JP2813774B2 - Optical code division multiplex transmission access system and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multiplex transmission system and apparatus in optical communication, particularly to communication requiring asynchronous multiple access and a pay-broadcast communication system and apparatus in which a receiver is specified.

[0002]

2. Description of the Related Art Conventional optical CDMA is described in the literature DBMo.
It was first proposed in rtimore, Electron. Lett., vol. 21, p. 42 (1985). FIG. 7 is a block diagram of a general system of optical CDMA, 701 is a light source, 702 is an optical modulator, 703 is a data signal generator, 704 is an optical encoder,
705 is an optical multiplexer, 706 is an optical fiber link, 707
Is an optical demultiplexer, 708 is an optical decoder, 709 is a light receiving element, 7
Reference numeral 10 denotes a threshold element. In this method, the light pulse intensity is ON.
It is called so-called incoherent optical CDMA, which expresses (1, 0) of a unipolar code by / OFF.

FIG. 8 shows the configuration of this incoherent optical CDMA system.
Is an optical encoder, 803 is an Mx1 optical multiplexer, 804 is an optical transmission line, 805 is a 1xM optical demultiplexer, 806 is an optical decoder,
807 is a light receiving element, and 808 is a threshold element.

On the transmitting side, a transmission signal from a transmitter (not shown) is converted into an optical signal by an electrical / optical converter 801 and is encoded by a pulse code assigned to each channel through an optical encoder 802. The light is multiplexed by the star coupler 803 and sent to the optical transmission path 804. On the receiving side,
The transmitted signal is distributed to each optical decoder 806 by the star coupler 805, a predetermined correlation is obtained by the optical decoder 806, and a peak value of the resulting autocorrelation is converted into an electric signal by the light receiving element 807. When the threshold value is processed by the threshold value element 808 and the level is equal to or higher than a certain level, "1" is reproduced as an output.

[0005] Another optical CDMA is described in reference ME.
There is a so-called coherent optical CDMA proposed in Marhic, IEEE J. Lightwave Technol. 11, 854 (1993).
This method expresses a bipolar code by changing the phase of an optical pulse. For example, a phase (0,
π) corresponds to the value (-1, 1) of the bipolar code.

A conventionally used optical encoder comprises an optical fiber delay line and a star coupler. In the case of incoherent optical CDMA, the number of optical fibers is equal to the unipolar code "1" of the chip. Only the number is needed.
The difference between the optical fiber lengths is set to an integral multiple of the chip rate Tc. The input optical pulse is distributed to each optical fiber by the star coupler and is delayed by propagating through each optical fiber. Therefore, when the input optical pulse is multiplexed again by the star coupler, it is converted into a predetermined pulse code. Note that the optical decoder has the same configuration as the optical encoder.

FIG. 9 shows an optical fiber ladder type optical encoder and optical decoder used for coherent optical CDMA. Reference numeral 901 denotes an optical fiber, and reference numeral 902 denotes a 3 dB coupler. This is a configuration in which 3 dB fiber couplers are connected in multiple stages, and the difference between the optical fiber lengths in each stage is set equal to the chip rate Tc.

[0008] The greatest difference between incoherent optical CDMA and coherent optical CDMA lies in the autocorrelation function. FIG.
Shows an example of 8 chips. The intensity value of the peak of the autocorrelation is (weight) 4 in the case of incoherent optical CDMA and 8 (the number of chips) in the case of coherent optical CDMA. Here, the weight is defined as the number of chips whose intensity is not zero in one code. From this result, the coherent optical CDMA generally has an advantage that the difference between the peak value and the side lobe value can be made larger, and a larger threshold margin can be obtained in the threshold processing.

