Patent Publication Number: US-2018054281-A1

Title: Method to transmit channel state information reference signals in large mimo systems

Description:
BACKGROUND 
     The third generation partnership project (3GPP), and specifically 3GPP LTE, aims to improve the universal mobile telecommunications system (UMTS) standard. The 3GPP LTE radio interface offers high peak data rates, low delays and an increase in spectral efficiencies. The LTE ecosystem supports both frequency division duplex (FDD) and time division duplex (TDD). This enables operators to exploit both paired and unpaired spectrums since LTE supports 6 bandwidths. 
     Multiple access schemes, as provided in systems such as LTE, also allow for performance enhancing scheduling strategies. For example, Frequency Selective Scheduling (FSS) can be used to schedule a user over sub-carriers (or part of the bandwidth) that provides maximum channel gains to that user (and avoid regions of low channel gain). The channel response is measured and the scheduler utilizes this information to intelligently assign resources to users over parts of the bandwidth that maximize their signal-to-noise ratios (and spectral efficiency). In other words, the end to end performance of a multi-carrier system like LTE relies significantly on sub-carrier allocation techniques and transmission modes. 
     In a downlink transmission of such a telecommunications system, a common reference signal (CRS) for user equipment (UE) performs channel estimation for demodulation of a physical downlink control channel (PDCCH) and other common channels, as well as to measure feedback. Additionally, a channel state information reference signal (CSI-RS) may be used to measure the channel status, especially when multiple transmission antennas exist. CSI-RS may measure parameters and feedback information such as precoding matrix indicator (PMI), channel quality indicator (CQI), and rank indicator (RI) of the precoding matrix. CSI-RS can support up to 8 transmission antennas, whereas CRS can only support 4 transmission antennas. 
     BRIEF SUMMARY 
     In one embodiment, the present technology relates to a method of transmitting a channel state information reference signal in a communications network, comprising computing a channel state information reference signal period based on an estimated Doppler metric corresponding to one or more user equipment in the network; grouping the one or more user equipment into ranges based on the estimated Doppler metric corresponding to a respective one of the one or more user equipment; configuring the one or more user equipment in each group to receive the channel state information reference signal with the corresponding channel state information reference signal period based on the Doppler metric; and transmitting the channel state information reference signal to the one or more user equipment according to the channel state information reference signal period. 
     In another embodiment, there is a base station for transmitting a channel state information reference signal in a communications network, comprising a memory storage comprising instructions; and one or more processors coupled to the memory that execute the instructions to compute a channel state information reference signal period based on an estimated Doppler metric corresponding to one or more user equipment in the network; group the one or more user equipment into ranges based on the estimated Doppler metric corresponding to a respective one of the one or more user equipment; configure the one or more user equipment in each group to receive the channel state information reference signal with the corresponding channel state information reference signal period based on the Doppler metric; and transmit the channel state information reference signal to the one or more user equipment according to the channel state information reference signal period. 
     In still another embodiment, there is a non-transitory computer-readable medium storing computer instructions for transmitting a channel state information reference signal in a communications network, that when executed by one or more processors, causes the one or more processors to perform the steps of computing a channel state information reference signal period based on an estimated Doppler metric corresponding to one or more user equipment in the network; grouping the one or more user equipment into ranges based on the estimated Doppler metric corresponding to a respective one of the one or more user equipment; configuring the one or more user equipment in each group to receive the channel state information reference signal with the corresponding channel state information reference signal period based on the Doppler metric; and transmitting the channel state information reference signal to the one or more user equipment according to the channel state information reference signal period. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the Background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are illustrated by way of example and are not limited by the accompanying figures for which like references indicate like elements. 
         FIG. 1  illustrates a wireless network for communicating data. 
         FIG. 2  illustrates an example of a physical layer diagram in accordance with an embodiment of the disclosure. 
         FIG. 3  illustrates a message sequence diagram between a base station and user equipment during downlink data transfer. 
         FIG. 4  illustrates a downlink radio frame to transmit a periodic channel state information reference signal. 
         FIG. 5  illustrates a grouping of user equipment into Doppler frequency zones. 
         FIG. 6A  illustrates a flow diagram of configuring user equipment to receive channel state information reference signals. 
         FIG. 6B  illustrates a flow chart for estimating a Doppler metric of user equipment. 
         FIG. 7  illustrates a flow diagram of reporting channel state information at user equipment. 
         FIGS. 8A and 8B  illustrate the impact of CSI-RS periodicity on average sector throughput with wideband and sub-band scheduling. 
         FIG. 9A  illustrates example user equipment that may implement the methods and teachings according to this disclosure. 
         FIG. 9B  illustrates example base station that may implement the methods and teachings according to this disclosure. 
