JPH09223997A - Tdma timing control system and its method for satellite communication network - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the synchronization among earth stations even in a multi- beam system that generally has no return path. SOLUTION: An earth station 2 generates the transmission timing of its own transmission burst based on a time piece device 221 and also detects the time when the signal is received from another station. The transmission timing of its on station and the information on the signal receiving time of another station are notified with each other among the stations 22 via the control burst. Then the time error of every station is calculated by solving the simultaneous equations where the time error of the device 221 of every station 2 and the propagation time required to a satellite 1 are defined as the unknown quantities. Based on this time error, the time of the device 221 is corrected. Thus the synchronization of time is secured among the stations 2.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a TDMA (Time Division M) in a satellite communication network.
More particularly, the present invention relates to a satellite communication network in which a plurality of earth stations and a large number of terminal stations are connected.

[0001]

2. Description of the Related Art A plurality of earth stations (LES; Land Earth Station) and a large number of terminal stations (TES; Terminal Earth) are provided via a communication satellite having a plurality of communication radio waves (hereinafter simply referred to as "beams").
Station), there is a satellite communication system that provides a communication path, and as an example, a mobile satellite communication system that provides communication services to mobile terminals will be described below.

FIG. 5 is a schematic block diagram of this type of satellite communication system. In FIG. 5, a plurality of beam areas (1
A large number of terminal stations (TES) 3 are present in each of the areas 4 that can be covered by one beam. These beam areas 4
A mobile link 6 composed of a plurality of beams is formed between each of the above and the satellite 1.

Further, a plurality of earth stations 2 are provided,
These earth stations 2 and the public switched telephone network (PSTN) 8 are connected to each other by a gateway 230. A feeder link (earth station line) with a plurality of beams is formed between each earth station 2 and the satellite 1.

The communication satellite 1 includes a feeder link 7 antenna 101 and a duplexer (DPX; Duplexer) 102.
And a low noise amplifier (LNA; Low Noise Am)
and the branching device (DIV; Divi)
der) 104 and a frequency converter (F / C; Frequ)
and the communication channel branching device (TCD; Traffic Channel)
Divider) 108 and a baseband switch (B
S; Baseband Switch) 109 and a speech channel synthesizer (TCC; Traffic Channel)
el Combiner) 110 and a beam former (B
It has an F; Beam Formatter) 114 and a beam antenna 115.

Due to the above respective elements, the forward (For)
A ward link is formed and a return (Retur)
n) The links are the return TCD 113 and the return BS 112.
A return TCC 111, a return combiner 107, and a high power amplifier (HPA; High Power Ampli).
file) 106.

An outline of frequency allocation in the satellite communication system shown in FIG. 5 is shown in FIG. 6 (A). The frequencies are roughly classified into a feeder link line (FL) 7 and a mobile link line (ML) 6, each of which is an uplink line (Uplink).
And the downlink (Downlink), F
Lu, FLd, MLu, and MLd are shown respectively.

In the communication satellite 1, the forward call channel is FLu → MLd and the return call channel is FLd → ML.
u functions as a relay device that performs frequency conversion.

In mobile satellite communication, the antenna gain and the transmission power of the terminal station are extremely limited, so it is necessary to increase the antenna gain of the communication satellite to compensate for them. Therefore, it is common to use a mobile link that has a large-diameter antenna and covers a wide area with multiple beams.

The feeder link usually uses a single beam in many cases, but it is effective to use a multi-beam feeder link for downsizing the earth station.

Regarding the signal multiplexing system, the time division as shown in FIG. 6B is made because the transmission / reception demultiplexer (DPX) of the terminal station is not necessary and the control channel during communication is easily monitored. Multiplexing (TDMA; Time Division)
The Multiple Access method is mainly used. In the TDMA method, since burst signals from a plurality of different stations are transmitted on the same frequency channel,
In order to avoid collision between bursts, it is essential for each transmitting station to perform transmission timing control.

The circuit shown in FIG. 7 has been conventionally used to establish burst synchronization for TDMA. In the figure, a global beam (Global) capable of returning and receiving the signals emitted from each earth station at each earth station.
The case of Beam) is shown.

Reference numeral 211 is an earth station antenna, 212 is a branching circuit, 213 is HPA, and 214 is an up converter (U /
C), 215 is a modulator, 222 is a reference oscillator, 227 is a demodulator, 228 is a down converter (D / C), 229.
Is the LNA.

Reference numeral 231 denotes a reception baseband processor (RX Baseband Processor), 2
Reference numeral 32 is a timing error detection unit, 233 is a transmission timing generation unit, and 234 is a transmission burst generation unit.

