Space optical transmission apparatus and space optical transmission system

A space optical transmission apparatus is provided which achieves high-speed simultaneous space optical transmission with respect to a plurality of terminals. In the space optical transmission apparatus, a light receiving section receives an optical signal from a terminal. A control section estimates how much optical axes of a master station and the terminal are deviated from each other, based on the received optical signal. The control section selects one of a plurality of light sources which requires a smallest amount of shift of an optical axis thereof, based on the estimated optical axis deviation amount, so as to communicate with the terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a space optical transmission apparatus and a space optical transmission system for transmitting an optical signal via free space. More particularly, the present invention relates to a space optical transmission apparatus and a space optical transmission system for achieving simultaneous communication with a plurality of terminals.

2. Description of the Background Art

In the field of free-space optical transmission, for example, Japanese Patent Laid-Open Publication No. 2003-309524 (hereinafter referred to as Patent Document 1) discloses a conventional space optical transmission system which achieves communication between a master station and a plurality of terminals. In the conventional space optical transmission system disclosed in Patent Document 1, an optical signal output from a light source provided in the master station is reflected and divided into a plurality of light beams, and angles of the reflected light beams are adjusted so that the light beams are directed toward the respective terminals.FIG. 8is a diagram illustrating the conventional space optical transmission system disclosed in Patent Document 1.

InFIG. 8, the conventional space optical transmission system is composed of a master station810and a plurality of terminals811a,811band811c. In the master station810, a light source812outputs an optical signal which is modulated using a signal to be transmitted. The optical signal output from the light source812is converted into a bundle of light beams substantially parallel to each other by an optical system813, such as a lens or the like. In the master station810, mirrors815a,815band815cfor reflecting a portion of the bundle of light beams are provided within the light bundle. Also in the master station810, angle adjusting mechanisms814a,814band814care provided so as to adjust angles of the mirrors815a,815band815c. For example, the angle of the mirror814ais adjusted so that a portion reflected by the mirror81aof the light bundle is emitted toward the terminal811a.

Patent Document 1 also discloses a space optical transmission system which employs a plurality of sets of a pair of a space optical transmission apparatus and a terminal.FIG. 9is a diagram illustrating the conventional space optical transmission system disclosed in Patent Document 1. In the conventional space optical transmission system ofFIG. 9, space optical transmission apparatuses911a,911band911care provided at a center base910, respectively corresponding to terminals921a,921band921cprovided in the vicinity of the center base910. Thus, in the conventional space optical transmission system, by associating the space optical transmission apparatuses911a,911band911cwith the terminals921a,921band921c, the viewing angle of transmitted light is reduced, so that levels of optical signals received by the terminals921a,921band921care prevented from being decreased.

However, in the conventional space optical transmission system ofFIG. 8, since an optical signal output from the single light source812is reflected and divided, a high-power light source is required, however, there is an upper limit of the power of the optical signal output from the light source812for the purposes of a reduction in power consumption and securing of safety of eyes. Therefore, in the conventional space optical transmission system, there is a limitation on the power of optical signals transmitted to the terminals811a,811band811c, so that high-speed space optical transmission cannot be performed with respect to the terminals811a,811band811c.

In addition, in the conventional space optical transmission system ofFIG. 9, a large number of space optical transmission apparatuses911a,911band911cneed to be provided in the vicinity of the center base910, so that the whole system tends to be complicated, and it is difficult to secure a place for providing the space optical transmission apparatuses911a,911band911c.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a space optical transmission apparatus and a space optical transmission system which achieve high-speed simultaneous space optical transmission with respect to a plurality of terminals while avoiding a complicated configuration of the whole apparatus and system.

