阅读说明：本技术 用于无线通信系统中的天线校准的方法和装置 (Method and apparatus for antenna calibration in a wireless communication system ) 是由 余少伟 肖磊 朱怀松 陈学勤 于 2017-06-19 设计创作，主要内容包括：本公开的实施例提供用于分布式天线系统(DAS)中的天线校准的方法、装置和计算机程序产品。实现一种在与多个分布式天线相连的数字单元中的方法,所述多个分布式天线包括第一天线、第二天线以及第三天线。所述方法包括：导致在相同的频率资源中从所述第一天线发送第一信号、从所述第二天线发送第二信号、以及从所述第三天线发送第三信号；基于在所述第三天线处接收的所述第一信号、在所述第三天线处接收的所述第二信号、在所述第一天线处接收的所述第三信号、以及在所述第二天线处接收的所述第三信号,获得所述第一天线与所述第二天线之间的接收机和发射机侧环回相位差；以及基于所获得的环回相位差,获得所述第一天线与所述第二天线之间的时间延迟差和初始相位差的估计。本公开的实施例可以提供具有低复杂性并且不需要耦合控制单元(CCU)的改进天线校准。(A method in a digital unit connected to a plurality of distributed antennas including a th antenna, a second antenna, and a third antenna, the method including causing a th signal to be transmitted from the th antenna, a second signal to be transmitted from the second antenna, and a third signal to be transmitted from the third antenna in the same frequency resources, obtaining a receiver and transmitter side loop back phase difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and obtaining a time delay difference and an initial phase difference between the th antenna and the second antenna based on the obtained phase difference.)

1, a method (600, 700, 900) in a digital unit (501) connected to a plurality of distributed antennas, the plurality of distributed antennas including a th antenna, a second antenna, and a third antenna, the method (600, 700, 900) comprising:

causing a signal to be transmitted (610, 730, 740, 750) from the th antenna, a second signal to be transmitted from the second antenna, and a third signal to be transmitted from the third antenna in the same frequency resource;

obtaining (620) a receiver and transmitter side loop back phase difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and

obtaining (630) an estimate of a time delay difference and an initial phase difference between the th antenna and the second antenna based on the obtained receiver and transmitter side loop-back phase difference.

2. The method (600, 700, 900) of claim 1, wherein causing to transmit (610, 730, 740, 750) comprises:

causing the th signal, the second signal, and the third signal to be transmitted (610, 730, 740, 750) during a period of time when no data is transmitted.

3. The method (600, 700, 900) of claim 2, wherein the period of time includes at least of:

guard periods in a time division duplex, TDD, frame structure, and

subframes during which no data and control signaling is sent.

4. The method (600, 700, 900) of claim 1, wherein the signal, the second signal, and the third signal transmitted from the th antenna, the second antenna, and the third antenna are generated based on a same constant amplitude sequence.

5. The method (600, 700, 900) of claim 1, wherein causing to transmit (610, 730, 740, 750) comprises:

resulting in the th signal, the second signal, and the third signal being transmitted in a time division multiplexed manner.

6. The method (600, 700, 900) of claim 1, wherein obtaining (620) the receiver and transmitter side loopback phase difference between the th antenna and the second antenna comprises:

obtaining (621) a loopback phase of the th antenna by comparing a phase of the third signal received at the th antenna with a phase of the th signal received at the third antenna;

obtaining (622) a second loop phase of the second antenna by comparing a phase of the third signal received at the second antenna with a phase of the second signal received at the third antenna; and

obtaining (623) the receiver and transmitter side loop back phase difference by determining a difference between the th loop back phase and the second loop back phase.

7. The method (600, 700, 900) of claim 1, wherein obtaining (630) the estimates of the time delay difference and the initial phase difference between the th antenna and the second antenna comprises:

estimating (630) the time delay difference and the initial phase difference according to a polynomial fit based on a least squares criterion or a recursive least squares criterion.

8. The method (600, 700, 900) of claim 1, wherein obtaining (620) the receiver and transmitter side loopback phase difference between the th antenna and the second antenna comprises:

obtaining the receiver and transmitter side loopback phase difference between the th antenna and the second antenna for subcarriers in the frequency resource.

