Precoding for advanced wireless communication systems

Base station (BS) capable of beamforming in a wireless communication includes a transceiver comprising an antenna array, the transceiver configured to measure SRS from a UE, using at least one portion of the antenna array, and a processor configured to select at least one UL beam vector based on an SRS measurement from a UL beam-codebook comprising a set of beam weight vectors, determine at least one DL beam weight vector corresponding to each of the selected at least one UL beam weight vector, transmit a beamformed CSI-RS by applying the at least one DL beam weight vector to the antenna array, receive a CSI feedback including a PMI from the UE, wherein the PMI is determined based on the beamformed CSI-RS, and construct a precoding channel matrix for the UE based on the PMI and the at least one DL beam weight vector.

TECHNICAL FIELD

The present disclosure relates generally to a codebook design and structure associated with a two dimensional transmit antenna array. Such two dimensional arrays are associated with a type of multiple-input-multiple-output (MIMO) system often termed “full-dimension” MIMO (FD-MIMO).

BACKGROUND

Wireless communication has been one of the most successful innovations in modern history. Recently, the number of subscribers to wireless communication services exceeded five billion and continues to grow quickly. The demand of wireless data traffic is rapidly increasing due to the growing popularity among consumers and businesses of smart phones and other mobile data devices, such as tablets, “note pad” computers, net books, eBook readers, and machine type of devices. In order to meet the high growth in mobile data traffic and support new applications and deployments, improvements in radio interface efficiency and coverage is of paramount importance.

SUMMARY

The present disclosure relates to a pre-5th-Generation (5G) or 5G communication system to be provided for supporting higher data rates beyond 4th-Generation (4G) communication system such as Long Term Evolution (LTE).

In a first embodiment, a base station (BS) capable of beamforming in a wireless communication is provided. The BS includes a transceiver comprising an antenna array, the transceiver configured to measure a sound reference signal (SRS) from a user equipment (UE), using at least one portion of the antenna array, and at least one processor configured to select at least one uplink (UL) beam vector based on an SRS measurement from a UL beam-codebook comprising a set of beam weight vectors, determine at least one downlink (DL) beam weight vector corresponding to each of the selected at least one UL beam weight vector, transmit a beamformed channel state information (CSI)-reference signal (RS) by applying the at least one DL beam weight vector to the antenna array, receive a CSI feedback including a Precoding Matrix Index (PMI) from the UE, wherein the PMI is determined based on the beamformed CSI-RS, and construct a precoding channel matrix for the UE based on the PMI and the at least one DL beam weight vector.

In a second aspect, a method for beamforming in a base station (BS) is provided. The method includes measuring a sound reference signal (SRS) from a user equipment (UE), using at least one portion of an antenna array, selecting at least one uplink (UL) beam vector based on an SRS measurement from a UL beam-codebook comprising a set of beam weight vectors, determining at least one downlink (DL) beam weight vector corresponding to each of the selected at least one UL beam weight vector, transmitting a beamformed channel state information (CSI)-reference signal (RS) by applying the at least one DL beam weight vector to the antenna array, receiving a CSI feedback including a Precoding Matrix Index (PMI) from the UE, wherein the PMI is determined based on the beamformed CSI-RS, and constructing a precoding channel matrix for the UE, based on the PMI and the at least one DL beam weight vector.

In a third aspect, a non-transitory computer-readable medium comprising program code for beamforming in a wireless communication in a base station (BS) is provided. The program code that, when executed by a processor, causes the processor to measure a sound reference signal (SRS) from a user equipment (UE), using at least one portion of an antenna array, select at least one uplink (UL) beam vector based on an SRS measurement from a UL beam-codebook comprising a set of beam weight vectors, determine at least one downlink (DL) beam weight vector corresponding to each of the selected at least one UL beam weight vector, transmit a beamformed channel state information (CSI)-reference symbol (RS) by applying the at least one DL beam weight vector to the antenna array, receive a CSI feedback including a Precoding Matrix Index (PMI) from the UE, wherein the PMI is determined based on the beamformed CSI-RS, and construct a precoding channel matrix for the UE, based on the PMI and the at least one DL beam weight vector.

DETAILED DESCRIPTION

FIG. 1illustrates an example wireless network100according to this disclosure. The embodiment of the wireless network100shown inFIG. 1is for illustration only. Other embodiments of the wireless network100could be used without departing from the scope of this disclosure.