The present invention is directed to a space optical transmission apparatus for performing space optical transmission with respect to an optical signal using at least one terminal. To achieve the above-described object, the apparatus comprises a plurality of light sources for outputting optical signals modulated using a data signal to be transmitted, a plurality of lenses for converting the optical signals output from the plurality of light sources into optical signals having an appropriate viewing angle, a direction control section for controlling directions of the optical signals output from the plurality of lenses, at least one reception section for receiving an upward signal or signals output by the at least one terminal, and a control section for controlling the plurality of light sources and the direction control section based on location information of the at least one terminal. The control section specifies the location information of the at least one terminal from the received upward signal or signals from the at least one terminal, and based on the specified location information of the at least one terminal, drives only a required one or ones of the plurality of light sources, and controls the direction control section so that an optical axis or axes of the optical signal or signals are directed toward the at least one terminal.

Preferably, the control section selects one or ones of the plurality of light sources which has smallest amount of shift of an optical axis thereof, based on the specified location information of the at least one terminal, and drives only the selected light source or sources.

Preferably, the optical signals output by the plurality of light sources cover a spatial area including at least areas varying among the light sources. Thereby, the space optical transmission apparatus can perform simultaneous space optical transmission with respect to a plurality of terminals, and communication in a wider area.

When the upward signal or signals output by the at least one terminal are an optical signal, the at least one reception section is at least one light receiving section for receiving the optical signal or signals output by the at least one terminal, and the control section detects a direction or directions of the at least one terminal based on the optical signal or signals received by the at least one light receiving section, and specifies the location information of the at least one terminal from the detected direction or directions of the at least one terminal.

When the upward signal or signals output by the at least one terminal are an optical signal, a viewing angle of the optical signal output via the direction control section is narrower than a viewing angle or angles of the optical signal or signals output by the at least one terminal. Thereby, the space optical transmission apparatus can obtain the location information of a terminal in a wider area, and a power of received light required for space optical transmission can be easily secured.

The upward signal or signals output by the at least one terminal may be a wireless signal. In this case, the at least one reception section receives the wireless signal or signals output by the at least one terminal. The control section detects a direction or directions of the at least one terminal based on the wireless signal received by the at least one reception section, and obtains the location information of the at least one terminal from the detected direction or directions of the at least one terminal. Also in this case, the space optical transmission apparatus can specify the location information of a terminal.

Preferably, wherein the at least one reception section receives an upward signal or signals including a relationship between relative locations of the space optical transmission apparatus and the at least one terminal, from the at least one terminal. The control section specifies the location information of the terminal from the relationship between relative locations of the space optical transmission apparatus and the at least one terminal, the relative relationship being included in the upward signal or signals received from the at least one terminal. Thereby, the space optical transmission apparatus can easily specify the location information of a terminal.

The at least one terminal may comprise a plurality of light receiving sections for converting an optical signal output from the space optical transmission apparatus into an electrical signal, a location information specifying section for specifying a relationship between relative locations of the space optical transmission apparatus and the terminal from the electrical signals output from the plurality of light receiving sections, and outputting location information of the terminal, and a transmission section for transmitting the location information output from the location information specifying section. In this case, the reception section receives the location information from the transmission section. The control section specifies the location information of the terminal based on the location information from the reception section.

Preferably, the space optical transmission apparatus further comprises a plurality of code generating sections for generating specific code sequences corresponding to the plurality of light sources, and a plurality of modulation sections for modulating the optical signals output by the light sources using the respective corresponding specific code sequences. The at least one reception section receives an upward signal including information about the light source corresponding to the specific code sequence or sequences from the at least one terminal. The control section specifies a light source generating the optical signal or signals received by the at least one terminal based on the information about the light source included in the upward signal or signals received by the at least one terminal, thereby specifying the location information of the at least one terminal. Thereby, the space optical transmission apparatus can easily specify the location information of a terminal.

Preferably, the space optical transmission apparatus further comprises a data switch section for selecting one or ones outputting an optical signal from the plurality of light sources based on the location information of the at least one terminal specified by the control section, and outputting the data signal toward the selected light source or sources. The space optical transmission apparatus may further comprises a data distributing section for distributing the data signal to the plurality of light sources. Thereby, the space optical transmission apparatus can perform broadcast-type space optical transmission with respect to a plurality of terminals with low power consumption.

