WO2011104955A1 - 信号処理回路とこの回路を有する通信装置 - Google Patents
信号処理回路とこの回路を有する通信装置 Download PDFInfo
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- WO2011104955A1 WO2011104955A1 PCT/JP2010/070699 JP2010070699W WO2011104955A1 WO 2011104955 A1 WO2011104955 A1 WO 2011104955A1 JP 2010070699 W JP2010070699 W JP 2010070699W WO 2011104955 A1 WO2011104955 A1 WO 2011104955A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2614—Peak power aspects
- H04L27/2623—Reduction thereof by clipping
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
- H04J11/0023—Interference mitigation or co-ordination
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2614—Peak power aspects
- H04L27/2623—Reduction thereof by clipping
- H04L27/2624—Reduction thereof by clipping by soft clipping
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
- H04W88/08—Access point devices
- H04W88/085—Access point devices with remote components
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
- H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
- H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
- H04B2201/70706—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation with means for reducing the peak-to-average power ratio
Definitions
- the present invention relates to a signal processing circuit that performs amplitude limitation on an IQ baseband signal by clipping processing or the like, and a communication apparatus including the circuit. More specifically, the present invention relates to an improvement of an amplitude limiting method for more appropriately limiting the amplitude of an IQ baseband signal input to a power amplification circuit in a wireless transmitter.
- the carrier phases overlap each other. Can result in signals with large peak power.
- the power amplifier power amplifier
- the power amplifier is required to have excellent linearity, when a signal having a level exceeding the maximum output is input, the output is saturated and nonlinear distortion increases.
- noise shaping-crest factor reduction (NS-CFR) and peak cancellation-crest factor reduction (PC-CFR) are known.
- the NS-CFR circuit performs band-limiting by filtering the peak component (increment from the threshold) of the IQ baseband signal whose instantaneous power exceeds the threshold with a low pass filter, FIR (Finite Impulse Response) filter, etc.
- FIR Finite Impulse Response
- the PC-CFR circuit sets in advance a cancellation pulse (basic function waveform) for preventing out-of-band radiation even when clipping, and the peak component of the IQ baseband signal whose instantaneous power exceeds the threshold (threshold).
- Patent No. 3954341 Patent No. 3853509 gazette Unexamined-Japanese-Patent No. 2004-135087 (FIG. 1-FIG. 6)
- the transmission power is set relatively high in order to communicate with a remote mobile phone in a certain frequency band, and in another frequency band
- the transmission power may be changed for each frequency band by setting the transmission power relatively low in order to communicate with the mobile phone.
- the average power of the IQ baseband signal input to the peak power suppression circuit also changes for each frequency band.
- the present invention provides a signal processing circuit and the like capable of appropriately limiting the amplitude without deteriorating the SNR, even for an IQ baseband signal in which the average power is different for each frequency band. To aim.
- a signal processing circuit is a signal processing circuit for reducing PAPR of a modulated wave signal input to a power amplification circuit, and the power for calculating instantaneous power of an IQ baseband signal of the modulated wave signal.
- the upper limit or lower limit of the instantaneous power or both of them may be equivalent to a predetermined threshold value using a calculation unit and a cancellation signal capable of canceling out the IQ baseband signal corresponding to the average power for each frequency band.
- a signal processing unit that limits the amplitude of the IQ baseband signal.
- the signal processing unit limits the amplitude of the IQ baseband signal using a cancellation signal that can be canceled corresponding to the average power for each frequency band of the IQ baseband signal.
- the power in each frequency band can be prevented from fluctuating more than necessary due to the cancellation signal. Therefore, even in the case of an IQ baseband signal in which the average power is different for each frequency band, the amplitude can be appropriately limited without deteriorating the SNR.
- the signal processing unit can be obtained by multiplying an increment of the IQ baseband signal from a first threshold defining the upper limit of the instantaneous power by a predetermined canceling pulse.
- the cancellation signal may be subtracted from the original IQ baseband signal to perform clipping processing to suppress the IQ baseband signal to instantaneous power equivalent to the first threshold.
- the signal processing unit multiplies a decrement of the IQ baseband signal from a second threshold that defines the lower limit of the instantaneous power by a predetermined cancellation pulse.
- the offset signal obtained may be added to the original IQ baseband signal, and boosting processing may be performed to raise the IQ baseband signal to instantaneous power equivalent to the second threshold.
- the signal processing unit may be capable of executing both the clipping process and the boosting process.
- the signal processing circuit of the present invention in the case of using a cancellation signal obtained by multiplying an increment or decrement from the above threshold value by a cancellation pulse, basic pulses determined for each frequency band are used for each frequency band. It is necessary to further include a pulse generation unit that generates the cancellation pulse by multiplying the relative proportions of the average powers to obtain the sum.
- the pulse generation unit is a ratio calculation unit that calculates the relative ratio for each frequency band, a waveform storage unit that stores the cancellation waveform for each frequency band, and The relative ratio may be multiplied by the corresponding offset waveform to obtain a summation.
- the multiplication and addition unit changes the calculated relative ratio. It is preferable to execute multiplication and summation using the relative ratio after the variation only, and to output the resultant cancellation pulse to the pulse holding unit. In this case, as long as the relative ratio does not change, the multiplication and addition unit does not execute multiplication and summation, and the pulse holding unit maintains the previous cancellation pulse. For this reason, it is possible to reduce the operation load of the circuit as compared to the case where the canceling pulse is generated each time.
- the relative ratio of the average power of a certain frequency band is that the square root of the instantaneous power of that frequency band is accumulated at a predetermined sampling period, and the accumulated value is divided by the sum of the accumulated values of each frequency band
- the circuit scale becomes large.
- division by a power of 2 only changes the position of the decimal point and practically does not require division.
- the ratio calculating unit accumulates the square root of the instantaneous power for each frequency band, and the sum of the accumulated values becomes a power of 2 ⁇ ⁇ ( ⁇ is a sufficiently small predetermined value).
- ⁇ is a sufficiently small predetermined value.
- the relative ratio can be calculated only by changing the position of the decimal point of each accumulated value, the relative ratio can be calculated accurately and quickly without increasing the circuit size.
- the ratio calculation unit performs the calculation of the relative ratio within a control period in which the average power of the IQ baseband signal may temporally fluctuate. Is preferred. The reason is that if the calculation of the relative ratio is performed within the control period, the relative ratio can be calculated in a stable state in which the average power of the IQ baseband signal does not fluctuate much, and an accurate relative ratio can be obtained. is there.
- the transmission power may fluctuate significantly for each time zone in response to the current call volume.
- the threshold for detecting the peak of the instantaneous power of the IQ baseband signal is a fixed value, it operates only in the time zone where the transmission power is large corresponding to the maximum traffic. For this reason, in the conventional peak power suppression circuit, there is a case where the power efficiency of the power amplifier can not be improved because it does not operate in a time zone where the average power of the IQ baseband signal is small.
- the signal processing circuit of the present invention further includes a threshold updating unit that updates the threshold used in the signal processing unit every control cycle in which the average power of the IQ baseband signal may temporally fluctuate. Is preferred.
- the threshold update unit updates the threshold used in the signal processing unit for each control cycle, for example, the instantaneous power can be reliably suppressed even in a time zone in which the average power of the IQ baseband signal is relatively small. be able to.
- the signal processing circuit of the present invention when used for a transmitter of the LTE (Long Term Evolution) system, it is preferable to adopt an OFDM (Orthogonal Frequency Division Multiplex) symbol period as the control period.
- OFDM Orthogonal Frequency Division Multiplex
- W-CDMA Wideband Code Division Multiple Access
- a control cycle of closed loop transmission power control may be adopted as the control cycle. The reason is that in LTE, the symbol period of OFDM is the smallest time unit in which the transmission power can greatly fluctuate, and in W-CDMA, the control period of closed loop transmission power control is the smallest in which transmission power can largely fluctuate. Because it is a unit of time.
- a communication apparatus includes a transmitter equipped with the signal processing circuit according to the present invention and the power amplification circuit disposed in the subsequent stage, and is similar to the signal processing circuit according to the present invention It produces an effect.
- the amplitude of the IQ baseband signal is limited by the canceling signal that can be canceled according to the average power of the IQ baseband signal for each frequency band, the average for each frequency band is obtained. Even for IQ baseband signals of different powers, the amplitude can be properly limited without degrading the SNR.
- FIG. 7 is a coordinate diagram of an IQ plane showing a relationship between an IQ baseband signal and a second threshold value when boosting processing is performed. It is a graph which shows the time change of the 2nd threshold that is updated with the instantaneous power of IQ baseband signal, and is updated sequentially.
