JP2003198505A - Transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maximize transmission line capacity and to improve a communication quality by determining an optimal modulated multilevel number corresponding to a dynamic change in tertiary mutual modulation distortion. <P>SOLUTION: A mutual modulation distortion detecting part 23a calculates a spectrum for each frequency from a photo-current RF signal, extracts the spectrum of a frequency located in the center of all bands and detects the distortion component information of the mutual modulation distortion. An evaluation function arithmetic part 23b operates an evaluation function on the basis of the distortion component information and calculates an optimal optical modulation factor mopt and entire transmission line capacity Ct including an optical fiber transmission line and a wireless transmission line for each modulation multilevel number. An optimal modulation multilevel number determining part 25a determines the optimal modulation multilevel number taking the maximum value of the entire transmission line capacity Ct. An RF modulation part 25b generates an RF modulated signal by applying RF modulation to transmission data while using the optimal modulation multilevel number. An optical signal transmitting part 26 generates and transmits an optical RF modulated signal by controlling the electric/optic conversion of the RF modulation signal while using the optimal optical modulation factor mopt. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission system, and more particularly to a transmission system for transmitting information wirelessly by an optical fiber.

[0002]

2. Description of the Related Art In recent years, the demand for wireless communication such as mobile communication and fixed wireless access (FWA) has spread to general consumers, and the number of wireless communication subscribers is rapidly increasing. It has increased. In this situation,
At present, a system called ROF (Radio on Fiber) is drawing attention as a transmission technique of wireless signal information using an optical fiber.

FIG. 15 is a diagram showing the overall configuration of the ROF system. The ROF system (optical fiber radio communication system) 100 includes a control station (CS) 20.
0, a base station (BS) 300, and a wireless terminal (WT) 400.

The control station 200 and the base station 300 are connected by an optical fiber (for upstream and downstream), and the base station 300 and the wireless terminal 400 are wirelessly connected through their respective antennas. Further, the control station 200 is connected to the network 5 including the Internet.

The ROF system utilizes the wide band property of an optical fiber to make RF (Radio Frequency) unique to various services.
ncy) signal information is superimposed on an optical carrier and transmitted through an optical fiber. It is also possible to transmit a plurality of optical signals of different wavelengths within the same optical fiber.

By using the ROF system 100 in which the optical fiber communication and the wireless communication are integrated as described above, the wireless services of different frequencies are integrated to realize a multi-service wireless transmission system for transmitting by one optical fiber. It will be possible.

On the other hand, in the conventional wireless transmission, a method in which the modulation multi-value number is fixed to a fixed value is mainly used regardless of the transmission line capacity, but in recent years, the modulation multi-value number is variable. A method of setting and controlling the transmission line capacity has been proposed.

For example, "adaptive modulation system" (Kozo Makizo, "Study on variable capacity microwave system", IEICE Transactions B-II, J 73-B-II, No.10, Oct. 1990). Then
The wireless modulation method is selected according to the situation of the wireless transmission path section characteristics to increase the transmission path capacity.

[0009]

However, the prior art as described above is an optimization considering only the wireless transmission path section between the base station and the wireless terminal, and the signal existing section is between the base station and the wireless terminal. In addition, it cannot be directly applied to the ROF system 100 that is extended to the section of the control station through the optical fiber.

On the other hand, intermodulation is a factor to be taken into consideration in noise and interference in the transmission system. This is a phenomenon in which when a plurality of signals are input to a non-linear circuit, a signal having a third frequency different from the frequency of the original signal is generated. The distortion component generated by this intermodulation deteriorates the transmission quality.

Since the ROF system 100 uses a non-linear circuit such as an LD (laser diode) when converting an RF signal into light, third-order intermodulation distortion (IM3: 3rd).
order Inter Modulation) occurs.

This third-order intermodulation distortion depends on the number of subcarriers (the number of subcarriers) proportional to the number of accommodated subscribers, and when the number of accommodated subscribers changes, the amount of third-order intermodulation distortion also changes. The transmission line capacity also changes dynamically. Therefore, there is a problem in that it is difficult to perform high-quality transmission especially in mobile wireless transmission in which traffic changes drastically.

Therefore, in the ROF system 100, the modulation multi-value number is variably set to maximize the transmission line capacity (total transmission line capacity of the optical fiber transmission line section and the wireless transmission path section) to improve the quality. To achieve this, it is necessary to perform optimization control that flexibly responds to changes in third-order intermodulation distortion.

The present invention has been made in view of the above circumstances, and determines the optimum modulation multi-level number in accordance with the dynamic change of the third-order intermodulation distortion to maximize the transmission line capacity, It is an object of the present invention to provide a transmission system with improved transmission quality.

[0015]

In order to solve the above problems, the present invention is directed to a transmission system 1 for transmitting wireless information by an optical fiber as shown in FIG. 1, in which an optical signal transmitted through an optical fiber transmission line is used. An optical signal receiving unit 21 that receives a signal, performs optical / electrical conversion to generate a photocurrent RF signal, and an RF demodulation that demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal. The unit 22, the intermodulation distortion detection unit 23a that calculates the spectrum for each frequency from the photocurrent RF signal, and detects the distortion component information of the intermodulation distortion from the spectrum of the frequency located at the center of the entire band, and the distortion component information. Based on the evaluation function, the optimum optical modulation index mopt and the total transmission line capacity Ct including the optical fiber transmission line and the wireless transmission line are calculated for each modulation multi-level number. An evaluation function calculating unit 23b, an RF reception control unit 23 composed of, processes the baseband signal to generate a transmission data, a baseband signal processing section 24 outputs the parameter values required for the evaluation function calculation,
An optimum modulation multi-value number determination unit 25a that determines the optimum modulation multi-value number that takes the maximum value of the total transmission line capacity Ct, and an optimum modulation multi-value number are used to perform RF modulation on the transmission data to generate an RF modulation signal. An RF transmission control unit 25 including an RF modulation unit 25b for controlling the electrical modulation of the RF modulation signal by using the optimum optical modulation degree mopt to generate and transmit the RF modulation optical signal. A transmission device 20 including a signal transmission unit 26, a radio reception unit 41 that receives a radio signal, and a terminal side RF modulation signal by modulating a baseband signal with the modulation multi-level number determined by the transmission device 20. Wireless terminal 4 configured to generate and wirelessly transmit the wireless terminal 4 and the wireless terminal 4. The wireless terminal 4 is connected to the wireless terminal 4 via a wireless transmission path, and is connected to the transmission device 20 via an optical fiber transmission path. A relay device that performs relay transmission between the transmission devices 20. Transmission system 1 is provided characterized by having a 30.

