GB2507782A - Three dimensional beamforming in mobile networks - Google Patents

Abstract

Three dimensional beamforming in mobile networks, comprising, on a network side: configuring at least one channel state information reference signal (CSI-RS) resource (S21), transmitting at least one CSI-RS using said at least one CSI-RS resource (S22), receiving at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator (CQI) indicative of a radio channel state, each respectively associated with said at least one CSI-RS (S23), and determining a final precoder for 3D beamforming based on said at least one matrix indicator (S24); and, on a terminal side: receiving the at least one CSI-RS (S31), selecting the at least one precoder from a precoder codebook (S32) and determining the at least one CQI (S33) based on measurements on receiving the CSI-RS, and transmitting the at least one matrix indicator (S34).

Description

METHODS AND APPARATUS FOR WIRELESS TRANSMTSSTON

Technical Field

The present invention relates to methods and apparatus for wireless S transmission. The present invention relates generally to 3D beamforming in mobile network scenarios. More specifically, the present invention exemplarily relates to measures (including methods, apparatus and computer program products) for realising 3D beamforming in mobile network scenarios.

Background

The following abbreviations which may be found in the specification and/or the drawing figures are defined as follows: 3GPP 3td Generation Partnership Project AAA active antenna array BS base station CoMP coordinated multipoint transmission and reception CSI channel state information CSI-RS channel state information reference signal CQI channel quality indicator eNB evolved NodeB ePDCCH enhanced physical downlink control channel FDD frequency division duplcx LoS line-of-sight LIE Long Ierm Evolution LIE-A Long Ierm Evolution Advanced MIMO multiple-input multiple-output MIU-MIMO multi-user multiple-input multiple-output PDSCH physical downlink shared chaiuiel PMI precoder matrix indicator PRB physical resource block RI rank indicator SYD singular-value decomposition Tx transmit UE user equipment UMaLoS urban macro line of sight

S

The present specification generally relates to beamforming technology in network deployments with respcct to wireless transmission between network entities.

Mobile data transmission and data services are constantly making progress.

With the increasing penetration of such services, a need for increased bandwidth and robustness for conveying the data is emerging.

A technique considering such demands is 3D beamforming. A decision to realise the same in 3' Generation Partnership Project (30PP) network deployments according to Rel-12 is pending. The 3D transmission beamforming is enabled since multiple antennas placed at a base station (BS) as e.g. an evolved NodeB (eNB) are traditionally transmitting in horizontal domain, while by mechanical tilt the array can point to different azimuths. Similarly in vertical domain, having multiple arrays and different mechanical tilts, one can achieve avery basic form of vertical sectorisation.

An enhanced 3D beamforming is a teclrnology brought by active antenna arrays (AAA) forming a grid (e.g. an N=2 times M=2 orthogonal grid as depicted in Figure 1) of antcnnas. Each of the antennas is implemented for example as cross-polarised antennas slants. The beamforming is then performed in the (traditional) horizontal domain as well as in the vertical domain.

Since a three dimensionally formed beam can be controlled in the horizontal as well as in the vertical domain, a thus generated beam is narrower and causes less interference to the other users, it is noted that an increased flash-light interference effect may occur on 3D beaniforming. i.e. the interference may be more strongly varying across subframes and physical resource block (PRB) pairs.

A narrower beam impacts positively also on the multi-user multiple-input multiple-output (MU-MIMO) scenario performance, as spatial separability is increased. In other words, when narrower beams are realised, a better spatial reuse S factor can be obtained.

In 3GPP Rel-lO work items, a channel state information reference signal (CSI-RS) resource has been introduced in order to allow estimation of channel for up to 8 transmit (Tx) antenna ports. In particular, by means of the CSI-RS, channel state feedback for up to eight transmit antenna ports can be obtained to assist the BS in precoding operations.

In 3GPP Rd-I 1 work items, it has been agreed that a mobile station such as e.g. a user equipment (UE) may be configured with a plurality of such CSI-RS resources in order to coordinate transmission (and assist precoding operations) from multiple transmission points. In particular, the BE may be configured to report feedback in the form of multiple CSI reports, each corresponding to one CSI-RS resource.

3D beamforming by means of active antenna arrays is implemented by setting respective antenna weights for each of the antennas forming the grid of an AAA.

In order to select antenna weights for forming the 3D (horizontal-vertical) beams towards the UE, some information is required at the base station about the radio channel.

While beamforming in the vertical domain can be performed by estimating the uplink angle of alTival at the fransmitter (i.e. the base station), in the horizontal domain the usage of codebooks has been shown to be more robust, especially in frequency division duplex (FDD) transmission systems.

Following this principle, new 3D codebooks could be designed, iii which case the UE would be required to select a precoding codeword (representing the antenna weights) from such a new codebook. Such a codebook would contain codewords representing the relevant 3D beamforming antenna weight combinations rather than S horizontal domain antenna weight combinations.

It is to be noted that the standardisation cffort to design a new codebook in 3GPP is high. Further, the increased size of such a 3D eodebook would imply an increase in complexity of selection of the codeword (precoder matrix indicator (PMI)) at the UE side. In addition, also the feedback size presumably increases.

A further proposed solution, estimation of a PMI in horizontal and vertical domain separately, is discussed in the following.

Published patent application CN102412885A and other proposals disclose a pair of CSI-RS resources that are configured such that one CSI-RS resource is configured for estimating the horizontal beam for the horizontal antenna array, and one CSI-RS resource is configured for estimating the vertical beam for vertical antenna array.

The UE derives a codeword separately for the horizontal and the vertical domain based on the channel measurements from the two configured CSI-RS resources. A final precoder W is then obtained as W = WH 0 Wv, where WH is the estimated precoder in honzontal and w, is the estimated precoder in vertical domain, and 0 denotes the known "Kroneckcr matrix product". The existing 2Tx, 4Tx and Tx 3OPP codebooks (for two, four and eight transmit antennas) are used to select the preferred PMI for estimated channel corresponding to each configured CSI-RS resource. Basically this approach assumes that the vertical tilt has no impact on the horizontal precoding weight and hence the vertical and horizontal prccoders can be chosen independently.