The principle of burst synchronization is simple, as shown in FIG.
As shown in (B) and (C), each earth station receives a TDMA signal including its own burst that is returned via a satellite, and measures the time difference between the reference burst transmitted by the reference station and its own return burst. Then, the timing error is detected by the deviation from the nominal setting value. Then, the output timing of the transmission timing generator 233 is controlled so as to correct the timing error.

Such a burst synchronization method is called a closed loop control method. With this method, synchronization can be easily established by the simplest negative feedback control except that the correction frequency is limited to the round-trip delay time to the satellite. However, it is obvious that the above method is limited to the case of the global beam capable of receiving the satellite return signal of the own station.

In FIGS. 6B and 6C, PR is a preamble part, UW is a unique word for synchronization, and DA is a DA.
TA indicates data respectively.

When the feeder link which is the earth station line is also multi-beam, the transmission burst of the own station cannot always be received by the own station, and the above closed loop control system cannot be applied. In such a case, a means for synchronizing the contents of the clock device of each earth station is required.

As such a system, Japanese Patent Laid-Open No. 5-1674
In Japanese Patent Publication No. 85, the method shown in FIG. 8 is proposed.
In FIG. 8, 41 is an antenna, 42 is a transmission / reception circuit, 4
3 is a variable reference clock generator, 44 is a D / A converter, 4
5a to c are interfaces, 46 is a CPU (arithmetic circuit), 47 is a GPS antenna, 48 is a GPS receiver, 4
9 is a counter.

The proposed system of FIG. 8 uses the GPS (Global)
al Positioning System) is intended to establish time synchronization with a satellite communication system by an external means by a receiver, and a signal from a GPS satellite operated and managed by the US Department of Defense is sent to the GPS antenna 47, GPS
The receiver 48 receives the absolute time from the received signal and controls the frequency of the reference clock A of the reference clock generator 43.

[0020]

As described above, the closed loop control method can be applied only to the global beam and cannot be applied to a general multi-beam satellite system. The reason is that the transmission burst of the local station does not return to the local station in the multi-beam system, so that the time error cannot be measured.

In the method using the GPS method shown in FIG. 8, the availability of the system is uncertain. The reason for this is that GPS is a system of the US military and may suddenly become difficult to use due to the outbreak of international conflict.

It is an object of the present invention to provide a TDMA timing control system and method in a satellite communication network that can establish synchronization within the satellite system without resorting to external systems such as GPS.

Another object of the present invention is to provide a TDMA timing control system and method in a satellite communication network that can function not only for global beams but also for general multi-beams.

[0024]

According to the present invention, a communication path between a plurality of earth stations and a large number of terminal stations is provided, and communication signals between the earth stations are provided within a given time slot. Each is configured to be multiplexed by time division multiplexing,
Each of the earth stations has a clocking means that can be corrected by a time error signal supplied from the outside and a transmission burst signal that reaches a communication satellite within a predetermined time slot controlled by the time by the clocking means. In addition, transmission timing control means for determining and controlling the transmission timing of the transmission burst signal, receiving means for receiving and reproducing burst signals from other earth stations, and each reception timing of these reception burst signals are detected by the clock means. Included in the timing detection means, transmission means for superimposing and transmitting the detected reception timing information to each other earth station in order to return to each other earth station, and the reception burst signal received and reproduced by the reception means. Based on the reception timing information at the earth station other than the burst signal of its own station, the time error of the time measuring means and the communication satellite A calculation means for calculating a time error by making a simultaneous equation of propagation time and the unknowns in, and time correction means to adjust the time of the time counting means based on the time error.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION First, an outline of the TDMA timing control system and method of the present invention will be described. Each earth station determines the transmission timing of its own transmission burst signal based on the time of its own clock device and detects the reception timing of the burst signal from another station. Then, each information of the transmission timing of the own station signal and the reception timing of the other station signal is transmitted and received between the earth stations via the control burst to notify each other, and the time error of the clock device of each earth station and the satellite The time error of each earth station is calculated by establishing a simultaneous equation with the propagation time as an unknown number and solving it.

By correcting the time of the clock device based on the calculated time difference, the time of all earth stations is synchronized.

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an earth station (LES) of an embodiment of the present invention.
2 is a block diagram of FIG. 2, and the same portions as those in FIG. 7 are denoted by the same reference numerals. The total number is N in the entire network.
(N is a natural number of 3 or more). In FIG. 1, 2
16 is a signal synthesizing unit, 217 is a data burst generating unit, 2
Reference numeral 18 is a control burst generator, 219 is a burst reception time detector, 220 is a transmission timing generator, 221 is a clock device (Time Base), and 222 is a reference oscillator.

