`
`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2007/0149206 A1
`(43) Pub. Date: Jun. 28, 2007
`
`Wang et al.
`
`(54)
`
`(75)
`
`METHOD AND SYSTEM FOR ADJUSTING
`UPLINK TRANSMISSION TIMING FOR
`LONG TERM EVOLUTION HANDOVER
`
`Inventors: Jin Wang, Central Islip, NY (US);
`Stephen E. Terry, Northport, NY (US)
`
`Correspondence Address:
`VOLPE AND KOENIG, P.C.
`DEPT. ICC
`UNITED PLAZA, SUITE 1600
`30 SOUTH 17TH STREET
`
`PHILADELPHIA, PA 19103 (US)
`
`(73)
`
`Assignee:
`
`INTERDIGITAL
`CORPORATION, Wilmington,
`(US)
`
`TECHNOLOGY
`DE
`
`(21)
`
`Appl. No.:
`
`11/612,837
`
`(22)
`
`Filed:
`
`Dec. 19, 2006
`
`Related US. Application Data
`
`(60)
`
`Provisional application No. 60/753,124, filed on Dec.
`22, 2005. Provisional application No. 60/839,267,
`filed on Aug. 21, 2006.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`H04Q 7/20
`(52) use. ............................................ 455/450; 455/502
`
`(57)
`
`ABSTRACT
`
`A method and system for adjusting uplink transmission
`timing when sending an initial
`transmission to a target
`cell/Node-B of an evolved universal terrestrial radio access
`
`network (E-UTRAN) immediately after handover from a
`source cell/Node-B of the E-UTRAN. In one embodiment,
`a user equipment (UE) autonomously computes and applies
`a timing advance (TA) value based on the current source
`cell/Node-B timing value, cell/Node-B beacon channel ref-
`erence signal measurements and knowledge of the relative
`time difference, (if any), between the source and target
`cells/Node-Bs.
`In another embodiment,
`the UE sends a
`scheduling request message or real data packets with the
`computed TA value applied to the uplink transmission
`timing to the E-UTRAN via pre-allocated non-contention
`based uplink radio resources. In an alternate embodiment,
`the UE sends a scheduling request message with the new
`computed TA value applied to the UL transmission timing to
`an E-UTRAN via a synchronous random access channel
`(RACH).
`
`CGMMAND
`HANDOVER
`MESSAGE
`
`
`
`
` SOURCE
`
`CELUNODEB
`
`i 3 i i i i i
`
`3 i i i i i i i 3
`
`APPLE 1024
`
`APPLE 1024
`
`
`
`
`
`.....
`
`EvoEVED UNEVERSAL
`
`TERRESTRiAL RADlQ
`AcoEss NETWORK
`(E-UTRAN)
`
`210
`
`USER
`
`EQUEPMENT
`
`
`(UE)
`51111191
`205
`
`
`
`:E
`TRANSMiSSEON TC) ‘
`
`UEADJUSTS TWHNG
`WGIET CELUNQDEMB
`ADVANCE (TA) VALUE 10
`{USENG FREE“ 7
`ACCESS TRANSMISSSGN
`ALL‘C’CATEE ”PUNK
`TO mam CELL/NQDE-E
`RAU‘O RESOURCES
`BASED ON REFERENCE
`OR “SENS
`SEGNALS OF SQURCE
`svmoHaoNous
`RANDQM ACCESS
`:
`:
`CELUNOBEE AND
`TARGET CELUNQDE—B
`BEACON CHANNELS
`
`Fl},
`
`TARGET
`GELUNODE—B
`
`Cit-IANNEL {RAG
`
`
`
`Patent Application Publication Jun. 28, 2007 Sheet 1 0f 3
`
`US 2007/0149206 A1
`
`
`
`
`
`
`USER
`EQUEPMENT
`
`(UE)
`
`
`
`
`COMMAND
`HANDOVER
`MESSAGE
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`TRANSMESSHON
`
`UE ADJUSTS TEMENG
`TARGET CELL/MODES
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`{USENC‘ RRE—
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`ALLQCATEB UPUNEE
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`SYNCHRQNOUS
`RANDOM ACCESS
`CHANNELTLRACHE
`
`ADVANCE {TA} VALUE TO
`ACCESS TRARSRTERTRN
`To TARGET CELL/NGDE—E
`BASED ON REFERENCE
`SEGNALS 3E: SQUREE
`CELL/mafia END
`TARGET CELL/NQDE—B
`BEACON CHANNELS
`
`EVOLVED UNEVERSAL
`TERRERTRTAL RADIO
`ACCESS NETWORK
`{BUTT-TAR)
`21g
`_____
`
`‘
`
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`TARGET
`GELUNODE-B
`Egg
`
`
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`
`
`
`
`
`Patent Application Publication Jun. 28, 2007 Sheet 2 0f 3
`
`US 2007/0149206 A1
`
`UNTVERSAL TERRESTRTAL RADIO ACCESS NETWORK {ENTRAN},
`
`ENABLE AND EEREORM TTNTTNG ADVANCE FOR A USER
`EQUEPMENT (LIE) EN A SQURCE CELUNDDE—B OF AN EVOLVED
`
`THE E~UTHAN MEASURES AND CALCULATES THE TIA/ENG ADVANCE
`
`'
`
`VALUE AND STGNALS THE TTNTTNG ADVANCE VALUE TO THE NE.
