(12) United States Patent
`Wallén et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 9,596,061 B2
`Mar. 14, 2017
`
`USOO9596,061 B2
`
`(54) REFERENCE SIGNAL COUPLING INA
`WIRELESS NETWORK
`
`(71) Applicant: Telefonaktiebolaget LM Ericsson
`(publ), Stockholm (SE)
`
`(72)
`
`Inventors:
`
`(*)
`
`Notice:
`
`Anders Wallén, Ystad (SE); Johan
`Bergman, Stockholm (SE); Erik
`Eriksson, Linköping (SE); Mattias
`Frenne, Uppsala (SE); Fredrik
`Nordström, Lund (SE)
`(73) Assignee: Telefonaktiebolaget LM Ericsson
`(publ), Stockholm (SE)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`Appl. No.:
`14/431,922
`
`PCT Fed:
`
`Jan. 27, 2015
`
`Mar. 27, 2015
`
`(21)
`(22)
`(86). PCT No.:
`S 371 (c)(1),
`(2) Date:
`(87) PCT Pub. No.: WO2015/119559
`PCT Pub. Date: Aug. 13, 2015
`
`(65)
`
`(60)
`
`(51)
`
`(52)
`
`Prior Publication Data
`US 2015/0358132 A1
`Dec. 10, 2015
`Related U.S. Application Data
`Provisional application No. 61/937,932, filed on Feb.
`10, 2014.
`
`Int. C.
`H04L 5/00
`H047 72/04
`U.S. C.
`CPC .......... H04L 5/0023 (2013.01); H04L 5/0048
`(2013.01); H04W 72/0446 (2013.01)
`
`(2006.01)
`(2009.01)
`
`(58) Field of Classification Search
`CPC. H04L 4/0023; H04L 4/0048; H04W 72/0446
`(Continued)
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`2011 0080965 A1 * 4/2011 Liu ..................... HO4L 25,0224
`375,260
`2011/0228735 A1* 9, 2011 Lee ....................... HO4L 5,0051
`370,329
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`WO WO 2013/141801 A1
`
`9, 2013
`
`OTHER PUBLICATIONS
`
`PCT Written Opinion of the International Searching Authority for
`International application No. PCT/SE2015/050080, Apr. 7, 2015.
`(Continued)
`Primary Examiner — Stephen J Clawson
`(74) Attorney, Agent, or Firm — Baker Botts, LLP
`(57)
`ABSTRACT
`According to some embodiments, a method of coupling
`reference signals of a wireless network comprises establish
`ing a wireless connection with a wireless device. The
`wireless connection comprises a first reference signal and a
`second reference signal and both the first and second refer
`ence signals are associated with one or more antenna ports.
`The method further comprises determining a mapping
`between the one or more antenna ports associated with the
`first reference signal and the one or more antenna ports
`associated with the second reference signal; communicating
`the mapping of antenna ports to the wireless device; and
`transmitting the first reference signal and the second refer
`ence signal to the wireless device according to the commu
`nicated mapping.
`
`16 Claims, 5 Drawing Sheets
`
`600
`establish wireless connection with a radio
`network node, the wireless connection
`comprising first and second reference signals
`
`se
`
`receive signaling related to antenna port
`mapping
`
`
`
`No
`
`DMRS Mapped to
`CRSP
`
`o
`
`perform channel estimation
`based on DMRS only
`
`perform channel estimation
`based on DMRS and CRS
`
`618
`
`
`
`demodulate data transmitted over
`the wireless connection based on
`the channel estimation
`
`Ex.1008
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`US 9,596,061 B2
`Page 2
`
`(58) Field of Classification Search
`USPC .......................................................... 370/329
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`2011/0256897 A1* 10/2011 Taoka .................. HO4B 7,0615
`455,509
`
`2/2013 Koorapaty et al.
`2013,0044701 A1
`2013/030 1542 A1 11/2013 Krishnamurthy et al.
`2014/0314041 A1* 10, 2014 Kim ...................... HO4L 5,004.8
`370,329
`
`OTHER PUBLICATIONS
`
`3GPPTM Work Item Description Title: Low cost & enhanced MTC
`UE for LTE-Core Part, RF-130848, 7 pages, Jun. 10-14, 2013.
`3GPP198 Work Item Description Title: Low cost & enhanced
`MTC UE for LTE-Feature Part, RF-130848, 5 pages, Jun. 10-14,
`2013.
