Wideband antenna calibration in time division duplex systems

Abstract

The present disclosure provides a method for antenna calibration in a time division duplex (TDD) wireless communication system. The method includes defining a calibration reference signal that fits within an available time gap outside of the active region of a TDD frame and that supports the desired bandwidth of a wideband OFDM system, while confirming to the emission requirements. The method further includes transmitting the calibration reference signal during the available time gap and performing antenna calibration based on the transmitted calibration reference signal without disrupting live traffic. The calibration reference signal may be transmitted or received through multiple antenna ports for transmit or receive calibration respectively, and phases of the feedback from the calibration signal are compared to determine the calibration coefficients.

Description

WIDEBAND ANTENNA CALIBRATION IN TIME DIVISION DUPLEX SYSTEMSFIELD OF INVENTION

[0001] The present disclosure relates to antenna calibration in wireless communication systems, particularly to calibrate Radio systems with large number of antennas or for advanced antenna systems operating in time division duplexing.BACKGROUND

[0002] In large scale antenna systems or in massive Multiple-Input Multiple-Output (MIMO) systems, the use of numerous antenna elements allows for the implementation of beamforming techniques, which can significantly improve signal quality and reduce interference. However, the effectiveness of these systems relies heavily on the accurate calibration of the antenna array to ensure optimal performance. Antenna calibration is essential for compensating for hardware imperfections, such as phase and amplitude mismatches between different antenna elements, which can arise due to manufacturing variations, temperature fluctuations, and other environmental factors.

[0003] Traditional antenna calibration methods often require dedicated calibration periods, during which the system is taken offline or operates at reduced capacity. This approach can lead to decreased network availability, dependence on other network elements, and reduced overall system efficiency. Additionally, some calibration techniques may necessitate the use of external reference signals orspecialized equipment, further complicating the calibration process and increasing system costs.

[0004] Time Division Duplex (TDD) systems, which alternate between uplink and downlink transmissions in the same frequency band, present both challenges and opportunities for antenna calibration. The reciprocity of the wireless channel in TDD systems can be leveraged for calibration purposes, but the limited time available between uplink and downlink transmissions poses constraints on when and how calibration can be performed.

[0005] Furthermore, the calibration process for wideband antenna systems presents additional challenges. In external calibration systems, the dependence on other network elements for calibration further complicates the process. This interdependence can introduce delays and can increase system complexity. It also limits the autonomy of individual network components, making it more challenging to implement localized or adaptive calibration strategies that could otherwise optimize performance on a more granular level.

[0006] As of today, self-calibration systems require either extensive offline sounding procedures, which consume substantial time and resources, or require the transmission of calibration tones during designated offline periods. While potentially more efficient in terms of time utilization, the latter approach is either not the best approach for large bandwidths or carries the risk of violating strict emission requirements set by regulatory bodies.

[0007] These limitations highlight the need for innovative self-calibration techniques that can operate seamlessly within the constraints of live network operations, particularly in TDD systems where the time available for calibration is inherently limited. Developing methods that can perform accurate calibration without significant system downtime, resource reservation, or emission requirement violations would represent a significant advancement in antenna array technology, potentially leading to more efficient, flexible, and high-performance wireless networks.

[0008] In TDD systems, which alternate between uplink and downlink transmissions within a shared radio frame, guard periods are incorporated to facilitate the transition between these transmission modes. While these guard periods present an opportunity for calibration signal transmission, the existing approach of using incrementally adjusted tone frequency to cover the desired channel bandwidth is fraught with limitations, particularly for wideband applications.

[0009] The maximum allowed Tx transition time past the end of DL frame and the emission during Tx OFF period, as specified by the 3rd Generation Partnership Project (3GPP), are critical parameters that wireless communication systems must adhere to for proper operation and regulatory compliance. The Tx transition time refers to the period allowed for a transmitter to switch between its " ON" and " OFF" states, while the emission requirements during the Tx OFF period ensure that the transmitter does not produce unwanted signals that could interfere with other wireless services.