`
`1
`
`‘ 305
`
`“ 310
`
`
`
`THE UE APPLIES THE TINTTNG ADVANCE VALUE OF STEP 3T 0 WHEN
`TRANSMTTTENG TO THE SQUHCE CELL/NODE—B.
`
`‘
`
`‘
`
`a.
`31"
`
`
`
`
`iiiiiYES
`
`.
`.
`A “ PERFORM '
`-
`a HAN
`
`~
`
`329
`
`THE SOURCE CELUNODE-B SENDS A HANDOVER COMMAND
`
`
`
`
`
`MESSAGE (OPTEGNALLY ENCLUDTNG RRE—ALLOCATEN URMNR RADEO
`RESCURGE OR SYNCHRQNOUS RANDOM ACCESS CHANNEL (EACH)
`
`
`TNEOHN’TATEON) T0 THE UE TO ENETEATE HANDOVEH.
`
`THE NE PERFORMS GNE OR NGRE MEASUREMENTS TC) DETERNTNE THE
`
`'
`
`NEEFERENCE TN PEOPAGATE'ON DELAYS BETWEEN THE SOURCE CELUNODEE AND
`
`ATARG
`
`CELL/NOD58 BASED ON BEACON CHANNEL R- EREN
`
`EGNALS.
`
`
`
`
`
`
`
`
`THE NE AUTONGMOUSLY CONTENTES A NEW TEMENG ATEVANCE VALUE BASED ON
`THE CURRENT SOURCE CELL/NQDE—B TTNTNG VALUES THE TV‘TEASUREMENTS
`
`PEREGRMED TN STEP 339 AND KNOWLEDGE (SEE THE HELATTVE TTTvTE DTEEEHENCE
`
`
`
`
`(TE ANY} BETWEEN THE SOURCE CELUNQDE‘B AND THE TARGET SELUNGDE—B.
`
`
`_
`
`. 325
`
`‘
`
`‘ 330
`
`335
`
`‘
`
`
`
`THE NE APPLEES THE NEW TTMENG ADVANEQE VALUE T0 ABJUST THE UPLTNK
`TRANSMTSSTON TEA/TING WE—TEN SENDTNG AN TNTTIAL THANSNTTSSTON TO THE ‘ ~
`
`340
`
`TARGET CELL/NODE—B ETTHEH THRQUGH THE USE OE EHE—ALLGCATED
`
`UPLTNK NfiN—CONTENTTON BASED RABTO HESQUHCES ON A SYNCHHONC‘US
`
`EACH, AS TNDECATED BY THE HANDOVEH COMMAND MESSAGE.
`
`
`
`
`
`
`
`
`
`
`Patent Application Publication Jun. 28, 2007 Sheet 3 0f 3
`
`US 2007/0149206 A1
`
`A USER EQUEPMENT {UE} AUTONOMGUSLY CGMPUTES A TEMTNG
`ADVANCE VALUE.
`
`‘ ‘
`;3
`
`THE UE SENDS A SCHEDULENG REQUEST MESSAGE THROUGH A
`
`SYNGH RONQUS RANDOM ACCESE CHANNEL (EACH) TO AN
`EVOLVED UNEVERSAL TERRESTREAL RADEO ACCESS NETWORK (E—
`UTRAN) WETH THE COMPUTED TTMENG ADVANGE VALUE APPLTED.
