Control Channel Dimensioning
RECOMMENDATION
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Copyright
© Ericsson AB 2009-2013. All rights reserved. No part of this document may be
reproduced in any form without the written permission of the copyright owner.
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continued progress in methodology, design and manufacturing. Ericsson shall
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of this document.
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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
4
2.3
Resource Element
4
2.4
Resource Element Group
4
2.5
Control Channel Elements
4
2.6
Resource Block
5
2.7
Scheduling Block
5
2.8
Resource Grid
6
3
Downlink Common Control Channels and Signals
7
3.1
Channels and Signals
7
3.2
Cell-Specific Reference Signals
7
3.3
Positioning Reference Signals
9
3.4
Physical Broadcast Channel
9
3.5
Primary and Secondary Synchronization Signal
10
3.6
Physical Control Format Indicator Channel
11
3.7
Physical HARQ Indicator Channel
12
3.8
Physical Downlink Control Channel
13
4
Dimensioning Downlink Control Channels
17
4.1
Resource map
17
4.2
Resource Use
17
4.3
Estimating Transmit Power
18
5
Uplink Common Control Channel Configuration
21
5.1
Channels and Signals
21
5.2
Demodulation Reference Signal
21
5.3
Sounding Reference Signal
21
5.4
Physical Uplink Control Channel
23
5.5
Physical Random Access Channel
30
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Control Channel Dimensioning
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Introduction
1
Introduction
This document describes control channel dimensioning recommendations for
the Long Term Evolution (LTE) Radio Access Network (RAN).
The document provides guidelines for dimensioning the common control
channels in LTE, including an estimate of spectral and power use of the
control channels. Channel configuration parameters are also recommended.
In addition, the document describes how the common control channels are
mapped 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
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An antenna port is defined by its associated reference
signal. The set of antenna ports supported depends on
the reference signal configuration in the cell:
•
Cell-specific reference signals support a
configuration of one, two, or four antenna ports
numbered 0, 1, 2, and 3
•
Positioning reference signals are transmitted on
antenna port 6
1
Control Channel Dimensioning
2
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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 10
subframes.
Slot
0.5 ms length. One subframe consists of two
slots.
OFDM symbol
Approximately 71.4 µs length. One slot consists
of 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 0
Tcp= 5.2 µs
Tu = 66.7 µs
OFDM symbol 1–6
Cyclic prefix
User data
Tcp = 4.7 µs
Tu = 66.7 µs
L0000222B
Figure 1
Time Domain Structure
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3
Control Channel Dimensioning
2.2
Frequency Domain Structure
Orthogonal Frequency-Division Multiplexing (OFDM) utilize a large number of
subcarriers. Each subcarrier is orthogonal to all other subcarriers. Subcarrier
spacing is equal to the subcarrier bandwidth, which is 15 kHz, see Figure 2.
One resource block
(12 subcarriers)
∆f = 15 kHz
DC-subcarrier
NRB resource blocks
(12 NRB + 1 subcarriers)
L0000212A
Figure 2
2.3
Frequency Domain Structure
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 for
transmission is referred to as a hole.
2.4
Resource Element Group
A Resource Element Group (REG) consists of four REs. In a REG, all REs are
located on the same OFDM symbol within 12 consecutive subcarriers, and
grouped together with at most one RE (or hole) intervening.
2.5
Control Channel Elements
The mapping of Physical Data Contol Channel (PDCCH) to REs is subject to 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:
4
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Resource Structure
4 RE
REG
9 REG
1 CCE
REG
1 CCE = 9 × 4 = 36 RE
L0000211A
Figure 3
2.6
CCE Configuration
Resource Block
A number of REs are grouped into a physical Resource Block (RB). An RB
is 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 in
the frequency domain, see Figure 4.
∆f= 15 kHz
One resource block
(12×7 = 84 resource elements)
One resource element
One slot,
7 OFDM symbols
L0000221A
Figure 4
2.7
Resource Block
Scheduling Block
A scheduling block consists of two RBs adjacent in time and with the same
subcarriers. A scheduling block is the smallest downlink unit that can be
scheduled to UE.
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5
Control Channel Dimensioning
2.8
Resource Grid
The mapping of channels and signals in each subframe is described by a
resource 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 frequency
domain, nRB , is given in Table 2.
Table 2
6
System Bandwidth to Resource Blocks Relation
Bandwidth [MHz]
Number of Resource Blocks,nRB
1.4
6
3
15
5
25
10
50
15
75
20
100
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Downlink Common Control Channels and Signals
3
Downlink Common Control Channels and
Signals
This chapter describes how downlink Layer 1 and Layer 2 common control
channels and signals are mapped to REs.
3.1
Channels and Signals
Four physical channels are specified to carry Layer 1 and Layer 2 control
information 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 downlink
physical signals are:
3.2
•
Cell-Specific Reference Signals (CRS)
•
Positioning Reference Signal (PRS)
•
Primary Synchronization Signals (PSS) and Secondary Synchronization
Signals (SSS)
Cell-Specific Reference Signals
To demodulate different downlink physical channels coherently, the UE requires
complex valued channel estimates for each subcarrier. Known cell-specific
reference symbols are inserted into the resource grid. The CRS is mapped to
REs spread evenly in the resource grid, in an identical pattern in every RB.
