Low-overhead uplink and downlink channel reciprocity air interface calibration method based on terminal assistance

Abstract

The invention discloses a low-overhead uplink and downlink channel reciprocity air interface calibration method based on terminal assistance, which comprises the following steps of: sending a frequency division orthogonal downlink reference signal sequence by a far-end wireless unit, estimating a downlink channel after a terminal receives a downlink reference signal, and calibrating the downlink channel according to a frequency domain pattern of the downlink reference signal, and mapping the uplink reference signal sequence to different frequency domain positions. A terminal weighs an uplink reference signal sequence according to downlink channels at different frequency domain positions and then sends the uplink reference signal sequence, and a base station side carries out channel estimation and frequency domain fitting interpolation on the uplink reference signal to obtain a full-band calibration coefficient of each antenna. According to the method, service transmission does not need to be interrupted, the self-calibration defect of the base station is overcome, the terminal does not need to report channel information, the continuity of communication between the base station and the terminal is guaranteed, reporting overhead does not exist, it is guaranteed that the orthogonality of the reference signals is not damaged by preprocessing, the calibration coefficient of the full frequency band can be obtained, the calibration coefficient can be filtered, and the accuracy of calibration is improved. And the calibration precision is improved.

Description

Technical Field

[0001] This invention relates to the field of wireless communication transmission technology, specifically a terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method. Background Technology

[0002] In Time Division Duplex (TDD) mode, the uplink and downlink channels are reciprocal. Therefore, the downlink channel can be obtained based on the detected uplink channel, thus enabling coherent downlink transmission. This reciprocity-based coherent downlink transmission is widely used in Massive Multiple-Input-Multiple-Output (MIMO), Coordinated Multi-Point (CoMP), Coherent Joint Transmission (CJT) in Multiple Transmission and Reception Point (Multi-TRP) systems, and Cell-free massive MIMO, a key technology in current 6G.

[0003] However, due to the mismatch in RF gain between the transceiver antennas, the channel is not reciprocal overall, thus requiring reciprocity calibration. Over-the-air (OTA) calibration eliminates the need for additional hardware loops, obtaining calibration coefficients directly from the OTA calibration signal. One OTA calibration method involves each remote radio unit on the base station side transmitting and receiving reference signals, known as base station self-calibration. Another OTA method involves the terminal and each remote radio unit on the base station side transmitting and receiving reference signals, with the terminal reporting downlink channels, and the base station calculating calibration coefficients using the uplink and downlink channels; this is known as terminal-assisted calibration. Base station self-calibration leads to service interruption, while terminal-assisted calibration requires terminal participation, necessitating consideration of terminal processing capabilities and reporting overhead. This invention proposes a low-overhead, low-complexity terminal-assisted calibration method. The terminal only needs to perform simple processing on the uplink reference signal sequence, without reporting downlink channels. After receiving the uplink reference signal, the base station can directly obtain the calibration coefficients for the subcarrier positions of each channel, and the full-band calibration coefficients can be obtained by fitting and interpolating each channel. Summary of the Invention

[0004] To address the problems of existing technologies, this invention provides a terminal-assisted, low-overhead uplink and downlink channel reciprocity air interface calibration method that does not require interruption of service transmission, overcomes the shortcomings of base station self-calibration, and eliminates the need for the terminal to report channel information.

[0005] This application provides a terminal-assisted, low-overhead uplink / downlink channel reciprocity air interface calibration method. The system includes multiple remote radio units, at least one auxiliary terminal, and a base station. The method includes the following steps:

[0006] The multiple remote wireless units are used to transmit frequency division orthogonal downlink reference signals;

[0007] The auxiliary terminal receives the downlink reference signal and performs channel estimation;

[0008] The auxiliary terminal, based on the frequency domain pattern of the downlink reference signal, uses the estimated downlink channel to weight the uplink reference signal sequence and then sends out the uplink reference signal sequence.

[0009] After receiving the uplink reference signal sequence, the base station performs channel estimation and obtains the calibration coefficients of each antenna at the partial subcarrier positions.

[0010] The base station performs fitting interpolation based on the calibration coefficients of the known partial subcarrier positions of each antenna to obtain the calibration coefficients of each antenna across the entire frequency band.

[0011] Optionally, in one embodiment of this application, there are N remote wireless units, each remote wireless unit has L antennas to be calibrated, that is, there are K=NL antennas to be calibrated on the base station side, including 1 auxiliary terminal, which uses 1 antenna to assist in calibration.