         FIG. 10  illustrates a block diagram of a network system that can be used to implement various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology, generally described, relates to technology for transmitting channel state information reference signals in large MIMO systems. 
     The technology groups UEs capable of receiving a CSI-RS based on a computed Doppler metric. Each UE having an estimated Doppler metric falling within a defined range will be placed in the same group. Each group of UEs may then be configured with a different CSI-RS period. That is, the CSI-RS period may be set based on the UE Doppler frequency (i.e., the base station computes the Doppler metric of the UE and sets the CSI-RS period based on the Doppler frequency). By grouping the UEs in this manner, a base station or serving cell may transmit a CSI-RS to a UE at a rate at which the UE&#39;s CSI is expected to change. Accordingly, the capacity of the system may be improved by utilizing system resources to otherwise transmit data. Additionally, inter cell interference may be reduced due to less frequent transmission of CSI-RS. 
     It is understood that the present embodiments of the invention may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art. Indeed, the described embodiments of the invention are intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be clear to those of ordinary skill in the art that the present invention may be practiced without such specific details or with equivalent implementations. 
       FIG. 1  illustrates a wireless network for communicating data. The communication system  100  includes, for example, UE  110 A- 110 C, radio access networks (RANs)  120 A- 120 B, a core network  130 , a public switched telephone network (PSTN)  140 , the Internet  150 , and other networks  160 . Additional or alternative networks include private and public data-packet networks including corporate intranets. While certain numbers of these components or elements are shown in the figure, any number of these components or elements may be included in the system  100 . 
     System  100  enables multiple wireless users to transmit and receive data and other content. The system  100  may implement one or more channel access methods, such as but not limited to code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), or single-carrier FDMA (SC-FDMA). 
     The UEs  110 A- 110 C are configured to operate and/or communicate in the system  100 . For example, the UEs  110 A- 110 C are configured to transmit and/or receive wireless signals or wired signals. Each UE  110 A- 110 C represents any suitable end user device and may include such devices (or may be referred to) as a user equipment/device (UE), wireless transmit/receive unit (WTRU), mobile station, fixed or mobile subscriber unit, pager, cellular telephone, personal digital assistant (PDA), smartphone, laptop, computer, touchpad, wireless sensor, or consumer electronics device. 
     In the depicted embodiment, the RANs  120 A- 120 B include one or more base stations  170 A,  170 B (collectively, base stations  170 ), respectively. Each of the base stations  170  is configured to wirelessly interface with one or more of the UEs  110 A,  110 B,  110 C (collectively, UEs  110 ) to enable access to the core network  130 , the PSTN  140 , the Internet  150 , and/or the other networks  160 . For example, the base stations (BSs)  170  may include one or more of several well-known devices, such as a base transceiver station (BTS), a Node-B (NodeB), an evolved NodeB (eNB), a Home NodeB, a Home eNodeB, a site controller, an access point (AP), or a wireless router, or a server, router, switch, or other processing entity with a wired or wireless network. 
     In one embodiment, the base station  170 A forms part of the RAN  120 A, which may include other base stations, elements, and/or devices. Similarly, the base station  170 B forms part of the RAN  120 B, which may include other base stations, elements, and/or devices. Each of the base stations  170  operates to transmit and/or receive wireless signals within a particular geographic region or area, sometimes referred to as a “cell.” In some embodiments, multiple-input multiple-output (MIMO) technology may be employed having multiple transceivers for each cell. 
     The base stations  170  communicate with one or more of the UEs  110  over one or more air interfaces (not shown) using wireless communication links. The air interfaces may utilize any suitable radio access technology. 
     It is contemplated that the system  100  may use multiple channel access functionality, including for example schemes in which the base stations  170  and UEs  110  are configured to implement the Long Term Evolution wireless communication standard (LTE), LTE Advanced (LTE-A), and/or LTE Broadcast (LTE-B). In other embodiments, the base stations  170  and UEs  110  are configured to implement UMTS, HSPA, or HSPA+standards and protocols. Of course, other multiple access schemes and wireless protocols may be utilized. 
     The RANs  120 A- 120 B are in communication with the core network  130  to provide the UEs  110  with voice, data, application, Voice over Internet Protocol (VoIP), or other services. As appreciated, the RANs  120 A- 120 B and/or the core network  130  may be in direct or indirect communication with one or more other RANs (not shown). The core network  130  may also serve as a gateway access for other networks (such as PSTN  140 , Internet  150 , and other networks  160 ). In addition, some or all of the UEs  110  may include functionality for communicating with different wireless networks over different wireless links using different wireless technologies and/or protocols. 
     In one embodiment, the base stations  170  comprise a carrier aggregation component (not shown) that is configured to provide service for a plurality of UEs  110  and, more specifically, to select and allocate carriers as aggregated carriers for a UE  110 . More specifically, the carrier configuration component of base stations  170  may be configured to receive or determine a carrier aggregation capability of a selected UE  110 . The carrier aggregation component operating at the base stations  170  are operable to configure a plurality of component carriers at the base stations  170  for the selected UE  110  based on the carrier aggregation capability of the selected UE  110 . Based on the selected UE(s) capability or capabilities, the base stations  170  are configured to generate and broadcast a component carrier configuration message containing component carrier configuration information that is common to the UEs  110  that specifies aggregated carriers for at least one of uplink and downlink communications. 
