Transmitter, receiver, and communication device

Abstract

To allow good antenna selection results to be obtained in a communication device that performs receiving antenna diversity using the OFDM system.SOLUTION: An OFDM modulation unit (12) of a transmitter (1) adds an antenna selection signal (41) having smaller amplitude fluctuations than an STF to a position before the STF of an OFDM signal. A receiver (2) includes an antenna changeover switch (23) that selects a receiving antenna to be used for communication from among a plurality of receiving antennas (24, 25), and an OFDM demodulation unit (22) that demodulates the OFDM signal received by the receiving antenna selected by the antenna changeover switch (23). The antenna changeover switch (23) selects the receiving antenna to be used for communication based on the antenna selection signal (41) added to the OFDM signal.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present disclosure relates to a communication device that performs reception antenna diversity in an OFDM system.

Background Art

[0002] As a modulation method in wireless communication, an Orthogonal Frequency Division Multiplexing (hereinafter referred to as "OFDM") method can be mentioned. In the OFDM method, by making subcarriers orthogonal, even when the spectra of the subcarriers overlap, the transmitted data can be demodulated, and there is an advantage of high frequency utilization efficiency.

[0003] Also, as a method for improving the transmission and reception performance in wireless communication, reception antenna diversity that receives signals using a plurality of antennas and transmission antenna diversity that transmits signals using a plurality of antennas are known.

[0004] For example, when performing selection diversity, which is one type of reception antenna diversity, in OFDM communication, an antenna with a larger time average of the RSSI (Received Signal Strength Indicator) of the first 4 symbols called the STF (Short Training Field) of the received signal having the data frame format of the OFDM method is selected as the antenna to be used for communication. However, in the OFDM method, since the amplitude of the signal fluctuates greatly, there is a possibility that the correct antenna cannot be selected by this method.

[0005] As a technique for obtaining good antenna selection results even in the OFDM method, Patent Document 1 below discloses an OFDM receiver provided with demodulation means for synthesizing signals received by two antennas, removing the guard interval signal inserted at the time of modulation, and extracting the valid symbol signal.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-111521 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] In the technology of Patent Document 1, since it is necessary to add a new device as effective symbol extraction means to the OFDM receiver, there are concerns about the complication and high cost of the OFDM receiver. Further, there are a selection diversity method and a combining diversity method for reception antenna diversity, but the technology of Patent Document 1 only corresponds to the combining diversity method and cannot be applied to the selection diversity method that can be implemented at a relatively low cost.

[0008] The present disclosure has been made to solve the above problems, and an object thereof is to enable a communication device that performs reception antenna diversity by an OFDM method to obtain good antenna selection results. [Means for Solving the Problems]

[0009] The transmitter according to the present disclosure includes an OFDM modulation unit that converts data to be transmitted into an OFDM signal modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method, and a transmission antenna that transmits the OFDM signal. The OFDM modulation unit adds an antenna selection signal having smaller amplitude fluctuations than the STF (Short Training Field) of the OFDM signal before the STF of the OFDM signal.

[0010] The receiver according to the present disclosure includes a plurality of receiving antennas that receive OFDM (Orthogonal Frequency Division Multiplexing) signals, an antenna switching switch that selects a receiving antenna to be used for communication from among the plurality of receiving antennas, and an OFDM demodulation unit that demodulates the OFDM signal received by the receiving antenna selected by the antenna switching switch. The antenna switching switch selects the receiving antenna to be used for communication based on an antenna selection signal added before the STF (Short Training Field) of the OFDM signal.

Effect of the Invention

[0011] According to the present disclosure, in a communication device that performs receiving antenna diversity in the OFDM system, good antenna selection results can be obtained.

Brief Description of the Drawings

[0012]

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Best Mode for Carrying Out the Invention

[0013] <Embodiment 1> FIG. 1 is a configuration diagram of a communication system according to Embodiment 1. In the following embodiments, for simplicity of explanation, the transmitter 1 that transmits a signal and the receiver 2 that receives a signal will be described as independent communication devices. However, one communication device may include both the transmitter 1 and the receiver 2 and perform both signal transmission and reception with other communication devices. Also, in the following embodiments, an example is shown in which both the transmitter 1 and the receiver 2 perform selection diversity using two antennas, but the number of antennas and the diversity method are not limited to this.

