Long Term Evolution Part 2: Network architecture and DL physical layer J. Hmlinen, 2015 Department of Communications and Networking

Long Term Evolution (LTE)

1 Introduction 1.1 Requirements and targets for LTE and LTE-Advanced 1.2 Overview of LTE and LTE-Advanced 1.3 After LTE?

2 System Architecture Evolution (SAE) 3 LTE downlink physical layer

3.1 Basic principles of OFDM 3.2 Implementation of OFDM by using IFFT/FFT processing 3.3 OFDMA for LTE downlink 3.4 Modulation 3.5 Downlink reference signals

Long Term Evolution (LTE)

4 LTE uplink physical layer 5 Radio protocols, transport and physical channels

5.1 Radio protocols 5.2 Transport and physical channels

6 LTE Radio Resource Management

2 System Architecture Evolution (SAE)

Background

The general reason to start architecture evolution was the drive towards flat Packet Switched (PS) optimized networks.

The design goals of LTE such as removal of soft handover also opened new opportunities for an enhanced architecture design. HSPA had also indicated that all radio functionalities can be efficiently located in the NodeB

The list of targets that shaped the architecture evolution: optimization for PS services without need to support the circuit switched mode of

operation optimized support for higher throughput and smaller packet delays improvement in the response times for activation and bearer set-up overall simplification of the system compared to the 3G cellular systems optimized inter-working with other wireless access networks

Basic LTE system architecture

UE Uu

X2

E-UTRAN S1-U

EPC

PCRF

S-GW

MME HSS

P-GW External networks,

operator IMS

eNode B

eNode B

S1-MME

SAE GW

S11

Services

Architecture is divided into four main domains: User Equipment (UE), Evolved UTRAN (E-UTRAN), Evolved Packet Core Network (EPC), Services domain.

S1-U: User Plane interface between eNB and S-GW S1-MME: Control Plane interface between eNB and MME X2: Interface between eNBs Uu: Radio Interface between UEs and eNB

Basic LTE system architecture: Domains

Three domains: UE, E-UTRAN and EPC form the so-called Internet Protocol (IP) Connectivity Layer. This part of the system is also called as Evolved Packet System (EPS). The main function of EPS is to provide IP based connectivity All services will be offered on top of IP

Maybe the biggest architectural change is that EPC does not contain a circuit switched domain.

Main functionalities of the EPC are equivalent to the packet switched domain of the existing 3GPP networks.

Basic LTE system architecture: Domains

As a logical element the SAE GW is a combination of the two gateways, Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW)

Since interfaces between S-GW and P-GW are defined in standards, it is possible that S-GW and P-GW are implemented either separately or together.

E-UTRAN contains only one element type: Evolved Node B (eNode B). All radio functionalities are controlled by eNode B. All radio related protocols

are terminated in eNode B. E-UTRAN network is just a mesh of eNodeBs connected to neighboring

eNodeBs through the X2 interface. Functionally eNodeB acts as a layer 2 bridge between UE and the EPC, by

being the termination point of all the radio protocols towards the UE. From functionality point of view the UE is similar like in 3G.

UE and eNodeB

eNode B performs Ciphering/deciphering of the User Plane data IP header compression/decompression Radio Resource Management (resource allocation, prioritizing, scheduling,

resource usage monitoring) eNode B is also involved with Mobility Management (MM).

The eNode B controls and analyses radio signal measurements carried out by the UE,

eNode B also makes signal measurements itself Based on measurement information eNode B makes decisions to handover

UEs between cells.

Mobility Management Entity (MME) and its functions

MME is the main control element in the EPC. It is typically a server in a secure location in the operators premises.

MME operates only in the control plane and is not involved with the user plane data.

MME also has a direct logical control plane connection to the UE. Connection is a primary control channel between the UE and the network.

Main functions of MME: Authentication and Security:

When a UE registers to the network for the first time, the MME initiates the authentication with the aid from the Home Subscription Server (HSS)

Mobility Management Entity (MME) and its functions Main functions of MME (cont):

Mobility Management: The MME keeps track of the location of all UEs in its service area. When a UE

register to the network, the MME signals the UE location to the HSS in the UEs home network.

