JPH0740687B2 - 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 system for transmitting digital signals in a multipath transmission line such as wireless transmission in urban areas.

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. 8 shows a phase transition of a transmission signal in the conventional digital signal transmission system. 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 BPSK (Binary Phase Shift Keying). 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. The wave that precedes in time is called a direct wave, and the wave that lags behind is called a delayed wave.

FIG. 9 is a diagram for explaining what a detection output signal looks like when a transmission signal as shown in FIG. 8 is subjected to delay detection under a two-wave multipath. FIG. 9 (a) shows
This shows the phase transition of the direct wave. On the contrary,
The phase transition of the delayed wave delayed by the propagation delay time difference τ is
It becomes like FIG. 9 (b). The detection output at a certain time point is the 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 product of the wave synthesis phase and that of B.

FIG. 10 illustrates the combined phase of the direct wave and the delayed wave in order to obtain the detection output at each time point A to C.
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 system of the present invention divides one time slot of data into a first half part and a second half part at a predetermined ratio,
Transmits a signal that has a phase transition between the first half part and the second half part in a certain direction by a predetermined angle without fail, and is transmitted in a phase difference between the first half part and the second half part after a predetermined time slot. It is used as a signal.

Operation 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 system according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a phase transition of a transmission signal of the digital signal transmission system of the present invention. The transmission signal of the digital signal transmission system 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 phase axis of FIG. 1 may be positive or advanced, and the transition direction and the transition amount of this phase transition φ are constant in all time slots. The ratio of T ₁ and T ₂ and φ may be arbitrary. Of course, T ₁ T ₂ may be equal.

Although the phase difference between the first half part of a certain time slot and the first half part after n time slots and the phase difference between the latter half parts are equal, this phase shift amount is indicated by θ.
Digital information is transmitted by the value of θ. For example, if 0 ° and 180 ° are used as possible values of θ, 1-bit information is transmitted by assigning 0 and 1 correspondingly. Also, as θ, 0
By using °, 90 °, 180 °, and 270 °, 2-bit information is transmitted. Furthermore, as the value of θ, 0 °, 45 °, 90 ° ...
... 8 phases, similarly 0 °, 22.5 °, 45 °, 67.5 ° ... 16 phases such as a power of 2 polyphase, and a non-power of 2 multiphase, and θ The interval between possible values may not be constant, and the value of θ may be any value as long as the value corresponds to the information to be transmitted. The value of n may be any number 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 °. .

Fig. 3 shows that when n = 1 and φ = 90 °, θ = 0 °, 90 °, 180 °,
An example of phase transition of a transmission signal for transmitting 2-bit data for one time slot corresponding to 270 ° is shown.

FIG. 4 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 = 180 °. .

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, n = 1, φ = 180 as an example of the phase transition of the transmission signal of the digital signal transmission system of the present invention.
.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 phase shift keying (DPSK), it is detected by differential detection using a delay line of n time slots. In the example of the transmission signal shown in FIG. 4,
It is detected by differential detection using a delay line of one time slot. 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 1 time slot delay device, 4 is a low pass filter, 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. 4 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 the 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 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. 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: Indefinite B: 1 + ρ ² + 2ρcosα ・ ・ ・ C …… 1 + ρ ² -2ρcosα ・ ・ ・ D …… 1 + ρ ² + 2ρcosα ・ ・ ・ E: Indefinite In sections A and E, depending on the data values of the time slots before and after, respectively. It becomes indefinite. 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 mentioned above, section B
Since D and section C produce complementary detection outputs, the eye does not close. 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. 4, n = 1, φ
Although the transmission signal when = 180 °, 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 one time slot delay unit 3 in FIG. 5 is set to n time slots, the above description is exactly the same. When φ is not 180 °,
The formula, the formula, and the formula corresponding to the formula are B …… 1 + ρ ² + 2ρcosα …… C …… 1 + ρ ² + 2ρcos (α ± φ) …… D …… 1 + ρ ² + 2ρcosα ……, and the sections 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. In addition, the compound of the formula is + 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. 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, except for the above, which 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 transmission transition φ can be freely selected as described above, its optimum value is 180 ° which was explained at the beginning when the error rate characteristic is taken into consideration. Because 4 formulas or 6
The smaller the correlation between formula and formula 5, the greater the diversity effect, but when φ is 180 °, the sign of the third term of formula 5 is 4
This is because it has the opposite sign to that of equation (6) or equation (6) and shows the minimum correlation value (also refer to IEICE Technical Report SAT86-23).

That is, the digital signal transmission system 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.

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 a phase transition is always performed in a certain direction between the first half portion and the second half portion by a predetermined angle.
By using a signal that has information to be transmitted in the phase difference between the first half part and the second half part after a predetermined time slot as the transmission signal, it is possible to perform digital transmission at higher speed than before in a multipath transmission line. become.

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 characteristic diagram showing phase transition of a transmission signal of a digital signal transmission system of the present invention, FIGS. 2 to 4 are characteristic diagrams showing phase transition of an example of the transmission signal, and FIG. 5 is FIG. 6 and 7 are characteristic diagrams for explaining that the digital signal transmission system of the present invention is resistant to multipath distortion, and FIG. 8 is a conventional configuration diagram of the detection circuit corresponding to the transmission signal shown in FIG. FIGS. 9 and 10 are characteristic diagrams showing the phase transition of the transmission signal of the digital signal transmission system, and FIGS. 9 and 10 are characteristic diagrams explaining that the conventional digital signal transmission system is weak against multipath distortion. 1 ... input terminal, 2 ... multiplier, 3 ... 1 time slot delay device, 4 ... low pass filter, 5 ... detection output terminal.

Claims (7)

[Claims] 1. In a transmission device for transmitting digital data, one time slot of data is divided into a first half portion and a second half portion at a predetermined ratio, and a predetermined angle is always provided between the first half portion and the second half portion in a certain direction. A phase transition, and using a transmission signal having information to be transmitted in a phase difference between the first half portion and the second half portion after a predetermined time slot, and delaying the signal by the time slot of the signal. A digital signal transmission method characterized by being detected by differential detection using a delay line that can be generated. 2. The digital signal transmission method according to claim 1, wherein the predetermined ratio is 1: 1. 3. The 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 according to claim 1, 2 or 3, wherein the phase difference is 0 ° and 180 °. 5. The digital signal transmission method according to claim 1, 2, or 3, wherein the phase difference is 0 °, 90 °, 180 °, 270 °. 6. The phase difference according to claim 1, wherein the phase difference is one of 360 degrees divided into eight.
The method of transmitting a digital signal according to item 3 or item 3. 7. The phase difference is any one of angles obtained by dividing 360 ° into 16 parts.
The method of transmitting a digital signal according to item 3 or item 3.