[0014] As shown in FIG. 1, the transmitter 1 includes a baseband signal generation unit 11, an OFDM modulation unit 12, an antenna switch 13, and two transmit antennas 14 and 15.

[0015] The baseband signal generation unit 11 generates a baseband signal (frequency: about several tens of kHz) including data to be transmitted. The OFDM modulation unit 12 modulates the baseband signal generated by the baseband signal generation unit 11 into an OFDM signal with a carrier frequency (frequency: several hundreds of MHz or more), and inputs it to the transmission antennas 14 and 15 via the antenna switch 13. The transmission antennas 14 and 15 transmit from the transmitter 1 by radiating the input signal as radio waves.

[0016] Note that the antenna switch 13 performs transmission antenna diversity using the transmission antennas 14 and 15. However, since the technology according to the present disclosure is characterized by the method of reception antenna diversity and the transmission antenna diversity may be the same as the conventional method, the description of the operation of the antenna switch 13 is omitted.

[0017] The receiver 2 includes an OFDM demodulation unit 22, an antenna switch 23, and two reception antennas 24 and 25. The reception antennas 24 and 25 receive the signal transmitted from the transmitter 1 by receiving the radio waves radiated from the transmission antennas 14 and 15 of the transmitter 1. The signals received by the reception antennas 24 and 25 are input to the OFDM demodulation unit 22 via the antenna switch 23. The OFDM demodulation unit 22 demodulates the input signal in the OFDM method and extracts the original data (data transmitted by the transmitter 1) from the demodulated signal.

[0018] FIG. 2 and FIG. 3 are diagrams for explaining the reception antenna diversity performed by the receiver 2. The OFDM demodulation unit 22 has a port for transmitting the modulated signal so that it can also be used as a modulation means (that is, the OFDM modulation unit 12) on the transmitter side. Here, a port for inputting the received signal (received signal) to be demodulated is defined as "Rx", and a port for transmitting the modulated signal (modulated signal) is defined as "Tx".

[0019] Figure 2 shows the case where, as a result of selection diversity, among the receiving antennas 24 and 25, the receiving antenna 24 is selected as the antenna to be used for communication. In this case, the antenna switch 23 connects the receiving antenna 24 to the Rx port of the OFDM demodulation unit 22 and connects the receiving antenna 25 to the Tx port of the OFDM demodulation unit 22. As a result, the received signal of the receiving antenna 24 is input to the Rx port and is demodulated by the OFDM demodulation unit 22. On the other hand, the received signal of the receiving antenna 25 is input to the Tx port, but since the Tx port is the transmission port for the modulated signal, nothing happens even if the received signal is input here.

[0020] When, as a result of selection diversity, among the receiving antennas 24 and 25, the receiving antenna 25 is selected as the antenna to be used for communication, as shown in Figure 3, the antenna switch 23 connects the receiving antenna 24 to the Tx port of the OFDM demodulation unit 22 and connects the receiving antenna 25 to the Rx port of the OFDM demodulation unit 22. As a result, the received signal of the receiving antenna 25 is input to the Rx port and is demodulated by the OFDM demodulation unit 22. Note that the OFDM demodulation unit 22 may not have a Tx port. In that case, processing such as termination is performed on the receiving antenna that is not connected to the Rx port.

[0021] FIG. 4 is a diagram showing a data frame format of a general OFDM signal. The leading data 31 is called STF (Short Training Field) and is used for frame detection and antenna selection. The data 32 after the STF is called LTF (Long Training Field) and is used for channel estimation and frequency offset correction. The subsequent data 33 is called PHR (PHY (Physical Layer) Header) and contains information such as the OFDM to be used. The last data 34 is called PHY Payload, and the data to be transmitted is put in this part. In the conventional selection diversity, the time-averaged value of the RSSI (Received Signal Strength Indicator) in the 4 symbols of the STF is calculated for each receiving antenna, and the antenna with a larger time-averaged value of the RSSI is selected to be used for communication.

[0022] FIG. 5 is a diagram showing an example of the time waveform of the RSSI when a general OFDM signal as shown in FIG. 4 is received by each of the receiving antennas 24 and 25 of the receiver 2. The solid waveform 24R in FIG. 5 indicates the RSSI of the receiving antenna 24, and the broken waveform 25R indicates the RSSI of the receiving antenna 25. Looking at the entire waveform in FIG. 5, since the time average of the RSSI is larger for the receiving antenna 24, it can be seen that the receiving antenna 24 should be selected as the antenna to be used for communication. However, in the STF part, the receiving antenna 25 has a larger time average of the RSSI. Therefore, when a waveform as shown in FIG. 5 is obtained, in the conventional selection diversity, the receiving antenna 25 is selected as the antenna to be used for communication. Thus, since the amplitude of the OFDM signal fluctuates greatly, the conventional selection diversity may not be able to select the correct antenna.

