JPH06141361A - Radio frequency optical transmission method and communication equipment applied with the method - Google Patents

Abstract

PURPOSE:To provide a transmission line design method for the radio frequency optical transmission line. CONSTITUTION:A line hit rate P is prepared in advance and a carrier versus noise ratio <GAMMAO> required for an optical link is calculated by the line hit rate P and the level PRF, of a radio frequency signal is automatically gain- controlled so as to keep the carrier versus noise ratio <GAMMAO>. Thus, the desired quality of the communication is secured. Furthermore, the carrier versus noise ratio <GAMMAO> is estimated smaller than that of a conventional method. Thus, the existing optical transmission technology is applied to the radio frequency optical transmission system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio frequency optical transmission method used in a so-called optical microcell system for connecting a central control station and a radio base station by using an optical fiber in mobile radio communication, and this method. The present invention relates to a communication device to which is applied.

[0002]

2. Description of the Related Art In order to respond to a rapid increase in demand for mobile communication, attention is being paid to a micro cell system in which the zone of the cellular system used in the current mobile communication is further miniaturized. By using this microcell system, the number of frequency repetitions can be increased, so that the frequency utilization efficiency is improved and the subscriber accommodation capacity is enhanced. In addition, the transmission power can be reduced, and it is expected that the portable device can be downsized and the power consumption can be reduced.

However, since the number of wireless base stations increases, the capital investment will increase significantly. In addition, it is expected that a mobile device (referred to as a mobile station) will have more opportunities to move between cells and control will be complicated. Therefore, in order to cover the above-mentioned drawbacks, adoption of an optical microcell system has been proposed.

In this system, as shown in FIG. 6, the wireless base station 42 and the mobile station 43 are connected by a wireless link, and the wireless base station 42 and the central control station 45 are connected by an optical fiber link. The radio base station 42 converts a radio frequency signal into an optical signal. The radio frequency signal sent from the mobile station 43 is analog-optically converted by the radio base station 42 without being demodulated to the baseband frequency, and the optical fiber 44 is transmitted.
Through the central control station 45. by this,
The modulation / demodulation device and the channel allocation control device required for the radio base station 42 can be collectively installed in the central control station 45, and the configuration of the radio base station 42 can be simplified and made inexpensive.

Although it is indispensable to design the line, it is essential to examine the transmission characteristics of the radio frequency optical transmission line.
Since it is a line that includes both optical and wireless links, a design method using a new concept that has never existed is necessary. Therefore, there is known a document that derives the line interruption rate considering the noise in the optical link (Park et al., "Study on the influence of optical link SNR on inter-cell diversity of optical microcell system") Technical report, RCS92-38 (1992)).

According to this document, C in a radio link
When NR (carrier-to-noise ratio) is represented by Γ _R and CNR in the optical link is represented by Γ _O , the time rate (the line interruption) in which the received CNR in the downlink from the central control station to the mobile station is below a certain threshold ν _R Rate) P is

[0007]

[Equation 1]

It is represented by (see the equation (10) in the same document).

[0009]

However, according to the above-mentioned document, even if the relationship between the line interruption rate and the CNR can be derived, how to design the line in order to obtain the required CNR. Nothing is mentioned about the guideline of what to do. Also, in another document (Shibuya et al., "Study on Characteristics of Optical Fiber Feeder in Microcell Mobile Communication System", Technical Report, RCS91-31 (1991)), the receiving CNR of the optical link is "demodulation of radio signal". Minimum required CN
Although it is written that it is necessary to satisfy "R" + "dynamic range", it is assumed that "minimum CNR required for demodulating a radio signal" is 15 dB and required "dynamic range" is 60 dB. This value is so large that it is difficult to realize with the current optical transmission technology.

