Patent Description:
The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio interface supports both Frequency division duplex (FDD) and Time division duplex (TDD). A multiple input multiple output (MIMO) technique is an advanced antenna technique for a wireless device to improve spectral efficiency and thereby boost overall system capacity of 3GPP LTE systems. MIMO may be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. Massive MIMO systems (i.e., <NUM>+ antennas) are currently under investigation for <NUM> systems.

Ideal linear precoding uses full channel-state information (CSI) at the transmitter. This may be possible only for TDD-based systems but not practical for FDD-based systems. Codebook based precoding allows the receiver to explicitly identify a precoding matrix/vector based on a codebook that should be used for transmission. In the 3GPP LTE standard, separate codebooks are defined for various combinations of the number of transmit antennas and the number of transmission layers. The latter is also called rank information (RI).

For reporting RI and associated precoding matrix index (PMI), the user equipment (UE) uses an exhaustive search over all the precoding codebook elements. The exhaustive search is difficult to implement with the increased number of transmit antennas in massive MIMO systems. For example, in systems with four transmit antenna ports, the UE needs to search over all the <NUM> precoding entities for finding rank information and precoding index. Similarly, for <NUM> transmit antennas, the UE needs to search over all the <NUM> elements defined in the 3GPP standard. With large scale MIMO, the codebook size becomes larger. Thus, the UE complexity increases with additional transmit antennas deployed at the eNode B. Also, frequent CSI computation drains the UE battery by increasing power consumption, consuming more memory, and using additional processing units at the UE.

<CIT> relates to a wireless communication system, and more particularly, to a method and apparatus for efficient feedback in a wireless communication system that supports multiple antennas.

<CIT> discloses a method for estimating channel state information (CSI) for a communication channel is disclosed.

<CIT> describes how a first N xM codebook of a first rank M may be used to generate a second Nx(N-M) codebook of a second rank (N-M).

The various implementations operate in wireless communication systems such as mmWave, Institute of Electrical and Electronics Engineers (IEEE) <NUM> standards-based systems, future <NUM>rd Generation Partnership Project (3GPP) <NUM> systems based on mmWave as well as other wireless systems. For simplicity, subsequent descriptions refer to communications at the user equipment (UE). However, other implementations may be applied to communications at the base station as well.

<FIG> is a diagram illustrating a wireless communication system, in accordance with various implementations. For example, the wireless communication system <NUM> may be a cellular system that enables a wireless communication device <NUM> to communicate with one or more base stations <NUM> (e.g., evolved Node B (eNB), access point, transmission node, RRU, RRH, nodes in distributed antenna systems (DAS), donor node controlling relay, base transceiver state (BTS), multi-standard radio (MSR)) over one or more wireless channels using a wireless communication technique (e.g., mmWave, time division duplex (TDD), frequency division duplex (FDD)).

The wireless communication device <NUM> may be a non-stationary device. For example, the wireless communication device <NUM> may include mobile radiotelephones, tablet computers, lap top computers, and other devices that may communicate over a wireless channel with the base station <NUM>. For consistency and simplicity, the wireless communication device <NUM> is subsequently referred to as user equipment (UE). The UE includes a transceiver and control circuitry coupled to a plurality of antenna elements through which beamforming may be accomplished.

The base station <NUM> may include a plurality of antennas coupled to a transceiver as well as control circuitry to control the base station operation. <FIG> and subsequent figures show only a single antenna for purposes of simplicity and clarity. However, a person of ordinary skill in the art would realize that for beamforming to be accomplished, the base station <NUM> comprises a plurality of antenna elements.

The base station <NUM> has a fixed location and may be part of a stationary base station network including other base stations that are coupled to a larger network. For example, the base station <NUM> may be part of a wired network that is coupled to the Internet. The UE <NUM> may then access the larger network by communicating over the wireless communication channels with the base station <NUM>.

