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Lecture 13

UMTS

Long Term Evolution

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Beyond 3G

� International Mobile Telecommunications (IMT)-2000

introduced global standard for 3G

� Systems beyond IMT-2000 (IMT-Advanced) are set to

introduce evolutionary path beyond 3G

� Mobile class targets 100 Mbps with high mobility and

nomadic/local area class targets 1 Gbps with low mobility

� 3GPP and 3GPP2 are currently developing

evolutionary/revolutionary systems beyond 3G

� 3GPP Long Term Evolution (LTE)

� 3GPP2 Ultra Mobile Broadband (UMB)

� IEEE 802.16-based WiMax is also evolving towards 4G

through 802.16m

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3GPP Evolution

�Release 99 (Mar 2000): UMTS/WCDMA

�Release-5 (Mar 2002): HSDPA

�Release-6 (Mar 2005): HSUPA

�Release-7 (2007): DL MIMO, IMS (IP Multimedia Subsystem), optimized realt time services (VoIP, gaming, push-to-talk)

�Long Term Evolution (LTE)

�3GPP work started in Nov 2004

�Standardized in the form of Release 8

�Spec finalized and approved in Jan 2008

�Target deployment starting from 2010

�LTE-Advanced study in progress

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IEEE 802.16 Evolution

�802.16 (2002): Line-of-isght fixed operation in 10 to 66 GHz

�802.16a (2003): Air interface support fro 2 to 11 GHz

�802.16d (2004): Minor improvements

�802.16e (2006): Support for vehicular mobility and asymmetrical links

�802.16m (2011): Higher data rate, reduced latency, efficient security mechanisms

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Requirements of LTE

� Peak data rate

� 100 Mbps DL/50 Mbps UP within 20 MHz bandwidth

� Up to 200 active users in a cell (5 MHz)

� Less than 5 ms user-plane latency

� Mobility

� Optimized for 0-15 Km/h

� 15-120 Km/h supported with high performance

� Supported up to 350 Km/h or even up to 500 Km/h

� Enhanced multimedia broadcast multicast service (E-MBMS)

� Spectrum flexibility: 1.25-20 MHz

� Enhanced support for end-to-end QoS

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LTE Enabling Technologies

�OFDM (Orthogonal Frequency Division Multiplexing)

�SC-FDMA (Single Carrier FDMA)

�MIMO (Multi-input Multi-output)

�Multicarrier channel-dependent resource scheduling

�Fractional frequency reuse

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LTE Enabling Technologies

�Single Carrier FDMA (SC-FDMA)

�A new multiple access technique which has similar

structure and performance to OFDMA

�Linearly pre-coded OFDMA

�Utilizes single carrier modulation and orthogonal frequency multiplexing using DFT-spreading in the transmitter and frequency domain equalization in the receiver

�Low PAPR respect to OFDMA

�H.G: Myung et al, “Single Carrier FDMA for Uplink wireless transmission”; IEEE Vehic. Tech.

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Key Features of LTE

� Multiple access scheme

� DL: OFDMA; UP: SC-FDMA

� Adaptive modulation and coding

� DL/UP modulations: QPSK, 16QAM and 64QAM

� Convolutional codes and Rel-6 turbo codes

� Advanced MIMO spational multiplexing techniques

� (2 or 4)x(2 or 4) downlink and uplink supported

� Multi-user MIMO

� TDD/FDD

� H-ARQ, mobility support, rate control, security, etc.

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Key Features of LTE

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Mobility Intra and Inter Technology

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LTE Standard Specifications

�Downloadable from 3GPP

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Network and Protocol

Architecture

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Protocol Architecture

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LTE Network Architecture

� E-UTRAN: Evolved Universal Terrestrial Radio Access Network

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LTE Network Architecture

�eNB: enhanced Node B

�All radio interface-related functions

�MME: Management Mobility Entity

�Manages Mobility, UE identity and security parameters

�S-GW: Serving Gateway

�Node that terminates the interface towards E-UTRAN

�P-GW: Packet Data Network Gateway

�Node that terminates the interface towards PDN

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Novel Components

�The overall system architecture was revisited:�New Radio-Access Network (RAN)

�New Core Network (referred as EPC)

�The RAN is responsible of:�Scheduling and radio-resource handling

�Retransmission protocols

�Coding and antenna schemes

�The EPC is responsible of:�Authentication

�Charging functionality

�Connections setup

Evolved Packet System (EPS)

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Core Network

�The EPC is a radical evolution from the other core network (GSM/GPRS, WCDMA/HSPA)

�Supports access to the packet-switched domain only (no access to the circuit-switched domain)

�Contains several nodes:�Mobility Management Entity (MME)

�The Serving Gateway (S-GW)

�The Packet Data Network Gateway (PDN/P-GW)

�Other nodes:�Policy and Charging Rules Function

�Home Subscriber Service

�Multimedia Broadcast Multicast Services

�All this nodes are logical nodes!

