JPH05344062A - Optical communication system - Google Patents

Abstract

PURPOSE:To provide an optical communication system able to receive a tracking signal with high sensitivity without deteriorating a bit error rate of a main signal. CONSTITUTION:A multi-value PPM modulation circuit 1 of a transmission circuit 10 converts a main signal Sm into a multi-value PPM signal Sp, a switch 4 is used to select the signal Sp or an inverted PPM signal Spi from an inverting circuit 2 based on a logic level of a tracking signal Ss and the PPM signal Sp or Spi is subject to E/O conversion and a transmission signal St is obtained. A received tracking signal Ssr is reproduced by a low pass filter 17 from the reception signal Sr from an 0/E 18 in a reception circuit 20. A timing recovery circuit 11 and a switch control circuit 12 are used to restore either of the reception signals Sr, Sri from a delay circuit 16 and an inversion circuit 15 based on the signal Ssr through changeover control of a switch 13 into a noninverting PPM signal Spr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system for spatially propagating a light beam for communication, and more particularly to an optical communication system for multiplex transmission of a tracking signal and a communication signal.

[0002]

2. Description of the Related Art A conventional optical communication system of this type will be described with reference to a block diagram of a transmission circuit for the optical communication system shown in FIG.

In this type of optical communication system, a tracking signal Ss prepared for tracking a light beam for communication is multiplexed with a main signal Sm for communication and converted into an optical signal St,
This optical signal St is sent from the transmission circuit 10A to the space propagation path 6. Since the tracking signal Ss needs to be received with particularly high sensitivity in a receiving circuit (not shown) that receives the optical signal St from the spatial propagation path 6, it is necessary that the tracking signal Ss is not only a low frequency signal but also a constant frequency signal. Is. Therefore, the frequency divider 3
2 is a clock signal Sc used inside the transmission circuit 10A
And the reduced clock signal is the tracking signal Ss.
It is said that. The PPM modulation circuit 31 uses the digital main signal S
Amplification of m by the tracking signal Ss produces a PAM signal Pa. The PAM signal Pa is an electrical / optical conversion circuit (E / O)
It is converted into an optical signal St by 32, and this optical signal St is sent to the space propagation path 6.

On the other hand, in a receiving circuit (not shown), the optical signal St received from the space propagation path 6 is converted into a pulse electric signal by an optical / electrical converting circuit (O / E), and the pulse electric signal is converted into a pulse electric signal. The tracking signal Ss is reproduced by extracting the average value, and the tracking operation is performed using the tracking signal Ss.

[0005]

In this conventional optical communication system, the degree of modulation with respect to the tracking signal cannot be increased so much as to prevent the tracking signal from interfering with the main signal.
It stopped at around 0%. Therefore, when the level of the received optical signal in the receiving circuit is lowered, the signal-to-noise ratio of the tracking signal is significantly deteriorated, and there is a drawback that tracking of the optical beam in long-distance communication is hindered.

[0006]

The optical communication system of the present invention comprises:
A transmission circuit that converts a pulse signal obtained by multiplexing a digital main signal with a tracking signal that is slower than the main signal into an optical signal by an electric-optical conversion circuit and sends the optical signal to a space propagation path, and the reception circuit received from the space propagation path. In an optical communication system including a receiving circuit that reproduces the main signal and the tracking signal from the received signal after converting the optical signal into a received signal by an optical / electrical conversion circuit, the transmitting circuit multivalues the main signal. PP
Multi-valued PPM modulation means for converting into M signal, and the multi-valued PP
M signal and PPM signal inverting means for generating the pulse signal in response to the tracking signal in response to the logical level of the tracking signal, the logical level of the multi-valued PPM signal being inverted or non-inverted. The receiving means extracts the average intensity of the received signal and reproduces the tracking signal, and tracking signal reproducing means, and detects the presence or absence of inversion of the logical level of the received signal from the reproduced tracking signal and the detection. Inverted PPM signal detecting means for generating a control signal according to the result, and in response to the control signal, the inverted logic level in the received signal is returned to the non-inverted logic level to output the multi-valued PP.
PPM signal re-inversion means for reproducing the M signal, and the PPM
And multi-valued PPM demodulation means for reproducing the multi-valued PPM signal from the signal re-inversion means to the main signal.

