Radio communication system, radio communication device, radio communication method, and radio communication program

Abstract

An objective of the present invention is to provide a radio communication system capable of suppressing a reduction in channel capacity caused by an error in the orbit prediction of an aerial base station when spatial multiplexing transmission is performed between the aerial base station and a ground station, the aerial base station moving around in an orbit in the sky and being capable of periodically communicating with the ground station. To this end, a processor circuit provided in one or both of the ground station and the aerial base station is configured to execute: capacity calculation processing for calculating a channel capacity between the ground station and the aerial base station on the basis of an orbit prediction of the aerial base station; and communication control processing for switching an antenna used in spatial multiplexing transmission between the ground station and the aerial base station on the basis of the channel capacity calculated in the capacity calculation processing. The communication control processing includes processing whereby switching of the antenna used for spatial multiplexing transmission between the ground station and the aerial base station, in a time period during which communication between the ground station and the aerial base station is possible, is suppressed to a greater extent in a first time period in which the degree of change over time in the channel capacity calculated in the capacity calculation processing is equal to or greater than a preset reference, than in a second time period other than the first time period, within the time period during which communication is possible.

Description

Wireless communication system, wireless communication device, wireless communication method, and wireless communication program

[0001] The present disclosure relates to a wireless communication system, a wireless communication device, a wireless communication method, and a wireless communication program.

[0002] A wireless communication system having a first wireless communication device and a second wireless communication device, wherein the first wireless communication device wirelessly communicates with the second wireless communication device using one or more first antennas, and the second wireless communication device wirelessly communicates with the first wireless communication device using one or more second antennas. A wireless communication system that controls the change of the first antenna that wirelessly communicates with the second wireless communication device among the first antennas of each of the plurality of first wireless communication devices, or the second antenna that wirelessly communicates with the first wireless communication device among the plurality of second antennas of the second wireless communication device, based on the transmission capacity at each time between the first antenna and the second antenna calculated using the movement schedule information indicating the position of the first wireless communication device having one or more first antennas at each time and the position of the second antenna is known (for example, see Patent Document 1). Also, as documents referring to the transmission time and channel capacity in low-orbit satellite MIMO transmission, for example, there are Non-Patent Documents 1 and 2.

[0003] International Publication No. 2021 / 250772

[0004] Kojima, Itoikawa, Yamashita, "A Study on Transmission Time Prediction in Low-Orbit Satellite MIMO Transmission Experiments", 2024 Institute of Electronics, Information and Communication Engineers Society Conference, B-3-24C. Kato, M. Nakadai, D. Goto, H. Shibayama and F. Yamashita, "Channel Capacity Analysis of Satellite MIMO System Depending on the Orbital Altitude", 37th AIAA International Communication Satellite Systems Conference (ICSSC 2019), Oct. 2019

[0005] However, as in the technology described in Patent Document 1, when calculating channel capacity based on movement schedule information indicating the time-dependent position of a wireless communication device, i.e., the orbital prediction of an upper-air station, and determining the antenna combination to be used for spatial multiplexing transmission between a ground station and an upper-air station, errors in the orbital prediction of the upper-air station become a problem. The orbital prediction of the upper-air station has errors compared to the actual orbit due to the influence of the gravitational pull of the sun and moon, solar flares, the atmosphere, etc. Furthermore, this error increases as time elapses since the issuance of the orbital information. As a result, errors also occur in the channel capacity calculated based on the inaccurate orbital prediction, and by switching the antennas used for spatial multiplexing transmission based on such an inaccurate channel capacity, it is possible to select an antenna combination that actually has a low channel capacity. If an antenna with a low actual channel capacity is switched due to an error, not only will the communication quality not improve because the optimal antenna cannot be selected, but it may also lead to a decrease in communication quality.

[0006] This disclosure was made to solve these problems. Its purpose is to provide a wireless communication system, wireless communication device, wireless communication method, and wireless communication program that can suppress channel capacity reduction caused by errors in the orbit prediction of the upper station when performing spatial multiplex transmission between a ground station having multiple antennas and an upper station having multiple antennas that orbits in the upper atmosphere and can periodically communicate with the ground station.

[0007] The wireless communication system according to this disclosure comprises a ground station having a plurality of antennas and an air station having a plurality of antennas that orbits in an upper orbit and is periodically able to communicate with the ground station, and is a wireless communication system capable of performing spatial multiplex transmission between the ground station and the air station, wherein one or both of the ground station and the air station are equipped with a processor circuit and a memory storing a program executed by the processor circuit, and the processor circuit is configured to perform a capacity calculation process that calculates the channel capacity between the ground station and the air station based on the orbital prediction of the air station, and a communication control process that switches the antenna used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, and the communication control process includes a process that suppresses the switching of the antenna used for spatial multiplex transmission between the ground station and the air station to a greater extent than in a second time period other than the first time period, in a first time period during which the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard within the communicationable time period between the ground station and the air station.

