JP4534037B2 - OFDM communication device - Google Patents

Description

  The present invention relates to an OFDM communication apparatus, and more particularly to an OFDM communication apparatus with a small delay while being digital communication.

  Research and development of remote control technology for robots, etc. is actively conducted. In particular, two-way robots that control tactile information (driving force and reaction force) by two-way transmission include telemedicine surgery, relief in collapsed houses due to earthquakes and disasters, searching for missing persons, and even mine removal, Expected to play an active role in the handling of hazardous materials in the event of a nuclear accident.

By the way, as information transmission means between these robots and the control device, conventionally, wired communication has been mainstream. This information transmission by wireless communication has many advantages such as the mobility of the robot, the expansion of the movement range by being released from the cable, the quick response in emergency such as the transportation and installation of the robot device. . On the other hand, there is a drawback due to the use of radio. In wireless communication, it is inevitable that noise is mixed on the transmission path, and countermeasures against transmission code errors caused by that are indispensable. As this measure,
1) An error detection code is used to detect the presence or absence of an error on the receiving side, and when an error is detected, an ARQ method (automatic retransmission request method) that returns a retransmission request to the transmitting side,
2) There is an FEC (Forward Error Correction) method in which an error correction code is used and an error is automatically corrected on the receiving side.

  In the latter system, interleaving (transmission code is shuffled in time and continuous burst errors are converted into random errors) is often used together to improve the correction capability. There is a drawback with time delay in transmission and reception, which has been the biggest factor hindering the application of wireless communication to real-time communication such as robot control.

  As a wireless communication means capable of reducing the transmission time delay, a method of applying an ACS (Adaptive sub-Carrier Selection adaptive channel selection) technique to an OFDM (Orthogonal Frequency Division Multiplexing) method has been proposed (for example, Patent Document 1). The OFDM system is a system that performs communication using a plurality of orthogonal subcarriers having different frequencies. The OFDM system is a wireless communication system that has high frequency utilization efficiency and is capable of transmitting high-speed data.

  FIG. 5 is a diagram illustrating a configuration of a conventional OFDM communication apparatus. In the conventional OFDM communication apparatus, the receiving unit includes a low noise amplifier (LNA) 31, a frequency converter (DC) 32, a Fourier transformer (FFT) 33, a demodulator 34, a deinterleaver 35, and a decoder 36. The unit includes an encoder 37, an interleaver 38, a modulator 39, an inverse Fourier transformer (IFFT) 40, a frequency converter (UC) 41, and a power amplifier (HPA) 42.

  Data input to the OFDM transmitter is subjected to error correction coding by an encoder 37 and an interleaver 38, and each modulator 39 is modulated by a modulator 39. The modulated subcarrier is frequency-division multiplexed and converted into an OFDM modulation waveform in the time domain by an inverse Fourier transformer 40. The obtained OFDM transmission wave is transmitted from the antenna through the frequency converter 41 and the power amplifier 42.

  In the receiving unit, the received signal received by the antenna passes through the low noise amplifier 31 and the frequency converter 32, and the received OFDM modulated wave is frequency-divided into each subcarrier by the Fourier transformer 33 and passes through the demodulator 34. Then, the data is subjected to error correction decoding by the deinterleaver 35 and the decoder 36.

  In wireless transmission, the code error rate is deteriorated due to the influence of the channel frequency characteristics due to fading in the channel, so the code error countermeasure technique shown in FIG. 5 is used. However, in the case of communication where the transmission speed is not so high as in robot control, an ACS communication system in which communication is performed by adaptively selecting a channel having a large carrier amplitude from a plurality of OFDM channels can be applied.

  FIG. 6 is a diagram illustrating the principle of the ACS communication method. As shown in FIG. 6, the radio wave transmitted from the OFDM transmitter 100 reaches the OFDM receiver 200 through various propagation paths. Therefore, it is received as a fading synthesized wave composed of a plurality of delayed waves subjected to the delay of each propagation path. As a result, the OFDM spectrum that was flat at the output end of the OFDM transmitter 100 is affected by the frequency characteristics of the fading propagation path in the OFDM receiver 200, and the amplitude of each subcarrier varies depending on the frequency. Here, a code error due to noise occurs in a channel in which the carrier amplitude is attenuated.