Preferably, the control section controls the direction control section so that the optical signals output by the plurality of light sources are emitted toward the at least one terminal which can perform space optical transmission. Thereby, the space optical transmission apparatus can easily specify the location information of a terminal.

Preferably, the space optical transmission apparatus may further comprise a data switch section for selecting one or ones outputting an optical signal from the plurality of light sources based on the location information of the at least one terminal, and outputting the data signal toward the selected light source or sources, and a variable-rate modulation section for changing a modulation scheme of the optical signal output by the light source, depending on a state of reception of the optical signal at the at least one terminal. Thereby, the space optical transmission apparatus can select an optimal modulation scheme, depending on a state of communication with a terminal.

The space optical transmission apparatus may further comprises a data distributing section for distributing the data signal to the plurality of light sources, and a variable-rate modulation section for changing a modulation scheme of the optical signal output by the light source, depending on a state of reception of the optical signal at the at least one terminal.

The control section may control the direction control section so that the optical signals output by the plurality of light sources are multiplexed and output to the at least one terminal. Thereby, the space optical transmission apparatus can easily secure a power of received light required for space optical transmission, thereby making it possible to achieve high-speed and high-quality space optical transmission.

Preferably, the upward signal received by the at least one terminal is a time division multiplexed signal. Alternatively, the upward signal or signals received by the at least one terminal may be a frequency division multiplexed signal. Alternatively, the upward signal or signals received by the at least one terminal may be a code division multiplexed signal.

Also, the present invention is directed to a space optical transmission system in which a space optical transmission apparatus and at least one terminal perform space optical transmission using an optical signal. To achieve the above-described object, the space optical transmission apparatus comprises a plurality of light sources for outputting optical signals modulated using a data signal to be transmitted, a plurality of lenses for converting the optical signals output from the plurality of light sources into optical signals having an appropriate viewing angle, a direction control section for controlling directions of the optical signals output from the plurality of lenses, at least one reception section for receiving an upward signal or signals output by the at least one terminal, and a control section for controlling the plurality of light sources and the direction control section based on location information of the at least one terminal. The control section specifies the location information of the at least one terminal from the received upward signal or signals from the at least one terminal, and based on the specified location information of the at least one terminal, drives only a required one or ones of the plurality of light sources, and controls the direction control section so that an optical axis or axes of the optical signal or signals are directed toward the at least one terminal. The terminal comprises a light receiving section for receiving the optical signal output by the space optical transmission apparatus, and a light source for outputting an optical signal toward the terminal.

As described above, according to the present invention, the space optical transmission apparatus selects one of a plurality of light sources based on the location information of a terminal to communicate therewith, thereby making it possible to achieve simultaneous space optical transmission with respect to a plurality of terminals. In addition, since the location information of a terminal to communicate therewith is specified, viewing angles of optical signals of a plurality of light sources can be narrowed. Thereby, a power of received light required for space optical transmission is easily secured, thereby making it possible to achieve high-speed and high-quality space optical transmission.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a block diagram illustrating an exemplary configuration of a space optical transmission system according to a first embodiment of the present invention. InFIG. 1, the space optical transmission system of the present invention is composed of a space optical transmission apparatus110and a first terminal130-1. Note that the space optical transmission apparatus is hereinafter referred to as a master station. Also, the first terminal130-1is simply referred to as a terminal130-1. The master station110communicates with the terminal130-1by space optical transmission. The master station110comprises an input terminal1, a plurality of light sources111a,111band111c, a plurality of lenses112a,112band112c, a direction control section113, a plurality of light receiving sections120a,120band120c(the light receiving section120cis not shown), and a control section121. The terminal130-1comprises a light receiving section131, a light source132, and an output terminal2.

An operation of the space optical transmission system of the first embodiment of the present invention will be described with reference toFIG. 1. InFIG. 1, when the terminal130-1wants to communicate with the master station110, an upward optical signal having a wide viewing angle is transmitted from the light source132to the master station110. Note that it is assumed that the terminal130-1knows an approximate location of the master station110. The upward optical signal output from the light source132is received by the light receiving sections120a,120band120cof the master station110. In the master station110, the light receiving sections120a,120band120ceach convert the received optical signal into an electrical signal, and input the electrical signal into the control section121.