[0008] The wireless communication device according to this disclosure functions as a ground station having multiple antennas, or an air station having multiple antennas that orbits in the air and periodically communicates with the ground station, and is capable of performing spatial multiplex transmission between the ground station and the air station, comprising a processor circuit and a memory storing a program executed by the processor circuit, wherein the processor circuit is configured to perform a capacity calculation process that calculates the channel capacity between the ground station and the air station based on the orbital prediction of the air station, and a communication control process that switches the antennas used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, and the communication control process includes a process that suppresses the switching of antennas used for spatial multiplex transmission between the ground station and the air station to a greater extent than in a second time period other than the first time period, in a first time period during which the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard within the communicationable time period between the ground station and the air station.

[0009] The wireless communication method according to this disclosure is a wireless communication method that performs spatial multiplex transmission between a ground station having a plurality of antennas and an air station having a plurality of antennas that orbits in the air and periodically communicates with the ground station, comprising: a capacity calculation step of calculating the channel capacity between the ground station and the air station based on the orbital prediction of the air station; and a communication control step of switching the antennas used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation step, wherein the communication control step includes a step of suppressing the switching of the antennas used for spatial multiplex transmission between the ground station and the air station to a degree greater than or equal to a preset standard in a first time period within the communicationable time period between the ground station and the air station, compared to a second time period other than the first time period.

[0010] The wireless communication program relating to this disclosure is a computer-readable wireless communication program for realizing the wireless communication device described above, and includes a program that causes a processor circuit of the wireless communication device to execute a capacity calculation process for calculating the channel capacity between the ground station and the air station based on the orbital prediction of the air station, and a communication control process for switching the antenna used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, wherein the communication control process includes a process that suppresses the switching of the antenna used for spatial multiplex transmission between the ground station and the air station to a greater extent than in a second time period other than the first time period, in a first time period within the communicationable time period between the ground station and the air station where the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard.

[0011] According to the wireless communication system, wireless communication device, wireless communication method, and wireless communication program described herein, when performing spatial multiplex transmission between a ground station having multiple antennas and an upper-air station having multiple antennas that orbits in the upper atmosphere and periodically communicates with the ground station, it is possible to suppress channel capacity reduction caused by errors in the orbit prediction of the upper-air station.

[0012] This is a diagram showing an example of the overall configuration of the wireless communication system according to Embodiment 1. This is a block diagram showing the functional configuration of the ground base station of the wireless communication system according to Embodiment 1. This is a diagram showing an example of the time change of channel capacity in the wireless communication system according to Embodiment 1. This is a diagram showing an example of the time change of channel capacity in the wireless communication system according to Embodiment 1. This is a diagram showing an example of antenna switching in the wireless communication system according to Embodiment 1. This is a flowchart showing an example of the communication control processing of the wireless communication system according to Embodiment 1. This is a flowchart showing an example of the communication control processing of the wireless communication system according to Embodiment 1. This is a flowchart showing an example of the communication control processing of the wireless communication system according to Embodiment 1.

[0013] The embodiments for implementing the wireless communication system, wireless communication device, wireless communication method, and wireless communication program related to this disclosure will be described with reference to the attached drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and redundant explanations are simplified or omitted as appropriate. For convenience, the positional relationships of each structure may be expressed based on the illustrated state in the following description. This disclosure is not limited to the embodiments described below, and any combination of embodiments, any modification of any component of each embodiment, or any omission of any component of each embodiment is possible without departing from the spirit of this disclosure.

[0014] Embodiment 1. Embodiment 1 of the present disclosure will be described with reference to Figures 1 to 8. Figure 1 is a diagram showing an example of the overall configuration of a wireless communication system. Figure 2 is a block diagram showing the functional configuration of a ground base station of a wireless communication system. Figures 3 and 4 are diagrams showing examples of the time variation of channel capacity in a wireless communication system. Figure 5 is a diagram showing an example of antenna switching in a wireless communication system. Figures 6 to 8 are flowcharts showing examples of communication control processing in a wireless communication system.

[0015] As shown in Figure 1, the wireless communication system 100 according to this disclosure comprises a plurality of first-layer airborne radio stations 110, a plurality of second-layer airborne radio stations 120, and a third-layer airborne radio station 130. The wireless communication system 100 also comprises a ground terminal station 180 and a plurality of ground base stations 140.

[0016] The first-layer airborne radio station 110 is a radio relay station that orbits in the upper atmosphere, including outer space, and periodically communicates with a ground base station 140, described later, which is installed in a predetermined ground area. Examples of the first-layer airborne radio stations 110 include HAPS (High Altitude Platform System), drones, unmanned aerial vehicles, aircraft, etc. Nearby first-layer airborne radio stations 110 are connected to each other by a first-layer internal circuit 2 and communicate wirelessly or by optical signal. The first airborne network 111 is formed by the first-layer airborne radio stations 110 connected via the first-layer internal circuit 2.

