Long Term Evolution Technology training PowerPoint Presentation

Long Term Evolution

Technology training(Session 3)

1

Outline

LTE TDD frame structureDownlink reference sequences

Uplink transmission

2

Allocatable resources

LTE – radio resource = “time-frequency chunk” + MCS

3

Time domain

1 frame = 10 sub-frames

1 subframe = 2 slots

1 slot = 7 (or 6) OFDM symbols

Frequency domain

1 OFDM carrier = 15KHz

Resource Block (RB) = 12 carriers in one TS (12*15KHz x 0.5ms)

Note: In LTE resource management is along three dimensions: Time, Frequency, Code

Bandwidth flexibility

LTE supports deployment from 6RBs to 110 RBs in 1 RB increments

6RBs = 6 x 12 x 15KHz = 1080KHz -> 1.4MHz (with guard band)

110RBs = 110 X 12 X 15KHz = 19800KHz -> 20MHz (with guard band)

Typical deployment channel bandwidths: 1.4, 3, 5, 10, 15, 20 MHz

Straight forward to support other channel bandwidths (due to OFDM)

UE needs to support up to the largest bandwidth (i.e. 20MHz)

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LTE duplexing

LTE may be:

Time Division Duplexed (TDD)

Frequency Division Duplex (FDD)

Half Duplex Frequency Division Duplexing HD-FDD

LTE supports different channel bandwidths

1.4, 3, 5, 10,

15 or

20

MHz in a signal channelBandwidth flexibility – facilitates deployment across wide spectrum range

LTE supports frequencies from 450MHz to 3.8GHzLTE-A supports carrier aggregationMultiple channels may be aggregated in data delivery5Duplexing schemes in LTE

Use of TDD

Unpaired spectrum allocation

Imbalance between DL and UL traffic

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3GPP TDD bands

Reported by Ericsson (annual report 2009)

KSA TDD bands

Time domain structure

Two time domain structures

Type 1: used for FDD transmission (may be full duplex or half duplex)

Type 2: used for TDD transmission

Both Type 1 and Type 2 are based on 10ms radio frame

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Radio frame : Type 1

Radio frame : Type 2

TDD frame configurations

Different configurations allow balancing between DL and UL capacity

Allocation is semi-static

Adjacent cells have same allocation

Transition DL->UL happens in the second subframe of each half-frame

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Note 1: By convention special sub-frame belongs to DL

Note

2

: TDD frame structure allows co-existence between LTE TDD and TD-SCDMA

Example – DL throughput calculation in TDD

Estimate PHY throughput range of DL in a TDD system using 15MHz of spectrum, TDD configuration 4:1 SISO antenna system and normal CP

15MHz = 75 resource blocks = 75 * 12 = 900 sub-carriers

In 4:1 TDD there are 6 downlink sub-frames per frame

For normal CP there are 14 OFDM symbols perf

subframe

Number of resource elements in a frame: 6*14*900 = 75,600

Lower bound:

All resource elements are carrying QPSK (2 bits/symbol)

Date rate: 75,600*2 bits / 10ms = 15.12 Mbps

Upper bound:All resource elements are carrying 64QAM (6 bits/symbol)Date rate: 75,600*6 bits / 10ms = 45.36 MbpsNote: Both lower and upper bounds may be slightly higher is special sub-frame is used for data transmission 9

Review questions

What is the smallest bandwidth that may be allocated on DL?

How many different MCS schemes may be used on the DL?

Can LTE be deployed in 1MHz of spectrum?

Can LTE TDD be deployed in 10MHz of spectrum?

How many different TDD configurations are standardized?

What is the reason for using TDD?

Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate DL throughput range for SISO configuration and extended CP.

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Channel state information

LTE implements radio resource management based on UE feedback

UE measures DL channel properties – using reference signals

Sends information back to

eNode

B

The information is referred to Channel State Information (CSI)

CSI contains variety of information (Power, CINR, Rank, PMI, …)

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Process of channel estimation

eNode

B sends a known reference sequence

Channel distorts the sequence

The mobile compares known sequence against the distorted version it receives

Based on the difference it calculates and sends back eh CSI

SISO / MIMO

eNodeB – mandatory support for

4 antennas (

Ntx

DL /

Nrx

UL = 4)

UE – mandatory support for 2

antennas (

Ntx UL /Nrx UL = 2)

UE - optional support for 4 antennas. When multiple antennas are used – multiple wireless channels are createdCSI needs to be estimated for each of the channelsSeparate reference (i.e. training) sequence is required for each channel12Different TX/RX configurations supported in LTEIn LTE terminology – antenna = “port”Note: under favorable propagation condition MIMO increases throughput by a factor min(

Ntx, Nrx

)

Downlink reference signals

For coherent demodulation – terminal needs channel estimate for each subcarrier

Reference signals – used for channel estimation

There are three type of reference signals

Cell specific DL reference signals

Every DL subframe

Across entire DL bandwidth

UE specific DL reference signals

Sent only on DL shared channel

Intended for individual UE’s

MBSFN reference signalsSupport multicast/broadcast13Note: Reference signals are staggered in time and frequency. This allows UE to perform 2-D complex interpolation of channel time-frequency response

Cell specific reference signals

DL transmission may use up to four antennas

Each antenna port has its own pattern of reference signals

Reference signals are transmitted at higher power in multi-antenna case

Reference signals introduce overhead

4.8% for 1 antenna port

9.5% for 2 antenna ports

14.3 % for 4 antenna ports

Reference symbols vary from position to position and from cell to cell – cell specific 2 dimensional sequence

Period of the sequence is one frame

14Four port TXTwo port TX

One port TX

There are 504 different Reference Sequences (RS)

They are linked to PHY-layer cell identities The sequence may be shifted in frequency domain – 6 possible shiftsEach shift is associated with 84 different cell identities (6 x 84 = 504)

Shifts are introduced to avoid collision between RS of adjacent cells

In case of multiple antenna ports – only three shifts are useful

For a given PHY Cell ID - sequence is the same regardless of the bandwidth used – UE can demodulate middle RBs in the same way for all channel bandwidths

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Cell specific reference signals (2)

Shifts for single port transmission

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)

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

Review questions

What is MIMO transmission?

