LONG TERM EVOLUTION (LTE)

AIR INTERFACE

Commonalities and Difference in TDD and FDD

TDD and FDD modes commonalities

TDD and FDD modes differencesregarding the air interface1. Same radio interface

schemesfor both uplink and downlink(OFDM and SC-FDMA)

2. Same subframe formats

3. Same network architecture

4. Same air interface protocols

5. Same physical channelsprocedures

1. Spectrum Allocation:TDD is using the same frequency bands forboth UL and DLFDD requires a paired spectrum with duplex separation in frequencyTDD requires an unpaired spectrum withsome guard bands in time to separateUL and DL

2. UE complexity:In FDD the UE is requiring an duplex filter(for UL – DL separation)In TDD Filter is not needed Lower complexity for TDD terminals

Basic principles – Air interface

Downlink OFDM OFDM = Orthogonal Frequency

Division Multiplexing OFDM = Parallel transmission on

multiple carriers Advantages of OFDM

Makes efficient use of the spectrum by allowing overlap

High transmission bitrates Eliminates ISI through use of a cyclic

prefix. Disadvantage of OFDM

High PAPR and lower power amplifier efficiency

Uplink (SC-FDMA) SC-FDMA = Single carrier FDMA Advantages

Signal has single carrier properties Low PAPR Similar hardware as OFDM Efficient power consumption

Disadvantage of SCFDMA Equalizer needed

DL modulation

UL modulation

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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. Cell search supports a scalable overall transmission bandwidth corresponding to 6 Resource Blocks and upwards.

• The UE has to decode the Primary synchronization signals

[PSS] as well as the Secondary synchronization signals [SSS] to decode the Physical Layer Cell ID.

eNB

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Cell Search

The LTE cell search involves the following :

Acquire Frequency and Symbol synchronization to a cell Acquire Frame Timing of the cell [ start of Downlink Frame ] Determine the Physical layer cell identity of the cell

eNB

The first steps after switching on the mobile are the following:

1. Primary Synchronization Signal PSS– from which the mobile can acquire frequency and time-slot synchronization. The synchronization is absolutely necessary, otherwise the mobile cannot read the rest of the physical channels. Also from the PSS the mobile is learning the cell identity – which could have values 0,1 or 2 in LTE. The cell identities are used to differentiate between different cells

2. Secondary Synchronizations Signal SSS – from which the mobile can learn what is the frame structure (10 ms in LTE). Also the physical cell id group with values from 1 to 168 is achieved. The physical cell id together with the group are used to separate the cells in LTE.

3. DL reference signals – they have almost the same functionality like the CPICH (common pilot channel) in UMTS. Used for channel estimation and measurements.

4. PBCH – Physical Broadcast Channel. From this channel the UE is learning about the system information. Please note that in LTE the PBCH is designed to have minimum possible information (for coverage reasons mainly). The PBCH contains only the MIB, Therefore the rest of system information which is organized in SIBs = System Information Blocks is now sent on the Physical Downlink Shared Channel PDSCH. From PBCH the UE is learning the system bandwidth 1.4, 3, … 20 MHz and the PHICH = Physical HARQ Indication Channel configuration.

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Reference signals

Reference signals :Cell search & initial acquisition DL channel quality estimation

Mainly there are two type of reference signals

• Cell specific DL reference signals Every DL subframeAcross entire DL bandwidth

• UE specific DL reference signalsIntended for individual UE’s

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• There are 504 different Reference Sequences (RS)• They are linked to PHY-layer cell identities • Shifts are introduced to avoid collision between RS of

adjacent cells• For a given PHY Cell ID - sequence is the same regardless

of the bandwidth used.

Cell specific reference signals

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UE Specific RS

• UE specific RS – used for beam forming• Provided in addition to cell specific RS• Sent over resource block allocated for DL-SCH

(applicable only for data transmission)

Note: additional reference signals increase overhead. One of the most beneficial use of beam forming is at the cell edge – improves SNR

Rank Index

Represents the number of layers to be used in downlink transmission.

Rank Index values are proposed by the UE based on the radio conditions.

eNodeB may or may not follow UE proposal.

2x2 MIMO• Spatial multiplexing : RI = 2• Transmit diversity : RI = 1• SDMA: RI = 1• Beam forming : RI = 14x4 MIMO• Spatial Multiplexing : RI = 2, 3, 4.• Transmit Diversity : RI = 1.• SDMA: RI = 1• Beam forming : RI = 1

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Physical cell identity (PCI)

PCI = PSS + 3* SSS

Primary synchronization signal :

• Frequency synchronization.• Time slot synchronization.• 3 PSS values (0,1,2)

Secondary synchronization signal :

• Radio frame synchronization.• TDD/FDD duplex information.• CP length.• 168 SSS values, (0 to 167).

3*168 = 504 different PCI values.

