JP2021069049A - Satellite communication device and transmission level control method - Google Patents

Abstract

To increase resistance to a weather condition.SOLUTION: A satellite communication device according to an embodiment includes a modulation unit, a transmission signal generation unit, a level setting unit, a control unit, a detection unit, and a calculation unit. The modulation unit generates a multi-value modulation signal from an encoded transmission signal. The transmission signal generation unit generates a transmission signal to be transmitted by uplink to an artificial satellite from the multi-value modulation signal. The level setting unit sets the transmission level of the transmission signal on the basis of the control amount. The control unit repeatedly controls the controlled variable to a target value in a first cycle. The detection unit repeatedly detects the reception level of a beacon signal received from the artificial satellite on the downlink. The calculation unit repeatedly calculates the target value in a second cycle which is shorter than the first cycle on the basis of the detected reception level.SELECTED DRAWING: Figure 4

Description

Embodiments of the present invention relate to satellite communication devices and transmission level control methods.

Various services such as SNG (Satellite News Gathering) are provided by using communication satellites (CS) and broadcasting satellites (BS). This type of satellite relays video / audio signals transmitted by uplink from a ground station or the like in a geostationary orbit, and retransmits them to the ground by downlink. The satellite communicates with the equipment on the ground side by radio waves in the Ku band (14 GHz / 12 GHz). Since the Ku band is greatly attenuated by raindrops, the satellite line may be disconnected due to clouds or rainfall. In order to deal with this, there is a technique called TPC (Transmission Power Control).

TPC is a technology that calculates the amount of rainfall attenuation according to the intensity of the reception level of the reference signal (beacon signal, etc.) transmitted from the satellite with constant power on the ground, and corrects the uplink transmission level accordingly. Is. By stabilizing the input level of the satellite to the transponder by TPC, the downlink reception level on the ground side can also be stabilized.

Japanese Unexamined Patent Publication No. 9-46286 Japanese Unexamined Patent Publication No. 4-275726 Japanese Unexamined Patent Publication No. 5-252084

In recent satellite communication systems, multi-value amplitude phase modulation methods such as 64APSK (Amplitude Phase Shift Keying) and 32APSK are used. Although these methods have a high transmission bit rate, they are vulnerable to sudden fluctuations in the reception level because the amplitudes and phase distances between symbols are close to each other. In particular, when the correction amount of the transmission level by the TPC becomes large due to a sudden change in the weather or the like, the demodulation function on the receiving side cannot follow the sudden change in the signal level, and a momentary reception interruption may occur. In order to avoid this, the transmission side takes a relatively long TPC correction cycle to suppress abrupt changes in the transmission level.

However, slowing the change in the transmission level becomes a problem when it is necessary to reduce the transmission level. That is, when the weather recovers rapidly. In such a case, if the increased transmission level cannot be suppressed promptly, the uplink transmission level will exceed the default value. If radio waves with a strength higher than the default value are transmitted to the satellite, the satellite (transponder, etc.) will be overloaded, which will cause deterioration of satellite performance and serious damage. It is required that the transmission level can be suppressed rapidly.

As described above, the satellite communication system adopting TPC has conflicting demands regarding the control of the transmission level, and the provision of a new technology capable of achieving both of these demands is required.
Therefore, an object is to provide a satellite communication device and a transmission level control method with improved resistance to weather conditions.

According to the embodiment, the satellite communication device includes a modulation unit, a transmission signal generation unit, a level setting unit, a control unit, a detection unit, and a calculation unit. The modulation unit generates a multi-value modulation signal from the encoded transmission signal. The transmission signal generation unit generates a transmission signal to be transmitted by uplink to an artificial satellite from a multi-value modulation signal. The level setting unit sets the transmission level of the transmission signal based on the control amount. The control unit repeatedly controls the controlled amount to the target value in the first cycle. The detection unit repeatedly detects the reception level of the beacon signal received from the artificial satellite on the downlink. The calculation unit repeatedly calculates the target value in the second cycle, which is shorter than the first cycle, based on the detected reception level.