[0007]

The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. FIG. 1A shows a transmission circuit, and FIG. 1B shows a reception circuit.

In the transmission circuit 10, the four-value PPM modulation circuit 1 (representative of multi-value modulation by four-value modulation) pulses the digital main signal Sm at a rate of one per frame determined by the frame pulse Sf. Of PPM signal Sp. From the main signal Sm to P by the 4-value PPM modulation circuit 1
The conversion (modulation) into the PM signal Sp is, for example, when the main signal Sm in one frame is "00", the PPM signal Sp is "10".
4 for "00", "01" for "0100", "10" for "0010", and "11" for "0001"
Causes position modulation of values. In the four-valued PPM modulation, the mark rate of one frame of the PPM signal Sp is always 1/4. The PPM signal Sp is branched into two,
One of the PPM signals Sp is directly led to one input terminal of the switch 4, and the other is made an inverted PPM signal Spi whose logic level is inverted by the inverting circuit 2 and is led to the other input terminal of the switch 4. The switch 3 switches the input end of the switch 3 according to the logic level of the tracking signal Ss obtained by dividing the clock signal Sc in the transmission circuit 10 by the frequency dividing circuit 3. As a result, the non-inverting PP is output to the output terminal of the switch 4.
A pulse train in which the M signal Sp and the inverted PPM signal Spi are exchanged at a constant cycle is obtained. This pulse train is converted into an optical signal St by an electric / optical conversion circuit (E / O) 5,
This optical signal St is transmitted to the space propagation path 6.

On the other hand, in the receiving circuit 20, first of all,
The electrical conversion circuit (O / E) 18 converts the optical signal St transmitted from the transmission circuit 10 through the space propagation path 6 into a reception signal Sr which is a pulsed electrical signal.

A part of the received signal Sr is passed through a low-pass filter 17 whose pass band is the signal band of the sub-signal Ss, and the low-pass filter 17 detects the average intensity of the received signal Sr and determines the magnitude. The reception tracking signal Ssr is reproduced from. This reception tracking signal S
The average value of sr is, as described above, the mark rate of the non-inverted four-valued PPM signal Sp is 1/4, and thus the mark rate of the inverted PPM signal Spi is 3/4, so that the reception sub-signal S
The amplitude modulation degree of sr is 50%. As described above, in the conventional technique, the modulation degree of the tracking signal, which is allowed to be only about 10% in order to avoid the bit error with respect to the main signal Sm, is increased by about 5 times in this optical communication system. In, the signal-to-noise ratio of the reception tracking signal Ssr can be increased five times.

Another part of the reception signal Sr is supplied to the timing reproduction circuit 11, which reproduces the reception clock signal Scr and the reception frame pulse Sfr by means known in the general PPM communication system. . The switch control circuit 12 responds to the reception tracking signal Ssr from the low-pass filter 17 and the reception frame pulse Sfr from the timing recovery circuit 11, and controls the rising and falling points of the reception tracking signal Ssr to control the control signal Sco. make. This control signal Sco is supplied to one input terminal thereof via the delay circuit 16 and further part of the received signal Sr, and to the other input terminal thereof after passing through the delay circuit 16 and then by the inverting circuit 15 described above. It is used for switching control of the switch 13 to which the inverted reception signal Sri in which the logical level of the reception signal Sr is inverted is supplied. The delay circuit 16 is a circuit for adjusting the timings of the reception signal Sr and the inverted reception signal Sri in the switch 13 and the control signal Sco. That is, this delay circuit 1
In order to prevent the extraction time of the reception tracking signal Ssr from being delayed by the internal delay of the low-pass filter 17 and the relative phase of the reception signal Sr (and the inverted reception signal Sri) and the control signal Sco from being shifted, It is inserted in order to delay the received signal Sr and restore both to the correct time relationship.

The switch 13 receives the control signal Sco at L level.
When the level is set, the reception signal Sr is selected and the control signal Sc
When o is at H level, the inverted reception signal Sri is selected. As a result, the reception PPM signal Spr of the non-inverted logic level is output to the output terminal of the switch 13. This reception P
The PM signal Spr is supplied to the four-level PPM demodulation circuit 14 together with the reception clock signal Scr and the reception frame pulse Sfr from the timing reproduction circuit 11, and the demodulation circuit 14 responds to these signals and the same reception main signal as the main signal Sm. Play Smr.