`
`4&5
`
`‘ 4T9
`
`‘
`
`‘ 435
`‘
`
`42%
`
`E
`
`A 425
`
`DOWNLENK EADTQ RESQURCEST
`
`THE E—UTAAN COMPUTES A AEETNEE (LE, MORE ACCURATE)
`TEMENG ADVANCE VALUE BASED ON SNFGRMATTQN EN THE
`SCHEDULENG REQUEST MESSAGE
`
`5F NECESSARY, THE E—UTRAN SENDS THE REETNED TTMTNG
`ADVANCE VALUE TC} THE UE IN A DQWNLENK SEGNALTNG MESSAGE
`
`AND ASSTGNS UPLENK AND/0R DOWNLTNK RADTO RESQURCES
`FOR USE BY THE UE FOR SUBSEQUENT DATA TRANSMTSSTONS
`
`$5
`
`THE UE ENETTATSE DATA TEANETATEETEN UETNE THE REFENED
`
`TEMENG ADVANCE VALUE AND THE ASSEGNEB UPLENK AND/0R
`
`
`
`US 2007/0149206 A1
`
`Jun. 28, 2007
`
`METHOD AND SYSTEM FOR ADJUSTING
`UPLINK TRANSMISSION TIMING FOR LONG
`TERM EVOLUTION HANDOVER
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] This application claims the benefit of US. Provi-
`sional Application No. 60/753,124 filed Dec. 22, 2005 and
`US. Provisional Application No. 60/839,267 filed Aug. 21,
`2006, which are incorporated by reference as if fully set
`forth.
`
`FIELD OF INVENTION
`
`[0002] The present invention relates to wireless commu-
`nication systems. More particularly, the present invention is
`related to a timing adjustment procedure for synchronizing
`data transmissions between a wireless transmit/receive unit
`
`(WTRU), (i.e., a user equipment (UE)), and a target cell/
`evolved Node-B (eNB) immediately after handover from a
`source cell/eNB to the target cell/eNB in a long term
`evolution (LTE) system.
`
`BACKGROUND
`
`[0003] The objective of evolved universal terrestrial radio
`access (E-UTRA) and evolved universal terrestrial radio
`access network (E-UTRAN) is to develop a radio access
`network (RAN) for providing a high-data-rate, low-latency
`and packet-optimized improved system capacity and cover-
`age. FIG. 1 shows a wireless communication system 100
`which includes at least one cell/Node-B 105 that commu-
`nicates with at least one UE 110. In order to achieve this
`
`objective, an evolution of the radio interface as well as the
`radio network architecture is being considered, such as a
`long term evolution (LTE) system. However, there are no
`existing dedicated channels in an LTE system, so all services
`are provided over shared and common channels. Further-
`more, system frame number-system frame number (SFN-
`SFN) measurements may not be available in the LTE system.
`This causes problems with synchronized communications
`between the UE 110 and the cell/Node-B 105 during han-
`dover in the LTE system.
`
`[0004] A timing advance (TA) enables the UE 110 to send
`its uplink (UL) bursts earlier than what the UE 110 perceives
`at the start of an UL timeslot for transmission, so that the UL
`bursts are received at the cell/Node-B 105 within a time
`window that allows accurate detection and minimizes, or
`eliminates, signal degradation. Single channel frequency
`division multiple access (SC-FDMA) is a new radio access
`technology that has a stringent performance requirement for
`UL synchronization. Thus, an appropriate and accurate TA is
`critical in LTE UL transmission.
`
`[0005] Handover requires that TA be adjusted for the LYE
`110 in the case where the UE 110 maintains shared channel
`
`connectivity or use of the synchronous PRACH in a target
`cell/Node-B with minimum delay which is especially impor-
`tant for time sensitive services such as voice over IP (VoIP)
`and interactive gaming, etc. The LTE system should avoid
`requiring an asynchronous random access channel (RACH)
`access burst to establish the TA during handover since this
`procedure increases the delay in establishing a connection in
`the target cell and is not an efficient use of physical resources
`relative to use of an UL shared channel. In the Third
`
`Generation Partnership Project (3GPP), TA during handover
`is achieved through measuring SFN-SFN timing difference
`between old and new radio links associated with old and new
`
`Node-Bs. However, in an LTE system, there is no new radio
`link set in parallel to the old radio link during handover, and
`an SFN-SFN for timing difference measurement may not
`exist. Thus, acquiring TA with less delay is desired during
`handover in an LTE system.
