Patent ID: 12218725

In the various figures, identical reference signs will be used for identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present application may be placed. It will be appreciated that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present application. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present application is defined by the appended claims.

For instance, it will be appreciated that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures.

Moreover, in the following detailed description as well as in the claims embodiments with different functional blocks or processing units are described, which are connected with each other or exchange signals. It will be appreciated that the present application covers embodiments as well, which include additional functional blocks or processing units that are arranged between the functional blocks or processing units of the embodiments described below.

Finally, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

FIG.1shows a communication system100comprising a communication device101according to an embodiment and a communication device131according to an embodiment configured to communicate via a wireless communication channel. In the following the communication device101will be also referred to as a user equipment101and the communication device131will be also referred to as a multiple-input multiple-output, MIMO, base station131. In an embodiment, the communication system100can be a wireless communication system100.

As can be taken fromFIG.1and as will be described in more detail further below, the user equipment101comprises a communication interface103configured to receive a current pilot signal over the wireless communication channel from the base station131. In an embodiment, the communication interface103of the user equipment101can comprise one or more antennas.

Furthermore, the user equipment101comprises a processor105configured to determine a current channel state information, CSI, on the basis of the current pilot signal and to determine a CSI reliability measure value on the basis of the current CSI and one or several previous CSI, wherein the processor105is configured to determine the previous CSI on the basis of previous pilot signals, i.e. previously received pilot signals.

As can be taken fromFIG.1and as will be described in more detail further below, the base station131comprises a communication interface133configured to send one or more pilot signals to the user equipment101and to receive feedback from the user equipment101, including or based on a channel state information, CSI, reliability measure value, the CSI delay ΔCSIto which the reliability measure corresponds. In an embodiment, the communication interface133of the base station131can comprise one or more antennas. In an embodiment, the communication interface133is a MIMO communication interface133.

Furthermore, the base station131comprises a processor135configured to apply an adaptively adjustable MIMO transmission scheme to data to be transmitted to the user equipment101. The processor135is further configured to adaptively adjust the MIMO transmission scheme on the basis of the CSI feedback and CSI reliability measure value and corresponding CSI delay value feedback from the user equipment101.

In an embodiment, the CSI reliability measure value is based on a current channel estimate or latest CSI and a previous CSI determined by the user equipment101on the basis of a respective pilot signal. In a further embodiment, the adaptively adjustable MIMO transmission scheme is a precoded space time/frequency block coding scheme associated with a space time/frequency block coding matrix and the processor135is configured to adaptively adjust the MIMO transmission scheme by weighting the columns or rows of the space time/frequency block coding matrix on the basis of a precoding-diversity weighting factor, as will be further discussed below.

According to an embodiment, either the user equipment101or the base station131can determine a precoding-diversity weighting factor on the basis of the CSI reliability measure value and optionally a SNR value, the number of base station antennas and/or the number of user equipment antennas, wherein the precoding-diversity weighting factor allows the base station131to adaptively adjust a MIMO transmission scheme for data to be transmitted to the user equipment101by weighting a beamforming precoding component and a diversity precoding component of the MIMO transmission scheme.

In an embodiment, the precoding-diversity weighting factor θ can be determined by the processor105of the user equipment101on the basis of the CSI reliability measure value ρ by using the scenario specific mapping between the CSI reliability measure value and the precoding-diversity weighting factor defined by the following equation.
θ=(1−1/T)ρ+1/T,wherein T denotes a system parameter integer value ≥2.

