WO2014130082A1

WO2014130082A1 - Method and apparatus for using demodulation reference signal in long term evolution advanced cellular networks

Info

Publication number: WO2014130082A1
Application number: PCT/US2013/057788
Authority: WO
Inventors: Long GAO; Sudhanshu Gaur; Joydeep Acharya
Original assignee: Hitachi, Ltd.
Priority date: 2013-02-21
Filing date: 2013-09-03
Publication date: 2014-08-28

Abstract

A wireless system comprises: an eNodeB including a controller and a memory, the controller being operable, for each subframe containing PDSCH of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, to select a demodulation reference signal (DMRS) pattern for DMRS transmission based on the subframe index, generate the PDSCH data with the selected DMRS pattern, and transmit the PDSCH data; and a UE including a UE controller and a UE memory, the UE controller being operable, upon receiving the PDSCH data of a subframe from the eNodeB, to identify the subframe index of the subframe, select a UE-selected DMRS pattern for channel estimation based on the subframe index, extract DMRS resource elements according to the UE-selected DMRS pattern, perform channel estimation using the extracted DMRS to obtain a channel estimate, and demodulate the PDSCH data based on the channel estimate.

Description

METHOD AND APPARATUS FOR USING DEMODULATION REFERENCE SIGNAL IN LONG TERM EVOLUTION ADVANCED CELLULAR

NETWORKS

BACKGROUND OF THE INVENTION [0001] The present invention relates generally to wireless systems and, more particularly, to the use of Demodulation Reference Signal (DMRS) in long term evolution (LTE) advanced cellular networks. [0002] When using the DMRS based transmission modes for Physical

Downlink Shared Channel (PDSCH) transmission in LTE-Advanced cellular network, there is potential collision between DMRS and Primary/Secondary Synchronization Signals (PSS/SSS) in central 6 Resource Blocks (RBs) in certain subframes.

BRIEF SUMMARY OF THE INVENTION [0003] Exemplary embodiments of the invention provide DMRS that can avoid colliding with PSS/SSS (referred to as SSs) from the serving cell and/or neighboring cells and achieve better demodulation performance than the DMRS specified in LTE-Advanced Rel-1 1 . In specific embodiments, a new method of using DMRS is proposed to avoid colliding with SSs and to achieve better demodulation performance for LTE-Advanced networks. In the proposed method, the subframes containing Physical Downlink Shared Channel (PDSCH) within each radio frame are divided into multiple exclusive sets. For Example, assume all subframes (indexed from 0 and 9) contain PDSCH within each radio frame. These subframes can be divided into two sets: {0, 5} and {1 , 2, 3, 4, 6, 7, 8, 9}. Different DMRS patterns are used for different sets of subframes. The new DMRS design can avoid colliding with synchronization signals and achieve better demodulation performance than the DMRS specified in Rel-1 1 . The technique can be used in current FDD/TDD (Frequency Division Duplex/Time Division Duplex) LTE-Advanced networks to improve the system performance. It can also be used in future LTE-Advanced cellular networks with new carrier deployment where only DMRS-based transmission will be supported.

[0004] In accordance with an aspect of the present invention, a wireless system comprises: an eNodeB including a controller and a memory, the controller being operable, for each subframe of a plurality of subframes containing PDSCH of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, to select a

demodulation reference signal (DMRS) pattern for DMRS transmission based on the subframe index, generate PDSCH data with the selected DMRS pattern, and transmit the PDSCH data; and a UE including a UE controller and a user memory, the UE controller being operable, upon receiving the PDSCH data of a subframe from the eNodeB, to identify the subframe index of the subframe, select a UE-selected DMRS pattern for channel estimation based on the subframe index, extract DMRS resource elements according to the UE-selected DMRS pattern, perform channel estimation using the extracted DMRS to obtain a channel estimate, and demodulate the PDSCH data based on the channel estimate.

[0005] In some embodiments, the subframes containing PDSCH of each radio frame are divided into multiple sets which have different subframe indices and use different DMRS patterns, respectively, corresponding to the different subframe indices. The different DMRS patterns are configured to avoid potential collision between the DMRS and synchronization signals in the subframe. The different DMRS patterns are configured to provide DMRS resource elements that do not overlap with resource elements for transmitting synchronization signals in the subframe so as to avoid potential collision between the DMRS and the synchronization signals in the subframe.

