JPH06350537A - Radio signal optical transmitting method and communication equipment using the same - Google Patents

Abstract

PURPOSE:To prevent transmission quality from being degraded by decreasing mutual modulation distortion by converting a received radio frequency signal into an IF signal and converting that IF signal into an optical signal. CONSTITUTION:A microwave from a mobile station is received through an antenna 11 and a circulator 12 by a base station 1, and that radio signal is sent through a BPF 13, amplifier 14 with AGC, mixer 15, BPF 16, amplifier 17, synthesizer 18, amplifier 19 and laser diode to an optical fiber 10. The radio signal through a PIN photodiode 31, amplifier 32 and branch filter 33 is supplied through an amplifier 34, BPF 35, mixer 36, BPF 37 and amplifier 38 to a signal processing circuit by a central control station 2 for receiving the optical signal from the fiber 10. On the other hand, a local oscillated signal through the branch filter 33 is inputted through an LPF 46, amplifier 45 and frequency divider 44 to a phase comparator 41. The output of a VCO 39 is inputted through a frequency divider 42 to the comparator 41, and the comparator outputs a voltage corresponding to phase difference between both the inputs and sends it to the VCO 39.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio signal optical transmission method used for mobile radio communication and a communication device to which the method is applied.

[0002]

2. Description of the Related Art At present, as a mobile communication system, a cellular system is used in which a plurality of base stations are dispersedly installed in a service area. In this cellular system, the area where the radio waves of the base station reach is called a zone or cell, and the communication between the portable device (called a mobile station) in any cell and the central control station is done via the base station. Do it.

In the current cellular system, there are areas where a sufficient received electric field cannot be obtained, such as gaps between cells, underground malls, and tunnels, and it is impossible to provide a satisfactory service to users in these areas. Therefore, a new small base station (hereinafter referred to as "business trip base station") is installed to cover such a blind area, and communication can be performed between the business trip base station and neighboring mobile stations. Is being considered. Here, it is necessary to install a large number of business trip base stations in correspondence with blind areas where there are many, and it is important to reduce the cost per station.

On the other hand, as a drastic measure, a microcell system in which a cellular cell used in the current mobile communication is further miniaturized is drawing attention as a next-generation mobile communication system. By using this micro cell system, the size of the cell can be reduced to eliminate blind areas, and the number of times the frequency is used geographically can be increased, so that frequency utilization efficiency is improved and subscriber capacity can be increased. You can 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.

On the other hand, since the number of base stations increases, it is the key to practical use to suppress the cost per station and to reduce the required capital investment. Therefore, as a method for realizing these inexpensive business trip base stations and microcell base stations,
The adoption of a wireless signal optical transmission system has been proposed. FIG. 2 shows a business trip base station 62 and a base station 61 that cover a blind area of radio waves.
Fig. 3 shows an example in which a wireless signal optical transmission system is adopted between
Shows an example in which a wireless signal optical transmission system is adopted between the central control station 63 and a plurality of base stations 64a and 64b.

According to this radio signal optical transmission system, the communication between the base station and the business trip base station in FIG. 2 and the communication between the base station and the central control station in FIG. Alternatively, it is performed by an optical signal obtained by analog AM-modulating a radio signal performed between the mobile station and the business trip base station. By adopting such a configuration, the base station and the business trip base station can omit the equipment for modulation / demodulation and channel control, and can simply modulate the signal from the mobile station as it is into an optical signal and relay it. Since the structure is improved, downsizing and cost reduction can be achieved. Further, since the channel allocation information of each base station is concentrated in the central control station, the control in the central control station, which is expected to be complicated, becomes easier.

[0007]

By the way, one of the problems of the radio signal optical transmission system is the problem of intermodulation distortion. As described above, the base station must convert a radio frequency signal containing multiple carriers into an optical signal.
Intermodulation distortion occurs due to the nonlinearity of the input / output characteristics of the laser diode, which is a light conversion element, and the transmission quality deteriorates.

In order to reduce the effect of such intermodulation distortion, the mobile station monitors the level of the pilot signal constantly output from the base station, and the mobile station monitors the level of the reception signal so that the reception level at the base station becomes constant. Controlling transmission power (APC: Automati
c Power Control), the base station corrects the level so that the level becomes uniform for each received carrier (ALC: Aut
omatic level control), a microwave band distortion generator having the inverse characteristic of the nonlinearity of the laser diode is inserted (predistorter), and the signals multiplexed in the microwave stage are frequency-converted and then optically converted (collectively). FM method) has been proposed.