[0009]

However, the "0"
In the incoherent optical CDMA using the unipolar code of “1” and the coherent optical CDMA using the bipolar codes of “1” and “−1”, the threshold processing has to be performed at the chip rate. It has been difficult to increase the bit rate due to the limitation of the response speed of electronic devices.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and is "0", "-1" or "1".
By using an optical duo-binary code based on a so-called duo-binary code of 1 ", when a code having the same chip rate as that of the conventional optical CDMA is used,
The frequency band of the code sequence can be compressed more than before. As a result, the required bandwidth of the threshold element can be reduced, and the demand for the response speed of the electronic device can be reduced.

In other words, if electronics having a response speed that can support optical CDMA using a conventional binary code are used, the chip rate can be increased by introducing a duobinary code, thereby increasing the number of users and increasing the number of information bits. Higher rates can be expected.

[0012]

FIG. 2 shows a system configuration according to the present embodiment, in which a transmitter 201, a pulse light source 202, and an optical modulator 2 are provided.
03, data generator 204, optical duobinary encoder 205, optical multiplexer 206, optical fiber transmission line 207, optical demultiplexer 208, receiver 209, optical duobinary decoder 210, light receiving element 211, and threshold element 212 Consists of

An optical pulse train output from the light source 202 is
The intensity is modulated by the optical modulator 203 with the payload data from the data generator 204, an optical signal representing data values “0” and “1” is generated by ON / OFF, and the optical signal is further transmitted to the optical duobinary encoder 205. The optical signal is subjected to duobinary encoding by passing through, and becomes a transmission signal.

After multiplexing transmission signals from a plurality of users and transmitting the multiplexed signals through the optical fiber transmission line 207, the signals transmitted on the receiving side are distributed to all users. Each user passes this through the optical duobinary decoder 210, converts the output into an electric signal by the light receiving element 211, and then performs threshold processing by the threshold element 212 to reproduce desired data.

First, the duobinary signal will be described. FIG. 3 shows an algorithm for generating the above duobinary code. The algorithm for generating the duobinary code is represented by the following equation. D is the original binary code
(K), and b (k) is defined by the following equation.

[0016]

## EQU1 ## b (k) = d (k) ∵b (k-1)

Here, ∵ represents XOR (exclusive OR). The duobinary code c (k) is obtained from the following equation.

[0018]

## EQU2 ## c (k) = b (k) + b (k-1)

FIG. 4 shows a duobinary code generated according to the above algorithm and the original data d (k),
5 shows an example of a signal waveform of b (k).

FIG. 5 shows an example of an optical duobinary pulse code having a code length of 8 and a weight of 4, and its autocorrelation waveform.

The "0" of the duobinary code turns off the light pulse. Autocorrelation spread is 15
(= 2N-1) chips, and the peak amplitude value is 8 for the duobinary signal. Since the threshold processing in decoding detects the presence or absence of the peak value of the autocorrelation function, a threshold element having a band sufficient for the chip rate (pulse width of one chip) Tc is required.

FIG. 6 shows an example of the configuration of an optical duobinary encoder, which comprises an optical tap 601, an optical waveguide 602, an optical phase shifter 603, and an optical multiplexer 604. Light tap 6
The waveguide length between 01 is set to the chip time Tc. The ratio of the optical tap 601 can be varied from 0 (pass) to 1 (total power reduction).

In the coding of "1" and "-1", the input optical pulse is tapped, and the optical phase shifter 6 is used only in the case of "-1".
03 gives a π phase shift. On the other hand, in the case of encoding of “0”, the tap ratio is set to 0 and the tap output is set to 0. The optical duobinary decoder has the same configuration as the optical duobinary encoder.

The present optical encoder / decoder is actually realized by a PLC (Planar Lightwave Circuit) using a quartz waveguide. In this case, a heater is locally loaded on the waveguide in the phase shifter. A method of changing the refractive index by controlling the temperature is employed.

[0025]

[Table 1]

Table 1 summarizes a comparison with the conventional optical CDMA system. The incoherent optical CDMA uses the unipolar code described in the related art, and the coherent optical CDMA is a relatively new system. The value of the bipolar code (1, -1) is changed to the phase (0, π) of the optical pulse. )
Is expressed as In any case, the threshold processing in the decoding is performed at the chip rate, and the band of the threshold element is compared under the condition of at most 1 / Tc.