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LTE Network Interfaces

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Nodes’ Functionalities

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Protocol Stacks

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Circuit-Switched Fall Back

(CSFB)

�LTE is a pure PS-switched network!

�natively supports VoIP using IMS services

�CSFB mechanism allows to handle voice calls by means of

3G/2G networks

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Message Sequence for CSFB

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Background on OFDMA

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OFDM Basic Concept

� OFDM is a special case of Frequency Division Multiplexing (FDM)

� For FDM� No special relationship between the carrier

frequencies

� Guard bands have to be inserted to avoid Adjacent Channel Interference (ACI)

� For OFDM� Strict relation between carriers: fk = k·∆f where

∆f = 1/TU

(TU - symbol period)

� Carriers are orthogonal to each other and can be packed tight

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How does OFDM work?

�In principle similar to CDMA, with continous orthogonal codes!

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OFDM Transmission model

Channel, h(t)

Modulatorand transmitter

Wireless channel

Receiver and demodulator

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Orthogonality – the essential property

� Example: Receiver branch k

� Ideal channel: No noise and no multipath

Tu = 1/∆f gives subcarrier orthogonality over one Tu

=> possible to separate subcarriers in receiver

( )

≠=

==⋅

⋅ ∑ ∫∫ ∑

−

=

⋅−π∆π−

−

=

∆π

qk,0

qk,adte

T

adteea

T

1 k1N

0q

T

0

tT

1kq2j

U

qT

0

ftk2j1N

0q

ftq2jq

U

c U

U

U c

Received signal, r(t)

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Example of OFDM

�Lets we have following information bits

�1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …

�Just converts the serials bits to parallel bits

C1 C2 C3 C4

1 1 -1 -1

1 1 1 -1

1 -1 -1 -1

-1 1 -1 -1

-1 1 1 -1

-1 -1 1 1

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Example of OFDM cont..

Modulated signal for C1 Modulated signal for C2

Modulated signal for C3 Modulated signal for C4

Modulate each column with corresponding sub-carrier using BPSK

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Example of OFDM cont..

�Final OFDM Signal = Sum of all signal

)2sin()()(1

0nttItV

N

nn

π∑−

==

Generated OFDM signal, V(t)

V(t)

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Multipath channel

],[ 00 τα

],[ 11 τα

Diffracted and Scattered Paths

Reflected Path

LOS Path

],[ kk τα

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Multipath channel (cyclic prefix)

Time[τ]

Amplitude [α]

Example multipath profile

τ0 τ1 τ2 The prefix is made cyclic to avoid inter-carrier-interference (ICI) (maintain orthogonality)

Multipath introduces inter-symbol-interference (ISI)

TU

Prefix is added to avoid ISITUTCP

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Multipath channel (cyclic prefix)

� Tcp should cover the maximum length of the time dispersion

� Increasing Tcp implies increased overhead in power and bandwidth (Tcp/ TS)

� For large transmission distances there is a trade-off between power loss and time dispersion

CP Useful symbol CP Useful symbolCP Useful symbol

TUTcp

TS

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Multipath channel (frequency diversity)

=

• The OFDM symbol can be exposed to a frequency selective channel

• The attenuation for each subcarrier can be viewed as “flat”– Due to the cyclic prefix there is no need for a complex equalizer

• Possible transmission techniques– Forward error correction (FEC) over the frequency band– Adaptive coding and modulation per carrier

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Frequency/subcarrier

Pilot carriers /reference signalsData carriers

Multipath channel (pilot symbols)

� The channel parameters can be estimated based on known symbols (pilot symbols)

� The pilot symbols should have sufficient density to provide estimates with good quality (tradeoff with efficiency)

� Different estimation methods exist� Averaging combined with interpolation

� Minimum-mean square error (MMSE)

Pilot symbol

Time

Frequency

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The Peak to Average Power

Problem� A OFDM signal consists of a number of independently

modulated symbols

� The sum of independently modulated subcarriers can have large amplitude variations

� Results in a large peak-to-average-power ratio (PAPR)

∑−

=

∆π⋅=1N

0k

tfk2jk

c

ea)t(x

PA

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The Peak to Average Power

Problem

� Example with 8 carriers and BPSK modulation � x(t) plotted

� It can be shown that the PAPR becomes equal to Nc

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The Peak to Average Power Problem