[0012] Optionally, in one embodiment of this application, multiple remote wireless units transmit frequency-division orthogonal downlink reference signals. Optionally, a comb pilot structure is adopted, that is, when there are M available subcarriers, the subcarriers occupied by the reference signal sequence transmitted by each antenna are as follows: for example, for antenna k, k = 1, 2, ..., K, the subcarrier index mapped to its reference sequence is... in This indicates rounding down. The reference sequence mapping pattern for each antenna on the base station side is then as follows:

[0013]

[0014] Where x k (k+Ki) represents the reference signal sequence value emitted by base station antenna k at subcarrier k+Ki.

[0015] Optionally, the terminal-side antenna 0 receives the downlink reference signal.

[0016]

[0017] Where y(m) is the reference signal value received by terminal antenna 0 at subcarrier m. Channel estimation is performed based on the frequency domain pattern of the base station pilot. For example, the value of the received sequence at subcarrier {k+Ki, i=0,1,…I} is divided by the pilot sequence value of antenna k {x(k+Ki), i=0,1,…I}, ultimately yielding the channel from the K antennas on the base station side:

[0018]

[0019] in h represents the division of corresponding elements of a matrix. 0,k (k+Ki) represents the estimated channel from base station antenna k at subcarrier k+Ki.

[0020] Terminal preparation uplink reference signal sequence:

[0021]

[0022] Where u(k+Ki) represents the reference signal sequence value at subcarrier k+Ki.

[0023] The uplink reference signal sequence is weighted according to the downlink channel at different subcarrier positions to obtain:

[0024]

[0025] The auxiliary terminal transmits {u(1+Ki) / h} via antenna 0. 0,1 (k+Ki),k=0,1…,K,i=0,1…,I} is emitted through one or more OFDM symbols.

[0026] Optionally, the uplink reference signal sequence received by the base station is:

[0027]

[0028] The base station performs channel estimation based on the subcarriers occupied by their respective antennas, for example, antenna k selects the received sequence of subcarrier {k+Ki, i=0,1,…I} and the original sequence, obtaining:

[0029]

[0030] Where h k,0 (k+Ki) represents the uplink channel of base station antenna k at subcarrier k+Ki, θ k (k+Ki)=h k,0 (k+Ki) / h 0,k (k+Ki) is the calibration coefficient of antenna k on the base station side at subcarrier k+Ki.

[0031] Optionally, the base station obtains the full-band calibration coefficients for each antenna by fitting and interpolating the calibration coefficients of the known partial subcarrier positions of each antenna. For example, it obtains the calibration coefficients {θ} of antenna k on the base station side at subcarrier k+Ki. k The fitting interpolation of (k+Ki),i=0,1,…I} yields:

[0032] The beneficial effects of this invention are as follows:

[0033] 1. By transmitting downlink reference signals and preprocessing uplink reference signals for auxiliary terminals, service transmission can be resumed without interruption, overcoming the shortcomings of base station self-calibration, and the terminal does not need to report channel information.

[0034] 2. It ensures the continuity of communication between the base station and the terminal without any reporting overhead. The reference signal uses frequency-division orthogonal pilots, ensuring that the orthogonality of the reference signal is not destroyed by preprocessing.

[0035] 3. Finally, the base station performs fitting interpolation on the frequency domain calibration coefficients. This not only yields the calibration coefficients for the entire frequency band, but also filters the calibration coefficients to improve calibration accuracy. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a flowchart of a terminal-assisted, low-overhead uplink / downlink channel reciprocity air interface calibration method.

[0038] Figure 2 This is a reference signal frequency domain resource mapping map.

[0039] Figure 3 The figure shows the performance gain of a terminal-assisted, low-overhead uplink / downlink channel reciprocity air interface calibration method. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] In time-division duplex systems, gain mismatch in the radio frequency channels leads to overall uplink and downlink channel non-reciprocity, affecting beamforming / precoding based on uplink and downlink channel reciprocity. Therefore, reciprocity compensation is needed for the uplink detected channel. Base station self-calibration requires each antenna on the base station side to transmit and receive calibration signals, such as using a one-to-many, many-to-one, or grouped transmission and reception scheme. This scheme is transparent to the terminal, but the mutual transmission between base station antennas causes interruption of service data transmission with the terminal. Terminal-assisted calibration avoids service data transmission interruption, but the terminal generally needs to feed back downlink channel information to the base station or report calibration coefficients, resulting in significant reporting overhead. This application proposes a low-overhead uplink and downlink channel reciprocity air interface calibration method based on terminal feedback, which has low computational complexity on the terminal side and requires no information reporting.

[0042] Figure 1 This is a flowchart of a low-overhead uplink / downlink channel reciprocity air interface calibration method according to an embodiment of this application.