As a result, the signal-to-interference noise power ratio of the present system is almost equal to the other two, but the band of the binary signal is 1
/ Tc (Tc: chip rate), whereas the bandwidth of the duobinary signal is reduced to about 2/3. Therefore, if the band of the threshold element is fixed, the code length can be increased by 1.5 times. As a result, the number of codes can be increased, and the number of users that can be accommodated is increased as compared with the other two. The threshold element band is compared under the condition of 1 / Tc at most.

[0028]

As described above, according to the present invention, by using the optical duobinary code, the frequency band of the code sequence can be compressed and the chip rate can be increased more than before, so that the number of users can be increased and information can be increased. It is possible to increase the bit rate and is expected to be a basic technology for a high-speed, large-capacity, asynchronous optical multiple access network suitable for multimedia communication.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating an example of an autocorrelation function of incoherent light CDMA and coherent light CDMA according to the present invention.

FIG. 2 is a block diagram illustrating an optical CDMA system configuration according to an embodiment of the present invention.

FIG. 3 is a block diagram illustrating an algorithm for generating a duobinary code.

4 is a conceptual diagram showing an example of a duobinary code generated according to the algorithm of FIG. 3 and signal waveforms of original data d (k) and b (k).

FIG. 5 is a conceptual diagram showing the relationship between an optical duobinary pulse code and its autocorrelation waveform.

FIG. 6 is a block diagram illustrating a configuration of an optical duobinary encoder.

FIG. 7 is a block diagram of a general system of optical CDMA.

FIG. 8 is a block diagram illustrating a configuration of an incoherent optical CDMA system.

FIG. 9 is a conceptual diagram showing a configuration of an optical fiber ladder type optical encoder and optical decoder used for coherent optical CDMA.

[Explanation of symbols]

 Reference Signs List 201 transmitter 202 pulse light source 203 optical modulator 204 data generator 205 optical duobinary encoder 206, 604 optical multiplexer 207 optical fiber transmission line 208 optical demultiplexer 209 receiver 210 optical duobinary decoder 211 light receiving element 212 threshold Element 601 Optical tap 602 Optical waveguide 603 Optical phase shifter

Continuation of the front page (56) References JP-A-1-282931 (JP, A) JP-A-5-300124 (JP, A) JP-A 1-126036 (JP, A) JP-A-2-127833 (JP) JP-A-10-4386 (JP, A) JP-A-1-144840 (JP, A) JP-A-1-140813 (JP, A) JP-A-51-49603 (JP, A) (58) Surveyed fields (Int. Cl. ⁶ , DB name) H04J 13/00 H04J 14/00 H04J 14/04 H04J 14/06

Claims (10)