`3GPPTM Work Item Description Title: Low cost & enhanced MT
`CUE for LTE-Performance Part, RF-130848, 5 pages, Jun. 10-14.
`2013.
`
`* cited by examiner
`
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`U.S. Patent
`
`Mar. 14, 2017
`
`Sheet 1 of 5
`
`US 9,596,061 B2
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`
`
`00||
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`U.S. Patent
`
`Mar. 14, 2017
`
`Sheet 2 of 5
`
`US 9,596,061 B2
`
`
`
`One OFDM symbol including cyclic prefix
`
`FIG. 2
`
`Sub-frame 1 ms
`
`(- - - - - - - - - - - - m - - - - - -
`
`Radio frame 10 ms
`
`- - - - - - - - - - - - -)
`
`FIG. 3
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`U.S. Patent
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`Mar. 14
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`9
`
`2017
`
`Sheet 3 of 5
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`US 9,596,061 B2
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`
`
`
`
`ÞVf7 '91. H.
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`U.S. Patent
`
`Mar. 14, 2017.
`
`Sheet 4 of 5
`
`US 9,596,061 B2
`
`500
`
`506
`
`508
`
`510
`
`establish wireless connection with a Wireless device, the Wireless
`connection comprising first and second reference signals
`
`determine a mapping between antenna ports associated with the
`first and second reference signals
`
`yz
`Signal DMRS to CRS antenna port mapping to UE
`
`512 v Transmit DMRS and data to UE according to signaled mapping
`
`FIG. 5
`
`6OO
`establish Wireless Connection with a radio
`network node, the Wireless Connection
`608 Tu-1
`comprising first and second reference signals
`
`610 - Y -
`
`receive signaling related to antenna port
`mapping
`
`u!
`-DMRS Mapped to N.
`or
`
`Yes
`
`NO
`
`4
`61
`perform channel estimation
`based on DMRS only
`
`?olo
`612
`perform channel estimation
`based on DMRS and CRS
`
`618
`
`
`
`demodulate data transmitted OVG
`the wireless Connection based On
`the channel estimation
`
`FIG. 6
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`U.S. Patent
`
`Mar. 14, 2017
`
`Sheet S of 5
`
`US 9,596,061 B2
`
`
`
`Transceiver
`810
`
`110
`
`l,
`
`
`
`
`
`
`
`TransCeiver
`
`
`
`710
`
`PrOCeSSOr
`
`720
`
`730
`
`FIG. 7
`
`
`
`Processor
`820
`
`Memory
`830
`
`NetWOrk
`Interface
`840
`
`
`
`FIG. 8
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`US 9,596,061 B2
`
`1.
`REFERENCE SIGNAL COUPLING IN A
`WRELESS NETWORK
`
`PRIORITY
`
`This nonprovisional application is a U.S. National Stage
`Filing under 35 U.S.C. S371 of International Patent Appli
`cation Serial No. PCT/SE2015/050080, filed Jan. 27, 2015,
`and entitled “REFERENCE SIGNAL COUPLING IN A
`WIRELESS NETWORK which claims priority to U.S.
`10
`Provisional Patent Application No. 61/937,932 filed Feb. 10,
`2014, both of which are hereby incorporated by reference in
`their entirety.
`
`TECHNICAL FIELD
`
`15
`
`Particular embodiments relate generally to reference sig
`nals in wireless communications networks, and more par
`ticularly to coupling reference signals in wireless commu
`nications networks.
`
`BACKGROUND
`
`2
`large for an M2M device in a remote location (such as an
`M2M sensor or metering device located in the basement of
`a building). In Such scenarios receiving a signal, including
`reference signals, from the base station may be challenging.
`For example, the path loss can be 20 dB worse than normal
`operation. Enhanced coverage in uplink and downlink may
`alleviate Such challenges. Examples of techniques in the UE
`and/or in the radio network node for enhancing the coverage
`include transmit power boosting, repetition of transmitted
`signal, applying additional redundancy to the transmitted
`signal, use of advanced/enhanced receiver, etc. In general,
`when employing coverage enhancing techniques, the M2M
`may be referred to as operating in "coverage enhancing
`mode. A low complexity UE (e.g., UE with one receiver)
`may also be capable of Supporting enhanced coverage mode
`of operation.