[0043] like Figure 1 As shown, the system consists of multiple remote radio units (RRUs), one auxiliary terminal (UE), and a base station (BS). The auxiliary UE is equipped with an auxiliary calibration antenna. There are N RRUs, and each RRU has L antennas to be calibrated. That is, there are a total of K = NL antennas to be calibrated on the base station side.

[0044] The low-overhead uplink / downlink channel reciprocity air interface calibration method based on terminal feedback includes the following steps:

[0045] In step S101, multiple remote radio units transmit frequency-division orthogonal downlink reference signals (or, in other words, orthogonal pilots). Optionally, a comb pilot structure is used. When there are M available subcarriers, the subcarriers occupied by the reference signal sequence transmitted by each antenna are as follows: For example, for antenna k, k = 1, 2, ..., K, the subcarrier index mapped to its reference sequence is... in This indicates rounding down. The reference sequence mapping pattern for each antenna on the base station side is then as follows:

[0046]

[0047] Where x k (k+Ki) represents the reference signal sequence value emitted by base station antenna k at subcarrier k+Ki.

[0048] The following example illustrates the role of the aforementioned reference signal in a multi-node cooperative system. For instance, with four RRUs, each with one antenna, the base station has four antennas. For a 100MHz bandwidth and a subcarrier spacing of 30kHz, there are 3276 subcarriers (or resource elements, REs) and 273 resource blocks (RBs), meaning each RB has 12 REs. For example... Figure 2 As shown, in this case, the frequency domain resource granularity of the pilot for each antenna is 3 REs, meaning each pilot is mapped to a value every 3 REs. The initial offset of the pilots for different antennas in the frequency domain is different; for example, the pilot of antenna k is mapped to the first RE with index k. Therefore, the design of the reference signal described above can ensure that the pilots emitted by different antennas are orthogonal in the frequency domain, i.e., occupying different frequency domain resources.

[0049] It should be emphasized that although the embodiments in this application only illustrate the mapping method of the downlink reference signal on one OFDM symbol, it can be extended to multiple OFDM symbols according to the actual situation. For example, if the base station antenna size is too large and the number of REs that each antenna can map to on the same OFDM symbol is too small, the reference signal sequence length can be guaranteed by mapping to two or more OFDM symbols. Alternatively, if the base station antenna size is not large, time diversity gain can also be obtained by transmitting multiple OFDM symbols.

[0050] In step S102, the terminal-side antenna 0 receives the downlink reference signal.

[0051]

[0052] Where y(m) is the reference signal value received by terminal antenna 0 at subcarrier m. Channel estimation is performed based on the frequency domain pattern of the base station pilot. For example, the value of the received sequence at subcarrier {k+Ki, i=0,1,…I} is divided by the pilot sequence value of antenna k {x(k+Ki), i=0,1,…I} to obtain the channel from the K antennas on the base station side:

[0053]

[0054] in h represents the division of corresponding elements of a matrix. 0,k (k+Ki) represents the estimated channel from base station antenna k at subcarrier k+Ki. 0,k (k+Ki) can be modeled as:

[0055] h 0,k (k+Ki)=r0(k+Ki)c 0,k (k+Ki)t k (k+Ki)

[0056] Where r0(k+Ki) is the RF channel receive gain of the terminal antenna 0 at subcarrier k+Ki, and c 0,k (k+Ki) is the air interface channel gain at subcarrier k+Ki, t k (k+Ki) represents the RF channel transmit gain of base station antenna k at subcarrier k+Ki. The RF channel gain can be modeled as:

[0057] t k (m)=α k,t exp(-ι2πf m τ k +ιφ k,t )

[0058] r k (m)=α k,r exp(ι2πf m τ k +ιφ k,r )

[0059] Where, α k,t α k,r f represents the amplitude of the transmit and receive gain of antenna k's RF channel. m Let τ be the frequency of subcarrier m. k φ is the timing deviation of antenna k. k,t φ k,r ι represents the initial phase of the transmit and receive gain of the RF channel of antenna k, where ι is an imaginary number.

[0060] In step S103, the terminal prepares the uplink reference signal sequence and performs frequency domain mapping based on the frequency domain pattern of the downlink reference signal:

[0061]

[0062] Where u(k+Ki) represents the reference signal sequence value at subcarrier k+Ki.

[0063] By weighting the uplink reference signal sequence using downlink channels at different subcarrier positions, we obtain:

[0064]

[0065] The auxiliary terminal transmits {u(1+Ki) / h} via antenna 0. 0,1 (k+Ki), k=1,2…,K,i=0,1…,I} are emitted through one or more OFDM symbols, u(1+Ki) / h 0,1 (k+Ki) represents the reference signal sequence value emitted by terminal antenna 0 at subcarrier k+Ki.

