JP2006186409A - Optical transmission system, optical transmitter and receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system exhibiting excellent maintainability by simplifying the arrangement, and to provide its optical transmitter and receiver. <P>SOLUTION: In the optical transmission system for transmitting an optical signal subjected to intensity modulation with a radio frequency signal bidirectionally, the optical signal in up-link is pulsated intermittently when it is required to transfer an alarm from a slave station device 20 to a mater station device 10. More specifically, when a fault is detected on the transmission side, emission/nonemission of the optical signal is repeated intermittently. On the reception side, instantaneous value and integrated value of light reception level of the optical signal are monitored and generation of an alarm is detected by the edge of a pulse waveform shown by the light reception level. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical transmission system that transmits an optical signal modulated with an analog signal such as a radio signal via an optical fiber, and an optical transmission device and an optical reception device that are used in the optical transmission system. In particular, the present invention relates to an optical transmission system in which an optical signal is intensity-modulated and transmitted by a radio frequency signal used in a radio section of a mobile communication system such as IMT-2000.

  In recent years, communication systems that transmit analog signals as analog have been reviewed. In particular, an optical analog transmission system using an optical fiber has attracted attention, and is applied as a relay system in a CATV (Cable Television) network or a mobile communication network. In particular, a system applied to a mobile phone network is called a ROF (Radio Over Fiber) system.

  Application of ROF systems to mobile communication networks based on various standards such as a CDMA (Code Division Multiple Access) system typified by IMT-2000 or a TDMA (Time Division Multiple Access) system has been studied. The ROF system realizes long-distance transmission by transmitting an optical signal modulated by an analog signal in a radio band (for example, 2 GHz band).

  In this type of system, a device that exchanges analog signals with a base station is referred to as a master station device. A device installed in a remote dead zone or the like via an optical fiber and sending / receiving an analog signal to / from the master station device is called a slave station device. The transmission direction from the master station device to the slave station device is referred to as downlink, and the reverse is referred to as uplink.

  By the way, in order to improve the maintainability of the system operation, it is necessary to notify the opposite device when a failure occurs. That is, when the slave station apparatus detects a downlink transmission path failure or its own fault, the slave station apparatus notifies the master station apparatus to that effect on the uplink. In the opposite case, the master station apparatus notifies the slave station apparatus of the occurrence of a failure as needed on the downlink. Various methods have been proposed to realize such processing.

  An optical communication system is disclosed in which a pilot signal is wavelength-multiplexed with a transmission signal, and the state of the transmission path can be detected by taking a ratio of the main signal and the pilot signal on the receiving side (see, for example, Patent Document 1). ). By applying this kind of technology, it is possible to notify between devices that a failure has been detected. However, in order to realize this method, a device such as an optical filter related to multiplexing and separation of pilot signals is required, so that the apparatus configuration becomes complicated and the cost increases. In Patent Document 1, the pilot signal is referred to as ASE (Amplified Spontaneous Emission) noise light.

  In addition, by combining and demultiplexing the monitoring optical signal in the optical multiplexer / demultiplexer that multiplexes the pumping light to the doped fiber, it is possible to transfer the monitoring information without causing output power reduction in the optical fiber amplifier. Is also disclosed (see, for example, Patent Document 2). However, even in the technique described in this document, since the monitoring information is transferred by an optical signal having a wavelength different from that of the main signal, it cannot be denied that the configuration of the relay device is particularly enlarged.

In addition, there is a system that notifies the downstream device of the occurrence of a failure by disconnecting the transmission signal when a failure occurs, but this method can distinguish between an upstream device failure and a transmission path failure. Can not.
JP 2002-280962 A JP-A-11-317706

As described above, the existing optical transmission system requires a device such as an optical filter in order to notify the occurrence of a failure. Therefore, there is a problem that the apparatus configuration becomes complicated and the cost increases. In addition, there is a problem that it is difficult to distinguish between a device failure and a transmission path failure, and it is difficult to maintain.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical transmission system that has a simple configuration and excellent maintainability, and an optical transmission device and an optical reception device thereof.

  In order to achieve the above object, according to one aspect of the present invention, an optical transmitter that transmits an optical signal that is analog-modulated based on a waveform of a radio frequency signal used for radio section communication of a mobile communication system to an optical fiber; In the optical transmission system including the optical receiver that receives the optical signal via the optical fiber, the optical transmitter generates the optical signal by performing electrical / optical conversion on the radio frequency signal. A light emitting element for sending a signal to the optical fiber; and an intermittent control means for intermittently sending an optical signal sent to the optical fiber when an alarm to be notified to the optical receiving device occurs. Detecting means for detecting a light receiving level of an optical signal arriving through the optical fiber, and detecting for detecting occurrence of the alarm in the optical transmission device when the light receiving level is intermittent. The optical transmission system characterized in that it comprises a means.

