Wireless communication device and distortion compensation method

Abstract

To prevent deterioration of a distortion compensation performance while suppressing an increase of a power consumption.SOLUTION: A radio communication device includes: a plurality of antenna elements; a plurality of power amplifiers provided to each of the plurality of antenna elements; a processor that outputs a transmission signal to the plurality of power amplifiers; and a plurality of feedback paths for feeding back a feedback signal from the plurality of power amplifiers to the processor. The processor integrally corrects variation of an analog characteristic in each of the plurality of feedback paths, and executes processing for updating a distortion compensation coefficient for compensating a non-liner distortion generated in the plurality of power amplifiers by using the transmission signal and the feedback signal, obtained by the correction.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a wireless communication device and a distortion compensation method.

Background Art

[0002] In recent years, from the viewpoints of miniaturization of a transmission device, reduction of operation costs, environmental problems, etc., the need for a power amplifier that operates with high efficiency has been increasing. When operating a power amplifier with high efficiency, in a region where the input power is relatively large, non-linear distortion occurs without the input power being linearly amplified. Therefore, an amplification method that combines digital predistortion for previously imparting distortion having an inverse characteristic of the non-linear distortion generated in the power amplifier to a transmission signal may be used. The distortion previously imparted to the transmission signal in digital predistortion is also called a distortion compensation coefficient, and is read from a look-up table or calculated using a polynomial. Then, the distortion compensation coefficient is appropriately updated so as to sufficiently compensate for non-linear distortion that varies according to the environment such as temperature.

[0003] On the other hand, in order to reduce interference to destinations other than the destination of the transmission signal and improve the communication system capacity, beamforming for forming a directional beam may be performed. When beamforming is performed, a phase difference is set in the signals transmitted from a plurality of antenna elements constituting an array antenna. Each of the plurality of antenna elements is provided with, for example, a phase shifter to control the phase of the signal, and the signal whose phase is controlled is amplified by a power amplifier corresponding to each antenna element.

[0004] Thus, even when a power amplifier is provided for each of a plurality of antenna elements, it has been proposed to perform digital predistortion on the transmission signal collectively. That is, after compensating the transmission signal for distortion by a distortion compensation coefficient, the transmission signal is split and amplified by a power amplifier for each antenna element, and transmission from each antenna element is being considered.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2019-154024 [Patent Document 2] Japanese Patent Application Laid-Open No. 2019-220816 [Patent Document 3] Japanese Patent Application Laid-Open No. 2018-195955 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] By the way, when digital predistortion is collectively performed on a plurality of power amplifiers, the distortion compensation coefficient is updated using the feedback signals fed back from the respective power amplifiers. At this time, the feedback signals from the respective power amplifiers are fed back via different feedback paths corresponding to a plurality of antenna elements.

[0007] However, since each feedback path corresponding to a plurality of antenna elements has different analog characteristics, there is a problem that an error occurs in the feedback signal of each feedback path due to the variation in the analog characteristics, and the distortion compensation performance deteriorates.

[0008] Specifically, there are variations in analog characteristics such as gain, phase, delay, and frequency characteristics in the feedback paths from the respective power amplifiers, and errors occur in the feedback signals due to these analog characteristics. Then, since the distortion compensation coefficient is updated from this feedback signal and the transmission signal, the accuracy of the distortion compensation coefficient decreases, and the distortion compensation performance deteriorates.

[0009] In order to correct the variations in the analog characteristics of such feedback paths, for example, it is conceivable to individually estimate the gain, phase, delay, and frequency characteristics in each feedback path and correct the analog characteristics for each feedback path. However, when correcting the gain, phase, delay, and frequency characteristics for each feedback path, the number of multipliers for correction increases, resulting in an increase in power consumption. In particular, recent array antennas include a large number of antenna elements, so the number of feedback paths also increases, and it is not practical to operate the multipliers corresponding to each feedback path.

[0010] The disclosed technology has been made in view of such points, and an object thereof is to provide a wireless communication device and a distortion compensation method that can suppress an increase in power consumption and prevent a decrease in distortion compensation performance.

Means for Solving the Problem

[0011] The wireless communication device disclosed in the present application, in one aspect, includes a plurality of antenna elements, a plurality of power amplifiers provided in the plurality of antenna elements, a processor that outputs a transmission signal to the plurality of power amplifiers, and a plurality of feedback paths that feedback a feedback signal from the plurality of power amplifiers to the processor. The processor collectively corrects variations in the analog characteristics in the plurality of feedback paths, and executes a process of updating a distortion compensation coefficient for compensating for non-linear distortion generated in the plurality of power amplifiers using the corrected transmission signal and feedback signal.

Advantages of the Invention

[0012] According to one aspect of the wireless communication device and the distortion compensation method disclosed in the present application, there is an effect that an increase in power consumption can be suppressed while preventing a decrease in distortion compensation performance.

Brief Description of the Drawings

[0013]

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[0014] Hereinafter, embodiments of the wireless communication device and the distortion compensation method disclosed in the present application will be described in detail with reference to the drawings. Note that the present invention is not limited by this embodiment.

[0015] (Embodiment 1) FIG. 1 is a diagram showing an example of a communication system according to Embodiment 1. In the communication system shown in FIG. 1, a plurality of RUs (Radio Units) 100 are connected to a CU / DU (Central Unit / Distributed Unit) 10, and the RU 100 and the UE (User Equipment) 20 perform wireless communication. Note that the CU / DU 10 does not necessarily have to be configured as an integrated device, and the CU and the DU may be configured as separate devices.

[0016] The CU / DU 10 is a device that executes baseband processing on signals. For example, it encodes information to generate a transmission baseband signal and transmits it to the RU 100, or decodes the received baseband signal received from the RU 100.

[0017] The RU 100 is wired-connected to the CU / DU 10, performs wireless transmission processing on the transmission baseband signal generated by the CU / DU 10, or performs wireless reception processing on the received signal from the UE 20 to generate a received baseband signal and transmits it to the CU / DU 10. Further, the RU 100 is a wireless communication device having a plurality of antenna elements. When performing wireless communication with the UE 20, antenna weights are applied to each of the plurality of antenna elements to perform beamforming. Furthermore, the RU 100 executes digital predistortion that compensates for non-linear distortion generated in the power amplifiers provided for each antenna element. In digital predistortion, a distortion compensation coefficient is multiplied by the transmission signal, and the update of the distortion compensation coefficient is executed after correcting the variations in the analog characteristics of the feedback path through which the feedback signals from each antenna element pass. The configuration and operation of the RU 100 will be described in detail later.

[0018] UE20 is a user terminal device such as a mobile phone or a smartphone, and performs wireless communication with RU100.

[0019] FIG. 2 is a block diagram showing the configuration of RU100 according to Embodiment 1. RU100 shown in FIG. 2 includes a communication interface unit (hereinafter abbreviated as "communication I / F unit") 110, a processor 120, a memory 130, a D / A (Digital / Analog) conversion unit 140, a phase shifter 150, a power amplifier 160, a phase shifter 170, a combining unit 180, and an A / D (Analog / Digital) conversion unit 190. In FIG. 2, the processing units related to the process of transmitting signals to UE20 are illustrated, and the illustration of the processing units related to the process of receiving signals from UE20 is omitted.

[0020] The communication I / F unit 110 is an interface that is wired-connected to CU / DU10, and transmits and receives baseband signals to and from CU / DU10. Specifically, the communication I / F unit 110 receives the transmitted baseband signal transmitted from CU / DU10 and transmits the received baseband signal to CU / DU10.

[0021] The processor 120 includes, for example, a CPU (Central Processing Unit), an FPGA (Field Programmable Gate Array), or a DSP (Digital Signal Processor), and overall controls RU100. Specifically, the processor 120 performs distortion compensation by multiplying the transmitted baseband signal received by the communication I / F unit 110 by a distortion compensation coefficient, and performs an update of the distortion compensation coefficient. At this time, the processor 120 corrects the variations in the analog characteristics in the feedback paths from the plurality of antenna elements collectively, and then updates the distortion compensation coefficient from the transmitted signal and the feedback signal. The internal configuration of the processor 120 will be described later.