FIG. 1 is a diagram showing an example of a satellite communication system to which the satellite communication device according to the embodiment can be applied. FIG. 2 is a conceptual diagram for explaining that the communication environment is affected by the weather conditions. FIG. 3 is a conceptual diagram for explaining that the beacon signal is transmitted from the satellite 100. FIG. 4 is a functional block diagram showing an example of the satellite communication device 200 provided in the base station 20. FIG. 5 is a block diagram showing an example of the data stored in the memory 70 and the functions realized by the processor 60. FIG. 6 is a diagram showing an example of a setting screen displayed on the LCD panel 90a of the operation unit 90. FIG. 7 is a flowchart showing an example of the processing procedure of the satellite communication device 200 according to the embodiment. FIG. 8A is a diagram showing an example of the reception level of the beacon signal observed in fine weather. FIG. 8B is a diagram showing an example of the reception level of the beacon signal observed when the weather deteriorates. FIG. 8C is a diagram showing an example of the reception level of the beacon signal observed when the weather recovers.

The ground-side device (earth station) of the satellite communication system includes a TPC device or a TPC function for compensating for the amount of rainfall attenuation in the uplink. The TPC calculates the amount of rainfall attenuation in the downlink from the reception intensity of a signal (beacon signal or the like) transmitted from the satellite at a constant level. Then, based on that value, the amount of rainfall attenuation on the uplink is obtained, and the amount of transmission power correction is calculated.

While there are devices that realize the TPC function with dedicated equipment (hardware), there are also cases where the TPC is realized only by software algorithms. This type of device calculates its own transmission level based on the reception level information of the beacon signal, and applies the transmission level to the transmission signal generator including the frequency conversion device, so that TPC can be performed without the need for dedicated equipment. Realize.

In recent satellite communication systems, multi-level amplitude phase modulation such as 64APSK and 32APSK is often used due to the needs for high image quality, high compression, narrowband transmission, etc., which makes them vulnerable to sudden fluctuations in reception level. I have to say that. In addition, due to the intensification of the weather in recent years (so-called guerrilla rainstorms), the possibility of block noise and momentary interruptions is increasing. In particular, in an application such as SNG that handles news materials, if noise is mixed in the image, it will be ruined and communication may be restarted from the beginning. Hereinafter, technologies that can solve such problems will be described.

FIG. 1 is a diagram showing an example of a satellite communication system to which the satellite communication device according to the embodiment can be applied. A satellite communication system includes a plurality of earth stations that communicate with each other via artificial satellites (satellite) in geostationary orbit. In this type of system, for example, a live image of a disaster site can be transmitted from a satellite communication device installed at the site and simultaneously received by earth stations such as a plurality of prefectural office locations. This makes it possible to know the disaster situation quickly and accurately.

In FIG. 1, the signal uplink-transmitted from the base station 20 to the satellite 100 is downlink-transmitted from the satellite 100 to the ground via the satellite 100. The signal relayed by the satellite 100 is received by the on-vehicle station 10 and the receiving station 30 (31 to 3N) on the ground. The in-vehicle station 10 is dispatched to, for example, a disaster site. The base station 20 is installed at the location of the prefectural office, and the receiving station 30 is installed in a mountainous area, a local government, or the like. All of the in-vehicle station 10, the base station 20, and the receiving station 30 can be equipped with a satellite communication device. In the embodiment, the satellite communication device provided in the base station 20 will be described.

As shown in FIG. 2, when clouds cover the sky above the base station 20, the communication link between the base station 20 and the satellite 100 is physically blocked, and the uplink signal from the base station 20 is sufficient. It will not reach satellite 100 at the level. Correspondingly, the power level of the downlink signal also decreases, and there is a possibility that reception failure may occur in the in-vehicle station 10 and the receiving station 30. In order to prevent reception failure, the base station 20 side corrects the transmission power by the amount of rainfall attenuation of the uplink signal and transmits the uplink signal.

As shown in FIG. 3, satellite 100 continuously downlinks the beacon signal at a defined level. By detecting the reception level of this beacon signal and constantly monitoring it, the ground station can grasp the amount of rainfall attenuation with and from the satellite 100. Based on the result, the TPC controls the transmission level of the uplink signal.

FIG. 4 is a functional block diagram showing an example of the satellite communication device 200 provided in the base station 20. The satellite communication device 200 includes a transmission unit 40, a reception unit 50, a processor 60, a memory 70, and an internal bus 80 connecting them. An operation unit 90 as a user interface is connected to the internal bus 80, and each function of the satellite communication device 200 can be set from the operation unit 90.

