02 LTE Air Interface GC

1 © Nokia Siemens Networks RA41202EN20GLA0

LTE RPESSLTE Air Interface


Module Objectives

After completing this module, the participant should be able to:

• Understand the basics of the OFDM transmission technology

• Explain how the OFDM technology avoids the Inter Symbol Interference

• Recognise the different between OFDM & OFDMA

• Identify the OFDM weaknesses

• Review the key OFDM parameters

• Analyze the reasons for SC-FDMA selection in UL

• Describe the LTE Air Interface Physical Layer

• Calculate the Physical Layer overhead

• Identify LTE Measurements

• List the frequency allocation alternatives for LTE

• Review the main LTE RRM features

• Identify the main voice solutions for LTE


Module Contents

• OFDM Basics

• OFDM & Multipath Propagation: The Cyclic Prefix

• OFDM versus OFDMA

• OFDM Weaknesses

• OFDM Key Parameters

• SC-FDMA

• LTE Air Interface Physical Layer

• Physical Layer Overhead

• LTE Measurements

• Frequency Variants

• RRM Overview

• VoIP in LTE


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


The Rectangular Pulse

Advantages:

+ Simple to implement: there is no complex filter system required to detect such pulses and to generate them.

+ The pulse has a clearly defined duration. This is a major advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference.

Disadvantage:

- it allocates a quite huge spectrum. However the spectral power density has null points exactly at multiples of the frequency fs = 1/Ts. This will be important in OFDM.

time

am

pli

tud

e

Ts

fs 1

Ts

Time Domain

frequency f/fs

sp

ec

tra

l p

ow

er

de

ns

ity

Frequency Domain

fs

FourierTransform

Inverse FourierTransform


TDMA

f

t

f

• Time Division

FDMA

f

f

t

• Frequency Division

CDMA

f

tcode

s

f

• Code Division

OFDMA

f

f

t

• Frequency Division

• Orthogonal subcarriers

Multiple Access Methods User 1 User 2 User 3 User ..

OFDM is the state-of-the-art and most efficient and robust air interface


OFDM Basics

• Transmits hundreds or even thousands of separately modulated radio signals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak ( centre frequency) of one subcarrier …

…intercepts the ‘nulls’ of the neighbouring subcarriers

15 kHz in LTE: fixed

Total transmission bandwidth


OFDM Basics

• Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission

• The throughput is the sum of the data rates of each individual (or used) subcarriers while the power is distributed to all used subcarriers

• FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined by the FFT size ( by the bandwidth)

Power

frequency

bandwidth


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


Tg: Guard period duration

ISI: Inter-Symbol Interference

Propagation delay exceeding the Guard Period

12

34

time

TSYMBOLTime Domain

time

time

Tg

1

2

3

time

4

Delay spread > Tg

ISI


The Cyclic Prefix OFDM symbol

OFDM symbol

OFDM symbol

OFDM symbol

Cyclic prefix

Part of symbol used for FFT processing in the receiver

• In all major implementations of the OFDMA technology (LTE, WiMAX) the Guard Period is equivalent to the Cyclic Prefix CP.

• This technique consists in copying the last part of a symbol shape for a duration of guard-time and attaching it in front of the symbol (refer to picture sequence on the right).

• CP needs to be longer than the channel multipath delay spread (refer to previous slide).

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


The OFDM Signal


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


OFDM

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Plain OFDM

time

sub

carr

ier

...

...

...

...

...

...

...

...

...

1 2 3 common info(may be addressed via Higher Layers)

UE 1 UE 2 UE 3

• OFDM stands for Orthogonal Frequency Division Multicarrier

• OFDM: Plain or Normal OFDM has no built-in multiple-access mechanism.

• This is suitable for broadcast systems like DVB-T/H which transmit only broadcast and multicast signals and do not really need an uplink feedback channel (although such systems exist too).

• Now we have to analyze how to handle access of multiple users simultaneously to the system, each one using OFDM.


OFDMA®

1

1

1

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2

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3

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Orthogonal FrequencyMultiple Access

OFDMA®

time

...

...

...

...

...

...

...

...

...

1

1

1 1

2

22

2 2

3 33 3 3

1

sub

carr

ier

1

1 1 1

111

3 3 3

33 3 3 3

3

Resource Block (RB)

1 2 3 common info(may be addressed via Higher Layers)

UE 1 UE 2 UE 3

OFDMA® stands for Orthogonal Frequency Division Multiple Access

• registered trademark by Runcom Ltd.

• The basic idea is to assign subcarriers to users based on their bit rate services. With this approach it is quite easy to handle high and low bit rate users simultaneously in a single system.

• But still it is difficult to run highly variable traffic efficiently.

• The solution to this problem is to assign to a single users so called resource blocks or scheduling blocks.

• such block is simply a set of some subcarriers over some time.

• A single user can then use 1 or more Resource Blocks.


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


Inter-Carrier Interference (ICI) in OFDM

• The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.

• If the receiver’s frequency slips some fractions from the subcarriers center frequencies, then we encounter not only interference between adjacent carriers, but in principle between all carriers.

• This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform.

• One possible cause that introduces frequency errors is a fast moving Transmitter or Receiver (Doppler effect).


f0 f1 f2 f3 f4

∆P

I3

I1I4

I0

ICI

= I

nte

r-C

arri

er I

nte

rfer

en

ce

Leakage effect due to Frequency Drift: ICI

Two effects begin to work:

1. -Subcarrier 2 has no longer its power density maximum here - so we loose some signal energy.

2.-The rest of subcarriers (0, 1, 3 and 4) have no longer a null point here. So we get some noise from the other subcarrier.


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


OFDMA Parameters in LTE

• Channel bandwidth: DL bandwidths ranging from 1.4 MHz to 20 MHz

• Data subcarriers: the number of data subcarriers varies with the bandwidth

– 72 for 1.4 MHz to 1200 for 20 MHz# subcarriers = (ch-BW – 0.1 ch BW)/15KHz for 3, 5, 10, 15, & 20. Not good for 1.4MHz# subcarriers = (ch-BW – 0.1 ch BW)/15KHz for 3, 5, 10, 15, & 20. Not good for 1.4MHz


OFDMA Parameters in LTE

• Frame duration: 10ms created from slots and subframes.

