Transmitting and receiving stations that switch between OTFS modulation and OFDM modulation

Abstract

To provide a transmitting station and a receiving station capable of switching between OTFS modulation and OFDM modulation in accordance with a Doppler frequency effect.SOLUTION: A transmitting station which wirelessly communicates with a receiving station on a predetermined carrier frequency includes: orthogonal frequency division multiplexing means for performing orthogonal frequency division multiplexing on a symbol sequence; orthogonal time frequency space multiplexing means for performing orthogonal time frequency space multiplexing on a symbol sequence; switching means for switching so as to input a symbol sequence in which a bit stream has been mapped into only one of the orthogonal frequency division multiplexing means and the orthogonal time frequency space multiplexing means; moving speed acquiring means for acquiring a relative moving speed with respect to the receiving station; switch controlling means for controlling the switching means on the basis of the moving speed; and switch notifying means for transmitting a switch notification as a control signal to the receiving station when the switching means is controlled.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the technology of OTFS (Orthogonal Time Frequency Space) modulation. [Background technology]

[0002] When there is a relative speed between the transmitting station and the receiving station, there is a phenomenon in which the frequency of a radio wave is received as a frequency different from the actual frequency. This phenomenon is generally called the Doppler effect. Due to the Doppler effect, the faster the transmitting station and the receiving station get closer to each other, the more the radio wave oscillations are compressed and the higher the frequency becomes, and conversely, the faster they get apart, the more the radio wave oscillations are stretched and the lower the frequency becomes.

[0003] OTFS modulation is a technique for communication that reduces the influence of the Doppler effect. OTFS modulation is a technique in which information symbols are mapped to resource elements in the delay-Doppler domain and converted into signals in the time-frequency domain using an ISFFT (Inverse Symplectic Fast Fourier Transform). When implementing OTFS, a technique that extends OFDM (Orthogonal Frequency Division Multiplexing) has also been proposed (see, for example, Patent Document 1). Specifically, one OTFS block is generated from multiple OFDM symbols.

[0004] OTFS modulation spreads information symbols mapped to resource elements in the time-frequency domain. Specifically, the symbols are spread across the signal bandwidth in the frequency domain and across one subframe in the time domain. As a result, OTFS modulation can achieve greater frequency and time diversity effects than OFDM modulation. In high-speed mobile environments where the Doppler effect is significant, OTFS modulation achieves a lower block error rate than OFDM modulation.

[0005] FIG. 1 is an example of a system in which the influence of the Doppler frequency occurs.

[0006] According to FIG. 1(a), the transmitting station 1 is a base station fixed on the ground, and the receiving station 2 is a satellite in space. The satellite as seen from the base station is moving at a high speed with a moving speed v. According to FIG. 1(b), the transmitting station 1 is a fixed base station, and the receiving station 2 is a railway vehicle. The railway vehicle as seen from the base station is moving at a high speed with a moving speed v. For example, according to the frequency band of 3 GHz to 7 GHz for mobile phones, in the communication between a fixed station and a mobile station such as a bullet train or an aircraft moving at about 200 km / h to 800 km / h, the communication rate is likely to deteriorate due to the influence of the Doppler effect. In order to cope with such a communication environment, it is preferable to apply OTFS modulation.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, when the influence of the Doppler effect is large, it is preferable to apply OTFS modulation. However, OTFS modulation has the problem of increasing power consumption. For example, the amount of calculation per 1-bit symbol during transmission and reception for OTFS modulation increases by the third power of the amount of calculation for OFDM modulation. This increases power consumption accordingly. In other words, when the influence of the Doppler effect is small, applying OFDM modulation rather than OTFS modulation reduces the amount of calculation, reduces power consumption, and increases communication capacity.

[0010] Therefore, an object of the present invention is to provide a transmitting station and a receiving station that can switch between OTFS modulation and OFDM modulation in accordance with the influence of the Doppler effect. [Means for solving the problem]

[0011] According to the present invention, in a transmitting station that wirelessly communicates with a receiving station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a moving speed acquisition means for acquiring a relative moving speed between the receiving station and the mobile station; a switching control means for controlling the switching means based on the moving speed; a switching notification means for transmitting a switching notification as a control signal to the receiving station when controlling the switching means; The present invention is characterized by having the following.

