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Cellular Networks - Part 4

4G: 3GPP LTE (E-UTRAN)

31/05/2013


References

•  Books –  LTE - The UMTS Long Term Evolution: From Theory to

Practice •  S. Sesia, I. Toufik, M. Bekar (Wiley)

•  3GPP Rel-11 –  TS 36.300 v11.2.0, July 2012

•  3GPP Specification details –  MAC

http://www.3gpp.org/ftp/Specs/html-info/36321.htm •  RLC http://www.3gpp.org/ftp/Specs/html-info/36322.htm •  RRC http://www.3gpp.org/ftp/Specs/html-info/36331.htm


Terms and Definitions

•  UE: User Equipment (Mobile) •  eNB: Evolved Node B (Base station) •  S-GW: Serving Gateway (Cellular network

edge router or MTSO) •  E-UTRA/N: Evolved UMTS Terrestrial Radio

Access/Network (Official name of LTE) •  EPS: Evolved Packet System (MTSO network) •  MME: Mobility Management Entity (also at

MTSO)


1999

Release 99

Release 4

Release 5

Release 6

LCR TDD

HSDPA

W-CDMA

HSUPA, MBMS

Release 7 HSPA+ (MIMO, etc.)

Release 8 LTE

Release 9

Release 10

LTE enhancements

Release 12

ITU-R M.1457 IMT-2000

Recommendation

ITU-R M.2012 [IMT.RSPEC] IMT-Advanced

Recommendation

LTE-Advanced

Further LTE enhancements

2001 2003 2005 2007 2009 2011 2013

---

Release 11

"   3GPP aligned to ITU-‐R IMT process "   3GPP Releases evolve to meet:

•  Future Requirements for IMT •  Future operator and end-‐user

requirements

only main RAN WI

listed

now 2015


What is LTE •  Long Term Evolution (LTE): based on OFDM/

OFDMA –  Evolution of UMTS. –  HSPA and HSPA+ dominant till 2015 –  LTE started to be deployed in 2011: BSs shipped in

March 2010 – waiting for user equipment (UE). –  LTE standard has been defined by European

Telecommunication Standardization Institute (ETSI) and the 3rd Generation Partnership Project (3GPP).

–  Specs available on the 3gpp.org


History of LTE

•  The work towards 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) started in 2004: HSDPA was not yet deployed at that time! >= 5 years from setting the system targets to commercial deployment using interoperable standards

•  First set of approved physical layer specifications; Sept. 2007

•  First full set of approved LTE specifications; Dec. 2007 •  Functional freeze: no new functionality can be introduced

anymore but the agreed content will be finalized; end 2008;

•  Backward compatibility of 3GPP release 8 protocol specifications core functionalities ready; March 2009.

•  Deep freeze: no change allowed, devices on the field •  First roll-out: March 2010 ….


Relative adoption of Technologies


LTE - Platform for the future


Promises of LTE •  Significantly increased peak data rates •  Increased cell edge bitrates •  Improved spectrum efficiency •  Improved latency •  Scalable bandwidth from below 1.5 MHz up to 20

MHz allocations. •  Reduced CAPEX and OPEX •  Acceptable system and terminal complexity, cost

and power consumption •  Compatibility with earlier releases and with other

systems •  Optimised for low mobile speed but supporting

high mobile speed


Data rates

•  Goal: significantly increased peak data rates, scaled linearly according to spectrum allocation

•  Targets: – Instantaneous downlink peak data rate

of 100Mbit/s in a 20MHz downlink spectrum (i.e. 5 bit/s/Hz)

– Instantaneous uplink peak data rate of 50Mbit/s in a 20MHz uplink spectrum (i.e. 2.5 bit/s/Hz)


Speeds

•  The Enhanced UTRAN (E-UTRAN) will: –  be optimised for mobile speeds 0 to 15 km/h

support, with high performance, speeds between 15 and 120 km/h

– maintain mobility at speeds between 120 and 350 km/h

•  and even up to 500 km/h depending on frequency band

–  support voice and real-time services over entire speed range

•  with quality at least as good as UTRAN


Comparison HSxPA/LTE

Requirement Current Release (Rel-6 HSxPA) LTE

Peak data rate 14 Mbps DL / 5.76 Mbps UL

100 Mbps DL / 50 Mbps UL

Spectral Efficiency 0.6 – 0.8 DL / 0.35 UL (bps/

Hz/sector)

3-4x DL / 2-3x UL improvement

Averaged user throughput 900 Kbps DL /150 Kbps UL 3-4x DL / 2-3x UL improvement

U-Plane Latency 50 ms 5 ms Call setup time 2 sec 50 ms Broadcast data rate 384 Kbps 6-8x improvement Mobility Up to 250 km/h Up to 350 km/h Multi-antenna support No Yes Bandwidth 5 MHz Scalable


Latency of different technologies


Comparison of downlink spectral efficiency


Comparison of uplink spectral efficiency


PHY of LTE


Spectrum allocation

•  LTE will likely start by using new 2600 MHz band

•  Plus farming to 900 and 1800 MHz bands

•  Europe there is in total a 565 MHz spectrum available for the mobile operators


Spectrum allocation

•  LTE flexibile bandwidth allocation promises easy replacement of dimissing GSM frequencies in the 900 and 1800 MHz bands


LTE Architecture


Basic principles of the Evolved Packet System

•  The EPS comprises the Evolved Packet Core (EPC) and the Evolved UTRAN (E-UTRAN)

•  EPS is designed to be a purely packet switched system –  IMS (IP Multimedia Subsystem) targeted as voice service

platform •  ETPS for 3GPP accesses similar to GPRS core,but more flat

–  Reduction of nodes in user plane path: 4-> 3 nodes –  GTP remain the main protocol for 3GPP accesses

•  EPS enables interworking with non-3GPP accesses (WLAN, WiMAX, CDMA2000,...) –  IP Mobility between 3GPP accesses and non-3GPP

accesses based on PMIPv6 (Proxy Mobile IPv6) or DSMIPv6 (Dual-stack Mobile IPv6)


EPS Architecture


EPS for 3GPP Accesses •  PDN GW: IP address alloction, charging and enforces QoS •  Serving GW: local mobility anchor for intra-3GPP HO

•  MME: Mobility management entity for intra-3GPP mobility, paging, authentication, bearer management etc.

