JP3365783B2 - Burst detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a burst mode demodulator used in a TDMA (Time Division Multiple Access) satellite communication system or the like, and more particularly to a burst detector thereof.

[0002]

2. Description of the Related Art The TDMA system is a multiple access system in which a number of stations use the same carrier frequency for transmission so that signals do not overlap in time. Each communication device communicates with another station using the assigned time slot, and performs time position control (burst synchronization control) so that the signal sent to the time slot does not collide with other signals. Therefore, TDM
In method A, bursts must always be detected well.

Further, in the TDMA satellite communication system and the like, as a burst format, a format in which a preamble is added before data and the preamble is composed of an unmodulated portion, a 0π alternating pattern and a unique word (UW) has been used. When using this format, it is necessary to detect the carrier from the unmodulated portion.

[0004]

However, in the conventional burst detector, the received burst-like phase modulation signal is converted into the intermediate frequency or the base band, and the signal level is judged by the threshold value. Therefore, the carrier-to-noise ratio (C
When / N) is low, there is a problem that it is difficult to detect the carrier wave properly.

The present invention has been made to solve the above problems, and an object thereof is to make it possible to preferably perform carrier wave detection even under a low C / N ratio. Another object of the present invention is to suitably detect the timing of the 0π alternating pattern and detect the correct preamble timing so as to prevent the occurrence of all burst errors.

[0006]

In order to achieve such an object, the burst detector of the present invention comprises a non-modulated part, 0.
A burst phase modulation signal in which a preamble composed of π alternating pattern and UW is added to the beginning of the data,
A means for generating an in-phase quasi-baseband signal and a quadrature quasi-baseband signal by performing frequency conversion with reference waves having a frequency substantially equal to that of the carrier wave of the modulation signal and different in phase from each other by π / 2, and these two types of quasi-baseband signals. Two types of narrowband lowpass filters that filter the baseband signals by narrowband lowpass, pass the non-modulated part and block the 0π alternating pattern, and these two types of narrowband lowpass filters Means for obtaining the sum of squares of the filtered output and averaging, a comparator for comparing the averaged signal with a threshold value, and a threshold value for detecting the arrival of the 0π alternating pattern is the arrival of an unmodulated portion. And a switch for switching and setting the threshold value to the comparator so that the value becomes smaller than the threshold value when detecting 0, and the arrival of the 0π alternating pattern is regarded as the arrival of the non-modulated portion as the output of the comparator increases. As a decline And detecting Re respectively.

According to a second aspect of the present invention, the pass band width of the narrow band low pass filter is the maximum value of the difference between the frequency of the reference wave and the frequency of the carrier wave of the burst phase modulation signal. Characterize. Claim 3 of the present invention provides a switching device
When the arrival of the unmodulated portion is detected, the threshold value is set to 0.
Cut to the threshold when detecting the arrival of π alternating patterns
It is characterized by changing and setting.

[0008]

In the present invention, first, the burst-like phase modulation signal is converted into the in-phase quasi-baseband signal and the quadrature quasi-baseband signal, and the narrow band low-pass filtering, the sum of squares operation, the averaging, and the threshold value are performed. The judgment is executed. The pass band width of the narrow band low-pass filter is the spectrum of the unmodulated portion that constitutes the preamble, that is, the spectrum of the carrier wave, and the spectrum of the 0π alternating pattern, that is, ± f.
It is set so that the spectrum of b / 2 (fb: symbol rate) is blocked. For example, as in claim 2, the difference in frequency of the reference wave with respect to the carrier wave (frequency offset)
Set to the maximum value of. 2 of the output of the narrow band low pass filter
When threshold values are determined after calculating the sum of multiplications and further averaging, the level of the signal obtained by averaging exceeds the threshold value in the non-modulation portion and falls in the 0π alternating pattern. Therefore, in the present invention, the arrival of the non-modulated portion can be detected as an increase in the output of the comparator and the arrival of the 0π alternating pattern can be detected as a decrease. Further, the level of the averaged signal sharply drops with the arrival of the 0π alternating pattern. In view of this, in the present invention, the threshold value for detecting the arrival of the 0π alternating pattern is set to a value smaller than the threshold value for detecting the arrival of the unmodulated portion. Therefore, low C /
The carrier wave detection probability does not decrease even when N is set. The arrival of the 0π alternating pattern is suitably detected, and the arrival timing of the preamble is reliably detected.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a burst detector according to an embodiment of the present invention. This burst detector inputs the burst-like phase modulation signal from the input terminal 10. As shown in FIG. 3A, the burst format of the burst phase-modulated signal used in this embodiment is such that a preamble is added before the data, and this preamble is composed of an unmodulated portion, 0π alternating pattern and UW. Format. The burst phase modulation signal is input to the frequency converters 12-1 and 12-2.