The control section121specifies a direction from which the upward optical signal comes from the terminal130-1(i.e., information about a location of the terminal130-1), based on the three electrical signals input from the light receiving sections120a,120band120c. For example, the control section121selects two of the three electrical signals input from the light receiving sections120a,120band120c, and calculates a difference between the two selected electrical signals, thereby estimating how much optical axes of the master station110and the terminal130-1are deviated from each other. Based on the estimated optical axis deviation amount, the control section121selects one of the light sources111a,111band111cwhich requires a smallest amount of shift of an optical axis thereof. It is here assumed that the light source111bis selected.

A data signal to be transmitted is input to the light source111bvia the input terminal1. The light source111boutputs an optical signal which is modulated using the input data signal. The optical signal output from the light source111bis converted into an optical signal having an appropriate viewing angle by the lens112b, is controlled by the direction control section113so that an optical axis thereof coincides with a direction of the terminal130-1, and is output into space.

Thus, the space optical transmission system drives only the light source111brequired for space optical transmission, and does not drive the other light sources111aand111c, resulting in low power consumption. In addition, if the viewing angle of the upward optical signal transmitted toward the master station110by the terminal130-1is broader than the viewing angle of a downward optical signal, the probability that the master station110can receive the upward optical signal is increased, thereby making it possible to easily achieve space optical transmission even in a wide area.

Next, an operation of the space optical transmission system when a second terminal130-2enters a communication area while the terminal130-1is communicating with the master station110, will be described with reference toFIG. 2.

FIG. 2is a block diagram illustrating an exemplary configuration of the space optical transmission system when the space optical transmission system simultaneously communicates with a plurality of terminals. The space optical transmission system ofFIG. 2has the same configuration as that ofFIG. 1, except that only the second terminal130-2is added to the space optical transmission system ofFIG. 1.

InFIG. 2, when the second terminal130-2wants to communicate with the master station110, the second terminal130-2transmits an upward optical signal having a wide viewing angle from a light source132toward the master station110, as is similar to the terminal130-1. Note that the second terminal130-2is hereinafter simply referred to as a terminal130-2. The master station110estimates a direction toward a location of the terminal130-2by receiving the upward optical signal of the terminal130-2. This operation of the terminal130-2is transmission of the upward optical signal toward the master station110, and therefore, does not have a bad influence on communication of other terminals. Thereafter, the master station110selects a light source (e.g., the light source111c) for communicating with the terminal130-2in a manner similar to that when the light source111bis selected, and performs space optical transmission with respect to the terminal130-2.

As described above, in the space optical transmission system of the first embodiment of the present invention, the master station110selects a light source to be used from the light sources111a,111band111cbased on information about the location of a terminal to be communicated with, thereby making it possible to achieve simultaneous space optical transmission with respect to a plurality of terminals. In addition, the information about the location of a terminal to be communicated with is specified, thereby making it possible to narrow the viewing angles of the optical signals output from the light sources111a,111band111c. Thereby, a power of received light required for space optical transmission is easily secured, thereby making it possible to achieve high-speed and high-quality space optical transmission.

Note that the number of the light receiving sections120a,120band120cis not limited to three, and may be four or more. Also, the number of the light sources111a,111band111cand the number of the lenses112a,112band112care not limited to three, and may be any number.

In the above description, the upward signal from a terminal to the master station110is assumed to be an optical signal. Instead of the optical signal, a wireless signal may be used. In this case, in the space optical transmission system, a plurality of wireless antennas may be provided in the master station110and may be adaptive array antennas which estimate directions from which wireless signals transmitted from terminals come. Even if the wireless signal is used as the upward signal in the space optical transmission system, an effect similar to that when an optical signal is used can be obtained. When the light receiving sections120a,120band120creceive a signal other than optical signals from a terminal, the light receiving sections120a,120band120care referred to as reception sections120a,120band120c.

