Backhaul enhancements for cooperative multi-point (CoMP) operations

Certain aspects of the present disclosure relate to techniques for backhaul enhancements for cooperative multi-point (CoMP) operations. An aggressor node may take pre-scheduling decisions in advance of beamformed data transmissions based on the pre-scheduling decisions. The aggressor node may communicate the pre-scheduling decisions to one or more victim nodes, for example, via a backhaul link between the aggressor node and the one or more victim nodes. A victim node may take scheduling decisions based at least on the pre-scheduling decisions of the aggressor node to coordinate beamformed transmissions from the victim node with the beamformed transmissions from the aggressor node.

BACKGROUND

Certain aspects of the disclosure generally relate to wireless communications and, more particularly, to techniques for leveraging a backhaul link between base stations for Cooperative Multi-Point (CoMP) operations.

A wireless communication network may include a number of base stations (BS) that can support communication for a number of user equipments (UEs). A UE may communicate with a base station via the downlink and uplink. The downlink (or forward link) refers to the communication link from the base station to the UE, and the uplink (or reverse link) refers to the communication link from the UE to the base station.

A base station may transmit data and control information on the downlink to a UE and/or may receive data and control information on the uplink from the UE. On the downlink, a transmission from the base station may observe interference due to transmissions from neighbor base stations. On the uplink, a transmission from the UE may cause interference to transmissions from other UEs communicating with the neighbor base stations. The interference may degrade performance on both the downlink and uplink.

SUMMARY

Certain aspects of the present disclosure provide a method for wireless communication by a first base station (BS). The method generally includes making pre-scheduling decisions, at the first BS, the pre-scheduling decisions involving selection of at least one of transmit beams or transmit power for subsequent transmissions from the first base station, and transmitting information regarding the pre-scheduling decisions to at least one second BS for use in coordinating transmissions with transmissions from the first base station.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes means for making pre-scheduling decisions involving selection of at least one of transmit beams or transmit power for subsequent transmissions from a first base station, and means for transmitting information regarding the pre-scheduling decisions to at least one second BS for use in coordinating transmissions with transmissions from the first base station.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes at least one processor and a memory coupled to the at least one processor. The at least one processor is generally configured to make pre-scheduling decisions involving selection of at least one of transmit beams or transmit power for subsequent transmissions from a first base station, and transmit information regarding the pre-scheduling decisions to at least one second BS for use in coordinating transmissions with transmissions from the first base station.

Certain aspects of the present disclosure provide a computer program product for wireless communication. The computer program product generally includes a computer-readable medium comprising code for making pre-scheduling decisions involving selection of at least one of transmit beams or transmit power for subsequent transmissions from a first base station, and transmitting information regarding the pre-scheduling decisions to at least one second BS for use in coordinating transmissions with transmissions from the first base station.

Certain aspects of the present disclosure provide a method for wireless communication. The method generally includes receiving information regarding pre-scheduling decisions from a first base station (BS) at a second BS, and making scheduling decisions at the second BS based at least on the received pre-scheduling decisions to coordinate transmissions from the second BS with transmissions from the first BS.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes means for receiving information regarding pre-scheduling decisions from a first base station (BS) at a second BS, and means for making scheduling decisions at the second BS based at least on the received pre-scheduling decisions to coordinate transmissions from the second BS with transmissions from the first BS.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes at least one processor and a memory coupled to the at least one processor. The at least one processor is generally configured to receive information regarding pre-scheduling decisions from a first base station (BS) at a second BS, and make scheduling decisions at the second BS based at least on the received pre-scheduling decisions to coordinate transmissions from the second BS with transmissions from the first BS.

Certain aspects of the present disclosure provide a computer program product for wireless communication. The computer program product generally includes a computer-readable medium comprising code for receiving information regarding pre-scheduling decisions from a first base station (BS) at a second BS, and making scheduling decisions at the second BS based at least on the received pre-scheduling decisions to coordinate transmissions from the second BS with transmissions from the first BS.

