JP5480411B2 - Modulation method and transmitter, OFDM modulation method and OFDM transmitter - Google Patents

Description

  The present invention relates to a modulation method for generating a communication signal and a transmission apparatus thereof. The present invention relates to an OFDM modulation method for generating an OFDM communication signal and an OFDM transmission apparatus thereof.

  In communication systems that communicate information using communication signals, noise is superimposed on the communication signal and an error occurs in many cases while the communication signal propagates from the transmission device to the reception device. Is received, the data transmission control for confirming the reception of the receiving device is performed on the transmitting device side.

  For example, in the 802.11a standard, this is performed as follows. FIG. 12 is a diagram illustrating a time chart of communication between transmitting and receiving apparatuses when communication is performed according to the 802.11a standard. FIG. 13 is a diagram illustrating a frame configuration of a data signal and an ACK signal. FIG. 14 is a diagram showing the configuration of the PLCP header part. FIG. 15 and FIG. 16 are diagrams showing time charts of communication between transmission / reception apparatuses when a data signal is retransmitted.

  As shown in FIG. 12, when the transmitting apparatus transmits a data signal 1 for information transmission and the receiving apparatus receives it correctly (normally), the receiving apparatus receives a notification (acknowledgment) to that effect. The device returns an ACK signal. Then, the transmission device receives the ACK signal from the reception device, confirms reception of the reception device, and transmits the data signal 2 in order to transmit the next information.

  In the 802.11a standard, the frame structure of a communication signal has a PLCP (Physical Layer Convergence Procedure) preamble portion of 16 μs (microseconds) followed by a PLCP of 4 μs as shown in FIGS. 13A and 13B. A header part, a data part following the header part, and a 6-bit PLCP tail part following the header part are configured. As shown in FIG. 13A, the data portion includes a 16-bit service (SERVICE), a 24-byte MAC (Medium Access Control) header, an 8-byte LLC header, and an IP maximum. The packet length accommodates 1500-byte IP packets and 4-byte frame check sequences. In the ACK signal, the data portion contains a 16-bit service, a 10-byte ACK, and a 4-byte frame check sequence. Further, as shown in FIG. 14, the PLCP header portion includes a 4-bit RATE indicating a transmission rate, a 1-bit reserved area (Reserved), a 12-bit LENGTH indicating a data length of the data portion, a 1-bit parity ( Parity), 6-bit Tail, and 16-bit service are included.

  This header part service is a data part service and is accommodated in the data part. The header information (SIGNAL) of RATE, reserved area, LENGTH, parity and tail excluding these services is OFDM-modulated by so-called inverse fast Fourier transform, and one OFDM symbol is added by adding a guard interval. It becomes.

  Here, assuming that the IP packet length is a maximum value of 1500 bytes in Ethernet (registered trademark) and the coding rate is 64 QAM, the transmission speed is 54 Mbps, and the occupation time of the data signal in this case is 248 μs. Yes, the occupancy time of the ACK signal is 23 μs.

  In the 802.11a standard, CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) is adopted for access control, and priority control for controlling the frame interval (IFS, InterFrame Space) is provided in order to improve communication efficiency. Has been done. There are three frame intervals. First, the short IFS (Short IFS) of 16 μs waiting for an ACK signal response (waiting for reception). Second, the channel is busy when performing carrier sense. 34 μs DCF · IFS (Distributed Coordination Function IFS), which is the waiting time required until it is determined that the state has changed from (occupied state) to idle (unoccupied state), and thirdly, a busy error is detected as a frame error. There is an extended IFS (EIFS = SIFS + ACK frame length + DIFS) of 73 μs, which is the waiting time, and the priority of SIFS is the highest, and the priority of DCF / IFS is low.

  If the communication period is defined as the time from transmission (transmission) of one data signal to transmission of the next data signal, in the 802.11a standard, there is each frame interval, as shown in FIG. In addition to the occupation time of the data signal and the ACK signal, the SIFS of 16 μs from the transmission of the data signal 1 to the reception of the ACK signal and the “101.5 μs DIFS from the reception of the ACK signal to the transmission of the next data signal 2 + back It is the time to add “off”. Therefore, in normal communication, the throughput is 31 Mbps.

  On the other hand, a case where an error has occurred will be described. For error detection, RATE, reservation, and LENGTH even parity (XOR operation) in the header part are used as parity.

  When the error detection is successful, as shown in FIG. 15, when the transmission apparatus transmits the data signal 1, the reception apparatus performs error detection using the header portion. When an error is detected, the receiving apparatus is immediately initialized to an idle state (waiting for reception) and performs carrier sense. Since the data signal 1 from the transmission device is present on the transmission path, the reception device determines that it is busy and maintains the idle state. During this time, the transmitting apparatus ends transmission of the data signal 1 of 248 μs, waits for the response of the ACK signal by “16 μs SIFS + 139.5 μs back-off”, and transmits the data signal 1 when there is no response of the ACK signal. The data signal 1 is retransmitted after determining failure, and the process is the same as in FIG. Therefore, when this error detection is successful and retransmission is executed only once, the throughput is 15.2 Mbps.

