A fully digital multibeam system and method based on band-limited predistortion

Abstract

This invention discloses a fully digital multibeam system based on band-limited predistortion. The system includes a digital beamforming network (1), a predistortion module (2), a fully digital multibeam array transmitter (3), an antenna array (4), and an OTA receiving link (5). This invention uses a band-limited digital predistortion model to sequentially predistort the channels in the array, making the beamformed signal of each channel have nonlinear characteristics opposite to the input-output characteristics of the channel's power amplifier. Then, all channels are activated, and the preprocessed signal is radiated into space through the fully digital array transmitter and antenna array. The target beam, linearized and unaffected by interference from other beams, can be decomposed using a decomposition algorithm. This invention can construct a simple, easily implemented, and computationally efficient band-limited digital predistortion model to achieve linearization of the fully digital multibeam system.

Description

Technical Field

[0001] This invention pertains to digital predistortion technology, and particularly relates to a fully digital multibeam system and method based on band-limited predistortion. Background Technology

[0002] Beamforming technology is one of the key solutions to overcome the severe path attenuation problem of millimeter waves. Through large-scale antenna arrays, high beam gain can be achieved, which helps improve communication speed and save energy. Generally, there are three types of beamforming technology: all-digital beamforming, analog beamforming, and hybrid beamforming. Among them, all-digital beamforming technology is widely used in multi-beam scenarios because it can flexibly control the steering of multiple beams and easily construct beamforming networks.

[0003] Similar to traditional communication systems, power amplifiers in massive MIMO arrays operate in high-efficiency mode to conserve energy, leading to beam nonlinearity, memory effects, and in-band and out-of-band distortion. This is especially true in massive MIMO arrays, where signal distortion on each channel results in more severe beam distortion in the final transmitted signal. Therefore, introducing appropriate digital predistortion (DPD) techniques is necessary in massive MIMO arrays. In recent years, the application of DPD techniques in multi-beam systems has attracted considerable attention. However, the limited bandwidth of MIMO systems has not yet been considered. Traditional DPD techniques typically require the communication system bandwidth to be several times greater than the input signal bandwidth. Currently, the commonly used 5G NR signal bandwidth is as high as 400MHz. According to traditional DPD techniques, this means a linearization bandwidth of 2GHz is required, significantly increasing the design complexity of MIMO systems. The high-speed data converters and ultra-wideband transmit and receive links required further increase the cost of DPD. Furthermore, even if the system bandwidth is sufficient, it is unnecessary to linearize the power amplifier (PA) to a 2GHz bandwidth; we can eliminate only a portion of the distortion within the center frequency band of the input signal. The portion exceeding this band can be eliminated using filters. Therefore, it makes sense to use band-limited techniques in MIMO systems. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a fully digital multibeam system based on band-limited predistortion, aiming to achieve digital predistortion of the fully digital multibeam system with a simple, interpretable and easy-to-implement algorithm.

[0005] Technical Solution: To achieve the above-mentioned objectives, the present invention provides a fully digital multibeam system based on band-limited predistortion, employing the following technical solution:

[0006] The system includes a digital beamforming network, a predistortion module, a fully digital multi-beam array transmitter, an antenna array, and an OTA (Over-The-Air) receiving link. In the fully digital multi-beam system, the final received beam will be distorted due to the nonlinearity of the power amplifiers on the channels and interference from other beams. In the fully digital multi-beam system, there are multiple channels. For a single channel, the original signal passes through the digital beamforming network, the predistortion module, and the fully digital multi-beam array transmitter, is radiated into space through the antenna array, and then the signal is received by the OTA receiving link, forming a feedback loop and constructing the predistortion of the single channel. After the predistortion of all channels is completed in sequence, all channels are turned on, and the digitally beamformed and predistorted signals are transmitted as baseband signals into their respective channels. After passing through the fully digital multi-beam array transmitter and the antenna array, multiple beams are radiated into space. Finally, the beams are received by the OTA receiving link, and after separation algorithm, the linearized target beam is obtained. Among them, the single-channel predistortion in the predistortion module uses a band-limited model.

