RU2788940C1 - Method for incoherent accumulation of light-location signals - Google Patents

Abstract

FIELD: optical signals reception.

SUBSTANCE: invention relates to the reception of optical signals, in particular to the technique of receiving signals using avalanche photodiodes, and can be used in location, communications and other photoelectronic systems. The method for incoherent accumulation of light-location signals, including a series of probing cycles, in each probing cycle, receiving a reflected signal and comparing the received signal with analog threshold levels, accumulating the sum of excesses of analog threshold levels, taking into account the weighting factor of the level, which is used to judge the presence of a signal by comparing the sum of excesses with threshold number, the reception of reflected signals is carried out using an avalanche photodiode in N channels of the reflected signal delay, characterized by the time duration of the channel τ and the delay measurement range Т=Nτ, where N is the number of channels, the avalanche multiplication coefficient of the photodiode, optimal in terms of signal-to-noise ratio, is preliminarily set M_opt, then, by controlling the bias voltage of the photodiode, the frequency f_m is reduced to the maximum allowable level f_m*, in this mode, the average duration of microplasma pulses t_m, the minimum amplitude U^m _min of microplasma pulses are determined, an additional threshold level U^p is set according to the condition U^p<U^m _min, and if in the current delay channel the emission of a mixture of signal and noise exceeds the threshold U^p, then in this accumulation cycle, signal processing in this channel is blocked. The maximum permissible level of the frequency of microplasma pulses f_m* can be set according to the dependence

, where

- the allowable amount of microplasmas in one accumulation channel in one cycle; K - the number of accumulation cycles; τ - time width of the accumulation channel; K_thr - the threshold value of the accumulated sum in one channel, at which a decision is made about the presence of a signal (threshold number); χ<<1 - coefficient providing the condition Q_n+Q_m≤Q; Q - the probability of signal skipping (probability of non-detection); Q_n - component of the signal skip probability due to fluctuation noise; Q_m - the probability of signal skipping due to the influence of microplasmas.

EFFECT: theoretically the ultimate sensitivity is ensured in all operating conditions, taking into account microplasma breakdowns and fluctuation noise.

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Description

The invention relates to laser technology, namely to laser ranging.

There is a method of incoherent accumulation of pulses when they are repeated many times, for example, to detect received signals during laser or radar sounding of remote objects [1-5]. This method consists in performing a series of probing cycles, in each probing cycle, the received signal is compared with an analog threshold (binary quantization is performed), the number of analog threshold exceedances is counted, and a decision is made on the presence of a signal if this number exceeds a predetermined number. This method does not allow realizing the potential probability of detecting signals due to the loss of information during binary quantization of the received signal.

The closest in technical essence to the proposed method is a method of incoherent accumulation of signals, including a series of probing cycles, in each probing cycle, receiving a reflected signal and comparing the received signal with one or more analog threshold levels, accumulating the sum of exceedances of analog threshold levels, according to which, after the completion of the series the presence of a signal is judged by comparing the sum of exceedances with a threshold number [6].

The advantages of this method are realized to the maximum if the reflected signal is received using an avalanche photodiode, which has the best threshold sensitivity compared to other receivers [7]. However, in the optimum sensitivity mode of avalanche multiplication, the formation of explosive (“telegraph”) noises caused by microplasma breakdowns (microplasmas) in the semiconductor junction of the photodiode is possible [8]. Microplasma current pulses have a rectangular shape and a constant amplitude, which increases as the reverse voltage increases. An increase in the amplitude is accompanied by an increase in the duration of the pulses and a decrease in the duty cycle [9]. In this mode, the noise of an avalanche photodiode consists of two independent components - normal noise [7] and explosive noise of microplasmas. The microplasma component of the photodiode noise is not statistically comparable with the normal component, and its participation in the process of controlling the photodiode bias is unpredictable [10].

The objective of the invention is to ensure the maximum probability of detecting light-location signals by the accumulation method in the presence of microplasmas arising from the avalanche multiplication of the signal in the photodiode.

This problem is solved due to the fact that in the known method of incoherent accumulation of light-location signals, including a series of probing cycles, in each probing cycle, receiving the reflected signal and comparing the received signal with analog threshold levels, accumulating the sum of exceedances of analog threshold levels, taking into account the weighting coefficient of the level, by which the presence of a signal is judged by comparing the sum of excesses with a threshold number, the reception of reflected signals is carried out using an avalanche photodiode in N channels of the reflected signal delay, characterized by the time duration of the channel t and the delay measurement range T=Nτ, where N is the number of channels, pre-set the optimal according to the signal-to-noise ratio, the avalanche multiplication coefficient of the photodiode M _opt , then, by controlling the bias voltage of the photodiode, the frequency f _m is reduced to the maximum allowable level f _m ^* , in this mode the average duration of microplasma pulses t _m , the minimum a amplitude U ^m _min of microplasma pulses, an additional threshold level U ^p is set according to the condition U ^p <U ^m _min , and if in the current delay channel the emission of a mixture of signal and noise exceeds the threshold U ^p , then signal processing in this channel is blocked in this accumulation cycle.