- FIG. 7 is a coordinate diagram of an IQ plane showing a relationship between an IQ baseband signal and first and second threshold values when both clipping and boosting processing are performed. It is a graph of the time domain which shows the variation of a basic balus.
- FIG. 1 is an overall configuration diagram of a wireless communication system according to a first embodiment to which the present invention can be suitably applied.
- a base station (BS: Base Station) 1 and a plurality of mobile terminals (MS: wirelessly communicating with the device 1 in the cell of the device 1). It consists of Mobile Station 2).
- the OFDM scheme is adopted as a modulation scheme between the base station apparatus 1 and the mobile terminal 2.
- This scheme is a multicarrier digital modulation scheme in which transmission data is placed on a large number of carriers (subcarriers), and since the subcarriers are orthogonal to each other, data can be arranged closely enough to cause overlap in the frequency axis. There is an advantage.
- the wireless communication system is a system for a mobile phone to which the LTE (Long Term Evolution) system is applied, and communication based on the LTE system is performed between each base station apparatus 1 and the mobile terminal 2.
- LTE Long Term Evolution
- the frequency band of the downlink frame can be set in units of 5 MHz, and when transmitting downlink signals to each mobile terminal 2 in the cell, transmission power for each frequency band It is possible to change.
- the case where the base station apparatus 1 transmits downlink frames in two types of frequency bands B1 and B2 is illustrated, and the transmission power of the first band B1 having a smaller frequency is set larger.
- the transmission power in the second band B2 having a larger frequency is set to be smaller.
- the communication area A1 to which the downstream signal of the first band B1 having the large transmission power can reach is far from the communication area A2 to which the downstream signal of the second band B2 having the small transmission power can reach. And it has become widespread.
- the radio communication system to which the present invention can be applied is not limited to the LTE system, and may be a W-CDMA system, but in the following, the case where the present invention is applied to the base station apparatus 1 of the LTE system I will assume and explain.
- FIG. 7 is a diagram illustrating the structure of an LTE downlink frame.
- the vertical axis direction indicates the frequency
- the horizontal axis direction indicates the time.
- a total of ten subframes (subframes # 0 to # 9) constituting a downlink (DL) frame are each formed of two slots (slot # 0 and slot # 1).
- One slot is composed of seven OFDM symbols (in the case of Normal Cyclic Prefix).
- a resource block which is a basic unit on data transmission is defined as 12 subcarriers in the frequency axis direction and 7 OFDM symbols (1 slot) in the time axis direction. Therefore, for example, when the frequency bandwidth of the DL frame is set to 5 MHz, 300 subcarriers are arrayed, and 25 resource blocks are arrayed in the frequency axis direction.
- a control channel for transmitting information necessary for downlink communication to the mobile terminal 2 by the base station device 1 is allocated.
- DL control information resource allocation information of the corresponding subframe
- reception success notification ACK: Acknowledgement
- HARQ Hybrid Automatic Report Request
- reception failure notification NACK: Negative Acknowledgment
- PBCH Physical Broadcast CHannel
- P-SCH Primary Synchronization CHannel
- S-SCH Secondary Synchronization CHannel
- resource blocks in other areas are DL shared communication channels (PDSCH: Physical Downlink Shared CHannel) for storing user data etc. Used.
- PDSCH Physical Downlink Shared CHannel
- the allocation of user data stored in the PDSCH is defined by the resource allocation information in the control channel allocated at the beginning of each subframe, and the mobile terminal 2 uses the resource allocation information to It can be determined whether the data is stored in a subframe.
- FIG. 2 is a functional block diagram showing the main part of the OFDM transmitter 3 of the base station device 1.
- the transmitter 3 includes a transmission processor 4 and a power amplification circuit 5.
- the transmission processor 4 is formed of, for example, an FPGA (Field Programmable Gate Array) internally having one or more memories and a CPU. ing.
- the above-mentioned FPGA can set in advance (configuration) configuration information for various logic circuits at the time of shipping of the processor, at the time of manufacturing of the base station apparatus 1 or the like, and each setting shown in FIG. Functional units 6 to 10 are configured. That is, the transmission processor 4 according to this embodiment includes, in order from the left, the S / P converter 6, the mapping unit 7, the IFFT (Inverse Fast Fourier Transform) unit 8, the signal processing unit 9, and the orthogonal modulation unit Contains 10
- the serial signal sequence input to the transmission processor 4 is converted into a plurality of signal sequences in the S / P (serial parallel) conversion unit 6, and each parallel signal sequence converted is converted in the mapping unit 7. It is converted into a plurality of subcarrier signals f1, f2,... Fn consisting of a combination of amplitude and phase.
- the subcarrier signals f 1, f 2,..., F n are converted by the IFFT unit 8 into I signals and Q signals as baseband signals orthogonal to each other on the time axis.
- the base station apparatus 1 transmits downlink signals in two types of frequency bands B1 and B2, and therefore, as shown in FIG. 2, subcarriers are included in the first band B1.
- the IFFT unit 8 outputs one signal I1, Q1 and second signals I2, Q2 whose subcarriers are included in the second band B2.
- the first signals I1 and Q1 and the second signals I2 and Q2 are input to the signal processing unit (signal processing circuit of the present embodiment) 9 in the subsequent stage, and predetermined processing is performed in the processing unit 9.
- the IQ signal (Iout, Qout) after signal processing is orthogonally modulated in the orthogonal modulation unit 10 to be a modulation wave signal, and this modulation wave signal is input to the power amplification circuit 5 in the subsequent stage.
- the instantaneous power P of the IQ baseband signal which is a combined signal on the time axis of the first signals I1 and Q1 and the second signals I2 and Q2, is higher than a predetermined threshold Pth.
- the IQ baseband signal is subjected to clipping processing so as not to be large, the details of which will be described later.
- the power amplification circuit 5 converts the modulated wave signal input from the quadrature modulation unit 10 into an analog signal, converts the analog signal after conversion into an RF frequency, and converts the power of the analog signal. And an amplified RF signal is transmitted from the antenna to the outside.
- the power amplifier circuit 5 of this embodiment may be a fixed voltage method in which the drain voltage of the power amplifier is constant, but from the viewpoint of achieving high efficiency of the high frequency amplifier, the ET (Envelope Tracking) method is adopted. It is preferable to do.
- the ET type power amplification circuit 5 extracts amplitude information (envelope) from the modulation wave signal input to the power amplifier, and applies a drain voltage corresponding to the amplitude information to the power amplifier to substantially saturate the power amplifier. Thus, the power loss occurring at the time of operation at the fixed voltage is reduced, and high efficiency of the power amplifier can be realized.
- FIG. 3 is a functional block diagram of the signal processing circuit 9 according to the first embodiment of the present invention.
- the composite signal of the first signals I1 and Q1 and the second signals I2 and Q2 will be simply referred to as “IQ baseband signal” or “IQ signal”.
- the instantaneous power of the first signals I1 and Q1 is P1
- the instantaneous power of the second signals I2 and Q2 is P2
- the signal processing circuit 9 of this embodiment includes power calculation units 13 to 15, a pulse generation unit 16, a signal processing unit 17, and delay units 18 and 19.
- the signal processing unit 17 of the present embodiment is obtained by multiplying the increments ⁇ I and ⁇ Q from the threshold value Pth by the predetermined cancellation pulse S.
- the PC-CFR circuit is configured to perform clipping processing to suppress the IQ baseband signal to the instantaneous power P equivalent to the threshold Pth by subtracting the cancellation signals Ic and Qc from the original IQ baseband signal.
- the signal processing unit 17 includes a difference rate calculating unit 20, a comparing unit 21, a pulse holding unit 22, and adders / subtractors 23 and 24.
- the difference rate calculation unit 20 uses the instantaneous power P calculated by the power calculation unit 13 and a predetermined threshold value Pth set in advance to increase the incremental rate ⁇ 1-SQRT (Pth / P) of the instantaneous power P with respect to the threshold value Pth.
- ⁇ Is calculated, and each component (I, Q) of the IQ baseband signal is multiplied by the increment rate ⁇ 1-SQRT (Pth / P) ⁇ through a multiplier. Therefore, the increments ⁇ I and ⁇ Q of the amount exceeding the threshold Pth of the IQ baseband signal are calculated based on the following equation.
- SQRT (•) is a function that takes the square root of the variable in parentheses (the same applies hereinafter).