It is generally said that the fluctuations in mobile radio waves include fluctuations in instantaneous values, fluctuations in the median of short intervals, and fluctuations in distance. The fluctuation of the instantaneous value is based on the Rayleigh fading due to the reflected and scattered waves, and the fluctuation speed is fast, and the automatic adjustment of amplification (AGC) cannot follow it. Rather, the fluctuation of the instantaneous value causes the line interruption. It will be. On the other hand, the fluctuation of the short-term median value is a fluctuation due to shadowing (becomes a shadow of a building) due to the movement of the mobile station, and is slower than the fluctuation of the instantaneous value. The distance variation is a variation caused by the movement of the mobile station and a change in the distance between the mobile station and the wireless station, and the variation is slow like the variation of the median of the short section.

Therefore, the inventor of the present application has to admit to some extent that the received CNR is momentarily deteriorated by the fluctuation of the instantaneous value, and the received CNR has a certain threshold ν.
By setting a time rate (line interruption rate) below _R , and using this line interruption rate as an evaluation standard, it is possible to obtain characteristics that can adequately follow slow fluctuations such as fluctuations in the median of short intervals and distance fluctuations. A radio frequency optical transmission method and a communication device to which the method is applied are proposed.

[0012]

A radio frequency optical transmission method according to claim 1, wherein a line interruption rate P, which is a time rate at which a carrier-to-noise ratio falls below a threshold value ν _R , is given in advance, and the line interruption is performed. The required carrier-to-noise ratio <Γ _O > of the optical link is calculated from the rate P and the carrier-to-noise ratio <Γ _R > of the wireless link to realize the required carrier-to-noise ratio <Γ _O > of the optical link. This is a method of calculating a necessary radio frequency signal level P _RF for optical conversion and automatically adjusting the amplification degree of the radio frequency signal so as to maintain the radio frequency signal level P _RF as a lower limit value.

A communication apparatus according to a second aspect has a mobile station, a radio base station for performing radio communication with the mobile station, and a central control station for performing optical communication with the radio base station, The radio base station is a radio base station antenna that transmits and receives radio waves to and from a mobile station, an amplifier that amplifies a radio frequency signal received by the radio base station antenna, and converts the amplified radio frequency signal into an optical signal. The amplifier has an automatic gain control means for automatically controlling the gain so as to maintain the lower limit of the input level P _RF of the optical conversion element determined by the line interruption rate P. is there.

[0014]

When the average CNR in the wireless link is <Γ _R >, and the average CNR in the optical link is <Γ _O >, the time when the received CNR in the downlink from the central control station to the mobile station is below a certain threshold ν _R Rate (line interruption rate) P _down
Is

[0015]

[Equation 2]

It is represented by In addition, the line interruption rate P _up in the uplink from the mobile station to the central control station is

[0017]

[Equation 3]

It is represented by In deriving Eqs. (2) and (3), it is assumed that the fluctuation of the radio field strength follows the Rayleigh distribution. Here, using the equations (2) and (3), the required average CNR <Γ _O > that the optical link must achieve to obtain a certain line interruption rate P _O can be obtained as follows.

[0019]

[Equation 4]

<Γ _RT > in the equation (4) is

[0021]

[Equation 5]

The average CNR required for obtaining the line interruption rate P _O of the model radio link that does not consider the noise generated in the optical link.
If equations (4) and (5) are used, the line interruption rate P _O and the threshold ν _R
Given, the required CNR <Γ _O > of the optical link can be calculated. For example, the line interruption rate P _O is 1
If 0 ^-3 and the threshold value ν _R are 10 dB, from equation (5), <
Γ _RT > is about 40 dB, and using the equation (4), the CNR <Γ _R > of the wireless link and the required CNR <Γ _O > of the optical link are obtained.
I understand the relationship.

