Virtual cable calibration at the device under test in OTA test setups

Abstract

An apparatus configured to : receive, from a test equipment, a first indication to estimate a channel matrix of a downlink channel; receive, from the test equipment, at least one downlink reference signal; estimate, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal; receive, from the test equipment, a second indication to determine a virtual cable calibration matrix; determine, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receive, from the test equipment, a third indication to perform virtual cable calibration on- at least one received baseband signal, before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration.

Description

VIRTUAL CABLE CALIBRATION AT THE DEVICE UNDER TEST IN OTA TEST SETUPSTECHNICAL FIELD

[0001] The example and non-limiting embodiments relate generally to testing of user equipment and, more particularly, to over-the-air testing .BACKGROUND

[0002] It is known, in testing of user equipment , to perform over-the-air testing in an anechoic chamber .SUMMARY

[0003] The following summary is merely intended to be illustrative . The summary is not intended to limit the scope of the claims .

[0004] In accordance with one aspect, an apparatus comprising : at least one processor; and at least one memory storing instructions that , when executed by the at least one processor, cause the apparatus at least to : receive, from a test equipment , a first indication to estimate a channel matrix of a downlinkchannel ; receive , from the test equipment , at least one downlink reference signal ; estimate , in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receive , from the test equipment , a second indication to determine a virtual cable calibration matrix; determine, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receive, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0005] In accordance with one aspect , a method comprising : receiving, with a device under test from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation , based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0006] In accordance with one aspect , an apparatus comprising means for : receiving, from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0007] In accordance with one aspect , a non-transitory computer- readable medium comprising program instructions stored thereon for performing at least the following : causing receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing receiving, from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on thevirtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0008] In accordance with one aspect , an apparatus comprising : at least one processor ; and at least one memory storing instructions that , when executed by the at least one processor, cause the apparatus at least to : transmit , to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmit , to the user equipment , at least one downlink reference signal ; transmit , to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmit , to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; receive, from the user equipment , an acknowledgement , wherein the acknowledgement is configured to confirm the virtual cable calibration; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0009] In accordance with one aspect , a method comprising : transmitting, with a test equipment to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmitting, to the user equipment , at least one downlink reference signal ; transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , beforedemodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0010] In accordance with one aspect , an apparatus comprising means for : transmitting, to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmitting, to the user equipment , at least one downlink reference signal ; transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0011] In accordance with one aspect , a non-transitory computer- readable medium comprising program instructions stored thereon for performing at least the following : causing transmitting, to a user equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing transmitting, to the user equipment , of at least one downlink reference signal ; causing transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; causing transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal, before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0012] According to some aspects, there is provided the subj ect matter of the independent claims . Some further aspects are defined in the dependent claims .BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The foregoing aspects and other features are explained in the following description, taken in connection with the accompanying drawings , wherein :

[0014] FIG . 1 is a block diagram of one possible and nonlimiting example system in which the example embodiments may be practiced;

[0015] FIG . 2 is a diagram illustrating features as described herein ;

[0016] FIG . 3 is a diagram illustrating features as described herein;

[0017] FIG . 4 is a flowchart illustrating steps as described herein;

[0018] FIG . 5 is a diagram illustrating features as described herein;

[0019] FIG . 6 is a flowchart illustrating steps as described herein; and

[0020] FIG . 7 is a flowchart illustrating steps as described herein .DETAILED DESCRIPTION OF EMBODIMENTS

[0021] The following abbreviations that may be found in the specification and / or the drawing figures are defined as follows :3 GPP third generation partnership project5G fifth generation5GC 5G core networkAMF access and mobility management functionAoA angle of arrival cRAN cloud radio access networkCSI channel state information cu central unitDRX discontinuous receptionDU distributed unitDUT device under test eNB (or eNodeB) evolved Node B (e.g., an LTE base station)EN-DC E-UTRA-NR dual connectivity en-gNB or En-gNB node providing NR user plane and control plane protocol terminations towards the UE, and acting as secondary node in EN-DCE-UTRA evolved universal terrestrial radio access, i.e., the LTE radio access technology gNB (or gNodeB) base station for 5G / NR, i.e., a node providing NR user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GCIE information elementI / F interfaceLI layer 1LMMSE linear minimum mean-squared errorLTE long term evolutionMAC medium access controlMIMO multiple input multiple outputMME mobility management entityMMSE minimum mean-squared errorNAS non-access stratum ng or NG new generation ng-eNB or NG-eNB new generation eNBNR new radioN / W or NW networkO-RAN open radio access networkOTA over the airPDCP packet data convergence protocolPDSCH physical downlink shared channelPHY physical layerRAN radio access networkRF radio frequencyRLC radio link controlRRC radio resource controlRRH remote radio headRRM radio resource managementRS reference signalRSRP reference signal received powerRSRP-B reference signal received power per branchRU radio unitRx receiverSDAP service data adaptation protocolSGW serving gatewaySMF session management functionSS system simulatorTE test equipmentTRP transmission reception pointTx transmiterUAI user equipment assistance informationUE user equipment (e.g., a wireless, typically mobile device)UPF user plane functionVCC virtual cable calibrationVNR virtualized network function

[0022] Turning to FIG . 1 , this figure shows a block diagram of one possible and non-limiting example in which the examples may be practiced . A user equipment (UE) 110 , radio access network ( RAN ) node 170 , and network element ( s ) 190 are illustrated . In the example of FIG . 1 , the user equipment (UE ) 110 is in wireless communication with a wireless network 100 . A UE is a wireless device that can access the wireless network 100 . The UE 110 includes one or more processors 120 , one or more memories 125 , and one or more transceivers 130 interconnected through one or more buses 127 . Each of the one or more transceivers 130 includes a receiver, Rx, 132 and a transmitter , Tx, 133. The one or more buses 127 may be address , data, or control buses , and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit , fiber optics or other optical communication equipment , and the like . A "circuit" may include dedicated hardware or hardware in association with software executable thereon . The one or more transceivers 130 are connected to one or more antennas 128 . The one or more memories 125 include computer program code 123 . The UE 110 includes a component 140 , comprising one of or both parts 140-1 and / or 140-2 , which may be implemented in a number of ways . The component 140 may be implemented in hardware as component 140-1 , such as beingimplemented as part of the one or more processors 120. The component 140-1 may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. In another example, the component 140 may be implemented as component 140-2, which is implemented as computer program code 123 and is executed by the one or more processors 120. For instance, the one or more memories 125 and the computer program code 123 may be configured to, with the one or more processors 120, cause the user equipment 110 to perform one or more of the operations as described herein. The UE 110 communicates with RAN node 170 via a wireless link 111.