- ⁇ I ⁇ 1-SQRT (Pth / P) ⁇
- I ⁇ Q ⁇ 1-SQRT (Pth / P) ⁇ ⁇ Q
- the comparison unit 21 compares the instantaneous power P calculated by the power calculation unit 13 with the threshold Pth, and issues an output command of the cancellation pulse S to the pulse holding unit 22 when the instantaneous power P is larger than the threshold Pth.
- the pulse holding unit 22 has a memory, such as a dual port RAM, for temporarily holding the cancellation pulse S output by the pulse generation unit 16 described later, and when receiving a command from the comparison unit 21, The cancellation pulses S held at that time are multiplied by the above-mentioned increments ⁇ I and ⁇ Q to calculate cancellation signals Ic and Qc.
- the delay units 18 and 19 preceding the adders / subtractors 23 and 24 delay the IQ baseband signal by the time of the arithmetic processing in the power calculation unit 13 and the signal processing unit 17.
- the adders / subtractors 23 and 24 respectively subtract the cancellation signals Ic and Qc from the respective components I and Q of the delayed IQ signal, and output Iout and Qout which are IQ signals after signal processing.
- the IQ baseband signal whose instantaneous power P exceeds the threshold Pth is corrected to a signal of instantaneous power corresponding to the threshold Pth.
- an IQ baseband signal whose instantaneous power P is equal to or less than the threshold Pth is output as it is without being corrected.
- FIG. 6 is a coordinate diagram of an IQ plane showing the relationship between the IQ baseband signal and the threshold value Pth when the clipping process is performed.
- the signal processing by the signal processing circuit 9 of the present embodiment is clipping processing for cutting the outer peripheral side of the instantaneous power P of the IQ baseband signal.
- PAPR with respect to the power amplifier of the power amplifier circuit 5 is reduced, so that the power efficiency of the power amplifier is improved.
- FIG. 4 is a functional block diagram of the pulse generation unit 16.
- the pulse generation unit 16 multiplies the basic pulses S1 and S2 obtained in advance for each of the first and second bands B1 and B2 by the relative ratio C1 and C2 of the average power for each of the bands B1 and B2, respectively, and adds them.
- the offset pulse S is generated by taking the above, and includes a ratio calculation unit 26, a waveform storage unit 27, and a multiplication and addition unit 28.
- the waveform storage unit 27 includes a storage device such as a memory that stores the basic pulses S1 and S2 for each of the frequency bands B1 and B2.
- a storage device such as a memory that stores the basic pulses S1 and S2 for each of the frequency bands B1 and B2.
- both basic pulses S1 and S2 are stored in advance in the waveform storage unit 27 in FIG. 4, only one of the basic pulses S1 (or S2) is stored in the waveform storage unit 27 and the remaining
- the waveform serving as the base may be generated by performing frequency conversion for each of the frequencies f1 and f2.
- Patent Document 3 Japanese Patent Application Laid-Open No.
- the basic pulses S1 and S2 have a plurality of (for example, N) pulses included in the frequency bands B1 and B2 used to transmit downstream signals.
- the carrier wave has an amplitude of 1 / N and a phase of 0, and is composed of a sinc waveform obtained by being input to the IFFT unit 8. In this case, only the real part I appears in the output of the IFFT unit 8 and the imaginary part Q becomes zero.
- the basic pulses S1 and S2 for the first and second bands B1 and B2 have the same IFFT unit 8 as in the case of the transmission signal for a plurality of subcarriers included in each of the bands B1 and B2. It is a waveform (Sinc waveform) of real part I obtained by performing an inverse Fourier transformation at. Therefore, the frequency bands of the basic pulses S1 and S2 coincide with the first and second bands B1 and B2, and the cancellation signal obtained by multiplying the increment of the IQ signal exceeding the threshold Pth by the basic pulses S1 and S2 is used. Even when the signal is clipped, unnecessary frequency components outside the bands B1 and B2 are not generated.
- the instantaneous power P1 of the first signals I1 and Q1 calculated by the power calculation unit 14 and the second signal I2 and Q2 instantaneous power P1 calculated by the power calculation unit 15 are input to the ratio calculation unit 26, respectively.
- the ratio calculator 26 calculates the relative ratios C1 and C2 of the average power for each of the frequency bands B1 and B2 based on the following equations using these instantaneous powers P1 and P2.
- C1 ⁇ 1 / P1 / ((P P1 + ⁇ P P2)
- C2 ⁇ P2 / ( ⁇ P1 ++ P2)
- the formula for calculating the relative ratios C1 and C2 accumulates the square roots PP1 and PP2 of the instantaneous powers P1 and P2 for each of the frequency bands B1 and B2 in a predetermined sampling period, and accumulates the accumulated values ⁇ P1 and ⁇ P2
- the sum of the accumulated values of the frequency bands B1 and B2 is divided by ⁇ ⁇ square root over (P) ⁇ + P ⁇ square root over (P2) ⁇ .
- FIG. 5 is a flowchart showing the above-mentioned operation logic executed by the ratio calculation unit 26. As shown in FIG. 5, the ratio calculation unit 26 first initializes the accumulated values Sum1 and Sum2 to zero (step ST1).
- the relative ratios C1 and C2 are calculated by dividing the accumulated values Sum1 and Sum2 by the power of 2 (step ST4).
- the relative ratios C1 and C2 can be calculated simply by changing the positions of the decimal points of the accumulated values Sum1 and Sum2, so that the relative ratios C1 and C2 can be accurately and quickly calculated without increasing the circuit size. Can.
- the ratio calculation unit 26 acquires the symbol period of the OFDM symbol, which is the smallest time unit in which the transmission power can greatly fluctuate, as a control period, and within this symbol period It is designed to perform calculations. In this way, since the relative ratios C1 and C2 can be calculated in a stable state in which the average power of the IQ baseband signal does not fluctuate much, there is an effect that accurate relative ratios C1 and C2 can be obtained.
- the multiplication and addition unit 28 includes two multipliers 29 and 30 and one adder 31. Among them, the multiplier 29 multiplies the relative ratio C1 corresponding to the first band B1 by the basic pulse S1 for the band B1, and the multiplier 30 converts the relative ratio C2 corresponding to the second band B2 to the relative ratio C2 Is multiplied by the basic pulse C2.
- the multiplication / addition unit 28 of the present embodiment compares the relative ratios C1 and C2 calculated by the ratio calculation unit 26 with a predetermined threshold to determine the variation thereof, and the relative ratios C1 and C2 exceed the threshold. Only when it fluctuates, multiplication and summation using the relative ratios C1 and C2 after the fluctuation are executed, and the cancellation pulse S generated as a result is output to the pulse holding unit 22. Therefore, as long as the relative ratios C1 and C2 do not change to some extent, the multiplication and addition unit 28 does not execute multiplication and summation, and the pulse holding unit 22 maintains the previous cancellation pulse S. Therefore, the operation load of the circuit can be reduced as compared with the case where the canceling pulse S is generated each time.
- the canceling pulse S corresponds to a ratio obtained by multiplying the relative ratio C1 of the average power of the first signals I1 and Q1 corresponding to the first band B1 by the basic pulse S1 for the band B1, and the second band B2. It is a composite pulse obtained by adding the relative ratio C2 of the average power of the second signals I2 and Q2 to the product of the basic pulse S2 for the band B2 multiplied. Therefore, even if the offset signals Ic and Qc obtained by multiplying the offset pulses S by the increments ⁇ I and ⁇ Q are subtracted from the original IQ baseband signal, the average power corresponding to the first and second bands B1 and B2 can be obtained. Thus, the amplitude of the IQ baseband signal is offset.
- the power in the first and second bands B1 and B2 does not decrease more than necessary due to the subtraction of the cancellation signals Ic and Qc, and for each of the frequency bands B1 and B2. Even in the case of IQ baseband signals with different average powers, clipping processing can be appropriately performed without deteriorating the SNR.
- FIG. 8 is a functional block diagram of the signal processing circuit 9 according to the second embodiment.
- the signal processing circuit 9 (FIG. 8) of this embodiment is different from the signal processing circuit 9 (FIG. 3) of the first embodiment in that an average calculation unit 33 and a threshold update unit 34 are further provided. It is in the point equipped.
- the configurations and functions common to the first embodiment are given the same reference numerals in the drawings, the description thereof is omitted, and the differences from the first embodiment are mainly described.
- the average calculating unit 33 acquires the symbol period of the OFDM symbol, which is the smallest time unit in which the transmission power may greatly fluctuate, as a control period for calculating the average power Pave of the IQ baseband signal. That is, the average calculation unit 33 acquires the instantaneous power P of the IQ baseband signal from the power calculation unit 13 and averages the instantaneous power P within the symbol period to obtain the IQ baseband per symbol period. The average power Pave of the signal is calculated and output to the threshold updating unit 34.
