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PDCCH Processing

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PDCCH Processing A CRC is attached to each DCI message payload. The identity of the terminal (or terminals) addressed

the Radio-Network Temporary Identifier (RNTI) is included in the CRC calculation.

After CRC attachment, the bits are coded with a rate-1/3 tail-biting convolutional code and rate matched to fit the amount of resources used for PDCCH transmission.

Tail-biting convolutional coding is similar to conventional convolutional coding with the exception that no tail bits are used.

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PDCCH Processing The sequence of bits corresponding to all the PDCCH

resource elements to be transmitted in the subframe including the unused resource elements, is scrambled by a

cell- and subframe-specific scrambling sequence to randomize inter-cell interference, followed by QPSK modulation and mapping to resource elements.

This structure is based on Control-Channel Elements (CCEs), which in essence is a convenient name for a set of 36 useful resource elements (nine resource-element groups).

The number of CCEs, one, two, four, or eight, required depends on the payload size of the control information (DCI payload) and the channel-coding rate.

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PDCCH Processing The number of CCEs depends on

the size of the control region the cell bandwidth the number of downlink antenna ports the amount of resources occupied by PHICH

The size of the control region can vary dynamically from subframe to subframe as indicated by the PCFICH.

A specific PDCCH can be identified by the numbers of the corresponding CCEs in the control region.

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PDCCH Processing As the number of CCEs may vary and is not signaled, the

terminal has to blindly determine the number of CCEs used for the PDCCH it is addressed upon.

The sequence of CCEs should match the amount of resources available in a given subframe - the number of CCEs varies according to the value transmitted on the PCFICH

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Blind Decoding of PDCCHs Each PDCCH supports multiple formats and the

format used is a priori unknown to the terminal. The terminal needs to blindly detect the format of the

PDCCHs.

In each subframe, the terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces.

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Blind Decoding of PDCCHs It is required to have mechanisms to limit the CCE

aggregations that the terminal is supposed to monitor.

From a scheduling point of view: these restrictions in the allowed CCE aggregations are undesirable as they may influence the scheduling flexibility and require additional processing at the transmitter side.

From a terminal-complexity point of view: the terminal to monitor all possible CCE aggregations, also for the larger cell bandwidths, is not attractive.

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Blind Decoding of PDCCHs To impose as few restrictions as possible on the

scheduler while at the same time limit the maximum number of blind decoding attempts in the terminal.

LTE defines search spaces, which describe the set of CCEs the terminal is supposed to monitor for scheduling assignments/ grants relating to a certain component carrier.

A search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to decode.

For multiple aggregation levels, corresponding to one, two, four, and eight CCEs, a terminal has multiple search spaces.

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Blind Decoding of PDCCHs The terminals will attempt to decode all the PDCCHs

that can be formed from the CCEs in each of its search spaces.

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Example 1. Terminal A in cannot be addressed on a PDCCH

starting at CCE number 20, whereas terminal B can. 2. If terminal A is using CCEs 1623, terminal B

cannot be addressed on aggregation level 4 as all CCEs in its level-4 search space are blocked by the use for the other terminals. each terminal in the system has a terminal-specific search

space at each aggregation level.

3. The terminal-specific search spaces partially overlap between the two terminals in this subframe (CCEs 2431 on aggregation level 8) but, as the terminal-specific search space varies between subframes, the overlap in the next subframe is most likely different.

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Blind Decoding of PDCCHs LTE has also defined common search spaces.

all terminals in the cell monitor the CCEs for control information.

It is primarily transmission of various system

messages, it can be used to schedule individual terminals as well.

Also, it can be used to resolve situations where scheduling of one terminal is blocked due to lack of available resources in the terminal-specific search space.

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Blind Decoding of PDCCHs The common search spaces are only defined for

aggregation levels of four and eight CCEs and only for the smallest DCI formats, 0/1A/3/3A and 1C. (Table 10-4)

Without support for DCI formats with spatial multiplexing in the common search space.

As the main function of the common search space is

to handle scheduling of system information intended for multiple terminals, and such information must be receivable by all terminals in the cell.

scheduling is used the common search space.

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Blind Decoding of PDCCHs The downlink DCI formats to decode in the terminal-

specific search space depend on the transmission mode configured for the terminal.

The DCI monitoring is described for the case of a single component carrier.

