JPS5836867B2 - Error control method by monitoring propagation path conditions - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an error control method by monitoring propagation path conditions,
In particular, it relates to an error control method by monitoring propagation path conditions for detecting propagation path disturbances in shortwave bands, etc., where conditions change rapidly, and controlling reception errors and the like accordingly.

In digital communications, reducing reception errors is an important issue, and error correction codes have traditionally been used for this purpose.

However, it is difficult to say that this method is sufficiently effective in propagation paths where conditions change rapidly, such as short-wave wireless lines.

This is because a code with only a fixed correction ability is used for a propagation path whose error rate changes from moment to moment.To solve this problem, it is necessary to detect the condition of the propagation path and then use the detection result to Therefore, it is necessary to change the way we use symbols.

In view of the above points, the present invention detects and utilizes the magnitude of disturbance in the propagation path at the same time as demodulating the digital information in the transmission of digital information using the phase keying (PSK) method, thereby reducing reception errors. This paper attempts to provide an error control method based on monitoring of propagation path conditions that aims to reduce errors.

Its characteristic feature is that the transmitter receives digital information to be transmitted (hereinafter referred to as transmission information).

) and a predetermined code sequence, the carrier is phase-modulated and transmitted so that the two can be identified and separated, and on the receiving side, after subtracting the phase shift due to the code sequence from the phase of the received wave, the transmission information is transmitted. At the same time, a correlation is detected between the phase difference obtained by eliminating the phase shift due to the transmitted information from the phase of the received wave and the same code sequence as that on the transmitting side generated on the demodulating side. The purpose of this method is to detect the magnitude of disturbances such as noise, interference, fusing, etc., and to perform error control using this detected value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an error control method based on channel condition monitoring according to the present invention will be described below with reference to the drawings.

On the transmitting side, a differential phase keying (DPSK) carrier is modulated with transmission information and a code sequence based on a predetermined pilot code (PN code) so as to have the phase relationship shown in FIG.

The solid line in the figure is the phase difference between adjacent elements in the case of normal four-phase differential phase modulation, and corresponds to transmission information.

In this example, the phase indicated by the broken line is further added, resulting in twice the number of phases, that is, 8-phase differential phase modulation.

As a result, the amount of information that can be transmitted per element increases by 1 bit, so this increased 1 bit is modulated with the pilot code.

Here, in normal 8-phase differential phase modulation, codes are assigned to each phase so that the code distance of the 3-bit code corresponding to the adjacent phase is 1, but in the case of Fig. 1, 4-phase difference Without changing the relationship between the phase and code assigned as dynamic phase modulation, when the pilot code is ``0'', the phase remains unchanged, and when the pilot code is ``1'', the phase is further shifted by 45 degrees. The numbers appended to the phases shown in the diagram are
The two pieces on the left side show the content of 2-bit information transmitted per element, and the one piece on the right side shows the content of the pilot code.

For example, when the 2 bits of transmission information are "01" and the pilot code is "1", the phase difference between the elements is 135°.

The transmitted wave s(t) modulated as described above is expressed by the following equation.

(However, ωC: carrier angular frequency, A(t): modulation amount due to transmission information, which takes values of 0, ■, 2, and 3, and B(t)
: The value of 0, 1, . . . 7 is the modulation amount by the pilot code.

) On the other hand, the received wave r(1) is a combined wave of the transmitted wave s(t) expressed by equation (1) and the noise n(t) expressed by the following equation.

n(t)-N(t)cos {ωct+θN(
t)} −=・(2) Therefore, the phase φ of the received wave r(t)
(1) is.

In this equation (3), the term θ'n(t) indicates phase fluctuation due to noise, and is as follows.

In equation (3) representing the phase φ(1), the pilot code component B(t) is a predetermined code sequence and is known to the demodulation side, so it can be eliminated; The remaining phase φ1(t) is as follows.

This modulation is the same as that of ordinary four-phase differential phase modulation, and since no influence of the pilot code remains, the transmitted information is restored from this component.

Note that the phase fluctuation θ',(t) due to noise differs depending on the value of B(t) for each element, but B
Since the statistical properties of θ'(t) when the value of (t) is 1 and the statistical properties of θ'n(t) when the value is 0 are the same, it can be said that there is no effect statistically.

