JP7704549B2 - Amplification device, transmission module, and transmission system - Google Patents

Description

The present invention relates to an amplifier device, a transmission module, and a transmission system.

Efforts have been made to develop RF transmission technology that uses low-noise, high-output, and high-linearity devices to suppress noise increases and distortion such as third-order intermodulation distortion.

For example, in the TV optical transmission system configuration of Patent Document 1, an electrical-optical conversion device in a TV optical transmission system for transmitting a TV-RF signal via optical fiber includes a current regulator that changes the current of a semiconductor laser diode light source. Also, an electrical-optical conversion device in a TV optical transmission system for transmitting a TV-RF signal via optical fiber includes an RF amplitude regulator that adjusts the TV-RF signal amplitude.

Specifically, Patent Document 1 has the feature that a current value with good linearity is set for an electro-optical conversion device by a current regulator for adjusting a point with good linearity, and the reception state can be adjusted to the best state. In addition, the modulation amplitude can also be set to the optimum range by an RF amplitude variable step switch that can adjust the RF signal amplitude adjustment amount in 1 dB units, and the reception state can be adjusted to the best state.

JP 2014-053879 A

In conventional technology, noise is reduced by utilizing the highly linear portion of at least a part of the high frequency band of the device, and the amplitude is adjusted to match the highly linear region where the optimal error rate is obtained, making it possible to handle high-output signals. However, without using the above configuration, it is difficult to transmit high-frequency signals without degradation using commercially available devices that lack frequency response characteristics in the high frequency band. Also, in transmission systems such as optical fibers, where noise is relatively high, it is difficult to suppress noise, and it can be difficult to transmit signals with a low SNR.

The present invention has been made in consideration of the problems inherent in the conventional technology. The object of the present invention is to provide an amplifier device or the like that can transmit high-frequency band signals without degrading their quality through a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band, and that can suppress noise in the transmission path when the SNR of the signal is low.

In accordance with an aspect of the present invention, an amplifier device that is disposed in front of a transmission path having a frequency domain in which the gain attenuates as the frequency increases inputs a signal containing frequency components in the frequency domain, amplifies the signal with an amplification factor that reduces the amount of attenuation of the frequency components in the transmission path, and outputs the amplified signal to the transmission path, preferably reducing degradation of the SNR of the signal in the transmission path.

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

A transmission system according to another aspect of the present invention preferably includes a transmission module according to the above aspect, a transmitting device arranged downstream of the transmission module, a transmission medium for inputting an output signal of the transmitting device, and a receiving device for inputting an output signal of the transmission medium.

The present invention makes it possible to provide an amplifier device or the like that can transmit a high-frequency band signal without degrading its quality over a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band, and that can suppress noise in the transmission path when the SNR of the signal is low.

FIG. 1 is a schematic diagram showing an example of a transmission system according to an embodiment of the present invention. 1 is a schematic diagram showing an example of frequency response characteristics of a device included in a transmission path according to the present embodiment. FIG. 1A is a schematic diagram showing an example of a proper operation of the light emitting element according to the present embodiment, and FIG. 1B is a schematic diagram showing an example of an operation in which clipping occurs in the light emitting element according to the present embodiment. 1 is a schematic diagram showing an example of frequency-dependent characteristics of third-order intermodulation distortion in a light-emitting element according to the present embodiment. FIG. 2 is a schematic diagram showing an example of a frequency response characteristic of the transmission system according to the present embodiment. 1 is a schematic diagram showing an example of a change in spectrum due to the influence of third-order intermodulation distortion of an OFDM signal according to the present embodiment; 2 is a schematic diagram for explaining an example of a frequency response characteristic of the transmission system according to the present embodiment. FIG. 1A is a schematic diagram showing an example of a spectral distribution of an OFDM signal before amplification according to the present embodiment, and FIG. 1B is a schematic diagram showing a spectral distribution of an OFDM signal after amplification according to the present embodiment, and a comparison example between the noise level of the amplified OFDM signal and the noise level of a transmission path. 10 is a graph showing an example of the relationship between the difference between the noise level of an amplified OFDM signal and the noise level of a transmission path, and the noise figure of the transmission path, according to the present embodiment. FIG. 13 is a schematic diagram showing another example of the transmission system according to the present embodiment.