The wireless network100includes an gNodeB (gNB)101, an gNB102, and an gNB103. The gNB101communicates with the gNB102and the gNB103. The gNB101also communicates with at least one Internet Protocol (IP) network130, such as the Internet, a proprietary IP network, or other data network.

Depending on the network type, other well-known terms may be used instead of “gNodeB” or “gNB,” such as “base station” or “access point.” For the sake of convenience, the terms “gNodeB” and “gNB” are used in this patent document to refer to network infrastructure components that provide wireless access to remote terminals. Also, depending on the network type, other well-known terms may be used instead of “user equipment” or “UE,” such as “mobile station,” “subscriber station,” “remote terminal,” “wireless terminal,” or “user device.” For the sake of convenience, the terms “user equipment” and “UE” are used in this patent document to refer to remote wireless equipment that wirelessly accesses an gNB, whether the UE is a mobile device (such as a mobile telephone or smartphone) or is normally considered a stationary device (such as a desktop computer or vending machine).

As described in more detail below, one or more of BS101, BS102and BS103include 2D antenna arrays as described in embodiments of the present disclosure. In some embodiments, one or more of BS101, BS102and BS103support the codebook design and structure for systems having 2D antenna arrays.

AlthoughFIG. 1illustrates one example of a wireless network100, various changes may be made toFIG. 1. For example, the wireless network100could include any number of gNBs and any number of UEs in any suitable arrangement. Also, the gNB101could communicate directly with any number of UEs and provide those UEs with wireless broadband access to the network130. Similarly, each gNB102-103could communicate directly with the network130and provide UEs with direct wireless broadband access to the network130. Further, the gNB101,102, and/or103could provide access to other or additional external networks, such as external telephone networks or other types of data networks.

FIGS. 2A and 2Billustrate example wireless transmit and receive paths according to this disclosure. In the following description, a transmit path200may be described as being implemented in an gNB (such as gNB102), while a receive path250may be described as being implemented in a UE (such as UE116). However, it will be understood that the receive path250could be implemented in an gNB and that the transmit path200could be implemented in a UE. In some embodiments, the receive path250is configured to support the codebook design and structure for systems having 2D antenna arrays as described in embodiments of the present disclosure.

The transmit path200includes a channel coding and modulation block205, a serial-to-parallel (S-to-P) block210, a size N Inverse Fast Fourier Transform (IFFT) block215, a parallel-to-serial (P-to-S) block220, an add cyclic prefix block225, and an up-converter (UC)230. The receive path250includes a down-converter (DC)255, a remove cyclic prefix block260, a serial-to-parallel (S-to-P) block265, a size N Fast Fourier Transform (FFT) block270, a parallel-to-serial (P-to-S) block275, and a channel decoding and demodulation block280.

In the transmit path200, the channel coding and modulation block205receives a set of information bits, applies coding (such as a low-density parity check (LDPC) coding), and modulates the input bits (such as with Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM)) to generate a sequence of frequency-domain modulation symbols. The serial-to-parallel block210converts (such as de-multiplexes) the serial modulated symbols to parallel data in order to generate N parallel symbol streams, where N is the IFFT/FFT size used in the gNB102and the UE116. The size N IFFT block215performs an IFFT operation on the N parallel symbol streams to generate time-domain output signals. The parallel-to-serial block220converts (such as multiplexes) the parallel time-domain output symbols from the size N IFFT block215in order to generate a serial time-domain signal. The add cyclic prefix block225inserts a cyclic prefix to the time-domain signal. The up-converter230modulates (such as up-converts) the output of the add cyclic prefix block225to an RF frequency for transmission via a wireless channel. The signal may also be filtered at baseband before conversion to the RF frequency.

A transmitted RF signal from the gNB102arrives at the UE116after passing through the wireless channel, and reverse operations to those at the gNB102are performed at the UE116. The down-converter255down-converts the received signal to a baseband frequency, and the remove cyclic prefix block260removes the cyclic prefix to generate a serial time-domain baseband signal. The serial-to-parallel block265converts the time-domain baseband signal to parallel time domain signals. The size N FFT block270performs an FFT algorithm to generate N parallel frequency-domain signals. The parallel-to-serial block275converts the parallel frequency-domain signals to a sequence of modulated data symbols. The channel decoding and demodulation block280demodulates and decodes the modulated symbols to recover the original input data stream.