Here, the optical signal receiving section 21 receives the signal transmitted through the optical fiber transmission line, performs optical / electrical conversion, and generates a photocurrent RF signal. The RF demodulation unit 22 demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal.
The intermodulation distortion detector 23a calculates a spectrum for each frequency from the photocurrent RF signal, and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located in the center of the entire band. The evaluation function calculation unit 23b calculates an evaluation function based on the distortion component information, and calculates the optimum optical modulation index mopt and the total transmission line capacity Ct including the optical fiber transmission line and the wireless transmission line for each modulation multilevel number. calculate. The baseband signal processing unit 24 processes the baseband signal, generates transmission data, and outputs a parameter value required when calculating the evaluation function.
The optimum modulation multi-level number determination unit 25a determines the optimum modulation multi-level number that takes the maximum value of the total transmission line capacity Ct. RF modulator 2
5b RF-modulates the transmission data by using the optimum modulation multi-level number to generate an RF modulation signal. Optical signal transmitter 26
Controls the electrical / optical conversion of the RF modulated signal using the optimum optical modulation degree mopt to generate and transmit the RF modulated optical signal. The wireless receiver 41 receives a wireless signal. The wireless transmission unit 42 modulates the baseband signal with the modulation multilevel number determined by the transmission device 20 to generate a terminal-side RF modulation signal,
Send wirelessly. The relay device 30 is connected to the wireless terminal 4 via a wireless transmission line and is connected to the transmission device 20 via an optical fiber transmission line to perform relay transmission between the wireless terminal 4 and the transmission device 20.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a principle diagram of a transmission system of the present invention. The transmission system 1 is an ROF system that transmits information transmitted wirelessly by an optical fiber, and includes a control station 2 including a transmission device 20, a base station 3 including a relay device 30, and a wireless terminal 4.

The control station 2 and the base station 3 are connected by upstream and downstream optical fibers, and the base station 3 and the wireless terminal 4 are wirelessly connected. Furthermore, the control station 2
A network 5 including the Internet is connected to the.

The transmission device 20 includes an optical signal receiver 21 and RF.
The demodulator 22, RF reception controller 23, baseband signal processor 24, RF transmission controller 25, and optical signal transmitter 26 are included.

The optical signal receiver 21 receives the upstream optical signal transmitted through the optical fiber transmission line,
Photocurrent R converted from light / electricity (O / E)
Generate an F signal.

The RF demodulation section 22 demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal. The RF reception controller 23 includes an intermodulation distortion detector 23a and an evaluation function calculator 23b. The intermodulation distortion detector 23a calculates a spectrum for each frequency from the photocurrent RF signal, and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located in the center of the entire band.

In the system of the present invention, the subscriber is not assigned to the frequency located in the center of the entire band. Further, in the present invention, since the third-order intermodulation distortion is targeted as the intermodulation distortion, the intermodulation distortion will be referred to as IM3 hereinafter.
Call.

The evaluation function calculator 23b calculates an evaluation function based on the distortion component information of IM3, and obtains the optimum optical modulation index mop.
t and the total transmission line capacity Ct including the optical fiber transmission line and the wireless transmission line are calculated for each modulation multi-level number. Details of the evaluation function and the calculation method will be described later.

The baseband signal processing section 24 processes the baseband signal, generates transmission data, and outputs a parameter value necessary for calculating the evaluation function. RF transmission controller 2
5 is an optimum modulation multi-value number determination unit 25a and an RF modulation unit 25b.
including. The optimum modulation multi-level number determination unit 25a determines the optimum modulation multi-level number (the optimum value of the QAM digital modulation multi-level number) that takes the maximum value of the total transmission line capacity Ct. That is, of the plurality of total transmission line capacities Ct calculated for each modulation multi-value number, the total transmission line capacity having the maximum value (Ct _max )
The modulation multi-value number corresponding to is the optimum modulation multi-value number.

The RF modulator 25b RF-modulates the transmission data using the optimum modulation multi-value number to generate an RF modulation signal. The optical signal transmission unit 26 controls the electrical / optical (hereinafter, E / O) conversion of the RF modulation signal by using the optimum optical modulation degree mopt to generate the RF modulation optical signal, and the optical fiber on the downstream side. To send through.

The wireless receiver 41 receives a wireless signal from the wireless terminal 4. The wireless transmission unit 42 modulates the baseband signal with the modulation multi-level number (optimum modulation multi-level number) determined by the transmission device 20, generates a terminal-side RF modulation signal, and wirelessly transmits it.

The relay device 30 is connected to the wireless terminal 4 via a wireless transmission line, and is connected to the transmission device 20 via an optical fiber transmission line. Then, relay transmission between the wireless terminal 4 and the transmission device 20 is performed. Next, the IM3 will be described. Frequency f ₁
And f ₂ are input to the non-linear circuit, the frequency f _{i of the} intermodulation wave generated is | m · f ₁ ± n · f ₂ | (m,
n is a positive integer), and m + n is called the order of the intermodulation wave. IM3 is an intermodulation wave of m + n = 3.

FIG. 2 is a diagram showing an image of IM3 generation for subcarriers. The vertical axis represents electric power and the horizontal axis represents frequency. Here, Nc = 3, where Nc is the number of subcarriers.
Think about the case. Further, the frequencies of the subcarriers are f ₁ , f ₂ , and f _3, and the bandwidth per subcarrier is BW.

With respect to the intermodulation waves I1 to I3 of IM3, the frequency of the intermodulation wave I1 is 2f ₂ −f ₃ , the frequency of the intermodulation wave I 2 is f ₁ −f ₂ + f ₃ , and the frequency of the intermodulation wave I 3 is 2
f ₂ −f ₁ (both m + n = 3).

Further, IM3 generated from the two-wave signal is replaced with I generated from the two-tone IM3 and the three-wave signal.
M3 is called a 3-tone IM3. The intermodulation waves I1 and I3 are two-tone IM3, and the intermodulation wave I2 is three-tone IM3.

As can be seen from the figure, the three-tone IM3 is
It occurs in the central part of the entire band, and the power spectrum value becomes larger (closer to twice) as compared with the two-tone IM3. Further, as the number of subcarriers (the number of subscribers) increases, the power spectrum value of the 3-tone IM3 in the central part of the entire band becomes higher. Therefore, when extracting the IM3 information, the three-tone IM3 that occurs in the center of the entire band assigned to the subscriber may be taken into consideration as the worst-case evaluation.