[0023] The RAN node 170 in this example is a base station that provides access by wireless devices such as the UE 110 to the wireless network 100. The RAN node 170 may be, for example, a base station for 5G, also called New Radio (NR) . In 5G, the RAN node 170 may be a NG-RAN node, which is defined as either a gNB or a ng-eNB. A gNB is a node providing NR user plane and control plane protocol terminations towards the UE, and connected via the NG interface to a 5GC (such as, for example, the network element (s) 190) . The ng-eNB is a node providing E-UTRA user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GC. The NG-RAN node may include multiple gNBs, which may also include a central unit (CU) (gNB-CU) 196 and distributed unit(s) (DUs) (gNB-DUs) , of which DU 195 is shown. Note that the DU may include or be coupled to and control a radio unit (RU) . The gNB-CU is a logical node hosting RRC, SDAP and PDCP protocols of the gNB or RRC and PDCP protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the Fl interface connected with the gNB-DU. The Fl interface is illustrated as reference 198, although reference 198also illustrates a link between remote elements of the RAN node 170 and centralized elements of the RAN node 170, such as between the gNB-CU 196 and the gNB-DU 195. The gNB-DU is a logical node hosting RLC, MAC and PHY layers of the gNB or en-gNB, and its operation is partly controlled by gNB-CU. One gNB-CU supports one or multiple cells. The gNB-DU terminates the Fl interface 198 connected with the gNB-CU. Note that the DU 195 is considered to include the transceiver 160, e.g., as part of a RU, but some examples of this may have the transceiver 160 as part of a separate RU, e.g., under control of and connected to the DU 195. The RAN node 170 may also be an eNB (evolved NodeB) base station, for LTE (long term evolution) , or any other suitable base station, access point, access node, or node.

[0024] The RAN node 170 includes one or more processors 152, one or more memories 155, one or more network interfaces (N / W I / F(s)) 161, and one or more transceivers 160 interconnected through one or more buses 157. Each of the one or more transceivers 160 includes a receiver, Rx, 162 and a transmitter, Tx, 163. The one or more transceivers 160 are connected to one or more antennas 158. The one or more memories 155 include computer program code 153. The CU 196 may include the processor (s) 152, memories 155, and network interfaces 161. Note that the DU 195 may also contain its own memory / memories and processor ( s ) , and / or other hardware, but these are not shown.

[0025] The RAN node 170 includes a component 150, comprising one of or both parts 150-1 and / or 150-2, which may be implemented in a number of ways. The component 150 may be implemented in hardware as component 150-1, such as being implemented as part of the one or more processors 152. The component 150-1 may be implementedalso as an integrated circuit or through other hardware such as a programmable gate array . In another example , the component 150 may be implemented as component 150-2 , which is implemented as computer program code 153 and is executed by the one or more processors 152 . For instance, the one or more memories 155 and the computer program code 153 are configured to, with the one or more processors 152 , cause the RAN node 170 to perform one or more of the operations as described herein . Note that the functionality of the component 150 may be distributed, such as being distributed between the DU 195 and the CU 196, or be implemented solely in the DU 195 .

[0026] The one or more network interfaces 161 communicate over a network such as via the links 176 and 131 . Two or more gNBs 170 may communicate using, e . g . , link 176. The link 176 may be wired or wireless or both and may implement , for example , an Xn interface for 5G, an X2 interface for LTE, or other suitable interface for other standards .

[0027] The one or more buses 157 may be address , data, or control buses , and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit , fiber optics or other optical communication equipment , wireless channels , and the like . For example , the one or more transceivers 160 may be implemented as a remote radio head (RRH) 195 for LTE or a distributed unit ( DU) 195 for gNB implementation for 5G, with the other elements of the RAN node 170 possibly being physically in a different location from the RRH / DU, and the one or more buses 157 could be implemented in part as , for example , fiber optic cable or other suitable network connection to connect the other elements ( e . g . , a central unit ( CU) , gNB-CU ) of the RAN node 170 to theRRH / DU 195. Reference 198 also indicates those suitable network link ( s ) .

[0028] It is noted that description herein indicates that "cells" perform functions, but it should be clear that equipment which forms the cell will perform the functions. The cell makes up part of a base station. That is, there can be multiple cells per base station. For example, there could be three cells for a single carrier frequency and associated bandwidth, each cell covering one-third of a 360 degree area so that the single base station's coverage area covers an approximate oval or circle. Furthermore, each cell can correspond to a single carrier and a base station may use multiple carriers. So if there are three 120 degree cells per carrier and two carriers, then the base station has a total of 6 cells.

[0029] The wireless network 100 may include a network element or elements 190 that may include core network functionality, and which provides connectivity via a link or links 181 with a further network, such as a telephone network and / or a data communications network (e.g., the Internet) . Such core network functionality for 5G may include access and mobility management function(s) (AMF(s) ) and / or user plane functions (UPF(s) ) and / or session management function (s) (SMF(s)) . Such core network functionality for LTE may include MME (Mobility Management Entity) / SGW (Serving Gateway) functionality. These are merely illustrative functions that may be supported by the network element (s) 190, and note that both 5G and LTE functions might be supported. The RAN node 170 is coupled via a link 131 to a network element 190. The link 131 may be implemented as, e.g., an NG interface for 5G, or an SI interface for LTE, or other suitable interface for other standards. Thenetwork element 190 includes one or more processors 175, one or more memories 171, and one or more network interfaces (N / W I / F(s) ) 180, interconnected through one or more buses 185. The one or more memories 171 include computer program code 173. The one or more memories 171 and the computer program code 173 are configured to, with the one or more processors 175, cause the network element 190 to perform one or more operations.

[0030] The wireless network 100 may implement network virtualization, which is the process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization involves platform virtualization, often combined with resource virtualization. Network virtualization is categorized as either external, combining many networks, or parts of networks, into a virtual unit, or internal, providing networklike functionality to software containers on a single system. For example, a network may be deployed in a tele cloud, with virtualized network functions (VNF) running on, for example, data center servers. For example, network core functions and / or radio access network(s) (e.g. CloudRAN, O-RAN, edge cloud) may be virtualized. Note that the virtualized entities that result from the network virtualization are still implemented, at some level, using hardware such as processors 152 or 175 and memories 155 and 171, and also such virtualized entities create technical effects.

[0031] It may also be noted that operations of example embodiments of the present disclosure may be carried out by a plurality of cooperating devices (e.g. cRAN) .

[0032] The computer readable memories 125, 155, and 171 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The computer readable memories 125, 155, and 171 may be means for performing storage functions. The processors 120, 152, and 175 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on a multicore processor architecture, as non-limiting examples. The processors 120, 152, and 175 may be means for performing functions, such as controlling the UE 110, RAN node 170, and other functions as described herein.

[0033] In general, the various example embodiments of the user equipment 110 can include, but are not limited to, cellular telephones such as smart phones, tablets, personal digital assistants (PDAs) having wireless communication capabilities, portable computers having wireless communication capabilities, image capture devices such as digital cameras having wireless communication capabilities, gaming devices having wireless communication capabilities, music storage and playback appliances having wireless communication capabilities, Internet appliances permitting wireless Internet access and browsing, tablets with wireless communication capabilities, as well as portable units or terminals that incorporate combinations of such functions.

