CN107257252B - Timing synchronization for Downlink (DL) transmission in coordinated multipoint (CoMP) systems - Google Patents

Abstract

Techniques for adjusting receiver timing of a wireless device in a coordinated multipoint (CoMP) system are disclosed. A method may include a wireless device receiving a plurality of node-specific Reference Signals (RSs) from a plurality of cooperating nodes in a coordination set of a CoMP system. The coordination group includes at least two coordination nodes. The wireless device can estimate a composite received RS timing from a plurality of received RS timings, the plurality of received RS timings generated from a plurality of node-specific RSs. The received RS timing represents timing from at least two cooperating nodes. The wireless device may adjust the receiver timing based on the composite received RS timing. The node-specific RSs may include channel state information reference signals (CSI-RSs).

Description

Timing synchronization for Downlink (DL) transmission in coordinated multipoint (CoMP) systems

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application attorney docket No. P41399Z, serial No. 61/556109, filed on 4/11/2011, and is incorporated herein by reference.

Background

Wireless mobile communication technology employs various standards and protocols to communicate data between nodes (e.g., transmission stations) and wireless devices. Some wireless devices communicate via the physical layer using Orthogonal Frequency Division Multiplexing (OFDM) in combination with a desired digital modulation scheme. Standards and protocols that use OFDM include the third generation partnership project (3GPP) Long Term Evolution (LTE), the Institute of Electrical and Electronics Engineers (IEEE)802.16 standards (e.g., 802.16e, 802.16m) (commonly known to the industry group as WiMAX (worldwide interoperability for microwave access)), and the IEEE 802.11 standard (commonly known to the industry group as WiFi).

In a 3GPP Radio Access Network (RAN) LTE system, a Node may be a combination of an evolved universal terrestrial radio access network (E-UTRAN) Node B (also commonly referred to as an evolved Node B, enhanced Node B, eNodeB, or eNB) and a Radio Network Controller (RNC), which communicates with a wireless device (e.g., a mobile device) referred to as User Equipment (UE). Downlink (DL) transmissions may be communications from the node station (or eNodeB) to the wireless device (or UE), and Uplink (UL) transmissions may be communications from the wireless device to the node.

In a homogeneous network, nodes, also referred to as macro nodes, may provide basic radio coverage to wireless devices in a cell. A cell may be an area in which a wireless device is operable to communicate with a macro node. Due to the increased usage and functionality of wireless devices, heterogeneous networks (hetnets) are used to handle the increased traffic load on macro nodes. Hetnets may include a layer of planned high-power macro nodes (or macro enbs) covered by a layer of lower-power nodes (micro enbs, pico enbs, femto enbs, or home enbs henbs) that may be deployed within the coverage area (cell) of the macro nodes in a less planned or even completely uncoordinated manner. The Lower Power Nodes (LPNs) may be collectively referred to as "low power nodes". Macro nodes may be used for basic coverage, while low power nodes may be used to fill coverage holes to increase capacity at hotspots or at boundaries between macro node's coverage areas, and to improve indoor coverage where building structures impede signal transmission. Inter-cell interference coordination (ICIC) or enhanced ICIC (eicic) may be used for resource coordination to reduce interference between nodes, such as macro nodes and low power nodes in a HetNet.

Drawings

The features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which together illustrate the features of the present disclosure, and in which:

fig. 1 illustrates a schematic diagram of Orthogonal Frequency Division Multiplexing (OFDM) symbol transmission from a macro node and a Low Power Node (LPN) in a coordination set, OFDM symbols received at a wireless device, and adjusting a Fast Fourier Transform (FFT) window using an earliest received Reference Signal (RS) timing, according to an example;

fig. 2 illustrates an Orthogonal Frequency Division Multiplexing (OFDM) symbol transmission from a macro node and a Low Power Node (LPN) in a coordination set, an OFDM symbol received at a wireless device, and a schematic diagram of adjusting a Fast Fourier Transform (FFT) window using a Reference Signal Received Power (RSRP) and a received Reference Signal (RS) timing, according to an example;

fig. 3 illustrates a schematic diagram of orthogonal frequency division complex periodic (OFDM) symbol transmission from a plurality of cooperating nodes in a coordination set, OFDM symbols received at a wireless device, and adjusting a Fast Fourier Transform (FFT) window using Reference Signal Received Power (RSRP) and received Reference Signal (RS) timing, according to an example;

fig. 4 illustrates a schematic diagram of Orthogonal Frequency Division Multiplexing (OFDM) symbol transmission from a plurality of cooperating nodes in a coordination group, an OFDM symbol received at a wireless device, and an Inverse Fast Fourier Transform (IFFT) window adjusting a first cooperating node using an adjustment timing, according to an example;

fig. 5 shows a block diagram of radio frame resources according to an example;

fig. 6 illustrates a flow diagram for timing synchronization of Downlink (DL) transmissions in a coordinated multipoint (CoMP) system, according to an example;

fig. 7 shows a block diagram of the physical layer of a transmitter and a receiver in an Orthogonal Frequency Division Multiplexing (OFDM) wireless network, according to an example;

figure 8 illustrates a flow diagram of a method for adjusting receiver timing of a wireless device in a coordinated multipoint (CoMP) system, according to one example;

figure 9 illustrates a flow diagram of a method for synchronizing a timing of a Downlink (DL) transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in a coordinated multipoint (CoMP) system, according to one example;

FIG. 10 shows a block diagram of a wireless device and a plurality of cooperating nodes, according to an example; and

fig. 11 shows a schematic diagram of a wireless device according to an example.

Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.

Detailed Description

Before the present invention is disclosed and described, it is to be understood that this invention is not limited to the particular structures, process steps, or materials disclosed herein but extends to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It is also to be understood that the terminology used herein is for the purpose of describing particular examples only and is not intended to be limiting. Like reference symbols in the various drawings indicate like elements. The numerals provided in the flowcharts and processes are provided for clarity in explaining the steps and operations and do not necessarily indicate a particular order or sequence.

Example embodiments

A preliminary overview of technical embodiments is provided below, and then specific technical embodiments are described in further detail later. This preliminary summary is intended to aid the reader in understanding the present technology more quickly, but is not intended to identify key features or essential features of the technology, nor is it intended to be limiting as to the scope of the claimed subject matter.