Particularly, the subframes containing PDSCH of each radio frame are divided into two sets which have two different subframe indices and use two different DMRS patterns, respectively, corresponding to the two different subframe indices. The DMRS pattern is selected from two different DMRS patterns for normal cyclic prefix length when the subframe has symbols of a normal cyclic prefix length; and the DMRS pattern is selected from two different DMRS patterns for extended cyclic prefix length when the subframe has symbols of an extended cyclic prefix length.

[0006] In specific embodiments, the controller of the eNodeB is operable to map the selected DMRS pattern to corresponding DMRS resource elements of the subframe. Demodulating the PDSCH data by the

UE controller comprises performing coherent demodulation based on the channel estimate. The memory stores a plurality of DMRS patterns to be selected by the controller; and the UE memory stores the same plurality of

DMRS patterns to be selected by the UE controller.

[0007] In some embodiments, the controller is configured, for each subframe, to: determine whether the subframe contains PSS/SSS

(Primary/Secondary Synchronization Signals) from the eNodeB or from one or more neighboring eNodeBs, wherein the subframe belongs to a first group if the subframe contains the PSS/SSS and the subframe belongs to a second group if the subframe does not contain the PSS/SSS; and configure, for the first group, a subframe offset for the one or more neighboring eNodeBs such that subframes carrying PSS/SSS from the eNodeB and subframes carrying PSS/SSS from the one or more neighboring eNodeBs are not aligned with each other, the subframe offset to be applied to the one or more neighboring eNodeBs.

[0008] In specific embodiments, the controller is configured, for each subframe, to: estimate a UE speed of the UE; and select a DMRS pattern based on the estimated UE speed. Different DMRS patterns are provided for different UE speeds.

[0009] Another aspect of the invention is directed to an eNodeB for transmitting PDSCH data to a UE in a wireless system. The eNodeB comprises a controller and a memory. The controller is operable, for each subframe of a plurality of subframes containing PDSCH of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, to select a DMRS pattern for DMRS transmission based on the subframe index, generate the PDSCH data multiplexed with the selected

DMRS pattern, and transmit the PDSCH data to the UE.

[0010] Another aspect of this invention is directed to a UE for receiving

PDSCH data from an eNodeB in a wireless system, each PDSCH data being generated for a subframe of a plurality of subframes of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, each PDSCH data being generated by the eNodeB using a DMRS pattern for DMRS transmission selected based on the subframe index. The UE comprises a UE controller and a UE memory. The UE controller is operable, upon receiving the PDSCH data of a subframe from the eNodeB, to identify the subframe index of the subframe, select a UE-selected DMRS pattern for channel estimation based on the subframe index, extract DMRS resource elements of the subframe according to the UE-selected DMRS pattern, perform channel estimation using the extracted DMRS to obtain a channel estimate, and demodulate the PDSCH data based on the channel estimate.

[0011] In some embodiments, the UE memory stores a plurality of DMRS patterns to be selected by the UE controller; and the plurality of DMRS patterns are the same DMRS patterns to be selected by the eNodeB.

[0012] These and other features and advantages of the present invention will become apparent to those of ordinary skill in the art in view of the following detailed description of the specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 shows a LTE radio frame.

[0014] FIG. 2 shows an example of PSS/SSS and DMRS patterns for normal CP in FDD systems.

[0015] FIG. 3 shows an example of PSS/SSS and DMRS patterns for extended CP in FDD systems.

[0016] FIG. 4 shows an example of a LTE-Advanced system.

[0017] FIG. 5 shows an example of an illustration of assigning different

DMRS patterns to different subframe sets. [0018] FIG. 6 is an example of a diagram illustrating the overall procedure of the proposed solution for LTE-advanced downlink.

[0019] FIG. 7 shows an example of a block diagram of the eNodeB in the proposed solution.

[0020] FIG. 8 shows an example of a block diagram of the UE in the proposed solution.

[0021] FIG. 9 shows an example of a flow diagram illustrating a process of the operation at the eNodeB.

[0022] FIG. 10 shows an example of DMRS Pattern A for normal CP (Cyclic Prefix) length.

[0023] FIG. 1 1 shows an example of DMRS Pattern A for extended CP length.

[0024] FIG. 12 shows an example of DMRS Pattern B for normal CP length.

[0025] FIG. 13 shows an example of DMRS Pattern B for extended CP length.