[0023] FIG. 6 is a diagram showing the data frame format of the OFDM signal transmitted by the transmitter 1 according to Embodiment 1, that is, the OFDM signal generated by the OFDM modulation unit 12. In the OFDM signal of Embodiment 1, an antenna selection signal 41 is added at the head (before the STF). The OFDM modulation unit 12 inserts a frequency shift keying (FSK) signal into the portion of the antenna selection signal 41.

[0024] In the reception antenna diversity in the receiver 2, among the reception antennas 24 and 25, the antenna with a larger time average of the RSSI of the antenna selection signal 41 (FSK signal) in the received signal is selected as the antenna to be used for communication. Since the FSK signal is a frequency modulation signal, its amplitude hardly varies. Therefore, by selecting the antenna to be used for communication based on the time average value of the RSSI of the FSK signal, better antenna selection than the conventional selection diversity can be expected.

[0025] In addition, since this embodiment is realized only by adding a signal to the data frame format of the conventional OFDM signal, it is not necessary to provide new devices for the transmitter 1 and the receiver 2, and it is applicable not only to the selection diversity method but also to the combining diversity method.

[0026] <Embodiment 2> In the data frame format (FIG. 6) of the OFDM signal in Embodiment 1, the modulation method of the data of the antenna selection signal 41 added at the head is the FSK method, and after the STF, it is the OFDM method, and the modulation methods are partially different. For example, when the OFDM demodulation unit 22 of the receiver 2 corresponds to both the modulation methods of the OFDM signal and the FSK signal, when receiving the OFDM signal in FIG. 6, since the antenna selection signal 41 at the head is modulated by the FSK method, there is a possibility that the data after the STF will also be demodulated by the FSK method.

[0027] Therefore, in Embodiment 2, a data frame format of an OFDM signal as shown in FIG. 7 is proposed. In the data frame format of FIG. 7, a modulation method setting signal 42 indicating the modulation method of the data after the STF is added between the antenna selection signal 41 and the STF. Information on whether the modulation method of the data after the STF is the OFDM method or the FSK method is inserted into the part of the modulation method setting signal 42. The OFDM demodulation unit 22 of the receiver 2 sets whether to demodulate the data after the STF by the OFDM method or the FSK method based on the modulation method setting signal 42 included in the received signal.

[0028] FIG. 8 is a flowchart showing the process in which the receiver 2 selects a receiving antenna in Embodiment 2. Here, it is assumed that the receiver 2 is compatible with both the OFDM method and the FSK method.

[0029] When the receiver 2 receives the antenna selection signal 41 included in the received signal (step S1), it calculates the time average of the RSSI of the antenna selection signal 41 for each receiving antenna (step S2). Then, it selects the receiving antenna with a larger time average value of the RSSI as the receiving antenna to be used for communication, controls the antenna switch 23, and connects the selected receiving antenna to the Rx port of the OFDM demodulation unit 22.

[0030] Thereafter, it acquires the information on the modulation method of the data after the STF from the data of the modulation method setting signal 42 (step S4), and based on that information, sets the operation mode of the OFDM demodulation unit 22 to the OFDM demodulation mode for demodulating by the OFDM method or the FSK demodulation mode for demodulating by the FSK method (step S5). Thereby, the OFDM demodulation unit 22 can correctly demodulate the data after the STF.

[0031] <Embodiment 3> In Embodiment 1, an example was shown in which an FSK signal was inserted into the portion of the antenna selection signal 41 added to the head of the OFDM signal. However, the signal inserted into the antenna selection signal 41 may be a signal with less amplitude fluctuation than the signal modulated by the OFDM method. Therefore, even if a CW (continuous wave) wave, which is an unmodulated signal, is inserted into the portion of the antenna selection signal 41, the same effect as in Embodiment 1 can be obtained.

[0032] FIG. 9 is a diagram showing the data frame format of the OFDM signal in Embodiment 3. In the data frame format of FIG. 9, a CW wave is inserted into the portion of the antenna selection signal 41.