`
`in SC-FDMA systems to
`[0006] TA is very important
`achieve the acceptable performance requirement. This
`becomes a problem during handover, as the UE 110 has to
`achieve fast synchronized communications with the cell/
`Node-B 105 after a network commanded handover is imple-
`mented, and the UE 110 has to achieve fast cell selection to
`maintain a satisfactory quality of service (QoS). Unsynchro-
`nized transmissions cause high UL interference and thus
`degrade the system performance. Thus, a fast timing adjust-
`ment mechanism for synchronizing transmission immedi-
`ately after handover would be advantageous for LTE.
`
`[0007] Because there is no dedicated channel established
`in an LTE system, only shared channels are to be used,
`which makes it difficult to maintain a tight synchronization.
`Thus, the handover of the UE 110 to a new cell/Node-B has
`to be performed using other channels such as an asynchro-
`nous primary RACH (PRACH) to acquire the TA between
`both cells/Node-Bs. By using the asynchronous PRACH for
`timing adjustment after handover, the UE 110 has to go
`through a contention based access procedure in order that
`the cell/Node-B 105 can successfully detect the PRACH
`sequence and then signal to the UE 110 the proper TA. This
`results in an unnecessary delay in establishing shared chan-
`nel connectivity in the target cell/Node-B. Thus, a respon-
`sive timing adjustment mechanism during handover would
`be advantageous for LTE to avoid the need for asynchronous
`RACH access procedure that incurs delay, (i.e., a handover
`“blackout period” is avoided).
`
`It would therefore be advantageous if a procedure
`[0008]
`existed relating to the timing adjustment for synchronized
`communications between the UE 110 and the cell/Node-B
`
`105 during a handover process that does not possess the
`limitations of conventional systems.
`
`SUMMARY
`
`[0009] The present invention is related to a method and
`system for adjusting UL transmission timing when sending
`an initial transmission to a target cell/Node-B of an E-UT-
`RAN immediately after handover from a source cell/Node-B
`of the E-UTRAN. In accordance with one embodiment of
`
`the present invention, the UE autonomously computes and
`applies a TA value based on beacon channel reference
`signals which are received from the source and target
`cells/Node-Bs and knowledge of the relative time difference,
`(if any), between the source and target cells/Node-Bs. In
`another embodiment, the UE sends a scheduling request
`message or real data packets with the computed TA value
`applied to the UL transmission timing to an E-UTRAN via
`pre-allocated non-contention based UL radio resources
`which are negotiated and reserved from the target cell/
`Node-B to the source cell/Node-B in advance of handover.
`
`In an alternative embodiment, the UE sends a scheduling
`request message with the new computed TA value applied to
`the UL transmission timing to an E-UTRAN via a synchro-
`
`
`
`US 2007/0149206 A1
`
`Jun. 28, 2007
`
`nous RACH. Then, the E-UTRAN computes a refined, (i.e.,
`more accurate), TA value in response to the scheduling
`request message and, if necessary, the E-UTRAN signals the
`refined TA value to the UE, and assigns UL and/or downlink
`(DL) radio resources to be used in the target cell/Node-B for
`the UE. If the refined TA value is signaled, the UE initiates
`data transmission using the refined TA value and the
`assigned radio resources after the EUTRAN signaling in the
`target cell is processed.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0010] A more detailed understanding of the invention
`may be had from the following description of a preferred
`embodiment, given by way of example and to be understood
`in conjunction with the accompanying drawings wherein:
`
`FIG. 1 shows a conventional wireless communica-
`[0011]
`tion system which includes at least one Node-B that com-
`municates with at least one UE;
`
`[0012] FIG. 2 shows a wireless communication system
`including a UE and a E-UTRAN with source and target
`cells/Node-Bs in accordance with the present invention;
`
`[0013] FIG. 3 is a flow diagram of an autonomous timing
`advance LTE handover procedure implemented in the sys-
`tem of FIG. 2 by accessing a target cell/Node-B using
`pre-allocated radio resources
`in accordance with one
`embodiment of the present invention; and
`
`[0014] FIG. 4 is a flow diagram of an autonomous timing
`advance LTE handover procedure implemented in the sys-
`tem of FIG. 2 in which the target cell/Node-B is accessed
`using synchronous RACH access in accordance with another
`embodiment of the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[0015] When referred to hereafter, the terminology “user
`equipment (UE)” includes but is not limited to a wireless
`transmit/receive unit (WTRU), a mobile station, a fixed or
`mobile subscriber unit, a pager, a cellular telephone, a
`personal digital assistant (PDA), a computer, or any other
`type of user device capable of operating in a wireless
`environment.