     In another embodiment, base stations  170  generate and transmit component carrier configuration information that is specific to the selected UE  110 . Additionally, the carrier aggregation component may be configured to select or allocate component carriers for the selected UE  110  based on at least one of quality of service needs and bandwidth of the selected UE  110 . Such quality of service needs and/or required bandwidth may be specified by the UE  110  or may be inferred by a data type or data source that is to be transmitted. 
     Although  FIG. 1  illustrates one example of a communication system, various changes may be made to  FIG. 1 . For example, the communication system  100  could include any number of UEs, base stations, networks, or other components in any suitable configuration. 
     It is also appreciated that the term UE may refer to any type of wireless device communicating with a radio network node in a cellular or mobile communication system. Non-limiting examples of a UE are a target device, device-to-device (D2D) UE, machine type UE or UE capable of machine-to-machine (M2M) communication, PDA, iPAD, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME) and USB dongles. 
     Moreover, while the embodiments are described in particular for downlink data transmission scheme in LTE based systems, they are equally applicable to any radio access technology (RAT) or multi-RAT system. The embodiments are also applicable to single carrier as well as to multicarrier (MC) or carrier aggregation (CA) operation of the UE in which the UE is able to receive and/or transmit data to more than one serving cells using MIMO. 
       FIG. 2  illustrates an example of a physical layer diagram in accordance with an embodiment of the disclosure. Transport block data is passed through a cyclic redundancy check (CRC)  200  for error detection. The CRC  200  appends a CRC code to the transport block data received from a media access control (MAC) layer before being passed through the physical layer. The transport block is divided by a cyclic generator polynomial to generate parity bits. These parity bits are then appended to the end of transport block. A detailed description of transport block and code segmentation may be found in the description below with reference to  FIG. 4 . 
     The physical layer comprises a channel coder  201 , a rate matcher  202 , a scrambler  204 , a modulation mapper  206 , a layer mapper  208 , a pre-coder  210 , a resource element mapper  212 , a signal generator (OFDMA)  214 , and a power amplifier (PA)  216 . 
     Channel coder  201  turbo codes the data with convolutional encoders having certain interleaving there-between, and the rate matcher  202  acts as a rate coordinator or buffer between preceding and succeeding transport blocks. The scrambler  204  produces a block of scrambled bits from the input bits. 
     Resource elements and resource blocks (RBs) define a physical channel. A RB is a collection of resource elements. A resource element is a single subcarrier over one OFDM symbol, and carries multiple modulated symbols with spatial multiplexing. In the frequency domain, a RB represents the smallest unit of resources that can be allocated. In LTE-A, a RB is a unit of time frequency resource, representing 180 KHz of spectrum bandwidth for the duration of a 0.5 millisecond slot. 
     Modulation mapper  206  maps the bit values of the input to complex modulation symbols with the modulation scheme specified. In one embodiment, the modulation scheme is Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM). In another embodiment, the modulation scheme is OFDM with aggressive PAPR reduction. 
     Spatial multiplexing creates multiple streams of data to individual UEs  110  on a single resource block (RB) effectively reusing each RB a number of times and thus increases spectral efficiency. Layer mapper  208  splits the data sequence into a number of layers. 
     Pre-coder  210  is based on transmit beam-forming concepts allowing multiple beams to be simultaneously transmitted in the M-MIMO system by a set of complex weighting matrices for combining the layers before transmission. Vector hopping is may be used for transmit diversity. The pre-coder  210  may, for example, vector hop with the weighting of the two antennas alternating between [+1, +1] T  and [+ 1 , − 1 ] T  from subframe to subframe, and resetting at the beginning of a new radio frame. 
     The resource element mapper  212  maps the data symbols, the reference signal symbols and control information symbols into a certain resource element in the resource grid. The signal generator  214  is coupled between the resource element mapper  212  and the PA array  216 , such that a generated signal is transmitted by the PA antenna array using common broadcast channels (e.g. PSS, SSS, PBCH, PDCCH and PDSCH) over a narrow sub-band resource. The signal generator  214 , which may also be referred to as the radio front end (RFE), converts digital signals to analog signals and up-converts, amplifies and filters the signals to radio frequency (RF) for transmission. 
     For example, LTE systems support transmission of a maximum of two codewords in the downlink channel, where a codeword is defined as an information block appended with a CRC. Each codeword is separately segmented and coded using turbo coding and the coded bits from each codeword are scrambled separately, as explained above. The complex-valued modulation symbols for each of the codewords to be transmitted are mapped onto one or multiple layers using layer mapper  208 . The complex-valued modulation symbols d (q) ( 0 ), . . . , d (q) (M (q)   symb 31 1) for codeword q are mapped onto the layers x(i)=[x (0) (i) . . . x (0−1) (i)] T , i=0, 1, . . . , M layer   symb −1, where u is the number of layers and M layer   symb  is the number of modulation symbols per layer. The codeword to layer mapping is shown in Table 1 below. 
     Once the layer mapping is completed, the resultant symbols are pre-coded using the pre-coder  210 . The pre-coded symbols are mapped to resource elements in the OFDM time frequency grid and the OFDM signal is generated at  214 . The resulting signal is passed to the antenna ports. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Codeword-to-Layer Mapping in LTE 
               