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Blind Decoding of PDCCHs For multiple component carriers, the above

procedures are applied in principle to each of the activated downlink component carriers. (Ch 12)

There is one UE-specific search space per aggregation level and per (activated) component carrier upon which PDSCH can be received (or PUSCH transmitted), although there are some carrier-aggregation-specific modifications.

UE-specific search space per aggregation level and component carrier used for the PDSCH/PUSCH.

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Blind Decoding of PDCCHs Where PDSCH/PUSCH transmissions on component

carrier 1 are scheduled using PDCCHs transmitted on component carrier 1.

For component carrier 2, a carrier indicator is assumed in the UE-specific search space as component carrier 2 is cross-carrier scheduled from PDCCHs transmitted on component carrier 1.

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Blind Decoding of PDCCHs For component carriers 3 and 4, the terminal will

handle the two search spaces independently, assuming (in this example) a carrier indicator for component carrier 4 but not for component carrier 3.

If the UE-specific and common search spaces relating to different component carriers happen to overlap for some aggregation level when cross-carrier scheduling is configured, the terminal only needs to monitor the common search space.

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Physical Hybrid ARQ Indicator Channel (PHICH)

The PHICH is used to transmit the hybrid-ARQ acknowledgements in response to UL-SCH transmission.

The hybrid-ARQ acknowledgement (one single bit of information per transport block) is repeated three times, followed by BPSK modulation on either the I or the Q branch and spreading with a length-four orthogonal sequence.

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PHICH

Multiple PHICHs mapped to the same set of resource elements constitute a PHICH group,

where PHICHs within the same PHICH group are separated through different orthogonal sequences.

Sequence index Orthogonal sequence seqPHICHn Normal cyclic prefix

4PHICHSF =N

Extended cyclic prefix

2PHICHSF =N

0 [ ]1111 ++++ [ ]11 ++ 1 [ ]1111 ++ [ ]11 + 2 [ ]1111 ++ [ ]jj ++ 3 [ ]1111 ++ [ ]jj + 4 [ ]jjjj ++++ - 5 [ ]jjjj ++ - 6 [ ]jjjj ++ - 7 [ ]jjjj ++ -

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PHICH PHICH duration in MBSFN and non-MBSFN

subframes

Non-MBSFN subframes MBSFN subframes

PHICH duration Subframes 1 and 6 in case of

frame structure type 2

All other cases On a carrier supporting

both PDSCH and PMCH

Normal 1 1 1

Extended 2 3 2

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PHICH After forming the composite signal representing the PHICHs in a

group, cell-specific scrambling is applied and the 12 scrambled symbols are mapped to three resource-element groups.

separated by approximately one-third of the downlink cell bandwidth.

In the first OFDM symbol in the control region, resources are first allocated to the PCFICH, the PHICHs are mapped to resource elements not used by the PCFICH

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PHICH In LTE, each downlink subframe is normally divided

into a control region, and a data region, consisting of the remaining part of the subframe.

control region consisting of the first few OFDM symbols,

The control region carries L1/L2 signaling necessary to control uplink and downlink data transmissions.

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PCFICH The PCFICH indicates the size of the control region

in terms of the number of OFDM symbols that is, indirectly where in the subframe the data region starts.

If PCFICH is incorrectly decoded, neither know how to process the control channels nor where

the data region starts for the corresponding subframe.

Two bits of information (32-bit codeword), corresponds to the three control-region sizes of one, two, or three OFDM symbols.

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PCFICH The coded bits are scrambled with a cell- and

subframe-specific scrambling code to randomize inter-cell interference, QPSK modulated, and mapped to 16 resource elements.

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PCFICH The mapping of resource elements in the first OFDM

symbol in the subframe is done in groups of four resource elements (resource-element groups) and separated in different frequencies.

a symbol quadruplet consisting of four (QPSK) symbols is mapped.

Transmit diversity for four antenna ports is specified

in terms of groups of four symbols (resource elements).

To avoid collisions in neighboring cells, the location of the four groups in the frequency domain depends on the physical-layer cell identity.

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PCFICH In first OFDM symbol; there are two resource

element groups per resource block, as every third resource element is reserved for reference signals (or non-used resource elements corresponding to reference symbols on the other antenna port).

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PCFICH In second OFDM symbol; (if part of the control

region) there are two or three resource-element groups depending on the number of antenna ports configured.