Each of the gNBs101-103may implement a transmit path200that is analogous to transmitting in the downlink to UEs111-116and may implement a receive path250that is analogous to receiving in the uplink from UEs111-116. Similarly, each of UEs111-116may implement a transmit path200for transmitting in the uplink to gNBs101-103and may implement a receive path250for receiving in the downlink from gNBs101-103.

Each of the components inFIGS. 2A and 2Bcan be implemented using only hardware or using a combination of hardware and software/firmware. As a particular example, at least some of the components inFIGS. 2A and 2Bmay be implemented in software, while other components may be implemented by configurable hardware or a mixture of software and configurable hardware. For instance, the FFT block270and the IFFT block215may be implemented as configurable software algorithms, where the value of size N may be modified according to the implementation.

AlthoughFIGS. 2A and 2Billustrate examples of wireless transmit and receive paths, various changes may be made toFIGS. 2A and 2B. For example, various components inFIGS. 2A and 2Bcould be combined, further subdivided, or omitted and additional components could be added according to particular needs. Also,FIGS. 2A and 2Bare meant to illustrate examples of the types of transmit and receive paths that could be used in a wireless network. Any other suitable architectures could be used to support wireless communications in a wireless network.

FIG. 3Aillustrates an example UE116according to this disclosure. The embodiment of the UE116illustrated inFIG. 3Ais for illustration only, and the UEs111-115ofFIG. 1could have the same or similar configuration. However, UEs come in a wide variety of configurations, andFIG. 3Adoes not limit the scope of this disclosure to any particular implementation of a UE.

The UE116includes an antenna305, a radio frequency (RF) transceiver310, transmit (TX) processing circuitry315, a microphone320, and receive (RX) processing circuitry325. The UE116also includes a speaker330, a main processor340, an input/output (I/O) interface (IF)345, a keypad350, a display355, and a memory360. The memory360includes a basic operating system (OS) program361and one or more applications362.

The main processor340is also coupled to the keypad350and the display unit355. The operator of the UE116can use the keypad350to enter data into the UE116. The display355may be a liquid crystal display or other display capable of rendering text and/or at least limited graphics, such as from web sites.

The memory360is coupled to the main processor340. Part of the memory360could include a random access memory (RAM), and another part of the memory360could include a Flash memory or other read-only memory (ROM).

AlthoughFIG. 3Aillustrates one example of UE116, various changes may be made toFIG. 3A. For example, various components inFIG. 3Acould be combined, further subdivided, or omitted and additional components could be added according to particular needs. As a particular example, the main processor340could be divided into multiple processors, such as one or more central processing units (CPUs) and one or more graphics processing units (GPUs). Also, whileFIG. 3Aillustrates the UE116configured as a mobile telephone or smartphone, UEs could be configured to operate as other types of mobile or stationary devices.

FIG. 3Billustrates an example gNB102according to this disclosure. The embodiment of the gNB102shown inFIG. 3Bis for illustration only, and other gNBs ofFIG. 1could have the same or similar configuration. However, gNBs come in a wide variety of configurations, andFIG. 3Bdoes not limit the scope of this disclosure to any particular implementation of an gNB. It is noted that gNB101and gNB103can include the same or similar structure as gNB102.

As shown inFIG. 3B, the gNB102includes multiple antennas370a-370n, multiple RF transceivers372a-372n, transmit (TX) processing circuitry374, and receive (RX) processing circuitry376. In certain embodiments, one or more of the multiple antennas370a-370ninclude 2D antenna arrays. The gNB102also includes a controller/processor378, a memory380, and a backhaul or network interface382.

The RF transceivers372a-372nreceive, from the antennas370a-370n, incoming RF signals, such as signals transmitted by UEs or other gNBs. The RF transceivers372a-372ndown-convert the incoming RF signals to generate IF or baseband signals. The IF or baseband signals are sent to the RX processing circuitry376, which generates processed baseband signals by filtering, decoding, and/or digitizing the baseband or IF signals. The RX processing circuitry376transmits the processed baseband signals to the controller/processor378for further processing.