When transmitting with several antennas, each antenna must transmit a unique
reference signal. When one antenna transmits its reference signal, the other
antenna must be silent. The mapping of the CRS on the resource grid therefore
depends on the antenna configuration, see Figure 5. The pattern of CRS can
be shifted in frequency compared to figure below. Which one of the six possible
frequency shifts to use depends on the Physical Cell Identity (PCI) sent on
PSS and SSS.
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7
Control Channel Dimensioning
frequency
One antenna port
time
Transmission of CRS on Antenna port 0
Two antenna ports
x
x
x
x
frequency
frequency
x
x
x
time
x
x
x
x
x
x
time
Antenna port 0
x
x
x
Antenna port 1
Transmission of CRS on
Antenna port 1
Transmission of CRS on
Antenna port 0
x No transmission
x No transmission
L0000213B
Figure 5
Example of Mapping CRS and Holes to One Scheduling Block
Holes are REs that must be silent because the CRS is transmitted on another
antenna port. With one antenna port, the number of REs in one scheduling
block occupied by the CRS and holes is 8. With two antenna ports the number
is 16.
The following table shows the total number of REs occupied by the CRS and
holes, nRE;CRS and nRE;CRS;hole respectively, for the bandwidths available:
Table 3
REs Occupied by CRS and Associated Holes in One Radio Frame for Each Antenna
Port
Bandwidth
nRB nRE;CRS
(one antenna
port)
[MHz]
8
nRE;CRS;hole
(one antenna
port)
nRE;CRS
(two antenna
ports)
nRE;CRS;hole
(two antenna
ports)
1.4
6
480
0
480
480
3
15
1200
0
1200
1200
5
25
2000
0
2000
2000
10
50
4000
0
4000
4000
15
75
6000
0
6000
6000
20
100
8000
0
8000
8000
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Downlink Common Control Channels and Signals
3.3
Positioning Reference Signals
Positioning reference signals are used for OTDOA User Plane Location Support.
Positioning reference signals are transmitted with a periodicity TP RS [ms],
as specified by prsPeriod. At each transmission occasion the position
reference signals are sent in nsubf;con consecutive DL subframes. The number
of consecutive DL subframes can be specified by nConsecutiveSubframes.
In the figure below an example of the transmission scheme for PRS subframes
is shown.
TPRS=160 ms
Radio frame #0
Radio frame #1
Radio frame #16
nsubf,con=4
Radio frame #17
PRS subframe
L0000484A
Figure 6
Example of Transmission of PRS Subframes with Four Consecutive
DL Subframes and a Periodicity of 160 ms.
To minimize the interference in the PRS subframes PDSCH is not scheduled in
any RB in those subframes. Also note that PBCH, PSS and SSS all have higher
priority than PRS. For a configuration with two antennas, PRS is transmitted
from one antenna at the time. The same antenna is used the entire PRS
occasion. 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 less resources available for PDSCH. The fraction
of subframes used for PRS can be calculated by the following formula:
ksubf;P RS
= nsubf;con
T
Equation 1
3.4
P RS
Fraction of Subframes Used for PRS
Physical Broadcast Channel
The PBCH carries part of the system information required by the UE to access
the network. In the frequency domain, PBCH occupies 72 subcarriers in the
middle of the band independent of deployed bandwidth. In the time domain,
PBCH is mapped in the first subframe, second slot on OFDM symbol 0, 1, 2
and 3 of every radio frame.
The information sent on the PBCH channel in one subframe is retransmitted in
the subsequent three radio frames. New data is transmitted only every fourth
radio frame, every 40 ms.
Within the area for PBCH mapping described above, some REs are designated
for the CRS and holes, as described in Section 3.2 on page 7. CRS and holes
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9
Control Channel Dimensioning
have priority over PBCH, so the REs designated for the CRS and holes are
excluded when mapping PBCH to REs. The system excludes REs as if four
antenna ports would have been configured, regardless of the actual number of
configured antenna ports, see Figure 7.
Radio frame,10 ms
Slot 0
Slot 1
frequency
72 subcarriers
in the middle
of the bandwidth
time
PBCH Channel
RE allocated for CRS in case of
4 Antenna ports
L0000215B
Figure 7
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:
nRE;PBCH
3.5
= 240
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 sequences
and SSS sending one of 168 binary sequences.
As with PBCH, PSS and SSS are mapped on 72 subcarriers in the middle of
the band. PSS is mapped on OFDM symbol 6 in the first slot of subframes 0
and 5 and SSS is mapped on OFDM symbol 5 in the first slot of subframes 0
and 5, see Figure 8.
Five subcarriers at each end of the 72 subcarriers designated for PSS and SSS
are reserved for future use. Nothing is transmitted there, so they are regarded
as holes. Reference signals (or holes related to reference signals) are never
mapped in the region designated for PSS and SSS.