• Subframe duration (TTI): 1 ms ( composed of two 0.5ms slots)

• Subcarrier spacing: Fixed to 15kHz ( 7.5 kHz defined for MBMS)

• Sampling Rate: Varies with the bandwidth but always factor or multiple of 3.84 to ensure

compatibility with WCDMA by using common clocking

Frame Duration

Subcarrier Spacing

Sampling Rate ( MHz)

Data Subcarriers

Symbols/slot

CP length

1.4MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

10 ms

15 kHz

Normal CP=7, extended CP=6

Normal CP=4.69/5.12 μs, extended CP= 16.67μs.

1.92 3.84 7.68 15.36 23.04 30.72

72 180 300 600 900 1200

10ms

For 3G Sampling Rate = 3.84MHz, for LTE multiple of 3.84MHz

Frame = 10ms devided intoo 10 Subframes (SF), each subframe devided into 2 slots, each slot contain symbols

For 3G Sampling Rate = 3.84MHz, for LTE multiple of 3.84MHz

Frame = 10ms devided intoo 10 Subframes (SF), each subframe devided into 2 slots, each slot contain symbols


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


Peak-to-Average Power Ratio in OFDMA

The transmitted power is the sum of the powers of all the subcarriers

• Due to large number of subcarriers, the peak to average power ratio (PAPR) tends to have a large range

• The higher the peaks, the greater the range of power levels over which the transmitter is required to work.

• Not best suited for use with mobile (battery-powered) devices

PAPRPAPR


SC-FDMA in UL

• Single Carrier Frequency Division Multiple Access: Transmission technique used for Uplink

• Variant of OFDM that reduces the PAPR:– Combines the PAR of single-carrier system with the

multipath resistance and flexible subcarrier frequency allocation offered by OFDM.

– It can reduce the PAPR between 6…9dB compared to OFDMA

– TS36.201 and TS36.211 provide the mathematical description of the time domain representation of an SC-FDMA symbol.

• Reduced PAPR means lower RF hardware requirements (power amplifier)

SC

-FD

MA

OF

DM

A


SC-FDMA and OFDMA Comparison (2/2)


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


LTE Physical Layer - Introduction

FDD

..

..

..

..

Downlink Uplink

Frequency band 1

Frequency band 2

.. ..Single frequency bandTDD

• It provides the basic bit transmission functionality over air

• LTE physical layer based on OFDMA DL & SC-FDMA in UL

– This is the same for both FDD & TDD mode of operation

• There is no macro-diversity in use

• System is reuse 1, single frequency network operation is feasible

– no frequency planning required

• There are no dedicated physical channels anymore, as all resource mapping is dynamically driven by the scheduler


LTE Physical Layer Structure – Frame Structure (FDD)

10 ms frame

0.5 ms slot

s0 s1 s2 s3 s4 s5 s6 s7s18 s19…..

1 ms sub-frame

SF0 SF1 SF2 SF9…..

sy4sy0 sy1 sy2 sy3 sy5 sy6

0.5 ms slot

SF3

SF: SubFrame

s: slot

Sy: symbol

• FDD Frame structure ( also called Type 1 Frame) is common to both UL & DL

• Divided into 20 x 0.5ms slots– Structure has been designed to facilitate short round trip time

- Frame length = 10 ms

- FDD: 10 sub-frames of 1 ms for UL & DL

- 1 Frame = 20 slots of 0.5ms each

- 1 slot = 7 (normal CP) or 6 OFDM symbols (extended CP)


LTE Physical Layer Structure – Frame Structure (TDD)

SF#0SF#0

. . .f

time

UL/DL carrier

radio frame 10 ms

subframe

Dw

PT

SD

wP

TS

GP

GP

Up

PT

SU

pP

TS SF

#2SF#2

SF#4SF#4

. . .

half frame

DwPTS: Downlink Pilot time Slot

UpPSS: Uplink Pilot Time Slot

GP: Guard Period to separate between UL/DL

Downlink Subframe

Uplink Subframe

Frame Type 2 (TS 36.211-900; 4.2)

• each radio frame consists of 2 half frames

• Half-frame = 5 ms = 5 Sub-frames of 1 ms• UL-DL configurations with both 5 ms & 10 ms DL-to-UL switch-point periodicity are supported • Special subframe with the 3 fields DwPTS, GP & UpPTS; length of DwPTS + UpPTS +GP = 1

subframe• DL / UL ratio can vary from 1/3 to 8/1 according to service requirements of the carrier

SF#0SF#0 D

wP

TS

Dw

PT

S

GP

GP

Up

PT

SU

pP

TS

SF#2SF#2

SF#4SF#4

subframe


Subframe structure & CP length

Short cyclic prefix:

Long cyclic prefix:

Copy= Cyclic prefix

= Data

5.21 s

16.67 s

• Subframe length: 1 ms for all bandwidths

• Slot length is 0.5 ms– 1 Subframe= 2 slots

• Slot carries 7 symbols with normal CP or 6 symbols with long CP– CP length depends on the symbol position within the slot:

▪ Normal CP: symbol 0 in each slot has CP = 160 x Ts = 5.21μs; remaining symbols CP= 144 x Ts = 4.7μs

▪ Extended CP: CP length for all symbols in the slot is 512 x Ts = 16.67µs

Ts: ‘sampling time’ of the overall channel basic Time Unit = 32.5 nsec

Ts =1 sec

Subcarrier spacing X max FFT size


Resource Block and Resource Element

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

Subcarrier 1

Subcarrier 12

180

KH

z

1 slot 1 slot

1 ms subframe

RB

Resource Element

• Physical Resource Block PBR or Resource Block RB:– 12 subcarriers in frequency domain x 1 slot period in time domain

– Capacity allocation based on Resource Blocks

Resource Element RE: – 1 subcarrier x 1 symbol period

– theoretical min. capacity allocation unit

– 1 RE is the equivalent of 1 modulation symbol on a subcarrier, i.e. 2 bits (QPSK), 4 bits (16QAM), 6 bits (64QAM).