Below, an example of an amplifier, a transmission module, and a transmission system according to the present embodiment will be described in detail with reference to the drawings. Note that the embodiments described below are comprehensive or specific examples. The numerical values, shapes, materials, components, the installation positions and connection forms of the components, steps, and the order of steps shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components that are not described in an independent claim that indicates a top concept will be described as optional components. Furthermore, the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios.

Furthermore, the following embodiments and their variations may contain similar components, and the same reference numerals will be used to denote similar components, and duplicate descriptions will be omitted.

(Description of the configuration and operation principle of the amplifier device, the transmission module, and the transmission system including the transmission module)
1 is a schematic diagram showing an image of a transmission system 1000a according to this embodiment. The transmission system 1000a includes an amplifier 120a, a transmitter 200, a transmission medium 300a, and a receiver 400 to which an RF signal SIa with a suppressed SNR is input. The RF signal SIa is preferably a radio signal including a carrier in a high frequency region such as a millimeter wave band, but is not limited to the radio signal. When the RF signal SIa is input in a transmission path including the transmitter 200, the transmission medium 300a, and the receiver 400 without the amplifier 120a according to this embodiment, the gain attenuates in the high frequency region of the RF signal SIa. A measurement result showing the frequency response characteristic of this embodiment is shown in FIG. 2.

The horizontal axis of FIG. 2 indicates the input power (dBm) of the RF signal SIa, and the vertical axis indicates the EVM (Error Vector Magnitude) of the output signal SOa of the receiving device 400. When the RF signal SIa includes a 6 GHz frequency component, the 6 GHz component has a good frequency response characteristic 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 frequency component has a good frequency response characteristic in that 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, so it is assumed that distortion is affecting the 6 GHz component with an input power of about -12 dBm or more in the transmission path.

On the other hand, for the 28 GHz frequency component of the RF signal SIa, the transmission path has a frequency response characteristic in which the EVM is degraded by about 8% or more in the input power range of about -32 dBm to about -20 dBm. That is, for the 28 GHz frequency component, even in the input power range of -32 dBm to about -20 dBm, gain loss occurs, and it is assumed that the EVM is degraded because an input signal with sufficient amplitude is not obtained. It is also assumed that the EVM is degraded due to the noise power of the transmission path because an input signal with sufficient amplitude is not obtained. In particular, in optical fiber radio, the noise power is large, so the SNR is significantly degraded due to insufficient frequency response characteristics, so conventionally, a transmission path was used for signals including a frequency range with good frequency response characteristics, for example, a frequency range up to about 6 GHz in FIG. 1. In addition, in order to transmit a signal including a 28 GHz component, a transmission path was configured by including an expensive, highly linear device whose frequency response characteristics are not insufficient even at 28 GHz.

However, in the frequency response characteristics of the RF signal SIa in Figure 1, even in areas where the link gain at 28 GHz is attenuated and the EVM is degraded, it is expected that the EVM will be improved by amplifying the 28 GHz component to compensate for at least a portion of the link gain attenuation.

Figure 5 shows the result of amplifying the power of the 28 GHz signal component input to the amplifier device 120a in the transmission system 1000a of Figure 1. If the power of the 28 GHz signal component is not amplified, Figure 2 predicts that the EVM of the 28 GHz signal component input to the transmission path will deteriorate from 0 dBm, similar to the 6 GHz component. However, Figure 5 shows that amplifying the 28 GHz signal component expands the range in which a good EVM state can be maintained.

