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`3G Evoluti“
`HSPA an LTE for
`Mobile Broadband
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`Egiik Dahlman
` Stefan Parkvall
`an Skéld
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`SAM SU NG 1036-0001
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`SAMSUNG 1036-0001
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`SAM SU NG 1036-0002
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`
`3G Evolution
`
`HSPA and LTE for Mobile Broadband
`
`Erik Dahlman, Stefan Parkvall, Iohan Skiild and Per Beming
`
`
`
`AMSTERDAM - BOSTON - HEIDELE-ERG - LONDON 0 NEW YORK I OXFORD
`PARIS - SAN DIEGO - SAN FRANCISCO - SINGAPORE ' SYDNEY * TOKYO
`Academic Press is an imprint ofE].scv1'¢r
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`SAM SU NG 1036-0003
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`SAMSUNG 1036-0003
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`Academic Press is an imprint ofElsevier
`84 Theobald’: Road, London WCIX SRR, UK
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`
`First edition 200?
`Reprinted 200? (twice), 2008
`
`Copyright 9 2007, Erik Dahlmsn, Stefan Parltvall, Johan Skold and Per Beming.
`Published by Elsevier Ltd. All rights reserved
`
`The right of Erik Dahlman. Stefan Parkvall, Johan Skold and Pet Beming to be
`identified as the authors of this work has been asserted in accordance with the
`Copyright, Designs and Patents Act 1988
`
`No part of this publication may be reproduced stored in a retrieval system
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`verification cfdiagnoses and drug dosages should be made
`
`British Library Cataloguing in Publication Data
`3G evolution: HSPA and LTE for mobile broadband
`2. Mobile
`l. Broadband Communication systems — Standards
`communication systems - standards
`3. Cellular telephone
`syseems — Standards
`L Dahlrnari. Erik
`62 l.3'3456
`
`Library of Congress Catalog Number: 2007925578
`ISBN: 91-'8-0-I2-339.533-2
`
`For information on all Academic Press publications
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`visit our website at books.elsev1'er.com
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`SAM SU NG 1036-0004
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`
`17
`LTE access procedures
`
`The previous chapters have described the LTE uplink and downlink transmission
`schemes. However, prior to transmission of data, the mobile terminal needs to
`connect to the network. In this chapter, procedures necessary for a terminal to be
`able to access an LTE-based network will be described.
`
`17.1 Cell search
`
`Celt search is the procedure by which the terminal finds a cell for potential con-
`nection to. As part of the cell-search procedure, the terminal obtains the identity
`of the cell and estimates the frame timing of the identified cell. Furthermore, the
`cell-search procedure also provides estimates of parameters essential for recep-
`tion of system information on the broadcast channel, containing the remaining
`parameters required for accessing the system.
`
`To avoid complicated cell planning, the number of physical layer cell identities
`should be sufficiently large. As mentioned in Chapter 16, LTE supports 510 dif-
`ferent cell identities, divided into 170 cell-identity groups of three identities each.
`
`In order to reduce the cell-search complexity, cell search for LTE is typically done
`in several steps, similarly to the three-step cell-search procedure of WCDMA.
`To assist the terminal in this procedure, LTE provides a primary synchronization
`signal and a secondary synchronization signal on the downlink. The primary and
`secondary synchronization signals are specific sequences, inserted into the last
`two OFDM symbols in the first slot of subframe zero and five as illustrated in
`Figure 17.1. In addition to the synchronization signals, the cell-Search procedure
`may also exploit the reference signals as part of its operation.
`
`17.1. 1 Cell-Search procedure
`
`In the first step of the cell-search procedure, the mobile terminal uses the primary
`synchronization signal to find the timing on a 5 ms basis. Note that the primary
`‘synchronization signal is transmitted twice in each frame. One reason is to simplify
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`10i'|'|5 radio {M1118
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`Figure 17.1 Primary ondsecondory synchronization signals (normal cyclicprefix length assumed).