Coordinated multipoint (CoMP) systems can be used to reduce interference from neighboring nodes in homogeneous networks and hetnets. In a coordinated multipoint (CoMP) system, nodes, referred to as cooperating nodes, may also be grouped together with other nodes, where nodes from multiple cells may transmit signals to and receive signals from wireless devices. The cooperating nodes may be nodes in a homogeneous network or macro nodes and/or Lower Power Nodes (LPNs) in a HetNet. Downlink CoMP transmission can be divided into two categories: coordinated scheduling or coordinated beamforming (CS/CB or CS/CBF), and joint processing or joint transmission (JP/JT). With CS/CB, a given subframe may be transmitted from one cell to a given wireless device (UE), and scheduling, including coordinating beamforming, dynamically coordinates between cells to control and/or reduce interference between different transmissions. For joint processing, joint transmission to a wireless device (UE) may be performed over multiple cells, where multiple nodes transmit simultaneously using the same time and frequency radio resources and/or dynamic cell selection.

In a non-CoMP system, timing synchronization at a wireless device (e.g., a UE) may be performed by using a Primary Synchronization Signal (PSS) and/or a cell-specific reference signal (CRS). As shown in fig. 1, in Downlink (DL) CoMP systems and deployments with distributed antennas at different geographical locations, timing estimation using PSS and/or CRS may not be accurate because the PSS and/or CRS transmission points (e.g., macro node 210 in macro cell 212) may not be the same as the Physical Downlink Shared Channel (PDSCH) transmission points (e.g., lower power node LPN 220 in LPN cell 222). In the Dynamic Point Selection (DPS) DL CoMP example using a common cell Identifier (ID) shown in fig. 1, DL transmissions 250 (including PSS and/or CRS) from the macro node to the wireless device (e.g., UE 230) and separate DL transmissions 260 (including data or PDSCH) from the LPN to the wireless device may be transmitted at substantially the same time. Due to different geographical locations of the nodes (e.g., macro node and LPN) and/or other factors, DL transmissions may arrive at the wireless device at different times. The wireless devices may synchronize to the PSS and/or CRS transmission points (e.g., macro nodes). For example, due to propagation delays, Orthogonal Frequency Division Multiplexing (OFDM) symbols in the macro node transmission 252 and substantially the same OFDM symbols in the LPN transmission 262 may be received by a wireless device (e.g., a UE) at different times. The OFDM symbol may include a Cyclic Prefix (CP). Since the UE is closer to the LPN than the macro node, the UE reception of the macro node DL transmission 254 may have a larger propagation delay 256 than the propagation delay 266 of the UE reception of the LPN DL transmission 264. If the PSS and/or CRS from the macro node is used for timing synchronization, the timing of the Fast Fourier Transform (FFT) window 280 used to sample the OFDMA symbols may be synchronized to the macro node DL transmission, which may not be the earliest transmission within the coordination group. Thus, the wireless device may apply transmissions from other nodes (in the coordinated group) having timing of the OFDM symbol advanced relative to the FFT sampling window. Moreover, in some cases, transmissions from the macro node may not have the strongest signal power (e.g., Reference Signal Received Power (RSRP)) and/or provide data transmissions (e.g., PDSCH). In these cases, inter-carrier interference (ICI) and inter-symbol interference (ISI)270 may occur due to incorrect settings of FFT timing at the wireless device. To reduce ICI and ISI and improve OFDMA symbol reception, the receiver timing may be adjusted, which may shift the FFT window. A plurality of FFT samples for an OFDM symbol may be captured in an FFT window used to receive the OFDM symbol. Although a macro node and LPN are shown in fig. 1-2, any type of node in a DL CoMP system may be used.

Timing synchronization of receiver timing of the wireless device can be modified to use timing estimates generated from node-specific reference signals of the CoMP measurement set, where the basic timing synchronization uses PSS and/or CRS. The node-specific reference signals may include channel state information reference signals (CSI-RS). The receiver timing may be the timing of processing within the receiver, such as when the receiver is looking for OFDM symbol boundaries or when the receiver is performing an FFT (e.g., sampling OFDM symbols). Because different CSI-RS configurations may be assigned to different geographically separated transmission points (e.g., macro node and LPN), timing estimation may be performed independently for each transmission point. Based on the multiple timing estimates from the CSI-RS, the wireless device may calculate the actual timing of data or PDSCH reception from multiple nodes.

In one example, a wireless device can receive a plurality of node-specific Reference Signals (RSs), such as CSI-RSs, from a plurality of cooperating nodes (e.g., macro nodes and LPNs) in a coordination set (e.g., a CoMP measurement set) of a CoMP system. The coordination group may include at least two cooperating nodes. The cooperating nodes may include a serving node, a macro node, or an LPN. The wireless device may receive a node-specific RS from at least two cooperating nodes. The wireless device may generate or calculate a receive RS timing from a node-specific RS of the cooperating node. The wireless device can estimate a composite received RS timing from the multiple received RS timings. The received RS timing may represent timing from at least two cooperating nodes. The wireless device may adjust the receiver timing based on the composite received RS timing. The adjusted receiver timing may be the time at which the receiver of the wireless device samples, performs, or processes an FFT for the received signal or OFDM symbol.

In one embodiment, the wireless device may determine the earliest received RS timing from a plurality of received RS timings representing various cooperating nodes. The estimated composite received RS timing for adjusting the receiver timing and/or FFT window may use or include the earliest received RS timing 282. The earliest received RS timing may represent the DL transmission with the shortest propagation delay relative to other cooperating nodes. Estimated composite received RS timing or actual PDSCH timing τ_PDSCHCan be set to all calculated timings in the CoMP measurement set

At the earliest timing in (1), by

Wherein, τ is_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is the channel state information reference signal (CSI-RS) timing calculated for each node in the CoMP measurement set, min () is a minimum function, and i is a positive integer representing the nodes in the CoMP measurement set (i.e., there are i nodes in the CoMP measurement set). At the wireless device, adjusting the receiver timing or FFT window based on the earliest received RS timing can reduce the timing of the signal relative to the FFT sampling interval advance of the wireless device. In one example, an estimated composite received RS timing using the earliest received RS timing may be used in joint transmission (JP/JT) for Joint Processing (JP), and then the FFT sample interval may be adjusted corresponding to the CSI-RS timing of the nearest node. In Joint Transmission (JT), PDSCH may be transmitted from multiple cooperating nodes of a cooperating cell.