Second Embodiment

FIG. 3is a block diagram illustrating an exemplary configuration of a space optical transmission system according to a second embodiment of the present invention. InFIG. 3, a master station310has a configuration different from that of the master station110ofFIG. 1in that the light receiving sections120band120care removed. The other parts of the master station310are indicated with the same reference numerals as those of the master station110ofFIG. 1and will not be described. A third terminal330comprises a plurality of light receiving sections131a,131band131c, a light source132, a location information specifying section331, and an output terminal2. Note that the third terminal330is hereinafter referred to as a terminal330.

An operation of the space optical transmission system of the second embodiment of the present invention will be described with reference toFIG. 3. InFIG. 3, when the terminal330wants to communicate with the master station310, an optical signal output from the master station310is received by the light receiving sections131a,131band131c. The light receiving sections131a,131band131ceach convert the received optical signal into an electrical signal, and inputs the electrical signal into the location information specifying section331. The location information specifying section331specifies a relationship between relative locations of the master station310and the terminal330(i.e., location information) from the input electrical signal. The light source132modulates an optical signal using the location information specified by the location information specifying section331, and transmits the modulated optical signal toward the master station310.

An upward optical signal transmitted from the terminal330is received by the light receiving section120aof the master station310. In the master station310, the light receiving section120aconverts the received optical signal into an electrical signal, and inputs the electrical signal into the control section121. The control section121detects the location information from the input electrical signal, and specifies a location of the terminal330. The control section121selects one of the light sources111a,111band111cwhich requires a smallest amount of shift of an optical axis thereof. It is here assumed that the light source111bis selected.

A data signal is input to the light source111bvia the input terminal1. The light source111boutputs an optical signal which is modulated using the input data signal. The optical signal output from the light source111bis converted into an optical signal having an appropriate viewing angle by the lens112b, is controlled by the direction control section113so that an optical axis thereof coincides with a direction of the terminal330, and is output into space.

Also, in the space optical transmission system of the second embodiment, even when a plurality of terminals are present, it is possible to communicate with the terminals by performing an operation similar to that of the first embodiment.

As described above, in the space optical transmission system of the second embodiment of the present invention, a terminal specifies the relationship between the relative locations of the terminal and the master station310. Therefore, the master station310can easily specify the location information of the terminal. Based on the location information of the terminal to be communicated with, the master station310selects one to be used of the light sources111a,111band111c, thereby making it possible to achieve simultaneous space optical transmission with respect to a plurality of terminals. Also, since the location information of a terminal to be communicated with is specified, the viewing angles of optical signals output from the light sources111a,111band111ccan be narrowed. Thereby, a power of received light required for space optical transmission is easily secured, thereby making it possible to achieve high-speed and high-quality space optical transmission.

In the above description, the upward signal from the terminal330to the master station310is assumed to be an optical signal. Instead of the optical signal, a wireless or wired signal may be used. In this case, in the space optical transmission system, any configuration which can transmit information from a terminal to the master station310may be used, including, for example, a configuration in which a wireless antenna is provided in the master station310where a wireless signal is used, a configuration in which a cable is used, and the like.

Third Embodiment

FIG. 4is a block diagram illustrating an exemplary configuration of a master station410according to a third embodiment of the present invention. The master station410ofFIG. 4is different from the master station310ofFIG. 3in that a data switch section411, a plurality of code generating sections412a,412band412c, and a plurality of modulation sections413a,413band413care further provided. The other parts of the master station410are indicated with the same reference numerals as those of the master station310ofFIG. 3and will not be described.

FIG. 5is a block diagram illustrating an exemplary configuration of a terminal530according to a third embodiment of the present invention. InFIG. 5, the fourth terminal530is different from the terminal130-1ofFIG. 1in that a demodulation section511is further provided. The other parts of the fourth terminal530are indicated with the same reference numerals as those of the terminal130-1ofFIG. 1and will not be described. Note that the fourth terminal530is hereinafter simply referred to as a terminal530.