[0022] The memory 130 includes, for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various information when processing is executed by the processor 120.

[0023] The D / A conversion unit 140 performs D / A conversion on the transmission signal whose distortion has been compensated by the processor 120. The analog transmission signal obtained by the D / A conversion is up-converted by an up-converter to become a radio frequency transmission signal.

[0024] The phase shifter 150 is provided corresponding to each of the plurality of antenna elements, and applies an antenna weight to the signal of each antenna element. That is, the phase shifter 150 sets a phase difference for the signal of each antenna element and performs beamforming to control the direction of the directive beam.

[0025] The power amplifier 160 is provided corresponding to each of the plurality of antenna elements, and amplifies the signal of each antenna element. That is, the power amplifier 160 amplifies the signal to which the antenna weight has been applied respectively and transmits it from the antenna element. Although non-linear distortion occurs in the signal during amplification by the power amplifier 160, in this embodiment, since distortion compensation is performed by the processor 120, when the signals transmitted from the respective antenna elements are combined in the wireless space, the non-linear distortion component included in the combined signal is reduced.

[0026] The phase shifter 170 is provided corresponding to each of the plurality of antenna elements, feeds back the signal output from the power amplifier 160 of each antenna element, and applies an inverse weight to the feedback signal (hereinafter abbreviated as "FB signal"). That is, the phase shifter 170 applies an inverse weight that cancels the antenna weight applied to each antenna element by the phase shifter 150 to the FB signal.

[0027] The combining unit 180 combines the FB signals from each of the plurality of antenna elements. That is, the combining unit 180 combines the FB signals output from the power amplifiers 160 of the respective antenna elements and to which reverse weights are applied. Since the FB signals combined by the combining unit 180 pass through different feedback paths, they are affected by variations in the analog characteristics of the feedback paths.

[0028] The A / D conversion unit 190 performs A / D conversion on the FB signal combined by the combining unit 180. Then, the A / D conversion unit 190 outputs the A / D-converted FB signal to the processor 120.

[0029] Next, the internal configuration of the processor 120 will be described. As shown in FIG. 2, the processor 120 includes a distortion compensation unit 121, a coefficient update unit 122, a variation correction unit 123, an analog characteristic estimation unit 124, and a filter coefficient calculation unit 125.

[0030] The distortion compensation unit 121 applies a distortion compensation coefficient to the transmission baseband signal to perform distortion compensation. That is, the distortion compensation unit 121 performs distortion compensation for compensating for non-linear distortion generated in the power amplifier 160, for example, using a look-up table or a polynomial. The distortion compensation unit 121 outputs the distortion-compensated transmission signal to the D / A conversion unit 140.

[0031] The coefficient update unit 122 performs an update process of the distortion compensation coefficient by comparing the transmission signal before distortion compensation and the FB signal. Specifically, the coefficient update unit 122 calculates a distortion compensation coefficient that minimizes the error between the transmission signal and the FB signal, for example, using the least mean square (LMS) algorithm. Then, the coefficient update unit 122 notifies the calculated distortion compensation coefficient to the distortion compensation unit 121. Since the transmission signal input to the coefficient update unit 122 does not include the non-linear distortion component generated in the power amplifier 160, a distortion compensation coefficient for compensating the non-linear distortion component can be calculated by performing the update process so as to minimize the error between the transmission signal and the FB signal.

[0032] The variation correction unit 123 corrects the variations in the analog characteristics in the feedback paths from the plurality of antenna elements. Specifically, the variation correction unit 123 has, for example, a FIR (Finite Impulse Response) filter, sets filter coefficients (hereinafter referred to as "batch filter coefficients") for collectively correcting the variations in the analog characteristics of the plurality of feedback paths in the FIR filter, and passes the transmission signal through this FIR filter. The FIR filter included in the variation correction unit 123 imparts characteristics corresponding to the analog characteristics of the plurality of feedback paths to the transmission signal. Therefore, the transmission signal input to the coefficient update unit 122 is imparted with characteristics corresponding to the analog characteristics of the plurality of feedback paths, similar to the FB signal passing through the feedback path, and the variations in the analog characteristics can be corrected. Also, since the variation correction unit 123 corrects the variations in the analog characteristics collectively using one FIR filter, the number of multipliers used for correction can be minimized, and an increase in power consumption can be suppressed.

[0033] The analog characteristic estimation unit 124 estimates the analog characteristics for each feedback path from the transmission signal before distortion compensation and the FB signal. Specifically, the analog characteristic estimation unit 124 estimates the gain, phase, delay, and frequency characteristics for the feedback paths related to each of the plurality of antenna elements. At this time, the analog characteristic estimation unit 124 acquires the FB signal for each feedback path, and uses the transmission signal before distortion compensation and the FB signal for each feedback path to estimate the gain, phase, delay, and frequency characteristics of each feedback path. When acquiring the FB signal for each feedback path, the gain of the phase shifter 170 of one feedback path may be set to 0 dB, and the gain of the phase shifter 170 of the other feedback paths may be set to -∞ dB, so that only the FB signal of the feedback path with the gain set to 0 dB is input to the analog characteristic estimation unit 124.

[0034] The filter coefficient calculation unit 125 calculates a batch filter coefficient to be set in the variation correction unit 123 based on the analog characteristics for each feedback path. Specifically, the filter coefficient calculation unit 125 calculates a path-specific filter coefficient corresponding to the analog characteristics for each feedback path from the gain, phase, delay, and frequency characteristics for each feedback path. That is, the filter coefficient calculation unit 125 calculates one path-specific filter coefficient for each feedback path. Then, the filter coefficient calculation unit 125 calculates the batch filter coefficient by obtaining the sum of the path-specific filter coefficients for each feedback path. The filter coefficient calculation unit 125 notifies the calculated batch filter coefficient to the variation correction unit 123.

[0035] Next, a distortion compensation method using the RU100 configured as described above will be described with reference to the flowchart shown in FIG. 3.

[0036] During a predetermined period such as when the RU100 is started up or when the filter coefficients are updated at a predetermined cycle, a process of setting the batch filter coefficient in the variation correction unit 123 is executed. The transmission baseband signal transmitted from the CU / DU10 during this period is received by the communication I / F unit 110 and input to the processor 120. Then, the transmission signal passes through the variation correction unit 123 and is input to the analog characteristic estimation unit 124.

[0037] Also, the transmission signal is distortion-compensated by the distortion compensation unit 121, D / A-converted and up-converted, then an antenna weight for beamforming is applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space. At this time, the signal after being amplified by the power amplifier 160 is fed back to the phase shifter 170, down-converted and A / D-converted, and then input to the analog characteristic estimation unit 124 as an FB signal.

[0038] In the analog characteristic estimation unit 124 to which the transmission signal and the FB signal are input, a process of estimating the analog characteristics of the feedback path using the transmission signal and the FB signal is executed (step S101). Specifically, the gain, phase, delay, and frequency characteristics of each feedback path corresponding to a plurality of antenna elements are estimated. That is, by adjusting the gain of the phase shifter 170, the FB signal of one feedback path is input to the analog characteristic estimation unit 124, and for this feedback path, the gain, phase, delay, and frequency characteristics are estimated. By repeating the estimation of the analog characteristics for each such feedback path, the analog characteristics of all the feedback paths are respectively estimated. The analog characteristic estimation process by the analog characteristic estimation unit 124 will be described in detail later.

[0039] When the analog characteristics for each feedback path are estimated, the filter coefficient calculation unit 125 calculates the path-specific filter coefficients corresponding to the analog characteristics for each feedback path (step S102). That is, the path-specific filter coefficients that represent the gain, phase, delay, and frequency characteristics for each feedback path by one FIR filter are calculated. Therefore, the same number of path-specific filter coefficients as the number of feedback paths are calculated by the filter coefficient calculation unit 125.

[0040] Then, the overall filter coefficient is calculated by the filter coefficient calculation unit 125 by obtaining the sum of the path-specific filter coefficients (step S103). The overall filter coefficient is the filter coefficient that represents the analog characteristics of a plurality of feedback paths by one FIR filter.