The base station <NUM> communicates over an area <NUM> substantially surrounding the base station antenna. This area <NUM> is typically referred to as a cell <NUM> and may comprise one or more sectors <NUM>, <NUM>, <NUM>. While three different sectors <NUM>, <NUM>, <NUM> are shown making up the cell <NUM> of <FIG>, other implementations may comprise different sector quantities.

While a plurality of UEs <NUM> may be within the cell <NUM> generated by the base station <NUM>, not all of the UEs <NUM> may be in contact with the base station <NUM>. UE <NUM> that is registered with the base station or is attempting to register with the base station <NUM> may be defined as "served UEs". Other UEs may be registered with other, neighboring base stations.

<FIG> is a block diagram of a wireless communication device <NUM>, in accordance with various implementations. The wireless communication device <NUM> may be in the example form of the UE (e.g., see <FIG>), a cellular base station (e.g., eNodeB, eNB), an access point (AP), or some other wireless station. For example, the wireless communication device <NUM> may be a computer, a personal computer (PC), a tablet PC, a hybrid tablet, a personal digital assistant (PDA), or part of any device configured to execute instructions (sequential or otherwise) that specify actions to be taken by the wireless communication device <NUM>.

The term "processor-based system" shall be taken to include any set of one or more communication apparatuses that are controlled by or operated by processing circuitry (e.g., a controller) to individually or jointly execute instructions to perform any one or more of the methodologies discussed herein. A set or sequence of instructions may be executed to cause the communication apparatus to perform any one of the methodologies discussed herein, according to an example implementation.

The wireless communication device <NUM> may include at least one controller <NUM> (e.g., a central processing unit (CPU), a graphics processing unit (GPU) or both, processor cores, compute nodes, etc.), and memory <NUM> that communicate with each other via a link <NUM> (e.g., bus). If the wireless communication device <NUM> is a UE, it may further include a display device <NUM> (e.g., video, LED, LCD) and an alphanumeric input device <NUM> (e.g., a keypad, keyboard). In one implementation, the display device <NUM> and the input device <NUM> may be incorporated in one unit as a touch screen display.

The wireless communication device <NUM> may additionally include a mass storage device <NUM> (e.g., a drive unit, hard disk drive, solid state drive, optical drive) and a network interface device <NUM>. The network interface device <NUM> may include one or more radios (e.g., transmitters and receivers (transceivers)) coupled to a plurality of antenna elements in order to communicate over a wireless network channel <NUM>, as illustrated in <FIG>. The one or more radios may be configured to operate using one or more communication techniques including the method for channel-state information determination disclosed herein. The network interface device <NUM> may also include a wired network interface (e.g., X2 backhaul link).

The storage device <NUM> includes a computer-readable medium <NUM> on which is stored one or more sets of data structures and instructions <NUM> (e.g., software) embodying or utilized by any one or more of the methodologies or functions described herein. The instructions <NUM> may also reside, completely or at least partially, within the memory <NUM> and/or within the controller <NUM> during execution thereof by the communication apparatus <NUM>. A plurality of sets precoding codebook elements including a rank information and a corresponding precoding matrix index may be stored in the memory of the UE.

While the computer-readable medium <NUM> is illustrated in an example implementation to be a single medium, the term "computer-readable medium" may include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more instructions <NUM>.

The UE may additionally include sensors <NUM> such as an accelerometer, a global positioning system (GPS) receiver, a gyroscope, and/or a compass to provide sensory input to the processor <NUM>. These sensors <NUM> may be used to determine a velocity (i.e., magnitude and direction) of the UE.

The UE described herein may implement a MIMO antenna configuration. One example of such a configuration is illustrated in <FIG>.