[0033] Normally, since the receiver 2 does not have a function of receiving an unmodulated signal, if only the CW wave antenna selection signal 41 is added to the OFDM signal, there is a possibility that the receiver 2 processes the antenna selection signal 41 as mere noise.

[0034] Therefore, in the data frame format of the OFDM signal of Embodiment 3, a CW wave start notification signal 43 for notifying the start of the CW wave is added before the antenna selection signal 41 containing the CW wave. Further, a CW wave end notification signal 44 for notifying the end of the CW wave is added after the antenna selection signal 41 containing the CW wave. Thereby, the receiver 2 that has received the OFDM signal can recognize that the CW wave between the CW wave start notification signal 43 and the CW wave end notification signal 44 is the antenna selection signal 41.

[0035] As a result, the receiver 2 that has received the OFDM signal in FIG. 9 can select the antenna to be used for communication based on the time average of the RSSI of the antenna selection signal 41 (CW wave) in the received signal, and the same effect as in Embodiment 1 can be obtained.

[0036] Note that the data frame format in FIG. 9 includes the modulation method setting signal 42 shown in the second embodiment. However, since the modulation method setting signal 42 is used when the part of the antenna selection signal 41 is an FSK signal, it may be omitted in this embodiment.

[0037] <Embodiment 4> As described above, it is difficult to correctly select an antenna in the receive antenna diversity performed by the OFDM method when the amplitude of the STF of the OFDM signal fluctuates greatly. Therefore, in the fourth embodiment, the OFDM modulation unit 12 of the transmitter 1 is provided with a function of suppressing the amplitude fluctuation of the STF of the OFDM signal.

[0038] FIG. 10 is a configuration diagram of a communication system according to the fourth embodiment. The communication system according to the fourth embodiment is the same as FIG. 1 except that a process of reducing the amplitude fluctuation of the STF is performed in the OFDM modulation unit 12 of the transmitter 1.

[0039] The data frame format of the OFDM signal in the fourth embodiment is shown in FIG. 11. The data frame format in FIG. 11 is the same as the general OFDM signal data frame format shown in FIG. 4. However, the STF is processed by the OFDM modulation unit 12 to reduce the amplitude fluctuation.

[0040] Here, a specific example of the "process of reducing the amplitude fluctuation of the STF" performed by the OFDM modulation unit 12 of the transmitter 1 will be described. For the primary modulation of OFDM, a modulation method typically called 16QAM is used. QAM stands for quadrature amplitude modulation, which is a modulation method that changes both the amplitude and phase of a carrier wave. In 16QAM, as shown in FIG. 12, 16 symbols are arranged on the complex plane (IQ plane), and a 4-bit signal is given to each symbol.

[0041] In each symbol on the IQ plane, the distance from the origin represents the amplitude, and the angle from the origin (the angle with respect to the +I axis) represents the phase. Therefore, when the symbol of the OFDM signal changes from the symbol at point A to the symbol at point B as shown in, for example, FIG. 13, or when it changes from the symbol at point A to the symbol at point C as shown in FIG. 14, if the distance from the origin of the symbol does not change, the amplitude of the OFDM signal does not change. However, when the symbol of the OFDM signal changes from the symbol at point A to the symbol at point D as shown in, for example, FIG. 15, i.e., when the distance from the origin of the symbol changes, the amplitude of the OFDM signal fluctuates. However, even when changing between symbols with the same distance from the origin (the same amplitude), there may be fluctuations in the amplitude. For example, the symbol at point E shown in FIG. 16 has the same amplitude as the symbol at point A, but when changing from the symbol at point A to the symbol at point E, since there is a change in amplitude as shown in FIG. 16, the amplitude of the OFDM signal changes.

[0042] From the above, in the process of generating the STF of the OFDM signal, the OFDM modulation unit 12 of the transmitter 1 can reduce the amplitude fluctuation of the STF by imposing constraints (for example, when selecting a symbol from another point from the symbol at point A, constraints such that point B or point C is selected) so that the change in the distance from the origin of the symbol does not occur.

[0043] According to the fourth embodiment, since the amplitude of the STF of the OFDM signal is processed to be small, the receiver 2 can realize good antenna selection even when selecting a receiving antenna based on the time average value of the RSSI of the STF in the same manner as the conventional selection diversity.

[0044] It should be noted that the embodiments can be freely combined, or each embodiment can be appropriately modified or omitted.