10
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Downlink Common Control Channels and Signals
Subframe 0 Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5
PSS + SSS
frequency
72 subcarriers
in the middle
of the bandwidth
time
PSS, OFDM symbol 6
SSS, OFDM symbol 5
L0000219A
Figure 8
PSS and SSS Mapping
The number of REs used by PSS and SSS per radio frame is independent of
bandwidth and the number of antenna ports:
nRE;PSS = 124
nRE;SSS = 124
3.6
Physical Control Format Indicator Channel
The Physical Control Format Indicator Channel (PCFICH) carries Control
Format Indicator (CFI), which informs about the number of OFDM symbols
used for PDCCHs in a subframe. PCFICH occupies four REGs (16 REs),
independent of system bandwidth. It is mapped on OFDM symbol 0 of the first
slot in all downlink subframes. The PCFICH is mapped to REGs to leave room
for the reference signals and holes as if two antenna ports were configured,
even when only one port is configured, see Figure 9.
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11
Control Channel Dimensioning
1st slot
2nd slot
PCFICH
CRS antenna port 0
frequency
CRS antenna port 1
time
L0000216B
Figure 9
Example of PCFICH Mapping
The number of REs used by PCFICH in one radio frame is independent of
bandwidth and the number of antenna ports:
nRE;PCFICH
3.7
= 160
Physical HARQ Indicator Channel
The Physical Hybrid ARQ Indicator Channel (PHICH) carries the hybrid ARQ
Acknowledgement (ACK) and Negative Acknowledgement (NACK) messages
for the uplink transmission. UE has an individual PHICH assigned. Multiple
PHICHs mapped to the same set of REs constitute a PHICH group, where the
individual PHICHs within the same PHICH group are separated by different
orthogonal sequences. Like PCFICH, PHICH is distributed in REGs across
the whole bandwidth. It is mapped on OFDM symbol 0 of the first slot in all
downlink subframes.
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Downlink Common Control Channels and Signals
1st slot
2nd slot
PHICH
CRS antenna port 0
frequency
CRS antenna port 1
time
L0000217B
Figure 10
Example of PHICH Mapping
The total number of REs that carry PHICH depend on the bandwidth, see
Table 4.
Table 4
3.8
Number of Resource Elements Used by PHICH in One Radio Frame
Bandwidth [MHz]
Bandwidth [RB]
nRE;PHICH
1.4
6
120
3
15
240
5
25
480
10
50
840
15
75
1200
20
100
1560
Physical Downlink Control Channel
The Physical Downlink Control Channel (PDCCH) is used for:
•
•
Downlink scheduling assignments, including
0 Physical Downlink Shared Channel (PDSCH) resource indication
0 Transport format indication
0 Hybrid-ARQ information and transport block size
0 Control information related to Multiple Input Multiple Output (MIMO)
0 PUCCH power control commands if applicable
Uplink scheduling grants, including
Physical Uplink Shared Channel (PUSCH) resource indication
Transport format indication
0
0
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13
Control Channel Dimensioning
0
0
Hybrid-ARQ information
PUSCH power control commands
PDCCH is transmitted in the beginning of each downlink subframe in REs
not used for reference signals, PHICH or PCFICH. Mapping the PDCCHs to
REs is based on CCEs, see Section 2.5 on page 4. The number of CCEs
required for a certain PDCCH depends on the PDCCH message size and on
the 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 equal
to CFI, see Section 3.6 on page 11, except for 1.4 MHz bandwidth where it is
equal to CFI +1. CFI can vary between subframes to match the estimated
demand of PDCCH in the subframe. The maximum number of OFDM symbols,
limited by the parameter pdcchCfiMode, is not exceeded.
Table 5
Possible Settings of pdcchCfiMode
pdcchCfiMode
Description
CFI_STATIC_BY_BW
Control region uses only CFI=1 for 15 and 20 MHz system
bandwidth, and uses only CFI=2 otherwise, which corresponds to
the hard coded setting in previous releases.
CFI_STATIC_1
PDCCH uses only CFI=1
CFI_STATIC_2
PDCCH uses only CFI=2
CFI_STATIC_3
PDCCH uses only CFI=3
CFI_AUTO_MAXIMUM_2
PDCCH dynamically adapts CFI up to the value of 2, depending
on the actual load in each subframe
CFI_AUTO_MAXIMUM_3
PDCCH dynamically adapts CFI up to the value of 3, depending
on the actual load in each subframe
To secure a sufficient amount of CCEs to transmit PDCCH for common channel
messages, CFI_STATIC_1 is not allowed for bandwidths less than 10 MHz.
However CFI=1 can be used in some subframes with pdcchCfiMode set to
CFI_AUTO_MAXIMUM_2 or CFI_AUTO_MAXIMUM_3 at low PDCCH demand.
To handle peaks of PDCCH load it is recommended for bandwidth of 10 MHz or
less to set pdcchCfiMode to CFI_AUTO_MAXIMUM_3.
