WO2014039056A1

WO2014039056A1 - Codebook construction using csi feedback for evolving deployment scenarios

Info

Publication number: WO2014039056A1
Application number: PCT/US2012/054409
Authority: WO
Inventors: Weidong Yang
Original assignee: Nokia Siemens Networks Oy
Priority date: 2012-09-10
Filing date: 2012-09-10
Publication date: 2014-03-13

Abstract

The specification and drawings present a new method, apparatus and software related product for using codebook constructions within CSI feedback framework for evolving deployment scenarios. A network element (e.g., eNB) may send to a UE a channel CSI-RS on a plurality of resources/frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword. In response, the eNB may receive from the UE an information report comprising selected one or more of the plurality of frequency subbands and related information on PMI/CQI/RI for each selected subband. Then the eNB can establish mapping between the each selected frequency subband and the one codeword for the each selected frequency subband and form by interpolation at least one further slowly changing precoder trajectory in the frequency domain based on the received information report.

Description

CODEBOOK CONSTRUCTION USING CSI FEEDBACK FOR EVOLVING

DEPLOYMENT SCENARIOS

Technical Field

The exemplary and non-limiting embodiments of this invention relate generally to wireless communications and more specifically to using codebook constructions using CSI feedback framework for evolving deployment scenarios (e.g., in LTE wireless systems).

Background Art

The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:

3 GPP third generation partnership project

BP bandwidth part

CoMP coordinated multi-point

CQI channel quality indicator

CSI channel state information

DL downlink

E-UTRA evolved universal terrestrial radio access

eNB evolved node B /base station in an E-UTRAN system

E-UTRAN Evolved UTRAN (LTE)

FDM frequency division multiplexing

FGI feature group indicator

HARQ hybrid automatic repeat request

IMR interference measurement resource

LDPC low-density parity check

LTE long term evolution

LTE-A long term evolution advanced

MIMO multiple input multiple output

PRB physical resource block

PDCCH physical downlink control channel

PDSCH physical downlink shared channel

PMI precoding matrix index

RAN radio access network RI rank index

SNR signal-to-noise ratio

UE user equipment

UL uplink

UTRAN universal terrestrial radio access network

CSI feedback has always been a central theme in broadband wireless communication systems. Accurate CSI feedback, HARQ transmission and turbo/LDPC codes have been a powerful combination which effective transmission at channel capacity within reach of communication engineers. In the design of CSI feedback schemes in a specific communication system such as LTE/LTE-A, there are always trade-offs between CSI feedback accuracy and corresponding overhead. This can be expressed as uplink resources needed to transmit a large amount of data for the accurate CSI feedback, downlink resources needed to enable accurate CSI feedback, and/or the power consumption/computational complexity at the UE for frequent CSI feedback calculations. For a variety of deployment scenarios which can be very difficult to foresee at the initial stage of standardization work such as LTE Release 8, no communication engineer (or a group such as 3 GPP RANI) can claim providing the right balance between accuracy and complexity/overhead for all possible scenarios. A change in one scenario versus other scenarios with time and/or weighting of different scenarios in the deliberation of CSI feedback schemes is not properly taken care. All that leaves the LTE/LTE-A standardization specification in a state of compromises which do not offer satisfactory solution for evolving deployment scenarios such as geographically separate antennas, elevation beamforming, and distributive antenna systems.

Specifically within 3GPP RANI, another trend can be also observed in the dynamics between infrastructure vendors and UE (chipset) vendors. The infrastructure vendors typically have been more supportive of introducing new or improved features in the CSI feedback such as new codebooks mainly motivated by a possible performance gain under one or more scenarios. On the other hand, the UE (chipset) vendors, given the fact they have to implement all the CSI feedback schemes included in the specification (by stages through FGIs), have little motivation for new/improved features in the CSI feedback. As the improvement brought by a new/improved feature can be limited to one scenario or not large enough to justify a change in the specification, and given the situation with the partnership/shared ownership between infrastructure vendors and the UE chipset vendors, the whole process of bringing in the new/improved features in the CSI feedback has been inefficient. Therefore, so far no one has a foresight to design a codebook which will match all the possible deployment scenarios. Even if some features may bring gains under one scenario, they may not be included in the specification due to the dynamics mentioned above. Consequently a CSI framework needed for evolving deployment scenarios should be future-proof, so that it does not need continuous modification of the LTE specification.