`
`SUMMARY
`
`According to Some embodiments, a method of coupling
`reference signals of a wireless network comprises establish
`ing a wireless connection with a wireless device. The
`wireless connection comprises a first reference signal and a
`second reference signal and both the first and second refer
`ence signals are associated with one or more antenna ports.
`The method further comprises determining a mapping
`between the one or more antenna ports associated with the
`first reference signal and the one or more antenna ports
`associated with the second reference signal; communicating
`the mapping of antenna ports to the wireless device; and
`transmitting the first reference signal and the second refer
`ence signal to the wireless device according to the commu
`nicated mapping.
`In particular embodiments, the mapping of antenna ports
`comprises a precoding weight associated with each of the
`antenna ports associated with the first reference signal.
`According to Some embodiments, a method of coupling
`reference signals of a wireless network comprises establish
`ing a wireless connection with a radio network node. The
`wireless connection comprises a first reference signal and a
`second reference signal and both the first and second refer
`ence signals are associated with one or more antenna ports.
`The method further comprises receiving a mapping between
`the one or more antenna ports associated with the first
`reference signal and the one or more antenna ports associ
`ated with the second reference signal; performing channel
`estimation based at least on the first reference signal, the
`second reference signal, and the received mapping of
`antenna ports; and demodulating data transmitted over wire
`less connection based on the channel estimation.
`In particular embodiments, performing channel estima
`tion comprises estimating an amplitude component of the
`channel based on the first reference signal and estimating a
`phase component of the channel based on both the first
`reference signal and the second reference signal.
`In particular embodiments, the mapping of antenna ports
`comprises a precoding weight associated with each of the
`antenna ports associated with the first reference signal.
`According to some embodiments, a network node for
`coupling reference signals of a wireless network comprises
`a processor operable to establish a wireless connection with
`a wireless device. The wireless connection comprises a first
`reference signal and a second reference signal and both the
`first and second reference signals are associated with one or
`more antenna ports. The processor is further operable to
`determine a mapping between the one or more antenna ports
`associated with the first reference signal and the one or more
`
`In a wireless network, a wireless device may communi
`cate with one or more radio network nodes to transmit and
`receive voice traffic, data traffic, control signals, and so on.
`Reference signals, or pilot signals, may be transmitted in
`communication systems to provide a phase reference that a
`receiver can use to synchronize timing of a transmission and
`adjust for frequency error between a transmitter and
`receiver. Reference signals may also provide a phase refer
`ence such that a receiver can estimate a propagation channel
`between a transmitter and the receiver to demodulate and
`decode a transmitted data message.
`In cellular systems, reference signals transmitted in a cell
`from a base station to user equipment (UE) may be referred
`to as common or dedicated reference signals. Common, or
`cell-specific, reference signals (CRS) may be used by all
`UES communicating with the cell and are typically broadcast
`with equal power in all directions within the cell. Dedicated,
`or user-specific, reference signals are typically received and
`used by a single user.
`Machine-to-machine (M2M) communication (also
`referred to as machine type communication (MTC)) estab
`lishes communication between machines and/or between
`machines and humans. The communications may comprise
`exchange of data, signaling, measurement data, configura
`tion information, etc. The device size may vary from that of
`a wallet to that of a base station. M2M devices are often used
`for applications like sensing environmental conditions (e.g.,
`temperature reading, etc.), metering or measurement (e.g.,
`electricity usage, etc.), fault finding or error detection, etc.
`Generally MTC devices are low cost and low complexity.
`A low complexity UE that may be used for M2M operation
`may implement one or more low cost features, such as
`Smaller downlink and uplink maximum transport block size
`(e.g., 1000 bits) and/or reduced downlink channel band
`width of 1.4 MHz for data channel (e.g., PDSCH). A low
`cost UE may also comprise of a half-duplex (HD-FDD) and
`one or more of the following additional features: single
`receiver (1 RX) at the UE, smaller downlink and/or uplink
`maximum transport block size (e.g., 1000 bits), and reduced
`downlink channel bandwidth of 1.4 MHZ for data channel.
`The low cost UE may also be referred to as a low complexity
`UE.
