02_NSN_Tele2_LTE_Background_and_Principles.pdf

1 Nokia Siemens Networks 2008 Customer Confidential

LTE Background/Principles for Link Budget

Stockholm, 26.01.2009

Piotr GodziewskiNetwork Engineering


Meeting Agenda

Requirements Peak data rate, capacity, latency

Radio access principles SC-FDMA/OFDMA Frame structure

Time and frequency resource grid Cyclic Prefix Channel estimation

Key features Multiple antennas Scheduling Adaptive Modulation and Coding HARQ Power Control

UE Categories


Requirements

3GPP 25.913

Peak user data rate (20MHz, 2RX div., 1TX at UE/eNB)

Spectrum efficiency (DL: 2TX-2RX, UL: 1TX-2RX)

U-plane latency (ping 32 bytes)

DL: 100 MbpsUL: 50 Mbps

DL: 34 times Release 6UL: 23 times Release 6


Meeting Agenda





UE Categories


Radio access principlesDownlink OFDMA

f

Subcarrier centre frequency at the null point of adjacent subcarrier

No Adjacent Carrier Interference

Each subcarrier can be separately modulated

Spectrum efficiency & coverage

Easy way to support multi user MIMO technique. Different MIMO schemes can be applied on various channel parts

Channel aware scheduling possible. Best subcarriers assignment based on feedback channel condition information

Subcarrier power boost possible. More efficient Power Control algorithms

FFT size and sampling frequency various for different bandwidth configurations.Max at 20 MHz FFT 2048, sampling 30.72 MHz

kHzN

ff sampling 15==

Time domain unit Tu = 0.032552 s

Example:Timeslot is defined as follows: 15360 x Tu = 0.5 ms


Radio access principlesUplink SC-FDMA Why change to Single Carrier FDMA?

Conventional OFDMA covers the whole bandwidth. PAPR is rising when variously modulated signals are transmitted in one symbol

Modified amplifiers required (higher cost, larger power consumption, hardware complexity)

Low PAPR characterized SC-FDMA + all conventional multi carrier scheme advantages

Not acceptable for UE


Radio access principlesUplink SC-FDMA

Low PAPR low hardware complexity

Single Carrier property

Flexible user bandwidth assignment

Multi Carrier property

Longer battery life due to low PAPR and possibility of accumulating UE power only on assigned subcarriers

M N

N-point IFFT refers to the whole channelM-point DFT refers to assigned bandwidth

User 2 f

User 1 f

f

Receiver


Frame structure

3GPP 36.211 defines Type 1 for full/half duplex FDD mode Type 2 for TDD mode


OFDM signal representation

Time/frequency resource grid

Covers two consecutive timeslots (time domain) and 12 subcarriers (frequency domain).No user multiplexing on PRB.7 x 12 = 86 Resource Elements x2 = 172 REsPRB bandwidth = 15 kHz x 12 = 180 kHz

Physical Resource Block

Smallest possible part of the grid carrying one modulated symbol.No data multiplexing on RE

Resource Element

Within one single OFDM symbol yesBetween Resource Elements from various OFDM symbols no!

Orthogonality

Inter Symbol Interference


Cyclic Prefix

Mechanism against Inter Symbol InterferenceNo orthogonality between subcarriers (k) coming from different OFDM symbol: (l-1)and (l). Time dispersion effects the orthogonality loss between subcarriers covered by the modulation window.

So important because of large side lobes of subcarriers large degradation effect even if small time dispersion.

f0 f1 f2 f3 f4

Solution is a guard period

Cyclic Prefix has been chosen for LTE air interface

12

3


Cyclic Prefix

Advantages Modulation window still operates with

the one and the same subcarrier Orthogonality preserved until the time delay does not exceed

CP length Separate configuration for DL and UL

Disadvantages Additional overhead (power

consumption and throughput) Requires trade-off between resource

loss and data corruption

Normal CP = (3GPP 36.211)

Regular deployments

Extended CP = (3GPP 36.211)

Hilly environmentsMBMS (Multimedia Broadcast Multicast Services)Other deployments for which very large delay is expected

Normal CP 7 symbols per 0.5 ms

Extended CP 6 symbols per 0.5 ms

12

3


Channel estimation

Channel state information has to be known for correct data demodulation

Reference signal Placed on predefined Resource Elements Position and sequence is known to the

receiver Possibility of channel estimation in the

vicinity of reference symbols(interpolation techniques in time and frequency domain)

Reference symbols density on a grid must be sufficient to allow a receiver to collect enough channel knowledge Additional overhead dedicated Resource

Elements cannot be used by data

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Channel estimationDL Reference signal configuration Reference signal configuration depends on the used antenna scheme

Each antenna port must have its own reference signal(channel estimation for every transmit branch separately)

Resource Elements for reference signal cannot be reused on other antenna ports

2 TX antennas

Subframe

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RS used by the antenna port 0

