Introduction to LTE
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Transcript of Introduction to LTE
Introduction to LTE
By:-
Nadisanka Rupasinghe,
Engineer – Planning and Optimization,
Etisalat Lanka (Private) Limited
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
Releases of 3GPP Specifications
Rel. 8 LTE EPC/SAE
Rel.9 Location Services
MBMS Multi-
Standard BS
Rel.10 LTE - A Carrier
Aggregation Relays
Rel.11 Enhanced
Carrier Aggregation
Intra Band Carrier
Aggregation
LTE/SAE Network
Architecture
X2 X2
X2
S1 S1 S1 S1
eNodeB
eNodeB
eNodeB
MME/S-GW MME/S-GW
E-UTRAN
Internet
P-GW
EPC S5
HSS
S6a
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)
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
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
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
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
• 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).
1 0 1
1
0
1
: Single Carrier
Transmission
: OFDM Transmission
t
Single carrier transmission Vs OFDM
Transmission
Why “Orthogonal” ?
Two modulated OFDM subcarriers 𝑥𝑘1 and 𝑥𝑘2 are mutually
orthogonal over the time interval m𝑇𝑢 ≤ t < (m+1)𝑇𝑢
Subcarriers “Orthogonal” in the time domain
In OFDM, Subcarriers are overlapped in Frequency
domain while maintaining orthogonality in time domain
• 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 ISI
• At the Receiver CP is removed and only the information
bearing part is further processed .
OFDM Symbol
CP Modulated Subcarriers
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.
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
𝑇1 𝑅1
𝑅2 𝑇2
Why MIMO?
Significant increase in Spectral efficiency and data
rates - Spatial Multiplexing
High QoS - Spatial diversity
Wide Coverage - Spatial diversity
SISO Channel Capacity :
𝐶 = 𝐵 log2(1 + 𝑆𝐼𝑁𝑅)
MIMO Channel Capacity (MIMO system with M×N antenna
configuration) :
𝑪 = 𝑩 𝒍𝒐𝒈𝟐(𝟏 + 𝑴𝑵 × 𝑺𝑰𝑵𝑹)
B : Channel Bandwidth
SINR : Signal to Interference plus Noise ratio
𝑇1 𝑅1
𝑅2 𝑇2
𝑦1
𝑦2 =
ℎ11 ℎ12
ℎ21 ℎ22
𝑥1
𝑥2
Channel impulse responses (ℎ𝑖𝑗) are determined by transmitting
reference signals from each transmitting antenna.
Received signal y at the receiver when signal x is transmitted,
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 ?
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.
User Multiplexing in SC-FDMA
Localized Transmission : Distributed Transmission :
User 1 User 2 User 3 User 1 User 2 User 3
LTE Physical Layer
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)
7 OFDM symbols Resource Grid
𝑁𝑆𝐶𝑅𝐵
𝑁𝑅𝐵𝐷𝐿
R
E
S
O
U
R
C
E
B
L
O
C
K
R
E
S
O
U
R
C
E
G
R
I
D
Resource Element
Time F
R
E
q
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.
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
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:
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.
Extended Multi Antenna Transmission :
DL Spatial Multiplexing expanded to support up to 8
transmission Layers.
Heterogeneous Deployments :
Ex : Pico Cell placed inside a Macro Cell
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