где

- допустимое количество микроплазм в одном канале накопления за один цикл; K - количество циклов накопления; т - временная ширина канала накопления; K_пор - пороговое значение накопленной суммы в одном канале, при котором принимается решение о наличии сигнала (пороговое число);

- коэффициент, обеспечивающий условие Q_ш+Q_м≤Q; Q - вероятность пропуска сигнала (вероятность необнаружения); Q_ш - составляющая вероятности пропуска сигнала, обусловленная флуктуационным шумом; Q_м - вероятности пропуска сигнала, обусловленная влиянием микроплазм.The maximum permissible level of the frequency of microplasma pulses f _m ^* can be set according to the dependence

where

- allowable amount of microplasmas in one accumulation channel in one cycle; K is the number of accumulation cycles; m is the time width of the accumulation channel; K _then - the threshold value of the accumulated amount in one channel, at which a decision is made about the presence of a signal (threshold number);

- coefficient providing the condition Q _w +Q _m ≤Q; Q is the probability of signal skipping (probability of non-detection); Q _w - component of the probability of signal skipping due to fluctuation noise; Q _m - the probability of skipping the signal, due to the influence of microplasmas.

To ensure the stability of the detection characteristics in a wide range of conditions, it is possible to pre-set the analog threshold levels relative to the zero level in the mode of automatic noise control by accumulating the sum of analog threshold exceedances in the absence of a signal, shifting the relative position of the zero level and threshold levels so that the accumulated total number of threshold exceedances levels was minimal, after which this relative position of the levels is maintained during the signal accumulation time.

In FIG. 1 shows an example of a signal-to-noise mixture and two analog thresholds set symmetrically about the zero level.

In FIG. 2 shows an example of signal blocking when microplasma pulses occur that exceed the threshold U ^p .

In FIG. 3 is a block diagram of the method.

The mixture of the received signal and noise (FIG. 1) forms an implementation of the random process, which is analyzed by comparison with one or more analog thresholds. In a two-level version, this implementation is compared with analog thresholds located symmetrically with respect to the zero level (Fig. 1). If in a given probing cycle in any time interval implementation 3 crosses the positive threshold level u ₊ up, then this is recorded by generating and adding the number +1 to the accumulated sum, and if the implementation crosses the negative threshold level u _- down, then the accumulated amount is added number minus 1. At the end of a series of K soundings, the accumulated sum k is compared with the threshold number K _then, and, if this number is exceeded, a decision is made about the presence of a signal in a given time interval.

The noise process is characterized by the rms value σ. In the example shown in FIG. 1, the signal amplitude A is equal to the rms value of the noise [11]. The values of the positive and negative thresholds u ₊ and u _- are u ₊ =+0.5 σ and u _- =-0.5 σ. One time step τ=10 ^-8 s. The duration of the signal at a level of 0.5 takes about three discretes. The bandwidth of the linear path is matched to the width of the signal spectrum. In the absence of a signal, the probabilities of crossing the positive and negative thresholds by the noise process up and down, respectively, are equal, therefore, in the absence of a signal, the average value M(n ₀ ) of the accumulated sum n0 is equal to zero.

An essential criterion is the accumulation efficiency E, which is an improvement in the signal-to-noise ratio at the input and output of the storage:

where M(k) - the average value of the accumulated amount k;

σ _K is the standard deviation of the accumulated amount k after K accumulation cycles;

A is the signal amplitude at the accumulator input;

σ is the rms value of the input noise.

The dependence of the accumulation efficiency on the relative value of the threshold levels u/σ at their symmetrical position from the zero level was studied [11] (Fig. 1). At the optimal position of the threshold levels, the two-level and four-level accumulation modes with a symmetrical placement of the threshold levels approach the theoretical limit in terms of efficiency

There is an optimal value of the avalanche multiplication factor M, which, in the absence of microplasmas, can be determined as follows. At the output of the avalanche photodiode, the equivalent square of the noise current acts:

where I ₀ ² is the square of the non-multiplied noise current;

e is the electron charge;

I ₁ - primary reverse current of the photodiode;

Δf - bandwidth of the receiving path to the input of the threshold device;

M - coefficient of avalanche multiplication;

M ^α - noise factor of avalanche multiplication;

α - coefficient determined by the material of the photodiode.