Although these are effective methods, respectively,
With this method, a pilot signal receiving circuit must be incorporated in each mobile station, which leads to an increase in the size and complexity of the mobile station. With this method, it is difficult to design an AGC (automatic gain control) amplifier that can follow the level fluctuation of the received carrier. The method (1) has a problem that it is difficult to make the inverse characteristic of the nonlinearity of the laser diode, and the method (2) causes a new problem of nonlinear distortion of the FM modulator.

Therefore, the present inventor has paid attention to the fact that the degree of nonlinearity of the laser diode varies depending on the frequency of the signal. It has been reported that the ratio (dB) of the third-order intermodulation distortion generated when two signals are input to the laser diode and the input signal level is improved when the input signal frequency is lower than when the input signal frequency is high. Yes (Yamane et al .; The Institute of Electronics, Information and Communication Engineers Spring Meeting B-99, March 30, 1993)
Five).

An object of the present invention is to pay attention to the characteristics of such a laser diode and to convert the frequency of a radio signal in a radio signal optical transmission system into an intermediate frequency signal.
It is an object of the present invention to provide a wireless signal optical transmission method and a communication device that reduce intermodulation distortion caused by nonlinearity of input / output characteristics of a laser diode and do not cause deterioration of transmission quality.

[0012]

A wireless signal optical transmission method according to claim 1, wherein a radio frequency signal obtained by receiving a radio wave is frequency-converted into an intermediate frequency signal, and the intermediate frequency signal is converted into an optical signal. It is a method of converting the optical signal into an original radio frequency signal at a station that has received the optical signal.

A communication apparatus according to a second aspect of the present invention comprises a base station that performs wireless communication with a mobile station, and another base station or a central control station that performs optical fiber communication with the base station. The base station includes a reference signal oscillator that oscillates a reference frequency signal for frequency conversion, a frequency conversion device that converts a radio frequency signal into an intermediate frequency signal by the reference frequency signal oscillated by the reference signal oscillator, and A multiplexer that matches a reference frequency signal oscillated by a reference signal oscillator with the intermediate frequency signal, and an optical conversion element that converts an output signal of the multiplexer into an optical signal, and the reference frequency signal and the intermediate frequency signal Optical fiber transmission at the same time.

[0014]

According to the wireless signal optical transmission method of claim 1,
The radio frequency signal obtained by receiving the radio wave is frequency-converted into an intermediate frequency signal, and this intermediate frequency signal is converted into an optical signal. There is an advantage that a frequency band with less distortion of the conversion element can be used.

According to another aspect of the communication device,
Since the base station is equipped with a frequency converter to convert to an intermediate frequency signal, it has the advantage of being able to use a frequency band with a small distortion of the optical conversion element, and also has a reference frequency signal oscillated by a reference signal oscillator. Is optically transmitted to the other station together with the intermediate frequency signal, so that the other station is completely the same without converting the intermediate frequency signal into the original radio frequency without providing a reference signal oscillator. A frequency reference signal is obtained.

[0016]

Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments. In the following embodiments, it is assumed that the wireless signal optical transmission is performed between the base station and the central control station, but the present invention is not limited to this, and the wireless signal optical transmission is performed between the base stations. It should be noted in advance that this may be done.

Further, in the following embodiments, only the uplink for wireless signal optical transmission from the base station to the central control station is described, but the downlink for wireless signal optical transmission from the central control station to the base station is described. A line is also possible with the same configuration. FIG. 1 shows a block configuration diagram of a base station 1 and a central control station 2 for performing radio signal optical transmission from the base station 1 to the central control station 2.

The base station 1 comprises a base station antenna 11 for receiving microwaves from a mobile station and a circulator 12, a band filter 13 for band-passing a radio signal passed through the circulator 12, and an automatic gain adjusting function. The amplifier 14 and the mixer 15 for converting into an intermediate frequency signal are provided. Further, the bandpass filter 16, the amplifier 17, the multiplexer 1
8, an amplifier 19, and an electric / optical conversion element (laser diode) 20.

