6558145 lte-final-v1

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3GPP Long Term EvolutionOverview and Technical Specifications1

All rights reserved © 2008, Faculty of Engineering @ Cairo University

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AGENDA:

Introduction LTE technologies LTE radio interface LTE physical layer

Frame structure Physical layer processing Physical layer procedures

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IntroductionLTE vs. HSPA Evolution

Among different 3G releases, LTE is release 8.

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IntroductionLTE Design Targets

Capabilities. System performance. Deployment-related aspects. Architecture and migration. Radio resource management. Complexity.

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LTE Design Targets Capabilities

Downlink and uplink peak data rates are 100 and 50 Mbit/s respectively for 20MHz bandwidth.

In other words downlink and uplink peak rates can be expressed as 5bit/s/Hz and 2.5 bit/s/Hz.

Supports both TDD and FDD. Control plane latency:

From an idle mode state : 100ms From a Cell_PCH state : 50ms

User plane latency: 5ms

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LTE Design Targets Capabilities

At least 200 mobile terminals in the active state for 5MHz bandwidth.

If bandwidth is more than 5MHz, at least 400 terminals should be supported.

Number of inactive terminals should be higher.

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LTE Design Targets System Performance

System performance design targets include: User throughput. Spectrum efficiency. Mobility. Coverage. Enhanced MBMS.

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LTE Design Targets User Throughput

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LTE Design Targets Mobility

Maximum performance is achieved at speeds 0-15 km/Hr

Up to 120 km/Hr, LTE should provide high performance.

Above 120 km/Hr, connection should be maintained.

Maximum speed is 350 km/Hr (may reach 500 km/Hr for wider bandwidths).

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LTE Design Targets Coverage

It is the maximum distance between the cell site and a mobile terminal in the cell.

It is also called cell radius. Cell radius is around 5km. For radius > 30km, user throughput is

degraded. Cells with radius > 100km should not

precluded.

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LTE Design Targets Enhanced MBMS

MBMS is defined to be Multimedia Broadcast/Multicast Service.

The requirement for the broadcast case is a spectral efficiency of 1 bits/s/Hz.

This corresponds to around 16 mobile-TV channels using in the order of 300 kbits/s each, in 5 MHz spectrum allocation.

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LTE Design Targets Deployment Related Aspects

Deployment scenarios. Coexistence and internetworking with other 3GPP

RATs such as GSM and WCDMA/HSPA. Spectrum deployment. Spectrum flexibility.

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LTE Design Targets Deployment Scenarios

Two possible scenarios: LTE system is deployed as a stand alone system. LTE system is deployed together with other

radio access technologies (with WCDMA/HSPA ands/or GSM systems)

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LTE Design Targets Interaction With Other RATs

Interruption requirements : longest acceptable interruption in the radio link when moving between the different radio access technologies, for both real-time and non-real-time services.

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LTE Design Targets Spectrum Flexibility & Deployment

LTE spectrum is to be deployed in existing IMT-2000 frequency bands.

This implies coexistence with the systems that are already deployed in those bands.

It should be possible to deploy LTE-based RA in both paired and unpaired spectrum allocations.

That is LTE should support both Frequency Division Duplex (FDD), and Time Division Duplex (TDD).

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FDD systems are deployed in paired spectrum allocations.

They have one frequency range intended for downlink transmission and another for uplink transmission.

TDD systems are deployed in unpaired spectrum allocations.

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LTE Design Targets Example

An example is the IMT-2000 spectrum at 2 GHz, that is, the IMT-2000 ‘core band’.

It consists of the paired frequency bands 1920– 1980MHz and 2110–2170MHZ intended for FDD-based radio access.

It has also the two frequency bands 1910–1920MHz and 2010–2025MHz intended for TDD based radio access.

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LTE Design Targets Example

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LTE needs to be scalable in the frequency domain and operate in different bands.

This flexibility requirement is stated as a list of LTE spectrum allocations (1.25, 1.6, 2.5, 5, 10, 15 and 20 MHz).

Furthermore, LTE should be able to operate in unpaired as well as paired spectrum.

LTE should also be possible to deploy in different frequency bands.

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Introduction LTE Design Targets


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LTE Design Targets Architecture & Migration

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AGENDA:



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LTE Technologies

Equalization against radio-channel frequency selectivity.

Uplink FDMA with flexible bandwidth assignment.

DFT Spread OFDM (DFTS-OFDM), called also (SC-FDMA).

CAZAC Sequences

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LTE TechnologiesEqualization

Three main equalization techniques were proposed for LTE Time domain linear equalization. Frequency domain equalization. Other equalization techniques.

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LTE TechnologiesTime Domain Equalization

A filter with an impulse response is applied to the received signal. ( )w

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( )w


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Matched filter Disadvantage: Maximizes SNR, but does not

compensate for any radio channel frequency selectivity.

