Wireless communication system, receiving device and estimation method

Abstract

This reception device comprises a Doppler shift estimation unit that receives waveform data transmitted by a wireless communication device and estimates a Doppler shift and a frequency offset affecting a reception signal indicated by the waveform data, the Doppler shift estimation unit being provided with: M first computation units (where M is an integer equal to or greater than 2) that divides the reception signal and multiplies each of M types of frequency offset correction coefficient candidates by the divided reception signals; M×N second computation units (where N is an integer equal to or greater than 2) that multiply N types of Doppler shift correction coefficient candidates by each output signal from the M first computation units; and a comparison unit that estimates, as a frequency offset estimation value and a Doppler shift estimation value used in compensation of the reception signal, the frequency offset correction coefficient candidate and Doppler shift correction coefficient candidate that are multiplied by any output signal from among output signals outputted from each of the M×N second computation units.　

Description

[Technical Field]

[0001] The present invention relates to a wireless communication system, a receiving device, and an estimation method. [Background technology]

[0002] With the development of IoT (Internet of Things) technology, the installation of IoT terminals equipped with various sensors in various locations is being considered. IoT terminals may be installed in places where it is difficult to install base stations, such as on marine buoys, ships, and mountainous areas. Therefore, a system has been proposed in which data collected by IoT terminals installed in various locations is relayed to base stations installed on the ground by relay devices installed on low-earth orbit satellites.

[0003] In a satellite sensing platform, signals transmitted from each IoT device experience different Doppler shifts depending on the device's location. As a result, signals with different Doppler shifts within a frame arrive at the receiving antenna of a low-earth orbit satellite at random times. Preambles also experience different Doppler shifts for each IoT device, making time synchronization processing based on correlation with known signals difficult. Non-Patent Document 1 proposes DFS (Doppler frequency shift) estimation using preambles and postambles. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] “Data-Aided Doppler frequency shift estimation and compensation for UAVs”, IEEE Internet of things journal., vol.7. no.1. jan. 2020. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in communication systems with low data rates, the overhead caused by inserting a preamble becomes a problem. Therefore, a method for estimating Doppler shift and frequency offset without a preamble is desirable. Conventionally, there has been a problem in that it is not possible to estimate Doppler shift and frequency offset without inserting a preamble or the like for multiple signals with very high speeds and different Doppler shifts for each terminal, such as in satellite communication.

[0006] In view of the above circumstances, an object of the present invention is to provide a technique capable of estimating Doppler shift and frequency offset for a plurality of signals that have undergone different Doppler shifts without inserting a preamble or the like. [Means for solving the problem]

[0007] One aspect of the present invention is a wireless communication system having a plurality of transmitting devices, a mobile wireless communication device, and a receiving device, wherein the plurality of transmitting devices are equipped with a transmitting unit that transmits wireless signals, the wireless communication device is equipped with one or more antennas that receive the wireless signals transmitted from the plurality of transmitting devices, and a waveform transmitting unit that transmits waveform data indicating a waveform of the received signal received by the one or more antennas to the receiving device, and the receiving device is equipped with a receiving unit that receives the waveform data transmitted from the wireless communication device, and a Doppler shift estimating unit that estimates a Doppler shift and a frequency offset that the received signal has undergone and that is indicated by the waveform data received by the receiving unit, the Doppler shift estimation unit includes M first calculation units that branch the received signal and multiply each of the branched received signals by M (M is an integer equal to or greater than 2) types of frequency offset correction coefficient candidates, M×N second calculation units that multiply each of output signals of the M first calculation units by N (N is an integer equal to or greater than 2) types of Doppler shift correction coefficient candidates, and a comparison unit that estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by any of the output signals output from the M×N second calculation units as an estimated value of a frequency offset and an estimated value of a Doppler shift to be used for compensating the received signal.

[0008] One aspect of the present invention is a receiving device in a wireless communication system having a plurality of transmitting devices, a mobile wireless communication device, and a receiving device, the receiving device comprising: a receiving unit that receives wireless signals transmitted from the plurality of transmitting devices via the wireless communication devices; and a Doppler shift estimation unit that estimates a Doppler shift and a frequency offset suffered by the received signals received by the receiving unit, wherein the Doppler shift estimation unit comprises M first calculation units that branch the received signals and multiply each of the branched received signals by M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates; M×N second calculation units that multiply each output signal of the M first calculation units by N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates; and a comparison unit that estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by one of the output signals output from each of the M×N second calculation units as an estimated value of a frequency offset and an estimated value of a Doppler shift to be used for compensating the received signals.