The mixer 15 includes a reference signal oscillator (crystal oscillator) 21, frequency dividers 22 and 26, and a phase comparator 2.
3, low pass filter 24, voltage controlled oscillator (VCO)
The reference frequency signal is supplied by a phase locked loop of 25. To explain the function of the phase locked loop, the signal oscillated by the crystal oscillator 21 is frequency-divided by the frequency divider 22 and enters the phase comparator 23. On the other hand, the output signal of the VCO 25 is input to the phase comparator 23 through the frequency divider 26, the phase difference between this and the signal frequency-divided by the frequency divider 22 is compared, and the voltage corresponding to the phase difference is phase-compared. It is output from the device 23. Only the direct current component of this voltage is extracted by the low pass filter 24 and input to the VCO 25,
The oscillation frequency of 5 is adjusted.

With the configuration of such a phase locked loop,
The output frequency of the VCO 25 can maintain the same frequency stability as the oscillation frequency of the crystal oscillator 21. On the other hand, the reference frequency signal oscillated by the crystal oscillator 21 is amplified by the amplifier 27, passed through the low pass filter 28, and input to the multiplexer 18. This is done because the reference frequency signal is placed at the time of optical transmission and the other station uses this reference frequency signal so that the stations can be easily synchronized.

The central control station 2 which receives an optical signal includes an optical / electrical conversion element (PIN photodiode) 31, an amplifier 3
2. The branching filter 33 is provided. The radio signal passed through the demultiplexer 33 is amplified by the amplifier 34, the bandpass filter 35, and the mixer 3.
6, the signal is supplied to a signal processing circuit (not shown) through the bandpass filter 37 and the amplifier 38. The contents of the signal processing will be briefly described. That is, the microwave band is converted into a base band, the signals of the respective channels are demodulated, and information processing and the like are performed based on the demodulated signals.

On the other hand, the local oscillation signal that has passed through the demultiplexer 33 is a low pass filter 46, an amplifier 45, and a frequency divider 44.
Through the phase comparator 41. Further, the output signal of the VCO 39 is input to the phase comparator 41 through the frequency divider 42, the phase difference between this and the signal frequency-divided by the frequency divider 44 is compared, and the voltage corresponding to the phase difference is phase-compared. It is output from the device 41. This voltage is low pass filter 40
Only the DC component is extracted by and input to the VCO 39.

With such a configuration of the phase locked loop,
Since the local oscillation frequency of the central control station 2 can be synchronized with the local oscillation frequency of the base station 1, there is no possibility that the two frequencies delicately differ from each other as in the conventional microwave multiplex communication. Base station 1 in the above configuration
Uplink data transmission from the central control station 2 to the central control station 2 will be described with reference to specific frequency values. However, it goes without saying that the present invention is not limited to the following specific frequency values.

In the mobile station, a microwave radio wave (frequency 1477 to 1501 MHz) containing data to be transmitted
) Is transmitted and the base station antenna 11 of the base station 1 receives the radio wave, the radio frequency signal is input to the amplifier 14 with the automatic gain adjustment function through the circulator 12 and the band filter 13. Here, the radio frequency signal kept at a level within a predetermined range as a result of the automatic gain control is converted into an intermediate frequency signal by the mixer 15.

In the phase locked loop, the crystal oscillator 21 oscillates a signal having a frequency of 12.8 MHz, the transmitted signal is frequency-divided by the frequency divider 22 into 128, and the phase comparator 23
Entered in. On the other hand, the oscillation center frequency of the VCO 25 is 1
It is set to 344 MHz, and the oscillation output signal of the VCO 25 is frequency-divided by the frequency divider 26 (128 × 106) and input to the phase comparator 23. The phase comparator 23 compares the phase difference with the signal frequency-divided by the frequency divider 22, inputs a voltage according to the phase difference to the VCO 25, and adjusts the oscillation frequency of the VCO 25 to 1344 MHz.
Therefore, the radio frequency signal having a frequency of 1477 to 1501 MHz is converted by the mixer 15 into an intermediate frequency signal having a frequency of 133 to 157 MHz.

When the optical signal transmitted from the base station 1 reaches the central control station 2 through the optical fiber 10, it is converted into an electric signal by the PIN photodiode 31, and the low noise amplifier 32, the demultiplexer 33, and the amplifier. After passing through the bandpass filter 34 and the bandpass filter 35, the frequency is converted in the mixer 36.
The phase locked loop of the central control station 2 receives the reference frequency signal having a frequency of 12.8 MHz, which is received through the demultiplexer 33,
The frequency is divided by 128 by the frequency divider 44 and supplied to the phase comparator 41, while the oscillation output signal of the VCO 39 is frequency-divided by (128 × 106) by the frequency divider 42 and supplied to the phase comparator 23. As a result, the voltage according to the phase difference is input to the VCO 39, and the oscillation frequency of the VCO 39 is maintained at 1344 MHz. Therefore, the radio frequency signal is converted by the mixer 36 into the high frequency signal having the original frequency of 1477 to 1501 MHz.

