Phase and delay shift alignment of wireless communication channel

Abstract

The present disclosure relates to a method of a receiving device (200) of estimating characteristics of a wireless communication channel established between the receiving device (200) and a transmitting device (100), and a receiving device (200) performing the method. The method comprises estimating (S101) a first channel matrix based on at least one reference signal received at at least two subcarrier frequencies over the wireless communication channel at a first time instance, estimating (S102) a second channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a second time instance, and evaluating (S103) the estimated first and second channel matrix for deriving phase and delay shift occurring at any of the receiving device (200) or the transmitting device (100) of the reference signal received at the second time instance with respect to the reference signal received at the first time instance, thereby enabling compensation for the derived phase and delay shift.

Description

PHASE AND DELAY SHIFT ALIGNMENT OF WIRELESS COMMUNICATION CHANNELTECHNICAL FIELD

[0001] The present disclosure relates to a method of a receiving device of estimating characteristics of a wireless communication channel established between the receiving device and a transmitting device, and a receiving device performing the method.BACKGROUND

[0002] Today’s wireless communication systems require some form of channel state information (CSI) to be able to efficiently transmit and receive data over wireless channels. The CSI typically contains information representing characteristics of wireless communication channels in a system, such as e.g. average received power, amplitude and phase as a function of time, frequency, antenna port, interference level, etc. At a transmitter side, the CSI is used to determine modulation order, coding scheme, beamforming weights, scheduling of transmissions, etc., while at a receiver side, the CSI is used to decode data from transmitted modulated radio signal.

[0003] A common method for acquiring CSI in time division duplex (TDD) systems is by transmission of known information symbols, so-called reference symbols, which the receiver can identify and utilize to assess how, and to which extent, the wireless channel affected the transmission. One type of reference signals is the commonly used sounding reference signals (SRS) used in the uplink in fourth generation (4G) Long Term Evolution (LTE) systems and fifth generation (5G) ‘new Radio (NR) systems. SRS transmissions are typically performed at regular intervals in time and frequency by the transmitter (e.g. a mobile phone; generally referred to as User Equipment (UE)) and allows the receiver (e.g. a radio base station) to make an assessment as to how the characteristics of the wireless communication channel varies in time and frequency. In the common scenario where the base station is equipped with multiple receive antennas for enabling receive diversity, this further allows characterization of how the channel varies over the antennas.

[0004] A signal transmitted over a wireless channel is a superposition of contributions from a multitude of radio waves carried over a plurality of propagation paths, which generally results in random-like variations in channel characteristicscommonly referred to as “fast fading”, even if there is some structure in the channel variations in that neighbouring time instants, frequency bins, or antennas experience correlation in the fading variations.

[0005] Upon applying beamforming, the radio base station uses channel estimates in the uplink at a certain time to make assessments on characteristics of the wireless channel at a subsequent time instant in order to adjust beamforming weights accordingly. In for example channel sounders, it is common to aggregate channel estimates over time to improve the quality of the channel estimations or even to predict how the channel will evolve in the near future.

[0006] The accuracy of channel estimate aggregation and prediction relies on the transmitter and receiver being synchronized and having a common and stable phase reference. If not, the phase of the wireless channel estimates will fluctuate randomly between consecutive samples which is detrimental to the performance of state-of-the-art channel estimation methods. Unfortunately, wireless communication system standards only guarantee that the UE transmissions are phase coherent within a slot. Any attempts over the years to provide for phase coherence over longer periods have generally not been supported by UE manufacturers since phase coherence is difficult to maintain when the UE changes its transmission power level, modulation format, or transmission beam. It is assumed that costly hardware in the form of calibration loops would be required to resolve this issue.

[0007] Movements of UEs and / or changes in the environment causes fluctuation in delay of signals transferred between a UE and a radio base station, and alignment of the delay between a UE and a radio base station is difficult to attain. Any delay alignment mismatch will result in a phase-rotation over subcarriers, which prevents wideband channel information aggregation.SUMMARY

[0008] One objective is to solve, or at least mitigate, the problems in the art and thus to provide an improved method of a receiving device of estimating characteristics of a wireless communication channel established between the receiving device and a transmitting device.

[0009] This objective is attained in a first aspect by a method of a receiving device of estimating characteristics of a wireless communication channel establishedbetween the receiving device and a transmitting device. The method comprises estimating a first channel matrix based on at least one reference signal received at at least two subcarrier frequencies over the wireless communication channel at a first time instance, estimating a second channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a second time instance, and evaluating the estimated first and second channel matrix for deriving phase and delay shift occurring at any of the receiving device or the transmitting device of the reference signal received at the second time instance with respect to the reference signal received at the first time instance, thereby enabling compensation for the derived phase and delay shift.

