Estimation and feedback of CSI reliability information Field of the invention The present invention relates to an apparatus, method and program for estimation and feedback of channel state information (CSI) reliability information.

The field of invention are mobile radio system concepts like 3GPP (3 ^rd Generation Partnership Project) LTE (Long Term Evolution), LTE-A (LTE-Advanced) and its evolution and especially future optimizations for CoMP (Cooperative Multipoint Transmission).

Background of the invention

In the project Artist4G (Advanced Radio Interface Technologies for 4G Systems) advanced CoMP is being investigated for downlink transmission and especially a novel inter- ference mitigation scheme has been developed based on joint transmission (JT) CoMP.

For practical JT CoMP systems, it is advisable to use robust precoders, which adapt their precoding strategy to the reliability of the channel state information.

JT CoMP requires as accurate as possible channel state information (CSI) for proper precoding. At the same time there are large differences between the achievable accuracy levels for channel components (CC) of different UEs as well as per UE (user equipment). The reason for the different accuracy levels between UEs is due to different noise levels or signal to noise ratios (SNR), different UE noise figures, different channel estimation algorithms etc. For example indoor UEs have a high likelihood to experience quite low SNRs due to the outdoor to indoor penetration loss. A further important differentiator is the residual interference floor level for the CSI reference signals, which can be optimized by suitable muting patterns and allocation of CSI RSs (reference signals) to eNBs as well as antenna ports (AP). Optimum channel estimation algorithms might exploit channel correlations in time and frequency providing more or less a so called interpolation gain (IPG), which leads to an x dB lower normalized mean square error (NMSE) for the estimated CSI compared to that on the received CSI RSs resource elements (RE). Different UEs might exploit more or less of the feasible interpolation gain.

The reason for the per UE variation of the CSI accuracy levels is due to different SINR (Signal to Interference plus Noise Ratio) conditions for strong and weak channel components (lower Rx power for weak CCs) and further on the different behavior of different channel components as such. For once it is well known that physical resource blocks (PRB) in deep notches are much more volatile than those PRBs with relative large Rx power. Additionally the variability of CCs depends on the UE mobility, which is mainly a differentiator between different UEs. But, in case of different relevant moving objects for different CCs there will be also a varying variance between the CCs of a single UE. In combination with the inevitably needed channel prediction these different variance levels will be turned into different reliability values.

It is noted that the most simple channel prediction assumes no time delay at all and 'predicts' no further channel variation. That is exactly done for the conventional CSI reporting e.g. in form of PMI (pre-coding matrix indicator) values for current LTE systems. In case of more advanced prediction techniques accordingly higher reliabilities will be achieved for the same prediction horizon of e.g. 5 or 10ms. If the prediction is done at UE side it will affect accordingly the reliability of reported CSI values.

The reporting as such together with the probably inevitable feedback compression might result in further reliability differences between different CCs, which should be taken into account as well.

As already mentioned above, for practical JT CoMP systems the reliability information is of highest importance. In case of the assumption of ideal CSI all channel components have the same infinite reliability leading to simple precoding schemes like zero forcing (ZF) or minimum mean square error (MMSE) like precoding. In reality, robust precoders have to ensure that unreliable channel components do not spoil the whole cooperation area. For a given reliability information, the optimum robust precoder will extract the maximum possible performance from the cooperation area.

Fig. 1 illustrates the Rx matrix R for a cooperation area serving 26 UEs from 36 overall antenna ports by simple ZF like precoding R = H'W, with W = pinv(H) and H' = H + ΔΗ. ΔΗ is the estimation error for the channel matrix leading to the off diagonal elements due to inter UE interference leakage. A robust precoder will optimize these off diagonal elements so that statistically the best overall performance - for example highest overall spec- tral efficiency - is achieved for the given reliability values for the different CCs. In contrast, a zero forcing precoder would assume the reported CSI values to be fully correct, resulting under ideal conditions in highest performance, but for realistic channels in typically much larger error terms.

Currently, different kind of measurements with according reporting like the reference signal received power (RSRP) and reference signal received quality (RSRQ) values are known in 3GPP LTE. RSRQ includes all interference from the whole network while RSRP is the received power for the common reference signals. Both are for wideband measurements. For the purpose of the present invention, they are not usable as CSI RSs needs a frequency selective reliability information. In addition only one aspect - i.e. the noise and overall interference floor - is taken into account, while the overall reliability will include many other aspects, as mentioned above, like prediction quality, to name only one.

In document [1 ], the importance of the reliability information is derived and an implementa- tion based on Kalman filtering is being proposed. Kalman filters derive automatically some form of reliability for the different channel components, which could be directly reported. At the same time this implementation in the UE is very specific and probably difficult to agree on in standardization.