As described above, the signal converted to the original frequency in the mixer 36 is processed as usual through the bandpass filter 37 and the amplifier 38. As described above, since the frequency of the signal converted by the laser diode 20 can be set to the intermediate frequency, it is possible to use a portion of the input / output characteristic of the laser diode having good linearity. For this reason, it becomes possible to reduce intermodulation distortion, which has been considered to be large in the past.

The present invention is not limited to the above embodiment. Although the microwave band is used as the radio wave in the embodiment, radio waves in other frequency bands other than this 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 system that transmits a signal through an optical fiber to a base station in a certain area and wirelessly transmits the signal to the user from that base station (FTTA).
(Fiber To The Area)) ”and“ a system for connecting each wireless LAN system in a building with an optical fiber ”, and can be applied to any system that combines wireless communication and optical communication. In addition, various modifications can be made without changing the gist of the invention.

[0029]

As described above, according to the wireless signal optical transmission method of the first aspect, a radio frequency signal obtained by receiving a radio wave is frequency-converted into an intermediate frequency signal, and the intermediate frequency signal is optically converted. Since the signal is converted into a signal, the frequency band in which the distortion of the optical conversion element is smaller can be used as compared with the case where the radio frequency signal is directly converted into the optical signal. Therefore, it is possible to provide a wireless signal optical transmission method in which the intermodulation distortion caused by the nonlinearity of the input / output characteristics of the laser diode is reduced and the transmission quality is not deteriorated.

According to the communication device of the second aspect,
In addition to the effect of the invention according to claim 1, the reference frequency signal oscillated by the reference signal oscillator is optically transmitted to the partner station together with the intermediate frequency signal. Therefore, in the partner station,
Even if the reference signal oscillator is not provided when converting the intermediate frequency signal to the original radio frequency, the reference frequency signal having the completely same frequency can be obtained. Therefore, compared to the case where expensive reference signal oscillators are independently provided in both stations,
An inexpensive and stable system can be constructed.

[Brief description of drawings]

FIG. 1 is a diagram showing a block configuration of a base station and a central control station for performing optical signal optical transmission from the base station to the central control station.

FIG. 2 is a configuration diagram showing an example in which a radio signal optical transmission system is adopted between a business trip base station and a base station which covers a blind area of a radio wave.

FIG. 3 is a configuration diagram showing an example in which a wireless signal optical transmission system is adopted between a central control station and a plurality of base stations.

[Explanation of symbols]

 1 Base Station 2 Central Control Station 10 Optical Fiber 11 Base Station Antenna 15 Mixer 18 Multiplexer 20 Laser Diode 21 Crystal Oscillator

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication location H04Q 7/04 A 7304-5K (72) Inventor Toshio Hokane 1-chome Shimaya, Konohana-ku, Osaka City No. 3 Sumitomo Electric Industries, Ltd. Osaka Works (72) Inventor Shozo Komaki 3-5-16-118 Moroguchi, Tsurumi-ku, Osaka (72) Inventor Katsutoshi Tsukamoto 4-17-20-Himurocho, Takatsuki City, Osaka Prefecture 201 (72) Inventor Norihiko Morinaga 4-6-1-1012 Yamada Nishi, Suita City, Osaka Prefecture

Claims (2)

[Claims] A radio frequency signal obtained by receiving radio waves is frequency-converted into an intermediate frequency signal, the intermediate frequency signal is converted into an optical signal and transmitted through an optical fiber, and the station receives the optical signal. A radio signal optical transmission method comprising converting the optical signal into a radio frequency signal. 2. A communication device using the wireless signal optical transmission method according to claim 1, wherein the communication device is a base station that performs wireless communication with a mobile station, and a base station and the base station. It is configured by another base station or a central control station that performs optical fiber communication, and the base station is a reference signal oscillator that oscillates a reference frequency signal for frequency conversion, and a radio by a reference frequency signal oscillated by the reference signal oscillator. A frequency conversion device for converting a frequency signal into an intermediate frequency signal, a multiplexer for combining the reference frequency signal oscillated by the reference signal oscillator with the intermediate frequency signal, and an output signal of the multiplexer for conversion into an optical signal. A communication device comprising an optical conversion element, and simultaneously transmitting a reference frequency signal and an intermediate frequency signal through an optical fiber.