� High efficiency power amplifiers are desirable� For the handset, long battery life

� For the base station, reduced operating costs

� A large PAPR is negative for the power amplifier efficiency

� Non-linearity results in inter-modulation� Degrades BER performance

� Out-of-band radiation

PA

PIN

POUT

IBO

AM/AM characteristic

OBO

Average Peak

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The Peak to Average Power Problem

� Different tools to deal with large PAPR

� Signal distortion techniquesClipping and windowing introduces distortion and out-of-band radiation, tradeoff with respect to reduced backoff

� Coding techniquesFEC codes excludes OFDM symbols with a large PAPR (decreasing the PAPR decreases code space). Tone reservation, and pre-coding are other examples of coding techniques.

� Scrambling techniquesDifferent scrambling sequences are applied, and the one resulting in the smallest PAPR is chosen

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Summary

� Advantages

� Splitting the channel into narrowband channels enables significant simplification

of equalizer design

� Effective implementation possible by applying FFT

� Flexible bandwidths enabled through scalable number of sub-channels

� Possible to exploit both time and frequency domain variations (time domain

adaptation/coding + freq. domain adaptation/coding)

� Challenges

� Large peak to average power ratio

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Radio Interface

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Frame Structure

�Two radio frame structures

�Type 1: FS1 FDD

�Type 2: FS2 TDD

�A radio frame has a duration of 10 ms

�A resource block (RB) spans 12 subcarriers over a slot duration of 0.5ms

�One subcarrier has a 15 KHz bandwidth, thus 180

KHz per RB

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Frame Structures

�FDD

�TDD

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Resource Grid

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LTE Bandwidth/Resource

Configuration

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Bandwidth Configuration

Example

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LTE Physical Channels

� A set of subcarriers lasting some symbols

� DL:

� Physical Broadcast Channel (PBCH)

� Physical Control Format Indicator Channel (PCFICH)

� Physical Downlink Control Channel (PDCCH)

� Physical Hybrid ARQ Indicator Channel (PHICH)

� Physical Downlink Shared Channel (PDSCH)

� Physical Multicast Channel (PMCH)

� UP:

� Physical Uplink Control Channel (PUCCH)

� Physical Uplink Shared Channel (PUSCH)

� Physical Random Access Channel (PRACH)

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LTE Transport Channels

�DL:

�Broadcast Channel (BCH)

�Downlink Shared Channel (DL-SCH)

�Paging Channel (PCH)

�Multicast Channel (MCH)

�UP:

�Uplink Shared Channel (UL-SCH)

�Random Access Channel (RACH)

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LTE Logical Channels

�Control channels for control-plane info

�Broadcast Control Channel (BCCH)

�Paging Control Channel (PCCH)

�Common Control Channel (CCCH)

�Multicast Control Channel (MCCH)

�Dedicated Control Channel (DCCH)

�Traffic channels for user-plane info

�Dedicated Traffic Channel (DTCH)

�Multicast Traffic Channel (MTCH)

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Channel Mappings

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LTE Layer 2

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RRC Layer

�Terminated in eNB on the network side

�Broadcast, Paging

�RRC connection management

�Radio Bearer management

�Mobility Functions

�UE measurement reporting and control

�RRC states:

�RRC_IDLE, RRC_CONNECTED

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Resource Scheduling of Shared

Channels

�Dynamic resource scheduler resides in eNB on MAC layer

�Radio resource assignment based on radio condition, traffic volume and QoS requirements

�Radio resource assignment consists of:

�Physical Resource Block (PRB)

�Modulation and Coding Scheme (MCS)

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Radio Resource Management

�Radio Bearer Control (RBC)

�Radio Admission Control (RAC)

�Connection mobility Control (CMC)

�Dynamic resource allocation (DRA) or packet scheduling

�Inter-cell interference coordination (ICIC)

�Load Balancing (LB)

�Other: ARQ, H-ARQ, Rate Control, DRX, QoS, Scurity

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LTE Uplink

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UL Resource BLock

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Uplink Channels

� Transport Channels (TrCH)

� UL-SCH Uplink Shared Channel

� RACH Random Access Channel

� Control Information

� UCI Uplink Control Information

� Mapping to Physical Channels

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Uplink Control Signalling

�Conveys L1 and L2 control information

�HARQ acknowledhements for DL-SCH blocks

�Channel quality reports: CQI, RI and PMI

�Scheduling requests

�Transmitted on

�PUCCH if no resources are allocated to UL-SCH

�Multiplexed with UL-SCH on the PUSCH (before

SC_FDMA) if there is a valid schedule grant

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Data and Control Information on PUSCH

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Channels and Signals

� A physical channel is defined as a set of resource elements carrying information originating at a higher layer or in support to the physical layer itself

� For the uplink, the following PHY channels are defined..

