting a TV-RF signal through an optical fiber, an RF amplitude regulator for adjusting the TV-RF signal amplitude is provided.

[0004] Specifically, Patent Document 1 has a feature that a good linearity current value can be set for the electro-optical conversion device by a current regulator for adjusting a point with good linearity, and the reception state can be optimally adjusted. Also, it has a feature that the reception state can be optimally adjusted by optimally setting the modulation amplitude range by an RF amplitude variable step switch that can adjust the RF signal amplitude adjustment amount in 1-decibel units.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the prior art, linearity in at least a part of the high-frequency band of a device is utilized to reduce noise. Also, the amplitude is adjusted according to the region with high linearity where an optimal error rate can be obtained, so as to be able to handle high-output signals. However, when the above configuration is not used, it has been difficult to transmit high-frequency signals without degrading them using commercially available devices with insufficient frequency response characteristics in the high-frequency band. Also, in a transmission system such as an optical fiber with relatively large noise, it has been difficult to suppress the noise, and there have been cases where it becomes difficult to transmit signals with a low SNR.

[0007] The present invention has been made in view of such problems of the prior art. And an object of the present invention is to provide an amplification device or the like that can transmit signals in a high-frequency band without degrading the quality of the signals in a transmission path including a device with insufficient frequency response characteristics in the high-frequency band, and can suppress the noise of the transmission path when the SNR of the signal is low.

Means for Solving the Problems

[0008] An amplification device arranged in the front stage of a transmission path having a frequency region where the gain decays as the frequency increases according to an aspect of the present invention inputs a signal including frequency components in the frequency region, amplifies the signal with an amplification degree that reduces the attenuation amount of the frequency components in the transmission path, outputs the amplified signal to the transmission path, and preferably reduces the degradation of the SNR of the signal in the transmission path.

[0009] A transmission module according to another aspect of the present invention preferably includes the amplification device of the above aspect, a feedback unit that outputs attenuation instruction information before distortion occurs in the frequency region in the transmission path, and an attenuation unit that attenuates the signal input to the amplification device in response to the attenuation instruction information.

[0010] A transmission system according to another aspect of the present invention preferably includes the transmission module of the above aspect, a transmission device arranged downstream of the transmission module, a transmission medium that inputs an output signal of the transmission device, and a reception device that inputs an output signal of the transmission medium.

Advantages of the Invention

[0011] According to the present invention, it is possible to provide an amplification device or the like that can transmit a signal in a high-frequency band through a transmission path including a device with insufficient frequency response characteristics in the high-frequency band without deteriorating the quality of the signal in the high-frequency band, and can suppress noise in the transmission path when the SNR of the signal is low.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0013] Hereinafter, an example of an amplifier, a transmission module, and a transmission system according to this embodiment will be described in detail with reference to the drawings. Note that the embodiments described below show comprehensive or specific examples. The numerical values, shapes, materials, components, installation positions and connection forms of the components, steps, order of steps, etc. shown in the following embodiments are merely examples and are not intended to limit the present disclosure. In addition, among the components in the following embodiments, the components not described in the independent claims indicating the most general concept are described as optional components. Furthermore, the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may be different from the actual ratios.

[0014] In addition, the following embodiments and their modifications may include similar components. The similar components are given common reference numerals and duplicate explanations are omitted.

[0015] (Explanation of the configuration and operating principle of the amplification device, transmission module, and transmission system including the transmission module) FIG. 1 is a schematic diagram showing an image of a transmission system 1000a according to the present embodiment. The transmission system 1000a includes an amplification device 120a to which an RF signal SIa with suppressed SNR is input, a transmission device 200, a transmission medium 300a, and a reception device 400. The RF signal SIa is preferably a wireless signal including a carrier wave in a high-frequency region such as a millimeter-wave band, but is not limited to such a wireless signal. In a transmission path including the transmission device 200, the transmission medium 300a, and the reception device 400 without the amplification device 120a according to the present embodiment, when the RF signal SIa is input, it is assumed that the gain attenuates in the high-frequency region of the RF signal SIa. The measurement results showing the frequency response characteristics of this form are shown in FIG. 2.

[0016] The horizontal axis of FIG. 2 is the input power (dBm) of the RF signal SIa, and the vertical axis shows the EVM (Error Vector Magnitude) in the output signal SOa of the reception device 400. When the RF signal SIa includes a 6 GHz component, the 6 GHz component has good frequency response characteristics with an EVM of less than about 3% in a dynamic range of about 20 dBm from about -12 dBm to about -32 dBm. Since the 6 GHz component has good frequency response characteristics in the dynamic range, it is assumed that there is almost no gain loss. However, when an input power of about -12 dBm or more is input to the transmission path, the EVM rapidly deteriorates. Therefore, it is assumed that distortion affects the 6 GHz component with an input power of about -12 dBm or more in the transmission path.

