control channel dimensioning

Control Channel Dimensioning

RECOMMENDATION

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Copyright

© Ericsson AB 2009-2014. All rights reserved. No part of this document may bereproduced in any form without the written permission of the copyright owner.

Disclaimer

The contents of this document are subject to revision without notice due tocontinued progress in methodology, design and manufacturing. Ericsson shallhave no liability for any error or damage of any kind resulting from the useof this document.

Trademark List

All trademarks mentioned herein are the property of their respective owners.These are shown in the document Trademark Information.

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Contents

Contents

1 Introduction 1

1.1 Limitations 1

1.2 Concepts 1

2 Resource Structure 3

2.1 Time Domain Structure 3

2.2 Frequency Domain Structure 5

2.3 Resource Element 5

2.4 Resource Element Group 6

2.5 Control Channel Elements 6

2.6 Resource Block 6

2.7 Scheduling Block 7

2.8 Resource Grid 7

3 Downlink Common Control Channels and Signals 9

3.1 Channels and Signals 9

3.2 Cell-Specific Reference Signals 9

3.3 UE-Specific Reference Signals 11

3.4 Positioning Reference Signals 13

3.5 Physical Broadcast Channel 13

3.6 Primary and Secondary Synchronization Signal 14

3.7 Physical Control Format Indicator Channel 15

3.8 Physical HARQ Indicator Channel 16

3.9 Physical Downlink Control Channel 17

4 Dimensioning Downlink Control Channels 23

4.1 Resource map 23

4.2 Resource Use 23

5 Uplink Common Control Channel Configuration 25

5.1 Channels and Signals 25

5.2 Demodulation Reference Signal 25

5.3 Sounding Reference Signal 25

5.4 Physical Uplink Control Channel 26

5.5 Physical Random Access Channel 34

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Introduction

1 Introduction

This document describes control channel dimensioning recommendations forthe Long Term Evolution (LTE) Radio Access Network (RAN).

The document provides guidelines for dimensioning the common controlchannels in LTE, including an estimate of spectral and power use of thecontrol channels. Channel configuration parameters are also recommended.In addition, the document describes how the common control channels aremapped to the resource elements and resource blocks in the resource grid.

1.1 Limitations

This guideline is valid for the current release of LTE.

1.2 Concepts

The following concept is used in control channel dimensioning.

Antenna ports An antenna port is defined by its associated referencesignal. The set of antenna ports supported depends onthe reference signal configuration in the cell:

• Cell-specific reference signals support aconfiguration of one, two, or four antenna portsnumbered 0, 1, 2, and 3

• UE-specific reference signals are transmitted onantenna port 5 or antenna port 7 and 8

• Positioning reference signals are transmitted onantenna port 6

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Resource Structure

2 Resource Structure

This chapter describes the resource structure for LTE control channels.

2.1 Time Domain Structure

In the time domain, the signal is structured in the following parts:

Table 1 Time Domain Signal Structure

Structure Element Description

Radio Frames 10 ms length

Subframes 1 ms length. One frame consists of 10subframes.

Slot 0.5 ms length. One subframe consists of twoslots.

OFDM symbol Approximately 71.4 µs length. One slot consistsof 7 OFDM symbols.

The following figure illustrates the time domain structure:

One radio frame (10 ms) = 10 subframes = 20 slots Subframe Subframe 1 Subframe 9

One subframe (1 ms) = 2 slots

One slot (0.5 ms) = 7 OFDM symbols

OFDM symbol 1–6

Tcp = 4.7 µs Tu = 66.7 µs

Cyclic prefixUser data

OFDM symbol 0

Tcp= 5.2 µs Tu = 66.7 µs

L0000222B

Figure 1 Time Domain Structure

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Downlink and uplink are transmitted on the same frequency. The resources aredivided in time on subframe level between uplink and downlink. Also in everyframe one or two subframes are configured as special subframes. A specialsubframe is used to switch from downlink to uplink. It is divided in three partsDwPTS, a guard period and UpPTS. The symbols in DwPTS are allocated fordownlink transmission and those in UpPTS for uplink transmission. Figure 2illustrates the radio frame structure with special subframes:

Subframe 7

Subframe 4

Subframe2

Subframe 3

Subframe 0

Dw

PTS

GP

UpP

TS

Dw

PTS

GP

UpP

TS

Subframe 1ms

One half frame 5 ms

One radio frame 10 ms

Subframe 8

Subframe 5

Specialsubframe

Specialsubframe

Subframe 9

L0000467B

Figure 2 Radio Frame Structure with 2 Special Subframes

Two of the uplink-downlink configurations specified by 3GPP are supported.These two uplink-downlink configurations are shown in Table 2. "DL", "UL"and "SS" denotes subframes used as downlink, uplink and special subframesrespectively.

Table 2 Supported TDD Uplink-downlink Configurations

Subframe NumberUplink-Downlink

Configuration

SpecialSubframePeriodicity 0 1 2 3 4 5 6 7 8 9

1 5 ms DL SS UL UL DL DL SS UL UL DL

2 5 ms DL SS UL DL DL DL SS UL DL DL

Subframe 0 and 5 are always reserved for downlink transmission. A specialsubframe is always followed by uplink transmission in the next subframe.

Table 3 shows the supported special subframe configurations.