Noise/Signal Ratio W Squared

where J _M ² =2eI ₁ Δf.

Zero Derivative Condition

or

where

The task of the present invention is solved due to the procedure of hardware interpretation of microplasmas not as false alarms, but as signal skipping factors. Due to this technique, a higher probability of microplasmas can be assumed and, thus, the avalanche multiplication factor can be maintained closer to the optimal level (6). Fig. 2 illustrates the principle of blocking a mixture of signal and jitter by detecting outliers that exceed the additional threshold UM.

In FIG. 3 shows an example of a two-threshold structure for implementing the method.

This location structure contains a transmitting channel 1, a photoreceiving channel 2, a key 3 and an additional threshold device 4, the output of which is connected to the control input of the key. At the output of the key there are threshold devices u ₊ 5 and u _- 6 shown in Fig. 1. Their outputs are connected to the inputs of a multichannel storage 7, at the output of which a digital block for detecting and temporarily fixing a signal 8 is installed. This structure is controlled and synchronized by the control unit 9.

On command from the control unit, the transmitting channel emits a series of K probing pulses to the target. Simultaneously, synchronization of the multichannel storage device 7 is started, switching its accumulation cells with a time resolution t (Fig. 1). The signals reflected by the target are received by the photodetector channel 2, at the output of which a mixture of the reflected signal and fluctuation noise is formed, through the open key 3 entering the inputs of the threshold devices 5 and 6. Emissions of noise in the mixture with the signal cause the operation of one of the threshold devices that form the pulse "1" if device 5 is triggered with threshold u ₊ and pulse "-1" if device 6 is triggered with threshold u _- . Pulses "1" and "-1" arrive according to their delay in the corresponding cell of the accumulator 7. If in the signal-noise mixture there is a microplasma pulse exceeding the threshold u ^M , an inhibition pulse is formed at the output of the additional threshold device 4, which closes the key 3, preventing thereby passing the microplasma pulse to the threshold devices 5 and 6. Upon completion of a series of K cycles of probing, the digital block for detecting and temporarily fixing the signal 8 searches for storage cells with a sum exceeding the digital threshold K _then . When a signal pulse is applied to the microplasma, the total surge is identified as a signal, i.e. no information loss occurs in this case. If the signal occupies several cells, then its timing is carried out according to the methods described, for example, in [11-13].

Для рассмотренного примера K=200 эта величина составит

то есть на два двоичных разряда больше по сравнению с двухуровневым накоплением.The required amount of equipment necessary for the implementation of the method has been estimated. For two-level accumulation with the specified data in each range channel, it is necessary to be able to accumulate until the digital threshold K _then is exceeded by (3-4) σ _K . With the optimal position of the analog levels |u ₊ |=|u _- |~0.5σ (Fig. 1) and the accumulation volume K=200, the standard deviation of the accumulated sum σ _K ~10, and the minimum volume of the adder K _max ~4σ _K =40 <2 ⁶ . The volume of the summing device during binary accumulation with the optimal position of the threshold level is estimated by the formula

For the considered example K=200, this value will be

that is, two more bits compared to two-level accumulation.

The signal skip, characterized by the probability Q _w and the appearance of microplasma, characterized by the probability Q _M , are mutually independent events [3], therefore, the given probability of the signal skip Q=1-D for the control time interval τ can be represented as the sum Q=Q _w + Q _m , where D is the probability of correct signal detection.

When choosing conditions

and

it is possible to almost completely eliminate the influence of microplasmas on the detection characteristics.

Condition (8) is equivalent to the relation

where m is the allowable amount of microplasmas in one accumulation channel per detection cycle;

K _then - the threshold value of the accumulated sum in one channel, at which a decision is made about the presence of a signal;

- коэффициент, обеспечивающий условие (7).

- coefficient providing condition (7).

The accumulated sum k is a random variable with the mathematical expectation corresponding to the level of the received signal and at the threshold value of this value k=K _then with the standard deviation

The critical amount of microplasmas in one accumulation channel m should not significantly affect the statistics of the accumulated amount due to the signal, which is indicated by expression (9).

Example.