RS used by the antenna port 1

REs that cannot be reused

Transmission on antenna port 0 Transmission on antenna port 1


Channel estimationDL Reference signal configuration Reference signal configuration depends on the used antenna scheme

Each antenna port must have its own reference signal(channel estimation for every transmit branch separately)

Resource Elements for reference signal cannot be reused on other antenna ports

4 TX antennas

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Channel estimationUL Reference signal configuration Reference signal is placed in every resource block carrying

controlling/user data on: 4th OFDM symbol in case of normal CP 3rd OFDM symbol in case of extended CP

Timeslot

REs occupied by PUSCH reference signal


Meeting Agenda





UE Categories


Multiple antennas

MIMO Multiple Input Multiple Output Using of multiple transmit/receive antennas to increase channel capacity or

channel condition (diversity techniques) Most representative transmission modes:

3GPP Mode 1

Single antenna port.1TX antenna transmitting always on port 0

3GPP Mode 2

Transmit diversity.Multiple antennas transmitting the same signal (single stream transmission).Improves SINR.

3GPP Mode 3

Open loop spatial multiplexing.Multiple antennas transmitting different signals (multiple streams transmission) without channel feedback for the used ports.Improves user data rate.

3GPP Mode 4

Closed loop spatial multiplexing.Multiple antennas transmitting different signals (multiple streams transmission) with channel feedback for the used ports from UEs.Improves user data rate.


Multiple antennas

MIMO configuration possibilities for DL channels Reference signal on every used antenna ports (mode independent) Synchronization signals (primary/secondary) always on antenna port 0 No spatial multiplexing for controlling channels

Other details TX diversity based on Alamouti scheme Closed loop MIMO precoding matrix and rank is chosen based on PMI (Precoding Matrix Indicator)

and RI (Rank Indicator) reported from UE Open loop MIMO without PMI feedback Adaptive/dynamic MIMO switching between different MIMO modes depending on radio link

conditions (on UE basis control) Multi Ratio Combining / Interference Rejection Combining receive diversity for uplink (eNB and UE)


Multiple antennas

Static or dynamic configuration per cell possible Dynamic (adaptive) MIMO

Switching points as SINR thresholds (O&M) allows for on UE (radio link) basis MIMO mode changes

Dual stream MIMO- high peak data rate- high SINR required

Single stream MIMO- improved SINR- better performance on the cell-edge

Both UEs experiencing good SINR- dual stream MIMO can be exploited


Multiple antennasSample solution 3-sector site 2TX-2RX (4RX optionally)

System module High Capacity System Module up to 3 cells @ 20 MHz bandwidth (2TX)

DL 150 Mbps / UL 50 Mbps per cell

2 x Triple RF Module Redundancy support (switching to 1TX in case of RF Module malfunction) 8/20/40/60 W per antenna connector (120 W per sector with 2TX div.) Feederless solution possible

Sector

1Sec

tor 3

Sector

2

Tx1/Rx1

Tx2/Rx2

Div Rx2

Rx3

Rx4 Tx/Rx2

Tx/Rx1

Rx3

Rx4

Extended coverage with one sector 2TX 60 + 60 W

Extended cell range(up to six cells @ 10 MHz per High Capacity System Module)


Multiple antennasSample antenna (co-location with GSM) Dual cross-polar Kathrein 800 10516

2TX GSM / 2TX LTE 2RX GSM / 4 RX LTE

MHA(optional)

Antennas

Feeders

Flexi LTE BTS

TX

RX

Single XXPol Panele.g. Kathrein 800 10516

Cross-polar advantage

Spatially separated MIMO branches suffer from path loss mismatch. Cross-polar provides better orthogonality for MIMO branches

Four antennas in one box


Scheduling

FDPS Frequency Domain Packet Scheduling Set 1 UEs with DL data available (need for resources) Set 2 based on time domain scheduling strategy (optimizing general time

domain performance, e.g. average data rate, retransmissions, etc.) Resolution: TTI (Time Transmission Interval = 1 ms);

possibility of persistence scheduling Set 3 based on frequency domain scheduling strategy

DL resolution: RBG (Resource Block Group) UL resolution: must fulfill the specific formula

integer:,,532#

kjiPRBPerUser kji =


Scheduling

Channel aware scheduling Radio link conditions must be known

DL: CQI reporting mode defines CQI reports resolution (3GPP 36.213) UL: Sounding Reference Signal required (known sequence is transmitted to eNB to

provide information about uplink channel condition) ~ 3.5% overhead

Frequency

Resource Block Group

Transmission on non-faded bandwidth parts

Carrier bandwidth

Frequency dependent fading signal

Metrics matrix for all UE/RBG combinations. Possibility of defining optimization criterion and strategy, e.g. Proportional Fairness


Adaptive Modulation and Coding

Link Adaptation AMC for PUSCH and PDSCH

Works on TTI basis The same MCS must be applied to all resource blocks belonging to one L2 PDU

transmitted to the user Retransmission always with initial transmissions MCS Co-operating with MIMO

