WO2011085082A2 - Augmented wireless feedback - Google Patents

Abstract

Methods and systems for transmitting feedback and precoding matrix indicators (PMI) include determining PMI and rank-1 or rank-2 feedback, determining a difference, referred to as a delta metric, between the PMI and its preferred feedback matrix, and transmitting the delta metric to a base station. The timing of delta metric transmissions can be based on detecting the exceeding of a threshold, intervals signaled by a base station, or other factors. Overhead may be reduced by manipulating a determined delta metric so that fewer complex values need to be transmitted and a base station can estimate values of the delta metric that are not transmitted. The delta metric may be mapped using phase and/or amplitude modulation methods. Delta metric transmissions may be repeated by alternately transmitting elements of the delta metric and complex conjugates of those elements.

Description

AUGMENTED WIRELESS FEEDBACK

CROSS-REFERENCE TO RELATED APPLICATIONS

|0001 ] This application claims the benefit of U.S. Provisional Application No.

61/297,523, filed January 22, 2010, and U.S. Provisional Application No. 61 /293,391 , filed

January 8, 201 0, both of which are entitled hereby incorporated by reference herein.

BACKGROUND

[0002] In order to support higher data rate and spectrum efficiency, the Third

Generation Partnership Project (3GPP) Long Term Evolution (LTE) system has been introduced into 3GPP Release 8 (R8). (LTE Release 8 may be referred to herein as LTE R8 or R8-LTE.) In LTE, transmissions on the uplink are performed using Single Carrier Frequency Division

Multiple Access (SC-FDMA). In particular, the SC-FDMA used in the LTE uplink is based on Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S- OFDM) technology. As used hereafter, the terms SC-FDMA and DFT-S-OFDM are used

interchangeably.

[0003] In LTE, a wireless transmit/receive unit (WTRU), alternatively referred to as a user equipment (UE), transmits on the uplink using only a limited, contiguous set of assigned sub-carriers in a Frequency Division Multiple Access (FDMA) arrangement. For example, i f the overall Orthogonal Frequency Division Multiplexing (OFDM) signal or system bandwidth in the uplink is composed of useful sub-carriers numbered 1 to 100, a first given WTRU may be

assigned to transmit on sub-carriers 1 -12, a second WTRU may be assigned to transmit on sub- carriers 1 3-24, and so on. Whi le the di fferent WTRUs may each transmit into only a subset of the available transmission bandwidth, an evolved Node-B (eNodeB) serving the WTR Us may receive the composite uplink signal across the entire transmission bandwidth. \

[0004] LTE Advanced (which includes LTE Release 10 (R 10) and may include future releases such as Release 1 1 , also referred to herein as LTE-A, LTE R 10, or R 10-LTE) is an

enhancement of the LTE standard that provides a fully-compliant 4G upgrade path for LTE and 3G networks. In LTE-A, carrier aggregation is supported, and, unlike in LTE, multiple carriers may be assigned to the uplink, downlink, or both.

[0005] In both LTE and LTE-A, there may be a need to provide feedback from a UE to a network node, such as an eNodeB. Such feedback may include feedback related to multiple- input multiple output (M IMO) operation, such as one or more precoding matrix indicators (PM I). Such feedback may be transmitted from a UE to a network node in an uplink channel, such as a physical uplink control channel (PUCCH). In current implementations, there is a trade-off between the accuracy of feedback provided by a UE and the amount of overhead required for transmitting feedback, i.e., the more accurate the feedback, the higher the overhead.

SUMMARY

[0006] Methods and systems for transmitting feedback and precoding matrix indicators (PMI) are disclosed. In an embodiment, PM1 and rank-1 feedback may be determined, and a difference, referred to as a delta metric, between the PMI and the preferred precoding matrix may be determined. The delta metric may then be transmitted to a base station, in some embodiments in conjunction with the transmitting of PMI. The delta metric may also be transmitted separately from PMI. In rank-2 embodiments, the matrices of the PMI and the preferred precoding matrix may be aligned prior to determining the delta metric. The timing of delta metric transmissions can be based on detecting the exceeding of a threshold, intervals signaled by a base station, or other factors as set forth herein. Overhead may be reduced by manipulating a determined delta metric so that fewer complex values need to be transmitted and a base station can estimate values of the delta metric that are not transmitted. The delta metric may be mapped using phase and/or amplitude modulation methods. Delta metric transmissions may be repeated by alternately transmitting elements of the delta metric and complex conjugates of those elements. These and additional aspects of the current disclosure are set forth in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The following detailed description of disclosed embodiments is better understood when read in conjunction with the appended drawings. For the purposes of illustration, there is shown in the drawings exemplary embodiments; however, the subject matter is not limited to the specific elements and instrumentalities disclosed. In the drawings:

[0008| Figure 1 A is a system diagram of an example communications system in which one or more disclosed embodiments may be implemented.

[0009J Figure 1 B is a system diagram of an example wireless transmit/receive unit (WTRU) that may be used within the communications system illustrated in Figure 1 A.

[0010] Figure 1 C is a system diagram of an example radio access network and an example core network that may be used within the communications system illustrated in Figure 1 A. [001 1 ) Figure 2 illustrates a non-limiting exemplary method of determining and transmitting a delta metric.

(0012 J Figure 3 illustrates another non-limiting exemplary method of determining and transmitting a delta metric.

|0013] Figure 4 illustrates a non-limiting exemplary method of calculating a delta metric.

|0014] Figure 5 illustrates another non-limiting exemplary method of determining and transmitting a delta metric.

[0015] Figure 6 il lustrates a plot of an exemplary non-limiting arc tangent function.

[0016] Figure 7 illustrates a non-limiting exemplary method of calculating an un- quantized precoding matrix.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0017] Figure 1 A is a diagram of an example communications system 100 in which one or more disclosed embodiments may be implemented. The communications system 100 may be a multiple access system that provides content, such as voice, data, video, messaging, broadcast, etc. , to multiple wireless users. The communications system 100 may enable multiple wireless users to access such content through the sharing of system resources, including wireless bandwidth. For example, the communications systems 100 may employ one or more channel access methods, such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single- carrier FDMA (SC-FDMA), and the like.