[0041] The batch filter coefficient is notified to the variation correction unit 123 and set in the FIR filter of the variation correction unit 123 (step S104). Thereby, the setting process in a predetermined period such as when the RU100 is started or when the filter coefficient is updated is completed. When the batch filter coefficient is set in the variation correction unit 123, normal signal transmission processing is executed thereafter. That is, the transmission signal is distortion-compensated by the distortion compensation unit 121 (step S105), D / A converted and up-converted, then antenna weights for beamforming are applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space.

[0042] Also, the signals amplified by the power amplifiers 160 of the respective antenna elements are respectively fed back via the feedback path, weighted inversely by the phase shifter 170, and then synthesized. The synthesized FB signal is input to the coefficient update unit 122. On the other hand, the same characteristics as the analog characteristics of the feedback path are collectively given to the transmission signal before distortion compensation by the variation correction unit 123, and the corrected transmission signal is input to the coefficient update unit 122. Then, by using the transmission signal and the FB signal by the coefficient update unit 122, the distortion compensation coefficient used by the distortion compensation unit 121 is updated.

[0043] In updating the distortion compensation coefficient, an FB signal obtained by synthesizing signals passing through different feedback paths and a transmission signal are used. Since the same characteristics as the analog characteristics of the feedback path are given to the transmission signal, the influence of the variation in the analog characteristics can be reduced. As a result, the accuracy of the distortion compensation coefficient can be improved, and a decrease in the distortion compensation performance can be prevented. Further, when giving the same characteristics as the analog characteristics of the feedback path to the transmission signal, it is only necessary to operate one FIR filter of the variation correction unit 123, so an increase in power consumption can be suppressed.

[0044] Next, the analog characteristic estimation process for estimating the analog characteristics of the feedback path will be specifically described with reference to the flowchart shown in FIG. 4. The analog characteristic estimation process described below is mainly executed by the analog characteristic estimation unit 124. Here, it is assumed that RU100 has L antenna elements (L is an integer of 2 or more), and there are also L feedback paths.

[0045] First, a variable i for counting the processed feedback paths is initialized to 1 (step S201), and the gain of the phase shifter 170 is set so that only the signal of the i-th feedback path is fed back (step S202). That is, the gain of the phase shifter 170 of the i-th feedback path is set to 0 dB, and the gain of the phase shifter 170 of the other feedback paths is set to -∞ dB. As a result, only the FB signal of the i-th feedback path is input to the analog characteristic estimation unit 124.

[0046] Then, the gain of the i-th feedback path is estimated (step S203). Specifically, the time-integrated power P T of the transmission signal and the time-integrated power P FB of the FB signal are calculated by the following formulas (1) and (2), respectively.

Equation

Equation

[0047] Then, the gain g of the feedback path is calculated by the following formula (3).

Equation

[0048] Next, the delay of the i-th feedback path is estimated (step S204). Specifically, a predetermined coefficient set is set in the delay FIR filter, and a delay corresponding to the coefficient set is imparted to the transmission signal. Here, for example, the coefficient set of coefficient set number 1 is set in the delay FIR filter, and a delay corresponding to this coefficient set is imparted to the transmission signal x(n), whereby a signal u(n) represented by the following formula (4) is obtained. [Number]

[0049] In formula (4), K is the number of taps of the delay FIR filter, and w(k) is the coefficient of the coefficient set being set. Then, the real part Re[Corr] and the imaginary part Im[Corr] of the correlation value between the signal u(n) and the FB signal y(n) are calculated by the following formulas (5) and (6), respectively. However, in formulas (5) and (6), Re[x] represents the real part of x, and Im[x] represents the imaginary part of x. [Number] [Number]

[0050] The power of the correlation value is calculated from the real part and the imaginary part of these correlation values, and the correlation value power corresponding to the coefficient set number 1 of the above-described delay FIR filter is calculated. Thereafter, the calculation of the correlation value power is repeated for each coefficient set of the delay FIR filter, and the coefficient set of the delay FIR filter that maximizes the correlation value power is specified. The specified coefficient set of the delay FIR filter corresponds to the delay of the i-th feedback path. Here, although a delay less than the clock is estimated, a delay in clock units may be estimated separately.

[0051] Next, the phase of the i-th feedback path is estimated (step S205). Specifically, the real part and the imaginary part of the correlation value between the transmission signal and the FB signal are calculated in the same manner as in the above equations (5) and (6). That is, in the above equations (5) and (6), the transmission signal x(n) is used instead of the signal u(n), and the real part Re[Corr] and the imaginary part Im[Corr] of the correlation value between the transmission signal x(n) and the FB signal y(n) are calculated.

[0052] Then, the phase θ of the feedback path is calculated by the following equation (7).

Equation

[0053] Next, the frequency characteristics of the i-th feedback path are estimated (step S206). Specifically, a predetermined coefficient set is set in the frequency characteristic FIR filter, and the transmission signal is given frequency characteristics corresponding to the coefficient set. Here, for example, the coefficient set of coefficient set number 1 is set in the frequency characteristic FIR filter, and the frequency characteristics corresponding to this coefficient set are given to the transmission signal x(n), whereby a signal u(n) similar to the above equation (4) is obtained. However, in the estimation of the frequency characteristics, K in the above equation (4) is the number of taps of the frequency characteristic FIR filter.

[0054] Then, the real part Re[Corr] and the imaginary part Im[Corr] of the correlation value between the signal u(n) and the FB signal y(n) are calculated in the same manner as in the above equations (5) and (6). The power of the correlation value is calculated from the real part and the imaginary part of these correlation values, and the correlation value power corresponding to the coefficient set number 1 of the above-described frequency characteristic FIR filter is calculated. Thereafter, the calculation of the correlation value power is repeated for each coefficient set of the frequency characteristic FIR filter, and the coefficient set of the frequency characteristic FIR filter that maximizes the correlation value power is specified. The specified coefficient set of the frequency characteristic FIR filter corresponds to the frequency characteristics of the i-th feedback path. Note that it is desirable to use a transmission signal corrected for delay in the estimation of the frequency characteristics. Also, the coefficients of the frequency characteristic FIR filter may be complex numbers.

[0055] Through the above processing, the analog characteristics of the i-th feedback path are estimated. Then, it is determined whether the variable i is greater than or equal to the number L of feedback paths (step S207). If the variable i is greater than or equal to L (step S207 Yes), it means that the analog characteristics have been estimated for all feedback paths, so the analog characteristic estimation process ends. On the other hand, if the variable i is less than L (step S207 No), since there is still a feedback path for which the analog characteristics have not been estimated, the variable i is incremented (step S208), and the above-described analog characteristic estimation process for the i-th feedback path is repeated. As a result, the gain, delay, phase, and frequency characteristics of all feedback paths are estimated respectively.

[0056] Next, a specific example of calculating the filter coefficients for each path will be described. The filter coefficients for each path are calculated by the filter coefficient calculation unit 125 when the analog characteristics for each feedback path are estimated.

[0057] As shown in FIG. 5, the filter coefficients for each path are the filter coefficients of the FIR filter corresponding to each of the L feedback paths from path 0 to path (L - 1), and correspond to the gain, phase, delay, and frequency characteristics in each feedback path. Here, consider the case of calculating the filter coefficients for each path such that a signal u(n) to which the analog characteristics of each feedback path are imparted can be obtained from the transmission signal x(n).

[0058] As shown in the left diagram of FIG. 5, if the gain-corrected output of path i is u i,1 (n), then u i,1 (n) can be expressed as in the following equation (8) using the real amplitude gain g i of path i.

Equation

[0059] Also, if the phase-corrected output of path i is u i,2Let it be (n), then u i,2 (n) is the real - valued phase θ of path i i and can be expressed as in the following formula (9).

Equation

[0060] And, let the delay - corrected output of path i be u i,3 (n), then u i,3 (n) is the coefficient set w i,1 (k1) of the delay FIR filter of path i and the number of taps K1 of the delay FIR filter, and can be expressed as in the following formula (10).

Equation

[0061] Similarly, let the frequency - characteristic - corrected output of path i be u i (n), then u i (n) is the coefficient set w i,2 (k2) of the frequency - characteristic FIR filter of path i and the number of taps K2 of the frequency - characteristic FIR filter, and can be expressed as in the following formula (11).