Therefore, a more general reliability feedback is desirable. Reference:

[1] D. Aronsson, "Channel estimation and prediction for MIMO OFDM systems - Key design and performance aspects of Kalman-based algorithms", Ph.D Thesis, Dept. of Engineering Sciences, Signals and Systems Group, Uppsala University, March

201 1.

Summary of the Invention

It is therefore an object of the present invention to find a suitable estimation as well as reporting scheme for the channel component individual reliability information. This is especially important in case of a large variation of the reliability between different channel components.

According to the present invention, there are provided apparatuses, methods and a program for estimation and feedback of channel state information (CSI) reliability information.

According to an aspect of the present invention there is provided a method comprising: estimating, at a user equipment, channel state information based on channel state information reference signals, obtaining processing capabilities of the user equipment at which the channel state information is estimated, calculating reliability information of the channel state information based on the estimated channel state information and the obtained processing capabilities.

According to further refinements of the invention as defined under the above aspect

- the channel state information comprises at least one of received signal power, channel state information reference signal interference power, signal to interference plus noise ratio, user equipment noise floor, coherence time, coherence bandwidth and interpolation gain;

- estimating the channel state information comprises measuring a received power of a certain channel component based on a specific channel state information reference signal, switching off the specific channel state information reference signal for the certain channel component, measuring inter channel state information reference signal interference power for the specific channel state information reference signal, and calculating a signal to interference plus noise ratio for the specific channel state information reference signal based on the received power and the inter channel state information reference signal interference power; - the reliability information is generated by forming a normalized mean square error value per channel component based on the channel state information.

- the method further comprises reporting the channel state information to a base station;

- the method further comprises reporting the reliability information to a base station;

- the reporting is done over a physical uplink control channel.

According to another aspect of the present invention there is provided a method comprising:

receiving channel state information from a user equipment, calculating reliability information of the channel state information.

According to further refinements of the invention as defined under the above aspect, the channel state information comprises at least one of received signal power, channel state information reference signal interference power, signal to interference plus noise ratio, user equipment noise floor, coherence time, coherence bandwidth and interpolation gain.

According to another aspect of the present invention there is provided an apparatus comprising:

a receiver/transmitter configured to communicate with at least another apparatus, a memory configured to store computer program code, and a processor configured to cause the apparatus to perform: estimating channel state information based on channel state information reference signals, obtaining processing capabilities of the user equipment at which the channel state information is estimated, calculating reliability information of the channel state information based on the estimated channel state information and the obtained processing capabilities.

According to further refinements of the invention as defined under the above aspect - the channel state information comprises at least one of received signal power, channel state information reference signal interference power, signal to interference plus noise ratio, user equipment noise floor, coherence time, coherence bandwidth and interpolation gain;

- estimating the channel state information comprises measuring a received power of a certain channel component based on a specific channel state information reference signal, switching off the specific channel state information reference signal for the certain channel component, measuring inter channel state information reference signal interference power for the specific channel state information reference signal, and calculating a signal to interference plus noise ratio for the specific channel state information reference signal based on the received power and the inter channel state information reference signal interference power;

- the reliability information is generated by forming a normalized mean square error value per channel component based on the channel state information.

- the processor is further configured to cause the apparatus to perform reporting the channel state information to a base station;

- the processor is further configured to cause the apparatus to perform reporting the reliability information to a base station; - the reporting is done over a physical uplink control channel.

According to another aspect of the present invention there is provided an apparatus comprising a receiver/transmitter configured to communicate with at least another apparatus, a memory configured to store computer program code, and a processor configured to cause the apparatus to perform: receiving channel state information from a user equipment, calculating reliability information of the channel state information.

According to further refinements of the invention as defined under the above aspect, the channel state information comprises at least one of received signal power, channel state information reference signal interference power, signal to interference plus noise ratio, user equipment noise floor, coherence time, coherence bandwidth and interpolation gain.

According to another aspect of the present invention there is provided a computer program product comprising code means adapted to produce steps of any of the methods as described above when loaded into the memory of a computer.

According to a still further aspect of the invention there is provided a computer program product as defined above, wherein the computer program product comprises a computer- readable medium on which the software code portions are stored.

According to a still further aspect of the invention there is provided a computer program product as defined above, wherein the program is directly loadable into an internal memory of the processing device. Brief Description of the Drawings

These and other objects, features, details and advantages will become more fully appar- ent from the following detailed description of embodiments of the present invention which is to be taken in conjunction with the appended drawings, in which:

Fig. 1 is a diagram illustrating the outcome of a zero forcing (ZF) precoder for a cooperation area serving 26 user equipments.

