JP2572765B2 - Transmission path diversity transmission method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in wireless communication in which transmission characteristics are significantly degraded due to fading.

[Conventional technology]

Generally, a plurality of transmission paths are formed in a wireless section of a wireless transmission system, and can be treated as a multipath transmission system.

FIG. 1 shows an outline of a system for performing digital transmission in such a system.

In FIG. 1, first, a certain code sequence is input from an input terminal 1 to a modulator 2. The modulator output is radiated from transmission point 3 into space. The radiated wave arrives at the receiving point 4 through two paths with propagation times τ ₁ and τ ₂ . The composite wave at the receiving point 4 is amplified by the receiver 5 and detected by the detector 6. The output of the detector 6 is identified by the determiner 7, and the same code sequence as the input is reproduced.

In such a transmission system, if the difference between the propagation times of τ ₁ and τ ₂ , Δτ = | τ ₂ −τ ₁ |, is less than one time slot of the modulated digital signal, the composite waves interfere with each other and receive. The term “one time slot of the digital signal” refers to a time width corresponding to a generation interval of a modulation basic pulse for generating a digital modulation wave, and the description of the time slot in this specification is not particularly specified. Unless otherwise indicated, it shall refer to the above time span.

Although FIG. 1 shows the synthesis of two waves, interference of three or more waves frequently occurs in a general transmission line, and is treated as a multiplex wave. Even when these multiplexed waves are combined, a large level fluctuation occurs as in the case of two waves. FIG. 2 shows the state of the fluctuation. Under such a level fluctuation, when the level is greatly reduced, a large number of transmission errors occur in bursts, and the transmission characteristics deteriorate.

[Problems to be solved by the invention]

Conventionally, in order to suppress the deterioration of the transmission characteristics as described above,
Diversity schemes have been considered.

For example, if two antennas are provided on the receiving side and demodulation is performed based on the antenna output having a high reception level, the transmission error is greatly improved.

However, this method requires two antennas as a configuration on the receiving side. For this reason, there is a disadvantage that it is difficult to apply to mobile communication or the like that requires a simple configuration on the receiving side.

As another method, there is known a transmission diversity system in which two antennas are installed on the transmitting side at a distance from each other, and the same modulated wave is transmitted to each antenna with only the center frequency intentionally offset. .

This method makes use of the fact that when two signals are synchronized with each other in a timing clock cycle, they become quadrature signals, and the detection output is the detection waveform of each wave weighted by the two waves. It becomes a composition. For this reason, a high-level signal dominate the detection characteristics, so that the transmission characteristics are improved.

However, this method has a drawback that it requires about twice the transmission band to form the orthogonal signal.

An object of the present invention is to provide a transmission diversity which solves such a drawback that the transmission band is wider than the signal band.

[Means for solving the problem]

According to the present invention, the above objects are achieved by the means as set forth in the claims.

That is, the present invention forms a diversity transmission system as follows.

(I) Transmit the modulated wave from the first transmitting antenna. (I
i) A delayed modulated wave obtained by delaying the modulated digital signal by one or more time slots is transmitted from the second transmitting antenna. (Iii) On the receiving side, a signal dispersed into a plurality of waves is received by one antenna while propagating through the multipath radio wave propagation path. (Iv) A multipath receiving device such as a waveform equalizer is installed on the receiving side, and a main wave component having the highest level among a plurality of waves included in the received wave is emphasized and extracted.

The present invention is different from the conventional technology in the following points.

(I) On the transmitting side, in addition to the directly modulated wave, a delayed modulated wave that is delayed by one or more time slots is generated and transmitted from the second transmitting antenna. In the conventional transmission diversity, the orthogonal signal transmitted from the second transmission antenna is obtained by offsetting the center frequency of the modulated wave to the same degree or broadening the band of the modulated wave almost twice. Bandwidth is almost doubled. In the present invention, such a spread of the transmission signal band does not occur.

(Ii) On the receiving side, after receiving with one antenna, the main wave component is emphasized and extracted using a multipath processing device such as a waveform equalizer. The difference from the conventional transmission diversity system is that the multipath processing device plays an essential role for obtaining the diversity effect.