Reference numeral 226 denotes a received data processing unit, 225 a fixed pattern (UW; Unique Word) detection unit, 2
Reference numeral 24 is a control burst reception processing unit, and 223 is a time base error detection / control unit.

The earth station 2 is connected to the public switched communication network (PSTN) 8 via the gateway 230 which is a gateway station, and all other earth stations (see FIG. 5) via the communication satellite 1.
(See) and communicate with each other.

Among the total N, at least one station is a reference station (r which generates a reference time of TDMA timing).
Effort earth station).

Each earth station 2 provides a communication path by a transmission data burst generation unit 217 connected to the gateway 230 and a reception data burst processing unit 226. In addition, the unique word detection unit 225, the control burst processing unit 22
4, time error detection / control unit 223, reference oscillator 222,
Time base 221, transmission timing generator 220, burst reception time detector 219, control burst generator 21
8 constitutes the control device 240. Further, a certain earth station (referred to as m-th station) exchanges various data for establishing synchronization with another earth station (referred to as n-th station) by the control device 240 in the own station. The data includes, for example, the time Tm of the time slot on the satellite allocated to the own station from the n station, the delay time Dm of UPLINK from the own station to the satellite 1 (this Dm is a pre-estimated value. , It is necessary to finally obtain the absolute delay time dm) and so on. On the other hand, from the m station corresponding thereto, data such as the reception detection timing τ [n / m 2] of the burst from the nth station is received. The symbol [n / m] indicates that the signal transmitted from the m station is received by the n station, and conversely, the symbol [m / n].
Shows the case where the m station receives the signal transmitted from the n station.

In FIG. 2, the nth earth station and the mth earth station are satellites 1.
It is the figure which showed the time of each part about the case where it opposes via. The operation will be described with reference to the operation processing flowcharts in the n stations of FIG. 2 and FIG. Now, the time obtained by counting the output of the reference oscillator 222 of the nth earth station with the time base 221 is represented by τ n. Actual time t
If the time error from is εn, then τn = t + εn (1)

On the transmitting side of the nth earth station, in response to the synchronous measurement start command (step 500), the time base unit 22
At 1, the transmission burst timing tn is determined (step 501). This time tn is the time T when the satellite should be reached.
It is a value obtained by subtracting the delay time Dn from n to the satellite and the delay time Sn in the local station transmitter. Since this value has a time error .epsilon.n, that is, the relationship of tn = Tn-Dn-Sn-.epsilon.n (2) is established. At this time, it is assumed that data of Sn, Dn, Rn (which is a delay time occurring in the receiving device of the own station) of the own station is inserted into the data of the own station control burst and transmitted to another station (step. 502).

Since this transmitted burst signal uses the estimated delay time Dn, it actually arrives on the satellite at time tn '= tn + dn (3). However, dn is the absolute signal propagation delay time from the nth earth station to the satellite, which is an unknown number.

On the other hand, the receiving side receives the received signal from the m station and measures its receiving timing τ [n / m] (step 503).

This τ [n / m] is notified to each station (step 505).

Then, Dm, Sm, and Rm inserted in the control burst from the m station are received (step 50).
4).

Further, the reception timing information τ [m / n] of the signal originating from the own station, which is inserted into the control burst from the m station, is received (step 506).

That is, τ [m / n] is the absolute signal delay time dm from the satellite to the mth earth station at the time tn 'on the satellite, the delay Rm of the receiver of the mth earth station, and the actual time. It means the time when the error ε m of is added, and the following equation is obtained.

Τ [m / n] = tn + dn + dm + Rm + εm (4) Similarly, the reception time τ [n / m] of the nth earth station is also represented by the following equation.

Τ [n / m] = tm + dm + dn + Rn + εn (5) As described above, this measurement operation is repeated until the correction start command is issued (step 510).

When there is a correction start command, the following correction operation is started (steps 507, 508, 50).
9).

First, by substituting the equation (2) into the equation (4), .epsilon.m-.epsilon.n + dn + dm = .tau. [M / n]-(Tn-Dn-Sn + Rm) (6) is obtained. Also, in equation (5), tm = Tm-Dm-Sm
Substituting-[epsilon] m (7) yields [epsilon]-[epsilon] m + dm + dn = [tau] [n / m]-(Tm-Dm-Sm + Rn) (8). Here, n, m = 1, 2, ..., N.
Expressions (6) and (8) are simultaneous linear equations in which 2N of {εn} and {dn} are unknowns. Since Sn and Rm can be measured in advance, they are known values, and εn and dn can be obtained by solving this simultaneous equation.