[0007] The all-digital multi-beam system achieves beamforming entirely in the digital domain, with the original N beams being x1, x2, ... x N The output signals of the digital beamforming network are u1, u2, ... u K ,in,

[0008] u1 = x1 + x2 + ... + x N ,

[0009]

[0010] Where d represents the distance between antenna elements, λ represents the wavelength, and θ i Let represent the direction of the i-th beam emission, and let at this time

[0011]

[0012] The output signals u1, u2, ... u of the digital beamforming network K In a fully digital multibeam system, the power amplifier will produce nonlinear distortion. In a fully digital multibeam system, the distortion generated by the power amplifier is: when signal u... k After passing through the power amplifier on the k-th channel, the signal u k It will be amplified, and the amplified signal is

[0013]

[0014] Among them, H k This represents the characteristic of the power amplifier on the k-th channel, i.e., the nonlinear gain.

[0015] The band-limited predistortion described herein is based on the traditional Volterra model, but with the addition of a digital filter to the Volterra operator to flexibly control the predistortion bandwidth.

[0016] In a generalized memory polynomial model, the traditional generalized memory polynomial model is as follows:

[0017]

[0018] Where x(n) and y(n) represent input and output, respectively, and K... a L a It is the nonlinear order and memory depth of the aligned term memory polynomial, K. b L b M b These represent the nonlinear order of the lag term, the memory depth, and the lag order, respectively; L c L c M c , respectively, represent the nonlinear order of the lead term, memory depth, and lead order. kl b klm c klm These are the coefficients of the alignment term, memory polynomial, lag term, and lead term, respectively; introducing the frequency response ω(n) of the band-limited filter, and * for convolution operation, we have the band-limited-generalized memory polynomial model:

[0019]

[0020] With the band-limited generalized memory polynomial model, predistortion of each channel of an all-digital multibeam system can be achieved with limited bandwidth.

[0021] The distortion of the finally received beam is not only due to the power amplifier on the channel, but also to interference from all other beams besides this beam. The amplified signal is denoted as U. k Ultimately, U1, U2, ... U K After being radiated into space by their respective antenna elements, the signal received at angle α in the far-field direction is:

[0022]

[0023] Where p k (α) represents the phase shift caused by the antenna array, expressed as: at this time:

[0024]

[0025] After the above band-limited predistortion:

[0026]

[0027] Where G represents the linearized power amplifier characteristics, i.e., the linear gain;

[0028] Assume that the angle α at which the received signal is transmitted is the direction of beam x1. That is:

[0029]

[0030] at this time

[0031]

[0032] From the above formula, it can be seen that although the signal is received in the direction of beam x1, the received signal is not only the main lobe of beam x1, but also includes the sidelobe signals of other beams.

[0033] When receiving beam signals, in the direction of the main lobe of a target beam, what is received is not only the main lobe signal of that target beam, but also the sidelobe signals of all other beams besides the target beam. This target beam will be affected by interference from all other beams except itself, which can be removed linearly. To decompose the beam into independent beams, in a two-beam scenario...

[0034] The signal received in the main lobe direction of beam 1:

[0035]

[0036] The signal received in the main lobe direction of beam 2:

[0037]

[0038] Where α1 and α2 are the receiving angles of the two beams.

[0039] Let Y = [y RX1 y RX2 ]

[0040] X = [x1 x2]

[0041]

[0042] Write the above equation as Y T =FX T Where T represents transpose.

[0043] To extract the desired beam, a matrix C is needed:

[0044]

[0045] Right now

[0046] so

[0047] According to the formula, matrix F can be obtained using the LS algorithm:

[0048] F=((X H X) -1 X H Y) T

[0049] H represents the conjugate transpose operation; based on the above, the linearized beam is separated.