The TX processing circuitry374receives analog or digital data (such as voice data, web data, e-mail, or interactive video game data) from the controller/processor378. The TX processing circuitry374encodes, multiplexes, and/or digitizes the outgoing baseband data to generate processed baseband or IF signals. The RF transceivers372a-372nreceive the outgoing processed baseband or IF signals from the TX processing circuitry374and up-converts the baseband or IF signals to RF signals that are transmitted via the antennas370a-370n.

The controller/processor378can include one or more processors or other processing devices that control the overall operation of the gNB102. For example, the controller/processor378could control the reception of forward channel signals and the transmission of reverse channel signals by the RF transceivers372a-372n, the RX processing circuitry376, and the TX processing circuitry374in accordance with well-known principles. The controller/processor378could support additional functions as well, such as more advanced wireless communication functions. For instance, the controller/processor378can perform the blind interference sensing (BIS) process, such as performed by a BIS algorithm, and decodes the received signal subtracted by the interfering signals. Any of a wide variety of other functions could be supported in the gNB102by the controller/processor378. In some embodiments, the controller/processor378includes at least one microprocessor or microcontroller.

The controller/processor378is also capable of executing programs and other processes resident in the memory380, such as a basic OS. The controller/processor378is also capable of supporting channel quality measurement and reporting for systems having 2D antenna arrays as described in embodiments of the present disclosure. In some embodiments, the controller/processor378supports communications between entities, such as web RTC. The controller/processor378can move data into or out of the memory380as required by an executing process.

The controller/processor378is also coupled to the backhaul or network interface382. The backhaul or network interface382allows the gNB102to communicate with other devices or systems over a backhaul connection or over a network. The interface382could support communications over any suitable wired or wireless connection(s). For example, when the gNB102is implemented as part of a cellular communication system (such as one supporting 5G, LTE, or LTE-A), the interface382could allow the gNB102to communicate with other gNBs over a wired or wireless backhaul connection. When the gNB102is implemented as an access point, the interface382could allow the gNB102to communicate over a wired or wireless local area network or over a wired or wireless connection to a larger network (such as the Internet). The interface382includes any suitable structure supporting communications over a wired or wireless connection, such as an Ethernet or RF transceiver.

The memory380is coupled to the controller/processor378. Part of the memory380could include a RAM, and another part of the memory380could include a Flash memory or other ROM. In certain embodiments, a plurality of instructions, such as a BIS algorithm is stored in memory. The plurality of instructions are configured to cause the controller/processor378to perform the BIS process and to decode a received signal after subtracting out at least one interfering signal determined by the BIS algorithm.

As described in more detail below, the transmit and receive paths of the gNB102(implemented using the RF transceivers372a-372n, TX processing circuitry374, and/or RX processing circuitry376) support communication with aggregation of FDD cells and TDD cells.

AlthoughFIG. 3Billustrates one example of an gNB102, various changes may be made toFIG. 3B. For example, the gNB102could include any number of each component shown inFIG. 3. As a particular example, an access point could include a number of interfaces382, and the controller/processor378could support routing functions to route data between different network addresses. As another particular example, while shown as including a single instance of TX processing circuitry374and a single instance of RX processing circuitry376, the gNB102could include multiple instances of each (such as one per RF transceiver).

FIG. 4illustrates logical port to antenna port mapping400, according to some embodiments of the current disclosure. In the figure, Tx signals on each logical port is fed into an antenna virtualization matrix (e.g., of a size M×1), output signals of which are fed into a set of M physical antenna ports. In some embodiments, M corresponds to a total number or quantity of antenna elements on a substantially vertical axis. In some embodiments, M corresponds to a ratio of a total number or quantity of antenna elements to S, on a substantially vertical axis, wherein M and S are chosen to be a positive integer.

FIG. 5illustrates an exemplary full-dimensional (FD) multiple input, multiple output (MIMO) or massive MIMO antenna array according to one embodiment of the present disclosure. The embodiment shown inFIG. 5is for illustration only. Other embodiments could be used without departing from the scope of the present disclosure.

As illustrated, the MIMO antenna array includes X-pol antenna element pairs comprising M rows and N columns of X-pol element pair. Each X-pol element pair comprises two antennas polarized into two directions, e.g., +45 degs and −45 degs.