  Note that the back-off is a waiting time until retransmission is given randomly (randomly) to each terminal in order to make the transmission opportunity between terminals fair. The backoff time is determined by multiplying the slot time by a random coefficient. The range of values that the random coefficient can take is called a contention window (CW), and increases with the power of 2 of the slot time. Since the probability of taking each value of the random coefficient is equal, the backoff time is calculated by multiplying the slot time by CW / 2, the slot time is 9 μs, and the CW is 15, 31, 63, and so on. For example, in the first retransmission, the back-off time is 139.5 μs.

  On the other hand, as shown in FIG. 14, since the parity is 1 bit, there is a possibility that error detection may fail with a probability of 1/2. If error detection by the header portion fails, the receiving apparatus continues demodulation processing of the received wave, and finally detects a packet error by a frame check sequence (FCS).

In addition, if the RATE RATE information and LENGTH LENGTH information are not correct, the data signal reception time (packet length) is erroneously calculated. Since the 4-bit RATE is fixed to 1 bit, the RATE information is determined by 3 bits, and since LENGTH is 12 bits, (the probability that the parity check is not correct) × ((valid RATE and Since all combinations of LENGTH bits)-(all correct combinations)) / (a combination of all valid RATE and LENGTH bits), (1/2) × (2 ^ 15-1) / 2 ^ 15 It is almost 1/2. Therefore, if the RATE RATE information and LENGTH LENGTH information are not incorrect, but the RATE RATE information is incorrect, the retransmission is performed as follows: time B (n) <time A <Case 1 at time C (n) and Case 2 at time C (n) <time A <time B (n + 1) (n is an integer of 1 or more).
Time A: Misrecognized packet length + EIFS + backoff time time B (n); (correct packet length + SIFS + backoff time) × n
Time C (n); (correct packet length × 2 + SIFS + backoff time) × n
In both cases 1 and 2, since the error detection is based on the frame error detection, the waiting time of the receiving apparatus is EIFS (73 μS).

  In case 1, as shown in FIG. 16B, since the retransmitted data signal 1 is detected during carrier sense, the time chart is the same as when error detection by the header portion fails, and the throughput is 15. 2 Mbps.

  In case 2, as shown in FIGS. 16A and 16C, since the retransmitted data signal 1 is not received, the retransmitted data signal 1 is received. Therefore, the throughput is 9 Mbps. turn into.

Masahiro Morikura, Shuji Kubota (supervised), Impress Standard Textbook Series “Revised 802.11 High-Speed Wireless LAN Textbook”, Impress R & D, February 21, 2006

  By the way, error detection of the header part, such as failure of error detection by the header part and error of RATE RATE information or LENGTH LENGTH information, can frequently occur when the SN ratio in a transmission path such as a radio channel deteriorates, for example. In such a case, as described above, the throughput is significantly reduced. The throughput decreases to, for example, a little less than 30% in the case of normal communication.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an OFDM modulation method and an OFDM transmitter capable of suppressing a decrease in throughput even when the SN ratio of a transmission path is deteriorated. That is.

As a result of various studies, the present inventor has found that the above object is achieved by the present invention described below. That is, in one aspect according to the present invention, in a modulation method for generating a communication signal having a frame structure including a preamble part, a header part after the preamble part, and a data part after the header part, only the header part is included. A spread process for performing spread modulation with a spread code, wherein the communication signal employs a block transmission system, and the spread process further includes modulation data based on header information to be accommodated in the header part in the block transmission system the data added with the gas chromatography de interval generates a plurality, characterized by spreading modulation by multiplying the sign of the respective data the spreading code, respectively.

  The modulation method having such a configuration can improve the robustness of the header portion by performing spread modulation on the header portion, and can suppress a reduction in throughput even when the SN ratio of the transmission path is deteriorated.

  For example, when the interference wave (noise) is an AM modulation signal and the spreading code is a Barker code, the carrier (carrier wave) frequency in the AM modulation signal is a subcarrier in the OFDM communication signal (OFDM signal). If the frequency coincides with the frequency, the phase of the spreading code is canceled depending on the points sampled when the despreading process (compression process) is performed using the Barker code, and the effect of the despreading is reduced. However, according to this configuration, the spread-modulated samples are arranged at intervals in the time direction, so that the phase of the AM modulation signal is difficult to erase the phase information of the Barker code. Therefore, according to this configuration, a more excellent diffusion modulation effect is achieved.

  In these modulation methods described above, the header section contains header information including control information for adaptive subcarrier modulation.

  According to this configuration, when the adaptive subcarrier modulation scheme is used for the communication signal and the control according to the adaptive subcarrier modulation is performed, the robustness of the control information of the adaptive subcarrier modulation can be improved. Control according to carrier modulation can be reliably performed.

  According to another aspect of the present invention, a transmission apparatus that transmits a communication signal generated by any one of the above modulation methods.

  In another aspect of the present invention, any one of the above-described modulation methods is an OFDM modulation method in which an OFDM scheme is adopted for the communication signal.