[0010] These parameters vary significantly across different wireless standards and frequency ranges as summarized below in table 1.5G NR (Sec 9.5.2 TS 38.104) LTE FR1 FR2 Standard (Sec 6.4.1,BS Type 1-C, BS BS Type 1- (BS type TS 36.104)Type 1-H O 2-O) Maxtransient 17 10 3 Emission period [µs]Requirements BW support 20 MHz 100 MHz 100 MHz 400 MHz Transmit -85dBm / -85 dBm / MHz -106 dBm / -36 dBm 1 OFF Power MHz MHz MHzTable 1This approach would enable a similar calibration method to be used across multiple wireless standards and frequency ranges, simplifying system design and improving overall efficiency.

[0011] The development of a timing strategy for antenna calibration that can work seamlessly across standards helps in achieving scalability across mobile networks, particularly in the context of massive MIMO and wideband applications. This strategy would need to balance the need for comprehensive calibration with the strict timing and emission requirements, potentially leveraging innovative signalprocessing techniques or novel calibration waveforms to achieve optimal performance across diverse wireless communication systems.

[0012] Moving beyond external calibration methods, self-calibration techniques have been developed to address some of these limitations. One such approach known in the art, focuses on calibration for LTE-based antenna array systems. This method employs a dual strategy: it utilizes feedback from downlink synchronization signals through a receive path for downlink calibration, while also leveraging the base station's knowledge of Sounding Reference Signals (SRS) for each user, which are stored in a buffer.

[0013] However, this calibration approach is not without its own set of challenges. A primary limitation lies in the nature of the downlink synchronization signal itself. In many cases, this signal does not span the entire channel bandwidth, resulting in what can be termed a partial-band calibration. This incomplete coverage of the frequency spectrum can lead to suboptimal calibration results, particularly in wideband systems where accurate calibration across the entire operational bandwidth is crucial for maintaining high performance.

[0014] These limitations underscore the ongoing challenges in developing robust, efficient, and universally applicable antenna calibration techniques for advanced wireless communication systems. The ideal solution would need to address the shortcomings of both external and self-calibration methods, providing full-bandwidth calibration capability without relying on external devices or significantly disrupting normal network operations.Another prior art as shown in Figure 1 i.e. US patent application US 2019 / 0044624 A1 by NEC Corporation discusses a method for calibrating antennas in Active Antenna Systems (AAS) during brief periods of signal change. It performs downlink (DL) and uplink (UL) calibration during specific time slots within a Special Sub Frame of the TDD frame structure. The system uses a calibration transmitter and receiver to measure and compensate for variations in amplitude and phase characteristics of the transmitter-receivers. The prior art lacks wideband calibration capabilities, making it unsuitable for newer technologies like 5G that require calibration across wide frequency ranges. The method necessitates a separate path to carry the common signal during the calibration of transmit and receive antennas, adding to the complexity and cost compared to more integrated approaches. Additionally, it fails to address RF emission compliance during calibration and lacks flexibility to adapt to varying transient periods across different wireless standards. These limitations significantly reduce the method's practical use and scalability in advanced wireless communication networks.These limitations highlight the ongoing challenges in developing comprehensive antenna calibration techniques that can meet the demands of modern wireless communication systems. As the industry moves towards more advanced, wideband, and flexible network architectures, there is a clear need for calibration methods that can address these shortcomings, offering wideband capability, hardware efficiency, emission compliance, and cross-standard adaptability.SUMMARY

[0015] The invention relates to an advanced method for calibrating antenna arrays in wireless communication systems, by using a unique calibration symbol that occupies the desired bandwidth of a wideband system such that it can fit within an available time gap outside the active transmission or reception time in a TDD system, thereby avoiding any disruption of live traffic while meeting emission requirements.

[0016] In another aspect of the present invention including determining the permissible duration of the calibration symbol and a suitable frequency domain composition based on the desired frequency bandwidth and the available time gap by computing the difference between an allocated ramp-down period and the actual ramp-down period of a power amplifier for a given output power. And, further, if the available time is N times smaller than the standard OFDM symbol, then a frequency domain calibration symbol using ‘1 / N’ frequency bins is up-sampled by a factor of ‘N’ which after an IFFT operation becomes ‘N’ identical time domain copies of ‘1 / N’ symbol time, and one of the latter is used for antenna calibration.

[0017] And, for receive antenna calibration, transmitting one of multiple similar time copies of the calibration reference signal just before the start of an uplink frame.