<FIG> is a block diagram of a MIMO antenna system, in accordance with various implementations. The MIMO system is an advanced antenna technique to improve spectral efficiency and thereby boosting the overall system capacity. The MIMO technique uses a commonly known notation (M × N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used for various technologies are: (<NUM> × <NUM>), (<NUM> × <NUM>), (<NUM> × <NUM>), (<NUM> × <NUM>), (<NUM> × <NUM>) and (<NUM> × <NUM>), (<NUM> × <NUM>), (<NUM> × <NUM>).

MIMO antenna systems can increase data carrying capacity of wireless systems. MIMO can also be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. For these reasons, MIMO is an integral part of the <NUM>rd and <NUM>th generation wireless systems.

<FIG> illustrates the general antenna mapping block <NUM> that maps the outputs of the data modulation blocks <NUM>, <NUM> to the different antenna ports <NUM>. The input to the antenna mapping block <NUM> thus comprises modulation symbols (e.g., QPSK, 16QAM, 64QAM, 256QAM) corresponding to the one or two transport blocks. To be more specific, there is one transport block per transmission time interval (TTI) except for spatial multiplexing, in which case there may be two transport blocks per TTI. The output of the antenna mapping block <NUM> is a set of symbols for each antenna port <NUM>. The symbols of each antenna port <NUM> are subsequently applied to respective orthogonal frequency division multiplexer (OFDM) modulators <NUM>-<NUM>. In other words, the symbols are mapped to the basic OFDM time-frequency grid corresponding to that particular antenna port. The OFDM modulators <NUM>-<NUM> then transmit the symbols from their respective antenna <NUM>-<NUM>.

Massive MIMO is a relatively recent research field in wireless communication where large numbers of antenna elements are deployed at the transmitter. Larger numbers of user terminals can be supported by combining massive MIMO technology with conventional time and frequency division multiplexing via orthogonal frequency division multiplexing. For this purpose, 3GPP may increase the number of antennas at the base station up to <NUM>/<NUM>/<NUM>.

<FIG> is a flow diagram showing the message sequence between a base station <NUM> and a UE <NUM>, in accordance with various implementations. The base station (e.g., eNB) <NUM> initially transmits cell specific and UE specific reference signals <NUM> to the UE <NUM>. The reference signals may include signals such as pilot signals.

The UE <NUM> uses those reference signals to determine condition of the channel as indicated by the channel-state information (CSI) <NUM>. The CSI may include such elements as, for example, channel quality indicator (CQI), precoding matrix index (PMI), and rank information (RI).

The CSI information is transmitted back to the eNB <NUM> over a feedback channel such as physical uplink control channel (PUCCH) for periodic reporting or physical uplink shared channel (PUSCH) for aperiodic reporting. The eNB <NUM> may then determine the parameters for downlink (DL) transmission (i.e., from the eNB to the UE). These parameters may include transmission power, modulation coding scheme (MCS), and physical resource blocks (PRBs).

The eNB <NUM> then transmits these parameters to the UE <NUM> over the DL control channel <NUM> (e.g., physical downlink control channel (PDCCH). However, before sending PDCCH, the eNB sends control format indicator information on PCFICH which is a physical channel providing the UEs with information necessary to decode the set of downlink control channels (PDCCH). After that, the eNB <NUM> may then transmit data traffic <NUM> to the UE <NUM>.

Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal. For example, downlink reference signals may include cell-specific reference signals (CRS), demodulation-reference signals (DM-RS), and channel-state information-reference signals (CSI-RS).

Cell-specific reference signals are transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The cell-specific reference signals can be used by the UE for channel estimation for coherent demodulation of any downlink physical channel except for PMCH and for PDSCH in the case of transmission modes <NUM>, <NUM>, or <NUM>. The cell-specific reference signals may also be used by the terminal to acquire channel-state information. Terminal measurements on cell-specific reference signals may be used as the basis for cell-selection and handover decisions.

Demodulation reference signals may also be referred to as UE-specific reference signals and are specifically intended to be used by UEs for channel estimation for PDSCH in the case of transmission modes <NUM>, <NUM>, <NUM> or <NUM>. The label "UE-specific" relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for PDSCH transmission to that terminal.