[0066] It should be noted that this application only discusses the case where the terminal is assisted by a single antenna, but it is also applicable to cases assisted by multiple antennas. A simple extension is: different auxiliary antennas... The sequences are transmitted using different OFDM symbols to ensure sequence orthogonality. For example, the terminal auxiliary antenna 0 antenna... The sequence is transmitted using OFDM symbol S0, and the terminal auxiliary antenna 1 antenna... The sequence is emitted using OFDM symbol S1.

[0067] In step S104, the base station receives the following uplink reference signal sequence:

[0068]

[0069] The base station performs channel estimation based on the subcarriers occupied by their respective antennas, for example, antenna k selects the received sequence of subcarrier {k+Ki, i=0,1,…I} and the original sequence, obtaining:

[0070]

[0071] in h represents the division of corresponding elements of a matrix. k,0 (k+Ki) represents the uplink channel of base station antenna k at subcarrier k+Ki, which can be modeled as:

[0072] h k,0 (k+Ki)=r k (k+Ki)c k,0 (k+Ki)t0(k+Ki)

[0073] Where r k (k+Ki) is the RF channel receive gain of base station antenna k at subcarrier k+Ki, c k,0 (k+Ki) is the air interface channel gain at subcarrier k+Ki. According to the air interface reciprocity of the time-division duplex system, c k,0 (k+Ki)=c 0,k (k+Ki), t k (k+Ki) is the RF channel transmit gain of base station antenna k at subcarrier k+Ki.

[0074] therefore

[0075]

[0076] This is the calibration coefficient of antenna k on the base station side at subcarrier k+Ki. At this time, the calibration coefficients of all antennas on the base station side are referenced to the auxiliary terminal antenna 0.

[0077] In step S105, the base station uses fitting interpolation based on the calibration coefficients of the known partial subcarrier positions of each antenna to obtain the calibration coefficients for the entire frequency band of each antenna. For example, it uses the calibration coefficients {θ} of antenna k on the base station side at subcarrier k+Ki. k The calibration coefficients for the antenna k full-band are obtained by fitting and interpolating the values ​​of (k+Ki), i=0,1,…I}: The key to fitting interpolation lies in solving the time delay difference τ. k,0 =τ k -τ0, initial phase difference and amplitude For the aforementioned comb-shaped pilot structure, the Fast Fourier Transform (FFT) algorithm is highly suitable because the frequency grid points of the pilots are equally spaced. In a specific embodiment, the time delay difference τ estimated using the FFT algorithm... k,0 for:

[0078]

[0079] Where Γ = {0, 1, ..., N} FFT -1},N FFT Let be the number of FFT points, and δ be the interval between adjacent frequency grid points of the pilot. For example, in a system with a subcarrier interval of Δ, δ = KΔ.

[0080]

[0081] Where i∈{0,1,2,…,I}, and for i>I, θ k (k+Ki)=0.

[0082] Therefore, estimate τ k,0 The first step is to work on θ k Each element of (k+Ki) is subjected to N FFT Point FFT transformation is used to obtain each F(λ), and then the λ that maximizes |F(λ)| is selected. * ,get final

[0083] Using the FFT algorithm to analyze the initial phase Make an estimate:

[0084]

[0085] in:

[0086]

[0087] After estimating Subsequently, the calibration coefficients across the entire frequency band of the base station antenna k can be expressed as:

[0088]

[0089] in

[0090] It should be noted that solving for the time delay difference τ k,0 Initial difference Amplitude α k,0 The algorithms are already very mature, including but not limited to the FFT algorithm, least squares, and quadratic interpolation.

[0091] like Figure 3 As shown, a performance gain diagram of terminal-assisted calibration applied to downlink precoding is presented. "Ideal calibration" refers to calibrating the spectral efficiency for downlink precoding when the RF channel gain is known. "No calibration" refers to using the uplink channel directly for precoding without calibrating the uplink channel before downlink precoding, resulting in spectral efficiency issues. "Terminal-assisted calibration" refers to the calibration scheme proposed in this invention. It can be seen that the terminal-assisted calibration performance proposed in this invention can approach ideal calibration, and the terminal does not need to report; it only needs to send the uplink reference signal.

[0092] The terminal-assisted, low-overhead uplink / downlink channel reciprocity air interface calibration method proposed in this application, through the transmission of downlink reference signals and the preprocessing of uplink reference signals by the auxiliary terminal, eliminates the need to interrupt service transmission and the terminal to report channel information. This ensures the continuity of communication between the base station and the terminal without reporting overhead. The reference signal uses frequency-division orthogonal pilots to ensure that the orthogonality of the reference signal is not destroyed by preprocessing. Finally, the base station performs fitting interpolation on the frequency domain calibration coefficients, which yields calibration coefficients across the entire frequency band and also filters the calibration coefficients to improve calibration accuracy.