[0026] FIG. 14 shows Table I for DMRS Pattern A for normal CP in the memory.

[0027] FIG. 15 shows Table II for DMRS Pattern A for extended CP in the memory.

[0028] FIG. 16 shows an example of a flow diagram illustrating a process of the operation at the UE.

[0029] FIG. 17 shows a simple two-cell scenario of a LTE-Advanced system to illustrate an example of PSS/SSS detection for a cell-edge UE. [0030] FIG. 18 shows an example of collision of PSS/SSS signals from two neighboring cells.

[0031] FIG. 19 shows an example of subframe shifting to avoid collision of PSS/SSS signals from two neighboring cells in the subframe level.

[0032] FIG. 20 shows an example of a flow diagram illustrating subframe grouping.

[0033] FIG. 21 shows an example of DMRS Pattern A for normal CP length to be used for low mobility UEs to avoid collision between DMRS and PSS/SSS.

[0034] FIG. 22 shows an example of DMRS Pattern A for extended CP length to be used for low mobility UEs to avoid collision between DMRS and PSS/SSS.

[0035] FIG. 23 shows an example of a flow diagram illustrating a process for the eNodeB to choose DMRS pattern A based on UE mobility speed.

[0036] FIG. 24 shows an example of a flow diagram illustrating eNodeB processing for subframe grouping and selection of UE-specific DMRS pattern based on UE mobility speed.

[0037] FIG. 25 shows an example of a flow diagram illustrating UE processing for subframe grouping and selection of UE-specific DMRS pattern based on UE mobility speed.

DETAILED DESCRIPTION OF THE INVENTION

[0038] In the following detailed description of the invention, reference is made to the accompanying drawings which form a part of the disclosure, and in which are shown by way of illustration, and not of limitation, exemplary embodiments by which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. Further, it should be noted that while the detailed description provides various exemplary embodiments, as described below and as illustrated in the drawings, the present invention is not limited to the embodiments described and illustrated herein, but can extend to other embodiments, as would be known or as would become known to those skilled in the art. Reference in the specification to "one embodiment," "this embodiment," or "these

embodiments" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, and the appearances of these phrases in various places in the specification are not necessarily all referring to the same embodiment. Additionally, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that these specific details may not all be needed to practice the present invention. In other circumstances, well-known structures, materials, circuits, processes and interfaces have not been described in detail, and/or may be illustrated in block diagram form, so as to not unnecessarily obscure the present invention.

[0039] Furthermore, some portions of the detailed description that follow are presented in terms of algorithms and symbolic representations of operations within a computer. These algorithmic descriptions and symbolic representations are the means used by those skilled in the data processing arts to most effectively convey the essence of their innovations to others skilled in the art. An algorithm is a series of defined steps leading to a desired end state or result. In the present invention, the steps carried out require physical manipulations of tangible quantities for achieving a tangible result. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals or instructions capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, instructions, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as

"processing," "computing," "calculating," "determining," "displaying," or the like, can include the actions and processes of a computer system or other information processing device that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system's memories or registers or other information storage, transmission or display devices.

[0040] The present invention also relates to an apparatus for performing the operations herein. This apparatus may be specially

constructed for the required purposes, or it may include one or more general- purpose computers selectively activated or reconfigured by one or more computer programs. Such computer programs may be stored in a computer- readable storage medium, such as, but not limited to optical disks, magnetic disks, read-only memories, random access memories, solid state devices and drives, or any other types of media suitable for storing electronic information. The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs and modules in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform desired method steps. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. The

instructions of the programming language(s) may be executed by one or more processing devices, e.g., central processing units (CPUs), processors, or controllers.

[0041] Exemplary embodiments of the invention, as will be described in greater detail below, provide apparatuses, methods and computer programs for using demodulation reference signal in LTE-Advanced cellular networks.

[0042] LTE Frame Structure

[0043] FIG. 1 shows a LTE radio frame. It contains 10 subframes, which are indexed from 0 and 9. Each subframe is further divided into two slots, each of which consists of 7 OFDM (Orthogonal Frequency-Division

Multiplexing) symbols for normal Cyclic Prefix (CP) length or 6 OFDM symbols for extended CP length. In the frequency domain, the LTE signal is divided into units of 12 subcarriers, each of which spans 1 80 kHz bandwidth with a subcarrier spacing of 15 kHz. Such a unit for a duration of one slot is defined as a Resource Block (RB). A RB is further divided into Resource Elements (REs). One RE is one OFDM subcarrier for a duration of one OFDM symbol and is the smallest unit in the LTE time-frequency resource grid.

