JP2012074982A - Communication device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform control so that a bit error rate in a multi-value intensity modulation becomes the same at all signal levels to prevent the bit error rate for a regular recipient from being deteriorated drastically.SOLUTION: A communication device having a driving circuit that outputs a driving signal for driving a laser, and a laser generating a multi-value signal to which intensity modulation is performed by the driving signal, has an equalizer changing the driving signal from the driving circuit according to a predetermined rule so that an interval of the multi-value signal changes ununiformly.

Description

  The present invention relates to a communication apparatus, and more particularly to control of a signal using light intensity multilevel modulation represented by Yuen quantum cryptography.

  Yuen quantum cryptography, also called optical communication quantum cryptography, is a communication technology that spreads optical quantum fluctuations (quantum shot noise) by modulation, making it impossible for an eavesdropper to receive an optical signal accurately. It is proposed to apply. In this common key quantum cryptography, a binary optical signal carrying binary transmission data is set as one set (referred to as a base), a plurality of M bases are prepared, and which base is used to transmit data. Randomly determined by pseudo-random numbers according to the encryption key. Actually, the optical M-value signal is designed so that the distance between the signals is so small that it cannot be identified by the quantum fluctuation, so that an eavesdropper cannot read the data information from the received signal at all.

  Since the optical modulator / demodulator of the authorized transmitter / receiver switches the binary M bases in accordance with a common pseudo-random number and communicates, the authorized receiver reads data by binary signal determination with a large inter-signal distance. Can do. Errors due to quantum fluctuations can be ignored, and accurate communication is possible between authorized senders and receivers. The cipher based on this optical modulation method is called Yuen quantum cryptography based on the Yuen-2000 cryptographic communication protocol (abbreviated as Y-00 protocol). It is disclosed in Patent Document 1 and Patent Document 2.

JP 2006-303927 A JP 2010-111462 A

  By the way, in the multi-value intensity modulation Y-00, even if all the multi-value intervals are the same, the amount of quantum noise (shot noise) varies depending on the signal level. For this reason, an eavesdropper will try to find a shortcut for decoding in a short time by intensively attacking a portion where the bit error rate (hereinafter referred to as BER) of the optical transmission data is good. That is, there is a possibility that the cause of the decryption of the transmission data by the multi-value intensity modulation is made. Therefore, a mechanism for correcting the optical power so that the code error rate is the same at any signal level is desired.

  An object of the present invention is to control the code error rate so as to be the same at all signal levels in multi-level intensity modulation, and to prevent the code error rate of a legitimate receiver from being greatly deteriorated.

  Preferably, in the communication apparatus having a drive circuit that outputs a signal for driving a laser and a laser that emits a multi-value signal that is intensity-modulated by the drive signal, the intervals of the multi-value signals are non-uniform. The communication apparatus includes an equalizer that changes a drive signal from the drive circuit according to a predetermined rule so as to change.

In a preferred example, the equalizer changes the output signal from the drive circuit so that the code error rate is uniform between the multilevel signals.
Preferably, the equalizer includes a binary converter that converts an analog signal output from the modulator drive circuit into binary digital, and an input signal including a plurality of upper bits and a plurality of lower bits of the digital signal. Has a correction table for converting the signal into an output signal.

  According to the present invention, the code error rate BER (Bit Error Rate) can be improved by changing the multilevel interval in accordance with the amount of quantum noise in the multilevel intensity modulation. Since the BER between the signals can be equalized, the trigger of decryption can be prevented and the safety is improved.

The figure which shows the mode of reception of the multi-value signal of normal multi-value intensity modulation Y-00. The figure which shows the mode of the signal which changed the space | interval of the multi-value signal by this invention nonuniformly. The figure which shows the principle which fluctuates the space | interval of a multi-value signal by one Example of this invention. The figure which shows the presence or absence of the nonlinear compensation in the relationship of the optical power with respect to a laser drive current. The figure which shows the relationship between a threshold current and optical power. The figure which shows the relationship between Signal number electric current and electric current amplitude Ii. The figure which shows the relationship between Signal number electric current and BER. The figure which shows the relationship between Signal number electric current and electric current amplitude Ii. The figure which shows the relationship between Signal number electric current and BER. The figure which shows the structural block of the communication apparatus by one Example. FIG. 3 is a diagram illustrating a configuration example of an equalizer according to the first embodiment. The figure which shows the structural example of the correction table by one Example. The figure which shows the example of the equalizer comprised with the hardware circuit by another Example.

An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 shows a state of reception of a multilevel signal of normal multilevel intensity modulation Y-00, noise, and an eavesdropper. The eavesdropper model of Y-00 assumes that the multi-value interval (that is, the interval between signals S0 to S4) and the quantum noise (shot noise) σ ² _i at all signal levels are equal (see Equation 1). However, since shot noise is proportional to optical power, if the multilevel interval is made the same, the amount of quantum noise at each signal level is different and the code error rate BER changes. Further, the threshold value Ithi (i is 1 to 4 in the example of FIG. 1) is identified by preparing (multi-value number -1) between signal levels.

Here, Formula 1 shows the formula of quantum noise. The quantum noise σ ² _i is proportional to the optical power P.

  Equation 2 shows the BER equation between each signal level.

  Equation 3 shows the BER considering all signal levels. It becomes the average value of BER between each signal level.

