JPH0740688B2 - Digital signal transmission method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital signal transmission method for transmitting a digital signal in a multipath transmission line such as wireless transmission in an urban area.

2. Description of the Related Art In recent years, even in the field of mobile communication, digitization is progressing due to improvement of confidentiality, sophistication of communication, and compatibility with peripheral communication networks. However, in urban areas where such demand is considered to be most concentrated, communication quality is significantly deteriorated due to multipath due to reflection and diffraction by buildings and other structures. In the case of digital transmission, if the propagation delay time difference between the waves forming the multipath cannot be ignored with respect to the data time slot, the waveform error and the tracking failure of the synchronous system will significantly deteriorate the code error rate characteristic.

Hereinafter, an example of the above-described conventional digital signal transmission method will be described with reference to the drawings.

FIG. 9 shows a phase transition of a transmission signal in the conventional digital signal transmission method of FIG. T indicates one time slot of data. When the data is 1, the phase shifts 180 °,
When the data is 0, no phase transition occurs. This signal format is called differentially encoded binary phase modulation.

To detect such a transmission signal, for example, differential detection having a delay line of one time slot can be performed.
Now, let us consider, as a typical example of multipath, what happens to the detection output signal under the multipath of two waves having a propagation delay time difference τ that cannot be ignored compared to time slots. In addition, the wave that precedes in time is a direct wave,
The delayed wave will be called the delayed wave.

FIG. 9 is a diagram for explaining what a detection output signal looks like when the transmission signal as shown in FIG. 8 is subjected to delay detection under a two-wave multipulse. Fig. 9 (a)
Shows the phase transition of the direct wave. On the other hand, the phase transition of the delayed wave delayed by the propagation delay time difference τ is as shown in FIG. 9 (b). Detection output at a certain point,
It is a vector inner product of the combined phase of the two waves at that time and the combined phase of the two waves one time slot before. For example, in FIG. 6 (c), the detection output in the section B is 2 at the time of B '.
It is the value of the vector inner product of the wave synthesis phase and that of B.

FIG. 10 is for obtaining the detection output at each time point A to C. 6 is a diagram illustrating a combined phase of a direct wave and a delayed wave.
The amplitude ratio of the direct wave and the delayed wave was ρ, and the phase difference was α. From FIG. 10, the detection output at each time point A to C in FIG. 9 (c) is as follows.

A: undefined B: 1 + ρ ² + 2ρ cosα C: undefined In sections A and C, the data values of the previous and subsequent time slots are undefined. After differential detection, a low-pass filter is usually inserted to remove unnecessary noise components, so the final detection output signal waveform is shown in FIG. 9 (c).
The waveform of the solid line is filtered to form a waveform as shown by the dotted line in FIG. 9C, which constitutes a part of the eye pattern. By the way, when ρ is close to 1 and α is around 180 °, the detection output in the section B, which is an effective detection output, becomes almost zero. Therefore, the eye is closed and the code error rate characteristic deteriorates. Further, at this time, since the invalid detection output of the sections A and C is much larger than the valid detection output of the section B,
The eye greatly fluctuates in the time axis direction, the reproduced clock cannot follow, and the code error rate further deteriorates. (For example, Onoe et al., "Code Error Rate Characteristic in Rayleigh Fading with Propagation Delay Time Difference", IEICE Tech.
8, 1982, or Takai et al., "Analysis of Instantaneous Code Error due to Multipath Propagation and Error Generation Mechanism Based on Bit Synchronous System", IEICE Technical Report, CS83-158, 1984) In the above-mentioned method, waveform distortion due to multipath is significant and the code error rate is significantly degraded as described above. In particular, examining the relationship between the signal S / N ratio and the error rate, there is a region where the error rate does not decrease even if the S / N ratio is improved. Such a code error is called an irreducible error. Due to such so-called irreducible errors, the actual data transmission rate in urban areas is greatly limited, and high-speed transmission is impossible.

In view of the above problems, the present invention provides a digital signal transmission method capable of high-speed digital transmission in a multipath transmission line such as an urban area.

Means for Solving the Problems In order to solve the above problems, the digital signal transmission method of the present invention divides one time slot of data into a first half and a second half at a predetermined ratio and divides the first half and the second half. Between,
The adjacent time slots alternately have a phase transition of a predetermined angle in the advance direction or the delay direction, and are transmitted by a predetermined even time slot to the phase difference between the first half and the second half respectively. A signal having information is used as a transmission signal.