[0009] One aspect of the present invention is an estimation method in a wireless communication system having a plurality of transmitting devices, a mobile wireless communication device, and a receiving device, in which the plurality of transmitting devices transmit wireless signals, the wireless communication devices transmit waveform data to the receiving device, indicating waveforms of received signals received by one or more antennas that receive the wireless signals transmitted from the plurality of transmitting devices, the receiving device receives the waveform data transmitted by the wireless communication devices, the receiving device branches the received signals indicated by the received waveform data, multiplies each of the branched received signals by M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates, multiplies each of the received signals multiplied by the M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates by N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates, and estimates, as an estimated value of a frequency offset and an estimated value of a Doppler shift to be used for compensating for the received signals, the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by any output signal of the received signals multiplied by the N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates. [Effects of the Invention]

[0010] According to the present invention, it is possible to estimate the Doppler shift and frequency offset for a plurality of signals that have undergone different Doppler shifts without inserting a preamble or the like. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a configuration diagram of a wireless communication system according to an embodiment. [Figure 2] FIG. 2 is a diagram illustrating a specific configuration example of a Doppler shift estimation unit in the embodiment. [Figure 3] FIG. 2 is a diagram illustrating an example of the configuration of a compensation unit according to an embodiment. [Figure 4] FIG. 2 is a diagram illustrating an example of the configuration of a replica generation unit in the embodiment. [Figure 5] FIG. 2 is a diagram illustrating an example of the configuration of a blanking processing unit in the embodiment. [Figure 6] 5A and 5B are diagrams for explaining processing performed by a blanking processing unit in the embodiment. [Figure 7] 10 is a flowchart showing the flow of a demodulation process performed by a base station in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1 is a configuration diagram of a wireless communication system 1 according to an embodiment. The wireless communication system 1 has a plurality of terminal stations 20, mobile relay stations 30, and base stations 40. The wireless communication system 1 may have any number of terminal stations 20, mobile relay stations 30, and base stations 40. It is assumed that the number of terminal stations 20 is large.

[0013] The terminal station 20 collects data such as environmental data detected by a sensor and transmits it wirelessly to the mobile relay station 30. For example, when the mobile relay station 30 instructs the terminal station 20 on a transmission timing, the terminal station 20 wirelessly transmits the collected data to the mobile relay station 30 at the instructed transmission timing. The terminal station 20 is, for example, an IoT (Internet of Things) terminal. The terminal station 20 is one aspect of a transmission device.

[0014] The mobile relay station 30 is an example of a wireless communication device that is mounted on a mobile body and whose communication area changes over time. The mobile relay station 30 of this embodiment is provided on a LEO (Low Earth Orbit) satellite. The altitude of a LEO satellite is 2000 km or less, and at an altitude of approximately 350 km, for example, it orbits the Earth once in approximately 1.5 hours. The terminal station 20 and the base station 40 are installed on the Earth, such as on land or sea. Hereinafter, a wireless signal transmitted from the terminal station 20 to the mobile relay station 30 will be referred to as a terminal uplink signal, and a signal transmitted from the mobile relay station 30 to the base station 40 will be referred to as a base station downlink signal.

[0015] Because the mobile relay station 30 mounted on the LEO satellite communicates while moving at high speed, the time during which each terminal station 20 and base station 40 can communicate with the mobile relay station 30 is limited. Specifically, from the ground, the mobile relay station 30 passes overhead every few minutes. The terminal station 20 collects data such as environmental data detected by sensors and stores the collected data. The terminal station 20 transmits a terminal uplink signal containing the collected data at a timing when communication with the mobile relay station 30 is possible. The mobile relay station 30 receives terminal uplink signals transmitted from each of the multiple terminal stations 20 while moving above the Earth. The mobile relay station 30 accumulates data received from each terminal station 20 via the terminal uplink signal and wirelessly transmits the accumulated data to the base station 40 via a base station downlink signal at a timing when communication with the base station 40 is possible. The base station 40 acquires the data collected by the terminal station 20 from the received base station downlink signal.

[0016] The mobile relay station 30 has an antenna used for wireless communication with the terminal station 20 and an antenna used for wireless communication with the base station 40. Therefore, the mobile relay station 30 can perform wireless communication with the terminal station 20 and wireless communication with the base station 40 in parallel.

[0017] Mobile relay stations can be mounted on geostationary satellites, drones, or unmanned aerial vehicles such as HAPS (High Altitude Platform Station). However, although relay stations mounted on geostationary satellites have a wide ground coverage area (footprint), their high altitude means that the link budget for IoT devices installed on the ground is very small. On the other hand, relay stations mounted on drones or HAPS have a high link budget but a narrow coverage area.

[0018] Furthermore, drones require batteries, and HAPS require solar panels. In this embodiment, a mobile relay station 30 is mounted on a LEO satellite. Therefore, the link budget is kept within limits, and since LEO satellites orbit outside the atmosphere, there is no air resistance and fuel consumption is low. The footprint is also larger than when a relay station is mounted on a drone or HAPS.