[0018] As shown in Figure 1 A, the communications system 100 may include wireless transmit/receive units (WTRUs) 102a, 102b, 1 02c, 102d, a radio access network (RAN) 1 04, a core network 106, a public switched telephone network (PSTN) 108, the Internet 1 10, and other networks 1 12, though it will be appreciated that the disclosed embodiments contemplate any number of WTRUs, base stations, networks, and/or network elements. Each of the WTRUs 102a, 102b, 102c, 102d may be any type of device configured to operate and/or communicate in a wireless environment. By way of example, the WTRUs 102a, 102b, 102c, 102d may be configured to transmit and/or receive wireless signals and may include user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a smartphone, a laptop, a netbook, a personal computer, a wireless sensor, consumer electronics, and the like. [0019] The communications systems 100 may also include a base station 1 14a and a base station 1 14b. Each of the base stations 1 14a, 1 14b may be any type of device configured to wirelessly interface with at least one of the WTRUs 102a, 102b, 102c, 102d to facilitate access to one or more communication networks, such as the core network 106, the Internet 1 10, and/or the networks 1 12. By way of example, the base stations 1 14a, 1 14b may be a base transceiver station (BTS), a Node-B, an eNode B, a Home Node B, a Home eNode B, a site controller, an access point (AP), a wireless router, and the like. While the base stations 1 14a, 1 14b are each depicted as a single element, it will be appreciated that the base stations 1 14a, 1 1 4b may include any number of interconnected base stations and/or network elements.

[0020J The base station 1 14a may be part of the RAN 104, which may also include other base stations and/or network elements (not shown), such as a base station controller (BSC), a radio network controller (RNC), relay nodes, etc. The base station 1 14a and/or the base station 1 14b may be configured to transmit and/or receive wireless signals within a particular geographic region, which may be referred to as a cell (not shown). The cell may further be divided into cell sectors. For example, the cell associated with the base station 1 1 4a may be divided into three sectors. Thus, in one embodiment, the base station 1 14a may include three transceivers, i.e. , one for each sector of the cell. In another embodiment, the base station 1 14a may employ multiple-input multiple output (MIMO) technology and, therefore, may utilize multiple transceivers for each sector of the cell.

[0021 ] The base stations 1 14a, 1 14b may communicate with one or more of the WTRUs 102a, 102b, 102c, 102d over an air interface 1 16, which may be any suitable wireless communication link {e.g., radio frequency (RF), microwave, infrared (IR), ultraviolet (UV), visible light, etc.). The air interface 1 16 may be established using any suitable radio access technology (RAT).

[0022] More specifically, as noted above, the communications system 100 may be a multiple access system and may employ one or more channel access schemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. For example, the base station 1 14a in the RAN 104 and the WTRUs 102a, 102b, 102c may implement a radio technology such as Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA), which may establish the air interface 1 16 using wideband CDMA (WCDMA). WCDMA may include communication protocols such as High-Speed Packet Access (HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed Downlink Packet Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA). |0023] In another embodiment, the base station 1 14a and the WTRUs 102a, 102b, 1 02c may implement a radio technology such as Evolved UMTS Terrestrial Radio Access (E-UTRA), which may establish the air interface 1 16 using Long Term Evolution (LTE) and/or LTE- · Advanced (LTE-A).

[0024] In other embodiments, the base station 1 14a and the WTRUs 102a, 102b, 102c may implement radio technologies such as IEEE 802. 16 (i. e. , Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000, CDMA2000 I X, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), Interim Standard 95 (1S-95), Interim Standard 856 (IS-856), Global System for Mobile communications (GSM), Enhanced Data rates for GSM Evolution (EDGE), GSM EDGE (GERAN), and the like.

[0025] The base station 1 14b in FIG. 1 A may be a wireless router, Home Node B, Home eNode B, or access point, for example, and may utilize any suitable RAT for facilitating wireless connectivity in a localized area, such as a place of business, a home, a vehicle, a campus, and the like. In one embodiment, the base station 1 14b and the WTRUs 102c, I 02d may implement a radio technology such as IEEE 802.1 1 to establish a wireless local area network (WLAN). In another embodiment, the base station 1 14b and the WTRUs 102c, I 02d may implement a radio technology such as I EEE 802.1 5 to establish a wireless personal area network (WPAN). In yet another embodiment, the base station 1 14b and the WTRUs 102c, 102d may utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE, LTE-A, etc. ) to establish a picocell or femtoce!l. As shown in FIG. 1 A, the base station 1 14b may have a direct connection to the Internet 1 10. Thus, the base station 1 14b may not be required to access the Internet 1 10 via the core network 106.

[0026] The RAN 104 may be in communication with the core network 106, which may be any type of network configured to provide voice, data, applications, and/or voice over internet protocol (VoIP) services to one or more of the WTRUs 102a, 102b, 102c, 102d. For example, the core network 106 may provide call control, billing services, mobile location-based services, pre-paid calling, Internet connectivity, video distribution, etc. , and/or perform high-level security functions, such as user authentication. Although not shown in FIG. 1 A, it will be appreciated that the RAN 104 and/or the core network 106 may be in direct or indirect communication with other RANs that employ the same RAT as the RAN 104 or a different RAT. For example, in addition to being connected to the RAN 104, which may be utilizing an E-UTRA radio technology, the core network 106 may also be in communication with another RAN (not shown) employing a GSM radio technology. [0027] The core network 106 may also serve as a gateway for the WTRUs 102a, 102b, 102c, 102d to access the PSTN 108, the Internet 1 10, and/or other networks 1 12. The PSTN 108 may include circuit-switched telephone networks that provide plain old telephone service (POTS). The Internet 1 10 may include a global system of interconnected computer networks and devices that use common communication protocols, such as the transmission control protocol (TCP), user datagram protocol (UDP) and the internet protocol (I P) in the TCP/I P internet protocol suite. The networks 1 12 may include wired or wireless communications networks owned and/or operated by other service providers. For example, the networks 1 1 2 may include another core network connected to one or more RANs, which may employ the same RAT as the RAN 104 or a different RAT.

[0028] Some or all of the WTRUs 102a, 102b, 102c, 102d in the communications system 100 may include multi-mode capabilities, i. e. , the WTRUs 102a, 102b, 102c, 102d may include multiple transceivers for communicating with different wireless networks over di fferent wireless links. For example, the WTRU 102c shown in FIG. 1 A may be configured to communicate with the base station 1 14a, which may employ a cellular-based radio technology, and with the base station 1 14b, which may employ an IEEE 802 radio technology.

[0029] Figure I B is a system diagram of an example WTRU 102. As shown in FIG. 1 B, the WTRU 102 may include a processor 1 1 8, a transceiver 120, a transmit/receive element 1 22, a speaker/microphone 124, a keypad 126, a display /touchpad 128, non-removable memory 130, removable memory 1 32, a power source 1 34, a global positioning system (GPS) chipset 1 36, and other peripherals 1 38. It will be appreciated that the WTRU 102 may include any subcombination of the foregoing elements while remaining consistent with an embodiment.