FIG. 2 shows the required average CN of the optical link with respect to the average CNR <Γ _R > of the wireless link based on the equation (4).
6 is a graph depicting R <Γ _O >. From the graph, if the CNR <Γ _R > of the wireless link is an infinite and “transparent” transmission line, in the case of the downlink, the required average CNR of the optical link
<Γ _O > becomes equal to the threshold level ν _R = 10 dB, and in the case of the uplink, <Γ _RT > = 40 dB when only the noise of the wireless link is considered without considering the noise of the optical link.
It turns out to be the same as. Also, it can be seen that the required average CNR of the optical link in the uplink may be designed to be larger by <Γ _RT > / ν _R dB than in the case of the downlink. Further, when <Γ _R > → <Γ _RT >, the required average CNR of the optical link diverges to infinity in both the downlink and the uplink, which can be naturally derived from the definition of <Γ _RT >.

Since the CNR <Γ _R > of the wireless link is a value given from the beginning by the wireless link design, the required average CNR <Γ _O > of the optical link can be obtained by using (4). Next, the received CNR in optical transmission is expressed by the following equation.

[0025]

[Equation 6]

In this equation, the denominator first, second, third,
The fourth term represents intensity noise of the light source, shot noise, receiver thermal noise, and intermodulation noise, respectively. The n in the numerator is the degree of light modulation, which is the modulation amplitude of the radio frequency signal divided by the average light intensity. I is the average output current of the photodetector, RIN is the noise intensity of the laser diode, e is the charge of the electron, k is the Boltzmann constant, T is the noise temperature, R _L is the load impedance of the receiving circuit, σ _IM is the intermodulation noise, and B Is the transmission bandwidth.

On the other hand, assuming that the threshold current of the laser diode is I _th , the bias current is I _b , and the input impedance of the laser diode is R _i , the relationship between the input level P _RF per carrier wave and the optical modulation factor m. Is

[0028]

[Equation 7]

(See FIG. 3. The degree of optical modulation m is a /
b)). Therefore, the required average C of the optical link
If NR <Γ _O > can be obtained, the lower limit of the optical modulation degree m can be found by applying the equation (6), and the input level of the laser diode can be derived using this optical modulation degree m. it can. Therefore, by automatically adjusting the amplification factor of the radio frequency signal in order to maintain this level P _RF as the lower limit value, it is possible to realize a radio frequency optical transmission method that does not impair the communication quality and a communication device to which this method is applied. You can

[0030]

Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments. FIG. 1 is a block diagram of a radio base station for carrying out radio frequency optical transmission of the present invention, FIG. 4 is a block diagram of a mobile station, and FIG. 5 is a block diagram of a central control station.

First, referring to FIG. 1, the radio base station 1 includes a radio base station antenna 2 for transmitting and receiving microwaves to and from a mobile station, a circulator (or duplexer) 3, a band filter 4, and an automatic gain. Amplifier 5 with adjustment function, electric / optical conversion element (laser diode) 6 for supplying optical signal to optical fiber 50, optical / electrical conversion element (photodiode) 7 for photoelectric conversion of optical signal from optical fiber 51, power An amplifier 8 and a bandpass filter 9 are provided.

Further, according to the block configuration of the mobile station shown in FIG. 4, the mobile station 10 includes a signal processing section 11, a modulator 12, and a frequency conversion section 1 for converting a base band into a microwave band.
3, power amplifier 14, bandpass filter 15, circulator (or duplexer) 16, mobile station antenna 17,
Bandpass filter 18, low noise amplifier 19, frequency converter 2
0 and a demodulator 21.

As shown in FIG. 5, the central control station 30 includes an optical / electrical conversion element (photodiode) 31 for converting an optical signal of the optical fiber 50 into an electric signal, a bandpass filter 32,
A low noise amplifier 33, a frequency converter 34 for converting a microwave band to a base band, a demodulator 35 provided for each channel, a signal processor 36, a modulator 37 for modulating with data for each channel, and a base band are provided. A frequency converter 38 for converting into a microwave band, an amplifier 39 with an automatic gain adjustment function, a bandpass filter 40, and an electric / optical conversion element (laser diode) 41 for inputting an optical signal to an optical fiber 51 are provided.