`
`handover from other radio-access technologies such as GSM to LTE. Thus, the
`primary synchronization signal can only provide the frame timing with a 5 ms
`ambiguity.
`
`The implementation of the estimation algorithm is vendor specific, but one pos-
`sibility is to do matched filtering between the received signal and the sequences
`specified for the primary synchronization signal. When the output of the matched
`filter reaches its maximum, the terminal is likely to have found timing on a 5 ms
`basis. The first step can also be used to lock the mobileterminal local-oscillator
`frequency to the base-station carrier frequency. Locking the local-oscillator fre-
`quency to the base—station frequency relaxes the accuracy requirements on the
`mobile—terminal oscillator, with reduced cost as a consequence.
`
`
`
`For reasons discussed below, three different sequences can be used as the primary
`synchronization signal. There is a one-to-one mapping between each of these three '
`sequences and the cell identity within the cell-identity group. Therefore, after
`the first step, the terminal has found the identity within the cell-identity group.
`Furthermore, as there is a one-to-one mapping between each of the identities in a
`cell-identity group and each of the three orthogonal sequence used when creating
`the reference signal as described in Chapter 16, the terminal also obtains partial
`knowledge about the reference signal structure in this step. The cell identity group,
`however, remains unknown to the terminal after this step.
`
`In the next step, the terminal detects the cell-identity group and determines the
`frame timing. This is done by observing pairs of slots where the secondary
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`synchronization signal is transmitted. Basically, if (s1, 52) is an allowable pair
`of sequences, where s1 and S2 represent the secondary synchronization signal in
`subframe zero and five, respectively, the reverse pair (.92, 31) is not a valid sequence
`pair. By exploiting this property, the terminal can resolve the 5 ms timing ambi-
`guity resulting from the first step in the cell-search procedure and determine the
`frame tinting. Furthermore, as each combination (s1 , 32) represents one of the cell-
`identity groups, also the cell identity group is obtained from the second cell-search
`step. From the cell identity group, the terminal also obtains knowledge about which
`pseudo-random sequence is used for generating the reference signal in the cell.
`
`the terminal receive the broad-
`I Once the cell-search procedure is complete,
`casted system information to obtain the remaining parameters, for example, the
`transmission bandwidth used in the cell.
`
`17.1.2 Time/frequency structure of synchronization signals
`The general time—frequency structure has already been briefly described above
`and is illustrated in Figure 17.1. As seen in the figure, the primary and secondary
`synchronization signals are transmitted in two subsequent OFDM symbols. This
`structure has been chosen to allow for coherent processing of the secondary syn-
`chronization signal at the terminal. After the first step, the primary synchronization
`signal is lcnown and can thus be used for channel estimation. This channel estimate
`can subsequently be used for coherent processing of the received signal prior to
`the second step in order to improve performance. However, the placement of the
`primary and secondary synchronization signals next to each other also implies that
`the terminal in the second step needs to blindly estimate the cyclic-prefix length.
`This, however, is a low-complexity operation.
`
`In many cases, the timing in multiple cells is synchronized such that the frame
`start in neighboring cells coincides in time. One reason hereof is to enable MBSFN
`operation. However, the synchronous operation also implies that transmission of
`the primary synchronization signals in different cells occur at the same time. Chan-
`nel estimation based on the primary synchronization signal will therefore reflect
`the composite channel from all cells if the same primary synchronization signal is
`used in all cells. Obviously, for coherent demodulation of the second synchroniza-
`tion signal, which is different in different cells, an estimate of the channel from the
`cell of interest is required, not an estimate of the composite channel from all cells.
`Therefore, LTE supports multiple sequences for the primary synchronization sig-
`nal. In case of coherent reception in a deployment with time-synchronized cells,
`neighboring cells can use different primary synchronization sequences to alleviate
`the channel-estimation problem describe above. Furthermore, as described above,
`theprimary synchronization signal also carries part of the cell identity.