            
           
           
               
               
               
            
               
                 Number of 
                 Number of 
                 Codeword-to-layer mapping 
               
               
                 layers 
                 codewords 
                 i = 0, 1, . . . , M symb   layer  − 1 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 1 
                 x (0)  (i) = d (0)  (i) 
                 M symb   layer  = M symb   (0)   
               
               
                 2 
                 2 
                 x (0)  (i) = d (0)  (i) 
                 M symb   layer  = M symb   (0)  = 
               
               
                   
                   
                 x (1)  (i) = d (1)  (i) 
                 M symb   (1)   
               
               
                 3 
                 2 
                 x (0)  (i) = d (0)  (i) 
                 M symb   layer  = M symb   (0)  = 
               
               
                   
                   
                 x (1)  (i) = d (1)  (2i) 
                 M symb   (1) /2 
               
               
                   
                   
                 x (2)  (i) = d (1)  (2i + 1) 
               
               
                 4 
                 2 
                 x (0)  (i) = d (0)  (2i) 
                 M symb   layer  = M symb   (0) /2 = 
               
               
                   
                   
                 x (1)  (i) = d (0)  (2i + 1) 
                 M symb   (1) /2 
               
               
                   
                   
                 x (2)  (i) = d (1)  (2i) 
               
               
                   
                   
                 x (3)  (i) = d (1)  (2i + 1) 
               
               
                 5 
                 2 
                 x (0)  (i) = d (0)  (2i) 
                 M symb   layer  = M symb   (0) /2 = 
               
               
                   
                   
                 x (1)  (i) = d (0)  (2i + 1) 
                 M symb   (1) /3 
               
               
                   
                   
                 x (2)  (i) = d (1)  (3i) 
               
               
                   
                   
                 x (3)  (i) = d (1)  (3i + 1) 
               
               
                   
                   
                 x (4)  (i) = d (1)  (3i + 2) 
               
               
                 6 
                 2 
                 x (0)  (i) = d (0)  (3i) 
                 M symb   layer  = M symb   (0) /3 = 
               
               
                   
                   
                 x (1)  (i) = d (0)  (3i + 1) 
                 M symb   (1) /3 
               
               
                   
                   
                 x (2)  (i) = d (0)  (3i + 2) 
               
               
                   
                   
                 x (3)  (i) = d (1)  (3i) 
               
               
                   
                   
                 x (4)  (i) = d (1)  (3i + 1) 
               
               
                   
                   
                 x (5)  (i) = d (1)  (3i + 2) 
               
               
                 7 
                 2 
                 x (0)  (i) = d (0)  (3i) 
                 M symb   layer  = M symb   (0) /3 = 
               
               
                   
                   
                 x (1)  (i) = d (0)  (3i + 1) 
                 M symb   (1) /4 
               
               
                   
                   
                 x (2)  (i) = d (0)  (3i + 2) 
               
               
                   
                   
                 x (3)  (i) = d (1)  (4i) 
               
               
                   
                   
                 x (4)  (i) = d (1)  (4i + 1) 
               
               
                   
                   
                 x (5)  (i) = d (1)  (4i + 2) 
               
               
                   
                   
                 x (6)  (i) = d (1)  (4i + 3) 
               
               
                 8 
                 2 
                 x (0)  (i) = d (0)  (4i) 
                 M symb   layer  = M symb   (0) /4 = 
               
               
                   
                   
                 x (1)  (i) = d (0)  (4i + 1) 
                 M symb   (1) /4 
               
               
                   
                   
                 x (2)  (i) = d (0)  (4i + 2) 
               
               
                   
                   
                 x (3)  (i) = d (0)  (4i + 3) 
               
               
                   
                   
                 x (4)  (i) = d (1)  (4i) 
               
               
                   
                   
                 x (5)  (i) = d (1)  (4i + 1) 
               
               
                   
                   
                 x (6)  (i) = d (1)  (4i + 2) 
               
               
                   
                   
                 x (7)  (i) = d (1)  (4i + 3) 
               
               
                   
               
            
           
         
       
     