Furthermore, the CSI reliability measure values can be determined by the processor105of the user equipment101corresponding to different CSI delay values ΔCSIas a correlation coefficient between the current channel estimate or the current CSI and a previous CSI separated by an amount of time equal to ΔCSIfrom the current channel estimate or the current CSI, wherein the CSI reliability measure value ρ(ΔCSI, new) corresponding to the CSI delay ΔCSIcan be determined on the basis of the following equation:

ρ⁡(ΔCSI,new)=α⁢h^t-ΔCSIH⁢hth^t-ΔCSI⁢ht+(1-α)⁢ρ⁡(ΔCSI,old)⁢⁢orρ⁡(ΔCSI,new)=α⁢h^t-ΔCSIH⁢h^th^t-ΔCSI⁢h^t+(1-α)⁢ρ⁡(ΔCSI,old),wherein ∥X∥ for some vector of matrix X stands for the value of a norm of X, ΔCSIdenotes the time delay between the reception of the previous pilot signal and the current pilot signal by the communications interface, α denotes an averaging coefficient in the range from 0 to 1, htor ĥtdenotes the vectorized form of a current channel response estimate or a quantized current channel response estimate, ĥt-ΔCSIH, denotes the vectorized form of a quantized previous channel response estimate and ρ(ΔCSI, old) denotes a previous CSI reliability measure value corresponding to CSI delay ΔCSI.

According to an embodiment, the processor105of the user equipment101is configured to store the value of ρ(ΔCSI, new) in an initially empty table whose first column is composed of the CSI delay values ΔCSIthat have been treated so far by the user equipment101and whose second columns is composed of the CSI reliability measure values corresponding to the CSI delay values of the first column.

Furthermore, the processor105of the user equipment101is configured to read the value of ρ(ΔCSI, old) from the table if the entry (ΔCSI, ρ(ΔCSI)) already exists in said table, or as a first alternative to assume a default value for ρ(ΔCSI, old) e.g. 0.5, or as a second alternative to use an interpolated value based on existing entries from the table e.g.

ρ⁡(ΔCSI,old)=ΔCSI-ΔCSI,1ΔCSI,2-ΔCSI,1⁢ρ⁡(ΔCSI,1)+ΔCSI,2-ΔCSIΔCSI,2-ΔCSI,1⁢ρ⁡(ΔCSI,2)where⁢(ΔCSI,1,ρ⁡(ΔCSI,1))⁢⁢and⁢⁢(ΔCSI,2,ρ⁡(ΔCSI,2))are two non-empty entries from the table such that ΔCSI,1≤ΔCSI≤ΔCSI,2.

At the base station131, according to an embodiment, the processor135is configured to determine the precoding-diversity weighting factor θ on the basis of the CSI delay value {tilde over (Δ)}CSIand the table mentioned above by using the scenario specific mapping between the CSI reliability measure value and the precoding-diversity weighting factor defined by the following equations:
θ=(1−1/T)ρ({tilde over (Δ)}CSI)+1/T,
{tilde over (Δ)}CSI=ΔCSI+tTr−tCSI,wherein T denotes a system parameter integer value ≥2, ΔCSIdenotes the time delay between the reception of the previous pilot signal and the current pilot signal by the user equipment101, tTrdenotes the moment of transmission and tCSIdenotes the moment of reception of the feedback from the user equipment101and ρ({tilde over (Δ)}CSI) is obtained from the above table by reading the entry ({tilde over (Δ)}CSI,ρ({tilde over (Δ)}CSI)) if it already exists in the table, or as a first alternative to assume a default value for ρ({tilde over (Δ)}CSI) e.g. 0.5, or as a second alternative to use an interpolated value based on existing entries from the table e.g.

ρ⁡(Δ~CSI)=Δ~CSI-ΔCSI,1ΔCSI,2-ΔCSI,1⁢ρ⁡(ΔCSI,1)+ΔCSI,2-Δ~CSIΔCSI,2-ΔCSI,1⁢ρ⁡(ΔCSI,2)where (ΔCSI,1,ρ(ΔCSI,1)) and (ΔCSI,2,ρ(ΔCSI,2)) are two non-empty entries from the table such that ΔCSI,1≤{tilde over (Δ)}CSI≤ΔCSI,2.