Inverse-channel filter: Disadvantage: Does not care about noise level

although it cancels channel frequency selectivity.

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*( ) ( )w h

( ) ( ) 1h w


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Minimum Mean Square Error (MMSE) Equalizer. Error between transmitted signal received signal

should be minimized.

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^2{[ ( ) ( )] }E s t s t


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LTE TechnologiesFrequency Domain Equalization

Reduces equalizer complexity. For each processing block of size N, the

frequency-domain equalization basically consists of: A size-N DFT/FFT. N complex multiplications (the frequency-

domain filter). A size-N inverse DFT/FFT.

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By means of cyclic prefix, we can design our equalizer such that it reverses the effect of the channel.

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*

20| |

kk

k

HW

H N


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LTE TechnologiesOther Techniques

Decision-Feedback Equalization (DFE) Implies that previously detected symbols are fed

back and used to cancel the contribution of the corresponding transmitted symbols to the overall signal corruption.

Maximum-Likelihood (ML) detection Also known as Maximum Likelihood Sequence

Estimation (MLSE)

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LTE TechnologiesFlexible BW Assignment

Orthognality between users is maintained either by TDMA or FDMA.

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TDMA FDMA


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As TDMA assigns the whole band to each user, it is used more than FDMA.

The mobile terminal generally can not transmit much power.

Some times due to channel conditions, data rate is limited by power rather than bandwidth. In these cases, although the whole band is assigned to the user it can not fully utilize it.

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This will lead to inefficient use of the whole bandwidth.

Proposed solution: Assign part of the

spectrum to another user. This is some sort of

mixing between TDMA and FDMA.

This will lead to a more efficient bandwidth utilization

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LTE TechnologiesDFT-Spread OFDM

SC-OFDMA combines: Small variations in the instantaneous power of the

transmitted signal (‘single carrier’ property). Possibility for low-complexity high-quality equalization

in the frequency domain. Possibility for FDMA with flexible bandwidth

assignment.

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LTE TechnologiesDFT-Spread OFDM

DFT/IDFT combination achieves multiplexing between users.

Padding with zeros in frequency domain reduces PAR (remember the delta function example).

Cyclic prefix simplifies the frequency domain equalization at the receiver side.

M controls LTE bandwidth.

s

MBW f

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s

MBW f

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LTE TechnologiesPAR Reduction

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LTE TechnologiesDFTS-OFDM Demodulator

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LTE TechnologiesDFTS-OFDM Demodulator

Equalization is also needed here.

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LTE TechnologiesUser Multiplexing

42Equal BW assignment Non-equal BW assignment


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With a raised cosine shaping instead of zero padding, PAR can be more and more reduced.

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LTE TechnologiesDFTS-OFDM With Spectrum Shaping


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LTE TechnologiesDFTS-OFDM With Spectrum Shaping

α is the roll off factor of the raised cosine. As α increases B.W. overhead increases.

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Constant or almost constant amplitude in line with the basic characteristics of the LTE uplink transmission scheme (low-PAR ‘single-carrier’).

Zero time-domain auto-correlation properties in order to allow for accurate uplink channel estimation.

One set of sequences with the CAZAC property is the set of Zadoff–Chu sequences.

In the frequency domain, a Zadoff–Chu sequence of length MZC can be expressed as:

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u is the index of the Zadoff–Chu sequence within the set of Zadoff–Chu sequences of length MZC.

LTE TechnologiesCAZAC Sequences


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AGENDA:



References

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LTE Radio InterfaceProtocol Stack

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Packet Data Convergence Protocol (PDCP) performs IP header

compression based on ROHC, a standardized header-compression algorithm used in WCDMA as well as several other mobile-communication standards.

Responsible for ciphering and integrity protection of the transmitted data Downlink LTE protocol stack

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LTE Radio Interface Protocol Stack

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Radio Link Control (RLC) Responsible for

segmentation/concatenat-ion, retransmission handling, and in-sequence delivery to higher layers.

RLC offers services to the PDCP in the form of radio bearers.

Downlink LTE protocol stack 48


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Medium Access Control (MAC) handles hybrid-ARQ

retransmissions Responsible for uplink

and downlink scheduling.

MAC offers services to the RLC in the form of logical channels.

The scheduling functionality is located in the eNodeB, which has one MAC entity per cell, for both uplink and downlink.



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Medium Access Control (MAC) DL/UL scheduling.

Dynamically determine, in each 1 ms interval, which terminal (s) that are supposed to receive DL-SCH transmission (transmit UL-SCH) and on what resources.

The scheduler is also responsible for selecting the transport-block size, the modulation scheme, and the antenna mapping (in case of multi-antenna transmission).

Downlink channel conditions can be measured by all mobile terminals in the cell by observing the reference signals transmitted by the eNodeB.