•  PCRF: QoS and charging rule provisioning


Network architecture

•  Ambitious design: –  packet-switched traffic –  seamless mobility –  quality of service (QoS) – minimal latency

•  Only two core nodes: –  Evolved Node-B (eNB) – Mobility management

entity/gateway (MME/GW)


Network architecture •  Major Change compared to

3G: –  radio network controller

(RNC) is eliminated from the data path;

–  its functions are now incorporated in eNB.

•  Single node in the access network has advantages: –  reduced latency –  distribution of the RNC

processing load into multiple eNBs.

–  Possibility of coordination among eNBs


Network architecture


User plane stack

•  Packet data convergence protocol (PDCP) and radio

link control (RLC) layers traditionally terminated in RNC are now terminated on eNB

•  PDCP: Sequence numbering, header compression, ordering

•  RLC: Segmentation, ARQ


Control plane stack

•  Non-access stratum (NAS) protocol:

–  terminated in the MME on the network side and at the UE on the terminal side

–  performs functions such as EPS (evolved packet system) bearer management, authentication and security control, etc.

•  Radio resources controller (RRC) protocol: system information broadcast, paging, radio bearer control, RRC connection management, mobility functions and UE measurement reporting and control.


Interfaces

•  MME/GW entities interconnected by means of S1 interfaces

•  eNBs interconnected by means of X2 interfaces


S1 Interface

•  S1 User plane interface (S1-U): interconnects eNB – S-GW

–  GTP-U (GPRS tunneling protocol – user data tunneling)

–  UDP/IP transport: non-guaranteed delivery of user plane PDUs between the eNB and the S-GW.

–  MME/GW entities interconnected by means of S1 interfaces

•  S1 control plane interface (S1-MME): interconnects eNB – MME

–  SCTP (stream control transmission protocol): ensures reliable, in-sequence transport of messages with congestion control.


X2 Interface

–  eNode B X2 Interface allows inter-eNode B handover –  X2 User plane interface (X2-U): interconnects eNB – S-GW –  X2 control plane interface (X2-MME): interconnects eNB – S-GW


S/P-Gateways •  Two logical gateway entities

–  serving gateway (S-GW) –  packet data network gateway

(P-GW) •  S-GW acts as a local mobility

anchor forwarding and receiving packets to and from the eNB serving the UE.

•  The P-GW interfaces with external packet data networks (PDNs) such as the Internet and the IMS. The P-GW also performs several IP functions such as –  address allocation, –  policy enforcement, –  packet filtering –  packet routing.


S/P-Gateways


MME •  MME: a signaling only entity and

hence user IP packets do not go through MME.

•  Separate network entity for signaling splits the network capacity for signaling and traffic: they can grow independently.

•  Main functions of MME: 1.  idle-mode UE reachability; 2.  control and execution of paging;

retransmission; 3.  tracking area list management; 4.  roaming; 5.  Authentication; 6.  Authorization; 7.  P-GW/S-GW selection; 8.  bearer management; 9.  security negotiations; 10. NAS signaling; 11. Etc...


MME


•  On QoS guarantees in LTE •  QoS and Bearer Service Architecture


QoS and Policy Control •  End-to-end Services such as VoIP, web browsing, video telephony

and video streaming have special QoS needs (delay, jitter, throughput)

•  EPS (evolved packet system) bearer management: QoS flows called EPS bearers are established between the UE and the P-GW

•  'Bearer' is basically a virtual concept and is a set of network configuration to provide special treatment to set of traffic

•  IP traffic is mapped to bearers by means of traffic flow templates (TFT)

•  One-to-one mapping between radio bearer and the S1 bearer


QoS and Policy control

•  In LTE, QoS is enforced at the granularity of the EPS bearers •  End to end Service: e.g. Video Streaming, requires QoS

guarantees •  UE <- -> PDN GW (for GTP-based EPC) •  The instance of a IP flow – the Video Stream - is logically

split into an External Bearer (in the external PDN) + an EPS bearer (in the LTE network)

•  The EPS bearer uniquely identifies traffic flows that receive a common QoS

•  A UE always has a Default Bearer, for all flows that do not require any special QoS treatment

•  Dedicated bearers are established for all service data flows that require special QoS treatment


•  Default Bearer

•  Dedicated Bearer


QoS and Policy Control

•  QoS is applied on Radio bearer, S1 bearer and S5/S8 bearer, collectively called as EPS bearer


QoS and Policy control

•  The EPS bearer QoS profile includes the parameters QCI, ARP, GBR –  QCI: QoS Class Indicator is a reference to access

node-specific parameters that control bearer level packet forwarding

–  ARP: Allocation and Retention Priority; pre-emption capability/vulnerability

–  GBR: Guaranteed Bit Rate •  When receiving an IP packet from the Internet:

–  P-GW performs packet classification based on certain predefined parameters (QoS implementation)

–  Then it sends with an appropriate EPS bearer. –  Based on the EPS bearer, eNB maps packets to the

appropriate radio QoS bearer.


•  The bit rate and QoS treatment parameters available to each of type bearer




•  Packet classification can be performed by means of IP-5-Tuple, DPI, … –  PDN GW (GTP) for downlink traffic –  UE for uplink

•  Downlink “Bearer binding” takes place in PDN GW for GTP based EPC


Policy and Charging Control (PCC) •  PCEF: Policy and Charging Enforcing Function enforces QoS policies on

bearers –  Decides which traffic is bound to which bearers –  Decides to setup dedicated bearers for certain traffic types –  Located in PDN-GW

•  PCRF: provides policies and rules for PCEF


Default QoS Classes

•  These values are UE-to-PCEF (PDN-GW) QoS values •  QCI QoS parameter have been mapped to scheduling and RRM parameters in the

eNodeB, such as : –  Scheduling delay budget, bandwidth –  HARQ and ARQ parameters


QoS and Policy Control – Idea and Reality


•  Downlink LTE

LTE: Layer-2 Structure


•  Uplink LTE

•  Downlink and Uplink have similar protocol stack

•  Downlink takes care of scheduling/priority of users

QoS in LTE: Layer 2 Structure


•  Network-initiated QoS control: it is the responsibility of the network to detect and infer what QoS resources are needed by the user or application.