`
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`Synchronization signal.
`time-domain representation
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`7 _.
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`_ _
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`Synchronization signal.
`frequency-domain representation
`
`illlillllllillIJ|||||||I|Ii|||||l||| iillllllillllli|||||lI||||iI|||||i|I
`-
`aosubcarriere
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`aasubcarriare
`-
`DC
`
`Figure 17.2 Generation ofthe synchronization signal in the frequency domain.
`
`From a TDD perspective, locating the synchronization signal at the end of the
`first slot in the subframe, instead of the second slot, is beneficial as it implies
`fewer restrictions on the creation of guard times between uplink and downlink.
`Alternatively, if the synchronization signals were located in the last slot of the
`subframe, there would be no possibility to obtain the guard time required for TDD
`by removing downlink OFDM symbols as discussed in Chapter 16. Also, note
`that, for TDD operation, the location of the synchronization signals implies that
`subfrarne zero and five always are downlink subframes.
`
`At the beginning of the cell-search procedure, the cell bandwidth is not neces-
`sarily known. In principle, detection of the transmission bandwidth could have
`been made part of the cell-search procedure. However, as this would complicate
`the overall cel1—scarch procedure, it is preferable to maintain the same cell—search
`procedure, regardless of the overall cell transmission bandwidth. The terminal
`can then be informed about the actual bandwidth in the cell from the broadcast
`
`channel. Therefore, to maintain the same frequency-domain structure of the syn»
`chronization signals, regardless of the cell system bandwidth, the synchronization
`signals are always transmitted using the 72 center subcarriers, corresponding to a
`bandwidth in the order of 1 MHz. Figure 17.2 illustrates a possible implementation
`for generation of the synchronization signals. Thirty-six subcarriers on each side
`of the DC subcarrier in the frequency domain are reserved for the synchronization
`signal. By using an IFFT, the corresponding time-domain signal can be generated.
`The size of the IFFT, as well as the number of subcarriers set to zero in Fig-
`ure 17.2, depends on the system bandwidth. Subcarriers not used for transmission
`of synchronization signals can be used for data transmission.
`
`17.1.3 initial and neighbor-cell Search
`
`Finding a cell to connect to after power up of the terminal is obviously an important
`case. However, equally important is the possibility to identify candidate cells for
`handover as part of the mobility support, when the terminal connection is moved
`from one cell to another. These two situations are usually referred to as initial celi
`Search and neighbor-cell search, respectively.
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`For initial cell search, the terminal does typically not know the carrier frequency
`of the cells it is searching for. To handle this case, the terminal needs to search for
`a suitable carrier frequency as well, basically by repeating the above procedure for
`any possible carrier frequency given by the frequency raster. Obviously, this may
`often increase the time required for cell search, but the search-time requirements for
`initial cell search are typically relatively relaxed. Implementation—specific methods
`can alsobeused to reduce the time from power-on until acell is found. For example,
`the terminal can use any additional information the terminal may have and start
`searching on the same carrier frequency it last was connected to.
`
`Neighbor-cell search, on the other hand, has stricter timing requirements. The
`slower the neighbor-cell search is, the longer it will take until the terminal is
`handed over to a cell with an in average better radio quality. This will obviously
`deteriorate the overall spectrum efficiency of the system. However, in the common
`case of intra-frequency handover, the terminal obviously does not need to search
`for the carrier frequency in the neighboring cell. Apart from omitting the search
`over multiple carrier frequencies, intra—frequency neighbor~cell search can use the
`same procedures as the initial cell search.
`
`Measurements for handover purposes are required also when the terminal is receiv-
`ing downlink data from the network. Hence, the terminal must be able to perform
`neighbor-cell search also in these cases. For intra-frequency neighbor-cell search,
`this is not a major problem as the neighboring candidate cells transmit at the
`same frequency as the terminal already is receiving data upon. Data reception
`and neighbor—cell search are simple separate baseband functions, operating on the
`same received signal.