The processor 60 is, for example, a CPU (Central Processing Unit), an MPU (Micro Processing Unit), or the like, and the memory 70 is realized as, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), or the like. That is, the satellite communication device 200 is a computer including a processor and a memory as hardware.

In FIG. 4, MPEG2-TS (Moving Picture Experts Group 2-Transport Stream) and H.M. A transmission signal including video, audio, data material, etc. encoded by a coding method such as 265 / HEVC (High Efficiency Video Coding) is input to the modulation unit (MOD) 45 of the transmission unit 40. The modulation unit 45 generates a multi-value modulation signal such as N-APSK from this transmission signal. Here, N is typified by 16, 32, 64, etc., but other modulation methods with a multi-valued number N are also possible. It is also possible to use a method such as QAM (Quadrature Amplitude Modulation).

The multi-valued modulated signal is input to the upconverter (U / C) 44 and converted to a frequency that is uplink-transmitted to the satellite 100. As a result, a transmission signal is generated.
Here, the upconverter 44 includes an attenuator (ATT) 44a. The attenuator 44a sets the transmission level of the transmission signal based on the control amount (TPC correction value (a)) given by the processor 60. The transmission signal is input from the upconverter 44 to the antenna device 41 via the power amplification unit (SSPA: Solid State Power Amplifier) 43 and the waveguide switching unit 42. The waveguide switching unit 42 is provided to share the antenna device 41 among the plurality of transmission units 40, and can be omitted if there is only one transmission unit 40.

The antenna device 41 includes a FEED 41a having a circulator function and an antenna portion 41b. The transmission signal from the power amplification unit 43 is transmitted by uplink from the antenna unit 41b to the satellite 100 via the FEED 41a.

On the other hand, the signal arriving from the satellite 100 on the downlink is received by the antenna unit 41b and input to the receiving unit 50 via the FEED 41a. This signal is amplified by the receiving amplifier (LNC: Low Noise Converter) 51 of the receiving unit 50, and then distributed to the beacon receiver 53 and the receiving device 54 by the distributor 52. The receiving device 54 receives and demodulates various data from the downlink signal.
The beacon receiver 53 constantly monitors the beacon signal from the satellite 100 and notifies the processor 60 of the beacon signal reception level (b) of the beacon signal.

FIG. 5 is a block diagram showing an example of the data stored in the memory 70 and the functions realized by the processor 60.

The processor 60 includes a level setting function 60a, a control function 60b, a detection function 60c, a calculation function 60d, and a setting value variable function 60e as processing functions related to the embodiment. These functions are realized by the processor 60 executing the instruction described in the program 70a stored in the memory 70. That is, the processor 60 functions as a level setting unit, a control unit, a detection unit, a calculation unit, and a set value variable unit by the program 70a.

The level setting function 60a is a function for setting / registering a reference level in fine weather (attenuation amount 0) when operating the TPC. For example, when the beacon level in fine weather is -60 dBm, 20 dB is set as the reference value (TPC reference value) of the attenuation amount of the attenuator, and 1.25 is set as the correction coefficient of TPC.

The control function 60b repeatedly gives the TPC correction value (a) to the separately calculated target value to the attenuator 44a in a preset stable period cycle (first cycle) to control the attenuator 44a. The stable period set value is input from the operation unit 90 using, for example, the user interface shown in FIG. 6, and is stored in the memory 70 as the stable period set value 70b.

The user interface of FIG. 6 is displayed on, for example, an LCD (Liquid Crystal Display) panel 90a of the operation unit 90. The LCD panel 90a displays an area for setting the calculation cycle, the number of moving averages, the minimum level change amount, and the stability period. An arrow mark (black-painted △ / black-painted ▽) is associated with each area, and the numerical value can be increased / decreased by touching the arrow mark. In the embodiment, the stable period set value = 2000 [msec: milliseconds].

The detection function 60c (FIG. 5) of the processor 60 repeatedly detects the reception level (b) of the beacon signal received from the satellite 100 on the downlink. The detected value is stored in the memory 70 as the reception level 70c.

The calculation function 60d has a calculation cycle (second cycle) shorter than the stable period set value (first cycle) based on the reception level (b) detected by the detection function 60c, and is a TPC correction value (a). The target value of is calculated repeatedly. The set value of the calculation cycle is input from the operation unit 90 using the LCD screen of FIG. 6, and is stored in the memory 70 as the calculation cycle set value 70d. In the embodiment, the calculation cycle set value = 800 [msec].
Here, the calculation function 60d calculates the target value based on the moving average value of the reception level (b) over the preset number of samples.