[0034] Having thus introduced one suitable but non-limiting technical context for the practice of the example embodiments ofthe present disclosure, example embodiments will now be described with greater specificity.

[0035] Features as described herein may generally relate to conductive testing. One of the biggest challenges for test and measurement equipment / device manufacturers is implementing standardized evaluation and verification methods for UE under repeatable and realistic conditions, which are also reliable with mass production processes. For LTE and 5G NR FR1 tests, conductive testing methods are the norm for multiple input multiple output (MIMO) devices. During testing, the antennas of the device under test (DUT) are disconnected from the antenna ports, and the DUT is directly connected to the test system using a coaxial cable.

[0036] The legacy conductive test system for FR1 requirements is illustrated in FIG. 2, where each UE antenna port (240, 250) is connected to the test system (210) via 50 Ohm cables. In the example of FIG. 2, DUT 230 is connected to the channel emulator 220 through coaxial cables (and not directly with the system simulator (SS) or test equipment (TE) 210) and the channel emulator 220 is connected with the SS / TE 210 through cables. The SS / TE models the transmission and reception points (TRP) while the channel (fading) emulator models the real-world wireless channel. It may be noted that the example of FIG. 2 is not limiting; for example, the channel emulator 220 may be implemented inside the SS / TE 210.

[0037] In the present disclosure, the terms UE and DUT may be used interchangeably.

[0038] In the present disclosure, a TRP may refer to a reception point, a transmission point, or both a reception point and a transmission point.

[0039] FIG. 2 illustrates an example of a 4x4 system with two H / V polarized antenna panels at the device under test (DUT) (230) . For the legacy conducted test system, the received signal can be modeled as: y = HLx (1)

[0040] where x is the transmitted signals from the transmission and reception points (TRPs) (here system simulator or test equipment) (220) , HLis the channel matrix (e.g., 2x2, 4x4, etc.) simulated by the channel emulator and y is the received signal at the DUT.

[0041] As the DUT antenna ports (240, 250) are connected to the channel emulator 220 and / or the test system (210) through cables, there is no crosstalk experienced in the channel of the testing chamber and, hence, the testing chamber's innate channel matrix (for an ideal scenario) can be modelled as an identity matrix of order 4, for example:’1 0 0 0 1 0Hcable 0 0 1 .0 0 0

[0042] where it has been assumed that the cables are perfect with unity gain (i.e., there is no crosstalk between the cables or from the connectors and no losses) . The crosstalk generated by the wireless channel for a DUT deployed in the field is, thus, modelledin the channel emulator HL( 220 ) for the purpose of conformance testing .

[0043] Conformance testing, in the context of 3GPP ( Third Generation Partnership Proj ect ) , refers to a set of standardized tests and procedures designed to ensure that telecommunication equipment and devices , such as mobile phones and network infrastructure , comply with the technical specifications and standards defined by 3GPP . These tests are essential for ensuring interoperability and compatibility among different manufacturers ' products within the 3GPP ecosystem.

[0044] Features as described herein may generally relate to over the air (OTA) testing, for example the multi-TRP / multi-probe radiated (OTA) test system. Over-the-air (OTA) testing is a technique for evaluating a cordless device ' s performance and reliability in the actual world . The testers use a test chamber with an open vacuum of space to represent real-world scenarios . It is put through a series of tests to determine how the device will respond to various systems . By analyzing the whole signal route and antenna functionality, OTA antenna testing helps to confirm that the equipment developed or chosen will function as expected . For testing UEs in the FR2 bands , conductive testing methods are not practical , as the large number of integrated antennas on the UE for spatial multiplexing and beamforming requires testing OTA, without cable connections .

[0045] 5G New Radio (NR) is the first standard to use the mmWave frequency region for higher data transfer rates . The highly integrated front-ends and array antennas necessitate advanced over-the-air (OTA) testing methods and new RF test metrics forassessing current and future mobile communication. Such test metrics include virtual cable calibration (VCC) , which may be mandatory or preferred for reproducible and reliable OTA throughput testing. For performance tests where fading is emulated, such as radio resource management (RRM) conformance and demodulation testing, the VCC method is crucial to assess defined antenna correlations with minimal crosstalk from the OTA link.

[0046] An example of the multi-TRP multi-Rx demodulation OTA test system is shown in FIG. 3, where the UE (350) is placed inside of the anechoic chamber (330) and the test system (comprising the SS / TE 310 and the channel emulator 320, which may be separate or part of each other) is connected to two (or more) dual-polarized probes (340) at different positions inside of that chamber. In this case, the received signal can be modeled as:

[0047] where x is the transmitted signals from the TRPs (here system simulator or test equipment) , HLis the same channel matrix (e.g., 2x2, 4x4, etc. ) as in the legacy conductive test system simulated by the channel emulator (320) , HOTAcomprises the OTA chamber and DUT (including spatial filters) , and y is the received signal at the DUT.

[0048] The innate channel matrix between the probe antennas and the baseband receiver branches of the DUT of such an OTA test chamber (Horj4) is harder to characterize. However, it is evident that different crosstalk factors will need to be taken into account, and they can broadly be clustered in inter polarization crosstalk factors and intra polarization crosstalk factors:„T1R1 „T2R1 „T2Rl-i pL1R1712 711 712 „T1R1 „T1R1 „T2R1 „T2R1 721 722 721 722HQTA — „T1R2 „T1R2 „T2R2 „T2R2 711 712 711 712„T1 R2 „T1R2 „T2R2 „T2R2 L721 722 721 722J

[0049] OTA testing introduces challenges . The transmitted signal propagating in the wireless channel ( i . e . OTA channel of the test chamber ) is affected by other signals and noise, becoming distorted . To have defined and reproducible conditions similar to conductive testing, the effects of the OTA channel of the test chamber must be eliminated . One approach to solve this issue is to calculate the unknown transfer matrix A by accounting for the complete OTA environment , including the transmitter and receiver antenna characteristics . This approach is complex and, in most cases , not possible : UE manufacturers are not required to give detailed information about their antenna characteristics , including the phase information required to apply this method .

[0050] For conformance testing, it is required to exactly mimic the behavior of real-world wireless channel (between the base station and the UE) inside the anechoic chamber in a controlled manner . Presently, this is done in FR1 conducted mode testing ( see FIG . 2 ) by using the channel fading emulator, which generates the desired wireless channel matrix HLduring the conformance testing . However, for FR2 , only over the air (OTA) mode testing is permitted, and hence the test setup shown in FIG . 3 needs to be adopted .

[0051] In OTA testing, the problem is that there is a wireless channel between the transmitting probes ( 340 ) of the test equipment installed inside the anechoic test chamber ( 330 ) and the DUT' s antenna panels / modules ( 360 ) , which is not present in the real-world environment (i.e. it is only present during OTA conformance testing) , and hence its effect (e.g. crosstalk / interf erence ) on the received signal is undesired, and may give incorrect test results. Therefore, the mitigation of the effect of anechoic chamber's OTA channel is required to achieve the correct test results. This can be done by implementing a wireless / virtual cable calibration at the test equipment.

