JPH0448294B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a TDMA satellite communication system using a plurality of carrier waves within the band of one satellite repeater, and particularly relates to a TDMA satellite communication system used for business satellite communication. .

[Conventional technology]

Satellite communications, which were initially used for international communications, have come to be used not only for domestic public communications, but also in the field of so-called business communications in private companies, as the cost of using them has decreased as technology advances.

One of the characteristics of business communications is that the traffic is diverse, including various data signals and image signals in addition to voice signals, and these are handled integrally as digital signals. Since it needs to be installed nearby, in order to avoid interference with existing terrestrial microwave relay lines, 14/11GHz (Ku band) or 30/20GHz
(Ka band) and other high frequency bands are used.

As a satellite communication method that carries digital signals,
SCPC (Single Channel Per Carrier) method uses a carrier wave for each unit channel (usually 64 kbps),
Multiplex multiple channels (1.5Mbps, 2.0Mbps, etc.)
MCPC (Multiple
There is a Channel Per Carrier method. However, in the former case, the maximum transmission capacity per channel is, for example,
There are problems such as being limited to 64kbps, and in the latter case, the transmission capacity of one earth station may not necessarily be 1.5Mbps or 2.0Mbps, and lacks flexibility when handling various digital signals in an integrated manner. There is a side.

TDMA is a method that solves these problems and handles digital signals efficiently and in an integrated manner.
(Time Division Multiple Access) method.

FIG. 2 shows the principle of the TDMA communication system. The figure shows three earth stations 30 for simplicity.
It is shown. One of the earth stations becomes a reference station,
A signal called a reference burst 10 is periodically transmitted to define a reference timing called a TDMA frame on the satellite. Each station learns the reference timing by receiving the reference burst 10, and stores data at a pre-assigned position with respect to this timing.
A signal for carrying traffic called a burst 20 is transmitted, and a repeater (usually called a transponder) mounted on the satellite is shared in a time-division manner.

In this method, all earth stations use carrier waves with a common frequency, so one receiver can receive all signals, and the transmission capacity of each station and the line capacity between each station can be changed flexibly. Moreover, since the length of the burst transmitted by each station can be changed flexibly, there are many advantages such as less savings in the size of the channel to be handled.

That is, the TDMA of the bursts transmitted by each earth station.
The position and width within a frame, the destinations of various traffic included therein, etc. are given by a set of data called a burst time plan. On the other hand, the received bursts and how to process their contents are also given in the burst time plan. Since such data can be stored in the memory circuit, the data of the new plan can be downloaded from the reference station to each related station and stored, and the entire system can be switched from the old plan to the new plan at the same time. It is also possible to change the network configuration instantly without affecting traffic. Therefore, by creating burst time plans as needed to accommodate changes in traffic demand and other conditions within the network, and distributing and using them where necessary, the network configuration can always be optimized. It is also possible to maintain A well-known example of a TDMA system using a burst time plan is Intelsat's TDMA system, which was put into practical use on international satellite communication lines in the fall of 1985.

The original TDMA system uses one carrier wave for one transponder on a satellite, using the entire transponder's bandwidth and all available power.

It is known that this method allows the most efficient use of the transponder's available power compared to other methods. This method provides a transmission capacity of approximately 60 to 120 Mbps for the entire system.
However, each station temporarily utilizes the entire band and power of the transponder, even though it is time-divided, so it requires that much capacity and a large earth station. Therefore, it is suitable for transmitting a large amount of traffic between a relatively small number of earth stations.

In contrast, in systems for business communication,
Since the transmission capacity per station is about several Mbps, the frequency band of the satellite transponder is divided into several parts and the TDMA system is used at a low speed of about several to 20 Mbps using a part of the frequency band.

In this case, the feature of effectively utilizing the power of the satellite transponder is lost, but the above-mentioned
The above-mentioned advantages of being able to use an earth station of the same size as the MCPC method or the SCPC method earth station that handles a large number of channels, and being able to flexibly construct the system, remain.