In practice however, as also shown below, the distribution of preferred precoding angles is dependent on several factors like e.g. terrain as well as LIE and eNB antenna height installation, and existing codebooks may therefore be improper.

Furthermore, the coffect supeiposition of horizontal and vertical beamforming phase offsets obeys the equation A = sin(O) cos(4,) + sin (4,), where U is an aziniuth angle and 4, is an elevation angle.

This equation shows that optimal horizontal precoder is dependent on the vertical tilt as well as on the horizontal tilt, which has not been taken into account when designing horizontal codebooks by the prior art techniques, where horizontal and vertical preeoders are estimated independently and final phase offset is then determined as A = sin(S) + sin (4i).

As stated above, to show the dependency of optimal horizontal precoder on the vertical tilt angle 4,, the inventors simulated closed loop 8x2,16x2 and 32x2 cross- polarised multiple-input multiple-output (MIMO) systems with fixed azimuthal line-of-sight (LoS) angle and 3D codebooks.

For example, the N=2 x M=2 eodeboolcs (according to a 2x2 antenna array) read i i [ 1 1 1 ii Cl = ei1t5i v Ii i I ii' sin8 cos Ø-I-jirsin 0 incorporating elevation in azimuthal direction (codebook Cl), and 1 [1 1 1 ii C2--1 j jJ' sin 9+jm sin ignoring elevation in azimuthal direction (codebook C2).

In both codebooks, the first tcim corresponds to bcamforming, the second term corresponds to coherent combination of polarisations, and ® denotes the icronecker matrix product.

The inventors simulated i03 realisations of single-carrier channel for each realisation of elevation LoS angle and fixed azimuth LoS angic U = and computed the average received power Rx* The results and relative losses are provided in the following table for M=2 vertical and N=2 horizontal x-pol (cross polariscd) pairs, showing the impact of elevation on azimuthal codebook design with N=2 horizontal cross polarised slants. In this table, UMaLoS denotes an urban macro line of sight model or radio channel typically having low angular spread of propagation due to a strong line of sight component, i.e. due to no or only few obstacles in the direct line between the transmitter and the recipient. It is derivable from this table that the dependency of the optimal horizontal precoder on the vertical tilt is noticeable starting from approximately 30 degree tilt, i.e. elevation angle. With height of the BS antenna of2lm and the UE antenna of 1.5rn, 30° (degree) and higher tilts colTespond to distances between the BS and the mobile station smaller than 34m.

-j. [O 0.0 5.7 11.5 17.2 22.9 28.7 34.4 40.1 -4 j4rad] 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 E[P], C1 6.07 5.98 5.93 6.03 5.98 5.97 5.86 5.92 UMa E[P], C2 6.07 5.98 5.93 6.01 5.94 5.86 5.65 5.51 LoS _________ _____ _____ _______ ______ ______ ____ ____ _____ Loss* I] 0.00 0.00 0.05 0.23 0.71 1.75 3.79 7.55 *the average singular-value decomposition (SVD) precoding power is 6.52 The case with M=2 vertical and N=4 horizontal x-pol pairs is displayed with the following table, showing the impact of elevation on azimuthal codebook design with N=4 horizontal cross polarised slants, Tn this case the impact of elevation on horizontal precoder is noticeable already from 17.2° of tilt, - [O 0.0 5.7 11.5 17.2 22.9 28.7 34.4 40.1 - IradI 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 E[PRX],Cl 11.65 11.49 tl,37 1.1.65 1168 tt,76 11,64 11,59 JjMa EIPRX1,C2 11.65 11.49 tl.35 11.53 11.30 10.78 9.70 8.27 I,oS __________ ______ _______ _______ ________ _______ ________ _______ ______ 1uss* [%] 0.00 -0.01 MS 0.97 3.40 9.18 20.00 40.15 *the average SVD precoding power is 12.85 The case with M=2 vertical and N=8 horizontal x-pol pairs is displayed with the following table, showing the impact of elevation on azimuthal codebook design with N=8 horizontal cross polarised slants. In this case the impact of elevation on horizontal precoder is noticeable already from 11.5° of tilt (elevation angle).

o - 1°1 0.0 5.7 11.5 17.2 22.9 28.7 34.4 40.1 - [rad] 0.00 0.10 0.29 0.30 0.40 0.50 (1.61) 0.70 E[PRXj,Cl 28.27 28.10 27.65 28.07 28.11 28.51 28.56 28.44 JIMa E[PRXj,Cz 28.27 28.09 27.37 26.67 24,01 19.33 12,46 5.59 LoS ________ _____ _____ ______ ______ ______ ______ ______ ______ Luss' [%l 0.00 0.05 1.03 5,24 17.07 47.46 129.27 408.50 *die average SYD precoding power is 32.30 The above simulations show that the beamforniing vectors standardised in current 3GPP codebooks arc not optimal anymore if vertical tilt is high enough. As a consequence, the beamforming vectors, forming a precoding codebook, shall be modified and shall depend on the vertical tilt angle 0.

Besides the above described proposal to determine an overall precoder by combining estimated horizontal and vertical precodci, which are estimated independently from each other from a respectively dedicated CSI-RS, further precoder construction possibilities are proposed, which are briefly discussed in the follo\ving.

S

It is possible to utilise uplink signals for estimating the angle of arrival. Tn this case, the base station (e.g. eNB) can determine the antenna weight of horizontally and vertically displaced antenna component directly based on the elevation and azimuth angle of arrival as = sin(S) cos() + sin ().

Further, it is proposed to design new codebooks that cover the whole 3D radio channel. In this case, the UE selee a codeword from the codebook and reports the selected codeword to the eNB, where the codeword represents the best precoder W (antenna weights) that would result in optimum performance from the UE's perspective. However, generation of new 3D beamforming codebooks requires inconvenient codeboolc standardisation processes and results in lack of backward compatibility with Rd-i 1 mobile devices.

In addition, the published patent application EP241 6603A I suggests assigning semi-static predefined/different tilt patterns to various subframes or frequency bands, carrier frequencies, etc. Based on feedback from a mobile station. the base station chooses which of those resources are assigned/scheduled to this mobile station. This scheme allows 3D beamforming, but implies scheduling restrictions, because the UE can be sehedulcd with preferred tilt only in the subfranies configured with that tilt according to a semi-static predetermined pattern.