In the system of the present invention, the central frequency fm (= (f ₁ + f
_Since no subscriber is assigned to _Nc ) / 2), there is no subcarrier at this frequency position.
Therefore, the power spectrum value of the component remaining at the frequency fm can be regarded as the power spectrum value of IM3. The power spectrum value of IM3 corresponds to D calculated by Expression (4a) described later, and corresponds to distortion component information.

Next, a method of calculating IM3 and a method of calculating the transmission line capacity and the optimum optical modulation index mopt in the evaluation function will be described. The evaluation function for calculating the transmission line capacity Cr in the wireless transmission line is Equation (1).

[0034]

[Equation 1]

However, 2n is a modulation multi-valued number. Specifically, 2n = 4, 16, 64, 256 as the modulation multilevel number of QAM (Quadrature Amplitude Modulation). Nc is the number of subcarriers (the number of ∝ subscriber terminals). BW is a signal bandwidth (unit: Hz) per subcarrier in the wireless transmission path section.

On the other hand, the transmission line capacity Copt in the optical transmission line
The evaluation function for calculating is Equation (2).

[0037]

[Equation 2]

However, BWmax is the upper limit value of the signal bandwidth per subcarrier in the optical fiber transmission line section which has been deteriorated by IM3 or LD relative noise intensity density. Iph is the current value of the photocurrent RF signal.
Na is an optical fiber transmission line section noise power, and is calculated by the following equation (3).

[0039]

[Equation 3]

In the equation (3), RIN is the LD relative noise intensity density, e is the charge amount, and <Ith ² > is the input equalization noise current density (thermal noise of the receiving circuit). Further, D in the formula (2) is I
It is a power spectrum value of M3, and is obtained from the equation (4a) as a function of the third-order coefficient (coefficient of IM3) a _{3 of} the LD input / output nonlinear characteristic curve and Nc.

[0041]

[Equation 4]

R in the equation (4a) is r = Nc / 2 when Nc is an even number, and r = Nc when Nc is an odd number.
(Nc / 2) +1. Further, D ₂ (Nc, r) is obtained by the equation (4b), and D ₃ (Nc, r) is obtained by the equation (4c) (N
Let c be k and r be i). Furthermore, γ in equation (2) is Q
Required CN to achieve specified error rate in AM modulation
R (Carrier Noise Ratio).

Here, the optical modulation degree m (the same value as the value obtained by dividing the modulated light amplitude by the LD average output light power) in the E / O conversion unit (corresponding to the LD) inside the optical signal transmission unit 26 is as follows. Adjustment to the optimum optical modulation index mopt in (5) is a necessary condition for calculating the optical fiber transmission line capacity Copt.

[0044]

[Equation 5]

The total transmission line capacity Ct, which is the total transmission line capacity including the optical fiber transmission line and the wireless transmission line.
The evaluation function for calculating is Equation (6). That is, the wireless transmission line capacity Cr and the optical fiber transmission line capacity Copt
Whichever is smaller is the total transmission line capacity Ct. m
in (A, B) means the smaller of A and B.

[0046]

[Equation 6]

The intermodulation distortion detector 23a and the evaluation function calculator 23b calculate the power spectrum value D of IM3, the total transmission line capacity Ct, and the optimum optical modulation degree mopt by using the equations as described above. Various parameter values required for calculation are passed from the baseband signal processing section 24.

Next, the relationship among the wireless transmission line capacity Cr, the optical fiber transmission line capacity Copt, and the total transmission line capacity Ct will be described. FIG. 3 is a diagram showing the relationship between the transmission line capacity and the number of modulation levels. The vertical axis represents the transmission line capacity (b / s), and the horizontal axis represents the QAM digital modulation multilevel number 2n.

From the equation (1), the radio transmission line capacity Cr is calculated by the radio signal band BW, the number of subcarriers Nc, and the modulation multilevel number 2n.
It can be seen that the value increases simply in proportion to and (curve (a) in FIG. 3).

On the other hand, from the equation (2), the optical transmission line capacity Copt
On the contrary, contrary to the case of the wireless transmission path, it is found that the deterioration is caused in inverse proportion to Nc and the modulation multilevel number 2n (FIG. 3).
Curve (b)). This is because the IM generated in the optical transmission line section with respect to the ratio of the increase in the transmission line capacity due to the increase in the modulation multi-value number.
This is because the deterioration rate of the transmission line capacity due to the power (= D) of 3 is higher. Also, the optical fiber transmission line capacity Copt
Depends on the optical modulation degree m of the E / O conversion section of the optical signal transmission section 26, and takes the maximum value at the optimum optical modulation degree mopt.

In the ROF system, since the same information of the RF modulated signal passes through both the wireless and optical fiber transmission lines, the total total transmission line capacity Ct of the wireless transmission line section and the optical fiber transmission line section is Cr. It will be limited by the minimum value with Copt (curve (c) in FIG. 3).

Here, for example, even if 256 QAM is adopted as the modulation method at the time of system design, the optimum modulation multi-level number that maximizes the total transmission line capacity is 64 instead of 256 due to the trade-off relationship shown in the figure. I understand. Therefore, in the conventional system design by the fixed modulation method, the originally desired transmission line capacity cannot be realized, and high quality transmission cannot be maintained.

According to the present invention, a high transmission quality is obtained by selecting the optimum digital modulation method that maximizes the total transmission line capacity Ct of the optical fiber transmission line section and the wireless transmission line section according to the dynamic change of the IM3 amount. The ROF system having the above is realized.

Next, the operation of the transmission device 20 will be described in detail (referred to as the first embodiment). FIG. 4 is a diagram for explaining the operation of the transmission device according to the first embodiment. The following steps S1 to S5 are processes on the upstream side,
Steps S6 to S8 are processes on the downstream side. Note that the configuration of the transmission device 20-1 is the same as that of FIG.

[S1] The optical signal Pout sent from the base station 3 to the control station 2 causes optical loss through the optical fiber transmission line and reaches the control station 2 as an optical signal Pa. The optical fiber loss is Floss and the optical power of the optical signal Pout is Pw.
Then, the optical power Pwa of the optical signal Pa is Pwa = P
w / Floss.