•  Terminal-initiated QoS control: it is the terminal that signals the network and requests that a dedicated bearer with the desired level of QoS be established.

•  This means that the terminal must be aware of the specifics of how QoS is handled in the access network and be able to interface with the network to convey the QoS request

•  Terminal-initiated QoS control, in public safety could be a good option specially in emergency scenarios

QoS Control – Network-Initiated vs. Terminal-Initiated


Medium Access Control (MAC)

•  MAC functions: –  Priority handling among

UEs (DL) + among UE logical channels (DL/UL).

–  mapping logical to/from transport channels (packets to/from bits);

–  multiplexing of RLC PDUs; –  Padding; –  Transport format

selection; –  Hybrid ARQ (HARQ).


MAC in LTE Protocol Stack


LTE Channel Architecture

•  RLC passes data to the MAC layer as logical channels

•  MAC layer formats and send the logical data as transport channel.

•  PHY encodes the transport channel data to physical channels


Downlink PDCP, RLC and MAC sub-layer


Uplink PDCP, RLC and MAC sub-layer


Radio Link Control (RLC)

•  RLC functions: –  ARQ in-sequence delivery and duplicate detection, etc;

•  The in-sequence delivery of upper layer PDUs is not guaranteed at handover;

–  RLC can be configured to either acknowledge mode (AM) or un-acknowledge mode (UM) transfers

•  UM mode can be used for radio bearers that can tolerate some loss;

•  In AM mode, ARQ functionality of RLC retransmits transport blocks that fail recovery by HARQ;

–  HARQ recovery failures: hybrid ARQ NACK to ACK error or because the maximum number of retransmission attempts is reached; relevant transmitting ARQ entities are notified and potential retransmissions and re-segmentation can be initiated.


States •  RRC connection established: UE moves from RRC IDLE to RRC

CONNECTED •  RRC connection released: a UE moves back from RRC CONNECTED to

RRC IDLE •  RRC IDLE state:

1.  receive broadcast/multicast data; 2.  monitors a paging channel to detect incoming calls; 3.  performs neighbor cell measurements and cell selection/reselection; 4.  acquires system information.

•  RRC CONNECTED: 1.  UE monitors control channels associated with the shared data channel to

determine if data is scheduled for it; 2.  channel quality feedback information, neighbor cell measurements and

measurement reporting and acquires system information; 3.  Unlike the RRC IDLE state, the mobility is controlled by the network in this state.


LTE Downlink Channels


DL Logical Channels


DL Transport Channel


DL Physical Channels


LTE Uplink Channels


UL Logical Channels


UL Transport Channels


UL Physical Channels


Random Access •  Random access is performed when

–  the UE turns on from sleep mode –  performs handoff from one cell to another –  when it loses uplink timing synchronization:

•  The UE needs to –  acquire downlink timing synchronization by receiving primary and secondary

synchronization sequences and the broadcast channel; –  receive system information including information on parameters specific to

random access; –  UE then perform the random access preamble transmission;

•  The eNB upon successfully receiving a random access preamble, replies with a random access response indicating

–  the successfully received preamble(s) along with the timing advance (TA) and uplink resource allocation information to the UE.

•  The UE matches the preamble number it used for random access with the preamble number information received from the eNB:

–  uses the TA information to adjust its uplink timing –  send uplink scheduling or a resource request


Random Access Procedure


H-ARQ

•  Very old result: feedback can increase system capacity (somewhere in the 70s): ACK and NACK techniques are widespread in the telecommunications community

•  However, ACK/NACK frames have a cost –  Redundancy: we have to spend some bandwidth to transmit

backwards ACK/NACK packets; typically this is a small overhead and some overhead is unavoidable;

–  Delay: this may be worse, especially when delay-sensitive traffic is to be accounted for.

•  FEC can be better for this type of traffic, as long as –  Enough redundancy can be transmitted over the channel before

the packets/frames deadlines; –  Enough computing power is available at the receiver side.

•  Hybrid ARQ is a simple idea: just use ACK and FEC combined, using ACKs only when recovery through FEC is not possible (read: it would cost too large delay)

•  Obviously: LTE supports Hybrid ARQ and this costs a lot of extra signaling!


TM-RLC

•  Transparent Mode RLC: RLC SDU directly

mapped into RLC PDU, restricted to –  broadcast system information messages, paging

messages, and some RRC messages under special circumstances (lack of Signaling Radio Bearers other than SRB0)

–  TM RLC is not used for user plane data transmission in LTE.


U-RLC

•  Unacknowledged Mode RLC: provides a

unidirectional data transfer service –  delay-sensitive and error-tolerant real-time

applications, especially VoIP, and other delay-sensitive streaming services.

–  Point-to-multipoint services such as MBMS (Multimedia Broadcast/Multicast Service) - no feedback path is available in the case of point-to-multipoint services


U-RLC


U-RLC functions

RLC events handled:

–  Segmentation and concatenation of RLC SDUs;

–  Reordering of RLC PDUs;

–  Duplicate detection of RLC PDUs;

–  Reassembly of RLC SDUs.


A-RLC

•  Acknowledged Mode RLC: provides a bidirectional data transfer service; AM RLC entities are configured with the ability both to transmit and to receive –  The most important feature of AM RLC is ‘retransmission’. An

Automatic Repeat reQuest (ARQ) operation is performed to support error-free transmission.

–  User plane: AM RLC is mainly utilized by error-sensitive and delay-tolerant non-real-time applications: browsing/file-downloading, streaming

–  Control plane: RRC messages typically utilize the AM RLC in order to take advantage of RLC acknowledgements and retransmissions to ensure reliability.