In certain embodiments, gNB is equipped with 2D rectangular antenna array comprising M rows and N columns with P=2 polarized, wherein each element is indexed with (m, n, p), and m=0, . . . , M−1, n=0, . . . , N−1, p=0, . . . , P−1. In one example (1-dimensional (1D) subarray partition), an antenna array comprising a column with a same polarization of a 2D rectangular array is partitioned into M groups of consecutive elements, and the M groups correspond to the M transceivers (TXRUs) in a column with a same polarization in the TXRU array inFIG. 5. In later embodiments, (M,N) is sometimes denoted as (NH, NV) or (N1, N2).

In some embodiments, a UE is configured with a CSI-RS resource comprising Q=MNP number of CSI-RS ports, wherein the CSI-RS resource is associated with MNP number of resource elements (REs) in a pair of PRBs in a subframe.

A UE is configured with a CSI-RS configuration via higher layer, configuring Q antenna ports—antenna ports A(1) through A(Q). The UE is further configured with CSI reporting configuration via higher layer in association with the CSI-RS configuration. The CSI reporting configuration includes information element (IE) indicating the CSI-RS decomposition information (or component PMI port configuration). The information element may comprise at least two integers, say N1and N2, which respectively indicates a first number of antenna ports for a first dimension, and a second number of antenna ports for a second dimension, wherein Q=N1·N2.

When the UE is configured with (N1, N2), the UE calculates CQI with a composite precoder constructed with two-component codebooks, N1-Tx codebook (codebook 1) and N2-Tx codebook (codebook 2). When W1and W2are respectively are precoders of codebook 1 and codebook 2, the composite precoder (of size P×(rank)) is the (columnwise) Kronecker product of W=W1⊗W2. If PMI reporting is configured, the UE will report at least two component PMI corresponding to selected pair of W1and W2.

In some embodiments, the BS includes a uniform rectangular array (URA) with M vertical, and N horizontal TXRUs, with antenna spacing of (dV, dH). The total number of TXRUs is denoted as NTXRU=MN. The UL and DL wavelengths are denoted as: λULand λDL; and the corresponding center frequencies are: fULand fDL

FIG. 6illustrates an exemplary flowchart for the hybrid CSI acquisition and MU-MIMO precoding according to one embodiment of the present disclosure. The embodiment of the method600shown inFIG. 6is for illustration only. One or more of the components illustrated inFIG. 6can be implemented in specialized processing circuitry configured to perform the noted functions or one or more of the components can be implemented by one or more processors executing instructions to perform the noted functions. Other embodiments are used without departing from the scope of the present disclosure.

The method600for the hybrid CSI acquisition and MU-MIMO precoding begins with step610. In step610, a base station (BS) configures and receives a sound reference signal (SRS) transmitted from UE, using at least one portion of the antenna array. In one embodiment, the BS can activate at lest one portion of the antenna array when receiving the SRS to generate one uplink (UL) beam vector.

In step620, the BS selects at least one uplink (UL) beam vector based on an SRS measurement from a UL beam-codebook comprising a set of beam weight vectors, each of which can produce a UL beam.

In one embodiment, the UL beam weight vector can be a vertical UL beam weight vector or a horizontal UL beam weight vector. Alternatively, the UL beam weight vector includes a vertical UL beam weight vector and a horizontal UL beam weight vector.

In another embodiment, the UL beam weight vector can be denoted as wUL=wHUL⊗wVUL, where ⊗ denotes Kronecker product. In cases where the beam weight vectors steer the beam into a pair of azimuth and elevation angles (ϕ, θ), the UL beam weights can be denoted as wUL=wHUL(ϕ)⊗wxUL(θ)

In one method, the beam weight vectors for azimuth and elevation dimensions are oversampled DFT vectors which can steer the beam to azimuth angle ϕ and elevation angle θ.

In another method, the beam weight vectors for azimuth and elevation dimensions are oversampled DFT vectors constructed with oversampling factors of OMand ON.

In another embodiment, the beam codebook is constructed with are oversampled DFT vectors constructed with oversampling factors of OMand ON, and then converted to oversampled DFT vectors which can steer the beam to azimuth angle ϕ and elevation angle θ, with applying codebook index to angle transformation:

Given codebook index kH, the elevation angle ϕ can be derived with solving the following equation for ϕ:

Given codebook index kV, the elevation angle θ can be derived with solving the following equation for θ:

In step630, the BS applies a frequency translation on a UL beam weight vector(s) corresponding to a UL beam(s) to predict a DL beam weight vector(s).