[0050] Beneficial effects: Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1) A band-limited predistortion-based all-digital multibeam system, which offers free and flexible beamforming for all-digital arrays;

[0052] 2) Linearizing each channel of the all-digital array enables full-angle linearization of multiple beams;

[0053] 3) A band-limited predistortion model is used, which can achieve linearization within a certain bandwidth when the bandwidth of the MIMO system is insufficient, and can also freely and flexibly control the linearization bandwidth. Attached Figure Description

[0054] Figure 1 The diagram shows a block diagram of the system in a specific embodiment of the present invention. In the specific implementation, a 4-channel, 2-beam all-digital multi-beam system is established. The diagram includes: a digital beamforming network 1, a predistortion module 2, an all-digital multi-beam array transmitter 3, an antenna array 4, and an OTA receiving link 5.

[0055] Figure 2 In a specific embodiment of the invention, an input signal with a center frequency of 4GHz and a bandwidth of 20MHz is mixed to 26GHz by a fully digital transmitter. The amplitude characteristic curve and phase characteristic curve of beam 1 before and after predistortion are shown.

[0056] Figure 3 In a specific embodiment of the invention, an input signal with a center frequency of 4GHz and a bandwidth of 20MHz is mixed to 26GHz by a fully digital transmitter. The amplitude characteristic curve and phase characteristic curve of beam two before and after predistortion are shown.

[0057] Figure 4 In a specific embodiment of the invention, an input signal with a center frequency of 4GHz and a bandwidth of 20MHz is mixed to 26GHz by a fully digital transmitter. The spectrum diagram of beam 1 before and after predistortion is shown.

[0058] Figure 5In a specific embodiment of the invention, an input signal with a center frequency of 4GHz and a bandwidth of 20MHz is mixed to 26GHz by a fully digital transmitter. The spectrum diagrams of beam two before and after predistortion are shown. Detailed Implementation

[0059] The technical solution of the present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0060] This specific embodiment discloses a fully digital multibeam system based on band-limited predistortion, such as Figure 1 As shown, the system includes a digital beamforming network 1, a predistortion module 2, a fully digital multi-beam array transmitter 3, an antenna array 4, and an OTA receiving link 5. In a fully digital multi-beam system, the final received beam will be distorted due to the nonlinearity of the power amplifier on the channel and interference from other beams. In a fully digital multi-beam system, there are multiple channels. For a single channel, the original signal passes through the digital beamforming network 1, the predistortion module 2, and the fully digital multi-beam array transmitter 3, is radiated into space through the antenna array 4, and then receives the signal via an OTA link. The receiving link 5 receives the signal, forming a feedback loop and constructing a single-channel predistortion. After completing the predistortion of all channels in sequence, all channels are turned on, and the digitally beamformed and predistorted signals are transmitted as baseband signals into their respective channels. After passing through the all-digital multi-beam array transmitter 3 and antenna array 4, multiple beams are radiated into space. Finally, the beams are received through the OTA receiving link 5, and after separation algorithm, the linearized target beam is obtained. Among them, the single-channel predistortion in the predistortion module 2 uses a band-limited model.

[0061] The specific implementation of beamforming in the digital domain of a band-limited predistortion-based all-digital multibeam system is as follows: Assume the original N signals (beams) are x1, x2, ... x N The output signals of the beamforming network are u1, u2, ... u K ,in,

[0062] u1 = x1 + x2 + ... + x N ,

[0063]

[0064] Where d represents the distance between antenna elements, λ represents the wavelength, and θ i Let represent the direction of transmission of the i-th beam. To simplify the following derivation, let . at this time

[0065]

[0066] In a fully digital multibeam system based on band-limited predistortion, the signal undergoes nonlinear distortion after passing through a power amplifier in the array. In a fully digital multibeam system, the distortion generated by the power amplifier can be derived theoretically as follows: when signal u... k After passing through the power amplifier on the k-th channel, the signal u k It will be amplified, and the amplified signal is

[0067]

[0068] Among them, H k This represents the characteristic of the power amplifier on the k-th channel, i.e., the nonlinear gain.