An uplink data transmission process in the above configuration will be described. In the mobile station 10, when the data to be transmitted is output from the signal processing unit 11, the data is modulated by the modulator 12 according to a predetermined modulation method, converted into the microwave frequency by the frequency conversion unit 13, and then the power amplifier. The signal is transmitted from the mobile station antenna 17 through the band filter 15 and the band filter 15.

When the radio base station antenna 2 of the radio base station 1 receives the radio wave, its radio frequency signal passes through the circulator 3 and the band filter 4 and is input to the amplifier 5 with the automatic gain adjustment function. Here, as a result of automatic gain control, a radio frequency signal kept at a level within a predetermined range is input to the laser diode 6. The method of automatic gain control will be described later.

When the optical signal sent from the radio base station 1 reaches the central control station 30 through the optical fiber 50, it is converted into an electric signal by the photodiode 31, passes through the band filter 32 and the low noise amplifier 33, It goes to the frequency converter 34 and is converted into a base band. Then, demodulation is performed for each channel, and data processing is performed by the signal processing unit 36.

In the downlink, the data transmitted from the signal processing unit 36 of the central control station 30 is modulated by the modulator 37 and converted into the microwave band by the frequency conversion unit 38. Then, it is amplified by the amplifier 39 and is automatically controlled in gain, and is converted into an optical signal by the laser diode 41 through the bandpass filter 40. The optical signal that has passed through the optical fiber 51 is converted into a radio frequency signal by the photodiode 7 in the radio base station 1, passes through the power amplifier 8, the bandpass filter 9 and the circulator 3 without being demodulated, and passes through the radio base station antenna. 2 is transmitted.

In the mobile station 10, when the radio wave transmitted by the radio base station antenna 2 is received, it passes through the band filter 18 and the low noise amplifier 19 and is converted into a base band in the frequency conversion section 20 to be demodulated. The signal is demodulated by 21 and processed by the signal processing unit 11. An example of line design using specific numerical values in the above station configuration will be described. Determination of Required CNR of Optical Link Since it can be considered that the characteristic evaluation method of the wireless link has been established in the past, the CNR <Γ _R > of the wireless link is 50.
Assume that it is given in dB. Threshold value ν _R is 10d
B, if the line interruption rate P is determined to be 10 ^-3 , <Γ _RT > becomes approximately 40 dB from the equation (5), and the required average CNR <Γ _O > of the optical link is obtained using the equation (4). To be That is, the required average CNR <Γ _O > of the uplink is 40.46.
dB, downlink required average CNR <Γ _O > is 10.4
It becomes 6 dB. This required average CNR <Γ _O > is the evaluation standard of the system. Determination of Lower Limit of Modulation Degree m The parameters of the optical link are given as follows.

Noise temperature T = 290 K, load impedance R _{L of} photodiodes 7, 31 =
50Ω, transmission bandwidth of one channel B = 300 kHz, relative noise intensity of laser diodes 6 and 41 RIN = −14
5 dB / Hz optical fiber transmission line loss L = 6.0 dB, laser diode 6, 41 current / optical conversion efficiency M = 0.
04W / A, light-to-current conversion efficiency r of the photodiodes 7 and 31 = 0.
8 A / W, drive current I _b −I _th = 30 for laser diodes 6 and 41
mA, laser diode 6, 41 input impedance R _i =
50Ω. From the above values, the average output current I of the photodiodes 7 and 31 can be calculated as I = (I _b −I _th ) Mr / L = 0.24 mA.

By substituting these values into the equation (6), the lower limit of the optical modulation factor m for realizing the CNR <Γ _O > is 8.16 × 10 ⁻³ for downlink and downlink. Is 2.58 × 10 ^-4 . Furthermore, the lower limit of the input level P _RF per carrier wave input to the laser diode can be obtained using the equation (7). As a result, in the uplink, P _RF = −28.2 dBm, and in the downlink, P _RF = −58.2 dBm.