In an embodiment, the processor135of the base station131is configured to adaptively adjust the MIMO transmission scheme by weighting a beamforming precoding component and a diversity precoding component of the MIMO transmission scheme on the basis of the precoding-diversity weighting factor.

Alternatively, the communications interface133of the base station131can receive feedback from the user equipment101and the feedback from the user equipment101comprises a precoding-diversity weighting factor based on the channel state information. CSI, reliability measure value. The processor135of the base station131is configured to adaptively adjust the MIMO transmission scheme by weighting a beamforming precoding component and a diversity precoding component of the MIMO transmission scheme on the basis of the precoding-diversity weighting factor.

In a further embodiment, the feedback from the user equipment101comprises precoding information derived from a precoding-diversity weighting factor based on the channel state information, CSI, reliability measure value and the processor135of the base station131is configured to adaptively adjust the MIMO transmission scheme by weighting a beamforming precoding component and a diversity precoding component of the MIMO transmission scheme on the basis of the precoding information reported in the user equipment feedback.

In an embodiment, the communications interface133of the base station131is configured to transmit the data to the user equipment101using a plurality of resource elements/blocks {1, . . . , B} and the processor135of the base station131is configured to derive a plurality of weights {θb}b=1 . . . Bon the basis of the precoding-diversity weighting factor θ to adaptively adjust the MIMO transmission scheme by weighting the columns or rows of the space time/frequency block coding matrix used for data transmission on the plurality of resource elements/blocks, wherein the plurality of weights {θb}b=1 . . . Bare derived from the precoding-diversity weighting factor θ using one of the following equations: θb=θ∓tbor θb=(1∓δb)×θ, where 0≤tb≤1 and 0≤δb≤1 are scenario dependent.

More specifically, the weighting of the columns or rows of the space time/frequency block coding matrix can be determined on the basis of the following equations:

Wb(θb)=W~⁢D⁡(θb),W~=[g^1⁢g^2⁢…⁢g^T],D⁡(θb)=diag⁢(θb,1-θbT-1,…,1-θbT-1)wherein θbdenotes the precoding-diversity weighting factor [for a resource element/block b], {tilde over (W)} is a MIMO precoding matrix whose columns ĝ1, ĝ2, . . . , ĝTare MIMO precoding vectors determined on the basis of CSI and D(θb) is a diagonal matrix that gives different weights to vectors ĝ1, ĝ2, . . . , ĝTon the basis of the value of θb.

In a further embodiment, the base station131can transmit the data to the user equipment101using a plurality of resource elements/blocks and a plurality of MIMO precoding vectors/matrices, and the processor135of the base station131is configured to apply to each member of the plurality of resource elements/blocks one of the vectors/matrices of the plurality of precoding vectors/matrices and to adaptively adjust the percentage of resource elements/blocks to which each one of the plurality of precoding vectors/matrices is applied on the basis of the CSI reliability measure value θ.

In a further embodiment, the processor135of the base station131is configured to extract the respective precoding vector/matrix from a subset of cardinality T of a predefined MIMO transmission codebook, and one of these vectors/matrices is used for data transmission on a portion of 100×0% of the plurality of resource elements/blocks whereas the remaining T−1 vectors/matrices are used for data transmission on the remaining 100×(1−θ)% of the plurality of resource elements/blocks.

FIG.2shows a schematic diagram illustrating a MU-MIMO system200which comprises a user equipment101and a base station131according to an embodiment, wherein the user equipment101and the base station131can communication via a MU-MIMO downlink channel251and the blocks drawn with dashed lines represent the new functionalities according to the embodiments of the application.

At the user equipment101, the channel estimates obtained based on cell-specific reference signals (CRS) pilots are used by the “channel state information reliability estimation” block201to derive the channel state information reliability metric (denoted ρ in the sequel). The output of the “channel state information reliability estimation” block201is used to update a look-up table (LUT)202whose entries comprise different values of the CSI reliability metric corresponding to different values of the CSI delay.