[0069] In a fully digital multibeam system based on band-limited predistortion, the characteristic of band-limited predistortion is the addition of a digital filter to the Volterra operator in the traditional Volterra model to flexibly control the predistortion bandwidth. Taking a generalized memory polynomial model as an example, the traditional generalized memory polynomial model can be written as:

[0070]

[0071] Where x(n) and y(n) represent input and output, K a L a It is the nonlinear order and memory depth of the aligned term memory polynomial, K. b L b M b K represents the nonlinear order of the lag term, the memory depth, and the lag order, respectively. c L c M c , respectively, represent the nonlinear order of the lead term, memory depth, and lead order. kl b klm c klm These are the coefficients of the alignment term, memory polynomial, lag term, and lead term, respectively. Introducing the frequency response ω(n) of the band-limited filter, and * for convolution, we have the band-limited-generalized memory polynomial model:

[0072]

[0073] With the band-limited generalized memory polynomial model, predistortion of each channel of an all-digital multibeam system can be achieved with limited bandwidth.

[0074] In a fully digital multibeam system based on band-limited predistortion, the distortion of the final received beam is not only due to the power amplifier on the channel, but also to interference from all other beams besides the receiving beam. As mentioned above, the amplified signal is denoted as U. k Ultimately, U1, U2, ... UK They are radiated into space through their respective antenna elements.

[0075] Finally, the signal received at angle α in the far-field direction is

[0076]

[0077] Where p k (α) represents the phase shift caused by the antenna array, expressed as:

[0078] at this time:

[0079]

[0080] After the above band-limited predistortion:

[0081]

[0082] Where G represents the linearized power amplifier characteristics, i.e., the linear gain;

[0083] Assume that the angle α at which the received signal is transmitted is the direction of beam x1. That is:

[0084]

[0085] at this time

[0086]

[0087] As can be seen from the formula, even though the signal is being received in the direction of beam x1, the received signal is not only the main lobe of beam x1, but also includes the sidelobe signals of other beams.

[0088] In a fully digital multibeam system based on band-limited predistortion, when receiving beam signals, in the direction of the main lobe of a target beam, what is received is not only the main lobe signal of the target beam, but also the side lobe signals of all other beams besides the target beam. This target beam will be affected by interference from all other beams except itself, which can be removed in a linear way: In order to decompose and obtain independent beams, take a scenario with two beams as an example.

[0089] The signal received in the main lobe direction of beam 1:

[0090]

[0091] The signal received in the main lobe direction of beam 2:

[0092]

[0093] Where α1 and α2 are the receiving angles of the two beams.

[0094] Let Y = [y RX1 y RX2 ]

[0095] X = [x1 x2]

[0096]

[0097] Therefore, the above equation can be written as Y. T =FX T , where T represents transpose.

[0098] To extract the desired beam, we need a matrix C:

[0099]

[0100] Right now

[0101] so

[0102] According to the formula, matrix F can be obtained using the LS algorithm:

[0103] F=((X H X) -1 X H Y) T

[0104] H represents the conjugate transpose operation. Based on the above theory, we can separate the linearized beam.

[0105] Taking an input signal with a center frequency of 4GHz and a bandwidth of 20MHz, mixed to 26GHz by a fully digital transmitter as an example, the band-limited predistortion proposed in this invention for linearizing the fully digital multibeam system achieves good results. The amplitude characteristic curves, phase characteristic curves, and power spectral density diagrams of the two beams after linearization and decomposition are shown below. Figure 2-5 As shown, using band-limited predistortion technology to linearize an all-digital multibeam system enables a fast, easy-to-operate, flexible, and easily understandable digital predistortion process.