[0044] PSS/SSS Specified in Rel-1 1

[0045] In LTE/LTE-Advanced cellular systems, synchronization signals are used for a UE to perform initial cell acquisition. Two types of

synchronization signals are defined in LTE-Advanced Rel-1 1 : Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). The PSS/SSS are always transmitted in central 6 RBs in frequency domain. In time domain, the PSS/SSS are transmitted in certain subframes within each radio frame. In FDD systems, the PSS/SSS are located in subframes 0 and 5. In TDD systems, the PSS is located in subframes 1 and 6 while the SSS is located in subframes 0 and 5. The exact locations of the PSS/SSS within subframes 0 and 5 for FDD systems are shown in FIG. 2 and FIG. 3 for the normal and extended CP cases, respectively. FIG. 2 shows an example of PSS/SSS and DMRS patterns for normal CP in FDD systems. FIG. 3 shows an example of PSS/SSS and DMRS patterns for extended CP in FDD systems.

[0046] DMRS Specified in Rel-1 1

[0047] In LTE/LTE-Advanced downlink, DMRS is used for UE PDSCH demodulation in Transmission Modes (TMs) 7-10. The DMRS-based TMs can support a maximum up to 4 layer transmissions, which use antenna ports

7-10. In those TMs, the eNodeB (E-UTRAN Node B or Evolved Node B) as a base station transmits DMRS in each scheduled RB for a UE. After receiving PDSCH data, the UE performs channel estimation based on DMRS and then coherent demodulation. The location of DMRS in each RB is fixed for FDD systems as shown in FIG. 2 and FIG. 3 for the normal and extended CP cases, respectively. In TDD systems, the location DMRS various over subframes as specified in 3GPP Technical Specification 36.21 1 v. 1 1 .1 .0, available online: http://www.3gpp.org/ftp/Specs/html-info/3621 1 .htm. From FIG. 2 and FIG. 3, one sees that the DMRS will collide with PSS/SSS if the eNodeB configures DMRS-based TMs in central 6 RBs.

[0048] Potential Collision Between DMRS and PSS/SSS

[0049] FIG. 4 shows an example of a LTE-Advanced system. It includes one eNodeB and multiple UEs. The eNodeB uses DMRS-based TMs to send PDSCH data to its associated UEs. In Rel-1 1 , the DMRS-based TMs are not allowed to be configured in the central 6 RBs when PSS/SSS are present due to the potential collision as shown in FIG. 2 and FIG. 3. In order to remove such restriction, a new DMRS design that can avoid colliding with PSS/SSS is needed.

[0050] Proposed Method of Using DMRS to Avoid Colliding with Synchronization Signals

[0051] In the proposed solution, the subframes containing PDSCH of each radio frame are divided into multiple sets which have different subframe indices and use different DMRS patterns, respectively, corresponding to the different subframe indices. FIG. 5 shows an example of an illustration of assigning different DMRS patterns to different subframe sets assuming all subframes contain PDSCH. In this example, the subframes within each radio frame are grouped into multiple exclusive sets based on their indices. For example, these subframes can be divided into two sets {0, 5} (for DMRS pattern A) and {1 , 2, 3, 4, 6, 7, 8, 9} (for DMRS pattern B) as shown in FIG. 5, depending on whether they carry PSS/SSS or not. The eNodeB uses different DMRS patterns for the two sets as shown in FIG. 5 such that the potential collision between DMRS and PSS/SSS is avoided. Since the DMRS-based TMs can be used in central 6 RBs for all subframes with the new DMRS design, the proposed solution can achieve better performance than the existing solutions.

[0052] FIG. 6 is an example of a diagram illustrating the overall procedure of the proposed solution for LTE-advanced downlink. On the transmitter side, the eNodeB selects the DMRS pattern based on subframe index and sends the PDSCH data multiplexed with the selected DMRS pattern. On the receiver side, the UE also chooses the DMRS pattern based on subframe index (such that the same DMRS pattern will be used for the eNodeB and UE in each subframe) and demodulates the received PDSCH data based on the selected DMRS pattern. Details of the proposed solution are explained as follows.