On the other hand, when both sides of equation (3) are multiplied by 4, the modulation amount (π/2)A(t) due to the transmission information component becomes 2πA(t), which is an integer multiple of 2π rad and is therefore canceled, leaving the remaining phase φ2 There is (t).

). Since B'(t) is a pilot code known on the demodulation side, the same code sequence is created on the demodulation side and φ2//(
When 1) is detected by correlation, the correlation detection output Φ becomes (where M: the correlation detection output value when there is no fluctuation, k: the cumulative number of elements in the correlation detection).

This equation (9) shows that the magnitude of phase fluctuation due to noise etc. can be obtained as the amount of level decrease from the output value M of correlation detection when there is no fluctuation.

Then, if the cumulative number of elements k is set to an appropriate size,
Since the variations in individual sample values are averaged out,
The value of the correlation detection output Φ can be used to represent the correctness of reception in that accumulation interval.

FIG. 2 shows transmission information D.

A demodulator for obtaining the correlation detection output Φ and the correlation detection output Φ based on the principle shown in the above formula is shown.

In this figure, a detector DET filters and extracts a signal component from a received wave Si arriving via a propagation path, detects it, detects the phase θi of each element of the received wave, and applies it to a subtracter SUB1.

Here, an element on the transmitting side has transmission information A, B and 3-bit information of a pilot code as shown in FIG. If phase modulated, the phase of the transmitted wave will be as shown in FIG. 3B, and when there is no disturbance in the propagation path, the received wave will be equal to this.

On the other hand, the code generator PG generates the same code as the pilot code generated on the transmitting side in synchronization with the received wave.

That is, the output of the code generator PG is as shown in Fig. 3C,
This output is applied to cumulative adder ADD1.

The output of the cumulative adder ADDI shown in FIG. 3D is applied to the subtracter SUBI.

This subtracter SUB1 outputs a signal as shown in FIG. 3E indicating the difference between the phase θi of the received wave and the phase of the output of the cumulative adder ADD1.

This removes the phase shift due to the pilot code.

The output of the subtracter SUB1 is applied to a subtracter SUB2 and a delay circuit DLI.

As a result, the subtracter SUB2 applies an output as shown in FIG. 3F indicating the difference between the current output of the subtracter SUB1 and the output corresponding to the previous element to the sign determination circuit DEC.

Then, the output of FIG. 3F is determined here, and the transmission information D
.

is restored. On the other hand, the phase θi of the received wave is determined by the multiplier M
Added to ULI.

This multiplier MUL1 multiplies the phase θ1 by four and removes the phase shift due to the transmitted information, and applies the output shown in FIG. 3G to the subtracter SUB3 and delay circuit DL2.

The subtracter SUB3 applies to the multiplier MUL2 an output as shown in FIG. 3H indicating the difference between the current output of the multiplier MUL1 and the output corresponding to the previous element.

This multiplier MUL2 multiplies the output of the subtracter SUB3 and the output of the code generator PG and adds the result to the cumulative adder ADD2, and the multiplication result is cumulatively added as shown in Figure 3, thereby detecting the correlation. Output Φ is obtained.

In this case, the correlation detection is repeated every period of the pilot symbol or every integer multiple thereof.

In the above configuration, if there are no disturbances such as noise in the propagation path, the output signals of each part in Fig. 2 will be as shown by the solid lines in Fig. 3, but there will be phase fluctuations due to the disturbances. In this case, it is indicated by a broken line.

It can be seen that at this time, the correlation detection output value decreases.

In other words, it is clear that the amount of decrease in the correlation detection output represents the magnitude of the disturbance in the propagation path, such as the signal-to-noise ratio, the fluctuation of the propagation path, etc. It can be regarded as indicating the degree of correctness of reception for each section.

Therefore, by comparing the obtained correlation detection output with an appropriately set threshold level, it is possible to determine whether or not the demodulation result is correct.

Note that all operations performed by the above circuit, except for the multiplier MUL2 and the cumulative adder ADD2, are operations modulo 2πrad, and although they can be constructed using digital circuits, it is also possible to use an amplifier with the characteristics shown in Figure 4. For example, analog circuits are also possible.

