JP2010016596A - Transmitter and transmission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide technique for limiting a range where a deterioration in communication quality due to peak suppression reaches only to a subcarrier that can admit a large deterioration when a plurality of modulation modes coexist in the same time slot in a multichannel transmission system. <P>SOLUTION: This transmitter for adaptively allocating a modulation mode to each subcarrier to transmit a signal in an orthogonal frequency division multiplexing system using a plurality of subcarriers is provided with: an acquiring part (120) for acquiring information on a modulation mode allocated to each subcarrier; and a peak reduction processing part (150) for reducing peak power of a signal to be transmitted by a subcarrier to which a modulation mode of a relatively low transmission speed is allocated among subcarriers constituting the signal to be transmitted on the basis of the acquired information. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a transmitter and a transmission control method, and more particularly to a transmitter and a transmission control method that efficiently reduce peak power of a transmission signal in multicarrier transmission such as OFDMA using an adaptive modulation technique.

  Systems that support multicarrier transmission schemes such as OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) use multiple different frequency bands (subcarriers). At the same time, a signal is transmitted. Since signals to be transmitted are generated independently from each other on each subcarrier, the phases of multiple signals match when the signals of each subcarrier are combined, and the combined signal has a very large peak (peak signal). May occur. In this case, since the ratio of peak power to average transmission power (Peak to Average Power Ratio: PAPR) increases, the transmitter power amplifier (PA) must be designed in consideration of the backoff amount equivalent to PAPR. Don't be. When using a power amplifier that does not secure a sufficient back-off amount for the PAPR of the transmission signal, the transmission efficiency deteriorates due to the distortion of the signal due to the influence of the nonlinear region of the power amplifier, an increase in the symbol error rate, etc. To do. If the PAPR of the signal to be transmitted is 10 dB, the peak power generated is 10 times the average power, so for example, a 100 W class power amplifier is required to transmit a 10 W signal. However, when the back-off amount of the power amplifier is increased, there is a problem that the power consumption, heat dissipation, and the like increase and the efficiency deteriorates, and it is preferable to reduce (suppress) the PAPR of the transmission signal in the multicarrier transmission system.

  On the other hand, in recent multi-carrier transmission schemes using OFDMA, etc., adaptive modulation and coding (AMC) technology is adopted, and in each time slot, different modulation schemes such as QPSK and 64QAM are assigned to each subcarrier. . Here, the demand for communication quality, such as modulation accuracy (EVM: Error Vector Magnitude), varies depending on the modulation method, and the higher the modulation method (higher transmission speed and bit rate) such as 64QAM, the more required Quality becomes severe. Although a peak signal may also occur in a transmission signal in a multicarrier transmission system employing such adaptive modulation, it is desirable to reduce PAPR for the reasons described above. However, there is a problem that the reduction of PAPR becomes a cause of deterioration of signal quality such as EVM.

  As a technique to solve the above-mentioned problem, when limiting the amplitude of the transmission signal to reduce the PAPR of the transmission signal, the conventional technology limits the influence on EVM by reducing the PAPR to a specific subcarrier. (Refer to Patent Document 1) and, when performing peak suppression in a multicarrier signal (transmission signal in a multicarrier transmission system), control a threshold value for peak suppression according to desired signal quality (Patent Document 2). Proposed.

Japanese Patent No. 3654526 JP 2008-22230 A

  However, Patent Document 1 does not describe the relationship between the modulation method or EVM and the desired quality. Even if the subcarriers where EVM deteriorates are limited, if the subcarrier contains a multilevel modulation signal such as 64QAM that is easily affected by the communication quality, that is, low resistance to EVM deterioration, The influence on the overall communication quality is increased. Further, Patent Document 2 does not limit the peak suppression range to a specific subcarrier, and when QPSK and 64QAM coexist, control according to 64QAM having lower resistance to EVM is performed. Therefore, efficient control is not always possible.