- the threshold update unit 34 adopts a value obtained by multiplying the average power Pave for each symbol period acquired from the average calculation unit 33 by a predetermined magnification as the threshold Pth in the symbol period. For example, when the ratio of the peak power Ppeak of the IQ baseband signal to the average power Pave is narrowed to 6 dB, the predetermined magnification is doubled.
- the threshold update unit 34 calculates the threshold Pth for each symbol period as described above, dynamically updates the threshold Pth, and outputs the updated threshold Pth to the difference ratio calculation unit 20 and the comparison unit 21. .
- the comparing unit 21 determines the magnitude of the instantaneous power P calculated by the power calculating unit 18 using the threshold Pth acquired from the threshold updating unit 34, and the instantaneous power P exceeds the updated threshold Pth.
- An output command of the cancellation pulse S is issued to the pulse holding unit 22.
- FIG. 9 is a graph showing temporal changes in the instantaneous power P of the IQ baseband signal and the threshold Pth sequentially updated.
- the threshold Pth used for clipping processing in the signal processing circuit 9 is sequentially calculated based on the average power Pave calculated for each symbol period (1/14 ms), and for each symbol period Updated to
- the symbol period of OFDM which is the smallest time unit in which the transmission power can fluctuate, is adopted as the control period for updating the threshold Pth.
- the control period for updating the threshold Pth is adopted as the control period for updating the threshold Pth.
- the threshold Pth may be adopted as a control cycle for updating.
- FIG. 10 is an entire configuration diagram of a wireless communication system according to a third embodiment of the present invention.
- an RRH (Remote Radio Head) 36 is connected to the base station device 1 via a CPR (Common Public Radio Interface), and the RRH 36
- a signal processing circuit 9 according to the third embodiment shown in FIG. 11 and the power amplification circuit 5 are provided.
- the base station apparatus 1 transmits a synchronization signal 38 for establishing synchronization with the RRH 36 to the RRH 36 through a fiber, and this synchronization signal 38 is synchronized with the symbol period of OFDM.
- the clock signal has a cycle of 1 ms.
- a cycle generation unit 37 to which the synchronization signal 38 is input is provided in the signal processing circuit 9 of the present embodiment.
- the period generation unit 37 generates a symbol period from the synchronization signal 38 acquired from the base station device 1 which is an external device, and outputs the generated symbol period to the pulse generation unit 16 and the average calculation unit 33. Since the other configuration is the same as that of the signal processing circuit 9 of the second embodiment (FIG. 8), the same reference numerals as in FIG. 8 are added to FIG. 11 and the detailed description will be omitted. As described above, in the present embodiment, since the synchronization signal 38 synchronized with the symbol period is acquired from the base station device 1 and the symbol period is generated based on the synchronization signal 38, the signal processing circuit 9 of the present invention is also applied to the RRH 36. Can be mounted.
- the signal processing unit 17 sets a clipping threshold (first threshold) Pth that defines the upper limit of the instantaneous power P.
- the “clipping process” is performed to suppress the IQ baseband signal larger than the above to the instantaneous power P corresponding to the threshold value Pth, but contrary to this processing, the instantaneous power P has a predetermined second threshold value Pth ′ ( ⁇ Pth A process (hereinafter, referred to as a "boosting process”) may be performed to raise the IQ baseband signal smaller than Q) to an instantaneous power P corresponding to the threshold value Pth '.
- the signal processing unit 17 that performs the “boosting process” reverses the operations of the difference rate calculating unit 20 and the comparing unit 21 to those in the clipping process (FIG. 3). It can be implemented with the same circuit configuration as FIG. 8 and FIG. Hereinafter, the operation of the signal processing unit 17 which performs the “boosting process” will be described by taking FIG. 3 as an example.
- the difference rate calculating unit 20 uses the instantaneous power P calculated by the power calculating unit 13 and the second threshold Pth ′ for boosting set in advance to obtain the instantaneous power. Decrement rate ⁇ SQRT (Pth '/ P) -1 ⁇ to second threshold Pth' of P is calculated, and this decrement rate ⁇ SQRT (Pth '/ P) -1 ⁇ is calculated by IQ via a multiplier. The components (I, Q) of the baseband signal are multiplied.
- the comparison unit 21 compares the instantaneous power P calculated by the power calculation unit 13 with the second threshold Pth ′, and outputs the cancellation pulse S when the instantaneous power P is smaller than the second threshold Pth ′.
- a command is issued to the pulse holding unit 22.
- the pulse holding unit 22 multiplies the decrements ⁇ I 'and ⁇ Q' by the held offset pulse S to cancel the cancellation signals Ic 'and Qc' respectively. calculate.
- the delay units 18 and 19 preceding the adders / subtractors 23 and 24 delay the IQ baseband signal by the time of the arithmetic processing in the power calculation unit 13 and the signal processing unit 17.
- the adders / subtractors 23 and 24 respectively add cancellation signals Ic 'and Qc' to the respective components I and Q of the delayed IQ signal, and output Iout and Qout which are IQ signals after signal processing.
- the IQ baseband signal whose instantaneous power P falls below the second threshold Pth ′ is corrected to a signal of instantaneous power corresponding to the second threshold Pth ′.
- the IQ baseband signal whose instantaneous power P is equal to or higher than the threshold Pth ′ is output without being corrected.
- the cancellation signals Ic ′ and Qc ′ obtained by multiplying the subtractions ⁇ S ′ and ⁇ Q ′ by the cancellation pulse S are added to the original IQ baseband signal, the first and second bands B1 and B2 The amplitude of the IQ baseband signal is canceled corresponding to the average power. Therefore, also in the signal processing circuit 9 performing the boosting process, the power in the first and second bands B1 and B2 does not increase more than necessary due to the addition of the cancellation signals Ic 'and Qc'. Even in the case of an IQ baseband signal having different average power for each B2, boosting processing can be appropriately performed without deteriorating the SNR.
- FIG. 12 is a coordinate diagram of an IQ plane showing the relationship between the IQ baseband signal and the second threshold Pth ′ when the boosting process is performed.
- the boosting process by the signal processing circuit 9 of this embodiment is different from the conventional clipping process in which the outer peripheral side of the instantaneous power P of the IQ baseband signal is cut. It is a process that cuts the inside and cuts it out.
- the PAPR of the modulation wave signal input to the power amplification circuit 5 is reduced, so that the power efficiency of the power amplifier is improved.
- the signal processing unit 17 may perform boosting processing.
- the threshold update unit 34 adopts a value obtained by multiplying the average power Pave for each symbol period acquired from the average calculation unit 33 by a predetermined magnification as a threshold Pth ′ for boosting in that symbol period.
- the predetermined magnification is 1/2.
- the threshold update unit 34 calculates the threshold Pth ′ for boosting for each symbol period as described above, dynamically updates the threshold Pth ′, and the updated threshold Pth ′ is used as the difference ratio calculation unit 20. Output to the comparison unit 21. Then, the comparison unit 21 determines the magnitude of the instantaneous power P calculated by the power calculation unit P using the threshold Pth ′ acquired from the threshold update unit 34, and the instantaneous power P is smaller than the updated threshold Pth ′. In this case, issue the output command.
- FIG. 13 is a graph showing temporal changes in the instantaneous power P of the IQ baseband signal and the second threshold Pth ′ which is sequentially updated.
- the threshold Pth ′ used for boosting processing in the signal processing circuit 9 is sequentially calculated based on the average power Pave calculated for each symbol period (1/14 ms), and the symbol It is updated every cycle.
- the signal processing circuit 9 of the third embodiment performs the boosting process, the same operation and effect as those of the clipping process can be achieved. That is, for example, even if the average power Pave of the IQ baseband signal fluctuates in response to the fluctuation of the traffic volume by the mobile terminal 2, the boosting process by the signal processing circuit 9 is always performed. The improvement of the power efficiency of the power amplifier due to the reduction can be effectively secured.
- a signal processing unit that performs the above-mentioned "boosting process” also in the signal processing circuit 9 of the third embodiment (FIG. 11) having a period generation unit 37 that generates a symbol period by the synchronization signal 38 from the base station device 1. 17 can be adopted, and in this case, it can be mounted on the RRH 36 as in the case of the third embodiment.
- the signal processing unit 17 performs both clipping processing and boosting processing on the IQ baseband signal. You may do so.