In another embodiment, the wireless device may determine a minimum received RS timing and a maximum received RS timing from a plurality of received RS timings representing various cooperating nodes. The estimated composite received RS timing may be a value or receiver RS timing substantially between the minimum received RS timing and the maximum received RS timing. As shown in fig. 3, the minimum received RS timing 362 can include the earliest received RS timing representing the DL transmission with the shortest propagation delay relative to other cooperating nodes. The maximum received RS timing 364 may comprise the latest received RS timing representing the DL transmission with the longest propagation delay relative to other cooperating nodes.

In another embodiment, the composite received RS timing for adjusting the receiver timing and/or the FFT window is determined or calculated by a combination of a Reference Signal Received Power (RSRP) for the cooperating node and a received RS timing generated from a node specific RS of the cooperating node. For example, the estimated composite received RS timing 284 or actual timing can be calculated using a weighted sum of the CSI-RS timings, as

Is represented by, wherein_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is each of the calculated channel state information reference signal (CSI-RS) timings of the CoMP measurement set,

is the CSI-RS antenna port received signal power, i is a positive integer representing a node in the CoMP measurement set, and f () is a monotonic function of its argument (i.e., function argument). Adjusting the receiver timing or FFT window based on RSRP can give weight or priority to OFDM symbols received from the channel or signal with the largest or strongest signal power. The composite received RS timing using a combination of the Reference Signal Received Power (RSRP) and the received RS timing for the cooperative node can be used in Dynamic Point Selection (DPS) or Dynamic Cell Selection (DCS) of Joint Processing (JP). In Dynamic Cell Selection (DCS), PDSCH is transmitted from a single cooperating node in the coordination set, which may be dynamically selected.

In another embodiment, a transmitting cooperating node or controller in the core network may select a selected cooperating node from a plurality of cooperating nodes for use in adjusting receiver timing of the wireless device for a reference cooperating node. The sending cooperative node may be the same cooperative node as the selected cooperative node or a different cooperative node. The sending cooperative node may communicate the selection of the selected cooperative node to the wireless device. The wireless device may receive a selection of the selected cooperating node from the cooperating node. The selection of the selected cooperating node may be transmitted or signaled in Downlink Control Information (DCI) for the wireless device. The wireless device may receive multiple node-specific RSs from various cooperating nodes. The wireless device may generate a synchronized RS timing from the node-specific RS from the selected cooperating node. The synchronized RS timing can be used to adjust the receiver timing of the wireless device (for timing synchronization) for received data or received Physical Downlink Shared Channel (PDSCH). The composite received RS timing can include synchronous RS timing. In this manner, a cooperating node (e.g., a transmitting cooperating node) or controller in the core network may select RS timing to be used for the composite received RS timing, which is used to adjust the receiver timing to receive the PDSCH.

Fig. 3 illustrates adjusting receiver timing of wireless devices in a coordinated multipoint (CoMP) system having two cooperating nodes 310A-B (e.g., first and second cooperating nodes) that transmit node-specific reference signals (NS-RSs) 350A-B to wireless devices 330 in a coordinated group 320. The wireless device may be initially synchronized to a PSS and/or a CRS transmission point (e.g., a second cooperating node). For example, due to propagation delays, the OFDM symbol in the first coordinated node transmission 352B and the substantially same OFDM symbol in the second coordinated transmission 352A may be received by the wireless device at different times. Since the wireless device is closer to the first cooperating node than the second cooperating node, the Wireless Device (WD) reception of the second Cooperating Node (CN) DL transmission 354A may have a greater propagation delay 356A than the propagation delay 356B of the Wireless Device (WD) reception of the first Cooperating Node (CN) DL transmission 354B. The timing of a Fast Fourier Transform (FFT) window 380 for sampling OFDM symbols may be synchronized to the macro node DL transmission if the PSS and/or CRS from the macro node is used for timing synchronization. The estimated composite received RS timing 384 for adjusting the receiver timing and/or the FFT window may use or include the earliest received RS timing, or may be determined or calculated by a combination of Reference Signal Received Power (RSRP) and received RS timing for the cooperating nodes. The cooperating node may transmit the node-specific RS to the wireless device before the wireless device generates the receive RS timing from the plurality of cooperating nodes.

Fig. 4 illustrates another example of synchronizing the timing of a DL transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in order to reduce ICI and ISI in a coordinated multipoint (CoMP) system. The OFDM symbols may be received from two cooperating nodes at substantially the same time. The adjusted timing 396 may be made in the transmitter timer of the cooperating node, which may shift the Inverse Fast Fourier Transform (IFFT) modulation window. An IFFT modulator or IFFT block may be used to generate the modulated signal. The wireless device may transmit timing feedback to the first cooperating node comprising the composite received RS timing or a first cooperating node received RS timing generated from a node-specific RS from the first cooperating node. The first coordinating node may receive timing feedback from the wireless device. The first cooperating node may modify the downlink transmission timing (e.g., the first cooperating node DL transmission 392) by adjusting the timing 396 using the composite received RS timing or the first cooperating node received RS timing. Modifying the downlink transmission timing may include offsetting (e.g., delaying or advancing) an Inverse Fast Fourier Transform (IFFT) timing of the downlink signal for the downlink transmission as a function of the composite received RS timing or the first cooperating node received RS timing. Variations in Wireless Device (WD) reception of the first Coordinated Node (CN) DL transmission 394 may reduce the time between the minimum received RS timing and the maximum received RS timing, which may align received OFDM symbols and reduce ICI and ISI. In another example, downlink transmissions from at least two of the plurality of cooperating nodes may be received by the wireless device at substantially the same time. In another example, DL transmissions of the cooperating nodes may be adjusted to synchronize reception of DL transmissions at the wireless device to a specified timing, such as an existing PSS and/or CRS.

In another example, the receiver timing of the wireless device can be adjusted using information from node-specific RSs from multiple cooperating nodes, and the transmitter timing of at least one cooperating node can be adjusted using timing feedback to reduce the time between the minimum received RS timing and the maximum received RS timing.

