LTE Radio Overview: Downlink

İrfan Ali 1
LTE Core Features
LTE Radio Primer
Irfan Ali
info@ikiteknoloji.com
LTE:
A feature based introduction

Irfan Ali 2İrfan Ali 2
Overview
• Downlink: How are control and data information sent to multiple mobiles?
Ø OFDM
Ø Downlink Radio Frame Structure
Ø In a sub-frame, how does a mobile know where to look for data.
Ø Logical Channels and Physical Channels
Ø Radio Protocol stack
Ø Channel signal strength measurement
• Uplink: How is control and data information received from multiple mobiles?
Ø SC-FDMA
Ø Uplink Radio Frame Structure
Ø Radio Frame Synchronization (Timing Advance)
Ø Physical Channels and Logical Channels
Ø Uplink Reference Signal Transmission

When does the base-station talk and when do the mobiles talk?
• The question is when and “where” in the time-frequency domain.
• LTE supports two duplexing modes:
Ø Frequency Division Duplexing (FDD)
Ø Time Division Duplexing (TDD)
Time
Frequency (MHz)
20 MHz
20 MHz
2110
2130
1920
1940
Downlink
Uplink
Frequency Division Duplexing

What if OFDM
• OFDM = Orthogonal Frequency Division Multiplexing
• What are orthogonal functions?
Ø Two functions h1(t) and h2(t) are orthogonal over an interval
[0,T], if
Ø Set of functions {h1(t) , h2(t) ,…, hn(t) } are mutually orthogonal, if
Ø If g(t) = a1h1(t) + a2h2(t) + … + anhn(t), for [0,T], then
ℎ",ℎ$ = ' ℎ" 𝑡 ℎ$ 𝑡 𝑑𝑡
*
+
= 0
ℎ-, ℎ. = 0, if 𝑖 ≠ 𝑘, and ℎ-,ℎ- = 𝑚-
𝑔, ℎ- = 𝛼-ℎ-,ℎ- = 𝛼- 𝑚-, for 𝑖 = 1. . 𝑛

• Harmonics (multiples) of cosine functions of frequency f0, are orthogonal over the base time-period, T = 1/f0
• Let f0 = 15 kHz , T = 66.67 µs
Orthogonal cosine functions
ℎ" = 2cos 2𝜋𝑓+ 𝑡
66.67 µs
Time
1.4
15kHz
x(t)
ℎ$ = 2cos 2𝜋.2𝑓+ 𝑡
66.67 µs
Time
1.4
30kHz
ℎB = 2cos 2𝜋.3𝑓+ 𝑡
66.67 µs
Time
1.4
45kHz
ℎ"$ = 2cos 2𝜋.12𝑓+ 𝑡
66.67 µs
Time
1.4
180kHz
𝑔 = −1.5 ℎ" + 𝑟𝑎𝑛𝑑(1,−1)ℎ$ + ⋯+ 𝑟𝑎𝑛𝑑(1,−1)ℎ"$
𝑔,ℎ"
g(t)
g(t)h1(t)
𝑔,ℎ"

What is OFDM
• Orthogonal Frequency Division Multiplexing
T 2T
15
30
45
60
180
3T 4T 5T 6T 7T 8T 9T 10T 11T 12T 13T 14T
66.67 µs
Time
1.4
30kHz
66.67 µs
Time
1.4
45kHz
66.67 µs
Time
1.4
180kHz
15 30 45 60 75-15-30-45-60-75
15 30 45 60 75-15-30-45-60-75
15 30 45 60 75-15-30-45-60-75
Frequency Domain
Frequency (kHz)
15 30 45 60 75-15-30-45-60-75
|X(f)|
66.67 µs
Time
1.4
15kHz
x(t)
frequency
(kHz)
Time
1 ms
Subframe