[0018] And for validating the calibration, the RMS value of the residual phase error is compared against a threshold. Based on this threshold the calibration is said to be success or failure.BRIEF DESCRIPTION OF FIGURESThe figures described below depict various aspects of the system and methods disclosed herein. It should be understood that each figure depicts an embodiment of a particular aspect of the disclosed system and methods, and that each of the figures is intended to accord with a possible embodiment thereof. Further, wherever possible, the following description refers to the reference numerals included in the following figures, in which features depicted in multiple figures are designated with consistent reference numerals.FIG. 1 shows an example of a frame structure and timing considerations in an LTE TDD system, particularly focusing on the calibration process within the Active Antenna System (AAS), according to one of the prior arts.FIG. 2 illustrates the generation of a fractional-length, full-bandwidth calibration symbol according to one embodiment of the present invention. The generated symbol is designed to fit into an available time gap before power amplifier ramp-down, in accordance with one embodiment of the present invention.FIG. 3 depicts a block diagram architecture of a transceiver, illustrating the signal paths for both transmit and receive antenna calibration processes according to one embodiment of the present invention.FIG. 4 illustrates antenna radiation patterns before and after calibration, according to one embodiment of the present invention.FIG. 5 shows a flowchart of a method for antenna calibration in a wireless communication system in accordance with one embodiment of the present invention.FIG. 5(A), 5(B), and 5(C) presents a detailed flowchart outlining the steps involved in the Tx calibration process, Rx calibration process and Validation process of the method of FIG. 5 respectively, under one embodiment of the present invention.FIG. 6 is a block diagram illustrating an example of a schematic hardware configuration of the network node according to the embodiments of the present disclosure.Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.DETAILED DESCRIPTION OF THE INVENTION

[0019] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention.

[0020] FIG. 2 illustrates the generation of a fractional-length, full-bandwidth calibration symbol in the available time gap before power amplifier ramp-down according to one embodiment of the present invention. This involves two steps: In Step 1, wide bandwidth calibration symbol is designed and in Step 2, the generated calibration symbol in placed in the time gap of the TDD frame.

[0021] In an example embodiment, generating a full bandwidth calibration symbol involves using a 4096-point IFFT to create the symbol, which may span the entire bandwidth of 100 MHz of an FR1 system. The frequency domain representation of the symbol is shown at the top-left of Step 1 (210), while the time domain representation is shown at 220. In 220, because of up-sampled frequency domain signal, there will be repetitions in time and one of the copies is used for calibration (230). In the example embodiment, 1 out of 16 copies is used for calibration, thereby making it fit into the available time gap (240) of 2.2µs.

[0022] Step 2 of the FIG. 2 shows the placement of the calibration symbols for Transmit and Receive calibration schemes. The example embodiment shows placement of the symbol in LTE / 5G NR frame structure. For Tx Calibration, the symbol is transmitted just after the end of the downlink subframe (250). For Rx Calibration, the symbol is transmitted just before the start of the uplink subframe (260).

[0023] FIG. 3 depicts a block diagram architecture of a transceiver, illustrating the signal paths for both transmit and receive antenna calibration processes according to one embodiment of the present invention. During transmit calibration,a sounding symbol is loaded from the memory in the frequency domain just after the end of the DL frame. The beamforming circuit (315) may be used to route the signal appropriately to different transmit paths, sending them through IFFT, up-sampling, DPD, and PA as shown in the shaded region. Directional couples (316) next to the antennas help collect appropriate feedback from the same that are collected through a combiner (313) which is received via a common Rx path for phase and amplitude comparison in the frequency domain.

[0024] During receive calibration, a similar sounding signal is played out through a common transmit path, which is fed to the same power combiner in the reverse direction, thus making it work like a power splitter (323). These signals pass through the directional couplers (326) into the receive paths and the amplitudes and phases are compared.

[0025] The path from the respective antenna ports to the common calibration port at the output of the combiner, and the reverse path, may not have the same characteristics, and thus have to be compensated for while measuring the individual phases by dividing by corresponding s-parameters. This needs to be taken care of for both transmit and receive calibration. Since the path involves direction couplers, the s-parameters are expected to differ for Transmit and Receive Calibration.