Mobility

•  Fast and seamless handovers (HO) are particularly important for delay-sensitive services such as VoIP

•  MME handovers much less frequent than eNB ones –  Critical for routing

•  X2 is the interface for handovers among eNBs


UE initiated handovers •  The network relies on the UE to detect

the neighboring cells for handovers: no neighbor cell information signaled from the network;

•  Handovers in the RRC CONNECTED state are network controlled and assisted by the UE;

•  UE sends a radio measurement report to the source eNB1 indicating: eNB2 > eNB1.

•  eNB1 sends the coupling information and the UE context to the target eNB2 (HO request) over the X2 interface

•  C-RNTI cell radio network temporary identifier associated to the UE RRC connection;

•  eNB2 signals to eNB1 that handover is possible

•  eNB1 commands the UE (HO command) to change the radio bearer to eNB2


Handover signaling •  UE performs synchronization to the target eNB

–  RACH following a contention-free procedure if a dedicated RACH preamble was allocated in the HO command

–  contention-based procedure if no dedicated preamble. •  Target eNB sends a path switch message to the MME to

inform that the UE has changed cell. •  MME sends a user plane update message to the S-GW. •  The S-GW

–  switches the downlink data path to the target eNB –  Sends one or more “end marker” packets on the old path to

the source eNB –  releases any user-plane/TNL resources towards the source

eNB. –  S-GW sends a user plane update response message to the

MME. •  The MME confirms the path switch message from the

target eNB with the path switch response message. •  After the path switch response message is received from

the MME, the target eNB informs success of HO to the source eNB by sending release resource message to the source eNB and triggers the release of resources.

•  On receiving the release resource message, the source eNB can release radio and C-plane related resources associated with the UE context (scheduler updates).


HO Schematic


LTE Enablers

•  OFDM (Orthogonal Frequency Division Multiplexing) for Down Link •  SC-FDMA (Single Carrier FDMA) for Up Link

–  Utilizes single carrier modulation and orthogonal frequency Multiplexing using DFT-spreading in the transmitter and frequency domain equalization in the receiver

–  A salient advantage of SC-FDMA over OFDM/OFDMA is low PAPR. •  Efficient transmitter and improved cell-edge performance •  MIMO (Multi-Input Multi-Output)

–  e.g., Open loop, Close loop, Diversity, Spatial multiplexing •  Multicarrier channel-dependent resource scheduling •  Fractional frequency reuse

–  Active interference avoidance and coordination


Downlink Access


Why OFDMA? •  3G leverages on CDMA: in the presence of multi-path

propagation codes are no longer orthogonal and interfere with each other resulting in inter-user and/or inter-symbol interference (ISI)

•  Linear minimum mean square error (LMMSE) receiver becomes complex for higher bandwidth

•  Lack of flexible bandwidth support as bandwidths supported can only be multiples of the chip rate

•  Solution: orthogonal frequency division multiplexing (OFDM) •  Notice: it was designed in 1966. •  No civil application was feasible before FFT-based architectures

and CPU power was available •  Famous communication protocols based on OFDM: ASDL, DVB-T,

Wireless LAN


OFDM – some basics

•  Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme –  First break the data into small portions –  Then use a number of parallel orthogonal sub-carriers to

transmit the data •  Conventional transmission uses a single carrier, which is

modulated with all the data to be sent

Single Carrier Company

Multi Carrier Company


OFDM – some basics

•  OFDM is a special case of Frequency Division Multiplexing (FDM)

•  For FDM –  No special relationship between the

carrier frequencies –  Guard bands have to be inserted to

avoid Adjacent Channel Interference (ACI)

•  For OFDM –  Strict relation between carriers: fk =

k·Δf where Δf = 1/TU (TU - symbol period)

–  Carriers are orthogonal to each other and can be packed tight

Tu = 1/Δf gives subcarrier orthogonality over one Tu => possible to separate subcarriers in receiver


OFDM – Signal Properties

Time domain

Frequency domain

Power Spectrum for OFDM symbol

frequency

Two characteristics are important for a Signal: • The time domain presentation: it helps recognize “how long the symbol lasts on air” • The frequency domain presentation: to understand the required spectrum in terms of bandwidth

FT

IFT


Multipath channel

],[ 00 τα

],[ 11 τα

Diffracted and Scattered Paths

Reflected Path

LOS Path

],[ kk τα


Multipath propogation and inter-symbol interference

•  The cancellation of inter‐symbol interference makes more complex the hardware design of the receivers.

•  In WCDMA for instance the RAKE receiver requires a huge amount of DSP capacity.

•  One the goals of future radio of systems is to simplify receiver design.

•  Inter‐symbol interference originating from the pulse form itself is simply avoided by starting the next pulse only after the previous one finished completely, therefore introducing a Guard Period (Tg) after the Pulse.

•  There is no inter‐symbol interference between symbols as long as the multi‐path delay spread (e.g. delay difference between first and last detectable path) is less than the guard period duration Tg.


Multipath channel – Cyclic Prefix

Time [τ]

Amplitude [α]

Example multipath profile

τ0 τ1 τ2 The prefix is made cyclic to avoid inter-carrier-interference (ICI) (maintain orthogonality)

Multipath introduces inter-symbol-interference (ISI)

TU

Prefix is added to avoid ISI TU TCP


Multipath channel (cyclic prefix) •  Tcp should cover the maximum length of

the time dispersion •  Increasing Tcp implies increased overhead

in power and bandwidth (Tcp/ TS) •  For large transmission distances there is a

trade-off between power loss and time dispersion

CP Useful symbol CP Useful symbol CP Useful symbol

TU Tcp

TS


Multipath channel (frequency diversity)

=

•  The OFDM symbol can be exposed to a frequency selective channel

•  The attenuation for each subcarrier can be viewed as “flat” •  Due to the cyclic prefix there is no need for a complex

equalizer •  Possible transmission techniques

•  Forward error correction (FEC) over the frequency band •  Adaptive coding and modulation per carrier