  According to another aspect of the present invention, there is provided an OFDM transmitter that transmits a communication signal generated by the above-described OFDM modulation method.

  Such a transmission apparatus, OFDM modulation method, and OFDM transmission apparatus can improve the robustness of the header part by spreading and modulating the header part, and suppress a decrease in throughput even when the SN ratio of the transmission path deteriorates. can do.

  The modulation method and transmission apparatus and the OFDM modulation method and OFDM transmission apparatus according to the present invention can suppress a decrease in throughput even when the SN ratio of the transmission path is deteriorated.

It is a figure which shows each structure of the OFDM transmitter in an embodiment, and an OFDM receiver. It is a figure which shows the frame structure of the data signal and ACK signal in embodiment. It is a figure which shows the time chart of communication between the OFDM transmission / reception apparatuses in embodiment. It is a figure which shows the time chart of communication between OFDM transmission / reception apparatuses in the case of resending a data signal in embodiment. It is a figure which shows the SN ratio versus error rate in BPSK modulation. It is a figure which shows the waveform of an OFDM signal when AM modulation wave is inject | poured into a specific frequency as interference wave (noise). It is a figure which shows each structure of the OFDM transmitter in a 1st modification, and an OFDM receiver. It is a figure which shows the frame structure of the data signal and ACK signal in a 1st modification. It is a figure which shows each structure of the OFDM transmitter in a 2nd modification, and an OFDM receiver. It is a figure which shows the frame structure of the data signal and ACK signal in a 2nd modification. It is a figure which shows the AM value at the time of despreading an AM modulation signal with a Barker code. It is a figure which shows the time chart of communication between the transmission / reception apparatuses in the case of performing communication by the 802.11a standard. It is a figure which shows each frame structure of a data signal and an ACK signal. It is a figure which shows the structure of a PLCP header part. It is a figure which shows the time chart of communication between transmission / reception apparatuses in the case of resending a data signal. It is a figure which shows the time chart of communication between transmission / reception apparatuses in the case of resending a data signal.

  Hereinafter, an embodiment according to the present invention will be described with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted. Moreover, when referring generically, it shows with the reference symbol which abbreviate | omitted the suffix, and when referring to an individual structure, it shows with the reference symbol which added the suffix.

An OFDM transmission apparatus and OFDM reception apparatus according to the present embodiment use a modulation method in which only the header part in a frame structure including a preamble part, a header part after the preamble part, and a data part after the header part is spread-modulated with a spreading code 2 is an example of each of a transmission device and a reception device that constitute a communication system that transmits and receives communication signals generated by. Wherein the communication signal, block transmission scheme is employed, a diffusion step of diffusing modulated by the spreading code, prior to addition of gas chromatography De intervals in the block transmission system, the header information to be contained in the header portion Based on this, the modulation data is subjected to spread modulation with the spread code. In this embodiment, the OFDM method is adopted as an example of the block transmission method, and the Barker code is adopted as an example of a spreading code.

  FIG. 1 is a diagram illustrating configurations of an OFDM transmission apparatus and an OFDM reception apparatus in the embodiment. FIG. 2 is a diagram illustrating a frame configuration of a data signal and an ACK signal in the embodiment.

  An OFDM transmitter is a communication signal of an OFDM (Orthogonal Frequency Division Multiplexing) system, which is a communication system that digitally modulates and multiplexes narrowband subcarriers (narrowband carrier waves) having an orthogonal relationship on the frequency axis. (OFDM signal) is generated and transmitted, and the OFDM receiver is a device for receiving this OFDM signal. These OFDM transmitters and OFDM receivers constitute an OFDM communication system. In this embodiment, adaptive subcarrier modulation is applied to this OFDM signal.

  For applied subcarrier modulation, transmission parameters such as the modulation method, transmission power, and coding rate are adaptively determined for each subcarrier according to the channel characteristics (communication status of the channel) between OFDM transceivers. It is a method to do. In this applied subcarrier modulation, prior to the start of communication of information data, first, an OFDM transmission device transmits an evaluation packet for evaluating transmission path characteristics to the OFDM reception device, and OFDM reception is performed based on the evaluation packet. The device estimates the so-called error vector magnitude (EVM), and determines transmission parameters such as subcarriers, modulation schemes, and coding rates used between OFDM transmitters and receivers in accordance with predetermined criteria set in advance based on the estimated EVM. Then, information regarding the determined transmission parameter is returned to the OFDM transmitter. Thus, the OFDM transmitter creates an OFDM signal packet according to the transmission parameters determined by the OFDM receiver, and starts communication of information data.

  For example, as shown in FIG. 1, such an OFDM transmitter SD includes primary modulation units 1 and 11, serial / parallel conversion units (S / P conversion units) 2 and 12, and OFDM modulation units 3 and 13. A guard interval adding unit (GI adding unit) 4, an up-sample / FIR unit 5, a digital / analog converting unit (DAC unit) 6, a Barker code generating unit 14, and a spreading modulation unit 15. And a preamble generation unit 21.