[0026] In an example embodiment, in a TDD mode, the PA is given a “Transient time allowance” to bring down the power from the “ON” level to the “OFF" level.However, in the actual system, the PA switching may not need the fully allocated “Transient allowance" (10μs for 5G NR FR1 and 3 μs for 5G NR FR2). This extratime is used for antenna calibration sounding keeping the total of PA ramp-down and calibration symbol to less than the allocated Transient period, shown in the equation below.tcal,symbol≤ tTransientPeriod− tPA,RampDownFor example, a TDD macro base station with 40W per port, operating with a channel bandwidth of 100 MHz and 30 kHz subcarrier spacing (SCS) takes about 5-6μs to ramp down from Tx ON to Tx OFF state, leaving about 4μs of spare time. A new-sounding OFDM waveform (compatible with the LTE / 5G symbol structure) can be defined to span the entire bandwidth but with a symbol duration small enough to fit this spare time of 4μs in the transient gap. A waveform with 256-point IFFT and channel BW of 100 MHz could be defined with a symbol duration of 2.23 μs duration (including CP) thus fitting within this available gap. In massive MIMO radios, as the power transmitted per port is less than 40 W, PA ramp-down time will be smaller. Thus, a longer calibration symbol, with finer SCS, can be designed, meeting the above equation.

[0027] The frequency domain Tx antenna sounding signal is loaded from the memory and sent through the beamforming module as shown in the Tx Calibration Path (315). Further, signals for multiple antenna ports may be assigned to separate subcarriers with the help of a transmit beamforming module during transmission. Instead of frequency domain Tx sounding signal, equivalent time domain signal can be loaded from the memory.

[0028] The need for a separate IFFT for the sounding signal (as SCS and symbol time duration are different) is avoided by reusing the same IFFT with up-sampled input in frequency domain and cropping signals in the time domain. For example, a 256-point IFFT is realized using the existing 4096-point IFFT by up-sampling the frequency domain signal by 16 (populating 1 of 16 subcarriers of the 4096-point IFFT) and selecting only 256 samples of the 4096 time-domain outputs for cyclic prefix addition. Moreover, the time domain signal is then passed through the existing transmitter chain i.e., respective TX up-sampling chains, DPDs, PAs, and the antenna ports.

[0029] Further, feedback from the direction couplers in individual antenna ports are combined and received through a common calibration path bypassing the LNA, using one of the existing down-sampling paths.

[0030] The received signal is multiplied with the conjugate of the transmitted signal for the subsequent inter-antenna phase comparisonPhAntRef= RxAntRef.* Tx'AntRefPhAnt2= RxAnt2.* TxAntRe^For reference antenna, only amplitude is compensated. Here the phase obtained from Antenna 1 will have a scaling factor only. These relative phases are then compared to get the frequency domain antenna calibration factorCalAnt1= PhAnt1.* Ph'Ant1 / |PhAnt1|CalAnt2= PhAntl.* PhAnt2 / \PhAnt2\As can be seen in the example diagram, calibration phases may not be available for frequencies covering all the resource blocks, hence those coefficients are derived through interpolation from the adjoining available frequencies. The correction factors are averaged over multiple iterations to minimize the impact of noise. The above steps are repeated for other remaining antennas. The averaged correction factors could be applied as part of the beamforming coefficients or may be applied separately in the frequency / time domain.

[0031] In an embodiment, for performing Receiver Calibration, the following steps are involved: just before the start of the first uplink OFDM symbol after the guard period in the TDD frame structure, the Rx calibration sounding signal is loaded in the frequency domain from the memory as shown in the Rx calibration Figure 3 (325).

[0032] During Rx calibration, a sounding signal that may fit into the available transition gap is similarly generated and the time domain signal is sent through one of the transmit paths that serves as the common Tx up-sampling path (with DPD and PA bypassed as they are inactive during the TDD guard period) to the antenna calibration port. The signal power gets divided at the power divider and a fraction of the power reaches all the antenna ports via the respective directional couplers.

[0033] After the respective down sampling and FFT operation, the phases / amplitude of the signals are compared, and the calibration coefficients are obtained as per the following equations. Note that unlike in TX calibration, here the same signal is received through all the ports.RRxCalAnt1= RxAnt1.* Rx'Ant1 / |Rx'Ant1|RxCalAnt2= RxAnt2.* RxAntl / \RxAnt2\RxCalAntN= RxAntN.* Rx'Ant1 / |Rx'AntN|

[0034] Obtained phases / amplitudes are applied as part of coefficients of beam combining circuit or multiplied as phasors. Further, the phase values are averaged over multiple iterations to reduce the impact of noise. If the calibration coefficients are spaced wide apart in frequency, then interpolation can be carried out and correction phases can be applied to each subcarrier.