One advantage of low-speed TDMA systems is that they are highly scalable. As can be seen from Figure 2, if three earth stations were operated using the TDMA method, each station would transmit radio waves for one-third of the time on average. Since normal lines are bidirectional, this means that each station only needs to receive a signal one-third of the time. In other words, the utilization efficiency of each station's equipment is only one-third. Therefore, if the available frequency band increases and each station can use it for two-thirds of the time when its equipment is idle, the transmission capacity of each station will triple, and the efficiency of equipment usage will also increase.
It will be close to 100%. This effect becomes more pronounced as the number of earth stations included in the system increases. Frequency hopping is a technology to achieve this. Frequency hopping is the process of selecting multiple carrier waves within the same transponder and
It is used while being switched periodically within the TDMA flake.

Figures 3a and 3b illustrate operation in frequency hopping. Here, it is assumed that there are three carrier waves 40, 1, 2, and 3, in the frequency band of the satellite transponder. In Figure a, three stations A, B, and C are respectively transmitting using specific carrier waves.
Here, the reference burst is omitted and only the data burst 20 is shown. However, bursts for different stations are treated as independent bursts.
On the other hand, the receiving side receives bursts destined for its own station while switching reception carrier waves using frequency hopping technology. Of course, one earth station cannot transmit on more than one carrier wave at the same time, nor can it receive more than one carrier wave at the same time, so when creating a burst time plan, each The position of the burst transmitted by a station and the destination of that burst must be determined with consideration given to ensuring consistency.

In FIG. 1B, the frequency of the received carrier wave is assigned to each station, and the transmitting side transmits using the corresponding carrier wave depending on the destination of the traffic to be sent.
That is, this is the case when frequency hopping technology is used on the transmitting side.

Although these diagrams are simplified, an actual system would be complicated because there are many earth stations and each station has a different transmission capacity. Furthermore, frequency hopping is often required to be performed not only on either the transmitting side or the receiving side, but on both sides.

However, in any case, the transmission capacity of each station can be expanded to nearly what it would be if the station transmitted signals almost continuously.

Note that the method (algorithm) for creating a burst time plan for a TDMA system using multiple carrier waves is already publicly known;
It is reported in the paper "THE DST-1100 TDMA SYSTEM" published in May 12-16 (West Germany).

[Problem that the invention seeks to solve]

As mentioned above, the TDMA satellite communication method can be made into a communication method suitable for business communication by using a low-speed TDMA method using a plurality of carrier waves. However, one remaining difficulty is that all earth stations in the system need to be of equal size, regardless of the earth station's transmission capacity. this is
This can be seen as a drawback in the SCPC method, in which the scale of each earth station is determined approximately in proportion to the transmission capacity of that earth station. For this reason, even if it is preferable to combine them into one communication system, the SCPC method is used for the star-shaped network part, and only for the mesh-shaped network part.
There are also examples of using the TDMA method. Note that the scale here refers to the degree of multiple functions such as the size of the earth station's antenna, the size of the output of the transmitter, and the sensitivity of the receiver.

One of the objects of the present invention is to realize a TDMA satellite communication system including earth stations of different sizes.

As mentioned above, various types of data are mixed and processed in business communications, and some of this data, such as voice signals, can be used even with a bit error rate of about 10 ^-4 during transmission, while others are compressed and have redundancy. There are cases where a bit error rate of about 10 ^-8 is required, such as data with a small amount of data. In conventional satellite communications, the transmission of audio signals was the main focus, so lines were designed for audio signals, and the requirement for high-quality transmission was met by adding an error correction function. However, in the case of business communications, an error correction function may be included from the beginning due to the economical design of the earth station, such as reducing the size of the required antenna. etc., there may be waste.

Another object of the invention is to provide the most efficient system for these different transmission quality requirements.

Another problem with business satellite communications has to do with the use of the Ku/Ka band. That is, this frequency band has a large attenuation against rain.

FIG. 4 shows the amount of attenuation due to rainfall and the annual probability that the amount of attenuation will exceed that amount in each frequency band. In reality, rainfall trends vary greatly from region to region, so such a map will look different from region to region. Therefore, this diagram is just one example of rainfall characteristics.