The CSI feedback process in LTE is reviewed in the following.

To allow a UE to feedback the desired precoder, a network element (e.g., e B) can transmit training signals s , which can be either CRS or CSI-RS, and a receiver model for CSI can be given by

r = Hs + n (i ) where r is a received signal, s is a training signals (e.g. CSI-RS signals), and H is a channel response of the wireless channel, n is a noise.

From Equation 1, the UE can obtain a channel estimate H of H from channel estimation performed with the known 5 , which can be either CRS or CSI-RS. The channel estimation is typically performed for each PRB or subband. The UE also needs to estimate a noise variance σ¹ from the CRS or an interference measurement resource (IMR) which is introduced in LTE Release 11. Then the UE can try each codeword in the codebook to calculate an expected throughput from the receiver model below as follows:

r = HW x + n (2), where W_j is a codeword in the codebook, and n is the noise with a noise variance σ . From this model, a Table 1 shown below can be built as follows:

Table 1. Expected throughput from the receiver model for different codewords.

In the LTE, the codebook actually used by the UE can be restricted by RRC signaling. If that is the case, then some columns in the Table 1 corresponding to the restricted codewords may be crossed out. It is possible then to identify an optimal precoder, i.e., the precoder providing the highest expected throughput can be identified for each subband, and the best subbands (i.e., the subbands with the high/highest informaion rates) can be also identified. It should be noted that some restrictions exist in LTE so that the UE can be required to feed back a single PMI or multiple PMIs in the feedback, and further the UE may be required to feedback a single CQI or multiple CQIs in the feedback. The actual feedback schemes in the LTE can be quite complicated. Nevertheless, the procedure for the CSI feedback is defined at a conceptual level.

Summary

According to a first aspect of the invention, a method comprising: sending, by a network element to a user equipment, a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword; receiving by the network element from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and mapping by the network element the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

According to a second aspect of the invention, an apparatus comprising: a processing system comprising at least one processor and a memory storing a set of computer instructions, in which the processing system is arranged to cause the apparatus to: send to a user equipment a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword; receive from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and map the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

According to a third aspect of the invention, a computer program product comprising a computer readable medium bearing computer program code embodied herein for use with a computer, the computer program code comprising: code for sending, by a network element to a user equipment, a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword; code for receiving by the network element from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and code for mapping by the network element the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

Brief Description of the Drawings:

For a better understanding of the nature and objects of embodiments of the invention, reference is made to the following detailed description taken in conjunction with the following drawings, in which:

Figure 1 is a diagram showing resources/frequency subbands vs. time originated at the network (eNB), according to an exemplary embodiment of the invention;

Figure 2 is a diagram showing slowly changing precoder trajectory in a frequency domain originated at the network (eNB), according to an exemplary embodiment of the invention;

Figure 3 is a diagram showing resources/frequency subbands vs. time including subbands 1 , 2 and 6 with corresponding codewords selected by a UE, according to an exemplary embodiment of the invention;

Figure 4 is a diagram showing slowly changing precoder trajectory in a frequency domain vs. expected throughput including subbands 1, 2 and 6 with corresponding codewords selected by a UE, according to an exemplary embodiment of the invention;

Figure 5 is a diagram showing resources/frequency subbands vs. time including subbands 1, 2 and 6 with corresponding codewords selected by a UE and subbands 3, 4 and 5 interpolated by the network (eNB), according to an exemplary embodiment of the invention;

Figure 6 is a flow chart demonstrating exemplary embodiments of the invention; and

Figure 7 is a block diagram of LTE wireless devices for practicing exemplary embodiments of the invention. Detailed Description

A new method, apparatus, and software related product (e.g., a computer readable memory) are presented for using codebook constructions within CSI feedback framework for evolving deployment scenarios (e.g., in LTE wireless systems). According to an embodiment of the invention, a network element (e.g., eNB) may send to a UE a channel state information reference signal (CSI-RS) on a plurality of resources/frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword/transform precoder (known or not known, or derived from the 3GPP Release 10). In response, the network element (eNB) may receive from the UE an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband. Then, the network element can establish mapping between the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

According to a further embodiment, if two or more of the plurality of frequency subbands are selected by the UE, the network element (eNB) may form by interpolation at least one further slowly changing precoder trajectory in the frequency domain, wherein the further plurality of frequency subbands include the selected two or more of the plurality of frequency subbands and one or more further frequency subbands with corresponding one or more codewords interpolated based on the received information report.