- the difference ratio calculation unit 20 uses two thresholds, ie, the first threshold Pth for clipping processing and the second threshold Pth ′ for boosting processing, to calculate instantaneous values for the respective thresholds Pth and Pth ′. Both an increment rate ⁇ 1-SQRT (Pth '/ P) ⁇ and a decrement rate ⁇ SQRT (Pth' / P) -1 ⁇ of power P are calculated.
- the difference rate calculation unit 20 multiplies each of the above rates by each component (I, Q) of the IQ baseband signal through a multiplier to obtain the increments ⁇ I, ⁇ Q and the decrements ⁇ I ′, Calculate ⁇ Q ′. Then, the comparing unit 21 compares the instantaneous power P calculated by the power calculating unit 13 with the first and second thresholds Pth and Pth ′, and the case where the instantaneous power P is larger than the first threshold Pth and When it is smaller than the second threshold Pth ′, an output command of the canceling pulse S is issued to the pulse holding unit 22.
- the pulse holding unit 22 When the pulse holding unit 22 receives the above output command from the comparison unit 21, the pulse holding unit 22 multiplies the above-mentioned increment .DELTA.I, .DELTA.Q or the decrement .DELTA.I ', .DELTA.Q' by holding the offsetting pulse S held, for clipping. A cancellation signal Ic, Qc or a cancellation signal Ic ', Qc' for boosting is calculated.
- the pulse holding unit 22 does not receive a command from the comparison unit 21, the pulse holding unit 22 multiplies the increment ⁇ I, ⁇ Q or the decrement ⁇ I ′, ⁇ Q ′ by zero.
- the cancellation signals Ic and Qc calculated by the following equations are input to the adders / subtractors 23 and 24 and the instantaneous power P is With regard to the IQ baseband signal that is below the threshold value Pth ′ of 2, the cancellation signals Ic ′ and Qc ′ calculated by the following equations are input to the adders / subtractors 23 and 24.
- the adders / subtractors 23 and 24 respectively subtract the offset signals Ic and Qc from the components I and Q of the delayed IQ signal, or add the offset signals Ic 'and Qc', respectively, to obtain a signal. It outputs Iout and Qout which are IQ signals after processing.
- the IQ baseband signal whose instantaneous power P exceeds the first threshold Pth is corrected to a signal of instantaneous power corresponding to the first threshold Pth, and the instantaneous power P is the second threshold Pth ′.
- the IQ baseband signal below is corrected to a signal of instantaneous power corresponding to the second threshold Pth ′.
- FIG. 14 is a coordinate diagram of an IQ plane showing the relationship between the IQ baseband signal and the first and second threshold values Pth and Pth ′ when both clipping and boosting processing are performed.
- the outer peripheral side of the instantaneous power P of the IQ baseband signal is cut, and the inside of the instantaneous power P is cut out.
- the PAPR of the modulation wave signal input to the power amplification circuit 5 can be further reduced compared to the case where only one of the processes is performed.
- FIG. 15 is a graph in the time domain showing variations of the basic pulses S1 and S2.
- (a) shows a Sinc waveform
- (b) a Chebyshev waveform
- (c) a Taylor waveform.
- the section including the maximum absolute value and the amplitude value becomes zero (section shown by hatching in FIG. 15) is the main lobe section
- the side lobe amplitudes are compared And the energy localization rate of the main lobe section can not be improved significantly.
- the values of the first few points (for example, a1 and a2) of the series an are used for the Chebyshev waveform, and the values for the subsequent points are used for the Sinc waveform. Both sidelobe amplitude suppression and attenuation characteristics are achieved. Therefore, the energy localization ratio of the main lobe section with respect to the total energy (square of the amplitude) in the predetermined time section T defining the basic pulses S1 and S2 is 91% in the case of the Sinc waveform, and the Chebyshev In the case of the waveform, it is 93%, and in the case of the Taylor waveform, it is about 95%, and the Taylor waveform is most advantageous.
- the energy localization rate is preferably 85% to 99%.
- the reason is that when the energy localization rate reaches 100%, the basic pulses S1 and S2 become impulses (delta function) and can not be applied to the present invention with band limitation, and the localization rate is less than 85%.
- the pulse shape is too blunt to be used.
- the basic pulses S1 and S2 have a waveform with an energy localization ratio of 85% to 99% of the main lobe section with respect to the total energy (square of amplitude) in a predetermined time section T (for example, one symbol period) It can be configured.