`
`[0016] When referred to hereafter, the terminology “cell/
`Node-B” includes but
`is not
`limited to a cell and/or a
`Node-B, an LTE eNB, a cell and/or a base station, a site
`controller, an access point
`(AP), or any other type of
`interfacing device capable of operating in a wireless envi-
`ronment.
`
`It should be understood by one of skill in the art
`[0017]
`that there are different types of handover, such as an intra-
`Node-B handover and an inter-Node-B handover. In the
`
`intra-Node-B handover case, because the handover happens
`between two cells within one Node-B, a handover occurs
`from a source cell to a target cell, but the handover is within
`a common Node-B and does not occur from a source
`
`Node-B to a target Node-B. In the inter-Node B handover
`case, a handover occurs from one cell, (i.e., a source cell),
`belonging to a source Node-B, to another cell, (i.e., a target
`cell), belonging to a target Node-B. In this case, the terms
`“cell” and “Node-B” are interchangeable. A handover from
`a source cell to a target cell may apply to both cases. When
`
`both the source and target cells are supported by a common
`Node-B it is more likely that these cells may be synchro-
`nized with each other.
`
`[0018] An application specific integrated circuit (ASIC)
`may be utilized to implement the present invention. The
`present invention is applicable to a radio resource manage-
`ment (RRM) and a radio resource controller for a WTRU,
`base station, network or system, at the physical layer, (digital
`baseband), or network layer, as software or as a digital signal
`processor (DSP). The present invention is applicable to the
`following air interfaces: wideband code division multiple
`access
`(WCDMA),
`frequency division duplex (FDD),
`CDMA2000 ((Ix Evolution-Data Only (IxEV—DO), Ix Evo-
`lution data and voice (IxEV—DV), CDMA, enhanced UL,
`high speed downlink packet access (HSDPA), and LTE
`based systems.
`
`[0019] The present invention is related to an LTE_Active
`state,
`for both intra/inter-Node-B handover cases. The
`present
`invention provides a method and procedure by
`which a UE can autonomously measure and calculate a TA
`value so that the synchronous transmission can be immedi-
`ately applied in the target cell following handover. Thus,
`application of the asynchronous PRACH procedure in the
`target cell to update the TA value can be avoided.
`
`[0020] During a non-handover situation, a TA value is
`determined by the E-UTRAN from the UL transmissions,
`and a TA adjustment value is signaled to the UE when
`necessary. When handover from a source, (i.e., current),
`cell/Node-B to a target, (i.e., new), cell/Node-B occurs, the
`UE can autonomously determine the TA value for starting
`transmissions in the target cell/Node-B, using either pre-
`allocated UL non-contention based radio resources or a
`
`synchronous RACH for access to the target cell/Node-B.
`Otherwise, if the TA is not adjusted for the target cell, no TA
`value is applied in the target cell/Node-B and the asynchro-
`nous PRACH procedure must be used for the first transmis-
`sion in the target cell.
`
`If absolute TA signaling is applied, the E-UTRAN
`[0021]
`must always know the applied TA value in the UE. When a
`new calculated TA is autonomously determined by the UE,
`the UE must report the TA after the autonomous adjustment.
`It is also possible for the E-UTRAN to request the applied
`TA in a measurement report. Once handover is complete, the
`nominal TA procedure applies again. If relative TA signaling
`is applied, it is not necessary to signal the new calculated TA
`to the E-UTRAN following autonomous TA adjustments by
`the UE.
`
`In accordance with the present invention, a han-
`[0022]
`dover refers specifically to a hard handover between syn-
`chronous cells/Node-Bs or between cells/Node-Bs where
`
`the relative time difference is known. The present invention
`provides a UE autonomous TA measurement and calculation
`method, as well as a procedure for LTE handover to achieve
`synchronous communication with reduced delay and less
`interference. The knowledge of the relative time difference
`(if any) between the source and target cells/Node-B should
`be signaled to the UE in order to compute a new TA value.
`In a preferred embodiment the relative time difference or
`indication that the cells are synchronized with each other is
`signaled in the handover command.
`
`[0023] Depending on which TA information element (IE)
`in a radio resource control (RRC) command is enabled,
`
`
`
`US 2007/0149206 A1
`
`Jun. 28, 2007
`
`either the pre-allocated UL non-contention based radio
`resource from a target cell/Node-B, or the synchronous
`RACH, will be used during the handover process to access
`the target cell/Node-B. Optionally,
`the E-UTRAN deter-
`mines which one of the two access functions will be used.