For bandwidths larger than 10 MHz CFI_AUTO_MAXIMUM_2 is recommended
since a higher number of symbols for PDCCH can increase the number of RB
pairs allocated to PUCCH. An increase in the number of RBs pairs allocated to
PUCCH leads to a reduced uplink peak rate, see Table 15.
The number of CCEs available for PDCCH depends on CFI, the bandwidth, and
the amount of resources occupied by PHICH and PCFICH. In many cases not
all CCEs are assigned to a PDCCH. Unused CCEs are part of the interleaving
and mapping process in the same way as any other CCE.
The following figure shows an example of how five PDCCH (and a few unused
CCEs) are aggregated and multiplexed with different formats:
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Downlink Common Control Channels and Signals
PDCCH 1
PDCCH 2
PDCCH 4
PDCCH 3
PDCCH 0
8-CCE
aggregations
0
16
8
4-CCE
aggregations
4
0
12
8
20
16
2-CCE
aggregations
0
2
4
6
8
10
12
14
16
18
20
22
1-CCE
aggregations
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
CCE transmitted
in the
control region
Unused CCEs
L0000210B
Figure 11
CCE Aggregation and PDCCH Multiplexing
The following table shows the maximum number of REs, nRE;PDCCH , used by
PDCCH in one frame, including holes associated with unused CCEs for each
setting of pdcchCfiMode:
Table 6
Maximum Number of REs Available for PDCCH in One Radio Frame
nRE;PDCCH
Bandwidth [MHz]
pdcchCfiMode
1.4
3
5
10
15
20
CFI_STATIC_BY_BW
1440
2520
4320
9000
4320
6120
CFI_STATIC_1
N/A
N/A
N/A
2880
4320
6120
CFI_STATIC_2
CFI_AUTO_MAXIMUM_2
1440
2520
4320
9000
13320
18000
CFI_STATIC_3
CFI_AUTO_MAXIMUM_3
2160
4320
7200
14760
22320
30240
The number of CCEs in a subframe can be calculated by dividing the number of
REs in the table above by 360.
Normally, some REGs per subframe are left unused. This is because the
unused REGs are too few to form a complete CCE. The unused REGs are
interleaved and mapped in the same way as the REGs grouped in a CCE.
The following table shows the total number of REs nRE;UN in unused REGs
for different bandwidth:
Table 7
Number of REs Not Used by PDCCH in One Radio Frame
nRE;UN
Bandwidth [MHz]
CFI
1.4
3
5
10
15
20
1
N/A
N/A
N/A
120
320
160
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15
Control Channel Dimensioning
nRE;UN
16
Bandwidth [MHz]
CFI
1.4
3
5
10
15
20
2
200
80
40
0
320
280
3
200
80
160
240
320
40
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Dimensioning Downlink Control Channels
4
Dimensioning Downlink Control Channels
This section gives methods to estimate the amount of air interface resource
and power used by control channels.
4.1
Resource map
Subcarrier
index
The following figure provides an example of how the mapping of common
control channels in the downlink, assuming a bandwidth of 5 MHz, CFI=1
and one antenna port. CFI=1 is used here as an example only, it is not to
be regarded as a recommendation.
Subframe 1–4, 6–9
Subframe 5
:
:
:
:
:
:
:
:
:
Frequency
288
Time
Subframe 0
180
168
Details of colors
PDSCH
PDCCH
PHICH
PCFICH
PBCH
SSS
PSS
CRS
Not Used
108
:
:
:
0
0
67
13
0
6 7
13
0
67
13
L0000214B
Figure 12
4.2
Example of Mapping Downlink Channels
Resource Use
Based on the numbers of REs nRE for the control channels, presented in
Section 3 on page 7, the resource use percentage of the total resource grid
is calculated.
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Control Channel Dimensioning
Table 8 gives the resource use for one and two antenna ports and 10 MHz
for CFI=2:
Table 8
Resource Use Percentage, 10 MHz Bandwidth
Not
used
CRS
PSS
SSS
PBCH
PCFICH
PHICH
PDCCH
Total
Control
Channel
PDSCH
One
Antenna
Port
1.3
4.8
0.1
0.1
0.3
0.2
1.0
10.7
17.2
81.5
Two
Antenna
Ports
4.8
4.8
0.1
0.1
0.3
0.2
1.0
10.7
17.2
77.9
The figures above assume that PRS transmission is not activated in the
cell. If PRS transmission is activated the available resources for PDSCH is
approximately reduced by a factor, (1 0 ksubf;P RS ),
4.3
Estimating Transmit Power
This section describes how to estimate the power consumption for the downlink
physical channels PBCH, PSS, SSS, PCFICH, PHICH, PDCCH, PDSCH and
the CRS according to the downlink configuration.
4.3.1
General Restrictions
Each antenna port has a maximum power Pant :
Pant
=
Pnom;ref =nap
Equation 2
Antenna Port Power
where
Pnom;ref is the sum of nominal power from all radio units in the cell at the
reference point [W]
nap is the number of antenna ports
The antenna power level Pant is the power for an OFDM-symbol on one
antenna port.