[0019] The base station 40 acquires from the mobile relay station 30 multiple received signals that have been subjected to different Doppler shifts due to transmission from each terminal station 20 to the mobile relay station 30, and estimates the Doppler shift and frequency offset of the multiple received signals without performing correlation detection or the like on the acquired multiple received signals. Hereinafter, the Doppler shift and frequency offset estimated by the base station 40 are referred to as the Doppler shift estimate and the frequency offset estimate. Furthermore, the base station 40 detects multiple received frames using the Doppler shift estimate and the frequency offset estimate. The base station 40 is one aspect of a receiving device.

[0020] The terminal station 20 and the base station 40 are installed at specific locations on the earth, such as on land or sea.

[0021] The configuration of each device will be explained. The terminal station 20 includes a data storage unit 21, a transmission unit 22, and one or more antennas 23. FIG. 1 shows a case where the terminal station 20 includes one antenna 23. The data storage unit 21 stores environmental data detected by a sensor. The transmission unit 22 communicates with the mobile relay station 30. The transmission unit 22 reads out the environmental data from the data storage unit 21 as terminal transmission data, and wirelessly transmits a terminal uplink signal in which the read terminal transmission data is set from the antenna 23.

[0022] The transmitter 22 transmits signals, for example, by LPWA (Low Power Wide Area). LPWA includes LoRaWAN (registered trademark), Sigfox (registered trademark), LTE-M (Long Term Evolution for Machines), NB (Narrow Band)-IoT, etc., but any wireless communication method can be used. The transmitter 22 may transmit signals to other terminal stations 20 by time division multiplexing, OFDM (Orthogonal Frequency Division Multiplexing), etc. The transmitter 22 may perform beamforming of signals to be transmitted from the multiple antennas 23 by a method predetermined for the wireless communication method used.

[0023] The mobile relay station 30 includes one or more antennas 31, a terminal communication unit 32, a data storage unit 33, a base station communication unit 34, and one or more antennas 35. In FIG. 1, the mobile relay station 30 includes one antenna 31 and one antenna 35.

[0024] The terminal communication unit 32 communicates wirelessly with the terminal station 20. The terminal communication unit 32 has a receiving unit 321 and a received waveform recording unit 322. The receiving unit 321 receives a terminal uplink signal via the antenna 31. The received waveform recording unit 322 samples the received waveform of the terminal uplink signal received by the receiving unit 321 and generates waveform data indicating the values obtained by the sampling. The received waveform recording unit 322 writes received waveform information, which sets the reception time of the terminal uplink signal at the antenna 31 and the generated waveform data, into the data storage unit 33. The received waveform information written by the received waveform recording unit 322 is stored in the data storage unit 33.

[0025] The base station communication unit 34 transmits the received waveform information to the base station 40 by a base station downlink signal of any wireless communication method.

[0026] The base station 40 includes an antenna 41, a receiver 42, a base station signal reception processor 43, a signal storage unit 44, and a terminal signal reception processor 45. The receiver 42 converts the base station downlink signal received by the antenna 41 into an electrical signal. The base station signal reception processor 43 demodulates and decodes the received signal converted into an electrical signal by the receiver 42 to obtain received waveform information. The base station signal reception processor 43 stores the received waveform information in the signal storage unit 44. The signal storage unit 44 stores the received waveform information obtained by the base station signal reception processor 43.

[0027] The terminal signal reception processing unit 45 includes a signal subtraction unit 46 , a Doppler shift estimation unit 47 , a signal detection unit 48 , a demodulation unit 49 , an inspection unit 50 , and an interference removal unit 51 .

[0028] The signal subtraction unit 46 receives the received signal that has been successfully demodulated by the interference removal unit 51 and the received waveform information stored in the signal storage unit 44. The signal subtraction unit 46 subtracts the received signal that has been successfully demodulated from the received waveform information that has been input. In this way, the signal subtraction unit 46 removes the component of the received signal that has been successfully demodulated from the received waveform information.

[0029] The Doppler shift estimation unit 47 estimates the Doppler shift and frequency offset based on received waveform information (hereinafter referred to as the "compensation-target received signal") from which components of the received signal that have been successfully demodulated by the signal subtraction unit 46 have been removed. The Doppler shift estimation unit 47 compensates for the compensation-target received signal using the estimated values of Doppler shift and frequency offset. Specifically, the Doppler shift estimation unit 47 multiplies the compensation-target received signal by the estimated value of Doppler shift and the estimated value of frequency offset to obtain the compensated received signal.

[0030] The signal detector 48 detects the compensated received signal on the frequency axis using a band-limiting filter.

[0031] The demodulator 49 performs demodulation processing on the compensated received signal detected by the signal detector 48 .