How is OFDM signal generated?
Modulate
Add
Cyclic
Prefix
Mix to
RF
PA
Add
Cyclic
Prefix
Digital to
Analog
Mix to
Baseband
Remove
Cyclic
Prefix
Serial to
Parallel
…
f0
2f0
3f0
Nf0
Parallel to
Serial
…
f0
2f0
3f0
Nf0
LNA
Input bit stream
De-Modulate
Output bit stream
Inverse
FFT
FFT
Remove
Cyclic
Prefix
Analog
to Digital

Why OFDM?
• The OFDM symbols duration is relatively long (66.67 µs), which
allows one to add time-gap (preamble) to handle relatively long
delay-spread of the channel (5 µs ~ 1.5 km) without loosing much
capacity.
Ø Reduced inter-symbol interference
• Multiple sub-carriers (rather than a single carrier) over large
bandwidths (20 MHz) enable to handle channel-fades over these
large bandwidths.
• Increased processing capability.

LTE Downlink Frame Structure
Channel
Bandwidth
MHz
# Resource
Blocks in
Frequency
Domain
Total
Subcarrier
Bandwidth
MHz
1.4
MHz
6 1.095
3 MHz 15 2.715
5 MHz 25 4.515
10 Mhz 50 9.015
15 Mhz 75 13.515
20 Mhz 100 18.015
1ms
Subframe
Radio Frame
10 ms
Radio Frame
10 ms
System Frame Number
SFN n
System Frame Number
SFN n+1
Time
Frequency

Synchronizing to DL Radio Frame
Radio Frame
10 ms
Radio Frame
10 ms
System Frame Number
SFN n
System Frame Number
SFN n+1
Frequency
Time
Secondary Synchronization
Signal (SSS)
Physical Broadcast
Channel (PBCH)
Primary Synchronization
Signal (PSS)
62subcarriers
72subcarriers
3MHz
Cell Reference Signal
MIB
• Downlink Bandwidth
• System Frame Number
• PHICH Configuration
PHICH Physical Hybrid ARQ Indicator Channel

Synchronization Signals
• The Primary Synchronization Signal is allocated to the central 62 subcarriers in the 1st and 6th
subframe of every Radio Frame. It’s in the 7th symbol in the subframe. Both transmissions are
identical.
• The Primary Synchronization Signal is used to:
Ø Achieve symbol, slot and subframe synchronization
Ø Determine the first part of Physical Layer Cell Identity (PCI): 3 values.
• The Secondary Synchronization Signal is allocated to the central 62 subcarriers in the 1st and 6th
subframe of every Radio Frame. It’s in the 6th symbol in the subframe.
• The 2 SSS transmissions within each radio frame use different sequences to allow the UE to
differentiate between the 1st and 2nd transmission, i.e. allowing the UE to achieve frame
synchronization.
• The Secondary Synchronization Signal is used to:
Ø Achieve frame synchronization
Ø Determine the second part of Physical Layer Cell Identity: 168 different values. This way the UE determines the
PCI of the cell, which is 1 of 504 different values

Downlink Physical Channels
Radio Frame
10 ms
Radio Frame
10 ms
System Frame Number
SFN n
System Frame Number
SFN n+1
Frequency
Time
Physical Broadcast
Channel (PBCH)
62subcarriers
72subcarriers
Physical Downlink Control
Channel (PDCCH)
Physical Downlink Shared
Channel (PDSCH)
3MHz
Physical Control Format
Indicator Channel (PCFICH)
Physical Hybrid ARQ
Indicator Channel (PHICH)

Physical Downlink Shared Channel (PDSCH)
• In the remaining 11 symbols of the
subframe
• Transfers :
Ø System Information Blocks (SIBs).
Ø Paging RRC message
Ø Other RRC messages
Ø Application data.
• QPSK (2 bits/RE), 16 QAM (4 bits/RE) or
64 QAM (6 bits/RE) modulation is used.
SIB-2
Paging UE-1
UE-2 RRC Message
UE-3 Data Radio Bearer
1.4MHz
Subframe (1ms)

