JPS6359612B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a co-frequency interference detection circuit for detecting co-channel interference caused by radio waves emitted from other stations using the same frequency, for example in a digital mobile communication system.

Conventionally, in digital mobile communications, there has been no effective method for efficiently detecting co-channel interference, and either the assignment of co-channels is prohibited, or the co-channel interference is prohibited only in areas separated by a sufficient distance that the level of interference waves is sufficiently low. There was no countermeasure against interference other than to avoid interference by methods such as allowing channels to be reused.

In addition, as a method of detecting interference at the expense of transmission efficiency, a unique word is added as a station identification code to the signal sequence to be transmitted, and this is detected on the receiving side, thereby making it possible to receive the desired wave. There is a way to determine whether or not. However, with this method, (i) transmission efficiency decreases due to the addition of a station identification code, (ii) it becomes necessary to provide a frame structure to the transmitted signal sequence, which adds new circuits for frame synchronization, etc. iii) Correct interference detection is possible only when the desired signal to interference wave power ratio (CIR) is sufficiently lower than 1, that is, when the interference wave is stronger and the interference wave is correctly received. It was hot. In particular, regarding (iii), it means that interference cannot be detected when the CIR is close to 1.
This shows a major drawback of this method in that it is not possible to take any interference countermeasures while the CIR degradation is small.

In order to eliminate these drawbacks, this invention aims to eliminate the amplitude of the composite received wave from the fact that the desired wave and the interference wave are usually modulated with different signals, and the instantaneous phases of the carrier waves are different from each other. Taking advantage of the fact that fluctuating beats occur,
The device is designed to detect interference by detecting the bead, and the drawings will be described in detail below.

FIG. 1 is a diagram showing temporal changes in received signal amplitude. In mobile communications, the received signal amplitude varies greatly at frequencies from several Hz to several tens of Hz due to fading caused by multiple wave propagation. The broken line in the figure shows the amplitude fluctuation due to this fading. When interference waves of the same frequency exist, amplitude fluctuations at a frequency close to the modulation frequency, that is, the baseband signal frequency, are added to the interference waves. The solid line in the figure shows the amplitude fluctuation due to fading and interference. The present invention is based on the principle of detecting interference by extracting only the fluctuation component due to interference from among the amplitude fluctuation components.
Usually, the modulation frequency is sufficiently higher than the fading frequency, so the two fluctuation components can be separated, which will be explained using the following equation.

FSK (Frequency Shitt Keying) for simplicity
Let's talk about the modulation method. Desired wave is e ₁ (t), interference wave is e ₂ (t), each amplitude is A ₁ , A ₂ , each modulation signal sequence is a ₁ (x), a ₂ (x), carrier wave angular frequency ωc ,
The angular frequency deviation is Ω, and the carrier phase difference between the two waves is φ.
(t), the following equation holds true.

e ₁ (t)=A ₁ e _xp [J(Ω∫ ^t _-∞ a ₁ (x)dx +ωct)] e ₂ (t)=A ₂ e _xp [J(Ω∫ ^t _-∞ a ₂ (x) dx + ωct + φ(t))] j: Imaginary unit Since the received signal is a combination of the desired wave and the interference wave, the received voltage S(t) is S(t)=Re[e ₁ (t) + e ₂ ( t)] becomes. However, Re[*] represents the real part of *. The instantaneous received power W(t) is expressed by the following formula.

W(t)=S ² (t)=A ² ₁ +A ² ₂ +2A ₁ A ₂ cos (φ ₁ −φ ₂ −φ(t))φ ₁ =Ω∫ ^t _-∞ a ₁ (x)dx, φ ₂ = Ω∫ ^t _-∞ a ₂ (x) dx Since the change in A ₁ , A ₂ , φ(t) is about the same speed as the fading pitch, the change in φ ₁ , φ ₂ (in MSK, the change is ±π/2). Therefore, the instantaneous received power W can be normalized by the time average value of about a fraction of the fading pitch = A ² ₁ + A ² ₂ , However, Λ=A ² ₂ /A ² ₁ , that is, the desired wave to interference wave power ratio (CIR). From the above equation, it can be seen that by envelope-detecting W/ and removing the DC component, an output voltage V(t)=2√/(1+Λ) corresponding to CIR can be obtained. FIG. 2 is a diagram showing a power spectrum of normalized amplitude fluctuation when either the desired wave or the interference wave is modulated with all marks and the other with all spaces. At this time, W/ is expressed by the following formula.

Therefore, there are only the DC component (1) and the W=2Ω component. FIG. 3 is a diagram showing changes in v(t) with respect to CIR and Λ. As can be seen from the figure, the present invention has a characteristic that the voltage v(t) of the interference detection signal increases rapidly in a region where Λ is small, and therefore has a sensitive interference detection ability.