As described above, a specific example in which the dynamic range of the 28 GHz signal component is assumed not to decrease even if the input power of the 28 GHz signal component is increased will be described in FIG. 6 using a light-emitting element. The black circle symbol in FIG. 6 is a third-order intermodulation distortion signal of the 6 GHz component, and the black square symbol is a third-order intermodulation distortion signal of the 28 GHz component. The 28 GHz component can only output an output power of about -40 dBm even if a large power of about 5 dBm is input, but the 6 GHz component can output an output power of about -45 dBm even if a small power of about -15 dBm is input. In addition, there is a difference of about 17 dB between the 6 GHz component signal and the 28 GHz component signal, which can obtain output power of about the same level. If this result is applied to FIG. 5, it can be understood that it is a reasonable estimate that the dynamic range of the 28 GHz component in FIG. 5 is a range shifted to the low power side by about 17 dB from the dynamic range of the 6 GHz component. In fact, the difference between the 6 GHz component and the 28 GHz component in the EVM degradation region with smaller input power in Figure 5 is approximately 18 dB, which is considered to be within the range where logical consistency with the results in Figure 6 can be achieved.

That is, when transmitting an RF signal using a high-frequency carrier such as a millimeter wave, if the frequency response characteristics of the transmission path are insufficient, the link gain is attenuated, the SNR is degraded, and the EVM is increased. With only the conventional transmission path, for a 6 GHz input signal, the dynamic range is about 30 dBm or more at an EVM of 8% or less. However, for a 28 GHz input signal, only half the dynamic range can be obtained at an EVM of 8% or less. However, if an amplifier device 120a is placed in front of the conventional transmission path as shown in FIG. 1 and the 28 GHz input signal is amplified, it may be possible to obtain a dynamic range of about 30 dBm or more at an EVM of 8% or less as shown in FIG. 5. Also, as can be inferred from FIG. 6, it is assumed that the 28 GHz input signal has a dynamic range similar to that of the 6 GHz input signal with a good EVM.

Next, we will explain an example of distortion caused by a transmission path when the input signal is amplified too much. Distortion caused by a transmission path is often caused by the nonlinear characteristics of modulators, demodulators, detectors, etc., but we will use Figure 3 to explain distortion caused by a light-emitting element (electrical/optical conversion unit) when optical fiber is used as the transmission medium.

The horizontal axis of FIG. 3(a) shows the current value (mA) of the input signal input to the light-emitting element, and the vertical axis shows 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 that the input signal SIc is not distorted and the output light SOc is linearly amplified and input to the optical fiber. However, as shown in FIG. 3(b), when an input signal SId with a large degree of amplification is input, clipping occurs and the amplitude of the output signal SOd is no longer a sine wave like 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 conventional technology, the amplification degree of the RF signal SIa needs to be set so that the output signal falls within a region where distortion such as clipping does not occur, so that the transmittable signal power region is limited. However, the amplifier device 120a according to this embodiment amplifies the signal to compensate for the link deficiency in the region where the frequency response characteristic of the light-emitting element is insufficient, making it possible to expand the signal power region. In other words, in the expanded signal power region, it becomes possible to suppress the degradation of the SNR and maintain a good EVM. Specifically, as described above, as shown in FIG. 4, even for a high-power input of a 28 GHz signal, third-order intermodulation distortion is suppressed, making it possible to expand the region with a good EVM.

That is, when optical fiber is used for the transmission path, third-order intermodulation distortion may increase in a light-emitting element that converts an electrical signal into an optical signal. Figure 3(b) shows a state in which an over-amplified electrical input signal exceeds the lower limit of the IP characteristic of the light-emitting element, causing clipping of the lower part of the output optical signal and large distortion. When clipping occurs, third-order intermodulation distortion increases rapidly and EVM deteriorates. It is assumed that the deterioration of EVM at 6 GHz with high input power in Figure 3 occurs due to distortion caused by such third-order intermodulation distortion. However, as explained in Figure 4, the link gain is reduced for high-frequency input signals such as 28 GHz. Therefore, it is possible to increase the amplification degree without causing deterioration of EVM until the shortfall due to link gain is fully satisfied by amplifying a high-frequency input signal such as 28 GHz.