Summary

According to a first aspect of the present invention, there is provided a method comprising configuring at least one channel state information rcferencc signal CST-RS resource, transmitting at least one CSI-RS using said at least one CST-RS resource, receiving at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and determining a final precoder for three-dimensional 3D beamfonning based on said at least one matrix indicator.

According to a second aspect of the present invention, there is provided a method comprising receiving at least one CSI-RS using at least one CSI-RS resource, selecting at least one precoder from a precoder codebook associated with said at least one CSI-RS based on measurements on said receiving, determining at least one S channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio channel state, and transmitting at Icast one matrix indicator according to said at least one precoder and said at Icast one channel quality indicator.

According to a third aspect of the present invention, there is provided an apparatus for use on a network side of a cellular system, the apparatus comprising a processing system arranged to cause the apparatus to perform configuring at lcast one CSI-RS resouce, transmitting at least one CSI-RS using said at least one CSI-RS resource, receiving at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and determining a final precoder for 3D beamforming based on said at least one matrix indicator.

According to a fourth aspect of the present invention, there is provided an apparatus for use on a terminal side of a cellular system, the apparatus comprising a processing system arranged to cause the apparatus to perform receiving at least one CSI-RS using at least onc CSI-RS rcsourcc, selecting at least one prceoder from a precoder codebook associated with said at least one CSI-RS based on measurements on said receiving, determining at least one channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio chaimcl state, transmitting at least one matrix indicator according to said at least one precoder and said at least one channel quality indicator.

The processing systems described above may comprise at least one processor, and at least one memory including computer program code, the at least one processor, with the at least one memory and the computer program code, being arranged to cause the apparatus to perform as described above.

There is also provided a computer program comprising computer-executable computer program code which, when the program is run on a computer (e.g. a computer of an apparatus according to any one of the aforementioned apparatus-related exemplary aspects of the present invention), is configured to cause the computer to carry out the method according to any one of the aforementioned method-related exemplary aspects of the present invention.

Such computer program may be stored on or in a computer program product which may comprise (or be embodied as) a (tangible) computer-readable (storage) medium or the like on which the computer-executable computer program code is stored, and/or the program may be directly loadable into an internal memory of the computer or a processor thereof Examples of some of the above aspects enable an efficient and accurate determination of overall (horizontal and vertical) precoders, increased robustness of transmissions, decreased interference with other transmissions, and bacl'ard compatibility with Rel-1 1 mobile devices, to thereby solve at least part of the problems and drawbacks identified in relation to the prior art.

By way of some examples of embodiments of the present invention, there is provided 3D beamforming in mobile network scenarios. More specifically, by way of some embodiments of the present invention, there are provided measures and mechanisms for realising 3D beamforming in mobile network scenarios.

Thus, improvement is achieved by methods, apparatus and computer program enabling/realising 3D beamforming in mobile network scenarios.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

S Brief Description of the Drawings

Figure 1 shows a schematic block diagram illustrating an active antenna array forming a w-id of N=2 times M=2 orthogonally disposed antennas implemented as eross-polarised antenna slants; Figure 2 shows a schematic diagram of an example of a procedure accoiding to some embodiments of the present invention; Figure 3 shows a schematic diagram of an example of a procedure according to some embodiments of the present invention; and Figure 4 shows a schematic block diagram illustrating exemplary apparatus according to some embodiments of the present invention.

Detailed Description

The present invention is described herein with reference to particular non-limiting examples and to what are presently considered to be conceivable embodiments of the prcscnt invention. A person skilled in the art will appreciate that the invention is by no means limited to these examples, and may be more broadly applied.

It is to be noted that the following description of the present invention and its embodiments mainly refers to specifications being used as non-limiting examples for certain exemplary network configurations and deployments. Namely, the present invention and examples of its embodiments are mainly described in relation to 3GPP specifications being used as non-limiting examples for certain exemplary network configurations and deployments. As such, the description of sonic embodiments given herein specifically refers to terminology which is directly related thereto. Such terminology is only used in the context of the presented non-limiting examples, and does not limit the invention in any way. Rather, any other eomnmnication or communication related system deployment, etc. may also be utilised as long as it is S compliant with the features described herein.

Hereinafter, various embodiments and implementations of the present invention and its aspects or embodiments are described using several variants and/or alternatives. It is generally noted that, according to certain needs and constraints, all of the described variants and/or alternatives may be provided alone or in any conceivable combination (also including combinations of individual features of the various variants and/or alternatives).

According to some embodiments of the present invention, in general terms, there are provided measures and mechanisms for (cnablingrealising) 3D beamfomiing in mobile network scenarios.

According to some embodiments of the present invention, multiple CSI-RS resources per mobile station are reused for 3D boamforming and a final 3D precoder is formed.

Figure 2 shows a schematic diagram of an example of a procedure according to some embodiments of the present invcntion.

As shown in Figure 2, a procedure according to some embodiments of the present invention comprises an operation of configuring (521) at least one CSI-RS resource, and an operation of transmitting (522) at least one CSI-RS using said at least one CSI-RS resource, an operation of receiving (523) at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CST-RS, and an operation of determining (S24) a final precoder for 3D beamforming based on said at least one matrix indicator.

In other words, in order to overcome the above issues, P CSI-RS resources are S configured and tile preferable one is selected based on a CQI feedback obtained from a UL per configured CSI-RS resource (i.e. associated with a respective CSI-RS resource), namely, a final prccoder is determined based on received matrix indicator and/or channel quality indicator and/or associated CST-RS resource.

According to sonic embodiments of the present invention, the above can be implemented in several ways, as described below. In the case of a "CSI-RS specific till", several CSI-RS configurations with different tilts (i.e. elevation angles) are configured. In the case of "CSI-RS per dimension", two CSI-RS configurations are required, one for the horizontal and one for the vertical dimension. In the case of a transmitted bitmap, one CSI-RS resource is configured and slants of antennas of an antenna anay are mapped to vertical and horizontal codebooks, wherein this mapping is indicated though two bitmaps.

According to a variation of the procedure shown in Figure 2, each of said at least one CSI-RS resources is associated with a different predetermined elevation angle, and exemplary additional operations are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to some embodiments of the present invention may comprise an operation of generating a vertical precoder for each of said at least one CSI-RS resources based on said respectively associated predeteimined elevation angle.