In this way, the optical modulation factor m and the lower limit of the input level P _RF of the laser diode can be obtained. At this level of input level, the effect of intermodulation can be ignored. Determination of Upper Limit of Modulation Degree m The upper limit of the modulation depth m is determined by the intermodulation distortion.

In general, intermodulation distortion is difficult to handle analytically, so a simple model will be considered in this embodiment. That is, it is assumed that the carriers (carrier waves) input to the laser diode are three waves, the carriers are arranged at equal intervals on the frequency axis, and the input levels of the carriers are all the same. In this case, the ratio between the carrier power C and the intermodulation distortion power IMD is C / IMD = 4 / 9A ₃ ² m ⁴ (8) for the carrier at the center frequency and C / IMD = 16 for the carriers at both ends. / 9A ₃ ² m ⁴ (9). However, A ₃ is a third-order nonlinear coefficient of the laser diode, which is a numerical value known by actual measurement. For example, assume that A ₃ = 0.1.

As can be seen from these equations (8) and (9), C
/ IMD is inversely proportional to the fourth power of the modulation factor m. Since the modulation factor m is proportional to the 1/2 power of the input level P _RF per carrier wave (see the equation (7)), C / IMD is proportional to the square of the carrier input level. That is, the intermodulation distortion characteristic deteriorates as the input level P _RF increases. Therefore, in order to maintain communication quality, there is an upper limit on the input level P _RF , and the modulation factor m
It turns out that there is an upper limit.

Since C / IMD becomes large especially in the carrier arranged in the central frequency, it is the central carrier that is easily affected by the intermodulation distortion. Therefore, it is preferable to replace the C / IMD of the center carrier with the required CNR <Γ _O > and set the modulation factor m. Therefore, the required average CNR <Γ _O > 40.46 dB for the uplink and the required average CNR <Γ _O > 10.46 dB for the downlink, which are determined in “Determination of required CNR for optical link”, are respectively expressed by the equation (8). When the modulation factor m is obtained by substituting the values, 0.
25, 1 or more in the downlink. In general, if the modulation factor m exceeds 1, overmodulation occurs, so it is necessary to set the upper limit of the modulation factor to 1 or less.

When the upper limit of the input level P _RF is calculated with m = 0.25 in the uplink and m = 0.3 in the downlink, P _RF = 1.53 dBm in the uplink and P _RF = 3 in the downlink. It becomes 0.06 dBm. Determining System Margin According to the above numerical example, the input level P _RF is set to fall within the range of −28.2 dBm to +1.53 dBm in the uplink, and P _RF is set to −58.2 dBm in the downlink.
To +3.98 dBm have been found to be necessary.

Therefore, the system margin is as follows: 1.53 dBm-(-28.2 dBm) = 29.7 dB in the uplink and 3.06 dBm-(-58.2 dBm) = 61.3 dB in the downlink. Become. The margin of the system is smaller in the uplink, but this means that the reception level of the radio signal at the radio base station fluctuates significantly due to fading, movement of the mobile station, presence of obstacles, etc. This is due to the stricter conditions required for links.

Due to the line design, in the uplink, the gain of the amplifier 5 is changed from −28.2 dBm per carrier wave to the laser diode 6 to +1.53.
It is necessary to automatically adjust the gain so that it falls within the range of dBm, and in the downlink, the gain of the amplifier 39 is adjusted so that the input signal level per carrier wave to the laser diode 41 is −.
It is necessary to automatically adjust to fall within the range from 58.2 dBm to +3.98 dBm.