DETAILED DESCRIPTION

Example Wireless Network

FIG. 1shows a wireless communication network100, which may be an LTE network. The wireless network100may include a number of evolved Node Bs (eNBs)110and other network entities. An eNB may be a station that communicates with user equipment devices (UEs) and may also be referred to as a base station, a Node B, an access point, etc. Each eNB110may provide communication coverage for a particular geographic area. In 3GPP, the term “cell” can refer to a coverage area of an eNB and/or an eNB subsystem serving this coverage area, depending on the context in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscription. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may allow restricted access by UEs having association with the femto cell (e.g., UEs in a Closed Subscriber Group (CSG), UEs for users in the home, etc.). An eNB for a macro cell may be referred to as a macro eNB (i.e., a macro base station). An eNB for a pico cell may be referred to as a pico eNB (i.e., a pico base station). An eNB for a femto cell may be referred to as a femto eNB (i.e., a femto base station) or a home eNB. In the example shown inFIG. 1, eNBs110a,110b, and110cmay be macro eNBs for macro cells102a,102b, and102c, respectively. eNB110xmay be a pico eNB for a pico cell102x. eNBs110yand110zmay be femto eNBs for femto cells102yand102z, respectively. An eNB may support one or multiple (e.g., three) cells.

The wireless network100may also include relay stations. A relay station is a station that receives a transmission of data and/or other information from an upstream station (e.g., an eNB or a UE) and sends a transmission of the data and/or other information to a downstream station (e.g., a UE or an eNB). A relay station may also be a UE that relays transmissions for other UEs. In the example shown inFIG. 1, a relay station110rmay communicate with eNB110aand a UE120rin order to facilitate communication between eNB110aand UE120r. A relay station may also be referred to as a relay eNB, a relay, etc.

The wireless network100may be a heterogeneous network (HetNet) that includes eNBs of different types, e.g., macro eNBs, pico eNBs, femto eNBs, relays, etc. These different types of eNBs may have different transmit power levels, different coverage areas, and different impact on interference in the wireless network100. For example, macro eNBs may have a high transmit power level (e.g., 20 watts) whereas pico eNBs, femto eNBs, and relays may have a lower transmit power level (e.g., 1 watt).

The wireless network100may support synchronous or asynchronous operation. For synchronous operation, the eNBs may have similar frame timing, and transmissions from different eNBs may be approximately aligned in time. For asynchronous operation, the eNBs may have different frame timing, and transmissions from different eNBs may not be aligned in time. The techniques described herein may be used for both synchronous and asynchronous operation.

A network controller130may couple to a set of eNBs and provide coordination and control for these eNBs. The network controller130may communicate with eNBs110via a backhaul. The eNBs110may also communicate with one another, e.g., directly or indirectly via wireless or wireline backhaul.

The UEs120may be dispersed throughout the wireless network100, and each UE may be stationary or mobile. A UE may also be referred to as a terminal, a mobile station, a subscriber unit, a station, etc. A UE may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a tablet, etc. A UE may be able to communicate with macro eNBs, pico eNBs, femto eNBs, relays, etc. InFIG. 1, a solid line with double arrows indicates desired transmissions between a UE and a serving eNB, which is an eNB designated to serve the UE on the downlink and/or uplink. A dashed line with double arrows indicates interfering transmissions between a UE and an eNB. For certain aspects, the UE may comprise an LTE Release 10 UE.

LTE utilizes orthogonal frequency division multiplexing (OFDM) on the downlink and single-carrier frequency division multiplexing (SC-FDM) on the uplink. OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM. The spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system bandwidth. For example, K may be equal to 128, 256, 512, 1024, or 2048 for system bandwidth of 1.25, 2.5, 5, 10, or 20 megahertz (MHz), respectively. The system bandwidth may also be partitioned into subbands. For example, a subband may cover 1.08 MHz, and there may be 1, 2, 4, 8, or 16 subbands for system bandwidth of 1.25, 2.5, 5, 10, or 20 MHz, respectively.