  Data to be transmitted (transmission data) is primarily modulated by the primary modulation unit 1 by 64QAM modulation, for example, and input to the S / P conversion unit 2. In the S / P conversion unit 2, the primary-modulated transmission data is divided into the number of subcarriers by serial / parallel conversion (serial / parallel conversion) and input to the OFDM modulation unit 3. For example, in the 802.11a standard, 52 subcarriers are used, but since 4 subcarriers are used for pilots, 48 subcarriers are used for data. In the OFDM modulation unit 3, each subcarrier is converted from the frequency domain to the time domain (OFDM modulation) by inverse fast Fourier transform, thereby becoming a multicarrier signal of transmission data.

  On the other hand, the header information is primarily modulated by the primary modulation unit 11 by a predetermined modulation method set in advance, for example, BPSK modulation, and input to the S / P conversion unit 12. In the S / P converter 12, the first-modulated header information is divided into the number of subcarriers by serial / parallel conversion (serial / parallel conversion), and is input to the OFDM modulator 13. The header information is information necessary for receiving and processing a packet signal in the physical layer, and includes, for example, the transmission speed and data length of the data unit 103. In this embodiment, the header information includes a control signal for adaptive subcarrier modulation indicating a transmission parameter. In the OFDM modulation unit 13, each subcarrier is converted from the frequency domain to the time domain (OFDM modulation) by inverse fast Fourier transform, thereby becoming a multicarrier signal of header information. The multi-carrier signal of the header information is input to the spread modulation unit 15, generated by the Barker code generation unit 14, and spread-modulated with the Barker code input to the spread modulation unit 15. The Barker code is one of spreading codes composed of [1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1], for example, 802. It is used as a spreading code in the .11b standard.

  The multi-carrier signal of the transmission data and the multi-carrier signal of the header information subjected to spread modulation are input to the GI adding unit 4, and a guard interval (GI) is added by the GI adding unit. The PLCP preamble generated by the generation unit 21 is added to form a frame. The framed OFDM signal is input to the upsampling / FIR unit 5, where the upsampling / FIR unit 5 is upsampled to a sampling frequency twice or more theoretically required as shown by the Nyquist theorem. 6 is input. In the DAC unit 6, the OFDM signal is converted from digital to analog, input to an unillustrated analog front end unit, and transmitted from an unillustrated transmitting antenna.

  Note that the OFDM signal obtained by spreading and modulating such header information may be shaped by a so-called root cosine filter in order to suppress the spread of the spectrum after spreading.

  On the other hand, as shown in FIG. 1, for example, the OFDM receiver RD includes demodulation units 31 and 41, parallel / serial conversion units (P / S conversion units) 32 and 42, OFDM demodulation units 33 and 43, and a GI. A removal unit 34, an FIR / down-sample unit 35, an analog / digital conversion unit (ADC unit) 36, a Barker code generation unit 44, and a despreading modulation unit 45 are configured. .

  A reception wave received by a reception antenna (not shown) is input to the ADC unit 36 via an analog front end unit (not shown), converted from analog to digital by the ADC unit 36, and input to the FIR / downsampling unit 35. The FIR / downsampler 35 downsamples the digital circuit to a predetermined sample frequency and inputs it to the GI remover 34. The GI remover 34 removes the GI. The data section 103 from which the GI has been removed is input to the OFDM demodulator 33, while the PLCP header section 102 from which the GI has been removed is input to the despreading modulator 45.

  The data unit 103 is converted from the time domain to the frequency domain (OFDM demodulation) by being fast Fourier transformed by the OFDM demodulator 33, thereby becoming each subcarrier signal, which is input to the P / S converter 32. Each of these subcarrier signals is converted into one received data by being subjected to parallel / serial conversion (parallel / serial conversion) by the P / S conversion unit 32, and is input to the demodulation unit 31. This received data is subjected to 64QAM demodulation processing according to the RATE information of the header information stored in the PLCP header section 102 by the demodulating section 31, and the data is demodulated.

  Further, the PLCP header section 102 (102-1 to 102-11) is a despreading modulation section 45, which is generated by the Barker code generation section 44 and compressed by the Barker code input to the despread modulation section 45 (despreading). Processed) and input to the OFDM demodulator 43. The compressed PLCP header 102 is converted from the time domain to the frequency domain (OFDM demodulation) by fast Fourier transform in the OFDM demodulator 43, thereby becoming each subcarrier signal, and to the P / S converter 42. Entered. Each of these subcarrier signals is converted into one reception data by being subjected to parallel / serial conversion (parallel / serial conversion) by the P / S conversion unit 42 and input to the demodulation unit 41. The received data is subjected to predetermined demodulation processing preset by the demodulator 41, for example, BPSK demodulation processing, and header information is demodulated.

  FIG. 3 is a diagram illustrating a time chart of communication between OFDM transmitting and receiving apparatuses in the embodiment. FIG. 4 is a diagram illustrating a time chart of communication between OFDM transmitting and receiving apparatuses when a data signal is retransmitted in the embodiment. FIG. 4A shows the transmitting side, and FIG. 4B shows the receiving side.