[0106] As described above, although the invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A fully digital multibeam system based on band-limited predistortion, characterized in that, The system includes a digital beamforming network (1), a predistortion module (2), a fully digital multi-beam array transmitter (3), an antenna array (4), and an OTA receiving link (5). In the fully digital multi-beam system, the final received beam will be distorted due to the nonlinearity of the power amplifier on the channel and interference from other beams. In the fully digital multi-beam system, there are multiple channels. For a single channel, the original signal passes through the digital beamforming network (1), the predistortion module (2), and the fully digital multi-beam array transmitter (3), and is radiated into space through the antenna array (4). The signal is then received through the OTA receiving link (5) to form a feedback loop, thus constructing the predistortion of the single channel. After completing the predistortion of all channels in sequence, all channels are turned on, and the digital beamforming signal and the predistorted signal are transmitted as baseband signals into their respective channels. After passing through the all-digital multi-beam array transmitter (3) and antenna array (4), multiple beams are radiated into space. Finally, the beams are received through the OTA receiving link (5), and after separation algorithm, the linearized target beam is obtained. Among them, the single-channel predistortion in the predistortion module (2) uses a band-limited model. The all-digital multibeam system achieves beamforming entirely in the digital domain, unlike the original... Each beam is The output signal of the digital beamforming network is ,in, , ， in, Indicates the distance between antenna elements. Indicates wavelength. Indicates the first The direction of beam emission, making ,at this time, ； The band-limited predistortion described herein is based on the traditional Volterra model, but with the addition of a digital filter to the Volterra operator to flexibly control the predistortion bandwidth. In a generalized memory polynomial model, the traditional generalized memory polynomial model is as follows: ， in, , Indicates input and output. , It refers to the nonlinear order and memory depth of the aligned term memory polynomial. , , These represent the nonlinear order of the lag term, the memory depth, and the lag order, respectively. , , These represent the nonlinear order of the lead term, the memory depth, and the lead order, respectively. , , These are the coefficients of the alignment term memory polynomial, the lag term, and the lead term; the frequency response introduced by the band-limited filter. , For convolution operations, we have the band-limited generalized memooth polynomial model: ， With the band-limited generalized memory polynomial model, predistortion of each channel of an all-digital multibeam system can be achieved with limited bandwidth; When receiving beam signals, in the direction of the main lobe of a target beam, what is received is not only the main lobe signal of that target beam, but also the sidelobe signals of all other beams besides the target beam. This target beam will be affected by interference from all other beams except itself. This interference is removed linearly: to decompose the beam into independent beams, in a two-beam scenario... The signal received in the main lobe direction of beam 1: The signal received in the main lobe direction of beam 2: ， in, , These are the receiving angles of the two beams; This represents the phase shift caused by the antenna array; Let G be the angle in the far-field direction, and G represent the power amplifier characteristics after linearization. make , ， ， Write the above formula as ,in, Indicates transpose. To extract the desired beam, a matrix is ​​needed. : ， Right now , so , According to the formula, the matrix It can be obtained using the LS algorithm: ， This represents the conjugate transpose operation; based on the above, the linearized beam is separated.

2. The all-digital multibeam system based on band-limited predistortion according to claim 1, characterized in that, The output signal of the digital beamforming network In a fully digital multibeam system, the power amplifier will produce nonlinear distortion. The distortion generated by the power amplifier in a fully digital multibeam system is: when the signal... Through the first After the power amplifier on each channel, the signal It will be amplified, and the amplified signal is: ， in, Indicates the first The characteristics of the power amplifier on each channel, i.e., the nonlinear gain.

3. A fully digital multibeam system based on band-limited predistortion according to claim 2, characterized in that, The distortion of the ultimately received beam is not only due to the power amplifier on the channel, but also to interference from all other beams besides the one in question. The amplified signal is denoted as... ;final, After being radiated into space by their respective antenna elements, the angle in the far-field direction... The signal received above is: ， in The phase shift caused by the antenna array is represented as: ; at this time: ， After the above band-limited predistortion: ， in This represents the characteristics of the power amplifier after linearization, i.e., the linear gain; Assuming the angle at which the signal is received is... It is a beam The direction of launch; that is: ， at this time, ， From the above formula, it can be seen that although this is in the beam The direction of signal reception is not limited to the beam. The main lobe also contains sidelobe signals from other beams.

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