`
`The case of inter-frequency handover, however, is more complicated since data
`reception and neighbor-cell search need to be carried out at different frequencies.
`Equipping the terminal with a separate RF receiver circuitry for neighbor-cell
`search, although in principle possible, is not attractive from a complexity per-
`spective. Therefore, gaps in the data transmission, during which the terminal can
`retune to a different frequency for inter-frequency measurement purposes, can be
`created. This is done in the same way as for HSPA, namely by avoiding scheduling
`the terminal in one or several downlink subframes.
`
`17.2 Random access
`
`A fundamental requirement for any cellular system is the possibility for the
`terminal to request a connection setup. This is commonly known as random
`access and serves two main purposes in LTE, namely establishment of uplinlc
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`Synchronize to
`downlink timing
`(from cell search}
`
`Adiust uplink
`timing
`
`Step 3: FIFIC signaling
`
`
`
`Only if UE is not known in Noc|eB
`(initial random access)
`
`TfffffIIfffff:ffffffffffffffffj__
`
`Figure 17.3 Overview oftke random access procedure.
`
`synchronization, and establishment of a unique terminal identity, the C-RNTI,
`known to both the network and the terminal. Thus, random access is used not only
`for initial access, that is, when moving from LTE_DETACIr{l-ED or LTE_IDLE
`to LTE_ACTIVE (see Chapter 15 for a discussion of different terminal states),
`but also after periods of uplink inactivity when uplink synchronization is lost in
`LTE_ACTIVE.
`
`The overall random-access procedure,
`four steps:
`
`illustrated in Figure 17.3, consists of
`
`1. ‘The first step consists of transmission of a random-access preamble, allowing
`the eNodeB to estimate the transmission timing of the tenninal. Uplink synchro-
`nization is necessary as the terminal otherwise cannot transmit any uplink data.
`2. The second step consists of the network transmitting a timing advance com-
`mand to adjust the terminal transmit timing, based on the timing measurement
`in the first step. In addition to establishing uplink synchronization, the second
`step also assigns uplink resources to the terminal to be used in the third step in
`the random access procedure.
`3. The third step consists of transmission of the mobile-terminal identity to the
`network using the UL-SCH similar to normal scheduled data. The exact content
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`of this signaling depends on the state of the terminal, in particular whether it is
`previously known to the network or not.
`4. The fourth and final step consists of transmission of a contention-resolution
`message from the network to the terminal on the DL-SCH. This step also
`resolves any contention due to multiple terminals trying to access the system
`using the same random-access resource.
`
`Only the first step uses physical-layer processing specifically designed for random
`access. The last three steps all utilizes the same physical-layer processing as used
`for nonnal uplink and downlink data transmission. In the following, each of these
`steps are described in more detail.
`
`17.2.1 Step 1: Ftandom access preamble transmission
`
`The first step in the random access procedure is the transmission of a random-
`cess preamble. The main purpose of the preamble is to indicate to the network
`e presence of a random-access attempt and to obtain uplink time synchronization
`ithin a fraction of the uplink cyclic prefix.
`
`n general, random-access-preamble transmissions can be either orthogonal or
`non-orthogonal to user data. In WCDMA, the preamble is non-orthogonal to the
`uplink data transmission. This provides the benefit of not having to semi-statically
`allocate any resources for random access. However,
`to control
`the random-
`access-to-data interference, the transmit power of the random-access preamble
`must be carefully controlled. In WCDMA, this is solved through the use of a
`power-ramping procedure, where the terminal gradually increases the power of
`the random-access preamble until it is successfully detected at the base station.
`Although this is a suitable solution to the interference problem, the ramping proce-
`dure introduces a delay in the overall random-access procedure. Therefore, from
`a delay perspective, a random-access procedure not requiring power ramping is
`beneficial.