  Accordingly, an object of the present invention is to relatively reduce the range of EVM degradation due to peak suppression when multiple modulation schemes such as QPSK and 64QAM are mixed in the same time slot in a multi-channel transmission scheme such as OFDMA. To provide a technique (transmitter and transmission control method) that is limited only to subcarriers that are highly tolerated, that is, tolerant of deterioration (for example, subcarriers to which a modulation scheme having a low bit rate such as QPSK is allocated). It is in.

In order to solve the above-described problem, a modulation scheme adaptively applied to the plurality of subcarriers (for each time slot) in an orthogonal frequency division multiple access (OFDMA) scheme using a plurality of subcarriers according to the present invention. Transmitters that assign signals and send signals
An acquisition unit for acquiring information on a modulation scheme allocated to subcarriers constituting a signal to be transmitted;
Based on the information acquired by the acquisition unit, processing for reducing peak power of a signal transmitted on a subcarrier to which a modulation scheme having a relatively low transmission rate is assigned among subcarriers constituting the signal to be transmitted is performed. And a peak reduction processing unit.

  The transmitter calculates a impulse response of a filter (FIR filter or the like) that passes a subcarrier assigned a modulation scheme having a relatively low transmission rate based on the information acquired by the acquisition unit. The peak reduction processing unit performs a convolution operation on the impulse response calculated by the calculation unit and a portion of the signal to be transmitted that exceeds a predetermined threshold, thereby calculating the peak power of the signal to be transmitted. Perform reduction processing.

In addition, a transmitter according to an embodiment of the present invention includes:
The peak reduction processing unit performs reduction processing of peak power of a signal transmitted on a subcarrier to which a modulation scheme having the lowest transmission rate is assigned among subcarriers constituting the signal to be transmitted.

In addition, a transmitter according to another embodiment of the present invention includes:
The acquisition unit
A modulation scheme assigned to subcarriers constituting the signal to be transmitted is detected from broadcast information (frame control header or the like) included in the signal to be transmitted.

Furthermore, a transmitter according to another embodiment of the present invention comprises:
A transmission data generation unit that generates the signal to be transmitted by allocating the same modulation scheme to a plurality of adjacent subcarriers (based on data received from an external device);
The acquisition unit acquires information on a modulation scheme assigned to the subcarrier from the transmission data generation unit.

  As described above, the solution of the present invention has been described as an apparatus. However, the present invention can be realized as a method, a program, and a storage medium that stores the program substantially corresponding to these, and the scope of the present invention. It should be understood that these are also included. In addition, each step of the following method and program uses an arithmetic processing unit such as a CPU or DSP as necessary in data processing, and the input data, processed / generated data, etc. are stored in the HDD, It is stored in a storage device such as a memory.

For example, in the orthogonal frequency division multiple access (OFDMA) system using a plurality of subcarriers realized as a method of the present invention, the modulation system is adaptively applied to the plurality of subcarriers (for each time slot). The transmission control method of assigning and transmitting signals is
Obtaining information on modulation schemes assigned to subcarriers constituting a signal to be transmitted;
Based on the information acquired in the step of acquiring, a peak for performing reduction processing of peak power transmitted on a subcarrier to which a modulation scheme having a relatively low transmission rate is allocated among subcarriers constituting the signal to be transmitted Reduction processing steps;
It is characterized by including.

  According to the present invention, when a plurality of modulation schemes such as QPSK and 64QAM are mixed in the same time slot in a multi-channel transmission scheme such as OFDMA, the range of EVM degradation due to suppression of peak power is limited to the degradation. It is possible to limit only to high subcarriers.