       FIG. 3  illustrates a message sequence diagram between a base station and user equipment during downlink data transfer. Although the figure is discussed with reference to a downlink channel, it is appreciated that communication may also be in an uplink channel. 
     As shown, base station (eNB)  170  communicates cell-specific/UE-specific reference (or pilot) signals at  301 . Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal (UE  110 ), including, but limited to the following. 
     One type of reference signal is a CRS, which is transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The cell-specific reference signals can be used by the UE  110  for channel estimation for coherent demodulation of any downlink physical channel with a few exceptions, for example, during various transmission modes. The cell-specific reference signals can also be used by the terminal to acquire CSI, as explained below ( 302 ). Additionally, terminal measurements on cell-specific reference signals are used as the basis for cell-selection and handover decisions. 
     Another type of reference signal is a demodulation reference signal (DM-RS). These reference signals (also referred to as UE-specific reference signals) are used by UEs  110  for channel estimation for physical downlink shared channel (PDSCH) in various transmission modes. 
     Still another type of reference signal is a CSI-RS, which may be used by UEs  110  to acquire CSI in the case when demodulation reference signals are used for channel estimation. CSI-RS have a significantly lower time/frequency density, thus implying less overhead, compared to the cell-specific reference signals. 
     Using one or more of the above-identified reference signals, the UE  110  computes the CSI and parameters needed for CSI reporting at  302 . The CSI report includes, for example, the CQI, PMI, and RI. 
     At  303 , the CSI report is sent to the base station  170  via a feedback channel, such as a physical uplink control channel (PUCCH) for periodic CSI reporting or a physical uplink shared channel (PUSCH) for aperiodic CSI reporting. Once received, the base station  170  scheduler may use the information to choose the parameters, such as the modulation and coding scheme (MCS), power and physical resource blocks (PRBs), for scheduling of the UE  110 . The base station  170  then sends the scheduling parameters to the UE  110  at  305  in the physical downlink control channel (PDCCH). 
     In one embodiment, before sending the parameters in the PDCCH, the base station  170  sends a control format indicator information on the physical control indicator channel (PCFICH), which is the physical channel providing the UEs  110  with information necessary to decode the set of PDCCHs. Subsequently, data transmission may occur between the base station  170  and the UE  110  at  306 . 
     As alluded to above, the PDCCH carries information about the scheduling grants. For example, the information may include the number of MIMO layers scheduled, transport block sizes, modulation for each code word, parameters related to hybrid automatic repeat request (HARQ), sub-band locations and PMI corresponding to the sub-bands. Typically, the following information is transmitted by the downlink control information (DCI) format: localized/distributed virtual resource block (VRB) assignment flag, resource block assignment, modulation and coding scheme, HARQ process number, new data indicator, redundancy version, transmit power control (TPC) command for PUCCH, a downlink assignment index, and a pre-coding matrix index and number of layers. 
     It is appreciated, however, that each of the DCI formats may not use all the information as detailed above. Rather, the contents of PDCCH depends on a transmission mode and the DCI format. 
     As discussed above, CSI may also be reported in the PUCCH in which information is carried about HARQ-ACK information corresponding to the downlink data transmission and channel state information. The channel state information may include RI, CQI and PMI. Either PUCCH or PUSCH can be used to carry this information. Various modes for PUCCH and PUSCH may be used, which modes generally depend on the transmission mode and the formats configured via higher layer signaling. 
       FIG. 4  illustrates a downlink radio frame used to convey transmitted periodic channel state information reference signals. In the illustrated embodiment, the downlink radio frame includes, for example, 10 subframes, where a subframe includes two slots in the time domain. A time required for transmitting one subframe is defined as a Transmission Time Interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms. One slot may include a plurality of OFDM symbols in the time domain and include a plurality of Resource Blocks (RBs) in the frequency domain. Since the 3GPP LTE system uses OFDMA in the downlink, the OFDM symbol indicates one symbol duration. The OFDM symbol may be called an SC-FDMA symbol or symbol duration. An RB is a resource allocation unit including a plurality of contiguous subcarriers in one slot. As appreciated, the structure of the radio frame is only exemplary. Accordingly, the number of subframes included in a radio frame, the number of slots included in a subframe or the number of symbols included in a slot may be changed in various manners. 
     As illustrated, the radio frame is divided into 10 subframes, subframe  0  to subframe  9 . A base station, such as base stations  170 , transmits a CSI-RS with a CSI-RS transmission period of 10 ms (i.e., in every 10 subframes). In this example, there is also a CSI-RS transmission offset of 3. Different base stations  170  may have different CSI-RS transmission offsets so that CSI-RSs transmitted from a plurality of cells are uniformly distributed in time. For example, if a CSI-RS is transmitted every 10 ms, its CSI-RS transmission offset may be one of 0 to 9. 
     A CSI-RS transmission offset indicates a subframe in which base station  170  starts CSI-RS transmission in every predetermined period. When the base station  170  signals a CSI-RS transmission period (and offset) to a UE  110 , the UE  110  may receive a CSI-RS from the base station  170  in subframes determined by the CSI-RS transmission period (and offset). The UE  110  may measure a channel using the received CSI-RS and thus may report such information as a CQI, a PMI, and/or an RI to the base station  170 , as noted above. 
     As the information related to the CSI-RS is cell-specific information common to UEs  110  within the cell, the CSI-RS transmission period (and offset) may be set separately for each individual CSI-RS configuration. In one embodiment, the CSI-RS transmission period (and offset) may be set as a group for each CSI-RS configuration, as explained below in more detail. 