Action The present invention, by using the transmission signal as described above,
When the differential detection is performed, two types of effective detection outputs can be obtained. Then, due to one kind of diversity effect by combining these outputs, the code error rate under multipath is significantly improved. Due to the effects as described above, digital transmission at a higher speed than ever can be performed on a multipath transmission line.

Embodiment A digital signal transmission method according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the phase transition of the transmission progress of the digital signal transmission method of the present invention. The transmission signal of the digital signal transmission method of the present invention will be described below with reference to FIG.

As shown in FIG. 1, one time slot of data is divided into a first half portion and a second half portion. The time of one time slot is shown as T, the time of the first half is T ₁ , and the time of the second half is T ₂ . Then, there is always a phase transition indicated by φ between the first half portion and the second half portion. The transition amount of this phase transition φ is constant in all time slots, but the transition direction is always different in adjacent time slots. That is, the first
In the figure, the positive phase axis may be either advanced or delayed, but the transition direction of the phase transition φ in the even-numbered time slots and the transition direction of the odd-numbered phase transition φ are different. . The ratio of T ₁ and T ₂ and φ may be arbitrary. Of course, T ₁ and T ₂ may be the same.

Although the phase difference between the first half part of a time slot and the first half part 2n time slots after that, and similarly, the phase difference between the latter half parts is equal, the phase amounts are represented by θ ₁ and θ _{2 in} FIG. Indicated by. Digital information is transmitted according to the value of the phase transition θ between the time slots. For example, if 0 ° and 180 ° are used as possible values of θ, 1-bit information is transmitted by assigning 0 and 1 correspondingly. If 0 °, 90 °, 180 °, 270 ° is used as θ, 2-bit information is transmitted. Furthermore, as the value of θ, 0 °, 45 °,
8 phases of 90 ° ……, 0 °, 22.5 °, 45 °, 67.5 ° ...
The multi-phase of powers of 2 such as 16-phases, the multi-phases of non-powers of 2 and the interval between the possible values of θ may not be constant, and the value of θ is Any value will do as long as the transmitted information corresponds. In addition,
The value of n may be arbitrary as long as it is a natural number.

As described above, there are various phase transitions of the transmission signal of the digital signal transmission method of the present invention depending on the values of T ₁ , T ₂ , φ, θ, and n. An example is shown in the figure.

FIG. 2 shows an example of phase transition of a transmission signal for transmitting 1-bit data for one time slot corresponding to θ = 0 ° and 180 ° when n = 1 and φ = 90 °. .

Similar to the case of FIG. 2, FIG. 3 shows the transmission signal for transmitting 1-bit data for one time slot corresponding to θ = 0 ° and 180 ° when n = 1 and φ = 90 °. The example of a phase transition is shown. In the transmission signal of the digital transmission method of the present invention, the phase difference between the first half and the second half of adjacent time slots, to be exact, the initial phase difference can be freely selected. The initial phase difference in the case of FIG. 3 deviates from the initial phase difference in the case of FIG. 2 by 45 ° or 135 °.

Fig. 4 shows that when n = 1 and φ = 90 °, θ = 0 °, 90 °, 18
Corresponding to 0 ° and 270 °, (0,0), (1,0),
2 for each time slot, such as (1,1) and (0,1)
The example of the phase transition of the transmission signal which transmits bit data is shown.

Next, the reason why the digital signal transmission method of the present invention is strong against multipath distortion will be described with an example.

In the following description, as an example of the phase transition of the transmission signal of the digital signal transmission system of the present invention, n = 1 and φ = 90
.Degree., .Phi. = 0.degree., 180.degree., That is, the transmission signal of FIG. A typical two-wave model is considered as a multipath model. The wave that precedes in time is called the direct wave, and the wave that is delayed is called the delayed wave.

Since the digital signal transmission system of the present invention is a kind of differential encoding phase modulation, it is detected by differential detection using a delay line of 2n time slots. In the example of the transmission signal shown in FIG. 2, it is detected by differential detection using a delay line of two time slots. An example of the structure of the detection circuit is shown in FIG. However, in FIG. 5, 1 is an input terminal, 2 is a multiplier, 3 is a 2 time slot delay device, 4 is a low-pass filter, and 5 is a detection output terminal.