9. The method (600, 700, 900) of claim 1, further comprising:

obtaining (640) a loopback amplitude difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna.

10. The method (600, 700, 900) of claim 8, wherein obtaining (640) the loop-back magnitude difference between the th antenna and the second antenna comprises:

obtaining (640) the loopback amplitude difference between the th antenna and the second antenna for subcarriers in the frequency resource.

11. The method (600, 700, 900) according to claim 1, wherein at least two of the th antenna, the second antenna and the third antenna are located at different remote radio units, RRUs, connected to the digital unit (501).

12. The method (600, 700, 900) of claim 1, further comprising:

adjusting (650) a phase or timing of a signal to be transmitted from either the th antenna or the second antenna based on the estimate of the time delay difference or the initial phase difference.

13, an apparatus (1100) connected to a plurality of distributed antennas including a th antenna, a second antenna, and a third antenna, the apparatus (1110) comprising a processor (1111) and a memory (1112), the memory (1112) containing instructions executable by the processor (1111), whereby the apparatus (1110) is operable to:

causing signals to be transmitted from the th antenna, a second signal to be transmitted from the second antenna, and a third signal to be transmitted from the third antenna in the same frequency resource;

obtaining a receiver and transmitter side loopback phase difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and

obtaining an estimate of a time delay difference and an initial phase difference between the th antenna and the second antenna based on the obtained receiver and transmitter side loopback phase difference.

14. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operative to cause the transmission of the signal, the second signal and the third signal during periods of no data being transmitted.

15. The apparatus (1100) of claim 14, wherein the period of time includes at least of:

guard periods in a time division duplex, TDD, frame structure, and

subframes during which no data and control signaling is sent.

16. The apparatus (1100) of claim 13, wherein the , second, and third signals transmitted from the , second, and third antennas are generated based on a same constant amplitude sequence.

17. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operative to cause the transmission of the signal, the second signal and the third signal in a time division multiplexed manner ().

18. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operable to obtain the receiver and transmitter side loopback phase difference between the th antenna and the second antenna by:

obtaining a loopback phase of the antenna by comparing the phase of the third signal received at the antenna with the phase of the signal received at the third antenna;

obtaining a second loop phase of the second antenna by comparing a phase of the third signal received at the second antenna with a phase of the second signal received at the third antenna; and

obtaining the receiver and transmitter side loop back phase difference by determining a difference between the th loop back phase and the second loop back phase.

19. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operable to obtain the estimate of time delay difference and initial phase difference between the th antenna and the second antenna by:

estimating the time delay difference and the initial phase difference according to a polynomial fit based on a least squares criterion or a recursive least squares criterion.

20. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operable to obtain the receiver and transmitter side loopback phase difference between the th antenna and the second antenna by:

obtaining the receiver and transmitter side loopback phase difference between the th antenna and the second antenna for subcarriers in the frequency resource.

21. The apparatus (1100) of claim 13, wherein the memory (1112) contains instructions executable by the processor (1111), whereby the apparatus (1110) is further operable to:

obtaining a loopback amplitude difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna.

22. The apparatus (1100) of claim 21, wherein the memory (722) contains instructions executable by the processor (721), whereby the apparatus (720) is further operable to obtain the loop-back amplitude difference between the th antenna and the second antenna by:

obtaining the loopback amplitude difference between the th antenna and the second antenna for subcarriers in the frequency resource.

23. The apparatus (1100) of claim 13, wherein at least two of the th antenna, the second antenna, and the third antenna are located at different remote radio units, RRUs, connected to the apparatus.

24. The apparatus (1100) of claim 13, wherein the memory (722) contains instructions executable by the processor (721), whereby the apparatus (720) is further operable to:

adjusting a phase or timing of a signal to be transmitted from the th antenna or the second antenna based on the estimate of the time delay difference or the initial phase difference.