A selected/intorpolated subband(s) with a single precoder or the interpolated slowly changing precoder trajectory (with multiple subbands) can be used in the data transmission by the network element such as eNB to the UE. The single/multiple precoder(s) can be chosen to maximize the projected channel throughput.

The design of a codebook with codewords/transform precoders by the network/network element (eNB) can be approached in different ways. In one example, the codebook designh can be treated as "Designing sets of N matrices that maximize the minimum subspace distance (where distance can be chosen in a number of different ways [36]) is known as Grassmannian subspace packing (see "Limited Feedback Unitary Precoding for Spatial Multiplexing Systems" by D. Love and R. Heath, IEEE Transactions On Information Theory, Vol. 51, No. 8, August 2005, pages 2967-2976).

It is obivous that with a larger number of precoding matrices, the minimum subspace distance among those matrices decreases from the design procedure. At the UE side, selecting the optimal codeword/precoder is equivalent to ^{^} quantizing" the unconstrained, ideal precoder to one precoder in the codebook. With a larger number precoders in the codebook, the "quantization" error decreases. Typically, when the size of a codebook is changed (e.g., more precoders are added to the existing codebook), both the eNB and the UE have to acquire that knowledge. Suppose a UE designed according to 3GPP Release 10 is released to the market, and if it is found later on that the codebook can be improved by adding more precoders to the existing codebook, there is no way to make it work as the 3GPP Release 10 UE is unaware of the new codebook.

According to an embodiment described herein, a new codebook say R₁₂ can be written as a union of _¾ and T₂R_l0 and T₃R_l0 , where R_w = {W W₂,..} is the 3 GPP Release 10 codebook defined in 3GPP TS 36.211 (which will be referred as "Release 10 codebook" hereafter), and T_k , k=l,2,3 are matrices, which will be called transform precoders in the following, and T_kR_l0 = {T_kW_x,T_kW₂,...} .

The construction of the new codebook is remniscent of the cosets in group theory: In Group theory, if G is a group and Q is a subgroup of G , and g is an element of G that is not in the subgroup Q, then gQ= {gq : q an element of Q} is a left coset of Q in G .

The intuitive justification to obtain a new codebook in this way is that one can use the Release 10 codebook as a building block, and then "rotate" (through a unitary matrix) or "twist" (through applying a non-unitary matrix) the building block ( by multipling a matrix from the left side towards 3GGP Release 10 codewords as shown above) to account for different antenna configurations or deployment scenarios.

Then a codeword in the new codebook R_u may be fully determined for example by a transform precoder (one of 7] , T₂ and T₃ in this case) and/or a codeword W. in the Release 10 codebook (a codeword like W_j will be referred to as "Release 10 codeword" hereafter). In the following, we can use the notation {T^W_j} to indicate a codeword in the new codebook.

Moreover, it is further noted that a codeword in the newbook is fully specified by the pair {T^W_j} . If one follows precendence from 3GPP Release 8/10, then the new release UE can be updated with the new codebook and then it can modify its codeword search algorithm/procedure to account for the new codebook. Also it needs to feedback the chosen pair {T^ W_j} to the network, and the network subsequently can apply the chosen pair from the UE's feedback in its data transmission to the UE. Once a new codebook is introduced in the frame of existing standards, substantial specification impacts will incure: for example, the UE has to be informed about it, the codeword search algorithm/procedure on the UE needs to be modified, and the CSI feedback scheme(s) has to be modified. Based on the exemplary embopdiments described herein, there is no need to make the knowledge of the new codebook available to the UE. Also even with introduction of the new codebook, there is no need to modify the UE's codeword search algorithm/procedure, and the CSI feedback scheme does not have to be changed. This provides the substantial benefits in terms of network deployment and UE implementation according to the exemplary embodiments described herein.

It is also possible that only a subset of R_w is needed as a building block. In the extreme case only one single codeword ini?₁₀ (e.g., the identity matrix) may be used as the building block, and in this case an arbitrary codebook can be obtained by choosing the transform precoders, and the new codebook may be given as:

The new codebook can satisfy some algebraic/geometric relation with the existing codebook. On different subbands (or PRBs), different trajectory precoder can be applied. Even though the term "precoder" is used herein, which typically suggests one out of a fixed number of codewords, actually no limit may be put to the new codebook which can consist of un-countable codewords, for example comprising a unitary matrix parameterized by one or more real variables.