[0032] The inspection unit 50 performs error detection on the demodulated and compensated received signal. For example, the inspection unit 50 performs error detection using CRC (Cyclic Redundancy Check). In this case, the inspection unit 50 extracts information corresponding to the CRC bits of the demodulated and compensated received signal. The inspection unit 50 performs error detection based on the extracted information corresponding to the CRC bits of the compensated received signal, and determines whether the demodulation result is correct. Note that the error detection performed by the inspection unit 50 may use an error detection method other than CRC.

[0033] The interference canceller 51 receives as input the error detection result from the inspector 50 and the compensated received signal that has been demodulated. The interference canceller 51 is made up of a replica generator 511 and a blanking processor 512. If the determination result from the inspector 50 indicates that the demodulation result is correct, the replica generator 511 generates a replica signal of the compensated received signal that has been demodulated. The replica generator 511 outputs the generated replica signal to the signal subtractor 46 as a successfully demodulated received signal. If the determination result from the inspector 50 indicates that the demodulation result is incorrect, the blanking processor 512 applies blanking to the signal band of the compensated received signal that has been demodulated to cancel the signal.

[0034] 2 is a diagram showing a specific example of the configuration of the Doppler shift estimation unit 47 in the embodiment. The Doppler shift estimation unit 47 includes M first calculation units 471-1 to 471-M, M processing units 472-1 to 472-M, a comparison unit 473, and a compensation unit 474. M is an integer of 2 or more.

[0035] The first calculation units 471-1 to 471-M receive the compensation target received signals. The first calculation units 471-1 to 471-M calculate M types of frequency offset correction coefficient candidates f a (a=1,...,M) to the compensation target received signal. For example, the first calculation unit 471-m (1≦m≦M) multiplies the frequency offset correction coefficient candidate f m In this way, the first calculation units 471-1 to 471-M multiply the compensation target received signal by the frequency offset correction coefficient candidates f a (a=1,...,M) is multiplied by the received signal to be compensated.

[0036] The processing units 472-1 to 472-M receive the output signals of the first calculation units 471-1 to 471-M. The output signals of the first calculation units 471-1 to 471-M are compensation-target received signals multiplied by different frequency offset correction coefficient candidates. For example, the processing unit 472-m receives the frequency offset correction coefficient candidate f mThe received signal to be compensated is input.

[0037] Here, the configuration of processing units 472-1 to 472-M will be described. Since each processing unit 472 has the same configuration, processing unit 472-M will be described as an example. Processing unit 472-M includes N second calculation units 476-M-1 to 476-MN, N power calculation units 477-M-1 to 477-MN, and comparison unit 478-M. Since each processing unit 472 has the same configuration, Doppler shift estimation unit 47 includes M×N second calculation units 476, M×N power calculation units 477, and M comparison units 478. N is an integer of 2 or greater.

[0038] The second calculation units 476-M-1 to 476-MN receive the output signal of the first calculation unit 471-M. The second calculation units 476-M-1 to 476-MN calculate N types of Doppler shift correction coefficient candidates d b (b=1,...,N) to the output signal of the first calculation unit 471-M. For example, the second calculation unit 476-Mn (1≦n≦N) multiplies the output signal of the first calculation unit 471-M by the Doppler shift correction coefficient candidate d n In this way, in one processing unit 472 (for example, processing unit 472-M), different Doppler shift correction coefficient candidates d b is multiplied by the output signal of the first calculation unit 471 (for example, the first calculation unit 471-M).

[0039] The power calculation units 477-M-1 to 477-MN receive as input the output signals of the second calculation units 476-M-1 to 476-MN. The output signals of the second calculation units 476-M-1 to 476-MN are compensation-target received signals multiplied by Doppler shift correction coefficient candidates. For example, the power calculation unit 477-Mn receives as input the compensation-target received signals multiplied by the Doppler shift correction coefficient candidates by the second calculation unit 476-Mn. The power calculation unit 477-Mn calculates the power of the output signal of the second calculation unit 476-Mn within the desired band.

[0040] The comparator 478-M receives as input the output values (power values) of the power calculators 477-M-1 to 477-MN. The comparator 478-M compares the output values of the power calculators 477-M-1 to 477-MN. As a result of the comparison, the comparator 478-M outputs to the comparator 473 the combination of correction coefficient candidates (frequency offset correction coefficient candidates and Doppler shift correction coefficient candidates) with the highest power and the power of that combination.