`
`
`
`In LTE, the transmission of the random—access preamble can be made orthogonal
`to uplink user-data transmissions and, as a consequence, no power ramping is
`necessary (although the specifications allow for ramping). Orthogonality between
`user data transmitted from other terminals and random-access attempts is obtained
`in both the time and frequency domains. The network broadcasts information to all
`terminals in which time-frequency resources random-access preamble transmis-
`sion is allowed. To avoid interference between data and random-access preambles,
`the network avoids scheduling any uplink transmissions in those time-frequency
`resources. This is illustrated in Figure 17.4. Since the fundamental time unit
`for data transmission in LTE is 1 ms,
`at subframe is reserved for preamble
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`Uplink resource reserved for
`random access preamble
`transmission
`
`Uplink resources
`used for data
`transmission
`
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`10 I113 frame
`
`Figure 17.4 Principal illustration of random-access-preamble transmission.
`
`transmissions. Within the reserved resources,
`transmitted.
`
`the random-access preamble is
`
`In the frequency domain. the random-access preamble has a bandwidth corre-
`sponding to six resource blocks (1.08 MHz). This nicely matches the smallest
`bandwidth in which LTE can operate, which is six resource blocks as discussed
`in Chapter 16. Hence, the same random-access preamble structure can be used,
`regardless of the transmission bandwidth in the cell. For deployments using larger
`spectrum allocations, multiple random-access resources can be defined in the
`frequency domain, providing an increased random-access capacity.
`
`A terminal carrying out a random-access attempt has, prior to the transmission of
`the preamble, obtained downlink synchronization from the cell-search procedure.
`However, the uplink timing is, as already discussed, not yet established. The start
`of an uplink frame at the terminal is defined relative to the start of the downlink
`frame at the terminal. Due to the propagation delay between the base station and the
`terminal, the uplink transmission will therefore be delayed relative to the downlink
`transmission tinting at the base station. Therefore, as the distance between the
`base station and the terminal is not known, there will be an uncertainty in the
`uplink timing corresponding to twice the distance between the base station an
`the terminal, amounting to 6.? ptsfkm. To account for this uncertainty and to avoi
`interference with subsequent subframes not used for random access, a guard tim
`is used, that is the length of the actual preamble is shorter than 1 ms. Figure 11.5
`illustrates the preamble length and the guard time. With the LTE preamble length
`of approximately 0.9 ms, there is 0.1 ms guard time allowing for cell sizes up to
`15 km. In larger cells, where the timing uncertainty may be larger than the basic
`guard time, additional guard time can be created by not scheduling any uplin
`transmissions in the subframe following the random-access resource.
`
`The preamble is based on Zadoff-Chu (ZC), sequences [131] and cyclic shifted
`sequences thereof. Zadoff-Chu sequences are also used for creating the uplink
`reference signals as described in Chapter 16, where the structure of these sequences
`
`
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`0.9 ms preamble
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`time
`
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`1 me random access eubframe
`
`E
`
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`
`Mediumdistance user 0ll1;l:lsB
`
`Far User
`
`Figure 17.5 Preamble timing at eNodeBfor drfiizrenr mndom-access users.
`
`(Not used. provides guard time}
`
`
`
`Figure 17.6 Random-access-preamble generation.
`
`is described. From each root Zadoff-Chu sequence X§'C)(k), m—1 cyclically
`shifted sequences are obtained by cyclic shifts of [Mm/mj each, where MZC
`is the length of the root Zadoff—Chu sequence.