The updated LUT202can be used at the user equipment101to feed the “precoding-diversity weighting factor computation” block203in order to derive the parameter (denoted θ in the sequel) needed to determine the level of tradeoff between MIMO beamforming and MUMO diversity while transmitting from the base station131. For instance, the highest possible value of θ, i.e. θ=1, (typically associated with the highest possible value of ρ, i.e. ρ=1) results in a pure MIMO beamforming transmission scheme, whereas the smallest possible value of θ i.e., θ=1/T for some integer T≥2 (typically associated with the smallest possible value of ρ i.e., ρ=0) results in a pure MIMO diversity transmission scheme.

The determined value can be sent to the base station131using a “precoding-diversity weighting factor feedback” message253. If this parameter is modified by the base station131before being used, then the updated value is signaled using a “precoding-diversity weighting factor signaling” message253. In case joint effective channel estimation across resource units with different levels of beamforming-diversity tradeoffs is needed, then the values of these levels need to be communicated by the “precoding-diversity weighting factor computation” block203to the “effective channel estimation” block204.

The output of the “channel state information reliability estimation” block201can also be fed back to the base station131using a “channel state information reliability feedback” message255. This message is used at the base station131to update a look-up table (LUT)234similar in its definition to its counterpart of the user equipment101. The output of the “precoding-diversity weighting factor computation” block231at the base station131is used to derive “precoder commands” that affect the computation of the MIMO precoders in the frame construction” block232and “precoded pilots commands” which affect the “pilot insertion” block233to make sure that the precoded pilots are precoded with the same MIMO precoders dictated by the “precoding-diversity weighting factor computation” block231.

FIG.3shows a schematic diagram summarizing a procedure300for estimating an effective channel in a communication network according to an embodiment, wherein the communication network comprises a user equipment101and a base station131. In the following, NTxand NRxdesignate the number of antenna elements at the base station131and the user equipment101respectively and the snr value represents the received signal-to-noise ratio. The procedure300shown inFIG.3comprises the following steps:

Step 1.a: First, the user equipment (UE)101estimates the downlink channel from cell specific reference signals e.g., CRS to get ĥt, a vectorized version of the UE channel matrix or a vectorized version of a quantization of this matrix. It then computes an estimation of the correlation coefficient ρ(ΔCSI) between channel estimates separated in time by the value ΔCSI, where ΔCSIis the delay between the two latest CRS receptions. One possible way of doing this is by keeping track of the Δ→ρ(Δ) mapping using a dynamic look-up table (LUT) (with entries initially set to zeros) and updating the ΔCSI→ρ(ΔCSI) entry by applying

ρ⁡(ΔCSI,new)=α⁢h^t-ΔCSIH⁢h^th^t-ΔCSI⁢h^t+(1-α)⁢ρ⁡(ΔCSI,old),wherein 0≤α≤1 is a predefined averaging coefficient.

Step 1.b: Secondly, if needed, the user equipment101applies a mapping to the estimated correlation to obtain the desired parameter θ, which can take the form θ=fs(ρ(ΔCSI), snr, NTx, NRx). Subscript s stands for the index of the current cell setting scenario among a set of predefined scenarios, e.g., urban micro cell, suburban macro cell, etc. These mappings will often be LUTs stored at the user equipment101. In case of multiple possible values of s, the particular LUT to be used can be signaled by the base station131using a dedicated signaling channel or it can be estimated at the user equipment101.

Step 2: The user equipment (UE)101either feeds back the value of ρ(ΔCSI) and ΔCSIor the value of θ, or both of them. The user equipment (UE)101also feeds back the pre-coding matrix index (PMI) or quantized channel state information (CSI) values depending on the mode of operation. It is worth noting that θ does not need to be communicated as often as the other indicators, and can have its own feedback period.