[0041] By performing the above processing, each processing unit 472 outputs the combination of correction coefficient candidates with the highest power and the power of the combination to the comparison unit 473. For example, the processing unit 472-1 outputs the combination with the highest signal power from among the combinations of the frequency offset correction coefficient candidate f1 and the different Doppler shift correction coefficient candidates to the comparison unit 473. For example, the processing unit 472-M outputs the combination with the highest signal power from the combination of the frequency offset correction coefficient candidate f2 and the different Doppler shift correction coefficient candidates to the comparison unit 473. M and the Doppler shift correction coefficient candidates different from each other, outputs the combination with the highest signal power to the comparison unit 473. The comparison unit 473 compares the powers output from each processing unit 472. As a result of the comparison, the comparison unit 473 calculates the combination of correction coefficient candidates (frequency offset correction coefficient candidate and Doppler shift correction coefficient candidate) with the highest power as the estimated value of frequency offset and estimated value of Doppler shift to be used for compensation of the received signal. The comparison unit 473 outputs the estimated value of frequency offset and estimated value of Doppler shift to the compensation unit 474.

[0042] The compensator 474 compensates for the compensation-target received signal by multiplying the compensation-target received signal by the estimated value of the frequency offset and the estimated value of the Doppler shift output from the comparator 473 .

[0043] 3 is a diagram illustrating an example configuration of the compensating unit 474 in the embodiment. The compensating unit 474 includes a multiplier 4741 and a multiplier 4742. The multiplier 4741 multiplies the received signal to be compensated for by the frequency offset estimate output from the comparator 473. In this way, the multiplier 4741 compensates for the frequency offset of the received signal to be compensated for. The multiplier 4741 outputs the received signal to be compensated for after frequency offset compensation to the multiplier 4742. The multiplier 4742 multiplies the received signal to be compensated for after frequency offset compensation by the Doppler shift correction coefficient candidate output from the comparator 473. In this way, the multiplier 4742 compensates for the Doppler shift suffered by the received signal to be compensated for.

[0044] Frequency offset correction coefficient candidate f a ,Doppler shift correction coefficient candidate d b The number of first calculation units 471 and the number of second calculation units 476 can be determined in advance as maximum or minimum values depending on the altitude and movement speed of the mobile relay station 30, but generally can be set arbitrarily.

[0045] Next, a specific flow of processing of the terminal signal reception processing unit 45 provided in the base station 40 will be described. Here, the input waveform of the Doppler shift estimation unit 47 is assumed to be x(t). The input waveform x(t) is a signal that has undergone a Doppler shift and a frequency offset due to high-speed movement of the mobile relay station 30. The Doppler shift d b and frequency offset f a is given as the following equation (1).

[0046]

number

[0047] x´ ab The Fourier transform of (t) is x´ ab The power calculation unit 477 calculates f=-(f w / 2)~(f w / 2) abCalculate f w is the bandwidth of the transmission signal, and is a value that is uniquely determined depending on the communication method, so it is set in advance depending on the communication method to be used.

[0048]

number

[0049] The comparison units 473 and 478 compare the power P ab are compared based on the following formula (3), and the combinations a' and b' that maximize the power are calculated.

[0050]

number

[0051] The comparison unit 478 compares the calculated frequency offset correction coefficient f sa´ (a is a subscript of s) and Doppler shift correction coefficient d sb´ (b is a subscript of s) is calculated based on the following equations (4) and (5). sa´ and Doppler shift correction coefficient d sb´ are the frequency offset and Doppler shift estimates.

[0052]

number

[0053]

number

[0054] Next, the compensating unit 474 compensates for the Doppler shift and frequency offset based on the following equation (6) using the estimated value of the frequency offset and the estimated value of the Doppler shift calculated by the comparing unit 478. The compensating unit 474 outputs the signal after compensation for the Doppler shift and frequency offset to the signal detecting unit 48.

[0055]

number

[0056] The signal detector 48 uses a band-pass filter to filter the signal output from the compensation unit 474, where f=-(f B / 2)~(f B / 2), where f B represents the frequency band to be extracted. The demodulation unit 49 restores the bit string by performing demodulation processing on the signal extracted by the signal detection unit 48. The demodulation unit 49 outputs the restored bit string to the inspection unit 50.

[0057] After inspecting the CRC bits in the bit string, the inspection unit 50 determines whether the demodulation result is correct. The inspection unit 50 outputs the determination result to the interference removal unit 51. Based on the determination result input from the inspection unit 50, the interference removal unit 51 performs one of the following processes.

[0058] (If the demodulation result is correct) 4 is a diagram illustrating an example of the configuration of the replica generation unit 511 in the embodiment. As shown in FIG. 4, the replica generation unit 511 modulates the bit string output from the demodulation unit 49, and generates the reciprocal (e- j2πfa´ ,e- j2πdb´t ) to perform channel estimation from the portion corresponding to the preamble included in the received frame. Replica generation unit 511 generates a received signal replica by multiplying the received signal by the channel. Replica generation unit 511 outputs the generated received signal replica to signal subtraction unit 46.