[0010] This objective is attained in a second aspect by a receiving device configured to estimate characteristics of a wireless communication channel established between the receiving device and a transmitting device, the receiving device comprising a processing unit and a memory, said memory containing instructions executable by said processing unit, whereby the receiving device is operative to estimate a first channel matrix based on at least one reference signal received at at least two subcarrier frequencies over the wireless communication channel at a first time instance, estimate a second channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a second time instance, and to evaluate the estimated first and second channel matrix for deriving phase and delay shift occurring at any of the receiving device or the transmitting device of the reference signal received at the second time instance with respect to the reference signal received at the first time instance, thereby enabling compensation for the derived phase and delay shift.

[0011] Embodiments are advantageous for numerous reasons. Firstly, the compensation with respect to the phase and delay shift is performed at the receiving device and is thus completely transparent to the transmitting device. Therefore, there is neither need for costly hardware changes and / or additions at the transmitting device, nor for future standard changes to enforce transmitter coherency. All transmitting devices including legacy 3G, 4G, and 5G devices are hence supported.

[0012] Secondly, much longer-term phase stability of channel estimates is achieved, which will enable coherent processing over longer time periods, e.g.multiple time slots. The increased time coherence will allow for better channel estimation and channel prediction as compared to the state of the art.

[0013] Thirdly, the approach is fairly straightforward to implement with limited implementation complexity.

[0014] In an embodiment, the method further comprises adjusting either of the estimated first or second channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment in phase and time of data received at the second time instance with data received at the first time instance to compensate for the derived phase and delay shift.

[0015] In an embodiment, the receiving device is configured to provide the derived phase and delay shift to another receiving device for enabling said another receiving device to compensate for the derived phase and delay shift.

[0016] In an embodiment, the method further comprises estimating a further channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a further time instance, the evaluating comprising evaluating the estimated second and said further channel matrix for deriving phase and delay shift occurring at any of the receiving device or the transmitting device of the reference signal received at the further time instance with respect to the reference signal received at the second time instance, thereby enabling compensation for the derived phase and delay shift.

[0017] In an embodiment, the adjusting comprises adjusting either of the estimated second or further channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment in phase and time of data received at the further time instance with data received at the second time instance to compensate for the derived phase and delay shift.

[0018] In an embodiment, the receiving device is configured to provide the derived phase and delay shift to another receiving device for enabling said another receiving device to compensate for the derived phase and delay shift.

[0019] In an embodiment, the evaluating comprises computing a measure of autocorrelation between the estimated first channel matrix and the estimated second channel matrix.

[0020] In an embodiment, the measure of autocorrelation is computed over the first and second channel matrices being estimated for a plurality of antennas at the receiving device and the transmitting device

[0021] In an embodiment, the measure of autocorrelation is computed by multiplying the estimated first channel matrix with a conjugate of the estimated second channel matrix.

[0022] In an embodiment, the phase shift of the reference signal of the second time instance with respect to the reference signal of the first time instance is derived by computing an argument of said measure of autocorrelation.

[0023] In an embodiment, the estimated second channel matrix is adjusted by multiplying the derived phase shift with the estimated second channel matrix.

[0024] In an embodiment, the deriving of the phase shift is configured to take into account variations in the estimated phase shifts over a number of subcarriers, by at least one of performing linear regression of the phase shifts or performing a Fourier transformation of the phase shifts, to provide a phase adjustment being added to the derived phase shift.

[0025] In an embodiment, the delay shift is derived from a slope of an approximation of a linear relationship between the derived phase shift and frequency difference for the at least two subcarrier frequencies.

[0026] In an embodiment, the delay shift is derived as the derived phase shift between the at least two subcarriers divided by the frequency difference between said at least two subcarriers.

[0027] In a third aspect, a computer program is provided comprising computerexecutable instructions for causing a receiving device to perform steps recited in the method of the first aspect when the computer-executable instructions are executed on a processing unit included in the receiving device.

[0028] In a fourth aspect, a computer program product is provided comprising a computer readable medium, the computer readable medium having the computer program according to the third aspect embodied thereon.

[0029] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a / an / the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0031] Figure 1 illustrates a communication network in which embodiments may be implemented;

[0032] Figure 2 shows a flowchart illustrating a method according to an embodiment;

[0033] Figure 3a shows a flowchart illustrating a method according to a further embodiment;

[0034] Figure 3b shows a flowchart illustrating a method according to a further embodiment

[0035] Figure 4 shows a flowchart illustrating a method according to still an embodiment;

[0036] Figure 5 illustrates an example of an estimated phase shift according to over a plurality of subcarriers;

[0037] Figure 6 illustrates a receiving device according to an embodiment; and

[0038] Figure 7 illustrates a further communication network in which embodiments may be implemented.DETAILED DESCRIPTION

[0039] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.