[0014]

As described above, according to the present invention, the main signal is converted into the multi-level PPM signal, and then the logic level of the multi-level PPM signal is inverted according to the logic level of the tracking signal, whereby the main signal is converted. Since the degree of modulation of the tracking signal in the receiving circuit can be deepened while keeping the crest value of the constant, the tracking signal with a good signal-to-noise ratio can be reproduced in the receiving circuit.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention. Figure 1
1A shows a transmission circuit, and FIG. 1B shows a reception circuit.

FIG. 2 is a block diagram of a conventional transmission circuit for an optical communication system.

[Explanation of symbols]

 1 4-level PPM modulation circuit 2,15 Inversion circuit 3,32 Frequency division circuit 4,13 Switch 5,33 Electric / optical conversion circuit (E / O) 6 Spatial propagation path 10,10A Transmission circuit 11 Timing recovery circuit 12 Switch control Circuit 14 4-level PPM demodulation circuit 16 Delay circuit 17 Low-pass filter 18 Optical-electrical conversion circuit (O / E) 20 Receiver circuit 31 Amplitude modulation circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshisada Koyama 5-7-1, Shiba, Minato-ku, Tokyo NEC Corporation (72) Inventor Hiroshi Arikawa 2-4-1-1, Hamamatsucho, Minato-ku, Tokyo (72) Inventor Shigeharu Nakamori 2-4-1 Hamamatsucho, Minato-ku, Tokyo Space Development Agency

Claims (2)

[Claims] 1. A transmission circuit for converting a pulse signal obtained by multiplexing a digital main signal with a tracking signal slower than the main signal into an optical signal by an electro-optical conversion circuit and transmitting the optical signal to a space propagation path, and the space propagation. In an optical communication system including a receiving circuit that reproduces the main signal and the tracking signal from the received signal after converting the optical signal received from the optical path into a received signal by an optical-electrical conversion circuit, the transmitting circuit, Multi-valued PPM modulation means for converting the main signal into a multi-valued PPM signal, and inversion of the logic level of the multi-valued PPM signal according to the logic level of the tracking signal in response to the multi-valued PPM signal and the tracking signal. Or a PPM signal inverting means for generating the pulse signal that remains non-inverted, wherein the receiving means extracts the average intensity of the received signal to reproduce the tracking signal. Signal reproduction means, inverted PPM signal detection means for detecting the presence or absence of inversion of the logical level of the received signal from the reproduced tracking signal, and generating a control signal according to the detection result, and in response to the control signal. PPM signal re-inversion means for reproducing the multi-valued PPM signal by returning the inverted logic level in the received signal to the non-inverted logic level, and reproducing the multi-valued PPM signal from the PPM signal re-inversion means for the main signal. An optical communication system, comprising: 2. A transmission circuit for converting a pulse signal obtained by multiplexing a digital main signal with a tracking signal slower than the main signal into an optical signal by an electro-optical conversion circuit and transmitting the optical signal to a space propagation path, and the space propagation. In an optical communication system including a receiving circuit that reproduces the main signal and the tracking signal from the received signal after converting the optical signal received from a path into a received signal by an optical-electrical conversion circuit, the transmitting circuit is A multi-level PPM modulating means for converting the main signal into a multi-level PPM signal for each frame in response to a frame signal generated from the main signal and the circuit clock signal, and in response to the multi-level PPM signal and the tracking signal. PPM signal inverting means for generating the pulse signal in which the logic level of the multi-level PPM signal is inverted or left non-inverted according to the logic level of the tracking signal. Including a tracking signal reproducing means for reproducing the tracking signal by extracting the average intensity of the received signal, a timing reproducing means for reproducing the frame signal and the circuit clock signal from the received signal, An inverted PPM signal detecting means for detecting whether or not the logical level of the received signal is inverted in response to the tracked signal, the frame signal and the clock signal, and generating the control signal according to the detection result; In response, the inverted logic level in the received signal is returned to the non-inverted logic level and the multivalued PP
PPM signal re-inversion means for reproducing the M signal, and the PPM
A multi-valued PPM demodulation means for reproducing the multi-valued PPM signal from the signal re-inversion means to the main signal, the pulse communication system.