[0059] (If the demodulation result is incorrect) 5 is a diagram showing an example of the configuration of the blanking processing unit 512 in the embodiment. As shown in FIG. 5, the blanking processing unit 512 performs blanking processing on the received signal, and then calculates the reciprocal (e- j2πfa´ ,e- j2πdb´t ) Here, the processing performed by the blanking processing unit 512 will be described with reference to FIG.

[0060] FIG. 6 is a diagram illustrating the processing performed by the blanking processor 512 in the embodiment. FIG. 6 shows three diagrams, from top to bottom: the top, middle, and bottom. The diagram in the top of FIG. 6 represents a signal after Doppler shift compensation. The diagram in the middle of FIG. 6 represents a blanked signal. The diagram in the bottom of FIG. 6 represents a signal after the blanked signal has been multiplied by the reciprocals of the estimated Doppler shift and the estimated frequency offset. As shown in the middle of FIG. 6, the blanking processor 512 performs blanking by setting to zero the frequency components corresponding to the transmission signal bandwidth within the observation band. Next, as shown in the bottom of FIG. 6, the blanking processor 512 multiplies the blanked signal by the reciprocals of the estimated Doppler shift and the estimated frequency offset, and outputs the result to the Doppler shift estimation unit 47 for subsequent received signal demodulation.

[0061] Next, the demodulation algorithm will be described. Fig. 7 is a flowchart showing the flow of the demodulation process performed by the base station 40 in this embodiment. At the start of the process in Fig. 7, the number of detected terminals T detis defined. The base station 40 sets the number of correctly demodulated signals i and the number of signals j removed by blanking to 0, i.e., i=0, j=0 (steps S101 and S102). The Doppler shift estimation unit 47 receives the compensation-target received signal as input and estimates the frequency offset and Doppler shift for the input compensation-target received signal (step S103). Thereafter, the Doppler shift estimation unit 47 compensates the compensation-target received signal using the estimated value of the frequency offset and the estimated value of the Doppler shift.

[0062] The signal detection unit 48 uses a band-limiting filter to detect the compensated received signal compensated by the Doppler shift estimation unit 47 (step S104). The signal detection unit 48 outputs the detected compensated received signal to the demodulation unit 49. The demodulation unit 49 performs demodulation processing on the compensated received signal output from the signal detection unit 48 (step S105). The demodulation unit 49 restores the bit string through the demodulation processing. The demodulation unit 49 outputs information on the restored bit string to the inspection unit 50.

[0063] The inspection unit 50 inspects the CRC bits based on the information of the bit string output from the demodulation unit 49, and determines whether the demodulation result is correct (step S106). If the demodulation result is correct (step S106-YES), the replica generation unit 511 performs channel estimation from the part of the bit string that corresponds to the preamble of the demodulated signal (step S107). The replica generation unit 511 generates a received signal replica using the estimated channel (step S108). The replica generation unit 511 outputs the generated received signal replica to the signal subtraction unit 46.

[0064] The signal subtraction unit 46 subtracts the received signal replica output from the replica generation unit 511 from the received signal received via the antenna 41 (step S109). The signal subtraction unit 46 outputs the received signal after subtraction to the Doppler shift estimation unit 47. The Doppler shift estimation unit 47 adds 1 to i and detects the next received signal (step S110).

[0065] In the process of step S106, if the demodulation result is incorrect (step S106-NO), blanking processing unit 512 performs blanking of the detected signal band (step S109). Blanking processing unit 512 outputs the signal after blanking processing to Doppler shift estimation unit 47. Doppler shift estimation unit 47 adds 1 to j and detects the next received signal (step S112).

[0066] After the processing of step S110 or step S112, the Doppler shift estimation unit 47 calculates i+j=T det (Step S113). Specifically, Doppler shift estimation unit 47 determines whether the sum of the number of correctly demodulated signals and the number of signals removed by blanking reaches the number of detected terminals. If the sum of the number of correctly demodulated signals and the number of signals removed by blanking reaches the number of detected terminals (Step S113-YES), base station 40 ends the process.

[0067] On the other hand, if the sum of the number of correctly demodulated signals and the number of signals removed by blanking does not reach the number of detected terminals (step S113-NO), the base station 40 executes the processes from step S102 onwards. Through the above procedure, the base station 40 performs interference removal, giving priority to demodulatable signals.

[0068] According to the wireless communication system 1 configured as described above, it is possible to estimate the Doppler shift and frequency offset for a plurality of signals that have undergone different Doppler shifts without inserting a preamble, etc. Specifically, the base station 40 includes a receiver 42 that receives waveform data transmitted by the mobile relay station 30, and a Doppler shift estimator 47 that estimates the Doppler shift and frequency offset suffered by the received signal indicated by the waveform data received by the receiver 42. The Doppler shift estimation unit 47 includes M first calculation units 471 that branch a received signal and multiply each of the branched received signals by M frequency offset correction coefficient candidates, M×N second calculation units 476 that multiply each of output signals of the M first calculation units 471 by N Doppler shift correction coefficient candidates, and a comparison unit 473 that calculates estimated values of frequency offset and Doppler shift to be used for compensating the received signal from the frequency offset correction coefficient candidate and Doppler shift correction coefficient candidate multiplied by one of the output signals output from each of the M×N second calculation units 476. This makes it possible to estimate the Doppler shift and frequency offset without using a preamble or the like. As a result, the signal frame can be demodulated.