In this case, when equations (6)-(8) are calculated and transformed so as to eliminate dn + dm, .epsilon.m-.epsilon.n = {T [m / n] -T [n / m]} / 2 ... (9) Here, T [m / n] = τ [m / n]-(Tn-Dn-Sn + Rm) (10) T [n / m] = τ [n / m]-(Tm-Dm-Sm + Rn ) ... (11).

Here, the station serving as the time reference is a base station, and the number of the base station is represented by r. Therefore, base station n = r
Then, since εr = 0, the equation (9) becomes the following equation.

Εm = {T [m / r] −T [r / m]} / 2 (12) (m = 1, 2, ..., N) As a result, εm can be calculated.

Therefore, since the time error of each station is obtained (step 508), the clock devices of each earth station can be synchronized.

Although the above equations (10) to (12) are solved for a general multi-beam, n = m may be set in the equation (6) for a global beam.
In this case, the local station burst is received by the local station. Therefore, εm = εn (13), dn = dm ...
Therefore, the relationship of (13) and (14) is substituted into the expression (6), and dn in this case is represented by dn *. As a result, the absolute propagation time can be directly calculated as dn * = T [n / n] / 2 (15).

Similarly, for the m earth station, dm * = T [m
/ M] / 2 (16)

The data of these equations (15) and (16) are
Used for mutual communication between m and n stations.

Therefore, in the case of the global beam,
From equations (6) and (10), εm-εn = {T [m / n]-(dn * + dm *) (17) (n, m = 1, 2, ..., N). That is, the difference in time difference between the stations is measured. Therefore, if εr = 0 with the specific station n = r as the reference station, εm = T [m / r] − (dm * + dr *) ... (18) (m = 1, 2, ..., N), and the time error of each station can be detected. Therefore, by controlling the time base (clock device) 221 so that ε m becomes 0, the clock devices of the respective earth stations can be synchronized (step 508).

FIG. 4 shows a circuit configuration for synchronizing the timepiece device by using the time error thus obtained. In FIG.
2201 is a counter, 2202 and 2204 are adders, 2
203 is a memory. The clock device of each earth station adds the output of the free-running counter 2201 and the correction value supplied from the memory 2203 by the adder 2202 to generate a time signal.

The correction value needs to be updated by the time error calculated by the above method. That is, the time error detection / control unit 223 receives the time error and the update timing signal, the adder 2204 calculates the difference between the correction value before that and the time error, and the memory 22 uses the update timing signal.
Update the contents of 03.

Further, the equation (6) + the equation (8) is calculated to obtain dn + dm = {T [m / n] + T [n / m]} / 2 (19). In equation (19), there are N unknowns and N equations.
There are (N-1) / 2. Therefore, if N = 3 or more earth stations exchange information and solve the above simultaneous equations, the true absolute propagation delay dn from each earth station to the satellite can be obtained (step 5).
07).

Then, by using dn thus obtained as a new propagation time Dn (step 509),
The output burst of each earth station can reach the satellite in the designated time slot, and thus burst synchronization can be achieved.

In the case of N = 2 stations, since there is only one equation for unknown number 2, d1 and d2 cannot be obtained. However, in this case, the signal of the earth station 1 (2) is the other party 2
Since it only goes to (1), it is not necessary to establish burst synchronization in the first place.

The above processing (steps 501 to 51 in FIG. 3)
0) is measured at regular intervals, for example, 0.5 seconds, and 0.5 seconds is a correction, and all earth stations are performed at 1-second intervals. As a result, in a communication system using a low-orbit (low-orbit) communication satellite that moves every moment (meaning a system in which dn changes every moment), correct synchronization is always possible.

Based on the time information of the clock device of each earth station and the propagation time information to the satellite thus synchronized, communication from each earth station to a terminal station existing in a beam area different from other earth stations is also performed. It can be performed by the TDMA method.

[0060]

As described above, according to the present invention, information such as TDMA burst transmission timing and detection reception timing of each station is mutually notified and calculated between the earth stations. Since the time error of the device is calculated and the time can be controlled based on the time error, there is an effect that each station can be synchronized only within the satellite communication system without depending on an external system such as GPS.

By synchronizing the time of each earth station, the propagation time to the satellite can be obtained,
In the multi-beam satellite communication system as well as the global beam, TDMA communication can be performed by establishing burst synchronization.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a satellite communication network of the present invention.

FIG. 2 is a diagram for explaining the timing of each part between the nth earth station and the mth earth station.

FIG. 3 is a diagram showing a processing flowchart of a timing control operation in the nth earth station and the mth earth station.