[0035] Receiver Antenna calibration is different from Transmitter Antenna Calibration in the following ways:

[0036] (1)The Rx calibration is performed during a transient period just before the start of the first uplink symbol, whereas the Tx calibration is performed just after the end of the last downlink symbol.

[0037] (2)The Rx calibration signal is sent keeping the DPD and the PA in switched OFF condition, as they both were switched off during the preceding guard period. This further helps in meeting the RF emission requirements.

[0038] (3)The power combiner module in the Tx calibration works as a power divider during Rx calibration.

[0039] (4) During Rx calibration, the feedback signals from different antennas in the uplink direction pass through the entire respective uplink receive chain (including all the LNA stages).

[0040] (5)The Rx calibration correction factors are applied to the combining module in the uplink path instead of the beamforming module in the downlink path for the Tx calibration.

[0041] FIG. 4 illustrates antenna radiation patterns before and after calibration. In an embodiment, the figure illustrates the significant impact of antenna calibration on radiation patterns in wireless communication systems. The comparative images before and after calibration demonstrate the importance of precise calibration in achieving optimal antenna functionality.

[0042] FIG. 5 shows the components of a method for antenna calibration in a wireless communication system by one embodiment of the present invention.

[0043] The method 500 may begin with step 510 or with step 520, which involves transmit or receive calibration respectively. This may be followed by a validation phase as shown in step 530. FIG. 5(A), 5(B), and 5(C) presents a detailed flowchart outlining the steps involved in the transmit calibration process, receive calibration process and validation process (500) under one embodiment of the present invention. The flowchart systematically guides through each stage of the calibration procedure, starting from the generation of calibration signals the Tx calibration process, followed by a validation process. A similar approach is taken for the Rx calibration and validation process.

[0044] In step 511, as also shown in figure 3, load the calibration symbol from the memory, and route it through multiplexer 315. The beamforming circuit may be used to route the calibration signal to appropriate transmit path thus avoiding anyseparate circuitry for antenna calibration. At step 512, the frequency domain signal is converted to the time domain using the respective IFFT of the transmit path and up-sampled and upconverted through the entire transmit paths.

[0045] At step 513, of the transmitter calibration process, feedback from all transmit antennas are collected. At step 514, the collected feedback signals are combined into a single signal using a power combiner, allowing a common receive path to carry signals from all paths that undergo the same distortions. Then the combined feedback signal is down-converted, down-sampled, and filtered using one of the existing receive paths. At step 515, the Fast Fourier Transform (FFT) of the receive path is used to extract the information about phase and amplitude characteristics in frequency domain.

[0046] The goal is to compare the phases and amplitudes by dividing the relevant subcarriers of the reference path frequency domain IQ with the relevant subcarriers of other path feedback of the calibration symbol. Here, the reference path is typically one of the existing transmit paths.

[0047] Step 516 plays a critical role in generating frequency domain error factors, which are used to compensate for the phase and amplitude mismatch in the transmit paths to improve overall system performance. This step is usually part of an iterative process (517) where the error factors are averaged over several iterations to minimize the effect of noise.

[0048] The interpolation process calculates these correction factors for the missing resource elements based on the error factors of the neighbouring resourceelements using linear or higher order interpolation, such as spline, at step 518. This step is optional and may be required when the error factors are separated by large number of subcarriers.

[0049] At step 519 of the calibration process, correction factors are applied to the transmit data.

[0050] FIG. 5(B) details about the steps involved for Receive Antenna Calibration. Just like in transmit calibration, a similar sounding symbol is loaded from the memory and sent through common transmit path in Step 521. Since, the signal need not be routed through multiple transmit paths, a predetermined time domain calibration symbol may be used.

[0051] In step 522, this signal is steered to a power splitter and then to the respective antenna ports via the direction couplers. The latter allows a small portion of the signal to be received through the receiver chain, down-convert, filter, downsample at step 523 and further convert to the frequency domain at step 524. Note that the same power combiner used in the transmit calibration functions as a power splitter in the reverse direction.