Further, each of said at least one CSI-RS is transmitted using said respective vertical prccodcr.

In other words, R CSI-RS resources are configured with different tilts, i.e. with specific baseband tilts = ], and the vertical precoding vector w1'() is formed. For simplicity is assumed to be a distance between neighbouring x-pol pairs. The horizontal CSI-RS are configured. Each vertical array is virtualised by wt'() and forms a single horizontal CSI-RS port.

According to a variation of the procedure shown in Figure 2, said precoder codebook is a horizontal precoder codebook, and exemplary additional operations and exemplary details of the determining operation are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to exemplary embodiments of the present invention may comprise an operation of selecting a CSI-RS associated with a highest channel quality identifier. Such exemplary determining operation according to exemplary embodiments of the present invention may comprise an operation of selecting a horizontal precoder from said horizontal precoder codebook according to said matrix indicator associated with said selected CSl-RS, and an operation of generating said final precoder based on said horizontal precoder and said vertical precoder of said selected CSI-RS.

That is, the horizontal precoder is selected in traditional way, the CSI process with highest CQI is selected with corresponding tilt and a horizontal precoder = [Wi W2 V/N_i B/NIT. Highest CQI process is optimal from the UEs view. However, from the network-side view a higher tilt may be more beneficial, if the CQI difference between these two processes is small enough. Accordingly, a selection of the honzontal precoder based on a combination of the CQI feedback and the respective tilt is also possible.

Two precoding matrix indices for vertical and horizontal precoding and only a single channel quality feed backIind icator are fed back to the transmitter.

According to a further variation, exemplary additional operations are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to some embodiments of the present invention may comprise an operation of allocating a predetermined number of said at least one CST-RS resource to a recipient e.g. a TiE. Consequently, only P CST-RS (from the possible R resources) resources are configured to each liE, in other words, as a thrthcr enhancement, the eNB could transmit R>=P CSI-RS resources and select a subset of P CSI-RS resources for each UE (e.g. according to rough angle of arrival estimation based on uplink signals).

According to some further embodiments of the present invention, each of said vertical precoders is generated using the formula = [i eit1) ... ei1T(M_i)() eiM1Tsuh1)]T, with wV() being said vertical precoder and = being said predetermined elevation angle, said horizontal preeoder is defined by w' = [Wi W2 WN_I WxIT selected from said horizontal beamforming codebook modified according to the tilt = [ø Ø1 (predetermined elevation angle) used with CSI-RS resource, and said final precoder (for data transmission) is determined using the formula W = vec(w' 0 w-'), with W being said final precoder and ® denoting a Kronecker product, i.e. based on the vertical and horizontal precoders.

According to a further variation of the procedure shown in Figure 2, said at least one CSI-RS resource comprises a horizontal CSI-RS resource and a vertical CSI-RS resource allocated to a recipient, i.e. a UE, and said at least one CSI-RS comprises a horizontal CSI-RS and a vertical CSI-RS. That is, the required two CSI-RS configurations, one for horizontal and one for vertical dimension, are configured to a certain UE as a horizontal and a vertical CS1-RS resource, and the reference signals are transmitted as a horizontal CSI-RS and a vertical CSI-RS.

According to a variation, said at least one matrix indicator is a horizontal matrix indicator indicative of a horizontal precoder selected from a horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precoder selected from a vertical precoder codeboolc, and said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicatoi and exemplary details of the determining operation are given, which are inherently independent from each other as suck Such exemplary determining operation according to exemplary embodiments of the present invention may comprise an operation of selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, an operation of selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and an operation of generating said final precoder based on said horizontal precoder and said vertical precoder.

That is, the pitcoders per horizontal and per vertical dimension are selected based on received matrix indices.

According to a further variation, an exemplary method according to some embodiments of the present invention may comprise an operation of estimating an average elevation angle based on received feedback, and an operation of determining a vertical beam based on said vertical precoder and an average elevation angle.

According to some embodiments of the present invenlion, the horizontal codebook may be modified based on said average elevation angle. That is, the horizontal precoder may be obtained from modified horizontal codebook that has been modified by said vertical beam.

That is, the vertical beam = Li e"0 ei(M_1ø) e01ø)]T is estimated from wP by estimating average tilt 0. The elevation angle (tilt) can be estimated by CSI feedback received for the CSI-RS resources configured for the vertical ports.

According to some thither embodiments of the present invention, said selected horizontal precoder is defined by wi = w1 w2 WN_1 W]T, said selected vertical pitcodcr is defined by V = [Wi 2 Wjg_1 WM]T, said vertical beam is determined using the formula = [i eiflM *.. eJ_i e''f, with being said average elevation angle. Horizontal beamforming codebook is then modified such that modified beamforming codewords are C', = fll () DC:. \vllere C; is the original beamforming vector i modified by vector m;(Ø) dependent on an average tilt angle, and a denoting the Hadamard product. Final precoder is determined using the formula W = vcc(w' ® WV), with W being said final precoder and 0 denoting a Kroneclcer product, i.e. based on the vertical and horizontal precoders.

According to a further variation of the procedure shown in Figure 2, one single CSI-RS is transmitted, said at least one matrix indicator is a horizontal matrix indicator indicative of a horizontal precoder selected from said horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precoder selected from said vertical precoder codebook, said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator, and exemplary additional operations and exemplary details of the deteimining operation are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to some embodiments of the present invention may comprise an operation of transmitting a first bitmap and a second bitmap, each associating a horizontal precoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna anay. An exemplary determining operation according to exemplary embodiments of the present invention may comprise an operation of selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, an operation of selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and an operation of generating said final precoder based on said horizontal precoder and said vertical precoder. is

According to such variation, only a single CS1-RS is allocated, and two bitmaps are signalled to a user. Bitmaps denote which antenna ports belong to which codebook. An example according to the active antenna array as shown in Figure 1 with a total of cight transmission ports is shown in the table below, pointing out the bitmaps for antenna port (lxi to TxS) to codebook (CB1 and CB2) allocation. In this example, the row corresponding to CB 1 indicates that the vertical PMI is estimated from CB1 on vertical ports Txi, Tx3, Tx5, 1x7 and the row corresponding to CB2 indicates that the horizontal PMI is estimated on horizontal ports lxi, 1x2, TxS and Tx6. Two precoding matrix indices for vertical and horizontal precoding and only a single channel quality feedback/indicator are fed back to the transmittcr.