The 6/4GHz frequency band shown as C-band in this figure is a frequency band used in international satellite communications, and is not affected by rain attenuation, but domestically there is a problem of interference with existing microwave relay lines. The usage has been saved since. Therefore, the Ku/Ka band with large rainfall attenuation will be used.

Measures to reduce rainfall attenuation include site diversity, which uses two earth stations installed at points several tens of kilometers apart with little correlation in rainfall;
There are methods such as controlling to increase the transmission power by the amount that the carrier wave is attenuated by rain, or allowing a margin in advance to account for rain attenuation at the line design stage.

However, site diversity does not meet the demands of business communications, which want to have an earth station nearby. Since there is a limit to the output power of the satellite transponder when controlling the transmission power, it is only possible to keep the transponder input constant.
Normally, rain attenuation in the downlink from the satellite to the earth station cannot be guaranteed.

In addition, if you are trying to design a line with a margin for power, as shown in Figure 4, for example, if you are trying to keep the line failure due to rain attenuation to less than 0.1% per year in the Ku band, you will need a margin of 10 dB. This is more than the capacity that can be transmitted under normal conditions.
This corresponds to only one-tenth of the data being transmitted, which is extremely uneconomical.

Although the above problem of rain attenuation is not related to any communication system, another object of the present invention is to solve this problem using a TDMA communication system using a plurality of carrier waves.

[Means for solving problems]

One feature of the present invention for solving the above problems
One is that although it is a TDMA system, the power level of the transmitted carrier wave can be changed depending on the conditions of the other earth station and the content to be transmitted. Originally, the transmission power level of each carrier wave should be determined at the line design stage, but in the present invention, the bandwidth allowed for the satellite transponder and the available power allocation conditions for the multiple carrier waves used in the system as a whole are determined. The number of carrier waves, the power level of each carrier wave, etc. can be changed flexibly according to the network situation, and this is to be thoroughly implemented within the network using a burst time plan. .

That is, the maximum N within the same transponder of a satellite
In a TDMA satellite communication system capable of using up to (N≧2) carrier waves, at least one earth station among multiple earth stations operating based on mutually compatible burst time plans On the transmitting side, multiple carrier waves can be accessed in a time-division manner by frequency hopping,
The carriers of M waves actually used (N≧M≧
1) and set the power level of each carrier individually.

Here, we basically assume a system as shown in FIG. 3b, in which a receiving carrier frequency is assigned to each earth station. Note that a system in which a receiving carrier frequency is assigned to each earth station as shown in FIG. 3a has a slightly different configuration, and therefore will not be described here.

The value of N is the carrier frequency that is possible if the allowed frequency band width and available power in the transponder are used to transmit traffic with normal quality between large earth stations in the network during clear weather. It is considered to be the maximum value of

If there are small earth stations mixed in the system, specify the carrier wave that the small earth stations should receive, and increase the transmission power of each earth station's carrier wave appropriately, so that the same network can be used. Operation becomes possible. In this case, the power distribution naturally changes and the number of carrier waves in the system must decrease.

If it is necessary to transmit traffic with higher quality, by specifying a specific carrier wave and increasing power allocation to that carrier wave, high quality transmission can be performed using that carrier wave. This case also becomes a factor in reducing the number of carrier waves in the entire system.

Further, as will be described later, if a margin is provided in power distribution in consideration of rain attenuation, the number of carrier waves may be reduced.

One way to set the power level of each carrier is to
It is possible to set the carrier wave permanently or at any time depending on the reception conditions of the earth station that receives the carrier wave.

A permanent power level setting condition is the size of the receiving earth station, that is, the signal receiving ability. The local rainfall characteristics may also be taken into consideration. These conditions are taken into account when creating burst time plan data, so once the data burst containing the traffic to be received by a certain earth station is determined, the frequency of its carrier wave is determined, and then the transmit power level of that carrier wave is determined. It will be decided. When the transmission quality requirements for the traffic to be transmitted are specified in the burst time plan, this also becomes a condition for setting the power level.

When the content of the traffic is not managed by the burst time plan, each earth station monitors the transmission quality requirements of the traffic that each earth station accepts, and the results are used to determine the requirements of each earth station for creating a burst time plan. It is possible to allocate the necessary power by submitting as follows.