FIG. 2shows a frame structure used in LTE. The transmission timeline for the downlink may be partitioned into units of radio frames. Each radio frame may have a predetermined duration (e.g., 10 milliseconds (ms)) and may be partitioned into 10 subframes with indices of 0 through 9. Each subframe may include two slots. Each radio frame may thus include 20 slots with indices of 0 through 19. Each slot may include L symbol periods, e.g., L=7 symbol periods for a normal cyclic prefix (as shown inFIG. 2) or L=6 symbol periods for an extended cyclic prefix. The 2L symbol periods in each subframe may be assigned indices of 0 through 2L−1. The available time frequency resources may be partitioned into resource blocks. Each resource block may cover N subcarriers (e.g., 12 subcarriers) in one slot.

In LTE, an eNB may send a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) for each cell in the eNB. The primary and secondary synchronization signals may be sent in symbol periods6and5, respectively, in each of subframes0and5of each radio frame with the normal cyclic prefix, as shown inFIG. 2. The synchronization signals may be used by UEs for cell detection and acquisition. The eNB may send a Physical Broadcast Channel (PBCH) in symbol periods0to3in slot1of subframe0. The PBCH may carry certain system information.

The eNB may send a Physical Control Format Indicator Channel (PCFICH) in the first symbol period of each subframe, as shown inFIG. 2. The PCFICH may convey the number of symbol periods (M) used for control channels, where M may be equal to 1, 2, or 3 and may change from subframe to subframe. M may also be equal to 4 for a small system bandwidth, e.g., with less than 10 resource blocks. The eNB may send a Physical HARQ Indicator Channel (PHICH) and a Physical Downlink Control Channel (PDCCH) in the first M symbol periods of each subframe (not shown inFIG. 2). The PHICH may carry information to support hybrid automatic repeat request (HARQ). The PDCCH may carry information on resource allocation for UEs and control information for downlink channels. The eNB may send a Physical Downlink Shared Channel (PDSCH) in the remaining symbol periods of each subframe. The PDSCH may carry data for UEs scheduled for data transmission on the downlink. The various signals and channels in LTE are described in 3GPP TS 36.211, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation,” which is publicly available.

A number of resource elements may be available in each symbol period. Each resource element may cover one subcarrier in one symbol period and may be used to send one modulation symbol, which may be a real or complex value. Resource elements not used for a reference signal in each symbol period may be arranged into resource element groups (REGs). Each REG may include four resource elements in one symbol period. The PCFICH may occupy four REGs, which may be spaced approximately equally across frequency, in symbol period0. The PHICH may occupy three REGs, which may be spread across frequency, in one or more configurable symbol periods. For example, the three REGs for the PHICH may all belong in symbol period0or may be spread in symbol periods0,1, and2. The PDCCH may occupy 9, 18, 32, or 64 REGs, which may be selected from the available REGs, in the first M symbol periods. Only certain combinations of REGs may be allowed for the PDCCH.

A UE may be assigned resource blocks in the control section to transmit control information to an eNB. The UE may also be assigned resource blocks in the data section to transmit data to the eNB. The UE may transmit control information in a Physical Uplink Control Channel (PUCCH)210a,210bon the assigned resource blocks in the control section. The UE may transmit only data or both data and control information in a Physical Uplink Shared Channel (PUSCH)220a,220bon the assigned resource blocks in the data section. An uplink transmission may span both slots of a subframe and may hop across frequency as shown inFIG. 2A.

A UE may be within the coverage of multiple eNBs. One of these eNBs may be selected to serve the UE. The serving eNB may be selected based on various criteria such as received power, pathloss, signal-to-noise ratio (SNR), etc.

A UE may operate in a dominant interference scenario in which the UE may observe high interference from one or more interfering eNBs. A dominant interference scenario may occur due to restricted association. For example, inFIG. 1, UE120ymay be close to femto eNB110yand may have high received power for eNB110y. However, UE120ymay not be able to access femto eNB110ydue to restricted association and may then connect to macro eNB110cwith lower received power (as shown inFIG. 1) or to femto eNB110zalso with lower received power (not shown inFIG. 1). UE120ymay then observe high interference from femto eNB110yon the downlink and may also cause high interference to eNB110yon the uplink.