Fig. 2 is a diagram illustrating an allocation of channel state information reference signals to cells of a cooperation area.

Fig. 3 is a diagram illustrating some aspects influencing the reliability of channel state in- formation.

Fig. 4 is a flowchart illustrating a method according to certain embodiments of the present invention.

Fig. 5 is a flowchart illustrating another method according to certain embodiments of the present invention.

Fig. 6 is a block diagram showing an example of an apparatus according to certain embodiments of the present invention.

Detailed Description

In the following, aspects/embodiments of the present invention are described by referring to general and specific examples of the aspects/embodiments, wherein the features of the aspects/embodiments can be freely combined with each other unless otherwise described. It is to be understood, however, that the description is given by way of example only, and that the described aspects/embodiments are by no means to be understood as limiting the present invention thereto.

The goal of the present invention is to provide reliability information to the eNBs (enhanced NodeB) required for robust precoding.

Thus, according to an embodiment of the present invention, it is proposed to define suitable measurement procedures for estimation of all the relevant parameters defining the reliability, as described above, per channel component and per PRB together with an ac- cording reporting scheme. As the reliability is a quite stable parameter, even per PRB reporting for a relative high number of channel components is feasible with only low to moderate overhead.

Fig. 3 illustrates an example of some aspects influencing the achievable reliability of a CSI, like SINR, coherence time, coherence bandwidth, interpolation gain (IPG), UE processing capabilities (algorithms), etc.

The basis of the reliability estimation is the estimation of the wideband SINR for a given channel component. This might be done in a similar manner as for the RSRQ values, i.e. in a first step the receive power of a certain channel component is being measured and in a second step the CSI RS for this specific channel component is switched off to allow the measurement of the overall interference floor for this CSI RS. This muting of certain channel components should be done sequentially in a harmonized manner to fully capture all relevant interferers on these resources.

Fig. 2 illustrates a potential cell specific CSI RS and muting pattern as being developed in the Artist4G project. In particular, Fig. 2 illustrated the allocation of CSI RS (different beam types are indicated by solid lines, dotted lines and dashed lines in Fig. 2) to cells of a cooperation area (CA) and muting patterns (hatchings of the circles). Inter CSI RS interfer- ence occurs only for the same muting pattern plus same beam types, as indicated by the bold hatched arrow. Each cooperation area (indicated by black triangles covering 3 sites each) uses a set of mutual orthogonal CSI RSs. These CSI RSs use specific muting patterns so that in each cooperation area only one site is active. The muting patterns are further harmonized between cooperation areas to minimize inter CSI RS interference with the goal to maximize channel estimation quality. In this scheme interfering cells using at a certain time instant the same CSI RSs are at least 2 inter site distances (ISD) apart from each other.

Estimation of the residual inter CSI RS interference requires a specific measurement phase. For that purpose one CA after the other switches off its own CSI RSs to allow the UEs of the other CAs a measurement of the interference power from all other active CSI RSs. This measurement will include inherently the noise figure of the according UEs, which might be higher or below the inter CSI RS interference floor. It will provide the first important parameter for the reliability, i.e. the mean SINR for the CSI RSs (see Fig. 3). To get a statistically meaningful result, preferably, the measurements may be repeated over several times, but as these measurements are seldom, the overhead will be small nonetheless. The measurement phases will be decided and controlled by the network and reported from the eNBs to the UEs.

Different channel components use different CSI RSs with accordingly different muting pat- terns and in the end different interference floor levels. So for estimation of the SINRs of the different channel components, the UEs have to measure all Rx powers from the own CA and to combine it with the appropriate interference floor levels (i.e. the residual mutual inter CSI RS interference), i.e. the UEs have to store internally a set of relevant interference floor and signal powers.

The achievable interpolation gain (IPG) of the channel estimation algorithm depends on the UE algorithm, but is also restricted by the coherence bandwidth as well as the coherence time of the radio channels. These values might vary significantly for different channel components. The coherence bandwidth and times have to be estimated from previous observations. The UEs have to combine all the available information like SINR, IPG, time and frequency coherence and potentially further inputs to generate a predefined reliability value. For example all effects might be combined to form artificial normalized mean square error (NMSE) values per channel component. Additionally for a predefined prediction time (e.g. for 5 or 10ms) a ANMSE might be reported or alternatively the time up to which the NMSE has been degraded by less than x dB, which may be a predetermined value.