CSI-reference signals are specifically intended to be used by UEs to acquire CSI in the case when demodulation reference signals are used for channel estimation. CSI-reference signals have a significantly lower time/frequency density thus implying less overhead as compared to the cell-specific reference signals.

In an LTE system, the downlink control channel (PDCCH) carries information about the scheduling grants. Typically this comprises a number of scheduled MIMO layers, transport block sizes, modulation for each code word, parameters related to HARQ, sub-band locations, and also a respective precoding matrix index corresponding to each sub-band.

Typically, the following information is transmitted by means of the downlink control information (DCI) format: localized/distributed VRB assignment flag, resource block assignment, modulation and coding scheme, HARQ process number, new data indicator, redundancy version, TPC command for PUCCH, downlink Assignment Index, precoding matrix index, and number of layers. Note that all DCI formats may not use all the information as shown above. In general, the contents of PDCCH depends on transmission mode and DCI format.

Also in an LTE system, the uplink control channel carries information about hybrid automatic repeat request-acknowledge (HARQ-ACK) information corresponding to the downlink data transmission, and channel state information. The CSI typically comprises RI, CQI, and PMI. Either PUCCH or PUSCH can be used to carry this information. Note that the PUCCH reporting is periodic and the periodicity of the PUCCH is configured by the higher layers, while the PUSCH reporting is aperiodic. Also note that there are various modes for PUCCH and PUSCH and in general it depends on the transmission mode and the formats is configured via higher layer signaling.

Ideal linear precoding uses full CSI at the transmitter. This may be possible only for TDD based systems but not practical for FDD based systems. Thus, codebook based precoding allows the receiver to explicitly identify a precoding matrix/vector based on a codebook stored in the UE that is used for transmission.

In the 3GPP LTE standard, separate codebooks are defined for various combinations of the number of transmit antennas and the number of transmission layers. The latter is also referred to as rank information (RI). For example, in 3GPP Release <NUM>, a total of <NUM> precoding vectors and matrices are defined as shown in Table <NUM> for <NUM> transmit antennas. Also for each rank in the codebook for the scenarios of RI=<NUM>, <NUM>, <NUM> and <NUM>, <NUM> elements per rank are defined. Table <NUM> is shown only for purposes of illustration and does not limit the present implementations to any certain information contained therein. The 3GPP standard does not specify the criteria for the UE to use to compute the RI and/or the optimum precoding matrices/vectors.

In a conventional CSI selection, an exhaustive search is performed of all of the precoding codebook elements. For example, the UE estimates the channel, computes the post-processing signal-to-interference-plus-noise ratio (SINR) for each entity in the precoding codebook, and the link quality metric (LQMs) (e.g., mutual information, capacity) for each entity in the codebook is computed. Then the precoding matrix index and the corresponding RI that maximizes the LQM is chosen.

For example, using mutual information in closed-loop SM, the UE needs to estimate a suitable CSI, (i.e., CQI/PMI/RI), in order to maximize the throughput and simultaneously maintain the block-error-rate (BLER) constraint that can be mathematically described by a joint (integer) optimization problem: <MAT>.

Unfortunately, this joint (discrete/integer) optimization problem does not have any closed-form solution. Hence, one normally tries to estimate a suitable PMI/RI (independent of CQI). Thereafter, a suitable CQI is estimated accordingly for the chosen PMI and RI. For example, consider a single-cell scenario having perfect time and synchronization, a received system model for (closed-loop) spatial multiplexing (SM) per sub-carrier (post-FFT) can be shown as: <MAT> where, Y ∈ CNr×<NUM> corresponds to a received signal vector, and H ∈ CNr×Nt describes an overall channel matrix incorporating the receiver and transmitter spatial correlation Rr ∈ · Nr×Nr and Rr ∈ · Nr×Nr, respectively. A complex zero-mean Gaussian noise vector n ∈ CNr×<NUM> is having covariance Rn. An unknown complex data/symbol vector is denoted by X ∈ · NL×<NUM> (having normalized power E{xxH}=Rx=I) corresponding to M-QAM (e.g., <NUM>-QAM) constellation. A (complex) precoder WPMI ∈ · Nt×NL is selected from a given/known codebook Π having NΠ number of precoders (where, PMI= {<NUM>,<NUM>,. NΠ -<NUM>}) for a given rank ≤ min{Nr,Nt}.