`Path loss between an M2M device and a base station can
`be large in some scenarios. For example, path loss may be
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`US 9,596,061 B2
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`3
`antenna ports associated with the second reference signal;
`communicate the mapping of antenna ports to the wireless
`device; and transmit the first reference signal and the second
`reference signal to the wireless device according to the
`communicated mapping.
`According to some embodiments, a wireless device for
`coupling reference signals of a wireless network comprises
`a processor operable to establish a wireless connection with
`a radio network node. The wireless connection comprises a
`first reference signal and a second reference signal and both
`the first and second reference signals are associated with one
`or more antenna ports. The wireless device also comprises
`an interface operable to receive a mapping between the one
`or more antenna ports associated with the first reference
`signal and the one or more antenna ports associated with the
`second reference signal. The processor is further operable to
`perform channel estimation based at least on one of the first
`reference signal, the second reference signal, and the
`received mapping of antenna ports. The processor is further
`operable to demodulate data transmitted over wireless con
`nection based on the channel estimation.
`Particular embodiments may exhibit some of the follow
`ing technical advantages. Particular embodiments may miti
`gate loss resulting from bad channel estimation quality
`during operation in low signal to noise ratio (SNR) condi
`tions by increasing the reference signal power available for
`performing channels estimation in a wireless device. Par
`ticular embodiments may improve channel estimation qual
`ity for a first reference signal by using power allocated to a
`second reference signal. For example, some advantages may
`be realized by a network node explicitly or implicitly
`informing a wireless device that the first and second refer
`ence signals are related to each other and that channel
`35
`estimation may be based on both reference signals. A
`wireless device may use the known relationship between the
`first and second reference signal to perform improved chan
`nel estimation by using the combined reference signals.
`Particular embodiments provide flexibility for selecting
`between improved robust channel estimates and versatile
`adaptation of multi-antenna transmission parameters.
`
`10
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`15
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`25
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`FIG. 8 is a block diagram illustrating an example embodi
`ment of a radio network node.
`
`DETAILED DESCRIPTION
`
`In particular networks, a UE might receive reference
`signals at a low signal to noise ratio (SNR). Demodulation
`results can be sensitive to channel estimation quality when
`operating in low SNR regions. Channel estimates at low
`SNR values may become noisy which may impact demodu
`lation performance. One reason for poor channel estimation
`quality is that power allocated to the reference signals may
`not be high enough under these low SNR conditions. Chan
`nel estimation quality for Some reference signals may be
`improved by boosting the pilot signal power. Increasing
`pilot signal power, however, may lower the power available
`for data symbols. A UE may also use averaging (or low-pass
`filtering) of channel estimates over several consecutive
`Subframes. This may be advantageous when reference sym
`bols are not multiplied by weights that vary between sub
`frames and when UEs are nearly stationary such that the
`propagation channel does not change significantly for sev
`eral subframes.
`To improve beam forming and/or diversity gain, an eNo
`deB may apply a (time-varying) precoder to its multiple
`antenna ports before transmitting data and/or dedicated
`reference signals. When operating in enhanced coverage
`mode, the eNodeB may realize particular advantages if the
`eNodeB does not change the precoder used for different
`reference signals. For example, not changing the precoder
`used for different reference signals may enable channel
`estimates to be low-pass filtered between subframes. Such
`filtering may alleviate performance loss due to bad channel
`estimation quality. Fewer resources, however, are still allo
`cated to Some reference signals compared to others.
`An object of the present disclosure is to obviate at least
`these disadvantages and provide an improved method to
`perform successful channel estimation in low SINR envi
`ronments. Particular embodiments described below may
`improve channel estimation quality for a first reference
`signal by using power allocated to a second reference signal.
`For example, some advantages may be realized by explicitly
`or implicitly informing a UE that the first and second
`reference signals are related to each other and that channel
`estimation may be based on both reference signals.
`Particular embodiments are described with reference to
`FIGS. 1-8 of the drawings, like numerals being used for like
`and corresponding parts of the various drawings. LTE is
`used throughout this disclosure as an example wireless
`system, but the ideas presented herein apply to other wire
`less communication systems as well.