количество каналов накопления N=10⁴; ширина канала τ=10^-8 с; коэффициент

Accumulation volume K=200; set threshold

the number of accumulation channels N=10 ⁴ ; channel width τ=10 ^- 8 s; coefficient

Under these conditions, the average number of microplasmas per channel for the entire accumulation time:

Average number of microplasmas per channel per cycle:

The duration of the accumulation cycle Т=Nτ=10 ⁴ ⋅10 ^-8 =10 ^-4 s. Permissible frequency of microplasmas:

Thus, the task of the invention is ensured - the achievement of theoretically limiting sensitivity in all operating conditions, taking into account microplasma breakdowns following with a frequency of up to 200 kHz.

The proposed method of incoherent signal accumulation provides the maximum probability of detecting signals with a minimum amount of equipment and can be implemented in portable laser rangefinders.

Sources of information

1. Ya.D. Shirman, V.N. Golikov "Fundamentals of the theory of detection of radar signals and measurement of their parameters." Ed. "Soviet radio", M., 1963, p. 179.

2. Ya.D. Shirman, V.N. Manzhos "Theory and technique of processing radar information against the background of interference." Ed. "Radio and communication", M., 1981, p. 81-83.

3. V.E. Gmurman, Probability Theory and Mathematical Statistics. Ed. "Higher School", M., 1977

4. V.G. Vilner Design of threshold devices with noise threshold stabilization. Optical-mechanical industry, 1984, No. 5.

5. Patent WO 2005/006016 A1 "Laser rangefinder and method thereof.

6. Patent of the Russian Federation No. 2359226 according to z-ke No. 2007137271 dated 10.10.2007. "Method of incoherent accumulation of light-location signals". - Prototype.

7. Vilner V.G., Leichenko Yu.A., Motenko B.N. Analysis of the input circuit of a photodetector with an avalanche photodiode and anti-noise correction. Optical-mechanical industry, 1981, No. 9, - p. 59.

8. Filachev A.M., Taubkin I.I., Trishenkov M.A. Solid state photoelectronics. Physical bases. Moscow, Fizmatgiz. 2007.

9. Vishnevsky A.I., Rudenko V.S., Platonov A.P. Power ion and semiconductor devices. Textbook for universities. Edited by V.S. Rudenko. Moscow, Higher School, 1975.

10. Shashkina A.S. Avalanche breakdown of a p-n junction in radio engineering problems - Scientific and Technical Bulletin of Information Technologies, Mechanics and Optics, 2016, vol. 16, no. 5, p. 864-871.

11. Vilner V.G. Evaluation of the possibilities of a light-location pulsed range meter with accumulation. Photonics, 2007, No. 6, p. 22-27.

12. RF patent No. 2390724. "Method for light-location determination of range by the method of incoherent accumulation".

13. RF patent No. 2469269. "Method of determining the range."

Claims (3)

1. A method for incoherent accumulation of light-location signals, including a series of probing cycles, in each probing cycle, receiving a reflected signal and comparing the received signal with analog threshold levels, accumulating the sum of exceedances of analog threshold levels, taking into account the weighting factor of the level, which is used to judge the presence of a signal by comparing the sum exceedances with a threshold number, characterized in that the reception of reflected signals is carried out using an avalanche photodiode in N channels of the reflected signal delay, characterized by the time duration of the channel τ and the delay measurement range T=Nτ, where N is the number of channels, pre-set the optimal signal / noise photodiode avalanche multiplication factor M _opt , then, by controlling the bias voltage of the photodiode, reduce the frequency f _m to the maximum allowable level f _m *, in this mode determine the average duration of microplasma pulses t _m , the minimum amplitude U ^m _min microplasma pulses, set an additional threshold level U ^p according to the condition U ^p <U ^m _min , and if in the current delay channel the emission of a mixture of signal and noise exceeds the threshold U ^p , then in this accumulation cycle, signal processing in this channel is blocked. 2. The method according to p. 1, characterized in that the maximum permissible level of the frequency of microplasma pulses f _m * is set according to the dependence

where

- allowable amount of microplasmas in one accumulation channel in one cycle; K is the number of accumulation cycles; τ - time width of the accumulation channel; K _then - the threshold value of the accumulated amount in one channel, at which a decision is made about the presence of a signal (threshold number);

- coefficient providing the condition Q _w +Q _m <Q; Q is the probability of signal skipping (probability of non-detection); Q _w - component of the probability of signal skipping due to fluctuation noise; Q _m - the probability of skipping the signal, due to the influence of microplasmas. 3. The method according to claim 1, characterized in that the analog threshold levels are preliminarily set relative to the zero level in the noise automatic adjustment mode by accumulating the sum of the excesses of the analog threshold levels in the absence of a signal, shifting the relative position of the zero level and the threshold levels so that the accumulated total number exceeding the threshold levels was minimal, after which this relative position of the levels is maintained during the signal accumulation time.

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