One method to allow a UE to choose the optimal precoder in R_l2 is to give the UE at least two (e.g., three) opportunities to feedback the PMI/RI/CQI as explained herein ("two" is chosen to be able to interpolate between two points by the network element later in the process).

At the k -th CSI feedback opportunity, the CSI-RS is precoded by T_k , and in general, the receiver model considering the precoded CSI-RS transmission (r, s and n are defined in Equation 1) may be given by

r = HT_ks + n (3).

If the wireless channel without precoding is W"inv(T_x) , i.e. H = W inv T) , then the receiver model without applying the transform precoder may be given by

r = W_J"inv(T_i )s + n (4).

When the transform precoder is applied, then the receiver model at the UE may be given by r = W_J"inv(T_l )T_ks + n (5).

When k = \ (i.e. on the first CSI feedback opportunity), the receiver model is reduced to r = W^inv{T_x )7 + n = W]'s + n (6).

The CW 1 = W_j , the codeword in the Release 10 codebook, can be selected by the UE. It should be noted that just the knowledge of the Release 10 codebook is adequate for the UE, no additional information on the new codebook is needed. When k = 2,3, a codeword in the Release 10 codebook will be selected, e.g., as CW2 and CW3 . The obtained expected throughput for {T₂,CW2} or {Τ ,ϋ 3} is typically less than that for {T_i,CW\ = W_j} under the channel realization (i.e., H =

The network can perform comparisons among at least two (e.g., three) CSI feedbacks to choose a winner: i.e., the CSI feedback with the highest CQI. From this, one can see that the codebook available to the network does not necessarily have to be the same as the codebook available at the UE. It is important that the network can modify the codebook available at its side without the knowledge of the UE.

If it is further found by the network/network element that 7j , T₂ and T₃ (shown as T1, T2 and T3 in Figures 1 and 2) can be connected on a smooth trajectory as shown in Figure 2. Then instead of using three different occassions of the CSI-RS transmission in the time domain, all transmissions can be conducted in the frequency domain. Then acomparison of CQIs and selection of a winner precoder can be all conducted on the UE side by exploiting frequency selective CSI feedback schemes such as best M and best BP (Bandwdith Part) in the LTE.

The best M is selected/chosen by the UE for subbands out of all the available subbands in the CSI subband feedback scheme. In the table below, the number "M" is specificed for various system bandwidths as follows:

The feedback schemes in the LTE may be useful in this case. In Figures 1 and 2, a slowly chaning trajectory has 6 points on it, and each point corresponds to a precoder, which are 5, ,

S₂ , S₆ . (shown as SI, S2, ..., S6 in Figures 1 and 2). Among them, S_x = 7] , S₂ = T₂ , and S₅ = Γ₃. S₂ can be thought as an interpolation between 7| and T₂ ; and S₄ can be thought as the interpolation between T₂ and T₂. S6 may be chosen/extrapolated from T₂ and T . With the interpolation points S₂ , S₄ and S₆ , effectively the new codebook may become the following union:

It is further noted that according to another embodiment, the codewords initially chosen by the network (eNB) may comprise subbands Γ, 2 6' on another subframe with corresponding codewords S'l, S'2, S'6 (they may or may not be from the Release 10 codebook ), as shown on the right in Figure 1. Generally the network can use new codewords, codewords from the Release 10 codebook and codewords derived from the Release 10 codebook such as transform precoders Jj , T₂ and T₃ as explained herein (where S1=T1,

S3=T2 and S5=T3). On the left and the right in Figure 1, two trajectories of network codewords are shown.

In the frequency selective CSI feedback from the UE, the index/indices of the preferred subband(s), PMI and CQI can be sent by the UE to the network. As the eNB is aware of the mapping between the transform preocoders (codewords) on the slowly changing trajectory and subbands, the index/indices of the preferred subband(s) also inform the eNB implicility about the preferred transform precoder(s)/codeword(s) as demonstrated in Figures 3 and 4. Then each codeword in that codebook on the network side can be determined by a codeword/ transform precoder (SI, S2, ...or S6) and a Release 10 codeword (i.e., W. ) selected by the UE from the codewords Wi, W₂, ..., W₆ (having ranks 1 and 2) as shown in Table 2 below. In Figures 3 and 4 the selected subbands by the UE are subbands 1, 3 and 5 with corresponding codewords CWl, CW3 and CW5.

