introduction to lte
TRANSCRIPT
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Introduction to
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For Downlink : OFDM and MIMO
For Uplink : SC - FDMA
Key technologies….
No more Codes
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Topics to discuss… System Architecture Evolution (SAE)
What is OFDM ?
What is MIMO ?
What is SC-FDMA ?
LTE Physical Layer
LTE Radio Access – An Overview
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IMT – Advanced Requirements
Support for at least 40 MHz Bandwidth
Peak Spectral Efficiencies : DL : 15 bits/s/Hz (600 Mbps) UL : 6.75 bits/s/Hz(270 Mbps)
Control Plane Latency < 100ms User Plane Latency < 10ms
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Releases of 3GPP Specifications
Rel. 8 LTE EPC/SAE
Rel.9 Location Services
MBMSMulti-
Standard BS
Rel.10 LTE - A Carrier Aggregation Relays
Rel.11Enhanced
Carrier Aggregation
Intra Band Carrier
Aggregation
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System Architecture Evolution (SAE)
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From 3G to 4G…
UTRAN in 3G,E-UTRAN in 4G
CN in 3G, EPC in 4G
NodeB in 3G, E-NodeB in 4G
No RNC as in 3G
RNC tasks perform by eNodeB and EPC
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LTE/SAE Network Architecture
X2 X2
X2
S1S1 S1 S1
eNodeB
eNodeB
eNodeB
MME/S-GW MME/S-GW
E-UTRAN
Internet
P-GW
EPCS5
HSS
S6a
MME S-GW
S11
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eNodeB :
Directly connected to the Core via S1 interface No RNC as in WCDMA eNodeBs interconnected via X2 interface Handovers are handled by eNodeBs it self, communicating via X2
interface This is an intelligent Node
Evolved UTRAN (E-UTRAN)
Evolved Packet Core (EPC)Supports only packet switched domain only
Mobility Management Entity (MME) :
Control Plane Node of the EPC handling connection/release of bearers to a terminal handling of IDLE to ACTIVE Transition handling of security keys
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Serving Gateway(S-GW) :
User plane node which connects EPC to E-UTRAN Acts as a mobility anchor when Terminals move between eNodeBs Mobility Anchor for other 3GPP technologies (GSM,HSPA) Collecting information for charging purposes
Packet Data Network Gateway (P-GW) : Connects EPC to the Internet Allocation of the IP address for a specific terminal QoS handling
Home Subscriber Service (HSS) : A database containing subscriber information
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What is Orthogonal Frequency Division Multiplexing (OFDM) ?
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OFDM
Why ?
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ISI – Inter Symbol Interference
Data Rate ISI
Time domain :
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Time Spreading (Freq. Selective Fading)
• When an impulse is transmitted , how
does the average power received by Mobile
vary as a function of time delay ζ ?
Power Delay Profile
Freq. Selective Fading : Ts < ζ0
Non Freq. Selective Fading : Ts > ζ0
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Power Delay Profile Spaced Freq. Correlation function
FT
Inside Coherence BW channel passes all freq. components with equal gain and linear phase
Freq. Selective Fading : W > f0
Non Freq. Selective Fading : W < f0
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• Symbol rate not increased in order to achieve high data rates.
• Instead of that Available BW breaks in to many narrower subcarriers and modulate generated symbols to these subcarriers.
• These subcarriers then combine linearly and transmit (OFDM symbol).
OFDM Modulation OFDM demodulation
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1 0 1
1
0
1
: Single Carrier Transmission
: OFDM Transmission
t
Single carrier transmission Vs OFDM Transmission
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Sub carrier Pulse shape and Spectrum
Subcarrier BW < Coherance BW
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Why “Orthogonal” ?
Two modulated OFDM subcarriers and are mutually orthogonal over the time interval m ≤ t < (m+1)
Subcarriers “Orthogonal” in the time domain
In OFDM, Subcarriers are overlapped in Frequency domain while maintaining orthogonality in time domain
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Overlapping subcarriers in Freq. domain
Overlapping Subcarriers Spectral Efficiency
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• Generated by Multiplexing several overlapping subcarriers and a Cyclic Prefix (CP).
• Cyclic Prefix added to the beginning of the OFDM symbol in order to eliminate IBI
• At the Receiver CP is removed and only the information bearing part is further processed .
OFDM Symbol
CP Modulated Subcarriers
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OFDM as a Multiple Access Scheme(OFDMA)
OFDMA : In each OFDM symbol interval, Different subsets of the overall set of available subcarriers are used for transmission to different terminals.