Equation

[0062] From formula (11), setting the filter coefficient of the FIR filter for each feedback path shown in the right figure of Figure 5 as w i (k), the number of taps as K - 1, K = K1+K2, k = k1 + k2, the FIR filter for each feedback path can be expressed as in the following formula (12).

Equation

[0063] The filter coefficient w i (k) is the product of the filter coefficients w i,1 (k1)w i,2Among (k2), it is the sum of those that satisfy k = k1 + k2. That is, for example, the filter coefficient w i (0) to w i (K - 2) can be obtained as follows.

Equation

[0064] In this way, one path - specific filter coefficient can be calculated for each feedback path corresponding to the gain, phase, delay, and frequency characteristics of each feedback path. Therefore, next, a specific example of calculating the batch filter coefficient from the path - specific filter coefficients for each feedback path will be described. The batch filter coefficient is calculated by the filter coefficient calculation unit 125 when the path - specific filter coefficients are calculated.

[0065] As shown in FIG. 6, the batch filter coefficient is the filter coefficient of a batch FIR filter that integrates the FIR filters of L feedback paths from path 0 to path (L - 1). Here, consider the case of calculating the batch filter coefficient such that a signal u(n) equivalent to the signal synthesized by passing the transmission signal x(n) through the FIR filters for each feedback path is obtained.

[0066] As shown in the left figure of FIG. 6, the FIR filter with the path - specific filter coefficients set generates a signal u i (n) with the analog characteristics for each feedback path from the transmission signal x(n). Let the path - specific filter coefficient of the FIR filter of path i be w i (k), and the number of taps of the FIR filter be K. Then, the output signal u i (n) of the FIR filter of path i can be expressed as in the following formula (13).

Equation

[0067] The batch FIR filter shown in FIG. 6 on the right integrates the FIR filters of each feedback path. Therefore, the output signal u(n) of the batch FIR filter can be expressed as in the following equation (14). [Number]

[0068] In equation (14), the filter coefficient w(k) of the batch FIR filter is the sum of the filter coefficients w i (k) of the FIR filter for each feedback path and is represented by the following equation (15). [Number]

[0069] The filter coefficient w(k) of this batch FIR filter is the batch filter coefficient for generating the signal u(n) from the transmission signal x(n). The batch filter coefficient calculated in this way is notified from the filter coefficient calculation unit 125 to the variation correction unit 123 and is set in the FIR filter possessed by the variation correction unit 123. Thereby, when the transmission signal is input to the variation correction unit 123, characteristics similar to the analog characteristics of the plurality of feedback paths can be imparted to the transmission signal, and it becomes possible to cancel the influence of the variation in the analog characteristics imparted to the FB signal.

[0070] As described above, according to the present embodiment, the analog characteristics of the plurality of feedback paths are estimated, the batch filter coefficient for correcting these analog characteristics is calculated, and the variation in the analog characteristics of the feedback paths is corrected collectively by one filter. Therefore, by operating the multiplier for one filter to correct the variation in the analog characteristics, the distortion compensation coefficient can be updated with high accuracy. As a result, it is possible to suppress an increase in power consumption and prevent a decrease in distortion compensation performance.

[0071] (Embodiment 2) The feature of Embodiment 2 is that correction is performed on the FB signal for the average of the analog characteristics of a plurality of feedback paths, and correction is performed on the transmission signal for the variation in the remaining analog characteristics.

[0072] The configuration of the communication system according to Embodiment 2 is the same as that of Embodiment 1 (FIG. 1), and thus its description is omitted. In Embodiment 2, the configuration of the processor 120 of the RU100 is different from that of Embodiment 1 (FIG. 2).

[0073] FIG. 7 is a block diagram showing the configuration of the processor 120 according to Embodiment 2. In FIG. 7, the same parts as those in FIG. 2 are denoted by the same reference numerals, and their description is omitted. The processor 120 shown in FIG. 7 has a variation correction unit 201, a filter coefficient calculation unit 202, and an average correction unit 203 instead of the variation correction unit 123 and the filter coefficient calculation unit 125 of the processor 120 shown in FIG. 2.

[0074] The variation correction unit 201 corrects the variation in the analog characteristics in the feedback paths from a plurality of antenna elements. Specifically, the variation correction unit 201 has, for example, an FIR filter, sets a batch filter coefficient in the FIR filter, and passes the transmission signal through this FIR filter. The FIR filter included in the variation correction unit 201 imparts to the transmission signal a characteristic corresponding to the variation excluding the average of the analog characteristics of a plurality of feedback paths. For this reason, the variation in the analog characteristics imparted to the FB signals passing through different feedback paths is imparted to the transmission signal input to the coefficient update unit 122, and the variation in the analog characteristics can be corrected. Further, since the variation correction unit 201 corrects the variation in the analog characteristics collectively using one FIR filter, the number of multipliers used for the correction can be minimized, and an increase in power consumption can be suppressed.

[0075] The filter coefficient calculation unit 202 calculates a batch filter coefficient to be set in the variation correction unit 201 and a filter coefficient to be set in the average correction unit 203 based on the analog characteristics for each feedback path. Specifically, the filter coefficient calculation unit 202 calculates the average value of the analog characteristics of the plurality of feedback paths and the remaining variation after dividing by the average value from the gain, phase, delay, and frequency characteristics for each feedback path. That is, the filter coefficient calculation unit 202 calculates a correction value corresponding to the average value for correcting the FB signal and a per-path filter coefficient corresponding to the variation for correcting the transmission signal. Then, the filter coefficient calculation unit 202 notifies the average correction unit 203 of the correction value corresponding to the average value and notifies the variation correction unit 201 of the batch filter coefficient calculated from the per-path filter coefficients.

[0076] The average correction unit 203 removes the average value of the analog characteristics of the plurality of feedback paths from the FB signal. That is, the average correction unit 203 acquires from the filter coefficient calculation unit 202 a correction value corresponding to the average value of the gain, phase, delay, and frequency characteristics of the plurality of feedback paths and performs correction regarding the average value on the FB signal.

[0077] Next, a distortion compensation method using the RU100 configured as described above will be described with reference to the flowchart shown in FIG. 8. In FIG. 8, the same parts as in FIG. 3 are denoted by the same reference numerals, and detailed description thereof is omitted.

[0078] During a predetermined period such as when the RU100 is started up or when the filter coefficients are updated at a predetermined cycle, a process of setting the batch filter coefficient in the variation correction unit 201 and setting the correction value in the average correction unit 203 is executed. The transmission baseband signal transmitted from the CU / DU10 during this period is received by the communication I / F unit 110 and input to the processor 120. Then, the transmission signal passes through the variation correction unit 201 and is input to the analog characteristic estimation unit 124.

[0079] Also, the transmission signal is distortion-compensated by the distortion compensation unit 121, D / A converted and up-converted, and then antenna weights for beamforming are applied by the phase shifter 150. After being amplified by the power amplifier 160, it is transmitted into the wireless space. At this time, the signal after being amplified by the power amplifier 160 is fed back to the phase shifter 170, down-converted and A / D converted, and then input to the analog characteristic estimation unit 124 as an FB signal.

[0080] In the analog characteristic estimation unit 124 to which the transmission signal and the FB signal are input, a process of estimating the analog characteristics of the feedback path is executed using the transmission signal and the FB signal (step S101).

[0081] When the analog characteristics for each feedback path are estimated, the filter coefficient calculation unit 202 calculates a correction value of the average of the analog characteristics for each feedback path (hereinafter referred to as "average correction value") and the remaining variation excluding the average (step S301).

[0082] Specifically, first, assuming that the amplitude gain estimated for each feedback path is g i the average amplitude gain g ave of all paths is represented by the following formula (16).

Equation

[0083] Since the average correction value regarding the gain is the reciprocal of this average amplitude gain g ave the average correction value g' regarding the gain is as shown in the following formula (17).

Equation

[0084] Also, the remaining variation g' i excluding the average regarding the gain is as shown in the following formula (18).

Number

[0085] In addition, in Expressions (17) and (18), the average correction value and the variation are multiplied by 1 / L in order to normalize the amplitude by the factor of L when the FB signals of L feedback paths are combined.