However, by selecting and using only a carrier having a large amplitude (ACS: adaptive subcarrier selection), transmission with few code errors is possible even in a propagation path with fading. When this ACS method is used, only a channel with a large carrier amplitude can be used, so that it is not necessary to use interleaving as a countermeasure against code errors, and the time delay required for transmission can be greatly reduced. Information that is required to have a short transmission delay is called HR (hard real time) information, and information that may have some transmission delay is called SR (soft real time) information. In the OFDM communication system, the transmission bandwidth is large, and it is possible to multiplex these information and transmit them simultaneously. For the HR information, the ACS is applied to reduce the code error and reduce the transmission delay. Code error countermeasures can be taken by ordinary interleaving.
JP 2003-273828 A

In order to apply this ACS technology to an actual OFDM communication apparatus, 1) to have means for acquiring radio wave propagation environment information, and 2) to solve two technical problems of notifying the other station of the selected ACS channel information. is required.

  An object of the present invention is to provide means for specifically executing these technical problems and to propose a practical OFDM communication apparatus.

In the OFDM communication apparatus of the present invention, a Fourier transformer that converts a received signal into a signal for each carrier wave and a signal for each carrier wave from the Fourier transformer are modulated by any of a plurality of modulation methods having different modulation multi-value numbers. A modulation method discriminator for discriminating whether or not a demodulator, a demodulator that demodulates each carrier wave from the Fourier transformer as a modulation method discriminated by the modulation method discriminator, and each carrier wave from the Fourier transformer according to its amplitude Separately, a channel estimator that uses a carrier wave having a large amplitude as an ACS channel, and a transmission signal that modulates the carrier wave that has been converted to an ACS channel by the channel estimator using a modulation method with a small number of modulation multilevels among the plurality of modulation methods. And a modulator that modulates a carrier wave set as a non-ACS channel by a modulation method having a large modulation multi-level number, and converts a modulation signal from the modulator into a signal in a time domain. That the inverse Fourier transformer, and the ACS memory for storing a history of ACS information which is information relating to ACS channel estimated by the channel estimator, the latest ACS information by the ACS information history stored in the ACS memory And a corrector that corrects an error and outputs the error to the modulator .

  Further, the ACS memory stores ACS information acquired from the modulation scheme of each carrier determined by the modulation scheme discriminator as a history of ACS information, so that the ACS information transmitted from the partner station is also used. The reliability of ACS information can be further improved.

  Further, the plurality of modulation schemes discriminated by the modulation scheme discriminator are a QPSK modulation scheme and a BPSK modulation scheme, and the modulation scheme with a small modulation multi-level number modulated by the modulator is a BPSK modulation scheme, Since the modulation scheme with a large modulation multi-level number is the QPSK modulation scheme, the error rate of the ACS channel can be reduced.

  According to the present invention, since it is possible to use a data frame for extracting radio wave propagation environment information, the reliability of ACS information is improved.

  In addition, since ACS information errors can be corrected based on the history of ACS information, wireless communication with few errors can be realized.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram illustrating a configuration of an OFDM communication apparatus according to Embodiment 1 of the present invention. FIG. 2 is a diagram showing a data frame configuration example of the present invention. FIG. 3 is a diagram for explaining an example of the modulation system of the present invention.

  As a method for performing bidirectional transmission in wireless communication, there are an FDD (Frequency Division Duplex) method and a TDD (Time Division Duplex) method, as shown in FIG. In the FDD scheme, communication is performed by assigning different frequencies to the uplink (SS (Subscriber Station) → MS (Master Station: Master Station)) and the downlink (MS → SS). In the TDD method, communication is performed using radio waves of the same frequency in the master station and slave stations, and the radio waves are transmitted in bursts by alternately dividing the time in the uplink (SS → MS) and downlink (MS → SS). It is a method to do. In the FDD system, since the frequency is different between upstream and downstream, data can be transmitted continuously in time. However, in order to synchronize data, data is framed in the same manner as in the TDD system. At the head of the frame, a known frame is placed by transmission / reception called a preamble, followed by a plurality of data frames.