[0040] These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by wayof example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0041] Figure 1 schematically illustrates a wireless communication system 300 comprising a group of wireless communication devices commonly referred to as User Equipment (UE) 100, 101, 102 communicating with a first network node in the form of a radio base station (RBS) 200. As further shown, the RBS 200 may also communicate with a neighbouring RBS 201. A UE may be embodied in the form of e.g. a smart phone, smart watch, tablet, desktop, Internet-of-Things (loT) device, gaming console, connected vehicle, etc. In the following, communication will be described from the perspective of the UE 100 and the RBS 200 but is equally applicable to the other devices.

[0042] Typically, the RBS 200 comprises a plurality of antennas for communicating with the UE 100 and performs beamforming towards the UE 100 in order to attain higher-quality communication with the UE 100, for instance as determined by a measure commonly known as Quality of Service (QoS).Beamforming generally improves signal-to-noise ratio of signals received at the UE 100 and the RBS 200 and mitigates, or ultimately even eliminates, undesirable interference from neighboring transmitting devices. The QoS of the channel may be based on numerous channel characteristics, such as for example data throughput, signal-to-interference-plus-noise ratio (SINR), attenuation, phase-shift, noise, delay, etc., of the channel.

[0043] In order to determine beamforming transmission parameters, characteristics of the communication channel setup between the UE 100 and the RBS 200 must be determined. As previously mentioned, this is commonly performed by having the UE 100 send a Sounding Reference Signal (SRS) to the RBS 200 which evaluates the SRS and derives so-called channel state information (CSI) for the communication channel between the UE 100 and the RBS 200 based on the SRS. The RBS 200 then evaluates the CSI and creates a so-called channel matrix utilized to model the channel. Finally, the RBS 200 may determine beamforming transmission parameters based on the channel matrix modelling the communication channel. The created channel matrix will thus serve as an estimation of the characteristics of thecommunication which enables the RBS 200 to transmit data towards the UE 100 using adequate beamforming transmission parameters for attaining a desired QoS.

[0044] The network node 200 may be composed of multiple physically separate components (e.g., an RBS component and a radio network controller (RNC) component, or a base transceiver station (BTS) component and a base station controller (BSC) component, etc.), which may each have their own respective components. In certain scenarios in which the network node 200 comprises multiple separate components (e.g., BTS and BSC components), one or more of the separate components may be shared among several network nodes. For example, a single RNC may control multiple RBSs. In such a scenario, each unique RBS and RNC pair, may in some instances be considered a single separate network node. In some embodiments, the network node 200 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components maybe duplicated (e.g., separate memory for different RATs) and some components may be reused (e.g., a same antenna may be shared by different RATs). The network node 200 may also include multiple sets of the various illustrated components for different wireless technologies integrated into the network nodes 200, for example Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE), New Radio (NR), WiFi, Zigbee, Z-wave, Long Range Wide Area Network (LoRaWAN), Radio Frequency Identification (RFID) or Bluetooth wireless technologies. These wireless technologies may be integrated into the same or different chip or set of chips and other components within the network node 200. The network node 200 will in the following be exemplified in the form of an RBS.

[0045] Now, as previously described, phase coherence and delay alignment are difficult to maintain over a plurality of time slots upon communicating data over the wireless communication channel setup between the UE 100 and the RBS 200.

[0046] In an embodiment to be described in the following, characteristics of the channel is estimated at the receiver side, i.e. at the RBS 200 in the uplink and / or at the UE 100 in the downlink, to compensate for - and thus resolve - any phase and delay mismatch occurring for the data communicated over the channel setup between the UE 100 and the RBS 200.

[0047] Alternatively, characteristics of a channel established between the RBS 200 and the neighbouring RBS 201 may be estimated and compensated for to attain synchronization between the RBS 200 and the neighbouring RBS 201. In such case, the estimated characteristics can e.g. be used at the RBS 200 to correct its clock or at the neighbouring RBS 201 to correct its clock in order to attain synchronization between the two RBSs 200, 201

[0048] The receiver performs the channel estimation to compensate for both the lack of transmitter coherence as well as mismatched delay between the UE 100 and the RBS 200 by utilizing second order statistics of the fast fading variations in time, to allow aggregation of the CSI in order to provide channel information with improved quality. Typically, over the time period of a few time slots, any phase and delay mismatch occurring is generally caused by the UE 100 and / or the RBS 200 not being synchronized to a far greater extent than being caused by e.g. fast fading of the wireless channel, as will be discussed in more detail in the following.