FIG. 6 is a diagram for explaining what the detection output signal looks like when the transmission signal of FIG. 2 is detected by the detection circuit of FIG. 5 under the two-wave multipath. Figure 6 (a)
Shows the phase transition of the direct wave. On the other hand, the phase transition of the delayed wave delayed by the propagation delay time difference τ which cannot be ignored compared to the time slot is as shown in FIG. 6 (b). The detection output at a certain time point is a vector inner product of the combined phase of the two waves at that time and the combined phase of the two waves two time slots before. For example, in FIG. 6 (c), the detection output in the section B is the value of the vector inner product of the two-wave combined phase at B'and that at B.

FIG. 7 shows a combined phase of the direct wave and the delayed wave in order to obtain the detection output at each time point A to E.
The amplitude ratio of the direct wave and the delayed wave was ρ, and the phase difference was α. Further, the phase axes of FIGS. 6A and 6B may be positive or may be in the advanced phase, but are set in the advanced direction. As shown in FIG. 7, when the waveform is not deformed by the low-pass filter 4 or the cutoff frequency is sufficiently higher than the data transmission rate, the demodulation outputs at the respective points A to E in FIG. become that way.

A: undefined B: 1 + ρ ² + 2ρ cosα ・ ・ ・ C: 1 + ρ ² + 2ρ cos (α-90) ・ ・ ・ D: 1 + ρ ² + 2ρ cosα ・ ・ ・ E: undetermined In sections A and E, the time slots before and after are respectively determined. It is undefined depending on the data value. Depending on the values of ρ and α, even if the detection signal in any of the sections B, D, and C becomes zero, the other does not become zero. In practice, the cutoff frequency of the low-pass filter 4 is selected low enough to prevent intersymbol interference. Therefore, the detection output signal after passing through the low-pass filter 4 is filtered on the waveform of the solid line in FIG. 6 (c), and a part of the eye pattern as shown by the dotted line in FIG. 6 (c). To form. As described above, since complementary detection outputs of the sections B and D and the section C are generated,
The eye never closes. Further, at least one of these effective detection outputs does not become smaller than the ineffective detection output in the section A or E, so that fluctuations of the eye in the time axis direction are reduced and the code due to the poor tracking of the reproduction clock is reduced. There is little deterioration in the error rate.

As described above, the digital signal transmission method of the present invention is less susceptible to waveform distortion due to multipath due to a kind of diversity effect by combining different detection outputs of the sections B and D and the section C. In this way, in the multipath transmission line, the code error rate characteristic is remarkably improved as compared with the conventional method, and high-speed digital transmission becomes possible.

In this explanation, as shown in FIG. 2, n = 1,
Although the transmission signal when φ = 90 °, θ = 0 °, 180 ° has been described as an example, the code error rate characteristic is remarkably improved for the transmission signals having other values by the same principle. For example, if n is not 1 and the delay time of the 2 time slot delay unit 3 of FIG. 5 is set to 2n time slots, the above invention is exactly the same. When φ is not 90 °,
The formula, the formula, and the formula corresponding to the formula are B …… 1 + ρ ² + 2ρ cosα …… C …… 1 + ρ ² + 2ρ cos (α ± φ) …… D …… 1 + ρ ² + 2ρ cosα ……, and also the intervals B and D And section C have different detection outputs, and due to a kind of diversity effect by combining the two, waveform distortion due to multipath is less likely to occur and the code error rate characteristic is significantly improved. The compound symbols in the equation are + when the phase transition φ is late and − when it is advanced. When θ is multi-phase such as 4-phase or 8-phase, a 90 ° phase shifter is further connected to the output of the 1-time-slot delay unit 3 in FIG. 5, and the output signal is used as a reference signal for differential detection on the orthogonal axes. There is a need to do. However, although the configuration of the detection circuit becomes complicated, the detection output of each detection axis has exactly two types of effective detection outputs, just like the above description, and due to a kind of diversity effect by combining both. , The bit error rate characteristic is significantly improved. T ₁ and T ₂
The lengths of the sections B, C, and D also change with respect to the ratio of, but other than that, it is exactly the same as the above description.

It should be noted that the transmission rate can be doubled or tripled with respect to the same time slot length by using multi-valued 4-phase and 8-phase, and a good error rate characteristic can be obtained in higher speed transmission. Can be demonstrated. Also, by intentionally changing the ratio of T ₁ and T ₂ from 1: 1, it is possible that either section B or section D and section C exist for a multipath transmission line having a longer delay time difference. The error rate characteristics can be improved (see IEICE technical report SAT86-23). This means, conversely, that higher-speed transmission can be performed on the same transmission line (with a constant delay time difference).

