Post on 05-Apr-2018
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Downlink Control Signaling
Genuino,Miguel Louis | Meregildo,Nervin
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Introduction
Three downlink control channels:
Physical Control Format Indicator Channel (PCFICH)- carries information on the number of OFDM symbols
used for PDCCH.
Physical Downlink Control Channel (PDCCH)
- used to inform the UEs about the resource allocationas well as modulation, coding, and hybrid ARQ control information.
Physical Hybrid ARQ Indicator Channel (PHICH)- used to carry hybrid ARQ ACK/NACK feedback for uplink
transmissions.
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Introduction
Design goal for control information transmission: High RELIABILITY
Why?
Data transmission FAILS when control Information is in error!
Why?
Control information carries UE identity as well as transmission
format and resource allocation information.
Broadcast with sufficiently high power. (Targets all UEs in the cell)
How?
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Introduction
Consider: UE waking up from sleep mode
1. The UE needs to acquire system parametersby receiving
broadcast and synchronization signals.
2. The network is unaware of this UEso the broadcast needsto be transmitted with enough power to reach the UE.
***The amount of power transmitted can be controlled if eNB has channel
quality information for the UEs.
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Introduction
Power Control
PBCH is always transmittedat the cell end coverage.
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Data Control Multiplexing
****Similar to data control multiplexing in the uplink, both time-multiplexing
and frequency-multiplexing can be considered for the downlink.
Difference
Uplink: TDM keeps single carrier frequency property of SC-FDMA achievinglow PAPR.
Downlink: multiple access scheme is OFDMA, low PAPR does not apply.- UE can decode control information in the beginning of the sub-
frame and can go to a short term sleep-mode if there is no data scheduled for
it in the sub-frame. (reducing delay)
- micro-sleep, short-term sleep, can save battery consumption!
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Data Control Multiplexing
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Resource Elements Group
****Downlink physical channels are mapped to resource units smaller than a
resource block.- to distribute the transmission over a larger bandwidth to capture
frequency diversity
Control information messages are generally much smaller than data messages!
Resource elements group unit of resource that is used for transmission of control information.
-- a resource block contains three of more REGs depending upon
whether the resource block carries reference signals or not.
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Resource Elements Group
***When reference signals
are present in the resource
block, 4 out of 12 carriersare used as reference
signals and the other 8
form 2 REGs.
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Control Format Indicator Channel
Control format indicator channel
- used to dynamically indicate resources used for downlink controlinformation transmission on a sub-frame-by-sub-frame basis.
***The CF indication is performed in units of OFDM symbols. Rememberthat the number of resource depends on the number of users scheduled and
the transmission formats of these users.
The IDEA
When OFDM symbols needed for downlink control information are less than
the maximum number of OFDM symbols that can be used for control, this
information can be conveyed to the scheduled UEs and the unused OFDM
symbols can instead be used for data transmissions.
UE first needs to decode CFI before decoding the downlink control information
based on CFI.
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Control Format Indicator Channel
Table 15.1 summarizes the
number of OFDM symbols
used for PDCCH in various
sub-frame types.
The 4 CFI codewords given
are obtained by using a
(3,2) simplex code with 10repetitions and appending 2
systematic bits at the end.
In order to randomize the
inter cell interference, the
block of coded bitsb(0)..b(31) is scrambled
with a cell-specific
sequence resulting to a
block of scrambled bits.
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Downlink Resource Allocation
Uplink (SC-FDMA) the allocated resources must be contiguous toguarantee single carrier property.
Downlink (OFDM) - non-contiguous resource blocks can be allocated thusproviding maximum scheduling flexibility.
- signaling overhead also increases!
- contiguous alternative can be used to have low
overhead but limiting the scheduling flexibility.
Three types of resource allocations:1. Resource allocation type 0
2. Resource allocation type 1
3. Resource allocation type 2
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Downlink Resource Allocation
Resource allocation type 0- allows allocation of non-touching group of resource blocks referred
to as resource block groups (RBGs) using bitmap where a RBG is a set of
consecutive physical resource blocks (PRBs).
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Downlink Resource Allocation
Resource allocation type 1- allows allocating resources on an RB level even for larger
bandwidths using the same number of resource allocation bits as in type 0
resource allocation.Resource allocation type 2
- used for uplink assignments that require a set of neighboringresource blocks as well as compact downlink assignments.
Example of type 2 signaling for
NRB=6
+ 1
2=
6 6 + 1
2= 21
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Hybrid ARQ Indicator
Hybrid ARQ Indicator- with corresponding physical channel referred to as physical hybrid
ARQ indicator channel provides ACK/NACK feedback for uplink data
transmissions on PUSCH.
- maximum of single codeword can be transmitted on PUSCH, 1-bitbinary is enough to denote 1 as positive acknowledgement and 0 negative
acknowledgement.
- for a single bit transmission, the bit is repeated three times asshown:
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Hybrid ARQ Indicator
***When transmitting a large message, the total number of coded bits is fairlylarge. These bits can be distributed over a large bandwidth. A single bit
transmission in a distributed fashion over a large bandwidth requires a large
number of repetitions thus making the inefficient use of the resources.
***PHICH is power controlled to make it reliable. If PHICH is transmitted on avery narrow bandwidth, it will create interference peaks in the neighboring
cells.
Solution:
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Hybrid ARQ Indicator
Short approach on Hybrid ARQ Indicator
The hybrid ARQ ACK/NACK information uses the concepts of groups where
ACK/NACKs within a group are code-multiplexed.
The code multiplexing approach allows for the carrying of multiple
ACK/NACKs on the same set of frequency diversity as well as randomize
intercell interference.