Physical Resource Blocks

....

12 subcarriers

Time

Frequency

0.5 ms slot

1 ms subframe or TTI

Resource block

During each TTI, resource blocks for different UEs are scheduled in the eNodeB

During each TTI, resource blocks for different UEs are scheduled in the eNodeB

• In both the DL & UL direction, data is allocated to users in terms of resource blocks (RBs).

• a RB consists of 12 consecutive subcarriers in the frequency domain, reserved for the duration of 0.5 ms slot.

• The smallest resource unit a scheduler can assign to a user is a scheduling block which consists of two consecutive resource blocks


LTE Channel Options

Bandwidth options: 1.4, 1.6, 3, 3.2, 5, 10, 15 and 20 MHzBandwidth options: 1.4, 1.6, 3, 3.2, 5, 10, 15 and 20 MHz

Subcarriers in frequency domain (15 kHz or 7.5 kHz subcarrier spacing)

Channel bandwidth (MHz)

Number of subcarriers

Number of resource blocks

1.4

72

6

3

180

15

5

300

25

10

600

50

15

900

75

20

1200

100


DL Physical Resource Block

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe

or TTI

DL reference signal

• Reference signals position in time domain is fixed (symbol 0 & 4 / slot for Type 1 Frame) whereas in frequency domain it depends on the Cell ID

• Reference signals are modulated to identify the cell to which they belong.

• This signal, consisting of a known pseudorandom sequence, is required for channel estimation in the UEs. (like CPICH in WCDMA).

• Note that in the case of MIMO transmission, additional reference signals must be embedded into the resource blocks.


DL Physical Channels

• PDSCH: Physical Downlink Shared Channel– carries user data, L3 Signalling, System Information Blocks & Paging

• PBCH: Physical Broadcast Channel– for Master Information Block only

• PMCH: Physical Multicast Channel– for multicast traffic as MBMS services

• PCFICH: Physical Control Format Indicator Channel – indicates number of OFDM symbols for Control Channels = 1..4

• PDCCH: Physical Downlink Control Channel– carries resource assignment messages for DL capacity allocations & scheduling

grants for UL allocations

• PHICH: Physical Hybrid ARQ Indicator Channel – carries ARQ Ack/Nack messages from eNB to UE in respond to UL transmission

There are no dedicated channels in LTE, neither UL nor DL.


UL Physical Channels

• PUSCH: Physical Uplink Shared Channel– Transmission of user data, L3 & L1 signalling (L1 signalling: CQI, ACK/NACKs, etc.)

• PUCCH: Physical Uplink Control Channel– Carries L1 control information in case that no user data are scheduled in this subframe

(e.g. H-ARQ ACK/NACK indications, UL scheduling request, CQIs & MIMO feedback).– These control data are multiplexed together with user data on PUSCH, if user data are

scheduled in the subframe

• PRACH: Physical Random Access Channel– For Random Access attempts; SIBs indicates the PRACH configuration (duration;

frequency; repetition; number of preambles - max. 64)


UL Physical Resource Block: DRS & SRS

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe or TTI

Frequency

Sounding Reference Signal on last OFDM symbol of 1 subframe;Periodic or aperiodic

transmission

Sounding Reference Signal on last OFDM symbol of 1 subframe;Periodic or aperiodic

transmission

Demodulation Reference Signal in subframes that carry

PUSCH

Demodulation Reference Signal in subframes that carry

PUSCH

Note: when the subframe contains the PUCCH, the Demodulation

Reference Signal is embedded in a different way

Note: when the subframe contains the PUCCH, the Demodulation

Reference Signal is embedded in a different way

• The Demodulation Reference Signal is transmitted in the fourth SC-FDMA symbol (counting from zero) in all resource blocks allocated to the PUSCH carrying the user data.

• This signal is needed for channel estimation, which in turn is essential for coherent demodulation of the UL signal in the eNodeB.

• The Sounding Reference Signal SRS provides UL channel quality information as a basis for scheduling decisions in the base station. This signal is distributed in the last SC-FDMA symbol of subframes that carry neither PUSCH nor PUCCH data. [SRS is always disabled in FDD RL20 and before.]

PUCCH: Physical UL Control Channel


b0 b1

QPSK

Im

Re10

11

00

01

b0 b1b2b3

16QAM

Im

Re

0000

1111

Im

Re

64QAM

b0 b1b2b3 b4 b5

• 3GPP standard defines the following options: QPSK, 16QAM, 64QAM in both directions (UL & DL)

– UL 64QAM not supported in RL10

• Not every physical channel is allowed to use any modulation scheme:

• Scheduler decides which form to use depending on carrier quality feedback information from the UE

Modulation Schemes

QPSK:

2 bits/symbol

16QAM:

4 bits/symbol

64QAM:

6 bits/symbol

Physical channel

Modulation

PDSCH QPSK, 16QAM, 64QAM

PMCH QPSK, 16QAM, 64QAM

PBCH QPSK

PDCCH, PCFICH

QPSK

PHICH BPSK

PUSCH QPSK, 16QAM, 64QAM

PUCCH BPSK and/or QPSK


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


DL Reference Signal Overhead

Reference Signal (RS)- If 1 Tx antenna*: 4 RSs per PRB

- If 2 Tx antenna*: there are 8 RSs per PRB

- If 4 Tx antenna*: there are 12 RSs per PRB

Example below: Normal CP (84 RE) & 2 Tx antenna*, DL RS overhead = 8 / 84 = 9.52 %

