JPH04345328A - Line changeover control circuit - Google Patents

Abstract

PURPOSE:To select the detection of signal quality deterioration due to fading in a shorter time in a digital radio system than that of a conventional system. CONSTITUTION:Noise level detection circuits 12a, 12b detect a noise level from each line from a recovered carrier obtained at plural demodulation circuits 11a, 11b. A comparator circuit 13 outputs a signal in which a noise level represents a minimum channel. A selection means 14 selects a demodulation signal of a channel with a minimum noise level from each output demodulation signal of the demodulation circuits 11a, 11b to switch the line.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a line switching control circuit, and more particularly to a line switching control circuit for switching radio lines in order to reduce fading in a digital radio system.

[0002] In recent years, development of digital wireless systems using microwave bands has been actively progressing, but fading that occurs in micro lines causes waveform distortion and degrades line quality, so there is a demand for more sophisticated systems. Accordingly, various fading compensation techniques have been developed. Such fading compensation techniques include: ■ space diversity (SD), which combines received waves and interference waves in such a way as to maximize received power or eliminate interference waves;
It has a variable resonator that can obtain a frequency characteristic that is completely opposite to the distortion in the frequency characteristic caused by fading, and the resonance frequency and selectivity of the variable resonator are varied to eliminate the distortion in the frequency characteristic caused by fading. Variable resonant automatic equalizer (R EQL) that performs equalization, ■ Transversal automatic equalizer (T E
QL) etc.

However, none of these fading compensation techniques could sufficiently compensate for the distortion in frequency characteristics caused by fading. A method has been developed. This frequency diversity method is a method in which the same information signal is simultaneously transmitted in parallel using different carrier frequencies, and the receiving side switches to a line with a smaller error rate to receive the signal.

Therefore, in this frequency diversity system, the relationship between the rate of deterioration of the error rate due to fading in the multiplex radio equipment on the receiving side and the line switching rate is important.

[0005]

2. Description of the Related Art FIG. 5 shows an example of a conventional line switching sequence. In the figure, it takes time t1 to detect that the bit error rate (BER), which is a line evaluation standard due to fading, has fallen below a predetermined value in the reception system of a digital radio system using a microwave band, and to determine a failure. .

[0006] Here, the format of the information signal transmitted and received in the above digital wireless system is shown in FIG. 6(A). In the same figure (A), B is a switching control bit, F is a frame synchronization bit, C is a dummy bit determination bit,
PH is a radio hop parity bit, PR is a radio route parity bit, and the other parts are information bits.

Furthermore, 88 bits starting from switching control bit B or dummy bit determination bit C constitute one subframe, and one multiframe is constituted by 12 subframes. Furthermore, there is one radio hop parity bit PH in every four subframes.

The information signal of the above format is 34.3
It is assumed that the digital information data is transmitted at 68 MB/s and is subjected to quadrature phase shift keying (QPSK),
Also, if the line switching threshold is set to an error rate of 1×10-4,
1/(34.368 x 106) = 2.9097 x 10
-8(s)2.9097×10-8×1×104 =0
．． 291 (ms) 0.291 × 2 = 0.5819 (
ms), so if one wireless hop parity bit PH is erroneous within 0.5819 ms from the first point in time of one multiframe shown in FIG. 6(B), the error rate will be 1×10 −4 . In addition, in the above formula, “2” multiplied by 0.291 is I
This means two signals: a signal and a Q signal.

However, in order to maintain the reliability of wireless equipment, conventionally the line is switched if 10 wireless hop parity bits are incorrect. Therefore, the time t1 shown in FIG. 5 is 5.
819 ms (=0.5819 ms×10).

When the receiving system determines that the above-mentioned BER has become below a predetermined value, it sends out a driving command for the receiving switch after confirming the normal operation of the backup channel (the time required for this is assumed to be t2). This drives the reception switch in the reception system and completes line switching to the backup channel (
The time required for this is assumed to be t3). Therefore, conventionally, the receiving system takes time T (=t1 +t2 +t3) from when it detects that the bit error rate has fallen below a predetermined value until it actually completes line switching to the protection channel.