25, computer-readable storage media (1112) having embodied thereon a computer program product (1114) comprising instructions that, when executed on at least processors (711), cause the at least processors (1111) to perform the method (600, 700, 900) according to any of claims 1-12 to item .

Technical Field

Non-limiting and example embodiments of the present disclosure relate generally to the field of wireless communications technology and, in particular, relate to methods, apparatus and computer program products for antenna calibration in a Distributed Antenna System (DAS).

Background

This section introduces aspects that may facilitate a better understanding of the disclosure. Therefore, the description of this section is to be read in this light, and not to be understood as an admission that it is and what is not prior art.

For example, in a Long Term Evolution (LTE)/LTE-Advanced (LTE-A) system developed by the third Generation partnership project (3GPP), MIMO has been adopted as a key feature.A distributed MIMO (D-MIMO) feature has been proposed to further enhance coverage and throughput for certain hot spots and/or indoor large scale user access scenarios (e.g., universities, Central Business areas (CBDs), subway stations, stadiums, airports, train stations waiting rooms, etc.).

The cellular system may utilize the RRUs in the DAS to provide more uniform coverage, reduced outage, and higher throughput, particularly in shaded and indoor locations, as examples, in LTE D-MIMO may be applied in the downlink to effectively reduce co-channel interference or increase spatial multiplexing gain, and thus may improve cell capacity, average throughput, and cell-edge user throughput.

In a DAS system where multiple RRUs are distributed at different sites, joint precoding or beamforming can be done to keep the same phase combining at the receiver side, furthermore, the DAS can be combined with Single User (SU) or paired multi-user (MU) MIMO communication by treating the RRUs AS a distributed antenna array and/or equipping the RRUs with multiple antennas.

Antenna calibration is required in order to minimize phase (and amplitude) differences between multiple transceiver chains in order to guarantee accurate BF/precoding and to obtain large diversity or multiplexing gains.

Disclosure of Invention

It will be appreciated that although certain embodiments of the present disclosure have been described with reference to LTE, LTE-a or 5G systems, embodiments are not so limited, but rather may be more generally applied to any wireless communication system in which similar issues exist.

Various embodiments of the present disclosure are generally directed to methods, apparatuses, and computer program products for antenna calibration for distributed antennas. Other features and advantages of embodiments of the present disclosure will be understood from the following description of the various embodiments, when read in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the embodiments of the disclosure.

In a th aspect of the disclosure, there is provided a method in a digital unit directly or indirectly connected to a plurality of distributed antennas including, but not limited to, a th antenna, a second antenna, and a third antenna, the method comprising causing transmission of a th signal from the th antenna, a second signal from the second antenna, and a third signal from the third antenna in the same frequency resources, obtaining a receiver and transmitter side loopback phase difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and obtaining an estimate of a time delay difference and an initial phase difference between the th antenna and the second antenna based on the obtained receiver and transmitter side phase difference.

In a further embodiment, the time period may include at least items of a Guard Period (GP) in a Time Division Duplex (TDD) frame structure, and subframes during which no data and control signaling is transmitted.

In another embodiments, the digital unit can result in transmitting the signal, the second signal, and the third signal in a Time Division Multiplexed (TDM) manner.

In embodiments, the th, second, and third signals transmitted from the th, second, and third antennas may be generated based on the same constant amplitude sequence.

In another embodiments, the digital unit may obtain the receiver and transmitter side loopback phase difference between the antenna and the second antenna by comparing the phase of the third signal received at the antenna to the phase of the signal received at the third antenna to obtain a loopback phase of the antenna, by comparing the phase of the third signal received at the second antenna to the phase of the second signal received at the third antenna to obtain a second loopback phase of the second antenna, and by determining a difference between the loopback phase and the second loopback phase.

In some embodiments, the digital unit may obtain the estimates of the time delay difference and the initial phase difference between the th antenna and the second antenna by estimating the time delay difference and the initial phase difference according to a polynomial fit based on a Least Squares (LS) criterion or a recursive least squares criterion.

In embodiments, the digital unit may obtain the receiver and transmitter side loopback phase difference between the th antenna and the second antenna by obtaining the receiver and transmitter side loopback phase difference between the th antenna and the second antenna for subcarriers in the frequency resource.