* with 1/2/4 Antenna PortsPRB: Physical Resource Block


Synchronization Signals Overhead

2 3 4 5 7 8 9 10

1 2 3 4 5 6 7

1 2 3 4 5 6

10ms Radio frame

1ms Subframe SSS

PSS0.5ms = 1 slot

Normal CP

Extended CP

PSS & SSS frame + slot structure in time domain

(FDD case)

checking for SSSat 2 possible positions

CP length

checking for SSSat 2 possible positions

CP length

Primary Synchronization Signal (PSS) - occupies 144 Resource Elements per frame (20 timeslots); i.e. (62 subcarriers + 10

empty Resource Elements) x 2 times/frameExample: Normal CP, 10 MHz bandwidth; PSS overhead = 144 / (84 × 20 × 50) = 0.17 %

Secondary Synchronization Signal (SSS)– Identical calculation to PSS; same overhead as for PSS


The combination of PDCCH, PCFICH & PHICH occupies the first 1, 2 or 3 symbols per TTI*

Resource Elements

reserved for

Reference Symbols

(2 antenna port case)

Control Channel

Region (1-3 OFDM symbols*)

12 s

ub

car

rier

s

Fre

qu

enc

y

Time Data Region

One subframe (1ms)

PDCCH, PCFICH & PHICH overhead (1/2)

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth


PDCCH, PCFICH & PHICH overhead (2/2)

The number of RE occupied per 1 ms TTI is given by (12 × y – x), where:• y depends upon the number of OFDM symbols per TTI (1, 2 or 3*) occupied by

Control Channels

• x depends upon the number of RE already occupied by the Reference Signal

• x = 2 for 1 Tx antenna (Antenna Port)

• x = 4 for 2 Tx antennas (Antenna Ports)

• x = 4 for 4 Tx antennas (Antenna Ports) when y = 1

• x = 8 for 4 Tx antennas (Antenna Ports) when y = 2 or 3

Example: in the case of normal CP, 2 Antenna Ports & 3 OFDM symbols occupied by Control Channels:

Control Channel Overhead = (12 × 3 - 4) / (12 × 7 × 2) = 19.05%

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth


PBCH Overhead

Occupies (288* – x) Resource Elements (REs) per 20 timeslots per transmit antenna

The value of x depends upon the number of REs already occupied by the Reference Signal:

x = 12 for 1 Tx antenna, x = 24 for 2 Tx antennas & x = 48 for 4 Tx antenna

- Example: normal CP, 2 Tx antennas, 10 MHz bandwidth;

PBCH Overhead = (288 – 24) / (84 × 20 × 50) = 0.31%

72 s

ub

car

rier

s

Repetition Pattern of PBCH = 40 ms

one radio frame = 10 ms

PBCH* PBCH uses central 72 Subcarrier over 4 OFDM symbols in Slot 1


UL Demodulation Reference Signal Overhead (1/2)

Demodulation Reference Signal (DRS)

• The DRS is sent on the 4th OFDM symbol of each RB occupied by the PUSCH.

PUCCH

PUCCH

PUSCH


Example:For 1.4 MHz Channel Bandwidth, the PUCCH occupies 1 RB per Slot. The number of RE per RB is 84 when using the normal CP. This means the DRS overhead* is: ((6-1) × 12)/(6 × 84) = 11.9 %

Channel BW PUCCH RB/slot DRS Overhead*

1.4 MHz 1 ((6-1) × 12) / (6 × 84) = 11.9 %

3 MHz 2 ((15-2) × 12) / (15 × 84) = 12.38 %

5 MHz 2 ((25-2) × 12) / (25 × 84) = 13.14 %

10 MHz 4 ((50-4) × 12) / (50 × 84) = 13.14 %

15 MHz 6 ((75-6) × 12) / (75 × 84) = 13.14 %

20 MHz 8 ((100-8) × 12) / (100 × 84) = 13.14 %

UL DRS Overhead (2/2)

* for normal CP


PRACH Overhead

PRACH• PRACH uses 6 Resource Blocks in the frequency domain.

• The location of those resource blocks is dynamic. Two parameters from RRC layer define it:

– PRACH Configuration Index: for Timing, selecting between 1 of 4 PRACH durations and defining if PRACH preambles can be send in any radio frame or only in even numbered ones

– PRACH Frequency offset: Defines the location in frequency domain

• PRACH Overhead calculation: 6 RBs * RACH Density / (#RB per TTI) x 10 TTIs per frame

– RACH density: how often are RACH resources reserved per 10 ms frame i.e. for RACH density: 1 (RACH resource reserved once per frame)

Channel BW PRACH Overhead

1.4 MHz (6 × 1) / (6 × 10) = 10 %

3 MHz (6 × 1) / (15 × 10) = 4 %

5 MHz (6 × 1) / (25 × 10) = 2.40 %

10 MHz (6 × 1) / (50 × 10) = 1.20 %

15 MHz (6 × 1) / (75 × 10) = 0.8 %

20 MHz (6 × 1) / (100 × 10) = 0.6 %


PUCCH Overhead

PUCCH• Ratio between the number of RBs used for PUCCH and the total number of RBs in frequency

domain per TTIChannel BW PUCCH RB/slot PUCCH Overhead

1.4 MHz 1 1 / 6 = 16.67 %

3 MHz 2 2 / 15 = 13.33 %

5 MHz 2 2 / 25 = 8 %

10 MHz 4 4 / 50 = 8 %

15 MHz 6 6 / 75 = 8 %

20 MHz 8 8 / 100 = 8%

Time

To

tal

UL

B

and

wit

h

PUCCH

PUCCH

PUSCH

1 subframe = 1ms

Fre

qu

enc

y

12 s

ub

car

rier

s


Physical Layer Overhead Example

Example of overhead:

• DL 2Tx – 2RX

• UL 1TX - 2RX

• PRACH in every frame

• 3 OFDM symbols for PDCCH


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


LTE Measurements

Physical layer measurements have not been extensively discussed in the LTE standardization. They could change.