[0030] The processor 1 1 8 may be a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of

microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Array (FPGAs) circuits, any other type of integrated circuit (IC), a state machine, and the like. The processor 1 18 may perform signal coding, data processing, power control, input/output processing, and/or any other functionality that enables the WTRU 102 to operate in a wireless environment. The processor 1 18 may be coupled to the transceiver 120, which may be coupled to the transmit/receive element 122. While FIG. 1 B depicts the processor 1 1 8 and the transceiver 120 as separate components, it will be appreciated that the processor 1 1 8 and the transceiver 120 may be integrated together in an electronic package or chip. |0031 ] The transmit receive element 122 may be configured to transmit signals to, or receive signals from, a base station (e.g., the base station 1 14a) over the air interface 1 16. For example, in one embodiment, the transmit/receive element 122 may be an antenna configured to transmit and/or receive RF signals. In another embodiment, the transmit/receive element 1 22 may be an emitter/detector configured to transmit and/or receive IR, UV, or visible light signals, for example. In yet another embodiment, the transmit/receive element 122 may be configured to transmit and receive both. RF and light signals. It will be appreciated that the transmit/receive element 122 may be configured to transmit and/or receive any combination of wireless signals.

[0032] In addition, although the transmit/receive element 122 is depicted in FIG. 1 B as a single element, the WTRU 102 may include any number of transmit/receive elements 1 22. More specifically, the WTRU 1 02 may employ MIMO technology. Thus, in one embodiment, the WTRU 102 may include two or more transmit/receive elements 1 22 (e.g., multiple antennas) for transmitting and receiving wireless signals over the air interface 1 16.

[0033] The transceiver 120 may be configured to modulate the signals that are to be transmitted by the transmit/receive element 122 and to demodulate the signals that are received by the transmit/receive element 122. As noted above, the WTRU 102 may have multi-mode capabilities. Thus, the transceiver 120 may include multiple transceivers for enabling the WTRU 102 to communicate via multiple RATs, such as UTRA and IEEE 802.1 1 , for example.

|0034] The processor 1 1 8 of the WTRU 102 may be coupled to, and may receive user input data from, the speaker/microphone 124, the keypad 126, and/or the display/touchpad 128 (e.g., a liquid crystal display (LCD) display unit or organic light-emitting diode (OLED) display unit). The processor 1 1 8 may also output user data to the speaker/microphone 124, the keypad 126, and/or the display/touchpad 128. In addition, the processor 1 1 8 may access information from, and store data in, any type of suitable memory, such as the non-removable memory 1 30 and/or the removable memory 1 32. The non-removable memory 1 30 may include random- access memory (RAM), read-only memory (ROM), a hard disk, or any other type of memory storage device. The removable memory 132 may include a subscriber identity module (SI M) card, a memory stick, a secure digital (SD) memory card, and the like. In other embodiments, the processor I 18 may access information from, and store data in, memory that is not physically located on the WTRU 102, such as on a server or a home computer (not shown).

[0035] The processor 1 18 may receive power from the power source 1 34, and may be configured to distribute and/or control the power to the other components in the WTRU 1 02.

The power source 134 may be any suitable device for powering the WTRU 102. For example, the power source 134 may include one or more dry cell batteries (e.g., nickel-cadmium (NiCd), nickel -zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion), etc. ), solar cells, fuel cells, and the like.

[0036] The processor 1 1 8 may also be coupled to the GPS chipset 1 36, which may be configured to provide location information (e.g., longitude and latitude) regarding the current location of the WTRU 102. In addition to, or in lieu of, the information from the GPS chipset 1 36, the WTRU 102 may receive location information over the air interface 1 16 from a base station (e.g., base stations 1 14a, 1 14b) and/or determine its location based on the timing of the signals being received from two or more nearby base stations. It will be appreciated that the WTRU 102 may acquire location information by way of any suitable location-determination method while remaining consistent with an embodiment.

[0037] The processor 1 1 8 may further be coupled to other peripherals 1 38, which may include one or more software and/or hardware modules that provide additional features, functionality and/or wired or wireless connectivity. For example, the peripherals 1 38 may include an accelerometer, an e-compass, a satellite transceiver, a digital camera (for photographs or video), a universal serial bus (USB) port, a vibration device, a television transceiver, a hands free headset, a Bluetooth® module, a frequency modulated (FM) radio unit, a digital music player, a media player, a video game player module, an Internet browser, and the like.

[0038] Figure 1 C is a system diagram of the RAN 104 and the core network 106 according to an embodiment. As noted above, the RAN 104 may employ an E-UTRA radio technology to communicate with the WTRUs 102a, 102b, 102c over the air interface 1 16. The RAN 104 may also be in communication with the core network 106.

[0039] The RAN 104 may include eNode-Bs 140a, 140b, 140c, though it will be appreciated that the RAN 104 may include any number of eNode-Bs while remaining consistent with an embodiment. The eNode-Bs 140a, 140b, 140c may each include one or more transceivers for communicating with the WTRUs 102a, 102b, 102c over the air interface 1 1 6. I n one embodiment, the eNode-Bs 140a, 140b, 140c may implement MIMO technology. Thus, the eNode-B 140a, for example, may use multiple antennas to transmit wireless signals to, and receive wireless signals from, the WTRU 102a.

|0040] Each of the eNode-Bs 1 40a, 140b, 140c may be associated with a particular cel l (not shown) and may be configured to handle radio resource management decisions, handover decisions, scheduling of users in the uplink and/or downlink, and the like. As shown in F IG. 1 C. the eNode-Bs 140a, 140b, 140c may communicate with one another over an X2 interface.

[00411 The core network 1 06 shown in FIG. 1 C may include a mobility management gateway (MME) 142, a serving gateway 144, and a packet data network (PDN) gateway 146. While each of the foregoing elements are depicted as part of the core network 106, it will be appreciated that any one of these elements may be owned and/or operated by an entity other than the core network operator.

|0042] The MME 142 may be connected to each of the eNode-Bs 142a, 142b, 142c in the RAN 104 via an S I interface and may serve as a control node. For example, the MM E 142 may be responsible for authenticating users of the WTRUs 102a, 102b, 102c, bearer

activation/deactivation, selecting a particular serving gateway during an initial attach of the WTRUs 102a, 102b, 102c, and the like. The MME 142 may also provide a control plane function for switching between the RAN 104 and other RANs (not shown) that employ other radio technologies, such as GSM or WCDMA.

[0043] The serving gateway 144 may be connected to each of the eNode Bs 140a, 140b, 140c in the RAN 1 04 via the S I interface. The serving gateway 144 may generally route and forward user data packets to/from the WTRUs 102a, 102b, 102c. The serving gateway 144 may also perform other functions, such as anchoring user planes during inter-eNode B handovers, triggering paging when downlink data is available for the WTRUs 102a, 102b, 102c, managing and storing contexts of the WTRUs 1 02a, 102b, 102c, and the like.