Table 3 Supported Special Subframe Configuration

Special SubframeConfiguration

DwPTS[symbols]

GuardPeriod[symbols]

UpPTS[symbols]

5 3 9 2

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Resource Structure

Special SubframeConfiguration

DwPTS[symbols]

GuardPeriod[symbols]

UpPTS[symbols]

6 9 3 2

7 10 2 2

The length of the guard period will put an upper limit for the cell range sinceround trip time needs to be shorter than the guard period.

2.2 Frequency Domain Structure

Orthogonal Frequency-Division Multiplexing (OFDM) utilize a large number ofsubcarriers. Each subcarrier is orthogonal to all other subcarriers. Subcarrierspacing is equal to the subcarrier bandwidth, which is 15 kHz, see Figure 3.

L0000212A

One resource block(12 subcarriers)

DC-subcarrier

∆f = 15 kHz

NRB resource blocks(12 NRB + 1 subcarriers)

Figure 3 Frequency Domain Structure

2.3 Resource Element

The smallest resource unit handled in LTE consists of the combination of:

• The smallest time domain unit, one OFDM symbol

• The smallest frequency domain unit, one subcarrier

This unit is called Resource Element (RE). An RE that is not used fortransmission is referred to as a hole.

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2.4 Resource Element Group

A Resource Element Group (REG) consists of four REs. In a REG, all REs arelocated on the same OFDM symbol within 12 consecutive subcarriers, andgrouped together with at most one RE (or hole) intervening.

2.5 Control Channel Elements

The mapping of Physical Downlink Control Channel (PDCCH) to REs is subjectto a certain structure. The structure is based on Control Channel Elements(CCE). Nine REGs are grouped in one CCE, as shown in the following figure:

L0000211A

4 RE

9 REG

1 CCE = 9 × 4 = 36 RE

1 CCE

REG

REG

Figure 4 CCE Configuration

2.6 Resource Block

A number of REs are grouped into a physical Resource Block (RB). An RBis defined as follows:

• In the time domain: 7 OFDM symbol times (one slot)

• In the frequency domain: 12 consecutive subcarriers

One RB consists of 84 REs. It covers 0.5 ms in the time domain and 180 kHz inthe frequency domain, see Figure 5.

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Resource Structure

L0000221A

∆f= 15 kHzOne resource block(12×7 = 84 resource elements)

One resource element

One slot,7 OFDM symbols

Figure 5 Resource Block

2.7 Scheduling Block

A scheduling block consists of two RBs adjacent in time and with the samesubcarriers. A scheduling block is the smallest downlink unit that can bescheduled to UE.

2.8 Resource Grid

The mapping of channels and signals in each subframe is described by aresource grid. The resource grid size is:

• One radio frame in the time domain

• The system bandwidth in the frequency domain

The system bandwidth expressed as total number of RBs in the frequencydomain, ��, is given in Table 4.

Table 4 System Bandwidth to Resource Blocks Relation

Bandwidth [MHz] Number of Resource Blocks,��10 50

15 75

20 100

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Downlink Common Control Channels and Signals

3 Downlink Common Control Channels andSignals

This chapter describes how downlink Layer 1 and Layer 2 common controlchannels and signals are mapped to REs.

3.1 Channels and Signals

Four physical channels are specified to carry Layer 1 and Layer 2 controlinformation for LTE.

• Physical Downlink Control Channel (PDCCH)

• Physical Control Format Indicator Channel (PCFICH)

• Physical Hybrid Automatic Repeat Request (HARQ) Indicator Channel(PHICH)

• Physical Broadcast Channel (PBCH)

In addition to the control channels there are also physical signals. The downlinkphysical signals are:

• Cell-Specific Reference Signals (CRS)

• Positioning Reference Signal (PRS)

• Primary Synchronization Signals (PSS) and Secondary SynchronizationSignals (SSS)

3.2 Cell-Specific Reference Signals

To demodulate different downlink physical channels coherently, the UE requirescomplex valued channel estimates for each subcarrier. Known cell-specificreference symbols are inserted into the resource grid. The CRS is mapped toREs spread evenly in the resource grid, in an identical pattern in every RB.

When transmitting with several antennas, each antenna must transmit a uniquereference signal. When one antenna transmits its reference signal, the otherantenna must be silent. The mapping of the CRS on the resource grid thereforedepends on the antenna configuration, see Figure 6. The pattern of CRS canbe shifted in frequency compared to figure below. Which one of the six possiblefrequency shifts to use depends on the Physical Cell Identity (PCI) sent onPSS and SSS.

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x

x

x

x

xx

x x

x

x

x

x

x

x

x

x

x

xx

x

xx

xxxx

x x

x x

x x

xx

x

xx

x

xx

xx

xx

x

x

x x

x x

x x

x x

x

x

x

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x

x

x x

x

xx

x x

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x x

xx

x

x

x

x

x

xx

x

x

x

x

x

x

x

Transmission of CRS

Transmission of CRSNo transmission

Transmission of CRSx

Antenna port 1

time

Antenna port 0

Antenna port 0

frequ

ency

One antenna port

Two antenna ports

No transmission x

L0000213C

time

frequ

ency

time

frequ

ency



Antenna port 1

time

Antenna port 0

frequ

ency

Four antenna ports

No transmission x

time

frequ

ency



Antenna port 3

time

Antenna port 2

frequ

ency

No transmission x

time

frequ

ency

Figure 6 Example of Mapping CRS and Holes to One Scheduling Block

With one antenna port, the number of REs in one scheduling block occupied bythe CRS is 8. With two antenna ports the number is 16 and for four antennaports 24.