FIG. 2 shows the state of a signal in which the interval of the multilevel signal according to the present invention is varied nonuniformly.
From Equation 2, σ ² _i becomes smaller on the side where the optical power P is low (S ₀ side). If the position of the multi-value interval and the threshold value Ithi is the same at each signal level (ie, (S _{i + 1} -Ithi) = (Ithi-S _i )), the lower the optical power, the lower the erfc in parentheses. Becomes larger and erfc () in Equation 2 becomes larger. That is, the lower the optical power, the worse the BER. Therefore, the present inventors considered that in order to make all BERi equal, it is better to narrow the multi-value interval as the optical power is lower. That is, it is meaningful to adjust the multi-value interval so that the multi-value interval is narrowed as the optical power is low and the multi-value interval is widened as the optical power is high so that the error rate does not change.

FIG. 3 illustrates the principle of unevenly varying the multi-level signal interval according to an embodiment of the present invention.
As an example, the input / output characteristics before laser compensation (indicated by the solid line A) are such that when the multilevel interval of the input signal is uniform, the interval of the output signal is also uniform. On the other hand, according to the present invention, as indicated by a two-dot chain line B, it is possible to make the multi-value interval of the output signal non-uniform by converting the linear input characteristic nonlinearly. Here, the conversion to nonlinear is referred to as nonlinear compensation.

FIG. 4 shows an example of the presence or absence of nonlinear compensation.
Equation 4 represents the characteristic of FIG. 4 without nonlinear compensation.

  Equation 5 represents the characteristic with nonlinear compensation of FIG.

  In FIG. 4, when there is no non-linear compensation, Ith = 20 mA and C = 0.15 mW / mA. With non-linear compensation, a0 = -11.9, a1 = 1.90, a2 = -0.133, a3 = 5.07e-3, a4 = -1.05e-4, a5 = 1.16e-6, a6 = -5.22e-9 It becomes.

FIG. 5 shows the relationship between threshold current and optical power.
Ith is the threshold current, and the multi-value current amplitude is Ii from the low signal number side, and the signal number is up to n.

FIG. 6 shows the relationship between the signal number and the current amplitude Ii.
The horizontal axis is the signal number, and the vertical axis is the current amplitude Ii. Signal number is up to 32. All current amplitudes are set constant.

  FIG. 7 shows the relationship between the signal number current and the BER under the conditions of FIG. It can be seen that the lower the signal number, the better the BER, and the BER relative to the signal number is not constant.

FIG. 8 shows the relationship between the signal number and the current amplitude Ii.
The horizontal axis is the signal number, the vertical axis is the current amplitude Ii, and the signal number is up to 32. In addition, the current amplitude Ii changes with respect to the signal number.

  FIG. 9 shows the BER under the conditions of FIG. It can be seen that the BER can be kept constant even if the signal number changes.

FIG. 10 shows a configuration of a communication apparatus according to an embodiment.
The communication apparatus includes a laser drive circuit 101 that outputs an electrical signal for driving a laser, an equalizer 102 that converts multi-value intervals in a non-uniform manner, in other words, a function for nonlinearly converting the input characteristics of the laser, A laser 103 for controlling the laser output (the intensity of the optical signal) is provided. Whereas a conventional communication apparatus is configured by a laser drive circuit and an intensity modulation laser, the communication apparatus of the present embodiment is configured by inserting an equalizer 102 between the laser drive circuit 101 and the laser 103 to form a linear shape. the input characteristics I _f, is converted into non-linear I _^'f, varying the laser output characteristics.
Since the conventional communication apparatus does not have an equalizer, the input characteristics of the laser remain linear, but according to the present embodiment, the output characteristics of the laser can be changed by the nonlinear conversion function by the equalizer 102. it can.

FIG. 11 shows a configuration example of the equalizer 102.
In this example, the equalizer 102 includes a binary converter 202 which converts the analog input I _f the binary digital (8 bits), the correction of converting the input I _f to the output I _^'f by the upper 4 bits and lower 4 bits The table 203 is configured.
An example of this correction table is shown in FIG. In the correction table, the value stored in the place where the lower 4 bits obtained by binary conversion of the analog input and the upper 4 bits coincide is output I ^′ _f .

FIG. 13 shows a configuration example of the equalizer 102 according to another embodiment.
In this example, using a hardware circuit, a polynomial approximation formula that makes the input / output characteristics of a laser nonlinear is realized by an equalizer.
The equalizer 102 realizes Expression 5 and converts the laser driving current. That is, the input I _f is multiplied by the counts d1 to dn (133) by a plurality (n−1) of multipliers 132, and the multiplication result is added by a plurality of n adders 134, whereby the output I ^′ _f Is obtained.
For example, when the upper 4 bits are “8” and the lower 4 bits are “7”, a current of I ^′ _f = 48.1 mA is directly applied to the modulation laser.

  As described above, according to the present embodiment, the multilevel interval can be changed non-uniformly according to the amount of quantum noise in multilevel intensity modulation, thereby improving the BER. By making the BER equal between the signals, the trigger of decryption can be prevented, and the safety is improved.

101: Laser drive circuit 102: Equalizer 103: Laser
104: Binary converter 105: Correction table.

Claims (3)

In a communication device having a drive circuit that outputs a signal for driving a laser and a laser that emits a multi-value signal that is intensity-modulated by the drive signal,
A communication apparatus comprising: an equalizer that changes a drive signal from the drive circuit according to a predetermined rule so that an interval of the multi-level signal changes non-uniformly. 2. The communication apparatus according to claim 1, wherein the equalizer changes an output signal from the drive circuit so that a code error rate is uniform between the multilevel signals. The equalizer converts an analog signal output from the modulator drive circuit into a binary digital signal, and converts an input signal into an output signal using a plurality of upper bits and a plurality of lower bits of the digital signal. The communication apparatus according to claim 1, further comprising a correction table that performs correction.

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