Planning guidelines:• Co PCI scenario must never

happen.• All 3 cells of a site should have

same SSS.• PSS should be defined 0 for first

sector, 1 for second sector, 2 for third sector.

• Some SSS should be reserved for indoor sites i.e. (151 – 167).

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Cyclic prefix

• Multipath propagation degrades orthogonality between carriers, to regain the orthogonality – cyclic prefix is used.

• Copying the last part of a symbol shape for a duration of guard-time and attaching it in front of the symbol.

• A receiver typically uses the last part of the following symbol to locate the start of the symbol and begin then with decoding.

• Normal CP : Length is 4.7 us. With normal CP, 7 symbols per time slot.• Extended CP : Length is16.7 us. 6 symbols per time slot.

Cell Selection

Initial Cell Selection:This procedure requires no prior knowledge of which RF channels are E-UTRA carriers. The UE shallscan all RF channels in the E-UTRA bands according to its capabilities to find a suitable cell. On each carrier frequency, the UE need only search for the strongest cell. Once a suitable cell is found this cell shall be selected.Stored Information Cell Selection:This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, from previously received measurement control information elements or from previously detected cells. Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the Initial Cell Selection procedure shall be started.Suitable cell:A "suitable cell" is a cell on which the UE may camp on to obtain normal service. Such a cell shall fulfil all the following requirements.- The cell is part of either: - the selected PLMN or the registered PLMN, or PLMN of the Equivalent PLMN list- The cell is not barred- The cell is part of at least one TA that is not part of the list of "forbidden tracking areas for roaming“.

Cell Reselection

Reselection priorities handling:1)Absolute priorities of different E-UTRAN frequencies or inter-RAT frequencies may be provided to the UE in the system information, in the RRCConnectionRelease message, or by inheriting from another RAT at inter-RAT cell (re)selection.2)In the case of system information, an E-UTRAN frequency or inter-RAT frequency may be listed without providing a priority (i.e. the field cellReselectionPriority is absent for that frequency). If priorities are provided in dedicated signalling, the UE shall ignore all the priorities provided in system information.3)If UE is in camped on any cell state, UE shall only apply the priorities provided by system information from current cell, and the UE preserves priorities provided by dedicated signalling unless specified otherwise.4)The UE shall delete priorities provided by dedicated signalling when:- the UE enters RRC_CONNECTED state; or a PLMN selection is performed on request by NAS.5)The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT frequencies that are given in system information and for which the UE has a priority provided.6)The UE shall not consider any black listed cells as candidate for cell reselection.

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RSRP

Reference signal received power:

Clutter Type Average Clutter Loss for 2300 MHz

Acceptable Outdoor RSRP Level

dBm

Indoor RSRP level as per definition of cell edge: -UL> 512

kbps

Dense Urban 24 -91 -115

Urban 20 -95 -115

Suburban 16 -99 -115

Rural 10 -105 -115

RSRP values:• Near : > -75 dBm, SINR > 25.• Middle : -85 dBm to -95 dBm, SINR 17 to 20.• Edge : -100 to -110 dBm, SINR 8 to 12.

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RSRQ & SINR

Reference signal received quality (RSRQ) :• Similar to Ec/Io in 3G.• 95% samples should be better than -12.

Signal to noise ratio (SINR) :• Excellent : > 25.• Good : Between 15 to 25.• Average : Between 5 to 15.• Poor : Less than 5.

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BLER & UE Tx power

BLER :• LTE works on 10% BLER.• 95% samples should be less than 10.

UE Tx power :• Class 3 UE has max Tx power as 23 dBm +/- 2.• 95% samples should be less than 15.• Poor : > 15.• Average : Between 5 to 15.• Good : Less than 5.

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UE states in LTE

LTE UE States:

1- Idle2- Active3- Detached

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UE states in LTE

Idle• Exact location is not known. Only TAC is known.• Bearer information is not known, only default bearer

is available.• RRC connection is released.• UE monitors paging channels for incoming calls.• UE does not inform network about the cell change.• Performs neighboring cell measurements and cell

selection/reselection.• Idle mode is UE power conservation state.• Reduces signaling overheads as compared to Active

state. Active

• Network knows the serving Cell ID.• UE transmits & receives data.

Detached • UE disconnected or powered off.

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Layer 3 states

Layer 3 Protocols :• 1- RRC (Radio resource

control)• 2- NAS (Non access

stratum) RRC States : Idle Connected

NAS States :• EMM (EPS Mobility Management)

DeregisteredRegistered

• ECM (EPS Connection Management) Idle

Connected

Handover in LTE

Objectives The eNodeB sends the measurement configuration to a UE, and

the UE performs measurements and completes the handover procedure under the control of the eNodeB to maintain seamless service.