[0053] FIG. 7 shows an example of a block diagram of the eNodeB in the proposed solution (e.g., the eNodeB as shown in FIG. 4). The eNodeB has three modules. The CPU module (controller) chooses the DMRS pattern based on the subframe index and informs the baseband processor of the selected DMRS pattern. The baseband DSP module maps the DMRS pattern to the corresponding REs according to the instruction from the CPU. The memory module stores the two DMRS patterns. [0054] FIG. 8 shows an example of a block diagram of the UE (e.g., UE1 and UE2 as shown in FIG. 4). The UE has the following four modules. The UE CPU module (UE controller) identifies the index of the current subframe and then chooses the corresponding DMRS pattern accordingly. The channel estimator extracts the DMRS REs based on the selected pattern by the CPU and performs channel estimation. The baseband DSP module performs coherent demodulation based on the channel estimate from the channel estimator. The UE memory module stores the same DMRS patterns as the eNodeB.

[0055] FIG. 9 shows an example of a flow diagram illustrating a process of the operation at the eNodeB. In the beginning of the subframe transmission, the CPU first checks the subframe index. If the subframe index is 0 or 5, the CPU selects Pattern A {0, 5} for DMRS transmission. Otherwise, the CPU chooses Pattern B {1 , 2, 3, 4, 6, 7, 8, 9} for DMRS transmission. Afterwards, the baseband DSP generates the PDSCH data multiplexed with the selected DMRS pattern.

[0056] FIG. 10 shows an example of DMRS Pattern A for normal CP (Cyclic Prefix) length. It can be applied to both FDD and TDD. FIG. 1 1 shows an example of DMRS Pattern A for extended CP length. It can also be applied to both FDD and TDD. Besides the above examples, any DMRS pattern that can avoid colliding with PSS/SSS could be a valid candidate. DMRS Pattern B uses the same pattern as specified in Rel-1 1 . See 3GPP Technical Specification 36.21 1 v. 1 1 .1 .0. As an example in FDD systems, DMRS Pattern B for the normal and extended CP cases are shown in FIG. 12 and FIG. 13, respectively. [0057] The DMRS Patterns are stored in the memory of the eNodeB using the following format. For each antenna port, the time-frequency location of each of the DMRS REs for a single RB is specified in terms of (OFDM symbol index, subcarrier index). For example, DMRS Pattern A for normal

CP as shown in FIG. 10 is stored in the memory as Table I in FIG. 14, where

OFDM symbols are indexed from left to right and the subcarriers are indexed from bottom. As another example, DMRS Pattern A for extended CP as shown in FIG. 1 1 is stored in the memory as Table II in FIG. 15. Note that all

RBs within each subframe have the same DMRS patterns but the DMRS pattern varies over subframes according to FIG. 5. FIG. 14 shows Table I for

DMRS Pattern A for normal CP in the memory. FIG. 1 5 shows Table II for

DMRS Pattern A for extended CP in the memory. The tables list antenna ports and corresponding time-frequency locations.

[0058] FIG. 16 shows an example of a flow diagram illustrating a process of the operation at the UE. After receiving the PDSCH data in each subframe, the CPU first identifies the subframe index. If the subframe index is

0 or 5, the CPU selects Pattern A for channel estimation. Otherwise, the CPU chooses Pattern B. Afterwards, the channel estimator extracts the DMRS according to the selected pattern by the CPU and performs channel estimation. Finally, the baseband DSP demodulates the PDSCH data based on the channel estimate from the channel estimator. The same DMRS

Patterns are stored in the memory of the UE as the eNodeB.

[0059] The new DMRS design in the invention can avoid colliding with synchronization signals and achieve better demodulation performance than the DMRS specified in Rel-1 1 . The invention can be used in current FDD/TDD LTE-Advanced networks to improve the system performance. It can also be used in future LTE-Advanced cellular networks with new carrier deployment where only DMRS-based transmission modes will be supported.

[0060] Subframe Grouping

[0061] In subframe grouping, we group the subframes into different sets to avoid collision with PSS/SSS from the serving cell or eNodeB and/or neighboring cell(s) or eNodeB(s). The above describes an example of how to group subframes into two sets to avoid collision with PSS/SSS from the serving cell. In this embodiment, another example illustrates subframe grouping to avoid collision with PSS/SSS from the serving cell and

neighboring cell(s).