Intra LTE measurements ( from LTE to LTE)• UE measurements

– CQI measurements – Reference Signal Received Power (RSRP)– Reference Signal Received Quality ( RSRQ)

• eNB measurements– Non standardized (vendor specific): TA, Average RSSI, Average SINR, UL CSI,

detected PRACH preambles, transport channel BLER– Standardized: DL RS Tx Power, Received Interference Power, Thermal Noise Power

Measurements from LTE to other systems• UE measurements are mainly intended for Handover.

– UTRA FDD: CPICH RSCP, CPICH Ec/No and carrier RSSI– GSM: GSM carrier RSSI– UTRA TDD: carrier RSSI, RSCP, P-CCPCH– CDMA2000: 1xRTT Pilot Strength, HRPD Pilot Strength

CSI: Channel State Information (received power per PRB)TA: Timing AdvanceCSI: Channel State Information (received power per PRB)TA: Timing Advance


UE Measurements: RSRP & RSRQ

RSRP (Reference Signal Received Power)

• Average of power levels (in [W]) received across all Reference Signal symbols within the considered measurement frequency bandwidth.

• UE only takes measurements from the cell-specific Reference Signal elements of the serving cell

• If receiver diversity is in use by the UE, the reported value shall be equivalent to the linear average of the power values of all diversity branches

• Reporting range -44…-133 dBm

RSRQ ( Reference Signal Received Quality)

• Defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RBs of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks

• Reporting range -3…-19.5dB


eNodeB Measurements

DL Reference Signal Transmitted Power• Average of power levels (in [W]) transmitted across all Reference Signal symbols

within the considered measurement frequency bandwidth

• Reference point for the DL RS TX power measurement: TX antenna connector

• The DL RS TX power signaled to the UE is not measured, it is just an eNB internal setting

Received Interference Power:• Received interference power, including thermal noise, within one PRBs bandwidth

Thermal noise power: No x W• Thermal noise power within the UL system bandwidth (consisting of variable # of

resource blocks) – ‘No’: white noise power spectral density on the uplink carrier frequency and ‘W’: denotes

the UL system bandwidth.

• Optionally reported with the Received Interference Power

• Reference point: RX antenna connector

• In case of receiver diversity, the reported value is the average of the power in the diversity branches


Module Contents

• OFDM Basics



• OFDM Weaknesses


• SC-FDMA





• RRM Overview

• VoIP in LTE


LTE Frequency Variants in 3GPP – FDD

1

2

3

4

5

7

8

9

6

2x25

2x75

2x60

2x60

2x70

2x45

2x35

2x35

2x10

824-849

1710-1785

1850-1910

1920-1980

2500-2570

1710-1755

880-915

1749.9-1784.9

830-840

BW[MHz] Uplink [MHz]

869-894

1805-1880

1930-1990

2110-2170

2620-2690

2110-2155

925-960

1844.9-1879.9

875-885

Downlink [MHz]

10 2x60 1710-1770 2110-2170

11 2x25 1427.9-1452.9 1475.9-1500.9

1800

2600

900

US AWS

UMTS core

US PCS

US 850

Japan 800

Japan 1700

Japan 1500

Extended AWS

Europe Japan Americas

788-798 758-768

777-787 746-756

Japan 800

US700

2x10

2x1013

12 2x18 698-716 728-746

14 US700

US700

815 – 830 860 – 875

704 – 716 734 – 746

2x15

2x1217

18

US700

Band

UHF (TV)832 – 862 791 – 821

830 – 845 875 – 890

2x30

2x1519

20

Japan 800

1626.5 – 1660.5 1525 – 1559 2x3424

1447.9 – 1462.9 1495.9 – 1510.92x1521


LTE Frequency Variants - TDD

33

34

35

36

37

39

40

38

1x20

1x60

1x15

1x20

1x40

1x60

1x100

1x50

1910 - 1930

1850 - 1910

2010 - 2015

1900 - 1920

1880 - 1920

1930 - 1990

2300 - 2400

2570 - 2620

BW[MHz] Frequency[MHz]

UMTS TDD 1

UMTS TDD 2

US PCS

US PCS

US PCS

Euro midle gap 2600

China TDD

China TDD

Band


Module Contents

• OFDM Basics




• OFDM Weaknesses

• SC-FDMA





• RRM Overview

• VoIP in LTE


RRM building blocks & functionsOverview

Scope of RRM:

•Management & optimized utilization of the radio resources:

• Increasing the overall radio network capacity & optimizing quality

•Provision for each service/bearer/user an adequate QoS (if applicable)

•RRM located in eNodeB

•MIMO Ctrl., LA & schedulers act on TTI basis.


LTE RRM: Scheduling (1/5)

• Motivation– Bad channel condition avoidance

OFDMA

The part of total available channel experiencing bad channel condition (fading)

can be avoided during allocation procedure.

CDMA

Single Carrier transmission does not allow to allocate only particular frequency parts. Every fading gap

effects the data.


Scheduler (UL/DL) (2/5)

• Cell-based scheduling (separate UL/DL scheduler per cell)

• Scheduling air interface resource on a 1ms × 12sub-carrier (PRB pair) basis

• Scheduler controls UEs & assigns appropriate grants per TTI

• Proportional Fair (PF) resource assignment among UEs

• Uplink:

• Channel unaware UL scheduling based on random frequency allocation

• Descending resource handling priority in UL for

1. Hybrid ARQ retransmission

2. Random access procedure

3. Signaling radio bearer with or without data radio bearer

4. Scheduling request

5. Conversational voice data

6. Data radio bearer

• Downlink:

• Channel aware DL scheduling - Frequency Domain Packet Scheduling (FDPS) - based on CQI with resources assigned in a fair manner


Downlink Scheduler (3/5) Algorithm

• Determine which PRBs are available (free) and can be allocated to UEs

• Allocate PRBs needed for common channels like SIB, paging, and random access procedure (RAP)

• Final allocation of UEs (bearers) onto PRB. Considering only the PRBs available after the previous steps