In another embodiments, the method may further include the step of obtaining a loop-back amplitude difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, in a further embodiment, the digital unit may obtain the loop-back amplitude difference between the th antenna and the second antenna by obtaining the loop-back amplitude difference between the th antenna and the second antenna for subcarriers in the frequency resources.

In some embodiments, at least two of the th antenna, the second antenna, and the third antenna may be located at different remote radio units, RRUs, connected to the digital unit.

In yet another embodiments, the method may further comprise step adjusting a phase or timing of a signal to be transmitted from the th antenna or the second antenna based on the estimate of the time delay difference or the initial phase difference.

In a second aspect of the disclosure, apparatuses directly or indirectly connected to a plurality of distributed antennas including (at least) a th antenna, a second antenna, and a third antenna include a transmission control unit configured to cause a th signal to be transmitted from the th antenna, a second signal to be transmitted from the second antenna, and a third signal to be transmitted from the third antenna in the same frequency resources, an obtaining unit configured to loop back a receiver-side and transmitter-side phase difference between the th antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and an estimating unit configured to obtain an estimated delay difference and an estimated initial phase difference between the th antenna and the second antenna based on the obtained receiver-side and transmitter-side phase difference.

In a third aspect of the disclosure there is provided an apparatus directly or indirectly connected to a plurality of distributed antennas including (at least) a th antenna, a second antenna, and a third antenna, the apparatus comprising a processor and a memory, the memory containing instructions executable by the processor whereby the apparatus is operable to cause a loop back of a signal from the th antenna, a second signal from the second antenna, and a third signal from the third antenna to be transmitted in the same frequency resources, obtain a receiver and transmitter side phase difference between the antenna and the second antenna based on the th signal received at the third antenna, the second signal received at the third antenna, the third signal received at the th antenna, and the third signal received at the second antenna, and obtain an estimated delay difference and an initial delay difference between the th antenna and the second antenna based on the obtained receiver and transmitter side phase difference.

In a fourth aspect of the present disclosure, a computer program product comprising instructions that, when executed on or more processors, cause the or more processors to perform the method according to aspect of the present disclosure is provided.

In a fifth aspect of the present disclosure there is provided an apparatus in a digital unit, the digital unit being directly or indirectly connected to a plurality of distributed antennas including (at least) a th antenna, a second antenna and a third antenna, the apparatus comprising processing means adapted to perform the method according to the aspect of the present disclosure.

In a sixth aspect of the present disclosure, computer-readable storage media are provided having embodied thereon a computer program product comprising instructions that, when executed on at least processors, cause the at least processors to perform the method according to aspect of the present disclosure.

In a seventh aspect of the present disclosure there is provided digital units comprising an apparatus according to the second or third aspect of the present disclosure.

In an eighth aspect of the present disclosure, network devices are provided that include the apparatus according to the second or third aspect of the present disclosure.

According to various aspects and embodiments as described above, antenna calibration with low complexity is achieved for distributed antenna systems without the presence of a Coupling Control Unit (CCU).

Drawings

The above and other aspects, features, and advantages of various embodiments of the present disclosure will become more fully apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals or letters are used to designate like or identical elements.

Fig. 1 illustrates an example wireless communication network in which embodiments of the present disclosure may be implemented;

2A-2B illustrate communication scenarios using DAS according to embodiments of the present disclosure;

figure 3 shows an auxiliary node based antenna calibration solution;

fig. 4 shows an antenna calibration solution based on CCUs for co-sited antenna arrays;

FIG. 5 shows a schematic block diagram of a calibration apparatus according to an embodiment of the present disclosure;

6A-6B illustrate a flow chart of an antenna calibration method according to an embodiment of the present disclosure;

fig. 7 shows a flow diagram of another antenna calibration methods according to embodiments of the present disclosure;

fig. 8 illustrates a frame structure for transmitting a calibration signal according to an embodiment of the present disclosure;

fig. 9 shows a flow diagram of yet another antenna calibration methods according to an embodiment of the present disclosure;

fig. 10 shows a schematic block diagram of an apparatus implemented as/in a digital unit according to an embodiment of the present disclosure; and

fig. 11 shows a simplified block diagram of an apparatus that may be implemented as/in a digital unit.