Frequency/subcarrier

Pilot carriers /reference signalsData carriers

Multipath channel (pilot symbols) •  The channel parameters can be estimated based on known

symbols (pilot symbols)

•  The pilot symbols should have sufficient density to provide estimates with good quality (tradeoff with efficiency)

•  Different estimation methods exist –  Averaging combined with interpolation

–  Minimum-mean square error (MMSE)

Pilot symbol

Time

Frequency


The Peak to Average Power Problem

•  A OFDM signal consists of a number of independently modulated symbols

•  The sum of independently modulated subcarriers can have large amplitude variations

•  Results in a large peak-to-average-power ratio (PAPR)

∑−

=

Δπ⋅=1N

0k

tfk2jk

c

ea)t(x

PA


The Peak to Average Power Problem •  High efficiency power amplifiers

are desirable –  For the handset, long battery life –  For the base station, reduced

operating costs

•  A large PAPR is negative for the power amplifier efficiency

•  Non-linearity results in inter-modulation

–  Degrades BER performance –  Out-of-band radiation

PA

PIN

POUT

IBO

AM/AM characteristic

OBO

Average Peak


The Peak to Average Power Problem •  Different tools to deal with large PAPR

–  Signal distortion techniques Clipping and windowing introduces distortion and out-of-band radiation, tradeoff with respect to reduced backoff

–  Coding techniques FEC codes excludes OFDM symbols with a large PAPR (decreasing the PAPR decreases code space). Tone reservation, and pre-coding are other examples of coding techniques.

–  Scrambling techniques Different scrambling sequences are applied, and the one resulting in the smallest PAPR is chosen


OFDM Synchronization •  Timing recovery

–  No problem if offset is within Δτ

•  Frequency synchronization –  A carrier synchronization error will

introduce phase rotation, amplitude reduction and ICI

–  Frequency offsets of up to 2 % of Δf is negligible

–  Even offsets of 5 – 10 % can be tolerated in many situations

τmax Δτ

CP Useful symbol Integration period, TU


Choosing the OFDM parameters •  Symbol time (TU) and subcarrier

spacing (Δf) are inverse –  TU = 1/Δf

•  Consequences of increasing the subcarrier spacing

–  Increase cyclic prefix overhead •  Consequences of decreasing the

subcarrier spacing –  Increase sensitivity to

frequency inaccuracy –  Increasing number of

subcarriers increases Tx and Rx complexity

Increasing subcarrier spacing

Decreasing subcarrier spacing

Increase sensitivity to frequency accuracy

TU

Increase CP overhead


OFDM - scoreboard •  Advantages

–  Splitting the channel into narrowband channels enables significant simplification of equalizer design

–  Effective implementation possible by applying FFT

–  Flexible bandwidths enabled through scalable number of sub-channels

–  Possible to exploit both time and frequency domain variations (time domain adaptation/coding + freq. domain adaptation/coding)

•  Challenges –  Large peak to average power ratio


Summary

Channel, h(t)

PA

CP

Frequency/subcarrier

Pilot carriers /reference signalsData carriers


OFDMA – Orthogonal Frequency Division Multiple Access

•  OFDM can be used as a multiple access scheme allowing simultaneous frequency-separated transmissions to/from multiple mobile terminals

•  The number of sub-carriers can be scaled to fit the bandwidth – Scalable OFDMA

•  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.





•  Time Division via OFDM •  Disadvantage is that every user gets the

same amount of capacity (sub carriers) and it is thus rather difficult to implement flexible (high and low) bit rate services

•  Furthermore it is nearly impossible to handle highly variable traffic efficiently without too much higher layer signaling and resulting delay and overhead-signaling.





•  The basic idea is to assign sub carriers 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 is to assign to a single user the 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 one or more resource blocks.





Difference between OFDM and OFDMA





Contiguous (localized) mapping Distributed (diversity) mapping


Subcarrier allocation techniques •  Contiguous or blockwise

mapping –  Adjacent sub-carriers

•  Frequency selective fading can erase a full block

•  For satisfactory performance it must be combined with dynamic scheduling or frequency hopping

•  Examples: –  E-UTRA

–  Mobile WiMAX – Band AMC


Subcarrier allocation techniques •  Distributed or diversity mapping

–  Carriers allocated to one user are spread across the total OFDM bandwidth

•  Permutation changes from time-slot to time-slot •  Robust against frequency selective fading


Channel dependent scheduling •  Exploits time-

frequency selective fading

•  The scheduled user is always allocated the best time-frequency block

•  Channel varies differently for different users


Synchronisation aspects

•  Impairments in time- and frequency synchronization reduces performance: ISI and ICI

•  Downlink –  Time- and frequency synchronization

•  Uplink –  Control is distributed between terminals

–  Frequency synchronization •  Impact on orthogonality between SCs belonging to different users

–  Timing synchronization •  Impact on inter-symbol interference (ISI)

–  Different received power at the base station •  Base station receiver dynamic range exceeded. Power control necessary


DFT-spread OFDMA •  Linear precoding of OFDMA symbols

•  N < NC subcarriers are allocated to one user –  An N-point Discrete Fourier Transform (DFT) is applied

–  New output symbols (Xk) are linear combinations of all N input symbols (xn)

•  Conventional OFDMA has a PAPR problem in the time domain.

•  Linear precoding with DFT moves the PAPR to the frequency domain

SC m

apping

+CP, D

/A+RF

Channel

RF+A/D

, -CP

NC-point D

FT

SC de-m

apping

NC-point ID

FT

NC NC N N N-point D

FT

N-point ID

FT

OFDMA DFT-spread

∑−

=

−⋅=

1

0

2N

n

knNj

nk exXπ


LTE Downlink: Conventinal OFDMA

•  LTE provides QPSK, 16QAM, 64QAM as DL modulation schemes

•  CP is used as guard interval, different configurations possible: •  Normal CP prefix with 5.2µs (first

symbol) / 4.7 µs (other symbols) •  Extended cyclic prefix with 16.7 µs

•  15 kHz subcarrier spacing •  Scalable bandwidth


OFDMA time-frequency multiplexing


Uplink Access


Uplink wishlist •  Orthogonal uplink transmission by different User

Equipment (UEs), to minimize intracell interference and maximize capacity;

•  Flexibility to support a wide range of data rates, and to enable data rate to be adapted to the SINR (Signal-to-Interference plus Noise Ratio).