The set value variable function 60e is a group of user interface functions for changing various set values. That is, the set value variable function 60e displays, for example, a user interface for variably setting either or both of the stable period and the calculation cycle on the LCD panel 90a. Further, the set value variable function 60e displays a user interface for setting a step (change amount) when controlling the TPC correction value (a) as a control amount to a target value on the LCD panel 90a.

As shown in FIG. 6, a region for setting the minimum level change amount is provided on the LCD panel 90a. The set value of the minimum level change amount is input from the operation unit 90 using the LCD screen of FIG. 6, and is stored in the memory 70 as the minimum level change amount set value 70e. In the embodiment, the minimum level change amount setting value = 0.5 [dB: decibel]. That is, the TPC correction value (a) is changed by 0.5 dB so as to approach the target value.

Further, the set value variable function 60e controls the LCD panel 90a to create a user interface for setting the number of samples (moving average number of times) of the reception level (b) for calculating the target value.

As shown in FIG. 6, an area for setting the moving average number of times is provided on the LCD panel 90a. The set value of the moving average number of times is input from the operation unit 90 using the LCD screen of FIG. 6, and is stored in the memory 70 as the moving average number of times set value 70f. In the embodiment, the moving average number of times setting value = 5 [times]. That is, the target value of the TPC correction value (a) is calculated based on the average value of the five consecutive samples of the beacon signal reception level (b) in chronological order.

Here, the stable period (first period) and the minimum level change amount are set on the receiving side of the multi-valued modulation signal based on the modulation multi-value number N of the multi-value modulation signal generated by the modulation unit 45. It is preferable that it is set to reduce the error of. That is, by lengthening the stable period as the number of multiple values increases, it is possible to suppress a steep change in the TPC correction and reduce the shock on the receiving side. The same effect can be obtained because the change in the TPC correction can be made gentle by reducing the minimum level change amount as the number of multi-values N increases. Next, the operation in the above configuration will be described.

FIG. 7 is a flowchart showing an example of the processing procedure of the satellite communication device 200 according to the embodiment. In FIG. 7, the satellite communication device 200 repeatedly acquires the reception level (beacon level) of the received beacon signal (block B1). As a result, as shown in FIGS. 8A to 8C, a time series of reception levels is acquired. FIG. 8A shows an example of the beacon level in fine weather, for example, a beacon level of −60 [dBm] is stably acquired. Here, the beacon level observation cycle is the calculation cycle of the control target value, and it is assumed that the number of samples equal to or larger than the number of moving averages is obtained. That is, in the embodiment, the beacon level is detected every 800 [msec], and it takes 4 seconds for the first time to acquire 5 samples, and then 5 samples are updated in an observation cycle of 800 [msec]. To do.

Next, the satellite communication device 200 executes a process of calculating a correction target value (block B2). In block B2, the satellite communication device 200 calculates the moving average value of the beacon level over 5 times. For example, in the state of FIG. 8A, the moving average value = −60 dBm, and the maximum beacon level (clear sky level) is obtained. Therefore, the maximum amount of attenuation (for example, 20 dB) is set in the attenuator 44a. This 20 dB is the correction target value.

FIG. 8B shows an example of the beacon level observed in a state where the weather is getting worse. FIG. 8B shows how the beacon level gradually decreases, and for example, a moving average value = −64.0 dBm is calculated. In this case, it is necessary to weaken the attenuation in the attenuator 44a and raise the output level of the transmission signal (attenuation amount DOWN).

Since the beacon level has dropped by 4 dBm from the clear sky level, this is multiplied by the default coefficient 1.25 (4 x 1.25 = 5) and this value is subtracted from 20 dB to obtain the target value of 15.0 dB. .. That is, in the case of FIG. 8B, 15.0 dB is calculated as the target value of the attenuation amount of the attenuator 44a. If the moving average value is lowered to −70.0 dBm, 20-{(70-60) × 1.25} = 7.5 dB is calculated as the target value of the attenuation of the attenuator 44a. This target value is calculated and updated at intervals of, for example, 800 msec.