The frequency converter 12-1 is directly connected to the frequency converter 12-2, and the frequency converter 12-2 is connected to the local oscillator 14 via a π / 2 phase shifter 16. Frequency converter 12-1
Receives the burst phase modulation signal from the input terminal 10 and also inputs the in-phase fixed reference wave from the local oscillator 14. Further, the frequency converter 12-2 transmits the in-phase fixed reference wave by π.
A quadrature fixed reference wave, which is a phase-shifted signal, is input.
The in-phase fixed reference wave and the quadrature fixed reference wave have substantially the same frequency as the carrier wave of the burst phase modulation signal.

A narrow band low-pass filter 18-1 or 18-2 and a squarer 20-1 or 20-2 are connected to the subsequent stages of the frequency converters 12-1 and 12-2, respectively. . The in-phase quasi-baseband signal obtained by the frequency converter 12-1 is filtered by the narrow band low-pass filter 18-1 and then squared by the squarer 20-1.
It is output to the adder 22. Similarly, the frequency converter 12-
The orthogonal quasi-baseband signal obtained by 2 is filtered by the narrow band low-pass filter 18-2, squared by the squarer 20 and output to the adder 22. The adder 22 adds the outputs of the squarer 20-1 and 20-2 and outputs the result to the averaging circuit 24. Averaging circuit 24
When the output of the adder 22 is averaged by, the averaged signal is output to the comparator 26, and the comparison result appears at the output terminal 28 to the demodulation unit.

The narrow band low-pass filters 18-1 and 18-2 have the characteristics shown in FIG. 2 (a). That is, the pass band width Δf is equal to the input terminal 1
A carrier wave of a burst phase modulation signal input from 0,
It is set to the maximum value of the frequency difference (offset of the local oscillator 14) from the fixed reference wave transmitted by the local oscillator 14. Since the spectrum of the unmodulated portion of the burst-like phase modulated signal is only the spectrum of the carrier frequency, the spectrum of the complex quasi-baseband signal obtained by combining the in-phase quasi-baseband signal and the quadrature quasi-baseband signal is shown in FIG. The line spectrum is as shown in. Therefore, the spectra of the non-modulated portions of the in-phase quasi-baseband signal and the quadrature quasi-baseband signal pass through the narrow band low-pass filters 18-1 and 18-2, respectively, and the squarers 20-1 and 20 -2 is output. Further, the spectrum of the burst-shaped phase modulation signal related to the 0π alternating pattern is a line spectrum that appears at ± fb / 2 with respect to the carrier wave. In the complex quasi-baseband signal, this spectrum is a narrow band low-pass filter 18-
It exists outside the pass band of 1 and 18-2. Therefore, when the 0π alternating pattern arrives, the averaging circuit 24
Output signal level decreases.

Therefore, as shown in FIG. 3B, the output level of the averaging circuit 24 is high in the non-modulation portion and low in the 0π alternating pattern portion. By determining this with a threshold value, the arrival timing of the non-modulated portion and the arrival timing of the 0π alternating pattern can be appropriately known even under low C / N. The comparator 26 determines whether the output of the averaging circuit 24 is a non-modulation portion or an 0π alternating pattern by threshold value determination, and outputs the determination result from the output terminal 28 to the demodulation unit. Further, in this embodiment, the threshold value used in the threshold value judgment in the comparator 26 is 0π when the arrival of the unmodulated portion is judged.
Different values are used when determining the arrival of the alternating pattern. That is, the threshold value of the non-modulated portion is determined by the relatively large threshold value 1, and the determination of the 0π alternating pattern is performed by the relatively small threshold value 2. The switching device 30 is a member used for this switching setting.