  In the OFDM transmitter SD having such a configuration, as shown in FIG. 2, the OFDM signal 100 such as a data signal or an ACK signal is a PLCP preamble containing a preamble signal for establishing reception synchronization, as in the background art. Part 101, PLCP header part 102 for accommodating header information, data part 103 for accommodating data to be transmitted, and PLCP tail part 104 for accommodating additional bits for terminating convolutional coding. However, since the header information is spread and modulated by the Barker code, the PLCP header portion 102 has a signal vector length that is 11 times longer, resulting in 11 OFDM symbols 102-1 to 102-11. The normal PLCP header part is 4 μS, but the PLCP header part 102 (102-1 to 102-11) of this embodiment does not change the sample rate so as not to widen the band. Since the header information is spread and modulated by the spread code, it becomes 11 times 44 μS. In general, the spread modulation is spread at a rate faster than a normal sample rate when no spread modulation is performed.

  Therefore, when communication is performed between the OFDM transmitter SD and the OFDM receiver RD without retransmitting the data signal, as shown in FIG. 3, the IP packet length is the maximum 1500 bytes in Ethernet (registered trademark). , The communication cycle is composed of “248 μS + 40 μS” data signal 1 transmission, 16 μS SIFS, “23 μS + 40 μS” ACK signal reception, and 101.5 μS back-off time. It will be 25.6 Mbps.

  On the other hand, in the case of retransmitting the data signal, since the header information is spread-modulated, an error in the RATE RATE information and the LENGTH LENGTH information error as described in the background section hardly occur. It is not necessary to assume each case shown in FIG. In this case, a case of retransmission due to an error in the data unit 103 is assumed, and as shown in FIG. 4, the OFDM receiver RV continues the demodulation process of the received wave and finally performs the frame check sequence (FCS). Although a packet error is detected, the data signal 1 by retransmission after the back-off time of 16 μS and 139.5 μS can be detected during carrier sense, and the time chart is shown in FIG. Therefore, the throughput is 13.7 Mbps.

  As described above, in the OFDM transmitter SD and the OFDM modulation method according to this embodiment, the header information is spread-modulated. Therefore, when data is normally transmitted and received, the throughput is slightly as described above due to the redundancy of the PLCP header unit 102. Although the data signal is retransmitted, the case shown in FIG. 16 of the background art is not assumed due to the improvement in noise resistance performance of the PLCP header unit 102, and the case shown in FIG. The background technology is improved from 9 Mbps to 13.7 Mbps described above. As described above, in the OFDM transmission device SD and the OFDM modulation method according to the present embodiment, the PLCP header 102 can be subjected to spread modulation to improve the robustness of the PLCP header 102, and even when the SN of the transmission path is deteriorated. It is possible to suppress a decrease in throughput. In the worst case, in the background art, the throughput is reduced by about 71% (= (1-9 / 31) × 100), but in the present embodiment, it is about 56% (= (1-13.7 / 31) × 100). Note that a slight decrease in throughput when data is normally transmitted and received is less when lower multilevel modulation (for example, BPSK or QPSK) is used.

  Also, when an adaptive subcarrier modulation scheme is used for the OFDM signal and control according to adaptive subcarrier modulation is performed, the control information for adaptive subcarrier modulation is included in the header information. Robustness can be improved, and control according to adaptive subcarrier modulation can be reliably performed.

  FIG. 5 is a diagram showing an SN ratio versus error rate in BPSK modulation. The horizontal axis in FIG. 5 is the SN ratio (SNR) expressed in dB, and the vertical axis is the bit error rate (BER). FIG. 6 is a diagram showing a waveform of an OFDM signal when an AM modulated wave is injected as a disturbance wave (noise) at a specific frequency. The horizontal axis in FIG. 6 is frequency, and the vertical axis is the signal level expressed in dB. FIG. 6A shows the case of the present embodiment in which the PLCP header portion 102 is spread-modulated, and FIG. 6B shows the case of the background art in which the PLCP header portion is not spread-modulated. As shown in FIG. 6, ◆ and ○ shown in FIG. 5 indicate bit error rates when an AM-modulated wave is injected as a disturbance wave (noise) at a specific frequency. 5 indicates the case of the present embodiment in which the PLCP header portion 102 is spread-modulated, and ◆ in FIG. 5 indicates the case of the background art in which the PLCP header portion is not spread-modulated.

  As shown in FIG. 5, for example, in order to achieve a bit error rate of 10 ^ -8, in the present embodiment in which the PLCP header 102 is spread-modulated, an error correction code having a coding gain of about 9 dB is used. By using it, the bit error rate of 10 ^ -8 can be achieved. However, in the case of the background art in which the PLCP header portion is not spread-modulated, the bit error rate 10 ^ -8 when the SN ratio is 0 dB exceeds the so-called Shannon limit and cannot be realized.