- Feature 2 When the basic pulses S1 and S2 are described mathematically, they consist of the waveform represented by the above-mentioned equation (1) having symmetry in the time domain.
- Feature 3 More specifically, the basic pulses S1 and S2 consist of a Sinc waveform, a Chebyshev waveform or a Taylor waveform.
- the Sinc waveform is a waveform of a real part (I signal) obtained by performing inverse Fourier transform on a plurality of carrier waves in a band with the amplitude being the same and the phase being zero.
- the signal processing circuit 9 can be adopted not only to the LTE system but also to a communication apparatus based on the W-CDMA system.
- the signal processing circuit 9 of the present invention when used for a transmitter of the W-CDMA system, closed loop transmission power is used as a control period for calculating the relative ratios C1 and C2 and updating the threshold values Pth and Pth '.
- the control cycle of control may be adopted.
- the signal processing circuit 9 that performs the clipping process based on PC-CFR is illustrated, but the present invention can be applied to the signal processing circuit 9 that performs the clipping process based on NS-CFR.
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Abstract
Description
その一方で、電力増幅器(パワーアンプ)には優れた線形性が要求されるが、最大出力を超えるレベルの信号が入力されると、出力が飽和して非線形歪みが増大する。
ピーク電力に対して非線形歪みを増大させないためには、ダイナミックレンジの広い電力増幅器が必要となるが、頻繁には出現しないピーク電力のために増幅器のダイナミックレンジを広げると、時間軸上の波形の平均電力と短時間のピーク電力との比(PAPR:Peak to Average Power Ratio)が大きくなり、電力効率が悪くなる。
かかるクリッピング処理は、時間軸上でインパルス状の信号を逆向きに印加する処理であるから、周波数軸上では、広い周波数帯域のノイズが印加されるのと同じこととなる。そのため、クリッピング処理のみを単純に行った場合には、帯域外にノイズを生じさせるという問題がある。
このうち、NS-CFR回路は、瞬時電力が閾値を超えるIQベースバンド信号のピーク成分(閾値からの増分)に対して、ローパスフィルタやFIR(Finite Impulse Response )フィルタ等でフィルタリングを行って帯域制限し、この帯域制限後のピーク成分を元のIQベースバンド信号から減算するものである(特許文献1参照)。
この場合、ピーク電力抑制回路に入力されるIQベースバンド信号の平均電力も、周波数帯域ごとに変化することになる。
このため、従来のピーク電力抑制回路では、平均電力が比較的低い周波数帯域については、相殺信号の減算によって必要以上にSNR(Signal to Noise Ratio )が低下して通信不能になるおそれがあった。
このため、周波数帯域ごとに平均電力が異なるIQベースバンド信号の場合でも、SNRを悪化させずに適切に振幅を制限することができる。
(5) 本発明の信号処理回路において、上記閾値からの増分又は減分に相殺用パルスを乗算した相殺信号を用いる場合には、前記周波数帯域ごとに求められた基本パルスにその周波数帯域ごとの平均電力の相対比率をそれぞれ乗算して総和をとることにより、前記相殺用パルスを生成するパルス生成部を、更に備えている必要がある。
この場合、相対比率が変動しない限り乗加算器が乗算及び総和を実行せず、パルス保持部が従前の相殺用パルスを維持する。このため、相殺用パルスを愚直に毎回生成する場合に比べて、回路の演算負荷を低減することができる。
その一方で、デジタル信号処理では演算処理が2進数で実行されるので、2のべき乗で除算するようにすれば、小数点の位置が変化するだけで実質的には除算の必要がない。
この場合、各累積値の小数点の位置を変化させるだけで相対比率を算出できるので、回路規模を増大させなくても、相対比率を正確かつ迅速に算出することができる。
その理由は、上記制御周期内で相対比率の算出を実行すれば、IQベースバンド信号の平均電力が余り変動しない安定状態で相対比率を算出することができ、正確な相対比率が得られるからである。
しかし、従来のピーク電力抑制回路では、IQベースバンド信号の瞬時電力のピークを検出する閾値が固定値であるため、最大通話量に対応して送信電力が大きい時間帯でしか動作しない。このため、従来のピーク電力抑制回路では、通話量が比較的少ないために、IQベースバンド信号の平均電力が小さい時間帯では動作せず、パワーアンプの電力効率を向上できないことがあった。
この場合、上記閾値更新部が、信号処理部で用いる閾値を上記制御周期ごとに更新するので、例えば、IQベースバンド信号の平均電力が比較的小さい時間帯でも、瞬時電力の抑制を確実に行うことができる。
その理由は、LTEでは、OFDMのシンボル周期は送信電力が大きく変動し得る最小の時間単位だからであり、W-CDMAでは、クローズドループ送信電力制御の制御周期が、送信電力が大きく変動し得る最小の時間単位だからである。
〔第1実施形態〕
〔無線通信システム〕
図1は、本発明を好適に適用可能な、第1実施形態に係る無線通信システムの全体構成図である。
図1に示すように、本実施形態の無線通信システムは、基地局装置(BS:Base Station)1と、この装置1のセル内で当該装置1と無線通信を行う複数の移動端末(MS:Mobile Station)2とから構成されている。
かかるLTE方式に基づく基地局装置1では、例えば5MHz単位でダウンリンクフレームの周波数帯域を設定可能であり、セル内の各移動端末2に下り信号を送信する場合において、その周波数帯域ごとに送信電力を変更可能になっている。
このため、図1に破線で示すように、送信電力が大きい第1帯域B1の下り信号が届く通信エリアA1は、送信電力が小さい第2帯域B2の下り信号が届く通信エリアA2よりも遠方でかつ広範囲になっている。
なお、本発明を適用可能な無線通信システムはLTE方式に限られるものではなく、W-CDMA方式であってもよいが、以下では、本発明をLTE方式の基地局装置1に適用した場合を想定して説明を進める。
図7は、LTEのダウンリンクフレームの構造を示す図である。図中、縦軸方向は周波数を示しており、横軸方向は時間を示している。
図7に示すように、ダウンリンク(DL)フレームを構成する合計10個のサブフレーム(subframe♯0~♯9)は、それぞれ2つのスロット(slot♯0とslot♯1)により構成されており、1つのスロットは7個のOFDMシンボルにより構成されている(Normal Cyclic Prefixの場合)。
従って、例えば、DLフレームの周波数帯域幅が5MHzに設定されている場合には、300個のサブキャリアが配列されるので、リソースブロックは、周波数軸方向に25個配置される。
この制御チャネルには、DL制御情報や、当該サブフレームのリソース割当情報、ハイブリッド自動再送要求(HARQ:Hybrid Automatic Report Request)による受信成功通知(ACK:Acknowledgement)、受信失敗通知(NACK:Negative Acknowledgement)等が格納される。
上記PDSCHに格納されるユーザデータの割り当てについては、各サブフレームの先頭に割り当てられている上記制御チャネル内のリソース割当情報で規定されており、移動端末2は、このリソース割当情報により、自己に対するデータがサブフレーム内に格納されているか否かを判断できる。
図2は、基地局装置1のOFDM送信機3の要部を示す機能ブロック図である。
この送信機3は、送信用プロセッサ4と電力増幅回路5とを備えており、送信用プロセッサ4は、例えば、1又は複数のメモリやCPUを内部に有するFPGA(Field Programmable Gate Array )により構成されている。
すなわち、本実施形態の送信用プロセッサ4は、左から順に、S/P変換部6、マッピング部7、IFFT(Inverse Fast Fourier Transform:逆高速フーリエ変換)部8、信号処理部9及び直交変調部10を含んでいる。
この各サブキャリア信号f1,f2,……fnは、IFFT部8によって時間軸上で互いに直交するベースバンド信号としてのI信号及びQ信号に変換される。
この第1信号I1,Q1と第2信号I2,Q2は、後段の信号処理部(本実施形態の信号処理回路)9に入力され、この処理部9において所定の信号処理が施される。
なお、本実施形態の信号処理回路9は、第1信号I1,Q1と第2信号I2,Q2の時間軸上の合成信号であるIQベースバンド信号の瞬時電力Pが、所定の閾値Pthよりも大きくならないように、当該IQベースバンド信号をクリッピング処理するものであるが、その詳細については後述する。
本実施形態の電力増幅回路5としては、パワーアンプのドレイン電圧が一定である固定電圧方式であってもよいが、高周波増幅器の高効率化を図る観点からは、ET(Envelope Tracking)方式を採用することが好ましい。