The MME requests the appropriate resources from eNode B and S-GW which it selects for the UE.

The MME keep tracking the UEs location either on the level of eNode B, if the UE remains connected or at the level of Tracking Area (TA) in case the UE goes to idle mode. An idle UE will report its location either periodically, or when it moves to another TA.

The MME also participates in control signaling between eNode Bs, S-GWs or MMEs in case of handover of an active mode UE.

Managing Subscription Profile and Service Connectivity: MME is responsible for retrieving UE subscription profile from its home network

when UE registers.

Serving Gateway (S-GW)

S-GW takes care of user plane tunnel management and switching, and relays data between eNodeB and P-GW.

The S-GW has a small role in control functions. When bearers for UEs are set up, cleared or modified the S-GW allocates its resources based on requests from MME, P-GW or PCRF.

Packet Data Network Gateway (P-GW)

P-GW is the edge router between the EPS and external packet data networks. P-GW is the highest level mobility anchor in the system, and usually it acts as

the IP point of attachment for the UE. Thus, typically the P-GW allocates the IP address to the UE, and the UE uses that to

communicate with other IP hosts in external networks, e.g. the internet. During mobility between eNodeBs, the S-GW acts as the local mobility anchor. The

MME commands the S-GW to switch the tunnel from one eNodeB to another. P-GW performs traffic gating and filtering functions as required by the service

in question. Both S-GW and P-GW are part of the network infrastructure maintained

centrally in operator premises. They also provide means for lawful interception

Policy and Charging Resource Function (PCRF), Home Subscription Server (HSS)

PCRF is the network element that is responsible for Policy and Charging Control (PCC).

HSS is the data repository for all permanent subscription data. Hence, HSS has the master copy of the subscriber profile

Main interfaces

X2 interface: This interface is used in mobility between the eNodeBs, and it includes functions for handover preparation, and overall maintenance of the relation between neighbouring eNodeBs. X2 is especially important interface because it connects logically eNodeB to

each other and E-UTRAN is basically just a mesh of eNodeBs connected via X2. S1-MME interface: Reference point for the control plane protocol between

E-UTRAN and MME. S1-U interface: Reference point between E-UTRAN and Serving GW for

the user plane tunnelling and inter eNodeB path switching during handover.

3 LTE Downlink physical layer

3.1 Basic principles of OFDM

The OFDM Subcarrier spectrum

0 ff f2f 2

( ) 2

//sin

ffff

fTu = /1

Time domain Frequency domain

Tu = per-subcarrier modulation-symbol time f = subcarrier spacing 1/ Tu = per-subcarrier modulation rate

OFDM principle

OFDM divides spectrum into large number of tightly packed narrow subcarriers

At the peak of each subcarrier, all other subcarriers have amplitude zero => avoids subcarrier interference

In e.g. 10 MHz bandwidth there can be 1024 subcarriers (LTE)

OFDMA has been recently adopted by many access technologies like LTE, WiMAX and WLAN.

OFDM is applied together with time division multiplexing

uTf /1=

OFDM: Multicarrier system with narrow tightly packed subcarriers

OFDM subcarriers have a frequency response resulting in overlap in the frequency domain. This overlap does however not cause interference due to the orthogonality of the subcarriers.

Conventional multicarrier approach

OFDM

OFDM: The use of DFT/FFT

The OFDM receiver uses a time and frequency synchronized DFT/FFT to convert the OFDM time waveform back into the frequency domain.

In this process the DFT/FFT picks up discrete frequency samples, corresponding to just the peaks of the carriers. At these frequencies, all other carriers pass through zero amplitude eliminating any interference between the subcarriers.