Txi Tx2 Tx3 Tx4 Tx5 Tx6 1x7 TxS CBI I 0 1 0 I 0 I 0 CB2 1 1 0 0 1 1 0 0 According to a further variation, the horizontal precoder eodebook may be modified based on said vertical precoder.

Figure 3 is a schematic diagram of an example of a procedure according to some embodiments of the present invention.

As shown in Figure 3, a procedure according to some embodiments of the present invention comprises an operation of receiving (531) at least one CST-RS using at least one CSI-RS resource, an operation of selecting (S32) at least one precoder from a precoder codebook associated with said at least one CSI-RS based on measurements on said receiving, an operation of determining (533) at least one channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio channel state, and an operation of transmitting (S34) at least one matrix indicator according to said at least one precoder and said at least one channel quality indicator.

According to a variation of the procedure shown in Figure 3, said at least one S CSI-RS resource comprises a horizontal CSI-RS resource and a vertical CSI-RS resource, and said at least one CSI-RS comprises a horizontal CSI-RS and a vertical CSI-RS.

According to a further variation, exemplary details of the selecting operation are given, which are inherently independent from caeh other as such. An exemplary selecting operation according to exemplary embodiments of the present invention may comprise an operation of choosing a horizontal precoder from a horizontal precoder codebook, and an operation of choosing a vertical precoder from a vertical precoder codebook. Further, the at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precoder, and the at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator.

According to a still further variation, exemplary additional operations are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to exemplary embodiments of the present invention may comprise an operation of obtaining a horizontal radio channel from said receiving of said horizontal CSI-RS, an operation of obtaining a vertical radio channel from said receiving of said vertical CSI-RS, and an operation of interpolating a full channel based on said horizontal channel and said vertical channel. FLirther, the single channel quality indicator is estimated based on said full channel.

That is, there are N vertical ports and M horizontal ports in an antenna array of a transmitter. The full radio channel is N x M. In order to get good CQI, this N x NI channel is to be constmcted. Hence sonic form of interpolation is needed.

Accordingly, in other words, a full channel is obtained by interpolating the channel obtained on the horizontal CSI-RS ports with the cbannel obtained on the vertical CSI-RS ports, and the single channel quality indicator is estimated on the obtained full channel.

Consequently, after receiving the at least one CSI-RS, based on measurements on the receiving horizontal and vertical PM[ are selected. A 3D PMI is constructed by applying the Kronecker product on the horizontal and vertical PMIs. Based on the 3D PMI, missing ports are interpolated. A single CQI is computed based on the 3D precoder and complete (interpolated) set of ports. Subsequently, two PMIs (horizontal and vertical PME) and one single CQI are fed back According to a variation of the procedure shown in Figure 3, one single CSI-RS is transmitted, and exemplary additional operations and exemplary details of the selecting operation are given, which are inherently independent from each other as such. According to such variation, an exemplary method according to some embodiments of the present invention may comprise an operation of receiving a fist bitmap and a second bitmap, each associating a horizontal precoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna array. An exemplary selecting operation according to exemplary embodiments of the present invention may comprise an opcration of choosing a horizontal precoder from said horizontal prccoder codebook, and an operation of choosing a vertical procoder from said vertical precoder codebook. Further, said at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precoder, and said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator.

According to a further variation, the horizontal precoder codebook may be modified based on said vertical precoder.

It is noted that Long Term Evolution (LTE) Re!-11 already supports configuring the UE with multiple CSI processes corresponding to up to three different CS!-RS resources. Further, the el'4B can transmit more than three CS!-RSs, but each lIE is only able to measure and report feedback for three CS!-RSs.

The original intention of introducing such configuration of the UE with multiple CSI processes corresponding to up to three different CS!-RS resourves into the specifications in Rel-li was coordinated multipoint transmission and reception (CoMP). However, according to some embodiments of the present invenlion, those different CSI-RS resources are utilised to provide 3D beamfomiing support.

Each of the up to three CSI-RS resources is precoded with a specific vertical precoder and associated with one of the CSI processes. The UE provides CQI/PML'rank indicator (RI) feedback for each of the CS! processes, hence enabling the eNB to select the best CSI-RS resource and thus the best vertical precoder to be used for enhanced physical downlink control channel (ePDCCH)/physical downlink shared channel (PDSCH) transmission to the UE.

Generally, the above-described procedures and functions may be implemented by respective functional elements, processors, or the like, as described below.

While in the foregoing some embodiments of the present invention are described mainly with reference to methods, procedures and functions, some corresponding embodiments of the present invention also cover respective apparatus, network nodes and systems, including both software, algorithms, and/or hardware thereof Some respective embodiments of the present invention are described below referring to Figure 4, while for the sake of brevity reference is made to the detailed

description with regard to Figures 1 to 3.

In Figure 4 below, which is noted to represent a simplified block diagram, the solid line blocks are basically configured to perform respective operations as described above. The cntirety of solid line blocks are basically configured to perform S the methods and operations as described above, respectively. With respect to Figure 4, it is to be noted that the individual blocks are meant to illustrate respective functional blocks implementing a respective function, process or procedure, respectively. Such functional blocks are implementation-independent, i.e. may be implemented by means of any kind of hardware or software, respectively. The arrows and lines interconnecting individual blocks arc meant to illustrate an operational coupling therebetween, which may be a physical and/or logical coupling, which on the one hand is implementation-independent (e.g. wired or wireless) and on the other hand may also comprise an arbitrary number of intermediary functional entities not shown.

Further, in Figure 4, only those functional blocks are illustrated that relate to any one of the above-described methods, procedures and functions. A skilled person will acknowledge the presence of any other conventional functional blocks required for an operation of respective structural arrangements., such as e.g. a power supply, a central processing unit, respective memories or the 111cc. Among others, memories are provided for storing proams or program instructions for controlling the individual functional entities to operate as described herein.

Figure 4 shows a schematic block diagram illustrating exemplary apparatus according to some embodiments of the present invention.