Detailed Description

For example, features shown or described as part of embodiments may be used with another embodiments to produce yet further embodiments.

References in this specification to " embodiments," " embodiments," " example embodiments," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not include the particular feature, structure, or characteristic in . moreover, the phrases do not refer to as in embodiments. furthermore, when a particular feature, result, or characteristic is described in connection with embodiments, it is considered within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described.

It will be understood that, although the terms "," second, "etc. may be used herein to describe various elements, these elements should not be limited by these terms.

As used herein, the singular forms "," "," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, it will also be understood that the terms "comprises," "comprising," and the like, when used herein, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of or more other features, elements, components, and/or groups thereof.

In the following specification and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

As used herein, the term "wireless communication network" refers to a network that conforms to any suitable wireless communication standard, including, for example, New Radio (NR), Long Term Evolution (LTE), LTE-Advanced (LTE-A), Wideband Code Division Multiple Access (WCDMA), High Speed Packet Access (HSPA), etc. moreover, communication between network devices in a wireless communication network may be performed according to any suitable generational communication protocol, including, but not limited to, Global System for Mobile communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), and/or other suitable communication protocols, such as the th generation (1G), the second generation (2G), 2.5G, 2.75G, the third generation (3G), the fourth generation (4G), 4.5G, the fifth generation (5G) communication protocol, Wireless Local Area Network (WLAN) standards (e.g., the IEEE 802.11 standard), and/or any other suitable wireless communication standard, and/or any other protocol now known or to be developed in the future.

As used herein, the term "network device" refers to a device in a wireless communication network via which a terminal device accesses the network and receives services therefrom. A network device may refer to a Base Station (BS) or an Access Point (AP), such as a NodeB (NodeB or NB), evolved NodeB (eNodeB or eNB), NR NB (also known as gNB), Remote Radio Unit (RRU), Radio Head (RH), Remote Radio Head (RRH), relay, low power node (e.g., femto, pico, etc.), depending on the terminology and technology applied.

The term "terminal device" refers to any terminal device capable of wireless communication. By way of example, and not limitation, a terminal device may be referred to as a User Equipment (UE), a Subscriber Station (SS), a portable subscriber station, a Mobile Station (MS), or an Access Terminal (AT). End devices may include, but are not limited to, mobile phones, cellular phones, smart phones, voice over IP (VoIP) phones, wireless local loop phones, tablet computers, wearable end devices, Personal Digital Assistants (PDAs), portable computers, desktop computers, image capture end devices (e.g., digital cameras), gaming end devices, music storage and playback appliances, in-vehicle wireless end devices, wireless endpoints, mobile stations, Laptop Embedded Equipment (LEEs), Laptop Mounted Equipment (LMEs), USB adapters, smart appliances, wireless client devices (CPEs), and the like. In the following description, the terms "terminal device", "terminal", "user equipment" and "UE" may be used interchangeably.

As yet another examples, in an Internet of things (IOT) scenario, a terminal device may represent a machine or other device that performs monitoring and/or measurements and transmits the results of these monitoring and/or measurements to another terminal devices and/or network devices in which case the terminal device may be a machine-to-machine (M2M) device, which may be referred to in the 3GPP context as a Machine Type Communication (MTC) device As specific examples of these machines or devices may be UE. devices that implement the 3GPP narrowband Internet of things (NB-IoT) standard are sensors, metering devices (e.g., power meters), industrial machines, or home or personal devices (e.g., refrigerators, televisions), personal wearable devices (e.g., watches), etc.

As used herein, DL transmission refers to transmission from a network device to a terminal device, and UL transmission refers to transmission in the opposite direction.