The following table shows the total number of REs occupied by the CRS,�� , assuming special subframe configuration 6 or 7 for the bandwidthsavailable:

Table 5 REs Occupied by CRS in One Radio Frame for Each Antenna Port

Uplink-Downlinkconfiguration

Bandwidth

[MHz]

��

(one antenna port)��

(two antenna ports)��

(four antenna ports)

1 10 50 2200 4400 6800

1 15 75 3300 6600 10200

1 20 100 4400 8800 13600

2 10 50 3000 6000 9200

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Table 5 REs Occupied by CRS in One Radio Frame for Each Antenna Port

Uplink-Downlinkconfiguration

Bandwidth

[MHz]

��

(one antenna port)��

(two antenna ports)��

(four antenna ports)

2 15 75 4500 9000 13800

2 20 100 6000 12000 18400

3.3 UE-Specific Reference Signals

For transmission modes based on beamforming, additional referencesignals called UE-specific reference signals are used for channel estimation.UE-specific reference signals are transmitted on either antenna port 5 iftransmission mode 7 (TM7) is used or antenna ports 7 and 8 if transmissionmode 8 (TM8) is used. UE-specific reference signals are only transmitted inRBs allocated to UEs using either TM7 or TM8. The mapping of UE-specificreference signals on the resource grid can be seen in Figure 7 and Figure 8.

L0000590A

frequ

ency

time

Transmission of UE-specific RS on Antenna port 5

Figure 7 Example of Mapping UE-Specific RS Transmitted on Antenna Port 5to One Scheduling Block

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L0000589A

frequ

ency

time

Transmission of UE-specific RS on Antenna port 7 and 8

frequ

ency

time

All other subframes Special subframes, configuration 6 and 7

Figure 8 Example of Mapping UE-Specific RS Transmitted on Antenna Port 7and 8 to One Scheduling Block

To avoid that UE-specific RS are mapped to the same REs as other controlchannels antenna port 5 is not allowed in scheduling blocks containing PBCHand antenna port 7 and 8 are not allowed in scheduling blocks containing PSSand SSS.

The total number of REs occupied by UE-specific RS �� in RBs whichare used for TM7:

Table 6 REs Occupied by UE-specific RS on Antenna Port 5 when using TM7

Subframe type ��

RBs in Downlink Subframes 12

RBs in Special SubframesConfiguration 6

6

RBs in Special SubframesConfiguration 7

9

The total number of REs occupied by UE-specific RS �� in RBs whichare used for TM8:

Table 7 REs Occupied by UE-specific RS on Antenna Port 7 and 8 whenusing TM8

Subframe type ��

RBs in All Subframes 12

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3.4 Positioning Reference Signals

Positioning reference signals are used for OTDOA User Plane Location Support.Positioning reference signals are transmitted with a periodicity �� [ms],as specified by prsPeriod. At each transmission occasion the positionreference signals are sent in �� consecutive DL subframes. The numberof consecutive DL subframes can be specified by nConsecutiveSubframes.In the figure below an example of the transmission scheme for PRS subframesis shown.

L0000485A

PRS subframe

D: Downlink subframe

Radio frame #0 Radio frame #1 Radio frame #16 Radio frame #17

U: Uplink subframe

S: Special subframe

D S U U D D S U U D D S U U D D S U U D

D S U D D D S U D D D S U D D D S U D D

D S U U D D S U U D D S U U D D S U U D

D S U D D D S U D D D S U D D D S U D D

nsubf,con=4

nsubf,con=4TPRS=160 ms

TPRS=160 ms

Figure 9 Example of Transmission of PRS Subframes with Four ConsecutiveDL Subframes and a Periodicity of 160 ms. Uplink-Downlinkconfiguration 1 (top), configuration 2 (bottom).

To minimize the interference in the PRS subframes, PDSCH is not scheduled inany RB in those subframes. Also note that PBCH, PSS and SSS have higherpriority than PRS. For a configuration with two antennas, PRS is transmittedfrom one antenna at the time. The same antenna is used the entire PRSoccasion. For more information, refer to OTDOA User Plane Location Support.

The more PRS subframes, the more accurate will the OTDOA positioning be.This comes at the expense of resources available for PDSCH. The fraction ofsubframes used for PRS can be calculated by the following formula:

��

��

Equation 1 Fraction of Subframes Used for PRS

3.5 Physical Broadcast Channel

The PBCH carries part of the system information required by the UE to accessthe network. In the frequency domain, PBCH occupies 72 subcarriers in themiddle of the band independent of deployed bandwidth. In the time domain,

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PBCH is mapped in the first subframe, second slot on OFDM symbol 0, 1, 2and 3 of every radio frame.

The information sent on the PBCH channel in one subframe is retransmitted inthe subsequent three radio frames. New data is transmitted only every fourthradio frame, every 40 ms.

Within the area for PBCH mapping described above, some REs overlap REsalready booked for CRS, as described in Section 3.2 on page 9. CRS havepriority over PBCH, so these REs have to be excluded when mapping PBCHto REs. In this process, REs are excluded as if four antenna ports wouldhave been configured, regardless of the actual number of configured antennaports, see Figure 10.

