JPS6314534B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Technical Field of the Invention The present invention relates to an initial acquisition method, and particularly to a PSK-
This relates to the initial acquisition method used in TDMA satellite communication systems.

(b) Prior art and problems In order for a large number of earth stations to communicate with each other using a common satellite repeater, a satellite repeater is installed on the time axis of the satellite repeater as shown in Figure 1a. Frame F is time-divided according to the number of earth stations and divided into time slots A, B, and C.
... will be established. Each earth station transmits a burst of radio waves that repeats at frame F intervals during the time slot assigned to it.

Radio waves from each earth station are relayed by this satellite repeater, and each station can receive radio waves from other stations to perform multiple access communications.

Here, in order to transmit burst-shaped radio waves within the allocated time slot, the transmission time is controlled based on the reference bursts transmitted at frame F intervals by a specific earth station (reference station).

FIG. 1b is a diagram showing the structure of the reference burst.

In the figure, CR is the unmodulated wave part used in the carrier wave recovery circuit to recover the reference carrier wave from the received wave, and BTR is the synchronization pattern used in the pit timing recovery circuit to recover the clock. ,
UK indicates a unique word signal for station identification and phase uncertainty removal, and OH indicates line status information, overhead information including data length, etc.

It should be noted that the data A, B, . . . of each station also includes the carrier wave regeneration circuit pattern CR, the bit timing regeneration circuit synchronization pattern BTR, etc.

And as mentioned above, it was received in burst form.
The reference carrier wave for reproducing data from the PSK wave,
The first thing a receiver needs to do is how to extract the information at high speed.

Various methods are known for this carrier wave regeneration. Typical examples include inverse modulation, remodulation,
There are frequency multiplication methods, etc. However, in any case, in order to extract a carrier wave component from a PSK wave without a carrier wave component, it is necessary to perform nonlinear operations such as modulation or multiplication as in the above example.

For example, consider inverse modulation as a phase lock loop circuit (abbreviated as PLL circuit) that includes a phase detector that has n stable points (n is equal to the number of modulation phases) within the 360 degree phase difference between two waves. This is nothing but multiplication of the so-called baseband signal.

In other words, in order to extract the carrier wave component from the PSK wave, multiplication with an order equal to the number of modulation phases is performed, and in the case of 4 phases, multiplication by 4 is performed, so the equivalent signal-to-noise ratio is lower than before multiplication. Deterioration of approximately 12dB.

Furthermore, the reception frequency fluctuates greatly due to fluctuations in the transmission frequency of the other station and fluctuations in the transmission frequency of the satellite repeater.

Therefore, during initial acquisition where the signal-to-noise ratio deteriorates due to multiplication and the received frequency fluctuates greatly, the PLL circuit included in the carrier wave recovery circuit becomes unable to track, so the carrier wave recovery circuit does not operate. There was a problem in that the reference carrier wave could not be extracted in some cases.

(c) Purpose of the Invention The present invention has been made in view of the above-mentioned problems of the prior art, and is intended to solve the problems of the prior art described above. The purpose is to provide an initial acquisition method that operates stably.

(d) Structure of the Invention The purpose of the invention is to detect, in the PSK-TDMA satellite communication system, the operating state of a carrier regeneration circuit included in the demodulation section of the receiving station when a unique word signal sent from the reference station is not detected by the receiving station. In the initial acquisition state, control is performed to have one stable pull-in point, and when the unique word signal can be detected periodically, control is performed to have a number of stable pull-in points equal to the number of modulation phases. This is achieved by providing an initial acquisition method that

(e) Embodiment of the invention Figure 2 shows PSK-TDMA for implementing the invention.
An example of a schematic block connection diagram of a demodulator is shown.

In the figure, 1 is a phase detector, 2 is a carrier wave recovery circuit, 3 is a bit timing recovery circuit, 4 is an instantaneous polarity determination circuit, 5 is a unique word detection circuit, and 6 to 8 are terminals, respectively.