Physical Downlink Control Channel (PDCCH)
• In the first 1-to-3 (configurable) symbols
of every subframe (1 ms)
• Transfers Downlink Control Information
(DCI).
• DCI messages consists of multiples
(i=1,2,3,4) of 36 resource elements.
• Three goals:
Ø Downlink resource allocation for
same subframe.
• Allocated as Resource Block Group
• RBG Size = 1, for 1.4 MHz,
• RBG Size = 4, for 20 MHz
• Bitmap used to indicate which RBG is
allocated to UE
Ø Uplink resource allocation
Ø Transmit Power Control
• QPSK modulation is used (2 bits/RE)
• DCI has a 16 bit CRCUplink
Resource
Allocation
for UE 2
1.4MHz
Subframe (1ms)
SIB-2
Paging UE-1
UE-2 RRC Message

How does a mobile know if there is a message for it in a subframe?
• There are four identities that a mobile
searches for in the Downlink Control
Information (DCI) in the PDCCH:
Ø UE’s unique cell-radio network
temporary identity (C-RNTI)
Ø Paging-RNTI, P-RNTI (0xFFFE) and
Ø System Information-RNTI, SI-RNTI
(0xFFFF).
Ø P-RNTI and SI-RNTI are the same for all
mobiles.
Ø The check for P-RNTI and SI-RNTI are not
performed in every subframe, but on
selected/ “paging-occasion” subframes,
(once every DRX cycle).
Ø During Random access
1. Random Access-RNTI (RA-RNTI): For Random
access response message.
2. Temporary C-RNTI: For RRC Connection Setup
message
• In the PDSCH, the MAC header tells the
mobile, if the message is an RRC message or a
data packet
Ø Logical Channel ID = 0..2 -> SRB 0..2
Ø Logical Channel ID = 3..10 -> DRBs
Uplink
Resource
Allocation
for UE 2
1.4MHz
Subframe (1ms)
SIB-2
Paging UE-1
UE-2 RRC Message
C-RNTI 2
C-RNTI 3
C-RNTI 2
P-RNTI
SI-RNTI

How does the mobile find out what information is being sent to it?
• If Logical Channel ID == 0 (SRB0) Common Control Channel (CCCH)
DL-CCCH-MessageType ::= CHOICE {
rrcConnectionReestablishment
rrcConnectionReestablishmentReject
rrcConnectionReject
rrcConnectionSetup }
• If Logical Channel ID == 1, 2 (SRB1 and SRB2) Dedicated Control Channel (DCCH)
DL-DCCH-MessageType ::= CHOICE {
csfbParametersResponseCDMA2000
dlInformationTransfer
handoverFromEUTRAPreparationRequest
mobilityFromEUTRACommand
rrcConnectionReconfiguration
rrcConnectionRelease
securityModeCommand
ueCapabilityEnquiry
counterCheck
ueInformationRequest-r9
spare6 NULL, spare5 NULL, spare4 NULL,
spare3 NULL, spare2 NULL, spare1 NULL }
• If Logical Channel ID == 3-10 (DRBs): Dedicated Traffic Chanel (DTCH). Data traffic
Sent before RRC Channel is setup
Sent after RRC Channel is setup

Downlink Protocol Layers and Channel Mapping in eNB
RRC
ROHC ROHC
Encryption Encryption
Sequence
Number
Sequence
Number
PDCP
RLC
MAC
PHY
RRC/
Data
Segmentation
Acknowledged
Mode (ARQ)
Segmentation
Acknowledged
Mode (ARQ)
DCCH
LCID1 LCID2
Integrity
Protection
Encryption
Sequence
Number
Integrity
Protection
Encryption
Sequence
Number
Segmentation Segmentation
UnAck
Mode
UnAck
Mode
LCID3 LCID4
DTCH
MIB SIB SRB0 SRB1 SRB2 DRB1 DRB2Page
PDSCH, C-RNTI
Transparent
Mode
(Buffer)
BCCH
PDSCH, SI-RNTIPBCH
Transparent
Mode
(Buffer)
PCCH
PDSCH, P-RNTI
Logical Channels
Transport Channels/
Physical Channels
Transparent
Mode
(Buffer)
CCCH
LCID0
PDSCH, Temporary C-RNTI
RRC Radio Resource Control
PDCP Packet Data Convergence Protocol
RLC Radio Link Control
MAC Medium Access Control
Scheduling
Priority Handling
HARQ
Multiplexing of MAC SDUs