RE allocated for CRS in case of4 Antenna ports

frequ

ency

time

Slot 0 Slot 1

PBCH Channel

Radio frame,10 ms

72 subcarriersin the middleof the bandwidth

L0000215B

Figure 10 PBCH Mapping

The number of REs used by PBCH in one radio frame is always 240,independent of bandwidth and number of configured antenna ports:

��

3.6 Primary and Secondary Synchronization Signal

The PSS and SSS are used for cell-search procedures and cell identification.Together they carry the PCI , PSS sending one of three orthogonal sequencesand SSS sending one of 168 binary sequences.

As with PBCH, PSS and SSS are mapped on 72 subcarriers in the middle ofthe band. PSS is mapped on OFDM symbol 2 in the first slot of subframes 1

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and 6 and SSS is mapped on last symbol in the second slot of subframes 0and 5, see Figure 11.

Five subcarriers at each end of the 72 subcarriers designated for PSS and SSSare reserved for future use. Nothing is transmitted there, so they are regardedas holes. Reference signals (or holes related to reference signals) are nevermapped in the region designated for PSS and SSS.

PSS

Subframe 0 Subframe 1 SubFrame 2 SubFrame 4 SubFrame 3 SubFrame 5

72 subcarriers in the middle of the bandwidth

SSS

SubFrame 6

frequ

ency

time

L0000468A

Figure 11 PSS and SSS Mapping for TDD

The number of REs used by PSS and SSS per radio frame is independent ofbandwidth and the number of antenna ports:

��

��

3.7 Physical Control Format Indicator Channel

The Physical Control Format Indicator Channel (PCFICH) carries ControlFormat Indicator (CFI), which informs about the number of OFDM symbolsused for PDCCHs in a subframe. PCFICH occupies four REGs (16 REs),independent of system bandwidth. It is mapped on OFDM symbol 0 of the firstslot in all downlink subframes. The PCFICH is mapped to REGs to leave roomfor the reference signals and holes as if two antenna ports were configured,even when only one port is configured, see Figure 12.

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frequ

ency

timeL0000216B

PCFICH

CRS antenna port 0

CRS antenna port 1

1st slot 2nd slot

Figure 12 Example of PCFICH Mapping

The number of REs used by PCFICH in one radio frame is independent ofbandwidth and the number of antenna ports:

Table 8 Number of Resource Elements Used by PCFICH per Radio Frame

Uplink-downlink Configuration ��

1 96

2 128

3.8 Physical HARQ Indicator Channel

The Physical Hybrid ARQ Indicator Channel (PHICH) carries the hybrid ARQAcknowledgement (ACK) and Negative Acknowledgement (NACK) messagesfor the uplink transmission. UE has an individual PHICH assigned. MultiplePHICHs mapped to the same set of REs constitute a PHICH group, where theindividual PHICHs within the same PHICH group are separated by differentorthogonal sequences. Like PCFICH, PHICH is distributed in REGs acrossthe whole bandwidth. It is mapped on OFDM symbol 0 of the first slot in alldownlink subframes.

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frequ

ency

timeL0000217B

PHICH

CRS antenna port 0

CRS antenna port 1

1st slot 2nd slot

Figure 13 Example of PHICH Mapping

PHICH is only mapped to subframes in downlink where HARQ signaling can beexpected. Therefore the number of downlink subframes that carry PHICH isequal to the number of configured uplink subframes.

Table 9 Presence of PHICH in Downlink Subframes

SubframeUplink-downlinkConfiguration 0 1 2 3 4 5 6 7 8 9

1 no yes - - yes no yes - - yes

2 no no - yes no no no - yes no

The total number of REs that carry PHICH in TDD will not only depend on thebandwidth but also the chosen uplink-downlink configuration, see Table 10.

Table 10 Number of Resource Elements Used by PHICH in One Radio Frame

�� Uplink-downlink Configuration

Bandwidth [MHz] 1 2

10 336 168

15 480 240

20 624 312

3.9 Physical Downlink Control Channel

The Physical Downlink Control Channel (PDCCH) is used for:

• Downlink scheduling assignments, including� Physical Downlink Shared Channel (PDSCH) resource indication

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� Transport format indication� Hybrid-ARQ information and transport block size� Control information related to Multiple Input Multiple Output (MIMO)� PUCCH power control commands if applicable

• Uplink scheduling grants, including� Physical Uplink Shared Channel (PUSCH) resource indication� Transport format indication� Hybrid-ARQ information� PUSCH power control commands

PDCCH is transmitted in the beginning of each downlink subframe in REsnot used for reference signals, PHICH or PCFICH. Mapping the PDCCHs toREs is based on CCEs, see Section 2.5 on page 6. The number of CCEsrequired for a certain PDCCH depends on the PDCCH message size and onthe channel coding rate. It is restricted to four different aggregation levels, 1,2, 4 or 8 CCEs per PDCCH.

The number of OFDM symbols available for PDCCHs in a subframe is equalto CFI, see Section 3.7 on page 15. The number of OFDM symbols is limitedby the parameter pdcchCfiMode. pdcchCfiMode has four static and twodynamic options. In the static options, CFI is fixed to the same value for all TTIsand subframes. In the dynamic options, CFI can vary between subframes tomatch the estimated demand of PDCCH in that subframe.

Table 11 Parameter Values of pdcchCfiMode

pdcchCfiMode Description

CFI_STATIC_BY_BW CFI=1 for system bandwidth 10 MHz and greater, CFI=2 otherwise,which corresponds to the hard coded setting in previous releases.