[0086] Next, let the coefficient set of the delay FIR filter estimated for each feedback path be p i Then, the average delay p ave of all paths is expressed by the following Expression (19).

Number

[0087] Let the total number of coefficient sets of the delay FIR filter be S1. Then, the average correction value p' regarding the delay is as shown in the following Expression (20).

Number

[0088] Also, the remaining variation p' excluding the average regarding the delay i is as shown in the following Expression (21).

Number

[0089] Next, let the phase estimated for each feedback path be θ i Then, the average phase θ ave of all paths is expressed by the following Expression (22).

Number

[0090] The average correction value regarding the phase is this average phase θ aveSince it is multiplied by (-1), the average correction value θ' for the phase is as shown in the following equation (23).

Equation

[0091] Also, the remaining variation θ' excluding the average for the phase i is as shown in the following equation (24).

Equation

[0092] Next, if the coefficient set of the frequency characteristic FIR filter estimated for each feedback path is q i then the average frequency characteristic q ave for all paths is represented by the following equation (25).

Equation

[0093] If the total number of coefficient sets of the frequency characteristic FIR filter is S2, the average correction value q' for the frequency characteristic is as shown in the following equation (26).

Equation

[0094] Also, the remaining variation q' excluding the average for the frequency characteristic i is as shown in the following equation (27).

Equation

[0095] The average correction values of the gain, delay, phase, and frequency characteristic calculated in this way are set in the average correction unit 203 (step S302). As a result, it becomes possible to correct the average of the analog characteristics of the plurality of feedback paths by correcting the FB signal by the average correction unit 203.

[0096] On the other hand, variations in gain, delay, phase, and frequency characteristics are used for calculating the filter coefficients for each path, similar to Embodiment 1. That is, the filter coefficient calculation unit 202 calculates the filter coefficients for each path corresponding to the variations in the analog characteristics for each feedback path (step S102). Then, the sum of the filter coefficients for each path is obtained by the filter coefficient calculation unit 202, thereby calculating the batch filter coefficient (step S103).

[0097] The batch filter coefficient is notified to the variation correction unit 201 and set in the FIR filter included in the variation correction unit 201 (step S104). Thereby, the setting process in a predetermined period such as at the startup of the RU100 or at the time of filter coefficient update is completed. When the batch filter coefficient is set in the variation correction unit 201, normal signal transmission processing is executed thereafter. That is, the transmission signal is distortion-compensated by the distortion compensation unit 121 (step S105), D / A converted and up-converted, then antenna weights for beamforming are applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space.

[0098] Also, the signals amplified by the power amplifiers 160 of the respective antenna elements are each fed back via the feedback path, and after reverse weights are applied by the phase shifter 170, they are combined. The combined FB signal is corrected using the average correction value by the average correction unit 203 and then input to the coefficient update unit 122. On the other hand, the same characteristics as the variations in the analog characteristics of the feedback path are collectively imparted to the transmission signal before distortion compensation by the variation correction unit 201, and the corrected transmission signal is input to the coefficient update unit 122. Then, the distortion compensation coefficient used by the distortion compensation unit 121 is updated by using the transmission signal and the FB signal by the coefficient update unit 122.

[0099] In updating the distortion compensation coefficient, an FB signal obtained by synthesizing signals passing through different feedback paths and a transmission signal are used. Since corrections are applied to the transmission signal and the FB signal, the influence of variations in analog characteristics can be reduced. As a result, the accuracy of the distortion compensation coefficient can be improved, and a degradation in distortion compensation performance can be prevented. Further, when imparting characteristics similar to the variations in the analog characteristics of the feedback path to the transmission signal, it is only necessary to operate one FIR filter included in the variation correction unit 201, so an increase in power consumption can be suppressed.

[0100] As described above, according to the present embodiment, the analog characteristics of a plurality of feedback paths are estimated, a correction value for the average of the analog characteristics and the variations are calculated, the FB signal is corrected for the average, and the transmission signal is collectively corrected for the variations by one filter. For this reason, the variations in the analog characteristics can be corrected by operating a multiplier for one filter, and the distortion compensation coefficient can be updated with high accuracy. As a result, it is possible to prevent a degradation in distortion compensation performance while suppressing an increase in power consumption.

[0101] (Embodiment 3) A feature of Embodiment 3 is that the phase shifters of the transmission path and the feedback path are calibrated.

[0102] Since the configuration of the communication system according to Embodiment 3 is the same as that of Embodiment 1 (FIG. 1), the description thereof is omitted. In Embodiment 3, the configuration of the RU100 is different from that of Embodiment 1 (FIG. 2).

[0103] FIG. 9 is a block diagram showing the configuration of the RU100 according to Embodiment 3. In FIG. 9, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted. The RU100 shown in FIG. 9 has a configuration in which a transmission phase shifter (PS) control unit 301 and a feedback (FB) phase shifter (PS) control unit 302 are added to the RU100 shown in FIG. 2.

[0104] The transmission PS control unit 301 calibrates the phase shifter 150 provided in the transmission path. That is, the transmission PS control unit 301 controls the gain and phase of the phase shifter 150. Specifically, the transmission PS control unit 301 turns on the transmission via the phase shifter 150 one by one for each of the L transmission paths, and controls the gain of the phase shifter 150 so that the power of the signal received by a predetermined UE20 becomes equal. Then, the transmission PS control unit 301 turns on the transmission via the phase shifter 150 two by two for each of the L transmission paths, and controls the phase of the phase shifter 150 so that the power of the signal received by a predetermined UE20 becomes maximum.

[0105] The transmission PS control unit 301 repeats the above gain and phase control for all the transmission paths. Note that after controlling the phase, the transmission PS control unit 301 may adjust the gain of the phase shifter 150 so that the power of the signal received by a predetermined UE20 becomes equal when the transmission via the phase shifter 150 is turned on one by one for each of the L transmission paths again.

[0106] The FBPS control unit 302 calibrates the phase shifter 170 provided in the feedback path. That is, the FBPS control unit 302 controls the gain and phase of the phase shifter 170. Specifically, the FBPS control unit 302 turns on the feedback via the phase shifter 170 one by one for each of the L feedback paths, and controls the gain of the phase shifter 170 so that the power of the FB signal becomes equal. Then, the FBPS control unit 302 turns on the feedback via the phase shifter 170 two by two for each of the L feedback paths, and controls the phase of the phase shifter 170 so that the power of the FB signal becomes maximum.

[0107] The FBPS control unit 302 repeats the above gain and phase control for all feedback paths. Note that after controlling the phase, the FBPS control unit 302 may adjust the gain of the phase shifter 170 so that the power of the FB signal becomes equal when the feedback passing through the phase shifter 170 one by one among the L feedback paths is turned on again.

[0108] In the present embodiment, during a predetermined period such as when the RU100 is started up or when the filter coefficients are updated at a predetermined cycle, after the calibration of the phase shifter 150 and the phase shifter 170 is executed, the analog characteristics of the plurality of feedback paths are estimated. Then, based on the estimated analog characteristics, the filter coefficients for each path and the batch filter coefficients are calculated in the same manner as in the first embodiment. Therefore, after making the analog characteristics of the plurality of feedback paths uniform to a certain extent, it is possible to further correct the variation in the analog characteristics, and it is possible to improve the accuracy of the transmission signal and the FB signal used for updating the distortion compensation coefficient.

[0109] As described above, according to the present embodiment, after calibrating the phase shifters provided in the transmission path and the feedback paths, the variation in the analog characteristics of the plurality of feedback paths is corrected. Therefore, the distortion compensation coefficient can be updated with higher accuracy, and a decrease in the distortion compensation performance can be prevented.

[0110] (Embodiment 4) The feature of the fourth embodiment is that the filter coefficients for each path corresponding to each of the plurality of feedback paths are derived using a convergence algorithm.

[0111] Since the configuration of the communication system according to the fourth embodiment is the same as that of the first embodiment (FIG. 1), the description thereof is omitted. In the fourth embodiment, the configuration of the RU100 is different from that of the first embodiment (FIG. 2).

