WO2024055150A1

WO2024055150A1 - Method, network node and computer program for over the air calibration of an active antenna system

Info

Publication number: WO2024055150A1
Application number: PCT/CN2022/118414
Authority: WO
Inventors: Hao Zhang; Ang FENG; Henrik Asplund; Christian Braun; Ming Li; Georgy LEVIN
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2022-09-13
Filing date: 2022-09-13
Publication date: 2024-03-21
Anticipated expiration: 2025-03-13

Abstract

There is provided techniques for over the air calibration of an active antenna system. A method is performed by a network node. The method comprises estimating uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the active antenna system from a user equipment. The method comprises precoding and transmitting downlink reference signals towards the user equipment from the active antenna system. The downlink reference signals are precoded according to information derived from the uplink channel properties. The method comprises receiving a report from the user equipment of measurements made by the user equipment on the downlink reference signals. The method comprises calibrating the active antenna system as a function of the measurements made by the user equipment.

Description

METHOD, NETWORK NODE AND COMPUTER PROGRAM FOR OVER THE AIR CALIBRATION OF AN ACTIVE ANTENNA SYSTEM

TECHNICAL FIELD

Embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for over the air calibration of an active antenna system.

BACKGROUND

Massive Multiple-Input Multiple-Output (MIMO) technologies can be used to boost capacity or enlarge coverage of wireless communication systems. These benefits are accomplished by beamforming, a functionality that can concentrate the radiation wave carrying transmitted signals at some specific directions to strengthen the signal power at the receiver side. To enable beamforming, the signals from multiple antenna elements need to be aligned coherently. In general terms, coherency refers to that the phase and amplitude response is the same for all antenna branches involved in the beamforming. Coherency is commonly difficult to be guaranteed by hardware alone. Therefore, Antenna Calibration (AC) is employed in Active Antenna Systems (AAS) , sometimes also referred to as advanced antenna systems. Two examples of AC are coupler-based AC and mutual coupling-based AC. Both these types of AC require either certain extra hardware in the AAS or calibration in the production of the AAC, resulting in high cost in addition to the higher hardware complexity. Further, a dedicated calibration signal is required in these types of AC. Transmission of this dedicated calibration signal might interrupt the normal wireless transmissions and thus lead to performance degradation. Over-the-Air (OTA) AC, also known as in-field AC, is a cheaper, and less complex in terms of hardware, way which uses OTA or user equipment (UE) assisted signals to implement AC at the AAS. OTA AC can be used with low cost and with less impact on the normal wireless transmissions.

Some techniques for OTA AC to calibrate the AAS assume that Channel State Information (CSI) from/to each antenna branch can be estimated through reference signals (RS) transmitted between the AAS at the network side and the UE.

However, some techniques for OTA AC introduce high overhead in the feedback channel from the UE to the network and require high computational complexity at the UE side. Further, the control and scheduling of reference signals are not suitable for AC.

Hence, there is still a need for an improved OTA AC of an AAS.

SUMMARY

An object of embodiments herein is to overcome the above noted issues, and in particular to provide OTA AC of an AAS that does not suffer from the above issues.

According to a first aspect there is presented a method for over the air calibration of an AAS. The method is performed by a network node. The method comprises estimating uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the AAS from a UE. The method comprises precoding and transmitting downlink reference signals towards the UE from the AAS. The downlink reference signals are precoded according to information derived from the uplink channel properties. The method comprises receiving a report from the UE of measurements made by the UE on the downlink reference signals. The method comprises calibrating the AAS as a function of the measurements made by the UE.

According to a second aspect there is presented a network node for over the air calibration of an AAS. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to estimate uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the AAS from a UE. The processing circuitry is configured to cause the network node to precode and transmitting downlink reference signals towards the UE from the AAS. The downlink reference signals are precoded according to information derived from the uplink channel properties. The processing circuitry is configured to cause the network node to receive a report from the UE of measurements made by the UE on the downlink reference signals. The processing circuitry is configured to cause the network node to calibrate the AAS as a function of the measurements made by the UE.

