JPH0756951B2 - Frequency control method for micro satellite communication system - Google Patents

Description

The present invention relates to a frequency control system for a satellite communication system using an extremely small antenna.

[Conventional technology]

A satellite communication system consisting of a large number of ultra-small earth stations with extremely small antennas with an antenna diameter of 1.2 to 1.8 m or less, and a central station that has a relatively large antenna and manages the entire system , Generally VSAT (Very Small Ape
rture terminal) system, the micro earth station is VS
AT stations and central stations are called HUB stations.

The VSAT system is usually used in a communication system in which the amount of communication per station is relatively small, and one satellite line is assigned to all VSAT stations as an inbound line from the VSAT station to the HUB station. Generally, the allocation method is TDMA (Time Divis
Ion Multiple Access) method may be used to divide one frame set on the time axis and allocate it to all VSAT stations. However, in the VSAT system, the number of participating stations is very large, so the line utilization efficiency is high. However, the slot Aloha method, which is a kind of random access method, is often used because the drawbacks such as the decrease in throughput, the decrease in throughput, and the increase in line connection time appear conspicuously.

Whether using the TDMA method or the slot aloha method, one satellite line is shared by all VSAT stations. Each VSAT station has a highly stable frequency oscillator in order to generate a carrier wave used for transmitting inbound signals, but naturally all frequencies are not exactly the same, and there is a frequency deviation between each VSAT station. . The demodulator of the HUB station must demodulate the burst-like inbound signal including this inter-station deviation, but if this inter-station deviation is too large, the demodulator's lead-in time for carrier recovery may be long. In an extreme case, the VSAT station cannot be demodulated and cannot be demodulated, so the VSAT station uses a very stable frequency oscillator for carrier generation, and the frequency deviation between each VSAT station is made as small as possible. .

Further, even with a highly stable frequency oscillator, the inter-station deviation cannot be made zero. Therefore, a carrier wave reproduction signal as shown in FIG. Let the part complete the pulling of the carrier wave. The carrier wave reproduction signal is often a continuous pattern of "1" or a continuous pattern of "0" that is easy to pull in. This length corresponds to the inter-station deviation allowed in each VSAT station, and the larger the permissible inter-station deviation, the longer the pattern needs to be, and the portion other than the data to be originally transmitted, that is, the overhead increases, Reduces the efficiency of.

Also, since the inbound line is shared by all VSAT stations,
In order to identify from which station the received signal is transmitted, the station number of the VSAT station is generally inserted after the carrier recovery signal as shown in FIG.

[Problems to be Solved by the Invention]

Since a frequency oscillator having a higher stability becomes larger and more expensive, it is not possible to use a frequency oscillator having an extremely high stability in view of economy and size of the device. However, if the stability is lowered, a long preamble is required, which causes a problem of lowering the line efficiency.

It is an object of the present invention to provide a frequency control method that does not require an expensive high stable frequency oscillator.

[Means for Solving the Problems]

The frequency control method of the present invention, means for detecting the frequency deviation of the carrier of the inbound signal received by the HUB station, means for returning this detection value to the VSAT station that sent the inbound signal, and the VSAT station for the detection value And means for correcting the frequency of the transmission carrier wave.

[Action]

In this control method, the HUB station measures the frequency deviation Δf from the reproduced carrier when demodulating the inbound signal.
Based on the station number in the demodulated received data, it is broadcast to the VSAT station through the outbound line. The VSAT station extracts data destined for itself from the received outbound signal, and adjusts the frequency of the carrier wave transmission frequency oscillator based on Δf in the data.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

Figure 1 (a) is a block diagram of a HUB station, and Figure 1 (b) is a VSA.
It is a block diagram of T station.

The HUB station includes a demodulator 1 and a modulator 2, and the demodulator 1
The carrier reproduces the carrier when demodulating the received IF signal, but at this time, it detects the frequency difference from its own frequency oscillator.
On the other hand, the transmission station number is extracted from the demodulated reception data.
These two pieces of information are sent as Δf information from the modulator 2 by being carried on the outbound signal.

The VSAT station includes a demodulator 3, a modulator 4 and a variable frequency oscillator 5. The demodulator 3 extracts the Δf information for its own station from the received data obtained by demodulating the received IF signal, and outputs the variable frequency oscillator. At 5, the oscillation frequency of the variable frequency oscillator for carrier wave is corrected by Δf and transmitted through the modulator 4.

To correct this frequency, the frequency of the frequency oscillator for carrier wave transmission is corrected. This frequency oscillator is not a fixed frequency type, but a voltage controlled crystal oscillator (VCXO; Volta
ge Controlled X'tal Oscillator).

Each time the HUB station receives an inbound signal from the VSAT station, the above loop is rotated and the correction is continued so that Δf is always zero. In this way, the frequency control operation of all VSAT stations can be performed without providing a separate satellite line.

〔The invention's effect〕

As described above, the present invention makes the frequency oscillator of the VSAT station variable, and by adopting a method that follows the control from the HUB station, it is possible to eliminate the need for an expensive and highly stable frequency oscillator, and Economicization can be measured. Since the VSAT system is often composed of an extremely large number of participating stations, the effect of the system as a whole is great.

In addition, even if the frequency oscillator changes over time, the frequency shift can be automatically corrected by remote control from the HUB station, which enables unattended operation of the VSAT station for a long period of time.

Furthermore, since an extra satellite line is not required for frequency control, the line utilization efficiency can be kept high.

In addition, since the transmission frequencies of the VSAT stations are synchronized, the length of the carrier recovery signal of the inbound signal can be shortened, so that the line utilization efficiency can be improved.

[Brief description of drawings]

FIG. 1 (a) is a block diagram of a HUB station according to the present invention.
FIG. 2B is a block diagram of the VSAT station, and FIG. 2 is a burst format diagram of the inbound signal. 1 ... Demodulator (frequency deviation detecting means), 2 ... Modulator (detection value returning means), 3 ... Demodulator, 4 ... Modulator,
5: Variable frequency oscillator (transmission frequency correction means).

Claims (1)

[Claims] 1. A micro earth station comprising a central station and a large number of micro earth stations, in which all satellites for satellite signals for inbound signals transmitted from the micro earth station to the central station in a burst form participate. In a microsatellite communication system in which an outbound signal from the central station to the microearth station is shared and is continuously transmitted in a broadcast mode by a separate satellite line, the frequency of the carrier wave of the inbound signal received by the central station is shared. It has means for detecting a deviation, means for returning the detected value to the micro earth station that has sent the inbound signal, and means for correcting the frequency of the transmission carrier wave by the micro earth station receiving the detected value. A frequency control method for a microsatellite communication system characterized by the above.