In case the UEs report the frequency selective channels explicitly, then the eNBs will be able to detect frequency notches and estimate the according coherence time per PRB by themselves. Otherwise, for a pure implicit reporting scheme, the UEs have to provide this important information as well, i.e. instead of a wideband reliability a PRB individual reliability has to be reported.

In summary, according to the present invention all means to estimate the reliability infor- mation for all channel components (signal power, mutual CSI RS interference power, UE noise floor, coherence time, coherence bandwidth, interpolation gain IPG, ...) are provided, based on CSI RS measurements and taking UE processing capabilities into account. Further, the per PRB reliability is calculated, which is, for example, very low in frequency notches. Further, a common reliability measure is defined as well as an according scheme for reporting of the reliability information, which defines for example, how often, for what bandwidth, for which CCs, in which order, etc. the information is reported.

Instead of the definition of a specific reliability measure, the UEs could report the estimations of relevant parameters like SINR, coherence time, coherence bandwidth etc. directly over the physical uplink control channel (PUCCH). The eNBs are then able to do the calculation of the reliability information per CC and per PRB according to their needs.

The benefit of the additional effort to estimate and report semi statically valid reliability values is a significantly increased robustness of the cooperation wide joint transmission precoding. Without such reliability information there is a great risk that unreliable channel components destroy the precoding of the whole cooperation area and that the performance with cooperation falls below that of conventional single cell transmission. A robust precoder ensures that always the maximum possible performance will be achieved given a certain channel knowledge.

For implementation, part of the reliability information might be estimated at eNB side and part of it might be provided from the UEs. For example an eNB having received the CSI for a frequency selective radio channel might combine the reported wideband reliability value with that derived from the relative power (e.g. for notches low prediction reliability can be assumed). Or the time dependent reliability for different prediction horizons can be estimated directly at the eNB, given that the UE has reported the general behavior of its channel predictor.

One natural candidate for reporting of reliability information is under the assumption of a Gaussian distribution the NMSE or similarly the variance σ of a channel component on a certain PRB. In case the error term has a different - or especially a significantly different - power density function (PDF) it might be needed to report the shape of the probability density function of the channel estimation errors as well. The other option - as already mentioned - is to report the variance levels from SINRs etc. while potential non Gaussian effects like the evolution of low power notches might be estimated directly at the eNB. The optimum way to calculate the reliability depends on Bayesian rules of logic and the maximum entropy principle on the cause of the unreliability. For additive white Gaussian noise or an interference floor it is a Gaussian distribution so that the mean value and the variance would have to be estimated and reported. Prediction errors for example behave differently and will probably need a different handling depending on the used algorithms for prediction.

As a most simple scheme one might consider reporting of only one or two bits indicating two or four reliability values per PRB or per PRB group like e.g.:

O'=unreliable and T=reliable or

0, 1 , 2, 3 relates to reliability R=0.25, 0.5, 0.75 and 1 .

Accuracy and granularity requirements for the reliability R regarding achievable robustness are for further study. The same is true for the reporting rate of R, which is expected in the order of seconds to hundreds of milliseconds. Fig. 4 is a flowchart illustrating processing of the apparatus according to certain embodiments of the present invention. In the present example, the steps as shown in Fig. 4 are executed by a user equipment.

According to the example of the present invention, first, in a step S41 , the apparatus, i.e. the user equipment, estimates channel state information based on channel state information reference signals. Then, in a step S42, the user equipment obtains processing capabilities of the user equipment at which the channel state information is estimated, and calculates, in a step S43, reliability information of the channel state information based on the estimated channel state information and the obtained processing capabilities.

The channel state information comprises at least one of received signal power, channel state information reference signal interference power, signal to interference plus noise ratio, user equipment noise floor, coherence time, coherence bandwidth and interpolation gain.

Further, when estimating the channel state information, the user equipment measures a received power of a certain channel component based on a specific channel state information reference signal, switches off the specific channel state information reference sig- nal for the certain channel component, measures inter channel state information reference signal interference power for the specific channel state information reference signal, and calculates a signal to interference plus noise ratio for the specific channel state information reference signal based on the received power and the inter channel state information reference signal interference power.

The reliability information may be generated by forming a normalized mean square error value per channel component based on the channel state information. The user equipment may further report the channel state information to a base station, or may report the reliability information to a base station. The reporting may be done over a physical uplink control channel.

Fig. 5 is a flowchart illustrating another processing of an apparatus according to certain embodiments of the present invention. In the present example, the steps as shown in Fig. 5 are executed by a base station, like e.g. an eNodeB.

According to the example of the present invention, first, in a step S51 , the apparatus, i.e. the base station receives channel state information from a user equipment in a step S51 , and then, in a step S52, calculates reliability information of the channel state information.