(57) [Claims] 1. A transmitting side, each bit of the binary digital signal representing the payload data, and encoded using a duobinary code sequence consisting of multi-level finite number of chips, duobinary marks by different codes < A plurality of encoded payload signals are multiplexed and transmitted to the same receiving side at the same time. At the receiving side, the same duobar used for encoding is used.
By correlating the Inary code with the received signal,
Optical CDMA (Code-Divive) characterized in that only specific payload data is decoded and selectively received.
(sion-Multiple-Access: code division multiple access) system. 2. On the transmitting side, a duo-binary using an optical code sequence expressing multi-levels of a code using phases, amplitudes, polarization directions, etc. which are attributes of a coherent optical pulse.
And over the coding, by multiplexing transmitted in the optical transmission line by combining a plurality of optical signals encoded in the receiving side, the light Deyuobai for each bit of the received optical signal
The bit value of the payload data is "1" only when the peak value exceeds a certain value by performing a correlation operation with the nally code and performing threshold processing on the electric signal obtained by the squared photodetection. When the peak value does not exceed a certain value, the bit value of the payload data is determined to be "0", and the original payload data is demodulated by not reproducing. Optical CDMA system. 3. The multi-level code sequence according to claim 2, wherein the multi-level code sequence is encoded using a duobinary code including a finite number of ternary chips of “0”, “−1”, or “1”;
The transmitting side multiplexes a plurality of payload signals encoded by the above-mentioned different codes, and simultaneously transmits the multiplexed payload signals to the same receiving side. On the receiving side, the same code as that used for encoding and the received signal are used. An optical CDMA system characterized in that only specific payload data is decoded by correlation and selectively received. 4. The duobinary code according to claim 3, wherein the ternary code “0” is a state where no optical pulse is present on the transmitting side, and “−1” and “1” are phases π each other.
The optical signal is encoded using an optical duobinary code expressed using only different optical pulses, and the multiplexed optical signals are multiplexed by multiplexing and transmitted through an optical transmission line. By performing a correlation operation with an optical code and optical means for each bit of the signal and performing threshold processing on the electric signal obtained by the squared photodetection, the bit value of the payload data becomes “only” when the peak value exceeds a certain value. If the peak value does not exceed a certain value, the bit value of the payload data is determined to be "0" and the original payload data is demodulated by not reproducing. Characteristic optical CDMA system. 5. The duo-binary code according to claim 4, wherein the ternary code “0” at the transmitting side has a light pulse phase of 0, “−1” has a light pulse phase of −π / 2, "
1 "is encoded using an optical duobinary code expressed using an optical pulse having an optical pulse phase of π / 2, and a plurality of encoded optical signals are multiplexed by multiplexing them to form an optical transmission line. Transmit, on the receiving side, the peak value exceeds a certain value by thresholding the electrical signal obtained by the squared optical detection by performing a correlation operation with the optical code and optical means for each bit of the received optical signal Only when the bit value of the payload data is determined to be "1" and reproduced, on the other hand, when the peak value does not exceed a certain value, the bit value of the payload data is determined to be "0" and the reproduction is not performed. An optical CDMA system characterized by demodulating the payload data of 6. An optical CD according to claim 2, wherein said optical transmission line is a single optical fiber.
MA method. 7. A method of generating an optical duobinary code according to claim 4 or 5, wherein a temporally coherent optical pulse is demultiplexed and divided into N (≧ 2) optical chip pulses. After a predetermined delay time difference is given between the optical chip pulses, the optical chip pulse is cut off for "0", and a phase difference of π is given for "-1" or "1". An optical CDMA system characterized by generating an optical chip pulse train corresponding to an optical code by recombining chip pulses. 8. As a method of performing correlation calculation and decoding by optical means, an optical chip pulse train corresponding to an optical duobinary code is demultiplexed and divided into optical chip pulse trains of N (≧ 2) chips. An optical CDMA system characterized by recovering the delay and phase of an optical chip pulse by performing an operation opposite to optical encoding on each optical chip pulse train, and then multiplexing all the optical chip pulses again. 9. An optical demultiplexer for demultiplexing a temporally coherent optical pulse, an optical tap for dividing into N (≧ 2) optical chip pulses, and a constant delay time difference between the optical chip pulses And an optical phase shifter that interrupts the optical chip pulse for "0" and provides a phase difference of π for "-1" or "1". An optical CDMA apparatus comprising an optical multiplexer for multiplexing, and an optical duobinary encoder for generating an optical chip pulse train corresponding to an optical code. 10. An optical demultiplexer that demultiplexes an optical chip pulse train corresponding to an optical code, an optical tap that divides the optical chip pulse train into N (≧ 2) chips, and an optical tap pulse train for each optical chip pulse train. The optical chip pulse is cut off for "0" and "-"
An optical delay line that gives a phase difference of 1 or 1 to π, and an optical multiplexer that recovers the delay and phase of the optical chip pulse and then multiplexes all the optical chip pulses again. An optical CDMA device comprising an optical duobinary decoder.