4.3.2
Resource Element Reference Power Level
The RE reference power PRE;ref [W] is power per RE when all REs have
equal power.
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Dimensioning Downlink Control Channels
PRE;ref
is calculated by dividing antenna port power by the number of
subcarriers:
PRE;ref =
Pant
12nRB
Equation 3
Reference Power
The total power for a specific control channel is directly proportional to the
number of REs used for that control channel.
4.3.3
Resource Element Power Levels
In the current LTE release, all REs are allocated PRE;ref with two exceptions:
In the case of two antenna ports, the REs used for reference signals (CRS and
PRS) are boosted with respect to all other REs. This is possible since holes
exist in the RE map for one antenna port when a reference signal is transmitted
on the other antenna. In every OFDM symbol where a reference signal is
transmitted, an unused RE (hole) exists from which an equal amount of power
can be "borrowed". A 3 dB boost for the reference signal is obtained for the
case of two antenna ports. This gives increased reliability for the reference
signal. Boosting is unavailable for the one antenna port configuration.
To optimize the PHICH performance PHICH is power controlled. The power
control algorithm allows the usage of a higher transmission power for users in
poor radio conditions as compared to users in good radio conditions. As a
consequence of the power control, REs belonging to one PHICH group can be
transmitted with a power different from PRE;ref . The total power used for the
PHICH divided by the total number of REs used for PHICH is still limited to
PRE;ref .
4.3.4
Example of per Antenna Power Consumption
The following table shows an example of power use for a 40 W LTE RBS with
channels allocated as described in Section 4.2 on page 17.
The power values of PHICH and PDCCH are calculated for all PHICHs and
PDCCHs respectively added together. Table 9 shows the average power
consumption during one radio frame for one and two antenna ports with a 10
MHz bandwidth:
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Control Channel Dimensioning
Table 9
Power Consumption during One Radio Frame, 10 MHz Bandwidth
CRS
(W)
PSS
(W)
SSS
(W)
PBCH
(W)
PCFICH
(W)
PHICH
(W)
PDCCH
(W)
Total
Control
(W)
PDSCH
(W)
Sum
(W)
One
Antenna
Port
1.90
0.06
0.06
0.11
0.08
0.40
4.29
6.90
32.59
39.49
Two
Antenna
Port
1.90
0.03
0.03
0.06
0.04
0.20
2.14
4.36
15.62
19.98
Note that the figures above assume that all PDCCH CCEs are utilized and PRS
transmission is not activated in the cell.
The total power does not equal Pantenna , since unused REs exist that do not
transmit any power. The unused REs are CRS holes, PSS and SSS REs
reserved for future use, and unused REGs.
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Uplink Common Control Channel Configuration
5
Uplink Common Control Channel
Configuration
This chapter describes how the uplink common control channels and signals
are mapped to the REs.
5.1
Channels and Signals
The following control channels for uplink are specified to carry the Layer 1 and
Layer 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 required
for the LTE uplink to enable coherent demodulation of different uplink physical
channels (PUSCH and PUCCH) on the receiver side. This reference signal is
more 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 used
for DMRS, see Figure 13. This means that in each RB 12 REs (approximately
14%) are used for transmission of DMRS.
PRACH
PUCCH
PUSCH
DMRS in PUSCH
SRS
Subframe 0
Subframe 1
Subframe 2
Subframe 3
Subframe 4
Subframe 5
Subframe 6
Subframe 7
Subframe 8
Subframe 9
L0000218C
Figure 13
5.3
Example of Mapping of Uplink Channels
Sounding Reference Signal
Sounding is a prerequisite for UL Frequency Selective Scheduling, see (FSS)
Uplink Frequency-Selective Scheduling. When sounding is activated a UE can
transmit Sounding Reference Signals (SRS) over the uplink system bandwidth.
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Control Channel Dimensioning
With the help of SRS the eNB can estimate the UL frequency dependant path
loss between the UE and the eNB.
As indicated in Figure 13, REs for SRS are allocated on every 5th subframe.
Such a subframe is referred to as a sounding subframe. In order to avoid
interference between SRS and PRACH, sounding subframes are not configured
in subframes with PRACH. Only the last symbol is allocated for SRS. This
means that the fraction of REs reserved for SRS is approximately 1.4%. Note
that these REs are always reserved in cells with the sounding functionality
enabled regardless of if there are UE using sounding. Several UE can transmit
SRS simultaneously in the same RB using different transmission combs and
cyclic shifts. Which SRS resource to use is signaled to the UE by RRC
signalling. A UE keeps its SRS resource as long as it is uplink synchronized.
The number of RBs over which SRS are transmitted is given by the following
table:
Table 10
Number of RBs over which SRS is Sent (SRS Bandwidth)
Bandwidth [MHz]
RBs
1.4
4
3
12
5
24
10
48
15
72
20
96
Depending on the PUCCH configuration, the SRS bandwidth can overlap with
RBs allocated for PUCCH. To avoid interference between SRS and information
carried over PUCCH format 1, a short PUCCH format 1 is used in sounding
subframes. The short PUCCH format 1 has the last symbol reserved for SRS.