All mobile terminals can share the same reference signal for channel-quality-estimation purposes.

Information about the downlink channel conditions, necessary for channel dependent scheduling, is fed back from the mobile terminal to the eNodeB via channel-quality reports (channel quality indicators).

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Medium Access Control (MAC) DL/UL scheduling.

Estimating the uplink channel quality is not as straightforward as is the case for the downlink.

Estimating the uplink channel quality require a sounding reference signal transmitted from each mobile terminal for which the eNodeB wants to estimate the uplink channel quality.

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Physical Layer (PHY) Handles

coding/decoding, modulation/demodulation, multi-antenna mapping,

The physical layer offers services to the MAC layer in the form of transport channels.



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LTE_DETACHED Used @ power up when the mobile terminal is not known

to the network. Before any further communication, the mobile terminal

need to register with the network using the random-access procedure.

LTE_ACTIVE Mobile terminal is active with transmitting and receiving

data.

LTE Radio InterfaceLTE States

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LTE_ACTIVE Mobile terminal is active with transmitting and receiving

data. IN_SYNC

Uplink is synchronized with eNodeB OUT_SYNC

Uplink is not synchronized with eNodeB. Mobile terminal needs to perform a random-access procedure

to restore uplink synchronization.


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LTE_IDLE Low activity state to reduce battery consumption. The only uplink transmission activity that may take

place is random access to move to LTE_ACTIVE. In the downlink, the mobile terminal can

periodically wake up in order to be paged for incoming calls

The network knows at least the group of cells in which paging of the mobile terminal is to be done.


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AGENDA:


Physical layer channels, services and Frame structure

Physical layer processing Physical layer procedures

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LTE Physical LayerSupported Channels

Downlink Physical Channels: Physical Downlink Shared Channel (PDSCH), Physical Multicast Channel (PMCH), Physical Downlink Control Channel (PDCCH), Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel

(PCFICH) Physical Hybrid ARQ Indicator Channel (PHICH).

Uplink Physical Channels : Physical Random Access Channel (PRACH), Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH). 5757


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LTE Physical LayerService provided to upper layers

Error detection on transport channel and indication to higher layers.

FEC encoding/decoding of the transport channel. Hybrid ARQ soft-combining. Rate matching of coded transport channel to physical

channels. Mapping of the coded transport channel onto physical

channels. Power weighting of physical channels. Modulation and demodulation of physical channels. Frequency and time synchronization. Radio characteristics measurements and indication to

higher layers. Multiple Input Multiple Output (MIMO) antenna

processing. Transmit Diversity (TX diversity). Beamforming.

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LTE Physical LayerOverall Time Domain Structure

Tframe =10 ms Consisting of ten equally sized subframes of

length Tsubframe =1 ms. Specification can be expressed as multiples

of a basic time unit Ts =1/30720000. Tframe =307200 ・ Ts and Tsubframe

=30720 ・ Ts.

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different subframes of a frame can either be used for downlink or uplink transmission.

in case of FDD all subframes of a carrier are either used for downlink transmission or

uplink transmission.

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In case of TDD subframe 0 and 5 are always assigned for downlink transmission while the remaining can be flexibly assigned to be used for either downlink or uplink transmission.

These subframes include the LTE synchronization signals.

The synchronization signals are transmitted on the downlink of each cell and are intended to be used for initial cell search as well as for neighbor-cell search.

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Type 1 LTE frame structure applicable for both FDD and TDD.

For LTE operating with TDD there is also an alternative or Type 2 frame structure , designed for coexistence with systems based on the current 3GPP TD-SCDMA-based standard.

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LTE Physical LayerDownlink Transmission scheme

The downlink physical resource: 1 ms sub-frame consists of two equally sized slots

of length Tslot =0.5 ms. symbol time Tu =1/∆f ≈ 66.7μs (2048 ・ Ts). Two cyclic-prefix lengths for LTE:

Normal CP Extended CP (MBSFN)

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LTE Physical LayerDownlink Transmission scheme The downlink physical resource

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The downlink physical resource Resource element corresponds to one OFDM

subcarrier during one OFDM symbol interval. OFDM subcarrier spacing ∆f=15 kHz. Sampling rate fs =15000 ・ NFFT (NFFT =2048) Ts defined is the sampling time of FFT-based

transmitter/receiver.

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The downlink physical resource Subcarriers are grouped into resource blocks. Resource block consists of 12 consecutive

subcarriers. An unused DC-subcarrier in the center of the

downlink spectrum. As it may coincide with the local-oscillator frequency at the base-station transmitter and/or mobile-terminal receiver.

Nsc =12 ・ NRB +1 6<NRB<110 1MHz<DL B.W.<20MHz

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The downlink physical resource

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Downlink reference signals DL reference symbols are the first and the third

last OFDM symbols of each slot and with a frequency-domain spacing of six subcarriers.