Fig. 1 shows an example wireless communication network 100 in which embodiments of the present disclosure may be implemented, as shown in fig. 1, the wireless communication network 100 may include or more network devices (also referred to as network nodes), such as network device 101, which may take the form of an eNB or a gNB, it will be understood that the network device 101 may also take the form of a Base Station (BS), a node b (nb), a virtual BS, a Base Transceiver Station (BTS), and/or a base station subsystem (BSs), an AP, etc. the network device 101 may be equipped with multiple distributed antennas (e.g., antenna 111 and 116) located at different sites (e.g., at multiple RRUs 121 and 123), using or multiple distributed antennas, the network device 101 may provide radio connectivity to sets of terminal devices or UEs 102-1, 102-2, …, 102-N (which are collectively referred to as "terminal device(s) 102", where N is a natural number), the multiple RRUs may be implemented by performing joint beamforming or precoding between multiple RRUs 123, and combining multiple rf beam forming devices 101 with the multiple rf beam forming devices 101 to implement in-phase data processing or combining multiple rf data processing units (DL) in a network device 101) that may also be implemented in a common to implement.

Using beamforming or precoding techniques, the network device can adaptively optimize the radiation beam pattern of the DAS by weighting the antenna array according to spatial channel correlation to achieve high gain and controlled beam width in the desired direction. In this way, the signal to interference and noise ratio (SINR) or signal to noise ratio (SNR) of the intended user is maximized while minimizing interference to unintended users. Therefore, inter-cell and intra-cell co-channel interference is suppressed, and cell-edge throughput and system capacity are improved.

Beamforming weights are generated based on characteristics of a composite spatial channel, which is a combination of a spatial wireless channel and an antenna apparatus internal chain. In TDD wireless communication systems like TD-SCDMA, TD-LTE and TD LTE-a, the spatial radio channel has good reciprocity in uplink and downlink. This means that the network side can obtain the channel state of the downlink radio channel by detecting the uplink radio channel. However, there is no reciprocity in the antenna arrangement internal chain. The antenna arrangement internal chain typically comprises an Intermediate Frequency (IF) chain(s), Radio Frequency (RF) circuitry and possibly a feeder cable connection between the RRU and the antenna, which may be different for different antennas and may result in different amplitude attenuation and phase shift. In addition, the amplitude attenuation and phase shift characteristics may vary with temperature, atmospheric humidity, and device aging. Therefore, antenna calibration is required to compensate for amplitude and phase shift differences between different antenna chains in order to maintain channel reciprocity in the uplink and downlink. It should be appreciated that even in FDD systems, antenna calibration is required to facilitate accurate BF weighting/precoding.

FIG. 2A illustrates a communication scenario using DAS, wherein multiple distributed antennas 201 & 210 are connected directly or indirectly to a Digital Unit (DU)220, the digital unit 220 may be part of a base station or shared by multiple base stations, the multiple antennas 201 & 210 may be used for data channel multiplexing and/or control channel combining, for example, antenna 201 & 204 may cover region 231, antenna 205 & 207 may cover region 232, and antenna 208 & 210 may cover region 233. region 231 & 233 may be served by or multiple base stations.

FIG. 2B illustrates another communication scenarios using DAS, in which several distributed Transmission Points (TPs) 241 & 248 are directly or indirectly connected to the DU 240. multiple TPs 241 & 248 may belong to or multiple base stations (not shown). in this example scenario, the TPs 241 & 243 serve the UE 251, the TPs 243 & 244 serve the UE 252, and the TPs 245 & 248 serve the UE 253, respectively.

In both example scenarios shown in fig. 2A-2B, it may not be feasible to apply a centralized antenna array calibration using a coupling network (e.g., a Coupling Control Unit (CCU)) because the transmitters (e.g., antenna units) are located at different sites.

Furthermore, the inventors of the present disclosure have recognized that UL and DL channel reciprocity cannot be guaranteed simply by performing centralized antenna array calibration (i.e., intra-transceiver antenna calibration) at each antenna site separately, as the antenna apparatus internal chains at the distributed sites may be different.

Therefore, there is a need for new methods and apparatus for antenna calibration in DAS without a coupling network in order to minimize amplitude and phase differences between the distributed antennas.

