JPH0638596B2 - Digital communication system - Google Patents

Description

TECHNICAL FIELD The present invention relates to a digital communication system that is resistant to multiple wave interference.

[Prior Art] In the conventional digital communication system, for example, in a mobile communication system, one digital signal modulated by one digital information is often received as a multiple wave through different transmission paths. There is a problem that the code error rate is significantly deteriorated due to mutual interference of waves.

This is the conventional BPSK (Binary Phase Shift Keying)
The signals will be described below. Fig. 2 (A) shows one digital information represented by binary information symbols "1" and "0", and Fig. 2 (B) shows the phase change of the BPSK signal obtained corresponding to this digital information. FIG. In other words, the BPSK signal in this case carries information on the change in phase. When performing delay demodulation, the phase of the BPSK signal changes by + π radian for binary information symbol “1” and remains unchanged for “0”. FIG. 3 shows a delay demodulation circuit for demodulating the BPSK signal and extracting the original digital information. I
The BPSK signal supplied to the N terminal (1) is divided into two, one to one input terminal of the multiplication circuit (2) and the other to the delay circuit (3) having a delay time of 1 time slot (T). Through the multiplication circuit
It is added to the other input terminal of (2), multiplied by the multiplication circuit (2), and then the LPF (4) removes the frequency component of twice the carrier frequency to obtain the original digital information, and the OUT terminal (5) Is output to. The above operation is well known, and as an example, "Digital Mobile Communication Technology" (Japan Industrial Technology Center) No. 2, 3,
2 (a) Differential detection section.

The BPSK signal supplied to the IN terminal (1) at this point is the first BPSK signal wave (hereinafter referred to as D wave) generated by the same digital information, and the arrival time is delayed by τ from this D wave. In the case of a signal in which the second BPSK wave (hereinafter referred to as the U wave) is overlapped (that is, multiple wave), the D wave is in the vicinity of the relative amplitude ratio δ of the U wave, and the D wave. D near the input terminal (1) when the phase difference φ between
The amplitude of the combined wave and U wave signal becomes smaller and the horizontal fluctuation becomes larger. As a result, the amplitude of the OUT terminal (5) becomes extremely small, and there is a problem that the code error rate is extremely poor. This will be described below. FIG. 4 is a diagram for explaining the temporal relationship between the D wave and the U wave. Here, T is the length of the time slot for transmitting one digital symbol of digital information. Section a is a section from the rise of the D wave to the rise of the U wave, section b is a section from the rise of the U wave to the point T / 2 of the D wave, and section c is a segment from the point T / 2 of the D wave to the U.
The section up to the point T / 2 of the wave, section d is the section from the point T / 2 of the U wave to the point T of the D wave.

The signal e (t) obtained at the OUT terminal (5) in each section is as follows.

In section a; In sections b, c, d; Here, δ: Relative amplitude ratio of U wave to D wave φ: Carrier wave phase difference between D wave and U As can be seen from this calculation, the signal e obtained at the OUT terminal (5) e
(t) depends on the symbol of the preceding bit in the section a, and δ
It is uncertain because the polarity is inverted depending on the value of, but in the sections b, c, and d, the polarity is uniquely determined corresponding to its own symbol.

[Problems to be solved by the invention]

However, as can be seen from the first equation (B), the section b,
The values of e (t) ^{in c and d,} e (t) ^{b, c, and d,} are almost zero when δ is near 1 and φ is near π, so the eye of the so-called eye pattern disappears. However, horizontal fluctuation (time fluctuation) of the detection signal is also generated.

That is, in multi-wave propagation, there is a problem that the code error rate becomes very poor near δ = 1 and φ = π.

The present invention was made in order to solve the problem of the deterioration of the code error rate as described above, and compared with a normal PSK modulation system such as the BPSK system, a digital code having an improved code error rate in multi-wave propagation. The purpose is to obtain a communication method.

[Means for solving problems]

As an example of the signal of the digital communication system according to the present invention, as shown in FIG. 1, the signal level in the latter half of each time slot is effectively set to zero. Hereinafter, this modulation method is referred to as a BPSK-RZ modulation method.

[Action]

In this way, by making the signal level in the latter half of each time slot substantially zero, e (t) ^{b, c and d} are δ = 1 and φ = π.
Even in the vicinity of, since the combined value of the D wave and the U wave can be maintained at a predetermined value without approaching zero,
The error rate near δ = 1 and φ = π can be significantly improved.

Example of Invention

FIG. 1 (C) shows an example of the BPSK-RZ signal according to the present invention. B
The PSK-RZ signal includes an ordinary BPSK signal shown in Fig. 1 (a) and an ON-OFF signal which is "1" only in the first half T / 2 section of each time slot shown in Fig. 1 (b). It is obtained by multiplication of.
That is, the BPSK-RZ signal is T / 2 in the first half of each time slot.
The section of has the same amplitude and phase as a normal BPSK signal, and the remaining T / 2 section is a signal in which the carrier amplitude is effectively zero.

When the multiplexed wave of the BPSK-RZ signal is reproduced by the delay demodulator shown in FIG. 3, the signal e (t) obtained at the OUT terminal (5) is given by the following equation.