– Pre-Scheduling: All UEs with data available for transmission based on the buffer fill levels

– Time Domain Scheduling: Parameter MAX_#_UE_DL decides how many UEs are allocated in the TTI being scheduled

– Frequency Domain Scheduling for Candidate Set 2 UEs: Resource allocation in Frequency Domain including number & location of allocated PRBs

Evaluation of available resources (PRBs/RBGs ) for dynamic allocation on PDSCH

Resource allocation and schedulingfor common channels

DL scheduling of UEs :Scheduling of UEs /bearers to PRBs /RBGs

Start

End

Pre -Scheduling :Select UEs eligible for scheduling

-> Determination of Candidate Set 1

Time domain schedulingof UEs according to simple criteria

-> Determination of Candidate Set 2

Start

End

Frequency domain schedulingof UEs /bearers

-> PRB /RBG allocation to UEs /bearers


Uplink Scheduler (4/5) Algorithm

• Evaluation of the #PRBs that will be assigned to UEs

• Available number of PRBs per user: resources are assigned via PRB groups (group of consecutive PRBs)

Time domain: • Max_#_UE_UL which can be scheduled per TTI time frame is restricted by an O&M

parameter and depends on the bandwidth: 7 UEs (5 MHz), 10 UEs (10MHz), 15 UEs (15MHz) and 16 UEs (20MHz)

Frequency Domain: • Uses a random function to assure equal distribution of PRBs over the available frequency

range (random frequency hopping)

a) b)

Feature ID(s): LTE45

Example of allocation in frequency domain:

Full Allocation: All available PRBs are assigned to the scheduled UEs per TTI

Fractional Allocation: Not all PRBs are assigned. Hopping function handles unassigned PRBs as if they were allocated to keep the equal distribution per TTI


• Flexi eNodeB takes into account the noise and interference measurements together with the UE Tx power density (= UE TX power per PRB) when allocating PRBs in the frequency domain

• Cell edge users are assigned to frequency sub-bands with low measured inter-cell interference

• Up to 10% gain for cell edge users in low and medium loaded networks

• Easier to implement than channel aware scheduling (no sounding reference signal used)

Improvement in UL coverage by optimizing the cell edge performance

eNode B measured interference

subband with low interference

subband with high interference

subband with medium interference

PRBs


RL30Uplink Scheduler (5/5) IAS: Interference Aware Scheduler UL


LTE RRM: Link Adaptation by AMC (UL/DL) (1/6)

• Motivation of link adaptation: Modify the signal transmitted to and by a particular user according to the signal quality variation to improve the system capacity & coverage reliability.

• It modifies the MCS (Modulation & Coding Scheme) & the transport block size (DL) and ATB (UL)

• If SINR is good then higher MCS can be used more payload per symbol more throughput.

• If SINR is bad then lower MCS should be used (more robust)

• Flexi Multiradio BTS performs the link adaptation for DL on a TTI basis

• The selection of the modulation & the channel coding rate is based:• DL data channel: CQI report from UE• UL: BLER measurements in Flexi LTE BTS

LTE31: Link Adaptation by AMC

Optimizing air interface efficiency


Link Adaptation / AMC for PDSCH (2/6)

Procedure:• Initial MCS is provided by O&M

(parameter INI_MCS_DL) & is set as default MCS

• If DL AMC is not activated (O&M parameter ENABLE_AMC_DL) the algorithm always uses this default MCS

• If DL AMC is activated HARQ retransmissions are handled differently from initial transmissions (For HARQ retransmission the same MCS has to be used as for the initial transmission)

• A MCS based on CQI reporting from UE , shall be determined for the PRBs assigned to UE as indicated by the DL scheduler

START

Retrieve Default MCS

Dynamic AMC active?

HARQ retransmission?

Determine avaraged CQI value for allocated PRBs

Use the same MCS as for initial transmission

Determine MCS

Use Default MCS

END

yes

no

no


Link Adaptation / AMC for PUSCH (3/6)

Functionality• UL LA is active by default but can be deactivated by O&M parameters. If not

active, the initial MCS is used all the time

• UE scope

• Two parallel algorithms adjust the MCS to the radio channel conditions:

– Inner Loop Link Adaptation (ILLA): ▪ Slow Periodic Link adaptation (20-500ms) based on BLER measurements

from eNodeB (based on SINR in future releases)

– Outer Loop Link Adaptation (OLLA): event based

▪ In case of long Link Adaptation updates and to avoid low and high BLER situations, the link adaptation can act based on adjustable target BLER:

- “Emergency Downgrade” if BLER goes above a MAX BLER threshold (poor radio conditions)

- “Fast Upgrade” if BLER goes below of a MIN BLER threshold (excellent radio conditions)


Downlink– fast

▪ 1 TTI

– channel aware▪ CQI based

– MCS selection▪ 1 out of 0-28

– output▪ MCS▪ TBS

– up to 64QAM support

Uplink– slow periodical

▪ ~30ms

– channel partly aware▪ average BLER based

– MCS adaptation▪ +/- 1 MCS correction

– output▪ MCS▪ ATB

– up to 16 QAM support

Comparison: DL & UL Link adaptation for PSCH (4/6)

MCS: Modulation & Coding SchemeTBS: Transport Block SizeATB: Automatic Transmission Bandwidth


Outer Link Quality Control (OLQC) (5/6)

Feature: CQI Adaptation (DL)• CQI information is used by the scheduler & link adaptation in such a way that a certain

BLER of the 1st HARQ transmission is achieved

• CQI adaptation is the basic mean to control Link Adaptation behaviour and to remedy UE measurement errors

• Only used in DL

• Used for CQI measurement error compensation

– CQI estimation error of the UE

– CQI quantization error or

– CQI reporting error

• It adds a CQI offset to the CQI reports provided by UE. The corrected CQI report is provided to the DL Link adaptation for further processing

• CQI offset derived from ACK/NACK feedback

Optimize the DL performance



Support of aperiodic CQI reports (6/6)