[0044] The serving gateway 144 may also be connected to the PDN gateway 146. which may provide the WTRUs 102a, 1 02b, 102c with access to packet-switched networks, such as the Internet 1 10, to facilitate communications between the WTRUs 102a, 102b, 102c and I P- enabled devices.

[0045] The core network 106 may facilitate communications with other networks. For example, the core network 106 may provide the WTRUs 102a, 102b, 102c with access to circuit- switched networks, such as the PSTN 108, to facilitate communications between the WTRUs 102a, 102b, 102c and traditional land-line communications devices. For example, the core network 106 may include, or may communicate with, an IP gateway (e.g., an IP multimedia subsystem (IMS) server) that serves as an interface between the core network 106 and the PSTN 108. In addition, the core network 106 may provide the WTRUs 102a, 102b, 102c with access to the networks 1 12, which may include other wired or wireless networks that are owned and/or operated by other service providers.

[0046] In an embodiment that utilizes M IMO or other multiple antenna transmission implementations, a predefined codebook may be used to form transmitted layers. A codebook may include a set of predefined precoding matrices. Each precoding matrix may be associated with an index. A UE configured with such a codebook may transmit an index of a precoding matrix to a base station (e.g. , an eNodeB) as one or more precoding matrix indicators (PM Is). PMI may be transmitted on an uplink channel such as a physical uplink control channel (PUCCH). By transmitting PMI, a UE may indicate to a base station the most suitable precoding matrix under current radio conditions.

[0047] Codebook-based feedback may use an effectively quantized version of the channel singular vectors and hence may introduce quantization errors. In such embodiments, some amount of throughput loss may occur, depending on the accuracy of the codebook and the application. Applications such as multiuser MIMO (MU-MIMO) and Coordinated Multipoint (CoMP) that rely on spatial nulling towards co-scheduled or other-cell UEs may benefit from more accurate (i.e. , not quantized) feedback. Feedback accuracy for MU-MIMO appl ications may grow linearly with the signal-to-noise ratio (SNR) in decibels (dB) if a fixed gap to capacity is maintained. For a given total feedback overhead the achievable MU-MIMO sum rate may be maximized when accurate feedback from fewer users is fed back as opposed to inaccurate feedback from more users. Thus, interference nulling due to accurate channel state information (CSI) may be more important than multiuser diversity.

[0048] In an embodiment, a UE may augment PMI feedback with the un-quantized delta (i. e., mathematical difference) between the fed back PMI and its preferred precoding matrix. Such means may enable the accuracy of the combined feedback to naturally grow with SNR. The delta signal may have a small average norm that may get smaller with improved codebook accuracy and may therefore be sent more reliably by appropriate boosting (e.g. , multiplying the delta signal by a certain gain to increase its power). The un-quantized delta may be sent if a certain metric is met. For example, un-quantized delta may be sent if the norm of the delta exceeds a certain threshold, hence reducing the feedback overhead to only cases where the increased overhead over baseline PMI feedback is justified.

[0049] Sending an un-quantized delta may be used where a UE. may have two or more channel feedback options with different accuracy. A UE may choose to feed back the more accurate of the feedback options if a particular metric is met so as to balance uplink overhead increase with downlink throughput improvement.

[0050] A delta may be quantized by using element-wise quantization or vector quantization with different accuracy. Such a delta may be transmitted in a digital fashion. Different levels and/or strengths of coding may be applied to the PMI and delta.

[0051] In an embodiment, a delta may be represented through decomposition according to the level of the detail of the spatial channel information. In particular, the spatial channel information may be transformed to a much small number of independent parameters. [0052] Note that in an embodiment, similar methods to any of those disclosed herein may be applied to the downlink where an eNodeB may feed back a precoder matrix to be used by a UE with multiple transmit antennas in its uplink transmission. For example, a UE may balance beamforming gain to its serving eNodeB while minimizing interference to a nearby eNodeB.

[0053] The embodiments disclosed herein may be used with potential enhanced LTE R 10 codebooks (e.g., differential codebooks, transformed codebooks, and/or multiple description codebooks) since the disclosed embodiments are not reliant on a specific codebook design.

[0054] The embodiments disclosed herein may allow the feedback accuracy to get as close as desired to 'optimal' by using repetition coding (e.g., over the un-quantized portion). This may not be possible with any pure codebook design due to the inherent quantization used in the process of calculating the PM1. Note that by default an eNodeB may average several feedback reports to improve accuracy if the product of mobility and feedback delay is low.

[0055] The embodiments disclosed herein may be applied in homogenous deployments as well as heterogeneous networks (HetNets) where the signal to interference-plus-noise ratio (SINR) or interference level may be much higher and accurate spatial nulling is more crucial, such as CSG Femto, Pico deployments with cell biasing or in the Relay backhaul channel.

[0056] The embodiments disclosed herein may comply with LTE's self contained feedback principle since the un-quantized delta may be sent together with the PMI in the same UL frame. The un-quantized delta may also be sent 'differentially' in successive frames i f the PMI is unchanged.

J0057] The presently disclosed embodiments may provide reduced sensitivity to un- calibrated antennas and reduced sensitivity to different antenna configurations (e.g., correlated, uncorrelated, polarized), an important benefit because the underlying codebook design may be a compromise between all the antenna types.

[0058[ The presently disclosed embodiments may also provide improved frequency granularity of the current LTE R8 PMI feedback structure without any changes to LTE R8. I n implementations with uncorrelated or semi-correlated antennas and medium to high delay spread impulse response, the channel singular vectors may change rapidly within one subband (5 resource blocks (RBs)) and may reduce the effectiveness of MU-MIMO or transmit nulling

(CoMP). While in some cases correlated antennas may be deployed at an eNodeB, LTE deployments may also use at least semi-correlated antennas (e.g., cross-polarized) to better support claimed achievable high downlink throughputs via spatial multiplexing. CoMP joint processing (JP) solutions may have low correlation due to the geographical separation between the eNodeBs, while intra-eNodeB CoMP may use sectored antennas pointing in di fferent directions). In an embodiment, one PMl per subband may be fed back but multiple un-quantized deltas corresponding to several subcarriers or RBs may be spread throughout the subband.