PUCCH format 2 is not modified in sounding subframes and interference
between SRS and PUCCH format 2 can occur. The impact is considered
negligible.
For system bandwidth of less than 10 MHz a non-frequency hopping SRS
transmission scheme is used while for larger bandwidths a frequency hopping
SRS scheme is used. In case of non-frequency hopping a UE being allocated
SRS resources transmits SRS every 20 ms. At each transmission occasion the
entire SRS bandwidth is covered. In case of the frequency hopping scheme, on
each transmission occasion SRS are sent on a subset of the SRS bandwidth.
Several transmission occasions are required to cover the entire SRS bandwidth.
The transmission schemes are summarized in the following table:
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Uplink Common Control Channel Configuration
Table 11
SRS Transmission Schemes
Band
width
[MHz
]
SRS
Frequency
hopping
Number of
SRS RBs each
transmission
occasion
Periodicity
for SRS
transmission
[ms]
Period for
covering SRS
bandwidth
[ms]
1.4
No
4
20
20
3
No
12
20
20
5
No
24
20
20
10
Yes
12
5
20
15
Yes
12
5
30
20
Yes
24
5
20
5.4
Physical Uplink Control Channel
5.4.1
General
The Physical Uplink Control Channel (PUCCH) carries uplink control
information from UE for which no PUSCH resource is granted in the same
subframe. For UE already granted a PUSCH, control signalling is multiplexed
with data onto PUSCH.
PUCCH is used for transmitting:
• Hybrid Automatic Repeat Request (HARQ) Acknowledgement/Negative
Acknowledgement (ACK/NACK)
• Scheduling Request (SR)
• Channel status reports, Channel Quality Indicator (CQI) and Rank Indicator
(RI)
The RBs allocated for PUCCH are placed at the band edges. The information
sent on PUCCH uses one RB in each of the two consecutive slots in a
subframe. The two RB used for PUCCH is here after called resource block pair
(RB-pair). RB-pairs (m in Figure 14) are allocated in the lower frequency band
edge in the first slot and in the upper band edge in the last slot or vice versa.
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Control Channel Dimensioning
m=1
m=0
m=3
m=2
m=2
m=3
m=0
m=1
12 Subcarriers
One Subframe
L0000220A
Figure 14
Mapping PUCCH Resources
To be able to share PUCCH in time domain, each PUCCH is assigned to a UE
with a periodicity deciding which subframes the UE can access PUCCH. The
default periodicity of CQI is 80 ms and for SR 10 ms.
PUCCH is not only specified by an RB-pair and a periodicity. To allow an
RB-pair to be shared by several UE, a resource on PUCCH is also specified
by a cyclic shift, and for SR and HARQ resources also an orthogonal cover
sequence.
Depending on the information to be carried on PUCCH, one of two formats
is used:
•
PUCCH Format 1 for SR and HARQ ACK/NACK
•
PUCCH Format 2 for CQI and RI
CQI and SR resources are allocated for a UE as long as the UE is uplink
synchronized. A UE is allowed to connect to a cell if there are free SR
resources, therefore if more CQI resources than SR resources are allocated
some CQI resource will be unused. UE already in connected mode will stay
connected when uplink synchronization is timed out and PUCCH resources
are released.
The parameters noOfPucchCqiUsers and noOfPucchSrUsers determine
the number of resources for CQI and SR per cell. To avoid PUSCH
from interfering PUCCH, it is recommended to use the same setting of
noOfPucchCqiUsers and noOfPucchSrUsers and thereby the same
number of PUCCH RBs in all cells.
To maximize PUSCH throughput, the number of RB-pairs should not be
over-dimensioned and preferably be an even number as an odd number will
leave one RB-pair unused by both PUCCH and PUSCH.
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Uplink Common Control Channel Configuration
5.4.2
Parameter Limitations
When calculating resource consumption the limitation for the two parameters
noOfPucchCqiUsers and noOfPucchSrUsers need to be considered:
•
Highest possible value of noOfPucchCqiUsers and noOfPucchSrUsers
per cell shown in Table 12.
•
Maximum number of RB pair used for PUCCH per DU shown in Table 13.
Table 12
Highest Value of SR and CQI Resources per Cell
DU Type
Number
of Rx
Antennas
Bandwidth
Maximum
noOfPucchSrUsers
Maximum
noOfPucch
CqiUsers
2,4
1.4 MHz
729
792
2,4
All other
Bandwidths
810
880
DUL20
DUS31
DUS41
The value of maximum noOfPucchSrUsers assumes default setting of
commonSrPeriodicity and that carrier aggregation is not activated.