[0049] A wireless radio channel may be modelled based on superposition of multiple propagation paths between the transmitter and receiver, where the complex amplitude of the radio channel for a specific transmitter (Tx) and receiver (Rx) antenna position (rtand rrrespectively), time instant (t), and frequency (f) can be denoted as H(f, t, rt, rr), being referred to as a channel matrix. Due to the superposition of multiple propagation paths, H will vary rapidly in each of these dimensions. These variations are commonly referred to as fast fading, characterized by rapidly varying phase variations of the channel.

[0050] Figure 2 illustrates a method according to an embodiment in which it is assumed that the transmitting device is embodied by the UE 100 while the receiving device is embodied by the RBS 200.

[0051] In a first step S101, upon the UE 100 sending a first reference signal (exemplified in the following by an SRS as previously discussed) at two or more subcarrier frequencies over the wireless communication channel in the uplink, the RBS 200 estimates a first channel matrix H(t) at a first time instance t based on the received first SRS.

[0052] Thereafter, in step S102, upon the UE 100 sending a second SRS at the same subcarrier frequencies over the channel, the RBS 200 estimates a secondchannel matrix H(t+At) at a second time instance t+At based on the received second SRS.

[0053] In S103, the RBS 200 evaluates the estimated first and second channel matrix H(t), H(t+At) for deriving phase and delay shift occurring at any of the UE 100 or the RBS 200 of the SRS received at the second time instance t+At with respect to the SRS received at the first time instance t, thereby enabling alignment of the estimated communication channel in phase and time.

[0054] As shown in Figure 3a, the RBS 200 may for instance in an embodiment adjust S104a either of the estimated first or second channel matrix H(t), H(t+At) to take into account said derived phase and delay shift occurring at any of the UE 100 for the RBS 200, wherein the adjusting enables in S105 alignment in phase and time of data received at the second time instance t+At with data received at the first time instance t. Thus, any phase and delay shift occurring at the UE 100 or the RBS 200 is advantageously compensated for at the RBS 200.

[0055] Note that while the second channel matrix may be adjusted in Figure 3a, an alternative is to not adjust only the first channel matrix but instead all channel matrixes preceding that first channel matrix.

[0056] As shown in Figure 3b, rather than performing steps S101-S103 for the channel setup between the UE 100 and the RBS 200, the same steps may in an alternative embodiment be performed for a channel set up between the RBS 200 and the neighbouring RBS 201. The derived phase and delay shift may either be used at the RBS 200 to correct its clock to attain synchronization with the neighbouring RBS 201, or sent to the neighbouring RBS 201 in in S104b, for performing synchronization of its clock, thereby enabling alignment in phase and time of data received at the second time instance with data received at the first time instance for the channel established between the two RBSs 200, 201.

[0057] Now, as is understood, the process of Figure 3a and 3b is repeated from S102 for a next time instance, where a next third channel matrix is estimated in S102 and evaluated in S103 with the adjusted second channel matrix estimated at the immediately preceding (i.e. second) time instance in S103, and so on. Thus, in such an embodiment, an iterative process is performed where a new channel matrix is estimated for a current time instance in S102 and evaluated in S103 along with theadjusted channel matrix of the immediately preceding time instance, and then proceeds to either 8104a (and possibly S105) or 8104b before advancing to yet a next time instance and repeating the process. Advantageously, with this embodiment, it is only necessary to evaluate the estimated current channel matrix with the adjusted immediately preceding matrix.

[0058] Embodiments described herein are advantageous for numerous reasons. Firstly, the compensation with respect to the phase and delay shift is performed at the receiving device and is thus completely transparent to the transmitting device.Therefore, there is neither need for costly hardware changes and / or additions at the transmitting device, nor for future standard changes to enforce transmitter coherency. All transmitting devices including legacy 3G, 4G, and 5G devices are hence supported.

[0059] Secondly, much longer-term phase stability of channel estimates is achieved, which will enable coherent processing over longer time periods, e.g. multiple time slots. The increased time coherence will allow for better channel estimation and channel prediction as compared to the state of the art.

[0060] Thirdly, the approach is fairly straightforward to implement with limited implementation complexity.

[0061] A more detailed embodiment of the estimation of the first and second channel matrix in S101 and S102, and the subsequent evaluation of the first and second channel matrix in S103 being performed in order to be able to compensate for phase and delay shift occurring at any of the UE 100 or the RBS 200 will be discussed in the following.