Using the PMI feedback from the UE, the Release 10 codeword W_} can be determiend by the eNB using the subband index of the preferred (selected by the UE) subband. Also using the knowledge the eNB has on the mapping between the codewords/transform preocoders on the slowly changing trajectory and subbands, the transform precoder (e.g., one of SI, S2, S6) can be also determined by the eNB (refer to Figures 4 and 5). Then the preferred codeword in the new codebook can be determined by the eNB as shown in Figure 5 and further described herein. Table 2. Mapping of subbands, network codewords/transform precoders and UE codewords selected from W_ls W , ..., W_6.

Figure 5 shows a diagram illustrating information available at the eNB after the CSI feedback from the UE showing resources/frequency subbands vs. time including subbands 1 , 2 and 6 with corresponding codewords selected by the UE, and subbands 3, 4 and 5 interpolated by the network from the selected by the UE subbands 1, 2 and 6 with corresponding codewords intpS3, intpS4, intpS5 (based on the Release 10 codebook or not) for the network and corresponding codewords intpCW3, intpCW4, intpCW5 (based on the Release 10 codebook) for the UE ,according to an exemplary embodiment of the invention.

It is further explained herein why a slowly changing trajectory is important: The motivation for interpolating between codewords is to introduce the varying codewords applied in the frequency domain in a gradual way. To illustrate this point, we can imagine what can happen if no restriction is put on the codewords which can be applied from one PRB to another PRB, e.g., an abrupt phase change can happen at the boundary of two neighboring PRBs. The observed channel response at the UE is the composition of the antenna response at the eNB/UE, where the codewords are applied at the eNB. The abrupt phase change at the PRB boundaries substantially will increase the delay spread the UE sees. (Multiplication in the frequency domain is equivalent to convolution in the time domain). Since we are interested to have a precoder/codeword selection scheme transparent to the UE, ideally the UE does not have to change its assumption on he delay spread. Consequently, if we want to control the delay spread increase, we need to have the change to be gradual, so there is no abrupt phase change at PRB/frequency subband boundaries, which justifies using interpolation between codewords. Consequently for the CSI feedback, the gradual change needs to be maintained in the frequency domain. Moreover, for data transmission (e.g., on PDSCH), the gradual change is also needed to ensure thaty channel estimation over adjacent PRBs/frequency subbands is still feasible.

In the case when a trajectory connecting all the transform precoders introduces changes which are too abrupt in the frequency domain, then, according to a further embodiment, the trajectory can be divided into two or more trajectories so in each trajectory, only a subset of transform precoders are transversed or connected with a gradual change from one transform precoder to another. Forming new codebook by the network (eNB) for each of the slowly changing precoder trajectories may be performed seperately but similarly, like for at least one such trajectory as described herein.

In the following, an interpolation example by the network element (eNB) is discussed for carrying out the trajectory interpolation using all the unitary matrices. In general, a 2 2 unitary matrix can be parameterized by

In the example of rank 2 feedback, at each subband where the UE feeds back the PMI, the composition of the artificially introduced gradual change and the selected rank 2 matriix (precoder), can be captured by a point in a 4D (4-dimensional) space

(θ_χ , θ₂ , α, ) (9),

The connecting curve is going through multiple points (corresponding to aforementioned "composition" at different subbands from the CSI feedback ) in the 4D space and may be used to determine the "state" of the desired composition at PRBs/subbands not included in the explicit feedback by the UE.

It is noted that mathematically there are space-filling curves (such as Peano curves in the 2D case) to fill an N-dimensional space by a single curve. As unitary matrices over the 2D or higher dimensional space can be parameterized by a vector (e.g. (θ_χ , <¾ , a, r). ), a trajectory can be constructed to go through all the vectors.

It is further noted that according to further embodiment, a rank of codewords, e.g., SI, S2, ...S6, CW1, CW2, ...CW6 and interpolated codewords shown in examples of Figures 1-5 may not be limited by values 1 or 2, but can be extended to be 3, 4,..., etc.

The elements contributing to different embodiments described herein can be summaraized as follows. • Depending on the deployment scenario and eNB antenna configuration, a new codebook can be built using new codewords, codewords from exiting Release 10 codebook and cosewords such as transform precoders derived from the existing Release 10 in some cases. The new codebook formed by the network (eNB) can be composed from a number of transform precoders such as T_t and a codeword W- (feedbacked by the UE) from an existing codebook in the 3 GPP Release 8/10.