An operation of the space optical transmission system of the third embodiment of the present invention will be described with reference toFIGS. 4 and 5. In the space optical transmission system of the third embodiment, as in the second embodiment, the terminal530is a main part which specifies a relative relationship (i.e., location information) between locations of the master station410and the terminal530.

InFIG. 4, the master station410comprises the code generating sections412a,412band412ccorresponding to the light sources111a,111band111cso as to make it easier for the terminal530to specify the location information. The code generating sections412a,412band412cgenerate specific code sequences corresponding to the light sources111a,111band111c, respectively. The code sequences are input to the modulation sections413a,413band413c. The modulation sections413a,413band413cmodulate optical signals output from the light sources111a,111band111c, using the input code sequences.

Specifically, the code sequences output from the code generating sections412a,412band412care converted into optical signals by the light sources111a,111band111cvia the modulation sections413a,413band413c. In the master station410, when the location information is specified, all of the light sources111a,111band111coutput optical signals41a,41band41c. Note that the master station410may transmit the code sequences either repeatedly or intermittently.

InFIG. 5, in the terminal530, a light receiving section131aconverts a received optical signal into an electrical signal, and inputs the electrical signal into the demodulation section511. The demodulation section511demodulates a code sequence included in the input electrical signal to specify one of the light sources111a,111band111cof the master station410which outputs the optical signal. It is here assumed that the light source111ais specified. The light source132modulates an optical signal using information about the light source111aspecified by the demodulation section511, and transmits the modulated optical signal toward the master station410.

The upward optical signal output by the terminal530is received by the light receiving section120aof the master station410. In the master station410, the light receiving section120aconverts the received optical signal into an electrical signal, and inputs the electrical signal into the control section121. The control section121detects information of a light source included in the input electrical signal, and based on the detected light source information, specifies one of the light sources111a,111band111cwhich outputs the optical signal. It is here assumed that the light source111ais specified. Thereby, the control section121can know that the optical signal output by the light source111ahas been received by the terminal530, and can specify the location information of the terminal530.

Note that the terminal530may modulate an optical signal using a code sequence demodulated by the demodulation section511, and may return the demodulated code sequence to the master station410. Also in this case, the master station410can specify the location information of the terminal530. Specifically, in the master station410, the control section121detects a code sequence included in an input electrical signal, and based on the detected code sequence, specifies one of the light sources111a,111band111cwhich outputs the optical signal. Thereby, the control section121can know that the optical signal output by the light source111ahas been received by the terminal530, and can specify the location information of the terminal530.

Next, a method with which the master station410transmits a data signal to the terminal530, will be described. In the master station410, a data signal is input via the input terminal1to the data switch section411. The data switch section411selects a terminal to which the data signal should be transmitted, depending on a content of the data signal, and outputs the data signal to an appropriate channel. It is here assumed that the terminal530is selected as the terminal to which the data signal should be transmitted, and a channel which employs the light source111ais selected as the appropriate channel.

The data signal output from the data switch section411is input to the modulation section413a. Therefore, the code sequence output from the code generating section412aand the data signal output from the data switch section411are input to the modulation section413a. When the location information of a terminal corresponding to the light source111ahas been specified in the control section121, the modulation section413aselects the data signal. On the other hand, when the location information of a terminal corresponding to the light source111ahas not been specified in the control section121(i.e., a terminal corresponding to the light source111ahas not been detected), the code sequence is selected. The signal selected by the modulation section413ais emitted into space via the light source111a, a lens112a, and the direction control section113.

As described above, in the space optical transmission system of the third embodiment of the present invention, the master station410assigns specific code sequences to the respective light sources111a,111band111c, and emits an optical signal including a code sequence into space. A terminal which receives the optical signal specifies a light source which has output the optical signal, from the code sequence, and outputs information about the specified light source toward the master station410. Thereby, the master station410can easily specify the location information of the terminal.

Note that, in the space optical transmission system of this embodiment, a data distributing section which distributes a data signal to each channel may be provided instead of the data switch section411ofFIG. 4, thereby making it possible to perform broadcast-type space optical transmission which communicates the same data over a wide area.