According to a third aspect there is presented a network node for OTA calibration of an AAS. The network node comprises an estimate module configured to estimate uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the AAS from a UE. The network node comprises a precode module configured to precode and transmitting downlink reference signals towards the UE from the AAS. The downlink reference signals are precoded according to information derived from the uplink channel properties. The network node comprises a receive module configured to receive a report from the UE of measurements made by the UE on the downlink reference signals. The network node comprises a calibrate module configured to calibrate the AAS as a function of the measurements made by the UE.

According to a fourth aspect there is presented a computer program for OTA calibration of an AAS, the computer program comprising computer program code which, when run on a network node, causes the network node to perform a method according to the first aspect.

According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

Advantageously, these aspects provide efficient OTA calibration of an AAS, without suffering from the above issues.

Advantageously, these aspects enable acquisition of accurate downlink CSI feedback for OTA AC with small overhead and lower computation load at the UE.

Advantageously, these aspects enable efficient reciprocity calibration for an AAS at one single transmission and reception point or an AAS spread between two or more transmission and reception points (TRP) , as used in distributed MIMO systems.

Advantageously, these aspects do not require any special configuration for the UE or for downlink CSI feedback.

Advantageously, these aspects support OTA AC for an AAS with large number of antenna elements with limited reference signal (RS) resource.

Advantageously, these aspects enable triggering of the reference signal transmission for OTA AC to not disturb regular transmissions.

Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, module, step, etc. " are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating a communication network according to embodiments;

Fig. 2 schematically illustrates uplink aspects of UE assisted OTA AC according to an embodiment;

Fig. 3 schematically illustrates downlink aspects of UE assisted OTA AC according to an embodiment;

Fig. 4 is a flowchart of methods according to embodiments;

Fig. 5 shows an example Doppler spectrum according to an embodiment;

Fig. 6 shows simulation results without OTA channel mitigation according to an embodiment;

Fig. 7 shows simulation results with OTA channel mitigation according to an embodiment;

Fig. 8 schematically illustrates a mapping between downlink reference signal resources and antenna elements according to a first embodiment;

Fig. 9 schematically illustrates a mapping between downlink reference signal resources and antenna elements according to a second embodiment;

Fig. 10 schematically illustrates scheduling of uplink reference signals and precoded downlink reference signals according to an embodiment;

Fig. 11 is a signaling diagram according to an embodiment;

Fig. 12 is a schematic diagram showing functional units of a network node according to an embodiment;

Fig. 13 is a schematic diagram showing functional modules of a network node according to an embodiment; and

Fig. 14 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional.

Fig. 1 is a schematic diagram illustrating a communication network 100 where embodiments presented herein can be applied. The communication network 100 comprises an access network 110 in which wireless access is provided to UE 170 by one or more AAS 140. The AAS 140 is operatively controlled by a network node 200. It is here noted that although illustrated as being arranged at one transmission and reception point, the AAS 140 might be distributed between two or more transmission and reception points within the access network 110. The network node 200 and one or more AAS 140 might collectively form an access network node, radio base station, base transceiver station, node B (NB) , evolved node B (eNB) , gNB, access point, or integrated access and backhaul (IAB) node. The UE 170 might be any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, smartphone, laptop computer, tablet computer, network equipped vehicle, network equipped sensor, Internet of Things (IoT) device, game controller, etc. As is understood, a plurality of UEs 170 might be served by one or more AAS 140 and/or network nodes 200. The access network 110 is via the network node 200 operatively connected to a service network 130, such as the Internet, via a core network 120. The UE 170 is thereby enabled to access service and data made accessible in the service network 130.

Assume that radio branches are to be calibrated in the AAS 140. Assume further that each radio branch is connected to one or more physical antennas (for example provided in a subarray arrangement) via a physical antenna port.

For an OTA AC procedure, the UE 170 should feedback its measurement of the downlink channel via CSI feedback to the network side. As noted above, there is still a need for an improved OTA AC of an AAS.