•  Sufficiently low Peak-to-Average Power Ratio (PAPR) of the transmitted waveform, to avoid excessive cost, size and power consumption of the UE Power Amplifier (PA).

•  Ability to exploit the frequency diversity afforded by the wideband channel (up to 20 MHz), even when transmitting at low data rates;

•  Support for frequency-selective scheduling;


SC-FDMA

•  Single Carrier Frequency Division Multiple Access (SC-FDMA) is used in the Uplink in order to multiplex UEs signals


SC-FDMA

•  The Localized FDMA scheme: each UE power amplifier then sees a single FFT- precoded transmission

•  A frequency-domain equalization (FDE) operation is performed using channel estimates obtained from pilots or reference signals received for each UE.


SC-FDMA •  Localized transmission

–  Need to feedback channel state information –  Mainly for low-to-medium mobility users

•  Distributed transmission –  Mainly for high mobility users

•  Orthogonal resource subspace division –  Transmission bandwidth is divided into localized band and distributed band –  Each band is further divided into several subbands for inter-cell interference

avoidance/concentration –  A subband out of each band in a cell is operated in whispering mode; UEs using

a channel belonging to the same subband in neighboring cells can be operated in speaking mode

L-subband 3L-subband 3 L-subband 3

frequency

* Different colors denote different UEs’ channel D-subband 1 D-subband 3

D-subband 2

Localized band Distributed band


SC-FDMA Parameters Transmission BW 5 MHz 10 MHz 15 MHZ 20 MHz

Subframe duration 0.5 ms

Subcarrier spacing 15 kHz

Sampling frequency 7.68 MHz 15.36 MHz 23.04 MHz 30.72 MHz

FFT size 512 1024 1536 2048

Number of occupied subcarriers 301 601 901 1201

Number of blocks of symbols per subframe 6 Long blocks + 2 Short blocks

CP length (us/samples) (4.04/31) × 7, (5.08/39) × 1

(4.1/63) × 7, (4.62/71) × 1

(4.12/95) × 7, (4.47/103) × 1

(4.13/127) × 7, (4.39/135) ×1


PAPR •  It is important to keep PAPR small to reduce the effect

of non linearities in the power amplifiers


PAPR

•  SC-FDMA maintains the flexibility in frequency while greatly reducing the PAPR compared to plain OFDMA


OFDMA v.s. SC-FDMA •  SC-FDMA maintains the flexibility in frequency while

greatly reducing the PAPR compared to plain OFDMA


Channel Structure and

Bandwidths


Spectrum usage •  The LTE system offers flexible bandwidth support for deployments in

diverse spectrum arrangements: –  bandwidths in increments of 180 kHz starting from a minimum

bandwidth of 1.08 MHz –  scheduling and transmission interval is defined as a 1 ms

subframe.

•  Two cyclic prefix lengths: normal cyclic prefix and extended cyclic prefix are defined to support small and large cells deployments respectively.

•  Subcarrier spacing of 15 kHz balances between cyclic prefix overhead and robustness to Doppler spread (orthogonality problems).

•  The uplink supports localized transmissions with contiguous resource block allocation due to single-carrier FDMA


Spectrum flexibility •  Channel bandwidth

represents the actual spectrum occupation

•  Transmission bandwidth configuration provides the utilization of the occupied bandwidth

•  For each configuration a given number of radio bearers are supported

•  Note: lower configurations (1.4 MHz) have lower efficiency

•  The carrier center frequency: 100 kHz


Bandwidth scalability

•  Scalable bandwidth 1.4 – 20 MHz using different number of subcarriers

•  Large bandwidth provides high data rates •  Small bandwidth allows simpler spectrum refarming, e.g.,

450 MHz and 900 MHz


LTE Frame structure

•  LTE frames are 10 msec in duration. They are divided into 10 subframes, each subframe being 1 msec long. Each subframe is further divided into two slots, each of 0.5 msec duration. Slots consist of either 6 or 7 OFDM symbols, depending on whether the normal or extended CP is employed.


LTE Slot

•  The LTE Slot carries – 7 symbols with short CP – 6 symbols with long CP


DL Frame structure type 1 (FDD)


DL Frame structure type 2 (TDD)


Time-frequency usage •  Uplink and downlink

subframe transmissions occur every 1 ms;

•  Every transmission consists of two consecutive time slots

•  Time durations are expressed in terms of sample period, which is 30.72 Msample/s;

•  iFFT sizes: from 128 to 2056

•  There exists an offset for the Uplink


Time slots •  1 time slot = 7 OFDM/

SC_FDM symbols=0.5 ms;

•  Time durations are expressed in terms of sample period, which is 30.72 Msample/s;

•  cyclic prefix lengths of (160 × Ts) and (144 × Ts) – relates to the cell size

•  Integer number of samples for IFFT sizes of 128, 256, 512, 1024 and 2048.


PRB

•  The Physical Resource Block (PRB) is the minimum resource block that can be handled both in the downlink and the uplink;

•  It is a square region of the OFDMA time-frequency domain; •  It is measured as number of consecutive subcarrier spacings (15 kHz

each) X number of consecutive OFDM symbols •  E.g. Downlink = 12 x 15 kHz over 7 OFDM symbols = 1.80 MHz over

0.5 ms


Reference Signals •  OFDM symbols can be

embedded with known reference signals

•  TDD is useful to enable power saving cycles since control information can be multiplexed in a single sub-frame

•  FDM of reference signals is good since they share transmission with data and power allocation is more flexible

•  Solution: hybrid •  Two types of signals:

–  Cell specific –  UE specific


Downlink Frame

•  A group of 20 slots (10 subframes) = radio frame of duration 10 ms

•  Primary synchronization signal (PSS) and secondary synchronization signal (SSS) are carried in the last and second last OFDM symbols respectively in slot number 0 and slot number 10.