`FIG. 1 is a block diagram illustrating an example of a
`wireless network, according to Some embodiments. Network
`100 includes radio network node 120 (such as a base station
`or eNodeB) and wireless devices 110 (such as mobile
`phones, Smart phones, laptop computers, tablet computers,
`MTC devices, or any other devices that can provide wireless
`communication). In general, wireless devices 110 that are
`within coverage of radio network node 120 communicate
`with radio network node 120 by transmitting and receiving
`wireless signals 130. For example, wireless devices 110 and
`radio network node 120 may communicate wireless signals
`130 containing Voice traffic, data traffic, reference signals,
`and/or control signals. Wireless signals 130 may include
`both downlink transmissions (from radio network node 120
`to wireless devices 110) and uplink transmissions (from
`wireless devices 110 to radio network node 120). Wireless
`
`40
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present inven
`tion and its features and advantages, reference is now made
`to the following description, taken in conjunction with the
`accompanying drawings, in which:
`FIG. 1 is a block diagram illustrating an example of a
`wireless network, according to some embodiments;
`FIG. 2 illustrates an example Orthogonal Frequency
`Division Multiplexed (OFDM) symbol;
`FIG. 3 illustrates an example radio frame;
`FIG. 4A illustrates an example antenna port mapping for
`cell specific reference signals;
`FIG. 4B illustrates an example antenna port mapping for
`user specific reference signals;
`FIG. 5 is a flowchart of an example method of coupling
`reference signals in a radio network node, according to some
`embodiments;
`FIG. 6 is a flowchart of an example method of coupling
`reference signals in a wireless device, according to some
`embodiments;
`FIG. 7 is a block diagram illustrating an example embodi
`ment of a wireless device; and
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`signals 130 may include reference signals 135. Wireless
`device 110 may detect reference signals 135 to perform
`channel estimation and data demodulation. Wireless signals
`130 comprise radio frames, such as the example radio frame
`illustrated in FIG. 3 described below.
`Radio network node 120 transmits and receives wireless
`signals 130 using antenna 140. In particular embodiments,
`radio network node 120 may comprise multiple antennas
`140. For example, radio network node 120 may comprise a
`multi-input multi-output (MIMO) system with two, four,
`eight, or any Suitable number of antennas 140.
`In network 100, each radio network node 120 may use any
`Suitable radio access technology, such as long term evolution
`(LTE), LTE-Advanced, UMTS, HSPA, GSM, cdma2000,
`WiMax, WiFi, and/or other suitable radio access technology.
`Network 100 may include any suitable combination of one
`or more radio access technologies. For purposes of example,
`various embodiments may be described within the context of
`certain radio access technologies. However, the scope of the
`disclosure is not limited to the examples and other embodi
`ments could use different radio access technologies.
`As described above, embodiments of a network may
`include one or more wireless devices and one or more
`different types of radio network nodes capable of commu
`nicating with the wireless devices. The network may also
`25
`include any additional elements suitable to Support commu
`nication between wireless devices or between a wireless
`device and another communication device (such as a land
`line telephone). A wireless device may include any suitable
`combination of hardware and/or software. For example, in
`particular embodiments, a wireless device. Such as wireless
`device 110, may include the components described with
`respect to FIG. 7 below. Similarly, a radio network node may
`include any suitable combination of hardware and/or soft
`ware. For example, in particular embodiments, a radio
`network node. Such as radio network node 120, may include
`the components described with respect to FIG. 8 below.
`In some embodiments, reference signal 135a may com
`prise a cell-specific reference signal (CRS). CRS 135 may be
`transmitted in all or almost all Subframes and may be used
`to Support channel estimation to demodulate different physi
`cal control and data channels. CRS 135a may also be used
`for measuring signal strength and quality within its own cell
`and neighboring cells.
`In some embodiments, target signal 135b may comprise a
`dedicated reference signal Such as a demodulation reference
`signal (DMRS or DM-RS). For example, DMRS 135b may
`comprise a demodulation reference signal for the Physical
`Downlink Shared Channel (PDSCH). As another example,
`DMRS 135c may comprise a demodulation reference signal
`for the Enhanced Physical Downlink Control Channel (EP
`DCCH).
`Reference signals 135 may be mapped to and transmitted
`from antenna ports. Antenna ports may represent logical
`antennas. An antenna port may map to one or more physical
`antenna. In multi-antenna transmission schemes, such as
`transmit diversity and Multiple-Input Multiple-Output
`(MIMO), multiple antenna ports may transmit multiple
`types of reference signals. In an LTE example, an antenna
`port may be mapped to a fixed set of resource elements (RE)
`in the OFDMA physical layer time-frequency grid. An LTE
`standard may define the reference symbols sent on each one
`of the REs.