  By taking such means, when an alarm (device failure or the like) to be notified to the optical receiver occurs, the analog optical signal is intermittently turned on / off. Thereby, in an optical receiver, two states can be distinguished. That is, when the optical signal is intermittent, on / off of the optical signal is detected by, for example, a latch that detects a pulse edge. If the optical signal off-period (non-light-emitting period) is made sufficiently shorter than the on-period (light-emitting period), the received light power of the optical signal becomes constant regardless of on / off. Can be prevented. That is, it is possible to notify only the alarm in the optical transmission device to the optical reception device.

  Moreover, since it is sufficient to turn on / off the optical signal by, for example, intermittently driving the driving current of the light emitting element in a pulse shape, a complicated circuit is not required. In addition, a pilot signal for notifying an alarm is not required. As a result, it is possible to clearly distinguish between a device failure and a transmission path failure by simplifying the configuration, and it is possible to improve maintenance operability.

  According to the present invention, it is possible to provide an optical transmission system having a simple configuration and excellent maintainability, and an optical transmission device and an optical reception device thereof.

  FIG. 1 is a system diagram showing an embodiment of an optical transmission system according to the present invention. The system of FIG. 1 is provided to assist a mobile communication system such as a cellular phone network, and expands the service area developed by the base station 40 using the optical fiber 3. This type of system is known as a ROF (Radio Over Fiber) system. In this type of optical analog transmission system, an optical fiber is rented from a network vendor (system provider or the like) to a user (communication company or the like) at a specified fee. The user connects the branch line fiber to the optical fiber and constructs a unique network. The optical fibers are connected to each other by fusion bonding or joining by optical parts such as connectors.

  In FIG. 1, a base station 40 is connected to the master station device 10 via a coaxial cable C. The master station device 10 acquires part of the radio frequency signal transmitted from the base station 40 via the coaxial cable C. The master station device 10 is connected to the slave station device 20 via the optical fiber 3. An optical repeater 50 may be provided between the master station device 10 and the slave station device 20. The radio frequency signal has a band used for radio section communication of the mobile communication system, and is modulated based on a scheme such as CDM (Code Division Multiplex) which is one of spread spectrum modulation schemes. The waveform pattern changes irregularly with time.

  The base station 40 and the mobile stations T1, T2 belong to a mobile communication system based on, for example, IMT-2000. The base station 40 is installed on the rooftop of a building (building 100) with a good view, for example, and develops a wireless zone. The mobile station T1 located in the radio zone is connected to the base station 40 via a radio channel of a carrier band assigned to this system.

  If a high-rise building (building 200) is constructed near the building 100, a radio frequency signal from the base station 40 does not reach immediately below the building 100, and a dead zone may be formed. Therefore, the dead zone can be eliminated by providing the slave station device 20 and opening an information communication path between the base station 40 and the slave station device 20 through the optical fiber 3. In addition, the slave station device 20 is installed in a building or the like, and is used to eliminate the dead zone.

FIG. 2 is a functional block diagram showing an embodiment of the master station device 10 and the slave station device 20 of FIG. In FIG. 2, the master station device 10 includes a transmitter 10A and an optical receiver 10B. Similarly, the slave station device 20 includes a reception unit 20A and an optical transmission device 20B.
A radio frequency signal (RF IN) introduced from the input terminal 11 to the master station device 10 is subjected to electrical / optical conversion in an LD (Laser Diode) module 13 of the transmission unit 10A, and thereby subjected to intensity modulation (Amplitude Modulation: AM). An analog optical signal is generated. This optical signal is transmitted to the slave station device 20 via the optical fiber 3 in the downlink, and is incident on the receiving unit 20A. This optical signal is optically / electrically converted by a PD (Photo Diode) module 24 to reproduce a radio frequency signal. This radio frequency signal is output from the output terminal 22.

  Uplink transmission is realized by the optical transmitter 20B and the optical receiver 10B. Similar to the downlink, also in the uplink, the radio frequency signal introduced from the input terminal 21 to the slave station device 20 is subjected to electrical / optical conversion in the LD module 23 to generate an optical signal. This optical signal is transmitted to the master station apparatus 10 via the optical fiber 3 via the uplink, and is optically / electrically converted by the PD module 14 to reproduce a radio frequency signal. This radio frequency signal is output from the output terminal 12.