[0012] According to another embodiment of the transmitting station of the present invention, The switching control means controls the switching means using a switching control table in which either the orthogonal frequency division multiplexing means or the orthogonal time-frequency space multiplexing means is associated in advance with each moving speed. It is also preferable.

[0013] According to another embodiment of the transmitting station of the present invention, the moving speed acquisition means stores in advance a moving speed table in which the relative moving speed of the receiving station as seen from the transmitting station is defined for each time period; The moving speed is acquired by referring to the moving speed table according to the time. It is also preferable.

[0014] According to another embodiment of the transmitting station of the present invention, The moving speed acquisition means receives the moving speed and moving direction from the receiving station, and estimates the relative moving speed of the receiving station as seen from the transmitting station based on the change per unit time in the distance and angle between the receiving station and the transmitting station. It is also preferable.

[0015] According to another embodiment of the transmitting station of the present invention, The moving speed acquisition means periodically receives position information from the receiving station, and estimates the relative moving speed of the receiving station as seen from the transmitting station based on the change per unit time in the distance and angle between the receiving station and the transmitting station. It is also preferable.

[0016] According to another embodiment of the transmitting station of the present invention, One of the transmitting station or the receiving station is a fixed station and the other is a mobile station. It is also preferable.

[0017] According to another embodiment of the transmitting station of the present invention, further comprising time synchronization means for synchronizing time with the receiving station; The switching control means switches between the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means in synchronization with the receiving station. It is also preferable.

[0018] According to another embodiment of the transmitting station of the present invention, Further comprising a Doppler frequency estimation means for estimating a Doppler frequency from the moving speed, The switching control means controls the switching means to: Switching to an orthogonal frequency division multiplexing means when the Doppler frequency is equal to or less than a predetermined threshold; Switching to orthogonal time-frequency space multiplexing means when the Doppler frequency is higher than a predetermined threshold. It is also preferable.

[0019] According to another embodiment of the transmitting station of the present invention, The switching control means predetermines, by simulation, a predetermined threshold value of the Doppler frequency at which the channel capacity per symbol in the orthogonal time-frequency space multiplexed signal becomes higher than the channel capacity per symbol in the orthogonal frequency division multiplexed signal. It is also preferable.

[0020] According to another embodiment of the transmitting station of the present invention, The switching control means pre-sets a predetermined threshold value of the Doppler frequency by simulation so that the orthogonal frequency division multiplexing means or the orthogonal time-frequency space multiplexing means that increases the channel capacity by three times or more is selected. It is also preferable.

[0021] According to another embodiment of the transmitting station of the present invention, The Doppler frequency estimation means estimates as follows: f'=fc·v·k f': Doppler frequency fc: carrier frequency v: Movement speed k: coefficient It is also preferable.

[0022] According to another embodiment of the transmitting station of the present invention, the orthogonal frequency division multiplexing means comprises an inverse Fourier transform unit for converting the signals into time domain signals; The orthogonal time-frequency space multiplexing means comprises an inverse symplectic Fourier transform unit for converting signals into time-frequency domain signals, and an inverse Fourier transform unit. It is also preferable.

[0023] According to the present invention, in a transmitting station that wirelessly communicates with a receiving station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a Doppler frequency estimation means for receiving a reference signal from a receiving station and estimating a Doppler frequency from amplitude and phase shifts in the reference signal; a switching control means for controlling the switching means based on the Doppler frequency; a switching notification means for transmitting a switching notification as a control signal to the receiving station when the switching means is controlled; The present invention is characterized by having the following.

[0024] According to the present invention, in a receiving station that wirelessly communicates with a transmitting station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a moving speed acquisition means for acquiring a relative moving speed between the transmitting station and the mobile station; a switching control means for controlling the switching means based on the moving speed; a switching notification means for transmitting a switching notification as a control signal to the transmitting station when the switching means is controlled; The present invention is characterized by having the following.