How many antenna ports are supported by

eNode

B?

If the transmission is in 4 by 2 MIMO mode, what is the maximum increase in data rate?

What are three types of reference sequences?

Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate

DL throughput

range for

2 by 2 MIMO configuration and extended CP.17

LTE UPLINK Transmission

Part 4

Peak to Average Power

OFDM – simple addition of multiple independently modulated carriers

OFDM – vary high Peak to Average Power Ratio (PAPR)

To make sure that TX amplifier is working in linear region – OFDM waveform requires large back off

Large back off = poor power efficiency

To improve PAPR – UL uses DFT spread OFDM

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Note: If uncompensated, PAPR for OFDM is 10log

10

(

Nsc), where Nsc = number of subcarriersBW (MHz)1.4

51015

Nsc

72

300

600

900

10log

10

(

Nsc

)

18.6

24.8

27.8

29.5

Note: very high PAPRs occur with very low probability

DFTS-OFDM

DFTS-OFDM = DFT Spread OFDM

Also known as s Single Carrier FDMA (SC-FDMA)

Used on RL of LTE

Advantages:

Lower PAPR than OFDM (4dB for QPSK and 2dB for 16-QAM)

Orthogonality between the users in the same cell

Low complexity TX/RX due to DFT/FFT

Disadvantage:

Needs an equalizer at the Node B RX

Need for some synchronization in time domain 20Outline of the DFTS-OFDMNote: In DFTS-OFDM, M < N

DFTS-OFDM TX/RX chain

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Note: the TX/RX of DFTS-OFDM is almost the same as OFDM. The DFT pre-coding / decoding and equalization are done in software

Uplink user multiplexing

Two ways of mapping the output of the DFT

Consecutive carriers: Localized DTFS-OFDM

Distributed carriers: Distributed DTFS-OFDM

Distributed OFDM has benefit of frequency diversity

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Note 1:

Mapping between output of the OFDM and carriers is performed by MAC scheduler

Note 2:

Spectrum bandwidth may be allocated in dynamic fashion

Localized DFTS-OFDM

Distributed DFTS-OFDM

Uplink frame format

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T

CP

: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols

T

CP-e

: 512 Ts (16.7 us) for all symbols

Need for two different CP:

To accommodate environments with large channel dispersion

To accommodate MBSFN (Multi-Cast Broadcast Single Frequency Network) transmission Note: UL and DL frame formats are identical

Modulation and coding on the UL

LTE defines UE categories

Higher category = better performance

Categories differ by

TX power,

Supported MCS

Supported MIMO modes

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Categories supported in LTE –

Rel

8Example: iPhone 6 features a Qualcomm MDM9625M LTE Category 4 modem that can offer peak download speeds of up to 150Mbps and peak upload speeds of up to 50Mbps in 20MHz LTE.

Example – UL throughput calculation in TDD

Estimate PHY throughput range of UL in a TDD system using 15MHz of spectrum, TDD configuration 4:1 SISO antenna system and normal CP. The UE is category 4.

15MHz = 75 resource blocks = 75 * 12 = 900 sub-carriers

In 4:1 TDD there are 2 downlink sub-frames per frame

For normal CP there are 14 OFDM symbols perf

subframe

Number of resource elements in a frame: 2*14*900 = 25,200

Lower bound:

All resource elements are carrying QPSK (2 bits/symbol)

Date rate: 25,200*2 bits / 10ms = 5.04Mbps

Upper bound:All resource elements are carrying 16QAM (4 bits/symbol)Date rate: 25,200*4 bits / 10ms = 10.08 MbpsNote: Both lower and upper bounds are slightly lower due to overhead25

Uplink reference signals (1)

Used for uplink channel estimation

Two types of sequences

Data demodulation Reference Signal (

DM-RS

)

Sounding Reference Signal (

SRS

)

DM-RSSent on each slot transmission to help demodulate data

Occupies center part of the slot transmission (symbols 4) in both transmission slotsUse same bandwidth as the UL data (multiples of 12 carrier RBs)Properties of DM-RS sequencesSmall power variations in frequency domainSmall power variations in time domain26

Uplink reference signals (2)

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SRS

Allow network to estimate channel quality across entire band

Used by MAC scheduler to perform frequency dependent scheduling

Optional implementation

UE can be configured to send SRS sequence at time intervals from 2ms to 160ms

Two modes of operation

Wideband SRS – UE send the sequence across the entire spectrum

Hopping SRS – UE sends narrowband sequence that hops across different parts of the spectrum

Review questions

What is the PAPR?

What is the UE category?

iPhone 6s is a category 5 device. What is its maximum supported data rate on DL? How about UL?

Consider 10MHz TDD deployment in 7:3 frame configuration. Estimate aggregate

UL

throughput range for 2 by 2 MIMO configuration and extended CP

.

What are two types of reference sequences used on the UL?

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