`FIG. 2 illustrates an example OFDM symbol. LTE uses
`OFDM in the downlink where each downlink symbol may
`be referred to as an OFDM symbol. Furthermore, LTE uses
`Discrete Fourier Transform (DFT)-spread OFDM, also
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`referred to as Single-Carrier FDMA (SC-FDMA), in the
`uplink, where each uplink symbol may be referred to as an
`SC-FDMA symbol. The basic LTE downlink physical
`resource may be illustrated as a time-frequency grid as
`shown in FIG. 2, where each resource element corresponds
`to one OFDM subcarrier during one OFDM symbol interval.
`In the time domain, LTE downlink transmissions may be
`organized into radio frames.
`FIG. 3 illustrates an example radio frame. A radio frame
`is 10 ms and each radio frame consists of ten 1 mS Sub
`frames. Resource allocation in LTE may be described in
`terms of resource blocks (RBs), where a resource block
`corresponds to one slot (0.5 ms) in the time domain and
`twelve contiguous Subcarriers in the frequency domain. A
`pair of two adjacent resource blocks in the time domain (1.0
`ms) may be referred to as a resource block pair. Resource
`blocks may be numbered in the frequency domain, starting
`with 0 at one end of the system bandwidth. Each slot
`typically corresponds to seven OFDM symbols for downlink
`(SC-FDMA symbols for uplink) for normal cyclic prefix and
`six OFDM symbols for downlink (SC-FDMA symbols for
`uplink) for extended cyclic prefix.
`LTE also includes the concept of virtual resource blocks
`(VRB) and physical resource blocks (PRB). The actual
`resource allocation to a UE is made in terms of VRB pairs.
`Resource allocations may be localized or distributed. Local
`ized resource allocation directly maps a VRB pair to a PRB
`pair, hence two consecutive and localized VRB are also
`placed as consecutive PRBs in the frequency domain. Dis
`tributed VRBs are not mapped to consecutive PRBs in the
`frequency domain, which provides frequency diversity for
`data channels transmitted using distributed VRBs.
`Downlink transmissions may be dynamically scheduled
`(i.e., in each subframe a base station transmits control
`information about which wireless devices will receive data
`and upon which resource blocks the data is transmitted).
`Downlink Control Information (DCI) may be carried by the
`Physical Downlink Control Channel (PDCCH). This control
`signaling may be transmitted in the first 1, 2, 3 or 4 OFDM
`symbols in each subframe, and the number n=1, 2, 3 or 4
`may be referred to as the Control Format Indicator (CFI).
`The downlink subframe may also contain common reference
`symbols, which are known to the receiver and used for
`coherent demodulation of, e.g., the control information.
`PDSCH may use different data transmission modes. For
`example, one mode may use a CRS for demodulation. In this
`mode, the UE may use the CRS symbols, among other
`things, to estimate a propagation channel from each transmit
`(TX) antenna port in an eNodeB to each receive (RX)
`antenna in a UE. In particular embodiments, the CRS may
`be transmitted on antenna ports p-0, peO, 1} or pe (O. 1, 2,
`3 depending on whether 1, 2, or 4 antenna port transmission
`is configured.
`FIG. 4A illustrates an example antenna port mapping for
`cell specific reference signals. FIG. 4A illustrates two
`example subframe patterns, each comprising time-frequency
`resource elements. The shaded time-frequency resource ele
`ments represent time-frequency resource elements allocated
`for transmitting CRS from each antenna port. A first pattern
`of time-frequency resource elements is allocated to transmit
`CRS from antenna port 0 and a second pattern of time
`frequency resource elements is allocated to transmit CRS
`from antenna port 1. In LTE, resource elements used for
`transmitting CRS on any antenna port are not typically used
`for any transmission on any other antenna port in the same
`slot.
`
`Ex.1008
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`US 9,596,061 B2
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`FIG. 4B illustrates an example antenna port mapping for
`user specific reference signals. FIG. 4B illustrates four
`example subframe patterns, each comprising time-frequency
`resource elements. The shaded time-frequency resource ele
`ments represent time-frequency resource elements allocated
`for transmitting DMRS from each antenna port. A first
`pattern of time-frequency resource elements is allocated to
`transmit DMRS from antenna ports 7 and 8 and a second
`pattern of time-frequency resource elements is allocated to
`transmit DMRS from antenna ports 9 and 10.