Step 3.a: The base station131receives and processes the channel state information (CSI) accuracy feedback as follows. Upon receiving ρ(ΔCSI) and ΔCSI, the base station131processes this value by taking into account the actual value {tilde over (Δ)}CSIof the CSI delay at the moment of the transmission where {tilde over (Δ)}CSI=ΔCSI+tTr−tCSIand where tTrstands for the moment of transmission and tCSIfor that of reception of the feedback from the user equipment101.

Step 3.b: In case of feedback of (ρ(ΔCSI),ΔCSI) from the user equipment101, the base station131uses the feedback values to update a dynamic LUT similar to its user equipment counterpart introduced in step 1.a by updating the ΔCSI→ρ(ΔCSI) entry as follows:

ρ⁡(ΔCSI,new)=α⁢h^t-ΔCSIH⁢h^th^t-ΔCSI⁢h^t+(1-α)⁢ρ⁡(ΔCSI,old).

Here, 0≤α≤1 is a predefined averaging coefficient. Next, the base station131gets the estimated CSI accuracy metric ρ({tilde over (Δ)}CSI) by accessing the entry corresponding to the delay {tilde over (Δ)}CSIfrom the abovementioned LUT or by using an interpolation method if this entry does not exist yet. It then applies a mapping to obtain the desired parameter θ, which can take the form θ=fs(ρ(ΔCSI), snr, NTx, NRx) and which can be stored as a set of LUTs corresponding to different values of the scenario index s. As already mentioned in step 1.a. In case of explicit feedback of δ by the user equipment101, the previous step can be skipped. Next, the base station131produces a set of weighting scalars {θb}b=1 . . . Band a mapping between said set of scalars and the resource units R available to the user. This plurality of weight scalars can be derived from θ by adding/subtracting to/from the reported value perturbations taken from a predefined set of values to compensate, for example, any potential imprecision produced while computing θ.

Step 4.a: The base station131enters the transmission phase, where the base station131transmits available data using an adaptive balanced MIMO beamforming and diversity scheme parameterized with on the different resource units with the scalars taken from {θb}b=1 . . . B. One example of such a transmission scheme is ST(F)BC precoded with a matrix whose columns: when used on resource unit b∈{1, . . . , B}, are scaled in accordance with the weight θbdetermined as explained above.

Step 4.b: If applicable e.g., in case precoder granularity is a resource element (RE) or a resource block (RB), the base station131signals the weighting scalars pattern {θb}b=1 . . . Bto the user equipment101using a dedicated downlink control message. Indeed, in such cases, these weights are needed at the user equipment to perform joint effective channel estimation across neighboring resource units.

Step 5: Upon reception of the downlink data transmission, the user equipment101can estimate the effective channels using precoded pilots and the signaled weight pattern if applicable and apply the appropriate MIMO receiver using the estimated effective channels.

Embodiments of the application offer in particular the following advantage: the possibility of achieving better reliability performance, e.g., lower block error rate values, compared to state-of-the-art MIMO solutions while the same cell-specific pilot overhead i.e., the same CS accuracy level is required. Indeed, tuning the balance between MIMO beamforming and MIMO diversity as enabled by the embodiments of the application thanks to the proposed CSI accuracy metric can only result, in principle, in a better reliability performances compared to existing solutions while achieving, at least, the same throughput performance.

Another advantage provided by embodiments of the application is the possibility of increasing the system throughput compared to existing solutions for MIMO transmission. Indeed, using the best balance between MIMO beamforming and MIMO diversity allows to achieve the same target reliability performance as state-of-the-art solutions while requiring a lower periodicity for transmitting cell specific pilots e.g., CSI-RS. The overhead associated with cell specific pilots is thus reduced and the effective system throughput is increased.

