Test Setups

Description

Technical field The present invention relates to concepts for performance testing in wireless communica- tion and wireless devices, in particular navigation devices and systems based on electro- magnetic waves, and to over-the-air tests. The present invention relates also to an appa- ratus and a method for the calibration of Over-the-Air (OTA) communication test setups for mmWave Frequency Bands.

Background of the invention

During the design, development and testing and performance evaluation, various aspects of a Device Under Test (DUT) have to be investigated. This is done by subjecting the DUT, in a laboratory, to various stimuli, signals and environmental conditions and inspect- ing the DUT’s reaction, proper functioning and performance under those conditions. Es- pecially relevant for this invention is the electromagnetic environment, such as the elec- tromagnetic waves transmitted by communication device/partner/counterpart as modified through the propagation channel, as well as interference, third party signals, etc.

Even if the aspects are referring primarily on the so called“downlink case”, where the DUT is a receiver, an extension of the presented concepts to the uplink case is appropri- ate, wherein e.g. the DUT is transmitting, it is straightforward for a person skilled in the art. Known measurement methods of this kind are tests as shown e.g. in US 9698920 B2 within an electromagneticaily- shielded measurement chamber, into which a Device Under Test has to be arranged (as disclosed e.g. in US9698920B2). In addition to the DUT, a set of illumination/probing antennas is placed within the same chamber). One or more master test signals are generated by one (or multiple) signal generators, which may be an LTE base station (eNB), an NR base station (gNB), a device generating GPS-, Beidou-, GLONASS-, Galileo- or some other form of GNSS-positioning signals or a combination of these examples. The master test signals are processed by a processing unit, which amongst other things applies propagation channel effects— including one or more of fad- ing, time dispersion and Doppler shift— to the master test signals and thus forms a set of test signals for each antenna. The individual test signals— or per-antenna test signals— are fed to the plurality of illumination antennas positioned inside the chamber and ar- ranged in such a manner that their radiation impinges uppon the DUT. The superposition of the radiated waves occurs at the antenna of the DUT whereupon a signal with the properties required to attain the test goal (i.e. modulated message content, direction of arrival, multipath effects, Doppler shift, interference etc.) is generated. The responses and/or performances of the DUT (e.g. bit error rates, signal robustness, noise immunity) are then observed.

The signals or the processing parameters for the processing unit may for example be found by channel measurements, as disclosed e.g. in US 9698920B2, or may be calculat- ed applying fading processes for each antenna, as described e.g. in US 201 10200084, or may be formed with other particular objectives, as described e.g. in US 9614627B2.

The mentioned methods have in common that the test signals of the individual OTA (Over-the-Air) illumination/probe antennas need to be radiated in a phase-coherent man- ner in order to reliably control the superimposed waves at the receiving antennas of the DUT. This is typically realized by sharing reference oscillators, or using multiple oscillators of high quality, with regards to frequency stability and phase noise, within the processing unit. The effects of downstream components like cables and antennas should be compen- sated by pre-calibration procedure before the actual test. When increasing the carrier fre- quency, negative effects like phase noise\drift and subsequently absolute phase coher- ence can become a serious issue for accurate and phase coherent OTA measurements.

Calibration of the test setup can also be detrimental being a time-consuming process and can sometimes provide inadequate compensation, especially during long-term measure- ments in which cables, antennas and RF amplifiers are not sufficiently stable with respect to phase and/or amplitude. These negative effects can occur due to thermal effects, for example where small changes in temperature may result in a severe measurement uncer- tainty, especially a phase uncertainty. Such corruptive effects can become more pro- nounced when the operating frequency is increased and hence the wavelength is de- creased as is the case with 5G systems operating in the centimeter and millimeter-wave bands.