[0017] On one hand, for the 28 GHz component of the RF signal SIa, the transmission path has a frequency response characteristic in which the EVM deteriorates to about 8% or more in the range where the input power is from about -32 dBm to about -20 dBm. That is, for the 28 GHz component, even in the range where the input power is from -32 dBm to about -20 dBm, gain loss occurs, and it is assumed that the EVM deteriorates because an input signal with sufficient amplitude cannot be obtained. Also, it is assumed that the EVM deteriorates due to the noise power of the transmission path because an input signal with sufficient amplitude cannot be obtained. Especially in optical fiber wireless, since the noise power is large and the SNR deteriorates significantly due to insufficient frequency response characteristics, conventionally, a signal including a frequency region with good frequency response characteristics, for example, a frequency region up to about 6 GHz in FIG. 1, has been used for the transmission path. Also, in order to transmit a signal including the 28 GHz component, the transmission path has been configured by including an expensive high-linearity device whose frequency response characteristics are not insufficient even at 28 GHz.

[0018] However, in the frequency response characteristic of the RF signal SIa in FIG. 1, even in the region where the link gain at 28 GHz decays and the EVM deteriorates, by amplifying the 28 GHz component so as to compensate at least a part of the decay amount of the link gain, an improvement in the EVM is expected.

[0019] FIG. 5 shows the result of amplifying the power of the 28 GHz signal component input to the amplifier 120a in the transmission system 1000a of FIG. 1. When the power of the 28 GHz signal component is not amplified, from FIG. 2, it is expected that the EVM will deteriorate from 0 dBm for the 28 GHz signal component input to the transmission path, similar to the 6 GHz component. However, it can be understood from FIG. 5 that when the 28 GHz signal component is amplified, the range in which a good state of the EVM can be maintained expands.

[0020] As described above, a specific example in which it is assumed that the dynamic range of the 28 GHz signal component does not decrease even when the input power of the 28 GHz signal component is increased will be described with reference to FIG. 6 using a light-emitting element. The black circle symbols in FIG. 6 are third-order intermodulation distortion signals of the 6 GHz component, and the black square symbols are third-order intermodulation distortion signals of the 28 GHz component. Even when a large power of around 5 dBm is input to the 28 GHz component, the output power can only be about -40 dBm. However, even when a small power of around -15 dBm is input to the 6 GHz component, an output power of around -45 dBm can be output. Also, there is a difference of about 17 dB between the signals of the 6 GHz component and the 28 GHz component that can obtain output powers around the same level. Applying this result to FIG. 5, it is understood that it is a reasonable presumption that the dynamic range of the 28 GHz component in FIG. 5 is a range in which the dynamic range of the 6 GHz component is shifted to the lower power side by about 17 dB. In fact, since the difference between the 6 GHz component and the 28 GHz component in the EVM degradation region on the lower input power side in FIG. 5 is about 18 dB, it is considered that it is within a range where logical consistency with the results of FIG. 6 can be achieved.

[0021] That is, when transmitting an RF signal using a high-frequency carrier such as millimeter waves, if the frequency response characteristics of the transmission path are insufficient, the link gain will attenuate, the SNR will deteriorate, and the EVM will increase. Only with a conventional transmission path, for a 6 GHz input signal, it has a dynamic range in the range of about 30 dBm or more with an EVM of 8% or less. However, for a 28 GHz input signal, with an EVM of 8% or less, only about half of the dynamic range can be obtained. However, if an amplification device 120a is arranged in front of the conventional transmission path as shown in FIG. 1 to amplify the 28 GHz input signal, as shown in FIG. 5, it may be possible to obtain a dynamic range of about 30 dBm or more with an EVM of 8% or less. Also, as inferred from FIG. 6, it is assumed that the 28 GHz input signal also has a dynamic range similar to that of the region where the EVM of the 6 GHz input signal is good.

[0022] Next, an example of distortion caused by the transmission path when the input signal is over-amplified will be described. Distortion caused by the transmission path often occurs due to non-linear characteristics of modulators, demodulators, detectors, etc. However, the distortion generated by the light-emitting element (electrical / optical conversion unit) when an optical fiber is used as the transmission medium will be described with reference to FIG. 3.