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

  FIG. 1 is an example of a block diagram of a transmitter according to the present invention. The transmitter 100 includes an antenna ANT, an acquisition unit 120, a calculation unit 130, a transmission unit 140, and a peak suppression unit 150. The acquisition unit 120 includes a modulation scheme detection unit 122. The peak suppressing unit 150 includes a delay unit 152, an amplitude limiting unit 153, a filter applying unit 156, and a subtracting unit 157, and the amplitude limiting unit 153 includes a limiter 154 and a subtracting unit 155. Transmitter 100 receives a signal to be transmitted, which is composed of subcarriers to which modulation schemes different from subcarriers are assigned, from an external device (not shown) that processes baseband signals to generate OFDMA signals. The peak suppression unit 150 performs processing to suppress (reduce) the peak voltage of the signal to be transmitted (details will be described later). The acquisition unit 120 acquires information on the modulation scheme assigned to each subcarrier of the signal to be transmitted. Based on the information detected by the acquisition unit 120, the calculation unit 130 obtains the shape (frequency characteristic) of the bandpass filter to be applied to the signal to be transmitted, and calculates the impulse response of the filter. Based on the impulse response calculated by the calculation unit 130, the filter application unit 156 included in the peak suppression unit 150 performs filtering of a signal to be transmitted.

  Processing in the peak suppression unit 150 will be described. The peak suppression unit 150 includes an amplitude control unit 153, a filter application unit 156, and a subtraction unit 157, and the amplitude limit unit 153 includes a limiter 154 and a subtraction unit 155. First, the limiter 154 limits the amplitude value in the signal to be transmitted so as not to exceed a predetermined threshold value. The predetermined threshold can be appropriately set based on transmission path characteristics estimated by the transmitter 100 based on a signal transmitted from the receiver (not shown) to the transmitter 100. The subtracting unit 155 performs a process of subtracting the signal whose amplitude value is limited by the limiter 154 from the original signal to be transmitted. Therefore, the signal (difference data) taken out by the amplitude limiter 153 is a signal of a signal whose amplitude value exceeds a predetermined threshold among signals to be transmitted.

  By subtracting the signal (difference data) extracted by the amplitude limiter 153 from the original transmission, the peak power of the signal to be transmitted can be suppressed. As described above, the EVM or the like can be performed by suppressing the peak power. There is a problem that gets worse. Therefore, the filter applying unit 156 performs filtering for limiting the range of deterioration to only predetermined subcarriers. For this reason, in the transmitter 100 according to the present invention, the calculation unit 130 is assigned a modulation scheme that is relatively allowed to deteriorate among subcarriers constituting a signal to be transmitted, that is, a modulation scheme that is highly resistant to the deterioration. The impulse response of the bandpass filter that passes the subcarrier is obtained. Then, the filter application unit 156 convolutes the signal extracted by the amplitude limiting unit 153, thereby limiting the peak suppression signal to subcarriers to which a modulation scheme that allows relatively deterioration is assigned. For example, when subcarriers to which modulation schemes of QPSK, 16QAM, and 64QAM are allocated are mixed in the time slot of the transmission signal, the modulation scheme to which deterioration is relatively allowed is QPSK having a relatively low transmission rate. The subtracting unit 157 generates a difference signal between the original signal to be transmitted delayed by the time required for the above filtering process by the delay unit 152 and the signal filtered by the filter applying unit 156. Therefore, the differential signal generated by the subtracting unit 157 is a signal in which the peak power is reduced with respect to the subcarriers to which the modulation scheme having high resistance to the reduction in peak power is assigned. The transmission unit 140 transmits the signal output from the peak suppression unit 150.

  There are many possible methods for the acquisition unit 120 to acquire information on the modulation schemes assigned to each subcarrier of the signal to be transmitted. For example, the modulation scheme detection unit 122 transmits the information from an external device. The control information (frame control header or the like) included in the baseband signal is read or the signal to be transmitted is subjected to FFT (Fast Fourier Transform), and the modulation scheme is estimated from the included reference signal (pilot signal).