[0025] According to the present invention, in a receiving station that wirelessly communicates with a transmitting station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a Doppler frequency estimation means for receiving a reference signal from a transmitting station and estimating a Doppler frequency from amplitude and phase shifts in the reference signal; a switching control means for controlling the switching means based on the Doppler frequency; a switching notification means for transmitting a switching notification as a control signal to the transmitting station when the switching means is controlled; The present invention is characterized by having the following. [Effects of the Invention]

[0026] According to the transmitting station and receiving station of the present invention, it is possible to switch between OTFS modulation and OFDM modulation in accordance with the influence of the Doppler effect. [Brief explanation of the drawings]

[0027] [Figure 1] This is an example of a system in which the Doppler frequency effect occurs. [Figure 2] FIG. 2 is a functional configuration diagram of a transmitting station that takes the initiative in switching in the present invention. [Figure 3] FIG. 2 is an explanatory diagram of switching control in the present invention. [Figure 4] FIG. 2 is a sequence diagram between a transmitting station and a receiving station according to the present invention. [Figure 5] FIG. 1 is a functional configuration diagram of a transmitting station that estimates a Doppler frequency from a reference signal. [Figure 6] FIG. 2 is a functional configuration diagram of a receiving station opposite to the transmitting station. [Figure 7] FIG. 2 is a functional configuration diagram of a receiving station that takes the initiative in switching in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0029] FIG. 2 is a functional block diagram of a transmitting station that takes the initiative in switching in the present invention.

[0030] Generally, the control of the communication system is performed by the base station side. Here, it is assumed that the transmitting station 1 is a fixed station serving as a base station, and the receiving station 2 is a mobile station. 2, the transmitting station 1 includes a symbol mapper 101, a CP (Cyclic Prefix) unit 102, an OFDM unit 11, an OTFS unit 12, a switching unit 13, a moving speed acquisition unit 14, a switching control unit 15, a Doppler frequency estimation unit 16, a time synchronization unit 17, and a switching notification unit 18. These functional components are realized by executing a program that causes a modem chip (computer) mounted in the transmitting station to function. The processing flow of these functional components can also be understood as a transmission method of the modem chip.

[0031] <Transmitting Station 1> [Symbol Mapper 101] The symbol mapper 101 converts the bit stream of transmission data into a sequence of N symbols mapped onto a complex (IQ) plane according to the modulation method. For example, in the case of 64QAM (Quadrature Amplitude Modulation), the bit stream is divided into 6-bit units and mapped onto the complex plane at 64 points. The horizontal axis of the complex plane represents the real part in amplitude units, and the vertical axis represents the imaginary part in amplitude units. The converted symbol sequence is output to the switching unit 13.

[0032] [Switching unit 13] The switching unit 13 switches the symbol string so that it is input to only one of the OFDM unit 11 and the OTFS unit 12. This switching is performed under the control of the switching control unit 15.

[0033] [OFDM part 11] The OFDM unit 11 orthogonally frequency-division multiplexes the symbol sequence. This is comprised of an inverse Fourier transform (IFFT (Inverse Fast Fourier Transform)) unit 110 that converts the symbol sequence into a time-domain signal. The inverse Fourier transform unit 110 converts the signal into a sine wave, for example, in 6-bit units.

[0034] [OTFS part 12] The OTFS unit 12 performs orthogonal time-frequency space multiplexing of the symbol sequence. This unit is composed of an inverse symplectic fast fourier transform (ISFFT) unit 120 that converts the symbol sequence into a time-frequency domain signal, and an inverse fourier transform unit 110. The OTFS unit 12 may be implemented by extending the OFDM unit 11 (see, for example, Patent Document 1).

[0035] [CP section 102] The CP unit 102 adds a redundant signal (cyclic prefix) to the beginning of a symbol input from the OFDM unit 11 or the OTFS unit 12. Specifically, a certain amount of data from the end of the symbol is inserted as a cyclic prefix to the beginning of the symbol, thereby suppressing inter-symbol interference and inter-carrier interference. The output signal from the CP unit 102 is D / A converted (Digital Analog Converter) and then transmitted from an antenna.

[0036] FIG. 3 is an explanatory diagram of the switching control in the present invention. FIG. 4 is a sequence diagram between a transmitting station and a receiving station in the present invention.

[0037] [Moving speed acquisition unit 14] The moving speed acquisition unit 14 acquires the relative moving speed between the receiving station 2. For example, there are the following three methods. <Method of storing data as a movement speed table> <Method of receiving moving speed and direction from receiving station> <Method of receiving location information from a receiving station>

[0038] <Method of storing as a moving speed table> (See S141 in FIG. 4) The moving speed acquisition unit 14 stores in advance a "moving speed table" in which the relative moving speed of the receiving station 2 as seen from the transmitting station 1 is defined for each time. This allows the moving speed to be acquired according to the time by referring to the moving speed table. For example, in the case of a medium or low earth orbit satellite that moves periodically, or a high-speed train such as a Shinkansen that moves based on a timetable, the position can be determined according to the time of day, and the moving speed as seen from the transmitting station 1 can also be determined in advance. The Doppler frequency can also be estimated according to the determined moving speed. For this purpose, the moving speed according to the time can be specified in a table.