       FIG. 5  illustrates a grouping of user equipment into Doppler frequency zones. In one embodiment, the CSI-RS period for each UE  110  is calculated based on the Doppler frequency. In order to calculate the CSI-RS period for a particular UE  110 , the UEs  110  are categorized (grouped) into zones based on the estimated or predicted Doppler frequency (or speed) of the UE  110 . Calculation of the Doppler frequency is discussed below with reference  FIG. 6B . However, as appreciated, there are many well-known techniques to compute Doppler frequency. 
     In the example embodiment of  FIG. 5 , the estimates/predicted Doppler frequencies are divided into three categories: low (zone  1 ), medium (zone  2 ) and high (zone  3 ). Each zone represents a range of Doppler frequencies corresponding to the speed of one or more UEs  110 . For example, zone  1  may include one or more low speed UEs  110 , zone  2  may include one or more medium speed UEs  110  and zone  3  may include one or more high speed UEs  110 . While the example of  FIG. 5  illustrates three zones, there is no limit on the amount of zones that may be employed. That is, any number of more or less zones may be employed. 
     In the specific example of  FIG. 5 , the Doppler frequency for each UE  110  has been estimated/predicted by a base station  170 . If f is the estimated/predicted Doppler frequency of a UE  110 , the Doppler frequency range (speed) may be divided into three categories (zones) as follows: 
     Low Doppler Frequency Range: 0&lt;f&lt;FL 
     Medium Doppler Frequency Range: FL≦f&lt;FH 
     High Doppler Frequency Range: FH≦f&lt;+Inf, 
     where the frequency thresholds FL (frequency low) and FH (frequency high) may be predetermined or predicted by simulation or analysis. 
     In one embodiment, the Doppler frequency zone thresholds may depend on scheduling strategies and feedback (reporting) modes (or a combination thereof). A strategy defining in which way resources in time and frequency are allocated to a set of UEs  110  is commonly referred to as a scheduling algorithm. For example, scheduling algorithms that prioritize users having a good channel or radio condition perform channel dependent scheduling. Proportional fair scheduling, on the other hand, adds control of an overall fairness in the radio communications network by prioritizing UEs  110  not only based on a channel quality of the user equipment but also on an average rate of a transmission. These strategies may also be employed to set the aforementioned thresholds for each of the zones ( FIG. 5 ). It is appreciated that the above-identified scheduling algorithms are non-limiting, and that other known scheduling algorithms may be employed. 
     Similarly, the information which is fed back to the base station  170  by the UE  110 , including for example CQI and PMI, may be used to define the thresholds for each of the zones ( FIG. 5 ). As discussed with reference to  FIG. 3 , the UE  110  may report the feedback information via a PUSCH or a PUCCH. The report types of the CQI/PMI for the PUSCH report mode and the PUCCH report mode are well known. 
     As one example of defining Doppler frequency zones, the base station  170  configures two sets of CSI-RS signals with periodicity values T 1  and T 2 , where T 1 &gt;T 2 . For example, T 1 =80 msec and T 2 =10 msec. As discussed below with reference to  FIGS. 8A and 8B , setting a CS-RS period to a high value does not degrade the average sector throughput. Accordingly, UEs  110  grouped in zone  3  (high frequency range) are set such that the CSI-RS period is equal to T 1 . The base station  170  may then transmit one set of CSI-RS to the UEs  110  to indicate the relevant parameters related to these CSI-RS. For UEs  110  grouped in zone  2  (medium frequency range), the CSI-RS period is set to T 2 . The base station  170  then transmits a different set of CSI-RS and indicates the relevant parameters related to these CSI-RS. 
     In another example, the base station  170  configures three sets of CSI-RS signals with periodicity values T 1 , T 2  and T 3 , where T 1 &gt;T 2 &gt;T 3 . For example T 1 =5 msec, T 2 =20 msec, and T 3 =80 msec. For High Doppler UEs  110  (in this example, UEs falling within zone  3 ), the CSI-RS period is set to T 3  and a set of CSI-RS is transmitted to the UEs  110  to indicate the relevant parameters related to these CSI-RS. For medium Doppler frequency UEs  110  (in this example, UEs falling within zone  2 ), the CSI-RS period is set to T 1  and a different set of CSI-RS is transmitted to the UE  110  to indicate the relevant parameters related to these CSI-RS. For low Doppler frequency UEs  110  (in this example, UEs falling within zone  1 ), the CSI-RS period is set to T 2  and a different set of CSI-RS is transmitted to the UEs  110  indicate the relevant parameters related to these CSI-RS. 
       FIG. 6A  illustrates a flow diagram of configuring user equipment to receive channel state information reference signals. In the disclosed embodiments, the methodology may be implemented by processor  904  of UE  900  or processor  958  of base station  950  ( FIG. 9 ), although such implementation is not limited thereto. 
     In a communications system, such as communications system  100 , the CSI-RSs may be transmitted periodically at every integer multiple of one subframe, or in a predetermined transmission pattern, to assist in reducing overhead of CSI-RS. The CSI-RS transmission period or pattern of the CSI-RSs may be configured, in one embodiment, by the base station  170  (or  900 ) based on a computed or measured UE  110  Doppler metric (speed), such as Doppler frequency, at  602 . 
     At  604 , the UEs  110  are grouped into ranges based on the estimated Doppler metric. That is, as described above with reference to  FIG. 5 , UEs falling within a same range are grouped together. For example, UEs  110  having a Doppler frequency between 0 and a threshold FL will be grouped together (zone  1 ), while UEs  110  having a Doppler frequency between threshold FL and threshold FH will be grouped together (zone  2 ). 
     After the UEs  110  are grouped according to Doppler frequency, the UEs  110  in each group are configured to receive the CSI-RS with the corresponding CSI-RS period based on the Doppler metric at  606 . Subsequently, at  608 , CSI-RS may be transmitted to the UEs  110  according to the CSI-RS period. 
       FIG. 6B  illustrates a flow chart for estimating a Doppler metric of user equipment. At  604 A, the Doppler metric is calculated for each UE  110 , according to various methodologies. In one embodiment, the Doppler frequency is estimated from the time-varying amount of a received downlink pilot symbol, and the moving speed of a mobile terminal is calculated from the estimated Doppler frequency and the center frequency. The relationship between the movement speed V and the Doppler frequency Fd, the center frequency Fc, and the velocity of light c is given by expression: V=cf d /f c . 
     In another embodiment, the base station  170  can compute the direct speed of the UE  110 , for example, by positioning or global positioning system (GPS) at multiple intervals. Then the Doppler frequency (Df) can computed as the average of the individual speed measurements, using the expression: 
     