[0023] The horizontal axis in FIG. 3(a) indicates the current value (mA) of the input signal input to the light-emitting element, and the vertical axis indicates the intensity of the output light output from the light-emitting element in power (mW). When the current value of the input signal SIc is small, the I-L characteristic is linear, so the input signal SIc is not distorted, and the output light SOc is linearly amplified without distortion and input to the optical fiber. However, as shown in FIG. 3(b), when an input signal SId with a large amplification degree is input, clipping occurs, and the amplitude of the output signal SOd is no longer a sine wave similar to the input signal SId. That is, the lower limit direction of the amplitude of the output signal SOd is saturated, that is, clipped, and a large distortion occurs in the output signal SOd. The clipped output signal SOd rapidly increases the third-order intermodulation distortion and rapidly deteriorates the EVM. Therefore, according to the prior art, since the amplification degree of the RF signal SIa needs to be set so that the output signal falls within a region where no distortion such as clipping occurs, the transmissible signal power region is limited. However, the amplification device 120a according to the present embodiment amplifies, for example, in a region where the frequency response characteristic of the light-emitting element is insufficient to compensate for the link deficiency, so that the signal power region can be expanded. That is, in the expanded signal power region, it is possible to suppress the deterioration of the SNR and maintain a good EVM. Specifically, as described above, as shown in FIG. 4, even for a large-power input of a 28 GHz signal, the third-order intermodulation distortion is suppressed, and it is possible to expand the region with good EVM.

[0024] That is, when an optical fiber is used for a transmission path, third-order intermodulation distortion may increase in a light-emitting element that converts an electrical signal into an optical signal. In Fig. 3(b), since an excessively amplified electrical input signal exceeds the lower limit value of the I-P characteristic of the light-emitting element, the lower part of the output optical signal is clipped, indicating that large distortion has occurred. When clipping occurs, the third-order intermodulation distortion increases rapidly, and the EVM deteriorates. The deterioration of the EVM at 6 GHz with high input power in Fig. 3 is assumed to occur due to distortion such as this third-order intermodulation distortion. However, as described with reference to Fig. 4, for input signals at high frequencies such as 28 GHz, the link gain is decreasing. Therefore, until the input signal at a high frequency such as 28 GHz is amplified to sufficiently satisfy the shortage due to the link gain, it is possible to increase the amplification degree without causing deterioration of the EVM.

[0025] Fig. 4 is a graph showing the frequency characteristics before and after third-order intermodulation distortion occurs in an OFDM signal in an amplifier or a light-emitting element. The horizontal axis of the graph in Fig. 4 represents frequency, and the vertical axis of the graph in Fig. 4 represents the absolute value of the output power in dBm. The left graph in Fig. 4 shows the frequency spectrum of the OFDM signal by two subcarrier signals f1 and f2 around 28 GHz, and it can be confirmed that the state where the noise and the frequency components of the OFDM signal are clearly separated. The right graph in Fig. 4 is a graph when third-order intermodulation distortion is generated in the OFDM signal by two subcarrier signals f1 and f2 around 28 GHz. It shows the noise band and the noise level generated by the third-order intermodulation distortion signals with frequencies (2f1 - f2) and (2f2 - f1). It is understood that the SNR decreases and the value of the EVM increases because the third-order intermodulation distortion signals of (2f1 - f2) and (2f2 - f1) are superimposed as noise on the OFDM signals of the subcarrier signals f1 and f2. Also, it can be understood that since the power leaking into the adjacent channels in the noise of the third-order intermodulation distortion signals also increases, the adjacent subcarrier signals are also greatly affected.

[0026] Next, with reference to FIGS. 7 to 9, a configuration for suppressing deterioration of the SNR of an input signal by amplifying, in the amplifier 120a, the amplification degree of an insufficient region where the response of the frequency response characteristic is insufficient up to the distortion generation limit will be described.

[0027] FIG. 7 is an example of a graph showing a situation where the EVM deteriorates due to noise or the like generated in the transmission path as the power of the input signal decreases, and a situation where the EVM deteriorates due to distortion or the like generated in the transmission path as the power of the input signal increases. The horizontal axis of FIG. 7 is the input power (dBm), which is a relative value indicated by the measurement system. The vertical axis of FIG. 7 is the EVM, and the region where the EVM exceeds 9% is omitted. As described above, when the power of the input signal is decreased below about -8 dBm, the influence of the noise generated in the transmission path appears, and the deterioration tendency of the EVM becomes remarkable. When the power of the input signal is decreased below -20 dBm, the EVM reaches a situation where it exceeds 8%. When the power of the input signal is increased above 6 dBm, the influence of the distortion generated in the transmission path appears, and the EVM tends to deteriorate. When the power of the input signal is increased above 14 dBm, the EVM reaches a situation where it exceeds 8%.