  Since the OFDM scheme uses a plurality of subcarriers, each modulation scheme can be changed. Therefore, different modulation schemes are used for the HR information and the SR information. As an example, FIG. 3 shows a case where HR information is transmitted using a BPSK (two-phase phase) modulation scheme and SR information is transmitted using a QPSK (four-phase phase) modulation scheme. BPSK is a scheme in which the phase of a carrier wave is assigned to 0 and π according to 0 and 1 of 1-bit data, and QPSK is a carrier phase of π / 4, 3π / 4, with respect to 00, 01, 11, and 10 of 2-bit data. This is a method corresponding to 5π / 4 and 7π / 4. Since the transmission power of each channel has the same value even if the modulation method is changed, the modulation signal point arrangement of BPSK and QPSK modulation is arranged on the same circle as shown in FIG. Note that the amplitude of the Q (quadrature phase) axis of the BPSK signal is zero. Thus, among the data frames of the OFDM frame, the ACS channel is BPSK modulated by HR information, and the non-ACS channel is QPSK modulated and transmitted. On the receiving side, the amplitude value of the Q (quadrature phase) axis component of each channel of the data frame is examined. If the amplitude value is smaller than Qth (1/2 of the Q component amplitude value of QPSK modulation), the BPSK modulated HR If the information is larger than Qth, it can be determined as SR information subjected to QPSK modulation and can be decoded.

  FIG. 1 shows the configuration of an OFDM communication apparatus that specifically realizes this in the TDD scheme. The OFDM communication apparatus according to the first embodiment includes a reception RF unit 11, a Fourier transformer (FFT) 12, a modulation scheme discriminator 13, a demodulator 14, a channel estimator 15, an ACS memory 16, a corrector 17, a modulator 18, It comprises an inverse Fourier transformer (IFFT) 19 and a transmission RF unit 20. The reception RF unit 11 is the same as the low noise amplifier 31 and the frequency converter 32 (see FIG. 5), and the Fourier transformer 12 is the same as the Fourier transformer 33. The modulation method discriminator 13 discriminates whether the modulation method of each frequency is BPSK modulation or QPSK modulation based on the amplitude value of the Q-axis component as described above, acquires ACS information from the discrimination result, and obtains ACS memory. 16 stores the previous burst ACS information. The demodulator 14 demodulates the modulation method determined by the modulation method determiner 13 and outputs HR information or SR information. As in the conventional OFDM communication apparatus, these pieces of information are decoded by deinterleaving the SR information as it is for the HR information. The channel estimator 15 estimates fading propagation path information from the received preamble and each data frame, and stores the channel having a large carrier amplitude as an ACS channel in the ACS memory 16 as ACS information of the current burst. At this time, since BPSK modulation or QPSK modulation is used in this embodiment, the amplitude of each carrier is the same throughout the burst, and the average value of the amplitude in the current burst can be set as the amplitude value of each carrier. If the accuracy can be somewhat sacrificed, the average calculation can be omitted by using the amplitude value of either the preamble or the data frame sample. The ACS memory 16 stores a series of bursts of ACS information. In the TDD system, since the frequency band used by the local station and the partner station is the same, the fading situation is the same. Therefore, the ACS information acquired from the partner station and the ACS information estimated by the own station can be used as a series of burst ACS information. The corrector 17 replaces the ACS information estimated from the current burst stored in the ACS memory 16 with the previous ACS information when there is an error when the ACS information is extremely different from the past ACS information. correct. The modulator 18 assigns the non-interleaved HR information to the ACS channel and performs BPSK modulation, and assigns the interleaved SR information to the non-ACS channel and performs QPSK modulation. As the ACS information at this time, the latest ACS information stored in the ACS memory 16 is error-corrected by the corrector 17. The inverse Fourier transformer 19 is the same as the inverse Fourier transformer 40, and the transmission RF unit 20 is the same as the frequency converter 41 and the power amplifier 42 (see FIG. 5).

  In the present embodiment, the ACS information is acquired by determining the modulation method of each received carrier, so that it is not necessary to prepare a data frame for transmitting the ACS information.

  Also, the reason why the HR information is BPSK modulated is that the BPSK modulation has a higher power per bit and therefore has a lower error rate. Thereby, HR information can be reliably transmitted with a small error rate.