[S2] The optical signal receiver 21 receives the optical signal Pa
Is received, O / E converted, and a photocurrent RF signal is generated. The current value Iph of the photocurrent RF signal is I
ph = η _PD · Pwa = η _PD · Pw / Floss. In addition,
η _PD is the light receiving sensitivity of the optical signal receiving unit 21, and generally indicates the ratio of O / E conversion. The unit is A / W.

[S3] The intermodulation distortion detector 23a performs frequency spectrum calculation on the photocurrent RF signal to calculate the spectrum value of the frequency of each subcarrier. Then, the entire band (Nc ×
The frequency component of the central frequency fm (= (f1 + f _Nc ) / 2) of BW) is extracted.

At the frequency position here, as described above,
There are no subcarriers because no subscribers have been assigned. Therefore, the power spectrum value of the component remaining in fm is used as the power spectrum value of IM3, and is expressed by equation (4)
Determined by a).

[S4] The evaluation function operation unit 23b uses the parameter values passed from the baseband signal processing unit 24 to calculate the QAM modulation multi-value number from the expressions (1), (2), and (5). The total transmission line capacity Ct corresponding to each case of 2n = 4, 16, 64, 256 is calculated. Further, the optimum light modulation degree mopt is calculated from the equation (5).

Then, the evaluation function calculator 23b temporarily holds each value of the total transmission line capacity Ct and the value of the optimum optical modulation degree mopt for the modulation multi-value number 2n. Then, each value of the total transmission line capacity Ct for the modulation multilevel number 2n is transmitted to the optimum modulation multilevel number determination unit 25a, and the optimum optical modulation degree mop is determined.
t is transmitted to the optical signal transmitter 26.

[S5] The RF demodulator 22 multi-value QAM demodulates the photocurrent RF signal to extract the baseband signals q1a (t), q2a (t), ... It is transmitted to the processing unit 24.

[S6] The optimum modulation multilevel number determination section 25a
A correspondence table between the modulation multilevel number 2n and the total transmission line capacity Ct is created, and the modulation multilevel number (optimum modulation multilevel number) corresponding to the maximum total transmission line capacity Ct _max in the total transmission line capacity Ct is selected. .

[S7] The RF modulator 25b transmits the baseband transmission data q1 (t), q2 (t), ... From the baseband signal processor 24 to each subscriber terminal.
Subcarrier frequencies f ₁ , f ₂ , f ₃ , and qNc (t)
, F _Nc are allotted, and digital multi-level QAM modulation by the optimum modulation multi-level number is applied to these to generate an RF modulation signal r.
1 (t), r2 (t), r3 (t), ..., rNc (t)
Is generated and output.

[S8] The optical signal transmitter 26 adds the DC bias current Ib to the current sum ν (t) = Σ [ri (t) · cos2πf _i ] of the RF modulation signal to adjust the optical modulation degree. At this time, mopt = | ν | / (Ib−
Ith) (Ith is the threshold current value of LD),
After adjusting Ib, Ib + ν (t) is output as the drive current of the E / O converter in the optical signal transmitter 26.

Then, the E / O converter in the optical signal transmitter 26 is driven by the current Ib + ν (t), and the optimum optical modulation degree mo is obtained.
The RF optical modulation signal maintained at pt is output. This optical signal will be transmitted in the order of optical fiber transmission line → base station 3 → wireless transmission line → wireless terminal 4.

Here, based on the LD input / output characteristic curve,
The a _{3 shown} in the equation (4a) will be described. 5 is L
It is a figure which shows a D input-output characteristic curve. The vertical axis represents the output light power of the LD, and the horizontal axis represents the drive current I of the LD.

The output optical power P indicates the power of the RF modulated optical signal, and the LD operates by the modulation drive current signal centered on the current I ₀ to generate the RF modulated optical signal. The output light power of the LD is ideally a linear output in the operation region above the LD threshold current I _th ,
A non-linear output is actually generated due to the non-uniformity of the electric field distribution of the LD characteristic of the LD and the non-linear phenomenon caused by the interaction between the injected carriers and the photons. At this time, when the modulation frequency component of the output light power is approximated by power series expansion for each order, the formula (7) is obtained.

[0068]

[Equation 7] P = P ₀ (1 + a ₁ · ν + a ₂ · ν ² + a ₃ · ν ³ + ...) (7) In this equation, the quadratic or higher order (the term of a ₂ · ν ² or higher) are all non-linear components. Become. Further, each coefficient a ₁ , a ₂ , a ₃ , ... Represents the intensity (amplitude) of each modulation component (to be exact, P ₀ respectively).
The multiplied value is the actual strength). A ₃ of formula (4a) is,
This corresponds to a ₃ in the formula (7).

Next, the transmission apparatus of the second embodiment will be described. FIG. 6 is a diagram showing the configuration of the transmission device according to the second embodiment. In the transmission device 20-2, the intermodulation distortion detection unit 23a does not detect IM3 from the photocurrent RF signal, but an electric signal obtained by converting the RF modulation optical signal generated by the optical signal transmission unit 26 into electricity again. E1 is received and IM3 is detected from this electric signal E1 (IM3 generated by a non-linear circuit such as an LD inside the optical signal transmitter 26 is detected). Note that the other configurations and operations are the same as those in the first embodiment, and therefore description thereof will be omitted.

Next, a transmission device according to the third embodiment will be described. FIG. 7 is a diagram showing the configuration of the transmission device according to the third embodiment. In the transmission device 20-3, the intermodulation distortion detection unit 23a does not detect IM3 from the photocurrent RF signal, but receives the RF modulation signal generated by the RF modulation unit 25b (reference numeral is E2 in the figure). Then, IM3 is detected from this signal E2 (RF modulator 2
IM3 caused by a non-linear circuit inside 5b is detected). Note that the other configurations and operations are the same as those in the first embodiment, and therefore description thereof will be omitted.

Next, a transmission device according to the fourth embodiment will be described. FIG. 8 is a diagram showing the configuration of the transmission device according to the fourth embodiment. In the transmission device 20-4, a radio modulation multi-level number extraction unit 23c is provided in the RF reception control unit 23 as a new constituent element with respect to the first embodiment.

The radio modulation multi-value number extraction unit 23c calculates and extracts the radio modulation multi-value number which is the modulation multi-value number only in the radio transmission path section from the photocurrent RF signal. The multi-level number is the optimum modulation multi-level number determining unit 25a.
Sent to.