[0062] Now, second order statistics in the form of the time-correlation of the wireless channel varies very slowly and can even be considered to be stationary over shorter time intervals. Hence, using the second order statistics for a single subcarrier, averaged over RBS antenna array aperture, can provide information of the UE noncoherency, whereas the knowledge of a small delay error results in a linear (increasing or decreasing) phase front over evenly spaced subcarriers. Thus, estimation of the slope of phase change in frequency domain, from second order statistics per subcarrier, allows for correction of the RBS-UE delay mismatch.

[0063] The time autocorrelation RH(t, t + At) of the channel matrix H is an example of a second order statistics and is defined asRH(t, t + At) = E{H(..., t,... )H(..., t + At,... )*}, (1)

[0064] where E{ } denotes the expectation operator and H* denotes the complex conjugate of H. It can be shown that RH(t, t + At) is independent of t for multipath fading channels, where a model referred to as “wide-sense stationary with uncorrelated scattering (WSSUS)” commonly is used to describe the behaviour of such channels, meaning that the directions and strengths of the multipath components are static. Most real -wo rid channels does not fully comply with the WSSUS model but can nevertheless be approximated by the WSSUS model if the movement of the transmitter or receiver is confined to a local area, implying that the eventual variations of the multipath due to changing directions or strength are very small. The size of the local area where such an approximation holds up is dependent on the propagation environment, but can practically be several meters wide. For a typical speed of movement of a mobile user in a wireless communication system, the time autocorrelation RH(t, t + At) can be approximated as being constant for fractions of a second to several seconds. This time span is much longer than the typical coherence time of UE transmissions, which transmission generally are only guaranteed to be coherent within a few ms.

[0065] In systems such as LTE and NR, the receiver (in this particular example being the RBS 200 for uplink communication and the UE 100 for downlink communication) forms estimates H f, t, rt, rr) of the wireless channel by comparing the received signal with a known transmitted signal. Hence, in case of using SRS to estimate the channel characteristics represented by the estimated channel matrix H (f, t, rt, rr), the transmitter sends the SRS (being a predefined signal with known properties) to the receiver, which receives a distorted version of the transmitted SRS. The distorted version of the SRS is compared with the known SRS that was transmitted. Channel characteristics may thus be derived based on the differences noted in the comparing of the two signals. These estimates will be affected by additive noise and estimation errors <j(t), but also of an eventual time-varying random phase error, <pnc(t), due to non-coherent UE transmissions, as well as an unpredictable delay error, 8sync(t), due to the UE-RBS synch mismatch which in frequency domainpSync f) = 2TT 5 is a linearly increasing / decreasing phase error over the subcarriers depending on the relative positive / negative synch error betweent,... ) and + At,...) resulting in a total phase error 0(, t) = <pnc(t) + <pSync f)> i-e-:H(f,t,rt,rr) = el0(J’t^H(f,t,rt,rr) + <r(t) (2)

[0066] Assuming that the noise is statistically independent in different time instants, i.e. that E a(t)cy(t + At)*} = 0, the time autocorrelation of H becomes:

[0067] Thus, even if the true channel autocorrelation RHis independent of time, the autocorrelation of the estimated channel is time-dependent due to the timevarying phase error. Note that the fact that the true channel autocorrelation RHis independent of time does not mean that the true phase of RHis known. It is an unknown parameter and can thus be selected arbitrarily. In the description below this true phase of RHis assumed to be zero

[0068] From (i)-(3), and the assumption that the phase of RHis zero, it can be concluded that consecutive estimates of R^ t, t + At) may be used to estimate the time- and frequency-varying phase error 0(, t) in an iterative process. Thus, determining the autocorrelation RH= E{H(t)H(t + At)*} or Rg = E{H(t)H(t + At)*} requires the calculation of an expectation value. In practice, the expectation value E{xy*} is estimated based on a finite set of N observations by using the arithmetic mean as E{xy*) «u- The more independent observations of H, the better this estimate becomes. For example, considering a single subcarrier in a multiple input-multiple output (MIMO) setup, channel coefficients for different RBS and UE antenna ports are used as different such observations. For example, if the channel is estimated for M transmit antennas, and N receive antennas, then the autocorrelation is estimated as:

[0069] where Hmn(t) is the channel estimate for the m:th transmit antenna, and the n:th receive antenna.

[0070] Figure 4 thus illustrates step S103 in more detail, where a measure of autocorrelation+ t) is computed as set out in equation (4) hereinabove between the estimated first channel matrix denoted Hmn(t) and the estimated second channel matrix denoted Hmn(t + At).

[0071] In this particular embodiment, the autocorrelation is computed over the first and second channel matrices being estimated for a plurality of antennas N, M at the receiving device and the transmitting device. Further, as can be concluded, the autocorrelation is computed by multiplying the estimated first channel matrix with a conjugate of the estimated second channel matrix.