Even small frequency synchronization error can be critical for OFDM

Sampling points

In the next slides we consider in more details the OFDM implementation using IFFT/FFT processing

OFDM: frequency offset No frequency offset (error)

OFDM: frequency offset Frequency offset (error) that is causing Inter Carrier Interference

3.2 Implementation of OFDM by using IFFT/FFT processing

The digital part of the OFDM transmission The OFDM transmission includes the following digital

phases: 1. OFDM modulation of complex modulation symbols

Here sequence of Nc complex modulation symbols (BPSK, QPSK, QAM symbols) are modulated to form a single OFDM symbol of length N.

During the process N-Nc zeros can be added to the original symbol sequence.

2. Cyclic Prefix (CP) insertation NCP last symbols of OFDM symbol are copied in front of OFDM

symbol. After this operation the length of the OFDM symbol is N + NCP samples.

the analog phase and impact of the mobile channel The analog phases of the transmission/reception chain are omitted

but the impact of mobile channel is taken into account: 3. The OFDM symbol is converted in DAC into analog signal

Details are omitted 4. Analog signal is transmitted through a mobile channel.

Resulting signal is of the form

where h(t) is the continuous-time impulse response, * is the convolution operation and z(t) represents the additive noise.

5. After ADC the received OFDM signal samples are passed to the digital part of the receiver.

)()()(~)(~ tzthtxtr +=

)(~ tx

the OFDM reception

The OFDM reception includes the following digital phases: 6. Cyclic Prefix (CP) removal

Here first NCP samples are removed and remaining N samples are passed to the OFDM demodulation.

7. OFDM demodulation After size N FFT Last N - Nc samples are ignored. These samples are

related to zeros that were added in the OFDM modulation. Remaining Nc complex demodulation outputs are obtained.

8. Signal equalization Usually only one-tap equalization is needed.

OFDM transmission/reception

Cyclic Prefix insertion

OFDM modulation (IFFT)

Channel h(t)

( ) 10)(

=

Nn

mnx ( ) 10)(~

+

=CPNN

nmnx

Noise z(t)

OFDM transmitter Mobile channel

DAC )(~ tx( ) 10)(

=cNn

mna )()()(~ tzthtx +

Cyclic prefix removal

OFDM demod. (FFT)

( ) 10)(~+

=CPNN

nm

nr ( ) 10)(

=

Nn

mnr

OFDM receiver

ADC ( ) 10)(

=cNn

mnb)()()(~)(~ tzthtxtr +=

OFDM modulation by using IFFT

)(1

)(1

)(0 ,,,

mN

mmc

aaa SP Size N IFFT

)(0ma)(

1ma

)(1

mNca

0

0

PS

)(0mx)(

1mx

)(1

mNx

)(1

)(1

)(0 ,,,

mN

mm xxx

OFDM modulation

Insertation of the cyclic prefix

Cyclic prefix insertion

OFDM modulation (IFFT)

)(1

)(1

)(0 ,,,

mN

mmc

aaa )(1

)(1

)(0 ,,,

mN

mm xxx )(1

)(0

)(1

)( ,,,,, mNmm

NmNN xxxx CP

Cyclic Prefix (CP)

N samples (modulation symbol time Tu)

N+NCP samples (time Tu+ TCP)

Tu TCP

Copy and insert CP

Tu TCP

Copy and insert CP

( ) 10)(~+

==CPNN

nmnx

Removal of the cyclic prefix

Cyclic prefix removal

OFDM demod. (FFT)

N samples (modulation symbol time Tu)

N+NCP samples (time Tu+ TCP)

( ) )( 1)(0)( 1)(10)( ,,,,,~ mNmmNmNNNN

nm

n rrrrr CPCP

+

= = ( ) )( 1)(010)( ,, mNmNn

mn rrr

= =

Matrix formulation of the received digital signal 1/4 After receiver ADC and removal of the CP we can write

samples related to mth OFDM symbol in the matrix form

We note that matrix x(m) here is not in a feasible form for FFT based demodulation.

+

Matrix formulation of the received digital signal 2/4 To achieve a suitable matrix formulation we add zeros to

channel vector and columns into matrix x(m). New columns of the matrix x(m) are permutations of the same

vector such that extended matrix is circulant and of size NxN. Thus, we obtain equation

+

Now we can effectively use FFT!