Downlink CSI feedback requires large overhead for accurate CSI to be reported by the UE for some OTA channels, such as non-line of sight channels with large angular spread. The OTA channel is the wireless channel between the AAS and the UE, from transmitter antenna (s) to receiver antenna (s) , not taking into consideration any internal channels in the transmitter or receiver.

Some techniques for OTA AC of requires advanced processing and high computation load at the UE side.

Having access only to a small number of logical ports for the downlink reference signals is a challenge when performing OTA AC for an AAS with large number of antenna elements.

The mapping between ports for the downlink reference signals and antenna elements is not designed for OTA AC. For example, one port for downlink reference signals might be mapped to multiple antenna elements, or to two polarizations.

Out in the field, the scheduling for downlink and uplink reference signals might not be predictable enough for OTA AC. For example, some reference signals might not be transmitted when there is no UE traffic.

The embodiments disclosed herein therefore relate to techniques for OTA calibration of an AAS 140. In order to obtain such techniques there is provided a network node 200, a method performed by the network node 200, a computer program product comprising code, for example in the form of a computer program, that when run on a network node 200, causes the network node 200 to perform the method.

In general terms, at least some of the herein disclosed embodiments relate to techniques for controlling and scheduling reference signal resources for UE assisted OTA AC. This can be used to reduce the overhead of the feedback channel from the UE to the network node as well as the computational complexity of the UE. In some aspects, this is achieved by exploiting reciprocity of the wireless channel between the AAS and the UE.

In general terms, at least some of the herein disclosed embodiments are based on precoding of downlink reference signals that exploits the reciprocity of the wireless channel, as well as scheduling of uplink and downlink reference signals for AC purposes.

Reference is next made to the block diagrams of Fig. 2 and Fig. 3. Fig. 2 shows uplink aspects of UE assisted OTA AC. Fig. 3 shows downlink aspects of UE assisted OTA AC.

Supposing that the uplink OTA channel is denoted

and the downlink OTA channel is denoted

the reciprocity of the OTA channel implies that

In the follow description, the term H _OTA is used to express both of them.

End-to-end channel information

in the uplink can be estimated through transmission of uplink reference signals, where the transmitted signal x by the UE is received at the AAS at the network side, denoted as y (n) , n=1, …N, where N is the number of antenna branches, or physical antennas, at the AAS.

is composed of three parts;

H _OTA and H _UE.

and H _UE represent the CSI of the radio hardware used in the AAS and the UE, respectively, for the uplink. In the below equations, the symbol

denotes the Hadamard division and the symbol “⊙” denotes the Hadamard multiplication, i.e., the element-wise division and multiplication of vectors and matrices. Hence,

where k denotes physical resource block (PRB) index. Since H _UE is common for all received signals at the network side (where the purpose of the AC is to find, and compensate, relative differences between radio branches at the AAS side) , the relative CSI can be simplified as

H _OTA is a matrix which contains elements for every physical antenna and every PRB. From the above follows that H _OTA (k, n) can be expressed as:

As shown in Fig. 3, there are multiple downlink reference signal ports x (n) , n=1, …, N, and each of them is mapped to one antenna element (via a physical antenna port) . Non-limiting examples of mappings between downlink reference signal ports and antenna elements in the AAS will be disclosed below.

Similar to the uplink channel, the downlink channel can be expressed as:

It is noted that for downlink channel, different antenna branches may exploit different reference signal resources to avoid underdetermined problem for channel estimation. The controlling and scheduling of downlink reference signal will be disclosed below.

Fig. 4 is a flowchart illustrating embodiments of methods for OTA calibration of an AAS 140. The methods are performed by the network node 200. The methods are advantageously provided as computer programs 1420.

S104: The network node 200 estimates uplink channel properties. The uplink channel properties are estimated from measurements made on uplink reference signals received over a wireless channel by the AAS 140 from the UE 170.

S106: The network node 200 precodes and transmits downlink reference signals towards the UE 170 from the AAS 140. The downlink reference signals are precoded according to information derived from the uplink channel properties.

S108: The network node 200 receives a report from the UE 170 of measurements made by the UE 170 on the downlink reference signals.