Triggers for Handover in LTE Coverage: Coverage-based handover connects a moving UE

to the cell with the best signal quality at any given moment, to guarantee that calls are not dropped during mobility. (Supports coverage-based handover and implemented in network)

Load: Load-based handover transfers UEs from a heavily loaded or congested cell to a less loaded cell, to maximize use of system resources. (Supports Load-based handover and currently not implemented in network)

Type of service: Cells which support high speed data services transfer UEs with only voice services to other RATs. (Not supported at present)

Types of Handover in LTE

Intra-frequency Handover Handover between two LTE cells on the same frequency. Intra-frequency handovers are triggered by UE measurements. As a UE

moves from its serving cell to a neighboring cell on the same frequency, it detects that signal quality is higher in the neighboring cell, and this triggers a coverage-based handover.

Inter-frequency Handover Handover between two LTE cells on different frequencies. Inter-frequency measurements are triggered by UE measurements. As a

UE moves from its serving cell to a neighboring cell on a different frequency, when signal quality in the serving cell drops below a certain threshold, this triggers coverage-based inter-frequency measurements.

Inter-RAT Handover Handover from LTE cells to GSM/WCDMA/TD-SCDMA/CDMA2000 cells. Inter-RAT measurements are triggered by UE measurements. As a UE

moves out of the area covered by the LTE system, when signal quality in the serving cell drops below a certain threshold, this triggers coverage-based inter-RAT measurements.

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Handover Events and Optimization

LTE Handover events:• A1• A2• A3• A4• A5

IRAT HO:• B1• B2

Basic optimization parameters:• Offset• Hysteresis• Time to trigger• Thresholds

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Event A1

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Event A2

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Event A2 examples

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Event A3

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A3 optimization

The smaller the value of a3offset+hysteresisa3 the faster we release the calls to neighboring cells.

The higher the value of a3offset+hysteresisa3 the more difficult we make it for calls do handover to other cells.

Time to trigger:

• If a3offset+ hysteresisa3 is relatively large (i.e.: 6dB or stronger), then a value of timetotriggera3 under 100 ms is acceptable.

• If a3offset+ hysteresisa3 is relatively small (i.e.: 2dB), then a value of timetotriggera3 should be around 320 to 640 ms.

The value allocated to timetotriggera3, hence, depends on:

• Parameter setting of a3offset and hysteresisa3,• Morphology (dense urban, urban, suburban, rural)• Speed of UE in the cells (freeways and or suburban roads).

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A3 examples

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Event A4

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Event A5

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Event B1

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Event B2

Event-Triggered Reporting in LTE

Events Event A1: Signal quality in the serving cell is above a threshold. When a UE reports that the serving cell

meets the triggering condition, the eNodeB stops inter-frequency or inter-RAT measurements.

Event A2: Signal quality in the serving cell is below a threshold. When a UE reports that the serving cell

meets the triggering condition, the eNodeB starts inter-frequency or inter-RAT measurements.

Event A3: Signal quality in intra-frequency neighboring cells is higher than that in the serving cell. When a

UE reports this event, the eNodeB sends an intra-frequency handover request.

Event A4: Signal quality in inter-frequency neighboring cells is above a threshold. When a UE reports this

event, the eNodeB sends an inter-frequency handover request.

Event B1: Signal quality in inter-RAT neighboring cells is above a threshold. When a UE reports this

event, the eNodeB sends an inter-RAT handover request.

Reporting Event-triggered periodic reporting

Complete LTE Handover Process

Three Phases of Handover Handover measurement: UEs perform measurements, which are

triggered as described in the previous slide.

Handover decision: Based on measurement reports from UEs, the eNodeB decides whether to initiate handovers.

Handover execution: The handover procedure is executed under the control of the eNodeB.

Note This presentation uses the common type intra-frequency handover for example.

Inter-frequency and inter-RAT handover procedures are similar.

Coverage-Based Intra-Frequency Handover: Decision and Execution

Decision1. The eNodeB generates a list of candidate cells that meet the condition for event A3

based on UE measurement reports.2. It then screens the list of candidate cells. Where measurement results are identical,

intra-eNodeB cells are prioritized over inter-eNodeB cells.

Execution The eNodeB triggers a handover to the target cell with the best signal quality.

There are four possible scenarios: Inter-eNodeB intra-MME handover in the presence of X2. Signaling messages and

packet data are transmitted over the X2 interface between the eNodeBs. Inter-eNodeB intra-MME handover in the absence of X2. Signaling messages and

packet data are transmitted over the S1 interface. Inter-eNodeB inter-MME handover in the presence of X2. Signaling messages are

transmitted over the S1 interface and EPC, and packet data is forwarded over the X2 interface.

Inter-eNodeB inter-MME handover in the absence of X2. Signaling messages and packet data are transmitted over the S1 interface and EPC.

Handover Events and Parameters

INTRA Freq HO- Event A3

INTER Freq HO- Event A3 (Start of Inter freq measurements A2 and A1 end of Inter Freq Measurements)

HO_Para_Inter_Intra

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