〔Example〕

 FIG. 3 shows an embodiment of the present invention.

In the figure, first, a transmission code sequence is input from an input terminal 8.
Input to modulator 9, the modulation output of which is radiated directly from transmitting antenna 10. Agree, the modulation output is
Is converted into a delay modulator delayed by one or more time slots (Td), and radiated from the transmission antenna 12. As described above, the “time slot” indicates a time width corresponding to a generation interval of a modulation basic pulse for generating a digital modulation wave to be transmitted. Therefore, the modulation scheme PSK
Is 1 symbol length for BPSK, 1 bit length for BPSK, 2 bit length for QPSK, and 4 bit length for 16QAM.

These signal waves are received by the receiving antenna 13. The received signal wave is a combination of the one from the transmitting antenna 10 and the one from the transmitting antenna 12, and the transmitting antenna 10 is set so that the correlation between these two waves becomes almost zero at the receiving point.
Then, the transmission antennas 12 are appropriately spaced. The received composite wave is amplified by a receiver 14 having an automatic gain control (AGC) circuit. The reception wave is a combination of the signal from the transmission antenna 10 and the signal delayed by Td from the transmission antenna 12, but since each component is a multiplex wave as described above, the level greatly varies. . Therefore, the larger one is used as the main wave component.

The amplified signal is detected by the detector 15. If the detected waveform is only the main wave, it will be a waveform in which one modulated wave is normally detected, but in the general case where components other than the main wave are at a level that cannot be ignored, the detected waveform will be greatly distorted I have.

In order to obtain a normal detection waveform from a reception wave having such a waveform distortion, a multipath processing device is used in a reception system.

In this embodiment, a waveform equalizer is used as a multipath processing device.
16 is used. In this apparatus, components other than the main wave are canceled by using the main wave. Since the distortion of such an output of the waveform equalizer is largely suppressed, normal discrimination, that is, data reproduction can be performed by the determiner 17.

In this embodiment, the modulated wave is delayed. However, when it is difficult to delay the modulated wave by hardware,
As shown in FIG. 4, one of the transmission input codes from the input terminal 18 is delayed by a delay circuit 19 in advance, and the modulators 20 and 21 are shifted.
As shown in the above, by using two, the respective outputs are amplified by the transmission power amplifiers 22 and 23 and transmitted from the transmission antennas 24 and 25, whereby a delayed wave can be easily obtained.

 The transmission characteristics in the above-described transmission system will be described in detail.

As shown in FIG. 3, the multiplex wave from the transmitting antenna 10 is composed of two waves and requires propagation times of τ ₁ and τ ₂ , and the multiplex wave from the transmitting antenna 12 is also composed of two waves and is represented by τ ₃ and τ ₄ . Assume that a propagation time is required. | Τ ₁ −τ ₂ | and | τ ₃ −
When τ ₄ | is sufficiently smaller than Td and the average level of each wave is almost equal, the carrier phase fluctuates randomly,
The levels fluctuate greatly because they strengthen or cancel each other, and the combined wave of the delayed waves of τ ₁ and τ ₂ from the transmitting antenna 10 and the combined wave of the delayed waves of τ ₃ and τ ₄ from the antenna 12 are: Rayleigh fading waves are independent of each other.

From the superimposition of the two synthesized waves that fluctuate in this way, the larger one is extracted by the waveform equalizer. Therefore, the receiving system shown in FIG. 3 is a two-branch selection diversity system for selecting one wave from the Rayleigh facing wave.

However, when the level of the two synthesized waves fluctuates and the level difference reverses, the extracted component is switched to the other, so that the timing clock fluctuates by the delay amount Td. At this moment, if the conventional waveform equalizer is used as it is, the waveform equalization process becomes unstable and the demodulated data becomes discontinuous. Therefore, it is necessary to perform these processes simultaneously.

Next, when the transmission rate (baud rate) 1 / T is high, | τ ₁ −τ ₂ | and | τ ₃ −τ ₄ to reduce T
| May be substantially equal to or greater than T. At this time, the waveform equalizer processes the received wave component having the highest level among the four waves as the main wave.