[0072] Hence, the time autocorrelation for each subcarrier, f, between the channel estimates in the current time instant and the channel estimates in a previous instant,fn) is computed in accordance with equation (4), and an estimate of the phase error, i.e. the phase shift, between these two time instants is determined as the argument of the autocorrelation:A0(f, tn) = arg(7?S(tn-x, tn)) (5)

[0073] Figure 4 thus illustrates step S103 in more detail incorporating the teachings of equations (4) and (5) in an embodiment, where a measure of autocorrelation Rfi(t, t + At) is computed in 8103a as set out in equation (4) hereinabove between the estimated first channel matrix denoted Hmn(t) and the estimated second channel matrix denoted Hmn(t + At).

[0074] In this particular embodiment, the autocorrelation is computed over the first and second channel matrices being estimated for a plurality of antennas N, M at the receiving device 200 and the transmitting device 100 (even if it may be computed for as few as only one antenna at the UE 100 and the RBS 200, respectively). Further, as can be concluded, the autocorrelation is computed by multiplying the estimated first channel matrix with a conjugate of the estimated second channel matrix.

[0075] Thereafter, in 8103b, the phase shift is derived in accordance with equation (5).

[0076] This estimated phase error can be used to compensate the channel estimates, H(f, tn) at each subcarrier f and time at tnby:

[0077] Thus, in this embodiment, the estimated second channel matrix is adjusted in 8104a by multiplying the derived phase shift with the estimated second channel matrix as proposed in equation (6). As previously described, the derived phase shift may alternatively be provided to neighbouring RBS 201 in 8104b for potential compensation at the neighbouring RBS 201.

[0078] Figure 5 illustrates an example of the estimated phase shift according to equation (5) over 136 subcarriers. Although synch errors show a clear tendency of having a linear phase dependency over the subcarriers, there are small variations in the estimations. Aware of that a delay error is expected to constitute a perfectly linear phase-front over the subcarriers, the accuracy of the correction can be improved. The slope can be determined using linear regression or Fourier transformation, providing a phase adjustment (p(J, tn) to make the correction perfectly linear by A0(, tn) = tn) + <p(f, tn). This will provide a further improved adjustment of the channel matrix by:

[0079] In other words, in an embodiment, the deriving of the phase shift is further configured to take into account variations in the estimated phase shifts over a number of subcarriers, by at least one of performing linear regression of the phase shifts or performing a Fourier transformation of the phase shifts, to provide a phase adjustment being added to the derived phase shift in accordance with equation (7).

[0080] In certain circumstances the delay variation 8 (t) can in a similar way be assumed perfectly linear over time. The linearity can be determined using linear regression or Fourier transformation, providing a further delay adjustment y(t) to make the correction linear by 86 (fn, t) = 86 (fn, t)+2nfnY(t). This will provide a further improved adjustment of the channel matrix by:

[0081] Advantageously, by consecutively calculating the autocorrelation and compensating the estimated channel matrices as time progresses, it is possible to achieve coherency over longer time periods.

[0082] In a further embodiment, with reference to any one of the flowcharts of Figure 2-4, the delay shift may be derived in S103 from a slope of an approximation of a linear relationship between the derived phase shift and frequency difference for the at least two subcarrier frequencies.

[0083] In a more detailed embodiment, with reference again to any one of the flowcharts of Figure 2-4, the delay shift may be derived in S103 by dividing the derived phase shift between the (at least) two subcarriers with the frequency difference between said (at least) two subcarriers.

[0084] Thus, both the derived phase and delay shift may subsequently be compensated for in 8104a and S105, or alternatively in 8104b.

[0085] Figure 6 illustrates a receiving device (e.g. RBS 200) configured to estimate characteristics of a wireless communication channel established between the receiving device 200 and a transmitting device (e.g. UE 100),. The steps of the method performed by the receiving device 200 are in practice performed by a processing unit 411 embodied in the form of one or more microprocessors arranged to execute a computer program 412 downloaded to a storage medium 413 associated with the microprocessor, such as a Random Access Memory (RAM), a Flash memory or a hard disk drive. The processing unit 411 is arranged to cause the receiving device 200 to carry out the method according to embodiments when the appropriate computer program 412 comprising computer-executable instructions is downloaded to the storage medium 413 and executed by the processing unit 411. The storage medium 413 may also be a computer program product comprising the computer program 412. Alternatively, the computer program 412 may be transferred to the storage medium 413 by means of a suitable computer program product, such as a Digital Versatile Disc (DVD) or a memory stick. As a further alternative, the computer program 412 may be downloaded to the storage medium 413 over a network. The processing unit 411 may alternatively be embodied in the form of a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a complex programmable logic device (CPLD), etc.The receiving device 200 further comprises a communication interface 414 (wired or wireless) over which it is configured to transmit and receive data.