A dominant interference scenario may also occur due to range extension, which is a scenario in which a UE connects to an eNB with lower pathloss and lower SNR among all eNBs detected by the UE. For example, inFIG. 1, UE120xmay detect macro eNB110band pico eNB110xand may have lower received power for eNB110xthan eNB110b. Nevertheless, it may be desirable for UE120xto connect to pico eNB110xif the pathloss for eNB110xis lower than the pathloss for macro eNB110b. This may result in less interference to the wireless network for a given data rate for UE120x.

In an aspect, communication in a dominant interference scenario may be supported by having different eNBs operate on different frequency bands. A frequency band is a range of frequencies that may be used for communication and may be given by (i) a center frequency and a bandwidth or (ii) a lower frequency and an upper frequency. A frequency band may also be referred to as a band, a frequency channel, etc. The frequency bands for different eNBs may be selected such that a UE can communicate with a weaker eNB in a dominant interference scenario while allowing a strong eNB to communicate with its UEs. An eNB may be classified as a “weak” eNB or a “strong” eNB based on the received power of signals from the eNB received at a UE (and not based on the transmit power level of the eNB).

According to certain aspects of the present disclosure, when a network supports enhanced inter-cell interference coordination (eICIC), the base stations may negotiate with each other to coordinate resources in order to reduce or eliminate interference by the interfering cell giving up part of its resources. In accordance with this interference coordination, a UE may be able to access a serving cell even with severe interference by using resources yielded by the interfering cell.

For example, a femto cell with a closed access mode (i.e., in which only a member femto UE can access the cell) in the coverage area of an open macro cell may be able to create a “coverage hole” (in the femto cell's coverage area) for a macro cell by yielding resources and effectively removing interference. By negotiating for a femto cell to yield resources, the macro UE under the femto cell coverage area may still be able to access the UE's serving macro cell using these yielded resources.

In a radio access system using OFDM, such as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), the yielded resources may be time based, frequency based, or a combination of both. When the coordinated resource partitioning is time based, the interfering cell may simply not use some of the subframes in the time domain. When the coordinated resource partitioning is frequency based, the interfering cell may yield subcarriers in the frequency domain. With a combination of both frequency and time, the interfering cell may yield frequency and time resources.

FIG. 3is a block diagram of a design of a base station or an eNB110and a UE120, which may be one of the base stations/eNBs and one of the UEs inFIG. 1. For a restricted association scenario, the eNB110may be macro eNB110cinFIG. 1, and the UE120may be UE120y. The eNB110may also be a base station of some other type. The eNB110may be equipped with T antennas334athrough334t, and the UE120may be equipped with R antennas352athrough352r, where in general T≧1 and R≧1.

At the eNB110, a transmit processor320may receive data from a data source312and control information from a controller/processor340. The control information may be for the PBCH, PCFICH, PHICH, PDCCH, etc. The data may be for the PDSCH, etc. The transmit processor320may process (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols, respectively. The transmit processor320may also generate reference symbols, e.g., for the PSS, SSS, and cell-specific reference signal. A transmit (TX) multiple-input multiple-output (MIMO) processor330may perform spatial processing (e.g., precoding) on the data symbols, the control symbols, and/or the reference symbols, if applicable, and may provide T output symbol streams to T modulators (MODs)332athrough332t. Each modulator332may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each modulator332may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. T downlink signals from modulators332athrough332tmay be transmitted via T antennas334athrough334t, respectively.

At the UE120, antennas352athrough352rmay receive the downlink signals from the eNB110and may provide received signals to demodulators (DEMODs)354athrough354r, respectively. Each demodulator354may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain input samples. Each demodulator354may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector356may obtain received symbols from all R demodulators354athrough354r, perform MIMO detection on the received symbols, if applicable, and provide detected symbols. A receive processor358may process (e.g., demodulate, deinterleave, and decode) the detected symbols, provide decoded data for the UE120to a data sink360, and provide decoded control information to a controller/processor380.