In one example, as shown in fig. 5, the OFDM symbols and node-specific RSs may represent elements of a radio frame structure transmitted on a Physical (PHY) layer in downlink transmission or uplink transmission between a node (or eNodeB) and a wireless device (or UE) using a general Long Term Evolution (LTE) frame structure. Although an LTE frame structure is shown, a frame structure for IEEE 802.16 standard (WiMax) using OFDM, IEEE 802.11 standard (WiFi), or another type of communication standard may also be used.

Fig. 5 shows a downlink radio frame structure type 2. In thatIn this example, a radio frame 100 of a signal for transmitting data may be configured to have a duration T of 10 milliseconds (ms)_f. Each radio frame may be segmented or partitioned into 10 subframes 110i, each 1ms long. Each subframe may be further subdivided into two slots 120a and 120b, each slot having a duration T_slotIs 0.5 ms. Each slot of a Component Carrier (CC) used by a transmitting station and a receiving station may include a plurality of Resource Blocks (RBs) 130a, 130b, 130i, 130m, and 130n based on a CC frequency bandwidth. The CC may have a carrier frequency with a bandwidth and a center frequency. Each RB (physical RB or PRB)130i may include 12-15kHz subcarriers 136 (on the frequency axis) and 6 or 7 Orthogonal Frequency Division Multiplexing (OFDM) symbols 132 (on the time axis) per subcarrier. If a short or normal cyclic prefix is employed, the RB may use 7 OFDM symbols. If an extended cyclic prefix is used, the RB may use 6 OFDM symbols. A resource block may be mapped to 84 Resource Elements (REs) 140i when a short or normal cyclic prefix is used, or may be mapped to 72 REs (not shown) using an extended cyclic prefix. The REs may be a unit of one OFDM symbol 142 by one subcarrier (i.e., 15kHz) 146. In case of Quadrature Phase Shift Keying (QPSK) modulation, each RE can transmit two bits of information 150a and 150 b. Other types of modulation may be used, such as 16 Quadrature Amplitude Modulation (QAM) or 64QAM to transmit a greater number of bits in each RE, or Binary Phase Shift Keying (BPSK) modulation to transmit a smaller number of bits (a single bit) in each RE. The RB may be configured for downlink transmissions from the eNodeB to the UE, or the RB may be configured for uplink transmissions from the UE to the eNodeB.

The reference signal may be transmitted through the OFDM symbol via a resource element in the resource block. The reference signal (or pilot signal or tone) may be a known signal used for various reasons, such as synchronization timing, estimating the channel and/or noise in the channel. The reference signal may be received and transmitted by the transmitting station and the mobile communication device. Different types of Reference Signals (RSs) may be used in the RB. For example, in an LTE system, downlink reference signal types may include cell-specific reference signals (CRS), multicast \ broadcast single frequency network (MBSFN) reference signals, UE-specific reference signals (UE-specific RS or UE-RS) or demodulation reference signals (DMRS), Positioning Reference Signals (PRS), and channel state information reference signals (CSI-RS).

The CRS may be transmitted in a downlink subframe in a cell supporting PDSCH. Data is transmitted from the eNodeB to the UE via the PDSCH. When a Physical Multicast Channel (PMCH) is transmitted in an MBSFN subframe, an MBSFN reference signal may be transmitted. The UE-RS or DMRS may be transmitted in a downlink subframe supporting PDSCH. UE-rss (dmrss) may be transmitted to a particular terminal (e.g., a mobile communication device) within resource blocks allocated for downlink shared channel (DL-SCH) transmissions, for beamforming a single UE using multiple antennas, and for PDSCH demodulation. PRSs may be transmitted in RBs in a downlink subframe configured for PRS transmission, but may not be mapped to a Physical Broadcast Channel (PBCH), a Primary Synchronization Signal (PSS), or a Secondary Synchronization Signal (SSS). The CSI-RS may be used for downlink channel quality measurements.

Fig. 6 illustrates an example flow diagram of timing synchronization 560 and additional timing synchronization 580 for Downlink (DL) transmissions in a coordinated multipoint (CoMP) system. Initially, a timing estimate for a receiver of a wireless device may be generated using PSS and/or CRS 562 from a cooperating node. The CoMP measurement set configuration 572 can be generated by at least one cooperating node. In another embodiment, the CoMP measurement set configuration 572 is received by at least one coordinating node from a controller in the core network. At least one segment of the CoMP measurement set configuration can be transmitted to the wireless device. The segment of the CoMP measurement set sent to the wireless device may include cooperating nodes in a coordination set for measurement of node-specific RSs (e.g., CSI-RSs). The additional timing synchronization can include timing estimates for each CSI-RS antenna port in CoMP measurement set 582 and a composite received RS timing 584 calculated from the estimated set of timings for adjusting or generating the actual timing of the receiver of the wireless device.

Based on controlling timing synchronization with PSS, SSS, and/or CRS signals only, additional timing synchronization using node-specific RS or CSI-RS may allow receiver timing to be adjusted to receive data OFDM symbols from different cooperating nodes such that most OFDM symbol boundaries fall within the guard interval of the OFDM symbol, which may reduce ICI and ISI. Receiver timing may include timing of processing within the receiver, timing when the receiver is looking for OFDM symbol boundaries, or when the receiver is performing or sampling an FFT. The additional timing synchronization uses several received reference signal timings from different cooperating nodes instead of only PSS, SSS and/or CRS signals from a single node. Each received Reference Signal (RS) timing may be from the ith cooperating node, where i is a positive integer representing a node in the CoMP measurement set. The OFDM symbol boundary may be in a signal received from an ith cooperating node, which may include a serving node. The value of the received RS timing may be measured or generated using a node-specific RS or CSI-RS from the ith cooperating node. The timing may include possible delays such as Transmitter (TX) delays, propagation delays, Receiver (RX) delays, and other processing delays.

Fig. 7 shows an OFDM demodulator, including an FFT demodulator in a Receiver (RX) for downlink in a wireless device; and an OFDM modulator including an IFFT modulator in a transmitter for downlink in the cooperative node. Additional timing synchronization may be used to adjust the timing of the FFT demodulator for the OFDM symbols.