OFDM demodulation by using FFT

)(1

)(1

)(0 ,,,

mN

mm rrr SP Size N FFT

)(0mr)(

1mr

)(1

mNcr

PS

)(0mb)(

1mb

)(1

mNcb

)(1

)(1

)(0 ,,,

mN

mmc

bbb

OFDM demodulation

)(mNcr

)(1mNr

)(mNcb

)(1

mNb

Not used

Matrix formulation of the received digital signal 3/4 Since matrix x is circulant we can use FFT to write the

received signal in the following form:

where F is the Fourier transform matrix with elements

and contains the eigenvalues of matrix x(m). We note that

( ) )()()()()()( mmHmmmm zhFDFzhxr x +=+=

( ))( 1)(1)(0 ,,, mNmm XXXdiag = xD

==

=

otherwise,010,1 )(1

0

/2)()( cmn

N

k

Nknjmk

mn

Nnaex

NX

=

=1

0

/2,

1 N

k

Nknjkn eN

F

Matrix formulation of the received digital signal 4/4 Now, after using the FFT to vector r(m) we obtain the

formulation

where

Thus, the OFDM converts the transmission over a frequency selective (multipath) channel into a transmission over Nc parallel flat fading channels:

( ) )()()()()()( mmmmmm ZHDFzFhDFrb xx +=+==

=

=

==1

0

/2)(1

0

/2)()( 11 CPN

k

Nknjmk

N

k

Nknjmk

mn ehN

ehN

H

10)()()()( += cmn

mn

mn

mn NnZaHb

Signal equalization

To equalize the output we simply multiply it by complex conjugate of H:

After this scaling the output can be passed to the detector and decoder.

This simple frequency domain representation of the received OFDM signal simplify the receiver implementation.

Note: Channel response needs to be estimated before operation (*) can be carried out.

( ) ( ) 10 )()()(2)()()()( +== cmnmnmnmnmnmnmn NnZHaHbHa(*)

)(mnH

Frequency domain OFDM signal model

)(0ma )(0

ma

)(0mH )(0

mZ )( )(0mH

)(0mb

)(1

mNca

)(1

mNca

)(1

mNc

H )(1

mNcZ

)()(1

mNc

H)(1

mNcb

Receiver Frequency domain channel

Illustration of the frequency domain channel response

50 100 150 200 250 300-30

-25

-20

-15

-10

-5

0

5

10

Chan

nel p

ower

resp

onse

[dB]

Number of the subcarrier

BW=5MHz FFT size = 512 Used subcarriers = 300 CP = 40 samples Power Delay Profile: Exponential PDP RMS delay spread = 0.5us Maximum delay = 4.7us

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time delay [us]

Powe

r [dB

]

3.3 OFDMA for LTE downlink

OFDM parameters for LTE

In LTE the following OFDM parameter selections has been done: The subcarrier spacing:

f =15kHz The cyclic prefix length:

Normal CP = 5.2s or 4.7s (see next slide) Extended CP = 16.7s

The number of subcarriers: Nc depends on the used bandwidth, between 72 and 1200 The size of FFT is power of two and depends on the used

bandwidth, between 128 and 2048

On the selection of the Cyclic Prefix length 1/2 The CP length should be longer than the maximum channel

delay spread Channel delay spread depends on the cell size and e.g. BS antenna

height and transmission power If CP is too short, it may limit the cell size, BS transmission power

and antenna configuration.