Figure 4 is a graph showing 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 Figure 4 indicates frequency, and the vertical axis of the graph in Figure 4 indicates the absolute value of output power in dBm units. The graph on the left side of Figure 4 shows the frequency spectrum of an OFDM signal by two subcarrier signals f1 and f2 around 28 GHz, and it can be seen that the frequency components of the OFDM signal are clearly separated from the background noise. The graph on the right side of Figure 4 is a graph showing the case where third-order intermodulation distortion is generated in an OFDM signal by two subcarrier signals f1 and f2 around 28 GHz. It shows the noise band and noise level generated by third-order intermodulation distortion signals with frequencies (2f1-f2) and (2f2-f1). It can be seen that the third-order intermodulation distortion signals (2f1-f2) and (2f2-f1) become noise and are superimposed on the OFDM signals of subcarrier signals f1 and f2, decreasing the SNR and increasing the EVM value. It can also be seen that the power leaking to adjacent channels in the noise of the third-order intermodulation distortion signals increases, greatly affecting adjacent subcarrier signals.

Next, a configuration for suppressing degradation of the SNR of the input signal by amplifying the amplification degree of the insufficient region of the frequency response characteristic to the distortion generation limit by the amplifier device 120a will be described with reference to Figures 7 to 9.

Figure 7 is an example of a graph showing a situation in which the EVM deteriorates due to noise generated in the transmission path when the power of the input signal is reduced, and a situation in which the EVM deteriorates due to distortion generated in the transmission path when the power of the input signal is increased. The horizontal axis of Figure 7 is the input power (dBm), which is a relative value indicated by the measurement system. The vertical axis of Figure 7 is the EVM, and the area where the EVM exceeds 9% is omitted. As described above, when the power of the input signal is reduced below approximately -8 dBm, the influence of noise generated in the transmission path appears, and the tendency of the EVM to deteriorate becomes noticeable. When the power of the input signal is reduced below -20 dBm, the EVM exceeds 8%. When the power of the input signal is increased above 6 dBm, the influence of distortion generated in the transmission path appears, and the EVM tends to deteriorate, and when the power of the input signal is increased above 14 dBm, the EVM exceeds 8%.

In other words, in FIG. 7, when the power of the input signal decreases, a region occurs in which the EVM rapidly deteriorates due to the noise figure of the transmission path. This region is also called the penalty region due to the noise figure of the transmission path. Furthermore, when the power of the input signal increases, the above-mentioned distortion 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 region of EVM deterioration due to distortion. In such an input power range, the amplifier device of this embodiment may be able to reduce the effects of noise occurring in the transmission path. Next, such a case will be described.

The amplifier of this embodiment is effective when the noise component level of the input signal is smaller than the noise level occurring in the transmission path, and the amplified noise component level is greater than the noise level without distortion occurring in the signal component even when the noise component is amplified. 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, the SNR of the OFDM signal is limited to about 40 dB, so the amplifier 120a according to this embodiment is likely to exhibit the above effect. Figure 8(a) is a schematic diagram showing the power spectrum of the OFDM input signal before being input to the amplifier 120a. Since information is transmitted in each of a plurality of subcarriers in OFDM, the convex portion in Figure 8(a) indicates the power of the signal component. In addition, the frequency band portions on both sides of the convex portion indicate the power of the noise component. Figure 8(b) is a schematic diagram showing the power spectrum of the amplified OFDM output signal output from the amplifier 120a. The difference between the noise level of the amplified OFDM output signal and the noise level occurring in the transmission path is indicated as ΔP. It can be seen that if the OFDM output signal is amplified so that ΔP is large within a range in which the signal components are not distorted, the effect of the noise level occurring in the transmission path is suppressed.

9 is a plot of 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 units by the following equation (2).
NF=10log((S_in/N_in)/(S_out/N_out))------(2)
(S_in is an input signal to the transmission system 1000a, N_in is an input noise to the transmission system 1000a, S_out is an output signal from the transmission system 1000a, and N_out is an output noise from the transmission system 1000a.)
Furthermore, 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)) (N_add is the amount of noise on the transmission path, and G is the amplification degree of the amplifier 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

Therefore, it can be understood that when (G×N_in) is greater than (N_add), the noise figure of the transmission path is reduced. In other words, when the SNR is limited, such as in the case of an OFDM signal, the input signal input to the transmission path is amplified by an amplifier device arranged in the front 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.