In third OFDM symbol (if part of the control region) there are always three resource-element groups per resource block.

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PCFICH The four resource-element groups are separated by

one-fourth of the downlink cell bandwidth in the frequency domain, with the starting position given by physical-layer cell identity.

16 QPSK symbols are used for the transmission of four resource-element groups

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Common Control Physical Channel (CCPCH)

CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than maximum

data rate is the chief consideration. QPSK is therefore the only available modulation

format. CCPCH is transmitted as close to the center

frequency as possible. CCPCH is transmitted exclusively on the 72 active subcarriers (=6 PRBs) centered on the DC subcarrier.

Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol (0..5/6) within the slot and l refers to the subcarrier.

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Downlink L1/L2 Control Signaling Downlink control signaling supports the transmission

of downlink and uplink transport channels. the corresponding information partly originates from the

physical layer (Layer 1) and partly from Layer 2 MAC.

Downlink L1/L2 control signaling consists of downlink scheduling assignments, including

information required for the terminal to be able to properly receive,

demodulate, decode the DL-SCH on a component carrier, uplink

scheduling grants informing the terminal about the resources transport format to use for uplink (UL-SCH) transmission hybrid-ARQ acknowledgements in response to UL-SCH

transmissions.

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Downlink L1/L2 Control Signaling The downlink L1/L2 control signaling is transmitted

within the first part of each subframe.

Each subframe can be divided into a control region followed by a data region, where the control region corresponds to the part of the subframe in which the L1/L2control signaling is transmitted.

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Downlink L1/L2 Control Signaling The control signaling can be dynamically adjusted

radio resources to match the instantaneous traffic situation.

For example, a small number of users being scheduled in a subframe, the required amount of control signaling is small and a larger part of the subframe can be used for data transmission.

The downlink L1/L2 control signaling consists of four different physical-channel types:

PCFICH, informing the terminal about the size of the control region (one, two, or three OFDM symbols).

There is one and only one PCFICH on each component carrier or, equivalently, in each cell.

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Downlink L1/L2 Control Signaling PDCCH carries signal downlink scheduling

assignments and uplink scheduling grants. Each PDCCH carries signaling for a single terminal, but can

also be used to address a group of terminals. Multiple PDCCHs can exist in each cell.

PHICH, used to signal hybrid-ARQ acknowledgements in response to uplink UL-SCH transmissions.

Multiple PHICHs can exist in each cell.

R-PDCCH (Relay Physical Downlink Control Channel), used for relaying.

the R-PDCCH is not transmitted in the control region.

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Downlink Scheduling Assignment Downlink scheduling assignments are valid for the same

subframe in which they are transmitted. The scheduling assignments use one of the DCI formats 1, 1A,

1B, 1C, 1D, 2, 2A, 2B, or 2C and the DCI formats used depend on the transmission mode configured

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Uplink The principle advantage of SC-FDMA over

conventional OFDM is a lower PAPR (by approximately 2 dB) than would otherwise be possible using OFDM.

Data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality.

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Uplink Physical Channels

Physical Uplink Shared Channel (PUSCH) Resources for the PUSCH are allocated on a sub-frame basis by

the UL scheduler. Subcarriers are allocated in multiples of 12 (PRBs) and may be

hopped from sub-frame to sub-frame. The PUSCH may employ QPSK, 16QAM or 64QAM modulation. CRC insertion: 24 bit CRC

Physical Uplink Control Channel (PUCCH) It is never transmitted simultaneously with PUSCH data. For frame structure type 2, the PUCCH is not transmitted in the

UpPTS field. PUCCH conveys control information including channel quality

indication (CQI), ACK/NACK, HARQ and uplink scheduling requests (SR).

The PUCCH transmission is frequency hopped at the slot boundary

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PUSCH The baseband signal representing the physical uplink

shared channel is defined in terms of the following steps:

scrambling modulation of scrambled bits to generate complex-valued

symbols transform precoding to generate complex-valued symbols mapping of complex-valued symbols to resource elements generation of complex-valued time-domain SC-FDMA signal

for each antenna port

Scrambling Modulation mapperTransform precoder

Resource element mapper

SC-FDMA signal gen.

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PUCCH

The physical uplink control channel supports multiple formats.

Formats 2a and 2b are supported for normal cyclic prefix only.