[0028] That is, in FIG. 7, when the power of the input signal decreases, a region where the EVM rapidly deteriorates due to the noise figure of the transmission path is generated. This region is also referred to as a penalty region due to the noise figure of the transmission path. When the power of the input signal increases, distortion as described above occurs, and the EVM rapidly deteriorates. Therefore, it is preferable to amplify the input signal between the penalty region due to the noise figure and the EVM deterioration region due to distortion. In such a range of input power, the amplifier of the present embodiment may be able to reduce the influence of the noise generated in the transmission path. Next, such a case will be described.

[0029] When the noise component level of the input signal is smaller than the noise level generated in the transmission path, and no distortion occurs in the signal component due to the amplification of the noise component, and the amplified noise component level becomes larger than the noise level, the amplification device of the present embodiment is effective. The SNR of the input signal as described above is often relatively small. For example, in the OFDM signal used in the fourth-generation mobile communication system, since the SNR of the OFDM signal is limited to around about 40 dB, the amplification device 120a according to the present embodiment is likely to exhibit the above effect. FIG. 8(a) is a schematic diagram showing the power spectrum of the OFDM input signal before being input to the amplification device 120a. In OFDM, information is transmitted in each of a plurality of subcarriers, so the convex portion in FIG. 8(a) indicates the power of the signal component. Also, the frequency band portions on both sides of the convex portion indicate the power of the noise component. FIG. 8(b) is a schematic diagram showing the power spectrum of the amplified OFDM output signal output from the amplification device 120a. The difference between the noise level of the amplified OFDM output signal and the noise level generated in the transmission path is indicated by ΔP. It is understood that when amplification is performed so that ΔP increases within the range where the signal component of the OFDM output signal is not distorted, the influence of the noise level generated in the transmission path is suppressed.

[0030] FIG. 9 plots the change in the difference ΔP between the noise level in the transmission path and the noise level after amplification of the input signal and the change in the noise level in the transmission path. The noise figure (NF) of the transmission path is expressed in dB by the following equation (2). NF = 10 log((S_in / N_in) / (S_out / N_out)) -----(2) (S_in is the input signal to the transmission system 1000a, N_in is the input noise to the transmission system 1000a. S_out is the output signal from the transmission system 1000a, N_out is the output noise from the transmission system 1000a.) Also, if F = (S_in / N_in) / (S_out / N_out), then Since F = (S_in / N_in) / (G × S_in / (N_add + G × N_in)) (where N_add is the noise volume of the transmission path and G is the amplification factor of the amplification device 120a), F = (N_add + G × N_in) / G × N_in F = ((N_add / G) + N_in) / N_in F = (N_add / (G × N_in) + 1

[0031] Therefore, it is understood that when (G × N_in) becomes larger than (N_add), the noise figure of the transmission path is reduced. That is, when the SNR is limited as in the case of an OFDM signal, the input signal input to the transmission path is amplified by an amplification device arranged in the previous stage of the transmission path, and when the amplified input noise power becomes equal to or greater than the noise power of the transmission path, it becomes possible to suppress the noise figure of the transmission path.

[0032] However, as described above, if the amplification factor of the amplification device is increased too much, the amplified signal may rapidly deteriorate in EVM due to the distortion characteristics of the transmission path. Therefore, it is necessary to operate the amplification device 120a with an amplification factor that is not affected by the distortion characteristics of the transmission path.

[0033] Based on the above findings, the transmission module 100 according to this embodiment having a configuration for applying feedback to the amplification factor of the amplification device according to this embodiment and the transmission system 1000b according to this embodiment including the transmission module will be described with reference to FIG. 10.

[0034] FIG. 10 is a schematic diagram showing an image of a transmission system 1000b according to the present embodiment. The transmission system 1000b includes a transmission module 100 into which an RF signal SIb with suppressed SNR is input, a driving unit 500 for driving an electric / optical conversion unit 600 such as a light-emitting element, the electric / optical conversion unit 600, a transmission medium 300b, and an optical / electric conversion unit 700. Note that the transmission medium 300b is an optical fiber. Although the noise in optical fiber radio has a very large value from -150 dBm / Hz to -140 dBm / Hz, in the transmission system 1000b according to the present embodiment, it is possible to effectively suppress the SNR. Note that the transmission media 300a and 300b may be collectively referred to as the transmission medium 300. Also, the transmission systems 1000a and 1000b may be collectively referred to as the transmission system 1000.