S110: The network node 200 calibrates the AAS 140 as a function of the measurements made by the UE 170.

Embodiments relating to further details of OTA calibration of an AAS 140 as performed by the network node 200 will now be disclosed.

There could be different purposes for the network node to precode the downlink reference signals according to information derived from the uplink channel properties. In some aspects, the precoding is performed to eliminate channel effects in measurements made by the UE on the downlink reference signals so that the downlink CSI feedback can be used for antenna calibration purposes. That is, in some embodiments, the downlink reference signals are precoded with an objective to eliminate effects (on the wireless channel) caused by the downlink channel properties. In some aspects, the precoding is performed with an objective to expose the uplink AC error

to the UE so that the AAS reciprocity error can be measured by the UE. In particular, in some embodiments, the downlink reference signals are precoded with an objective for the UE 170 to measure a reciprocity error of the AAS 140. In some aspects, the precoding is performed so that the downlink reference signals as received by the UE appear to come from the line of sight boresight direction. In particular, in some embodiments, the downlink reference signals are precoded with an objective to appear to have been transmitted towards the UE 170 from the AAS 140 in a boresight line of sight direction.

Aspects of precoding of the downlink reference signals will be disclosed next.

In general terms, as follows from the above description with reference to Figs. 2 and 3, there are three parameters that needs to be estimated for the AC to be performed. These parameters are uplink AC error

downlink AC error

and reciprocity error

That is, in some examples, the uplink channel properties between the UE 170 and the AAS 140 are composed of a first part H _UE representing channel impairments caused by the UE 170, a second part H _OTA representing channel impairments caused by the wireless channel from the UE 170 to the AAS 140, and a third part

representing uplink channel impairments caused by the AAS 140. H _UE is common or same for all BS antennas so it can be ignored in following procedures for the purpose of BS AC.

Since

is already obtained, to have full calibration, either

or

is needed next. In what follows will be disclosed three embodiments for how to estimate these parameters. In some aspects, the reciprocity of the OTA channel is exploited to reduce the overhead in the feedback channel from the UE and the complexity at the UE side.

It is noted that

is a measure of the BS reciprocity, where

To fulfill reciprocity requirements,

must be constant for all antennas in the AAS.

According to a first aspect, the channel properties correspond to the full uplink channel properties except the impairments caused by the UE. This corresponds to an embodiment where the information is derived from the second part H _OTA and the third part

and whilst ignoring the first part H _UE. In further detail, let S (k, n) = [S ₁ (k, n) , S ₂ (k, n) , …, S _M (k, n) ] , where S (k, 1) , S (k, 2) , …, S (k, N) are orthogonal signals, denoting the root sequence of the downlink reference signals which contains multiple reference signal ports on every PRB. N is the number of reference signal ports. M is the number of resource elements for each reference signal port. S (k, n) can be divided by the OTA CSI, i.e., H _OTA (k, n) and the UL AC error, i.e.,

to generate the precoded downlink reference signal sequence X (k, n) . After precoding, the network node transmits the downlink reference signals towards the UE (as illustrated in Fig. 3) . Denote by Y (k, n) = [Y ₁ (k, n) , Y ₂ (k, n) , …, Y _M (k, n) ] the received downlink reference signals by the UE. This leaves only the reciprocity error

in the channel estimation for the UE. Finally, the UE feeds back the reciprocity error to the network node. This procedure can be summarized in equations as follows:

According to a second aspect, the channel properties correspond to the uplink OTA part. This corresponds to an embodiment where the information is derived from the second part H _OTA whilst ignoring the first part H _UE and removing the third part

In further detail, S (k, n) can be divided by H _OTA (k, n) to generate the precoded downlink reference signal sequence X (k, n) . After the precoding, the network node transmits the downlink reference signals towards the UE. This leaves only the factor

in the channel estimation for the UE. Finally, the UE feeds back the downlink AC error, in which the impact of downlink wireless channel is removed. This procedure can be summarized in equations as follows:

It is noted that this is not to beamform the downlink reference signals, because the precoding is done on each reference signal, and the reference signals should be orthogonal to each other. In some examples, each downlink reference signal port is precoded with different information. Then, the network node transmits these precoded downlink reference signal at corresponding ports towards the UE for that the UE can extract each reference signal separately for estimation only of the AAS AC error.