In the figure, the received wave applied to the terminal 6 is divided into three parts, the first received wave is extracted with a reference carrier component by the carrier regeneration circuit 2, and this reference carrier component is added to the phase detector 1. . On the other hand, phase detector 1
The second received wave applied to is demodulated using the reference carrier component, and then applied to the instantaneous polarity determination circuit 4. Note that the clock is extracted from the third received wave applied to the bit timing recovery circuit 3,
This clock is applied to the instantaneous polarity determining circuit 4, where the received data is extracted from the demodulated wave.

In the initial acquisition state, when the unique word signal included in the reference burst sent from the reference station is not detected, the unique word detection signal from the unique word detector 5 is not sent to the carrier wave regeneration circuit 2. 2 has a stable point of 1
Operates in one state.

On the other hand, when a unique word signal is detected, normal operation is performed using this unique word detection signal.
The carrier regeneration circuit 2 operates in a state having two stable points (in the case of a 4-phase PSK wave).

FIG. 3 is a more specific block connection diagram of the carrier wave recovery circuit 2 shown in FIG. 2, and the dotted line portions are the portions added to implement the present invention.

In the figure, 10 is a delay circuit, 11, 14, 23 are phase detectors, 12, 15 are low-pass filters, 1
3 and 16 are voltage comparators, 20 and 21 are phase modulators, 17 and 22 are 90 degree hybrid circuits, 2
4 is a retrigger type monostable multivibrator circuit, 18 is a voltage controlled oscillator, 19 is a peak holding type loop filter, 25 and 26 are AND circuits, and 30 to 32 are terminals.

Each of these blocks is connected as follows.

The terminal 30 is connected to the phase modulator 2 via the delay circuit 10.
0 and 21, and via the phase detector 11, low-pass filter 12, voltage comparator 13, and AND circuit 25, the terminal 1 of the phase modulator 20, and further the phase detector 14 and the low-pass filter. It is connected to the terminal 1 of the phase modulator 21 via the voltage converter 15, the voltage comparator 16, and the AND circuit 26, respectively.

Terminals 1 and 2 of the hybrid circuit 17
are terminals 2 of phase detectors 11 and 14, respectively, and
Terminal 3 is connected to phase detector 2 via terminal 32, voltage controlled oscillator 18, and peak holding loop filter 19.
3, respectively.

Furthermore, terminals 1 and 3 of the hybrid circuit 22 are connected to terminals 3 of phase modulators 20 and 21, respectively, and terminal 2 is connected to terminals 32 and 3 via a phase detector 23, respectively.
1 is a retrigger type monostable multivibrator circuit 2
4 to other terminals of AND circuits 25 and 26.

The operation of each block connected in this way is as follows.

The received wave applied to the terminal 30 is divided into three parts, the first received wave is sent to the phase detector 11, the low-pass filter 12, and the voltage comparator 13, and the second received wave is sent to the phase detector 14, A digital signal is extracted by demodulation by a low-pass filter 15 and a voltage comparator 16, and this digital signal is passed through AND circuits 25 and 2.
Added to 6.

On the other hand, the third received wave is applied to the phase modulators 20 and 21 through a delay circuit 10 having a delay time equal to that of the low-pass filters 12 and 15.

Here, when the unique word signal is not detected, the unique word detection signal is not applied to the terminal 31, so the retrigger type monostable multivibrator 24 and the AND circuits 25 and 26 do not operate, and therefore the demodulated digital No signal is applied to phase modulators 20 and 21 for inverse modulation. Therefore, the third received wave that has passed through the delay circuit 10 passes through the phase modulators 20 and 21 and the hybrid circuit 22, and is directly applied to the phase detector 23 of the PLL circuit.