On the uplink, at the UE120, a transmit processor364may receive and process data (e.g., for the PUSCH) from a data source362and control information (e.g., for the PUCCH) from the controller/processor380. The transmit processor364may also generate reference symbols for a reference signal. The symbols from transmit processor364may be precoded by a TX MIMO processor366if applicable, further processed by modulators354athrough354r(e.g., for SC-FDM, etc.), and transmitted to the eNB110. At the eNB110, the uplink signals from the UE120may be received by the antennas334, processed by the demodulators332, detected by a MIMO detector336if applicable, and further processed by a receive processor338to obtain decoded data and control information sent by the UE120. The receive processor338may provide the decoded data to a data sink339and the decoded control information to the controller/processor340.

The controllers/processors340and380may direct the operation at the eNB110and the UE120, respectively. The controller/processor340, receive processor338, and/or other processors and modules at the eNB110may perform or direct operations and/or processes for the techniques described herein. The memories342and382may store data and program codes for the eNB110and the UE120, respectively. A scheduler344may schedule UEs for data transmission on the downlink and/or uplink.

Coordinated Multi-Point (CoMP) Transmission and Reception

In a multi-cell configuration, a UE (e.g., UE120) may communicate with multiple base stations or cells (e.g., base stations110) simultaneously to transmit and/or receive data. In one aspect, a multi-cell configuration may be a coordinated multi-point (CoMP) configuration wherein joint processing and/or coordinated scheduling (CS)/coordinated beamforming (CBF) may employed. For example, in a joint processing CoMP configuration on the downlink, physical downlink shared channel (PDSCH) data (e.g., downlink user data) may be available at multiple points. PDSCH transmissions may be joint transmissions in which PDSCH data is transmitted from multiple points. In addition, PDSCH transmission may be based upon a dynamic cell selection scheme in which PSDCH data is transmitted from one point at a given time.

For example, turning toFIG. 1, a joint processing downlink CoMP configuration may be implemented by a CoMP cooperating set including macro eNB110band pico eNB110xserving UE120x. The CoMP cooperating set generally includes a plurality of cells or base stations (e.g. macro, pico, femto etc.), potentially geographically separated, which coordinate to provide wireless communication services to a UE. In an example, eNBs110band110xmay coordinate to jointly transmit downlink data to UE120x. In another example, eNBs110band110xmay coordinate such that the base station having the greatest instantaneous channel quality transmits downlink data to UE120x.

eNBs110band110xmay further coordinate to implement scheduling and/or beamforming decisions. For instance, downlink data for UE120xmay be available at the UE's serving cell (e.g. pico cell102x), however, scheduling decisions impacting UE120xaccount for information provided by other members of the CoMP cooperating set, such as eNB110b. eNB110xmay schedule UE120xto avoid potential downlink interference from transmissions in non-serving cell102band/or eNB110xmay suppress transmissions in the non-serving cell102bbased upon information provided by eNB110bvia a backhaul link (not shown) between the eNBs110band110x.

It is to be appreciated that CoMP techniques may be applied on uplink transmissions in addition to downlink transmissions. For instance, eNBs110band110xmay coordinate to receive uplink transmissions from UE120xin a joint manner. In an example, eNBs110band110xmay receive an uplink data packet from the UE120x. Both eNBs110band110xmay independently attempt to demodulate and decode the uplink packet. eNB110band/or eNB110xmay exchange decoded data packets and employ packet combining to improve reliable reception of the uplink packet. In another aspect, eNBs110band110xmay exchange information via the backhaul link, wherein the exchanged information may be utilized to enhance scheduling decisions. For example, UE120xmay reside at a cell edge between serving cell102xand non-serving cell102b. eNB110bmay provide information regarding UEs within the non-serving cell102band in proximity to UE120x. eNB110xmay utilize the information to schedule UE120xon the uplink such that interference to/from neighboring cells is reduced.