[0112] FIG. 10 is a block diagram showing the configuration of the RU100 according to Embodiment 4. In FIG. 10, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted. The RU100 shown in FIG. 10 has a per-path coefficient calculation unit 401 and a batch coefficient calculation unit 402 instead of the analog characteristic estimation unit 124 and the filter coefficient calculation unit 125 of the RU100 shown in FIG. 2.

[0113] The per-path coefficient calculation unit 401 calculates per-path filter coefficients corresponding to the analog characteristics of each of a plurality of feedback paths using a convergence algorithm such as, for example, the LMS algorithm. Specifically, the per-path coefficient calculation unit 401 has, for example, the configuration shown in FIG. 11. As shown in FIG. 11, the per-path coefficient calculation unit 401 has a variable FIR 411 and an error calculation unit 412.

[0114] The variable FIR 411 is a FIR filter with variable filter coefficients, filters the transmission signal, and outputs it to the error calculation unit 412. At this time, the variable FIR 411 filters the transmission signal while changing the filter coefficients so that the error calculated by the error calculation unit 412 becomes small. Then, the variable FIR 411 outputs the filter coefficients when the error is minimized as the per-path filter coefficients to the batch coefficient calculation unit 402.

[0115] The error calculation unit 412 calculates the error between the transmission signal output from the variable FIR 411 and the FB signal. If characteristics equivalent to the analog characteristics imparted to the FB signal in the feedback path are imparted to the transmission signal by filtering with the variable FIR 411, the error calculated by the error calculation unit 412 approaches 0.

[0116] With this configuration, the per-path coefficient calculation unit 401 outputs, to the batch coefficient calculation unit 402, per-path filter coefficients that minimize the error between the transmission signal and the FB signal for each feedback path.

[0117] Returning to FIG. 10, the batch coefficient calculation unit 402 calculates a batch filter coefficient by obtaining the sum of the path-specific filter coefficients for each feedback path. The batch coefficient calculation unit 402 notifies the variance correction unit 123 of the calculated batch filter coefficient.

[0118] Next, the distortion compensation method using the RU100 configured as described above will be described with reference to the flowchart shown in FIG. 12. In FIG. 12, the same parts as those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0119] During a predetermined period such as when the RU100 is started up or when the filter coefficients are updated at a predetermined cycle, a process of setting the batch filter coefficient in the variance correction unit 201 is executed. The transmission baseband signal transmitted from the CU / DU10 during this period is received by the communication I / F unit 110 and input to the processor 120. Then, the transmission signal passes through the variance correction unit 123 and is input to the path-specific coefficient calculation unit 401.

[0120] Also, the transmission signal is distortion-compensated by the distortion compensation unit 121, D / A converted and up-converted, then antenna weights for beamforming are applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space. At this time, the signal after being amplified by the power amplifier 160 is fed back to the phase shifter 170, down-converted and A / D converted, and then input to the path-specific coefficient calculation unit 401 as an FB signal.

[0121] In the path-specific coefficient calculation unit 401 to which the transmission signal and the FB signal are input, the transmission signal and the FB signal are used to calculate the path-specific filter coefficient for each feedback path (step S401). Specifically, by adjusting the gain of the phase shifter 170, the FB signal for each feedback path is input to the path-specific coefficient calculation unit 401, and the error between the transmission signal and the FB signal that has passed through the variable FIR 411 is calculated by the error calculation unit 412.

[0122] Here, the transmitted signal y'(n) obtained by filtering the transmitted signal x(n) input to the variable FIR 411 by the variable FIR 411 is represented by the following formula (28).

Equation

[0123] In formula (28), w(k) is the filter coefficient of the variable FIR 411, and K is the number of taps of the variable FIR 411. Since such a transmitted signal y'(n) is output from the variable FIR 411, the error e(n) between the transmitted signal y'(n) and the FB signal y(n) is calculated by the error calculation unit 412 as shown in the following formula (29).

Equation

[0124] The calculated error e(n) is fed back to the variable FIR 411, and the filter coefficient w(k) of the variable FIR 411 is updated so that the error e(n) becomes smaller. That is, the filter coefficient w(k) is updated by the following formula (30).

Equation

[0125] However, in formula (30), μ is the step size parameter, and x * represents the complex conjugate of x. In this way, the process of updating the filter coefficient of the variable FIR 411 so that the error becomes smaller is repeated, and the filter coefficient corresponding to the minimum error is specified as the path-specific filter coefficient. The calculation of the path-specific filter coefficient is repeatedly executed for all feedback paths, and the path-specific filter coefficients for each feedback path are output to the batch coefficient calculation unit 402.

[0126] Then, the batch coefficient calculation unit 402 calculates the sum of the filter coefficients for each path, thereby calculating the batch filter coefficient (step S103). The batch filter coefficient is notified to the variation correction unit 123 and set in the FIR filter included in the variation correction unit 123 (step S104). Thereby, the setting process in a predetermined period such as when the RU100 is started up or when the filter coefficient is updated is completed. When the batch filter coefficient is set in the variation correction unit 123, normal signal transmission processing is executed thereafter. That is, the transmission signal is distortion-compensated by the distortion compensation unit 121 (step S105), D / A converted and up-converted, then antenna weights for beamforming are applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space.

[0127] In addition, the signals amplified by the power amplifiers 160 of the respective antenna elements are respectively fed back via the feedback paths, weighted in the reverse direction by the phase shifter 170, and then combined. The combined FB signal is input to the coefficient update unit 122. On the other hand, the same characteristics as the analog characteristics of the feedback path are collectively imparted to the transmission signal before distortion compensation by the variation correction unit 123, and the corrected transmission signal is input to the coefficient update unit 122. Then, the coefficient update unit 122 updates the distortion compensation coefficient used by the distortion compensation unit 121 by using the transmission signal and the FB signal.

[0128] In the update of the distortion compensation coefficient, the FB signal obtained by combining the signals passing through different feedback paths and the transmission signal are used. Since the same characteristics as the analog characteristics of the feedback path are imparted to the transmission signal, the influence of the variation in the analog characteristics can be reduced. As a result, the accuracy of the distortion compensation coefficient can be improved, and the deterioration of the distortion compensation performance can be prevented. Furthermore, when imparting the same characteristics as the analog characteristics of the feedback path to the transmission signal, only one FIR filter included in the variation correction unit 123 needs to be operated, so an increase in power consumption can be suppressed.

[0129] As described above, according to the present embodiment, the filter coefficients for each path corresponding to a plurality of feedback paths are calculated by a convergence algorithm, the batch filter coefficients are calculated from the filter coefficients for each path, and the variations in the analog characteristics of the feedback paths are corrected collectively by one filter. Therefore, the multiplier for one filter can be operated to correct the variations in the analog characteristics, and the distortion compensation coefficient can be updated accurately. As a result, it is possible to suppress an increase in power consumption while preventing a decrease in distortion compensation performance.

[0130] (Embodiment 5) The feature of Embodiment 5 is that the variations in the analog characteristics of the feedback path are corrected for the FB signal.

[0131] The configuration of the communication system according to Embodiment 5 is the same as that of Embodiment 1 (FIG. 1), and thus the description thereof is omitted. In Embodiment 5, the configuration of the processor 120 of the RU100 is different from that of Embodiment 1 (FIG. 2).

[0132] FIG. 13 is a block diagram showing the configuration of the processor 120 according to Embodiment 5. In FIG. 13, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted. The processor 120 shown in FIG. 13 has an inverse characteristic conversion unit 501 and a variation correction unit 502 instead of the variation correction unit 123 of the processor 120 shown in FIG. 2.

[0133] The inverse characteristic conversion unit 501 converts the batch filter coefficients calculated by the filter coefficient calculation unit 125 into inverse characteristics. Then, the inverse characteristic conversion unit 501 notifies the variation correction unit 502 of the batch filter coefficients converted into inverse characteristics.