In case commonSrPeriodicity is changed or carrier aggregation is activated,
maximum noOfPucchSrUsers must be adjusted by using Equation 4:
nSR;res =
nSR;res;max
10
Equation 4
0 nCA;res
3 TSR
Adjustment of Highest Value of SR Resources per Cell
where
nSR;res;max
TSR
nCA;res
Table 13
is the maximum number of SR resources in Table 12.
is the periodicity for SR in milliseconds, specified by
operator parameter commonSrPeriodicity. Default
value is 10 ms.
is resources needed for carrier aggregation. Set to 8 if
Carrier Aggregation is activated in the cell, otherwise 0.
Maximum Number of RB Pair Used for PUCCH per DU
DU Type
Number of Rx
Antennas
Maximum Number of
RB Pair per DU
DUL20
2
24
4
12
DUS31
2, 4
24
DUS41
2, 4
36
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Control Channel Dimensioning
For some configuration special considerations must be taken then calculating
resource consumption:
5.4.3
Combined
Cell
In case of combined cell is used, noOfPucchSrUsers and
noOfPucchCqiUsers are set on cell level, that is PUCCH
configuration will be the same for all sector carriers. As each
sector carrier will require its own PUCCH RB pairs, when
dimensioning each sector carrier shall be viewed as a separate
cell. For example a DU configured with three combined cells
each with two sector carriers and DU configured with six cells will
use the same amount of RB pairs assuming same parameter
setting and bandwidth.
Different
bandwidths
In a DU configured with cells using different bandwidths,
noOfPucchSrUsers and noOfPucchCqiUsers can be set
differently per bandwidth. The settings of noOfPucchSrUsers
and noOfPucchCqiUsers must be chosen so that the total
number of RB pairs used for PUCCH in the DU do not exceed the
maximum number of RB pairs inTable 13.
Different
number
of RX
antennas
If cells with different number of RX antennas are configured in a
DU, the highest number of RX antennas used in a cell should be
chosen for the maximum number of RB pairs inTable 13.
PUCCH Capacity in DU
The number of PUCCH resources that simultaneously can be allocated to UE
are limited by DU. The PUCCH resources are pooled within a DU which means
that the number of used resources can vary between cells. Table 14 describes
the maximum number of SR and CQI resources that simultaneously can be
allocated to UE in a DU. The values in Table 14 do not limit the setting of
noOfPucchCqiUsers and noOfPucchSrUsers, the resulting numbers of SR
and CQI resources in all cells can exceed the number of allocated resources a
DU can handle.
Table 14
DU Type
DUL20
DUS31
DUS41
26
Maximum Number of Allocated SR and CQI Resources per DU
Band
width
[MHz]
SR resources per DU
CQI resources per DU
2Rx
4Rx
2Rx
4Rx
1.4 MHz
1224-x
819-x
1224
792
All other
Bandwi
dths
1360-x
910-x
1360
880
1.4 MHz
1638-y
1638-y
1584
1584
All other
Bandwi
dths
1820-y
1820-y
1760
1760
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As a number of HARQ resources need to be reserved for each cell configured
in the DU, the number of SR resources that can be allocated in a DU will
decrease if more cells are configured in a DU. Therefore the number of SR
resources in Table 14 must be decreased depending on number of configured
cells, ncells , by using Equation 5.
x = Min (12; 6 3 ncells) 3 nSF;P U CCH
y = Min (24; 6 3 ncells) 3 nSF;P U CCH
Equation 5
Adjustment of SR Resources per DU
where nSF;P U CCH is number of subframes with PUCCH, equal to 9 for 1.4
MHz otherwise 10.
5.4.4
Calculation of the Number of PUCCH RB-pairs
The number of RB-pairs for PUCCH can be calculated for a given a setting of
noOfPucchSrUsers and noOfPucchCqiUsers.
The number of RB-pairs for format 1 is shared between SR and HARQ
resources. In the current release of LTE, up to 36 scheduling requests and
HARQ resources can be used per RB-pair. The number of pairs for these
resources must be calculated together by:
nRB;F ormat1 =
Equation 6
nP U CCH;SR + nP U CCH;HARQ + nP U CCH;CA0HARQ
36
RB-pairs for SR and HARQ Resources
where
nP U CCH;SR
is resources for SR per subframe
nP U CCH;HARQ
is the HARQ resources per subframe
nP U CCH;CA0HARQ is additional HARQ resources per subframe. Set to 8 if
Carrier Aggregation is activated in the cell, otherwise 0.
d e
indicates round up to next higher integer
The SR resources per subframe, nP U CCH;SR , are calculated by:
nP U CCH;SR =
Equation 7
nSR;res
10
TSR nSF;P U CCH
Scheduling Request Resources
where
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27
Control Channel Dimensioning
nSR;res
is the desired number of SR resources on the
PUCCH channel, specified by operator parameter
noOfPucchSrUsers
TSR
is the periodicity for SR in milliseconds, specified by
operator parameter commonSrPeriodicity. Default
value is 10 ms.
nSF;P UCCH
number of subframes with PUCCH, equal to 9 for 1.4 MHz
otherwise 10
Increasing SR periodicity commonSrPeriodicity to 20 ms will result in a
doubling of the SR capacity, but it shall be noted that increasing SR periodicity
will lead to higher latency.