Therefore, there is a four-branch selection diversity effect. When each component selected in this way is formed from a multiplex having a smaller delay difference, each component has the same distribution as Rayleigh fading.

However, when each component can be regarded as one wave that cannot be resolved any more, the level distribution of those components is often a log-normal distribution, and the fluctuation has a smaller dynamic range than Rayleigh fading and a faster fluctuation. It is also calm. In such a case, not only the processing of waveform equalization becomes easy but also the diversity effect becomes large.

However, this four-branch effect is | τ ₁ −τ ₂ |, |
τ ₃ −τ ₄ | needs to be greater than T,
This delay difference changes according to the state of the radio wave propagation path, and is a stochastic phenomenon.

Therefore, when Td is not delayed on the transmitting side, the number of branches fluctuates stochastically between one branch and four branches, and the transmission characteristics are not very stable.

However, when the transmission side delays Td as in the present invention, the transmission characteristic fluctuates between two and four branches, and the stability of transmission characteristics increases. In order to further suppress this deterioration, a method of further increasing the number of delayed waves having different delay amounts on the transmission side is also conceivable. In this case, since a more accurate waveform equalizer is required, it is necessary to consider the number of delay waves in consideration of the cost for the performance.

As described above, the multipath processing device has been described by taking the waveform equalizer as an example.However, the delay amount of each component is observed, the delay amount is adjusted, and each wave is synthesized, and the components other than the main component are relatively compared. The transmission path diversity method described above is also known, and the RAKE reception is widely used in the spread modulation method. The basic pulse generation interval T of the modulated wave is the chip interval of the spread modulation code. Therefore, one time slot in this case is the chip length.

Further, depending on the processing method of the multi-pass processing device, extracting one wave from two waves at the same level may require extremely high processing accuracy or may require time for convergence of the algorithm.

In such a case, it is conceivable to change the ratio of two waves obtained by adding a weighting circuit to the delayed wave generating circuit.

Note that the closer the correlation value of the received wave from each antenna is to 0, that is, the less the correlation, the greater the diversity effect described above.
The same effect can be expected even if it increases to about 0.7.

〔The invention's effect〕

As described above, the present invention is transmission diversity that does not substantially increase the signal transmission bandwidth, and thus can realize a transmission system with a high frequency utilization efficiency.

Also, the diversity effect can be obtained with a single antenna on the receiving side, and the multipath processing device can be easily integrated into an IC, so that the receiver can be easily downsized and simplified.

Therefore, in the field of mobile communication and broadcasting, there is an advantage that a receiver that is small, suitable for carrying, has good transmission characteristics, and can be easily realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional wireless transmission system, FIG. 2 is a diagram showing a level change at a receiving point during fading, FIG. 3 is a diagram showing an embodiment of the present invention, and FIG. It is a figure showing other examples. 1 ... input terminal, 2 ... modulator, 3 ... transmission point, 4 ...
Reception point, 5: Receiver, 6: Detector, 7: Judgment device,
8 ... input terminal, 9 ... modulator, 10, 12 ... transmission antenna, 11 ... delay circuit, 13 ... transmission antenna, 14 ... receiver, 15 ... detector, 16 ... waveform equalization Container, 17 …… determiner,
18 input terminal, 19 delay circuit, 20, 21 modulator, 2
2,23 …… Transmission power amplifier, 24,25 …… Transmission antenna

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-15341 (JP, A) JP-A-58-95446 (JP, A) JP-A-61-214634 (JP, A) 1988 Electronic Information Proceedings of the Communication Society Spring National Conference [Part B-1] B-724 (63-3-15) 475

Claims (1)

(57) [Claims] A transmitter radiates a modulated wave from a first transmitting antenna, radiates a delayed wave obtained by delaying the modulated wave by one time slot or more from a second transmitting antenna, and transmits a modulated wave to a receiving side. 1. A transmission path diversity transmission system comprising a multipath processing device that emphasizes and extracts a main wave component included in a transmission path diversity.