According to a third aspect, channel properties correspond to uplink impairments caused by the AAS. This corresponds to an embodiment where the information is derived from the third part

(whilst ignoring the first part H _UE and removing the second part H _OTA) . In further detail, S (k, n) can be divided by

to generate the precoded downlink reference signal sequence X (k, n) . After the precoding, the network node transmits the downlink reference signals towards the UE. This leaves the OTA channel and

in the channel estimation for the UE. Finally, the UE feeds back the OTA CSI and reciprocity error. This procedure can be summarized in equations as follows:

In some embodiments, the downlink reference signals are precoded according to information derived from the uplink channel properties under an assumption of channel reciprocity of the wireless channel. In some aspects, to use the uplink channel properties for precoding downlink reference signals, the channel reciprocity should be consistent. A channel can be reciprocal if the rate of change between the time of uplink (UL) and downlink (DL) transmission is small. To determine the channel rate of change, the Doppler shift of H _OTA can be computed as below. Here, the variable t is the time interval of the received uplink reference signal. Let

H _OTA= [H _OTA (k, n, 1) , H _OTA (k, n, 2) , …, H _OTA (k, n, t) ]

H _dop (k, n, i) =FFT (H _OTA)

where FFT (H _OTA) denotes the Fast Fourier Transform of H _OTA.

For every PRB k and every antenna port n, the Doppler spectrum P is estimated as follows:

for i=0: N _fft-1

Here, N _fft is the number of FFT bins. The Doppler shift is denoted by I which is the index of the peak of the Doppler spectrum P. Fig. 5 shows an example Doppler spectrum of the OTA CSI, where it can be observed that Doppler shift can be accurately evaluated by the index of the peak.

If the Doppler shift is small enough, then the downlink reference signals can be precoded according to information derived from the uplink channel properties under an assumption of channel reciprocity of the wireless channel. Hence, in some embodiments, the assumption of channel reciprocity is confirmed by a Doppler shift, I representing a peak in the Doppler spectrum, of the second part H _OTA being smaller than a threshold value. The threshold value might be determined according to simulations, or tests.

Simulation results will be disclosed next with reference to Fig. 6 and Fig. 7. Fig. 6 shows simulation results without OTA channel mitigation. Fig. 7 shows simulation results with OTA channel mitigation using the above disclosed second embodiment. Each curve represents the CSI phase response of one antenna element. Fig. 6 shows the CSI as affected by the OTA channel and the AC error, where the former contains angular spread (AS) and delay spread (DS) from the wireless channels. To feedback accurate CSI in this case, a large overhead is needed for the feedback channel and a high computation load is needed for the UE. In Fig. 7, on the other hand, shows the CSI as affected only by the AC error only, yielding a much lower AS and DS. In turn, this significantly reduces the overhead for the feedback channel and the computation load for the UE.

The above disclosed first embodiment yields even better results, i.e., lower frequency variation, than the second embodiment.

As disclosed above, there could be different ways in which the downlink reference signals are sent using the AAS. In general terms, some kind of mapping between reference signal resources and antenna elements is used for this purpose. Aspects of such mappings will be disclosed next. If there is a one-to-one mapping between reference signal resources and antenna elements, then the CSI for every antenna element of the AAS can be acquired from the CSI feedback from the UE.

The AAS 140 might comprises antenna branches of one or more polarizations. In some embodiments, per each time instant the downlink reference signals are transmitted, one downlink reference signal resource is transmitted on each antenna branch of each polarization.