In these approaches, a third party secondary node 301 must be introduced as a reference and the varying radio channel between the secondary node 301 and the target distributed antennas 302 and 303 may introduce much uncertainty, furthermore, since the transceiver chain of the secondary node 301 is contained in the transceiver calibration chain, the calibration results may be inaccurate and the methods and apparatus for calibrating the antennas become complex.

In the D-MIMO system, however, the RRU cluster (also referred to as D-MIMO cluster) associated with DU can be distributed at different sites and each RRU in the D-MIMO cluster can be connected by cable without a CCU (403 and 404 shown in fig. 4) and in this scenario, the solution shown in fig. 4 can be hardly utilized because it requires the cluster of RRUs 401 and 402 to be installed together, and furthermore the optimal solution in this solution is not the cost of the external merger 410.

Furthermore, the inventors of the present disclosure have recognized that small delay differences between antenna systems may result in large phase shifts of high frequency subcarriers in OFDM systems. Field tests have shown that the errors of the beamformed patterns are typically limited by the telecom operator to less than 5 degrees. In other words, for a 20MHz TD-LTE system, the delay difference between the antenna elements must be less than 1/32T (sample duration). None of the currently available distributed antenna calibration solutions can meet this stringent requirement for calibration accuracy with reasonable complexity.

In embodiments of the present disclosure, methods and apparatus for antenna calibration in DAS without a coupling network (e.g., CCU) have been proposed. Using embodiments of the present disclosure, calibration accuracy and system robustness of a DAS can be improved with low complexity. The embodiment of the disclosure can be applied to a D-MIMO system based on a multipoint joint precoding technology.

For simplicity, calibration means for three groups of antennas are schematically shown in fig. 5, wherein DUs 501 are connected with three groups of RRUs 502-504, respectively, e.g. by optical fibers 510-530, antenna radiation units 541-546 are connected with corresponding RRUs (502, 503 or 504), e.g. by cables, antenna calibration of any of the antennas 541-546 can be controlled by the DUs 501, in some embodiments both the DUs 501 and the antennas 541-546 can be considered as parts of the network device 500, the network device 500 can be a base station (which can be a virtual concept), it should be understood that in some other embodiments the network device 500 can comprise more or fewer RRUs or antennas or antenna arrays, furthermore, in some scenarios, the DUs 501 can be shared by multiple network devices and can be connected directly or indirectly to antennas (or RRUs) associated with more than network devices.

Fig. 6 shows a flow diagram of a method 600 in a digital unit according to embodiments of the present disclosure in embodiments, distributed antenna calibration is divided into an initial calibration phase and a periodic calibration phase, and the method 600 may be used for the initial calibration phase and/or the periodic calibration phase.

The Digital Unit (DU) performing the method 600 is directly or indirectly connected to a plurality of distributed antennas including, for example, a th antenna, a second antenna, and a third antenna, for example, the DU may be a UD associated with the network device 101 shown in fig. 1 or a DU 501 shown in fig. 5 for ease of discussion, the method 600 will be described below with reference to the DU 501 and the scenario shown in fig. 5, however, embodiments of the present disclosure are not so limited and may be more generally applied in other scenarios where similar issues exist.

As shown in FIG. 6, at block 610, the DU 501 causes a th signal to be transmitted from a th antenna, a second signal to be transmitted from a second antenna, and a third signal to be transmitted from a third antenna in the same frequency resources each of the th signal, the second signal, and the third signal may be referred to as a calibration signal.

In embodiments, prior to the operation of block 610, DU 501 is synchronized to a Global Positioning System (GPS) or other absolute network clock when the system boots, and RRU 502 and 504 recover timing from, for example, the CPRI interconnected with DU 501.

In some embodiments, after preheating the RRU 502 and DU 501, the operations of block 610 may be triggered when the RRU 502 and 504 are switched to the calibration state and an initial calibration command is received.

Alternatively or additionally, the operations of block 610 may be triggered when RRU 502 and 504 are switched to the periodic calibration state and a periodic calibration command is received after the initial calibration is completed.