[0059] While some exemplary embodiments disclosed herein describe a UE feeding back rank- 1 or rank-2 PMl (e.g. , as in LTE R8), the presently disclosed embodiments are not limited to a UE that is limited to rank- 1 or rank-2. For example, in MU-MIMO and CoM P implementations of the present subject matter, if a UE decides that rank-2 provides higher SU- M IMO capacity than rank- 1 , the present embodiments allow for improved rank- 1 or rank-2 feedback. This in turn may provide the eNodeB with improved feedback even if rank- 1 is eventually employed because information on the null space may be conveyed. If a UE decides that rank- 1 is optimal for SU-MIMO there may be less need to feed back information about the null space and other applications such as MU-MIMO or CoMP may not require higher than rank- 1 feedback.

[0060] Figure 2 illustrates a non-limiting exemplary method 200 of providing rank- 1 feedback. At block 210, the best PMl for a given subband may be computed, for example, using a processor such processor 1 1 8 of Figure 1 B. This PMl may be denoted by Q herein. At block 220, the best or preferred precoding matrix may be computed, for example, using a processor such processor 1 1 8 of Figure 1 B. This feedback may be denoted by t herein. The preferred precoding matrix may be computed using any means, such as using high accuracy codebooks or differential codebooks, or other means including those disclosed herein. At block 230, a delta metric between V and Q may be computed. The delta metric may yield a signal with smal l norm when Q is close to V.

[0061 ] In an embodiment, the delta metric between V and Q may be computed using an element-wise difference of the complex elements may be used. K may be phase aligned to Q to generate

and compute the element-wise difference E = V_uliwi -Q.

[0062] In an embodiment, the delta metric between V nd Q may be computed using a normalized element-wise difference E = ( V_alij,_n -Q)IQ-

[0063] In an embodiment, the delta metric between and Q may be computed using separate element-wise magnitude and phase difference E = _e ~^&) and E₂ = _e ^{/( |}"^~ -^> where

V_u = p^/ _e-""^«"^,<^^| o> j_{s a} phase aligning of such that the mean phase difference is zero. I n this embodiment, the signals represented in £, and E₂ may have lower norm than E due to the separation of the amplitude and phase.

[0064] Referring again to Figure 2, at block 240, Q may be fed back, for example, as fed back in LTE R8, and E may be fed back as described in more detail herein. Such feedback may be transmitted by a transceiver such as, for example, transceiver 120 of Figure 1 B.

[0065] For an eNodeB with N antennas, E may contain N complex values. Feedback overhead may be reduced by one complex element by using the unitary householder matrix that generated Q as a new basis to represent E, The householder matrix, denoted by W, associated with Q may provide the new representation as E' - W'^■ E where the first element is real and negative. A UE may send the last three complex elements of E' and the eNodeB may estimate the first element such that the resulting precoder has norm 1 . For smal l TV such as N=2 or 4, such reduction in overhead may be substantial.

[00661 Figure 3 illustrates a non-l imiting exemplary method 300 of providing rank-2 feedback. At block 310, the best PMI in a given subband may be computed, for example, using a processor such processor 1 1 8 of Figure 1 B. This PMI may be denoted by Q herein. At block 320, the best or preferred precoding matrix may be computed, for example, using a processor such processor 1 1 8 of Figure I B. This precoding matrix may be denoted by V herein. The preferred precoding matrix may be computed using any means, such as using high accuracy codebooks or differential codebooks, or other means including those disclosed herein. At block 330, the matrices and Q may be aligned, for example, using a processor such processor 1 1 8 of Figure I B.

|0067] In an embodiment, the singular vectors of matrices V and Q may be aligned independently as in rank- 1 . When using this embodiment in rank-2 feedback, the element-wise difference E may not be small. This is because the columns of Q may not be good

approximations of the singular vectors.

[0068] In an embodiment, the singular vectors of matrices V and Q may be aligned using a 2x2 unitary matrix U. In such an embodiment, alignment may be achieved by searching over all the unitary matrices such that || Q - VU || is minimized. This is a well known orthogonal

Procrustes problem, the solution to which may be given by U = V_ipU_p" where U_jp∑ V_p' - Q^H V is the SVD factorization of the 2x2 inner product of Q and V and may be simply calculated using the algorithm provided herein. While the element-wise difference E = VU - Q in this case may have much lower norm the eNodeB may not be able to compute accurate rank- 1 estimation because the singular vectors in V are, mixed up. In embodiments using rank-2 feedback, an eNodeB may accurately calculate the optimal rank- 1 feedback for MU-MI MO or CoMP applications.

|0069| In such an embodiment, a 2-antenna rank-2 PMI (/,· may be used to approximate U. The resulting norm of E may be lower than in embodiments where the singular vectors are aligned independently as in rank- 1 , and may be reasonably close to the use of a 2x2 unitary matrix with a 2-bit codebook. One such codebook may be the current LTE R8 2-antenna rank-2 codebook augmented with the element

^" 1 f

-J J.

Specifically, a UE may phase align the columns of V independently (as in embodiments where the singular vectors are aligned independently as in rank- 1 ) relative to QU, where ί/,· may be chosen such that the norm of the matrix E = V_uli - QU_t may be minimized. Note that the specific metric may be UE implementation specific and may not need standardization.

|0070| Referring again to Figure 3, at block 340 the delta metric between K and Q may be computed, for example, using a processor such processor 1 1 8 of Figure 1 B, by computing the element-wise difference E = V_{a i},_n - QU_m , where m may be the preferred index found at block

330.

[0071 ] At block 350, Q may be fed back, for example, as done in LTE R8. U_m and the first or both columns of E may also be fed back. Such feedback may be transmitted by a transceiver such as, for example, transceiver 120 of Figure I B. The eNodeB may signal the UE to feed back only the first column of E in order to reduce feedback overhead in cases where only rank- 1 feedback is required for MU-MIMO or CoMP applications. If the UE is scheduled in SU- M IMO mode, the eNodeB may use Q, and if the UE is in MU-MIMO mode the eNodeB may get an accurate estimation of the rank- 1 feedback.

[0072) As with the rank- 1 feedback embodiments describes above, a change of basis may enable the reduction of the transmission by one complex element per column. Due to the unitary constraint a second complex element belonging to the second column may not need to be transmitted as the eNodeB can solve two equations with two unknowns. Denoting by ff the LTE R8 householder matrix associated with Q, if may be transformed using U_m. The columns of W may be swapped so that the first two columns equal Q (note that by the construction of the LTE R8 rank-2 codebook, the first columns of Q and W may already be equal). Then the first two columns of IV ay be multiplied by U„, and the last two columns of ff may be multipl ied by U„, to get a new unitary basis U, o .

W = [Q null(Q)]

0 U,

In the new coordinates E'— W'^■ E , the first element of the first column may be real and negative as may be the second element of the second column, therefore they both may not need to be transmitted. Furthermore the first element of the second column may not need to be transmitted.