[0011]

However, since the conventional line switching has a complicated switching sequence as described above, it takes several tens of milliseconds to complete the line switching. This time T poses no problem when switching if the radio propagation path is relatively stable, but depending on the fading frequency (speed), errors exceeding the line standard may occur when switching lines.

[0012] In other words, fading is a phenomenon that occurs when multiple radio waves (direct waves and interference waves) received by the same antenna through different propagation paths interfere with each other, so the received power is not added at a certain frequency. , is subtracted at other frequencies, resulting in a frequency characteristic as shown in FIG. In this case, at frequency f1, the bit error rate indicated by II is 10-4, which is greater than the bit error rate 10-4 of the line standard indicated by I.
yields 3. Therefore, when receiving a signal in the band including frequency f1, line switching information begins to be detected with a bit error rate of 10-4, and within the time from detection until line switching control is completed, the bit error rate exceeds 10-3. As a result, more errors than the line standard occur when switching lines.

[0013] The present invention has been made in view of the above points.
It is an object of the present invention to provide a line switching control circuit that solves the above problems by detecting signal quality deterioration due to fading in a shorter time than before.

[0014]

Means for Solving the Problems FIG. 1(A) shows a diagram of the principle configuration of the invention according to claim 1. In the figure, a plurality of demodulation circuits 11a and 11b receive received signals of a frequency diversity system that transmits signals obtained by modulating the same information data at different carrier frequencies on a plurality of channels, and demodulate the received signals for each channel.

The noise level detection circuits 12a and 12b receive the reproduced carriers extracted through the demodulation circuits 11a and 11b as input signals, and detect the noise levels near the reproduced carriers, respectively. The comparison circuit 13 compares each output detection signal of the noise level detection circuits 12a and 12b, and outputs a signal indicating the channel with the minimum noise level.

Based on the output signal of the comparator circuit 13, the selection means 14 selects the demodulated signal of the channel with the minimum noise level from among the demodulated signals output from the demodulating circuits 11a and 11b, and switches the line.

[0017]

[Operation] When the reproduced carrier extracted from the demodulation circuits 11a and 11b is demodulated from the carrier of the received signal that is not affected by fading (at the time of normal signal), the reproduced carrier is as shown in FIG. 1(B).
As shown by the solid line, the noise level is large at the reproduced carrier frequency fC, and the noise at frequencies around fC is extremely small. On the other hand, in the case of a reproduced carrier demodulated from a received signal affected by fading, the noise level near the reproduced carrier frequency fC becomes considerably large, as shown by the broken line in FIG. 1(B). That is, the noise level near the reproduced carrier frequency is proportional to the magnitude of fading.

Therefore, the present invention focuses on the above points, and the comparison circuit 13 constantly compares the noise levels near the reproduced carrier of each channel, and the selection means 14 selects the demodulated signal of the channel with the lowest noise level. . As a result, it is possible to always switch to the channel with the least influence of fading in a shorter time than in the past.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows a block diagram of one embodiment of the present invention. This embodiment is an example in which the present embodiment is applied to a receiving device that receives a transmission signal consisting of QPSK-modulated data in the format shown in FIG. It is.

[0020] The transmission signal of the first channel (plot channel) of the first carrier frequency of the 6 GHz band;
The transmission signal of the second channel (main channel) of the second carrier frequency in the GHz band is a signal in which data with the same information content is transmitted in parallel, received by a single antenna of the receiving device of this embodiment, and then The frequency-selected high-frequency received signal of the first channel is input to the RRF section 22a via the terminal 21a, and the high-frequency received signal of the second channel is inputted to the RRF section 22b via the terminal 21b.

The RRF sections 22a and 22b each convert the input high frequency received signal into an intermediate frequency signal (for example, a frequency of 70 MHz).
IF signals) and input them to demodulation circuits (DEM) 24a, 24b through IF amplifiers 23a, 23b. The demodulation circuits (DEM) 24a and 24b correspond to the above-mentioned demodulation circuits 11a and 11b, respectively, and demodulate the 70 MHz intermediate frequency IF signal to obtain the original data, while also generating a reproduced carrier.