[0017] The second-layer upper-air radio station 120, like the first-layer upper-air radio station 110, is a radio relay station that orbits in the upper atmosphere, including outer space, and periodically communicates with the ground base station 140, described later, which is installed in a predetermined ground area. Examples of second-layer upper-air radio stations 120 include LEO (Low Earth Orbit) satellites that move in sun-synchronous orbits, sun-synchronous quasi-recurrent orbits, circular orbits, polar orbits, etc., at an orbital altitude of 2000 km or less. Nearby second-layer upper-air radio stations 120 are connected by a second-layer internal circuit 4 and communicate wirelessly or using optical signals. The second upper-air network 121 is formed by the second-layer upper-air radio stations 120 connected via the second-layer internal circuit 4.

[0018] Furthermore, the second-layer airborne radio station 120 receives data transmitted from the ground terminal station 180 via the ground station inter-station line 1. The second-layer airborne radio station 120 then relays the data via the second airborne network 121 to another second-layer airborne radio station 120 that can communicate with an airborne radio station of a different layer, or to another second-layer airborne radio station 120 that can communicate with the ground base station 140.

[0019] A second-layer airborne radio station 120, capable of communicating with airborne radio stations on different layers, transmits the received data to the airborne radio station on the different layer via the first inter-network link 3 or the second inter-network link 8. Furthermore, a second-layer airborne radio station 120, capable of communicating with a ground base station 140, transmits the received data to the ground base station 140 via the data channel 5.

[0020] The third-layer airborne radio station 130 is, for example, a GEO (Geostationary Orbit) satellite. The third-layer airborne radio station 130 can communicate with one or more of the second-layer airborne radio stations 120 belonging to the second airborne network 121.

[0021] In this disclosure, when it is not necessary to distinguish between the first-layer airborne radio station 110 and the second-layer airborne radio station 120, they are simply referred to as airborne radio station 160. As mentioned above, airborne radio station 160 is an airborne station that orbits in the air and is able to periodically communicate with ground base station 140. Although Figure 1 illustrates an example in which multiple airborne radio stations 160 constitute a three-layer non-terrestrial network, the number of layers of airborne radio stations in the wireless communication system 100 is not limited to three. The wireless communication system 100 may further include a fourth-layer airborne radio station, a fifth-layer airborne radio station, and so on, as needed. Furthermore, airborne radio stations 160 do not necessarily have to be arranged according to layer.

[0022] The ground base station 140 receives data transmitted from the airborne radio station 160 via the data channel 5 and transmits it to the ground network. In other words, the ground base station 140 functions as a gateway station for the ground network. The ground base station 140 may also be a portable radio station such as a Very Small Aperture Terminal (VSAT).

[0023] The ground terminal station 180 is a smartphone, IoT terminal, Bluetooth® communication device, etc., that communicates wirelessly with the airborne radio station 160 via the ground station inter-station line 1. The ground terminal station 180 transmits data such as internet traffic to the airborne radio station 160.

[0024] Thus, in the wireless communication system 100 according to this embodiment, the ground terminal station 180 transmits data to the airborne radio station 160. The transmitted data is relayed by the airborne radio station 160 and transmitted to the ground base station 140. Although not explained here, it is also possible for the ground base station 140 to transmit data to the ground terminal station 180 via relay by the airborne radio station 160.

[0025] In the wireless communication system 100 according to this embodiment, each airborne radio station 160 is equipped with multiple antennas. Each ground base station 140 is also equipped with multiple antennas. By using their respective multiple antennas, the airborne radio stations 160 and ground base stations 140 can perform MIMO (Multi Input Multi Output) transmission, that is, spatial multiplexing transmission.

[0026] Furthermore, in the wireless communication system 100 according to this embodiment, as shown in Figure 2, the ground base station 140 includes a track information acquisition unit 141, a communication time calculation unit 142, a channel capacity calculation unit 143, and a communication control unit 144.

[0027] The ground base station 140 is equipped with a computer having a processor and memory as hardware, for example. The processor is also called a CPU (Central Processing Unit), central processing unit, processing unit, arithmetic unit, microprocessor, microcomputer, or DSP. The memory includes, for example, non-volatile or volatile semiconductor memory such as RAM, ROM, flash memory, EPROM and EEPROM, or magnetic disks, flexible disks, optical disks, compact disks, minidiscs and DVDs.

[0028] The ground base station 140 stores a program as software in its memory. The connection destination control device 520 then executes the program stored in memory using its processor, performing pre-configured processes. As a result of the cooperation between hardware and software, the ground base station 140 realizes the functions of the orbit information acquisition unit 141, the communication time calculation unit 142, the channel capacity calculation unit 143, and the communication control unit 144.