[0073] For small N such as N=2 or 4, such reduction in overhead may be substantial as only 1 element of E needs to be fed back for N=2 and 5 elements for N=4.

[0074] Note that while embodiments disclosed herein may be described in relation to implicit feedback, such embodiments may easily be converted to explicit feedback embodiments that may be used with UEs having 2 antennas. Explicit feedback mechanisms may convey the entire channel // without a specific transmission assumption. For example, the channel / may be fed back by eNodeB requests from a UE to always feed back rank-2 PMI and delta. In an embodiment, a UE may additionally feed back the singular values ratio quantized into 1 out of N values, as an analog number, or as a hybrid of quantized and delta as may be done with the PM I.

|0075] For UEs with 4 receive antennas, a similar method may be applied using rank-4 but practically rank-2 feedback conveys most of the information because the UE's antennas are expected to be correlated and therefore the third and fourth singular values are expected to be small.

|0076] In an embodiment, the transmit correlation matrix R = H H may be used as a good representation of the average channel in a given band S. Figure 4 illustrates method 400 of providing either rank- l or rank-2 feedback using this transmit correlation matrix. At block 410, a UE may determine whether to provide rank- l feedback or rank-2 feedback. If the UE determines that it is to provide rank- l feedback (where Q denotes a codebook based precoder), at block 420 the UE may calculate a delta E as E = gR - QQ' where g may be a positive number used by the UE to minimize the norm of E (e.g. , Frobenius norm). As an example, finding the real valued g that minimizes the following quadratic equation may lead to minimizing the Frobenius norm of E: Ag² + Bg + ] where A = tr ce(R² ) and B = -trace{RQQ'+ QQ' R) . The eNodeB may receive Q and E and may reconstruct R (up to a certain insignificant gain). Note that due to the unitary properties of the matrices involved, only the diagonal values and upper triangular half may need to be fed back. The number of complex analog values in this embodiment may be N²I2, where N is the number of eNodeB antennas. [0077] If the UE determines that it is to provide rank-2 feedback, at block 430 the UE may first construct Q_att = QU_m as set forth above, which may be the best approximation of the rank-2 precoder using the rank-2 codebook. Alternatively the UE can use Q as well. The UE may also construct the ratio of the approximated singular values λ =

2) ¾,,,„(:, 2) ' approximate it to Am bits (e.g., I bit approximation to the values [0.25 0.5]), feed it back, and create the delta E as E = g ? - &„„(:, !)0,« (:, ·) '- Ίβ^Ο, 2)&_//*.(:> ²)' ^where> ^{as before}' S may be a positive number used by the UE to minimize the norm of E.

|0078] Figure 5 illustrates method 500 of providing feedback for any rank while reducing feedback overhead of E using parameterization. At block 510, the best PM l and rank in a given subband may be computed. This may be denoted by Q herein. At block 520, the preferred precoder may be computed. The preferred precoder may be denoted by t herein. At block 530, the unitary matrix ^ may be decomposed in terms of the products of a series Givens matrices

where A/*,,- is an NxN unitary matrix parameterized by only two parameters (a, b), where a is in the position (k, k) and ( , ), and b is in (k, i) and -b* is in ( , k), k<i. Since M is a unitary matrix, Iaf^;; * l&P■ Ϊ . The other diagonal elements of matrix are 1 and the remaining elements are 0.

[0079] The diagonal value a may be obtained as a real number by operations such as a Cayley transform, householder reflection, and/or Givens rotations so that the diagonal elements of V are set to be real. A parameter Θ can be defined where Θ is the phase of b. The magnitude of b may be derived from the square root of ( 1 - a²). Therefore, a complex iVx/V unitary matrix V may now be represented by (N²-N) real parameters.

[0080] At block 540, the same procedure may then be applied to decompose Q. The two parameters may be denoted as (c, β) for each matrix of the products. The delta E may contain real values which are the difference between (α, Θ) and (c, β). At block 550, Q may be fed back, in an embodiment, as done in LTE R8. E may also be fed back at block 550.

[0081 ] In an embodiment, a UE may be configured to transmit E only if necessary and thus reduce the feedback overhead (e.g. , when compared to sending E all the time) with negligible or controlled loss of downlink throughput. A UE may use a predefined metric (e.g. , SU-MIMO capacity loss, norm of E, absolute value of the inner product of Q and V) and a threshold to determine whether to feed back E. The eNodeB may decide the threshold based on the application, UE geometry, cell loading and/or other parameters, and may signal the threshold to the UE. A predefined N bit table may be used or the UE may start from a default or signaled value and then be given periodic up/down commands to change that value.

[0082] In an embodiment, a UE may feed back E at a frequency less than it feeds back PMl as controlled by the eNodeB. For example, PMl may be fed back every 2ms, but the UE may feed back E every 4ms.

[0083] A UE may feed back E only for a fixed size subset of subbands used to feed back PMl. The size of the subset may be signaled by an eNodeB, and the UE may signal the specific indices of the subset.

[0084] The UE may combine modes 1 -2 and 2-2 in LTE 8 such that PMl may be fed back as in mode 1 -2 and E may be fed back for all the subbands used for CQI feedback in mode 2-2. ^'

[0085| An eNodeB may give a UE reporting opportunities to report E and the UE may report E only on those allocated resources. The method by which the eNodeB decides when and how many resources to allocate to the UE may depends on the UE geometry, speci fic application, UL traffic and other factors as determined by the eNodeB.

[0086] In an embodiment, a UE may report back E only if a certain metric is exceeded and only when allocated a reporting opportunity.

|0087] In an embodiment, a UE may transmit either PMl or E to reduce overhead. For example, in frame 1 a UE may transmit PMl and in frames 2, 3, and 4, the UE may transmit E relative to the PMl fed back in frame 1 . In frame 5, the UE may feed back a new PM l. The U E may decide to feed back a new PMl every time its algorithm chooses a new PMl, or the UE may decide to feed back a new PM l only when the norm of E exceeds a certain threshold due to the old PM l becoming stale relative to the current channel. The UE may use one bit to signal whether it fed back PMl or E.

100881 Particularly with the application of differential codebooks, an eNodeB may instruct the UE to feed back during the first TV (e.g. , 1 or 2) feedback reports. Differential codebooks may progressively improve feedback accuracy, hence after several refinements the gain from feeding back E may not be necessary.

[0089] Assuming the norm of E to be the metric (normalized by norm of Q which is 1 ), a UE may feed back E only if the metric exceeds the signaled threshold. In SU-M I MO embodiments, an eNodeB may use the highest threshold because gains from sending E may not be high and LTE R8 PMl may be sufficient. In MU-M IMO embodiments, an eNodeB may determine the threshold based on the UE geometry such that a high threshold may be used for cell edge or low geometry UEs and a low threshold may be used for high geometry UEs.