In view of the above, the thus described apparatus 40 and 50 arc suitable for use in practising some embodiments of the present invention, as described herein.

The thus described apparatus 40 may represent a (part of a) network entity, such as a base station or access node or any network-based controller, e.g. an cNB, operable in at least one of a LTE and a LTE-A cellular system, and may be configured to perform a procedure and/or functionality as described in conjunction with Figure 2.

The thus described apparatus 50 may represent a (part of a) device or terminal such as a mobile station MS or user equipment UE or a modem (which may bc installed as part of a MS or UB, but may be also a separate module, which can be attached to various devices), is operable in at least one of a LIE and a LTE-A cellular system, and may be configured to perform a procedure and/or functionality as described in conjunction with Figure 3.

As indicated in Figure 4, according to some embodiments of the present invention, the apparatus 40 comprises a processor 41, a memory 42 and an interface 43, which are connected by a bus 44 or the like. Further, according to some embodiments of the present invention, the apparatus 50 comprises a processor 51, a memory 52 and an interface 53, which are connected by a bus 54 or the like, and tim apparatus 40, 50 may be connected via a link 45.

The processor 41/51 and/or the interface 43/53 may also include a modem or the like to facilitate communication over a (hardwire or wireless) link, respectively.

The interface 43/53 may include a suitable transceiver coupled to one or more antennas or communication means for (hardwire or wireless) communications with the linked or connected device(s), respectively. The interface 43/53 is generally configured to communicate with at least one other apparatus, i.e. the interface thereof The memory 42/52 may store respective programs assumed to include proam instructions or computer program code that, when executed by the respective processor, enables the respective electronic device or apparatus to operate in accordance with some embodiments of the present invention.

In general terms, the respective devices/apparatus (and/or parts thereof) may represent means for performing respective operations and/or exhibiting respective t'unctionalities, and/or the respective devices (and/or parts thereof) may have functions for performing respective operations and/or exhibiting respective functionalities.

When in the subsequent description it is stated that the processor (or some other means) is configured to perform some function, this is to be construed to be equivalent to a description stating that at least one processor, 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 constrtted to be equivalently implementable by specifically configured means for performing the respective function (i.e. the expression "processor configured to [cause the apparatus to] perform xxx-ing" is construed to be equivalent to an expression such as "means for xxx-ing").

According to some embodiments of the present invention, an apparatus representing the network entity 40 comprises at least one processor 41, at least one memory 42 including computer program code, and at least one interface 43 configured for communication with at least another apparatus. The processor (i.e. the at least one processor 41, with the at least one memory 42 and the computer proam code) is configured to perform configuring at least one CSI-RS resource, to perform transmitting at least one CSI-RS using said at least one CSI-RS resource, to perform receiving at least one matrix indicator indicative of a precoder selected from a precodcr codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and to perform determining a final precoder for 3D beamforming based on said at least one matrix indicator.

In its most basic form, stated in other words, the apparatus 40 may thus comprise respective means for configuring, means for transmitting, means for receiving, and means for determining.

As outlined above, the apparatus 40 may comprise one or more of respective means for generating, means for selecting, and means for allocating.

According to some embodiments of the present invention, an apparatus representing the terminal 50 comprises at least one processor 51, at least one memory 52 including computer program code, and at least one interface 53 configured for communication with at least another apparatus. The processor (i.e. the at least one processor 51, with the at least one memory 52 and the computer program code) is configured to perform receiving at least one CSI-RS using at least one CSI-RS resource, to perform selecting at least one preeoder from a precoder codebook associated with said at least one CSI-RS based on measurements on said receiving, to perform determining at least one channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio channel state, and to perform transmitting at least one matrix indicator according to said at least one precoder and said at least one channel quality indicator.

h its most basic form, stated in other words, the apparatus 50 may thus comprise respective means for receiving, means for selecting, means for determining and means for transmitting.

As outlined above, the apparatus 50 may comprise one or more of respective means for choosing, means for obtaining, and means for interpolating.

For further details regarding the operability/functionality of the individual apparatus, reference is made to the above description in connection with any one of Figures 2 and 3, respectively.

According to exemplary embodiments of the present invention, a system may comprise any eoneeivabe combination of the thus depicted devices/apparatus and other network elements, which arc configured to cooperate with any one of them.

As explained above, while there are publications suggesting configuring fixed tilts to resources according to a predetermined pattern and scheduling the mobile stations to the resources that are suitable for the respective mobile station based on feedback obtained from the respective mobile station, according to some embodiments of the present invention P (a certain number P of) CSI-RS resources are configured, which allow the mobile station to be scheduled in the arbitrary resource with the mobile station's preferred tilt.

Accordingly, according to some embodiments of the present invention, the base station does not need to signal to mobile station which tilt has been used in the transmitted sub*ame and the whole process is thus transparent to The UE.

Further, the CSI-RS specific tilt method according to some embodiments of the present invention is backward compatible to ReI-1 I legacy UEs, and hence provides 3D bcamforming support also for Release 11 TiEs. Also, according to some embodiments of the present invention, the eNB is allowed to accommodate its transmission to the surrounding environment Moreover, according to exemplary embodiments of the present invention, reliable CQI for full array 3D beamforming is provided, and the codebook does not need to be re-considered, e.g. taking the distribution of elevation angle into account According to some embodiments of the present invention, the feedback (i.e. CQ1) reported per CSI-RS resource is the correct CQI that can be used for data transmission, while according to prior art methods two CQIs are reported which have to be combined/scaled, such that the final CQI estimate is only a guess, especially in non-correlated scenarios. Hence, the CQI estimation is simpler and more accurate in the scheme according to some embodiments of the present invention.

For the purpose of the present invention as described herein above, it should benoted that -method steps likely to be implemented as software code portions and being run using a processor at a network server or network entity (as examples of devices, apparatus and/or modules thereof; or as examples of entities including apparatus and/or modules therefore), are software code independent and can be specified using S any known or future developed proamming 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 apparatus, or any module(s) thereof; (e.g. devices carrying out the functions of the apparatus 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), BCL (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 network entity or network register, or any one of their respective units/means) can be implemented as individual devices, units or means, but this does not exclude that they arc implemented in a distributed fashion throughout the system, as long as the functionality of the device, unit or means is preserved; -an apparatus like the user equipment and the network entity/network register 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 S independently of each other but in a same device housing, for example.