[0069] (Variation 1) In the above-described embodiment, the comparison unit 473 calculates the frequency offset correction coefficient f based on the equations (4) and (5). sa´ and Doppler shift correction coefficient d sb´ The comparison unit 473 calculates the frequency offset correction coefficient f by either the (second calculation method) or (third calculation method) shown below. sa´ and Doppler shift correction coefficient d sb´ This will be explained in detail below.

[0070] (Second calculation method) The comparison unit 473 performs demodulation processing on each output signal output from each of the M×N second calculation units 476, and then performs error detection, and estimates the frequency offset correction coefficient candidate and Doppler shift correction coefficient candidate multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection as the estimated value of the frequency offset and the estimated value of the Doppler shift. For example, the comparison unit 473 ab After performing demodulation processing on (t), error detection is performed using CRC bits. The comparison unit 473 calculates, as estimated values of the frequency offset and the Doppler shift, combinations a″ and b″ of frequency offset correction coefficient candidates and Doppler shift correction coefficient candidates (e.g., a and b) multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection.

[0071] (Third calculation method) The comparison unit 473 performs demodulation processing on output signals having power exceeding a predetermined threshold among the powers calculated by the N power calculation units 477, and then performs error detection, and estimates the frequency offset correction coefficient candidate and Doppler shift correction coefficient candidate multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection as the estimated value of the frequency offset and the estimated value of the Doppler shift. For example, the comparison unit 473 calculates the power P ab Among them, x' with power exceeding a predetermined threshold ab After performing demodulation processing on (t), error detection is performed using CRC bits. The comparison unit 473 calculates, as estimated values of the frequency offset and the Doppler shift, combinations a″ and b″ of frequency offset correction coefficient candidates and Doppler shift correction coefficient candidates (e.g., a and b) multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection.

[0072] (Variation 2) In the above-described embodiment, an example has been described in which the Doppler shift estimation unit 47 is configured to include multiple comparison units. Specifically, an example has been described in which the Doppler shift estimation unit 47 is configured to include M comparison units 478 and one comparison unit 473 in multiple stages. The Doppler shift estimation unit 47 may be configured to include one comparison unit 473 instead of M comparison units 478. In such a configuration, the one comparison unit 473 receives as input the power values output from the power calculation units 477 of the processing units 472 and compares the input power values output from the power calculation units 477 of the processing units 472. The one comparison unit 473 estimates the combination of correction coefficient candidates (frequency offset correction coefficient candidates and Doppler shift correction coefficient candidates) with the highest power as the frequency offset and Doppler shift to be used for compensation of the received signal.

[0073] Some or all of the processing performed by base station 40 in the above-described embodiment may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" as used herein includes hardware such as an operating system (OS) and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as a flexible disk, a magneto-optical disk, a read-only memory (ROM), a compact disc-ROM (CD-ROM), and a storage device such as a hard disk built into a computer system.

[0074] Furthermore, the term "computer-readable recording medium" may include a medium that dynamically stores a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, or a medium that stores a program for a fixed period of time, such as a volatile memory within a computer system that serves as a server or client in such a case. The program may also be one that realizes part of the above-mentioned functions, or one that can realize the above-mentioned functions in combination with a program already stored in the computer system, or one that can be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0075] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0076] The present invention can be applied to a technology for communicating with a mobile object equipped with a mobile relay station. [Explanation of symbols]

[0077] 1...wireless communication system, 20...terminal station, 21...data storage unit, 22...transmission unit, 30...mobile relay station, 40...base station, 31...antenna, 32...terminal communication unit, 33...data storage unit, 34...base station communication unit, 35...antenna, 41...antenna, 42...reception unit, 43...base station signal reception processing unit, 44...signal storage unit, 45...terminal signal reception processing unit, 46...signal subtraction unit, 47...Doppler shift estimation unit, 48...signal detection unit, 49...demodulation unit, 50...inspection unit, 51...interference removal unit, 471, 471-1 to 471-M...first calculation unit, 472, 472-1 to 472-M...processing unit, 473, 478, 478-M...comparison unit, 474...compensation unit, 476, 476-M-1 to 476-MN... second calculation unit, 477, 477-M-1 to 477-MN... power calculation unit, 511... replica generation unit, 512... blanking processing unit