In view of the conventional solutions, there is a desire for a concept for OTA (Over-the-Air) measurements to overcome the above mentioned drawbacks, which provides for an im- proved tradeoff between cost and accuracy. Summary of the invention

An embodiment may refer to an apparatus for over the air (OTA) testing a wireless device, wherein the apparatus comprises at least one signal generator/tester and a signal pro- cessing unit. The signal generator/tester may be configured to generate and receive test signals for/from the signal processing unit. The signal processing unit may be connected to a plurality of illumination antennas for the transmission and/or reception of the test sig- nals, being configured to process the test signals. The apparatus for OTA testing may be configured to determine a calibration information on the basis of a single or a plurality of calibration signals which is/are wirelessly transmitted and/or received by the apparatus during a transmission or reception of test signals to or from the wireless device under test, and to adjust the signal generation and/or the signal processing on the basis of the cali- bration information derived from the single or the plurality of calibration signal(s). The ap- paratus for OTA testing may be configured to adjust the signal generation and/or the sig- nal processing in dependence on a response message, received by the apparatus from the wireless device or from an auxiliary receiver via a return path, using the calibration information, wherein the response message is responsive to the transmission of a test signal or a calibration signal by the apparatus and/or to extract the calibration information for the adjustment of the signal generation and/or signal processing from a transmit signal received by the apparatus from the wireless device or from an auxiliary transmitter. Ad- vantageously the signal transmitted for determining calibration information may be trans- mitted over the air (OTA) in either direction to and from the apparatus for testing a certain wireless device, enabling an accurate test result.

According to a second embodiment the apparatus may be configured to detect a degrada- tion of the transmitted test signal(s), and/or to detect a degradation of the received test signal(s), and/or to detect a degradation of the transmitted calibration signal(s), and/or to detect a degradation of the received calibration signal(s) and

to adjust the signal generation or the signal processing unit in order to compensate for the degradation. Advantageously the signal generation or signal processing can therefore be executed in an optimized mode, being able to reduce degradation factors of the signal path.

According to a third embodiment, the apparatus may be configured for a continuous cali- bration or for calibration at defined time instances of a measurement system during a test run. Advantageously the apparatus may therefore provide a continuous calibration or a calibration at defined time instances throughout the test and measurement process. This may ensure that the calibration of the measurement system\processing unit is always ac- curate and current, even during prolonged test runs.

According to another embodiment, the processing unit may be configured to divide the test signals generated by the signal generator/tester into a plurality of subbands In the frequency domain, to obtain subband signals, to individually process the subband signals to Include channel propagation effects, to leave at least one subband unprocessed by means of not including channel propagation effects, or any other kinds of modifications of the signal and to transmit the subband signals over the illumination antennas to the wire- less device under test. In this case the one or more signal generators are configured to configure the wireless device with channel state information (CSI) reference signal setting and reporting configuration for the unprocessed or processed subbands, so that the wire- less device under test reports a channel state information to the apparatus. The apparatus may be configured to selectively determine the calibration information on the unprocessed subbands on the basis of channel state information reported from the device under test.

According to another embodiment, the apparatus may be configured to determine the cal- ibration information on the unprocessed subbands on the basis of the channel state infor- mation obtained by the device under test using reference signals adapted for data de- modulation and/or channel estimation by the wireless device under test which are includ- ed in one or more subbands that are left unprocessed by the processing unit.

According to another embodiment, the apparatus may be configured to temporally vary which of the one or more subbands are left unprocessed by the signal processing unit and to determine the calibration information associated with the respective subbands on the basis of channel state information on the subbands reported from the device under test.

According to another embodiment, the apparatus may be configured to instruct the wire- less device under test to selectively report channel state information to the apparatus for the one or more subbands which are left unprocessed by the processing unit.

According to another embodiment, the apparatus may be configured to receive a channel state information report which contains amplitude and phase information or relative ampli- tude and phase information from the wireless device under test and to determine the call- bration information on the basis of the channel state information reported by the wireless device under test.

According to another embodiment, one or more signal generators or the processing unit may be configured to analyze the results of the measurements provided by the wireless device under test and to calculate calibration information to adjust the processing unit and/or signal parameters to compensate for signal degradation.

According to another embodiment, the processing unit may be configured to be fed with a plurality of signals received by a plurality of illumination antennas, The processing unit may be configured to divide the signals into a plurality of subbands, to obtain subband signals, and to individually process the subband signals to include channel propagation effects leaving at least one subband unprocessed, The apparatus may be configured to selectively determine the calibration information on the basis of a reception of the calibra- tion signa!(s) of the unprocessed subband(s) and to apply to the processing unit calibra- tion information derived from the unprocessed subbands.