`
`The UE calculates the timing difference from the source and
`target cells/Node-Bs by measuring reference signals on
`beacon channels received from different cells/Node-Bs. The
`
`UE then autonomously determines the TA to apply in UL
`transmission to a new target cell/Node-B upon handover to
`avoid the asynchronous PRACH procedure requirement.
`The UE can use an assigned UL channel with TA applied for
`direct transmission for a resource request, or it can use a
`synchronous RACH for a resource request and then start
`data transmission after radio resource allocation from the
`
`target cell/Node-B is completed. When the E-UTRAN
`directs the UE to handover to a new target cell/Node-B, the
`E-UTRAN will direct the UE to apply the computed TA in
`the new cell/Node-B. At all other times, it is the E-UTRAN
`that determines the TA value. This avoids the need for
`
`requiring an asynchronous RACH access procedure to a
`target cell/Node-B, or a source cell/Node-B SFN—SFN
`reporting associated with the E-UTRAN handover com-
`mand.
`
`[0024] FIG. 2 shows a wireless communication system
`200 including a UE 205 and an E-UTRAN 210 in accor-
`dance with the present
`invention. The E-UTRAN 210
`includes a source cell/Node-B 215 and a target cell/Node-B
`220.
`
`[0025] UE Autonomous TA Measurement During LTE
`Handover
`
`If the UE 205 performs autonomous TA during
`[0026]
`handover, it must determine the value of its one-way propa-
`gation delay. Let L denote the radio frame length, ti denote
`the clock time at the cell/Node-B i, pi denote the one-way
`propagation delay from the cell/Node-B i to the UE 205, and
`( )L denote the module operation by L. Since, through cell
`search, the UE 205 only knows the sum of (ti)L and pi for a
`cell/Node-B i that the UE 205 is not connected to, the UE
`205 has to know either (ti)L or pi to solve the other.
`
`Suppose the distance between the UE 205 and the
`[0027]
`cell/Node-B i is D. The coarse DL timing that the UE 205
`detects, (in the first cell search step), for the cell/Node-B i,
`is (ti)L+Pi+'cDL, where ‘UDL is the multipath that generates a
`peak for timing detection. The propagation delay pi=Di/c is
`therefore not affected by the frequency. The ‘UDL part
`depends on both frequency and environment. After refined
`timing detection, (the second or third step of cell search), at
`least part of multipath delay can be resolved.
`
`[0028] Let "EDL denote the residual multipath delay which
`is shorter than ‘CDL. Then,
`the fine DL timing becomes
`(ti)L+pi+'EDL. If "EDL is very small, it can be argued that fine
`DL timingz(ti)L+pi, which is independent of frequency. It
`can temporarily be assumed that "EDL is very small in the
`following analysis.
`
`In order for the UE 205 to align its UL transmission
`[0029]
`with other UEs at the cell/Node-B i, the UE 205 needs to
`perform TA by the amount of 2pi. In this way,
`the UL
`transmitted signals of the UE 205 are received at the time of
`RT(i), which is given by:
`RT(i)=(l;)L+Pi—2Pi+Pi+TUL=(li)L+TULa
`
`Equation (1)
`
`where ‘UUL is the maximum multipath delay in the UL and
`depends on frequency as well.
`
`[0030] A cyclic period (CP) is used in an OFDMA system
`to avoid inter-timeslot interference. Thus, it functions as a
`guard period. The use of a CP, (that covers the length of
`”CUL), ensures that the UL receives signals from UEs which
`are aligned in time and keeps the orthogonality among them.
`
`[0031] According to the preferred embodiment, there are
`two options to realize a TA calculation at the UE 205.
`
`In one option, if the source and target cells/Node-
`[0032]
`Bs 215 and 220 in the E-UTRAN 210 are not synchronized,
`(so far it is the assumption in LTE), the source cell/Node-B
`i signals the UE 205 the clock difference module by frame
`length, (i.e., (ti-ti)L), between the source cell/Node-B i and
`the target cell/Node-B j when the cell/Node-B i signals the
`UE to handover to the target cell/Node-B j. By knowing
`(tj)L, pj
`is solved. If the cells/Node-Bs 215 and 220 are
`synchronized, then (ti)L=(tJ-)L. The TA is solved as well.
