mobile network evolution gsm and · pdf filemobile network evolution gsm and umts u gsm ......

Integrated Communication Systems Group

Ilmenau University of Technology

Mobile Network Evolution

GSM and UMTSu GSM

u Cell layoutu Radio Accessu Architectureu Call setupu Mobility managementu Security

u GPRSu Architectureu Protocolsu QoS

u EDGEu Architectureu Modulation & coding

u UMTSu Architectureu Packet handlingu Resource management and

QoSu LTE

u Features and requirementsu Architectureu Packet handling and

resource managementu References

2


Advanced Mobile Communication Networks

possible radio coverage of the cell

idealized shape of the cellcell

Segmentation of the area into cells

GSM: cellular network

u use of several carrier frequenciesu different frequencies in neighboring cellsu cell radius varies from some 100 m up to 35 km depending on

user density, geography, transceiver power etc.u hexagonal shape of cells is idealized (cells overlap, shapes depend

on geography)u if a mobile user changes cells

-> handover of the connection to the neighbor cell

3



Cellular systems: Frequency planning IFrequency reuse only beyond a certain distance between base stationsTypical (hexagon) model:

reuse-3 cluster: reuse-7 cluster:

Other regular pattern: reuse-19

è Frequency reuse pattern determines the experienced SIR

Fixed frequency assignment:u certain frequencies are assigned to a certain cellu problem: different traffic load in different cells

Frequency Hopping:u Improves quality for slow moving or stationary users (frequency diversity)u Reduces impact of intercell interference by statistical averaging

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7f2

f1f3

f2

f1f3

f2

f1f3

4



GSM: Air InterfaceFDMA (Frequency Division Multiple Access) / FDD (Frequency Division Duplex)

123124. . .

890 MHz 915 MHz

123124. . .

935 MHz 960 MHz

200 kHz

Uplink Downlink

frequency

TDMA (Time Division Multiple Access)

time

Downlink

87654321

4,615 ms = 1250 bit

Uplink

87654321

5



Framing Modulation(GMSK)

GSM: Voice Coding

Voice coding Channelcoding Framing Modulation

(GMSK)

114 bit/slot114 + 42 bit

Guard (8.25 bits): avoid overlap with other time slots (different time offset of neighboring slot)Training sequence: select the best radio path in the receiver and train equalizerTail: needed to enhance receiver performanceFlag S: indication for user data or control data

1 2 3 4 5 6 7 8GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

6



GSM Architecture

GSMRAN

Base stationBase stationcontroller

Base station

Base station

MSC

ISDN

GSM Core (Circuit switched)

HLRAuCEIR

GMSC

TransmissionATM based

GSM

7



GSM system: Network elementsGSM is a PLMN (Public Land Mobile Network)u several providers setup mobile networks following the GSM standard

within each country

GSM Elements:u MS (mobile station)u Radio Access Network (Base station subsystem - BSS): covers

all radio aspectsu BTS (base transeiver station)u BSC (base station controller)

u Core Network: call forwarding, handover, switchingu MSC (mobile services switching center)u LR (location register): HLR and VLRu OMC (operation and maintenance centre)u AuC (authentication centre)u EIR (equipment identity register)

8



Mobile Terminated Call (MTC)

PSTNcallingstation GMSC

HLR VLR

BSSBSSBSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

1316

10 10

11 11 11

14 15

17

1: calling a GSM subscriber2: forwarding call to GMSC3: signal call setup to HLR4, 5: request MSRN from VLR6: forward responsible

MSC to GMSC7: forward call to

current MSC8, 9: get current status of MS10, 11: paging of MS12, 13: MS answers14, 15: security checks16, 17: set up connection

9



RA

RA

RA RA

RA

RA RA

RA

RA

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

Location Management / Mobility ManagementThe issue: Compromise between

u minimizing the area where to search for a mobile

u minimizing the number of location updates

Solution 1:Large paging area

Solution 2:Small paging area

PagingSignalling Cost

Paging Area UpdateSignalling Cost

TOTALSignalling Cost

∑

∑+

=

10



HandoverThe problem:

Change the cell whilecommunicating

Reasons for handover:u Quality of radio link

deterioratesu Communication in other cell

requires less radio resourcesu Supported radius is

exceeded (e.g. Timing advance in GSM)

u Overload in current cellu Maintenance

Link

qua

lity

Link to cell 1 Link to cell 2 time

cell 1

cell 2

Handover margin (avoid ping-pong effect)

cell 1 cell 2

11



Handover procedure (change of BSC)

HO access

BTSold BSCnew

measurementresult

BSCold

Link establishment

MSCMSmeasurementreport

HO decisionHO required

BTSnew

HO request

resource allocationch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear command

clear complete clear complete

„Make-before-break“ strategy

make

break

12



GSM - authentication

A3

RANDKi

128 bit 128 bit

RAND

SRES* =? SRES

A3

RAND Ki

128 bit 128 bit

SRES 32 bit

SRES

Authentication Request (RAND)

Authentication Response (SRES 32 bit)

AuC

MSC

SIM

Ki: individual subscriber authentication key SRES: signed response

SRES* 32 bit

Challenge-Response:• Authentication center provides RAND to Mobile• AuC generates SRES using Ki of subscriber and

RAND via A3• Mobile (SIM) generates SRES using Ki and RAND• Mobile transmits SRES to network (MSC)• network (MSC) compares received SRES with one

generated by AuC

13



GSM - key generation and encryption

A8

RANDKi

128 bit 128 bit

Kc64 bit

A8

RAND Ki

128 bit 128 bit

SRES

RAND

encrypteddata

MS with SIM

AuC

BTS

SIM

A5

Kc64 bit

A5MS

data data

cipherkey

Ciphering:• Data sent on air interface ciphered for security• A8 algorithm used to generate cipher key• A5 algorithm used to cipher/decipher data• Ciphering Key is never transmitted on air

14



2G to 3G Evolution: GSM - GPRS - UMTS

GPRS Core (PacketSwitched)

SGSN

GGSN

Inter-net

GSMRAN


Base station

Base station

MSC

ISDN


HLRAuCEIR

GMSC

TransmissionATM based

GSM+GPRS

15



Circuit vs. Packet Switched Communication

Connection (e.g. voice, CS data) => principle for GSM & UTRAN design• clearly defined start and end times• no burstiness=> dedicated channels

minutesconnection

setupconnection

release

Packet session => supported by GPRS core, IMS, SAE, HSPA, LTE• packet arrival times are typically unknown to the system• traffic is highly bursty=> shared channels & packet scheduling

hours

seconds

16



GPRS (General Packet Radio Service)u Introducing packet switching in the networku Using shared radio channels for packet transmission over the air:

u multiplexing multiple MS on one time slotu flexible (also multiple) allocation of timeslots to MS

(scheduling by PCU Packet Control Unit in BSC or BTS) u using free slots only if data packets are ready to send

(e.g., 115 kbit/s using 8 slots temporarily)u standardization 1998, introduction 2001u advantage: first step towards UMTS, flexible data services

GPRS network elementsu GSN (GPRS Support Nodes): GGSN and SGSNu GGSN (Gateway GSN)

u interworking unit between GPRS and PDN (Packet Data Network)u SGSN (Serving GSN)

u supports the MS (location, billing, security)u HLR (GPRS Register – GR)

u maintains location and security information

17



carrierTS0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Multiplexing

Multislot capability

GPRS: Multiplexing and multislot allocation

18



GPRS servicesEnd-to-end packet switched traffic (peak channel rates)u 28 kbps (full use of 3 time slots, CS-1: FEC) u 171.2 kbps (full use of 8 time slots, CS-4: no FEC)Average aggregate throughput of a cell (Source: H. Menkes, WirelessWeb, Aug. 2002)u 95 kbps (for both up and downlink)u Assumptions: 4/12 reuse, realistic RF conditions, random trafficu Worse figures for individual TCP trafficAdaptive Coding Schemes (adaptive Forward Error Control – FEC)u CS 1: 9.05 Kbps/slotu CS 2: 13.4 Kbps/slotu CS 3: 15.6 Kbps/slotu CS 4: 21.4 Kbps/slot (no FEC)Problems and limitsu IP-based network => high latency, no guaranteesu Limited data rate: 28 kbps (3 slot/CS-1) - 64.2 kbps (3 slot/CS-4)u Latency/flow control problems with TCP