To support CoMP operations, UE120xmay report feedback information to members of a CoMP reporting set. The CoMP reporting set may include all members of the CoMP cooperating set or a portion thereof. The feedback information may include channel state information (CSI), and/or sounding reference signals which may be utilized for uplink scheduling and/or channel estimation via channel reciprocity. In accordance with an aspect, sounding reference signal transmissions may be configured with a set of parameters and transmit power may be controlled such that the sounding reference signal is received by members of a CoMP cooperating set with a reasonable carrier-to-interference ratio.

Example Backhaul Enhancements for Cooperative Multi-Point (COMP) Operations

In certain aspects, performance of a UE at cell edges may be significantly improved through downlink CoMP in which multiple nodes (or base stations/eNBs) cooperate for serving a UE. In a coordinated scheduling (CS) and coordinated beam-forming (CBF), only the serving cell has the data packets for the UE. CS and CBF generally involve suitable UE selection by a base station, beam selection, power control (for example, Boolean, wherein interferer is transmitting using full power or silenced on some resources), and improved link adaptation.

Aspects of the present disclosure provide methods for feedback enhancements for CS/CBF schemes. A “victim” BS (e.g., pico in a macro-pico scenario) may make scheduling decisions conditioned on one or more interfering “aggressor” BSs (e.g., macros). Namely, a victim BS may implicitly or explicitly know the scheduling decisions (e.g., beams and transmission powers) of neighboring aggressor BSs and may take this information into considering when coordinating beamformed transmissions with neighbor BSs.

In certain aspects, all or a subset of nodes make scheduling decisions prior to transmission. Generally, interference experienced by a UE depends on transmission powers and beams employed by interferers. The UE periodically reports CSI (or CQI), which reflects scheduling decisions (e.g., transmission powers and beams) of interfering, neighbor cells. The UE may determine CSI based on data or reference signals (RS) (e.g., CSI-RS) transmitted by the interferers on resource elements (REs). However, the interference experienced by the UE may be unpredictable as interfering cells may change beams and transmit power on a transmission time interval (TTI) basis. This may create a mismatch in the reported CSI based on RS, and the actual interference experienced during data transmissions (e.g. PDSCH data transmissions) due to the one or more interferers changing beams and/or transmission powers for transmission.

A more accurate knowledge of interference at the time of transmission by a serving BS (e.g., estimated interference matches actual interference at the time of the transmission) may increase performance. Increased performance may occur due to improved beam selection, link adaption, and multiuser diversity (MUD) gain. For example, a more accurate knowledge of interference experienced at a UE by one or more neighbor base stations may improve a UE's CSI estimate. Additionally, a more accurate knowledge of interference may allow beam selection such that the transmit signal from the serving BS and interference from the neighbor base stations are orthogonal.

In certain aspects, one or more aggressor BSs (e.g., aggressor macros) may take scheduling decisions (or pre-scheduling decisions) in advance (e.g., x ms in advance) of an actual transmission based on the scheduling decisions. For example, an aggressor BS may select transmission beam and transmission power for a future transmission x ms later using the selected transmission beam and transmission power. In an aspect, the scheduling decisions may apply to multiple subframes in the future. The aggressor may transmit reference signals (e.g., CSI-RS) using the selected transmission beam and transmission power for CSI reporting by CoMP UEs. In an aspect, each victim BS (e.g. victim pico) may take scheduling decisions based on the aggressor's scheduling decisions and/or CSI reported by the UEs served by the victim BS. In an aspect, the x ms time may be selected to account for the time taken by a UE to estimate and report CSI based on received CSI-RS from an aggressor, time taken by the victims to decode the reported CSIs from CoMP UEs, CSI-RS reporting periodicity etc.