`[0033]
`In another option,
`the UE 205 measures signal
`strength of the reference signals (pilots), synchronization
`channels (SCH) or other DL channels. Based on the mea-
`surement, the UE 205 determines its distance from the target
`cell/Node-B 220 in the E-UTRAN 210 and computes the
`propagation delay. However, usually it is known that dis-
`tance can not be accurately and reliably derived from signal
`strength or path loss measurement. Signal strength fluctuates
`with fading, which can be mitigated, (however, not elimi-
`nated), by collecting measurements over a long time inter-
`val.
`
`In order to calculate the TA adjustment, the UE
`[0034]
`must be signaled either the relative time difference between
`the source and target cells/node-Bs, or must be informed that
`the cells are synchronized.
`
`[0035] UE Autonomous TA Procedure in LTE Handover
`
`[0036] A UE autonomous TA procedure is initiated upon
`reception of a handover command from the E-UTRAN 210,
`or fast cell selection coordinated between the UE 205 and
`
`the source and target cells/Node-Bs 215 and 220. The UE
`205 detects the time difference in reception of the reference
`signal from beacon channels of the source and target cells/
`Node-Bs 215 and 220. The time offset is added to the last TA
`
`value in the source cell/Node-B 215 upon handover to the
`target cell/Node-B 220.
`
`[0037] Referring to FIG. 2, the UE 205 uses a reference
`signal from a beacon channel of the source cell/Node-B 215
`and a reference signal of a beacon channel of the target
`cell/Node-B 220 to infer the difference in range between the
`UE 205 and the source and target cells/Node-Bs 215 and
`220. The reference signals may be any type of signal with
`reference characteristics. The UE 205 is then able to autono-
`
`mously determine the amount of TA to apply to the target
`cell/Node-B 220 upon handover by adjusting the source cell
`TA by the relative difference between the source and target
`cell reference signals. The beacon channel may be a broad-
`cast channel, a synchronization channel (SCH), and the like.
`
`FIG. 3 is a flow diagram of a UE autonomous TA
`[0038]
`LTE handover procedure 300 implemented in the system
`200 of FIG. 2 in accordance with the present invention. In
`step 305, TA for the UE 205 is enabled and performed in the
`source cell/Node-B 215 of the E-UTRAN 210. This is
`
`
`
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`enabled by RRC signaling from the network (E-UTRAN)
`side. In step 310, the E-UTRAN measures and calculates the
`TA value and signals the TA value to the UE 205. In step
`315, the UE 205 applies the TA value of step 310 when
`transmitting to the source cell/Node-B 215. By using this TA
`value, the UE 205 is able to adjust its UL transmission
`timing. In step 320, the E-UTRAN 210 determines when it
`is time to perform a handover from the source cell/Node-B
`215 to the target cell/Node-B 220. When the E-UTRAN 210
`determines that a handover is to be performed in step 320,
`the source cell/Node-B 215 of the E-UTRAN 210 sends a
`
`handover command message 225, (i.e., RRC signaling), to
`the UE 205 to initiate handover of the UE 205 (step 325).
`The handover command message 225 includes an indication
`of the relative time difference between the source and target
`cells or an indication that the cells are synchronized, and
`may include pre-allocated UL radio resource information
`which is used to establish initial transmission 230 to the
`
`target cell/Node-B 220. The autonomous TA procedure can
`be explicitly or implicitly inferred from the handover com-
`mand message 225. The handover command message
`enables an initial transmission 230 from the UE 205 to the
`
`target cell/Node-B 220 to occur during handover, either
`through the use of pre-allocated UL radio resources from the
`target cell/Node-B 220 or through the use of a synchronous
`RACH. When the initial
`transmission 230 to the target
`cell/Node-B 220 uses pre-allocated UL radio resources,
`information regarding the pre-allocated UL radio resources
`is contained inside the handover command message 225.
`This RRC signaling may also indicate that a different
`non-UE autonomous TA measurement approach should be
`used during handover. In this case, the RRC signaling must
`also explicitly or implicitly specify if no UE autonomous TA
`adjustment process is required.
`[0039]
`Still referring to FIGS. 2 and 3, in step 330, the UE
`205 performs one or more measurements to determine the
`difference in propagation delays between the source cell/
`Node-B 215 and the target cell/Node-B 220 based on
`reference signals transmitted on beacon channels of the
`source cell/Node-B 215 and the target cell/Node-B 220. In
`step 335, the UE 205 autonomously computes a new TA
`value based on the current source cell TA value, the mea-
`surements performed in step 330, and knowledge of the
`relative time difference between the source cell/Node-B 215
`
`and the target cell/Node-B 220 or knowledge that the source
`cell/Node-B 215 and the target cell/Node-B 220 are syn-
`chronized, (i.e., there is no significant relative timing dif-
`ference between the source cell/Node-B 215 and the target
`cell/Node-B 220). In step 340, the UE applies the new TA
`value to adjust the UL transmission timing when sending an
`initial transmission 230 to the target cell/Node-B 220 using
`either pre-allocated uplink non-contention based radio
`resources or a synchronous RACH, as directed by the
`handover command message 225.