The amount of HARQ resources required per subframe, nP UCCH;HARQ , is
linked to the amount of CCEs that can be allocated for PDCCH in the downlink
see Section 3.8 on page 13. The required HARQ resources depend on the
allocated bandwidth and are set as shown in Table 15:
Table 15
HARQ Resources
nP UCCH;HARQ
pdcchCfiMode
Bandwidth [MHz]
CFI_STATIC_
BY_BW
CFI_STATIC_1
CFI_AUTO_MAXI
MUM_2
CFI_STATIC_2
CFI_AUTO_MAXI
MUM_3
CFI_STATIC_3
1.4
5
N/A
5
7
3
8
N/A
8
13
5
13
N/A
13
22
10
27
11
27
44
15
16
16
41
66
20
22
22
55
88
The number of resource blocks per slot allocated for format 2 is calculated by:
nRB;F ormat2 =
Equation 8
nCQI;res
10
3
ncap TCQI nSF;P UCCH
RB-pairs for CQI Resources
where
28
nCQI;res
is the wanted number of CQI resources on the PUCCH,
specified by operator parameter noOfPucchCqiUsers
ncap
TCQI
is the CQI resources per RB-pair, equal to 4
is the periodicity for CQI reporting in milliseconds. All UE
are allocated the same periodicity of 80 ms
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Uplink Common Control Channel Configuration
The total capacity allocated for the PUCCH in terms of RB-pairs or RBs per
slot, PRB;PUCCH is given by:
nRB;P UCCH
= nRB;Format1 + nRB;F ormat2
Equation 9
RB-pairs for PUCCH
It should be verified that the total number of RB-pairs per DU does not exceed
the number in Table 13.
5.4.5
Calculation of Parameter Settings
Given a desired number of RB-pairs for format 1 and format 2, the setting of
noOfPucchSrUsers and noOfPucchCqiUsers can be calculated as:
n
nSR;res = (36nRB;format1 0 nPUCCH;HARQ ) TSR SF;P UCCH
10
Equation 10
Calculation of SR Resources from a Wanted Number of RB-pairs
for Format 1
n
nCQI;res = nRB;format2 ncap TCQI SF;P UCCH
10
Equation 11
Calculation of CQI Resources from a Wanted Number of
RB-pairs for Format 2
It should be verified that the number of SR resources and CQI resources does
not exceed the number in Table 12.
5.4.6
Example Calculation of Parameter Settings
Assuming that the configuration to be used is
•
Six cells
•
DUS41
•
4 RX diversity
•
pdcchCfiMode=CFI_AUTO_MAXIMUM_3
•
Network bandwidth of 10 MHz
What is the highest setting for noOfPucchSrUsers and noOfPucchCqiUse
rs?
Table 13 gives that DUS41 can support 36 RB-pairs which gives that each
cell can at most use 6 RB-pairs. As a first attempt the available RB-pairs are
divided equally between Format 1 and Format 2.
By using Equation 10 noOfPucchSrUsers is calculated to:
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29
Control Channel Dimensioning
nSR;res
= (36 3 3 0 44) 3 10 3 1010 = 640
By using Equation 11 noOfPucchCqiUsers is calculated to:
nCQI;res
= 3
3 4 3 80 3 1010
= 960
nCQI;res is limited to 880 according to Table 12.
As nSR;res is lower than nCQI;res as a second attempt one RB-pair is moved to
be used by Format 2 instead of Format 1:
nSR;res
= (36
3 4 0 44) 3 10 3 1010 = 1000
nSR;res is limited to 810 according to Table 12.
nCQI;res
= 2
3 4 3 80 3 1010
= 640
The second attempt gives the best solution as it allows most SR resources.
Therefore noOfPucchSrUsers is set to 810 and noOfPucchCqiUsers
is set to 640.
5.5
Physical Random Access Channel
The Physical Random Access Channel (PRACH) is used for random access
and allows the RBS to estimate the delay between the RBS and UE.
For cell ranges up to 15 kilometers the PRACH has a bandwidth of 72
subcarriers. In the time domain the length is 1 ms, which is equivalent to one
subframe. In cells with cell range exceeding 15 kilometers the length in the
time domain is doubled to 2 ms or two consecutive subframes. Cell ranges
exceeding 15 kilometers are only allowed with the feature Maximum Cell Range.
The PRACH resource is allocated once every radio frame in subframe 1, 4 or
7 and placed adjacent to the PUCCH lower frequency band allocation, see
Figure 13.
To avoid collisions between PRACH and PUCCH in a 1.4 MHz system PUCCH
resources are not scheduled in subframes where PRACH is allocated.
The number of RBs used by PRACH per radio frame, nRB;P RACH is
independent of bandwidth and given in the table below:
Table 16
30
Number of Resource Blocks Used by PRACH per Radio Frame
Cell Range
nRB;PRACH
Cell range ≤ 15 km
12
Cell range >15 km
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