Although the phase transition φ can be freely selected as described above, it should be determined by a trade-off between the transmission bandwidth (bandwidth limitation) and the error rate characteristic. In other words, φ is 180 from the error rate characteristic.
The closer to ゜, the better. Because 4 or 6 formula,
The smaller the correlation with Equation 5, the greater the diversity effect, but when φ is 180 °, the sign of the third term of Equation 5 is the opposite sign to that of Equation 4 or Equation 6 and shows the minimum correlation value. This is also the case (similarly, IEICE technical research report
See SAT86-23). On the other hand, the larger the phase transition φ, the more significant the characteristic deterioration due to the band limitation.

That is, the digital signal transmission method of the present invention using the transmission signal having the phase transition as shown in FIG. 1 is T ₁ , T ₂ ,
Φ, θ, and n can be freely selected, and regardless of the difference in their respective values, by combining two types of effective detection outputs that are different from each other, a kind of diversity effect can be used in the conventional multipath transmission line. The code error rate characteristic is remarkably improved as compared with the above method, and high-speed digital transmission becomes possible.

EFFECTS OF THE INVENTION As described above, according to the present invention, one time slot of data is divided into a first half portion and a second half portion at a predetermined ratio, and an advancing direction or a lag phase is alternately provided in adjacent time slots between the first half portion and the latter half portion. By using, as a transmission signal, a signal that has a phase transition of a predetermined angle in a direction and has information to be transmitted in the phase difference between the first half part and the second half part after a predetermined even time slot, respectively, as a transmission signal. High-speed digital transmission becomes possible in the path transmission line compared to the conventional one.

In particular, the present invention makes it possible to arbitrarily set the magnitude of the phase transition of a predetermined angle within a time slot and the position within the time slot represented by a predetermined ratio, which contributes to the improvement of characteristics in a multipath transmission line. The optimum parameters can be set according to other constraint conditions such as transmission path conditions. Furthermore, in addition to the multi-phase information phase, higher speed and higher quality can be achieved. Transmission becomes possible.

[Brief description of drawings]

FIG. 1 is a phase transition diagram of a transmission signal of the digital signal transmission method of the present invention, FIGS. 2 to 4 are phase transition diagrams of an example of the transmission signal, and FIG. 5 is a transmission signal shown in FIG. FIG. 6 and FIG. 7 are block diagrams of an example of a corresponding detection circuit for explaining that the digital signal transmission method of the present invention is resistant to multipath distortion, and shows a waveform diagram of a detection output signal and a composite phase of multipath waves. The vector diagram shown in FIG. 8 is a phase transition diagram of the transmission signal of the conventional digital signal transmission method, and FIGS. 9 and 10 are the detection output signals for explaining that the conventional digital signal transmission method is vulnerable to multipath distortion. It is a vector diagram which shows a waveform diagram and the synthetic | combination phase of a multipath wave. 1 ... input terminal, 2 ... multiplier, 3 ... 2 time slot delay device, 4 ... low pass filter, 5 ... detection output terminal.

Claims (7)

[Claims] 1. A transmission device for transmitting digital data, wherein one time slot of data is divided into a first half portion and a second half portion at a predetermined ratio, and the first half portion and the latter half portion are alternately arranged in adjacent time slots. A transmission signal having a phase transition of a predetermined angle in the advance direction or the lag direction and having information to be transmitted in a phase difference between the first half part and the second half part after a predetermined even time slot, respectively. And a delay line capable of delaying the signal by the predetermined even time slot is used for detection by differential detection. 2. The digital signal transmission method according to claim 1, wherein the predetermined ratio is 1: 1. 3. A digital signal transmission method according to claim 1, wherein the predetermined ratio is different in the first half and the second half. 4. The digital signal transmission method according to any one of claims (1) to (3), wherein the phase difference is 0 ° and 180 °. 5. The digital signal transmission according to any one of claims (1) to (3), wherein the phase difference is 0 °, 90 °, 180 °, 270 °. Method. 6. The digital signal transmission method according to claim 1, wherein the phase difference is any one of 360 ° divided into eight angles. . 7. The digital signal transmission method according to claim 1, wherein the phase difference is one of angles obtained by dividing 360 ° into 16 parts. .