[0039] For example, let's assume that transmitting station 1 is a fixed terrestrial station on Earth and receiving station 2 is a satellite. Transmitting station 1, a fixed terrestrial station, rotates around the Earth at 1,700 km / h, while receiving station 2, a satellite, moves at 7 to 8 km / h at an altitude of 500 km above the Earth (a low-orbit satellite). Depending on the angle of receiving station 2 as seen from transmitting station 1, the distance between transmitting station 1 and receiving station 2 changes, and so does the speed of movement.

[0040] <Method of receiving moving speed and moving direction from receiving station> (See S142 in Figure 4) The moving speed acquisition unit 14 receives the "moving speed and moving direction" as control information from the receiving station 2. This makes it possible to estimate the relative moving speed from the moving trajectory that changes per unit time in the distance and angle between the transmitting station 1 and the receiving station 2.

[0041] <Method for receiving location information from a receiving station> (See S143 in Figure 4) The movement speed acquisition unit 14 periodically receives "position information (latitude, longitude, altitude)" as control information from the receiving station 2, and can estimate the relative movement speed from the movement trajectory that changes per unit time in the distance and angle between the transmitting station 1 and the receiving station 2.

[0042] [Time synchronization section 17] GNSS The time synchronization unit 17 synchronizes the time with the receiving station 2 with high precision. Specifically, it generates Coordinated Universal Time (UTC) by receiving radio waves such as a Global Navigation Satellite System (GNSS) protocol. Alternatively, it may use Simple Network Time Protocol (SNTP), Network Time Protocol (NTP), or Precision Time Protocol (PTP). By synchronizing the time between the transmitting station 1 and the receiving station 2 with high accuracy, the switching control unit 15 synchronizes the switching between the OFDM unit 11 and the OTFS unit 12 with the receiving station 2. In addition, the moving speed acquisition unit 14 can derive the moving speed of the receiving station 2 according to the time with high accuracy.

[0043] [Doppler frequency estimation unit 16] The Doppler frequency estimation unit 16 estimates the Doppler frequency from the moving speed. Specifically, the Doppler frequency estimation unit 16 estimates the maximum Doppler frequency as follows. f'=fc·v·k f': maximum Doppler frequency fc: carrier frequency v: Movement speed k: coefficient Here, the coefficient k is calculated as follows: k=cosθ / c θ: Radio wave reception angle (angle of the mobile station as seen from the fixed station) c: speed of light

[0044] [Switching control unit 15] The switching control unit 15 controls the switching unit 13 in the following two ways. <Switching control based on movement speed using a movement speed table> <Switching control based on estimated Doppler frequency>

[0045] <Switching control based on movement speed using a movement speed table> The switching control unit 15 has a "switching control table" in which each moving speed is previously associated with either the OFDM unit 11 or the OTFS unit 12. The switching control unit 15 controls the switching unit 13 by referring to the switching control table in accordance with the moving speed input from the moving speed acquisition unit 14. Here, the switching control table of the switching control unit 15 is defined so that the Doppler frequency is estimated from the moving speed, and switching is performed between the OFDM unit 11 and the OTFS unit 12 according to the Doppler frequency.

[0046] Here, the switching control table differs depending on the carrier frequency fc. If the carrier frequency is constant, the faster the moving speed v, the higher the Doppler frequency f'. On the other hand, even if the moving speed v is constant, the higher the carrier frequency fc, the higher the Doppler frequency f'.

[0047] The switching control unit 15 switches the switching unit 13 in the following manner according to the Doppler frequency. If the Doppler frequency is below a predetermined threshold: Switch to OFDM unit 11 If the Doppler frequency is higher than the threshold: Switch to OTFS section 12

[0048] Here, the predetermined threshold value of the Doppler frequency is set in advance through simulation. The switching control unit 15 sets the Doppler frequency at a value at which the channel capacity per symbol in the OTFS unit 12 becomes higher than the channel capacity per symbol in the OFDM unit 11. Specifically, it is also preferable that the switching control unit 15 sets a predetermined threshold value for the Doppler frequency in advance by simulation so that the OFDM unit 11 or the OTFS unit 12 with the channel capacity three times or more is selected.