  Furthermore, by adding the guard interval after spreading, the optimum guard interval length can be selected according to the delay profile of the target transmission line, and efficient transmission can be realized. For example, when a significant delayed wave is present up to 5 μS as a result of measuring the delay profile, the guard interval length may be set to 5 μS.

  In the above-described embodiment, the Barker code is used as the spreading code of the spread modulation. However, the present invention is not limited to this. For example, an M-sequence code, a Gold code, a bulk code, and a Walsh-Hadamard code are used. It is possible to use.

  FIG. 7 is a diagram illustrating each configuration of the OFDM transmitter and the OFDM receiver in the first modification. FIG. 8 is a diagram illustrating a frame configuration of a data signal and an ACK signal in the first modification.

  In the above-described embodiment, the header information is subjected to inverse fast Fourier transform by the OFDM modulation unit 13 to perform spread modulation on the OFDM modulated header information and then GI is added. However, the header information is subjected to inverse fast Fourier transform to perform OFDM modulation. Spread modulation may be performed after adding GI to the modulated PLCP header 102.

  In the first modification, the header information to be accommodated in the PLCP header section 102 may be subjected to inverse fast Fourier transform, and further subjected to spread modulation with a spreading code.

  For example, as shown in FIG. 7, the OFDM transmitter SDA in the first modification has primary modulation units 1 and 11, S / P conversion units 2 and 12, OFDM modulation units 3 and 13, Guard interval adding units (GI adding units) 51 and 52, an upsampling / FIR unit 5, a DAC unit 6, a Barker code generating unit 14, a spread modulation unit 15, and a preamble generating unit 21. ing.

  That is, this OFDM transmitter SDA is different from the OFDM transmitter SD shown in FIG. 1 in that the output of the OFDM modulator 3 is replaced with the GI adder 4 to which the output of the OFDM unit 3 and the output of the spread modulator 15 are input. The GI adding unit 51 and the GI adding unit 52 that receives the output of the OFDM modulation unit 13 and outputs the output to the spread modulation unit 15 are provided.

  In the OFDM transmission device SDA having such a configuration, a guard interval (GI) is added to the multicarrier signal of transmission data generated by OFDM modulation by the OFDM modulation unit 3 by the GI addition unit 51. Further, a guard interval (GI) is added by the GI adding unit 52 to the multi-carrier signal of the header information generated by OFDM modulation by the OFDM modulating unit 13. The header information multicarrier signal to which the GI is added is input to the spread modulation unit 15, generated by the Barker code generation unit 14, and spread-modulated by the Barker code input to the spread modulation unit 15. Then, the OFDM symbol of the header information of the spread modulation output from the spread modulation unit 15 and the OFDM symbol of the transmission data output from the GI addition unit 51 are generated by the preamble generation unit 21 at the beginning of the time. The added PLCP preamble is added to be framed.

  For this reason, as shown in FIG. 8, the OFDM signal 100A generated by the OFDM transmitter SDA includes a PLCP preamble section 101, a PLCP header section 201, a data section 103, and a PLCP tail section, as in the background art. 104, but the PLCP header section 201 includes a GI 202 and header information (SIGNAL) 203 that are spread-modulated by a Barker code. This PLCP header part 201 is 11 times longer than the conventional PLCP header part, and becomes 44 μS.

  In this first variation, the number of subcarriers Ns remains as it is, and the sampling frequency fs of the fast Fourier transform is fs / P, where P is the spreading length, and the number of OFDM symbols in the PLCP header section 201 is one. It is.

  On the other hand, the OFDM receiver RDA in the first modified embodiment includes, for example, as shown in FIG. 7, demodulation units 31 and 41, P / S conversion units 32 and 42, OFDM demodulation units 33 and 43, and a GI removal unit. 53, 54, FIR / downsampling unit 35, ADC unit 36, Barker code generation unit 44, and despreading modulation unit 45.

  In the OFDM receiver RD shown in FIG. 1, the output of the FIR / downsampling unit 35 is input to the OFDM demodulator 33 and the despreading modulator 45 via the GI remover 34. In the OFDM receiver RDA, The output of the FIR / downsampler 35 is input to the OFDM demodulator 33 via the GI remover 53, and the output of the FIR / downsampler 35 is sent to the OFDM demodulator via the despreading modulator 45 and the GI remover 54. 43 is input.

  In the OFDM receiver RDA having such a configuration, the received signal downsampled to a predetermined sampling frequency of the digital circuit by the FIR / downsampler 35 is input to the GI remover 53, and the GI remover 53 removes the GI. Then, the signal is input to the OFDM demodulator 33 and is demodulated by the OFDM demodulator 33. Also, the received signal downsampled to a predetermined sampling frequency of the digital circuit by the FIR / downsampler 35 is input to the despreading modulator 45, generated by the Barker code generator 44 in the despreading modulator 45, Compressed by the Barker code input to the despreading modulation unit 45, input to the GI removal unit 54, GI removed by the GI removal unit 54, input to the OFDM demodulation unit 43, and OFDM demodulated by the OFDM demodulation unit 43 The

  Even with such a configuration, the OFDM transmitter SDA and the modulation method thereof can improve the robustness of the PLCP header unit 201 by spreading and modulating the PLCP header unit 201, and even when the SN ratio of the transmission path is deteriorated. It is possible to suppress a decrease in throughput. Moreover, since GI becomes 11 times longer, it becomes possible to cope with a longer delayed wave.