� PUSCH: Phy Uplink Shared Channel

� PUCCH: Phy Uplink Control Channel;

� PRACH: Phy Random Access Channel

� ..plus the following signals

� Souding Reference Signal (SRS)

�Not associated with any other transmission

� Demodulation Reference Signal (DRS)

�associated with PUSCH or PUCCH, for channel estimation

�Desired features: small power variations in time and frequency

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Sounding Reference Signal

� eNodeB needs channel quality information in order to assign resources

� From DRS eNobeB can only get channel estimates on UE allocated spectrum

� No information available out of assigned spectrum

� SRS overcomes this problem!

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Sounding Reference Signal (2)

� May cover large frequency span (not assigned to UE)

� Multiple of 4 resource blocks span

� Can be transmitted from every 2ms to every 160ms

� Transmitted on last symbol of subframe (at most every 2 subframes)

� Multiple UEs can transmit simultaneously thanks to cyclic shifts (orth codes)� Wideband: one transmission covers band of interest

� Frequency hopping: narrowband, location changes with time

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Physical Uplink Control Channel

�PUCCH:

�Conveys uplink control information

�Used when UE has no valid schedule grant

�Never transmitted simulanteously with PUSCH

�Transmitted with frequency hopping on band edges to leave contiguos bandwidth to PUSCH

�Multiple PUCCH over a RB by means of orthogonal codes

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Physical Uplink Shared Channel

� PUSCH:

� Carries data and control information, by means of the following

processing chain

� Allocated spectrum to a UE can changge every subframe

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SC-FDMA Modulation in LTE UP

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Uplink Summary

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LTE Downlink

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Downlink Channels

� Transport Channels (TrCH)

� DL-SCH Downlink Shared Channel

� BCH Broadcast Channel

� PCH Paging Channel

� MCH Multicast Channel

� Control Information

� CFI Control Format Indicator

� HI HARQ Indicator

� DCI Downlink Control Information

� Mapping to Phy Channels

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Downlink Channels and Signals

� Phy Channel: set of resource elements carrying information from higher layers

� Phy DL Shared Ch PDSCH; Phy DL Control CH PDCCH; Phy Multicast

Ch PMCH; Phy Broadcast Channel PBCG; Phy Control Format Indicator

Channel PCFIC; Phy HARQ Indicator Channel PHICH

� Phy Signal: set of resource elements used for physical layer information

� Reference Signals

� Synchronization Signals

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Phy Donwlink Channels

�PCFICH: Specifies how many OFDM symbols are used for PDCCH transmission

�PDCCH: carries control information including scheduling assignments;

�PHICH: hybrid ARW and NACK indicators for Ues

�PDSCH: main downlink channel to transport data blocks to the mobiles

�PBCH: broadcast information with a coded block of 1920 samples every 40 ms

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Donwlink Reference Signals

�Used for channel estimations and to obtain quality measurements at the UE side

�Cell-specific:

�Structure depends on the cell ID

�UE- specific

�Used for beamforming

�Arranged across time and frequency

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DL Reference Signal

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Synchronization Signals

�Always transmitted in the same place regardless of the total bandwidth

�First 72 carriers around DC carrier

�OFDM symbols 5 and 6 of first slot in subframe 0 and 5

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Downlink Processing Chain

�The general structure of downlink physical channels processing is the following one (for PDSCHs)

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Transmission schemes (1)

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Transmission Schemes (2)

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Transmission Schemes (3)

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Downlink Summary

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Cell Search

� UE acquires time and frequency synchronization with a cell and detects the cell ID of that cell

� Based on BCH and hierarchical Synchronization signals

� Primary-SCH and Secondary-SCH are transmitted twice per radio frame for FDD

� Cell search:

� 5 ms timing identified using P-SCH

� Radio timing and group ID from S-SCH

� Full cell ID from DL RS

� Decode BCH

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Random Access

�Open loop power controlled with power ramping

�RACH signal bandwidth: 1.08 MHz (6RBs)

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LTE Release 10 and beyond

�Carrier aggregation to give up to 100MHz bandwidth

�Downlink transmission with 8 antennas and layers

�Uplink multi-antenna up to 4 antennas

�Coordinated Multi Point transmission

�Relaying from Relay Nodes to eNB

�Latency Improvements

�Self Optimising Networks enhancements

�Home eNobeB (femtocells)