However, as mentioned above, if the amplification level of the amplifier device is increased too much, the EVM of the amplified signal may suddenly deteriorate due to the distortion characteristics of the transmission path, so it is necessary to operate the amplifier device 120a at an amplification level that is not affected by the distortion characteristics of the transmission path.

Based on the above findings, the transmission module 100 according to this embodiment, which has a configuration for applying feedback to the amplification degree of the amplifier 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.

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

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

In the prior art, a transmission system including a drive unit 500, an electrical/optical conversion unit 600, an optical fiber as a transmission medium 300b, and an optical/electrical conversion unit 700 transmits signals having frequencies below the cutoff frequency, which is the response limit of the transmission system. This is because the frequency response characteristics of the transmission system are insufficient for frequency components above the cutoff frequency, resulting in a decrease in link gain and a large degradation of SNR. In particular, in optical fiber radio, the noise of the transmission system is relatively large, so if the frequency response characteristics are insufficient, the SNR may deteriorate significantly. Therefore, the transmission system has been used in a frequency range where the frequency response characteristics of the transmission system can be fully utilized. Alternatively, a transmission system has been constructed using an expensive device that shifts the cutoff frequency over a wide range.

However, by providing an amplifier 120b having an amplification degree that compensates for the insufficient gain in the region where the frequency response characteristic is insufficient, in 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 optical fiber radio, by placing an amplifier 120b having an amplification degree corresponding to the insufficiency of the frequency response characteristic of the electrical/optical conversion unit 600 in the front stage of the transmission system, it becomes possible to compensate for the decrease in the link gain. That is, in the past, a signal having a frequency component that matches the cutoff frequency of the electrical/optical conversion unit 600, or an expensive light emitting unit that shifts the cutoff frequency to a wide range, was used. However, in this embodiment, an amplifier 120b having an amplification degree at frequencies equal to or higher than the cutoff frequency of the electrical/optical conversion unit 600 is used. Therefore, it may be possible to construct a transmission system 1000b that can transmit signals having frequency components equal to or higher than the cutoff frequency by appropriately controlling the SNR.

That is, the amplifier device 120b has the function of amplifying the signal components of the RF signal SIb in a region where the gain with respect to frequency is attenuated in the transmission path including the driver unit 500, the electrical/optical conversion unit 600, the transmission medium 300b, and the optical/electrical conversion unit 700. Therefore, in the conventional technology, the amplifier device 120b functions to compensate for the insufficient gain of the signal in the transmission path, which requires flat high-frequency characteristics, particularly in the electrical/optical conversion unit 600.

Furthermore, in conventional technology, when a signal with a low SNR is used as an input signal, there is a possibility that the SNR of the output signal may deteriorate due to noise in the transmission system, resulting in cases where accurate information cannot be transferred in one go. However, by amplifying an input signal with a low SNR and amplifying it so that the noise component of the amplified signal is larger than the noise in the transmission system, it is possible to suppress deterioration of the SNR of the output signal and transmit an output signal with an SNR that is appropriately controlled relative to the input signal.

By installing the amplifier device 120b, the present disclosure can achieve the above-mentioned effects, but if the signal is distorted in the transmission system after being amplified, it becomes difficult to obtain an appropriate output signal. Therefore, in the transmission system 1000b, the input signal is amplified until the amplified signal is about to be distorted in the transmission system, and the above-mentioned effects can be maximized. That is, the feedback unit 130 detects the level at which the amplified signal is affected by distortion in the transmission path, and attenuates the input signal input to the amplifier device 120b when it detects or estimates the possibility of distortion occurring in the amplified signal. With this configuration, it becomes possible to transmit a signal with an appropriately controlled SNR by configuring the transmission module 100 according to this embodiment to feedback with the maximum input power that can be suppressed so that distortion does not occur in the transmission signal.

The feedback section 130 detects whether the amplified signal amplified by the amplifier device 120b has reached a level where distortion such as clipping occurs in the electrical/optical conversion section 600 due to excessive amplification. For example, if the output signal has not reached that level, the feedback section 130 outputs instruction information to the variable attenuation section 110 to decrease the degree of attenuation, and if there is a possibility that the output signal will reach that level, it outputs instruction information to the variable attenuation section 110 to increase the degree of attenuation. In this way, the feedback section 130 controls the variable attenuation section 110, making it possible for the amplifier section 121b to operate at a constant degree of amplification to compensate for insufficient gain in the transmission path.