[0035] The transmission module 100 includes a variable attenuation unit 110, an amplification unit 121b that amplifies a signal attenuated or not attenuated by the variable attenuation unit 110, and a feedback unit 130 that notifies in advance the occurrence of distortion in the transmission path of the output signal of the amplification unit 121b. Note that the amplification devices 120b and 120a including the amplification unit 121b may be collectively referred to as the amplification device 120.

[0036] In the prior art, in a transmission system including the driving unit 500, the electric / optical conversion unit 600, an optical fiber as the transmission medium 300b, and the optical / electric conversion unit 700, a signal having a frequency below the cutoff frequency, which is the response limit of the transmission system, was transmitted. This is because frequency components above the cutoff frequency have insufficient frequency response characteristics of the transmission system, resulting in a decrease in link gain and a large deterioration in SNR. In particular, in optical fiber radio, since the noise of the transmission system is relatively large, if the frequency response characteristics are insufficient, the SNR may deteriorate significantly. Therefore, the transmission system has been used at frequencies in a region where the frequency response characteristics of the transmission system can be fully utilized. Or, a transmission system using an expensive device with a wide-transition cutoff frequency has been constructed.

[0037] However, by providing an amplifier 120b having an amplification factor that compensates for the insufficient gain in the region where the frequency response characteristic is insufficient at the front stage of the transmission system, it becomes possible to compensate for the decrease in the link gain of the transmission system. In particular, in fiber optic radio, by arranging an amplifier 120b having an amplification factor corresponding to the insufficient frequency response characteristic of the electrical / optical conversion unit 600 at the front stage of the transmission system, it becomes possible to compensate for the decrease in the link gain. That is, conventionally, a signal having a frequency component matching the cut-off frequency of the electrical / optical conversion unit 600 or an expensive light emitting unit with a wide-area transition of the cut-off frequency has been used. However, in the present embodiment, an amplifier 120b having an amplification factor at frequencies equal to or higher than the cut-off frequency of the electrical / optical conversion unit 600 is used. Therefore, it may be possible to construct a transmission system 1000b capable of appropriately controlling the SNR of a signal having a frequency component equal to or higher than the cut-off frequency and transmitting it.

[0038] That is, the amplifier 120b has a function of amplifying the signal component of the RF signal SIb in the region where the gain with respect to the frequency in the transmission path including the drive unit 500, the electrical / optical conversion unit 600, the transmission medium 300b, and the optical / electrical conversion unit 700 attenuates. Therefore, in the prior art, the amplifier 120b functions to compensate for the insufficient gain of the signal in the transmission path that requires flat high-frequency characteristics, particularly in the electrical / optical conversion unit 600.

[0039] Also, in the prior art, when a signal with a small SNR is used as the input signal, there is a possibility that the SNR of the output signal deteriorates due to the noise of the transmission system, and accurate information cannot be transferred at once. However, by amplifying an input signal with a small SNR so that the noise component of the amplified signal becomes larger than the noise of the transmission system, it is also possible to suppress the deterioration of the SNR of the output signal and transmit an output signal with an appropriately controlled SNR with respect to the input signal.

[0040] By installing the amplification device 120b, it becomes possible to achieve the effects as described above according to the present disclosure. However, if the signal is distorted in the transmission system after amplification, it becomes difficult to obtain an appropriate output signal. Therefore, in the transmission system 1000b, the input signal is amplified until immediately before the amplified signal is distorted in the transmission system, and a configuration is adopted that can maximize the above effects. That is, the feedback unit 130 is configured to detect the level at which the amplified signal is affected by the distortion of the transmission path, and when it detects or estimates that distortion may occur in the amplified signal, it attenuates the input signal input to the amplification device 120b. According to such a configuration, by configuring the transmission module 100 according to the present embodiment to perform feedback with the maximum input power that can suppress the occurrence of distortion in the transmission signal, it becomes possible to transmit a signal with an appropriately controlled SNR.

[0041] The feedback unit 130 detects whether the amplified signal amplified by the amplification device 120b has reached a level at which it is amplified too much and distortion such as clipping occurs in the electro-optical conversion unit 600. For example, when the output signal has not reached the level, the feedback unit 130 outputs instruction information to lower the attenuation degree to the variable attenuation unit 110, and when the output signal may reach the level, it outputs instruction information to increase the attenuation degree to the variable attenuation unit 110. In this way, by the feedback unit 130 controlling the variable attenuation unit 110, the amplification unit 121b can operate to compensate for the insufficient gain of the transmission path with a constant amplification degree.