A wireless communication system may be subdivided into various portions called layers. In the LTE system, the communication layer may include a Physical (PHY), a Medium Access Control (MAC), a Radio Link Control (RLC), a Packet Data Convergence Protocol (PDCP), and a Radio Resource Control (RRC) layer. As shown in fig. 7, the physical layer may comprise the basic hardware transport components of the wireless communication system 400. A basic multiple-input multiple-output (MIMO) system is used to simply illustrate the basic hardware transmission components, but the components may also be applicable to complex MIMO systems, SISO systems, or similar systems. For example, in a MIMO system, at the transmitter 410, the binary input data 420 may be protected by encoding using a channel encoder 422, interleaved using an interleaver 424 to combat fading, and mapped using a mapper 426 to improve reliability. The mapped data may be separated into layers for antenna ports by a Transmitter (TX) beamformer 434 and the layers may be OFDM modulated into OFDM symbols using modulators 428A-B. The modulator may use an Inverse Fast Fourier Transform (IFFT) algorithm to compute an Inverse Discrete Fourier Transform (IDFT) to generate a modulated signal (vector x for each antenna port). The modulated signals may be converted to analog signals by digital-to-analog converters (DACs) 430A-B. The analog signals may be transmitted via Radio Frequency (RF) transmitters (Tx)432A-B configured to transmit the signals to transmitter antennas 440A-B, which may be operable to communicate the signals. The analog signal will follow a path called channel 450. The physical layer may include other components (not shown) such as a serial-to-parallel (S/P) converter, a parallel-to-serial (P/S) converter, a Cyclic Prefix (CP) inserter and deleter, a guard band inserter and deleter, and other required components.

The signal transmitted through the channel 450 may be subject to noise 452 and interference 454. Noise and interference are shown as an addition 456 to the channel signal, which may be received via receiver antennas 490A-B and one or more Radio Frequency (RF) receivers (Rx)482A-B of receiver 460. The channel signals combined with noise and interference may be converted to digitally modulated signals by analog-to-digital converters (ADCs) 480A-B. The digital signals may be OFDM demodulated using demodulators 478A-B. The demodulator may use a Fast Fourier Transform (FFT) algorithm to compute a Discrete Fourier Transform (DFT) to produce a demodulated signal (vector y for each antenna port). Channel estimator 462 may use the demodulated signal to estimate channel 450 and noise and interference present in the channel. The channel estimator may include or be in communication with a feedback generator that may generate a Physical Uplink Shared Channel (PUSCH) feedback report, such as a Channel Quality Indicator (CQI) report, a Precoding Matrix Indicator (PMI) report, or a transmission Rank Indicator (RI) report. The CQI may be used to assist the MIMO transmission mode. The demodulated signals may be combined using a MIMO decoder 484, demapped using a demapper 476, deinterleaved using a deinterleaver 474, and decoded by a channel decoder 472 to generate binary output data 470, which binary output data 470 may be used by other layers of the receiving station.

As shown in the flowchart of fig. 8, another example provides a method 500 for adjusting receiver timing of a wireless device in a coordinated multipoint (CoMP) system. The method may be performed as instructions on a machine, where the instructions are included on at least one computer-readable medium or one non-transitory machine-readable storage medium. The method includes an operation of receiving, at a wireless device, a plurality of node-specific Reference Signals (RSs) from a plurality of cooperating nodes in a coordination set of a CoMP system, wherein the coordination set includes at least two cooperating nodes, as shown in block 510. Operations then follow for estimating a composite received RS timing from a plurality of received RS timings generated from a plurality of node-specific RSs, wherein the received RS timings represent timings from at least two cooperating nodes, as shown in block 520. The next operation of the method may be to adjust the receiver timing based at least in part on the composite received RS timing, as shown in block 530.

The node-specific RSs may include channel state information reference signals (CSI-RSs). The adjusted receiver timing may be a time for a receiver of the wireless device to process a Fast Fourier Transform (FFT) of the received signal.

In one embodiment, the operation of estimating the timing of the synthesized received RS may further comprise: the earliest received RS timing is selected for the composite received RS timing. The composite received RS timing may be determined by

Is represented by, wherein_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is each of the calculated channel state information reference signal (CSI-RS) timings of the CoMP measurement set, min () is a minimum function, and i is a positive integer representing a node in the CoMP measurement set.

In another embodiment, the operation of estimating the composite received RS timing may further comprise: a receiver RS timing is selected that is substantially between the minimum received RS timing and the maximum received RS timing. The minimum received RS timing may include a timing generated from a first received node-specific RS of the first cooperative node, and the maximum received RS timing may include a timing generated from a last received node-specific RS of the last cooperative node. In one example, the user may be enabled to negotiateThe combined Received Signal Received Power (RSRP) of the peer node and the received RS timing generated from the node-specific RS of the cooperating node are combined to determine a composite received RS timing. In another example, the composite receive RS timing can be determined by

Is represented by, wherein_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is each of the calculated channel state information reference signal (CSI-RS) timings of the CoMP measurement set,

is the CSI-RS antenna port Reference Signal Received Power (RSRP), i is a positive integer representing a node in the CoMP measurement set, and f () is a monotonic function of its argument.

The method can also include the wireless device transmitting timing feedback including the composite received RS timing to the cooperating node. In another example, the method can also include the wireless device transmitting timing feedback to the cooperating node including received RS timing generated from a node-specific RS from the cooperating node. The node-specific RSs may include Primary Synchronization Signals (PSS), Secondary Synchronization Signals (SSS), cell-specific reference signals (CRS), or channel state information reference signals (CSI-RS).

As shown in the flow diagram in fig. 9, another example provides a method 600 for synchronizing a timing of a Downlink (DL) transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in a coordinated multipoint (CoMP) system. The method may be performed as instructions on a machine, where the instructions are included on at least one computer readable medium or one non-transitory machine-readable storage medium. The method includes an operation of receiving timing feedback from a wireless device at a first cooperating node, wherein the timing feedback includes at least one received Reference Signal (RS) timing generated from a node-specific RS of at least one cooperating node, as shown in block 610. Operations next proceed to modify the downlink transmission timing at the first cooperating node by adjusting the timing using the timing feedback, as shown in block 620.

The timing feedback includes a composite received RS timing or a first cooperating node received RS timing. The composite received RS timing may be estimated from a plurality of received RS timings representing timings from at least two cooperating nodes. The first cooperating node receive RS timing may be generated from a node-specific RS from the first cooperating node. These receive RS timings may be generated from a plurality of node-specific RSs.