In the above description, the upward signal from the terminal530to the master station410is assumed to be an optical signal. Instead of the optical signal, a wireless or wired signal may be used. In this case, in the space optical transmission system, any configuration which can transmit information from a terminal to the master station410may be used, including, for example, a configuration in which a wireless antenna is provided in the master station410where a wireless signal is used, a configuration in which a cable is used, and the like.

Fourth Embodiment

FIG. 6is a block diagram illustrating an exemplary configuration of a master station610according to a fourth embodiment of the present invention. The master station610ofFIG. 6is different from the master station310ofFIG. 3in that a data switch section411and a plurality of variable-rate modulation sections613a,613band613care further provided. The other parts of the master station610are indicated with the same reference numerals as those of the master station310ofFIG. 3and will not be described.

In the space optical transmission system of the fourth embodiment, the master station610is characterized by multiplexing and transmitting an optical signal toward a terminal (not shown). The master station610specifies the location information of the terminal using any method described in the first to third embodiments. In the master station610, the control section121determines whether or not an optical signal is to be multiplexed before transmission to the terminal, depending on the location information of the terminal, the data type, the type of a service to be provided, or the like. It is here assumed that the control section121determines that the optical signals41a,41band41care to be multiplexed before transmission to the terminal.

The control section121controls the data switch section411to output a data signal to the variable-rate modulation sections613a,613band613c. Also, the control section121determines a rate at which the data signal is modulated, depending on the location information of a terminal, a data type, the type of a service to be provided, or the like. The control section121can change the determined rate, depending on a state of communication with a terminal, or the like. The variable-rate modulation sections613a,613band613cchange the modulation rates of optical signals output by the light sources111a,111band111cinto the modulation rate determined by the control section121. Alternatively, the variable-rate modulation sections613a,613band613cmay modulate the data signal output by the data switch section411at the rate determined by the control section121.

Note that the control section121may determine a modulation scheme of a data signal instead of determining the modulation rate of a data signal. In this case, the variable-rate modulation sections613a,613band613cchange the modulation schemes of optical signals output by the light sources111a,111band111cinto the modulation scheme determined by the control section121. Alternatively, the variable-rate modulation sections613a,613band613cmay modulate the data signal output by the data switch section411using the modulation scheme determined by the control section121.

The control section121controls the light sources111a,111band111cand the direction control section113to multiplex a plurality of optical signals before transmission to terminals. A plurality of data signals output from the variable-rate modulation sections613a,613band613care converted into a plurality of optical signals by the light sources111a,111band111c. The optical signals output by the light sources111a,111band111care multiplexed and emitted into space by the direction control section113via the lenses112a,112band112c.

As described above, according to the space optical transmission system of the fourth embodiment of the present invention, the master station610can multiplex and transmit a plurality of optical signals to terminals. Thereby, a power of received light required for space optical transmission is easily secured, thereby making it possible to achieve high-speed and high-quality space optical transmission.

Note that, in the space optical transmission systems according to the first to fourth embodiments of the present invention, the master station can also be composed of a plurality of direction control sections.FIG. 7is a block diagram illustrating an exemplary configuration of a space optical transmission system in which a master station110acomprises a plurality of direction control sections. Also in the master station110aofFIG. 7, the direction of an optical signal is controlled by any of a plurality of direction control sections113a,113band113c, and the resultant optical signal is emitted into space.

The first to fourth embodiments of the present invention have been described, assuming that transmission from a plurality of terminals to a master station is performed using time division multiplexing. Alternatively, any other multiplexing transmission technique may be used, including frequency division multiplexing, code division multiplexing, and the like.

The features of the space optical transmission systems according to the first to fourth embodiments of the present invention may be used in combination. For example, the master station110of the first embodiment (seeFIG. 1) and the terminal330of the second embodiment (seeFIG. 3) can be used in combination.

The space optical transmission system of the present invention can be applied to, for example, a system in which simultaneous space optical transmission is achieved between a master station and a plurality of terminals.