• On the network side, some CSI resources may be used to transmit to the UE to enable CSI observation (e.g. CSI-RS resource in 3 GPP LTE release 10/11);

• On the UE side, the UE can autonomously select the preferred subband(s) (e.g. best M from the 3GGP Release 8/10) and/or subband(s) in 3 GPP Release 11/12 where the UE is allowed to selectively feedback the preferred subband(s), or under the control of eNB to periodically scan the whole transmission bandwidth to feed back the preferred subbands (e.g. BP best from the 3GGP Release 8/10). Some other schemes may be also used, where the UE has a role to feedback only a portion of the observed bandwidth indicating its preference to the eNB.

• On the CSI resources, the network can transmit with a slowly changing precoder trajectory in the frequency domain which does not impact substantially the channel estimation performance/assumption on the UE side. The slowly changling precoder trajectory goes over the transform precoders. In other words, the transform precoders are points in the slowly changing precoder trajectory.The slowly chaning precoder trajectory serves as an interpolator to connect the transform precoders in a smooth way. The precoders applied by the eNB on subbands may be called trajectory precoders S_i , 1 < i≤ N_subband . On the eNB side, a mapping is esbalished between the index of a subband i and the trajectory precoder S. which is applied to that subband.

• In the feedback from a UE, the index (indices) of the selected subband(s) and accompanying PMI/CQI/RI are indicated to the eNB. The selection of subband(s) by the UE is used by the eNB to introduce new codebooks without the knowledge of the UE to implicitly indicate to the network the trajectory precoder. From the CSI feedback, the eNB acquires the following information:

- the preferred codeword W_j from the 3GGP Release 8/10,

- the preferred trajectory precoder S_j from the index of the preferred subband.

• On the network side, the selected subbands and PMI/CQI/RI are used to form an interpolation for other subbands not selected in the CSI report to deduce the best precoder for each subband to fit a desired precoder trajectory for the whole bandwidth.

• A single precoder or slowly changing precoder trajectory from the previous step can be used in the data transmission to the UE. The single/multiple precoder(s) can be chosen to maximize the projected channel throughput (refer to Figure 5). For example, {intpS3, intpCW3} is the codeword on subband 3 interpolated from adjacent points {SI, CW1}, {S2, CW2}, {S6, CW6}. The same goes for {intpS4, intpCW4}, {intpS5, intpCW5} . Further extrapolation instead of interpolation is also possible to determine the codeword on subbands beyond the subband 6 in the example shown in figure 5. Additionally, past information on those subbands can be also used.

Figure 6 shows an exemplary flow chart demonstrating implementation of

embodiments of the invention by the network element (e.g., eNB). It is noted that the order of steps shown in Figure 6 is not absolutely required, so in principle, the various steps may be performed out of the illustrated order. Also certain steps may be skipped, different steps may be added or substituted, or selected steps or groups of steps may be performed in a separate application.

In a method according to the exemplary embodiment shown in Figure 6, in a first step 40, the network element (eNB) forms at least one slowly changing precoder trajectory in a frequency domain with a plurality of frequency subbands/PRBs and corresponding network codeword/transform precoders, wherein each of the plurality of frequency subbands is embedded with one codeword. In a next step 42, the network element sends to a UE a channel state information reference signal (CSI-RS) on a plurality of frequency subbands/PRBs formed using at least one slowly changing precoder trajectory.

In a next step 44, the network element receives from the UE an information report(s) comprising selected two or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband.

According to a further embodiment the information report may be received by the network element using one message or alternatively multiple messages/reports from the user equipment, wherein each report of multiple reports may be dedicated to information related to only one selected subband. For example, if best M approach is used, two or more subbands selected by the UE can be feed back to the eNB in one information report (CSI report). In another approach such as BP best, different subbands may be feed back to the eNB in a time sequence so that each CSI report may comprise information about only one selected subband. In both examples, the eNB is able to collect the channel state information over two or more subbands from one information report (as in the case of best approach) or multiple information reports (as in the case of BP best approach).

In this way, the selection of the network codeword(s) by the UE is implicitly carried out in the selection of subband(s) by the UE. Also frequency selective CSI feedback schemes in general, and the frequency selective CSI feedback schemes such as best M and BP best in LTE in particular, provide an excellent framework to accomplish the selection of the network codeword by the UE without the UE knowing it is doing that, which gives the eNB the freedom to choose and modify the network codewords according to different deployment scenarios without a need for informing the UEs about the new codebook and the corresponding changes in the UE implementation.