In summary, embodiments of the application can compute channel state information reliability metrics at the user equipment101(UE) and feed the obtained channel state information reliability metrics to the base station131. The corresponding method comprises a step of estimating the correlation coefficient ρ(Δ) between different instances of the estimated channel vectors at the user equipment separated by possibly different values of CSI delays Δ by constantly updating a LUT corresponding to the Δ→ρ(Δ) mapping, and a step of feeding back the latest estimated correlation coefficient along with the value of the delay that was assumed while computing it.

Furthermore, embodiments of the application can process at the user equipment101the obtained channel state information reliability/accuracy metrics to produce, with the help of a LUT, a plurality of scalar weights to be used on a corresponding plurality of time-frequency resource units to accordingly tune an adaptive balanced MIMO beamforming and diversity transmission scheme, by using possibly instruction signaled by the base station131e.g., which LUT to use from a plurality of predefined LUTs. Embodiments of the application can also generate these instruction messages at the base station131.

Furthermore, embodiments of the application can feedback the obtained scalar weights from the user equipment101to the base station131and process the feedback channel state information reliability metrics from one or several user equipments. This processing at the base station131translates the reported values into a plurality of scalar weights to be used on a corresponding plurality of time-frequency resource units to accordingly tune an adaptive balanced MIMO beamforming/diversity transmission scheme. Finally, the determined scalar weights can be signaled to the intended user equipment101.

FIG.4shows a schematic diagram summarizing a procedure for determining an adaptive balanced scheme of MIMO beamforming and ST(F)BC diversity with explicit CSI reliability feedback according to an embodiment. The procedure shown inFIG.4comprises the following steps at the user equipment101and the base station131respectively:

At the user equipment101, the user equipment101estimates vector channel from cell specific downlink pilot (step401).

The user equipment101estimates CSI correlation coefficients between ΔCSI-separated instances of the estimated channel (step402) and feeds the values to the base station131(step405).

If applicable, the user equipment101receives mapping of weights to resource blocks from the base station131(step403) and performs joint effective channel estimation across the resource blocks using the received weight mapping (step404).

The user equipment101applies the appropriate MIMO receiver using the estimated effective channels (step407).

At the base station131, the base station131receives the CSI correlation coefficients from the user equipment101(step411).

The base station131derives a plurality of weights θ and scales the columns of the precoding matrices on different resource blocks with the plurality of weights (step413).

The base station131precodes data transmitted on different resource blocks with the above mentioned weighted precoding matrices and signals the weights being used or the index of their pattern (step415).

According to this embodiment, the CSI reliability feedback along with the value of {tilde over (Δ)}CSI=ΔCSI+tTr−tCSIare used to derive a plurality of weights

{θb}b=1⁢…⁢B⁢(1T≤θb≤1for some integer T≥2) through the mapping δ=fs(ρ(ΔCSI), snr, NTx, NRx) to be used on B RBs as follows.

On the b-th RB, matrix Wb(θb) is used to precode a T×T ST(F)BC matrixcbe.g., an Alamouti code matrix with T=2, to produce cb=Wbcb,where
Wb(θb)={tilde over (W)}D(θb),

W~=[g^1⁢g^2⁢…⁢g^T],D⁡(θb)=diag⁢(θb,1-θbT-1,…,1-θbT-1).Here, ĝ1is the precoder vector dictated by the latest (delayed) CSI feedback and ĝ2, . . . , ĝTare T−1 precoding vectors that are orthogonal to ĝ1. Note that the power allocation to the different precoding vectors i.e., the scaling of vectorsg1on the one hand andg2, . . . , ĝTon the other, is dictated by the value of θbwhich, in turn, is dictated by the CSI reliability feedback. Consider for example the mapping