  Incidentally, the optical transmission device 20B includes an LD driving unit (LD SD) 25 and a multivibrator 26. The LD driving unit 25 applies a driving current to the light emitting element 23 a of the LD module 23 to drive the light emission. The drive current is a current obtained by adding a constant bias current to the radio frequency signal. The multivibrator 26 pulsates and generates a pulse waveform when an alarm signal is given. This pulse waveform is given to the LD drive unit 25.

  When the pulse waveform is given, the LD driving unit 25 intermittently turns on / off the bias current of the light emitting element 23a based on the pulse waveform. As a result, the light emitting element 23a is turned on / off and outputs an optical signal intermittently. The alarm signal is generated when an alarm to be transmitted from the slave station device 20 to the master station device 10 (device failure of the slave station device 20 or downlink transmission path failure detected in the slave station device 20) occurs. Generated by a failure monitor (not shown) provided in the slave station device 20.

  On the other hand, the optical receiver 10 </ b> B includes an alarm detection circuit 15. The alarm detection circuit 15 detects two different alarms based on the state of the light reception level of the optical signal that arrives via the optical fiber 3 via the uplink. That is, the alarm detection circuit 15 detects the occurrence of an alarm in the optical transmission device 20B when the received light level of the incoming optical signal is intermittent. When the time average of the received light level is equal to or less than a predetermined threshold, the disconnection of the uplink optical fiber 3 is detected.

  FIG. 3 is a functional block diagram showing an embodiment of the alarm detection circuit 15 of FIG. In FIG. 3, the uplink optical signal is incident on the light receiving element 14a from the incident end 14b. This optical signal is output from the output terminal 12 in FIG. 2 after optical / electrical conversion, and the received light level is converted into a voltage value by the current-voltage converter (IV) 4 in FIG. This light reception level is branched into two systems, one being input to the comparator 7 and the other being input to the comparator 6 via the low pass filter (LPF) 5.

  The comparator 7 compares the light reception level of the optical signal with a predetermined value, and gives the result to the trigger circuit (TRIG) 8. The trigger circuit 8 is latched by the edge of the pulse waveform indicated by the output of the comparator 7, and outputs a binary signal having different values before and after the latch from the output terminal 30 (output 2). That is, the trigger circuit 8 detects a short-term change point of the light reception level, and holds the output level for a certain period when a change occurs.

  The low-pass filter 5 integrates (averages) the received light level and inputs the time average value of the received light level to the comparator 6. The comparator 6 compares the output of the low-pass filter 5 with a specified threshold value, and outputs a binary signal based on the result from the output terminal 2 (output 1). Of these, output 2 corresponds to an alarm signal. Next, the operation of the above configuration will be described.

  FIG. 4 is a waveform diagram for explaining the operation when an alarm is generated in the slave station device 20 of FIG. When an alarm is generated in FIG. 4, the alarm signal is changed from L (Low) to H (High), whereby the multivibrator 26 is activated and a pulse waveform (OSC output) is generated. This pulse waveform is superimposed on the drive current of the light emitting element 23a, and the optical signal has a pulse shape in which the specific light emission period occurs intermittently. The characteristics of the multivibrator 26 are set appropriately in advance so that the light emission period is sufficiently longer than the specific light emission period.

  When this pulsed optical signal reaches the master station device 10, the alarm detection circuit 15 detects a change point (for example, a falling edge) of the optical signal, and the output 2 changes to the “H” level. Accordingly, the master station device 10 detects that an alarm has occurred in the slave station device 20. That is, an alarm signal is transmitted from the slave station device 20 to the master station device 10. In this state, the state of the output 1 of the alarm detection circuit 15 is maintained at the “L” level (not detected).

  FIG. 5 is a waveform diagram for explaining the operation of the alarm detection circuit 15 shown in FIGS. In FIG. 5, since the light receiving level (PD current) of the light receiving element 14 a is pulsed, a pulsed voltage is output from the current-voltage converter 4. The comparator 7 identifies the state of the pulse voltage based on a predetermined threshold value (one-dot chain line in the figure). When the trigger circuit 8 detects a change point (for example, a falling edge) of the pulse voltage, the trigger circuit 8 holds the output 2 at the “H” level and detects that an alarm is generated in the slave station device 20.