In a next step 46, the network element forms by interpolation at least one further slowly changing precoder trajectory in the frequency domain, wherein the further plurality of frequency subbands include the selected two or more of the plurality of frequency subbands and one or more further frequency subbands with corresponding one or more codewords interpolated based on the received information report, wherein mapping is established between the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

In a next step 48, the network element sends data to the UE on one or more subbands/PRBs of the further plurality of frequency subbands/PRBs to maximize a projected throughput using the interpolated at least one further slowly changing precoder trajectory.

It is further noted that according to a further embodiment, steps 42-48 can be repeated indefinitely using the output of step 46 for step 42.

Figure 7 shows an example of a block diagram demonstrating LTE devices including a network element (e.g., eNB) 80 comprised in a network 100, and UE 82 communicating with the eNB 80, according to an embodiment of the invention. Figure 7 is a simplified block diagram of various electronic devices that are suitable for practicing the exemplary embodiments of this invention, and a specific manner in which components of an electronic device are configured to cause that electronic device to operate. The UE 82 may be a mobile phone, a camera mobile phone, a wireless video phone, a portable device or a wireless computer, etc.

The eNB 80 may comprise, e.g., at least one transmitter 80a at least one receiver 80b, at least one processor 80c at least one memory 80d and a subband selection and codebook interpolation application module 80e. The transmitter 80a and the receiver 80b may be configured to provide a wireless communication with the UE 82 (and others not shown in Figure 7), e.g., through a corresponding link 81, according to the embodiments of the invention. The transmitter 80a and the receiver 80b may be generally means for transmitting/receiving and may be implemented as a transceiver, or a structural equivalence thereof. It is further noted that the same requirements and considerations are applied to transmitter and receiver of the UE 82.

Various embodiments of the at least one memory 80d (e.g., computer readable memory) may include any data storage technology type which is suitable to the local technical environment, including but not limited to semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory, removable memory, disc memory, flash memory, DRAM, SRAM, EEPROM and the like. Various embodiments of the processor 80c include but are not limited to general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and multi-core processors. Similar embodiments are applicable to memories and processors in other wireless devices such as UE 82 shown in Figure 7.

The subband selection and codebook interpolation application module 80e may provide various instructions for performing steps 40-48 shown in Figure 6. The module 80e may be implemented as an application computer program stored in the memory 80d, but in general it may be implemented as software, firmware and/or hardware module or a combination thereof. In particular, in the case of software or firmware, one embodiment may be implemented using a software related product such as a computer readable memory (e.g., non-transitory computer readable memory), computer readable medium or a computer readable storage structure comprising computer readable instructions (e.g., program instructions) using a computer program code (i.e., the software or firmware) thereon to be executed by a computer processor. Furthermore, the module 80e may be implemented as a separate block or may be combined with any other module/block of the device 80, or it may be split into several blocks according to their functionality.

The UE 82 may have similar components as the eNB 80, as shown in Figure 7, so that the above discussion about components of the eNB 80 is fully applicable to the components of the UE 82.

A CSI feedback application module 87 in the UEs 82 may assist the eNB 80 to perform step 44 (in response to step 42) shown in Figure 6. The module 87 may be implemented as an application computer program stored in the memory 83 of UE, but in general it may be implemented as software, firmware and/or hardware module or a combination thereof. In particular, in the case of software or firmware, one embodiment may be implemented using a software related product such as a computer readable memory (e.g., non-transitory computer readable memory), computer readable medium or a computer readable storage structure comprising computer readable instructions (e.g., program instructions) using a computer program code (i.e., the software or firmware) thereon to be executed by a computer processor. Furthermore, the module 87 may be implemented as a separate block or may be combined with any other module/block of the device 82, or it may be split into several blocks according to their functionality.

It is noted that various non-limiting embodiments described herein may be used separately, combined or selectively combined for specific applications.

Further, some of the various features of the above non-limiting embodiments may be used to advantage without the corresponding use of other described features. The foregoing description should therefore be considered as merely illustrative of the principles, teachings and exemplary embodiments of this invention, and not in limitation thereof.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the invention, and the appended claims are intended to cover such modifications and

arrangements.