On the other hand, the output wave of the voltage controlled oscillator 18 is also applied to the phase detector 23, where the phase is compared with the unmodulated wave portion CR in the reference burst. in this case,
Since the nonlinear operation called inverse modulation is not performed,
This PLL circuit performs a normal pull-in operation with one stable point F within the 360-degree phase difference between the two waves, as shown by the dotted line in Figure 4, and the reproduced carrier wave is transferred to the terminal 3.
2.

On the other hand, the bit timing reproduction pattern BTR
Since the unique word signal UW part is PSK modulated, the phase detector 23 compares the PSK wave with the output wave of the voltage controlled oscillator 18, so the output voltage of the phase detector 23 is almost 0, and the voltage controlled oscillator 18 starts free running.

Therefore, by providing the loop filter 19 included in this PLL circuit with a voltage holding function,
If the output voltage obtained by phase comparison with the non-modulated wave portion CR is held only during the bit timing recovery pattern BTR, the phase change of the output wave of the voltage controlled oscillator 18 will be similar to the unique word signal. can be reduced to an almost negligible value.

Next, when a unique word signal is detected, the unique word detection signal is applied to the terminal 31, so that the demodulated digital signal passes through the AND circuits 25 and 26 to the phase modulators 20 and 21.
The PSK wave applied through the delay circuit 10 is inversely modulated into a non-modulated wave, and the phase of this non-modulated wave is compared with the output wave of the voltage controlled oscillator 18 by a phase detector 23. At this time, the PLL circuit performs a conventional operation having four stable points E, F, G, and H (in the case of a four-phase PSK wave) within 360 degrees, as shown by the solid line in FIG.

FIG. 5 shows the PLL circuit in FIG. 3 replaced with a single tuning circuit 27 and a voltage comparator 28.
Although it does not track like a PLL circuit, it is functionally equivalent.

Note that the same symbols as in FIG. 3 indicate the same parts.

(f) Effect of the invention As explained above, according to the present invention, PSK-
In the initial acquisition state in which the unique word signal of the reference burst sent from the reference station is not detected in the demodulated signal of the TDMA satellite communication receiving station, the carrier wave recovery circuit of the demodulator is set in a state where there is one stable point. By controlling the normal carrier wave regeneration when the unique word signal can be periodically received, stable initialization can be achieved even when the signal-to-noise ratio of the received wave is poor and the reception frequency fluctuates greatly. It becomes possible to capture.

There has been a proposal to switch the phase detection characteristics of the PLL circuit between the 1 stable point type and the n stable point type by sending out a gate pulse that matches the carrier synchronization pattern CR from the TDMA control unit, but it is not possible to switch the phase detection characteristics of the PLL circuit between the 1 stable point type and the n stable point type. In this case, since the gate pulse cannot be obtained from the TDMA control section, it is difficult to apply it to a transmission line with a poor signal-to-noise ratio.

On the other hand, the present invention can be applied to a transmission line with a poor signal-to-noise ratio even without a gate pulse from the TDMA control section, and can also be used in combination with the gate pulse method described above.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining time allocation in the TDMA system, and Figure 2 is a diagram for explaining the time allocation of the TDMA system.
An example of a schematic block connection diagram of the TDMA demodulator is shown below.
3 shows a more specific block connection diagram of the carrier wave recovery circuit shown in FIG. 2, FIG. 4 shows a diagram for explaining the operation of FIG. 3, and FIG. 5 shows another embodiment. . In the figure, 1 is a phase detector, 2 is a carrier recovery circuit, 3 is a bit timing recovery circuit, 4 is an instantaneous polarity determination circuit, and 5 is a unique word detection circuit.

Claims (1)

[Claims] 1 In the PSK-TDMA communication system, when the operating state of the carrier regeneration circuit included in the demodulation section of the receiving station is in the initial acquisition state where the unique word signal sent from the reference station is not detected by the receiving station, one pull-in stable state is detected. When the unique word signal is in a state where it can be detected periodically, the initial acquisition method is characterized in that the unique word signal is controlled so as to have a number of stable pull-in points equal to the number of modulation phases.