The post-processing SINR per ith spatial layer for a given PMI, assuming linear-MMSE detector employed at the receiver, reads <MAT> where [A]i,i corresponds to an ith diagonal element of a matrix A.

In order to estimate a suitable PMI/RI, a link-quality metric (LQM) is computed. The LQM (e.g., mean mutual information) may be denoted as mMI (per sub-band/wide-band) and is computed as given below: <MAT>
Where, I (SINRi[k]) is a mutual information that is a function of post-processing SINRi[k] as given in Table <NUM> for ith spatial layer and kth resource-element. The number of resource-elements employed for the computation of the aforementioned LQM is given by a parameter K (depending on the wide-band/sub-band PMI estimate).

After having the estimate of mMI (per sub-band/wide-band), the PMI and RI may be estimated jointly employing unconstrained optimization which can be given as: <MAT>.

In a capacity approach, instead of using the LQM, the capacity information is used to maximize the capacity for every combination of RI, PMI as follows: <MAT>.

Note that the received SINR at the output of the MIMO detector (MMSE, MLD etc.) is a function of channel matrix H, precoding matrix, the noise power spectral density, and the co -channel interference power.

The present implementations reduce the complexity in finding CSI (e.g., rank information, precoding matrix index, channel quality indicator) using an iterative search method (incremental/decremental search) for finding the rank information and associated precoding matrix index, thereby avoiding the conventional exhaustive search for finding this information. The present implementations are based upon the premise that the RI changes very little during normal cellular communications. In fact, the RI mostly changes in steps of ± <NUM> for most Doppler frequency ranges unless the UE is moving at a velocity greater than a predetermined threshold relative to the eNB (e.g., highway velocities). Thus, in the present implementations, the prior RI of past reports is used to compute the RI for future use.

<FIG> is a flowchart showing a method for channel-state information determination in wireless networks, in accordance with various implementations. In block <NUM>, an initial channel-state information is determined for a wireless channel based on a search of a plurality of sets of precoding codebook elements stored in a multiple input multiple output UE. Each set of the plurality of sets precoding codebook elements may include a rank information and a corresponding precoding matrix index such that determining the initial channel-state information comprises determining the rank information and the corresponding precoding matrix index for the wireless channel.

In block <NUM>, a subsequent channel-state information is determined based on the initial channel-state information. The plurality of sets of precoding codebook elements are stored in a predetermined order in the UE and determining the subsequent channel-state information includes selecting the set of precoding codebook elements, including the subsequent rank information that is immediately previous to the set of precoding codebook elements including the initial channel-state information. In another implementation not claimed as such, determining the subsequent channel-state information may include selecting the set of precoding codebook elements, including the subsequent rank information that is immediately subsequent to the set of precoding codebook elements including the initial channel-state information.

In yet another implementation not claimed as such, a velocity of the UE relative to the serving base station may be determined (e.g., using a GPS receiver) and the subsequent channel-state information may be determined based on the search of the plurality of sets of precoding codebook elements or the initial channel-state information when the velocity is less than a threshold (e.g., less than highway speed). The subsequent channel-state information may also be determined (e.g., see <FIG>) based on the initial channel-state information when the velocity is greater than the threshold. In block <NUM>, the subsequent channel-state information may be reported to the serving base station over the wireless channel.

