Comm 4 Track

7/31/2019 Comm 4 Track

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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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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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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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***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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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.

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