[0093] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on its differences from other embodiments. In particular, for the device embodiments, the above descriptions are merely preferred embodiments of the present invention. Since they are fundamentally similar to the method embodiments, the descriptions are relatively simple, and relevant parts can be referred to the descriptions of the method embodiments. The above descriptions are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention, without departing from the principle of the present invention, should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method, using a terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration system composed of multiple remote wireless units, one or more auxiliary terminals and a base station, characterized in that The following steps are involved: 1) Multiple remote wireless units send frequency-division orthogonal downlink reference signals; 2) The auxiliary terminal receives the downlink reference signal and performs channel estimation; 3) The auxiliary terminal weights the uplink reference signal sequence using the estimated downlink channel according to the frequency domain pattern of the downlink reference signal and then sends the uplink reference signal sequence; 4) The base station receives the uplink reference signal sequence, performs channel estimation, and obtains the calibration coefficients of each antenna at some subcarrier positions; 5) The base station performs fitting interpolation based on the calibration coefficients of some subcarrier positions known for each antenna to obtain the calibration coefficients for the full frequency band of each antenna.

2. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: In the terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration system, the base station has N remote wireless units, each remote wireless unit has L antennas to be calibrated, that is, there are a total of K=NL antennas to be calibrated on the base station side, including 1 auxiliary terminal, and the terminal uses 1 antenna to assist in calibration.

3. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: In step 1), multiple remote wireless units send frequency-division orthogonal downlink reference signals using a comb-shaped pilot structure, that is, when there are M subcarriers available for OFDM symbols, the subcarriers occupied by the reference signal sequence sent by each antenna are as follows: for antenna k, k = 1, 2, ..., K, the subcarrier index to which its reference sequence is mapped is {k+Ki, i = 0, 1, ... I}, in represents rounding down, then the reference sequence mapping pattern of each antenna on the base station side is: where x k (k+Ki) represents the reference signal sequence value sent by base station antenna k at subcarrier k+Ki.

4. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: The downlink reference signal in step 1) is mapped to two or more OFDM symbols to ensure the reference signal sequence length or to obtain time diversity gain by sending multiple OFDM symbols.

5. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: Step 2) The downlink reference signal received by the auxiliary terminal is: Where y(m) is the reference signal value received by the terminal antenna 0 at subcarrier m. According to the frequency domain pattern of the base station pilot, the channel is estimated. The value of the received sequence at the subcarrier {k+Ki,i=0,1,…I} is divided by the pilot sequence value of antenna k {x(k+Ki),i=0,1,…I}, and finally the channel from K antennas on the base station side is obtained: in Represents the division of corresponding elements of the matrix, h 0,k (k+Ki) represents the estimated channel from base station antenna k at subcarrier k+Ki.

6. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: The uplink reference signal sequence described in step 3) is: Where u(k+Ki) represents the reference signal sequence value at subcarrier k+Ki; The uplink reference signal sequence is weighted according to the downlink channels at different subcarrier positions to obtain: The auxiliary terminal transmits {u(1+Ki) / h 0,1 (k+Ki), k=1,2…,K,i=0,1…,I} is transmitted through one or more OFDM symbols.

7. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 6, characterized in that: For multiple antennas, different auxiliary antennas The sequences are sent using different OFDM symbols to ensure sequence orthogonality.

8. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: In step 4), the uplink reference signal sequence received by the base station is: The base station side performs channel estimation based on the subcarriers occupied by each antenna, specifically the received sequence and original sequence of the subcarrier {k+Ki,i=0,1,…I} for antenna k, and obtains: where h k,0 (k+Ki) represents the uplink channel of antenna k at the base station side at subcarrier k+Ki, θ k (k+Ki)=h k,0 (k+Ki) / h 0,k (k+Ki) is the calibration coefficient of the base station side antenna k at subcarrier k+Ki.

9. The terminal-assisted low-overhead uplink and downlink channel reciprocity air interface calibration method according to claim 1, characterized in that: Step 5) The specific process is as follows: the base station fits and interpolates the calibration coefficients of the known partial subcarrier positions of each antenna to obtain the calibration coefficients of the full-band of each antenna, and calculates the calibration coefficients of the antenna k at the subcarrier k+Ki on the base station side {θ k (k+Ki), i=0,1,…I} are fitted and interpolated to obtain:

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