`[0040] There are two options to use pre-allocated UL radio
`resource information to access the target cell/Node-B 220
`during handover. One option for the UE 205 is to use the
`pre-allocated UL radio resource by sending a resource
`request message and/or traffic data to the target cell/Node-B
`220. In this case, the target cell/Node-B 220 must respond to
`the UE 205 with the newly allocated radio resource and if
`necessary a refined TA value for supporting its subsequent
`data transmission 230 to the target cell/Node-B 220. The
`other option is to use the pre-allocated UL radio resource
`
`included in the handover command message for direct data
`transmission. For the above two options, the amount of the
`pre-allocated radio resource will be different for different
`purposes that is to be used during handover. The selected
`option is signaled from the E-UTRAN 210 to the UE 205
`inside the DL RRC signaling, during call setup, or inside the
`handover command message 225 as described above. In
`doing so, the adjustment of UL transmission timing syn-
`chronization to the target cell/Node-B 220 may be achieved
`immediately after the handover, without requiring an asyn-
`chronous RACH access procedure.
`[0041] Optionally, in the case absolute TA values are used,
`it is necessary for the UE 205 to report the autonomously
`computed TA value to the target cell/Node-B 220 when
`sending the initial transmission 230 to the target cell/Node-B
`220. The UE 205 is not required to inform the target
`cell/Node-B 220 exactly what the new TA value is in the
`case relative TA value signaling is applied.
`[0042]
`Synchronous RACH Access Procedure During
`LTE Handover
`
`FIG. 4 is a flow diagram of a synchronous RACH
`[0043]
`access LTE handover procedure 400 in accordance with
`another embodiment of the present invention. After the UE
`autonomously computes the timing advance value (step
`405), the UE sends a scheduling, (i.e., resource), request
`message through a synchronous RACH channel
`to the
`E-UTRAN 210 with the computed TA value applied (step
`410). In step 415, the E-UTRAN 210 computes a refined,
`(i.e., more accurate), TA value based on information in the
`scheduling request message received from the UE 205. If
`necessary, the the E-UTRAN 210 sends the refined TA value
`to the UE 205 in a DL signaling message, and assigns UL
`and/or DL radio resources for the UE 205 for subsequent
`data transmissions (step 420). In step 425,
`the UE 205
`initiates data transmission by using the refined TA value and
`the assigned UL/DL radio resources.
`[0044] Although the features and elements of the present
`invention are described in the preferred embodiments in
`particular combinations, each feature or element can be used
`alone without the other features and elements of the pre-
`ferred embodiments or in various combinations with or
`
`without other features and elements of the present invention.
`The methods or flow charts provided in the present invention
`may be implemented in a computer program, software, or
`firmware tangibly embodied in a computer-readable storage
`medium for execution by a general purpose computer or a
`processor. Examples of computer-readable storage mediums
`include a read only memory (ROM), a random access
`memory (RAM), a register, cache memory, semiconductor
`memory devices, magnetic media such as internal hard disks
`and removable disks, magneto-optical media, and optical
`media such as CD-ROM disks, and digital versatile disks
`(DVDs).
`Suitable processors include, by way of example, a
`[0045]
`general purpose processor, a special purpose processor, a
`conventional processor, a digital signal processor (DSP), a
`plurality of microprocessors, one or more microprocessors
`in association with a DSP core, a controller, a microcontrol-
`ler, Application Specific Integrated Circuits (ASICs), Field
`Programmable Gate Arrays (FPGAs) circuits, any other type
`of integrated circuit (IC), and/or a state machine.
`[0046] A processor in association with software may be
`used to implement a radio frequency transceiver for use in
`
`
`
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`Jun. 28, 2007
`
`a wireless transmit receive unit (WTRU), user equipment
`(UE), a terminal, a base station, a radio network controller,
`or any host computer. The WTRU may be used in conjunc-
`tion with modules, implemented in hardware and/or soft-
`ware, such as a camera, a Video camera module, a Video-
`phone, a speakerphone, a V