FIG. 4illustrates an example timeline400of coordinated beam forming (CBF) between an aggressor macro node and a victim pico node. For illustrative purposes only, the example assumes a 5 ms CSI reporting periodicity from CoMP UEs and 5 ms CSI-RS transmission periodicity from the macro node. The macro node makes scheduling decisions every 5 ms. For example, the macro node makes scheduling decisions at subframe n (n+5, n+10, etc.). At subframe n+1 (n+6, n+11, etc.), the macro node transmits CSI-RS on pre-determined REs, for example, using beams and transmission power selected at step 1. UEs served by the pico, may then calculate Channel Quality Indicator (CQI) (based on CSI-RS at step 2) and report to the pico at subframe n+5 (n+10, etc.). Subsequently, the macro node B may transmit data (e.g., PDSCH data) according to the scheduling decisions made at subframe n, carried out at time n+9 to n+13. The pico node may make scheduling decisions based on the reported CQI and/or pre-scheduling information from the macro node, and transmit at time n+9 to n+13 (persistency is not required), for example, with beams selected to account for the beams selected by the macro node.

In certain aspects, the macro node may transmit information regarding the pre-scheduling decisions to the pico node (e.g., via a backhaul link). The information may include all information required by the destination pico node to determine interference caused by the source macro node to UEs served by the destination pico node. The information regarding the pre-scheduling may comprise at least one of a downlink transmission power, a transmit beam, a transmission rank, and time-frequency resources these decisions refer to. In an aspect, the pico node may take scheduling decisions based on the received pre-scheduling information and/or CSI reported by served UEs.

In some cases, the pre-scheduling decisions (e.g., transmit beams and powers) may be constant across sets of N consecutive physical resource blocks (PRBs), wherein N is a PRB bundling size. Thus, the interference may change slowly across those PRBs, for example, only due to frequency-selecting fading. In an aspect, the pre-scheduling information transmitted from the macro node to the pico node may include the PRB bundling size N.

In certain aspects, the serving pico may pass on the information regarding the PRB bundling size N to its served UEs (e.g., via a Radio Resource Control (RRC) Information Element (IE)), which may use this information to determine interference. For example, the UE may use the information to decide how many consecutive PRBs to average when estimating interference. In an aspect, if a UE has multiple aggressors and each of them is using a different PRB bundling size, the RRC message may include a list of bundling sizes (optionally with cell IDs of corresponding aggressor cells). Alternatively, the serving cell may select one common value, for example, the minimum among the bundling sizes used by most dominant aggressors for that UE. In a further aspect, the PRB bundling sizes of neighboring nodes may be selected such that the bundling is substantially the same for the nodes. The PRB bundling may be selected via backhaul negotiation between the neighboring nodes.

In an aspect, the larger is the N, the better may be the interference estimate at the UEs. However, a larger N may entail performance degradation for the aggressor BSs, depending on number of UEs, transmission mode, traffic, frequency selectivity of the channel etc.

An aggressor node (e.g., macro node) may independently decide its PRB bundling size N and inform the neighboring victim nodes (e.g., pico nodes). The macro node may also receive information from the at least one other BS (e.g., victim pico node) and select the PRB bundling size N based on the information received from the at least one other BS. The information received from the at least one other BS may comprise at least one of active CoMP UEs and an expected interference estimation accuracy for the active CoMP UEs.

In certain aspects, the victim picos may forward (e.g., via a backhaul link) the CSIs reported by their respective served UEs to the aggressor node(s). The CSI may be used by an aggressor node to determine the UEs associated to a victim node, that are affected (e.g., in terms of interference) by the aggressor node. In an aspect, only those UEs taking part in the CoMP configuration, for example, which reported CSI from the aggressor even though connected with the victim, may be considered for CSI forwarding. The UEs considered for CSI forwarding may include all RRC_connected UEs, scheduled UEs (e.g., if the CSI forwarding message is transmitted along with scheduling information of the victim node), or a subset of UEs (e.g., selected in round-robin fashion).

In an aspect, the encoding used to forward the CSIs may be same as for over-the-air (OTA) CSI reporting from a UE to eNB. The frequency granularity of the CSI forwarding messages may also be same as for the OTS CSI reporting. In an aspect, if backhaul capacity is a concern, only a subset of subbands may be reported. The time granularity for the CSI forwarding messages may be per transmission time interval (TTI), for example, if the CSI forwarding message is transmitted along with scheduling information of the victim node. Alternatively, the time granularity may be periodic, wherein the periodicity may depend on CSI speed of change. The periodicity may also depend on periodicity for OTS CSI reporting.