[0052] The Rel-18 multi-Rx work item deals with up to 4 DL layers being received at the UE from two different directions / angles of arrival. These 4 DL layers may be transmitted by the 4 singlepolarized probes during the conformance test, thereby making the anechoic chamber's OTA matrix to be of size 4x4. Indian Provisional Patent Application No. 202341053907 (filed August 11, 2023) , which is hereby incorporated by reference in its entirety, proposed a procedure to send the estimate of the anechoic chamber's OTA matrix coefficients back to the test equipment so that it can implement a virtual cable calibration (VCC) matrix at its side. However, this method will not be optimal for more advanced UEs supporting multi-TRP multi-panel DL reception (e.g. more than 2 TRPs and 2 UE panels) or more MIMO layers because the size of the OTA matrix / MIMO matrix increases with the increase in the number of TRPs and / or UE panels and / or layers. This will increase the feedback overhead required to send the OTA matrix estimate to the test equipment for VCC implementation.

[0053] An alternative approach derives the VCC matrix using only the reference signal received power per branch (RSRP-B) feedback parameters, which can be retrieved from an FR2 UE per the 3GPP standards. This enables having a quasi-conducted or "virtual cable" connection or "virtual cabling" in an FR2 radiated testenvironment . The approach lays the foundation for practical 5G UE performance testing for maximum throughput , as well as testing under various channel conditions , such as fading . An equalized channel is also mandatory for conformance tests with fading, which are needed for RRM tests .

[0054] The current approach to find the virtual cable calibration matrix based on reference signal received power per branch ( RSRP- B) is potentially taking hours , which is too long to find enough DUT / probe positions for OTA conformance testing with various angles of arrival (AoA) . Also, for advanced UEs supporting multi- TRP multi-panel DL reception or a large number of MIMO layers ( e . g . for much more than 2 TRPs and 2 UE panels ; relevant for future 3GPP release and / or 6G) , the method proposed in Indian Provisional Patent Application No . 202341053907 may not be optimal , as it will increase the feedback overhead (because of the increased size of the OTA matrix / MIMO matrix) required to send the OTA matrix estimate to the test equipment for VCC implementation .

[0055] It is desirable to find an optimal , quicker, and easier method to implement a virtual calibration matrix mitigating the effect of the OTA matrix, and potentially with larger isolation .

[0056] In an example embodiment, a DUT may estimate the anechoic chamber' s OTA matrix, HOTA, using the downlink training / ref erence symbols .

[0057] In an example embodiment , the DUT may calculate the VCC matrix, Hvcc, from the estimated matrix HOTA. HOTAmay be defined as the estimate of HOTA. The VCC matrix may also be estimated based on received reference signals .

[0058] In an example embodiment, the DUT may store the calculated VCC matrix (Hvcc} at the DUT, for example to be used for VCC at a later point in time during the OTA conformance test.

[0059] In an example embodiment, the DUT may apply the VCC matrix to the received baseband signals (e.g. information signal vector (s) received on the physical downlink shared channel (PDSCH) ) before the demodulation operation.

[0060] In an example embodiment, the test equipment (TE) or system simulator (SS) may provide the configuration of the downlink training / ref erence symbols to the DUT so that the DUT can estimate HOTAbefore the start of the actual conformance test.

[0061] In an example embodiment, the test equipment or system simulator may indicate, to the DUT, to calculate the VCC matrix (Hvcc) from the estimated matrix HOTAand to store the calculated VCC matrix (Hvcc) at the DUT, for example to be used for VCC at a later point in time during the OTA conformance test, through a new signaling .

[0062] In an example embodiment, the test equipment or system simulator may indicate, to the DUT, to apply the VCC matrix to the received baseband signals (e.g. before the demodulation operation) through a new signaling.

[0063] Referring now to FIG. 4, illustrated is a high-level message flow explaining the messages that may be exchanged between the TE / SS, and the expected DUT action to implement VCC at the DUT.

[0064] At 405 , UE side VCC calibration process may be started by the TE / SS and the DUT . At 410 , the TE / SS may transmit , to the DUT, a configuration required to estimate HOTA. The test-equipment may configure the DUT to estimate HOTAby providing the required configuration parameters through an RRC message, for example , RRC Reconfiguration message . Furthermore , the test equipment may also conf igure / set its parameters for the same , for example , by setting the 'Channel Emulator' matrix ( see FIG . 3 ) to an identity matrix of a suitable order .

[0065] At 415 , the TE / SS may transmit , to the DUT , a trigger for HQTA estimation, and may send reference signals . The test equipment may trigger the HOTA estimation either through a newly defined RRC signaling or through dedicated non-access stratum (NAS ) signaling for conformance testing ( i . e . , by defining a new RAN5 conformance test function for the same ) and transmit the DL reference signals . The reference signals may typically be used to estimate the wireless channel matrix / coef f icients , and / or to estimate a VCC matrix .

[0066] At 420 , the DUT may estimate the HOTA matrix . Optionally, at 425 the DUT may transmit , to the TE / SS , a confirmation or acknowledgement . The DUT may send an acknowledgement message to the test equipment confirming the completion of estimation phase . This acknowledgement may be explicitly sent in the UL, for example , either through 'UE Assistance Information' message or a RAN5 conformance test acknowledgement message . Alternatively, this acknowledgement may be implicitly indicated to the test equipment , for example, by providing the channel state information (CSI ) report to the test equipment , on reception of which the test equipment will infer that the HOTAis estimated at the DUT .

[0067] The DUT may estimate the complete DL channel (which includes the combined effect of the channel emulator, connecting cables, and the OTA test chamber) . However, the estimated channel may correspond to only the OTA test chamber portion of the DL channel (between the test probes and DUT) . This is because the channel emulator is set to an identity matrix during this estimation, and cables are assumed to be perfect, hence the channel corresponding to the cables may also be modeled as an identity matrix. Therefore, in this case, the combined DL channel may correspond to only the OTA channel portion between the test probes and the DUT (see, e.g., FIG. 3) .

[0068] Once the HOTAis estimated at the DUT and is (optionally) informed to the test equipment, at 430, the TE / SS may transmit, to the DUT, an indication to calculate and store the VCC matrix HVccusing the estimate of HOTA. Optionally, the TE / SS may transmit, to the DUT, an indication of the method to be used for the calculation of the VCC matrix (for example, zero-forcing or linear minimum mean-squared error (LMMSE) or some other method) , for example in case more than one method for deriving the VCC matrix at the DUT is standardized. These indication ( s ) may be communicated to the DUT either through a new RRC signaling message / inf ormation element, or through a new RAN5 conformance test function.

[0069] At 435, the DUT may calculate the VCC matrix Hvccusing the estimate of HOTA- The VCC matrix Hvcc may be determined using an indicated method. The VCC matrix Hvccmay be stored for later use. For example, the coefficients of this matrix may be stored.