A frequency-domain spacing of three subcarriers between the first and second reference symbols.

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Reference-signals sequences and physical-layer cell identity: 510 reference signal sequences, corresponding

to 510 different cell identities. Reference-signal sequence is the product of a

two-dimensional pseudo-random sequence and a two-dimensional orthogonal sequence.

170 pseudo-random sequences corresponding to one out of 170 cell-identity groups.

3 orthogonal sequences corresponding to a specific cell identity within each cell-identity group.

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Reference-signal frequency hopping: Frequency-domain positions of the reference

symbols may also vary between consecutive subframes. First reference symbols: p(k) = (p0 + 6 ・ i + offset(k))

mod 6 Second reference symbols: p(k) = (p0 + 6 ・ i + 3 +

offset(k)) mod 6

Frequency-hopping pattern has a period of length 10

170 different frequency-hopping patterns defined, where each pattern corresponds to one cell-identity group.

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Reference signals for MCBSFN:

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Reference signals for multi-antenna transmission:

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LTE Physical LayerUplink Transmission scheme

The uplink physical resource: ∆f =15 kHz In contrast to the downlink, no unused DC-

subcarrier is defined for the uplink.

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The uplink physical resource:(cont.) The presence of a DC-carrier in the center of the

spectrum would have made it impossible to allocate the entire system bandwidth to a single mobile terminal and still keep the low-PAR single-carrier property of the uplink transmission.

The DFT-based pre-coding, the impact of any DC interference will be spread over the block of M modulation symbols and will therefore be less harmful compared to normal OFDM transmission.

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The uplink physical resource

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The uplink physical resource:(cont.) Overall time–frequency resource assigned to a

mobile terminal must always consist of consecutive subcarriers.

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The uplink physical resource:(cont.) Inter-slot frequency hopping implies that the

physical resources used for uplink transmission in the two slots of a subframe do not occupy the same set of subcarriers.

Frequency hopping provides additional frequency diversity, assuming that the hops are in the same order as or larger than the channel coherence bandwidths.

provides interference diversity (interference averaging),assuming that the hopping patterns are different in neighbor cells.

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Uplink reference signals: Reference signals frequency multiplexed with

data from the same mobile terminal is not possible.

Importance of low power variations for uplink transmissions.

Reference signals are time multiplexed with uplink data.

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Reference signals for channel sounding: UL channel-dependent scheduling by assigning

uplink resources to a mobile terminal depending on the instantaneous channel quality.

Estimates of the frequency-domain channel quality is needed.

DL by Reporting the estimated channel quality to the network by means of a Channel Quality Indicator (CQI).

Wide band reference signals

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Reference signals for channel sounding:(cont.)

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AGENDA:



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Physical-layer blocks which are dynamically controlled by the MAC layer are shown in grey.

Semi-statically configured physical-layer blocks are shown in white.

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LTE Physical LayerDownlink Channel Processing


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Physical-layer blocks which are dynamically controlled by the MAC layer are shown in grey.

while semi-statically configured physical-layer blocks are shown in white.

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LTE Physical LayerUplink Channel Processing

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LTE Physical LayerTransport Channel Processing

Multiplexing and channel coding Different channels may undertake

different combination of these block as we will see later on.

84All rights reserved © 2008, Faculty of Engineering @ Cairo University

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CRC attachment Cyclic Redundancy Check (CRC) is calculated and

appended to each transport block. The CRC allows for receiver-side detection of residual errors in the decoded transport block.

CRC parity bits are generated by one of the following cyclic generator polynomials:

The bits after CRC attachment are denoted by

where B = A+ L. The relation between ak and bk is:

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CRC attachment DL-SCH and MCH CRC parity length is L = 24 BCH CRC parity length is L = 16 DL-CCH CRC parity length is L = 16

16 however parity bit attachment differs from that defined before and shall be performed as CRC calculation gives a sequence of bits

The relation between ak and ck is where K = A+ L and is the UE

Identity sequence

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CRC attachment UL-SCH: The parity bits are computed and attached to the UL-SCH

transport block by setting L to 24 bits. UL-CSH: N/A UL-RCH:N/A

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Code Block Segmentation The input bit sequence to the code block segmentation is denoted by,

where B > 0. If B is larger than the maximum code block size Z, segmentation of the input bit sequence is performed and an additional CRC sequence of L = 24 bits is attached to each code block. The maximum code block size is Z = 6144.

Total number of code blocks C is determined by:

If B<=Z

L = 0 C = 1 B’ = BElse L = 24 C = ceil ( B/(Z-L) ) B’ = B + C*LEnd This block is used only in M-CH & UL/DL-SCH.