Ts = Sampling time

OFDM symbol CP

TCP = NCP Ts Tu = N Ts (=1/f )

Effective channel impulse response

On the selection of the Cyclic Prefix length 2/2 The longer the CP the larger is the rate loss:

The relative rate loss due to CP is given by CP = TCP /(Tu + TCP)

Due to these constraints, in LTE two CP lengths are used: Normal CP for small and medium size cells Extended CP for large cells and/or extreme radio environments with

long channel spreads

On the selection of the subcarrier spacing Constraints for the OFDM subcarrier spacing are:

The subcarrier spacing should be as small as possible. Then Tu =1/f is large and CP overhead is small

The smaller the subcarrier spacing is, the more sensitive the system is for frequency errors

Frequency errors can be due to Channel Doppler spread: High mobile speed => large Doppler spread

=> intercarrier interference Inaccurate frequency synchronization: This can result from transmitter

and receiver inaccuracies

LTE subcarrier spacing (15kHz) represent the best found trade-off

LTE bandwidth options

LTE provides very good frequency flexibility => effective refarming of frequency bands is easier.

Bandwidth 1.4MHz 3 MHz 5MHz 10MHz 15MHz 20MHz Subcarriers 72 180 300 600 900 1200 FFT (DFT) size 128 256 512 1024 1536 2048 Sampling rate [MHz] 1.92 3.84 7.68 15.36 23.04 30.72 Samples per slot 960 1920 3840 7680 11520 15360

Narrowband options to be used for refarming of e.g. GSM bands

Compatible with WCDMA/HSPA

High data rate options

LTE time slot structure: Normal CP

LTE baseline configuration (7 symbols/time slot) The CP time duration for the first symbol is 5.2 us. For other symbols the CP time duration is 4.7 us The OFDM symbol time duration is always 66.7 us Number of samples depends on the sampling frequency. Here sampling

frequency is 30.72MHz related to the 20MHz bandwidth

CP OFDM symbol CP OFDM symbol CP OFDM symbol CP OFDM symbol CP OFDM symbol

LTE time slot: 0.5 ms, 15360 samples (20MHz bandwidth)

Slot structure for normal CP, seven OFDM symbols: 160 samples (5.2 s)

144 samples (4.7 s)

2048 samples (66.7 s)

LTE time slot structure: Extended CP

This configuration is used in large cells where long signal delay spreads occur (6 symbols/time slot)

The CP time durations for all symbols are 16.7 us The OFDM symbol time duration is always 66.7 us Number of samples depends on the sampling frequency. Here sampling

frequency is 30.72MHz related to the 20MHz bandwidth

LTE time slot: 0.5 ms, 15360 samples (20MHz bandwidth)

CP OFDM symbol CP OFDM symbol CP OFDM symbol CP OFDM symbol

Slot structure for extended CP, six OFDM symbols: 512 samples (16.7 s)

2048 samples (66.7 s)

LTE time frame structure LTE frame:

Contains 10 subframes of 1ms Each subframe contains 2 time slots (0.5ms) Time slot contains 6-7 OFDM symbols Frame length is 10ms

LTE time-frequency structure 1/4

Resources are grouped into two dimensional Resources Blocks (RBs). A pair of Resource Blocks is the minimum unit a scheduler can allocate. RB contains 12 subcarriers (180kHz in total) and 7 consecutive symbols

(normal CP) or 6 symbols (extended CP)

Time

Freq

uenc

y

12 subcarriers

2*7 symbols (1ms in total) 2 RBs form a resource block pair

Here channels of 12 adjacent subcarriers are strongly correlating in frequency and almost constant in time unless receiver moves really fast.

LTE time-frequency structure 2/4

In downlink LTE supports the so-called localized OFDMA where adjacent symbols and subcarriers are scheduled together. Subcarriers within RB are strongly correlated in frequency and temporal

channel changes are small during RB time duration. This decreases reference signal overhead that is needed for channel estimation.

Distributed OFDMA was not taken into LTE specifications. There subcarriers from different parts of the frequency spectrum could be allocated to the user. Then, reference symbols would be needed in each subcarrier => high reference signal overhead.

Number of resource blocks attached to a user defines the user data rate This number varies depending on the service, data rate, scheduler buffer status

and channel conditions.