Furthermore, in the case of an RF signal SIb with a low SNR, there is concern that the SNR may deteriorate due to noise present in the transmission path including the driving unit 500, the electrical/optical conversion unit 600, the optical fiber as the transmission medium 300b, and the optical/electrical conversion unit 700. However, in this embodiment, by setting the amplification degree of the amplifier device 120b to be equal to or greater than the noise power of the transmission path, it is possible to suppress the deterioration of the SNR of the RF signal SIb.

The above configuration makes it possible to provide a transmission module and a transmission system that can transmit signals in the high frequency band without degrading their quality, even if the transmission module and system include a device that lacks frequency response characteristics in the high frequency band, and can suppress noise when the SNR of the signal is low.

(supplementary explanation)
As described above, when the frequency response characteristic of the transmission path for transmitting an RF signal is insufficient, it is possible to compensate for the link gain of the transmission system by placing an amplifier 120 that compensates for the insufficient gain at the front stage of the transmission path as shown in Figures 1 and 10. If 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, if Si(t) is input to compensate for the insufficiency of the frequency response characteristic of H(n), it is possible to obtain So(t). Conventionally, the frequency response characteristic of H(n) has been improved by using expensive devices. However, when considering a system in which a modulation/demodulation circuit is added to the transmission path, it is understood that it is sufficient to input Si(t) to compensate for the insufficiency of the frequency response characteristic of H(n).

Also, for example, consider the OFDM RF signal, which is widely adopted in terrestrial digital television broadcasting, wireless LAN, etc. In OFDM, a pilot symbol with a predetermined value is placed in a 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. Since the input QAM symbol A0,n is known in the pilot symbol part, the transmission path frequency response H'n of the data part is estimated by estimating Hn from the demodulation 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, in the past, the high frequency response characteristics of the device were improved because the level on the high frequency side of the transmission path of the device was attenuated and the EVM deteriorated. However, by amplifying the high-frequency level at the front end of the transmission path, it becomes possible to estimate the transmission path frequency response H'n of the data portion, and as a result, it becomes possible to transmit signals with a higher frequency than the cutoff frequency of the frequency response characteristics of the transmission path without deteriorating the EVM.

For signals with a suppressed SNR, such as OFDM signals, the noise figure, which is the degradation of SNR due to the transmission path, can be suppressed by an amplifier. Figure 8 is a schematic diagram showing how the noise figure in the transmission path is reduced in an OFDM signal with an SNR suppressed to about 40 dB. It can be seen from Figure 8 that the effect of noise in the transmission path is reduced by amplifying the noise level of the OFDM signal as an input signal so that it is higher than the noise level in the transmission path. Here, the difference between the noise level in the transmission path and the noise level of the amplified input signal is ΔP.

(Variation 1)
In the above embodiment, the description has focused on solid transmission media such as optical cables and coaxial cables as the signal transmission media. However, the transmission media according to this embodiment are not limited to solids. In other words, the present embodiment is applicable not only to transmission media connected by wire from a transmitting station, but also to relaying and transmitting signals emitted into the air as wireless signals. Specifically, it is possible to use an amplifier or transmission module capable of performing the functions according to this embodiment in the relay device.

(Variation 2)
Although the input signal input to the transmission system 1000 according to the present embodiment has been described as an OFDM signal, the input signal is not limited to the OFDM signal. In particular, it is possible to use a radio signal with a limited SNR, which is expected to be highly effective in suppressing degradation of the SNR, as the input signal. That is, it is effective to apply the present embodiment to a signal including a radio signal having an SNR such that the noise component amplified by the amplifier device according to the present 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. In addition, in a transmission path having a device that becomes expensive if the frequency response characteristic is made flat, it is possible to apply the present embodiment to a signal having a frequency component in a frequency region whose frequency response characteristic is not flat. The frequency of the carrier wave of the radio signal may be a frequency band of the microwave band, the millimeter wave band, or more.