θ=fs(ρ⁡(Δ~CSI),snr,NTx,NRx)=1-T-1μ⁡(ρ⁡(Δ~CSI),snr,NTx,NRx)+T-1α⁢snr⁢(withμ⁢(ρ⁡(Δ~CSI),snr,NTx,NRx)=snr⁡(α⁡(T+α⁢η)⁢(2⁢M-1)+❘"\[LeftBracketingBar]"ρ❘"\[RightBracketingBar]"2⁢NTx⁢NRx+2⁢α⁡(T+α⁢snr)⁢(2⁢T-1)⁢❘"\[LeftBracketingBar]"ρ❘"\[RightBracketingBar]"2⁢NTx⁢NRx+❘"\[LeftBracketingBar]"ρ❘"\[RightBracketingBar]"4⁢(NTx⁢NRx)2+α2(T+α⁢snr)2)4⁢(T+α⁢snr)2andα=1-❘"\[LeftBracketingBar]"ρ⁡(Δ~CSI)❘"\[RightBracketingBar]"2).

If the feedback reliability value is high (corresponding to a CSI feedback that is relatively accurate), the above-mentioned mapping will typically produce values of θbthat are close to one i.e., most of the transmit power goes to the precoder vector ĝ1corresponding to the (relatively accurate) CSI feedback. However, if the CSI reliability value is low (corresponding to a CSI feedback that is relatively outdated), the mapping will typically produce values of θbthat are close to

1Ti.e., the transmit power is equally divided among the T precoder vectors as there is no preferred one among them due to the low reliability of the CSI feedback.

FIG.5shows a schematic diagram illustrating an embodiment of a communication system500comprising a transmitter such as a base station131and a receiver such as a user equipment101for the pre-coded data transmission, assuming a transmission taking place on B RBs using beamforming and ST(F)BC with precoding-diversity weighting factors {θ1, θ2, . . . , θB} that have been determined as explained above inFIG.4.

FIG.6shows a schematic diagram summarizing a procedure for an adaptive balanced scheme of MIMO beamforming and ST(F)BC diversity with implicit CSI reliability feedback according to an embodiment. The procedure shown inFIG.6comprises the following steps at the user equipment101and the base station131respectively.

At the user equipment101, the user equipment101estimates vector downlink channel from cell specific reference signals (step601).

The user equipment101estimates correlation coefficients between ΔCSI-separated instances of the estimated channel (step603)

The user equipment101obtains the θ parameter by applying a mapping to the estimated correlation coefficients (step605).

The θ parameter is used at the user equipment101to scale the columns of the precoding matrix that is fed back to the base station131after quantizing it using some predefined codebook (step607).

At the base station131, the base station131receives the precoding matrix sent from the user equipment101(step609) and uses it as columns of the precoding matrices (step611).

The base station131then pre-codes data transmitted on different resource blocks with the weighted precoding matrices (step613).

This embodiment resembles the previous one except that the θparameter

(1T≤θ≤1for some integer T≥2) computed at the user equipment101is not explicitly fed back using a dedicated feedback channel. Instead, this parameter is used at the user equipment101to scale the columns of the precoding matrix that will be fed back to the base station131(after quantizing it using some predefined codebook) and which will be used at the base station131to pre-code the ST(F)BC codewords before their transmission. This pre-coded data transmission can be performed by the communication network as inFIG.5.

FIG.7shows a schematic diagram summarizing a procedure for determining an adaptive balanced scheme of MIMO beamforming and precoder cycling diversity with explicit CSI reliability feedback according to an embodiment. The procedure shown inFIG.7comprises the following steps at the user equipment101and the base station131respectively:

At the user equipment101, the user equipment101estimates downlink channel from cell specific pilots (step701).

The user equipment101estimates correlation coefficients between ΔCSI-separated instances of the estimated channel (step703) and feeds the values to the base station131(step705).

The user equipment101can also feed the precoder vector ĝ1and possibly other precoding vectors that are orthogonal to ĝ1 (step707).

At the base station131, the base station receives estimated correlation coefficients and precoder vectors from the user equipment101(step709).

The base station obtains the θ parameter from (ΔCSI), ΔCSIand ΔCSIusing stored mapping. (step711).