On the other hand, in the optimum modulation multi-value number determining section 25a,
It has a correspondence table of the modulation multi-value number and the total transmission path capacity Ct calculated by each modulation multi-value number, and the whole transmission path corresponding to the current value of the wireless modulation multi-value number on this correspondence table. When the capacity is Ctr and the preset specified deterioration amount is Δ, and Ct _max −Ctr> Δ, the modulation multi-value number corresponding to the _maximum total transmission line capacity Ct _max is selected from the table. Then, let this be the optimum modulation multi-valued number, and
If t _max −Ctr ≦ Δ, the current total transmission line capacity C
The modulation multi-value number corresponding to tr is selected from the table and is set as the optimum modulation multi-value number.

The reason why such control is performed is that even if the difference between Ct _max and Ctr is small, if the modulation method is changed frequently, the control method between the control station 2 and the wireless terminal 4 is established until the modulation method is established. This is because it takes time and the line communication quality cannot be maintained. In order to maintain quality, it is necessary to reduce the overhead time required for control. Therefore, in the fourth embodiment, when the deterioration amount with respect to the transmission line capacity exceeds the specified value, the modulation method is switched to reduce the overhead time.

Next, the wireless terminal 4 will be described. FIG. 9 is a diagram showing the configuration of the wireless terminal 4. The wireless terminal 4 includes a wireless reception unit 41, a wireless transmission unit 42, and a terminal side baseband signal processing unit 43. The wireless reception unit 41 includes a terminal-side RF demodulation unit 41a and an optimum modulation multi-level number detection unit 41b, and the wireless transmission unit 42 includes a terminal-side RF modulation unit 42a.

[S11] The RF modulated optical signal transmitted from the control station 2 becomes a radio signal at the base station 3 and is received through an antenna (not shown) of the radio terminal 4. The terminal-side RF demodulation unit 41a of the wireless terminal 4 demodulates the received signal to generate a baseband signal, and transmits this to the terminal-side baseband signal processing unit 43.

[S12] Optimal modulation multi-value number detection section 41b
Detects the optimum modulation multi-level number set by the transmission device 20 in the control station 2. [S13] The terminal side baseband signal processing section 43 processes the baseband signal.

[S14] The terminal-side RF modulation section 42a modulates the baseband signal processed by the terminal-side baseband signal processing section 43 with the optimum modulation multilevel number detected by the optimum modulation multilevel number detection section 41b. Then, an RF modulation signal (terminal side RF modulation signal) is generated. And the terminal side RF modulation signal is
It is wirelessly transmitted to the base station 3 through the antenna. Also,
This radio signal is converted into an optical signal by the base station 3 and transmitted to the control station 2.

Next, the transmission device 20-1 of the first embodiment
A specific configuration example and operation regarding the above will be described. FIG. 10 is a diagram illustrating a configuration example of the transmission device 20-1 according to the first embodiment.

The PIN photodiode 21a corresponds to the optical signal receiving section 21, and the RF quadrature demodulation circuit 22a includes the RF signal.
It corresponds to the demodulation unit 22. The intermodulation distortion detector 23a
FT (Fast Fourier Transform) arithmetic processing circuit 23a-
1. IM3 component calculation processing circuit 23a-2 is included. The evaluation function calculation unit 23b includes a transmission line capacity calculation processing circuit 23b-
1. Optimal light modulation degree calculation processing circuit 23b-2, parameter storage memory 23b-3, registers 23b-1a and 23b
-2a is included.

Further, the optimum modulation multi-level number determination unit 25a includes a correspondence table storage memory 25a-1 and a selector 25a-2.
The multilevel QAM modulation circuit 25b-1 corresponds to the RF modulation unit 25b. Further, the optical signal transmission unit 26 includes a bias current adjustment circuit 26a, a bias application unit 26b, and an E /
It includes an LD 26c that performs O conversion.

Here, the PIN photodiode 21a
Performs O / E conversion of the received optical signal. The FFT operation processing circuit 23a-1 performs a fast Fourier transform operation on the received electric signal to calculate a frequency spectrum component. The IM3 component calculation processing circuit 23a-2 calculates the power spectrum value D of IM3 regarding the frequency fm at the center of the entire band.

The transmission line capacity calculation processing circuit 23b-1 calculates the total transmission line capacity Ct for the QAM modulation multi-valued number based on the power spectrum value D. The optimum light modulation degree calculation processing circuit 23b-2 calculates the optimum light modulation degree mopt based on the power spectrum value D. Parameter storage memory 23
b-3 stores various parameters required for the calculation passed from the baseband signal processing unit 24, and the transmission line capacity calculation processing circuit 23b-1 and the optimum optical modulation degree calculation processing circuit 23b.
Output to -2.

The register 23b-1a holds the calculation result of the transmission line capacity calculation processing circuit 23b-1, and the register 23b-1a
b-2a holds the calculation result of the optimum light modulation degree calculation processing circuit 23b-2.

The RF quadrature demodulation circuit 22a uses 2n-valued QA.
The received electrical signal that has been M-modulated is orthogonally demodulated to generate a baseband signal and transmitted to the baseband signal processing unit 24.

The correspondence table storage memory 25a-1 receives the total transmission path capacity Ct held in the register 23b-1a, and stores a correspondence table between the modulation multi-value number and the total transmission path capacity Ct.

The selector 25a-2 is in the correspondence table.
, The maximum total transmission line capacity Ct _maxPick out that
The optimum modulation multilevel number corresponding to this is set to the multilevel QAM modulation circuit 25.
Output to b-1. The multi-level QAM modulation circuit 25b-1 is
Transmission data transmitted from the baseband signal processing unit 24
To generate an RF modulation signal by modulating the
It

The bias current adjusting circuit 26a adjusts the bias current value based on the optimum optical modulation factor mopt so that this mopt can be maintained. The bias applying unit 26b is
A bias current is applied to the RF modulation signal to generate an LD drive signal. LD26c is RF based on LD drive signal.
Generate and transmit a light modulated signal.

Next, the transmission device 20-2 of the second embodiment
A specific configuration example and operation regarding the above will be described. FIG. 11 is a diagram illustrating a configuration example of the transmission device 20-2 according to the second embodiment.

In the transmission device 20-2, a monitor photodiode (monitor PD) 26d is newly installed inside the optical signal transmitter 26 of the transmission device 20-1 described above with reference to FIG. The output signal E1 of the monitor PD 26d is the FFT.
After inputting to the arithmetic processing circuit 23a-1, IM3 is detected. Other configurations and operations are the same as those in the first embodiment, and therefore description thereof will be omitted.