Figure 4 shows that when both the desired wave and the interference wave are modulated with a PN (pseudorandom) code, the power spectrum of the normalized amplitude fluctuation spreads as shown in this figure, but at a sufficiently high frequency compared to the fluctuation due to fading. Fluctuation components exist and interference detection is possible.

FIG. 5 shows an embodiment of the invention. The level detecting circuit in this embodiment is a square law detector 11, and the beat detecting circuit is a portion consisting of a high frequency converter 12, a low frequency converter 13, and a divider 14. The received signal applied to the input terminal 10 is passed through the square law detector 1
1, the received signal is converted into a signal 17 representing the instantaneous received power, that is, a signal representing the amplitude (level) of the received signal is obtained and applied to the high frequency converter 12 and the low frequency converter 13. The output of the high frequency converter 12 becomes a signal 18 (corresponding to W-) representing the instantaneous received power after removing both the direct components of the received power W and the fluctuation component (corresponding to) due to fading, and the low frequency converter 13
The outputs become (corresponding to) a signal 19 representing the average received power including a fluctuation component due to fading, and are respectively added to the divider 14. The output signal of the divider 14 is as shown in the following equation.

The output signal is converted into an envelope voltage by an envelope detector 15. The output of the envelope detector is as follows.

The above equation is nothing but the interference detection signal voltage V(t) corresponding to the above-mentioned CIR. Interference detection signal voltage V
(t) is output to the interference detection output terminal 16.

FIG. 6 shows another embodiment of the invention. The level detection circuit in this embodiment is a portion consisting of a logarithmic amplifier 20 and an envelope detector 21, and the beat detection circuit is a portion consisting of a high-frequency wave generator 12, a low-frequency wave generator 13, and a subtracter 22. In this embodiment, the square law detector 11 of FIG. 5 is replaced with a logarithmic amplifier 20 and an envelope detector 21 connected to its output side.
Also, the divider 14 is replaced with a subtracter 22, and the circuit is operated in dB values. In this embodiment, since the dB value of the average received power is also obtained at the same time, it is advantageous when used together with control that requires diversity equal level detection.

As an example of actually using the present invention, a method can be considered in which, when the voltage of the interference detection signal exceeds a specified level, it is determined that interference is occurring, and corresponding measures are taken. Further, although the above explanation has been made regarding FSK, beats due to same frequency interference occur also in digital communication systems using other modulation methods such as BPSK, QPSK, MSK, etc., so the interference detection circuit of the present invention is effective. take action. In this manner, the present invention detects the magnitude of co-frequency interference and the presence or absence of co-frequency interference. When only the presence or absence of interference is to be detected, only the presence or absence of the output of the high frequency amplifier 12 may be detected.

As explained above, the present invention utilizes the fact that when there is a co-frequency interference wave, the amplitude of the received signal fluctuates at a frequency close to the modulation frequency (baseband signal frequency), which is sufficiently high compared to the fading frequency. Because it is an interference detection circuit that detects, (i) an interference detection signal corresponding to the desired signal to interference signal power ratio (CIR) can be obtained without being affected by fluctuations in the reception level, and (ii) FSK, PSK, and MSK. can be applied to any digital angle modulation method such as (iii) has sensitive interference detection characteristics and operates extremely effectively even in a region with poor CIR (near 0 dB); (iv) has no interference with the modulated signal sequence. It has advantages such as not imposing any constraint conditions and not affecting transmission efficiency.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the temporal change in the amplitude of the received signal,
2 and 4 are diagrams each showing an example of the power spectrum of a received signal, and FIG. 3 is a diagram showing the relationship between the desired wave to interference wave power ratio and the interference detection output level,
FIGS. 5 and 6 are block diagrams showing embodiments of the present invention, respectively. 10...Received signal input terminal, 11...Square detector, 12...High frequency device, 13...Low frequency device,
14...Divider, 15...Envelope detector, 16...
...Interference detection output terminal, 17... Signal representing received power, 18... Signal representing instantaneous received power with average received power removed, 19... Signal representing average received power, 20... Logarithmic amplifier, 21... Envelope detector, 22... Subtractor, 23... Interference detection output terminal (dB value), 24... Output terminal for a signal representing average received power (dB value).

Claims (1)

[Claims] 1. In a receiving device used in digital communication that uses the angle modulation method, interference occurs due to co-frequency interference between the level detection circuit that detects the amplitude of the received signal and the signal output from the level detection circuit that indicates the received signal amplitude. What is claimed is: 1. A co-frequency interference detection circuit, comprising: a beat detection circuit that extracts fluctuating components of a frequency close to a baseband signal, and detects co-frequency interference.