In one example, synthesizing the received RS timing may include receiving the RS timing from a first cooperating node generated from a node-specific RS from the first cooperating node. The node-specific reference signals include channel state information reference signals (CSI-RS). The downlink transmission includes data or a Physical Downlink Shared Channel (PDSCH). Modifying the downlink transmission timing may further include offsetting an Inverse Fast Fourier Transform (IFFT) timing of the downlink signal for the downlink transmission based on the composite received RS timing or the first cooperating node received RS timing. The method may further comprise: a first cooperating node (e.g., a sending cooperating node) selects a selected cooperating node from a plurality of cooperating nodes. The node-specific RS from the selected cooperating node may be used by the wireless device to generate a synchronized RS timing, and the synchronized RS timing may be used for timing synchronization of received data or received Physical Downlink Shared Channel (PDSCH). The first cooperating node may communicate the selection of the selected cooperating node to the wireless device. The synchronized RS timing can be used to adjust the receiver timing of the wireless device for received data or received PDSCH. The method may further comprise: the first cooperating node transmits the node-specific RS to the wireless device prior to receiving the timing feedback.

Figure 10 illustrates example coordinating nodes 710A-B and an example wireless device 720 in a coordinated multipoint (CoMP) system. The cooperating nodes may include macro nodes (e.g., macro enbs) or low power nodes (e.g., micro enbs, pico enbs, femto enbs, or henbs).

A wireless device 720 (e.g., a UE) may communicate with the cooperating nodes 710A-B. The wireless device can include a timing estimation device 718 for estimating receiver timing of the wireless device in a coordinated multipoint (CoMP) system. The timing estimation apparatus may include a downlink receiving module 722 and a timing estimator 724. In some embodiments, the timing estimation device may include a timing adjustment module 726 and an Uplink (UL) transmission module 728. The wireless device may include a transceiver configured to receive DL transmission information from a cooperating node and transmit UL transmission information to the cooperating node.

Downlink receiving module 722 may be configured to receive, at a wireless device, a plurality of node-specific Reference Signals (RSs) from a plurality of cooperating nodes in a coordination set of a CoMP system. The coordination group may include at least two cooperating nodes. The downlink receiving module may be further configured to receive a selection of the selected cooperating node. The selected cooperating node may be selected by a controller in the core network or one of the cooperating nodes. The node-specific RS from the selected cooperating node may be used by the wireless device to generate a synchronized RS timing, and the synchronized RS timing may be used to timing synchronize or adjust the receiver timing of the wireless device for received data or received Physical Downlink Shared Channel (PDSCH). The timing estimator 724 may be configured to estimate a composite received RS timing from a plurality of received RS timings generated from a plurality of node-specific RSs. The received RS timing may represent timing from at least two cooperating nodes. The node-specific RS includes a channel state information reference signal (CSI-RS). In one example, the timing estimator can be configured to select an earliest received RS timing for synthesizing the received RS timings. The composite received RS timing may be determined by

Is represented by, wherein_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is each of the calculated channel state information reference signal (CSI-RS) timings of the CoMP measurement set, min () is a minimum function, and i is a positive integer representing a node in the CoMP measurement set. In another example, the timing estimator can be configured to select between substantially a minimum received RS timing and a maximum received RS timing using the composite received RS timingThe receiver RS timing between the received RS timings. In another example, the timing estimator may be configured to determine the composite received RS timing from a combination of a Reference Signal Received Power (RSRP) for the cooperating node and a received RS timing generated from a node-specific RS of the cooperating node. Composite receive RS timing

Wherein, τ is_PDSCHIs the Physical Downlink Shared Channel (PDSCH) timing,

is each of the calculated channel state information reference signal (CSI-RS) timings of the CoMP measurement set,

is the CSI-RS antenna port Reference Signal Received Power (RSRP), i is a positive integer representing a node in the CoMP measurement set, and f () is a monotonic function of its argument.

The timing adjustment module 726 may be configured to adjust the receiver timing based on the composite received RS timing. The adjusted receiver timing may be a time for a receiver of the wireless device to process a Fast Fourier Transform (FFT) of the received signal. This time may represent the boundary of the FFT window. The uplink transmission module 728 may be configured to transmit timing feedback to the cooperating nodes including the composite received RS timing or the received RS timing generated from the node-specific RSs from the cooperating nodes. The wireless devices may include User Equipment (UE) and a Mobile Station (MS). The wireless device may be configured to connect to at least one of a Wireless Local Area Network (WLAN), a Wireless Personal Area Network (WPAN), and a Wireless Wide Area Network (WWAN). The wireless device may include an antenna, a touch sensitive display screen, a speaker, a microphone, a graphics processor, an application processor, internal memory, or a non-volatile memory port.

Each cooperating node 710A-B may include a timing synchronization device 708A-B for synchronizing a timing of a Downlink (DL) transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in a coordinated multipoint (CoMP) system. The timing synchronization apparatus may include downlink transmit modules 712A-B, uplink receive modules 714A-B, and timing modification modules 716A-B. In one example, the timing synchronization device can include a selection module (not shown). In another example, the selection module may be included in a controller in the core network. The cooperating nodes may be in a coordination set 740 of the CoMP system and communicate with each other via a backhaul link 750. The backhaul link may include X2 signaling or backhaul link signaling via a wired connection, a wireless connection, or an optical fiber connection. The communication between the cooperating nodes may include CoMP measurement set information.

The uplink receiving modules 714A-B may be configured to receive timing feedback from the wireless devices. The timing feedback may include at least one received Reference Signal (RS) timing generated from a node-specific RS of the at least one cooperating node. The timing feedback may include a synthetic received Reference Signal (RS) timing or a first cooperating node received RS timing. The composite received RS timing can be estimated from a plurality of received RS timings representing timings from at least two cooperating nodes, and the received RS timing can be generated from a plurality of node-specific RSs. The first cooperating node receive RS timing may be generated from a node-specific RS from the first cooperating node. The timing modification module 716A-B may be configured to modify the downlink transmission timing at the first cooperating node by adjusting the timing using timing feedback. The node-specific reference signals include channel state information reference signals (CSI-RS). The timing modification module may be further configured to offset an Inverse Fast Fourier Transform (IFFT) timing of the downlink signal for the downlink transmission by synthesizing the received RS timing or the cooperating node received RS timing. The downlink transmission modules 712A-B may be configured to transmit the node-specific RS to the wireless device. The selection module may be configured to select the selected cooperative node from a plurality of cooperative nodes. The node-specific RS from the selected cooperating node may be used by the wireless device to generate a synchronized RS timing, and the synchronized RS timing may be used for timing synchronization of received data or a received Physical Downlink Shared Channel (PDSCH). The downlink transmitting module may be further configured to transmit the selection of the selected cooperating node to the wireless device. The synchronized RS timing can be used to adjust the receiver timing of the wireless device for received data or received PDSCH. The cooperating nodes may include macro nodes, Low Power Nodes (LPNs), macro evolved Node bs (macro enbs), micro enbs, pico enbs, femto enbs, or home enbs (henbs).