  Next, the filtering process by the band pass filter applied to the signal to be transmitted, in which the calculation unit 130 calculates the impulse response, will be described with reference to the drawings. FIG. 2 is a schematic diagram illustrating the filtering process according to the present invention. FIG. 2 shows an example of a transmission frame by a multicarrier transmission scheme such as OFDMA. In the multicarrier transmission method, resources in the frequency axis and time axis directions can be divided and assigned to a plurality of users, and a modulation method can be adaptively assigned according to data transmitted by the user. In this way, a plurality of modulation methods are mixed in the frame. Here, the slot in the figure means a unit for allocating resources in the time axis direction. For example, in the Nth slot, there are QPSK, 16QAM, and 64QAM modulation signals, and the QPSK modulation signal having the smallest multi-value number (low transmission speed) exists near the center in the frequency axis direction.

  The acquisition unit 120 acquires information on the modulation scheme (FIG. 2) assigned to each subcarrier using the method described above. This information is acquired for each time slot. For example, in the Nth slot, it is a subcarrier to which QPSK is assigned that has a high tolerance level against EVM degradation caused by peak power reduction processing. That is, if the signal for reducing the peak power is limited to the subcarrier region, the influence on the communication quality of the entire signal to be transmitted can be minimized. Therefore, the frequency response of the filter used for the peak reduction process may be a shape indicated by FT1 in FIG. The filter applying unit 156 performs a convolution operation between the signal that has passed through the amplitude limiting unit 153 and the impulse response of the filter FT1, and the subtracting unit 157 subtracts an extra signal (exceeding a predetermined threshold) from the original signal to be transmitted. Signal) is a signal transmitted on a subcarrier to which a modulation scheme having high resistance against the influence of peak power suppression is assigned. Here, as shown in FIG. 2, the modulation scheme allocation to each subcarrier may vary from slot to slot, so that the shape of the filter to be applied is different for slot N + 1 and slot N + 2, respectively. Different like FT2 and FT3. Therefore, by appropriately calculating the impulse response of each filter for each slot, it is possible to always minimize the adverse effects due to the degradation of EVM.

  The peak reduction process described above will be described using a flowchart. FIG. 4 is an example of a flowchart of peak reduction processing by the transmitter of the present invention. The transmitter 100 repeats the loop (steps S12 to S15) for each time slot. First, information on the modulation scheme assigned to the subcarrier is acquired (step S12). Thereafter, based on the acquired information, an impulse response of a filter that allows passage of a signal of a subcarrier assigned a modulation scheme having a relatively low transmission rate among the subcarriers of the time slot is calculated (step S13). Next, based on the calculated impulse response, the above-described peak reduction processing is performed (step S14), and a signal with reduced peak power is transmitted (step S15). The loop is repeated at each time slot until the data to be transmitted is completed (step S16).

  In the above-described embodiment, a mode is described in which a signal (modulated IQ data is mapped to each subcarrier) received by modulating an already transmitted data and assigning it to a subcarrier from an external device that performs baseband signal processing is described. did. However, the transmitter according to the present invention may receive data before allocation to subcarriers (before mapping) and generate a signal in which a predetermined modulation scheme is allocated to a predetermined subcarrier region in its own device. Furthermore, it is possible to adopt a configuration that notifies the acquisition unit 120 of the assignment result of the modulation scheme. FIG. 3 shows a schematic block diagram of the transmitter of the present invention in this case. In the transmitter 100A, components having the same functions as those of the transmitter 100 in FIG. A transmitter 100A illustrated in FIG. 3 further includes a transmission data generation unit 110 in the transmitter 100 illustrated in FIG. The transmission data generation unit 110 includes a data allocation unit 112 and an IFFT (inverse Fourier transform) unit 114. The data allocation unit 112 receives IQ data modulated by a different modulation method for each packet from an external device, and maps these data on subcarriers. At that time, in each slot, a modulation scheme having a relatively low transmission rate is assigned to predetermined subcarriers, and the assignment information is transmitted to the acquisition unit 120. Thereafter, the calculation unit 130 calculates the impulse response of the filter based on the allocation information transmitted from the acquisition unit 120, and the peak suppression unit 150 performs a peak reduction process. Other functional units and peak reduction processing are the same as those in the above-described embodiment, and thus description thereof is omitted. In addition, the allocation information may be transmitted directly from the transmission data generation unit 110 to the calculation unit 130 without providing the acquisition unit 120. In this embodiment, since the modulation system is not detected, there is an advantage that the processing load is reduced in addition to the advantage that the adverse effect due to the degradation of EVM is minimized.