[0062] Consider a simple two-cell scenario as shown in FIG. 17. In this example, a cell-edge UE is trying to detect PSS/SSS signals from cell 1 and cell 2 for cell selection/reselection. If the radio frames of these two cells are aligned as shown in FIG. 18, the PSS/SSS signals will collide with each other. Thus, it is difficult for the UE to detect PSS/SSS signals from both cells. In this embodiment, a solution to solve the PSS/SSS collision of neighboring cells involves the following:

[0063] Step 1 : Use subframe shifting to avoid PSS/SSS collision in the subframe level. For example, cell 2 can configure a subframe offset as shown in FIG. 19 such that the subframes carrying PSS/SSS from two cells are not aligned with each other.

[0064] Step 2: Group subframes into two sets based on whether the

PSS/SSS signals are transmitted from the serving cell and neighboring cell(s).

Consider cell 1 in the example as shown in FIG. 19, the PSS/SSS from itself is transmitted in subframes 0 and 5. With subframe shifting, the PSS/SSS is transmitted from cell 2 in subframes 4 and 9. Thus, subframes can be divided into two sets: {0, 4, 5, 9} and {1 , 2, 3, 6, 7, 8}. Based on the same principle, the subframes for cell 2 can be divided into two sets: {0, 1 , 5, 6} and {2, 3, 4, 7, 8, 9}.

[0065] Step 3: Use different DMRS patterns for different sets of subframes such that the potential collision between DMRS and PSS/SSS is avoided. The scheme of using different DMRS patterns for different sets of subframes as described above can be applied here.

[0066] FIG. 20 shows an example of a flow diagram illustrating subframe grouping. This involves, for a given serving cell, figuring out the subframe offset of neighboring cell(s) and identifying the subframes containing PSS/SSS from the serving cell and neighboring cell(s) (see Step 1 ). Then, the identified subframes are grouped into one set and the remaining subframes are grouped into another set (see Step 2).

[0067] UE-Specific DMRS

[0068] The approach of UE-specific DMRS is such that the DMRS patterns for different UEs could be different depending on their speeds. In the above examples, the scheduled UEs use the same DMRS pattern (chosen from either pattern A or B depending on subframe index) for demodulation. In this embodiment, we propose UE-specific DMRS pattern whereby different

UEs scheduled in the same subframe could use different DMRS patterns based on their mobility speeds. In principle, the pattern with higher DMRS RE density will be used for the UEs with higher speeds. For example, DMRS pattern A in the above examples (see FIGS. 1 0 and 1 1 ) can be used by high mobility UEs. For low mobility UEs, the DMRS patterns in FIG. 21 and FIG. 22 can be used in normal CP and extended CP cases, respectively, in order to avoid collision between DMRS and PSS/SSS.

[0069] FIG. 23 shows an example of a flow diagram illustrating a process for the eNodeB to choose DMRS pattern A based on UE mobility speed. It uses the same procedure to choose DMRS pattern B. As seen in FIG. 23, the process estimates the UE speed and determines which UE speed category to which the UE speed belong. For low speed category, the DMRS low mobility pattern (A or B) is used for the low mobility UE. For high speed category, the DMRS high mobility pattern (A or B) is used for the high mobility UE. Although FIG. 23 shows only two speed categories, the invention can be used for three or more speed categories to provide three or more different DMRS mobility patterns (A or B) for three or more different mobility UEs. All DMRS patterns are stored in the UE memory. The eNodeB informs UE which DMRS pattern to use for each configured subframe set by RRC signaling.

[0070] FIG. 24 shows an example of a flow diagram illustrating eNodeB processing for subframe grouping and selection of UE-specific DMRS pattern based on UE mobility speed. For subframe grouping, the eNodeB determines whether the subframe contains PSS/SSS either from the serving cell or from neighboring cell(s). If the answer is yes, DRMS pattern A will be used to avoid collision between DMRS and PSS/SSS. If the answer is no, DRMS pattern B will be used. For each subframe group, the eNodeB determines the speed category to which the UE belongs (e.g., high mobility or low mobility) and, for each speed category, the DMRS pattern (A or B) will be used. Finally, the eNodeB generate data with the selected DMRS pattern for the subframe.