For τ / T <0.5; For 1> τ / T>0.5; As can be seen from the above calculation, in the case of τ / T <0.5, the phase difference between the D wave and the U wave in the section a and the section c, and in the case of 1> τ / T <0.5, the phase difference between the D wave and the U wave in the section b and the section d. Regardless of how you use the normal BPSK
The problem of disappearing the so-called eye pattern and increasing the horizontal fluctuation, which has been a problem in the modulation method, is eliminated.

If there is no U wave and no multiple wave interference, then δ = 0, so that both sections 2 and 3 are represented by sections a and b. Then Section c, d Then, 2e (t) = 0, and the BPSK-RZ signal can be detected normally. Therefore, according to the present invention, PSK signals can be correctly communicated even in a section where there is no multiple wave interference. 5 (A) and 5 (B) are diagrams showing an example of characteristics of the BPSK-RZ modulation method. FIG. 5 (A) shows the change in bit error rate when the SN ratio is changed, and FIG.
(B) shows a change in bit error rate with respect to a change in the delay time difference between the D wave and the U wave at a constant SN ratio.

The details of each item are as follows.

P _D and P _U are changed by D-wave and U-wave co-ray distribution respectively.
Wave U wave average power, Eo is signal energy per bit, No is noise power per unit number of waves, and fd is maximum frequency of fading dip.

The characteristic of ordinary BPSK is shown by the curve BPSK in FIG. 5 (A). It can be seen that this method is considerably improved over the ordinary BPSK method.

If the length of the T / 2 section that makes the amplitude substantially zero deviates from T / 2, the characteristics will deteriorate compared to the ideal state, but still good characteristics can be obtained compared to the conventional BPSK method. It has been confirmed that the effects of the present invention are maintained.

So far, I have explained BPSK (two-phase modulation), but Q
Similar improvement can be obtained for PSK (four-phase modulation) by using the QPSK-RZ modulation method in which the amplitude in the latter half of one time slot is zero. Fig. 6 compares the characteristics of the BPSK-RZ modulation method and the QPSK-RZ modulation method, and shows the same improvement characteristics.

Note that the ON-OFF signal has a baseband filter (bandwidth
It has been confirmed that when the band is limited in Bb), the change in characteristics is extremely small as shown in FIG.

The same idea can be extended and applied not only to binary information symbols but also to multiphase modulation corresponding to multilevel information symbols. Also, the period of zero amplitude is set to T / 2 in the latter half of the time slot.
However, the same effect can be obtained if the amplitude is zero in any part of one time slot.

〔The invention's effect〕

As described above, according to the present invention, in various PSK modulation systems, the signal configuration is set so that the amplitude of any T / 2 section of one time slot is effectively zero. It is possible to provide a digital communication system with an improved code error rate by solving the problem that the demodulation output becomes extremely small regardless of the values of the amplitude ratio and the phase difference.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a (BPSK-RZ) signal according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of a normal BPSK modulation system, and FIG. 3 is a diagram showing a configuration example of a delay demodulation circuit, FIG. 4 is a diagram for explaining the relationship between the D wave and the U wave, and FIG. 5 is the BPSK according to the present invention.
FIG. 6 is a diagram showing an example of the characteristics of the −RZ modulation system, FIG. 6 is a diagram comparing the characteristics of the QPSK-RZ modulation system which is another embodiment of the present invention with the characteristics of the BPSK-RZ modulation system, and FIG. 7 is ON. It is a figure which shows the change of the error characteristic when the band of the -OFF signal is limited. Third
In the figure, (1) is an IN terminal, (2) is a multiplication circuit, (3) is a delay circuit, (4) is an LPF, and (5) is an OUT terminal. Further, among the symbols shown in the respective figures, the same symbols indicate the same contents or equivalent contents.

Front page continuation (72) Inventor ▲ Yoshi ▼ Susumu 10-7 Kohata, Hata, Uji-shi, Kyoto (72) Inventor Tsutomu Takeuchi 10-63 Yamanawa, Terado-cho, Hyuga-shi, Kyoto (72) Inventor Sirikyat Ali Yaui Shitakun, Sakyo-ku, Kyoto City, Kyoto Prefecture ▲ Kichi ▼ 4 Kaguraoka-cho, No. 3 Kurahashi Mansion 3 (56) Bibliographic Reference Sho 55-53347 (JP, U) US Patent 4229821 (US, A)

Claims (3)

[Claims] 1. A signal in which a phase of a carrier wave is changed corresponding to each symbol of digital information transmitted every one time slot is used, and a main transmission line and a signal substantially corresponding to the main transmission line are provided between transmission and reception. In a digital communication system in which sub-transmission lines having different delay times are interposed, the transmitting side has an amplitude and a phase determined in correspondence with the digital information in an arbitrary approximately T / 2 period of one time slot period (T). In the remaining period, a signal whose amplitude is controlled to be substantially zero is generated and transmitted, and the receiving side performs demodulation with a delay demodulation circuit including a delay circuit with a delay time T, A digital communication system characterized in that the deterioration of the code error rate caused by the interposition of a plurality of transmission lines having different delay times is improved. 2. A two-phase PSK (BPSK) signal is used as a signal in which the phase of a carrier wave changes corresponding to each symbol of digital information transmitted for each time slot. A digital communication system according to the first item of the range. 3. A four-phase PSK (QPSK) signal is used as a signal in which the phase of a carrier wave changes corresponding to each symbol of digital information sent for each time slot. A digital communication system according to the first item of the range.