19



EDGE (Enhanced Data Rates for GSM Evolution)

u EDGE can carry data speeds up to 236.8 kbit/s for 4 timeslots

u theoretical maximum is 473.6 kbit/s for 8 timeslots

u Adaptation of modulation dependingon quality of radio path

u GMSK (GSM standard – 1 bit per symbol)u 8-PSK (3 bits per symbol)

u Adaptation of coding scheme dependingon quality of radio path (9 coding schemes)

u Gain: data rate (gross) up to 69,2kbps (compare to22.8kbps for GSM)

u Edge is a complex extension of GSM!

NodeB

UE 1

UE 2

Near-far problem

20



EDGE – Adaptive Modulation and Coding SchemesScheme Modulation Maximum

rate [kb/s]Code Rate Family

MCS-9 59.2 1.0 AMCS-8 54.4 0.92 AMCS-7 44.8 0.76 BMCS-6 29.6 / 27.2 0.49 AMCS-5

8PSK

22.4 0.37 BMCS-4 17.6 1.0 CMCS-3 14.8 / 13.6 0.80 AMCS-2 11.2 0.66 BMCS-1

GMSK

8.8 0.53 C

21



Payload for GPRS and EDGE

22



Differences of GSM/GPRS Compared to WLAN SystemsSpectrum management and utilization

u coverage and interference management due to cell planningu capacity and load management due to admission control and QoS supportu flexible cell size simplifies cost-efficient nationwide coverage

Mobility managementu fast, lossless HO due to make-before-break

General control structures and control philosophyu high reliability and QoS guaranties due to centralized/infrastructure-

based management and control of all resourcesEnergy

u high energy cost on network side, low cost on mobile due to passiv cellcamping instead of active association, paging mode and sleep cycles

Customer relationsu monthly/bi-yearly contracts, pay per serviceu security due to preshared credentials

Implementationu simple implementation of TDMA, e.g. with GNUradio and SDR

23



2G to 3G Evolution: GSM - GPRS – UMTS R99


SGSN

GGSN

Inter-net

GSMRAN


Base station

Base station

UTRAN

Radio networkcontroller

Base station Base station

Base station

MSC

ISDN


HLRAuCEIR

GMSC

ATM based

GSM+GPRS+UMTS R99

24



2G to 3G Evolution: GSM - GPRS - UMTS R5 - IMS


SGSN

GGSN

Inter-net

GSMRAN


Base station

Base station

UTRAN

Radio networkcontroller

Base station Base station

Base station

IP based

3G Core

GERANGERAN + UMTS R5 + IMS

25



UMTS – Macro Diversity in CDMA

BS 1BS 2

UE

CDMA is a single frequency system

Mobile user may be servedu by multiple base stations

=> soft handover=> combining in RNC

u by multiple cells from the same base station=> softer handover=> combining in base station

Due to increased redundancy, the combining of signals improves quality (BER) or allows to reduce required TX power to achieve given BER requirements.

Note that this is an extension of the make-before-break strategy where radio links are continuously added and deleted.

26



UMTS – Cell BreathingCDMA systems: cell size depends on the actual loading

u Additional traffic will cause higher interferenceu If the interference becomes too strong, users at the cell edge can no

longer meet SINR requirements to continue communication

27



UMTS – Mobility Management in Packet Domain

L1

RLC

PDCP

MAC

E.g., IP,PPP

Application

L1

RLC

PDCP

MAC

ATM

UDP/IP

GTP-U

AAL5

Relay

L1

UDP/IP

L2

GTP-U

E.g., IP,PPP

3G-SGSNUTRANMSIu-PSUu Gn Gi

3G-GGSN

ATM

UDP/IP

GTP-U

AAL5

L1

UDP/IP

GTP-U

L2

Relay

28



End-to-End Resource Management in UMTS (contr. plane)A sophisticated QoS architecture

Transl. Transl.