CFI_STATIC_1 CFI=1 statically

CFI_STATIC_2 PDCCH uses only CFI=2 statically

CFI_STATIC_3 PDCCH uses only CFI=3 statically

CFI_AUTO_MAXIMUM_2 Dynamic adaptation up to CFI=2

CFI_AUTO_MAXIMUM_3 Dynamic adaptation up to CFI=3

For Uplink-downlink configuration 1 it is recommended to set pdcchCfiModeto CFI_AUTO_MAXIMUM_2. To secure a sufficient amount of PUCCH resourcesit is recommended for Uplink-downlink configuration 2 to set pdcchCfiMode toCFI_STATIC_BY_BW, see Section 5.4.3 on page 30.

The number of CCEs available for PDCCH depends on CFI, bandwidth, and theamount of resources occupied by PHICH and PCFICH. In many cases someCCEs are left unused by the PDCCH. Unused CCEs are part of the interleavingand mapping process in the same way as any other CCE.

The following figure shows an example of how five PDCCH (and a few unusedCCEs) are aggregated and multiplexed with different formats:

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8 16

12 16 20

10 12 14 16

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

18 20 220 2 4 6 8

4 80

8-CCEaggregations

4-CCEaggregations

2-CCEaggregations

1-CCEaggregations

CCE transmitted in thecontrol region

PDCCH 3 PDCCH 2 PDCCH 1

Unused CCEs

0

PDCCH 0

PDCCH 4

L0000210B

Figure 14 CCE Aggregation and PDCCH Multiplexing

The following table shows the maximum number of REs, �� , used byPDCCH in one frame, including holes associated with unused CCEs for eachsetting of pdcchCfiMode:

Table 12 Maximum Number of REs Available for PDCCH in One Radio FrameUplink-downlink Configuration 1 for One or Two Antenna Ports

�� Bandwidth [MHz]

10 15 20

CFI_STATIC_BY_BW 1872 2880 3960

CFI_STATIC_1 1872 2880 3960

CFI_STATIC_2CFI_AUTO_MAXIMUM_2

5544 8280 11160


7920 11880 15984

Table 13 Maximum Number of REs Available for PDCCH in One RadioFrame Uplink-downlink Configuration 1 for Four Antenna Ports


10 15 20


CFI_STATIC_1 1872 2880 3960


4248 6480 8784


6696 10080 13608

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Table 14 Maximum Number of REs Available for PDCCH in One Radio FrameUplink-downlink Configuration 2 for One or Two Antenna Ports


10 15 20


CFI_STATIC_1 2736 4320 5760


7632 11520 15480


11232 16920 22680

Table 15 Maximum Number of REs Available for PDCCH in One RadioFrame Uplink-downlink Configuration 2 for Four Antenna Ports


10 15 20


CFI_STATIC_1 2736 4320 5760


5904 9000 12312


9576 14400 19512

The number of CCEs in a radio frame can be calculated by dividing the numberof REs in the table above by 36, note that the number of CCEs is higher insubframes without PHICH compared to subframes mapped with PHICH.

Normally, some REGs per subframe are left unused. This is because theunused REGs are too few to form a complete CCE. The unused REGs areinterleaved and mapped in the same way as the REGs grouped in a CCE.The following table shows the total number of REs �� in unused REGsfor different bandwidth:

Table 16 Number of REs Not Used by PDCCH in One Radio FrameUplink-downlink Configuration 1 for One or Two Antenna Ports


CFI 10 15 20

1 96 144 120

2 24 144 120

3 48 144 96

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Table 17 Number of REs Not Used by PDCCH in One Radio FrameUplink-downlink Configuration 1 for Four Antenna Ports


CFI 10 15 20

1 96 144 120

2 120 144 96

3 72 144 72

Table 18 Number of REs Not Used by PDCCH in One Radio FrameUplink-downlink Configuration 2 for One or Two Antenna Ports


CFI 10 15 20

1 168 112 200

2 72 112 80

3 72 112 80

Table 19 Number of REs Not Used by PDCCH in One Radio FrameUplink-downlink Configuration 2 for Four Antenna Ports


CFI 10 15 20

1 168 112 200

2 72 112 80

3 72 112 80

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Dimensioning Downlink Control Channels

4 Dimensioning Downlink Control Channels

This section gives methods to estimate the amount of air interface resourceused by control channels.

4.1 Resource map

The following figure provides an example of the mapping of common controlchannels in the downlink, assuming a bandwidth of 5 MHz, CFI=2, two antennaports and uplink-downlink configuration 1 with special subframe 6.

L0000469A

Details of colors PDSCH PDCCH PHICH PCFICH PBCH SSS PSS CRS Not Used

Subframe 0 Subframe 1, 6 Subframe 4, 9 Subframe 5

Freq

uenc

y

Time

288

168

108

180

0

Sub

carr

ier

inde

x

Figure 15 Example of Mapping Downlink Channels

4.2 Resource Use

The percentage of resources used, relative to the total amount availableis calculated based on the numbers of REs �� for the control channelspresented in Section 3 on page 9.