In the example illustrated in Fig. 8, the downlink reference signal resources (e.g., 32 CSI-RS ports) are mapped to one of two polarizations of the antenna elements for each time instant. At a first time instant the 32 CSI-RS ports are transmitted in the antenna elements of polarization 1, and at a second time instant the 32 CSI-RS ports are transmitted in the antenna elements of polarization 2. For each time instant the UE estimates and feeds back the CSI to the network node based on the received CSI-RS. Then the network node can estimate the AC errors per each polarization from the received channel information H _p1 (n) and H _p2 (n) , n=1, 2, …32, respectively.

In another case where the number of physical antenna branches is comparatively high, e.g., the AAS comprises 128 antenna branches or more, as shown in Fig. 9, a switch is made between different subsets of antenna elements. As an example, per each time instant the 32 CSI-RS ports are mapped to one such subset to provide direction of departure (DoD) processing. If the 32 CSI-RS ports are mapped to a stochastic subarray arrangement, the DoD information would be lost. Therefore, in some embodiments, the antenna branches of the AAS 140 are divided in partly overlapping subsets. In some embodiments, the downlink reference signals are transmitted in downlink reference signal resources, where, per each time instant the downlink reference signals are transmitted, one downlink reference signal resource is transmitted per each of the antenna branches of only one of the subsets. A switch is made from one subset to another between consecutive time instants in which the downlink reference signals are transmitted.

Aspects of scheduling transmission of the uplink reference signals and transmission of the precoded downlink reference signals will be disclosed next.

In some aspects, the uplink reference signals and the precoded downlink reference signals are only transmitted if either the UE traffic is not predicable or there is no UE traffic at all. Hence, in some embodiments, the network node is configured to perform (optional) step S102:

S102: The network node schedules transmission of the uplink reference signals and transmission of the precoded downlink reference signals. The uplink reference signals and the precoded downlink reference signals are scheduled to be transmitted when traffic for the UE 170 is either unpredictable or absent.

As illustrated in Fig. 10, the uplink reference signals are scheduled to be transmitted before the precoded downlink reference signals. In Fig. 10, SRS is short for sounding reference signal and is an example of an uplink reference signal, Polx is short for polarization x (where x = 1, 2, and where polarization 1 and polarization 2 in some examples are orthogonal relative each other) , Bx is short for beam y (where y = 1, 2, 3) , and “DL CSI Feedback” represents channel feedback sent by the UE. In this respect, the precoded downlink reference signals (represented by the CSI-RS) might need to be transmitted at multiple time instants together with a defined mapping to the antenna elements, to complete all the antenna branches and polarizations. In this respect, two examples are illustrated in Fig. 10, where Scheduling 1 is suitable for an AAS with a comparatively small number of antenna elements, whereas Scheduling 2 is suitable for a comparatively larger number of antenna elements. For Scheduling 2 the precoded downlink reference signals and the uplink reference signals need to be transmitted repeatedly (in three different beams) to keep the OTA channel reciprocity. Hence, Scheduling 2 continues with transmission of CSI-RS in beam 2 using polarization 2 after the last illustrated “DL CSI Feedback” , and so on until CSI-RS has been transmitted in all three beams using polarization 2. Aspects of quality checking of report from the UE of measurements made by the UE will be disclosed next.

For checking if the received measurements are usable for antenna calibration purposes, the network node might check the Channel Quality Indicator (CQI) , if reported from the UE to determine if the CSI feedback is usable for AC purposes. In one example, the network node determines the CSI quality for UEs configured with a lowMobilityEvaluation criterion. In another example, the network node determines the CSI quality for UEs configured with both a lowMobilityEvaluation criterion and a cellEdgeEvaluation criterion whereas the combineRelaxedMeasCondition criterion is not configured, and the UE (s) has fulfilled only the lowMobilityEvaluation criterion. In another example, the network node determines that the measurements are usable for antenna calibration purposes if the UE reports a signal to interference ratio (SNR) being higher than some threshold value, where the threshold value can be predefined or configured by the network node.

One embodiment for OTA calibration of an AAS 140 as performed by the network node and the UE will be disclosed next with reference to the signalling diagram of Fig. 11. Although not explicitly disclosed, it is here assumed that scheduling of transmission of the uplink reference signals and transmission of the precoded downlink reference signals is ongoing by the network node.