Fig. 11 provides an example illustration of a wireless device, such as a User Equipment (UE), Mobile Station (MS), mobile wireless device, mobile communication device, tablet, handset, or other type of mobile wireless device. A wireless device may include one or more antennas configured to communicate with a Node, such as a macro Node, a Low Power Node (LPN), or a transmission station, such as a Base Station (BS), an evolved Node B (eNB), a baseband unit (BBU), a Remote Radio Head (RRH), a Remote Radio Equipment (RRE), a Relay Station (RS), a Radio Equipment (RE), or other type of Wireless Wide Area Network (WWAN) access point. The wireless device may be configured to communicate using at least one wireless communication standard including 3GPP LTE, WiMAX, High Speed Packet Access (HSPA), bluetooth, and WiFi. Wireless devices may communicate using separate antennas for each wireless communication standard or using a common antenna for multiple wireless communication standards. The wireless devices may communicate in a Wireless Local Area Network (WLAN), a Wireless Personal Area Network (WPAN), and/or a WWAN.

Fig. 11 also provides an illustration of a microphone and one or more speakers that may be used for audio input and output from the wireless device. The display screen may be a Liquid Crystal Display (LCD) screen or other type of display screen, such as an Organic Light Emitting Diode (OLED) display. The display screen may be configured as a touch screen. The touch screen may use capacitive, resistive, or other types of touch screen technology. The application processor and the graphics processor may be coupled to internal memory to provide processing and display capabilities. The non-volatile memory port may also be used to provide data input/output options to a user. The non-volatile memory port may also be used to extend the memory function of the wireless device. The keyboard may be integrated with or wirelessly connected to the wireless device to provide additional user input. A virtual keyboard may also be provided using a touch screen.

The various techniques, or certain aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, non-transitory computer-readable storage medium, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the various techniques. In the case of program code execution on programmable computers, the computing device can include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. The volatile and nonvolatile memory and/or storage elements may be RAM, EPROM, flash drives, optical drives, magnetic hard drives, or other media for storing electronic data. The base station and wireless device may also include a transceiver module, a counter module, a processing module, and/or a clock module or timer module. One or more programs that may implement or utilize the various techniques described herein may use an Application Programming Interface (API), reusable controls, and the like. Such programs may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the program(s) can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations.

It should be appreciated that many of the functional units described in this specification have been labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module.

Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. A module may be passive or active, including an agent operable to perform a desired function.

Reference throughout this specification to "an example" means that a particular feature, structure, or characteristic described in connection with the example is included in at least one embodiment of the present invention. Thus, the appearances of the phrase "in one example" appearing in various places throughout the specification are not necessarily all referring to the same embodiment.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and distinct member. Thus, no member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. Furthermore, various embodiments and examples of the present invention may be referred to herein, along with alternatives for the various components thereof. It should be understood that such embodiments, examples, and alternatives are not to be construed as actual equivalents of each other, but are to be considered as separate and independent representations of the present invention.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of layouts, distances, network examples, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, arrangements, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the foregoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and implementation details may be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is intended that the invention be limited only by the claims set forth below.

Claims (25)

1. A method for adjusting timing in a coordinated multipoint (CoMP) system, comprising:

receiving, at a User Equipment (UE), a plurality of node-specific Reference Signals (RSs) from a plurality of nodes in a coordination set of the CoMP system, wherein the coordination set includes at least two nodes;

generating a plurality of node-specific RS timings received by antenna ports for the at least two nodes;

estimating an average timing delay from a plurality of received RS timings, wherein the average timing delay comprises a Transmitter (TX) delay, a propagation delay, a Receiver (RX) delay, or other processing delay; and

transmitting an indication of the average timing delay to at least one node of the plurality of nodes, the at least one node configured to adjust downlink transmission timing in accordance with a given timing value based at least in part on the average timing delay.

2. The method of claim 1, wherein receiving a plurality of node-specific RSs from a plurality of cooperating nodes uses a channel state information reference signal (CSI-RS) antenna port or a UE-specific reference signal (UE-specific RS or UE-RS) antenna port associated with a downlink transmission, and the downlink transmission comprises data or a Physical Downlink Shared Channel (PDSCH).

3. The method of claim 1, wherein the node-specific RS comprises a channel state information reference signal (CSI-RS).

4. The method of claim 1, wherein the at least one node is configured to adjust the downlink transmission timing by adjusting an Inverse Fast Fourier Transform (IFFT) timing of a downlink signal for the downlink transmission as a function of the given timing value.

5. The method of claim 1, wherein the node-specific RSs comprise Primary Synchronization Signals (PSS), Secondary Synchronization Signals (SSS), cell-specific reference signals (CRS), user equipment-specific reference signals (UE-specific RSs or UE-RSs), or channel state information reference signals (CSI-RSs).

6. The method of claim 1, wherein the plurality of nodes are cooperative nodes.

7. A method for synchronizing timing of a Downlink (DL) transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in a coordinated multipoint (CoMP) system, comprising:

receiving timing feedback from a User Equipment (UE) at the first cooperating node, wherein timing feedback comprises at least one received Reference Signal (RS) timing generated from a RS of at least one cooperating node of the CoMP system;

modifying downlink transmission timing at the first cooperating node by adjusting timing based at least in part on the timing feedback; and

transmitting, by the first coordinating node, a downlink signal to a wireless device using the downlink transmission timing.

8. The method of claim 7, wherein the timing feedback comprises a composite received RS timing or a first cooperating node received RS timing, wherein the composite received RS timing is estimated by a plurality of received RS timings representing timings from at least two cooperating nodes or by a first cooperating node received RS timing generated from a plurality of node-specific RSs from the first cooperating node, and the composite received RS timing and the first cooperating node received RS timing are generated from the plurality of node-specific RSs.