RRC
ROHC ROHC
Sequence
Number
Sequence
Number
PDCP
RLC
MAC
PHY
RRC/
Data
Segmentation
Acknowledged
Mode (ARQ)
Segmentation
Acknowledged
Mode (ARQ)
DCCH
LCID1 LCID2
Integrity
Protection
Encryption
Sequence
Number
Integrity
Protection
Encryption
Sequence
Number
UnAck
Mode
UnAck
Mode
LCID3 LCID4
DTCH
PDSCH, C-RNTI
Transparent
Mode
(Buffer)
BCCH
PDSCH, SI-RNTIPBCH
Transparent
Mode
(Buffer)
PCCH
PDSCH, P-RNTI
Logical Channels
Transport Channels/
Physical Channels
Transparent
Mode
(Buffer)
CCCH
LCID0
Scheduling
Priority Handling
HARQ
Channel (PDCCH)
Channel (PDSCH)
Physical Broadcast
Channel (PBCH)
Source: Netmanias

RRC
ROHC ROHC
Sequence
Number
Sequence
Number
PDCP
RLC
MAC
PHY
RRC/
Data
Segmentation
Acknowledged
Mode (ARQ)
Segmentation
Acknowledged
Mode (ARQ)
DCCH
LCID1 LCID2
Integrity
Protection
Encryption
Sequence
Number
Integrity
Protection
Encryption
Sequence
Number
UnAck
Mode
UnAck
Mode
LCID3 LCID4
DTCH
PDSCH, C-RNTI
Transparent
Mode
(Buffer)
BCCH
PDSCH, SI-RNTIPBCH
Transparent
Mode
(Buffer)
PCCH
PDSCH, P-RNTI
Logical Channels
Transport Channels/
Physical Channels
Transparent
Mode
(Buffer)
CCCH
LCID0
Scheduling
Priority Handling
HARQ
RLC
Header
n n + 1 n + 2 n + 3
RLC
Header
RLC SDU
RLC PDU

RRC
ROHC ROHC
Sequence
Number
Sequence
Number
PDCP
RLC
MAC
PHY
RRC/
Data
Segmentation
Acknowledged
Mode (ARQ)
Segmentation
Acknowledged
Mode (ARQ)
DCCH
LCID1 LCID2
Integrity
Protection
Encryption
Sequence
Number
Integrity
Protection
Encryption
Sequence
Number
UnAck
Mode
UnAck
Mode
LCID3 LCID4
DTCH
PDSCH, C-RNTI
Transparent
Mode
(Buffer)
BCCH
PDSCH, SI-RNTIPBCH
Transparent
Mode
(Buffer)
PCCH
PDSCH, P-RNTI
Logical Channels
Transport Channels/
Physical Channels
Transparent
Mode
(Buffer)
CCCH
LCID0
Scheduling
Priority Handling
HARQ

Example of IDs in the PDCCH and message in PDSCH
MIB
SIB-1
SIB-2
SIB-5
Random Access Preamble
Random Access Response
RRC Connection Request
RRC Connection Setup
RRC Connection SetupComplete
DL Info Transfer (NAS: Authn Req)
Channel (PDCCH)
Channel (PDSCH)
Physical Broadcast
Channel (PBCH)
ID in PDCCH MAC Packet
SI-RNTI SIB
RA-RNTI RAR
C-RNTI
LCID
1
LCID
29
Timing
Advance
NAS Message
Authn Request
RAPID, Uplink Grant, TC-RNTI
TC-RNTI
LCID
0
LCID
31
LCID
28
UE Contention
Resolution ID
RRC Connection
Setup
Pad
RRC Connection Req Msg
…
RRC Connection Request
UE-Identity rand
Establish-cause mo-data, mo-signaling,
mt-Access, …
Info Content
MIB Downlink Channel Bandwidth, PHICH Configuration, SFN
SIB 1 PLMN ID, Tracking Area Code, Cell Selection Parameters, Frequency band,
cell barring, Schedulinginfo for other SIBs
SIB2 Access Class Barring, Channel (RACH, BCCH, ..) parameters, UE timers,
UL Carrier Frequency
SIB3 Cell Selection Parameters
SIB4 Inter Frequency neighbour cell info
SIB5 Intra Frequency neighbour cell info