Here, the coefficient 1.25 will be described. That is, the attenuation amount of the beacon level and the attenuation amount of the attenuator 44a are usually different values. This is because the slope of attenuation differs depending on the frequency. For example, it is known that the amount of deterioration calculated for the beacon signal received in the 12 GHz band is corrected to be approximately 1.25 times in the 14 GHz band, which is the transmission band. As described above, the conversion of the attenuation amount due to the difference in the attenuation characteristics of the signal between the reception band and the transmission band is processed by, for example, the level setting function 60a.

There is a certain relationship between the moving average value and the amount of attenuation. For example, the moving average value is tabulated in advance and stored in the memory 70. That is, when the value of the moving average value becomes low, the amount of attenuation is also reduced (the output level is increased). This correspondence can be calculated in advance by, for example, simulation.

Next, the satellite communication device 200 executes a process of controlling the TPC level (block B3). This block includes a process of controlling the attenuation of the attenuator 44a to the calculated target value and a process of setting the transmission level of the uplink transmission signal to the satellite 100 based on the attenuation of the attenuator 44a. ..

That is, as an example, the satellite communication device 200 gradually changes the attenuation amount of the attenuator 44a from 20.0 dB to 15.0 dB in steps (steps) of 0.5 dB every 2000 msec. Here, 2000 msec is the stable period shown in FIG. 6, and 0.5 dB is the minimum level change amount.

FIG. 8C shows an example of the beacon level observed in a state where the weather is recovering. In FIG. 8C, a state in which the beacon level gradually returns to the original level is shown, and for example, a moving average value = -62.0 dBm is calculated. In this case, it is necessary to immediately strengthen the attenuation at the attenuator 44a so that the output level of the transmission signal does not exceed the default value (attenuation amount UP). In the case of FIG. 8C, 20-{(62-60) × 1.25} = 17.5 dB is calculated as the target value of the attenuation of the attenuator 44a.

In the process from the state of FIG. 8B to the state of FIG. 8C, the target value of the attenuation of the attenuator 44a changes from 15.0 dB to 17.5 dB. Here, too, the satellite communication device 200 gradually changes the attenuation of the attenuator 44a from 15.0 dB to 17.5 dB in steps of 0.5 dB every 2000 msec. Then, the procedures of blocks B1 to B3 are repeatedly executed.

As described above, in this embodiment, the attenuation amount of the attenuator 44a is controlled to the control target value relatively slowly in the first cycle and in a small step. By doing so, even when the number of modulated multi-values of the multi-valued modulated signal increases, the change in the signal level can be softened, the shock on the receiving side can be reduced, and the occurrence of transmission error can be suppressed.

On the other hand, the calculation of the control target value itself is executed faster than the first cycle and is constantly updated. As a guide, if the attenuation amount is controlled in a cycle of 2000 msec, the target value is repeatedly calculated in a cycle of 800 msec. As a result, it is possible to follow a rapid change in the weather at a sufficiently high speed, and it is possible to prevent the uplink transmission level from exceeding the default value.

In the existing technology, the algorithm design is such that negative feedback acts only after the uplink transmission level exceeds the default value. Therefore, for example, a transmission signal of a predetermined level or higher is transmitted to the satellite 100 for several seconds, which may put an excessive load on the satellite 100.

On the other hand, according to the embodiment, the correction target value for TPC is calculated at high speed by the moving average value and constantly updated. As a result, it is possible to improve the ability to follow sudden changes in the weather. In addition, regarding control to the control target value, by ensuring the minimum amount of level change that can withstand the multi-value modulation method and the stable period, it is possible to make it resistant to sudden level fluctuations of the received signal. Become.

That is, according to the embodiment, it is possible to achieve both suppression of abrupt changes in TPC correction and responsiveness of TPC correction to changes in weather. Therefore, stable satellite communication is possible without momentary interruption even in multi-level amplitude phase modulation. In addition, it is expected that the algorithm will be applied and applied even if the technology for increasing the number of values is further advanced in the future. Furthermore, safe satellite communication is possible even against sudden changes in the weather such as guerrilla rainstorms. By applying the algorithm of the embodiment, it is possible to respond according to the characteristics of the weather in the area.
From these facts, it becomes possible to provide a satellite communication device and a transmission level control method having improved resistance to weather conditions.

The present invention is not limited to the above embodiment. For example, in the above description, an example in which the function related to the embodiment is implemented in the base station 20 is shown. Instead of this, it is of course possible to mount the functional block shown in FIG. 5 on the in-vehicle station 10 or the receiving station 30.