`As a particular LTE example, antenna port 8, when
`present, may use the same set of resource elements as
`antenna port 7, but with different precoding (i.e., they are
`code division multiplexed). Similarly, antenna ports 9 and
`10 (when present) may use a different set of resource
`elements in a similar manner. If a resource element is used
`in a slot for DMRS transmission on antenna port 7 or 8, then
`it is typically not used for any transmission on antenna port
`9 or 10, and vice versa.
`In particular embodiments, time-frequency resource ele
`ments for some reference signals may be statically allocated,
`Such as by a standards specification, and time-frequency
`resource elements for other reference signals may be
`dynamically allocated. Dynamic allocation may be based on
`network conditions or any other suitable criteria.
`To optimize signal to noise ratio (SNR) for data trans
`mission in multi-antenna transmission schemes, data sym
`bols may be multiplied with a precoding vector or matrix.
`For TX diversity, a precoding vector may rotate the phases
`of signals from the TX antennas Such that the signals add
`constructively at the RX antennas at the UE and may
`produce a beam forming gain. Similarly, a precoding matrix
`may attempt to maximize a spatial multiplexing gain for
`MIMO transmission.
`Because CRS symbols are common to all users in a cell,
`however, the CRS symbols may not be precoded for a
`specific user. Instead, for example, an eNodeB may signal to
`a UE a particular precoder used for data transmission to that
`UE. The UE may use the signaled precoder to undo the
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`precoding when reconstructing the transmitted data sym
`bols. In some embodiments, a UE may inform an eNodeB
`about the UE's preferred precoder. In some embodiments,
`the preferred precoder may be selected from a codebook of
`possible precoders based on measurements related to the
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`propagation channel.
`As another example, a data transmission mode may use
`user-specific reference signals. When using user-specific
`reference signals, an eNodeB may perform phase rotations
`to improve beam forming and spatial multiplexing gains both
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`on reference signals and data symbols. In this example, a UE
`may perform channel estimation and demodulate the data
`signal without knowing the precoder used by a transmitting
`eNodeB. An eNodeB may not be restricted to use only the
`precoders defined in codebooks, and may also change pre
`coders each Subframe. An advantage is that transmissions
`may be adapted to maximize link-level performance. Such
`an advantage may also be recognized in Scenarios with a
`rapidly changing propagation channel. In some embodi
`ments, an eNodeB may receive measurements from a UE.
`The eNodeB may use the measurements to determine pre
`coder selections.
`In some embodiments, user-specific reference signals
`may be used with certain transmission modes of the data
`channel PDSCH, and with the Enhanced physical control
`channel EPDCCH. The DMRS for PDSCH may be trans
`mitted on antenna ports p=5, p=7, p=8, or one or several of
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`pe 7... 14. The DMRS for EPDCCH may be transmitted
`on one or several of pe107 . . . 110}.
`A particular goal of enhanced coverage for MTC UEs is
`to improve the SNR region in which the MTC UEs may
`Successfully communicate with a radio network node. The
`targeted improvement may be different for different physical
`channels. In some cases, the desired improvement may be on
`the order of 15 dB. Such improvements may be obtained by
`repetition. For example, a message may be transmitted over
`several 1 mS subframes instead of a single subframe trans
`mission. EPDCCH may benefit from coverage improvement
`using repeated transmissions using user-specific reference
`signals. DMRS may also benefit from using repeated trans
`missions.
`Demodulation results may become sensitive to channel
`estimation quality when operating in low SNR conditions.
`Channel estimates at low SNR values may become noisy
`which may impact demodulation performance. One reason
`for poor channel estimation quality is that power allocated to
`the reference signals may not be high enough for these low
`SNR operating points. Channel estimation quality for CRS
`based transmissions may be improved by boosting the pilot
`signal power. Increasing pilot signal power, however, may
`lower the power available for data symbols.
`A UE may also use averaging (or low-pass filtering) of
`channel estimates over several consecutive subframes. This
`may be advantageous when CRS symbols are not multiplied
`by weights that vary between subframes and when UEs
`requiring operation in an enhanced coverage mode are
`almost stationary, Such that the propagation channel does not
`change significantly for several Subframes. Detrimental
`effects of deteriorating chan