  On the other hand, the pulse voltage is integrated by the low-pass filter 5 and the time average of the pulse voltage is output (LPF output). The LPF output is identified by the comparator 6 based on the threshold value. In FIG. 5, since the LPF output does not cross the threshold value, the state of the output 1 is maintained at the “L” level (no decrease in optical input). Conversely, when the LPF output crosses the threshold value, the light receiving level of the light receiving element 14a is excessively lowered. This is detected by the comparator 6, and the state of the output 1 becomes “H” level, and an alarm indicating that a failure has occurred in the uplink optical fiber 3 is indicated. As described above, the state of the output 1 reflects the normal / disconnected states of the optical fiber 3. The threshold values used in the comparator 6 and the comparator 7 may be different from each other.

  FIG. 6 is a functional block diagram illustrating an example of an existing optical transmission system for comparison. In FIG. 6, the slave station device 20 includes a combiner 27, and the master station device 10 includes a separator 16. The alarm signal is frequency-synthesized with the radio frequency signal in the synthesizer 27, and is then subjected to electrical / optical conversion to reach the master station device 10. The master station device 10 separates the radio frequency signal and the alarm signal by the separator 16. In this way, the warning signal is transferred on the uplink by superimposing the warning signal on the radio frequency signal as a pilot signal. That is, the presence or absence of an alarm is transmitted depending on the presence or absence of a pilot signal. However, such a configuration requires a device such as a filter, which complicates the apparatus configuration.

  FIG. 7 is a functional block diagram showing another example of an existing optical transmission system for comparison. In FIG. 7, when an alarm signal is generated, the LD driving unit 25 is stopped, and the current supply is stopped to forcibly stop driving the light emitting element 23a. Then, there is no optical signal on the uplink, the alarm detection circuit 15 detects that and outputs an alarm signal. However, with such a configuration, it is impossible to distinguish between an abnormality in the optical fiber 3 and an alarm in the slave station device 20.

  On the other hand, in the present embodiment, in the optical transmission system that bidirectionally transmits the optical signal intensity-modulated by the radio frequency signal, when the alarm needs to be transferred from the slave station device 20 to the master station device 10, The uplink optical signal is intermittently pulsed. That is, when a failure is detected on the transmission side, light emission / non-light emission of the optical signal is repeated intermittently. On the receiving side, the instantaneous value and integral value of the light reception level of the optical signal are monitored, and the occurrence of an alarm is detected by the edge of the pulse waveform indicated by the light reception level.

  Since it did in this way, generation | occurrence | production of the alarm in the sub_station | mobile_unit apparatus 20 can be detected by monitoring the instantaneous value of a light reception level. Further, the presence or absence of a failure in the optical fiber 3 can be detected by monitoring the light reception level integrated value. As described above, since it is possible to reliably distinguish and detect a device failure and a transmission path failure, it is possible to improve the system maintainability.

  In addition, since the above-described operation can be realized by a circuit such as the multivibrator 26 and the low-pass filter 5, it is possible to facilitate the configuration and reduce the cost. Further, by making the light emission period sufficiently longer than the non-light emission period, it is possible to prevent the reception performance of the reception apparatus from being adversely affected. Furthermore, since the waveform of the optical signal is analog, the influence on the transmission characteristics due to the interruption of the optical signal can be minimized. Accordingly, it is possible to provide an optical transmission system that has a simple configuration and excellent maintainability, and an optical transmission device and an optical reception device thereof.

  The present invention is not limited to the above embodiment. For example, in the above embodiment, the alarm notification in the uplink has been described, but the present invention can be similarly applied to the alarm notification in the downlink. Alternatively, the light emitting element 23a may emit light continuously and an external modulator may be used to obtain a pulsed optical signal. Further, a plurality of patterns of pulse waveforms may be generated, and alarm information may be encoded and transferred by each pattern. In this way, more detailed information can be transmitted.

  Furthermore, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

1 is a system diagram showing an embodiment of an optical transmission system according to the present invention. The functional block diagram which shows embodiment of the main | base station apparatus 10 and the sub_station | mobile_unit apparatus 20 of FIG. The functional block diagram which shows embodiment of the alarm detection circuit 15 of FIG. The wave form diagram for demonstrating operation | movement when the alarm generate | occur | produces in the sub_station | mobile_unit apparatus 20 of FIG. FIG. 4 is a waveform diagram for explaining the operation of the alarm detection circuit 15 of FIGS. 2 and 3. The functional block diagram which shows an example of the existing optical transmission system for a comparison. The functional block diagram which shows the other example of the existing optical transmission system for a comparison.