According to another embodiment, the apparatus may be configured to sequentially or in parallel determine the calibration information on the basis of a reception of the calibration signal(s) by different illumination antennas.

According to another embodiment, the signal generator/tester may be configured to con- figure the wireless device with a reference signal setting containing the configuration, e.g. time/frequency location, signal type, of the reference/calibration signals, wherein the ref- erence/calibration signals are used for the determination of the calibration information by the apparatus. The reference signals may be e.g. sounding reference signals, SRS, etc. containing the configuration e.g. time/frequency location, signal type,

According to another embodiment, the apparatus may be configured to vary over time which one or more subbands are left unprocessed by the processing unit and to deter- mine the calibration information associated with one or more respective subbands, which are left unprocessed by the processing unit, on the basis of a reception of calibration sig- nals associated with the respective subbands,

According to another embodiment, the apparatus may be configured to perform meas- urements in terms of amplitude and phase or relative amplitude and phase on signals re- ceived by the apparatus on the one or more non-processed subbands and to determine calibration information on the basis of the amplitude information and the phase information or relative amplitude and phase information derived from the signals received by the ap- paratus in the non-processed subbands.

According to another embodiment, the apparatus may be configured to use the calibration information to perform a compensation for signal degradation by adjusting parameters of the processing unit and/or signal parameters of the received signals.

According to another embodiment, the processing unit may be configured to add at least one calibration signal to the test signal, such that the calibration signal(s) i s/a re left unpro- cessed by means of not including channel propagation effects, or any other kinds of modi- fications, or wherein the apparatus is configured to add a calibration signal to the test sig- nal provided by the processing unit.

According to another embodiment, the apparatus comprises at least one additional re- ceiver with a pick up antenna to receive the calibration signal(s). Such an additional re- ceiver may be advantageously included in the apparatus, for example, coupled to an out- put of the processing unit.

According to another embodiment, the apparatus may be configured to provide one or more calibration signals to include one or a combination of a continuous carrier at a cer- tain frequency, one or more modulated signals, one or more signals with a time pattern, one or more multi-tone signals, wherein the apparatus is configured to provide different, distinguishable calibration signals for different illumination antennas.

According to another embodiment, the apparatus may be configured to provide a plurality of calibration signals being mutually orthogonal or quasi-orthogonal in time and/or fre- quency and/or any other dimension. Advantageously such an apparatus may be config- ured to use the information about signals which exceed communication signals processed by the wireless device under test in a normal operation mode for performing a compensa- tion for signal degradation, realizing a calibration by adjusting parameters of the pro- cessing unit e.g. the measurement system and/or signal parameters of received signals.

According to another embodiment, the apparatus may be configured to provide the one or more calibration signals such that they lie outside of a frequency range used by the com- munication signals. Advantageously such communication signals may be test signals used for communication between the apparatus for over the air testing and the wireless device.

According to another embodiment, the apparatus may be configured to provide the cali- bration signals in such a way that interference to the communication signal is avoided.

According to another embodiment, the apparatus may be configured to provide the cali- bration signals such that they use CDMA or PRN or similar spreading codes being adapted for being separable from the communication signals. Advantageously, the appa- ratus may be configured to provide the calibrating signals being able to avoid altering ef- fects on test signals.

According to another embodiment, the apparatus may be configured to provide the cali- bration signals for avoiding interference onto the communication signal.

According to another embodiment, the apparatus may be configured to provide the com- munication signals such that they represent an information signal of a cell-based commu- nication system and to provide the one or more calibration signals to be given by refer- ence signals from neighboring cells of the cell-based communication system. Advanta- geously, the one or more calibrating signals may lie in a frequency band associated with a neighboring cell of the cell-based communication system and/or such that the one or more calibrating signals comprise a signal strength which is smaller than or equal to a signal strength as specified for a neighboring cell of the cell based communication system, and/or such that the one or more calibrating signals comprise a modulation type as speci- fied for a neighboring ceil of the cell based communication system, e.g. LTE.

According to another embodiment, the apparatus may be configured to adapt the calibra- tion signals to be positioned close to, just outside spectrum occupied by the communica- tion signals sent to/received from the wireless device DUT.