S201. The UE transmits uplink reference signals that are received at the AAS.

S202a, S202b, S202c. The network node performs uplink processing on the received uplink reference signals.

S203a, S203b. The network node performs downlink processing by precoding and transmitting downlink reference signals towards the UE from the AAS. The downlink reference signals are precoded according to information derived from the uplink channel properties. The precoded downlink reference signals are received by the UE.

S204a, S204b: The UE makes measurements on the received downlink reference signals to perform channel estimation and compression on the received precoded downlink reference signals and feeds back reports of the measurements to the network node.

S205. The network node checks the received measurements and performs AC as a function of the measurements.

Fig. 12 schematically illustrates, in terms of a number of functional units, the components of a network node 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU) , multiprocessor, microcontroller, digital signal processor (DSP) , etc., capable of executing software instructions stored in a computer program product 1410 (as in Fig. 14) , e.g., in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC) , or field programmable gate array (FPGA) .

Particularly, the processing circuitry 210 is configured to cause the network node 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the network node 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 200 may further comprise a communications interface 220 at least configured for communications with other entities, functions, nodes, and devices, as illustrated in Fig. 1. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the network node 200 e.g., by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the network node 200 are omitted in order not to obscure the concepts presented herein.

Fig. 13 schematically illustrates, in terms of a number of functional modules, the components of a network node 200 according to an embodiment. The network node 200 of Fig. 13 comprises a number of functional modules; an estimate module 210b configured to perform step S104, a precode module 210c configured to perform step S106, a receive module 210d configured to perform step S108, and a calibrate module 210e configured to perform step S110. The network node 200 of Fig. 13 may further comprise a number of optional functional modules, such as a schedule module 210a configured to perform step S102. In general terms, each functional module 210a: 210e may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the network node 200 perform the corresponding steps mentioned above in conjunction with Fig 13. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 210a: 210e may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 210a: 210e and to execute these instructions, thereby performing any steps as disclosed herein.

The network node 200 may be provided as a standalone device or as a part of at least one further device. For example, the network node 200 may be integrated with the AAS. Alternatively, functionality of the network node 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time.

Thus, a first portion of the instructions performed by the network node 200 may be executed in a first device, and a second portion of the of the instructions performed by the network node 200 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network node 200 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 12 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a: 210e of Fig. 13 and the computer program 1420 of Fig. 14.

Fig. 14 shows one example of a computer program product 1410 comprising computer readable storage medium 1430. On this computer readable storage medium 1430, a computer program 1420 can be stored, which computer program 1420 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1420 and/or computer program product 1410 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 14, the computer program product 1410 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1410 could also be embodied as a memory, such as a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM) , or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1420 is here schematically shown as a track on the depicted optical disk, the computer program 1420 can be stored in any way which is suitable for the computer program product 1410.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

A method for over the air calibration of an active antenna system (140) , the method being performed by a network node (200) , the method comprising:

estimating (S104) uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the active antenna system (140) from a user equipment (170) ;

precoding (S106) and transmitting downlink reference signals towards the user equipment (170) from the active antenna system (140) , wherein the downlink reference signals are precoded according to information derived from the uplink channel properties;

receiving (S108) a report from the user equipment (170) of measurements made by the user equipment (170) on the downlink reference signals; and