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Table 20 gives the resource use for one and two antenna ports, 20 MHz, CFI=1and Uplink-downlink Configuration 1 with special subframe 7:

Table 20 Resource Use Percentage, 20 MHz Bandwidth

Notused

CRS PSS SSS PBCH PCFICH PHICH PDCCH TotalControlChannel

PDSCH

OneAntennaPort

1.5 4,8 0.1 0.1 0.3 0.1 0.7 4.3 10.5 88.0

TwoAntennaPorts

5.0 4,8 0.1 0.1 0.3 0.1 0.7 4.3 10.5 84.5

The figures above assume that PRS transmission is not activated in thecell. If PRS transmission is activated the available resources for PDSCH isapproximately reduced by a factor � ,��

��

��

�

Equation 2 PDSCH usage reduction due to PRS

where �� is defined in sectionSection 3.4 on page 13and �� is thefraction of subframes used for DL transmission..

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Uplink Common Control Channel Configuration

5 Uplink Common Control ChannelConfiguration

This chapter describes how the uplink common control channels and signalsare mapped to the REs.

5.1 Channels and Signals

The following control channels for uplink are specified to carry the Layer 1 andLayer 2 control information for LTE:

• Physical Uplink Control Channel (PUCCH)

• Physical Random Access Channel (PRACH)

The uplink physical signals are the Demodulation Reference Signal (DMRS)and the Sounding Reference Signal (SRS).

5.2 Demodulation Reference Signal

Similar to the downlink, reference signals for channel estimation are requiredfor the LTE uplink to enable coherent demodulation of the uplink physicalchannels PUSCH and PUCCH on the receiver side. This reference signal ismore specifically referred to as the uplink Demodulation Reference Signal(DMRS). The DMRS is time multiplexed with both PUCCH and PUSCH.

When DMRS is multiplexed with PUSCH, the middle symbol in each slot is usedfor DMRS, see Figure 16. This means that in each RB 12 REs (approximately14%) are used for transmission of DMRS.

PRACH

PUCCH

PUSCH

Subframe 0 Subframe 1 Subframe 9Subframe 2 Subframe 4 Subframe 5 Subframe 6 Subframe 7 Subframe 8Subframe 3

DMRS in PUSCH

L0000470A

Figure 16 Example of mapping of Uplink Channels with Uplink-downlinkConfiguration 1

5.3 Sounding Reference Signal

Sounding is a prerequisite for UL Frequency Selective Scheduling (FSS), seeUplink Frequency-Selective Scheduling. When sounding is activated a UE can

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urn:x-ericsson:r2:reg-doc:*-*:*:*#pZeroNominalPucch?title=Uplink Frequency-Selective Scheduling


transmit a Sounding Reference Signal (SRS) over the uplink system bandwidth.With the help of SRS the eNB can estimate the UL frequency dependent pathloss between the UE and the eNB.

As indicated in Figure 17, REs for SRS are allocated on 2 UpPTS symbols inevery special subframe (subframe 1 and 6). Since these symbols are not usedfor anything else, no capacity loss occurs due to sounding. Several UE cantransmit SRS simultaneously in the same UpPTS and RB combination usingdifferent transmission combs and cyclic shifts. Which SRS resource to use issignaled to the UE by RRC signalling. A UE keeps its SRS resource as longas it is uplink synchronized.

Subframe 7

Subframe 4

Subframe2

Subframe 3

Subframe 0

Dw

PTS

GP

UpP

TS

Dw

PTS

GP

UpP

TS

Subframe 8

Subframe 5

2 symbolsfor SRS

2 symbolsfor SRS

Subframe 9

L0000535A

Figure 17 Mapping of SRS

The number of RBs over which the SRS is transmitted is given by the followingtable:

Table 21 Number of RBs over which SRS are Transmitted (SRS Bandwidth)

Bandwidth [MHz] RBs

10 48

15 72

20 96

A UE which is allocated sounding resources transmits SRS in one UpPTS every5th ms. At each transmission occasion the UE sends SRS over 24 consecutiveRBs. To cover the entire SRS bandwidth (96 RBs), 4 SRS transmissionoccasions are required.

5.4 Physical Uplink Control Channel

5.4.1 General

The Physical Uplink Control Channel (PUCCH) carries uplink controlinformation from UE for which no PUSCH resource is granted in the samesubframe. For a UE already granted a PUSCH, control signalling is multiplexedwith data onto PUSCH.

PUCCH is used for transmitting:

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• Hybrid Automatic Repeat Request (HARQ) Acknowledgement/NegativeAcknowledgement (ACK/NACK)

• Scheduling Request (SR)• Channel status reports, Channel Quality Indicator (CQI), Precoding Matrix

Indicator (PMI) and Rank Indicator (RI)

The RBs allocated for PUCCH are placed at the band edges. The informationsent on PUCCH uses one RB in each of the two consecutive slots in asubframe. The two RB used for PUCCH is here after called resource block pair(RB-pair). RB-pairs (m in Figure 18) are allocated in the lower frequency bandedge in the first slot and in the upper band edge in the last slot or vice versa.

m=2

m=1m=0

m=3

m=1

m=2m=3

m=0

One Subframe

12 Subcarriers

L0000220A

Figure 18 Mapping PUCCH Resources

To be able to share PUCCH in the time domain, each PUCCH is assigned toa UE with a periodicity specifying in which subframes the UE can access thePUCCH. The default periodicity of CQI is 80 ms and for SR 20 ms.

PUCCH is not only specified by an RB-pair and a periodicity. To allow anRB-pair to be shared by several UE, a resource on PUCCH is specified by acyclic shift, and for SR and HARQ resources also one of a series of orthogonalcover sequences.