According to another embodiment, the apparatus may be configured to provide the cali- bration signals to be positioned symmetrically on both sides outside the spectrum occu- pied by the communication signals sent to the wireless device DUT. According to another embodiment, the apparatus may be alternatively configured to pro- vide the calibration signals to be positioned asymmetrically on both sides outside the spectrum occupied by the communication signals sent to the wireless device DUT.

According to another embodiment, the apparatus may be configured in a way, such that when the communication signals are not transmitted for a specific period of time, or when parts of the spectrum of the communication signals are not inspected or processed by the wireless device for a specific period of time, calibration signals are inserted in said specific period of time. Advantageously in such a period of time for example, a subband may re- main unprocessed, being used in this time frame as a calibration / reference signal.

According to another embodiment, the apparatus may comprise a dedicated calibration observation antenna/pick up antenna being configured for receiving and/or transmitting calibration signals used for determining the calibration information. Advantageously the apparatus may therefore provide for dedicated conditions, which can be suitable elected, while a test run may be conducted e.g. inside a controllable chamber.

According to another embodiment, the apparatus may be configured for using an antenna of the wireless device being configured for receiving and/or transmitting calibration signals through the use of one or more RF splitters, the calibration signals being used for deter- mining the calibration information. Advantageously the calibration signals may therefore be used for determining the calibration information of a single or a plurality of signal paths connected to the signal processing unit.

According to another embodiment, the apparatus may be configured for using an auxiliary receiver being configured for receiving and for analyzing the received calibration signals, to derive calibration information for a single or plurality of signal paths of the apparatus.

According to another embodiment, the apparatus may be configured for using an auxiliary receiver being configured to feedback analyzed data to the apparatus, wherein the appa- ratus is configured to adjust parameters of the measurement system and/or signal param- eters of a single or plurality of transmitted communication signals in dependence on the calibration information.

According to another embodiment, the apparatus may be configured for using an auxiliary receiver being configured to feedback analyzed data to the apparatus, wherein the appa- ratus is configured to at least partially compensate the degradation of transmitted signals in dependence on the calibration information.

According to another embodiment, the apparatus may be configured to receive infor- mation about one or more calibration signals from the wireless device under test (DUT).

According to another embodiment, the apparatus may be configured to receive infor- mation about calibration signal(s), which i s/a re separate from the communication signals, from the wireless device under test.

According to another embodiment, the apparatus may be configured to set the wireless device under test into a mode, in which the wireless device under test provides infor- mation about signals apart from communication signals processed by the wireless device under test in a normal operation mode, and wherein the apparatus is configured to use the information about these signals which exceed communication signals processed by the wireless device under test in a normal operation mode for performing a compensation for a signal degradation by adjusting parameters of the processing unit and/or signal parame- ters of received signals.

According to another embodiment, in the apparatus a plurality of signal paths may be connected to a plurality of illumination antennas for transmitting and/or receiving , wherein in each of the transmitting paths an inband or outband Individual calibration signal is transmitted and/or wherein in the plurality of receiving paths the plurality of calibration signals from the wireless device under test are received and analyzed.

According to another embodiment, the apparatus may be configured to provide signals to a plurality of illumination antennas which are configured to create a spatial directivity or beam pattern or wavefiled by application of signals having specific phases and amplitudes to each or some of the illumination antennas separately or jointly. Advantageously each illumination antenna may radiate a unique reference signal at the same time, e.g. per transmission channel/antenna/polarization.

According to another embodiment, the apparatus may be configured to receive with an auxiliary receiver connected to a pick up antenna around the DUT the plurality of calibra- tion signals over the air instead of using the wireless device under test to measure the plurality of transmitted calibration signals from the apparatus and/or applying an auxiliary transmitter connected to an pick up antenna to transmit calibration signals over the air to be received at the apparatus over the plurality of signal paths.

According to another embodiment, the wireless device may be configured to receive a communication signal comprising a plurality of subbands, wherein the wireless device is configured to selectively provide an explicit channel state information on the basis of one or more subbands of a communication signal and/or

when the wireless device is configured to provide a complete channel estimation infor- mation to an apparatus for OTA testing.