In general, it is to be noted that respective fimctionat blocks or elcmen 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 realised in individual functional blocks or by individual devices, or one or more of the method steps can be realised 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 proam 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.

The present invention also covers any conceivable combination of method steps and operations described above, and any conceivable combination of nodes, apparatus, modules or elements described above, as long as the above-described concepts of methodology and structural anangement are applicable.

In view of the above, there are provided measures for 3D beamforming in mobile network scenarios. Such measures exemplarily comprise configuring at least one CSI-RS resource, transmitting at least one CSI-RS using said at least one CSI-RS resouzve, receiving at least one matrix indicator indicative oft precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and determining a final precoder for 3D beamforming based on said at least one matrix indicator.

The above embodiments are to be understood as illustrative examples of the invention, Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims (9)

1. CLANS1. A method comprising: configuring at least one channel state infbrniation reference signal CSI-RS resource, transmitting at least one CSI-RS using said at least one CSI-RS resource, receiving at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and determining a final pivcoder for three-dimensional 3D beamforming based on said at least one matrix indicator.
2. 2. A method according to claim!, wherein: each of said at least one CSI-RS resources is associated with a different predetermined elevation angle, the method comprising: generating a vertical precoder for each of said at least one CSI-RS resources based on said respectively associated predetermined elevation angle, wherein each of said at least one CSL-RS is transmitted using said respective vertical precoder.
3. 3. A method according to claim 2, comprising: selecting a CSI-RS associated with a highest channel quality identifier, wherein said precoder codebook is a horizontal precoder codebook, and in relation to said determining, the method comprises: selecting a horizontal precoder from said horizontal precoder codebook according to said matrix indicator associated with said selected CSI-RS, and generating said final precoder based on said horizontal precoder and said vertical precoder of said selected CSI-RS.
4. 4. A method according to claim 2 or claim 3, comprising: allocating a predetermined number of said at least one CST-RS resource to a recipient.
5. 5. A method according to any of claims 2 1o4, wherein: each of said vertical precoders is generated using the formula w1'@) = Ft ei1) . .. t)s[n(') ejMj)IT with wt'() being said vertical precoder and = j being said predetermined elevation angle, said horizontal precoder is defined by wt' = [Wi W2 WN_i WNIT, and said final precoder is determined using the formula W = vec(wL ® w"), with W being said final precoder and 0 denoting a Kroneclccr product.
6. 6. A method according to claim 1, wherein: said at least one CSI-RS resource comprises a horizontal CSI-RS resource and a vertical CSI-RS resource allocated to a recipient, and said at least one CSI-RS comprises a horizontal CSI-RS and a vertical CSI-RS.
7. 7. A method according to claim 6, wherein: said at least one matrix indicator is a horizontal matrix indicator indicative of a horizontal precodcr selected from a horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precoder selected from a vertical precoder codebook, said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined uti lising said horizontal matrix indicator and said vertical matrix indicator, and in relation to said determining, the method comprises: selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and generating said final precoder based on said horizontal precoder and said vertical precoder.
8. 8. A method according to claim!, wherein: one single CSI-RS is lransmitted, the method comprising: transmitting a first bitmap and a second bitmap, each associating a horizontal precoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna aray, and said at least one matrix indicator is a horizontal matrix indicator indicative of a horizontal precoder selected from said horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precoder selected from said vertical precoder codebook, said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator, and in relation to said determining, the method comprises: selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and generating said final precoder based on said horizontal precoder and said vertical precodet
9. 9. A method according to any of claims 3, 7 and 8, wherein: said horizontal prccoder codebook is modified based on said vertical precoder.ia A method comprising: receiving at least one CSI-RS using at least one CSI-RS resource, selecting at least one precoder from a precoder codebook associated with said at least one CSL-RS based on measurements on said receiving, determining at least one channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio channel state, and transmitting at least one matrix indicator according to said at least one precoder and said at least one channel quality indicator.11. A method according to claim 10, whereiit said at least one CSI-RS resource comprises a horizontal CSI-RS resource and a vertical CSI-RS resouive, and said at least one CSI-RS comprises a horizonlal CSI-RS and a vertical CSI-RS.12. A methodaccordingtoclaim IOorclaim l1,wherein: in relation to said selecting, the method comprises: choosing a horizontal precoder from a horizontal precoder codebook, choosing a vertical precoder from a vertical precoder codebook, wherein said at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precoder, and said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined uti lising said horizontal matrix indicator and said vertical matrix indicator.13. A method according to claim 12, comprising: obtaining a horizontal radio channel from said receiving of said horizontal CSI-RS, obtaining a vertical radio channel from said receiving of said vertical CSI-RS, interpolating a fbll channel based on said horizontal channel and said vertical channel, wherein said single channel quality indicator is estimated based on said MI channel.14. A method according to claim 12 or claim 13, wherein: one single CSI-RS is received, the method comprising: receiving a fist bitmap and a second bitmap, each associating a horizontal piecoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna array, and in relation to said selecting, the method comprises: choosing a horizontal precoder from said horizontal precoder codebook, choosing a vertical precoder from said vertical precoder codeboolç wherein said at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precoder, and said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined uti using said horizontal matrix indicator and said vertical matrix indicator.15. A method according to claim 14, wherein said horizontal precoder codebook is modified based on said vertical precoder.16. Apparatus for use on a network side of a cellular system, the apparatus comprising: a processing system arranged to cause the apparatus to perform: configuring at least one CSI-RS resource, transmitting at least one CSI-RS using said at least one CSI-RS resource, receiving at least one matrix indicator indicative of a precoder selected from a precoder codebook and at least one channel quality indicator indicative of a radio channel state, each respectively associated with said at least one CSI-RS, and determining a final precoder for 3D beamforming based on said at least one matrix indicator.17. Apparatus according to claim 16. wherein: each of said at least one CSI-RS resources is associated with a