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Channel Coding There are two convolution coding types used with LTE

Tail biting convolution coding Turbo coding

The bit sequence input for a given code block to channel coding is denoted by

where K is the number of bits to encode. After encoding the bits are denoted by

where D is the number of encoded bits per output stream and i indexes the encoder output stream. The relation between and between K and D is dependent on the channel coding scheme.

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Channel Coding Tail biting convolutional coding

A tail biting convolutional code with constraint length 7 and coding rate 1/3 is defined.

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Channel Coding Turbo coding

The scheme of turbo encoder is a Parallel Concatenated Convolutional Code (PCCC) with two 8-state constituent encoders and one turbo code internal interleaver. The coding rate of turbo encoder is 1/3.

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Channel Coding DL-SCH and MCH LTE uses turbo code rate 1/3

BCH LTE uses tail biting convolutional code rate 1/3

DL-CCH LTE uses tail biting convolutional code rate 1/3

CFI LTE defines block code as shown below.

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Channel Coding UL-SCH: Turbo coding with rate 1/3 UL-CCH: not specified yet according to release 8, Sep07

Three forms of channel coding are proposed, one for the channel quality information (CQI), another for HARQ-ACK (acknowledgement) and another for channel quality information (CQI) and HARQ-ACK.

UL-RCH: N/A

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Rate matching

Rate matching for DL-SCH and UL-SCH is defined per coded block and consists of interleaving the three information bit streams, followed by the collection of bits and the generation of a circular buffer.

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Rate matching Sub-block interleaver The bits input to the block interleaver are denoted by where D is the

number of bits.

The output bit sequence from the block interleaver is derived as follows:

Construct R x C matrix such that C=32 and D <= R * C. If R × C > D, then ND = (R × C – D) dummy bits are padded. Write the input bit sequence into the R × C matrix row by row starting with

bit y0 in column 0 of row 0.

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Rate matching Sub-block interleaver The bits input to the block interleaver are denoted by where D is the

number of bits.

The output bit sequence from the block interleaver is derived as follows:

Construct R x C matrix such that C=32 and D <= R * C. If R × C > D, then ND = (R × C – D) dummy bits are padded. Write the input bit sequence into the R × C matrix row by row starting with

bit y0 in column 0 of row 0.

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Rate matching Sub-block interleaver

For Perform the inter-column permutation for the matrix based on the pattern that in the below table, where P(j) is the original column position of the j-th permuted column. After permutation of the columns, the inter-column permuted R × C matrix is equal to

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Rate matching The output of the block interleaver is the bit sequence read out column by column

from the inter-column permuted R × C matrix. The output of the sub-block interleaver is denoted by

Where P, the permutation function, is defined in the previous Table .

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Rate matching Bit collection, selection and transmission

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Code Block concatenation. Simply this block concatenate the different code blocks (C code block) that are

segmented before in the code block segmentation the output of this block is fed to the scrambling and modulation blocks as described in the following section.

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Downlink modulation and antenna coding

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Downlink modulation and antenna coding Scrambling Block of bits delivered by the code block concatenation is multiplied (XOR

operation) by a bit-level scrambling sequence By applying different scrambling sequences for neighbor cells, the interfering

signal(s) after de-scrambling are randomized, ensuring full utilization of the processing gain provided by the channel code.

In case of MBSFN-based transmission using the MCH transport channel,the same scrambling should be applied to all cells taking part in a certain MBSFNtransmission (cell-common scrambling)

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Downlink modulation The downlink data modulation transforms a block of scrambled bits to

corresponding block of complex modulation symbols. modulation schemes supported for the LTE are QPSK, 16QAM, and 64QAM,

corresponding to two, four, and six bits per modulation symbol. All these modulation schemes are applicable in case of DL-SCH transmission.

For other transport channels certain restrictions may apply. As an example, only QPSK modulation can be applied in case of BCH transmission, and DL-CCH

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Antenna mapping Layer mapping and pre-coding

Antenna Mapping jointly processes the modulation symbols corresponding to transport blocks, and maps the result to the different antennas.

LTE supports up to four transmit antennas Antenna mapping can be configured in different ways to provide

different multi-antenna schemes including transmit diversity, and spatial multiplexing

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Antenna mapping Layer mapping and pre-coding

The layer mapping provides de-multiplexing of the modulation symbols of each codeword (coded and modulated transport block) into one or multiple layers.

The pre-coding extracts exactly one modulation symbol from each layer, jointly processes these symbols, and maps the result in the frequency and antenna domain.

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Antenna mapping Example: spatial multiplexing

In case of spatial multiplexing there is, in the general case, two code words, NL layers, and NA antennas, with NL ≥ 2 and NA ≥ NL.

The following Figure illustrates the case of three layers (NL =3) and four transmit antennas (NA =4).