According to another embodiment, the wireless device may be configured to receive a configuration information defining for which one or more subbands of the communication signal to provide the explicit channel state information or complete channel estimation information, wherein the wireless device may be configured to obtain the explicit channel state information or the complete channel estimation on the basis of pilot symbols and/or reference symbols included in the one or more subbands of the communication signal to be used for the provision of the explicit channel state information or for the provision of the complete channel estimation.

According to another embodiment, the wireless device may be configured to obtain the explicit channel state information while receiving data using one or more other subbands of the communication signal which are not used for obtaining explicit channel state infor- mation.

According to another embodiment, the wireless device may be configured to receive scheduling information describing on the basis of which one or more subbands the explicit channel state information should be provided at different times, such that the wireless device provides the explicit channel state information on the basis of different subbands at different times. Advantageously scheduling information may be e.g, synchronization in- formation describing on the basis of which one or more subbands the explicit channel state information should be provided at different times

According to another embodiment, the wireless device may be configured to perform, at a request, measurements of an amplitude or a relative amplitude and of a phase or a rela- tive phase, of the calibration signal, and to feedback measurement results as the explicit channel state information. According to another embodiment, the wireless device may be configured to be switcha- ble between a normal operation mode, in which the wireless device evaluates communi- cation signals in accordance with a predefined communication protocol, to receive com- munication data, and a test mode in which the wireless device provides information about signals which exceed communication signals processed by the wireless device in the normal operation mode.

According to another embodiment, the wireless device may be configured to evaluate out- of-band signals or signals which are orthogonal to signal contents processed in the normal mode of operation.

According to another embodiment, the wireless device may be configured to use infor- mation about signals which exceed communication signals processed by the wireless de- vice in the normal operation mode for enabling a determination of calibration information in an apparatus communicating with the wireless device.

An embodiment may refer to method, which may comprise determining a calibration in- formation on the basis of a signal which is Wirelessly transmitted by an apparatus during a transmission of communication signals to a wireless device under test or which is wire- lessly transmitted by the wireless device during a transmission of communication signals from the wireless device to the apparatus, wherein the method may comprise adjusting a signal generation or a signal processing in the apparatus on the basis of the calibration information.

An embodiment may refer to a computer program for carrying out the above mentioned method, wherein the computer program is provided for being executed on a computer.

Brief description of the drawings

The drawings are not necessarily to scale; instead, emphasis is generally being placed on illustrating the principles of the invention. In the following description, various embodi- ments of the invention are described with reference to the following drawings, in which:

FIG. 1 shows schematically an embodiment according to claim 1 ; FIG. 2 illustrates schematically a known measurement setup for an over-the-air testing a wireless device;

FIG. 3 Illustrates schematically a measurement setup for an over-the-air testing a wireless device, according to a specific embodiment of the present invention; and

FIG. 4 illustrates schematically an example of a spectral arrangement of a test signal and additional calibration signals, according to a specific embodiment of the pre- sent invention.

Detailed Description of the Embodiments

The subject matter is now described with reference to the drawings, wherein like refer- ence numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject matter. It is understood that subject matter embod- iments can be practiced without these specific details.

In particular, the description starts with presenting a specific example,

sets out advantages and then provides further embodiments rendering clear that the ad- vantages may also be achieved by slightly deviating from the specific example outlined before.

FIG. 1 illustrates schematically an embodiment of an apparatus 100 for testing a wireless device 105, wherein the apparatus 100 is configured to determine a calibration information on the basis of a signal 110 which is wirelessly transmitted by the apparatus during a transmission of test information to the wireless device 105 under test (DUT) or which is wirelessly transmitted by the wireless device 105 during a transmission of test information 120 from the wireless device 105 to the apparatus 100, wherein the apparatus is config- ured to adjust a signal generation or a signal processing on the basis of the calibration information.

FIG. 2 illustrates schematically a known measurement setup for an over-the-air testing a wireless device105 under test (DUT). Such testing environments may be conducted by providing an electromagnetically- shielded measurement chamber 21 1 , which may be an anechoic or a non-anechoic chamber.

A wireless device 105, which may be the Device Under Test (DUT), may be placed in the chamber 21 1 as shown in Fig. 2. In addition to the DUT, a set of illumination/probing an- tennas may be placed also within the same chamber 21 1 .