Depending on the information to be carried on PUCCH, one of two formatsis used:

• PUCCH Format 1 for SR and HARQ ACK/NACK

• PUCCH Format 2 for CQI, PMI and RI

The parameters noOfPucchCqiUsers and noOfPucchSrUsers determinethe number of resources for CQI and SR per cell. To avoid PUSCH frominterfering with PUCCH, it is recommended to use the same number of

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PUCCH RBs in all cells. This can be achieved by the same setting ofnoOfPucchCqiUsers and noOfPucchSrUsers in all cells.

To maximize PUSCH throughput, the number of RB-pairs should not beover-dimensioned. An even number of RB-pairs is preferable, as an oddnumber will leave one RB-pair unused by both PUCCH and PUSCH.

A UE is allowed to connect to a cell only if there are free SR resources. Ifmore CQI resources than SR resources are allocated some CQI resource willbe unused. CQI and SR resources are allocated for a UE as long as the UE isuplink synchronized. UE already in connected mode will stay connected evenwhen uplink synchronization is timed out and PUCCH resources are released.

If one or more cells in an eNodeB are experiencing CQI and SR congestioneven with the largest possible CQI and SR allocations, it is recommended toactivate the feature Random Access Re-sync and PDCCH ordered re-sync.With this feature the UE will lose the uplink synchronization after a time ofinactivity and the CQI and SR resources are released and can be allocated toanother UE.

5.4.2 Parameter Limitations

When performing the calculations in Section 5.4.3 on page 30 or Section 5.4.4on page 32, two limitations for the two parameters noOfPucchCqiUsers andnoOfPucchSrUsers need to be considered. These limitations are:

• Maximum allowed value of noOfPucchCqiUsers and noOfPucchSrUsers per cell.

• Maximum number of RB pair used for PUCCH per DU.

Maximum allowed value of noOfPucchCqiUsers and noOfPucchSrUsers

Table 22 shows the maximum allowed values of noOfPucchCqiUsers andnoOfPucchSrUsers:

Table 22 Maximum Allowed of SR and CQI Resources per Cell

DU Type Uplink/Downlink

Configuration

Numberof Rx

Antennas

Maximum noOfPucchSrUsers

MaximumnoOfPucchCqiUsers

1 2,4 1328 704DUS31

2 2,4 664 352

2 1472 800

4 1504 8001

8 864 480

2,4 768 400

DUS41

28 416 240

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The value of maximum noOfPucchSrUsers assumes default setting ofcommonSrPeriodicity.

In case commonSrPeriodicity is changed or if a cell is considered as aprimary cell in carrier aggregation, maximum noOfPucchSrUsers must beadjusted by using Equation 3. From PUCCH dimensioning perspective, a cell isconsidered as a primary cell in CA if the following is true:

• noOfChannelSelectionSets >0 and

• dlChannelBandwidth >= 5000 and

• Carrier Aggregation featureState = ON on the eNodeB and

• the cell has one sCellCandidate set to ALLOWED

��

��

��

��

��

��

Equation 3 Adjustment of Maximum Allowed Value of SR Resources per Cell

where

�� is the maximum number of SR resources in Table 22.

�� is the periodicity for SR in milliseconds, specified byoperator parameter commonSrPeriodicity. Defaultvalue is 20 ms.

�� number of subframes with PUCCH, equal to 4 and 2 foruplink-downlink configuration 1 and 2, respectively.

�� is the number of HARQ resources reserved for 2CC DLCarrier Aggregation in primary cell. It is calculated as 2� noOfChannelSelectionSets and is 12 when defaultvalue for noOfChannelSelectionSets is used.

Maximum number of RB pair used for PUCCH per DU

Table 23 shows the maximum allowed number of RB pair used for PUCCHper DU:

Table 23 Maximum Number of RB Pair Used for PUCCH per DU

DU Type Number of Rx Antennas Maximum Number of RB Pairper DU

DUS31 2, 4 27

2, 4 48DUS41

8 36

For some configurations special considerations must be taken then calculatingresource consumption:

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CombinedCell

In case of combined cell, noOfPucchSrUsers andnoOfPucchCqiUsers are set on cell level, that is the PUCCHconfiguration will be the same for all sector carriers in a combinedcell. As each sector carrier will require its own PUCCH RB pairs,a combined cell with n sector carriers will use n times more PUCHRB pairs than a single sector carrier cell, if sets same valuesfor noOfPucchSrUsers,noOfPucchCqiUsers and otherparameters. Therefore, a DU configured with three combinedcells each with two sector carriers and a DU configured with sixsingle sector carrier cells will use the same amount of RB pairsassuming the same parameter setting and bandwidth.

Differentnumberof RXantennas

If cells with different number of RX antennas are configured in aDU, the highest number of RX antennas used in a cell should bechosen for the maximum number of RB pairs inTable 23.

5.4.3 Calculation of the Number of PUCCH RB-pairs

The number of RB-pairs for PUCCH can be calculated for a given a setting ofnoOfPucchSrUsers and noOfPucchCqiUsers.

The number of RB-pairs for format 1 is shared between SR and HARQresources. In the current release of LTE, up to 36 scheduling requests andHARQ resources can be used per RB-pair. The number of RB-pairs for theseresources must be calculated together by:

��

��

��

�

Equation 4 RB-pairs for SR and HARQ Resources

where

�� is the number of resources for SR per subframe.

�� is the number of HARQ resources per subframe.