[0070] Optionally, at 440, the DUT may transmit, to the TE / SS, a confirmation or acknowledgement (e.g. either through 'UEAssistance Information' message or a RAN5 conformance test acknowledgement message) . The confirmation or acknowledgement may confirm the calculation and storing of the VCC matrix for future use .

[0071] At 445, the TE / SS may transmit, to the DUT, an indication to apply the VCC matrix Hvccon the received DL signals at the DUT, for example either through a new RRC signaling message / inf ormation element or through a new RAN5 conformance test function.

[0072] At 450, the DUT may multiply the received DL baseband symbol vector (before demodulation) by the VCC matrix Hvcc- For example, if y is the received baseband symbol vector (before demodulation) , then the symbol vector obtained after performing VCC at the DUT will be Hvcc*y, where denotes the matrix multiplication operation.

[0073] At 455, the DUT may transmit, to the TE / SS, a confirmation or acknowledgment, for example either through 'UE Assistance Information' message or a RAN5 conformance test acknowledgement message. The confirmation or acknowledgment may confirm the implementation of VCC at the DUT, after which the TE may start the actual conformance testing.

[0074] At 460, the OTA conformance test may begin. At 465, the OTA conformance test may end.

[0075] In an alternative example embodiment, the indication to trigger the estimation of H0Taat the DUT (415) , the indication to calculate / derive and store Hvccat the DUT (430) , and the indication to apply VCC at the DUT (445) , may all be communicated to the DUT in a single message, for example (substantially) simultaneouslyor in parallel. Alternatively, the indications may be compressed together. A technical effect of sending the indications together may be to save the message exchange time and / or reduce overall signaling overhead.

[0076] In an alternative example embodiment, the indication to calculate / derive and store Hvccat the DUT (430) and the indication to apply VCC at the DUT (445) may comprise a same indication / signaling . In other words, a single indication may serve to cause the DUT to both derive VCC matrix and use it for VCC starting from slot [ x] .

[0077] In an example embodiment, virtual cable calibration may be applied at the DUT for OTA conformance testing, for example as illustrated in FIG. 5. The DUT may comprise one or more panels (510, 520, 530) connected to RF chains (540) . The baseband (550) may comprise a virtual cable calibration at the DUT module, block, or function (560) and a demodulation and other processing modules, blocks, or functions (570) .

[0078] Before the virtual cable calibration is performed at the DUT as illustrated in FIG. 5, it may be required that the DUT first estimate the DL channel, i.e., HOTA, (e.g. between test probes and UE antenna panels) , and then calculate the VCC matrix, i.e. , HVCCrfor example by using the estimate of HOTA, and store Hvccin the DUT, for example to be used for VCC at a later point in time (e.g. just before starting the OTA conformance testing) .

[0079] In an additional example embodiment, the DUT may use separate demodulators per RF frontend, or per panel. In such a case, a VCC block may be needed per f rontend / panel / demodulator .Steps according to example embodiments of the present disclosure may then apply to each VCC block .

[0080] A technical effect of example embodiments of the present disclosure may be to provide an optimal solution by implementing VCC at the CUT . A technical effect of example embodiments of the present disclosure may be to provide an easy and fast approach for implementing virtual cable solution OTA conformance testing . A technical effect of example embodiments of the present disclosure may be to not require exhaustive search for obtaining the optimal weights .

[0081] A technical effect of example embodiments of the present disclosure may be to improve testing time , and thus testing cost .

[0082] A technical effect of example embodiments of the present disclosure may be to eliminate the quantization error observed in the feedback process , as OTA matrix coefficients are not feedback to the test equipment .

[0083] A technical effect of example embodiments of the present disclosure may be to ensure easy debugging in case of errors , as in the example of FIG . 4 each indication may be sent to the DUT in the DL only after the acknowledgement for the previous indication is received at the TE .

[0084] A technical effect of example embodiments of the present disclosure may be to save the signaling message exchange time , resources and energy in the alternative where all the indications are sent to the DUT in the DL in a single message .

[0085] FIG . 6 illustrates the potential steps of an example method 600. The example method 600 may include : receiving, froma test equipment , a first indication to estimate a channel matrix of a downlink channel , 610 ; receiving, from the test equipment , at least one downlink reference signal , 620 ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal , 630 ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix , 640 ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix, 650 ; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix, 660 ; and performing conformance testing . based, at least partially, on the virtual cable calibration, 670 . The example method 600 may be performed, for example , with a UE, DUT , etc .

[0086] FIG . 7 illustrates the potential steps of an example method 700 . The example method 700 may include : transmitting, to a user equipment , a first indication to estimate a channel matrix of a downlink channel , 710 ; transmitting, to the user equipment , at least one downlink reference signal , 720 ; transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix, 730 ; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal, before demodulation, based, at least partially, on the virtual cable calibration matrix, 740 ; and performing conformance testing based, at least partially, on the virtual cable calibration, 750 . The example method 700 may be performed, for example , SS , etc .

[0087] In accordance with one example embodiment , an apparatus may comprise : at least one processor; and at least one memory storing instructions that , when executed by the at least one processor, cause the apparatus at least to : receive, from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receive , from the test equipment, at least one downlink reference signal ; estimate , in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receive, from the test equipment , a second indication to determine a virtual cable calibration matrix; determine , in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receive, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0088] The first indication to estimate the channel matrix may be received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0089] The example apparatus may be further configured to : transmit , to the test equipment , a confirmation that the channel matrix has been estimated .

[0090] The confirmation may be transmitted via at least one of : an explicit indication, user equipment assistance information, aconformance test acknowledgement message , an implicit indication, or a channel state information report .

[0091] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0092] The virtual cable calibration matrix may be determined based, at least partially, on the indicated method for determining the virtual cable calibration matrix .

[0093] The second indication may be received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0094] The example apparatus may be further configured to : transmit , to the test equipment , a confirmation that the virtual cable calibration matrix has been determined .

[0095] The third indication may be received via at least one of : radio resource control signaling, or a user equipment special conformance test function .

[0096] The first indication, the second indication, and the third indication may be received in parallel via a same message .

[0097] The first indication, the second indication , and the third indication may be received as a compressed message .

[0098] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0099] The example apparatus may be further configured to : in response to the third indication, multiply the at least one received baseband signal , before demodulation, by the virtual cable calibration matrix .

[0100] The example apparatus may be further configured to : receive , from the test equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be estimated based, at least partially, on the configuration .

[0101] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the apparatus inside a test chamber .

[0102] The example apparatus may be further configured to : transmit , to the test equipment , an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0103] In accordance with one aspect , an example method may be provided comprising : receiving, with a device under test from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at leastpartially, on the estimated channel matrix; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0104] The first indication to estimate the channel matrix may be received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0105] The example method may further comprise : transmitting, to the test equipment , a confirmation that the channel matrix has been estimated .

[0106] The confirmation may be transmitted via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .

[0107] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0108] The virtual cable calibration matrix may be determined based, at least partially, on the indicated method for determining the virtual cable calibration matrix .