The following describes the features of the amplifier device, transmission module, and transmission system 1000 according to this embodiment.

According to the first aspect of the present disclosure, the amplifier device 120 is disposed in front of a transmission path having a frequency domain in which the gain attenuates as the frequency increases, and preferably inputs a signal including frequency components in that frequency domain. The amplifier device 120 preferably amplifies the signal with an amplification factor that reduces the amount of attenuation of the frequency components in the transmission path, outputs the amplified signal to the transmission path, and reduces degradation of the SNR of the signal in the transmission path.

According to the present disclosure, it is possible to transmit a high-frequency band signal without degrading its quality through a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band, and to suppress noise in the transmission path when the SNR of the signal is low.

The amplification degree of the amplifier device 120 according to the second aspect of the present disclosure is preferably such that the amplified signal does not generate distortion in the transmission path.

According to the present disclosure, it is possible to transmit a signal in a high frequency band without degrading its quality through a transmission path that includes a device that lacks frequency response characteristics in the high frequency band. For example, if distortion due to nonlinear characteristics occurs in the transmission path, the SNR of the signal will deteriorate significantly due to third-order intermodulation distortion, etc. However, by increasing the amplification degree of the amplifier 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.

The amplification degree of the amplifier device 120 according to the third aspect of the present disclosure is preferably such that the power of the noise component of the signal is greater than the power of the noise in the transmission path.

According to the present disclosure, for a transmission path including a device that lacks frequency response characteristics in the 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 degree of the amplifier device 120.

The distortion of the amplifier device 120 according to the fourth aspect of the present disclosure is distortion caused by the nonlinear characteristics of the devices used in the transmission path, and the signal is preferably a radio signal including a carrier wave having a frequency equal to or higher than the millimeter wave band.

According to the present disclosure, it is possible to improve the EVM for a transmission path including a device that lacks frequency response characteristics in a high frequency band by amplifying the signal in the high frequency band to a level where the signal is not distorted. In particular, if the high frequency band is the millimeter wave band or higher, it becomes possible to effectively utilize inexpensive devices that do not have sufficient frequency response characteristics in the millimeter wave band.

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

According to the present disclosure, it is possible to transmit a high-frequency band signal without degrading its quality through a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band, and to suppress noise in the transmission path when the SNR of the signal is low.

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

According to the present disclosure, for a transmission path including a device with insufficient frequency response characteristics in the high frequency band, it may be possible to at least partially compensate for the frequency region of some subcarriers of an OFDM signal, even if the frequency response characteristics are insufficient. This compensation may make it possible to transmit OFDM signals that could not be used before with improved EVM. Furthermore, when the SNR of the OFDM signal is low, it may be possible to suppress noise in the transmission path.

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

According to the present disclosure, for a transmission path including a device with insufficient frequency response characteristics in the high frequency band, it may be possible to improve values such as EVM by increasing the amplification level to a level where distortion does not occur in the signal in the high frequency band. Furthermore, when the SNR of the signal is low, it may be possible to suppress noise in the transmission path by using the amplification level.

A transmission system 1000 according to an eighth aspect of the present disclosure preferably includes an amplifier device 120 according to any one of the first to sixth aspects, a transmitter device 200 disposed downstream of the amplifier device, and a transmission medium 300 that inputs an output signal from the transmitter device 200. Furthermore, the transmission system 1000 preferably includes a receiver device 400 that inputs an output signal from the transmission medium 300.

According to the present disclosure, it may be possible to transmit a high-frequency band signal without degrading its quality through a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band. In addition, it may be possible to suppress noise in the transmission path when the signal SNR is low.

A transmission system 1000 according to a ninth aspect of the present disclosure preferably includes a transmission module 100 according to the seventh aspect, a transmitting device 200 disposed downstream of the transmission module 100, and a transmission medium 300 that inputs an output signal from the transmitting device 200. Furthermore, the transmission system 1000 preferably includes a receiving device 400 that inputs an output signal from the transmission medium 300.