Functionality

• Aperiodic CQI reports scheduled in addition to periodic reports

– Periodic CQI reports on PUCCH

– Aperiodic CQI reports on PUSCH

Description

• Controlled by the UL scheduler– Triggered by UL grant indication (PDCCH)

• Basic feature

UL grant + CQI indicator


Benefits

• Not so many periodic CQIs on PUCCH needed

• Allow frequent submission of more detailed reports (e.g. MIMO, frequency selective parts)


LTE RRM: Power Control (1/5)

Downlink:

• There is no adaptive or dynamic power control in DL but semi-static power setting

• eNodeB gives flat power spectral density (dBm/PRB) for the scheduled resources:

– The power for all the PRBs is the same

– If there are PRBs not scheduled that power is not used but the power of the remaining scheduled PRBs doesn’t change:

▪ Total Tx power is max. when all PRBs are scheduled. If only 1/2 of the PRBs are scheduled the Tx power is 1/2 of the Tx power max ( i.e. Tx power max -3dB)

• Semi-static: PDSCH power can be adjusted via O&M parameters

– Cell Power Reduction level CELL_PWR_RED [0...10] dB attenuation in 0.1 dB steps

Improve cell edge behaviour, reduce inter-cell interference & power consumption



Power Control (2/5)Downlink Power Boosting for Control Channels

RL30

• Offsets determine power shifts for subcarriers which carry PCFICH/PHICH or cell-specific Reference Signal

Benefits:

• Better PCFICH detection avoids throughput degradation due to lost subframes

• Higher reliability of PHICH avoids unnecessary retransmissions causing capacity degradation and additional UE power consumption

• Better channel estimation avoids throughput degradation and improves HO performance

Cons:

• Small degradation on PDSCH subcarriers: Subcarrier power boosting only allowed if the excess power is withdrawn from the remaining subcarriers


Example of Reference Signals power boosting


Power Control (3/5)

Uplink:

• UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

• Closed Loop PC component f(i): Makes use of feedback from the eNB. Feedback are TCP commands send via PDCCH to instruct the UE to increase or decrease its Tx power

Improve cell edge behaviour, reduce inter-cell interference and power consumption

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _010 dBmifiPLjjPiMPiP TFPUSCHPUSCHCMAXPUSCH

1) Initial TX power level

2) SINR measurment

3) Setting new power offset4) TX power level adjustment with the new offset

• UL Power control is Slow power control:

– No need for fast power control as in 3G: if UE Tx power was high it incremented the co-channel for other UEs.

– In LTE all UEs resources are orthogonal in frequency & time

TPC: Transmit Power Control


Power Control (4/5)

Uplink (cont.):

• UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

• PCMAX: max. UE Tx power according to UE power class; e.g. 23dBm for class 3

• MPUSCH: # allocated PRBs. The UE Tx Power is increased proportionally to the # of allocated RBs. Remaining terms of the formula are per RB

• P0_PUSCH: eNB received power per RB when assuming path loss 0 dB. Depends on α

• α: Path loss compensation factor. Three values:– α= 0, no compensation of path loss

– α= 1, full compensation of path loss (conventional compensation)

– α ≠ { 0 ,1 } , fractional compensation

• PL: DL Path loss calculated by the UE

• Delta_TF: increases the UE Tx power to achieve the required SINR when transmitting a large number of bits per RE. It links the UE Tx power to the MCS.

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _010 dBmifiPLjjPiMPiP TFPUSCHPUSCHCMAXPUSCH


Conventional & Fractional Power Control (5/5)

• Conventional PC schemes:– Attempt to maintain a constant SINR at the receiver – UE increases the Tx power to fully compensate for increases in the path loss

• Fractional PC schemes:– Allow the received SINR to decrease as the path loss increases. – UE Tx power increases at a reduced rate as the path loss increases. Increases in

path loss are only partially compensated.– [+]: Improve air interface efficiency & increase average cell throughputs by reducing

Intercell interference

• 3GPP specifies fractional power control for the PUSCH with the option to disable it & revert to conventional based on α

Conventional Power Control: α=1

If Path Loss increases by 10 dB the UE Tx power increases by 10 dB

Fractional Power Control: α ≠ { 0 ,1}

If Path Loss increases by 10 dB the UE Tx power increases by < 10 dB

UE Tx Power UE Tx

Power

UL SINR

UL SINR


LTE RRM: Radio Admission Control (RAC)

Objective: To admit or reject requests for establishment of Radio Bearers (RB) on a cell basis

• Based on number of RRC connections and number of active users per cell

– Non QoS aware

– Both can be configured via parameters▪ RRC connection is established when the SRBs have been admitted & successfully

configured

▪ UE is considered as active when a Data Radio bearer (DRB) is established

– Upper bound for maximum number of supported connections depends on the BB configuration of eNB :

▪ RL10: support for 200, 400 & 800 active users respectively in 5, 10 & 20 MHz

▪ RL20&RL30: up to 840 active users in 20MHz

• Handover RAC cases have higher priority than normal access to the cell


LTE RRM: MIMO / Antenna Control (1/5) Transmit diversity for 2 antennas

Benefit: Diversity gain, enhanced cell coverage

• Each Tx antenna transmits the same stream of data with Receiver gets replicas of the same signal which increases the SINR.