[0052] In step 525, the amplitude and phase of these signals are compared against that of the reference path, to get the relative phases and amplitude differences.

[0053] In an embodiment, the calibration coefficients are obtained as per the following equations.RxCalAnt1= RxAnt1.* Rx'Ant1 / |Rx'Ant1|RxCalAnt2= RxAnt2.* Rx'Ant1 / |Rx'Ant2|RxCalAntN= RxAntN.* Rx'Ant1 / |Rx'AntN|

[0054] In step 526, these error factors are averaged over several iterations to minimize the effect of noise. In step 527, the conjugate values of the error factors are interpolated to get correction factors with finer frequency resolution. In the final step 528, the incoming uplink data is multiplied in the frequency domain by these correction factors, along with the beam combining coefficients, to compensate for any receive path mismatch.

[0055] FIG 5(C) explains the steps for Validation, which can be performed without the need for test instrument. The validation for the transmit antenna calibration is carried out soon after the Tx calibration. This is then followed by the Rx antenna calibration and the subsequent validation of the Rx Calibration. The transmit / receive calibration status may be notified to other network units for any action thereafter.

[0056] In step 531, the calibration symbols are generated, either transmit or receive calibration, and symbols are sounded as described in the earlier steps. In step 532, the calibration coefficients are obtained, either for transmit or receive calibration. In step 533, the phase and amplitude correction values are applied in the frequency domain as part of Tx / Rx Calibration.

[0057] In step 534, feedback signals are again analysed for residual phase or amplitude offsets. The obtained residual phases are calculated as follows:PhAnt2Cor= RxAnt2.* Tx'Ant1TxCalAnt2= PhAnt1.* Ph'Ant2cor / |PhAnt2cor|

[0058] In step 535, the RMS value of the residual phase error is calculated and compared against a threshold “AT”.RMSerrtx= (1 / N) ΣNn=1(TxCalAnt2- (1 + j * 0)) * (TxCalAnt2- (1 + j * 0))'n=l where N represents number of subcarriers.

[0059] For the validation of receive calibration, the residual phase error can be obtained from the following equations:RxCalAnt2= RxAnt2.* RxAnti / \RxAnt2\RMSerrrx= (1 / N) ΣNn=1(RxCalAntn- (1 + j * 0)) * (RxCalAntn- (1 + j * 0))'n=l

[0060] The Rx RMS error is compared against a threshold (AR). If the phase and amplitude errors are estimated correctly in calibration phase, then the RMS error should be minimum. If the RMS error is below the threshold, then the antenna calibration is declared as successful otherwise it is a failure. The calibration status is informed to the other entities not limited to the Distributed Unit (DU), and the Element Management System (EMS).

[0061] The above method can be extended to Full Duplex (FD) and Sub Band Full Duplex (SBFD) systems as well. In FD and SBFD, there can be additional transitions from DL to UL and calibration symbols can be loaded in these transitions as well. This will further reduce the time to calibrate. If this transient period isdifferent from the TDD transient period as mentioned in the Table 1, an appropriate calibration symbol can be designed and calibration can be carried out.

[0062] Calibration can be performed during Discontinuous Transmission (DTX) period as well. Tx and Rx calibration can be carried out just before entering into DTX mode or coming out of the DTX mode respectively. The calibration symbol duration in DTX need not be same as that used in the TDD transient period. Using DTX mode, the live calibration can be carried out in the FDD system as well.

[0063] The above methodology may leverage existing 4G / 5G system infrastructure and protocols for calibrating the antennas, while not being limited to these specific technologies. By utilizing established cellular network frameworks, the calibration process can potentially benefit from mature signalling mechanisms, resource allocation schemes, and synchronization methods already in place. However, the approach may be adaptable to other wireless communication standards, both current and future. The calibration technique could be applied to various antenna array configurations and may be extensible to emerging technologies like 6G or other advanced wireless systems. This flexibility allows the method to evolve alongside advancements in wireless communication, potentially offering a scalable solution for antenna calibration across different generations of mobile networks and beyond.

[0064] FIG. 6 is a block diagram illustrating an example of a schematic hardware configuration of the network node according to the embodiments of the presentdisclosure. Referring to FIG.6, the network node 600 includes a network interface 610, a processor 620, a memory 630, and a storage 640.