For example, if a vertical UL beam vector is selected in step620, the BS applies the frequency translation on a vertical UL beam weight vector to predict a vertical DL beam weight vector. Also, if a horizontal UL beam vector is selected in step620, the BS applies the frequency translation on a horizontal UL beam weight vector to predict a horizontal DL beam weight vector. In addition, if vertical and horizontal UL beam vectors are selected in step620, the BS applies the frequency translations on the vertical and horizontal UL beam weight vectors to predict vertical and horizontal DL beam weight vectors, respectively.

Given an oversampled UL DFT vector wUL=wHUL(ϕ)⊗wVUL(θ) which can steer the beam to azimuth angle ϕ and elevation angle θ, the frequency translation can be applied to find a corresponding DL beam weight vector wDL=wHDL(ϕ)⊗wDL(θ).

The DL beam weight vector to be used for the CSI-RS in the DL carrier is determined by applying elementwise power to the ratio of the DL and UL center frequency values, i.e.,

fDLfUL
and normalizing the component weight vectors so that the norm of wDLis one.

Given an oversampled UL DFT vector wUL=wHUL(kH)⊗wVUL(kV), the frequency translation can be applied to find a corresponding DL beam weight vector wDL=wHDL(kH)⊗wVDL(kV). The corresponding DL beam weight vector(s) corresponding to the UL beam weight vector(s) is determined by applying elementwise power to the ratio of the DL and UL center frequency values, i.e.,

fDLfUL
and normalizing the component weight vectors so that the norm of wDLis one.

In step640, the BS transmits beamformed CSI-RS to the UE, wherein the CSI-RS is transmitted on the antenna arrays by applying the DL beam weight vector(s) to the antenna array.

For example, if a vertical DL beam weight vector is predicted in step630, the BS transmits a beamformed CSI-RS to the UE by applying the vertical DL beam weight vector to the antenna array. Also, if a horizontal DL beam weight vector is predicted in step630, the BS transmits a beamformed CSI-RS to the UE by applying the horizontal DL beam weight vector to the antenna array. In addition, if vertical and horizontal DL beam weight vectors are predicted in step630, the BS transmits a beamformed CSI-RS to the UE by applying the vertical and horizontal DL beam weight vectors to the antenna array.

In some embodiments, the beamforming weights for the CSI-RS are applied to both dimensions. For example, CSI-RS beams are constructed narrow in both azimuth or elevation dimensions.

The narrow CSI-RS beam along a dimension is constructed by applying beam weights over the antenna elements or TXRUs of the 2D antenna array/panel. The beam weights may be applied across all the TXRUs or antenna elements of the 2D antenna panel. In this case, the UL beam weight vector can be denoted as wUL=wHUL⊗wVUL; and the DL beam weight vector can be denoted as wDL=wHDL⊗wVDLwhere ⊗ denotes Kronecker product.

In cases where the beam weight vectors steer the beam into a pair of azimuth and elevation angles (ϕ, θ), the UL and the DL beam weights can be denoted as wUL=wHUL(ϕ)⊗wVUL(θ) and wDL=wHDL(ϕ)⊗wVDL(θ), respectively.

In one method, the beam weight vectors for azimuth and elevation dimensions are oversampled DFT vectors which can steer the beam to azimuth angle ϕ and elevation angle θ.

In another method, the beam weight vectors for azimuth and elevation dimensions are oversampled DFT vectors constructed with oversampling factors of OMand ON.

In some embodiments, the beamforming weights for the CSI-RS are applied to a single dimension. For example, CSI-RS beams are constructed narrow in either azimuth or elevation dimension, wide in the other dimension.

The narrow CSI-RS beam along a dimension is constructed by applying beam weights across the antenna elements or TXRUs comprising each column or each row of the 2D antenna array. In one method, wUL=wHUL(ϕ) to steer the beam to an azimuth angle4); in another method, wUL=wVUL(θ) to steer the beam to an elevation angle θ. The frequency translation to the DL vectors can be similarly conducted similarly as the above 2D steering case.

The DL beam weight vector is used for the CSI-RS in the DL carrier. DL beam weight vector wDL=wHDL(ϕ)⊗wVDL(θ).