[0071] FIG. 25 shows an example of a flow diagram illustrating UE processing for subframe grouping and selection of UE-specific DMRS pattern based on UE mobility speed. For subframe grouping, the eNodeB informs the

UE of subframes containing PSS/SSS either from the serving cell or from neighboring cell(s) by RRC signaling. If the answer is yes, DRMS pattern A will be used to avoid collision between DMRS and PSS/SSS. If the answer is no, DRMS pattern B will be used. For each subframe group, the eNodeB determines the speed category to which the UE belongs (e.g., high mobility or low mobility) and, for each speed category, the DMRS pattern (A or B) will be used. Next, the UE estimates channel based on the selected DMRS pattern.

Finally, the UE demodulates the data packet of the subframe.

[0072] Of course, the communications systems shown in FIGS. 4 and

17 are purely exemplary of systems in which the present invention may be implemented, and the invention is not limited to a particular hardware or software configuration. The computers and storage systems implementing the invention can also have known I/O devices (e.g., CD and DVD drives, floppy disk drives, hard drives, etc.) which can store and read the modules, programs and data structures used to implement the above-described invention. These modules, programs and data structures can be encoded on such computer-readable media. For example, the data structures of the invention can be stored on computer-readable media independently of one or more computer-readable media on which reside the programs used in the invention. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include local area networks, wide area networks, e.g., the Internet, wireless networks, storage area networks, and the like.

[0073] In the description, numerous details are set forth for purposes of explanation in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that not all of these specific details are required in order to practice the present invention. It is also noted that the invention may be described as a process, which is usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged.

[0074] As is known in the art, the operations described above can be performed by hardware, software, or some combination of software and hardware. Various aspects of embodiments of the invention may be implemented using circuits and logic devices (hardware), while other aspects may be implemented using instructions stored on a machine-readable medium (software), which if executed by a processor, would cause the processor to perform a method to carry out embodiments of the invention.

Furthermore, some embodiments of the invention may be performed solely in hardware, whereas other embodiments may be performed solely in software.

Moreover, the various functions described can be performed in a single unit, or can be spread across a number of components in any number of ways.

When performed by software, the methods may be executed by a processor, such as a general purpose computer, based on instructions stored on a computer-readable medium. If desired, the instructions can be stored on the medium in a compressed and/or encrypted format.

[0075] From the foregoing, it will be apparent that the invention provides methods, apparatuses and programs stored on computer readable media for using demodulation reference signal in LTE-Advanced cellular networks. Additionally, while specific embodiments have been illustrated and described in this specification, those of ordinary skill in the art appreciate that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments disclosed. This disclosure is intended to cover any and all adaptations or variations of the present invention, and it is to be understood that the terms used in the following claims should not be construed to limit the invention to the specific

embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with the established doctrines of claim interpretation, along with the full range of equivalents to which such claims are entitled.

Claims

WHAT IS CLAIMED IS:

1 . A wireless system comprising:

an eNodeB including a controller and a memory, the controller being operable, for each subframe of a plurality of subframes containing PDSCH (Physical Downlink Shared Channel) of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, to select a demodulation reference signal (DMRS) pattern for DMRS transmission based on the subframe index, generate the PDSCH data with the selected DMRS pattern, and transmit the PDSCH data; and

a UE including a UE (user equipment) controller and a UE memory, the UE controller being operable, upon receiving the PDSCH data of a subframe from the eNodeB, to identify the subframe index of the subframe, select a UE- selected DMRS pattern for channel estimation based on the subframe index, extract DMRS resource elements according to the UE-selected DMRS pattern, perform channel estimation using the extracted DMRS to obtain a channel estimate, and demodulate the PDSCH data based on the channel estimate.

2. The wireless system according to claim 1 ,

wherein the subframes containing PDSCH of each radio frame are divided into multiple sets which have different subframe indices and use different DMRS patterns, respectively, corresponding to the different subframe indices; and wherein the different DMRS patterns are configured to avoid potential collision between the DMRS and synchronization signals in the subframe.

3. The wireless system according to claim 2,

wherein the different DMRS patterns are configured to provide DMRS resource elements that do not overlap with resource elements for transmitting synchronization signals in the subframe so as to avoid potential collision between the DMRS and the synchronization signals in the subframe.

4. The wireless system according to claim 1 ,

wherein the subframes containing PDSCH of each radio frame are divided into two sets which have two different subframe indices and use two different DMRS patterns, respectively, corresponding to the two different subframe indices.