Explanation of symbols

  C: Coaxial cable, T1, T2 ... Mobile station, 2 ... Output terminal, 3 ... Optical fiber, 4 ... Current / voltage converter, 5 ... Low pass filter, 6, 7 ... Comparator, 8 ... Trigger circuit, 10 ... Master station device DESCRIPTION OF SYMBOLS 10A ... Transmission part, 10B ... Optical receiver, 11 ... Input terminal, 12 ... Output terminal, 13 ... LD module, 14 ... PD module, 14a ... Light receiving element, 14b ... Incident end, 15 ... Alarm detection circuit, 16 ... Separator, 20 ... Slave station device, 20A ... Receiver, 20B ... Optical transmitter, 21 ... Input terminal, 22 ... Output terminal, 23 ... LD module, 23a ... Light emitting element, 24 ... PD module, 25 ... LD driver , 26 ... Multivibrator, 27 ... Synthesizer, 30 ... Output terminal, 40 ... Base station, 50 ... Optical repeater, 100, 200 ... Building

Claims (11)

An optical transmitter for transmitting an optical signal analog-modulated based on a waveform of a radio frequency signal used for radio section communication of a mobile communication system to an optical fiber, and an optical receiver for receiving the optical signal via the optical fiber In an optical transmission system comprising:
The optical transmitter is
A light emitting element that electrically / optically converts the radio frequency signal to generate the optical signal and sends the optical signal to the optical fiber;
And an intermittent control means for intermittently transmitting an optical signal sent to the optical fiber when an alarm to be notified to the optical receiver occurs.
The optical receiver is
Detecting means for detecting a light receiving level of an optical signal coming through the optical fiber;
An optical transmission system comprising: detection means for detecting occurrence of the alarm in the optical transmission device when the light reception level is intermittent. 2. The optical transmission system according to claim 1, wherein the detection unit detects a failure having a factor different from that of the alarm when a time average of the light reception level is equal to or less than a predetermined threshold value. The intermittent control means includes
An oscillator that generates a pulse waveform;
The optical transmission system according to claim 1, further comprising drive control means for driving the light emitting element on / off based on the pulse waveform. The intermittent control means includes
An oscillator that generates a pulse waveform;
The optical transmission system according to claim 1, further comprising: an external modulator that externally modulates output light of the light emitting element in a pulse shape based on the pulse waveform. The detection means includes
Latch means for outputting a first binary signal which is latched by an edge of a pulse waveform indicated by the light receiving level and has different values before and after the latch;
A low-pass filter that averages the received light level;
The optical transmission system according to claim 1, further comprising a comparator that compares the output of the low-pass filter with a specified threshold value and generates and outputs a second binary signal based on the result. In an optical transmitter for transmitting an optical signal that is analog-modulated based on a waveform of a radio frequency signal used for radio section communication of a mobile communication system, to an optical fiber,
A light emitting element that electrically / optically converts the radio frequency signal to generate the optical signal and sends the optical signal to the optical fiber;
An optical transmission device comprising: an on / off control means for interrupting an optical signal transmitted to the optical fiber when an alarm to be notified to an optical receiving device connected via the optical fiber occurs. . The intermittent control means includes
An oscillator that generates a pulse waveform;
The optical transmission device according to claim 6, further comprising drive control means for driving the light emitting element on / off based on the pulse waveform. The intermittent control means includes
An oscillator that generates a pulse waveform;
The optical transmission device according to claim 6, further comprising: an external modulator that externally modulates output light of the light emitting element in a pulse shape based on the pulse waveform. In an optical receiver that receives an optical signal, which is analog-modulated based on a waveform of a radio frequency signal used for radio section communication of a mobile communication system, via an optical fiber,
Detecting means for detecting a light receiving level of an optical signal coming through the optical fiber;
An optical receiving apparatus comprising: a detection unit configured to detect occurrence of the alarm in an optical transmission apparatus connected via the optical fiber when the light reception level is intermittent. 10. The optical receiver according to claim 9, wherein the detecting unit detects a fault having a factor different from that of the alarm when a time average of the light reception level is equal to or less than a predetermined threshold. The detection means includes
Latch means for outputting a first binary signal which is latched by an edge of a pulse waveform indicated by the light receiving level and has different values before and after the latch;
A low-pass filter that averages the received light level;
The optical receiver according to claim 9, further comprising a comparator that compares the output of the low-pass filter with a predetermined threshold value and generates and outputs a second binary signal based on the result.

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