<FIG> is a flowchart showing a method for channel-state information determination in wireless networks, in accordance with various implementations. This method illustrates one method for determining the initial channel-state information.

In block <NUM>, the channel conditions are estimated based on reference signals transmitted over the channel. This estimating may be accomplished based on pilot signals and data signals. In block <NUM>, a signal-interference plus noise ratio (SINR) is determined for each set of precoding codebook elements of the plurality of sets of precoding codebook elements. In block <NUM>, a link quality metric (LQM) is determined for each set of precoding codebook elements of the plurality of sets of precoding codebook elements. In block <NUM>, the rank information and the corresponding precoding matrix index are determined that provide a largest LQM from a plurality of LQMs as discussed previously.

The methods for CSI determination may provide advantages over the convention exhaustive search method for determining CSI. For example, the percentage degradation with respect to full search is zero, while the complexity can be reduced significantly. Battery life on the UE side is improved since the power used to compute RI/PMI is less compared to the conventional exhaustive search method. The UE hardware resources (e.g., memory and processing units) can be partly relieved to allow the UE to efficiently execute additional procedures (e.g. inter-RAT measurements) in parallel with the CSI estimation.

In one implementation, a user equipment includes radio transceiver means coupled to a plurality of antenna elements, wherein the radio transceiver means is a multiple input multiple output (MIMO) radio; and control means, coupled to the radio transceiver means and antenna elements, the control means configured to determine a first rank information for a wireless channel based on a search of a plurality of precoding codebook elements stored in a multiple input multiple output UE and determine a second rank information based on the first rank information. In one implementation, the control means is further configured to report the first and second rank information and respective corresponding precoding matrix indices to a base station over the wireless channel. In one implementation, the control means is further configured to report a third rank information, subsequent to the second rank information, to the base station based on the search of the plurality of precoding codebook elements. The control means may further be configured to estimate a channel condition of the wireless channel based on reference symbols and/or data received over the wireless channel, to determine a signal-to-interference plus noise ratio (SINR), for a first precoding matrix index corresponding to the first rank information, based on the channel condition, to determine a mutual information or capacity of the wireless channel based on the SINR and to select the second rank information and corresponding second precoding matrix index based on the mutual information or the capacity. An implementation of the transceiver may also include a reporting means configured to report the second precoding matrix index, the second rank information, and a channel quality indicator, associated with the second precoding matrix index, to a base station communicating with the UE over the wireless channel.

In an implementation, the control means may further be configured to estimate a channel condition of the wireless channel from reference symbols and/or data received over the wireless channel and to determine a signal-to-interference plus noise ratio (SINR), for each precoding matrix index corresponding to each respective precoding codebook element of the plurality of precoding codebook elements, based on the channel condition. The control means may further be configured to determine a mutual information or capacity of the wireless channel based on the SINR, to select a precoding matrix index based on the mutual information or the capacity and to report rank information and channel quality indicator, associated with the selected precoding matrix index, to a base station communicating with the UE over the wireless channel.

Implementations may be implemented in one or a combination of hardware, firmware and software. Implementations may also be implemented as instructions stored on a computer-readable storage device, which may be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transitory mechanism for storing information in a form readable by a computer. For example, a computer-readable storage device may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. In some implementations, a system may include one or more processors and may be configured with instructions stored on a computer-readable storage device.

Claim 1:
A method comprising:
determining (<NUM>) in a multiple input multiple output user equipment, UE, an initial channel-state information for a wireless channel based on a search of a plurality of sets of precoding codebook elements stored in the UE; and
determining (<NUM>) a subsequent channel-state information based on the initial channel-state information;
wherein the plurality of sets of precoding codebook elements is stored in a predetermined order in the UE and
characterized in that
determining the subsequent channel-state information comprises selecting the set of precoding codebook elements, including a subsequent rank information, that is immediately previous to the set of precoding codebook elements including the initial channel-state information.