5. The wireless system according to claim 4,

wherein the DMRS pattern is selected from two different DMRS patterns for normal cyclic prefix length when the subframe has symbols of a normal cyclic prefix length; and

wherein the DMRS pattern is selected from two different DMRS patterns for extended cyclic prefix length when the subframe has symbols of an extended cyclic prefix length.

6. The wireless system according to claim 1 , wherein the controller of the eNodeB is operable to map the selected DMRS pattern to corresponding DMRS resource elements of the subframe.

7. The wireless system according to claim 1 ,

wherein demodulating the PDSCH data by the UE controller comprises performing coherent demodulation based on the channel estimate.

8. The wireless system according to claim 1 ,

wherein the memory stores a plurality of DMRS patterns to be selected by the controller; and

wherein the UE memory stores the same plurality of DMRS patterns to be selected by the UE controller.

9. The wireless system according to claim 1 , wherein the controller is configured, for each subframe, to:

determine whether the subframe contains PSS/SSS

(Primary/Secondary Synchronization Signals) from the eNodeB or from one or more neighboring eNodeBs, wherein the subframe belongs to a first group if the subframe contains the PSS/SSS and the subframe belongs to a second group if the subframe does not contain the PSS/SSS; and

configure, for the first group, a subframe offset for the one or more neighboring eNodeBs such that subframes carrying PSS/SSS from the eNodeB and subframes carrying PSS/SSS from the one or more neighboring eNodeBs are not aligned with each other, the subframe offset to be applied to the one or more neighboring eNodeBs.

10. The wireless system according to claim 1 , wherein the controller is configured, for each subframe, to:

estimate a UE speed of the UE; and

select a DMRS pattern based on the estimated UE speed;

wherein different DMRS patterns are provided for different UE speeds.

1 1 . An eNodeB for transmitting data to a UE (user equipment) in a wireless system, the eNodeB comprising:

a controller; and

a memory;

wherein the controller is operable, for each subframe of a plurality of subframes containing PDSCH (Physical Downlink Shared Channel) of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, to select a demodulation reference signal (DMRS) pattern for DMRS transmission based on the subframe index, generate the PDSCH data multiplexed with the selected DMRS pattern, and transmit the PDSCH data to the UE.

12. The eNodeB according to claim 1 1 ,

13. The eNodeB according to claim 12,

14. The eNodeB according to claim 1 1 ,

15. The eNodeB according to claim 14,

16. The eNodeB according to claim 1 1 , wherein the controller is operable to map the selected DMRS pattern to corresponding DMRS resource elements of the subframe.

17. The eNodeB according to claim 1 1 , wherein the controller is

configured, for each subframe, to:

determine whether the subframe contains PSS/SSS

18. The eNodeB according to claim 1 1 , wherein the controller is

configured, for each subframe, to:

estimate a UE speed of the UE; and

select a DMRS pattern based on the estimated UE speed;

wherein different DMRS patterns are provided for different UE speeds.

19. A UE (user equipment) for receiving PDSCH (Physical Downlink

Shared Channel) data from an eNodeB in a wireless system, each PDSCH data being generated for a subframe of a plurality of subframes of a radio frame of a plurality of radio frames to be transmitted, each subframe having a subframe index, each PDSCH data being generated by the eNodeB using a demodulation reference signal (DMRS) pattern for DMRS transmission selected based on the subframe index, the UE comprising:

a UE controller; and

a UE memory;

wherein the UE controller is operable, upon receiving the PDSCH data of a subframe from the eNodeB, to identify the subframe index of the subframe, select a UE-selected DMRS pattern for channel estimation based on the subframe index, extract DMRS resource elements of the subframe according to the UE-selected DMRS pattern, perform channel estimation using the extracted DMRS to obtain a channel estimate, and demodulate the PDSCH data based on the channel estimate.

20. The UE according to claim 19,

wherein the subframes containing PDSCH of each radio frame are divided into multiple sets which have different subframe indices and use different DMRS patterns, respectively, corresponding to the different subframe indices; and

wherein the different DMRS patterns are configured to avoid potential collision between the DMRS and synchronization signals in the subframe.

21 . The UE according to claim 20,

22. The UE according to claim 19,

23. The UE according to claim 22,

24. The UE according to claim 19,

wherein the UE memory stores a plurality of DMRS patterns to be selected by the UE controller; and

wherein the plurality of DMRS patterns are the same DMRS patterns to be selected by the eNodeB.