Fig. 6 is a block diagram showing an example of an apparatus according to certain embodiments of the present invention. The apparatus may be a user equipment or a base station.

As shown in Fig. 6, according to an embodiment of the present invention, the apparatus 60, i.e. the user equipment or the base station, comprises a receiver/transmitter 61 , a memory 62 and a processor 63. The receiver/transmitter 61 configured to communicate with at least another apparatus in the network and to transmit and receive signals, and the memory 62 is configured to store computer program code.

According to an example of the present invention, the apparatus is a user equipment and the processor 63 is configured to cause the apparatus to perform estimating channel state information based on channel state information reference signals, obtaining processing capabilities of the user equipment at which the channel state information is estimated, calculating reliability information of the channel state information based on the estimated channel state information and the obtained processing capabilities. According to another example, the apparatus is a base station and the processor 63 is configured to perform receiving channel state information from a user equipment, and calculating reliability information of the channel state information.

In the foregoing exemplary description of the apparatus, i.e. the user equipment or the base station, only the units that are relevant for understanding the principles of the invention have been described using functional blocks. The apparatus may comprise further units that are necessary for its respective operation. However, a description of these units is omitted in this specification. The arrangement of the functional blocks of the apparatus is not construed to limit the invention, and the functions may be performed by one block or further split into sub-blocks.

When in the foregoing description it is stated that the apparatus, i.e. the user equipment or the base station (or some other means) is configured to perform some function, this is to be construed to be equivalent to a description stating that a (i.e. at least one) processor or corresponding circuitry, potentially in cooperation with computer program code stored in the memory of the respective apparatus, is configured to cause the apparatus to perform at least the thus mentioned function. Also, such function is to be construed to be equivalent^ implementable by specifically configured circuitry or means for performing the re- spective function (i.e. the expression "unit configured to" is construed to be equivalent to an expression such as "means for").

For the purpose of the present invention as described herein above, it should be noted that

- method steps likely to be implemented as software code portions and being run using a processor at an apparatus (as examples of devices, apparatuses and/or modules thereof, or as examples of entities including apparatuses and/or modules therefore), are software code independent and can be specified using any known or future developed programming language as long as the functionality defined by the method steps is preserved;

- generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the embodiments and its modification in terms of the functionality implemented;

- method steps and/or devices, units or means likely to be implemented as hardware components at the above-defined apparatuses, or any module(s) thereof, (e.g., devices carrying out the functions of the apparatuses according to the embodiments as described above) are hardware independent and can be implemented using any known or future developed hardware technology or any hybrids of these, such as MOS (Metal Oxide Semiconductor), CMOS (Complementary MOS), BiMOS (Bipolar MOS), BiCMOS (Bipolar CMOS), ECL (Emitter Coupled Logic), TTL (Transistor-Transistor Logic), etc., using for example ASIC (Application Specific IC (Integrated Circuit)) components, FPGA (Field- programmable Gate Arrays) components, CPLD (Complex Programmable Logic Device) components or DSP (Digital Signal Processor) components;

- devices, units or means (e.g. the above-defined apparatuses, or any one of their respective units/means) can be implemented as individual devices, units or means, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device, unit or means is preserved;

- an apparatus may be represented by a semiconductor chip, a chipset, or a (hardware) module comprising such chip or chipset; this, however, does not exclude the possibility that a functionality of an apparatus or module, instead of being hardware implemented, be implemented as software in a (software) module such as a computer program or a computer program product comprising executable software code portions for execution/being run on a processor;

- a device may be regarded as an apparatus or as an assembly of more than one apparatus, whether functionally in cooperation with each other or functionally independently of each other but in a same device housing, for example.

In general, it is to be noted that respective functional blocks or elements according to above-described aspects can be implemented by any known means, either in hardware and/or software, respectively, if it is only adapted to perform the described functions of the respective parts. The mentioned method steps can be realized in individual functional blocks or by individual devices, or one or more of the method steps can be realized in a single functional block or by a single device.

Generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the present invention. Devices and means can be implemented as individual devices, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device is preserved. Such and similar principles are to be considered as known to a skilled person. Software in the sense of the present description comprises software code as such comprising code means or portions or a computer program or a computer program product for performing the respective functions, as well as software (or a computer program or a computer program product) embodied on a tangible medium such as a computer-readable (storage) medium having stored thereon a respective data structure or code means/portions or embodied in a signal or in a chip, potentially during processing thereof.

It is noted that the embodiments and general and specific examples described above are provided for illustrative purposes only and are in no way intended that the present invention is restricted thereto. Rather, it is the intention that all variations and modifications which fall within the scope of the appended claims are covered.