図3は、本発明の第1実施形態に係る信号処理回路9の機能ブロック図である。
なお、以下において、第1信号I1,Q1と第2信号I2,Q2の合成信号を単に「IQベースバンド信号」或いは「IQ信号」というものとする。
また、第1信号I1,Q1の瞬時電力をP1とし、第2信号I2,Q2の瞬時電力をP2とし、IQベースバンド信号の瞬時電力をP(=P1+P2)とする。
このうち、電力算出部13は、IQベースバンド信号のI成分(=I1+I2)とQ成分(=Q1+Q2)の2乗和よりなる瞬時電力Pを算出する。
具体的には、この信号処理部17は、差分率算出部20、比較部21、パルス保持部22及び加減算器23,24を含む。
従って、IQベースバンド信号の閾値Pthを超えた分の増分ΔI,ΔQが、次式に基づいて算出される。なお、この場合、SQRT(・)は、括弧内の変数の平方根を取る関数である(以下、同様)。
ΔI={1-SQRT(Pth/P)}×I
ΔQ={1-SQRT(Pth/P)}×Q
パルス保持部22は、後述するパルス生成部16が出力する相殺用パルスSを一時的に保持する、デュアルポートRAM等よりなるメモリを有しており、比較部21から指令を受けた場合は、その時点で保持している相殺用パルスSを上記増分ΔI,ΔQに乗算して相殺信号Ic,Qcを算出する。
従って、瞬時電力Pが閾値Pthを超えているIQベースバンド信号については、次の式に基づいて算出された相殺信号Ic,Qcが加減算器23,24に入力される。
Ic=ΔI×S={1-SQRT(Pth/P)}×I×S
Qc=ΔQ×S={1-SQRT(Pth/P)}×Q×S
この減算により、瞬時電力Pが閾値Pthを超えるIQベースバンド信号については、閾値Pth相当の瞬時電力の信号に補正される。また、瞬時電力Pが閾値Pth以下のIQベースバンド信号については、補正されずにそのまま出力される。
この図6に示すように、本実施形態の信号処理回路9による信号処理は、IQベースバンド信号の瞬時電力Pの外周側をカットするクリッピング処理である。このため、電力増幅回路5のパワーアンプに対するPAPRが低下するので、パワーアンプの電力効率が向上する。
図4は、パルス生成部16の機能ブロック図である。
このパルス生成部16は、第1及び第2帯域B1,B2ごとに予め求められた基本パルスS1,S2に、その帯域B1,B2ごとの平均電力の相対比率C1,C2をそれぞれ乗算して総和をとることにより、前記相殺用パルスSを生成するものであり、比率算出部26、波形記憶部27及び乗加算部28を有している。
この基本パルスS1,S2は、特許文献3(特開2004-135078号公報)の場合と同様に、下り信号の送信に使用する周波数帯域B1,B2に含まれる複数本(例えば、N本とする。)の搬送波を、振幅を1/Nにしかつ位相を0にして、前記IFFT部8に入力して得られたSinc波形よりなるものである。この場合、IFFT部8の出力には実部Iだけが出現し、虚部Qはゼロになる。
従って、基本パルスS1,S2の周波数帯域は第1及び第2帯域B1,B2と一致しており、閾値Pthを超えるIQ信号の増分に、基本パルスS1,S2を乗算した相殺信号を用いてIQ信号をクリッピングしても、帯域B1,B2外の不要な周波数成分は発生しない。
C1=Σ√P1/(Σ√P1+Σ√P2)
C2=Σ√P2/(Σ√P1+Σ√P2)
上記累積値の除算を含む演算をリアルタイムで正確に処理するためには、有効桁数を大きく取る必要があり回路規模が大きくなるが、累積値の総和(Σ√P1+Σ√P2)が2のべき乗の場合には、小数点の位置が変化するだけで実質的に除算の必要がない。
図5は、比率算出部26が実行する上記演算ロジックを示すフローチャートである。
図5に示すように、比率算出部26は、まず、累積値Sum1,Sum2をそれぞれゼロに初期化する(ステップST1)。
また、比率算出部26は、上記累積値Sum1,Sum2の総和Tが2のべき乗±δ(δは十分に小さい所定値)の範囲になったか否かを判定し(ステップST3)、この判定結果が否定的である場合にはステップST2に戻って累積を繰り返す。
この場合、各累積値Sum1,Sum2の小数点の位置を変化させるだけで相対比率C1,C2を算出できるので、回路規模を増大させなくても、相対比率C1,C2を正確かつ迅速に算出することができる。
Sum2= 681(2進数表示で「1010101001」)
T=1024(2進数表示で「1000000000」)となる。
この場合、これを用いて相対比率C1,C2を算出すると、
C1=343/1024=0.0101010111
C2=681/1024=0.1010101001
C1+C2 =1.0000000000
となり、各累積値Sum1,Sum2の小数点以下における0と1の順序は変化しないので、その小数点の位置を変化させるだけで相対比率C1,C2を算出することができる。
このようにすれば、IQベースバンド信号の平均電力が余り変動しない安定状態で相対比率C1,C2を算出できるので、正確な相対比率C1,C2が得られるという効果がある。
また、加算器31は、各乗算器29,30の乗算結果を加算して相殺用パルスSを生成し、このパルスSを信号処理部17のパルス保持部22に出力する。すなわち、乗加算器16は、次の式に基づいて相殺用パルスSを生成する。
S=C1×S1+C2×S2
このため、相対比率C1,C2がある程度変動しない限り、乗加算器28が乗算及び総和を実行せず、パルス保持部22が従前の相殺用パルスSを維持する。従って、相殺用パルスSを愚直に毎回生成する場合に比べて、回路の演算負荷を低減することができる。
上記相殺用パルスSは、第1帯域B1に対応する第1信号I1,Q1の平均電力の相対比率C1に、その帯域B1用の基本パルスS1を乗算したものと、第2帯域B2に対応する第2信号I2,Q2の平均電力の相対比率C2に、その帯域B2用の基本パルスS2を乗算したものとを、加算した合成パルスになっている。
このため、上記相殺用パルスSを増分ΔI,ΔQに乗算した相殺信号Ic,Qcを元のIQベースバンド信号から減算しても、第1及び第2帯域B1,B2ごとの平均電力に対応してIQベースバンド信号の振幅が相殺されることになる。
図8は、第2実施形態に係る信号処理回路9の機能ブロック図である。
図8に示すように、本実施形態の信号処理回路9(図8)が第1実施形態の信号処理回路9(図3)と異なる点は、更に、平均算出部33と閾値更新部34を備えている点にある。
以下、第1実施形態と共通する構成及び機能は図面に同一符号を付して説明を省略し、第1実施形態との相違点について重点的に説明する。
すなわち、平均算出部33は、電力算出部13からIQベースバンド信号の瞬時電力Pを取得しており、その瞬時電力Pを上記シンボル周期内で平均化することにより、シンボル周期ごとのIQベースバンド信号の平均電力Pave を算出し、これを閾値更新部34に出力する。
閾値更新部34は、上記のようにしてシンボル周期ごとに閾値Pthを算出して当該閾値Pthを動的に更新し、その更新した閾値Pthを、差分率算出部20と比較部21に出力する。
図9に示すように、本実施形態では、信号処理回路9におけるクリッピング処理に用いる閾値Pthが、シンボル周期(1/14ms)ごとに算出した平均電力Pave に基づいて逐次算出され、そのシンボル周期ごとに更新される。
もっとも、第1実施形態の場合と同様に、LTEでは、リソースブロック(図7参照)がユーザ割当の最小単位になっているので、このリソースブロックの送信周期である7OFDMシンボル(1スロット)を、閾値Pthを更新する制御周期として採用することにしてもよい。
図10は、本発明の第3実施形態に係る無線通信システムの全体構成図である。
図10に示すように、本実施形態の無線通信システムでは、基地局装置1に、CPRI(Common Public Radio Interface)を介してRRH(Remote Radio Head)36が接続されており、このRRH36には、図11に示す第3実施形態に係る信号処理回路9と前記電力増幅回路5とが設けられている。
図11に示すように、本実施形態の信号処理回路9では、上記同期信号38が入力される周期生成部37が設けられている。
上記の通り、本実施形態では、シンボル周期と同期する同期信号38を基地局装置1から取得し、その同期信号38に基づいてシンボル周期を生成するので、RRH36にも本発明の信号処理回路9を搭載することができる。
前記第1~第3実施形態の信号処理回路9(図3、図8及び図11)では、信号処理部17が、瞬時電力Pの上限を規定するクリッピング用の閾値(第1の閾値)Pthよりも大きいIQベースバンド信号を、当該閾値Pth相当の瞬時電力Pに抑制する「クリッピング処理」を行うが、この処理とは逆に、瞬時電力Pが所定の第2の閾値Pth’(<Pth)よりも小さいIQベースバンド信号を、当該閾値Pth’相当の瞬時電力Pに底上げする処理(以下、これを「ブースティング処理」という。)を行うものであってもよい。
以下、図3を例にとって、上記「ブースティング処理」を行う信号処理部17の動作について説明する。
ΔI’={SQRT(Pth’/P)-1}×I
ΔQ’={SQRT(Pth’/P)-1}×Q
パルス保持部22は、比較部21から上記出力指令を受けた場合には、保持している相殺用パルスSを上記減分ΔI’, ΔQ’に乗算して相殺信号Ic’,Qc’をそれぞれ算出する。
従って、瞬時電力Pが第2の閾値Pth’を下回っているIQベースバンド信号については、次の式に基づいて算出された相殺信号Ic’,Qc’が加減算器23,24に入力される。
Ic’=ΔI’×S={SQRT(Pth’/P)-1}×I×S
Qc’=ΔQ’×S={SQRT(Pth’/P)-1}×Q×S
この加算により、瞬時電力Pが第2の閾値Pth’を下回るIQベースバンド信号については、第2の閾値Pth’相当の瞬時電力の信号に補正される。また、瞬時電力Pが閾値Pth’以上のIQベースバンド信号については、補正されずにそのまま出力される。
従って、ブースティング処理を行う信号処理回路9の場合も、第1及び第2帯域B1,B2における電力が相殺信号Ic’,Qc’の加算によって必要以上に増大することがなく、周波数帯域B1,B2ごとに平均電力が異なるIQベースバンド信号の場合でも、SNRを悪化させずに適切にブースティング処理することができる。
この図12に示すように、本実施形態の信号処理回路9によるブースティング処理は、IQベースバンド信号の瞬時電力Pの外周側をカットする従来のクリッピング処理とは逆に、その瞬時電力Pの内側をカットしてくり抜くような処理となる。このように瞬時電力Pを底上げする「ブースティング処理」の場合も、電力増幅回路5に入力する変調波信号のPAPRが低下するので、パワーアンプの電力効率が向上する。