Next, the transmission device 20-3 of the third embodiment
A specific configuration example and operation regarding the above will be described. FIG. 12 is a diagram illustrating a configuration example of the transmission device 20-3 according to the third embodiment.

The transmission device 20-3 is different from the transmission device 20-1 described with reference to FIG. 10 in that it has a multi-level QAM modulation circuit 25b-1.
The output signal E2 of the above is input to the FFT operation processing circuit 23a-1. After that, the IM3 is detected. Other configurations and operations are the same as those in the first embodiment, and therefore description thereof will be omitted.

Next, the transmission device 20-4 of the fourth embodiment
A specific configuration example and operation regarding the above will be described. 13 and 14 are diagrams showing a configuration example of the transmission device 20-4 according to the fourth embodiment.

The transmission device 20-4 is different from the transmission device 20-1 described above with reference to FIG. 10 in that a radio modulation multi-level number extraction unit 23c is newly provided inside the RF reception control unit 23, and the optimum modulation multi-level number is further provided. A deterioration amount determination unit 25a-4 and a threshold value storage memory 25a-3 are newly provided inside the determination unit 25a. Other components are the same as those of the transmission device 20-1.

Here, the PIN photodiode 21a
Performs O / E conversion of the received optical signal. The FFT operation processing circuit 23a-1 performs a fast Fourier transform operation on the received optical signal to calculate a frequency spectrum component. The IM3 component calculation processing circuit 23a-2 calculates the power spectrum value D of IM3 regarding the frequency fm at the center of the entire band.

The radio modulation multi-value number extraction unit 23c selects a modulation multi-value number when only the radio transmission path section is taken into consideration from the propagation characteristics of the radio transmission path section (technique of the radio modulation multi-value number extraction section 23c. For example, JP-A-7-25011
The technique described in Japanese Patent No. 6 can be applied).

The transmission line capacity calculation processing circuit 23b-1
Based on the power spectrum value D of M3, the total transmission line capacity Ct for the QAM modulation multi-valued number is calculated. The optimum light modulation degree calculation processing circuit 23b-2 calculates the optimum light modulation degree mopt based on the power spectrum value D of IM3.

The parameter storage memory 23b-3 stores various parameters necessary for the calculation passed from the baseband signal processing section 24, and the transmission line capacity calculation processing circuit 23b-
1 and the optimum light modulation degree calculation processing circuit 23b-2. The register 23b-1a holds the calculation result of the transmission line capacity calculation processing circuit 23b-1, and the register 23b-2a
Holds the calculation result of the optimum light modulation degree calculation processing circuit 23b-2.

The RF quadrature demodulation circuit 22a uses a 2n-valued QA.
The M-modulated received electrical signal is quadrature-demodulated to generate a baseband signal and transmitted to the baseband signal processing section 24. The correspondence table storage memory 25a-1 is a register 2
The total transmission line capacity Ct held in 3b-1a is received, and the correspondence table between the modulation multi-value number and the total transmission line capacity Ct is stored.

The threshold value storage memory 25a-3 stores the specified deterioration amount Δ of the deterioration of the transmission line capacity. Deterioration amount determination unit 25
a-4 uses the correspondence table to compare the difference (Ct _max -Ctr) in the total transmission line capacity with the specified deterioration amount Δ and make a determination. The selector 25a-2 receives the comparison determination result,
The optimum modulation multilevel number is determined and the multilevel QAM modulation circuit 25b is determined.
Output to -1.

The multilevel QAM modulation circuit 25b-1 modulates the transmission data transmitted from the baseband signal processing section 24 by the optimum modulation multilevel number to generate an RF modulation signal. The bias current adjusting circuit 26a adjusts the bias current value based on the optimum optical modulation factor mopt so that this mopt can be maintained. The bias applying unit 26b applies a bias current to the RF modulation signal to generate an LD drive signal. LD
26c generates and transmits an RF light modulation signal based on the LD drive signal.

As described above, according to the present invention, R
By flexibly responding to the change in the IM3 generation amount in the OF system, the optimum modulation multilevel number is determined, and the total transmission path capacity between the control station 2 and the wireless terminal 4 is maximized to achieve high quality communication. It will be possible to do.

In addition, in contrast to the embodiment shown above,
A transmission device can be configured by arbitrarily combining the respective embodiments. For example, the second embodiment in which the IM3 is detected from the electric signal E1 from the monitor PD 26d and the fourth embodiment in which the difference value of the total transmission line capacity is compared with the specified deterioration amount to determine the optimum modulation multilevel number. You may comprise a transmission apparatus combining the form of this and.

(Supplementary Note 1) In a transmission system for transmitting information by radio using an optical fiber, an optical signal transmitted through an optical fiber transmission line is received, and is optically / electrically converted,
An optical signal receiver that generates a photocurrent RF signal, an RF demodulator that demodulates the photocurrent RF signal and frequency-converts it into a baseband band, and generates a baseband signal. Calculating the spectrum, from the spectrum of the frequency located in the center of the entire band, the intermodulation distortion detection unit that detects the distortion component information of the intermodulation distortion, and calculate the evaluation function based on the distortion component information, and the optimum optical modulation degree. An RF reception control unit including an evaluation function calculation unit that calculates the total transmission line capacity including the optical fiber transmission line and the wireless transmission line for each modulation multi-level number, and processes the baseband signal. , A baseband signal processing unit that generates transmission data and outputs a parameter value necessary for calculating an evaluation function, and an optimum modulation multi-value number that takes the maximum value of the total transmission line capacity. And an RF transmission control unit configured to determine an optimum modulation multilevel number determining unit and an RF modulation unit that performs RF modulation on the transmission data by using the optimum modulation multilevel number to generate an RF modulation signal. , Controlling the electrical / optical conversion of the RF modulated signal by using the optimum optical modulation degree,
An optical signal transmission unit that generates and transmits a modulated optical signal, a transmission device that includes the transmission device, and a wireless reception unit that receives a wireless signal,
A wireless terminal configured to modulate a baseband signal with a modulation multi-level number determined by the transmission device to generate a terminal-side RF modulation signal and wirelessly transmit the wireless terminal, the wireless terminal and the wireless transmission. And a relay device which is connected to the transmission device by an optical fiber transmission line and performs relay transmission between the wireless terminal and the transmission device.