The base station131establishes a mapping from {ĝ1, ĝ2, . . . , ĝT} to transmission resource elements or resource blocks so that ĝ1is used on θ% of them and

g^t(2≤t≤T)⁢on⁢(1-θ)T-1⁢%of them (step713).

The base station131then pre-codes data transmitted on different resource blocks with the above resource-to-precoder mapping and signals the weights being used or the index of their pattern (step715).

In this embodiment, a percentage equal to

θ(1T≤θ≤1for some integer T≥2) of the resource elements (REs), resource blocks (RBs) or resource block groups (RBGs) out of the total time-frequency resources available for the MIMO transmission is done using a beam/precoder based on the reported (delayed) CSI and a percentage equal to 1−θ of these resources is done using beams/precoders belonging to a subset of cardinality T−1 selected from a predefined MIMO transmission codebook and which are orthogonal/quasi-orthogonal to the beam/precoder based on the reported CSI. As in the two above-mentioned embodiments, parameter θ is computed (seeFIG.7) as a function of ρ({tilde over (Δ)}CSI) which is itself a function of the actual CSI delay {tilde over (Δ)}CSIat the moment of transmission and of the CSI reliability metric ρ(ΔCSI) measured assuming a CSI delay value ΔCSIthat could be different from {tilde over (Δ)}CSI. An example of such a function is θ=(1−1/T)ρ({tilde over (Δ)}CSI)+1/T.

It is worth noting that this mapping results in θ=1/T, i.e., full randomization and maximum diversity, when the reliability metric is the smallest i.e., ρ({tilde over (Δ)}CSI)=0. However, when the reliability metric is at its highest possible value i.e., ρ({tilde over (Δ)}CSI)=1, then the mapping results in θ=1 i.e., no randomization as only the beam/precoder based on the reported CSI is used for MIMO transmission on the considered time-frequency resources.

FIG.8shows a schematic diagram illustrating an embodiment of a communication system800comprising a transmitter such as a base station131and a receiver such as a user equipment101for the pre-coded data transmission, assuming a transmission taking place on B RBs using beamforming/precoder cycling with precoding-diversity weighting factor θ that has been determined as inFIG.7. The indexes shown inFIG.8are selected such that |{b∈{1, . . . , B}|ib(θ)=1}|≈θ, where notation |ε| stands for the cardinality of the set ε.

FIG.9shows a schematic diagram illustrating a method900of operating a user equipment101for wireless communications with a base station131over a wireless communications channel.

The method900comprises the following steps performed by the user equipment101: receiving901a current pilot signal over the wireless communications channel from the base station131; determining903a current channel response estimate on the basis of the current pilot signal; determining905a current channel state information, CSI, on the basis of current channel response estimate: and determining907a CSI reliability measure value on the basis of the current channel estimate or the latest available CSI and a previous CSI, wherein the previous CSI is determined on the basis of a previous pilot signal, i.e. previously received pilot signal.

FIG.10shows a schematic diagram illustrating a method1000of operating a MIMO base station131for wireless communications with a user equipment101over a wireless communications channel.

The method1000comprises the following steps performed by the MIMO base station131: sending1001one or more pilot signals to the user equipment101; receiving1003feedback from the user equipment101, including or based on a channel state information, CSI, reliability measure value, wherein the CSI reliability measure value is based on a current CSI and a previous CSI determined by the user equipment on the basis of a respective pilot signal; adaptively adjusting1005a MIMO transmission scheme for data to be transmitted to the user equipment101on the basis of the feedback from the user equipment101; and applying1007the adaptively adjusted MIMO transmission scheme to the data and to some of the reference signals to be transmitted to the user equipment101.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include”, “have”, “with”, or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise”. Also, the terms “exemplary”, “for example” and “e.g.” are merely meant as an example, rather than the best or optimal. The terms “coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the application beyond those described herein. While the present application has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present application. It is therefore to be understood that within the scope of the appended claims and their equivalents, the application may be practiced otherwise than as specifically described herein.