[0042] Also, in the case of an RF signal SIb with a low SNR, there is a concern about the degradation of the SNR due to noise present in the transmission path including the drive unit 500, the electro-optical conversion unit 600, the optical fiber as the transmission medium 300b, and the opto-electrical conversion unit 700. However, in the present embodiment, by setting the amplification degree of the amplification device 120b to be equal to or higher than the noise power of the transmission path, it becomes possible to suppress the decrease in the SNR of the RF signal SIb.

[0043] According to the above configuration, even if a device with insufficient frequency response characteristics in a high-frequency band is included, it is possible to transmit a signal in the high-frequency band without degrading the quality of the signal, and to provide a transmission module and a transmission system capable of suppressing noise when the SNR of the signal is low.

[0044] (Supplementary Explanation) As described above, when the frequency response characteristics of the transmission path for transmitting the RF signal are insufficient, as shown in FIGS. 1 and 10, if an amplification device 120 for compensating for the insufficient gain is arranged in the front stage of the transmission path, it is possible to compensate for the link gain of the transmission system. Assuming that the input signal is Si(t) and the transfer coefficient of the transmission path is H(n), the output signal So(t) is expressed by Equation (1). So(t)=H(n)*Si(t)----------------(1) Therefore, it is possible to obtain So(t) by inputting Si(t) that is only insufficient in the frequency response characteristics of H(n). Conventionally, the frequency response characteristics of H(n) have been improved by using expensive devices. However, considering a system with a complex modulation / demodulation circuit added to the transmission path, it is understood that it is only necessary to input Si(t) that is insufficient in the frequency response characteristics of H(n).

[0045] Also, for example, consider an RF signal using OFDM, which is widely adopted in terrestrial digital television broadcasting, wireless LAN, etc. In OFDM, pilot symbols with predetermined values are arranged in part of the transmission symbol. When the QAM symbol A0,n propagates through the transmission path, the received symbol becomes rn = Hn * A0,n, and amplitude fluctuation and phase rotation may occur. In the pilot symbol part, since the input QAM symbol A0,n is known, the transmission path frequency response H’n of the data part is estimated by estimating Hn from the demodulated symbol. Next, the received symbol of the data part is equalized by A’0,n = rn / H’n. QAM demodulation can be performed using the obtained equalized QAM symbol A’0,n. However, conventionally, since the level on the high-frequency side of the device in the transmission path decays and the EVM deteriorates, the high-frequency response characteristics of the device have been improved. However, by amplifying the level on the high-frequency side before the transmission path, it becomes possible to estimate the transmission path frequency response H’n of the data part, and as a result, it becomes possible to transmit a signal with a frequency higher than the cut-off frequency of the frequency response characteristics of the transmission path without deteriorating the EVM of the signal.

[0046] Note that for a signal with a suppressed SNR, such as an OFDM signal, the noise figure, which is the degradation of the SNR due to the transmission path, can be suppressed by an amplifier. Fig. 8 is a schematic diagram showing the state where the noise figure in the transmission path is reduced in an OFDM signal with an SNR suppressed to about 40 dB. It can be understood from Fig. 8 that by amplifying the noise level of the OFDM signal as the input signal to a level higher than the noise level in the transmission path, the influence of the noise in the transmission path is reduced. Here, the difference between the noise level in the transmission path and the noise level of the input signal after amplification is defined as ΔP.

[0047] (Modification Example 1) In the description of the above embodiment, the description centered on the solid transmission medium of cables such as optical cables and coaxial cables as the signal transmission medium. However, the transmission medium according to this embodiment is not limited to solids. That is, it is applicable not only to the transmission medium connected by wire from the transmitting station, but also to the case of relaying and transmitting the signal radiated into the air as a wireless signal. Specifically, it is also possible to use an amplification device or a transmission module capable of exerting the function according to this embodiment in the relay device.

[0048] (Modification 2) Also, although the input signal input to the transmission system 1000 according to this embodiment has been described by taking an OFDM signal as an example, the input signal is not limited to the OFDM signal. In particular, it is also possible to use a wireless signal with a limited SNR that is expected to have a high effect in suppressing the degradation of the SNR as the input signal. That is, it is effective to apply this embodiment to a signal including a wireless signal having an SNR such that the noise component amplified by the amplification device according to this embodiment becomes larger than the noise component of the transmission system and the signal component is not distorted by the amplification degree in that case. Further, in a transmission path having a device whose cost becomes high when the frequency response characteristic is flattened, it is possible to apply this embodiment to a signal having a frequency component in a frequency region where the frequency response characteristic is not flat. Note that the frequency of the carrier wave of the wireless signal may be in a frequency band of a microwave band or a millimeter wave band or higher.