One or more master test signals may be generated by one or multiple signal generators 201 , which may be e.g, an LTE base station, an NR base station, a device generating GPS, Beidou, GLONASS, Galileo or some other form of GNSS positioning signals or a combination of these examples. The master test signals are processed by a signal pro- cessing unit 203, which amongst other things applies propagation channel effects, includ- ing e.g, one or more of fading, time dispersion and Doppler shift, to the master test signals and thus forming a set of test signals for each illumination antenna 205, The individual test signals may be fed to the plurality of illumination antennas 205 positioned inside the chamber 21 1 and arranged in such a manner that their radiation is directed towards the wireless device 105 (DUT).

The superposition of the radiated waves occurs at an antenna of the wireless device 105 (DUT), whereupon a signal with the properties required to attain the test goal e.g, modu- lated message content, direction of arrival, multipath effects, Doppler shift, interference etc. may be generated. The responses and/or performances of the wireless device 105 e.g. bit error rates, signal robustness, noise immunity may be observed thereupon. The signals or the processing parameters for the signal processing unit 203 may for example be found by channel measurements, or may be calculated applying fading processes for each antenna 205, or may be formed with other particular objectives.

The mentioned methods have in common that the test signals of the individual illumination antennas should be radiated in a phase-coherent manner in order to reliably control the superimposed waves at the receive antennas of the wireless device 105. This may be typically realized by sharing reference oscillators, or using multiple oscillators of high qual- ity within the signal processing unit 201. The effects of downstream components like ca- bles and antennas may be compensated by pre-calibration procedure before the actual test.

When increasing the carrier frequency, effects like phase noise\drift and subsequently absolute phase coherence may become a serious Issue for accurate and phase coherent OTA measurements. Calibration of the test setup may be a time-consuming process and can sometimes provide inadequate compensation, especially during long-term measure- merits in which cables, antennas and RF amplifiers are not sufficiently stable with respect to phase and/or amplitude.

FIG. 3 Illustrates schematically an apparatus 100 according to a specific embodiment of the present invention, provided for an over-the-air testing a wireless device 105, according to a specific embodiment of the present invention. The shown OTA Test setup may com- prise a signal generator/tester 201 connected with a signal processing unit 203.

The apparatus 100 may be configured to provide additional calibrating signals and an ad- ditional reference receiver 309. Only three OTA illumination antennas 205 are shown in an exemplary manor, being connected to the signal processing unit 203.

The apparatus 100 may be set up to execute a method of continuous calibration through- out the test and measurement process of an wireless device 105. This procedure is aimed to ensure that the calibration of a signal processing unit 203 is always accurate and cur- rent, even during prolonged test runs. In addition the apparatus is set up for allowing less expensive components to be used since small errors in phase and/or amplitude may be compensated automatically without causing detrimental effects to the test results.

According to the embodiment depicted in Fig. 1 , in addition to the per-antenna test signal per-antenna means a single antenna/radiation element 205, or an array antenna formed by several radiating elements 205 are fed with a common test signal to be radiated by some or all of them. By application of specific phases and attenuation to each or some of these radiating elements separately or jointly, a spatial directivity or beam pattern may be created, each illumination antenna 205 radiates a unique reference signal at the same time, e.g. per transmission channel/antenna/polarization,

A calibrating (reference) signal may be added either by the signal processing unit 203, or by an additional reference signal generator. Any modifications to the test signals from the signal generator/tester (emulator) 201 , e.g. convolution with a propagation channel, are not applied to the reference signals. The reference signals may include a simple continu- ous carrier of a certain frequency, amplitude and phase, more complex modulated signals, signals with a time pattern, multi-tone signals or any combination of these examples.

The unique and individual calibrating (reference) signals sent by the OTA illumination (probe) antennas 205 are preferably mutually orthogonal, or quasi-orthogonal. This or- thogonality may be achieved - depending on the radio signals used for the test - using separate e.g. out-of-band frequencies, suitable transmission times, or CDMA spreading codes. In addition, the calibrating (reference) signals may be chosen in such a way that they have no effect on the test signal(s) used to test the wireless device (DUT) 105 nor do they affect the performance of the DUT in any way, Methods similar to those described above, e.g. frequency-, time- or code diversity, may be used to achieve this as shown exemplary for frequency division multiplexing In Fig, 4.