[0029] The functions realized by the components described in this disclosure may be implemented in a circuit or processing circuit including a general-purpose processor, application processor, integrated circuit, ASICs (Application Specific Integrated Circuits), CPU (a Central Processing Unit), conventional circuitry, and / or a combination thereof, programmed to realize the described functions. A processor includes transistors and other circuits and is considered a circuit or processing circuit. A processor may also be a programmed processor that executes a program stored in memory.

[0030] In this disclosure, circuit, “part,” “unit,” or “means” is hardware programmed to perform the functions described, or hardware that performs such functions. Such hardware may be any hardware disclosed herein, or any hardware programmed to perform, or known to perform, such functions.

[0031] If the hardware is a processor that is considered to be of the type of circuit, then the circuit, “part,” “unit,” or “means” is a combination of the hardware and the software used to constitute the hardware and / or processor.

[0032] The orbital information acquisition unit 141 acquires orbital information from the airborne radio station 160, for example, via the control channel 6. However, the source of orbital information is not limited to the airborne radio station 160. The orbital information acquisition unit 141 may also acquire orbital information from other sources, such as a satellite control center (not shown) that controls the operational routes of the airborne radio station 160. The orbital information includes the orbital forecast of the airborne radio station 160. The communication availability time calculation unit 142 calculates the communication availability time between the ground terminal station 180 and the airborne radio station 160 based on the orbital information acquired by the orbital information acquisition unit 141, particularly the orbital forecast of the airborne radio station 160.

[0033] Furthermore, the channel capacity calculation unit 143 calculates the channel capacity between the ground base station 140 and the airborne radio station 160 based on the orbital information acquired by the orbital information acquisition unit 141, particularly the orbital prediction of the airborne radio station 160. The channel capacity between the ground base station 140 and the airborne radio station 160 referred to here is the MIMO channel capacity for each combination of transmitting and receiving antennas, that is, for each combination of the antenna of the ground base station 140 and the antenna of the airborne radio station 160.

[0034] Figures 3 and 4 show examples of channel capacity between a ground base station 140 and an airborne radio station 160. These figures show an example where the airborne radio station 160 is a LEO satellite of a second-layer airborne radio station 120 at an altitude h of 600 [km]. Figure 3 shows the case where the transmitting antenna spacing and receiving antenna spacing d are 0.5 [m]. Figure 4 shows the case where the transmitting antenna spacing and receiving antenna spacing d are 1.0 [m].

[0035] In these figures, the horizontal axis represents the Acquisition Of Signal (AOS) time, i.e., the elapsed time since the start of the communication-enabled period, and the vertical axis represents the channel capacity. In these figures, the thick dashed line represents the transmission capacity by multibeam, the thick dashed line represents the minimum theoretical capacity by MIMO, and the thick solid line represents the maximum theoretical capacity by MIMO. Note that the minimum theoretical capacity by MIMO, shown by the thick dashed line, is equal to the channel capacity by SISO (Single Input Single Output).

[0036] The thin dashed line indicates the channel capacity between the ground base station 140 and the aerial radio station 160. As the aerial radio station 160 moves from the time of visibility start (AOS) to the time of visibility end (LOS: Los Of Signal), the channel capacity fluctuates due to changes in the distance between the ground base station 140 and the aerial radio station 160, changes in the relative relationship between the antennas, etc. As shown in these figures, during the communication-enabled time period, the channel capacity between the ground base station 140 and the aerial radio station 160 repeatedly increases and decreases between the minimum theoretical capacity and the maximum theoretical capacity.

[0037] The communication control unit 144 switches the antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 to ensure the maximum channel capacity, based on the channel capacity between the ground base station 140 and the airborne radio station 160 calculated by the channel capacity calculation unit 143. Figure 5 shows an example of switching the antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160. In the figure, the dashed line shows the channel capacity when the antenna spacing d is 1.0 [m], and the dashed line shows the channel capacity when the antenna spacing d is 0.5 [m]. As shown by the solid line in the figure, the communication control unit 144 controls the switching of the antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 so that the channel capacity is greater than or equal to the value of the antenna spacing d of 1.0 [m] and 0.5 [m].

[0038] In the wireless communication system 100 according to this embodiment, the communication control unit 144 of the ground base station 140 performs a first antenna switching control and a second antenna switching control for switching antennas used in spatial multiplex transmission between the ground base station 140 and the airborne radio station 160. The communication control unit 144 performs the first antenna switching control in the first time period. The communication control unit 144 also performs the second antenna switching control in the second time period.

[0039] The first time zone is the period during which communication is possible between the ground base station 140 and the airborne radio station 160, in which the degree of change in channel capacity over time, as calculated by the channel capacity calculation unit 143, is equal to or greater than a preset standard. The second time zone is the period during which communication is possible between the ground base station 140 and the airborne radio station 160, excluding the first time zone.