[0049] The features of the amplification device, the transmission module, and the transmission system 1000 according to this embodiment will be described below.

[0050] The amplification device 120 arranged in the front stage of a transmission path having a frequency region in which the gain attenuates as the frequency increases according to the first aspect of the present disclosure preferably inputs a signal including the frequency component of the frequency region. The amplification device 120 preferably amplifies the signal with an amplification degree that reduces the attenuation amount of the frequency component in the transmission path, outputs the amplified signal to the transmission path, and reduces the degradation of the SNR of the signal in the transmission path.

[0051] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, signals in the high-frequency band can be transmitted without degrading the signal quality, and when the SNR of the signals is low, noise in the transmission path can be suppressed.

[0052] The amplification factor of the amplification device 120 according to the second aspect of the present disclosure is preferably an amplification factor at which the signal amplified in the transmission path does not generate distortion.

[0053] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, it becomes possible to transmit signals in the high-frequency band without degrading the signal quality. For example, if distortion due to non-linear characteristics occurs in the transmission path, the SNR of the signal is significantly degraded due to third-order intermodulation distortion or the like. However, by increasing the amplification factor of the amplification device 120 to the maximum level at which such distortion does not occur, it is also possible to improve the EVM of a signal with insufficient gain.

[0054] The amplification factor of the amplification device 120 according to the third aspect of the present disclosure is preferably an amplification factor at which the power of the noise component of the signal is greater than the power of the noise in the transmission path.

[0055] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, it may be possible to suppress noise in the transmission path when the SNR of the signal is low. For example, it may be possible to suppress noise in the transmission path in inverse proportion to the amplification factor of the amplification device 120.

[0056] The distortion of the amplification device 120 according to the fourth aspect of the present disclosure is distortion generated by the non-linear characteristics of the device used in the transmission path, and the signal is preferably a wireless signal including a carrier wave having a frequency of millimeter wave band or higher.

[0057] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, it is possible to improve the EVM by amplifying the signal in the high-frequency band to such an extent that the signal is not distorted. In particular, if the high-frequency band is a millimeter-wave band or higher, it becomes possible to effectively utilize an inexpensive device that does not have sufficient frequency response characteristics in the millimeter-wave band.

[0058] The distortion of the amplifier 120 according to the fifth aspect of the present disclosure may be distortion generated by clipping of a device used in the transmission path.

[0059] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, it is possible to transmit the signal in the high-frequency band without degrading the quality of the signal, and suppress the noise of the transmission path when the SNR of the signal is low.

[0060] The signal input to the amplifier 120 according to the sixth aspect of the present disclosure is an OFDM signal generated by OFDM, and it is preferable that at least a part of the subcarrier signals of the OFDM signal is included in the frequency domain.

[0061] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high-frequency band, even if the frequency domain of a part of the subcarriers of the OFDM signal is an area where the frequency response characteristics are insufficient, it may be possible to compensate at least a part thereof. By this compensation, it may be possible to improve the EVM and transmit an OFDM signal that could not be used heretofore. Also, when the SNR of the OFDM signal is low, it may be possible to suppress the noise of the transmission path.

[0062] The transmission module 100 according to the seventh aspect of the present disclosure preferably includes an amplification device 120 according to any one of the first to sixth aspects, and a feedback unit 130 that outputs attenuation instruction information before distortion occurs in the frequency domain in the transmission path. Further, the transmission module 100 preferably further includes a variable attenuation unit 110 that attenuates the signal input to the amplification device in response to the attenuation instruction information.

[0063] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high frequency band, it may be possible to improve values such as EVM by increasing the amplification factor to such an extent that distortion does not occur in the signal in the high frequency band. Also, when the SNR of the signal is low, it may be possible to suppress the noise in the transmission path by the amplification factor.

[0064] The transmission system 1000 according to the eighth aspect of the present disclosure preferably includes an amplification device 120 according to any one of the first to sixth aspects, a transmission device 200 disposed downstream of the amplification device, and a transmission medium 300 that inputs the output signal of the transmission device 200. Further, the transmission system 1000 preferably includes a receiving device 400 that inputs the output signal of the transmission medium 300.

[0065] According to the present disclosure, for a transmission path including a device having insufficient frequency response characteristics in a high frequency band, it may be possible to transmit the signal in the high frequency band without degrading the quality of the signal. Also, when the SNR of the signal is low, it may be possible to suppress the noise in the transmission path.