Of course, the calculation cycle of the target value (800 msec), the number of moving averages (5 times), the minimum level change amount (0.5 dB), and the stability period (2 seconds) can all be set to different values. It flexibly responds to various conditions such as system hardware, multi-value modulation method, and weather conditions, and can be applied in a wide range of conditions. For example, if the capacity of the processor 60 is sufficient, the calculation cycle of the target value may be shortened to 400 msec, 200 msec, or the like. Further, these values may be set in advance by default according to the modulation multi-value number N.

Further, it is possible to switch and set the modulation multi-value number N and the modulation method even during the operation of the system. At that time, it is operationally preferable to switch the modulation multi-value number N and the modulation method between the first news material and the second news material which are different from each other.

Although embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. This novel embodiment can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. This embodiment and its modifications are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

10 ... In-vehicle station, 20 ... Base station, 30 ... Receiving station, 40 ... Transmitting unit, 41 ... Antenna device, 41a ... FEED, 41b ... Antenna unit, 42 ... Waveguide switching unit, 43 ... Power amplification unit, 44 ... Upconverter, 44a ... attenuator, 45 ... modulator, 50 ... receiver, 51 ... LNC, 52 ... distributor, 53 ... beacon receiver, 54 ... receiver, 60 ... processor, 60a ... level setting function, 60b ... control function , 60c ... detection function, 60d ... calculation function, 60e ... variable setting value function, 70 ... memory, 70a ... program, 70b ... stable period setting value, 70c ... reception level, 70d ... calculation cycle setting value, 70e ... minimum level change Amount setting value, 70f ... Movement average number of times setting value, 80 ... Internal bus, 90 ... Operation unit, 90a ... LCD panel, 100 ... Satellite, 200 ... Satellite communication device.

Claims (13)

A modulator that generates a multi-valued modulated signal from a coded transmission signal,
A transmission signal generator that generates a transmission signal uplink-transmitted to an artificial satellite from the multi-value modulation signal,
A level setting unit that sets the transmission level of the transmission signal based on the control amount, and
A control unit that repeatedly controls the control amount to the target value in the first cycle,
A detector that repeatedly detects the reception level of the beacon signal received from the artificial satellite on the downlink,
A satellite communication device including a calculation unit that repeatedly calculates the target value in a second cycle shorter than the first cycle based on the detected reception level. The satellite communication device according to claim 1, further comprising a user interface for variably setting at least one of the first cycle or the second cycle. The first cycle is set according to any one of claims 1 or 2, wherein the first period is set to reduce an error on the receiving side of the multi-valued modulated signal based on the number of modulated multi-values of the multi-valued modulated signal. Satellite communication device. The satellite communication device according to claim 1, wherein the control unit controls the control amount to the target value in each first cycle with a variable amount of change. The satellite communication device according to claim 4, further comprising a user interface for variably setting the amount of change. The satellite according to claim 4 or 5, wherein the amount of change is set to reduce errors on the receiving side of the multi-valued modulated signal based on the number of modulated multi-values of the multi-valued modulated signal. Communication device. The satellite communication device according to claim 1, wherein the calculation unit calculates the target value based on a moving average value of the reception level over a variable number of samples. The satellite communication device according to claim 7, further comprising a user interface for variably setting the number of samples. The process of setting the transmission level of the transmission signal generated from the multi-value modulation signal and transmitted by uplink to the artificial satellite based on the control amount, and
The process of repeatedly controlling the controlled amount to the target value in the first cycle, and
The process of repeatedly detecting the reception level of the beacon signal received from the artificial satellite on the downlink, and
A transmission level control method comprising a process of repeatedly calculating the target value in a second cycle shorter than the first cycle based on the detected reception level. The transmission level control method according to claim 9, wherein the first cycle is set to reduce errors on the receiving side of the multi-valued modulated signal based on the number of modulated multi-values of the multi-valued modulated signal. .. The transmission level control method according to claim 9, wherein the control amount is a variable amount of change and is controlled to the target value in each first cycle. The transmission level control method according to claim 11, wherein the amount of change is set to reduce an error on the receiving side of the multi-valued modulated signal based on the number of modulated multi-values of the multi-valued modulated signal. The transmission level control method according to claim 9, wherein the target value is calculated based on a moving average value of the reception level over a variable number of samples.

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