[0109] The second indication may be received via at least one of : radio resource control signaling, or non-access stratumsignaling using a user equipment special conformance test function .

[0110] The example method may further comprise : transmitting, to the test eguipment , a confirmation that the virtual cable calibration matrix has been determined .

[0111] The third indication may be received via at least one of : radio resource control signaling, or a user eguipment special conformance test function .

[0112] The first indication, the second indication, and the third indication may be received in parallel via a same message .

[0113] The first indication, the second indication, and the third indication may be received as a compressed message .

[0114] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0115] The example method may further comprise : in response to the third indication, multiplying the at least one received baseband signal , before demodulation, by the virtual cable calibration matrix .

[0116] The example method may further comprise : receiving, from the test equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be estimated based, at least partially, on the configuration .

[0117] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the device under test inside a test chamber .

[0118] The example method may further comprise : transmitting, to the test equipment , an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0119] In accordance with one example embodiment , an apparatus may comprise : circuitry configured to perform: receiving, from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; circuitry configured to perform: receiving, from the test equipment , at least one downlink reference signal ; circuitry configured to perform: estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; circuitry configured to perform : receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix; circuitry configured to perform: determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; circuitry configured to perform: receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and circuitry configured to perform : performing conformance testing based, at least partially, on the virtual cable calibration .

[0120] In accordance with one example embodiment , an apparatus may comprise : processing circuitry; memory circuitry including computer program code, the memory circuitry and the computer program code configured to, with the processing circuitry, enable the apparatus to : receive , from a test equipment , a first indication to estimate a channel matrix of a downlink channel ;receive, from the test equipment, at least one downlink reference signal; estimate, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal; receive, from the test equipment, a second indication to determine a virtual cable calibration matrix; determine, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receive, from the test equipment, a third indication to perform virtual cable calibration on at least one received baseband signal, before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0121] As used in this application, the term "circuitry" may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable) : (i) a combination of analog and / or digital hardware circuit (s) with sof tware / f irmware and (ii) any portions of hardware processor (s) with software (including digital signal processor ( s ) ) , software, and memory (ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit (s) and or processor (s) , such as a microprocessor (s) or a portion of a microprocessor ( s ) , that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation." This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers animplementation of merely a hardware circuit or processor ( or multiple processors ) or portion of a hardware circuit or processor and its ( or their) accompanying software and / or firmware . The term circuitry also covers , for example and if applicable to the particular claim element , a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device , or other computing or network device .

[0122] In accordance with one example embodiment , an apparatus may comprise means for : receiving, from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation , based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0123] The first indication to estimate the channel matrix may be received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0124] The means may be further configured for : transmitting, to the test equipment , a confirmation that the channel matrix has been estimated .

[0125] The confirmation may be transmitted via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .

[0126] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0127] The virtual cable calibration matrix may be determined based, at least partially, on the indicated method for determining the virtual cable calibration matrix .

[0128] The second indication may be received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0129] The means may be further configured for : transmitting, to the test equipment , a confirmation that the virtual cable calibration matrix has been determined .

[0130] The third indication may be received via at least one of : radio resource control signaling, or a user equipment special conformance test function .

[0131] The first indication, the second indication, and the third indication may be received in parallel via a same message .

[0132] The first indication, the second indication, and the third indication may be received as a compressed message .

[0133] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0134] The means may be further configured for : in response to the third indication, multiplying the at least one received baseband signal , before demodulation, by the virtual cable calibration matrix .

[0135] The means may be further configured for : receive , from the test equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be estimated based, at least partially, on the configuration .

[0136] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the apparatus inside a test chamber .

[0137] The means may be further configured for : transmitting, to the test equipment , an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0138] A processor, memory, and / or example algorithms (which may be encoded as instructions , program, or code ) may be provided as example means for providing or causing performance of operation .

[0139] In accordance with one example embodiment , a non- transitory computer-readable medium comprising instructions stored thereon which, when executed with at least one processor, cause the at least one processor to : cause receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; cause receiving, from the test equipment , of at least one downlink reference signal ; estimate , in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; cause receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix; determine , in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; cause receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0140] In accordance with one example embodiment , a non- transitory computer-readable medium comprising program instructions stored thereon for performing at least the following : causing receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing receiving , from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on theestimated channel matrix; causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0141] In accordance with another example embodiment , a non- transitory program storage device readable by a machine may be provided, tangibly embodying instructions executable by the machine for performing operations , the operations comprising : causing receiving, from a test equipment, of a first indication to estimate a channel matrix of a downlink channel ; causing receiving, from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0142] In accordance with another example embodiment , a non- transitory computer-readable medium comprising instructions that , when executed by an apparatus , cause the apparatus to perform at least the following : causing receiving, from a test equipment , ofa first indication to estimate a channel matrix of a downlink channel ; causing receiving, from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0143] A computer implemented system comprising : at least one processor and at least one non-transitory memory storing instructions that , when executed by the at least one processor, cause the system at least to perform: causing receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing receiving, from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing receiving, from the test equipment, of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially,on the virtual cable calibration matrix ; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0144] A computer implemented system comprising : means for causing receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; means for causing receiving, from the test equipment , of at least one downlink reference signal ; means for estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; means for causing receiving, from the test equipment, of a second indication to determine a virtual cable calibration matrix; means for determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; means for causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and means for performing conformance testing based, at least partially, on the virtual cable calibration .

[0145] In accordance with one example embodiment , an apparatus may comprise : at least one processor ; and at least one memory storing instructions that , when executed by the at least one processor, cause the apparatus at least to : transmit , to a user equipment, a first indication to estimate a channel matrix of a downlink channel ; transmit , to the user equipment , at least one downlink reference signal ; transmit , to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ;transmit , to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0146] The first indication to estimate the channel matrix may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0147] The example apparatus may be further configured to : receive , from the user equipment , a confirmation that the channel matrix has been estimated .

[0148] The confirmation may be received via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .

[0149] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0150] The second indication may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0151] The example apparatus may be further configured to : receive , from the user equipment , a confirmation that the virtual cable calibration matrix has been determined .

[0152] The third indication may be transmitted via at least one of : radio resource control signaling, or a user equipment special conformance test function .

[0153] The first indication, the second indication, and the third indication may be transmitted in parallel via a same message .

[0154] The first indication, the second indication, and the third indication may be transmitted as a compressed message .

[0155] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0156] The example apparatus may be further configured to : transmit , to the user equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be configured to be estimated based, at least partially, on the configuration .

[0157] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the user equipment inside a test chamber .

[0158] The example apparatus may be further configured to : receive , from the user equipment , an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0159] In accordance with one aspect , an example method may be provided comprising : transmitting, with a test equipment to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmitting, to the user equipment , at least one downlink reference signal ; transmitting, to the user equipment, a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0160] The first indication to estimate the channel matrix may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0161] The example method may further comprise : receiving, from the user equipment , a confirmation that the channel matrix has been estimated .