[0040] The communication control unit 144, for example, formulates a communication plan and controls the switching of antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 according to this formulated communication plan. In formulating the communication plan, the communication control unit 144 first identifies a first time period and a second time period based on the degree of time change in channel capacity for each combination of antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160.

[0041] The degree of change in channel capacity over time can be evaluated, for example, by the second time derivative of the channel capacity. In this case, the communication control unit 144 identifies the time period in which the second time derivative of the channel capacity calculated by the channel capacity calculation unit 143 is greater than or equal to a preset reference value as the first time period. Alternatively, the degree of change in channel capacity over time can also be evaluated by the variance of the amount of change in channel capacity over time. In this case, the communication control unit 144 identifies the time period in which the variance of the amount of change in channel capacity over time calculated by the channel capacity calculation unit 143 is greater than or equal to a preset reference value as the first time period.

[0042] The communication control unit 144 then identifies the first time period it has identified as the second time period, which is the time period that has been excluded from the communication-enabled time period calculated by the communication-enabled time period calculation unit 142. In other words, the communication control unit 144 identifies the second time period as the time period during which communication is possible between the ground base station 140 and the airborne radio station 160, in which the degree of change in channel capacity over time calculated by the channel capacity calculation unit 143 is less than a preset standard.

[0043] The communication control unit 144 formulates a communication plan so as to perform the first antenna switching control in the first time slot thus specified and perform the second antenna switching control in the second time slot. The second antenna switching control performed by the communication control unit 144 in the second time slot is the control for switching antennas by ordinary MIMO described so far. That is, in the second antenna switching control, as described above, the communication control unit 144 switches the antennas used for spatial multiplexing transmission between the ground base station 140 and the aerial radio station 160 so as to ensure the maximum channel capacity based on the channel capacity for each combination of the antennas of the ground base station 140 and the aerial radio station 160.

[0044] In contrast, the first antenna switching control performed by the communication control unit 144 in the first time slot is a control for suppressing the switching of the antennas used for spatial multiplexing transmission between the ground base station 140 and the aerial radio station 160 more than the second antenna switching control. Some examples of the first antenna switching control in this embodiment will be described. First, the first example of the first antenna switching control is a control for not performing any switching of the antennas used for transmission between the ground base station 140 and the aerial radio station 160. In other words, in this first example, the communication control unit 144 switches from MIMO transmission to SISO transmission.

[0045] Next, the second example of the first antenna switching control is a control for reducing the frequency or number of times of switching the antennas used for spatial multiplexing transmission between the ground base station 140 and the aerial radio station 160. In this case, for example, even at the timing when the antenna switching would be performed in the second antenna switching control, the communication control unit 144 does not perform the antenna switching once every n times (n is an integer of 2 or more). Also, for example, the communication control unit 144 may perform the antenna switching only once every n times (n is an integer of 2 or more) at the timing when the antenna switching would be performed in the second antenna switching control.

[0046] And the third example of the first antenna switching control is control that evaluates the channel capacity increase effect due to antenna switching and does not perform antenna switching when the increase effect is small. In this case, at the timing of performing antenna switching in the case of the second antenna switching control, the communication control unit 144 calculates the difference between the channel capacity before antenna switching and the channel capacity after antenna switching assuming that the antenna is switched. Then, if the difference in channel capacity before and after antenna switching is less than or equal to a preset reference value, the communication control unit 144 does not perform antenna switching for transmission between the ground base station 140 and the airborne radio station 160. On the other hand, if the difference in channel capacity before and after antenna switching is greater than the preset reference value, the communication control unit 144 switches the antenna used for spatial multiplexing transmission between the ground base station 140 and the airborne radio station 160.

[0047] In the wireless communication system 100 configured as described above, based on the orbit prediction of the airborne radio station 160, the channel capacity for each combination of antennas of the ground base station 140 and the airborne radio station 160 is calculated. Then, based on the calculated channel capacity for each combination of antennas, the antenna used for spatial multiplexing transmission between the ground base station 140 and the airborne radio station 160 is switched so as to ensure the maximum channel capacity. However, there is an error from the actual orbit in the orbit prediction due to the influence of the gravitational force of the sun and the moon, solar flares, the atmosphere, etc. Also, this error increases as time elapses from the issuance of the orbit information.

[0048] Therefore, an error also occurs in the channel capacity calculated based on the orbit prediction with an error, and by switching the antenna used for spatial multiplexing transmission based on such a channel capacity with an error, there is a possibility of selecting a combination of antennas with a small actual channel capacity. If the antenna is switched to one with a small actual channel capacity due to an error, not only can the optimal antenna not be selected and the communication quality cannot be improved, but there is also a risk of causing a deterioration in the communication quality.