[0086] The receiving device 200 according to embodiments may be provided as a standalone device or as a part of at least one further device. Alternatively, functionality of the receiving device 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to a radio cell than instructions that are not required to be performed in real time.

[0087] Thus, a first portion of the instructions performed by the receiving device 200 may be executed in a first device, and a second portion of the of the instructions may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the receiving device 200 may be executed.

[0088] Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a device residing in a cloud computational environment. Therefore, although a single processing unit 411 is illustrated in Figure 6, the processing unit 411 may be distributed among a plurality of devices, or nodes.

[0089] Figure 7 illustrates a network in the form of an Open RAN 301 (O-RAN), in which embodiments may be implemented.

[0090] With reference to the O-RAN 301, the role of a Non-Real Time RAN intelligent controller (RIC) 202 is among other things, such as providing a service management and orchestration framework, to serve one or more radio base stations 350 (i.e. RAN sites) referred to as O-eNB via 01 interface and to provide high-level control signals to Near-Real Time RICs 400 via Al interface; such signals include but not limited to policy-based guidance, machine-learning (ML) model management, and enrichment of data. The role of Near- Real Time RICs 400 is to perform low-level control signals to O-RAN compatible network elements including the one or more O-eNBs 350, O-CU-CP 500, O-CU-UP 600 and 0-DU 700, via E2 interface. Further included is an 0-RU 800 connected to the 0-DU 700 via a control, user andsynchronization (CUS) plane as well as via a management (M) plane, and an O-Cloud 900, i.e. a cloud platform.

[0091] The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

[0092] Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

CLAIMS1. A method of a receiving device (200) of estimating characteristics of a wireless communication channel established between the receiving device (200) and a transmitting device (100), comprising:estimating (S101) a first channel matrix based on at least one reference signal received at at least two subcarrier frequencies over the wireless communication channel at a first time instance;estimating (S102) a second channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a second time instance; andevaluating (S103) the estimated first and second channel matrix for deriving phase and delay shift occurring at any of the receiving device (200) or the transmitting device (100) of the reference signal received at the second time instance with respect to the reference signal received at the first time instance, thereby enabling compensation for the derived phase and delay shift.

2. The method of claim 1, further comprising:adjusting (S104a) either of the estimated first or second channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment (S105) in phase and time of data received at the second time instance with data received at the first time instance to compensate for the derived phase and delay shift.

3. The method of claim 1, wherein the receiving device (200) is configured to provide (S104b) the derived phase and delay shift to another receiving device (201) for enabling said another receiving device (201) to compensate for the derived phase and delay shift.

4. The method of any one of the preceding claims, further comprising:estimating (S102) a further channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a further time instance; the evaluating comprising:evaluating (S103) the estimated second and said further channel matrix for deriving phase and delay shift occurring at any of the receiving device or the transmitting device of the reference signal received at the further time instance withrespect to the reference signal received at the second time instance, thereby enabling compensation for the derived phase and delay shift.

5. The method of claims 2 and 4, the adjusting comprising:adjusting (8104a) either of the estimated second or further channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment (S105) in phase and time of data received at the further time instance with data received at the second time instance to compensate for the derived phase and delay shift.

6. The method of claims 3 and 4, wherein the receiving device (200) is configured to provide (8104b) the derived phase and delay shift to another receiving device (201) for enabling said another receiving device (201) to compensate for the derived phase and delay shift.

7. The method of any one of the preceding claims, the evaluating (S103) comprising:computing (8103a) a measure of autocorrelation between the estimated first channel matrix and the estimated second channel matrix.

8. The method of claim 7, wherein the measure of autocorrelation is computed (8103a) over the first and second channel matrices being estimated for a plurality of antennas at the receiving device and the transmitting device9. The method of claims 7 or 8, wherein the measure of autocorrelation is computed (8103a) by multiplying the estimated first channel matrix with a conjugate of the estimated second channel matrix.

10. The method of any one of claims 7 or 8, wherein the phase shift of the reference signal of the second time instance with respect to the reference signal of the first time instance is derived by computing (8103b) an argument of said measure of autocorrelation.

11. The method of any one of the preceding claims, wherein the estimated second channel matrix is adjusted (8104a) by multiplying the derived phase shift with the estimated second channel matrix.

12. The method of any one of the preceding claims, wherein the deriving (S102) of the phase shift further is configured to take into account variations in the estimated phase shifts over a number of subcarriers, by at least one of performing linear regression of the phase shifts or performing a Fourier transformation of the phase shifts, to provide a phase adjustment being added to the derived phase shift.