PUCCH

format

Modulation

scheme

Number of bits per

subframe, bitM

1 N/A N/A

1a BPSK 1

1b QPSK 2

2 QPSK 20

2a QPSK+BPSK 21

2b QPSK+QPSK 22

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Uplink Physical Signals

Uplink physical signals are used within the PHY and do not convey information from higher layers.

Two types of UL physical signals are defined: Demodulation reference signal, associated with

transmission of PUSCH or PUCCH Sounding reference signal, not associated with

transmission of PUSCH or PUCCH

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Uplink Reference Signals

The demodulation signal facilitates coherent demodulation.

It is transmitted in the fourth SC-FDMA symbol of the slot and is the same size as the assigned resource.

There is also a sounding reference signal used to facilitate frequency dependent scheduling.

Both variants of the UL reference signal are based on Zadhoff-Chu sequences.

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Downlink Channel Coding

Different coding algorithms are employed for the DL physical channels.

The PDSCH uses up to 64-QAM modulation. Rate 1/3 turbo coding has been selected for the PDSCH.

For the common control channel (CCPCH), modulation is restricted to QPSK.

For control channels, coverage is the paramount requirement.

Convolutional coding has been selected for use with the CCPCH, though a final determination regarding code rate has not yet been made.

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Channel Coding and Interleaving

Channel coding scheme for transport blocks Turbo Coding with a coding rate of R=1/3, Two 8-state constituent encoders and A contention-free quadratic permutation

polynomial (QPP) turbo code internal interleaver. Maximum information block size of 6144 bits Error detection is supported by the use of 24-

bit CRC

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Channel Coding

TrCH Coding scheme Coding rate

UL-SCH

DL-SCH

PCH

MCH

Turbo coding 1/3

BCH

Tail biting

convolutional

coding

1/3

Usage of channel coding scheme and coding rate for TrCHs

Control Information Coding scheme Coding rate

DCI

Tail biting

convolutional

coding

1/3

CFI Block code 1/16

HI Repetition code 1/3

Block code variable

UCI Tail biting

convolutional

coding

1/3

Usage of channel coding scheme and coding rate for control information

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Channel Coding Channel coding for DL-SCH (as well as for PCH and

MCH) is based on Turbo coding. The encoding consists of two rate-1/2, eight-state

constituent encoders, implying an overall code rate of 1/3, in combination with QPP-based interleaving.

QPP : Quadrature Permutation Polynomial.

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Channel Coding The QPP interleaver provides a mapping from the input (non-

interleaved) bits to the output (interleaved) bits according to the function:

i is the index of the bit at the output of the interleaver, c(i) is the index of the same bit at the input of the interleaver, K is the code-block/interleaver size.

The values of the parameters f1 and f2 depend on the code-block size K.

The range of code-block sizes is from a minimum of 40 bits to a maximum of 6144 bits, together with the associated values for the parameters f1 and f2.

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Downlink Physical-Layer Processing (10) LTE downlink, there are four different types of

transport channels defined, the Downlink Shared Channel (DL-SCH), the Multicast Channel (MCH), the Paging Channel (PCH), the Broadcast Channel (BCH).

DL-SCH is mapping to the resource elements of the OFDM timefrequency grid.

DL-SCH is used for transmission user data and dedicated control information, as well as part of the downlink system information.

The DL-SCH physical-layer processing is to a large extent applicable also to MCH and PCH transport channels, although with some additional constraints.

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Processing Steps Within each Transmission Time Interval (TTI), corresponding to

one subframe of length 1 ms, up to two transport blocks of dynamic size are delivered to the physical layer and transmitted over the radio interface for each component carrier.

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Processing Steps In the case of no spatial multiplexing there is at most

a single transport block in a TTI. In the case of spatial multiplexing, with transmission

on multiple layers in parallel to the same terminal, there are two transport blocks within a TTI.

CRC Insertion Per Transport Block a 24-bit CRC is calculated for and appended to each

transport block.

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Code-Block Segmentation and Per-Code-Block CRC Insertion The LTE Turbo-coder internal interleaver is only

defined a maximum block size of 6144 bits. If the transport block, including the transport-block CRC,

exceeds this maximum code-block size, code-block segmentation, is applied before the Turbo coding.