[0049] In particular, during periods when the degree of time variation of channel capacity for each antenna combination used in spatial multiplexing transmission between the ground base station 140 and the airborne radio station 160 is greater than the standard, in other words, during periods when the time variation of channel capacity is steeper than the standard, the discrepancy between the calculated channel capacity due to errors in orbital prediction and the actual channel capacity tends to be larger compared to periods when the time variation of channel capacity is gentler. For this reason, the channel capacity calculated by MIMO may be lower than the channel capacity calculated by SISO. Also, during periods when the time variation of channel capacity is steep, there may be periods when the communication line is unstable due to fluctuations in channel capacity. Furthermore, during periods when the time variation of channel capacity is steep, there may be periods when an increase in channel capacity cannot be expected even if the antenna is switched, due to errors in orbital prediction.

[0050] Therefore, in the wireless communication system 100 according to this embodiment, in the first time period described above, that is, in the time period when the degree of time change of channel capacity for each combination of antennas used in spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 is greater than the standard, in other words, in the time period when the time change of channel capacity is steeper than the standard, a first antenna switching control is performed to suppress the switching of antennas used in spatial multiplex transmission between the ground base station 140 and the airborne radio station 160, compared to the second time period when this is not the case.

[0051] For example, in Figures 4 and 5, the time periods enclosed by thick borders represent the first time period in which the time change of channel capacity is steeper than the standard. During such time periods, the communication control unit 144 performs a first antenna switching control that suppresses the switching of antennas used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160.

[0052] In this way, by suppressing antenna switching during periods when the discrepancy between calculated channel capacity and actual channel capacity due to orbital prediction errors tends to be large, a deterioration in communication quality can be suppressed. Furthermore, by suppressing antenna switching during periods when channel capacity changes rapidly over time, instability of the communication line due to channel capacity fluctuations can be suppressed. Moreover, according to the third example of the first antenna switching control described above, by evaluating the channel capacity increase effect of antenna switching and refraining from switching antennas if the increase effect is small, unnecessary antenna switching in cases where an increase in channel capacity cannot be expected even if the antenna is switched can be suppressed.Therefore, when performing spatial multiplex transmission between a ground station with multiple antennas and an upper-air station with multiple antennas that orbits in the upper atmosphere and can periodically communicate with the ground station, it is possible to suppress the reduction in channel capacity caused by orbital prediction errors of the upper-air station.

[0053] Next, an example of communication control processing in the wireless communication system 100 according to this embodiment will be explained with reference to the flowcharts in Figures 6 to 8. Figure 6 shows the first example of the first antenna switching control described above.

[0054] First, in step S110, the orbit information acquisition unit 141 of the ground base station 140 acquires orbit information of the airborne radio station 160. Then, the channel capacity calculation unit 143 of the ground base station 140 calculates the channel capacity for each combination of antennas of the ground base station 140 and the airborne radio station 160 based on the orbit prediction of the airborne radio station 160.

[0055] In the subsequent step S111, the communication control unit 144 of the ground base station 140 formulates a communication plan. At this time, the communication control unit 144 identifies a first time period in which the degree of change in channel capacity over time is equal to or greater than a preset standard, based on the channel capacity calculated in step S110. The communication control unit 144 also identifies a second time period in the communication-enabled time period that is other than the first time period. The communication control unit 144 then formulates a communication plan in which it performs first antenna switching control in the identified first time period and second antenna switching control in the second time period.

[0056] In the example shown in Figure 6, the first antenna switching control is the first example described above. That is, during the first time period, the first antenna switching control, i.e., antenna switching by SISO, is planned. Then, during the second time period, the second antenna switching control, i.e., antenna switching by normal MIMO, is planned.

[0057] Then, in step S112, the communication control unit 144 of the ground base station 140 transmits channel capacity control information based on the communication plan formulated in step S111 to the airborne radio station 160 via the control channel 6. The channel capacity control information specifies whether to perform first antenna switching control (i.e., SISO) or second antenna switching control (i.e., normal MIMO).

[0058] Meanwhile, in step S120, the airborne radio station 160 acquires data using sensors and a camera (not shown) mounted on the airborne radio station 160. In the following step S121, the airborne radio station 160 receives channel capacity control information transmitted from the ground base station 140 in step S112. Furthermore, in step S122, the airborne radio station 160 determines the data transmission timing based on the channel capacity control information received in step S121. Then, in step S123, at the data transmission timing determined in step S122, the airborne radio station 160 transmits data to the ground base station 140 via the data channel 5, according to the antenna switching control specified by the channel capacity control information received in step S121, that is, whether it is MIMO or SISO.

[0059] Furthermore, in step S130, the ground base station 140 receives the data transmitted from the airborne radio station 160 in step S123 via the data channel 5. At this time, the ground base station 140 communicates with the airborne radio station 160 according to the communication plan formulated in step S111, that is, the antenna switching control specified by the channel capacity control information in step S112. Then, in the subsequent step S131, the ground base station 140 processes the data received in step S130.