[0049] <Switching control based on estimated Doppler frequency> FIG. 5 is a functional block diagram of a transmitting station that estimates the Doppler frequency from a reference signal.

[0050] 5, unlike FIG. 2, the transmitting station 1 receives a reference signal from the receiving station 2. Then, the Doppler frequency estimating unit 16 of the transmitting station 1 can estimate the Doppler frequency from the amplitude and phase displacement (e.g., phase rotation) of the reference signal (see, for example, Non-Patent Document 1). In this case, it takes more time to measure the Doppler frequency than in the case of the switching control table based on moving speed using the moving speed table described above. As a result, the switching control unit 15 compares the Doppler frequency with a predetermined threshold value and switches to either the OFDM unit 11 or the OTFS unit 12.

[0051] [Switching notification unit 18] When the switching unit 13 is controlled, the switching notification unit 18 transmits a “switching notification” as a control signal to the receiving station 2. This allows the transmitting station 1 and the receiving station 2 to synchronize the switching of the OFDM unit 11 or the OTFS unit 12.

[0052] <Receiving station 2> FIG. 6 is a functional configuration diagram of a receiving station opposite to the transmitting station in FIG.

[0053] 6, the receiving station 2 has a symbol demapper 201, a CP unit 202, an OFDM unit 21, an OTFS unit 22, a switching unit 23, a moving speed acquisition unit 24, and a switching control unit 25. These functional components are realized by executing a program that causes a modem chip (computer) mounted in the receiving station to function. The processing flow of these functional components can also be understood as a receiving method of the modem chip.

[0054] 6, the receiving station 2 receives a "switching notification" transmitted from the transmitting station 1 via the switching notification receiving unit 29. The switching notification specifies either the OFDM unit 21 or the OTFS unit 22 to be switched to, and also specifies the switching timing. This allows the switching control unit 25 to switch to either the OFDM unit 21 or the OTFS unit 22 in synchronization with the transmitting station 1.

[0055] FIG. 7 is a functional block diagram of a receiving station that takes the initiative in switching in the present invention.

[0056] 7 shows switching initiated by the receiving station, as opposed to switching initiated by the transmitting station in FIG. 2. The processing of each functional component is exactly the same as in FIG. When the receiving station 2 receives a reference signal from the transmitting station 1, the Doppler frequency estimating unit 26 can also estimate the Doppler frequency from the amplitude and phase shift of the reference signal, and can control the switching unit 23 based on the Doppler frequency.

[0057] In the above-described embodiment, the transmitting station 1 is a fixed station and the receiving station 2 is a mobile station. However, this is not limiting, and the transmitting station 1 may be a mobile station and the receiving station 2 may be a fixed station. Furthermore, both the transmitting station 1 and the receiving station 2 may be mobile stations, as long as the relative moving speeds can be obtained.

[0058] As described above in detail, the transmitting station and receiving station of the present invention can switch between OTFS modulation and OFDM modulation depending on the influence of the Doppler frequency.

[0059] With respect to the various embodiments of the present invention described above, various changes, modifications, and omissions that fall within the scope of the technical spirit and aspects of the present invention may be easily made by those skilled in the art. The above description is merely illustrative and is not intended to be limiting in any way. The present invention is limited only by the claims and their equivalents. [Explanation of symbols]

[0060] 1. Transmitting station, base station 101 Symbol Mapper 102 CP section 11 OFDM Department 110 Inverse Fourier transform section 12 OTFS Department 120 Inverse symplectic Fourier transform section 13 Switching section 14 Movement speed acquisition section 15 Switching control section 16 Doppler frequency estimation unit 17 Time Synchronization Unit 18 Switching notification section 2 Receiving station, terminal 201 Symbol Demapper 202 CP Department 21 OFDM Department 22 OTFS Department 23 Switching section 24 Movement speed acquisition section 25 Switching control section 26 Doppler frequency estimation unit 27 Time Synchronization Unit 28 Switching notification section 29 Switching notification receiving unit

Claims

1. In a transmitting station that communicates wirelessly with a receiving station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a moving speed acquisition means for acquiring a relative moving speed between the receiving station and the mobile station; a switching control means for controlling the switching means based on the moving speed; a switching notification means for transmitting a switching notification as a control signal to the receiving station when controlling the switching means; A transmitting station comprising:

2. The switching control means controls the switching means using a switching control table in which either the orthogonal frequency division multiplexing means or the orthogonal time-frequency space multiplexing means is associated in advance with each moving speed.