9. The method of claim 8, wherein the composite received RS timing represents an average delay between RSs received by antenna ports of the at least two cooperating nodes, wherein the average delay comprises a Transmitter (TX) delay, a propagation delay, a Receiver (RX) delay, or other processing delay.

10. The method of claim 7, wherein the node-specific reference signals comprise channel state information reference signals (CSI-RS) and the downlink transmissions comprise data or a Physical Downlink Shared Channel (PDSCH).

11. The method of claim 8, wherein modifying downlink transmission timing further comprises:

offset an Inverse Fast Fourier Transform (IFFT) timing of a downlink signal for the downlink transmission as a function of the composite receive RS timing or the first cooperating node receive RS timing, wherein offsetting the IFFT timing comprises delaying or advancing the IFFT timing.

12. The method of claim 7, further comprising, prior to receiving the timing feedback:

selecting, at the first cooperating node, a selected cooperating node from a plurality of cooperating nodes, wherein a node-specific RS from the selected cooperating node is used by the UE to generate a synchronous RS timing and the synchronous RS timing is used for timing synchronization of received data or a received Physical Downlink Shared Channel (PDSCH);

transmitting the selection of the selected cooperating node to the UE, wherein the synchronous RS timing is used to adjust the UE's receiver timing for the received data or the received PDSCH; and

transmitting a node-specific reference signal (UE-specific RS or UE-RS) from the first cooperating node to the UE using a CSI-RS antenna port or a UE-specific RS antenna port associated with a downlink transmission, and the downlink transmission comprising data or a Physical Downlink Shared Channel (PDSCH).

13. At least one non-transitory machine readable medium comprising a plurality of instructions adapted to be executed to implement the method of any of claims 1-12.

14. An apparatus for adjusting timing in a coordinated multipoint (CoMP) system, comprising:

means for receiving, at a User Equipment (UE), a plurality of node-specific Reference Signals (RSs) from a plurality of nodes in a coordination set of the CoMP system, wherein the coordination set includes at least two nodes;

means for generating a plurality of node-specific RS timings received by antenna ports for the at least two nodes;

means for estimating an average timing delay from a plurality of received RS timings, wherein the average timing delay comprises a Transmitter (TX) delay, a propagation delay, a Receiver (RX) delay, or other processing delay; and

means for transmitting an indication of the average timing delay to at least one node of the plurality of nodes, the at least one node configured to adjust downlink transmission timing as a function of a given timing value based at least in part on the average timing delay.

15. The apparatus of claim 14, wherein receiving a plurality of node-specific RSs from a plurality of cooperating nodes uses a channel state information reference signal (CSI-RS) antenna port or a UE-specific reference signal (UE-specific RS or UE-RS) antenna port associated with a downlink transmission, and the downlink transmission comprises data or a Physical Downlink Shared Channel (PDSCH).

16. The apparatus of claim 14, wherein the node-specific RS comprises a channel state information reference signal (CSI-RS).

17. The apparatus of claim 14, wherein the at least one node is configured to adjust the downlink transmission timing by adjusting an Inverse Fast Fourier Transform (IFFT) timing of a downlink signal for the downlink transmission as a function of the given timing value.

18. The apparatus of claim 14, wherein the node-specific RSs comprise a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), a cell-specific reference signal (CRS), a user equipment-specific reference signal (UE-specific RS or UE-RS), or a channel state information reference signal (CSI-RS).

19. The apparatus of claim 14, wherein the plurality of nodes are cooperative nodes.

20. An apparatus for synchronizing timing of a Downlink (DL) transmission of a first cooperating node relative to a downlink transmission of a second cooperating node in a coordinated multipoint (CoMP) system, comprising:

means for receiving timing feedback from a User Equipment (UE) at the first cooperating node, wherein timing feedback comprises at least one received Reference Signal (RS) timing generated from a RS of at least one cooperating node of the CoMP system; and

means for modifying downlink transmission timing at the first cooperating node by adjusting timing based at least in part on the timing feedback; and

transmitting, by the first cooperating node, a downlink signal to a wireless device using the downlink transmission timing.

21. The apparatus of claim 20, wherein the timing feedback comprises a composite received RS timing or a first cooperating node received RS timing, wherein the composite received RS timing is estimated from a plurality of received RS timings representing timings from at least two cooperating nodes or a first cooperating node received RS timing generated from a plurality of node-specific RSs from the first cooperating node, and the composite received RS timing and the first cooperating node received RS timing are generated from the plurality of node-specific RSs.

22. The apparatus of claim 21, wherein composite received RS timing represents an average delay between RSs received by antenna ports of the at least two cooperating nodes, wherein the average delay comprises a Transmitter (TX) delay, a propagation delay, a Receiver (RX) delay, or other processing delay.

23. The apparatus of claim 20, wherein the node-specific reference signals comprise channel state information reference signals (CSI-RS), and the downlink transmissions comprise data or a Physical Downlink Shared Channel (PDSCH).

24. The apparatus of claim 21, wherein means for modifying downlink transmission timing further comprises:

means for offsetting an Inverse Fast Fourier Transform (IFFT) timing of a downlink signal for the downlink transmission as a function of the composite receive RS timing or the first cooperating node receive RS timing, wherein offsetting the IFFT timing comprises delaying or advancing the IFFT timing.

25. The apparatus of claim 20, further comprising means for performing the following steps prior to receiving the timing feedback:

selecting, at the first cooperating node, a selected cooperating node from a plurality of cooperating nodes, wherein a node-specific RS from the selected cooperating node is used by the UE to generate a synchronous RS timing and the synchronous RS timing is used for timing synchronization of received data or a received Physical Downlink Shared Channel (PDSCH);

transmitting the selection of the selected cooperating node to the UE, wherein the synchronous RS timing is used to adjust the UE's receiver timing for the received data or the received PDSCH; and

transmitting a node-specific reference signal (UE-specific RS or UE-RS) from the first cooperating node to the UE using a CSI-RS antenna port or a UE-specific RS antenna port associated with a downlink transmission, and the downlink transmission comprising data or a Physical Downlink Shared Channel (PDSCH).

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