The tracking speed of the AGC of the amplifier is set so that it can sufficiently follow the fluctuation of the short-range median value and the fluctuation of the distance. This condition is, for example, AGC voltage generation circuit, P
It can be realized by combining it with a variable attenuation circuit composed of IN diodes. Then, the gain of the AGC is adjusted so that the output of the amplifier falls within the range obtained in the above "determination of system margin". As a result, even if the mobile station moves through the service zone, it is possible to keep the line interruption rate based on the fluctuation of the instantaneous value below the set value, and to secure the desired communication quality.

The present invention is not limited to the above embodiment. Although microwaves are used as the radio waves in the embodiments, radio waves of other frequencies can be used. Further, although the laser diode is used as the light conversion element, other light conversion elements such as an LED (light emitting diode) can be used in addition to this.
In addition to the optical microcell system, the present invention is a system (FTTA that transmits a signal to a wireless base station in a certain area through an optical fiber and wirelessly transmits the signal to the user (FTTA).
(Fiber To The Area)) ”and“ a system for connecting each wireless LAN system in a building to each other by an optical fiber ”, and can be applied to any system combining wireless communication and optical communication. In addition, various modifications can be made without changing the gist of the invention.

[0050]

As described above, according to the present invention, the line interruption rate P is given in advance, and the required carrier-to-noise ratio <Γ _O > of the optical link is calculated based on the line interruption rate P and the like. Since the level P _RF of the radio frequency signal capable of realizing this carrier-to-noise ratio <Γ _O > is maintained by the automatic gain control, the desired communication quality can be secured. Further, the carrier-to-noise ratio <Γ _O > can be estimated to be smaller than the conventional one, so that the current optical transmission technology can be applied to the radio frequency optical transmission system.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a radio base station for carrying out radio frequency optical transmission of the present invention,

FIG. 2 is a graph depicting the required average CNR <Γ _O > of an optical link against the average CNR <Γ _R > of a wireless link.

FIG. 3 is a graph showing an input current-output light intensity characteristic of a laser diode.

FIG. 4 is a block diagram of a mobile station.

FIG. 5 is a block diagram of a central control station.

FIG. 6 is a schematic diagram of an optical microcell system.

[Explanation of symbols]

 1 Radio Base Station 2 Radio Base Station Antenna 5 Amplifier 6 Optical Conversion Element 10 Mobile Station 30 Central Control Station

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Shozo Komaki 3-5-16-118 Moroguchi, Tsurumi-ku, Osaka City, Osaka Prefecture (72) Katsutoshi Tsukamoto 4-17-20-201, Himuro Town, Takatsuki City, Osaka Prefecture

Claims (2)

[Claims] 1. A radio frequency optical transmission method for amplifying a radio frequency signal obtained by receiving a radio wave and converting the amplified radio frequency signal into an optical signal as it is for optical transmission, wherein a carrier-to-noise ratio is provided. Is given in advance a line interruption rate P that is a time rate below the threshold ν _R, and the line interruption rate P and the carrier-to-noise ratio of the wireless link <
Γ _R > and the required carrier-to-noise ratio of the optical link <Γ _O
Is calculated to calculate the radio frequency signal level P _RF for optical conversion necessary to realize the required carrier-to-noise ratio <Γ _O > of the optical link, and the radio frequency signal level P _{RF is set} to the lower limit. A radio frequency optical transmission method, wherein the amplification factor of a radio frequency signal is automatically adjusted so as to maintain the value. 2. A communication device to which the radio frequency optical transmission method according to claim 1 is applied, wherein the communication device is a mobile station, a radio base station for performing radio communication with the mobile station, and a radio base station. And a central control station for performing optical communication with the wireless base station, wherein the wireless base station amplifies a radio frequency signal received by the radio base station antenna for transmitting and receiving radio waves to and from the mobile station. And an optical conversion element for converting the amplified radio frequency signal into an optical signal, wherein the amplifier has a gain so as to maintain the lower limit of the input level P _RF of the optical conversion element determined by the line interruption rate P. A communication device having automatic gain control means for automatically controlling