different predetermined elevation angle, the processing system being arranged to cause the apparatus to perform: generating a vertical preeoder for each of said at least one CSI-RS resources based on said respectively associated predetermined elevation angle, wherein each of said at least one CSI-RS is transmittcd using said respective vertical precod er.18. Apparatus according to claim 17, the processing system being arranged to cause the apparatus to perform: selecting a CSI-RS associated with a highest channel quality identifier, wherein said precoder codebook is a horizontal precoder codebook, and in relation to said determining, the processing system being arranged to cause the apparatus to perform: selecting a horizontal precoder from said horizontal precoder codebook according to said matrix indicator associated with said selected CSI-RS, and generating said final precodcr based on said horizontal prceodcr and said vertical preeoder of said selected CSI-RS.1 9. Apparatus according to claim 17 or claim 18, the processing system being arranged to cause the apparatus to perform: allocating a predetermined number of said at least one CSI-RS resource to a recipient.20. Apparatus according to any of claims 17 to 19, wherein: each of said vertical precoders is generated using the formula w1'(q5) = Li eifh() . .. t)sin() eiM()I 2' with w'1'() being said vertical precoder and = ] being said predetermined elevation angle, said horizontal precoder is defined by w" = [w1 w2 W1y_1 Il/NIT, and said final precoder is determined using the formula W = vec(wh 0w"), with W being said final precoder and 0 denoting a Kronecker product.21. Apparatus according to claim 16, wherein: said at icast onc CSI-RS resource compriscs a horizontal CSI-RS rcsourcc and a vertical CSI-RS resource allocated to a recipient, and said at least one CSI-RS comprises a horizontal CSI-RS and a vertical CSI-RS.22. Apparatus according to claim 21, wherein: said at least one matrix indicator is a honzontal matrix indicator indicative of a horizontal precoder selected from a horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precodcr selected from a vertical precoder codebook, said at least one channel quality indicator is a single channel quality indicator indicative of a radio channcl state dctcrmincd uti using said horizontal matrix indicator and said vertical matrix indicator, and in relation to said determining, thc processing system being alTanged to cause the apparatus to perfonn: selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and generating said final precoder based on said horizontal precoder and said vertical precoder.23. Apparatus according to claim 16, wherein: one single CST-RS is transmitted, the processing system being arranged to cause the apparatus to perform: transmitting a flrst bitmap and a second bitmap, each associating a horizontal precoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna array, and said at least one matrix indicator is a horizontal matrix indicator indicative of a horizontal precoder selected from said horizontal precoder codebook and a vertical matrix indicator indicative of a vertical precoder selected from said vertical precoder codebook, said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator, and in relation to said determining, the processing system being arranged to cause the apparatus to perform: selecting said horizontal precoder from said horizontal precoder codebook according to said horizontal matrix indicator, selecting said vertical precoder from said vertical precoder codebook according to said vertical matrix indicator, and generating said final precoder based on said horizontal precoder and said vertical precoder.24. Apparatus according to any of claims 18, 22 and 23, wherein: said horizontal precoder codebook is modified based on said vertical precoder.25. Apparatus according to any of claims 16 to 24, wherein: the apparatus is operable as or at a base station, evolved NodeB, or access node of a cellular system, and/or the apparatus is operable in at least one of a Long Term Evolution LTE and a Long Term Evolution Advanccd LIE-A cellular system.26. Apparatus for use on a terminal side of a cellular system, the apparatus comprising: a processing systcm arranged to causc the apparatus to perform: receiving at least one CST-RS using at least one CST-RS resource, selecting at least one precoder from a precoder codebook associated with said at least one CSL-RS based on measurements on said receiving, determining at least one channel quality indicator associated with said at least one CSI-RS based on measurements on said receiving indicative of a radio channel state, transmitting at least one matrix indicator according to said at least one precoder and said at least one channel quality indicator.27. Apparatus according to claim 26, wherein: said at least one CSI-RS resource comprises a horizontal CSI-RS resource and a vertical CSI-RS rcsouive, and said at least one CSI-RS comprises a horizontal CSI-RS and a vertical CSI-RS.28. Apparatus according to claim 26 or claim 27, wherein: in relation to said selecting the processing system being arranged to cause the apparatus to perform: choosing a horizontal precoder from a horizontal precoder codebook, choosing a vertical precoder from a vertical precoder codebook, wherein said at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precodcr, and said at least one channcl quality indicator is a single channel quality indicator indicative of a radio channel state determined utilising said horizontal matrix indicator and said vertical matrix indicator.29. Apparatus according to claim 28, the processing system being arranged to cause the apparatus to perform: obtaining a horizontal radio channel from said receiving of said horizontal CSI-RS, obtaining a vertical radio channel from said receiving of said vertical CSI-RS, interpolating a MI channel based on said horizontal channel and said vertical channel, wherein said single channel quality indicator is estimated based on said Mi channel.30. Apparatus according to claim 26 or claim 27, wherein: one single CSI-RS is received, the processing system being arranged to cause the apparatus to pcrfornt receiving a fist bitmap and a second bitmap, each associating a horizontal precoder codebook and a vertical precoder codebook with respective at least two antenna ports of an antenna array, and in relation to said selectin& the pmcessing system being arranged to cause the apparatus to perform: choosing a horizontal precoder from said horizontal precoder codebook, choosing a vertical precoder from said vertical precoder codebook, wherein said at least one matrix indicator is a horizontal matrix indicator according to said horizontal precoder and a vertical matrix indicator according to said vertical precoder, and said at least one channel quality indicator is a single channel quality indicator indicative of a radio channel state determined uti using said horizontal matrix indicator and said vertical matrix indicator.31. Apparatus according to claim 30, whcrcin said horizontal prccodcr codebook is modified based on said vertical precoder.32. Apparatus according to any of claims 26 to 31, wherein: the apparatus is operable as or at a terminal, user equipment mobile station or modem, and/or the apparatus is operable in at least one of a LTE and a LTE-A cellular system.33. A computer program comprising computer-executable computer program code which, when the program is run on a computer, is configured to cause the computer to carry out the method according to any of claims ito 9 or claims 10 to 15.34. A method of wireless transmission, substantially in accordance with any of the examples as described herein with reference to and illustrated by the accompanying drawings.35. Apparatus for wireless transmission, substantially in accordance with any of the examples as described herein with reference to and illustrated by the accompanying drawings.