In step650, the UE receives a beamformed CSI-RS from the BS. Based on beamformed CSI-RS, the UE derives CSI feedbacks from, for example, a codebook and provides the CSI feedbacks to the BS. The CSI feedbacks include a Precoding Matrix Index (PMI), a Channel Quality Indication (CQI) and a rank indicator (RI). For this operation, the UE may have been configured with, for example, the transmission mode 9.

For example, if a beamformed CSI-RS was produced with the vertical DL beam weight vector in step640, the PMI includes indications on a horizontal DL beam vector and a co-phase value. Also, if a beamformed CSI-RS was produced with a horizontal DL beam vector in step640, the PMI includes indications on a vertical DL beam vector and a co-phase value. In addition, if a beamformed CSI-RS was produced with vertical and horizontal DL beam vectors in step640, the PMI includes an indication on a co-phase value.

In step660, using the feedback CSI comprising PMI, CQI and RI together with the DL beam weight vector(s) predicted in step630, the BS can identify a vertical DL beam vector, a horizontal DL beam vector, and a co-phase value. With these two beam vectors and one co-phase value, the BS reconstructs a precoder matrix for the UE. For example, the precoder matrix can be calculated as follows:

W=1p⁢⁡[wH⊗wV⁢⁢ϕn⁢wH⊗wv⁢⁢];(11)
wherein p is a normalization factor to make total transmission power 1, and wVis a vertical DL beam vector, wHis a horizontal DL beam vector, and a co-phase value.

In step670, the BS performs scheduling, precoding and link adaptation operations, using the reconstructed precoding channel matrices for the UE.

In the 3 carrier aggregation (3CA) system, SRS is only available at one carrier frequency called primary cell (Pcell) while the other two carrier frequencies only have UE feedback available and called secondary cells (Scell). SRS can be used to find CSI at Pcell. For the 3CA system, the disclosure provides the three options below for construction CSI at Scell where UE feedback only is available. Let HsrsϵCNTxRU*#SCsbe the received SRS at Pcell at all available subcarriers (SCs) for all TXRUs of the BS. The CSI at Scell can be constructed using one of three methods.

The first option utilizes wideband (WB) version of Hsrswhich is denoted by hsrsWBas WB CSI for Scell. In this option, hsrsWBis the dominant eigen vector of the empirical covariance of the columns of Hsrs=[hsrs1, hsrs2, . . . , hsrs#SCs] where empirical conariance matrix is denoted by Hsrscov. Specifically,
Hsrscov=Σi=1#SCs[hsrsi*(hsrsi)*]
and
hsrsWB=v1(Hsrscov)  (12)
where v1.) represents the most dominant eigen vector of the matrix it operates on.

The second option utilizes hsrsWBderived in the first option, but the second option applies subband PMI feedback from each UE to construct subband CSI to be used in Scell. We denote subband CSI used in Scell by HsrsSB=[HsrsSB,1, HsrsSB,2, . . . , HsrsSB,#RBGs]ϵCNTXRU*#RBGswhere feedback reporting may occur at each resource block group (RBG). If PMI feedback (containing only cophase information) for this UE is P=[p1, p2, . . . p#RBG]ϵC2*#RBGs, then

As the third option, if UE is configured as the transmission mode 9 (TM9), a method similar to hybrid precoding can be used. Using precoded CSI-RS, the third option is able to get both the direction of the horizontal channel and the cophase information from each UE. Vertical channel can be obtained using dominant Eigenvector of the empirical covariance of vertical SRS signals across available subcarriers using all columns of the BS. Empirical covariance is calculated as done in method 1 using only vertical channels (i.e., hsrsi(1:M), hsrsi(M+1:2M), . . . , hsrsi(MN−M+1:MN)). In particular

Hsrscov=∑k=12⁢N⁢∑i=1#⁢SCs⁢[hsrsi⁡(M2⁢(k-1)+1⁢:⁢M2⁢k)*(hsrsi⁡(M2⁢(k-1)+1:M2⁢k))*](14)
Then, the vertical direction of the channel is calculated to be
hverticalWB=v1(Hsrscov)  (15)
Following that, the CSI can be constructed exactly as in hybrid precoding method.

To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. § 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim. Use of any other term, including without limitation “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” “system,” “processor,” or “controller,” within a claim is understood by the applicants to refer to structures known to those skilled in the relevant art and is not intended to invoke 35 U.S.C. § 112(f).