(Supplementary Note 2) The intermodulation distortion detection unit remains as the distortion component information of the intermodulation distortion at the frequency at the center of the entire band with respect to the entire band assigned to the subscriber.
The transmission system according to appendix 1, wherein the distortion component information of the third-order intermodulation distortion formed by three tones is detected.

(Supplementary Note 3) The intermodulation distortion detector detects the distortion component of the intermodulation distortion from an electric signal obtained by converting the RF modulated optical signal into electric instead of the photocurrent RF signal or the RF modulated signal. The transmission system according to appendix 1, wherein the transmission system detects information.

(Supplementary Note 4) A radio modulation multi-value number extraction unit for calculating and extracting a radio modulation multi-value number which is a modulation multi-value number only in a radio transmission path section from the photocurrent RF signal is further provided. The optimum modulation multilevel number determination unit creates a table of the calculated total transmission line capacity and the modulation multilevel number, and sets the maximum total transmission line capacity to Ct _max , and the wireless modulation multilevel number on the table. When the current total transmission line capacity corresponding to the value is Ctr and the specified deterioration amount is Δ, Ct _max −Ct
In the case of r> Δ, the modulation multi-value number corresponding to the total transmission line capacity Ct _max is set as the optimum modulation multi-value number, and Ct _max −Ctr ≦ Δ
In the case of 1, the modulation multi-value number corresponding to the current total transmission line capacity Ctr is set as the optimum modulation multi-value number.
The described transmission system.

(Supplementary Note 5) An optical signal for receiving a light signal transmitted through an optical fiber transmission line and optically / electrically converting the light signal to generate a photocurrent RF signal in a transmission device for transmitting information by radio through an optical fiber. A receiving unit, an RF demodulation unit that demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal, and calculates a spectrum for each frequency from the photocurrent RF signal to center the entire band. From the spectrum of the frequency to be located, the intermodulation distortion detection unit that detects the distortion component information of the intermodulation distortion, and the evaluation function is calculated based on the distortion component information, the optimum optical modulation degree, for each modulation multi-value number, the optical An RF reception control unit including an evaluation function calculation unit that calculates a total transmission line capacity including a fiber transmission line and a wireless transmission line, and processes the baseband signal. A baseband signal processing unit that generates transmission data and outputs a parameter value necessary for calculating an evaluation function, and an optimum modulation multilevel number that determines the optimum modulation multilevel number that takes the maximum value of the entire transmission line capacity. An RF transmission control unit including a determination unit and an RF modulation unit that performs RF modulation on the transmission data to generate an RF modulation signal by using the optimum modulation multilevel number, and the optimum optical modulation degree. To control the electrical / optical conversion of the RF modulated signal to
An optical signal transmission unit that generates and transmits a modulated optical signal, and a transmission device.

(Supplementary Note 6) The intermodulation distortion detection unit remains as the distortion component information of the intermodulation distortion at the frequency at the center of the entire band with respect to the entire band assigned to the subscriber.
6. The transmission device according to appendix 5, wherein distortion component information of third-order intermodulation distortion formed of three tones is detected.

(Supplementary Note 7) The intermodulation distortion detector detects the distortion component of the intermodulation distortion from the electric signal obtained by converting the RF modulated optical signal into electric instead of the photocurrent RF signal or the RF modulated signal. 6. The transmission device according to appendix 5, which detects information.

(Supplementary Note 8) A radio modulation multi-value number extraction unit for calculating and extracting a radio modulation multi-value number which is a modulation multi-value number only in a radio transmission path section from the photocurrent RF signal, The optimum modulation multi-level number determination unit creates a table of the calculated total transmission line capacity and the modulation multi-level number, and sets the maximum total transmission line capacity to Ct _max , and sets the radio modulation multi-level number on the table. When the current total transmission line capacity corresponding to the value is Ctr and the specified deterioration amount is Δ, Ct _max −Ct
In the case of r> Δ, the modulation multi-value number corresponding to the total transmission line capacity Ct _max is set as the optimum modulation multi-value number, and Ct _max −Ctr ≦ Δ
In this case, the modulation multilevel number corresponding to the current total transmission line capacity Ctr is set as the optimum modulation multilevel number.
The described transmission device.

(Supplementary Note 9) In a wireless terminal for wireless communication, a wireless receiving section for receiving a wireless signal and a baseband signal are modulated by a modulation multi-level number determined by a transmission device installed on the station side. And a wireless transmission unit that wirelessly transmits a side RF modulated signal.

(Supplementary Note 10) In an RF receiver for controlling signal reception, an optical signal transmitted through an optical fiber transmission line is received, converted into an electrical signal, and converted into a photocurrent R.
An optical signal receiving unit that generates an F signal and an intermodulation distortion detection that calculates a spectrum for each frequency from the photocurrent RF signal and detects the distortion component information of the intermodulation distortion from the spectrum of the frequency located in the center of the entire band. Section, and an evaluation function calculation for calculating an evaluation function based on the distortion component information, and calculating an optimum optical modulation degree and an overall transmission line capacity including an optical fiber transmission line and a wireless transmission line for each modulation multi-value number. An RF receiving apparatus, comprising:

(Supplementary Note 11) In the RF transmitter for signal transmission control, the optimum modulation multi-value number which has the maximum value with respect to the total transmission line capacity including the optical fiber transmission line and the wireless transmission line, which is obtained at the time of calculating the evaluation function, is calculated. Using the optimum modulation multi-value number determining unit for determining and the optimum modulation multi-value number, R
An RF modulator that performs F modulation to generate an RF modulated signal;
Using the optimum optical modulation degree obtained at the time of calculating the evaluation function, R
An RF transmitting apparatus, comprising: an optical signal transmitting unit that controls electrical / optical conversion of an F modulated signal to generate and transmit an RF modulated optical signal.

[0115]

As described above, in the transmission system of the present invention, in the transmission device, from the distortion component information of the intermodulation distortion,
The optimum degree of optical modulation and the total transmission line capacity including the optical fiber transmission line and the wireless transmission line are calculated to determine the optimum modulation multi-value number that maximizes the total transmission line capacity. Then, the RF modulation signal is generated using the optimum modulation multi-valued number, the electrical / optical conversion of the RF modulation signal is controlled using the optimum optical modulation degree, and R
An F-modulated optical signal is generated and transmitted. Also, in wireless terminals,
The baseband signal is modulated by the multilevel modulation number determined by the transmission device to generate the terminal side RF modulation signal. As a result, the optimum modulation multi-level number is determined according to the dynamic change of the intermodulation distortion, and the total transmission line capacity between the control station and the wireless terminal can be maximized, so that high quality communication can be performed. It will be possible.