Code-block segmentation implies that the transport

block is segmented into smaller code blocks. matching the set of code-block sizes supported by the Turbo

coder. the specification includes the possibility to insert dummy

filler bits at the head of the first code block.

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Code-Block Segmentation and Per-Code-Block CRC Insertion Transport block implies that an CRC is calculated for

and appended to rear. CRC is small for the transport block.

The code block also adds additional error-detection capabilities (using CRC) and thus further reduces the risk for undetected errors in the code block.

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Localized and Distributed Resource Mapping In some cases downlink channel-dependent

scheduling is not suitable to use or is not practically possible:

For low-rate services such as voice and the feedback signaling may lead to extensive relative overhead.

At high, it may be difficult to track the instantaneous channel conditions to the accuracy required for channel-dependent scheduling to be efficient.

LTE allows for such distributed resource-block

allocation by resource allocation types 0 and 1

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Distributed Resource-Block Allocation Drawbacks:

For types 0 and 1, the minimum size of the allocated resource can be as large as four resource-block pairs and may thus not be suitable when resource allocations of smaller sizes are needed.

Both these resource-allocation methods are associated with a relatively large PDCCH payload.

Resource-allocation type 2 always allows for the allocation of a single resource-block pair and is also associated with a relatively small PDCCH payload size.

Only allows for the allocation of resource blocks that are contiguous in the frequency domain.

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Virtual Resource Block (VRB) A Virtual Resource Block (VRB) has used in

distributed resource-block allocation In resource-allocation type 2 and in a single resource block

pair.

The key to distributed transmission then lies in the mapping from VRB pairs to Physical Resource Block (PRB) pairs that is, to the actual physical resource used for transmission.

Two types of VRBs: 1. Localized VRBs : there is a direct mapping from VRB pairs to

PRB pairs

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Distributed VRBs 2. Distributed VRBs : Consecutive VRBs are not mapped to PRBs that are

consecutive in the frequency domain; This provides frequency diversity between consecutive VRB

pairs. Even a single VRB pair is distributed in the frequency domain.

A. The spreading in the frequency domain is done by means of a block-based interleaver operating on resource-block pairs.

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Distributed VRBs B. A split of each resource-block pair such that the two

resource blocks of the resource-block pair are transmitted with a certain frequency gap in between. This also provides frequency diversity for a single VRB

pair. This step can be seen as the introduction of frequency hopping on a slot basis.

Whether the VRBs are localized or distributed is indicated on the associated PDCCH in type 2 resource allocation. dynamically switch between distributed and localized

transmission and also mix distributed and localized transmission for different terminals within the same subframe.

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Distributed VRBs The exact size of the frequency gap depends on the

overall downlink cell bandwidth according to Table 10.1.

Based on two criteria:

1.The gap should be of the order of half the downlink cell bandwidth in order to provide good frequency diversity also in a single VRB pair.

2.The gap should be a multiple of P2, where P is the size of a resource-block group and used for resource allocation types 0 and 1.

1.This constraint is to ensure a smooth coexistence in the same subframe between distributed transmission as described above and transmissions based on downlink allocation types 0 and 1.

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Distributed VRBs

PDCCH ProcessingPDCCH ProcessingPDCCH ProcessingPDCCH ProcessingPDCCH ProcessingBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsExampleBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsBlind Decoding of PDCCHsPhysical Hybrid ARQ Indicator Channel (PHICH)PHICHPHICHPHICHPHICHPCFICHPCFICHPCFICHPCFICHPCFICHPCFICHCommon Control Physical Channel (CCPCH)Downlink L1/L2 Control SignalingDownlink L1/L2 Control SignalingDownlink L1/L2 Control SignalingDownlink L1/L2 Control SignalingDownlink Scheduling AssignmentUplinkUplink Physical ChannelsPUSCHPUCCHUplink Physical SignalsUplink Reference SignalsDownlink Channel CodingChannel Coding and Interleaving Channel CodingChannel CodingChannel CodingDownlink Physical-Layer Processing (10)Processing StepsProcessing StepsCode-Block Segmentation and Per-Code-Block CRC InsertionCode-Block Segmentation and Per-Code-Block CRC InsertionLocalized and Distributed Resource MappingDistributed Resource-Block AllocationVirtual Resource Block (VRB)Distributed VRBsDistributed VRBsDistributed VRBsDistributed VRBs