[0066] The transmission system 1000 according to the ninth aspect of the present disclosure preferably includes the transmission module 100 according to the seventh aspect, a transmission device 200 disposed downstream of the transmission module 100, and a transmission medium 300 that inputs the output signal of the transmission device 200. Further, the transmission system 1000 preferably includes a receiving device 400 that inputs the output signal of the transmission medium 300.

[0067] According to the present disclosure, for a transmission path including a device with insufficient frequency response characteristics in a high-frequency band, signals in the high-frequency band can be transmitted without degrading the signal quality, and when the SNR of the signal is low, noise in the transmission path can be suppressed.

[0068] In the transmission system 1000 according to the tenth aspect of the present disclosure, it is preferable that the transmission medium 300 is an optical fiber, the transmission device includes an electrical / optical conversion unit 600, and the reception device includes an optical / electrical conversion unit 700.

[0069] According to the present disclosure, for a transmission path including a device with insufficient frequency response characteristics in a high-frequency band, signals in the high-frequency band can be transmitted without degrading the signal quality, and when the SNR of the signal is low, noise in the transmission path can be suppressed. In particular, when the transmission medium 300 is an optical fiber, there is a problem that the noise figure of fiber optic wireless is significantly larger than that of an electric wire cable. However, according to the present disclosure, it may be possible to effectively suppress the noise figure of fiber optic wireless.

[0070] (Supplement of the embodiment) The block diagrams used in the description of the above-described embodiments show blocks of functional units. The method for realizing each functional block is not particularly limited. For example, each functional block may be realized using one physically or logically combined device, or may be realized using two or more physically or logically separated devices directly or indirectly connected to each other.

[0071] Also, the information, parameters, etc. described in the present disclosure may be represented using absolute values, relative values from a predetermined value, or corresponding other information.

[0072] The names used for the above-described parameters are not limiting names in any way. Furthermore, the mathematical formulas using these parameters may be different from those explicitly disclosed in the present disclosure. Various names are not limiting names in any way.

[0073] Each aspect / embodiment described in this disclosure may be used alone, in combination, or switched and used during execution.

[0074] Although the embodiments have been described in detail with reference to the drawings, the present invention is not limited by the content described in the above embodiments. Further, the components described above include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the configurations described above can be combined as appropriate. Also, various omissions, substitutions, or changes in the configuration can be made without departing from the gist of the present invention.

Description of Reference Numerals

[0075] 100 Transmission Module 110 Variable Attenuator 120, 120a, 120b Amplifier 130 Feedback Unit 200 Transmitter 300, 300a, 300b Transmission Medium 400 Receiver 600 Electrical / Optical Conversion Unit 700 Optical / Electrical Conversion Unit

Claims

1. An amplification device arranged in the front stage of a transmission path having a frequency region where the gain decays as the frequency increases, The amplification device inputs a signal including frequency components in the frequency region, amplifies the signal with an amplification degree that reduces the attenuation amount of the frequency components in the transmission path, outputs the amplified signal to the transmission path, and reduces the deterioration of the SNR of the signal in the transmission path, The amplification degree is an amplification degree in which the power of the noise component of the amplified signal is greater than the power of the noise in the transmission path within a range where the amplified signal does not generate distortion in the transmission path.

2. The distortion is distortion generated by the non-linear characteristics of the device used in the transmission path, and the signal is a wireless signal including a carrier wave having a frequency in the millimeter wave band or higher. The amplification device according to claim 1.

3. The distortion is distortion generated by clipping of the device used in the transmission path. The amplification device according to claim 1 or 2.

4. The signal is an OFDM signal generated by OFDM, and at least a part of the sub-carrier signals of the OFDM signal is included in the frequency region. The amplification device according to any one of claims 1 to 3.

5. An amplification device according to any one of claims 1 to 4, A feedback unit that outputs attenuation indication information before distortion occurs in the frequency region in the transmission path, A transmission module comprising: an attenuation unit that attenuates the signal input to the amplification device in response to the attenuation indication information.

6. An amplification device according to any one of claims 1 to 4, A transmission device arranged in the subsequent stage of the amplification device, A transmission medium that inputs the output signal of the transmission device, A transmission system comprising: a receiving device that inputs the output signal of the transmission medium.

7. A transmission module according to claim 5, A transmission device arranged in the subsequent stage of the transmission module, A transmission medium that inputs the output signal of the transmission device, A transmission system comprising: a receiving device that inputs the output signal of the transmission medium.

8. The transmission medium is an optical fiber, the transmission device includes an electrical / optical conversion unit, and the receiving device includes an optical / electrical conversion unit. The transmission system according to claim 6 or 7.

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