[0134] The variation correction unit 502 corrects the variations in the analog characteristics in the feedback paths from the plurality of antenna elements. Specifically, the variation correction unit 502 has, for example, a FIR filter, sets the batch filter coefficients notified from the inverse characteristic conversion unit 501 in the FIR filter, and passes the FB signal through this FIR filter. The FIR filter included in the variation correction unit 502 imparts the inverse characteristics of the analog characteristics of the plurality of feedback paths to the FB signal. Therefore, in the FB signal input to the coefficient update unit 122, the analog characteristics of the feedback path are removed, and the variations in the analog characteristics can be corrected. Further, since the variation correction unit 502 corrects the variations in the analog characteristics collectively using one FIR filter, the number of multipliers used for the correction can be minimized, and an increase in power consumption can be suppressed.

[0135] Next, a distortion compensation method using the RU100 configured as described above will be described with reference to the flowchart shown in FIG. 14. In FIG. 14, the same parts as those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof is omitted.

[0136] During a predetermined period such as when the RU100 is started up or when the filter coefficients are updated at a predetermined cycle, a process of setting the batch filter coefficients in the variation correction unit 502 is executed. The transmission baseband signal transmitted from the CU / DU10 during this period is received by the communication I / F unit 110 and input to the processor 120. Then, the transmission signal is input to the analog characteristic estimation unit 124.

[0137] Also, the transmission signal is distortion-compensated by the distortion compensation unit 121, D / A converted and up-converted, then an antenna weight for beamforming is imparted by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space. At this time, the signal after being amplified by the power amplifier 160 is fed back to the phase shifter 170, down-converted and A / D converted, then passes through the variation correction unit 502 as an FB signal, and is input to the analog characteristic estimation unit 124.

[0138] In the analog characteristic estimation unit 124 to which the transmission signal and the FB signal are input, a process of estimating the analog characteristics of the feedback path using the transmission signal and the FB signal is executed (step S101). When the analog characteristics for each feedback path are estimated, the filter coefficient calculation unit 125 calculates the filter coefficients for each path corresponding to the analog characteristics for each feedback path (step S102). Then, the filter coefficient calculation unit 125 calculates the batch filter coefficient by obtaining the sum of the filter coefficients for each path (step S103).

[0139] The batch filter coefficient is output to the inverse characteristic conversion unit 501, and the inverse characteristic conversion unit 501 converts the batch filter coefficient into the inverse characteristic (step S501). That is, a batch filter coefficient for collectively canceling the analog characteristics imparted to the FB signal in a plurality of feedback paths is derived. The batch filter coefficient with the inverse characteristic is notified to the variation correction unit 502 and set in the FIR filter possessed by the variation correction unit 502 (step S502). Thereby, the setting process in a predetermined period such as at the time of starting up the RU100 or updating the filter coefficient is completed. When the batch filter coefficient with the inverse characteristic is set in the variation correction unit 502, normal signal transmission processing is executed thereafter. That is, the transmission signal is distortion-compensated by the distortion compensation unit 121 (step S105), D / A converted and up-converted, then an antenna weight for beamforming is imparted by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space.

[0140] Also, the signals amplified by the power amplifiers 160 of the respective antenna elements are each fed back via a feedback path, and after being given an inverse weight by the phase shifters 170, they are combined. The combined FB signal is collectively given the inverse characteristic of the analog characteristic of the feedback path by the variation correction unit 502, and the corrected FB signal is input to the coefficient update unit 122. On the other hand, the transmission signal before distortion compensation is also input to the coefficient update unit 122, and the coefficient update unit 122 updates the distortion compensation coefficient used by the distortion compensation unit 121 by using the transmission signal and the FB signal.

[0141] In the update of the distortion compensation coefficient, an FB signal obtained by combining signals passing through different feedback paths is used. Since the inverse characteristic of the analog characteristic of the feedback path is given to this FB signal, the influence of the variation in the analog characteristic can be reduced. As a result, the accuracy of the distortion compensation coefficient can be improved, and a decrease in the distortion compensation performance can be prevented. Further, when giving the inverse characteristic of the analog characteristic of the feedback path to the FB signal, it is only necessary to operate one FIR filter provided in the variation correction unit 502, so an increase in power consumption can be suppressed.

[0142] As described above, according to the present embodiment, the batch filter coefficients for correcting the analog characteristics of a plurality of feedback paths are converted into inverse characteristics, and the FB signal is collectively corrected using the inverse characteristic batch filter coefficients. Therefore, the multiplier for one filter can be operated to correct the variation in the analog characteristic, and the distortion compensation coefficient can be updated with high accuracy. As a result, it is possible to prevent a decrease in the distortion compensation performance while suppressing an increase in power consumption.

[0143] (Embodiment 6) The feature of Embodiment 6 is that batch filter coefficients are calculated for each beam direction, and the variation in the analog characteristic of the feedback path is corrected using the batch filter coefficients corresponding to the beam direction.

[0144] The configuration of the communication system according to Embodiment 6 is the same as that of Embodiment 1 (FIG. 1), and thus its description is omitted. In Embodiment 6, the configuration of the processor 120 of the RU 100 is different from that of Embodiment 1 (FIG. 2).

[0145] FIG. 15 is a block diagram showing the configuration of the processor 120 according to Embodiment 6. In FIG. 15, the same parts as those in FIG. 2 are denoted by the same reference numerals, and their descriptions are omitted. The processor 120 shown in FIG. 15 has a configuration in which a filter coefficient storage unit 601 is added to the processor 120 shown in FIG. 2.

[0146] The filter coefficient storage unit 601 stores the batch filter coefficients calculated by the filter coefficient calculation unit 125 in association with the beam directions. That is, the filter coefficient storage unit 601 stores the batch filter coefficients in association with the directions of the transmission beams formed by the phase shifters 150 during a predetermined period such as when the RU 100 is started up or when the filter coefficients are updated at a predetermined cycle. Then, during normal signal transmission processing, the filter coefficient storage unit 601 acquires the information on the direction of the transmission beam and notifies the variation correction unit 123 of the batch filter coefficients corresponding to the beam direction.

[0147] Next, a distortion compensation method by the RU 100 configured as described above will be described with reference to the flowchart shown in FIG. 16. In FIG. 16, the same parts as those in FIG. 3 are denoted by the same reference numerals, and their detailed descriptions are omitted.

[0148] During a predetermined period such as when the RU 100 is started up or when the filter coefficients are updated at a predetermined cycle, a process of storing the batch filter coefficients in the filter coefficient storage unit 601 is executed. The transmission baseband signal transmitted from the CU / DU 10 during this period is received by the communication I / F unit 110 and input to the processor 120. Then, the transmission signal passes through the variation correction unit 123 and is input to the analog characteristic estimation unit 124.

[0149] Also, the identification number m in the beam direction is initialized to 1 (step S601), and the antenna weight corresponding to the m-th beam direction is set in the phase shifter 150 (step S602). As a result, the RU100 forms a transmission beam facing the m-th beam direction.

[0150] The transmission signal is distortion-compensated by the distortion compensation unit 121, D / A converted and up-converted, then the antenna weight corresponding to the m-th beam direction is applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted in the m-th beam direction. At this time, the signal amplified by the power amplifier 160 is fed back to the phase shifter 170, down-converted and A / D converted, and then input to the analog characteristic estimation unit 124.

[0151] In the analog characteristic estimation unit 124 to which the transmission signal and the FB signal are input, a process of estimating the analog characteristics of the feedback path is executed using the transmission signal and the FB signal (step S101). When the analog characteristics for each feedback path are estimated, the path-specific filter coefficients corresponding to the analog characteristics for each feedback path are calculated by the filter coefficient calculation unit 125 (step S102). Then, the sum of the path-specific filter coefficients is obtained by the filter coefficient calculation unit 125, thereby calculating the batch filter coefficient (step S103).

[0152] The calculated batch filter coefficient is stored in the filter coefficient storage unit 601 in association with the m-th beam direction (step S603). Then, it is determined whether the identification number m is greater than or equal to the total number M of beam directions (step S604). If the identification number m is less than M (step S604No), the identification number m is incremented (step S605). Then, in the same manner as above, the batch filter coefficient when a transmission beam facing the m-th beam direction is formed is calculated and stored by the filter coefficient storage unit 601.