• Synchronization signals are transmitted only via the 1st antenna

• eNode B sends different cell-specific Reference Signals (RS) per antenna

• It can be enabled on cell basis by O&M configuration

• Processing is completed in 2 phases:• Layer Mapping: distributing a stream of data into two streams• Pre-coding: generation of signals for each antenna port


S1

S2

Spatial multiplexing (MIMO) for 2 antennas (2/5)

Benefit: Doubles peak rate compared to 1Tx antenna

• Spatial multiplexing with 2 code words

• Supported physical channel: PDSCH

Two code words (S1+S2) are

transmitted in parallel to 1 UE double peak rate

Layer Mapping

L1

L2

Precoding

Map onto Resource Elements

×

Map onto Resource Elements

OFDMA

OFDMA

Modulation

Modulation

Code word 1

Code word 2

×

Scale

××

W2

W1

• 2 code words transferred when channel conditions are good

• Signal generation is similar to Transmit Diversity: i.e. Layer Mapping & Precoding

• Can be open loop or closed loop depending if the UE provides feedback

RL20: LTE703: DL adaptive closed loop MIMO


Precoding (3/5)

• Precoding generates the signals for each antenna port

• Precoding is done multiplying the signal with a precoding matrix selected from a predefined codebook known at the eNB and at the UE side

• Closed loop: UE estimates the radio channel, selects the best precoding matrix (the one that offers maximum capacity) & sends it to the eNB

• Open loop: no need for UEs feedback as it uses predefined settings for Spatial Multiplexing & precoding

Pre-coding codebook for 2 Tx antenna case


DL adaptive MIMO for 2 antennas (4/5)

Benefit: High peak rates (2 code words) & good cell edge performance (single code word)

• 2 TX antennas

• Dynamic selection between • Transmit diversity • Spatial Multiplexing

• Supported physical channel: PDSCH

• Dynamic switch considers the UE specific link quality, UE capability, etc.

• Enabled/disabled on cell level (O&M)• If disabled case either static spatial

multiplexing or static Tx diversity can be selected for the whole cell (all UEs)

2 code words (A+B) are transmitted in parallel to 1 UE which doubles the peak rate

1 code word A is transmitted via 2

antennas to 1 UE; improves the LiBu*

AB

A

(RL10) LTE70: DL Adaptive Open Loop MIMO

(RL20) LTE703: DL Adaptive Closed Loop MIMO, utilising PMI report for precoding

* LiBu: Link Budget


MIMO, DL channels & RRM Functionality (5/5)

Available MIMO options vs. channel type

• Options for Transmit Diversity (2 Tx):– Control Channels– PDSCH

• Options for spatial Multiplexing:– Only DL PDSCH

• MIMO is SW feature

Channel can be configured to use MIMO mode

Channel cannot be configured to use MIMO mode

In UL, Flexi eNodeB has 2Rx Div. :

• Maximum Ratio Combining

Benefit: increase coverage by increasing the received signal strength and quality

RRM MIMO Mode Control Functionality • Refers to switch between:

Tx Diversity (single stream) MIMO Spatial Multiplexing (single / dual stream) 1x1 SISO / 1x2 SIMO

• Provided by eNB only for DL direction


LTE RRM: Connection Mobility Control Handover Types• Intra-RAT handover

– Intra eNodeB and Inter eNodeB handover

– Above handovers can also be Inter-frequency handovers (RL20) i.e. to support different frequency bands and deployments within one frequency band but with different center frequencies

– Data forwarding over X2 for inter eNodeB HO

– HO via S1 interface (RL20): HO in case of no X2 interface configured between serving eNB and target eNB

• Inter-RAT handover

– LTE to WCDMA: RL30

– WCDMA to LTE: RL40

– LTE to CDMA2000: RL40 (CDMA2000 to LTE not assigned)

– LTE GSM and GSM LTE: not assigned


Intra frequency handover via X2

• Basic Mobility Feature

• Event triggered handover based on DL measurements (ref. signals)

• Network evaluated HO decision

• Operator configurable thresholds for

• coverage based & • best cell based handover

• Data forwarding via X2

• Radio Admission Control (RAC) gives priority to HO related access over other scenarios S1

S1 X2

MMES-GW

P-GW


A reliable and lossless mobility


Intra LTE Handover via S1

Extended mobility option to X2 handover

• Handover in case of

• no X2 interface between eNodeBs, e.g. multi-vendor scenarios

• eNodeBs connected to different CN elements


• coverage based (A5) and

• best cell based (A3) handover

• DL Data forwarding via S1


RL20

• Admission Control gives priority to HO related access over other scenarios

• Blacklists


Inter Frequency Handover

Multi-band mobility

• Network controlled

• Event triggered based on DL measurement RSRP and RSRQ

• Inter frequency measurements triggered by events A1/A2


coverage based (A5),

best cell based (A3) handover

• Service continuity for LTE deployment in different frequency bands as well as for LTE deployments within one frequency band but with different center frequencies

• Blacklists


RL20


Module Contents

• OFDM Basics




• OFDM Weaknesses

• SC-FDMA





• RRM Overview

• VoIP in LTE


VoIP in LTE

• Voice is still important in LTE

• CS voice call will not be possible in LTE since there is no CS core interface

• Voice with LTE terminals has a few different solutions

• The first voice solution in LTE can rely on Call Setup FallBack redirection where LTE terminal will be moved to 2G/3G to make CS call

• The ultimate LTE voice solution will be VoIP + IMS

RL20

(RL20) LTE10: EPS Bearers for Conversational Voice(RL20) LTE562: Call Setup FallBack (CSFB)


LTE Voice Evolution

VoIPLTEHSPAI-HSPA2G/3G

EPC

MSS

LTE broadband for high speed data Fast-Track VoLTE IMS for enriched IP

multimedia services

LTEHSPAI-HSPA

• Simple upgrade of MSS with NVS (VoIP) function

• Fully IMS compatible reuse of CS infra-structure for LTE VoIP capable handsets

• SRVCC (HO LTE VoIP to 3G CS)

• IMS-centric service architecture

• Rich Communication Services with full multimedia telephony

• Support for any access• SRVCC (HO LTE VoIP

to 3G VoIP)

NVS

LTEHSPAI-HSPA2G/3G

EPCMSS

EPC

VoIP

NVSIMS

• Main focus on LTE data• CS Fallback to 2G/3G

CS access for voice • Re-use existing MSC

Server system for voice

Evolution to IMSVoIP solution

Introduce NVSVoIP solution

MSS: Mobile Softwitching solution

NVS: Nokia Siemens Networks Voice Server

IMS: IP Multimedia Subsystem

02 LTE Air Interface GC

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