The same MCS for every data stream Differentially modulated data streams (only with closed-loop MIMO)

Rate control for PUCCH (per TTI) Control Channel Elements aggregation (1 CCE = 9 REs) based on wideband CQI

reports Only QPSK, but PDCCH robustness can still be controlled

16QAM/64QAM can be enabled/disabled by O&M switches(on cell basis)Up to the operator which MCS shall be used as default one(if AMC disabled)


HARQ

Hybrid ARQ Entity operating in Layer 1 DL ACK/NACK messages sent via PHICH

(Physical HARQ Indicator Channel) UL ACK/NACK messages multiplexed on

PUCCH (Physical Uplink Control Channel) Failed packets are stored in decoder

buffer in order to combine them with the retransmitted ones

HARQ buffer stores transport channels which have not been yet acknowledged ACK empty buffer NACK MAC layer informs Radio

Resource Management about retransmission

The same MCS and antenna mode Chase Combining or

Incremental Redundancy

Packet L1 NACK ReTX


HARQ

HARQ bufferfor retransmissions

ACK/NACK

Logical Channels

PCH BCH DL-SCH

PDCCH PBCH PDSCH

Transport Channels

Physical Channels

PUCCH

Advantage

Fast L1/L2 retransmissions w/o involving RLC.ARQ (RLC layer) launched when HARQ maximum number of retransmission has been exceeded.

Hybrid ARQ is implemented only for PDSCH and PUSCH (user data)

DL example


Power allocation

Downlink Total TX power is equally shared between

subcarriers No subcarrier power boosting

Could be suitable for control channel coverage control

Uplink Total TX power is accumulated only on allocated

subcarriers Open/Closed Loop Power Control

f

PTX

Total TX power

f

PTX

Total TX power

0 subcarriers(not used for transmission)

Max power at antenna connector

eNB SW license for 8,20,40,60 WUE 23 dBm [2 dBm]

Total TX power


UL Power Control

Common formula for PUSCH and PUCCH)](log10,min[ 010max iMCS fPLPMPP ++++=

Maximum UE output power

Number of allocated resource blocks

Cell specific parameter broadcasted on PBCH [0.0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]UE path loss

MCS-dependent power offset set by eNB (compensation of RF imperfections)

user0

nominal00 PPP +=

Broadcasted on PBCH [-126 dBm, 24 dBm] (1 dB step)

Sent via RRC [-8 dB, 7 dB] (1 dB step)

Closed loop PC commands (eNB UE) via RRC using:- UL scheduling grant or- special PDCCH format (TPC-PDCCH)2 bits message: [-1, 0, 1, 3]


UL Power Control

UL PC for PUCCH Different P0, , i and MCS are defined for PUCCH

P0 for PUCCH [-127dBm, -96dBm] = 0i also supports 1 bit format [-1, 1]MCS generally not needed (one modulation possible only)

Different power offset for various messages (ACK/NACK, CQI, SR) can be applied

P0, optimal values depends on propagation scenario, transmission bandwidth, etc.

Closed Loop Power Control Compensation of Open Loop PC errors More advanced PC schemes useful for interference coordination or

QoS-aware scheduling


UE categories

All categories support 20 MHz 64QAM mandatory in downlink, but no in uplink (except Class 5) 2x2 MIMO mandatory in other classes except Class 1

Peak rate DL/UL

RF bandwidth

Modulation DL

Modulation UL

Rx diversity

BTS tx diversity

MIMO DL

Class 1 Class 2 Class 3 Class 4 Class 5

10/5 Mbps 50/25 Mbps 100/50 Mbps 150/50 Mbps 300/75 Mbps

20 MHz 20 MHz 20 MHz 20 MHz 20 MHz

64QAM 64QAM 64QAM 64QAM 64QAM

16QAM 16QAM 64QAM 16QAM 16QAM

Yes Yes YesYes Yes

1-4 tx

Optional 2x2 4x42x2 2x2

1-4 tx 1-4 tx 1-4 tx 1-4 tx


Thank you for attention

LTE Background/Principles for Link BudgetMeeting AgendaRequirements Meeting AgendaRadio access principlesDownlink OFDMARadio access principlesUplink SC-FDMARadio access principlesUplink SC-FDMAFrame structureOFDM signal representationCyclic PrefixCyclic PrefixChannel estimationChannel estimationDL Reference signal configurationChannel estimationDL Reference signal configurationChannel estimationUL Reference signal configurationMeeting AgendaMultiple antennasMultiple antennasMultiple antennasMultiple antennasSample solutionMultiple antennasSample antenna (co-location with GSM)SchedulingSchedulingAdaptive Modulation and CodingHARQHARQPower allocationUL Power ControlUL Power ControlUE categories

02_NSN_Tele2_LTE_Background_and_Principles.pdf

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