  FIG. 4 is a diagram showing a configuration of an OFDM communication apparatus according to Embodiment 2 of the present invention. In this embodiment, the present invention is applied to an FDD system. In the FDD system, the frequency band used by the own station and the other station is different, so the ACS information acquired from the other station is used exclusively for demodulation and cannot be used to confirm the accuracy of the ACS information estimated by the own station. . Therefore, since the ACS information obtained by discriminating the modulation scheme by the modulation scheme discriminator 13 in the first embodiment is not stored in the ACS memory 16, these are newly used as the modulation scheme discriminator 23 and the ACS memory 26. Except for this point, the configuration is the same as that of the first embodiment.

  Also in this embodiment, the ACS information is acquired by determining the modulation method of each received carrier, so that it is not necessary to prepare a data frame for transmitting the ACS information.

  Also, the reason why the HR information is BPSK modulated is that the BPSK modulation has a higher power per bit and therefore has a lower error rate. Thereby, HR information can be reliably transmitted with a small error rate.

  In addition, this invention is not limited to the said Example.

  In the first embodiment (TDD system), the error in the ACS information of the current burst acquired by channel estimation is corrected. However, the error of the ACS information of the previous burst acquired by the modulation system discrimination may be corrected, Further, errors in both the previous burst and the current burst may be corrected from the history of ACS information including the current burst and the previous burst by majority logic. Further, the technique for correcting the ACS information error using the history of ACS information is a technique independent of the technique for transmitting the ACS information by the modulation method, and therefore the ACS information is not necessarily transmitted by the modulation method. Alternatively, a frame for ACS information transmission may be separately prepared, and the ACS information received in the frame may be error-corrected using the estimated ACS information and / or the history of ACS information.

  If the accuracy can be sacrificed somewhat, the ACS information estimated by the channel estimator without using the corrector can be used as it is without error correction.

It is a figure which shows the structure of the OFDM communication apparatus by Example 1 of this invention. It is a figure which shows the example of a data frame structure of this invention. It is a figure explaining the example of a modulation system of this invention. It is a figure which shows the structure of the OFDM communication apparatus by Example 2 of this invention. It is a figure which shows the structure of the conventional OFDM communication apparatus. It is a figure which shows the principle of an ACS communication system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Reception RF part 12 Fourier transformer 13 Modulation system discriminator 14 Demodulator 15 Channel estimator 16 ACS memory 17 Corrector 18 Modulator 19 Inverse Fourier transformer 20 Transmission RF part 23 Modulation system discriminator 26 ACS memory 31 Low noise amplifier 32 frequency converter 33 Fourier transformer 34 demodulator 35 deinterleaver 36 decoder 37 encoder 38 interleaver 39 modulator 40 inverse Fourier transformer 41 frequency converter 42 power amplifier 100 OFDM transmitter 200 OFDM receiver

Claims (3)

A Fourier transformer that converts the received signal into a signal for each carrier;
A modulation method discriminator for discriminating which one of a plurality of modulation methods having different modulation multilevel numbers is modulated for each carrier signal from the Fourier transformer;
A demodulator that demodulates each carrier wave from the Fourier transformer as the modulation scheme determined by the modulation scheme discriminator;
A channel estimator in which each carrier from the Fourier transformer is divided by its amplitude and a carrier having a large amplitude is an ACS channel;
A carrier wave converted to an ACS channel by the channel estimator by a transmission signal is modulated by a modulation method having a small modulation multi-level number among the plurality of modulation methods, and a carrier wave set to a non-ACS channel has a large modulation multi-level number. A modulator that modulates with a modulation scheme;
An inverse Fourier transformer that converts the modulated signal from the modulator into a time domain signal ;
An ACS memory that stores a history of ACS information that is information about the ACS channel estimated by the channel estimator;
An OFDM communication apparatus, comprising: a corrector that corrects an error in the latest ACS information based on a history of ACS information stored in the ACS memory and outputs the error to the modulator . 2. The OFDM communication apparatus according to claim 1 , wherein the ACS memory stores ACS information acquired from the modulation scheme of each carrier determined by the modulation scheme discriminator as a history of ACS information. The plurality of modulation schemes determined by the modulation scheme discriminator are a QPSK modulation scheme and a BPSK modulation scheme,
3. The OFDM according to claim 1 or 2 , wherein the modulation scheme with a small modulation multi-level number modulated by the modulator is a BPSK modulation scheme, and the modulation scheme with a large modulation multi-level number is a QPSK modulation scheme. Communication device.

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