[0060] Next, Figure 7 shows a second example of the first antenna switching control described above. First, in step S210, the orbit information acquisition unit 141 of the ground base station 140 acquires orbit information of the airborne radio station 160. Then, the channel capacity calculation unit 143 of the ground base station 140 calculates the channel capacity for each combination of antennas of the ground base station 140 and the airborne radio station 160 based on the orbit prediction of the airborne radio station 160.

[0061] In the subsequent step S211, the communication control unit 144 of the ground base station 140 formulates a communication plan. At this time, the communication control unit 144 identifies a first time period in which the degree of change in channel capacity over time is equal to or greater than a preset standard, based on the channel capacity calculated in step S210. The communication control unit 144 also identifies a second time period in the communication-enabled time period that is other than the first time period. The communication control unit 144 then formulates a communication plan in which it performs first antenna switching control in the identified first time period and second antenna switching control in the second time period.

[0062] In the example shown in Figure 7, the first antenna switching control is the second example described above. That is, in the first time period, the frequency or number of times the first antenna switching control, i.e., the switching of antennas used in spatial multiplex transmission, is planned to be reduced compared to the second time period. Then, in the second time period, the second antenna switching control, i.e., normal MIMO antenna switching, is planned.

[0063] Then, in step S212, the communication control unit 144 of the ground base station 140 transmits channel capacity control information based on the communication plan formulated in step S211 to the airborne radio station 160 via the control channel 6. The channel capacity control information specifies whether to perform first antenna switching control (i.e., reducing the frequency of antenna switching) or second antenna switching control (i.e., normal MIMO).

[0064] Meanwhile, in step S220, the airborne radio station 160 acquires data using sensors and a camera (not shown) mounted on the airborne radio station 160. In the following step S221, the airborne radio station 160 receives channel capacity control information transmitted from the ground base station 140 in step S212. Furthermore, in step S222, the airborne radio station 160 determines the data transmission timing based on the channel capacity control information received in step S221. Then, in step S223, at the data transmission timing determined in step S222, the airborne radio station 160 transmits data to the ground base station 140 via the data channel 5 using the antenna switching control specified by the channel capacity control information received in step S221.

[0065] Furthermore, in step S230, the ground base station 140 receives the data transmitted from the airborne radio station 160 in step S223 via the data channel 5. At this time, the ground base station 140 communicates with the airborne radio station 160 according to the communication plan formulated in step S211, that is, the antenna switching control specified by the channel capacity control information in step S212. Then, in the subsequent step S231, the ground base station 140 processes the data received in step S230.

[0066] Next, Figure 8 shows a third example of the first antenna switching control described above. First, in step S310, the orbit information acquisition unit 141 of the ground base station 140 acquires orbit information of the airborne radio station 160. Then, the channel capacity calculation unit 143 of the ground base station 140 calculates the channel capacity for each combination of antennas of the ground base station 140 and the airborne radio station 160 based on the orbit prediction of the airborne radio station 160.

[0067] In the following step S311, the communication control unit 144 of the ground base station 140 formulates a communication plan. At this time, the communication control unit 144 identifies a first time period in which the degree of change in channel capacity over time is equal to or greater than a preset standard, based on the channel capacity calculated in step S310. The communication control unit 144 also identifies a second time period in the communication-enabled time period that is other than the first time period. The communication control unit 144 then formulates a communication plan in which it performs first antenna switching control in the identified first time period and second antenna switching control in the second time period.

[0068] In the example shown in Figure 8, the first antenna switching control is the third example described above. That is, during the first time period, the first antenna switching control is performed, which evaluates the effect of channel capacity increase due to antenna switching, and if the increase effect is small, the antenna is not planned to be switched. Then, during the second time period, the second antenna switching control is performed, which is planned to be performed using normal MIMO antenna switching.

[0069] Then, in step S312, the communication control unit 144 of the ground base station 140 transmits channel capacity control information based on the communication plan formulated in step S311 to the airborne radio station 160 via the control channel 6. The channel capacity control information specifies whether to perform first antenna switching control (control based on evaluation of the effect of increasing channel capacity by antenna switching) or second antenna switching control (i.e., normal MIMO).

[0070] Meanwhile, in step S320, the airborne radio station 160 acquires data using sensors and a camera (not shown) mounted on the airborne radio station 160. In the following step S321, the airborne radio station 160 receives channel capacity control information transmitted from the ground base station 140 in step S312. Furthermore, in step S322, the airborne radio station 160 determines the data transmission timing based on the channel capacity control information received in step S321. Then, in step S323, at the data transmission timing determined in step S322, the airborne radio station 160 transmits data to the ground base station 140 via the data channel 5 using the antenna switching control specified by the channel capacity control information received in step S321.

[0071] Furthermore, in step S330, the ground base station 140 receives the data transmitted from the airborne radio station 160 in step S323 via the data channel 5. At this time, the ground base station 140 communicates with the airborne radio station 160 according to the communication plan formulated in step S311, that is, the antenna switching control specified by the channel capacity control information in step S312. Then, in the subsequent step S331, the ground base station 140 processes the data received in step S330.