Depending on the type of communication system under test, it may be advantageous to use specific reference signals as calibrating signals associated with that system. For ex- ample in LTE, the signals could be similar to the reference signals from neighboring cells,

An example for GNSS signals could be satellite identifiers or spreading codes (PRNs) that are not in in general use.

As depicted in Fig. 4 above the calibrating (reference) signals may be positioned left and right from the spectrum occupied by the test signal sent to the wireless device (DUT) 105. The calibrating signals may be positioned symmetrically or asymmetrically around the test signal(s). Moreover, the number of calibrating (reference) signals may be identical or not at either side of the spectrum of the test signal(s).

In particular, a specific distribution of the calibrating (reference) signals may be chosen such that the so-called image carriers are not colliding with the image signal in direct up- and down-conversion, allowing the reference antenna to detect unwanted mixing signals in the reference signal domain space. Equivalently, a real valued reference signal may be transmitted by loading e.g. OFDM subcarriers in an appropriate fashion like in DMT modu- lation.

Alternative options for the calibrating (reference) signal design may include a distribution of the calibration (reference) signal in time and/or frequency domain having a minimum Impact on the test signal for the test of the wireless device (DUT) 105. Such specific time variants, shown in Fig. 4, may exploit knowledge about the test signal such that if the test signal is silenced for specific period of time, or parts of the spectrum of the test signal (e.g, unused subbands, pilots) are not inspected / processed by the DUT receiver suitable ref- erence signals could be inserted, as shown exemplarily in Fig. 4. In a period of time t ₀ to t ₁ for example, subband 1 remains unprocessed, being used in this time frame as a calibra- tion (reference) signal.

The additional calibration (reference) signals used only for the calibration of the signal processing unit 203 (measurement system) are received using a reference observation antenna 305 inside the chamber 21 1 . On the transmit side the reference signals are mul- tiplexed with the test signals and therefore undergo the same signal degradation as the test signals. The altered reference signals may be received and analyzed by a reference observation receiver 309. The calibration (reference) signals are analyzed for degrada- tions (e.g. phase, amplitude, delay) for each chain. This information is fed back to the sig- nal processing unit 203 which then may compensate for the degradation by adjusting pa- rameters of the measurement system and/or signal parameters of the transmit signals. For example, if the received amplitude of the received reference signal for a particular channel is too high, e.g. due to temperature drift of an amplifier, the signal processing unit 203 may decrease the amplitude in digital or analog domain of the per-antenna-signal for that particular OTA illumination antenna 205.

In another embodiment the use of a separate reference observation antenna may be omit- ted, and instead uses an existing antenna, e.g. those of the wireless device (OUT) 105, through the use of RF splitters.

In a further embodiment of the invention the separate reference receiver 309 from Fig. 3 may also be omitted. The additional reference signals may therefore be received by the DUT. The wireless device (DUT) 105 may either use a special test mode, or pure sample- pass through, or by evaluating the signals as part of the DUT test signal. In this case, the wireless device (DUT) 105 uses a communication interface to transmit the appropriate feedback to the processing unit. This communication interface may be wired or wireless, and may be in-band or out-of-band with respect to the communication system under test. A further embodiment of the invention may use the reference signals inherently contained in the communication signals generated by the signal emulator as calibration signals for the calibration of the processing unit 203 (measurement system). For example, when an LTE or 5G base station is used as signal generator/tester 201 (signal emulator), in com- munication signals there a number of reference signals (e.g., CRS, CSI-RS, PT-RS, etc.) embedded which may be used for different purposes. Some of the reference signals such as CSI-RS are distributed over the whole system bandwidth and each reference signal may be associated with a single RF antenna port of the signal generator/tester 201 (signal emulator).

In a further embodiment one may assume that there is a plurality of N RF antenna ports at the signal emulator and each RF antenna port is associated with a single reference signal, such as channel state information (CSI-RS). Moreover, each RF-antenna port of the sig- nal generator/tester may be associated with a single OTA illumination (probe) antenna 205, wherein the OTA illumination (probe) antennas may be marked individually.