[0090] In CoM P coordinated beamforming (CBF) embodiments, a UE may feed back a rank- 1 PMI for the serving cel l and interfering cells. The PMI for the interfering cells may serve as an approximation of the transmit spatial correlation and may be used for transmit null ing using, for example, signal-to-leakage-and-noise ratio (SLNR) criterion. Since transmit nulling may require more accurate feedback, the UE may only need to send E for interfering cells that are A^" dB stronger than the serving cell. The eNodeB may signal the parameter A^" and a threshold for sending E. A reasonable value for X may be, for example, - l OdB. The UE may feed back E only for the interfering cells that are dB stronger than the serving cell and only if the norm of E for those cells exceeds the signaled threshold.

(0091 ) In UL CoMP embodiments, an eNodeB may signal a UE a PM I and E such that a tradeoff between maximizing gain to itself and interference to other eNodeBs is achieved.

[0092] In CoMP JP embodiments, a UE may send one PMI per eNodeB and a concatenating PMI (drawn from a 2-antenna codebook) to tie up the per-eNodeB PMIs. The criterion for choosing PM I may be implementation specific, but the UE may use the formula set forth below to calculate the global joint strongest singular vector. As in the MU-M 1MO embodiments, the UE may calculate E for each eNodeB using the PMI for that eNodeB and the respective elements in the global joint strongest singular vector, and send E only for those eNodeBs for which the norm exceeds the threshold. In addition, the UE may calculate E for the concatenating PMI and send it as well using the same threshold.

[0093] LTE UL was designed to have low cubic metric. A cubic metric increase of only 0.25dB may be observed when the un-quantized values are mixed with regular quadrature amplitude modulation (QAM) carrying data or control feedback information. Thus, the exact level of cubic metric may depend not only on the modulation used for carrying digital data but also on the percentage of the RJB dedicated to carrying E and the specific modulation of f as well.

[0094] The use of either of two mappings may allow controlled cubic metric increase or no cubic metric increase at all. In an embodiment, phase modulation may be used for mapping.

Analog phase modulation may not increase the cubic metric above that of quadrature phase-shi ft keying (QPSK). Hence E may be mapped to subcarriers by mapping phases and amplitudes of each element to separate carriers using phase modulation. Boosting the phases of E_\ and £₂

(described above in an embodiment for providing rank- 1 feedback) may be applied in order to improve robustness. Note that the element of E_\ should not span a ful l circle to prevent 2n errors. The boosting parameter may depend on the expected norm of E_\ and E₂ which itself may depend on the codebook and the antenna configuration and may be signaled in the DL by the eNodeB.

[0095] In an embodiment, amplitude modulation may be used for mapping. E may be mapped to subcarriers using the formula A + mE, where A is a constant and m is a boosting gain.

The modulation index may be defined by the ratio P^ea^ ^vq^^e °f ^m^ _ancj _may _conlro] the variation of the signal around its mean and its cubic metric. The eNodeB may control both A and m and may signal A and m to the UE. Higher boosting gain m may provide better immunity to noise at the expense of higher cubic metric for a given A . The boosting gain value may depend on the codebook, the antenna configuration, the UL modulation, and/or the percentage of E mixed in with data. For example, the norm of E may be smaller by about l OdB for λ/2 spaced antennas in spatial channel model (SCM) Urban Macro and hence a mapping gain m of 6dB may be possible with minor increase in cubic metric. In such an embodiment, superposition mapping may be used on top of the QPSK subcarriers carrying PM1/CQI with appropriate power normalization. In this embodiment, the QPSK subcarriers may replace A.

[0096] In an embodiment, the cubic metric may be reduced by reducing the amplitude variation of the signal. This may be done by using various amplitude compressing schemes. An amplitude compression scheme may takes a signal and reduce its amplitude dynamic range around its mean or around a certain threshold such that values that are far from that threshold are either clipped or reduced in power. For example, we can use the arc tangent function as shown in Figure 6 on the signal amplitude (note the signal phase remains unchanged).

[0097] In an embodiment, a UE may feed back PMI for M best subbands but may feed back E for M'<M bands. Thus, signaling of the M' subset may need to be accommodated. The feedback channel may need to accommodate a possibly random number M' of feedbacks of E. While an eNodeB may not know M ' exactly, it may know the expected value because it may control the threshold the UE uses. Hence, in order to allocate an appropriate number of resources, the current LTE PUCCH design may be used where multiple UEs share a RB and spread the PMI and E of several subbands across the RB. Alternatively, an eNodeB may signal M' to the UE and the UE may then feed back up to M ' E. In embodiments where amplitude modulation is used for mapping, a UE may spread multiple E belonging to different subbands across the same subcarriers. In embodiments where phase modulation is used for mapping, a U E may use different subcarriers for each subband feedback. [0098] In an embodiment, a signal of E may be power controlled so that higher geometry UEs can enjoy higher accuracy E. This is different than power control for other CQI or HARQ feedback. For example, an eNodeB may have the UE transmit at fixed power when sending E or with several targets for SINR based on UE geometry.

[0099] In some embodiments, to improve received signal quality, each element of E may be repeated at the transmitter. When the analog error signals {i. e. , elements of E) are mapped directly to subcarriers (frequency domain), conjugated repeating may be applied instead of simple repeating. In conjugate repeating, the analog error signal itself and its complex conjugate may be alternated. For example, for a repetition factor of 4, signal sequence {e, conj(e), e, conj(e)} may be transmitted, where e is the analog error signal to be fed back.

[0100] In embodiments where two cell edge UEs simultaneously send back analog feedback e l and el to their respective eNodeBs on the same RB, and the repeating factor is 2 for both, each eNodeB will receive interference on top of its own signal. In such embodiments, where a single antenna (may be extended to multiple antenna) at the UE and the eNodeB may be used, with a signal channel h and interference channel g, the signal model for simple repetition may be

The receiver may not be able to suppress interference (e2). However, with the conjugate repetition, the signal model become

In this embodiment, the eNodeB receiver may be able to separate interference and signal using linear receivers.

[0101] By implementing the embodiments disclosed herein, which in many implementations are a simple augmentation of LTE R8 PMI, improved CSI feedback accuracy may be provided, enabling efficient tradeoff between feedback accuracy and tradeoff for different applications.