[0072] Furthermore, some or all of the orbit information acquisition unit 141, the communication time calculation unit 142, the channel capacity calculation unit 143, and the communication control unit 144 may be provided in the airborne radio station 160 instead of the ground base station 140. In this case, the airborne radio station 160 is equipped with the aforementioned computer, for example, similar to the ground base station 140. In other words, one or both of the ground base station 140 and the airborne radio station 160 are equipped with a processor circuit and a memory that stores the program executed by the processor circuit.

[0073] The processor circuit is configured to perform a capacity calculation process that calculates the channel capacity between the ground base station 140 and the airborne radio station 160 based on the orbital prediction of the airborne radio station 160, and a communication control process that switches the antenna used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 based on the channel capacity calculated in the capacity calculation process. This communication control process includes a process that suppresses the switching of the antenna used for spatial multiplex transmission between the ground base station 140 and the airborne radio station 160 more than in a second time period other than the first time period, in a first time period during which the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard within the communicationable time period between the ground base station 140 and the airborne radio station 160.

[0074] This disclosure can be used in a wireless communication system, wireless communication device, wireless communication method, and wireless communication program that can perform spatial multiplex transmission between a ground station having multiple antennas and an air station having multiple antennas that orbits in the upper atmosphere and can periodically communicate with the ground station.

[0075] 1 Ground station inter-station link 2 Layer 1 inter-station link 3 Layer 1 inter-network link 4 Layer 2 inter-station link 5 Data channel 6 Control channel 8 Layer 2 inter-network link 100 Wireless communication system 110 Layer 1 airborne radio station 111 Layer 1 airborne network 120 Layer 2 airborne radio station 121 Layer 2 airborne network 130 Layer 3 airborne radio station 140 Ground base station 141 Orbit information acquisition unit 142 Communication availability time calculation unit 143 Channel capacity calculation unit 144 Communication control unit 160 Airborne radio station 180 Ground terminal station

Claims

1. A wireless communication system comprising: a ground station having multiple antennas; and an air station having multiple antennas, orbiting in an upper orbit and periodically communicating with the ground station, wherein spatial multiplex transmission is possible between the ground station and the air station, and one or both of the ground station and the air station are equipped with a processor circuit and a memory storing a program executed by the processor circuit, the processor circuit is configured to perform: a capacity calculation process that calculates the channel capacity between the ground station and the air station based on the orbital prediction of the air station; and a communication control process that switches the antenna used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, wherein the communication control process includes a process to suppress the switching of the antenna used for spatial multiplex transmission between the ground station and the air station more than in a second time period other than the first time period within the communicationable time period between the ground station and the air station during a first time period within the communicationable time period where the degree of time change of the channel capacity calculated in the capacity calculation process is above a preset standard.

2. A wireless communication device that functions as a ground station having multiple antennas, or an air station having multiple antennas that orbits in the air and periodically communicates with the ground station, and is capable of performing spatial multiplex transmission between the ground station and the air station, comprising a processor circuit and a memory storing a program executed by the processor circuit, wherein the processor circuit is configured to perform a capacity calculation process that calculates the channel capacity between the ground station and the air station based on the orbital prediction of the air station, and a communication control process that switches the antennas used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, wherein the communication control process includes a process that suppresses the switching of antennas used for spatial multiplex transmission between the ground station and the air station more than in a second time period other than the first time period within the communicationable time period between the ground station and the air station in a first time period within the communicationable time period where the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard.

3. A wireless communication method for performing spatial multiplex transmission between a ground station having multiple antennas and an air station having multiple antennas that orbits in the air and periodically communicates with the ground station, comprising: a capacity calculation step of calculating the channel capacity between the ground station and the air station based on the orbital prediction of the air station; and a communication control step of switching the antennas used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation step, wherein the communication control step includes a step of suppressing the switching of the antennas used for spatial multiplex transmission between the ground station and the air station to a degree greater than or equal to a preset standard in a first time period within the communicationable time period between the ground station and the air station, compared to a second time period other than the first time period.

4. A computer-readable wireless communication program for realizing the wireless communication device described in claim 2, the program includes a program that causes a processor circuit of the wireless communication device to execute: a capacity calculation process that calculates the channel capacity between the ground station and the air station based on the orbital prediction of the air station; and a communication control process that switches the antenna used for spatial multiplex transmission between the ground station and the air station based on the channel capacity calculated in the capacity calculation process, wherein the communication control process includes a process that suppresses the switching of the antenna used for spatial multiplex transmission between the ground station and the air station more than in a second time period other than the first time period within the communicationable time period between the ground station and the air station during a first time period in which the degree of time change of the channel capacity calculated in the capacity calculation process is greater than or equal to a preset standard.

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