�� is the number of HARQ resources reserved for 2CC DLCarrier Aggregation in primary cell. It is calculated as 2� noOfChannelSelectionSets and is 12 when defaultvalue for noOfChannelSelectionSets is used. Fornon-primary cells the value is 0.

� � indicates round up to next higher integer.

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The number of SR resources per subframe, ��, are calculated by:

��

��

��

��

��

�

Equation 5 Scheduling Request Resources

where

�� is the desired number of SR resources on thePUCCH channel, specified by operator parameternoOfPucchSrUsers.

�� is the periodicity for SR in milliseconds, specified by theoperator parameter commonSrPeriodicity. Defaultvalue is 20 ms.

�� number of subframes with PUCCH, equal to 4 and 2 foruplink-downlink configuration 1 and 2, respectively.

By decreasing the SR periodicity, latency will be decreased at the cost of anincreased number of PUCCH RB-pairs or lower SR capacity.

The amount of HARQ resources required per subframe, ��, islinked to the amount of CCEs that can be allocated for PDCCH in the downlinksee Section 3.9 on page 17. The maximum number of CCEs �� dependson the bandwidth and is given in Table 24:

Table 24 Maximum Number of CCEs

�� pdcchCfiMode

Bandwidth [MHz] CFI_STATIC_BY_BW

CFI_STATIC_1 CFI_AUTO_MAXIMUM_2CFI_STATIC_2

CFI_AUTO_MAXIMUM_3CFI_STATIC_3

10 11 11 27 44

15 16 16 41 66

20 22 22 55 88

�� is also affected by the chosen uplink-downlink configuration.If more downlink than uplink subframes are allocated, an uplink subframeneeds to cater for ACK/NACKs associated to more than one DL subframe.The maximum number of downlink subframes that can be ACK/NACKed inone uplink subframe, that is, the maximum ACK/NACK bundling window sizeis shown in Table 25:

Table 25 Maximum Bundling Window Size

Uplink-downlink Configuration ��

1 2

2 4

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The number of resources needed for HARQ-ACK�� is nowdetermined by:

��

Equation 6 HARQ Resources with Bundling

The number of RB-pairs allocated for format 2 is calculated by:

��

��

��

��

��

�

Equation 7 RB-pairs for CQI Resources

where

�� is the desired number of CQI resources on the PUCCH,specified by the operator parameter noOfPucchCqiUsers.

�� is the number of CQI resources per RB-pair, equal to 4.

�� is the periodicity for CQI reporting in milliseconds. All UEare allocated the same periodicity of 80 ms.

The total capacity allocated for PUCCH in terms of RB-pairs or RBs per slot,�� is given by:

��

Equation 8 RB-pairs for PUCCH

It must be verified that the total number of RB-pairs per DU does not exceedthe number in Table 23.

5.4.4 Calculation of Parameter Settings

Given a desired number of RB-pairs for format 1 and format 2, the setting ofnoOfPucchSrUsers and noOfPucchCqiUsers is calculated as:

��

��

Equation 9 Calculation of SR Resources from a Desired Number of RB-pairsfor Format 1

��

��

Equation 10 Calculation of CQI Resources from a Desired Number ofRB-pairs for Format 2

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It must be verified that the number of SR resources and CQI resources doesnot exceed the number in Table 22.

5.4.5 Example Calculation of Parameter Settings

Assume that the following configuration is used:

• Three cells

• DUS41

• 2 RX diversity

• Uplink-downlink Configuration 2

• pdcchCfiMode=CFI_AUTO_MAXIMUM_2

• Network bandwidth of 20 MHz

What is the highest setting for noOfPucchSrUsers and noOfPucchCqiUsers?

Table 23 shows that DUS41 can support 48 RB-pairs. Each cell can thereforeuse 16 RB-pairs. As a first attempt 8 RB-pairs are allocated to both Format1 and Format 2.

By using Equation 9 noOfPucchSrUsers is calculated to:

��

��

By using Equation 10 noOfPucchCqiUsers is calculated to:

��

��

�� is not limited according to Table 22.

�� is limited to 400 according to Table 22.

Since �� is limited by Table 22 one less RB-pair is allocated for Format2 and one more RB-pair is allocated for Format 1:

By using Equation 9 noOfPucchSrUsers is calculated to:

��

��

By using Equation 10 noOfPucchCqiUsers is calculated to:

��

��

�� still exceeds the limit 400 resources in Table 22.

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The second attempt gives the best solution as it allows most SR resources.Therefore noOfPucchSrUsers is set to 416 and noOfPucchCqiUsers is setto 400 due to limitation in Table 22.

5.5 Physical Random Access Channel

The Physical Random Access Channel (PRACH) is used for random access. Itallows the RBS to estimate the delay between the RBS and UE.

The PRACH has a bandwidth of 72 subcarriers and in the time domain thelength is 1 ms, which is equivalent to one subframe. In cells with cell rangeexceeding 15 kilometers the length in the time domain is doubled to 2 ms or twoconsecutive subframes. Cell ranges exceeding 15 kilometers are only allowedwith the feature Maximum Cell Range.

The PRACH resource is allocated once every radio frame and placed adjacentto the PUCCH lower frequency band allocation, see Figure 16.

The number of RBs used by PRACH per radio frame, �� isindependent of bandwidth and given in the table below:

Table 26 Number of Resource Blocks Used by PRACH per Radio Frame

Cell Range ��

Cell range ≤ 15 km 12

Cell range >15 km 24

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control channel dimensioning

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