[0102] In many embodiments, an un-quantized precoding matrix may be calculated. In one such embodiment, method 700 of Figure 7 may be used to calculate an un-quantized precoding matrix. At block 710, the channel singular vectors over several subcarriers across the band may be calculated. In some implementations, one subcarrier per RB may be sufficient. At block 720, a determination may be made as to whether rank- 1 or rank-2 feedback is to be provided. If rank- 1 feedback is to be provided, at block 730 the phase aligned singular vectors may be averages, using, for example, the formula

where j may be any subcarrier within the subband S and v_A may be the strongest singular vector on subcarrier k. If rank-2 feedback is to be provided, at block 740 the phase aligned strongest and second singular vectors may be averaged separately using the formula used at block 730. The resulting vectors may be denoted by v_t and v₂. At block 750, the resulting vector or vectors may be normalized. In rank-2 feedback embodiments, the resulting precoder may not be unitary since orthogonality may have been lost in the averaging process at block 740. A simple Gram- Schmidt orthogonal ization of the second singular vector may be performed using

[0103] In an embodiment, a closed form solution for the calculation of channel singular vectors and singular values for general MIMO channels with two antennas at one end may be used. In such an embodiment, a 2xW channel may be denoted by H, where TV is any integer number, //' ^"may be of size Nx2. In this embodiment, an effort may be made to find the right singular vectors which are of size 2 and use them to find the left singular vectors which are the right singular vectors of H. Denote ^H = V∑U^H where U may be generally written as

cos O sin # ^

. sin tf e" - cos de'

By the definition of SVD the first column of U may be calculated according to

J cos Θ \

0,0 = arg max || H" . || where ||x|| stands for the Eucl idean norm. The maximum Euclidean norm may be the maximal singular value σ, . Denoting by /?, the i'th row of H and developing the above expression, h^'h

e¹⁹ =— T-— may be derived. Substitution provides that

# = arg max | /7, |² cos² Θ+ \ ₂ |² sin ² Θ + 2 \ h₂ ^' | sin #cos # .

Θ [0104] Differentiating and equating to zero provides that

H

A CORDIC rotation or LUT can be used to calculate cos #, sin # . Using UH = V∑ the singular vectors may be obtained by normalizing v, = A,^* cos d + h₂' sin Θ e^J* and

[0105] Although features and elements are described above in particular combinations, one of ordinary skill in the art will appreciate that each feature or element can be used alone or in any combination with the other features and elements. In addition, the methods described herein may be implemented in a computer program, software, or firmware incorporated in a computer- readable medium for execution by a computer or processor. Examples of computer-readable media include electronic signals (transmitted over wired or wireless connections) and computer- readable storage media. Examples of computer-readable storage media include, but are not limited to, a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, UE, terminal, base station, RNC, or any host computer.

Claims

What is Claimed is:

1. A method for transmitting feedback comprising:

determining, at a wireless transmit and receive unit (WTRU), a rank-1 precoding matrix indicator (PMI);

determining, at the WTRU, a preferred precoding matrix;

determining, at the WTRU, a delta metric based on the rank-1 PMI and the preferred precoding matrix; and

transmitting the PMI and the delta metric to a base station.

2. The method of claim 1, wherein determining the delta metric comprises:

determining a first delta metric based on the rank- 1 PMI and the preferred precoding matrix; and

determining a product of the first delta metric and a householder matrix associated with the rank- 1 PMI to generate the delta metric.

3. The method of claim 2, wherein transmitting the delta metric comprises transmitting second, third, and fourth elements of the delta metric, wherein the base station estimates a first element of the delta metric.

4. The method of claim 1, wherein determining the delta metric comprises determining an element- wise difference between elements of the rank-1 PMI and the preferred precoding matrix.

5. The method of claim 1, wherein transmitting the delta metric comprises alternately transmitting the delta metric and a complex conjugate of the delta metric.

6. The method of claim 1, wherein determining the delta metric comprises determining a separate element-wise magnitude difference and phase difference between elements of the rank-1 PMI and the preferred precoding matrix.

7. The method of claim 1 , further comprising determining to transmit the delta metric based on determining that a threshold has been exceeded.

8. The method of claim 1 , wherein transmitting the delta metric to the base station comprises repeatedly transmitting at the delta metric using conjugated repetition.

9. A method for transmitting feedback comprising:

determining, at a wireless transmit and receive unit (WTRU), a rank-2 precoding matrix indicator (PMI);

determining, at the WTRU, a preferred precoding matrix;

aligning a matrix of the rank-2 PMI and the preferred precoding matrix;

determining, at the WTRU, a delta metric based on the rank-2 PMI and the preferred precoding matrix; and

transmitting the rank-2 PMI and at least one column of the matrix of the delta metric to a base station.

10. The method of claim 9, wherein transmitting the at least one column of the matrix of the delta metric comprises transmitting two columns of the matrix of the delta metric.

1 1. The method of claim 9, wherein transmitting the at least one column of the matrix of the delta metric comprises alternately transmitting an element of the at least one column of the matrix of the delta metric alternately and a complex conjugate of the element of the at least one column of the matrix of the delta metric.

12. The method of claim 9, wherein transmitting the at least one column of the matrix of the delta metric comprises:

determining an amount of gain; and

boosting a signal comprising the delta metric by the amount of gain.

13. The method of claim 9, further comprising decomposing the delta metric using at least one independent parameter.

14. The method of claim 13, further comprising representing complex elements of the delta metric as a result of an operation, wherein the operation is at least one of transformation, reflection, and rotation.

15. A wireless transmit and receive unit (WTRU) configured to transmit uplink control information, comprising:

a processor configured to:

determine a precoding matrix indicator (PMI);

determine a preferred precoding matrix; and

determine a delta metric based on the PMI and the preferred precoding matrix ; and

a transceiver configured to:

transmit the delta metric and the PMI.

16. The WTRU of claim 15, wherein the processor is further configured to determine a complex conjugate of an element of the delta metric, and wherein the transceiver is further configured to alternately transmit the element of the delta metric and the complex conjugate of the element of the delta metric.

17. The WTRU of claim 15, wherein the processor is further configured to:

determine a first delta metric between the PMI and the preferred precoding matrix; and

determine a product of the first delta metric and a householder matrix associated with the PMI to generate the delta metric.

18. The WTRU of claim 15, further comprising decomposing the delta metric using at least one independent parameter.

19. The WTRU of claim 15, wherein the transceiver is further configured to receive an indication of delta metric transmission frequency from a base station, and wherein the transceiver configured to transmit the delta metric comprises the transceiver configured to transmit the delta metric at the delta metric transmission frequency.

20. The WTRU of claim 15, wherein the processor is further configured to determine an amount of gain, and wherein the transceiver is configured to transmit the delta metric by boosting a signal comprising the delta metric by the amount of gain.