LTE time-frequency structure 3/4

LTE time-frequency structure 4/4

52

Bandwidth 1.4MHz 3 MHz 5MHz 10MHz 15MHz 20MHz Subcarriers 72 180 300 600 900 1200 FFT (DFT) size 128 256 512 1024 1536 2048 Sampling rate [MHz] 1.92 3.84 7.68 15.36 23.04 30.72 Samples per slot 960 1920 3840 7680 11520 15360 Resource Blocks 6 15 25 50 75 100

Narrowband options to be used for refarming of e.g. GSM bands

Compatible with WCDMA/HSPA

High data rate options

In performance studies it is usually assumed that 2 RBs are used for control purposes

WCDMA/HSPA vs LTE: OFDMA benefit

WCDMA/HSPA spectrum allocation dont allow wider carrier bandwidth than 5 MHz HSPA receiver with equalizer shows good performance at 5 MHz When selecting 4G radio access method WCDMA for bandwidths

wider than 5MHz were investigated. However, if carrier bandwidth is increased to e.g. 10-20MHz, then WCDMA performance suffers due to increased multipath components and complexity of required equalizer in the receiver grows rapidly with bandwidh.

OFDMA used in LTE is feasible for wide bandwidths If carrier bandwidth is increased, then OFDMA time frequency

resources remain orthogonal => OFDMA gives very good scaling property for LTE.

Even carrier bandwidths 40MHz or wider could be introduced for LTE but it has not seen reasonable due to implementation reasons.

3.4 Modulation

LTE Modulation (DL and UL) Available modulations for user data transmission are QPSK (2bits/symbol),

16QAM (4bits/symbol) and 64QAM (6bits/symbol) In uplink 64QAM is not mandatory capability for UE. When using 16QAM or 64QAM the UE applies lower TX power than in case of

using QPSK due to increased peak-to-average power ratio BPSK is used on some control channels

3.5 Downlink reference signals

LTE downlink reference signals

Mobile radio channel is subject to multipath fading which causes Inter-Symbol Interference (ISI).

OFDM is robust against ISI due to use of CP. Yet, practical mobile systems apply coherent detection where channel knowledge is exploited.

Thus, for efficient detection of information the amplitude and phase of the complex channel should be known as accurately as possible.

Most common way to arrange the channel estimation is to use reference signals (RS)

Reference signals carry symbols that are known for both transmitter and receiver

Let us briefly consider LTE reference signals in more details.

LTE downlink reference signals

In LTE downlink 5 different RS types has been defined. In the following discussion we focus on cell specific RSs while other RS types are omitted Cell specific RSs (also called as common RSs): These RSs are available (and

the same) for all users in the cell.

In LTE reference symbols are placed in time-frequency domain in predefined locations Location of RSs depend on the number of TX antennas (called as antenna

ports) and CP type (normal/extended). See examples on the next slides

RSs are QPSK modulated and formed using length-31 Gold Codes In LTE Rel8 cell specific RSs are available for up to 4 antenna ports

In 3GPP terminology antenna port can be implemented using one or more physical antenna elements.

Cell specific reference signals: one antenna port

R0 R0

R0 R0

R0 R0

R0 R0

Time

Freq

uenc

y

Cell specific reference signal locations in a resource element when using one antenna port and normal CP.

Cell specific reference signals: two antenna ports

E R0 E R0

R0 E R0 E

E R0 E R0

R0 E R0 E

Antenna port 0

R1 E R1 E

E R1 E R1

R1 E R1 E

E R1 E R1

Cell specific reference signal locations in resource elements when using two antenna ports and normal CP. E refers to empty RE. Use of empty REs prevent interference towards reference symbols

Antenna port 1

Remark: Illustrated REs are transmitted at the same time and frequency from separate antenna ports.

Remark: Cell specific RSs for 4 antenna ports are omitted in this presentation

Cell specific RS design principles

Notes on cell specific RSs: UE can use cell specific RSs to estimate the

channel. Cell specific RS are equidistant in time-

frequency domain => Optimal estimation of the channel in between

RSs locations Density of RSs has been selected based on

Expected Doppler spread (terminal speed) Channel frequency selectivity (radio

environment) UE uses cell specific RSs also to define

Channel State Information (CSI) feedback.