Setting the power level at any time is mainly a rain attenuation countermeasure. As mentioned above, transmission power control technology that compensates for rain attenuation has already been put into practical use in the uplink from the earth station to the satellite. The problem is compensation for rain attenuation in the downlink. Two methods are possible here. One method is that each earth station monitors the quality of its received signal, i.e., the bit error rate, and when an earth station detects that the quality of its received signal has deteriorated beyond a standard, it sends an alarm signal and Each receiving earth station voluntarily changes the power level of the carrier wave it is receiving according to a prescribed procedure.

Another method is to have a reference station intervene, and the reference station receives the alarm signal and changes the transmission power of each station as a command from the reference station. In this case, by using the monitoring function of the reference station, the above-mentioned transmission power control function can be executed in the form of a command from the reference station, and the related equipment of each station is simplified.

The premise that this method is possible is that burst
When creating a time plan, it is necessary to take into account power margins for rainfall. However, this power margin can be applied to any of a plurality of carrier waves. Therefore, by adopting this method, it is possible to eliminate the waste of securing power for all lines during clear weather, and it is possible to achieve a remarkable effect in terms of effective use of satellite power. can.

〔Example〕

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

FIG. 1 illustrates the principle of the present invention and is also the most typical embodiment. In the figure, there are 1,
It is assumed that there are 3 carrier waves 40, 2 and 3.
Each carrier wave is assigned a purpose of use.

In Fig. 1a, f3 is for transmitting signals that require normal transmission quality such as voice signals, f2 is for transmitting signals that require high transmission quality such as special data, and f1 is for transmitting signals such as earth stations of different scales or rainfall, etc. This is for transmitting signals destined for specific earth stations with different reception conditions than others. The power level of each carrier wave is set according to its purpose. Of course, the total power of the entire carrier must be within the power allocated to this system at the satellite transponder.

FIG. 1b shows, for example, a burst transmitted from station A. In this way, each station transmits bursts with different carrier frequencies and power levels, but these are planned in a burst time plan so that they are consistent as a whole, as shown in Figure 1a. As long as the system satisfies the allocated bandwidth and power conditions.

FIG. 5 shows an example of the configuration of a TDMA satellite communication system to which the present invention is applied. In the figure, earth station A30 and earth station B30' are shown to represent many earth stations, and earth station A30 is a reference station. The earth station A30 has a terrestrial line 102 and a terrestrial line connection device 36.
The traffic to be transmitted is transmitted by the communication terminal device 34 to the satellite 54 through the earth station device 32. In addition, the traffic to be received is routed through the terrestrial line 1 via the reverse route.
02. The monitoring and control device 38 is a device that includes a small computer function, and has control functions, data processing functions, etc., as well as monitoring the operating status and line status of each device in the earth station. Earth station B3
The configuration of 0' is basically the same as above, and is indicated by a number with (').

The reference station device 52 is a device for adding a function as a reference station to the earth station A30, and is connected to the central operation monitoring and control facility 50 located within the same earth station or at a remote location via a dedicated data line 104, and is capable of creating reference bursts, It performs data collection through transmission, reception of status information reported from each earth station in the system, and monitoring of each earth station burst, and plays the role of a front end processor for the central operation monitoring and control facility 50.

The central operation monitoring and control facility 50 has a monitoring and control function related to the operation of the entire system, and normally collects necessary information via the reference station device 52, but if this is not possible, it directly communicates with each earth station using the public telephone line 106. You can also access and collect information. In addition, it has a function of creating a burst time plan, and the created burst time plan is distributed via the satellite to each earth station through the reference station device 52, and downloaded into an appropriate storage circuit. As mentioned above, in the present invention, when creating a burst time plan, the scale of each earth station,
Reception conditions, transmission quality required for the traffic to be transmitted, etc. are also taken into consideration.