Claims

1. A wireless communication system having a plurality of transmitting devices, a mobile wireless communication device, and a receiving device, the plurality of transmitting devices each include a transmitting unit that transmits a radio signal; The wireless communication device one or more antennas for receiving the radio signals transmitted from the plurality of transmitting devices; a waveform transmitting unit that transmits waveform data indicating a waveform of a received signal received by the one or more antennas to the receiving device; Equipped with The receiving device a receiving unit that receives the waveform data transmitted by the wireless communication device; a Doppler shift estimation unit that estimates a Doppler shift and a frequency offset that have been applied to the received signal indicated by the waveform data received by the receiving unit; Equipped with The Doppler shift estimation unit M first calculation units that split the received signal and multiply each of the split received signals by M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates; M×N second calculation units that multiply each output signal of the M first calculation units by N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates; a comparison unit that estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by any one of the output signals output from each of the M×N second calculation units as an estimated value of the frequency offset and an estimated value of the Doppler shift used to compensate for the received signal; A wireless communication system comprising:

2. the Doppler shift estimation unit further includes N power calculation units that calculate powers of the output signals output from the M×N second calculation units, the comparison unit compares the powers calculated by the N power calculation units, and estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by the signal with the maximum power as the estimated value of the frequency offset and the estimated value of the Doppler shift.

10. The wireless communication system of claim 1.

3. the comparison unit performs demodulation processing on each output signal output from each of the M×N second calculation units, and then performs error detection, and estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection as the frequency offset estimate value and the Doppler shift estimate value.

10. The wireless communication system of claim 1.

4. the Doppler shift estimation unit further includes N power calculation units that calculate powers of the output signals output from the M×N second calculation units, the comparison unit performs demodulation processing on an output signal having a power exceeding a predetermined threshold among the powers calculated by the N power calculation units, and then performs error detection, and estimates, as the estimated value of the frequency offset and the estimated value of the Doppler shift, the candidate frequency offset correction coefficient and the candidate Doppler shift correction coefficient multiplied by the output signal whose demodulation result is determined to be correct as a result of the error detection.

10. The wireless communication system of claim 1.

5. The Doppler shift estimation unit further compensates the received signal using the estimated value of the frequency offset and the estimated value of the Doppler shift. A wireless communication system according to any one of claims 1 to 4.

6. The receiving device a signal detection unit that detects a signal in a predetermined band from the received signal compensated by the Doppler shift estimation unit; a demodulation unit that restores a bit string by demodulating the signal detected by the signal detection unit; an inspection unit that performs error detection based on the bit string demodulated by the demodulation unit; a replica generation unit that, when the demodulation result is correct as a result of the error detection by the check unit, performs channel estimation by multiplying the bit string by the frequency offset and the reciprocal of the Doppler shift after modulating the bit string, and generates a received signal replica by multiplying the estimated channel by the received signal; a blanking processing unit that, when the demodulation result is incorrect as a result of error detection by the checking unit, performs blanking processing on the received signal, then multiplies the received signal by the frequency offset and the reciprocal of the Doppler shift, and inputs the result to the Doppler shift estimating unit; The wireless communication system of claim 5 further comprising:

7. A receiving device in a wireless communication system having a plurality of transmitting devices, a mobile wireless communication device, and a receiving device, a receiving unit that receives wireless signals transmitted from the plurality of transmitting devices via the wireless communication device; a Doppler shift estimation unit that estimates a Doppler shift and a frequency offset of a received signal received by the receiving unit; Equipped with The Doppler shift estimation unit M first calculation units that split the received signal and multiply each of the split received signals by M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates; M×N second calculation units that multiply each output signal of the M first calculation units by N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates; a comparison unit that estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by any one of the output signals output from each of the M×N second calculation units as an estimated value of the frequency offset and an estimated value of the Doppler shift used to compensate for the received signal; A receiving device comprising:

8. 1. An estimation method in a wireless communication system having a plurality of transmitting devices, a moving wireless communication device, and a receiving device, comprising: the plurality of transmitting devices transmit radio signals; the wireless communication device transmits to the receiving device waveform data indicating waveforms of received signals received by one or more antennas that receive the wireless signals transmitted from the plurality of transmitting devices; the receiving device receives the waveform data transmitted by the wireless communication device; the receiving device branches the received signal indicated by the received waveform data, multiplies each of the branched received signals by M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates, multiplies each of the received signals multiplied by the M (M is an integer of 2 or more) types of frequency offset correction coefficient candidates by N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates, and estimates the frequency offset correction coefficient candidate and the Doppler shift correction coefficient candidate multiplied by an output signal of any of the received signals multiplied by the N (N is an integer of 2 or more) types of Doppler shift correction coefficient candidates as an estimated value of a frequency offset and an estimated value of a Doppler shift to be used for compensation of the received signal.

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