[0065] Advantages

[0066] The antenna calibration method described in this invention offers several key benefits that address critical challenges in modern wireless communication systems. (1) Low complexity and cost: By utilizing existing system components and leveraging unused time gaps in the TDD frame structure, this calibration method minimizes the need for additional specialized hardware or complex signal processing algorithms. (2) Zero network downtime: One of the most significant advantages of this calibration technique is its ability to operate without interrupting normal network operations. By fitting the calibration process into unused time gaps, the system can maintain continuous service availability. (3) One of the key features of this calibration approach is the use of a calibration symbol that spans the entirety of the desired channel bandwidth. This will make antenna calibration fast and will ensure accurate and consistent performance across all frequency components. (4) Autonomous calibration capability: The method's independence from external system components such as the Distributed Unit (DU) or Element Management System (EMS) provides a high degree of autonomy to the Radio Unit (RU).

[0067] It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to thoseof skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.

Claims

We Claim:

1. A method for antenna calibration in a time division duplex (TDD) wireless communication system, comprising:defining a calibration symbol that occupies the desired bandwidth of a wideband system that can fit within an available time gap outside the transmission or reception time duration in a TDD system without disrupting live traffic.

2. The method of claim 1, further comprising:determining the permissible duration of the calibration symbol and a suitable frequency domain composition based on the available time gap by computing the difference between an allocated ramp-down period and the actual ramp-down period of a power amplifier for a given output power; anddevising the defined calibration symbol with a duration of 1 / N of a normal Orthogonal Frequency Division Multiplexing (OFDM) symbol, wherein generating comprises up-sampling frequency bin data by a factor of N to occupy a desired bandwidth and produce N copies of 1 / N length identical symbols.

3. The method of claim 2, further comprising:for transmit antenna calibration, transmitting one of multiple repeated time copies of the calibration reference signal, wherein the time copies are configured to fit within an available time gap just before a delayed start of power amplifier rampdown, thereby avoiding violation of emission requirements.

4. The method of claim 2, further comprising:for receive antenna calibration, transmitting one of multiple similar copies of the calibration reference signal just before the start of an uplink frame.

5. The method of claim 3, wherein for transmit antenna calibration comprising:transmitting the calibration reference signal through multiple antenna ports; andreceiving feedback through a common path to compare phases of the calibration reference signal from different antenna ports.

6. The method of claim 4, wherein for receive antenna calibration comprising:transmitting the calibration reference signal through a common calibration path; andreceiving feedback through multiple receive paths to compare phases of the calibration reference signal after final reception through different antenna ports.

7. The method of claim 1, wherein performing antenna calibration comprises calculating calibration coefficients based on the phase comparisons; and applying the calibration coefficients to a beamforming (for transmit calibration) or a combining circuit for (receive calibration).

8. The method of claim 1, further comprising validating the antenna calibration by:retransmitting the calibration reference signal with the applied calibration coefficients; andcalculating a residual phase error; and comparing the residual phase error to a predetermined threshold.

9. The method of claim 1, further comprises:implementing the calibration using existing signal processing capabilities of the radio unit without requiring additional specialized calibration hardware.

10. The method of claim 1, wherein the method is applicable to separate transmit and receive antenna arrays in full duplex or sub-band full duplex systems, provided the transmit antenna array has a feedback path to a receiver.

11. The method of claim 1, wherein the available time gap for antenna calibration is within any period of battery saving inactivity period of a Discontinuous transmission (DTX) system12. The wireless communication device, comprising:a processor including at least one memory, the processor is configured for defining a calibration symbol that occupies the desired bandwidth of a wideband system that can fit within an available time gap outside the transmission or reception time duration in a TDD system suitable for confirming to the emission requirements without disrupting live traffic;determining the permissible duration of the calibration symbol and a suitable frequency domain composition based on the available time gap by computing the difference between an allocated ramp-down period and the actual ramp-down period of a power amplifier for a given output power; and devising the defined calibration symbol with a duration of 1 / N of a normal Orthogonal Frequency Division Multiplexing (OFDM) symbol, wherein generating comprises upsampling frequency bin data by a factor of N to occupy an entire bandwidth and produce N copies of 1 / N length identical symbols.

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