`
`Cyclically shifted ZC sequences possess several attractive properties. The ampli-
`tude of ‘the sequences is constant, which ensures efficient power amplifier
`utilization and maintains the low PAR properties of the single-carrier uplink. The _
`sequences also have ideal cyclic auto~correlation, which is important for obtain-
`ing an accurate timing estimation at the eN0deB. Finally, the cross~correlation
`between different preambles based on cyclic shifts of the same ZC sequence is
`zero at the receiver as long as the time cyclic shift LN/in] used when generating the
`preambles is larger than the maximum round-trip propagation time plus the maxi-
`mum delay spread of the channel. Therefore, thanks to the ideal cross-correlation
`property. there is no intra-cell interference from multiple random-access attempts
`using preambles derived from the same Zadoff—Chu root sequence.
`
`The generation of the random-access preamble is illustrated in Figure 17.6.
`Although the figure illustrates generation in the time-domain, frequency-domain
`generation can equally well be used in an implementation. Also,
`to allow for
`frequency-domain processing at the base station (discussed further below), a cyclic
`prefix is included in the preamble generation.
`
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`1ms subfrarne
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`
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`UE far from NOCIBB
`
`UEcIosetoNodeB
`
`Frequancy-domain representation
`of Zadofl-Chu root sequence
`
`Sequence idetected. delay estimate 1-
`
`Figure 17.? Random-access-preamble detection in the frequency domain.
`
`Preamble sequences are partitioned into groups of 64 sequences each. As part of
`the system configuration, each cell is allocated one such group by defining one or
`several root Z.adoff—Chu sequences and the cyclic shifts required to generate the
`set of preambles. The number of groups is sufficiently large to avoid the need for
`careful sequence planning between cells.
`
`When performing a random-access attempt, the terminal selects one sequence at
`random from the set of sequences allocated to the cell the terminal is trying to
`access. As long as no other terminal is performing a random-access attempt using
`the same sequence at the same time instant, no collisions will occur and the attempt
`will, with a high likelihood, be detected by the network.
`
`The base-station processing is implementation specific, but thanks to the cyclic
`prefix included in the preamble, low-complexity frequency-domain processing is
`possible. An example hereof is shown in Figure 11?. Samples over a window are
`collected and convened it into the frequency-domain representation using an FFI‘.
`The window length is 0.8 ms, which is equal to the length of the ZC sequence
`without a cyclic prefix. This allows to handle timing uncertainties up to 0.1 ms
`and matches the guard time defined.
`
`The output of the FFT, representing the received signal in the frequency domain,
`is multiplied with the complex-conjugate frequency-domain representation of the
`root Zacloff—Chu sequence and the results is fed through an IFFI‘. By observing
`the IFPT outputs, it is possible to detect which of the shifts of the Zadoff—Chu root
`sequence has been transmitted and its delay. Basically, a peak of the IFFT output in
`
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`interval i corresponds to the E-th cyclically shifted sequence and the delay is given
`by the position of the peak within the interval. This frequency-domain implementa-
`tion is computationally efficient and allows detection of multiple random-access
`attempts using different cyclic shifted sequences generated from the same root
`Zadoff—Chu sequence; in case of multiple attempts there will simply be a peak in
`each of the corresponding intervals.
`
`17.2.2 Step 2: Random access response
`
`In response to the detected random access attempt, the network will, as the sec-
`ond step of the random-access procedure, transmit a message on the DL-SCH,
`containing:
`
`- The index of the random-access preamble sequence the network detected and
`for which the response is valid.
`0 The timing correction calculated by the random-access-preamble receiver.
`o A scheduling grant, indicating resources the terminal shall use for the trans-
`mission of the message in the third step.
`0 A temporary identity used for further communication between the terminal and
`the network.
`
`In case the network detected multiple random-access attempts (from different
`terminals), the individual response messages of multiple mobile terminals can be
`combined in a single transmission. Therefore, the response message is scheduled
`on the DL-SCH and indicated on a L1/L2 control channel using an identity reserved
`for random-access response. All terminals which have transmitted a preamble
`monitors the LUL2 control channels for random-access response. The timing of _
`the response message is not fixed in the specification in order to be able to respond
`to sufficiently many simultaneous accesses. It also provides some flexibility in the
`base-station implementation.