2. A transmitting station according to claim 1.

3. the moving speed acquisition means stores in advance a moving speed table in which the relative moving speed of the receiving station as seen from the transmitting station is defined for each time period; The moving speed is acquired by referring to the moving speed table according to the time.

2. A transmitting station according to claim 1.

4. The moving speed acquisition means receives the moving speed and moving direction from the receiving station, and estimates the relative moving speed of the receiving station as seen from the transmitting station based on the change per unit time in the distance and angle between the receiving station and the transmitting station.

2. A transmitting station according to claim 1.

5. The moving speed acquisition means periodically receives position information from the receiving station, and estimates the relative moving speed of the receiving station as seen from the transmitting station based on the change per unit time in the distance and angle between the receiving station and the transmitting station.

2. A transmitting station according to claim 1.

6. One of the transmitting station or the receiving station is a fixed station and the other is a mobile station.

2. A transmitting station according to claim 1.

7. further comprising time synchronization means for synchronizing time with the receiving station; The switching control means switches between the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means in synchronization with the receiving station.

2. A transmitting station according to claim 1.

8. Further comprising a Doppler frequency estimation means for estimating a Doppler frequency from the moving speed, The switching control means controls the switching means to: Switching to an orthogonal frequency division multiplexing means when the Doppler frequency is equal to or less than a predetermined threshold; Switching to orthogonal time-frequency space multiplexing means when the Doppler frequency is higher than a predetermined threshold.

2. A transmitting station according to claim 1.

9. The switching control means predetermines, by simulation, a predetermined threshold value of the Doppler frequency at which the channel capacity per symbol of the orthogonal time-frequency space multiplexed signal becomes higher than the channel capacity per symbol of the orthogonal frequency division multiplexed signal.

9. A transmitting station according to claim 8.

10. The switching control means pre-sets a predetermined threshold value of the Doppler frequency by simulation so that the orthogonal frequency division multiplexing means or the orthogonal time-frequency space multiplexing means that increases the channel capacity by three times or more is selected.

10. A transmitting station according to claim 9.

11. The Doppler frequency estimation means estimates as follows: f' = fc v k f': Doppler frequency fc: carrier frequency v: Movement speed k: coefficient A transmitting station according to any one of claims 8 to 10.

12. the orthogonal frequency division multiplexing means comprises an inverse Fourier transform unit for converting the signals into time domain signals; The orthogonal time-frequency space multiplexing means comprises an inverse symplectic Fourier transform unit for converting signals into time-frequency domain signals, and the inverse Fourier transform unit.

2. A transmitting station according to claim 1.

13. In a transmitting station that communicates wirelessly with a receiving station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a Doppler frequency estimation means for receiving a reference signal from a receiving station and estimating a Doppler frequency from amplitude and phase shifts in the reference signal; a switching control means for controlling the switching means based on the Doppler frequency; a switching notification means for transmitting a switching notification as a control signal to the receiving station when the switching means is controlled; A transmitting station comprising:

14. In a receiving station that communicates wirelessly with a transmitting station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a moving speed acquisition means for acquiring a relative moving speed between the transmitting station and the mobile station; a switching control means for controlling the switching means based on the moving speed; a switching notification means for transmitting a switching notification as a control signal to the transmitting station when the switching means is controlled; A receiving station comprising:

15. In a receiving station that communicates wirelessly with a transmitting station at a predetermined carrier frequency, an orthogonal frequency division multiplexing means for orthogonally frequency division multiplexing the symbol sequence; an orthogonal time-frequency space-multiplexing unit for orthogonally time-frequency space-multiplexing the symbol sequence; a switching means for switching the symbol sequence onto which the bit stream is mapped so as to be input to only one of the orthogonal frequency division multiplexing means and the orthogonal time-frequency space multiplexing means; a Doppler frequency estimation means for receiving a reference signal from a transmitting station and estimating a Doppler frequency from amplitude and phase shifts in the reference signal; a switching control means for controlling the switching means based on the Doppler frequency; a switching notification means for transmitting a switching notification as a control signal to the transmitting station when the switching means is controlled; A receiving station comprising:

Images