Such an amplifier can be implemented using an operational amplifier and a diode.

Further, in FIG. 2, a clock portion necessary for arithmetic processing is omitted.

According to the embodiments described above, it is possible to obtain a correlation output that indicates the magnitude of disturbance in the propagation path without affecting the information to be transmitted, and thereby the degree of correctness of the reception result can be obtained. , that is, the error rate can be determined.

If reception error control is performed using this, reception errors can be reduced.

Now, in the embodiment described above, the ability of the error correction code can be effectively used by performing error correction coding on the transmission information and changing the error control operation according to the correlation detection output result.

In other words, an error correction code has a property that when the bit error rate before correction exceeds a certain limit value, the error rate after correction becomes worse than before correction. By comparison with the shoulder level, if the propagation path condition is determined to be good, error correction is performed; if the propagation path condition is determined to be poor, no correction is performed, or error detection is performed and the error detected characters The drawbacks of error correction codes can be eliminated by outputting them with an error indication.

This is particularly effective when applied to a propagation path where the conditions change rapidly, such as shortwave.

In addition, if error correction codes and correlation detection output are used in combination for error control, which detects reception errors on the receiving side and sends the results back to the transmitting side to retransmit the erroneously received characters, it is possible to improve efficiency. transmission becomes possible.

In other words, the correlation detection output is compared with a predetermined threshold level, and when the propagation path conditions are good and the error rate is small, error correction is performed using an error correction code, and even if the character contains error bits, it is not retransmitted. When the propagation path conditions are bad and the error rate is high, the frequency of errors exceeding the correction ability of the code will increase, so error detection is performed and if the character contains error bits, Request retransmission.

Through the above processing, the retransmission rate can be reduced without worsening the missed error rate in error detection on the receiving side.

Furthermore, when a code capable of correcting errors of 2 bits or more is used as an error correction code, it is also possible to set the threshold level in multiple stages and change the number of bits for error correction depending on the level. .

For example, if it is a code that can be corrected by 2 bits, (a) Correlation detection output decrease amount is small... Correct all errors. A three-step process is possible: correction, retransmission for 2-bit error (c) large drop in correlation detection output... retransmission of all characters.

In the above embodiment, a case where differential phase modulation is performed has been described, but it is also possible to replace the phase difference with an absolute phase and perform synchronous phase modulation.

Furthermore, if the number of phases is twice the number of phases assigned to transmission information, it can be applied to phase modulation with an arbitrary number of phases with different amounts of information per element.

As described above, according to the present invention, in the transmission of digital information using a phase modulation method, the magnitude of disturbance in the propagation path is detected at the same time as the digital information is demodulated, and this detection result is used for reception error control. By using this method, we can obtain an error control method by monitoring the propagation path condition that can reduce reception errors.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the phase of a modulated carrier according to an embodiment of the error control method based on channel condition monitoring according to the present invention, FIG. 2 is a block diagram showing a demodulation device in the embodiment, and FIG. FIG. 4 is a waveform diagram showing the effect thereof, and FIG. 4 is a characteristic diagram showing the characteristics required when the demodulator is constructed with an analog circuit. ADDI, ADD 2...Accumulative adder, DEC
...Sign determination circuit, DET...Detector, DLI, DL2...Delay circuit, MULI, M
UL2... Multiplier, PG... Code generator, SUBI, SUB2, SUB3... Subtractor.

Claims (1)

[Claims] 1 The transmitting side transmits a transmission wave that is phase-modulated using transmission information and a predetermined code sequence so that the two can be separated and identified, and the receiving side calculates the phase deviation due to the code sequence from the phase of the received wave. The transmitted information is demodulated by subtracting the phase of the same code sequence as the code sequence generated by the code sequence, and the phase difference is obtained by eliminating the phase deviation due to the transmitted information from the phase of the received wave. Correlation with the phase of the code sequence generated on the receiving side is detected, and if the detected correlation value is higher than a preset first level, error correction is performed on the code obtained by demodulation. carried out, and the correlation detection value is set to the first level and a second level lower than this.
If the value is between the level of An error control method based on monitoring of propagation path conditions, which performs error control by instructing the transmitting side to retransmit the character when the level is smaller than the level of .