`
`As long as the terminals that performed random access in the same resource used
`different preambles, no collision will occur and from the downlink signaling it is
`clear to which terminal(s) the information is related. However, there is a certain
`
`probability of contention, that is multiple terminals using the same random access
`preamble at the same time. In this case, multiple terminals will react upon the same
`downlink response message and a collision occurs. Resolving these collisions
`is part of the subsequent steps as discussed below. Contention is also one of
`the reasons why hybrid ARQ is not used for transmission of the random-access
`response. A terminal receiving a random-access response intended for another
`terminal will have incorrect uplink timing. If hybrid ARQ would be used, the
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`timing of the ACKINAK for such a terminal would be incorrect and may disturb
`uplink control signaling from other users.
`
`Upon reception of the random-access response in the second step, the terminal
`will adjust its uplink transmission timing and continue to the third step.
`
`17.2.3 Step 3: Terminal! identification
`
`After the second step, the uplink of the terminal is time synchronized. However,
`before user data can be transmitted toifrom the terminal, a unique identity within
`the cell (C-RNTI) must be assigned to the terminal. Depending on the terminal
`state, there may also be a need for additional message exchange.
`
`In the third step, the terminal transmits the neoes sary messages to the network using
`the resources assigned in the random-access response in the second step. Trans-
`mitting the uplink message in the same manner as scheduled uplink data instead of
`attaching it to the preamble in the first step is beneficial for several reasons. Firstly,
`the amount of information transmitted in absence of uplink synchronization should
`be minimized as the need for a large guard time makes such transmissions rela-
`tively costly. Secondly, the use of the ‘normal’ uplink transmission scheme for
`message transmission allows the grant size and modulation scheme to be adjusted
`to, for example, different radio conditions. Finally, it allows for hybrid ARQ with
`soft combining for the uplink message. The latter is an important aspect, especially
`in coverage-limited scenarios, as it allows for the use of one or several retransmis-
`sions to collect sufficient energy for the uplink signaling to ensure a sufficiently
`high probability of successful transmission. Note that RLC retransmissions are
`not used for the uplink RRC signaling in step 3.
`
`An important part ofthe uplink message is the inclusion ofaterminal identity as this
`identity is used as part of the contention-resolution mechanism in the fourth step.
`In case the terminal is in LTE_ACTlVE state, that is, is connected to a known cell
`and therefore has a C-RN'I'I assigned, this C-RNTI is used as the terminal identity
`in the uplink message. Otherwise, a core-network terminal identifier is used and
`the radio-access network needs to involve the core network prior to responding to
`
`the uplink message in step 3.
`
`17.2.4 Step 4: Contention nesoiution
`
`The last step in the random access procedure consists of a downlink message for
`contention resolution. Note that, from the second step, multiple terminals per-
`
`forming simultaneous random-access attempts using the same preamble sequence
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`in the first step listen to the same response message in the second step and there-
`fore have the same temporary identifier. Hence, in the fourth step, each terminal
`receiving the downlink message will compare the identity in the message with
`the identity they transmitted in the third step. Only a terminal which observes a
`match between the identity received in the fourth step and the identity transmitted
`as part of the third step will declare the random access procedure successful. If
`the terminal has not yet been assigned a C-RNTI, the temporary identity from the -
`second step is promoted to the C-RNTI; otherwise the terminal keeps its already
`assigned C-RNTI.
`
`The contention-resolution message is transmitted on the DL-SCH, using the tem-
`porary identity from the second step for addressing the terminal on the L1/L2
`control channel. Since nplink synchronization already has been established, hybrid
`ARQ is applied to the downlink signaling in this step. Terminals with a match
`between the identity they transmitted in the third step and the message received in
`the fourth step will also transmit a hybrid-ARQ acknowledge in the uplink.
`
`Termin