•  The PSS and SSS are carried in the frequency domain using 62 subcarriers out of a total of 72 subcarriers (1.08 MHz).

•  DC subcarrier is not used for any transmission.


SC-FDMA Frame Structure

•  Frame duration: 10 msec •  One frame consists of 20 UTPs (Uplink Traffic Packet, UTP and sub-frame

are the same in this context) –  UTP: 0.5 msec –  UTP: 6 regular symbol blocks + 2 half-length symbol blocks

UTP #0 UTP #1 UTP #2 UTP #19

Tframe=10msec

TUTP=0.5msec

CP LB #1 SB

#1CP

CP LB #2 C

P LB #3 CP LB #4 C

P LB #5 SB #2

CP

CP LB #6


Pilot Channel •  Pilot

–  For uplink channel quality measurement (channel sounding)

–  For channel estimation and coherent detection at receiver side

•  TDM pilot structure –  Easy to keep low PAPR

characteristic –  Pilot symbols are carried

on two short blocks –  Support both localized and

distributed channels •  Alternating transmission

for fitting into short block structure

N subcarriers for regular blocks (long blocks)

N/2 subcarriers: even-numbered pilot subcarriers are transmitted via SB #1

N/2 subcarriers: odd-numbered pilot subcarriers are transmitted via SB #2


PSS and SSS frame and slot structure

•  OFDM systems are very sensitive when it comes to carrier frequency offset (CFO) and errors in sample timing.

•  In order to transfer data correctly the UE must perform a synchronization with the serving cell. With the help of the

•  Primary Synchronization Signal (PSS) the UE can estimate the CFO and the OFDM symbol timing. Furthermore the beginning of an LTE radio frame (BOF) must be found to allow any communication.

•  The Second Synchronization Signal can be used to identify the cell-ID which is needed to register the UE with the eNB what is required to receive incoming phone calls.

•  The full synchronization and cell identification procedure needs to be complete as fast as possible.


Physical Channel Structure


Physical Channel Procedure – 1/2


Physical Channel Procedure – 2/2


Cell Search

•  Cell Search: UE acquires time and frequency synchronization with a cell and detects the cell ID –  Based on BCH signal and hierarchicxal SCH signals

•  P-SCH and S-SCH are transmitted twice per radio frame for FDD

•  Cell search procedure: –  5 ms timing identified using P-SCH –  Radio timing and group ID found from S-SCH –  Full cell ID found from DL RS –  Decode BCH


UE Measurements

•  In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbour cells.

•  In LTE, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality)


LTE Bit rate calculation •  1 Radio Frame = 10 Sub-frame •  1 Sub-frame = 2 Time-slots •  1 Time-slot = 0.5 ms (i.e., 1 Sub-frame = 1 ms) •  1 Time-slot = 7 Modulation Symbols (when normal CP length is used) •  1 Modulation Symbols = 6 bits; if 64 QAM is used as modulation scheme •  Radio resource is managed in LTE as resource grid.... •  1 Resource Block (RB) = 12 Sub-carriers •  Assume 20 MHz channel bandwidth (100 RBs), normal CP •  Therefore, number of bits in a sub-frame •  = 100RBs x 12 sub-carriers x 2 slots x 7 modulation symbols x 6 bits=

100800 bits •  Hence, data rate = 100800 bits / 1 ms = 100.8 Mbps\ •  * If 4x4 MIMO is used, then the peak data rate would be 4 x 100.8 Mbps

= 403 Mbps. •  * If 3/4 coding is used to protect the data, we still get 0.75 x 403 Mbps

= 302 Mbps as data rate.


Intercell interference

management for downlink (Virtual MIMO)


Virtual MIMO

•  Downlink inter-cell interference mitigation – Coordinated symbol repetition – Transmission and Detection – Resource partitioning and allocation – Simulation results


Coordinated symbol repetition

–  Inter-cell interference mitigation based on coordinated symbol repetition for cell-edge UEs and control channels

–  The resources for symbol repetition of one cell/sector are set to exactly collide with those of other cell/sectors.

•  Identical repetition-resource allocation among different cell/sectors

BS BSUE

R(f1,t1)

R(f2,t2)

S1 R(f1,t1)

R(f2,t2)

S2


Coordinated symbol repetition •  The transmission and reception is equivalent to a MIMO

system (thus, called virtual MIMO) •  Symbol detection using ZF, MMSE, IC etc

Serving Cell Interfering Cell

f1, f2

Cell-edge UE

S1 S2

“2 X 2 Virtual MIMO”


Repetition-resource allocation pattern

Cluster type - Localized data subchannels

Comb type - Control channels

- Distributed data subchannels

Block-random type

Repetition factor G


Joint detection on repeated symbols

•  Received signal •  Repetition factor G •  Number of cell/sectors J (G ≥ J)

1 11 11 12 12 1 1 1 1

2 21 21 22 22 2 2 2 2

1 1 2 2

...

...: : : . : : :

...

J J

J J

G G G G G GJ GJ J G

R h c h c h c s nR h c h c h c s n

R h c h c h c s n

=

⎛ ⎞ ⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟= +⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠

R Hs +n

scrambling/orthogonal codes

data symbols from J cell/sectors

received signals


Joint detection on repeated symbols

•  Combining weights

11MMSE: MMSE JSNR

−+ +⎡ ⎤+ Ι⎢ ⎥⎣ ⎦

W = H H H

1ZF: ZF

−+ +⎡ ⎤⎣ ⎦W = H H H

S =WR


Code sequences for detection performance improvement

•  To enhance symbol detection, double-layered sequences are multiplied to repetition symbols

•  Cell-specific scrambling sequences as signature randomizers e.g. M-ary random phasors

»  Easy cell planning »  Improve diversity among repetition symbols

•  Sector-specific orthogonal codes »  Minimize correlation between the desired symbol

and interfering symbols from neighboring sectors within the same cell.