13. The method of any one of the preceding claims, the delay shift being derived (S103) from a slope of an approximation of a linear relationship between the derived phase shift and frequency difference for the at least two subcarrier frequencies.

14. The method of any one of the preceding claims, the delay shift being derived (S103) as the derived phase shift between the at least two subcarriers divided by the frequency difference between said at least two subcarriers.

15. A computer program (412) comprising computer-executable instructions for causing the receiving device (200) to perform steps recited in any one of claims 1-14 when the computer-executable instructions are executed on a processing unit (411) included in the receiving device (200).

16. A computer program product comprising a computer readable medium (413), the computer readable medium having the computer program (412) according to claim 15 embodied thereon.

17. A receiving device (200) configured to estimate characteristics of a wireless communication channel established between the receiving device (200) and a transmitting device (100), the receiving device (200) comprising a processing unit (411) and a memory (413), said memory containing instructions (412) executable by said processing unit (411), whereby the receiving device (200) is operative to:estimate (S101) a first channel matrix based on at least one reference signal received at at least two subcarrier frequencies over the wireless communication channel at a first time instance;estimate (S102) a second channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a second time instance; andevaluate (S103) the estimated first and second channel matrix for deriving phase and delay shift occurring at any of the receiving device (200) or thetransmitting device (100) of the reference signal received at the second time instance with respect to the reference signal received at the first time instance, thereby enabling compensation for the derived phase and delay shift.

18. The receiving device (200) of claim 17, further being operative to:adjust (8104a) either of the estimated first or second channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment (S105) in phase and time of data received at the second time instance with data received at the first time instance to compensate for the derived phase and delay shift.

19. The receiving device (200) of claim 17, further being operative to provide (8104b) the derived phase and delay shift to another receiving device (201) for enabling said another receiving device (201) to compensate for the derived phase and delay shift.

20. The receiving device (200) of any one of claims 17-19, further being operative to:estimate (S102) a further channel matrix based on at least one reference signal received at said at least two subcarrier frequencies over the wireless communication channel at a further time instance; the evaluating comprising:evaluate (S103) the estimated second and said further channel matrix for deriving phase and delay shift occurring at any of the receiving device or the transmitting device of the reference signal received at the further time instance with respect to the reference signal received at the second time instance, thereby enabling compensation for the derived phase and delay shift.

21. The receiving device (200) of claims 18 and 20, further being operative to:adjust (8104a) either of the estimated second or further channel matrix to take into account said derived phase and delay shift, wherein the adjusting enables alignment (S105) in phase and time of data received at the further time instance with data received at the second time instance to compensate for the derived phase and delay shift.

22. The receiving device (200) of claims 19 and 20, further being operative to provide (8104b) the derived phase and delay shift to another receiving device (201)for enabling said another receiving device (201) to compensate for the derived phase and delay shift.

23. The receiving device (200) of any one of claims 17-22, further being operative to, upon evaluating (S103) the estimated first and second channel matrix:computing (8103a) a measure of autocorrelation between the estimated first channel matrix and the estimated second channel matrix.

24. The receiving device (200) of claim 23, further being operative to compute (8103a) the measure of autocorrelation over the first and second channel matrices being estimated for a plurality of antennas at the receiving device and the transmitting device25. The receiving device (200) of claims 23 or 24, further being operative to compute (8103a) the measure of autocorrelation by multiplying the estimated first channel matrix with a conjugate of the estimated second channel matrix.

26. The receiving device (200) of claims 23 or 24, further being operative to derive the phase shift of the reference signal of the second time instance with respect to the reference signal of the first time instance is by computing (8103b) an argument of said measure of autocorrelation.

27. The receiving device (200) of any one of claims 17-26, further being operative to adjust (8104a) the estimated second channel matrix by multiplying the derived phase shift with the estimated second channel matrix.

28. The receiving device (200) of any one of claims 17-27, further being operative to derive (S102) the phase shift by taking into account variations in the estimated phase shifts over a number of subcarriers, by at least one of performing linear regression of the phase shifts or performing a Fourier transformation of the phase shifts, to provide a phase adjustment being added to the derived phase shift.

29. The receiving device (200) of any one of claims 17-28, further being operative to derive (S103) the delay shift from a slope of an approximation of a linear relationship between the derived phase shift and frequency difference for the at least two subcarrier frequencies.

30. The receiving device (200) of any one of claims 17-29, further being operative to derive the delay shift as the derived phase shift between the at least two subcarriers divided by the frequency difference between said at least two subcarriers.

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