The switch 30 is provided with an input terminal 32-1 for threshold 1 and an input terminal 32-2 for threshold 2. The switcher 30 sets the threshold value 1 in the comparator 26 in advance, and sets the threshold value 2 in the comparator 26 after the arrival of the non-modulated portion is detected by the threshold value judgment. The output of the averaging circuit 24 in the case of the 0π alternating pattern is
Since the output is significantly lower than the output of the averaging circuit 24 in the non-modulated portion, the threshold 2 is set sufficiently smaller than the threshold 1 to detect the arrival timing of the 0π alternating pattern in FIG. ) Can be suitably performed.

Therefore, according to this embodiment, low C / N
The carrier wave can be detected even under the
The arrival timing of the π alternating pattern can be preferably detected.
Also, the preamble timing can be detected with certainty, and all burst errors can be prevented.

[0017]

As described above, according to the present invention,
Since a narrow band low-pass filter that passes the non-modulated portion and blocks the 0π alternating pattern is provided and the sum of squares of the output is averaged and then the threshold value is determined, a low C / N ratio is obtained.
The carrier wave can be suitably detected even below. Also, 0
Since the threshold value for detecting the arrival of the π alternating pattern is set to a value smaller than the threshold value for detecting the arrival of the unmodulated portion, the arrival timing of the 0π alternating pattern can be accurately detected, and the preamble timing The effect of preventing all burst errors by reliable detection can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a burst detector according to an embodiment of the present invention.

FIG. 2 is a diagram showing a spectrum relationship in this embodiment, FIG. 2 (a) shows characteristics of a narrow band low-pass filter, and FIG. 2 (b) is a complex quasi-baseband relating to an unmodulated portion. FIG. 2C is a diagram showing the spectrum of the signal, and FIG. 2C is a diagram showing the spectrum of the complex quasi-baseband signal related to the 0π alternating pattern.

3A and 3B are diagrams showing an output of an averaging circuit and an output of a comparator in this embodiment, FIG. 3A shows a burst format, FIG. 3B shows an output of the averaging circuit, and FIG. c)
FIG. 3 is a diagram showing the output of each comparator.

[Explanation of symbols]

10 input terminals 12-1, 12-2 Frequency converter 14 Local oscillator 16 π / 2 phase shifter 18-1, 18-2 Narrow band low pass filter 20-1, 20-2 squarer 22 adder 24 Averaging circuit 26 Comparator 28 output terminals 30 switch 32-1 and 32-2 threshold input terminals

Claims (3)

(57) [Claims] 1. A burst phase modulation signal in which a preamble composed of an unmodulated portion, an 0π alternating pattern , and a unique word is added to the beginning of data has a frequency substantially equal to that of the carrier of the modulation signal and is mutually different. Phase is π / 2
A means for generating an in-phase quasi-baseband signal and a quadrature quasi-baseband signal by performing frequency conversion with different reference waves, and narrowband low-pass filtering of these two types of quasi-baseband signals, respectively, and passing through an unmodulated portion. And two types of narrow band low-pass filters that block the 0π alternating pattern, a means for averaging the squared sums of the filtered outputs of these two types of narrow band low-pass filters, and averaging The comparator that compares the received signal with the threshold value and the threshold value when the arrival of the 0π alternating pattern is smaller than the threshold value when the arrival of the unmodulated portion is detected. A burst detector comprising: a switch for switching and setting a value, and detecting the arrival of an unmodulated portion as an increase in the output of the comparator and the arrival of a 0π alternating pattern as a decrease. 2. The burst detector according to claim 1, wherein the pass band width of the narrow band low pass filter is the maximum value of the difference between the frequency of the reference wave and the frequency of the carrier wave of the burst phase modulation signal. A burst detector characterized in that 3. The burst detector according to claim 1 or 2.
When the switch detects the arrival of the unmodulated part,
Is a threshold for detecting the arrival of the 0π alternating pattern.
Burst detection characterized by switching to value
vessel.