[0153] Then, when the identification number m becomes M or more (Yes in step S604) and the batch filter coefficients are stored in association with all beam directions, normal signal transmission processing is executed thereafter. At this time, when the antenna weights for beamforming are set in the phase shifter 150, the batch filter coefficients corresponding to the direction of the transmission beam are notified from the filter coefficient storage unit 601 to the variation correction unit 123 and set in the FIR filter included in the variation correction unit 123 (step S606).

[0154] Then, the transmission signal is distortion-compensated by the distortion compensation unit 121 (step S105), D / A converted and up-converted, then the antenna weights for beamforming are applied by the phase shifter 150, amplified by the power amplifier 160, and then transmitted into the wireless space.

[0155] Also, the signals amplified by the power amplifiers 160 of the respective antenna elements are each fed back via a feedback path, given inverse weights by the phase shifter 170, and then combined. The combined FB signal is input to the coefficient update unit 122. On the other hand, the transmission signal before distortion compensation is corrected by the batch filter coefficients corresponding to the beam direction by the variation correction unit 123, and the corrected transmission signal is input to the coefficient update unit 122. Then, by using the transmission signal and the FB signal by the coefficient update unit 122, the distortion compensation coefficients used by the distortion compensation unit 121 are updated.

[0156] In updating the distortion compensation coefficient, an FB signal obtained by synthesizing signals passing through different feedback paths and a transmission signal are used. Since the transmission signal is given the same characteristics as the analog characteristics of the feedback path, the influence of variations in the analog characteristics can be reduced. As a result, the accuracy of the distortion compensation coefficient can be improved, and a decrease in distortion compensation performance can be prevented. Further, when giving the same characteristics as the analog characteristics of the feedback path to the transmission signal, it is only necessary to operate one FIR filter included in the variation correction unit 123, so an increase in power consumption can be suppressed. Also, since correction of variations is performed according to the beam direction, appropriate correction corresponding to changes in the weights in the phase shifters 150 and 170 can be realized.

[0157] As described above, according to the present embodiment, batch filter coefficients for correcting the analog characteristics of a plurality of feedback paths are stored in association with the beam direction, and the variations in the analog characteristics of the feedback path are corrected using the batch filter coefficients corresponding to the transmission beam. For this reason, variations in the analog characteristics can be appropriately corrected corresponding to the beam direction.

[0158] In each of the above embodiments, path-specific filter coefficients are calculated for all of the plurality of feedback paths, and batch filter coefficients are calculated from these path-specific filter coefficients. However, it is not always necessary to calculate the path-specific filter coefficients for all of the feedback paths. The path-specific filter coefficients may be calculated for some of the feedback paths, and the batch filter coefficients may be calculated from these path-specific filter coefficients.

[0159] Also, in the process of estimating the analog characteristics of the feedback path, it is not always necessary to estimate the gain, delay, phase, and frequency characteristics of each feedback path one by one. That is, for example, FB signals of two or more feedback paths may be collectively fed back, and the analog characteristics of the feedback circuit corresponding to this FB signal may be collectively estimated.

[0160] The above-described Embodiments 1 to 6 can be implemented in appropriate combinations. For example, when correcting each of the FB signal and the transmission signal by combining Embodiments 2 and 3, calibration of the phase shifter may be performed. Further, for example, when combining Embodiments 4 and 6, after deriving the filter coefficients for each path using a convergence algorithm, the batch filter coefficients for each beam direction are calculated, and the variation in analog characteristics may be corrected using the batch filter coefficients corresponding to the transmission beam.

Description of Reference Numerals

[0161] 110 Communication I / F unit 120 Processor 121 Distortion compensation unit 122 Coefficient update unit 123, 201, 502 Variation correction unit 124 Analog characteristic estimation unit 125, 202 Filter coefficient calculation unit 130 Memory 140 D / A conversion unit 150, 170 Phase shifter 160 Power amplifier 180 Combining unit 190 A / D conversion unit 203 Average correction unit 301 Transmission PS control unit 302 FBPS control unit 401 Coefficient calculation unit for each path 402 Batch coefficient calculation unit 411 Variable FIR 412 Error calculation unit 49 Inverse characteristic conversion unit 601 Filter coefficient storage unit

Claims

1. A plurality of antenna elements, a plurality of power amplifiers provided for the plurality of antenna elements, a processor that outputs a transmission signal to the plurality of power amplifiers, and a plurality of feedback paths that feedback a feedback signal from the plurality of power amplifiers to the processor, wherein the processor estimates analog characteristics of the plurality of feedback paths, calculates a correction value corresponding to an average value of the analog characteristics of the plurality of feedback paths and a per-path filter coefficient corresponding to a variance excluding the average value based on the estimated analog characteristics for each feedback path, passes the feedback signal through a filter that corrects using the correction value corresponding to the average value of the analog characteristics in the plurality of feedback paths to correct the feedback signal, passes the transmission signal through one FIR (Finite Impulse Response) using a batch filter coefficient calculated from the calculated per-path filter coefficients to correct the transmission signal by collectively correcting the variance excluding the average value from the analog characteristics in the plurality of feedback paths, updates a distortion compensation coefficient for compensating for non-linear distortion occurring in the plurality of power amplifiers using an error between the corrected transmission signal and the feedback signal corresponding to the corrected transmission signal. A wireless communication device characterized by executing the process.

2. The correcting process collectively corrects variations related to at least any one of gain, delay, phase, and frequency characteristics in the plurality of feedback paths. The wireless communication device according to claim 1, characterized by this.

3. The processor further executes a process of calculating a filter coefficient for collectively correcting the estimated analog characteristics. The wireless communication device according to claim 1, characterized by this.

4. The calculating process calculates a per-path filter coefficient corresponding to the analog characteristics of each of the plurality of feedback paths, and calculates a batch filter coefficient by obtaining the sum of the per-path filter coefficients. The wireless communication device according to claim 3, characterized by this.

5. The correcting process corrects the transmission signal by filtering through the one FIR filter with a filter coefficient set so as to obtain characteristics equivalent to the analog characteristics in the plurality of feedback paths. The wireless communication device according to claim 1, characterized in that...

6. The process of correcting is as follows: By passing the feedback signal through one FIR filter with the filter coefficients of the inverse characteristics of the analog characteristics in the plurality of feedback paths for feeding back the feedback signal from the plurality of power amplifiers to the processor, the inverse characteristics are imparted to the feedback signal. The wireless communication device according to claim 1, characterized in that...

7. The processor: Using a convergence algorithm from the transmission signal and the feedback signals of the respective plurality of feedback paths, calculates the path-specific filter coefficients corresponding to the analog characteristics of the respective plurality of feedback paths, Calculates a batch filter coefficient by obtaining the sum of the path-specific filter coefficients. The wireless communication device according to claim 1, further characterized in that it further executes the process.

8. The processor: Calculates, for each direction of the transmission beam, a filter coefficient for collectively correcting the analog characteristics in the plurality of feedback paths, Stores the calculated filter coefficients in association with the direction of the transmission beam. The wireless communication device according to claim 1, further characterized in that it further executes the process.

9. A distortion compensation method executed by a wireless communication device having a plurality of antenna elements, a plurality of power amplifiers provided in the plurality of antenna elements, a processor for outputting a transmission signal to the plurality of power amplifiers, and a plurality of feedback paths for feeding back a feedback signal from the plurality of power amplifiers to the processor, wherein: Estimates the analog characteristics of the plurality of feedback paths, Based on the analog characteristics of each of the estimated feedback paths, calculates a correction value corresponding to the average value of the analog characteristics of the plurality of feedback paths and path-specific filter coefficients corresponding to the remaining variations excluding the average value, Corrects the feedback signal by passing the feedback signal through a filter that corrects using the correction value corresponding to the average value of the analog characteristics in the plurality of feedback paths. A calculated batch filter coefficient calculated from the filter coefficients for each path, which collectively corrects the variations obtained by removing the average value from the analog characteristics in the plurality of feedback paths, is used to pass a transmission signal through one FIR (Finite Impulse Response) to correct the transmission signal. Using the error between the corrected transmission signal and the feedback signal corresponding to the corrected transmission signal, update the distortion compensation coefficient for compensating the non-linear distortion generated in the plurality of power amplifiers. A distortion compensation method characterized by having the process.

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