Resource partitioning and allocation

•  Logical resource partitioning –  Two large resource blocks

»  Type-A resources for traffic channels »  Type-B resources for control channels

–  Type-A resource block »  Subblock A1 for interference-free UEs »  Subblock A2 for interference-susceptible UEs


– Every cell adopts the same resource allocation scheme.

– The sizes of subblocks A1 and A2 can be adjusted dynamically by taking into account the interference-susceptible traffic.

Resource partitioning and allocation


Resource allocation (geographical)

Traffic channels Control channels


LTE Release 8 Key Features: Summary •  High spectral efficiency

–  OFDMA in Downlink: robust against multipath interference –  DFT-Spread-OFDM (“Single-Carrier FDMA”) in Uplink: low Peak to Average

Power Ratio –  Multi-antenna application

•  Very low latency for setup and handover •  Support of variable bandwidth: 1.4, 3, 5, 10, 15 and 20 MHz •  Simple protocol architecture: Shared channel based, PS mode only

with VoIP capability •  Simple Architecture: eNodeB as the only E-UTRAN node

–  Smaller number of RAN interfaces •  eNodeB ↔ MME/SAE-Gateway (S1) •  eNodeB ↔ eNodeB (X2)

•  FDD and TDD within a single radio access technology •  Inter-working with other systems, e.g. cdma2000 •  Support of Self-Optimizing Network (SON):

Self configuration, Basic self-optimization •  Home eNode B (HeNB): closed access mode only •  Reduced deployment/operational cost (CAPEX and OPEX)

eNB

MME / S-GW MME / S-GW

eNB

eNB

S1 S1

S1 S1

X2

X2X2

E-UTRAN


From LTE to LTE-Advanced –  REL-9: mainly addition of LCS (Location service) & MBMS (Multimedia

Broadcast Multicast Service) & enhancement of others (e.g. SON, HeNB) –  Main motivation to introduce LTE-A in REL-10: –  IMT-Advanced standardization process in ITU-R for 4G –  Additional IMT spectrum band identified in WRC07 –  LTE-Advanced (REL-10/11 ...) is an evolution of LTE (REL-8/9),

i.e. LTE-Advanced is backwards compatible with LTE –  è Smooth and flexible system migration from Rel-8 LTE to LTE-Advanced

LTE Rel-8 cell

LTE Rel-8 terminal LTE-Advanced terminal

LTE-Advanced cell

LTE Rel-8 terminal LTE-Advanced terminal

An LTE-Advanced terminal can work in an LTE Rel-8 cell

An LTE Rel-8 terminal can work in an LTE-Advanced cell

LTE-Advanced contains all features of LTE Rel-8

& 9 and additional features for further

evoluton

LTE target:: peak data rates:

DL: 100Mbps UL: 50Mbps TS 25.913

LTE-A target:: peak data rates:

DL: 1Gbps UL: 500Mbps

TS 36.913


Main Features in LTE-A Release 10 •  Support of wider bandwidth (Carrier Aggregation)

•  Use of multiple component carriers (CC) to extend bandwidth up to 100 MHz

•  Common L1 parameters between component carrier and LTE Rel-8 carrier

è  Improvement of peak data rate, backward compatibility with LTE Rel-8

•  Advanced MIMO techniques •  Extension to up to 8-layer transmission in downlink

(REL-8: 4-layer in downlink) •  Introduction of single-user MIMO with up to 4-layer

transmission in uplink •  Enhancements of multi-user MIMO è  Improvement of peak data rate and capacity

•  Heterogeneous network and eICIC (enhanced Inter-Cell Interference Coordination)

•  Interference coordination for overlay deployment of cells with different Tx power

è  Improvement of cell-edge throughput and coverage •  Relay

•  Relay Node supports radio backhaul and creates a separate cell and appears as Rel. 8 LTE eNB to Rel. 8 LTE UEs

è  Improvement of coverage and flexibility of service area extension

100 MHz

f CC

Relay Node Donor eNB

UE

UE

eNB

macro eNB

micro/pico eNB


LTE/LTE-A REL-11 features •  Coordinated Multi-Point Operation (DL/UL) (CoMP):

–  cooperative MIMO of multiple cells to improve spectral efficiency, esp. at cell edge •  Enhanced physical downlink control channel (E-PDCCH): new Ctrl channel with higher capacity •  Further enhancements for

–  Minimization of Drive Tests (MDT): QoS measurements (throughput, data volume) –  Self Optimizing Networks (SON): inter RAT Mobility Robustness Optimisation (MRO) –  Carrier Aggregation (CA): multiple timing advance in UL, UL/DL config. in inter-band CA

TDD –  Machine-Type Communications (MTC): EAB mechanism against overload due to MTC –  Multimedia Broadcast Multicast Service (MBMS): Service continuity in mobility case –  Network Energy Saving for E-UTRAN: savings for interworking with UTRAN/GERAN –  Inter-cell interference coordination (ICIC): assistance to UE for CRS interference reduction –  Location Services (LCS): Network-based positioning (U-TDOA) –  Home eNode B (HeNB): mobility enhancements, X2 Gateway

•  RAN Enhancements for Diverse Data Applications (eDDA): –  Power Preference Indicator (PPI): informs NW of mobile’s power saving preference

•  Interference avoidance for in-device coexistence (IDC): –  FDM/DRX ideas to improved coexistence of LTE, WiFi, Bluetooth transceivers, GNSS

receivers in UE •  High Power (+33dBm) vehicular UE for 700MHz band for America for Public Safety •  Additional special subframe configuration for LTE TDD: for TD-SCDMA interworking •  In addition: larger number of spectrum related work items: new bands/band

combinations

Optical fiber

Coordination


High Level Directions: Rel-12 and beyond


LTE - Expectations

Source: Sharp Corporation 3GPP workshop 2012


High Level Direction: Denser Network and Bandwidth Extension

Source:


High Level Direction: Spectrum and Transmission Efficiency


Future Cellular Deployments

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