  FIG. 9 is a diagram illustrating each configuration of the OFDM transmitter and the OFDM receiver in the second modification. FIG. 10 is a diagram illustrating a frame configuration of a data signal and an ACK signal in the second modification.

  In the above-described embodiment, the GI is added after spreading and modulating the signal obtained by performing the inverse fast Fourier transform on the header information to be accommodated in the PLCP header portion 102. However, a plurality of OFDM symbols of the header information are copied, and the header information Spread modulation may be performed by multiplying each OFDM symbol and each code constituting the Barker code.

  For example, as shown in FIG. 9, the OFDM transmitter SDB in the second modified example includes primary modulation units 1 and 11, S / P conversion units 2 and 12, OFDM modulation units 3 and 13, Guard interval addition units (GI addition units) 61 and 62, a copy unit 63, a count unit 64, an upsampling / FIR unit 5, a DAC unit 6, a Barker code generation unit 14, a spread modulation unit 65, And a preamble generation unit 21.

  That is, the OFDM transmitter SDB is different from the OFDM transmitter SD shown in FIG. 1 in that the output of the OFDM modulator 3 is replaced with the GI adder 4 to which the output of the OFDM unit 3 and the output of the spread modulator 15 are input. The GI adding unit 61 and the GI adding unit 52 that receives the output of the OFDM modulation unit 13 and outputs the output to the spread modulation unit 65 via the copy unit 63 are provided. And a counting unit 64 that is connected to the unit 14 and counts (counts) the number of codes output from the Barker code generation unit 14.

  In the OFDM transmitter SDB configured as described above, a guard interval (GI) is added to the multicarrier signal of the transmission data generated by OFDM modulation by the OFDM modulator 3 by the GI adder 61. Further, a guard interval (GI) is added by the GI adding unit 52 to the multi-carrier signal of the header information generated by OFDM modulation by the OFDM modulating unit 13. The header information multicarrier signal to which the GI is added is an OFDM symbol of the header information, and is sequentially copied (replicated) by the copy unit 63, and the OFDM symbol of the header information of the copy is input to the spread modulation unit 65. The On the other hand, each code constituting the Barker code is sequentially input from the Barker code generation unit 14 to the spread modulation unit 65 in accordance with the timing at which the OFDM symbol of the header information of this copy is input from the copy unit 63 to the spread modulation unit 65. Is done. Then, in the spread modulation unit 65, as shown in FIG. 10, the OFDM symbol 301 of the header information of the copy input from the copy unit 63 and the code constituting the Barker code input from the Barker code generation unit 14 are multiplied. .

  For example, as described above, the barker code is a spreading code composed of each code of [1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1]. The first "1" and the first copied OFDM symbol 301-1 of the header information are multiplied by the spread modulation unit 65, and the second "-1" and the second copied The OFDM symbol 301-2 of the header information is multiplied by the spread modulation unit 65, and the third “1” and the OFDM symbol 301-3 of the third copied header information are multiplied by the spread modulation unit 65. In the same manner, multiplication is performed up to the eleventh.

  The counting unit 64 counts the number of codes output from the Barker code generation unit 14, and outputs a control signal to the copy unit 63 when the count value becomes 11, which is the number of codes of the Barker code. The copying unit 63 stops outputting the OFDM symbol of the header information.

  The 11 OFDM symbols of the header information of the spread modulation output from the spread modulation unit 65 and the OFDM symbol of the transmission data output from the GI addition unit 61 are preceded by a preamble generation unit. The PLCP preamble generated at 21 is added to form a frame.

  For this reason, as shown in FIG. 10, the OFDM signal 100B generated by the OFDM transmitter SDB includes a PLCP preamble section 101, a PLCP header section 301, a data section 103, and a PLCP tail section, as in the background art. 104, the PLCP header section 301 includes each code [1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1] of the Barker code. Each of the header information is multiplied by OFDM symbols 301-1, 301-2, 301-3,. Therefore, this PLCP header section 301 is provided with 11 conventional PLCP header sections each multiplied by each Barker code, which is 44 μS.

  On the other hand, the OFDM receiver RDB in the second modified form includes, for example, as shown in FIG. 9, demodulating units 31, 41, P / S converting units 32, 42, OFDM demodulating units 33, 43, and a GI removing unit. 66, 67, a counter unit 68, an FIR / downsampling unit 35, an ADC unit 36, a Barker code generation unit 44, and a despreading modulation unit 69.