FIG. 6 is an example of a control circuit configuration used in the present invention. This circuit is mainly included in the communication terminal device 34. That is, the frame period counter 20
A counter 4 determines the reference timing on the transmitting side, and generates various timing signals at the TDMA frame period to control the operations of various circuits inside the communication terminal device. The burst time plan downloaded from the reference station via the satellite is processed by the supervisory control device 38, for example, and then loaded into the storage circuit 202. Memory circuit 2
The contents of 02 are sequentially read out according to the frame period counter 204. In addition to being used for data burst creation in burst shaping circuit 206, it is also added to frequency hopping control circuit 214 and used to control oscillator 212 for frequency hopping. Oscillator 212 includes up to N frequency synthesizers, the output of one of which is applied to modulator 208 in response to control information from frequency hopping control circuit 214 to define the frequency of the modulator's output. Traffic data supplied from the terrestrial line 102 passes through the terrestrial line connecting device 36, is converted into a data burst by a burst forming circuit 206, and is modulated into a carrier wave by a modulator 208. Normally, the modulator output is applied to the earth station device 32, subjected to frequency conversion and large power amplification, and then transmitted to the satellite.
An attenuator 210 is provided at the output of the earth station device 32, and the output power of the carrier wave from the earth station device 32 is controlled by controlling the amount of attenuation using the output of the frequency hopping control circuit 214. In the present invention, since the transmission output has a one-to-one correspondence with the carrier wave, the same data as the control data for carrier wave hopping can be used for output control.

〔Effect of the invention〕

As explained above, in the TDMA satellite communication system that uses multiple carrier waves, the present invention independently specifies the carrier waves to be transmitted depending on the size of the receiving station, the reception condition, the content of the traffic to be transmitted, etc. This technology solves many problems in business communications by transmitting electricity using low power, and makes it possible to efficiently use the available power of the satellite, which has a significant effect on reducing the overall cost of satellite communications.

[Brief explanation of the drawing]

1A and 1B are diagrams showing an embodiment of the present invention together with its principle, FIG. 2 is a diagram showing the principle of TDMA communication system, and FIGS. 3A and 3B are diagrams showing the principle of TDMA communication system, respectively.
Figure 4 is a diagram showing the principle of the communication system, Figure 4 is a diagram showing the amount of attenuation due to rain, Figure 5 is a block diagram showing an example of a communication system to which the present invention is applied, Figure 6 is the control circuit configuration for realizing the present invention. FIG. 2 is a block diagram showing an example. 10...Reference burst, 20...Data burst, 30...Earth station, 40...Carrier wave, 32...
...Earth station equipment, 34...Communication terminal equipment, 36...
Terrestrial line connection device, 38...Monitoring control device, 50
... Central operation monitoring and control facility, 52 ... Reference station equipment, 54 ... Satellite, 102 ... Terrestrial line, 202
... Memory circuit, 204 ... Frame period counter, 206 ... Burst forming circuit, 208 ... Modulator, 210 ... Attenuator, 212 ... Oscillator, 2
14...Frequency hopping control circuit.

Claims (1)

Claims: 1. A system comprising a plurality of earth stations operating according to mutually compatible burst time plans, with at least one earth station acting as a reference station and at least one earth station serving as a transmitter. In a TDMA satellite communication system, it is possible to access multiple carrier waves in a time-divisional manner by frequency hopping, and up to N (N≧2) carrier waves can be used within the same transponder of a satellite.
A specific value indicating the total power level of the carrier waves that can be used is assigned, and the carrier wave of the M waves ( N≧M≧1)
This method uses multiple carrier waves, which is characterized by selecting the carrier wave and setting the power level of each carrier wave individually.
TDMA method. 2. The TDMA system using a plurality of carrier waves according to claim 1, wherein the power level of each carrier wave is set permanently or as needed depending on the reception conditions of the earth station that receives the carrier wave. 3. The TDMA system using a plurality of carrier waves according to claim 2, wherein at least a part of the setting conditions for the power level of each carrier wave is given to related earth stations by a burst time plan. 4. Using a plurality of carrier waves according to claim 2, in which at least a part of the setting conditions for the power level of each carrier wave is determined based on information from a reference station or an earth station receiving the carrier wave. there was
TDMA method. 5. Using a plurality of carrier waves according to claim 1, wherein at least a part of the setting conditions for the power level of the carrier wave transmitted by at least one earth station at any time is determined in relation to the content of the transmitted signal. there was
TDMA method.