The N communication test signals generated by the signal emulator may be fed to the processing unit 203, where the signals are divided into several bandwidth parts (BWPs) and individually further processed to include channel propagation effects like fading, mul- tipath and spatial characteristics, etc.. At least one bandwidth part BWP, used for the cali- bration reference signals, is not processed. The N signals of all bandwidth parts may be finally fed to the OTA illumination antennas 205. The transmitted communication signals containing the processed as well as the non-processed calibration (reference) signals may be received by the wireless device 105 (DUT). The wireless device 105 (DUT) may be configured by the signal generator/tester 201 with a CSI-RS resource setting to provide a channel state information CSI-report in terms of amplitude and phase, e.g. an explicit CSI, in a specified format based on the configured a channel state information reference signals (CSI-RS) and configured un-processed band- width parts BWPs. The wireless device 105 (DUT) may perform measurements of ampli- tude and phase of the reference signals of the configured un-processed BWPs and feed- ing back this information to the signal generator/tester 201 (emulator) or to the processing unit 203.

The signal generator/tester (emulator) 201 or the processing unit 203 itself may analyze the feedback information, i.e. amplitude and phase measurements of the CSI-RS in the non-processed BWP, provided by the wireless device 105 and may calculate calibration information used by the processing unit 203 to adjust the processing unit 203 (measure- ment system) and/or signal parameters to compensate for the degradation of the trans- mited signals.

According to another embodiment of the invention, the reference signals inherently con- tained in the uplink communication signals generated by the wireless device 105 (DUT) may be used for the calibration of the measurement system. In uplink communications signals, there are number of embedded reference signals such as a Sounding Reference Signal (SRS) and a Demodulation Reference Signal (DMRS). The SRS signals may be used for uplink channel estimation and are distributed over the whole system bandwidth or at least a system bandwidth part. Sounding Reference Signals and/or Demodulation Ref- erence Signals may be used for channel estimation and for coherent demodulation in up- link transmission. The signal generator/tester (emulator) 201 may configure the wireless device 105 (DUT) with a SRS resource configuration indicating the SRS ports, system bandwidth, etc.

The uplink communication test signals generated by the wireless device 105 may be re- ceived by the OTA illumination antennas 205 and fed to the OTA processing unit 203 and signal generator/tester (emulator) 201 . Within the processing unit 203, the received sig- nals may be divided into several bandwidth parts (BWPs) and individually further pro- cessed to include channel propagation effects like fading, multipath and spatial character- istics, etc., as discussed above. At least one bandwidth part BWP remains not processed.

The signal generator/tester (emulator) 201 may then perform measurements in terms of amplitude and phase on the received SRS signals on the non-processed BWP(s), and may send a measurement report including phase and amplitude information derived from the received SRS signals for the non-processed bandwidth part BWP to the processing unit 203 measurement system. The OTA measurement system may use the information from the measurement report to compensate for the signal degradation by adjusting pa- rameters of the processing unit 203 measurement system and/or signal parameters of the received signals.

Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, one or more of the most important method steps may be executed by such an apparatus.

Some embodiments according to the invention comprise a data carrier having electroni- cally readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the present invention can be implemented as a computer pro- gram product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier.

Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or nontransitionary.

A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods de- scribed herein. The data stream or the sequence of signals may for example be config- ured to be transferred via a data communication connection, for example via the Internet, A further embodiment comprises a processing means, for example a computer, or a pro- grammable logic device, configured to or adapted to perform one of the methods de- scribed herein.

A further embodiment comprises a computer having installed thereon the computer pro- gram for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatus or a system con- figured to transfer (for example, electronically or optically) a computer program for per- forming one of the methods described herein to a receiver. The receiver may, for exam- ple, be a computer, a mobile device, a memory device or the like. The apparatus or sys- tem may, for example, comprise a file server for transferring the computer program to the receiver.

In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods de- scribed herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

The apparatus described herein may be implemented using a hardware apparatus, or using a computer, or using a combination of a hardware apparatus and a computer.

The apparatus described herein, or any components of the apparatus described herein, may be implemented at least partially in hardware and/or in software.

The methods described herein may be performed using a hardware apparatus, or using a computer, or using a combination of a hardware apparatus and a computer.

The methods described herein, or any components of the apparatus described herein, may be performed at least partially by hardware and/or by software.

The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, there- fore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.