[0162] The confirmation may be received via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .

[0163] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cablecalibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0164] The second indication may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0165] The example method may further comprise : receiving, from the user equipment , a confirmation that the virtual cable calibration matrix has been determined .

[0166] The third indication may be transmitted via at least one of : radio resource control signaling, or a user equipment special conformance test function .

[0167] The first indication, the second indication, and the third indication may be transmitted in parallel via a same message .

[0168] The first indication, the second indication, and the third indication may be transmitted as a compressed message .

[0169] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0170] The example method may further comprise : transmitting, to the user equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be configured to be estimated based, at least partially, on the configuration .

[0171] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the user equipment inside a test chamber .

[0172] The example method may further comprise : receive , from the user equipment , an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0173] In accordance with one example embodiment , an apparatus may comprise : circuitry configured to perform : transmitting, with a test equipment to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; circuitry configured to perform: transmitting, to the user equipment , at least one downlink reference signal ; circuitry configured to perform: transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; circuitry configured to perform: transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and circuitry configured to perform: performing conformance testing based, at least partially, on the virtual cable calibration .

[0174] In accordance with one example embodiment , an apparatus may comprise : processing circuitry; memory circuitry including computer program code, the memory circuitry and the computer program code configured to, with the processing circuitry, enable the apparatus to : transmit , to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmit , to the user equipment , at least one downlink reference signal ; transmit , to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; transmit , to the user equipment , athird indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0175] In accordance with one example embodiment , an apparatus may comprise means for : transmitting, to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmitting, to the user equipment , at least one downlink reference signal ; transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0176] The first indication to estimate the channel matrix may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0177] The means may be further configured for : receiving, from the user equipment , a confirmation that the channel matrix has been estimated .

[0178] The confirmation may be received via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .

[0179] The second indication to determine the virtual cable calibration matrix may comprise at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .

[0180] The second indication may be transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

[0181] The means may be further configured for : receiving, from the user equipment , a confirmation that the virtual cable calibration matrix has been determined .

[0182] The third indication may be transmitted via at least one of : radio resource control signaling, or a user equipment special conformance test function .

[0183] The first indication, the second indication, and the third indication may be transmitted in parallel via a same message .

[0184] The first indication, the second indication, and the third indication may be transmitted as a compressed message .

[0185] The second indication and the third indication may comprise one of : different indications , or a same indication .

[0186] The means may be further configured for : transmitting, to the user equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix may be configured to be estimated based, at least partially, on the configuration .

[0187] The channel matrix of the downlink channel may comprise a channel matrix of an over the air channel between test probes and the user equipment inside a test chamber .

[0188] The means may be further configured for : receiving, from the user equipment, an acknowledgement , wherein the acknowledgement may be configured to confirm the virtual cable calibration .

[0189] In accordance with one example embodiment , a non- transitory computer-readable medium comprising instructions stored thereon which, when executed with at least one processor, cause the at least one processor to : cause transmitting, to a user equipment , of a first indication to estimate a channel matrix of a downlink channel ; cause transmitting, to the user equipment , of at least one downlink reference signal ; cause transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; cause transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and perform conformance testing based, at least partially, on the virtual cable calibration .

[0190] In accordance with one example embodiment , a non- transitory computer-readable medium comprising program instructions stored thereon for performing at least the following : causing transmitting, to a user equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing transmitting, to the user equipment , of at least one downlinkreference signal ; causing transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix ; causing transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0191] In accordance with another example embodiment , a non- transitory program storage device readable by a machine may be provided, tangibly embodying instructions executable by the machine for performing operations , the operations comprising : causing transmitting, to a user equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing transmitting, to the user equipment , of at least one downlink reference signal ; causing transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0192] In accordance with another example embodiment , a non- transitory computer-readable medium comprising instructions that , when executed by an apparatus , cause the apparatus to perform at least the following : causing transmitting, to a user equipment , ofa first indication to estimate a channel matrix of a downlink channel ; causing transmitting, to the user equipment , of at least one downlink reference signal ; causing transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing transmitting, to the user equipment, of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0193] A computer implemented system comprising : at least one processor and at least one non-transitory memory storing instructions that , when executed by the at least one processor, cause the system at least to perform: causing transmitting, to a user equipment, of a first indication to estimate a channel matrix of a downlink channel ; causing transmitting, to the user equipment , of at least one downlink reference signal ; causing transmitting, to the user equipment , of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

[0194] A computer implemented system comprising : means for causing transmitting, to a user equipment , of a first indication to estimate a channel matrix of a downlink channel ; means forcausing transmitting, to the user equipment , of at least one downlink reference signal ; means for causing transmitting, to the user equipment, of a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; means for causing transmitting, to the user equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and means for performing conformance testing based, at least partially, on the virtual cable calibration .

[0195] The term "non-transitory, " as used herein, is a limitation of the medium itself ( i . e . tangible , not a signal ) as opposed to a limitation on data storage persistency (e . g . , RAM vs . ROM) .

[0196] It should be understood that the foregoing description is only illustrative . Various alternatives and modifications can be devised by those skilled in the art . For example , features recited in the various dependent claims could be combined with each other in any suitable combination ( s ) . In addition, features from different embodiments described above could be selectively combined into a new embodiment . Accordingly, the description is intended to embrace all such alternatives , modification and variances which fall within the scope of the appended claims .

Claims

CLAIMSWhat is claimed is:

1. An apparatus comprising: at least one processor; and at least one non-transitory memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: receive, from a test equipment, a first indication to estimate a channel matrix of a downlink channel; receive, from the test equipment, at least one downlink reference signal; estimate, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal; receive, from the test equipment, a second indication to determine a virtual cable calibration matrix; determine, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receive, from the test equipment, a third indication to perform virtual cable calibration on at least one received baseband signal, beforedemodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .2 . The apparatus of claim 1 , wherein the first indication to estimate the channel matrix is received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .3 . The apparatus of claim 1 or 2 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : transmit , to the test equipment , a confirmation that the channel matrix has been estimated .4 . The apparatus of claim 3 , wherein the confirmation is transmitted via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .5 . The apparatus of any of claims 1 through 4 , wherein the second indication to determine the virtual cable calibration matrix comprises at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .6 . The apparatus of claim 5 , wherein the virtual cable calibration matrix is determined based, at least partially, on the indicated method for determining the virtual cable calibration matrix .7 . The apparatus of any of claims 1 through 6 , wherein the second indication is received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .8 . The apparatus of any of claims 1 through 7 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : transmit , to the test equipment , a confirmation that the virtual cable calibration matrix has been determined .9 . The apparatus of any of claims 1 through 8 , wherein the third indication is received via at least one of : radio resource control signaling, ora user equipment special conformance test function .10 . The apparatus of any of claims 1 through 9 , wherein the first indication, the second indication, and the third indication are received in parallel via a same message .11 . The apparatus of any of claims 1 through 10 , wherein the first indication, the second indication, and the third indication are received as a compressed message .12 . The apparatus of any of claims 1 through 9 , wherein the second indication and the third indication comprise one of : different indications , or a same indication .13 . The apparatus of any of claims 1 through 12 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : in response to the third indication, multiply the at least one received baseband signal , before demodulation, by the virtual cable calibration matrix .14 . The apparatus of any of claims 1 through 13 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : receive , from the test equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix is estimated based, at least partially, on the configuration .15 . The apparatus of any of claims 1 through 14 , wherein the channel matrix of the downlink channel comprises a channel matrix of an over the air channel between test probes and the apparatus inside a test chamber .