FIG. 4 is a diagram showing a specific configuration of a time base 221 in the control circuit 240.

FIG. 5 is a block diagram showing a conventional mobile satellite communication network.

FIG. 6A is a diagram showing the frequency arrangement of FIG. 5;
(B) is a diagram illustrating a TDMA signal. (C)
FIG. 6 is a diagram illustrating UW detection timing.

FIG. 7 is a block diagram showing a configuration of the earth station of FIG.

8 is a block diagram showing a configuration of another earth station using the GPS receiver in FIG.

[Explanation of symbols]

 1 communication satellite 2 earth station 3 mobile station 4 beam area 6 mobile link 7 feeder link 8 public switched telephone network 211 earth station antenna 212 demultiplexer 213 high power amplifier 214 upconverter 215 modulator 216 combiner 217 data burst generator 218 Control burst generation unit 219 Burst reception time detection unit 220 Transmission timing detection unit 221 Clock device (time base) 222 Reference oscillator 223 Time difference error calculation / control unit 224 Control burst processing unit 225 Unique word detection unit 226 Received data burst processing unit 227 Demodulation 228 Down converter 229 Low noise amplifier 230 Gateway station (gateway)

Claims (11)

[Claims] 1. A TDMA timing control system in a satellite communication network in which a plurality of earth stations communicate with each other by a TDMA communication method, wherein the earth station determines a transmission timing based on its own time base, and other earth stations. A means for transmitting the data of its own station, a means for detecting the reception timing of the reception signal received by the other earth station and the data of the other station, and information for each of the transmission timing and the reception timing. A TDMA timing control system in a satellite communication network, comprising: a means for exchanging between the other earth stations and a means for correcting the time of the time base based on the information. 2. The TDMA in the satellite communication network according to claim 1, wherein the correcting means is obtained by solving a simultaneous equation in which the time error of the time base and the propagation time to the satellite are unknowns. Timing control system. 3. The T in the satellite communication network according to claim 1, wherein the transmission timing and the reception timing are both transmitted by a control burst signal.
DMA timing control system. 4. The data comprises a delay time from a local station to a satellite, a delay time generated by a transmission of the local station, and a delay time generated by a reception of the local station.
A TDMA timing control system in the satellite communication network according to a few items. 5. The earth station has at least one TD.
The TDMA in the satellite communication network according to claim 1, wherein the TDMA is a station serving as a base for MA timing.
Timing control system. 6. The TDMA timing control system in a satellite communication network according to claim 1, wherein the satellite communication network is a satellite communication network using global beams or multi-beams. 7. The TDMA in a satellite communication network according to claim 1, 5, or 6, wherein said earth station is further connected to a public switched telephone network (PSTN) via a gateway which is a gateway station. Timing control system. 8. A TDMA timing control in a satellite communication network for providing a communication path between a plurality of earth stations, wherein communication signals between the earth stations are multiplexed by a time division multiplexing method within a given time slot. In the system, each of the earth stations has a clocking means that can be corrected by a time error signal supplied from the outside, and a transmission burst signal to a communication satellite within a predetermined time slot controlled by the time by the clocking means. Transmission timing control means for determining and controlling the transmission timing of the transmission burst signal so as to arrive, receiving means for receiving and reproducing burst signals from other earth stations, and each reception timing of the reception burst signals. Timing detection means for detecting the received timing information and returning the detected reception timing information to other earth stations. A transmitting unit that superimposes and transmits the self-station burst signal to return, and a self-station burst signal included in the reception burst signal received and reproduced by the receiving unit based on reception timing information at another earth station. It has a calculating means for calculating a time error by establishing a simultaneous equation in which a time error and a propagation time to the communication satellite are unknowns, and a time correcting means for adjusting the time of the time measuring means based on the time error. TDMA timing control system in a satellite communication network. 9. The TDMA timing control system in a satellite communication network according to claim 8, wherein the calculating means is configured to solve the simultaneous equations to calculate the propagation time. 10. The transmission timing control means is configured to determine the transmission timing of the transmission burst signal based on the time corrected by the time correction means and the propagation delay time. A TDMA timing control system in a satellite communication network according to claim 8. 11. A TDMA timing control method in a satellite communication network for communicating between a plurality of earth stations by a TDMA communication method, said earth station determining a transmission timing based on a time base of the own station, and said determining. Transmitting the transmission timing to another earth station, detecting the reception timing of a reception signal from the other earth station, detecting the data of the other earth station from the reception signal, In the satellite communication network, comprising the step of notifying each of the information of the transmission timing and the reception timing between the other earth stations, and the step of correcting the time of the time base based on the information. TDMA timing control method.