Claims

CLAIMS: What is claimed is:

1. A method comprising:

sending, by a network element to a user equipment, a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword;

receiving by the network element from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and

mapping by the network element the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

2. The method of claim 1, further comprising:

sending data by the network element to the user equipment on the selected one or more subbands using the received information report and said mapping.

3. The method of claim 1 ,wherein a number of the selected one or more of the plurality of frequency subbands is more than one, and the method further comprises:

forming by interpolation at the network element at least one further slowly changing precoder trajectory in the frequency domain, wherein the further plurality of frequency subbands include the selected two or more of the plurality of frequency subbands and one or more further frequency subbands with corresponding one or more codewords interpolated based on the received information report.

4. The method of claim 3, further comprising:

sending data by the network element to the user equipment on one or more subbands of the further plurality of frequency subbands using the interpolated at least one further slowly changing precoder trajectory to maximize a projected throughput.

5. The method of claim 3, further comprising:

sending, by the network element to the user equipment, a further channel state information reference signal on the further plurality of frequency subbands formed using the at least one further slowly changing precoder trajectory in a frequency domain;

receiving by the network element from the user equipment a further information report comprising further selected two or more of the further plurality of frequency subbands and futher related information on precoding matrix index/channel quality indicator/rank indicator for each further selected subband; and

forming by interpolation at the network element at least one still further slowly changing precoder trajectory in the frequency domain, wherein the still further plurality of frequency subbands include the further selected two or more of the plurality of frequency subbands and one or more still further frequency subbands with corresponding further one or more codewords interpolated based on the further received information report.

6. The method of claim 3, wherein a number of subbands in the further plurality of frequency subbands is different than in the plurality of frequency subbands.

7. The method of claim 1, wherein the information report is received by the network element using multiple messages from the user equipment.

8. The method of claim 1, wherein the one codeword or each of the one or more codewords has a rank of two.

9. The method of claim 1, wherein at least one of the embedded codewords is a transform precoder.

10. The method of claim 1 , wherein the user equipment is a mobile phone, a camera mobile phone, a wireless video phone, a portable device or a wireless computer, and wherein the network element is an eNB.

11. The method of claim 1 , wherein the at least one further slowly changing precoder trajectory is formed using a point in a four-dimensional space for a particular subband using unitary matrices.

12. The method of claim 1, wherein each of the corresponding one or more codewords interpolated by the network comprises one codeword interpolated for recognition by the user equipment and another codeword is interpolated for recognition only by the network.

13. An apparatus comprising:

a processing system comprising at least one processor and a memory storing a set of computer instructions, in which the processing system is arranged to cause the apparatus to: send to a user equipment a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword;

receive from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and

map the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.

14. The apparatus of claim 13, wherein the one codeword or each of the one or more codewords has a rank of two.

15. The apparatus of claim 13, wherein at least one of the embedded codewords is a transform precoder.

16. The apparatus of claim 12, wherein a number of subbands in the further plurality of frequency subbands is different than in the plurality of frequency subbands.

17. The apparatus of claim 12, wherein the user equipment is a mobile phone, a camera mobile phone, a wireless video phone, a portable device or a wireless computer and wherein apparatus comprises an eNB.

18. The method of claim 1, wherein a number of the selected one or more of the plurality of frequency subbands is more than one, and the processing system is further arranged to cause the apparatus to:

form by interpolation at least one further slowly changing precoder trajectory in the frequency domain, wherein the further plurality of frequency subbands include the selected two or more of the plurality of frequency subbands and one or more further frequency subbands with corresponding one or more codewords interpolated based on the received information report.

19. The apparatus of claim 18, wherein the processing system is further arranged to cause the apparatus to:

send data to the user equipment on one or more subbands of the further plurality of frequency subbands using the interpolated at least one further slowly changing precoder trajectory to maximize a projected throughput.

20. A computer program product comprising a computer readable medium bearing computer program code embodied herein for use with a computer, the computer program code comprising:

code for sending, by a network element to a user equipment, a channel state information reference signal on a plurality of frequency subbands formed using at least one slowly changing precoder trajectory in a frequency domain, wherein each of the plurality of frequency subbands is embedded with one codeword;

code for receiving by the network element from the user equipment an information report comprising selected one or more of the plurality of frequency subbands and related information on precoding matrix index/channel quality indicator/rank indicator for each selected subband; and

code for mapping by the network element the each selected frequency subband and the one codeword for the each selected frequency subband embedded in the at least one slowly changing precoder trajectory based on the received information report.