       
         
           
             
               
                 D 
                 f 
               
               = 
               
                 
                   1 
                   N 
                 
                  
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     N 
                   
                    
                   
                     Di 
                     * 
                     
                       
                         f 
                         c 
                       
                       / 
                       C 
                     
                   
                 
               
             
             , 
           
         
       
     
     where D i  is the individual speed measurement in m/sec, f c  is the carrier frequency and C is the velocity of light in free space. N is the number of speed measurements. 
     In yet another embodiment, a rate of change of the uplink channel may be used to estimate Doppler frequency (speed). In this case, the base station  170  estimates the uplink channel such that he rate of change of the uplink channel predicts a measurement of the Doppler frequency for the UE  110 . 
     Once the Doppler metrics are calculated for the UEs  110 , they may be divided into categories (groups) for creating zones at  604 B, as discussed above with reference to  FIG. 5 . 
       FIG. 7  illustrates a flow diagram of reporting channel state information at user equipment. Once the UE  110  receives the reporting periods of the CSI-RS from the base station  170 , at  702 , the UE  110  will estimate the channel from the respective CSI-RS during those periods at  704 . 
     Once all of the elements of the channel matrix is formed, the UE  110  will compute the parameters related to CSI, at  706 , for example CQI, RI, PMI, best sub-band indices, etc. The UE  110  then reports these values to the base station  170  either periodically using PUCCH or aperiodically using PUSCH, at  708 , as explained above. 
     In one embodiment, the UE  110  can recommend to the base station  170  whether it is in low Doppler region, medium Doppler region or High Doppler region to thereby assist the base station  170  in determining the Doppler metric and the CSI-RS reporting period for the corresponding UE  110 . 
     In another embodiment the UE determines the Doppler region and recommends the CSI-RS reporting period to the base station  170 . 
       FIGS. 8A and 8B  illustrate the impact of CSI-RS periodicity on average sector throughput with wideband and sub-band scheduling. In closed loop MIMO systems having different CSI-RS periods, performance loss occurs as a result of varying Doppler frequencies (speed) between UEs  110  and base stations  170 . 
       FIG. 8A  shows the throughput performance of a downlink channel in a MIMO system having two transmit antennas with wideband scheduling (in this example, in transmission mode  9 ). The percentage of degradation in average sector throughput is plotted in the vertical axis against the CSI-RS period (in msec) along the horizontal axis. Three different UE Doppler frequencies (speeds) are plotted in the graph of  FIG. 8A , namely the low Doppler frequency, medium Doppler frequency and high Doppler frequency. As the CSI-RS period increases, the average sector throughput decreases. However, accordingly to the graph, the impact for low Doppler frequency UEs and for high Doppler frequency UEs is below 8% when approaching a CSI-RS of 80 msec. This is a result of slow speed UEs having slower channel changes. For high speed Doppler frequencies, on the other hand, the channel changes are fast enough such that the performance loss (degradation) is nearly the same for different CSI-RS periods. For medium Doppler frequency UEs, the percentage loss (degradation) in average sector throughput is severe. The severity is due to low CSI-RS periods in which the CQI reported by a UE is valid, but as the CSI-RS period increases the channel is outdated. 
     Following the examples set forth above with respect to  FIG. 5 , to have a performance loss (degradation) of less than 5% for each of the Doppler frequency ranges, the periods should be set to 20 msec for low Doppler UEs, 10 msec for medium Doppler UEs, and 80 msec for high Doppler UE, as illustrated. 
       FIG. 8B  shows the throughput performance of a downlink channel in a MIMO system having two transmit antennas with sub-band scheduling. Similar to  FIG. 8A , the low Doppler frequency, medium Doppler frequency and high Doppler frequency are also impacted by the changing periodicity of the CSI-RS. However, in the case of  FIG. 8B , the percent of loss (degradation) is severe for each of the Doppler frequencies. For example, to ensure a performance loss of less than 10%, the periods should be set to 20 msec for low Doppler UEs, 5 msec for medium Doppler UEs and 80 msec for high Doppler UEs. 
     Accordingly, as explained above, the CSI-RS period in the disclosed technology is set based on the estimated/predicted UE Doppler frequency (i.e., the base station computes the Doppler metric of the UE and sets the CSI-RS period based on the Doppler frequency or range of frequencies). 
       FIG. 9A  illustrates example user equipment that may implement the methods and teachings according to this disclosure. As shown in the figure, the UE  900  includes at least one processor  904 . The processor  904  implements various processing operations of the UE  900 . For example, the processor  804  may perform signal coding, data processing, power control, input/output processing, or any other functionality enabling the UE  900  to operate in the system  100  ( FIG. 1 ). The processor  904  may include any suitable processing or computing device configured to perform one or more operations. For example, the processor  904  may include a microprocessor, microcontroller, digital signal processor, field programmable gate array, or application specific integrated circuit. 
     The UE  900  also includes at least one transceiver  902 . The transceiver  902  is configured to modulate data or other content for transmission by at least one antenna  910 . The transceiver  902  is also configured to demodulate data or other content received by the at least one antenna  910 . Each transceiver  902  may include any suitable structure for generating signals for wireless transmission and/or processing signals received wirelessly. Each antenna  910  includes any suitable structure for transmitting and/or receiving wireless signals. It is appreciated that one or multiple transceivers  902  could be used in the UE  900 , and one or multiple antennas  910  could be used in the UE  900 . Although shown as a single functional unit, a transceiver  902  may also be implemented using at least one transmitter and at least one separate receiver. 
     The UE  900  further includes one or more input/output devices  908 . The input/output devices  908  facilitate interaction with a user. Each input/output device  908  includes any suitable structure for providing information to or receiving information from a user, such as a speaker, microphone, keypad, keyboard, display, or touch screen. 