[0045] <Appendix> Hereinafter, aspects of the present disclosure will be summarized and described as appendices.

[0046] (Appendix 1) An OFDM modulation unit that converts data to be transmitted into an OFDM signal modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method, A transmission antenna that transmits the OFDM signal, Comprising, Before the STF (Short Training Field) of the OFDM signal, the OFDM modulation unit adds an antenna selection signal with smaller amplitude fluctuation than the STF, Transmitter.

[0047] (Appendix 2) The antenna selection signal is a frequency modulation signal, The transmitter according to Appendix 1.

[0048] (Appendix 3) After the antenna selection signal of the OFDM signal, the OFDM modulation unit further adds a modulation method setting signal indicating the modulation method of the data after the STF, The transmitter according to Appendix 2.

[0049] (Appendix 4) The antenna selection signal is an unmodulated signal, The transmitter according to Appendix 1.

[0050] (Appendix 5) Before and after the antenna selection signal of the OFDM signal, the OFDM modulation unit further adds signals indicating the start and end of the unmodulated signal, The transmitter according to Appendix 4.

[0051] (Appendix 6) An OFDM modulation unit that converts data to be transmitted into an OFDM signal modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method, A transmission antenna that transmits the OFDM signal, Comprising, The OFDM modulation unit performs processing to reduce the amplitude variation of the STF (Short Training Field) for the OFDM signal. Transmitter.

[0052] (Appendix 7) A plurality of receiving antennas for receiving an OFDM (Orthogonal Frequency Division Multiplexing) signal, An antenna switching switch for selecting a receiving antenna to be used for communication from among the plurality of receiving antennas, An OFDM demodulation unit for demodulating the OFDM signal received by the receiving antenna selected by the antenna switching switch, Comprising: The antenna switching switch selects the receiving antenna to be used for communication based on an antenna selection signal added before the STF (Short Training Field) of the OFDM signal. Receiver.

[0053] (Appendix 8) The transmitter according to any one of Appendices 1 to 5, The receiver according to Appendix 7, A communication device comprising:

Explanation of Signs

[0054] 1 Transmitter, 2 Receiver, 11 Baseband signal generation unit, 12 OFDM modulation unit, 13 Antenna switching switch, 14 Transmitting antenna, 15 Transmitting antenna, 22 OFDM demodulation unit, 23 Antenna switching switch, 24 Receiving antenna, 25 Receiving antenna, 31 STF, 32 LTF, 33 PHR, 34 PHY Payload, 41 Antenna selection signal, 42 Modulation method setting signal, 43 CW wave start notification signal, 44 CW wave end notification signal.

Claims

1. An OFDM modulation unit that converts data to be transmitted into an OFDM signal modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method, A transmission antenna that transmits the OFDM signal, Comprising, Before the STF (Short Training Field) of the OFDM signal, the OFDM modulation unit adds an antenna selection signal with smaller amplitude fluctuations than the STF, Transmitter.

2. The antenna selection signal is a frequency modulation signal, The transmitter according to claim 1.

3. The OFDM modulation unit further adds a modulation method setting signal indicating the modulation method of the data after the STF after the antenna selection signal of the OFDM signal, The transmitter according to claim 2.

4. The antenna selection signal is an unmodulated signal, The transmitter according to claim 1.

5. The OFDM modulation unit further adds signals indicating the start and end of the unmodulated signal before and after the antenna selection signal of the OFDM signal, The transmitter according to claim 4.

6. An OFDM modulation unit that converts data to be transmitted into an OFDM signal modulated by an OFDM (Orthogonal Frequency Division Multiplexing) method, A transmission antenna that transmits the OFDM signal, Comprising, The OFDM modulation unit performs a process of reducing the amplitude fluctuation of the STF (Short Training Field) on the OFDM signal, Transmitter.

7. A plurality of receiving antennas that receive an OFDM (Orthogonal Frequency Division Multiplexing) signal, An antenna switching switch that selects a receiving antenna to be used for communication from among the plurality of receiving antennas, An OFDM demodulation unit that demodulates the OFDM signal received by the receiving antenna selected by the antenna switching switch, Comprising, The antenna switching switch selects the receiving antenna to be used for communication based on an antenna selection signal added before the STF (Short Training Field) of the OFDM signal, Receiver.

8. The transmitter according to any one of claims 1 to 5, The receiver according to claim 7, A communication device comprising.

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