FIG. 3 shows a block diagram of one embodiment of the demodulation circuit 24a (24b). In the figure, an intermediate frequency signal input to an input terminal 31 is branched into two by a hybrid circuit 32 and input to mixers 33 and 34, respectively. On the other hand, 70 taken out from the voltage controlled oscillator (VCO) 35
The reproduced carrier near MHz is branched into two by the hybrid circuit 36 and phase-shifted so as to have a phase difference of 90 degrees, one of which is input to the mixer 33 and the other to the mixer 34 .

As a result, the demodulated signal of the in-phase signal I is taken out from the mixer 33, and the demodulated signal of the orthogonal signal Q is taken out from the mixer 34. These in-phase signal I and quadrature signal Q
The demodulated signals are respectively input to the baseband multiplier 39 through amplifiers 37 and 38 and multiplied, and then converted into a synchronous signal that is at a high level when synchronized and an asynchronous signal that is at a high level when not synchronized. Frequency converted.

The synchronization signal is also applied as a control voltage to the VCO 35 through an amplifier 40 and a low-pass filter 41, respectively, to synchronize the phase of its output oscillation signal with the carrier frequency of the input intermediate frequency signal. That is, each circuit 3 above
3 to 41 constitute a phase locked loop circuit (PLL circuit).

On the other hand, the output signal of the mixer 42 is sent to the amplifier 43.
The signals are input to a low-pass filter 44 and a switch circuit 45 through the filter. The switch circuit 45 is taken out from the baseband multiplier 39, and an asynchronous signal amplified by an amplifier 46 is applied as a switching signal to turn the circuit on when it is asynchronous and off when it is synchronous. As a result, the switch circuit 45 short-circuits the low-pass filter 44 during non-synchronization, and uses the signal from the amplifier 43 as a control voltage for AFC (automatic frequency control) to control the VCO3.
5 to quickly roughly adjust the output oscillation frequency of the VCO 35 to around a predetermined frequency (70 MHz). In addition, V
When the PLL circuit is synchronized, an AFC control voltage is applied to the CO 35 from the low-pass filter 44, and its output oscillation frequency is controlled. As a result, a reproduced carrier signal whose phase is synchronized with the carrier frequency of the input intermediate frequency signal is extracted from the VCO 35.

Further, from the baseband multiplier 39, signals of frequencies (approximately 17 MHz) related to the respective transmission speeds of the orthogonal signal Q and the in-phase signal I are taken out, unnecessary frequency components are removed by the bandpass filter 47, and After the waveform is shaped into a rectangular wave by the comparator 48, the comparator 4
The signal is input to the clock terminal 9 and the digital processing section 50.

The comparator 49 receives the above-mentioned orthogonal signal Q.
A demodulated signal of the in-phase signal I is also input, and the demodulated signal whose waveform is shaped according to the input clock is sent to the digital processing unit 5.
Enter 0. The digital processing section 50 outputs the signal from the amplifier 46 as it is as a demodulation circuit alarm signal, and also outputs demodulated data.

Demodulation circuit 24a, 2 having the above configuration and operation
Of the demodulated data and reproduced carrier taken out from 4b, the reproduced carrier is supplied to noise level detection circuits (DET) 25a and 25b, as shown in FIG. 2, and the demodulated data is inputted to the reception switch circuit 27.

The noise level detection circuits (DET) 25a and 25b correspond to the above-mentioned noise level detection circuits 12a and 12b, respectively, and have the same configuration, for example, as shown in the block diagram of FIG. 4. In FIG. 4, the 70 MHz reproduced carrier is input to the input terminal 61. This reproduced carrier is amplified by an amplifier 62 and then input to a mixer 63, where it is input to a 70MHz oscillator 64.
The frequency is converted to 70MHz from

The output signal of the mixer 63 is passed through a low-pass filter 65.
Only low frequencies are extracted. Here, while the frequency of the reproduced carrier fluctuates by an amount corresponding to the noise level, the 70MHz from the 70MHz oscillator 64 is a noise-free frequency with extremely high frequency stability, so both of these signals are mixed by the mixer 63. By passing the frequency-converted signal through the low-pass filter 65, the noise superimposed on the reproduced carrier is extracted from the low-pass filter 65 in the baseband band.