Cell Reference Signals (CRS)
• Cell Reference Signals
Ø Known reference signals are inserted at regular intervals within the OFDM time-
frequency grid.
Ø There are four resource elements per resource block that are dedicated to
Reference Signal.
7 symbols = 0.5 ms
(Slot)
12subcarriers=180kHz
Æ The location of Reference Signals depends
on the Physical layer cell identity of the cell.
Æ The Primary and Secondary Synchronization
Signals the Physical Layer Cell Identity Resource Elements used
for Reference Signal

Reference Signal Received Power (RSRP)
• The RSRP is the average power (in watts) received from a single Reference Signal (RS)
resource element
• RSRP measures only the RS power and excludes all noise and interference power.
• Knowledge of absolute RSRP enables mobile to calculate downlink path-loss.
• The maximum RSRP is based on maximum input power to UE of -25dBm (0.0032
mWatts). In 1.4 MHz BW with 6 RBs (72 Resource Elements), max RSRP is -44 dBm.
• The minimum value is -140 dBm (has 6 dBm of margin from minimum possible
received power at UE).
7 symbols = 0.5 ms
(Slot)
Resource Elements used
for Reference Signal𝑅𝑆𝑅𝑃 =
1
𝐾
P 𝑃QR,.
S
.T"
where, Prs,k is the estimated received power (in Watts) of
the kth Reference Signal Resource element

Measurement 2: Reference Signal Received Quality (RSRQ)
• RSRP does not give an indication of signal quality, i.e. the strength of the reference signal compared
to overall energy in the channel (aka received signal strength indicator (RSSI))
• The RSSI parameter represents the entire received power including the wanted power from the
serving cell as well as all co-channel power and other sources of noise.
• Measuring RSRQ becomes particularly important near the cell edge when decisions need to be
made, regardless of absolute RSRP, to perform a handover to the next cell.
• The maximum value of RSRQ is -3 dB. (One reference signal has 50% energy in the RB)
• The minimum value of reported RSRQ is -19.5 dB. (One reference signal RE has only 1% of energy in
RB)
where NRB is the number of Resource blocks
(NRB= 6 for 1.4MHz Bandwidth)
RSRQ = RSRP
(RSSI / NRB)
7 symbols = 0.5 ms
(Slot)
Resource Elements used
for Reference Signal
RSSI is measured only in OFDM symbol
containing the RS

Summary: DL Radio Frame
Radio Frame
10 ms
Radio Frame
10 ms
System Frame Number
SFN n
System Frame Number
SFN n+1
Frequency
Time
Signal (SSS)
Physical Broadcast
Channel (PBCH)
Signal (PSS)
62subcarriers
72subcarriers
Channel (PDCCH)
Channel (PDSCH)
3MHz
Physical Hybrid ARQ

OFDM in multi-color
Frequency
Time
Signal (SSS)
Physical Broadcast
Channel (PBCH)
Signal (PSS)
62subcarriers
72subcarriers
Channel (PDCCH)
Channel (PDSCH)
3MHz
Physical Hybrid ARQ
Radio Frame
10 ms
Radio Frame
10 ms

References
• Specifications:
Ø TS 36.300: RAN Architecture
Ø TS 36.331: RRC
Ø TS 36.323: PDCP
Ø TS 36.322: RLC
Ø TS 36.321: MAC
• Other References:
Ø LTE in Bullets
Ø www.sharetechnote.com
Ø www.youtube.com/lte4g

LTE Radio Overview: Downlink

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