Layer mapping de-multiplexes the modulation symbols of the two code-words.

the first codeword is mapped to the first layer while the second codeword is mapped to the second and third layer. Thus, the number of modulation symbols of the second codeword should be twice that of the first codeword to ensure the same number of symbols on each layer.

Pre-coding then applies the pre-coding matrix W of size NA ×NL to the each layer vector vi.

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Antenna mapping Example: spatial multiplexing

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Resource mapping Maps the symbols to be transmitted on each antenna to the resource elements of

the set of resource blocks assigned for the transmission of the transport block(s). the selection of resource block can, be based on estimates of the channel quality

of the different resource blocks as seen by the target mobile terminal. downlink scheduling is carried out on a sub-frame (1 ms) basis. Thus, as a

downlink resource block is defined as a number of sub-carriers during one 0.5 ms slot, the downlink resource-block assignment is always carried out in terms of pairs of resource blocks.

some of the resource elements within a resource block will not be available for transport channel mapping as they are already occupied by:

Downlink reference symbols including. Downlink L1/L2 control signaling.

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Resource mapping

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Resource mapping Downlink L1/L2 control signaling

L1/L2 control channels are mapped to the first (up to three) OFDM symbols within each subframe.

Physical resource to which the L1/L2 control signaling is mapped consists of a number of control-channel elements, where each control channel element consists of a predefined number of resource elements.

Modulated symbols of each L1/L2 control channel is then mapped to one or several control-channel elements depending on the size.

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Uplink modulation and antenna coding

Scrambling UL-SCH & UL-CCH use UE specific scrambling sequences prior to

modulation. UL-RCH: N/A

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Modulation UL-SCH supports all modulation schemes

UL-CCH uses only BPSK or QPSK according to the following table.

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Transform pre-coding ( UL-SCH only) Each block of complex-valued modulated symbols is divided

into M symb / M scPUSCH, each corresponding to one SC-FDMA

symbol.

Transform pre-coding shall be applied according to:

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α2,α3,α5 is a set of non-negative integers

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Mapping to physical resources UL-SCH

The mapping to resource elements (k, l), not used for transmission of reference signals, shall be in increasing order of first the index l, then the slot number and finally the index k which is given by.

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f hop(.) denotes the frequency-hopping pattern.

k0is given by the scheduling decision.

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Mapping to physical resources UL-CCH

The mapping to resource elements (k, l) not used for transmission of reference signals shall start with the first slot in the sub-frame. The set of values for index k shall be different in the first and second slot of the sub-frame, resulting in frequency hopping at the slot boundary.

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AGENDA:



Cell search Random access Paging Power control

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Physical Layer Procedures Cell search Cell search is the procedure by which a UE acquires

time and frequency synchronization with a cell and detects the physical layer cell ID of that cell.

LTE supports 510 different cell identities, divided into 170 cell-identity groups, each group containing three unique identities.

A cell is uniquely defined by a number in the range of 0 to 169, representing the cell-identity group, and a number in the range of 0 to 2, representing the cell identity within the cell-identity group.


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Physical Layer Procedures Cell search LTE provides a primary synchronization signal and a

secondary synchronization signal on the downlink to assist in the cell search procedure .

Synchronization signals are specific sequences, inserted into the last two OFDM symbols in the first slot of sub-frame zero and five (slots number 0 and 10).


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Primary and secondary synchronization signals:

Physical Layer Procedures Cell search


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Time/frequency structure of synchronization signals At the beginning of the cell-search procedure, the cell

bandwidth is not necessarily known. To maintain the same cell-search procedure, regardless

of the overall cell transmission bandwidth, the synchronization signals are transmitted using the 72 center subcarriers, corresponding to a bandwidth of 1.08 MHz.

Thirty-six subcarriers on each side of the DC subcarrier in the frequency domain are reserved for the synchronization signal.



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Secondary synchronization signal generation: The sequence used for the second synchronization signal is an

interleaved concatenation of two length-31 binary sequences obtained as cyclic shifts of a single length-31 sequence generated by

. The concatenated sequence is scrambled with a

scrambling sequence given by the primary synchronization signal. The combination of two length-31 sequences defining the secondary

synchronization signal differs between slot 0 and slot 10.


)61(),...,0( dd


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First step The mobile terminal uses the primary synchronization

signal to find frame timing with 5 ms ambiguity. One possible implementation is to do matched filtering

between the received signal and the sequences specified for the primary synchronization signal and get the maximum of the MF output.

Products of first step Frame timing on a 5 ms basis. Finding the identity within the cell-identity group. As there is a one-to-one mapping between each of the

identities in a cell-identity group and each of the three orthogonal sequences used when creating the reference signal, the terminal obtains partial knowledge about the reference signal structure in this step.



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Second step: Observe pairs of slots where the secondary

synchronization signal is transmitted. If (s1, s2) is an allowable pair of sequences, where s1 and

s2 represent the secondary synchronization signal in subframe zero and five, respectively, the reverse pair (s2, s1) is not a valid sequence pair.