  The advantages of the synchronous control method according to the present invention will be described again. According to the present invention, when a plurality of modulation schemes such as QPSK and 64QAM are mixed in the same time slot in a multi-channel transmission scheme such as OFDMA, a sub-range in which large degradation is allowed within a range where degradation of EVM due to peak suppression extends. It is possible to limit only to carriers. In addition, since the filter is adaptively generated based on the allocation information of the modulation scheme to the subcarrier acquired for each time slot, high-quality communication can be performed over all communication. Further, since the allocation information of the modulation scheme of the signal to be transmitted is detected by the own apparatus, there is an advantage that there is no restriction on the external apparatus that processes the baseband signal. In addition, since the amplitude (peak power) is suppressed for a modulation system with a small multi-level number, demodulation errors on the receiving side are compared with those for a modulation system with a large multi-level number that uses amplitude information for data discrimination. Will be less.

  Although the present invention has been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications and corrections based on the present disclosure. Therefore, it should be noted that these variations and modifications are included in the scope of the present invention. For example, the functions included in each means, each step, etc. can be rearranged so as not to be logically contradictory, and a plurality of means, steps, etc. can be combined into one or divided. .

FIG. 2 is a schematic block diagram of a transmitter according to the present invention. It is the schematic explaining the filtering process by this invention. FIG. 2 is a schematic block diagram of a transmitter according to the present invention. It is an example of the flowchart of the peak reduction process by the transmitter of this invention.

Explanation of symbols

100, 100A transmitter 110 transmission data generation unit 112 data allocation unit 114 IFFT
DESCRIPTION OF SYMBOLS 120 Acquisition part 122 Modulation system detection part 130 Calculation part 140 Transmission part 150 Peak suppression part 152 Delay part 153 Amplitude restriction | limiting part 154 Limiter 155,157 Subtraction part 156 Filter application part ANT Antenna FT1-FT3 Filter characteristic

Claims (5)

A transmitter for adaptively allocating modulation schemes to the plurality of subcarriers and transmitting signals in an orthogonal frequency division multiple access scheme using a plurality of subcarriers;
An acquisition unit for acquiring information on a modulation scheme allocated to subcarriers constituting a signal to be transmitted;
Based on the information acquired by the acquisition unit, processing for reducing peak power of a signal transmitted on a subcarrier to which a modulation scheme having a relatively low transmission rate is assigned among subcarriers constituting the signal to be transmitted is performed. A peak reduction processing unit;
A transmitter comprising: The transmitter of claim 1, wherein
The peak reduction processing unit performs processing for reducing peak power of a signal to be transmitted on a subcarrier assigned a modulation scheme having the lowest transmission rate among subcarriers constituting the signal to be transmitted.
A transmitter characterized by that. The transmitter according to claim 1 or 2,
The acquisition unit
Detecting a modulation scheme assigned to subcarriers constituting the signal to be transmitted from broadcast information included in the signal to be transmitted;
A transmitter characterized by that. The transmitter according to claims 1-3.
A transmission data generation unit that generates the signal to be transmitted by allocating the same modulation scheme to a plurality of adjacent subcarriers;
The acquisition unit acquires information on a modulation scheme assigned to the subcarrier from the transmission data generation unit.
A transmitter characterized by that. A transmission control method for adaptively allocating modulation schemes to the plurality of subcarriers and transmitting signals in an orthogonal frequency division multiple access scheme using a plurality of subcarriers,
Obtaining information on modulation schemes assigned to subcarriers constituting a signal to be transmitted;
Based on the information acquired in the step of acquiring, a peak for performing reduction processing of peak power transmitted on a subcarrier to which a modulation scheme having a relatively low transmission rate is allocated among subcarriers constituting the signal to be transmitted Reduction processing steps;
Including a transmission control method.

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