This noise is input to the diode detector 67 through the amplifier 66, where it is level detected and then
The noise level detection signal is outputted from an output terminal 69 through a DC amplifier 68.

The noise level detection signals from the noise level detection circuits 25a and 25b having such a configuration and operation are as follows.
As shown in FIG. 2, the signals are input to the comparison circuit 26, where the levels are compared. The comparison circuit 26 corresponds to the comparison circuit 13 and has a comparator configuration using an operational amplifier, and the first noise level detection signal from the noise level detection circuit 25a is higher than the second noise level detection signal from the noise level detection circuit 25b. When the level is higher than the noise level detection signal, for example, a low level comparison signal is output, and when the level relationship is opposite to the above, a high level comparison signal is output.

The above comparison signal is applied as a switching signal to the receiving switch circuit 27, and when the comparison signal is at a low level, the demodulated data from the demodulating circuit 24b is selectively outputted from the receiving switch circuit 27, and when the comparison signal is at a high level. In this case, the demodulated data of the demodulation circuit 24a is selectively outputted from the reception switch circuit 27. Therefore, from among the demodulated data of the received signals of the first and second channels, the receiving switch circuit 27 always selects and outputs the demodulated data of the received signal of the channel with the smaller noise level, that is, the channel that is least affected by fading. (In other words, the micro line will be switched).

Therefore, according to this embodiment, line switching is performed at the level shown in III of the fading characteristics shown in FIG. Since line switching can be performed earlier by the time t1, high-quality reception can be achieved without being affected by fading at a higher rate than in the past.

It should be noted that the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications such as data modulation methods, transmission speeds, signal formats, etc. can be considered.

[0036]

As described above, according to the present invention, it is possible to switch to the line of the channel least affected by fading in a shorter time than before. This system has the advantage of being able to switch to another line without being affected by high-speed fading, which sometimes occurs, and thus allowing higher quality reception than before.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the principle of the present invention.

FIG. 2 is a block diagram of one embodiment of the invention.

FIG. 3 is a block diagram of one embodiment of a demodulation circuit.

FIG. 4 is a block diagram of one embodiment of a noise level detection circuit.

FIG. 5 is a diagram showing an example of a conventional line switching sequence.

FIG. 6 is an explanatory diagram of a conventional transmission signal format, etc.

FIG. 7 is a diagram showing the relationship between line switching and fading.

[Explanation of symbols]

11a, 11b Demodulation circuit 12a, 12b Noise level detection circuit 13 Comparison circuit 14 Selection means

Claims (3)

[Claims] [Claim 1] The same information data on multiple channels,
In a frequency diversity digital wireless system that modulates and transmits signals using different carrier frequencies, a plurality of demodulation circuits (11a, 11b) that separately demodulate signals received for each of the plurality of channels;
A plurality of noise level detection circuits (12a, 11b) each receive, as an input signal, the reproduced carriers of the received signals of the plurality of channels extracted through the demodulation circuits (11a, 11b), and respectively detect noise levels in the vicinity of the reproduced carriers.
2b) and the noise level detection circuit (12a, 12b)
A comparison circuit (13) that compares each output detection signal of the plurality of demodulation circuits (13) and outputs a signal indicating the channel with the lowest noise level;
11a, 11b), selecting means (14) for selecting the demodulated signal of the channel with the lowest noise level from among the output demodulated signals of the output demodulated signals (11a, 11b) to switch the line. 2. The noise level detection circuit (12a,
12b) is an oscillator (64) that oscillates and outputs the same frequency as the intermediate frequency, and a reproduced carrier of the intermediate frequency of the received signals of the plurality of channels from the demodulation circuit (11a, 11b) and the output of the oscillator (64). a frequency conversion circuit (63, 65) that converts the oscillation frequency and extracts baseband noise;
2. The line switching control circuit according to claim 1, further comprising a detection circuit (67) for detecting the level of the output noise of step 5). 3. The line switching control circuit according to claim 1, wherein the received signals of the plurality of channels are signals obtained by quadrature phase modulating information data at a carrier frequency in a microwave band.