  In the OFDM receiver RD shown in FIG. 1, the output of the FIR / downsampling unit 35 is input to the OFDM demodulator 33 and the despread modulator 45 via the GI remover 34. In the OFDM receiver RDB, The output of the FIR / downsampler 35 is input to the OFDM demodulator 33 via the GI remover 66 and the output of the FIR / downsampler 35 is the OFDM demodulator via the despreading modulator 69 and the GI remover 67. The count unit 68 that counts the number of codes that are input to the output 43 and output from the Barker code generation unit 14 is connected to the Barker code generation unit 14.

  In the OFDM receiver RDB having such a configuration, the received signal down-sampled to a predetermined sampling frequency of the digital circuit by the FIR / down-sampling unit 35 is input to the GI removing unit 66, and the GI is removed by the GI removing unit 66. Then, the signal is input to the OFDM demodulator 33 and is demodulated by the OFDM demodulator 33. The received signal downsampled to a predetermined sample frequency of the digital circuit by the FIR / downsampler 35 is input to the despreading modulation unit 69, generated by the Barker code generation unit 44 by the despreading modulation unit 69, Compression processing is performed with each code of the Barker code input to the despreading modulation unit 69, input to the GI removal unit 67, GI is removed by the GI removal unit 67, input to the OFDM demodulation unit 43, and the OFDM demodulation unit 43 OFDM demodulated.

  Even with such a configuration, the OFDM transmitter SDB and the modulation method thereof have robustness of the PLCP header unit 301 by performing spread modulation on the PLCP header unit 301 (301-1, 301-2, 301-3,...). Thus, even when the SN ratio of the transmission path is deteriorated, it is possible to suppress a decrease in throughput.

  Further, when the interference wave (noise) is an AM modulated signal, if the frequency of the carrier (carrier wave) in the AM modulated signal matches the frequency of the subcarrier in the OFDM signal, despreading processing (compression processing) is performed using the Barker code. Depending on the points sampled in some cases, the phase of the spreading code is canceled and the effect of despreading is diminished. However, in the OFDM transmitter SDB and its modulation method according to the second modification, the spread modulated samples are arranged at intervals in the time direction, so that the phase of the AM modulation signal is difficult to erase the phase information of the Barker code. Become. Therefore, the OFDM transmitter SDB and the modulation method thereof according to the second modification have a better spread modulation effect. An example is shown in FIG.

FIG. 11 is a diagram illustrating the AM value when the AM modulated signal is despread with the Barker code. The horizontal axis in FIG. 11 is the frequency, and the vertical axis is the AM value after despreading.
The AM modulation signal is assumed to have a sufficiently low modulation frequency compared to the carrier frequency, and only the carrier frequency is targeted, and expressed as a function sin (2π · k · Δf · n · Ts). Here, Ts is 1/20 MHz in the case of the 802.11a standard, and Δf is fs / NFFT = 20 MHz / 64.

  FIG. 11A shows despread values obtained by the above embodiment with Σsin (2π · k · Δf · n · Ts) * Barker (n), and FIG. 11B shows Σsin (2π K · Δf · (1 + 80 (n−1)) · Ts) * Barker (n) is the despread value obtained by the above-described second modification. Here, Barker (n) is a code value of the nth Barker code. Therefore, in this embodiment and the second modification, Σ calculates the sum of 1 to 11 for n.

  As can be seen from a comparison between FIG. 11A and FIG. 11B, the frequency of the zero cross where the influence of the AM modulation signal is reduced after despreading is higher in FIG. 11B than in FIG. 11A. There are many. Therefore, the second modification has improved robustness against noise of AM modulation signals having various frequencies.

  Here, in the embodiment and the first modification thereof, the sample frequencies fs of the OFDM modulation sections 3 and 13 and the OFDM demodulation sections 33 and 43 are fs and fs / P, respectively, but the OFDM modulation section in the second modification form 3, 13 and the OFDM demodulator 33, 43 sample frequency fs is fs.

  In order to express the present invention, the present invention has been properly and fully described through the embodiments with reference to the drawings. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not covered by the claims. To be construed as inclusive.

SD, SDA, SDB OFDM transmitters RV, RVA, RVB OFDM receivers 14 and 44, Barker code generation unit 15, 65 Spreading modulation unit 45, 69 Despreading modulation unit 63 Copying unit 64, 68 Counter unit 100, 100A, 100B OFDM signal 102, 201, 301 PLCP header part

Claims (5)

In a modulation method for generating a communication signal having a frame structure comprising a preamble part, a header part after the preamble part, and a data part after the header part,
Including a spreading step of spreading and modulating only the header part with a spreading code;
The communication signal employs a block transmission system,
The diffusion process, the data obtained by adding a gas chromatography intervals in addition said block transmission scheme to the modulation data based on header information to be contained in the header section generates a plurality, and each code of the spreading code and each of these data A modulation method characterized by spreading modulation by multiplying each. The modulation method according to claim 1, wherein the header section contains header information including control information for adaptive subcarrier modulation.   A transmission apparatus that transmits a communication signal generated by the modulation method according to claim 1.   3. The modulation method according to claim 1, wherein an OFDM system is adopted for the communication signal. An OFDM transmitter for transmitting a communication signal generated by the OFDM modulation method according to claim 4.

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