この場合には、閾値更新部34は、平均算出部33から取得したシンボル周期ごとの平均電力Pave に所定の倍率を乗算した値を、そのシンボル周期におけるブースティング用の閾値Pth’として採用する。例えば、IQベースバンド信号の平均電力Pave と谷間電力(逆ピーク電力)Pvalleyの比率を6dBに絞る場合には、上記所定の倍率は1/2倍となる。
そして、比較部21は、閾値更新部34から取得した閾値Pth’を用いて、電力算出部Pが算出した瞬時電力Pの大小を判定し、瞬時電力Pが更新後の閾値Pth’よりも小さい場合に前記出力指令を発する。
図13に示すように、この場合には、信号処理回路9におけるブースティング処理に用いる閾値Pth’が、シンボル周期(1/14ms)ごとに算出した平均電力Pave に基づいて逐次算出され、そのシンボル周期ごとに更新される。
すなわち、例えば、移動端末2による通話量の変動に対応して、IQベースバンド信号の平均電力Pave が変動しても、信号処理回路9によるブースティング処理が常に行われることになるので、PAPRの低減よるパワーアンプの電力効率の向上を、有効に確保することができる。
更に、前記第1~第3実施形態の信号処理回路9(図3、図8及び図11)において、信号処理部17が、IQベースバンド信号に対してクリッピング処理とブースティング処理の双方を行うようにしてもよい。
この場合には、差分率算出部20が、クリッピング処理用の第1の閾値Pthと、ブースティング処理用の第2の閾値Pth’の2つの閾値を用いて、各閾値Pth,Pth’に対する瞬時電力Pの増分率{1-SQRT(Pth’/P)}と減分率{SQRT(Pth’/P)-1}の双方を算出する。
そして、比較部21が、電力算出部13で算出された瞬時電力Pと第1及び第2の閾値Pth,Pth’とを比較し、瞬時電力Pが第1の閾値Pthよりも大きい場合と、第2の閾値Pth’よりも小さい場合に、相殺用パルスSの出力指令をパルス保持部22に発する。
また、パルス保持部22は、比較部21から指令を受けてない場合は、上記増分ΔI,ΔQ又は減分ΔI’,ΔQ’にゼロを乗算する。
Ic=ΔI×S={1-SQRT(Pth/P)}×I×S
Qc=ΔQ×S={1-SQRT(Pth/P)}×Q×S
Ic’=ΔI’×S={SQRT(Pth’/P)-1}×I×S
Qc’=ΔQ’×S={SQRT(Pth’/P)-1}×Q×S
この減算又は加算により、瞬時電力Pが第1の閾値Pthを超えるIQベースバンド信号については、その第1の閾値Pth相当の瞬時電力の信号に補正され、瞬時電力Pが第2の閾値Pth’を下回るIQベースバンド信号については、その第2の閾値Pth’相当の瞬時電力の信号に補正される。
この図14に示すように、クリッピング処理とブースティング処理の双方を行う場合には、IQベースバンド信号の瞬時電力Pの外周側がカットされ、かつ、その瞬時電力Pの内側がくり抜かれるので、いずれか一方の処理のみを行う場合に比べて、電力増幅回路5に入力する変調波信号のPAPRを更に低下させることができる。
図15は、基本バルスS1,S2のバリエーションを示す時間領域のグラフであり、図15において、(a)はSinc波形、(b)はチェビシェフ波形、(c)はテーラー波形である。
これらの波形は、数学的には、すべて次の式(1)で表すことができ、Sinc波形の場合にはan=nπとなっている。
これに対して、チェビシェフ波形では、振幅値=0となるxの解を構成する数列anの値を調整することで、サイドローブの振幅を小さくできるが、この場合には振幅が減衰しなくなる。
従って、基本パルスS1,S2を定義する所定の時間区間Tにおける全エネルギー(振幅の2乗)に対する、メインローブ区間のエネルギー局在率を比較すると、Sinc波形の場合には91%であり、チェビシェフ波形の場合には93%であり、テーラー波形の場合には約95%になり、テーラー波形が最も有利となる。
その理由は、エネルギー局在率が100%になると、基本パルスS1,S2がインパルス(デルタ関数)となって、帯域制限がある本発明に適用できなくなり、局在率が85%未満の場合は、パルス形状が鈍化し過ぎて使用できなくなるからである。
特徴1:基本パルスS1,S2は、所定の時間区間T(例えば、1シンボル周期)における全エネルギー(振幅の2乗)に対するメインローブ区間のエネルギー局在率が、85%~99%の波形により構成できる。
特徴3:より具体的には、基本パルスS1,S2は、Sinc波形、チェビシェフ波形又はテーラー波形よりなる。このうち、Sinc波形は、帯域内の複数本の搬送波を、振幅が同一でかつ位相をゼロにして逆フーリエ変換して得られる実部(I信号)の波形よりなる。
今回開示した実施形態は例示であって制限的なものではない。本発明の権利範囲は特許請求の範囲によって示され、特許請求の範囲の構成と均等の範囲内での全ての変更が含まれる。
例えば、上記実施形態では、基地局装置1が2つの周波数帯域B1,B2を使用する場合を例示したが、3つ以上の周波数帯域を使用する場合でも本発明の信号処理回路9を構成することができる。
このW-CDMA方式では、クローズドループ送信電力制御によって基地局装置1の送信電力を制御するようになっており、この制御周期が送信制御の最小時間単位となっている。具体的には、この制御周期は、1無線フレーム周期10msの15分の1(=約0.667ms)である。
また、上記実施形態では、PC-CFRに基づくクリッピング処理を行う信号処理回路9を例示したが、NS-CFRに基づくクリッピング処理を行う信号処理回路9にも、本発明を適用することができる。
2 移動端末
3 送信機
4 送信用プロセッサ
5 電力増幅回路
9 信号処理部(信号処理回路)
13 電力算出部
14 電力算出部
15 電力算出部
16 パルス生成部
17 信号処理部
20 比較部
21 差分率算出部
22 パルス保持部
23,24 加減算器
26 比率算出部
27 波形記憶部
28 乗加算部
33 平均算出部
34 閾値更新部
B1 第1帯域
B2 第2帯域
Pth 第1の閾値(クリッピング用)
Pth’ 第2の閾値(ブースティング用)
ΔI 増分
ΔQ 増分
Ic 相殺信号
Qc 相殺信号
ΔI’ 減分
ΔQ’ 減分
Ic’ 相殺信号
Qc’ 相殺信号
S 相殺用パルス
S1 基本パルス
S2 基本パルス
C1 相対比率
C2 相対比率
Claims (11)
- 電力増幅回路に入力する変調波信号のピーク電力対平均電力比(Peak-to-Average Power Ratio:以下、特許請求の範囲において、「PAPR」という。)を低減するための信号処理回路であって、
前記変調波信号のIQベースバンド信号の瞬時電力を算出する電力算出部と、
前記IQベースバンド信号をその周波数帯域ごとの平均電力に対応して相殺可能な相殺信号を用いて、前記瞬時電力の上限又は下限若しくはこれらの双方が所定の閾値相当となるように前記IQベースバンド信号の振幅を制限する信号処理部と、
を備えていることを特徴とする信号処理回路。 - 前記信号処理部は、前記瞬時電力の上限を規定する第1の閾値からの前記IQベースバンド信号の増分に所定の相殺用パルスを乗算して得られる前記相殺信号を、元の前記IQベースバンド信号から減算して、当該IQベースバンド信号を前記第1の閾値相当の瞬時電力に抑制するクリッピング処理を行う請求項1に記載の信号処理回路。
- 前記信号処理部は、前記瞬時電力の下限を規定する第2の閾値からの前記IQベースバンド信号の減分に所定の相殺用パルスを乗算して得られる前記相殺信号を、元の前記IQベースバンド信号に加算して、当該IQベースバンド信号を前記第2の閾値相当の瞬時電力に底上げするブースティング処理を行う請求項1に記載の信号処理回路。
- 前記信号処理部は、前記クリッピング処理と前記ブースティング処理の双方を実行可能である請求項2又は3に記載の信号処理回路。
- 前記周波数帯域ごとに求められた基本パルスにその周波数帯域ごとの平均電力の相対比率をそれぞれ乗算して総和をとることにより、前記相殺用パルスを生成するパルス生成部を、更に備えている請求項1~4のいずれか1項に記載の信号処理回路。
- 前記パルス生成部は、前記周波数帯域ごとの前記相対比率をそれぞれ算出する比率算出部と、前記周波数帯域ごとの前記基本パルスを記憶する波形記憶部と、算出された前記相対比率を対応する前記基本パルスにそれぞれ乗算して総和をとる乗加算部と、を有する請求項5に記載の信号処理回路。
- 前記信号処理部は、前記相殺用パルスを保持するパルス保持部を有しており、
前記乗加算部は、算出された前記相対比率が変動した場合にのみその変動後の当該相対比率を用いた乗算及び総和を実行し、その結果生成された前記相殺用パルスを前記パルス保持部に出力する請求項6に記載の信号処理回路。 - 前記比率算出部は、前記周波数帯域ごとの瞬時電力の平方根を累積し、その累積値の総和が2のべき乗±δ(δは十分に小さい所定値)になった場合に、当該2のべき乗で前記累積値を除算することにより、前記相対比率を算出する請求項6又は7に記載の信号処理回路。
- 前記比率算出部は、前記IQベースバンド信号の平均電力が時間的に変動する可能性のある制御周期内に、前記相対比率の算出を実行する請求項6~8のいずれか1項に記載の信号処理回路。
- 前記信号処理部で用いる前記閾値を前記IQベースバンド信号の平均電力が時間的に変動する可能性のある制御周期ごとに更新する閾値更新部を、更に備えている請求項1~9のいずれか1項に記載の信号処理回路。
- 請求項1~10のいずれか1項に記載の前記信号処理回路と、その後段に配置された電力増幅回路とが搭載された送信機を有することを特徴とする通信装置。
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Also Published As
Publication number | Publication date |
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EP2541817A4 (en) | 2017-07-12 |
EP2541817A1 (en) | 2013-01-02 |
KR20130009948A (ko) | 2013-01-24 |
US8787495B2 (en) | 2014-07-22 |
JP2011176577A (ja) | 2011-09-08 |
TWI511490B (zh) | 2015-12-01 |
US20120321014A1 (en) | 2012-12-20 |
JP5201158B2 (ja) | 2013-06-05 |
KR101643030B1 (ko) | 2016-07-26 |
CN102783060A (zh) | 2012-11-14 |
CN102783060B (zh) | 2016-08-03 |
TW201210230A (en) | 2012-03-01 |
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