     In addition, the UE  900  includes at least one memory  906 . The memory  906  stores instructions and data used, generated, or collected by the UE  900 . For example, the memory  906  could store software or firmware instructions executed by the processor(s)  904  and data used to reduce or eliminate interference in incoming signals. Each memory  906  includes any suitable volatile and/or non-volatile storage and retrieval device(s). Any suitable type of memory may be used, such as random access memory (RAM), read only memory (ROM), hard disk, optical disc, subscriber identity module (SIM) card, memory stick, secure digital (SD) memory card, and the like. 
       FIG. 9B  illustrates example base station that may implement the methods and teachings according to this disclosure. As shown in the figure, the base station  950  includes at least one processor  958 , at least one transmitter  952 , at least one receiver  954 , one or more antennas  960 , and at least one memory  956 . The processor  958  implements various processing operations of the base station  950 , such as signal coding, data processing, power control, input/output processing, or any other functionality. Each processor  958  includes any suitable processing or computing device configured to perform one or more operations. Each processor  958  could, for example, include a microprocessor, microcontroller, digital signal processor, field programmable gate array, or application specific integrated circuit. 
     Each transmitter  952  includes any suitable structure for generating signals for wireless transmission to one or more UEs or other devices. Each receiver  954  includes any suitable structure for processing signals received wirelessly from one or more UEs or other devices. Although shown as separate components, at least one transmitter  952  and at least one receiver  954  could be combined into a transceiver. Each antenna  960  includes any suitable structure for transmitting and/or receiving wireless signals. While a common antenna  960  is shown here as being coupled to both the transmitter  952  and the receiver  954 , one or more antennas  960  could be coupled to the transmitter(s)  952 , and one or more separate antennas  860  could be coupled to the receiver(s)  954 . Each memory  956  includes any suitable volatile and/or non-volatile storage and retrieval device(s). 
       FIG. 10  is a block diagram of a network system that can be used to implement various embodiments. Specific devices may utilize all of the components shown, or only a subset of the components, and levels of integration may vary from device to device. Furthermore, a device may contain multiple instances of a component, such as multiple processing units, processors, memories, transmitters, receivers, etc. The network system may comprise a processing unit  1001  equipped with one or more input/output devices, such as network interfaces, storage interfaces, and the like. The processing unit  1001  may include a central processing unit (CPU)  1010 , a memory  1020 , a mass storage device  1030 , and an I/O interface  1060  connected to a bus. The bus may be one or more of any type of several bus architectures including a memory bus or memory controller, a peripheral bus or the like. 
     The CPU  1010  may comprise any type of electronic data processor. The memory  1020  may comprise any type of system memory such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof, or the like. In an embodiment, the memory  1020  may include ROM for use at boot-up, and DRAM for program and data storage for use while executing programs. In embodiments, the memory  1020  is non-transitory. The mass storage device  1030  may comprise any type of storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via the bus. The mass storage device  1030  may comprise, for example, one or more of a solid state drive, hard disk drive, a magnetic disk drive, an optical disk drive, or the like. 
     The processing unit  1001  also includes one or more network interfaces  1050 , which may comprise wired links, such as an Ethernet cable or the like, and/or wireless links to access nodes or one or more networks  1080 . The network interface  1050  allows the processing unit  901  to communicate with remote units via the networks  1080 . For example, the network interface  1050  may provide wireless communication via one or more transmitters/transmit antennas and one or more receivers/receive antennas. In an embodiment, the processing unit  1001  is coupled to a local-area network or a wide-area network for data processing and communications with remote devices, such as other processing units, the Internet, remote storage facilities, or the like. 
     There are many benefits to using embodiments of the present disclosure. For example, in the disclosed technology, the base station or the serving cell transmits the CSI-RS to a UE at a rate at which the UE&#39;s CSI is expected to change. Otherwise, these resources can be used for transmitting data to thereby improve the capacity of the system. In addition, the inter cell interference is reduced due to less frequent transmission of CSI-RS. 
     It is understood that the present subject matter may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this subject matter will be thorough and complete and will fully convey the disclosure to those skilled in the art. Indeed, the subject matter is intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the subject matter as defined by the appended claims. Furthermore, in the following detailed description of the present subject matter, numerous specific details are set forth in order to provide a thorough understanding of the present subject matter. However, it will be clear to those of ordinary skill in the art that the present subject matter may be practiced without such specific details. 
     In accordance with various embodiments of the present disclosure, the methods described herein may be implemented using a hardware computer system that executes software programs. Further, in a non-limited embodiment, implementations can include distributed processing, component/object distributed processing, and parallel processing. Virtual computer system processing can be constructed to implement one or more of the methods or functionalities as described herein, and a processor described herein may be used to support a virtual processing environment. 
     Aspects of the present disclosure are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatuses (systems) and computer program products according to embodiments of the disclosure. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable instruction execution apparatus, create a mechanism for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The aspects of the disclosure herein were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure with various modifications as are suited to the particular use contemplated. 
     For purposes of this document, each process associated with the disclosed technology may be performed continuously and by one or more computing devices. Each step in a process may be performed by the same or different computing devices as those used in other steps, and each step need not necessarily be performed by a single computing device. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.