According to the present disclosure, it is possible to transmit a high-frequency band signal without degrading its quality through a transmission path that includes a device that lacks frequency response characteristics in the high-frequency band, and to suppress noise in the transmission path when the SNR of the signal is low.

The transmission medium 300 of the transmission system 1000 according to the tenth aspect of the present disclosure is preferably an optical fiber, the transmitting device includes an electrical/optical conversion unit 600, and the receiving device includes an optical/electrical conversion unit 700.

According to the present disclosure, it is possible to transmit a high-frequency band signal without degrading its quality through a transmission path including a device with insufficient frequency response characteristics in the high-frequency band, and to suppress noise in the transmission path when the SNR of the signal is low. In particular, when the transmission medium 300 is optical fiber, there is an issue that the noise figure of optical fiber radio is significantly larger than that of an electric wire cable, but according to the present disclosure, it may be possible to effectively suppress the noise figure of optical fiber radio.

(Supplementary description of the embodiment)
The block diagrams used in the description of the above-mentioned embodiments show functional blocks. The method of realizing each functional block is not particularly limited. For example, each functional block may be realized using one device that is physically or logically coupled, or may be realized using two or more devices that are physically or logically separated and directly or indirectly connected to each other.

In addition, the information, parameters, etc. described in this disclosure may be expressed using absolute values, may be expressed using relative values from a predetermined value, or may be expressed using other corresponding information.

The names used for the parameters described above are not limiting in any way. Furthermore, the formulas etc. using these parameters may differ from those explicitly disclosed in this disclosure. The various names are not limiting in any way.

Each aspect/embodiment described in this disclosure may be used alone, in combination, or switched depending on the implementation.

Although the embodiments have been described in detail with reference to the drawings, the present invention is not limited to the contents described in the above embodiments. Furthermore, the components described above include those that a person skilled in the art would easily imagine and those that are substantially the same. Furthermore, the configurations described above can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the configuration can be made without departing from the spirit of the present invention.

REFERENCE SIGNS LIST 100 Transmission module 110 Variable attenuation section 120, 120a, 120b Amplification device 130 Feedback section 200 Transmitter 300, 300a, 300b Transmission medium 400 Receiving device 600 Electrical/optical conversion section 700 Optical/electrical conversion section

Claims (8)

An amplifier device disposed in a front stage of a transmission path having a frequency domain in which the gain attenuates as the frequency increases,
the amplifier device receives a signal including a frequency component in the frequency domain, amplifies the signal with an amplification factor that reduces an amount of attenuation of the frequency component in the transmission path, and outputs the amplified signal to the transmission path, thereby reducing degradation of an SNR of the signal in the transmission path;
An amplifier device, wherein the amplification degree is such that 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 in which the amplified signal does not cause distortion in the transmission path. The amplifier device according to claim 1, wherein the distortion is generated by the nonlinear characteristics of a device used in the transmission path, and the signal is a radio signal including a carrier wave having a frequency equal to or higher than the millimeter wave band. The amplifier device according to claim 1 or 2, wherein the distortion is caused by clipping of a device used in the transmission path. 4. The amplifier according to claim 1, wherein the signal is an OFDM signal generated by OFDM, and at least some subcarrier signals of the OFDM signal are included in the frequency domain. An amplifier device according to any one of claims 1 to 4,
a feedback unit that outputs attenuation instruction information before distortion occurs in the frequency domain in the transmission path;
a transmission module comprising: an attenuation unit that attenuates the signal input to the amplifier device in response to the attenuation instruction information. An amplifier device according to any one of claims 1 to 4,
A transmitting device disposed downstream of the amplifier;
a transmission medium for inputting an output signal of the transmitting device;
a receiving device that receives an output signal of the transmission medium. A transmission module according to claim 5;
A transmitting device disposed downstream of the transmission module;
a transmission medium for inputting an output signal of the transmitting device;
a receiving device that receives an output signal of the transmission medium. The transmission system according to claim 6 or 7, wherein the transmission medium is an optical fiber, the transmitting device includes an electrical/optical conversion unit, and the receiving device includes an optical/electrical conversion unit.

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