[Brief description of drawings]

FIG. 1 is a principle diagram of a transmission system of the present invention.

FIG. 2 is a diagram showing an image of IM3 generation with respect to subcarriers.

FIG. 3 is a diagram showing a relationship between a transmission line capacity and a modulation multilevel number.

FIG. 4 is a diagram for explaining the operation of the transmission device according to the first embodiment.

FIG. 5 is a diagram showing an LD input / output characteristic curve.

FIG. 6 is a diagram showing a configuration of a transmission device according to a second embodiment.

FIG. 7 is a diagram showing a configuration of a transmission device according to a third embodiment.

FIG. 8 is a diagram showing a configuration of a transmission device according to a fourth embodiment.

FIG. 9 is a diagram showing a configuration of a wireless terminal.

FIG. 10 is a diagram illustrating a configuration example of a transmission device according to the first embodiment.

FIG. 11 is a diagram illustrating a configuration example of a transmission device according to a second embodiment.

FIG. 12 is a diagram illustrating a configuration example of a transmission device according to a third embodiment.

FIG. 13 is a diagram illustrating a configuration example of a transmission device according to a fourth embodiment.

FIG. 14 is a diagram illustrating a configuration example of a transmission device according to a fourth embodiment.

FIG. 15 is a diagram showing an overall configuration of an ROF system.

[Explanation of symbols]

1 Transmission system 2 control station 3 base stations 4 wireless terminals 5 network 20 Transmission device 21 Optical signal receiver 22 RF demodulator 23 RF reception controller 23a Intermodulation distortion detector 23b Evaluation function calculator 24 Baseband signal processor 25 RF transmission controller 25a Optimum modulation multi-value number determination unit 25b RF modulator 26 Optical signal transmitter 30 relay device 41 Wireless receiver 42 Wireless transmitter Ct Total transmission line capacity mopt Optimal light modulation

Claims (5)

[Claims] 1. A transmission system for transmitting information wirelessly by an optical fiber, receiving an optical signal transmitted through an optical fiber transmission line,
An optical signal receiving unit that performs optical / electrical conversion to generate a photocurrent RF signal; an RF demodulation unit that demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal; An intermodulation distortion detection unit that calculates a spectrum for each frequency from the current RF signal and detects distortion component information of intermodulation distortion from the spectrum of frequencies located in the center of the entire band, and an evaluation function is calculated based on the distortion component information. Then, R is composed of an optimum optical modulation degree and an evaluation function calculation unit for calculating the total transmission line capacity including the optical fiber transmission line and the wireless transmission line for each modulation multi-valued number.
An F reception control unit, a baseband signal processing unit that processes the baseband signal, generates transmission data, and outputs a parameter value necessary for calculating an evaluation function, and an optimum modulation that takes the maximum value of the total transmission line capacity. An optimum modulation multi-value number determining unit that determines a multi-value number, and R that performs RF modulation on the transmission data by using the optimum modulation multi-value number to generate an RF modulation signal
An RF transmission control unit including an F modulation unit, and an optical signal transmission that controls the electrical / optical conversion of the RF modulation signal by using the optimum optical modulation degree to generate and transmit the RF modulation optical signal. And a radio receiving unit for receiving a radio signal, and a baseband signal is modulated by a modulation multi-level number determined by the transmission device to generate a terminal side RF modulated signal, A wireless terminal including a wireless transmitting unit for transmitting, a wireless transmission path connected to the wireless terminal, an optical fiber transmission path connected to the transmission apparatus, and relay transmission between the wireless terminal and the transmission apparatus. And a relay device for performing the transmission. 2. The intermodulation distortion detection unit is formed of three tones that remain at the frequency at the center of the entire band as the distortion component information of the intermodulation distortion with respect to the entire band assigned to the subscriber. The transmission system according to claim 1, wherein distortion component information of third-order intermodulation distortion is detected. 3. The intermodulation distortion detector detects the distortion component information of intermodulation distortion from an electric signal obtained by converting the RF modulated optical signal into electricity or the RF modulated signal instead of the photocurrent RF signal. The transmission system according to claim 1, wherein the transmission system detects. 4. A radio modulation multi-value number extraction unit for calculating and extracting a radio modulation multi-value number that is a modulation multi-value number only in a radio transmission path section from the photocurrent RF signal, the optimum modulation The multilevel number determination unit creates a table of the calculated total transmission line capacity and the modulation multilevel number, and sets the maximum total transmission line capacity to Ct _max , and sets the value of the wireless modulation multilevel number on the table. When the corresponding current total transmission line capacity is Ctr and the specified deterioration amount is Δ, and Ct _max −Ctr> Δ, the modulation multi-level number corresponding to the total transmission line capacity Ct _max is the optimum modulation multi-level number. When Ct _max −Ctr ≦ Δ,
2. The transmission system according to claim 1, wherein the modulation multilevel number corresponding to the current total transmission line capacity Ctr is set as the optimum modulation multilevel number. 5. A transmission device for transmitting information by radio with an optical fiber, receiving an optical signal transmitted through an optical fiber transmission line,
An optical signal receiving unit that performs optical / electrical conversion to generate a photocurrent RF signal; an RF demodulation unit that demodulates the photocurrent RF signal and frequency-converts it into a baseband band to generate a baseband signal; An intermodulation distortion detection unit that calculates the spectrum for each frequency from the current RF signal and detects the distortion component information of the intermodulation distortion from the spectrum of the frequencies located in the center of the entire band, and the evaluation function is calculated based on the distortion component information. Then, an RF reception control unit including an optimum optical modulation degree and an evaluation function calculation unit that calculates the total transmission line capacity including the optical fiber transmission line and the wireless transmission line for each modulation multi-level number, Processing the baseband signal to generate transmission data,
A baseband signal processing unit that outputs a parameter value required at the time of calculating an evaluation function, an optimum modulation multilevel number determination unit that determines an optimum modulation multilevel number that takes the maximum value of the entire transmission line capacity, and the optimum modulation multilevel And an RF transmission control unit configured to perform RF modulation on the transmission data to generate an RF modulation signal using a number, and an electric power of the RF modulation signal using the optimum optical modulation degree. /
An optical signal transmitter that controls optical conversion to generate and transmit an RF modulated optical signal.