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What is Multiple-Input Multiple-Output (MIMO) ?
𝑇 1
𝑇 2
𝑇 𝑛
𝑅1
𝑅2
𝑅𝑛
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Main Transmission Techniques
Spatial Diversity : Signal copies are transmitted at multiple antennas or received at more than one antenna
.
Spatial Multiplexing : Transmit independent and separately encoded data streams over different antennas
𝑇 1𝑅1
𝑅2𝑇 2
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Why MIMO?
Significant increase in Spectral efficiency and data rates - Spatial Multiplexing
High QoS - Spatial diversityWide Coverage - Spatial diversity
SISO Channel Capacity :
MIMO Channel Capacity (MIMO system with M×N antenna configuration) :
B : Channel BandwidthSINR : Signal to Interference plus Noise ratio
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𝑇 1𝑅1
𝑅2𝑇 2
=
Channel impulse responses (are determined by transmitting reference signals from each transmitting antenna.
Received signal y at the receiver when signal x is transmitted,
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What is Single Carrier FDMA (SC – FDMA)?
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SC – FDMA (DFTS-OFDM)
One of the main drawbacks in OFDM : Large instantaneous power variations in the Transmitting signal
This leads to High Peak-to-Average-Power Ratio (PAPR) in the Power Amplifier.
Power Amplifier Efficiency
Power Amplifier Cost
Hence Multicarrier OFDM is not a Viable solution for Low power Mobiles
Why not Multi Carrier OFDM in Uplink ?
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In OFDM, each subcarrier carries information relating to one specific Symbol
In SC-FDMA, each subcarrier contains information of All Transmitted symbols.
Hence no need of transmitting with High Power. Signal energy is distributed among sub carriers.
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User Multiplexing in SC-FDMA Localized Transmission : Distributed Transmission :
User 1 User 2 User 3 User 1 User 2 User 3
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LTE Physical Layer
Overall RAN Protocol Architecture
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LTE Physical Layer Processing
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Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0
Subcarrier BW (kHz) 15
PRB BW (kHz) 180
No. of available RBs 6 12 25 50 75 100
Available DL BW and Physical Resource Blocks (PRBs)
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1 Frame (10 ms)
0 1 2 n 18 19
1 Slot (0.5 ms)
1 Sub Frame (1 ms)
0 1 3 6542
7 OFDM symbols
Generic Frame Structure
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7 OFDM symbolsResource Grid
𝑁 𝑆𝐶𝑅𝐵
𝑁𝑅𝐵𝐷𝐿
RESOURCE
BLOCK
RESOURCE
GRID
Resource Element
TimeFREq
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Physical Resource Block (PRB) allocation is done by the scheduling function in eNodeB
PRB is the smallest element of resource allocation assigned by the base station scheduler.
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LTE Radio Access : An Overview
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Channel dependent Scheduling and Rate adaptation : Depending on the channel conditions, time – frequency resources
are allocated to users by the scheduler Scheduling decisions taken once every 1ms with frequency
domain granularity of 180 kHz. Scheduler allocates resources depending on the Channel State
Information(CSI) provided by the UE
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Inter Cell interference Coordination (ICIC) :
1
3
2
Inner Region
Outer Region
In LTE, Frequency Reuse Factor equals to one (full spectrum availability at each Cell)
This leads to high performance degradation specially the Users in cell edge.
ICIC reduce ICI at cell edge applying certain restrictions on resource assignment.
Adaptive Fractional Frequency Reuse Coordination:
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Multicast / Broadcast Single frequency Network (MBSFN)
As Identical information is transmitted from transmitters (time aligned), UEs in Cell edge can utilize received power of several surrounding cells to detect / decode broadcasted data.
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Special Features in LTE – A (Rel.10)
Carrier Aggregation :
Relaying:
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Extended Multi Antenna Transmission :
DL Spatial Multiplexing has been expanded to support up to 8 transmission Layers.
Heterogeneous Deployments :
Ex : Pico Cell placed inside a Macro Cell
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References :
. “4G LTE/LTE-Advanced for Mobile Broadband” by Erik Dhalman, Stefan Parkvall, Johan Skold
“Overview of the 3GPP Long Term Evolution Physical Layer ” by Jim Zyren, Dr.Wes McCoy
“Wireless Communication” by Andrea Goldsmith
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THANK YOU!
Nadisanka RupasingheEngineer – Network Optimization