16. The apparatus of any of claims 1 through 15 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : transmit , to the test equipment , an acknowledgement , wherein the acknowledgement is configured to confirm the virtual cable calibration .17 . A method comprising : receiving, with a device under test from a test equipment , a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix ;receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .18 . The method of claim 17 , wherein the first indication to estimate the channel matrix is received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .

19. The method of claim 17 or 18 , further comprising : transmitting, to the test equipment , a confirmation that the channel matrix has been estimated .20 . The method of claim 19 , wherein the confirmation is transmitted via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .21 . The method of any of claims 17 through 20 , wherein the second indication to determine the virtual cable calibration matrix comprises at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, or a fifth indication to store the virtual cable calibration matrix .22 . The method of claim 21 , wherein the virtual cable calibration matrix is determined based, at least partially, on the indicated method for determining the virtual cable calibration matrix .23 . The method of any of claims 17 through 22 , wherein the second indication is received via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .24 . The method of any of claims 17 through 23 , further comprising : transmitting, to the test equipment , a confirmation that the virtual cable calibration matrix has been determined .25 . The method of any of claims 17 through 25 , wherein the third indication is received via at least one of : radio resource control signaling, ora user equipment special conformance test function .

26. The method of any of claims 17 through 25 , wherein the first indication, the second indication, and the third indication are received in parallel via a same message .27 . The method of any of claims 17 through 26 , wherein the first indication, the second indication, and the third indication are received as a compressed message .28 . The method of any of claims 17 through 25 , wherein the second indication and the third indication comprise one of : different indications , or a same indication .29 . The method of any of claims 17 through 28 , further comprising : in response to the third indication, multiplying the at least one received baseband signal , before demodulation, by the virtual cable calibration matrix .30 . The method of any of claims 17 through 29 , further comprising : receiving, from the test equipment , a configuration for the at least one downlink reference signal , wherein the channel matrix is estimated based, at least partially, on the configuration .31 . The method of any of claims 17 through 30 , wherein the channel matrix of the downlink channel comprises a channelmatrix of an over the air channel between test probes and the device under test inside a test chamber .32 . The method of any of claims 17 through 31 , further comprising : transmitting, to the test equipment , an acknowledgement , wherein the acknowledgement is configured to confirm the virtual cable calibration .33 . An apparatus comprising means for : receiving, from a test equipment, a first indication to estimate a channel matrix of a downlink channel ; receiving, from the test equipment , at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; receiving, from the test equipment , a second indication to determine a virtual cable calibration matrix; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; receiving, from the test equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix ; andperforming conformance testing based, at least partially, on the virtual cable calibration .34 . An apparatus comprising means for performing a method according to any one of claims 17 through 32 .35 . A non-transitory computer-readable medium comprising program instructions stored thereon for performing at least the following : causing receiving, from a test equipment , of a first indication to estimate a channel matrix of a downlink channel ; causing receiving, from the test equipment , of at least one downlink reference signal ; estimating, in response to the first indication, the channel matrix based, at least partially, on the at least one downlink reference signal ; causing receiving, from the test equipment , of a second indication to determine a virtual cable calibration matrix ; determining, in response to the second indication, the virtual cable calibration matrix based, at least partially, on the estimated channel matrix; causing receiving, from the test equipment , of a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation,based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

36. An apparatus comprising : at least one processor ; and at least one non-transitory memory storing instructions that , when executed by the at least one processor, cause the apparatus at least to : transmit , to a user equipment , a first indication to estimate a channel matrix of a downlink channel ; transmit , to the user equipment , at least one downlink reference signal ; transmit , to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmit , to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and perform conformance testing based, at least partially, on the virtual cable calibration .37 . The apparatus of claim 36 , wherein the first indication to estimate the channel matrix is transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .38 . The apparatus of claim 36 or 37 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : receive , from the user equipment , a confirmation that the channel matrix has been estimated .39 . The apparatus of claim 38 , wherein the confirmation is received via at least one of : an explicit indication, user equipment assistance information, a conformance test acknowledgement message , an implicit indication, or a channel state information report .40 . The apparatus of any of claims 36 through 39 , wherein the second indication to determine the virtual cable calibration matrix comprises at least one of : a fourth indication of a method for determining the virtual cable calibration matrix, ora fifth indication to store the virtual cable calibration matrix .41 . The apparatus of any of claims 36 through 40 , wherein the second indication is transmitted via at least one of : radio resource control signaling, or non-access stratum signaling using a user equipment special conformance test function .42 . The apparatus of any of claims 36 through 41 , wherein the at least one memory stores instructions that , when executed by the at least one processor, cause the apparatus to : receive , from the user equipment , a confirmation that the virtual cable calibration matrix has been determined .43 . The apparatus of any of claims 36 through 42 , wherein the third indication is transmitted via at least one of : radio resource control signaling, or a user equipment special conformance test function .44 . The apparatus of any of claims 45 through 43 , wherein the first indication, the second indication, and the third indication are transmitted in parallel via a same message .45 . The apparatus of any of claims 45 through 44 , wherein the first indication, the second indication, and the third indication are transmitted as a compressed message .

46. The apparatus of any of claims 45 through 43, wherein the second indication and the third indication comprise one of: different indications, or a same indication.

47. The apparatus of any of claims 45 through 46, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus to: transmit, to the user equipment, a configuration for the at least one downlink reference signal, wherein the channel matrix is configured to be estimated based, at least partially, on the configuration.

48. The apparatus of any of claims 45 through 47, wherein the channel matrix of the downlink channel comprises a channel matrix of an over the air channel between test probes and the user equipment inside a test chamber.

49. The apparatus of any of claims 45 through 48, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus to: receive, from the user equipment, an acknowledgement, wherein the acknowledgement is configured to confirm the virtual cable calibration.

50. A method comprising: transmitting, with a test equipment to a user equipment, a first indication to estimate a channel matrix of a downlink channel;transmitting, to the user equipment , at least one downlink reference signal ; transmitting, to the user equipment , a second indication to calculate a virtual cable calibration matrix based, at least partially, on the estimated channel matrix; transmitting, to the user equipment , a third indication to perform virtual cable calibration on at least one received baseband signal , before demodulation, based, at least partially, on the virtual cable calibration matrix; and performing conformance testing based, at least partially, on the virtual cable calibration .

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