In certain aspects, a special beamformed CSI-RS (e.g., RQI-RS) configuration may be used for CSI estimation by victim UEs. This special CSI-RS configuration may be shared among the BSs, for example via a backhaul link between the BSs. In an aspect, at the first transmission of such CSI-RS after pre-scheduling by an aggressor macro node, the macro node may transmit a pseudo-random sequence on the CSI-RS REs according to the beams and transmission powers selected by the pre-scheduling macro node. The victim UEs may use these REs to estimate interference from the macro node.

In certain aspects, certain nodes in a CoMP configuration may behave both as victim and aggressor nodes. For example, a pico node may be an aggressor to another pico node (e.g., inter-pico interference), and a victim for another pico or macro node. In inter-pico interference scenarios, the interference estimated by a UE associated to a pico node may need to include interference from neighbor pico nodes as well (besides interference from aggressor macro nodes). This may be accomplished by using different muted CSI-RS patterns for neighboring nodes. Generally, serving cell does not transmit on certain REs corresponding to a particular muted CSI-RS patterns, while all other cells are transmitting. So the UE associated to the serving cell may determine the interference from all the interferers based on the beams and the TX powers used by them. The rationale typically is that the energy on the muted tones captures as much of the overall interference as possible (ideally, from all nodes except the serving node). In an aspect, adjacent nodes may mute on different CSI-RS configurations.

However, in certain aspects, only a finite set of CSI-RS configurations are available. Thus, CSI-RS configurations may be re-used. In an aspect, a static coloring through operation and maintenance (OAM) may be used, such that neighboring nodes mute on different CSI-RS configurations. Alternatively, autonomous coloring via X2 negotiation (e.g., via backhaul links between nodes) may be used to select different muted CSI-RS configurations for neighboring nodes. In an aspect, information exchange between nodes may include presence/absence of CoMP UEs and Reference Signal Received Power (RSRP) reported by CoMP UEs from non-serving cells (e.g. non-serving pico cells to determine impact of pico interference). Based on this information, each node may build an interference graph for the UEs and may select an appropriate CSI-RS configuration and inform the configuration to other nodes (e.g., via the backhaul links).

In certain aspects, if an aggressor node has delay-sensitive traffic, it may decide to set aside some non-CoMP resources (e.g., in the frequency and/or time domain) where scheduling may be instantaneous. Such resources may be used to schedule delay-sensitive traffic. The victim nodes may need to know whether and which resources used by the aggressor node is non-CoMP. In certain aspects, each aggressor node may define a resource assignment including CoMP and non-CoMP resources and inform victim nodes of the resource assignment, for example, via backhaul links between the aggressor node and the victim nodes. In an aspect, the aggressor node may receive information from one or more victim nodes and use the received information for defining the resource assignment. The information may include number of CoMP active UEs and average loading on CoMP resources. In an aspect, the resource assignment may adaptively be changed, for example, based on the information received from the victim nodes.

FIG. 5illustrates example operations500that may be performed by an aggressor evolved node B (eNB) in accordance with certain aspects of the disclosure. The operations500may begin, at502, by making pre-scheduling decisions, at a first base station (BS), the pre-scheduling decisions involving selection of at least one of transmit beams or transmit power for subsequent transmissions from the first base station. At504, information regarding the pre-scheduling decisions may be transmitted to a second base station for use in coordinating transmissions with transmissions from the first base station.

FIG. 6illustrates example operations600that may be performed by a victim evolved node B (eNB) in accordance with certain aspects of the disclosure. The operations600may begin, at602, by receiving information regarding pre-scheduling decisions from a first base station (BS) at a second base station. At604, scheduling decisions may be made at the second BS based at least on the received pre-scheduling decisions to coordinate transmissions from the second BS with transmissions from the first BS.