calibrating (S110) the active antenna system (140) as a function of the measurements made by the user equipment (170) .
The method according to claim 1, wherein the downlink reference signals are precoded according to information derived from the uplink channel properties under an assumption of channel reciprocity of the wireless channel.
The method according to claim 1 or 2, wherein the downlink reference signals are precoded with an objective to eliminate effects on the wireless channel caused by the downlink channel properties.
The method according to any preceding claim, wherein the downlink reference signals are precoded with an objective for the user equipment (170) to measure a reciprocity error of the active antenna system (140) .
The method according to any preceding claim, wherein the downlink reference signals are precoded with an objective to appear to have been transmitted towards the user equipment (170) from the active antenna system (140) in a boresight line of sight direction.
The method according to any preceding claim, wherein the uplink channel properties between the user equipment (170) and the active antenna system (140) are composed of a first part H _UE representing channel impairments caused by the user equipment (170) , a second part H _OTA representing channel impairments caused by the wireless channel from the user equipment (170) to the active antenna system (140) , and a third part
representing uplink channel impairments caused by the active antenna system (140) .
The method according to claim 6, wherein the information is derived from uplink channel properties whilst ignoring the first part H _UE.
The method according to claim 6 or 7, wherein the information is derived from the second part H _OTA and the third part
The method according to claim 6 or 7, wherein the information is derived from the second part H _OTA .
The method according to claim 6 or 7, wherein the information is derived from the third part
The method according to any of claims 6 to 10, wherein the assumption of channel reciprocity is confirmed by a Doppler shift, I representing a peak in a Doppler spectrum, of the second part H _OTA being smaller than a threshold value.
The method according to any preceding claim, wherein the method further comprises:

scheduling (S102) transmission of the uplink reference signals and transmission of the precoded downlink reference signals, wherein the uplink reference signals and the precoded downlink reference signals are scheduled to be transmitted when traffic for the user equipment (170) is either unpredictable or absent.
The method according to any preceding claim, wherein the active antenna system (140) comprises antenna branches of at least one polarization, wherein the downlink reference signals are transmitted in downlink reference signal resources, and wherein, per each time instant the downlink reference signals are transmitted, one downlink reference signal resource is transmitted per each of the antenna branches of each polarization.
The method according to any of claims 1 to 12, wherein the active antenna system (140) comprises antenna branches, and wherein the antenna branches are divided in partly overlapping subsets.
The method according to claim 14, wherein the downlink reference signals are transmitted in downlink reference signal resources, and wherein, per each time instant the downlink reference signals are transmitted, one downlink reference signal resource is transmitted per each of the antenna branches of only one of the subsets, with a switch from one subset to a next between consecutive time instants the downlink reference signals are transmitted.
A network node (200) for over the air calibration of an active antenna system (140) , the network node (200) comprising processing circuitry (210) , the processing circuitry being configured to cause the network node (200) to:

estimate uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the active antenna system (140) from a user equipment (170) ;

precode and transmitting downlink reference signals towards the user equipment (170) from the active antenna system (140) , wherein the downlink reference signals are precoded according to information derived from the uplink channel properties;

receive a report from the user equipment (170) of measurements made by the user equipment (170) on the downlink reference signals; and

calibrate the active antenna system (140) as a function of the measurements made by the user equipment (170) .
A network node (200) for over the air calibration of an active antenna system (140) , the network node (200) comprising:

an estimate module (210b) configured to estimate uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the active antenna system (140) from a user equipment (170) ;

a precode module (210c) configured to precode and transmitting downlink reference signals towards the user equipment (170) from the active antenna system (140) , wherein the downlink reference signals are precoded according to information derived from the uplink channel properties;

a receive module (210d) configured to receive a report from the user equipment (170) of measurements made by the user equipment (170) on the downlink reference signals; and

a calibrate module (210e) configured to calibrate the active antenna system (140) as a function of the measurements made by the user equipment (170) .
The network node (200) according to claim 16 or 17, further being configured to perform the method according to any of claims 2 to 15.
A computer program (1420) for over the air calibration of an active antenna system (140) , the computer program comprising computer code which, when run on processing circuitry (210) of a network node (200) , causes the network node (200) to:

estimate (S104) uplink channel properties from measurements made on uplink reference signals received over a wireless channel by the active antenna system (140) from a user equipment (170) ;

precode (S106) and transmitting downlink reference signals towards the user equipment (170) from the active antenna system (140) , wherein the downlink reference signals are precoded according to information derived from the uplink channel properties;

receive (S108) a report from the user equipment (170) of measurements made by the user equipment (170) on the downlink reference signals; and

calibrate (S110) the active antenna system (140) as a function of the measurements made by the user equipment (170) .
A computer program product (1410) comprising a computer program (1420) according to claim 19, and a computer readable storage medium (1430) on which the computer program is stored.