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Products of second step: The terminal can resolve the 5 ms timing ambiguity

resulting from the first step and determine the frame timing.

As each combination (s1, s2) represents one of the cell identity groups, the cell identity group is obtained from the second cell-search step.

From the cell identity group, the terminal knows which pseudo-random sequence is used for generating the reference signal in the cell.

Once the cell-search procedure is complete, the terminal receive the broadcasted system information to obtain the remaining parameters, for example, the transmission bandwidth used in the cell.



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After the first step, the primary synchronization signal is known and can be used for channel estimation.

The primary and secondary synchronization signals are transmitted in two subsequent OFDM symbols.

This channel estimate can be used for coherent processing of the received signal prior to the second step in order to improve performance.

Synchronization signals are located at the end of the first slot in the sub-frame, instead of the second slot to have fewer restrictions on the creation of guard times between uplink and downlink in case of TDD.



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Initial cell search For initial cell search, the terminal does not know the

carrier frequency of the cells it is searching for. The terminal needs to search for a suitable carrier

frequency, by repeating the above procedure for any possible carrier frequency.

Initial cell search has relaxed search-time requirements. The terminal can use any additional information it has

(for example, starts searching on the same carrier frequency it was last connected to).



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Neighbor-cell search Neighbor-cell search has stricter timing requirements. In the case of intra-frequency handover, the terminal

does not need to search for the carrier frequency in the neighboring cell.

Not major problem as the neighboring candidate cells transmit at the same frequency as the terminal already is receiving data upon.

Data reception and neighbor-cell search are simple separate baseband functions, operating on the same received signal.



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Neighbor-cell search (contd.): In the case of inter-frequency handover, data

reception and neighbor-cell search need to be carried out at different frequencies.

Equipping the terminal with a separate RF receiver circuitry for neighbor-cell search, is a complex solution.

A possible solution is to create gaps in the data transmission, during which the terminal can retune to a different frequency.

This is done by avoiding scheduling the terminal in one or several downlink subframes.



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AGENDA:




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Random access is the process by which a terminal requests a connection setup.

Purposes of random access: Establishment of uplink synchronization.

Establishment of a unique terminal identity, C-RNTI (Cell Radio Network Temporary Identifier), known to both the network and the terminal

When? Initial access when moving from LTE_DETACHED or

LTE_IDLE to LTE_ACTIVE. After periods of uplink inactivity when uplink

synchronization is lost in LTE_ACTIVE.

Physical Layer Procedures Random access


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Step 1: Random access preamble transmission

Indicates to the network the presence of a random-access attempt and obtains uplink time synchronization.

Random-access-preamble transmissions can be either orthogonal or non-orthogonal to user data.

To avoid interference between data and random-access preamble, the transmission of the random-access preamble in LTE is made orthogonal to uplink user-data transmissions.

The network broadcasts information to all terminals about which time-frequency resources random-access preamble transmission is allowed.

The network avoids scheduling any uplink transmissions in these time-frequency resources.

A sub-frame is reserved for preamble transmissions.



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Step 1: Random access preamble transmission

The random-access preamble has a bandwidth corresponding to six resource blocks (1.08 MHz).

The same random-access preamble structure can be used, regardless of the transmission bandwidth in the cell.

Prior to the transmission of the preamble, the terminal has already obtained downlink synchronization from the cell-search procedure.

The start of an uplink frame at the terminal is defined relative to the start of the downlink frame at the terminal.



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Step 1: Random access preamble transmission Due to the propagation delay between the base station

and the terminal, the uplink transmission will be delayed relative to the downlink transmission timing at the base station.

As the distance between the base station and the terminal is not known, there will be an uncertainty in the uplink timing corresponding to twice the distance between the base station and the terminal.

To account for this uncertainty and to avoid interference with subsequent subframes not used for random access, a guard time is used.



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Step 1: Random access preamble generation

The preamble is based on Zadoff–Chu (ZC) sequences and cyclic shifted ZC sequences.

From each root Zadoff–Chu sequence X(u)ZC(k) , m−1

cyclically shifted sequences are obtained by cyclic shifts of each, where MZC is the length of the root Zadoff–Chu sequence.

The amplitude of ZC sequences is constant, which ensures efficient power amplifier utilization and maintains the low PAR properties of the single-carrier uplink.

ZC sequences also have ideal cyclic auto-correlation, which is important for obtaining an accurate timing estimation at the eNodeB.

Due to the zero cross-correlation property of cyclically shifted ZC sequences, there is no intra-cell interference from multiple random-access attempts using preambles derived from the same Zadoff–Chu root sequence.



Step 1: Random access preamble generation

Preamble sequences are partitioned into groups of 64 sequences each.

Each cell is allocated one such group by defining one or several root Zadoff–Chu sequences and the cyclic shifts required to generate the set of preambles.

When performing a random-access attempt, the terminal selects one sequence at random from the set of sequences allocated to the cell the terminal is trying to access.

As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will be detected by the network.

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Step 1: Random access preamble detection

Samples over a window of length 0.8 ms are collected and converted into the frequency-domain representation using an FFT.

The output of the FFT is multiplied with the complex-conjugate frequency-domain representation of the root Zadoff–Chu sequence and the results is fed through an IFFT.

By observing the IFFT outputs, it is possible to detect which of the shifts of the Zadoff–Chu root sequence has been transmitted and its delay.

A peak of the IFFT output in interval i corresponds to the i-th cyclically shifted sequence and the delay is given by the position of the peak within the interval.

In case of multiple random-access attempts using different cyclic shifted sequences generated from the same root Zadoff–Chu sequence, there will be a peak in each of the corresponding intervals.



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Step 1: Random access preamble detection



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Step 2: Random access response In response to the detected random access attempt, the

network will transmit a message on the DL-SCH containing: The index of the random-access preamble sequence the

network detected and for which the response is valid. The timing correction calculated by the random-access-

preamble receiver. A scheduling grant, indicating resources the terminal shall

use for the transmission of the message in the third step. A temporary identity used for further communication

between the terminal and the network.



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Step 2: Random access response

In case the network detected multiple random-access attempts (from different terminals), the individual response messages for multiple mobile terminals can be combined in a single transmission on the DL-SCH and indicated on a L1/L2 control channel using an identity reserved for random-access response.

As long as the terminals that performed random access in the same resource used different preambles, no collision will occur.

When multiple terminals are using the same random access preamble at the same time , contention will occur.

Upon reception of the random-access response in the second step, the terminal will adjust its uplink transmission timing and continue to the third step.



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Step 3: Terminal identification

Before user data can be transmitted to/from the terminal, a unique identity within the cell (C-RNTI) must be assigned to the terminal.

In the third step, the terminal transmits the necessary messages to the network using the resources assigned in the random-access response in the second step.

The terminal sends its message on the UL-SCH similar to normal scheduled data.

The use of the ‘normal’ uplink transmission scheme for message transmission allows the grant size and modulation scheme to be adjusted to different radio conditions.

The exact content of this message depends on the state of the terminal, whether it is previously known to the network or not.



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Step 4: Contention resolution

The network transmits a contention-resolution message to the terminal on the DL-SCH.

This step resolves any contention due to multiple terminals trying to access the system using the same random-access resource.

Each terminal receiving the downlink message will compare the identity in the message with the identity they transmitted in the third step.

Only a terminal which observes a match between the identity received in the fourth step and the identity transmitted as part of the third step will declare the random access procedure successful.

Since uplink synchronization already has been established, hybrid ARQ is applied to the downlink signaling in this step.



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AGENDA:




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Paging is used for network-initiated connection setup.

An efficient paging procedure should allow the terminal to sleep with no receiver processing most of the time and to wake up only at predefined time intervals to monitor paging information from the network.

In LTE, no separate paging-indicator channel is used.

Physical Layer Procedures Paging


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A Discontinuous cycle is defined, which allows the terminal to sleep most of the time and only briefly wake up to monitor the L1/L2 control signaling.

If the terminal detects a group identity used for paging when it wakes up, it will process the corresponding paging message transmitted in the downlink.



The paging message includes the identity of the terminal(s) being paged.

The terminal that does not find its identity will discard the received information and sleep according to the DRX cycle.

The uplink timing is unknown during the DRX cycles no ACK/NACK signaling can take place (No hybrid ARQ can be used for paging messages).

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AGENDA:




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Downlink power control determines the energy per resource element (EPRE).

Energy per resource element denotes the energy prior to CP insertion and also denotes the average energy taken over all constellation points for the modulation scheme applied.

Uplink power control determines the average power over a DFT-S OFDM symbol in which the physical channel is transmitted.

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Physical Layer Procedures Power control


Uplink power control: Uplink power control controls the transmit power of the different

uplink physical channels. A cell wide overload indicator (OI) is exchanged for inter-cell power

control. An indication X is exchanged to indicate PRBs that an eNodeB

scheduler allocates to cell edge UEs and that will be most sensitive to inter-cell interference.

Physical uplink shared channel power control. Physical uplink control channel power control. Sounding Reference Symbol power control.

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Downlink power control: The eNodeB determines the downlink transmit energy per resource

element. A UE shall assume downlink reference symbol EPRE is constant

across the downlink system bandwidth and constant across all subframes until different RS (Reference signal) boosting information is received.

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6558145 lte-final-v1

Technology

Transcript of 6558145 lte-final-v1