Adm.Contr

.Adm.Contr

.Adm.Contr

.Adm.Contr

.Adm.Contr

.

RAB Manager

UMTS BS Manager

UMTS BS Manager

UMTS BS Manager

Subscr. Control

Adm./Cap. Control

MT Gateway CN EDGE UTRAN

Ext. Service Control

Local Service Control

Iu BS Manager

Radio BS Manager

Iu NS Manager

UTRA ph. BS M

Radio BS Manager

UTRA ph. BS M

Local BS Manager

Adm./Cap. Control

Adm./Cap. Control

Adm./Cap. Control

Iu BS Manager

Iu NS Manager

CN BS Manager

Ext. BS Manager

CN BS Manager

service primitive interface protocol interface

BB NS Manager

BB NS Manager

TE Ext. Netw.

For details see 3GPP TS 23.207

29



End-to-End Resource Management in UMTS (user plane)

ResourceManager

Mapper

Classif.

Cond.

ResourceManager

ResourceManager

Mapper

ResourceManager

Mapper

ResourceManager

ResourceManager

Cond.

Classif.

Cond.

MT GatewayCN EDGEUTRAN

BB network serviceIu network serviceUTRA phys. BS

data flow with indication of direction

TE Ext.Netw.

Local BS External BS

30



Evolution from GSM to UMTS and LTE GSM: voice-dominated, dedicated channels, heavy statesGPRS: add support for packet data on shared channels; add IP-based core

networkEDGE: increased packet data capacity of GSM systemsUMTS: separate voice and packet data support; focus on dedicated channels and

heavy states, complicated RAN architecture and protocols due to macro diversity (soft handover) and QoS requirements

HSPA: improved support for packet data; emphasis on shared channels and fast radio resource management

IMS: support for IP-based services, e.g. voice (VoIP)LTE: strong packet data support (latency, throughput, control overhead), limited

state; simplified protocols; PS only, i.e. no CS core network

Evolution of technologyu from circuit switching to packet switchingu from slow, explicit setup and release of resources to fast channel-condition-

and demand-specific resource management

31



Evolution towards LTE – Architecture

• LTE radio system is a packet-only network - there is no support for circuit-switched services (no MSC)

• LTE started on a clean state - everything was up for discussion including the system architecture and the split of functionality between Radio Access Network (RAN) and Core Network (CN)

• 3GPP (3rd Generation Partnership Program) study items • „3G Long-term Evolution” (LTE) for new Radio Access and • “System Architecture Evolution” (SAE) for Evolved Network

32



LTE: Evolved Packet System (EPS) Architecture

eNB

eNB

eNB

MME/S-GW MME/S-GW

X2

EPCE-UTRAN

S1

S1

S1S1

S1S1

X2

X2

EPC = Evolved Packet Core

2 instead of 4 user plane entities

Key changesueNB: merging of base

station and RNC functionality

uS-GW: merger of SGSN and GGSN functionality

33



LTE: Requirements and Performance Targets

34



LTE Key Features (Release 8)

u Support for both FDD and TDD

u Adaptive modulation and codingu DL modulations: QPSK, 16QAM, and 64QAMu UL modulations: QPSK and 16QAM

u Hybrid ARQ in addition to ARQ

u Multi-antenna solutionsu (2 or 4) x (2 or 4) downlink and uplink supportedu Multi-layer transmission with up to four streamsu Multi-user MIMO also supported

u Implicit support for interference coordination

35



Multi-antenna Solutions

IEEE Communications Magazine • April 200948

ple-input multiple-output [MIMO], as well asmulti-user MIMO) with up to four antennas, andbeamforming. Which of the schemes (or whichcombination of the schemes) to use depends onthe scenario (Fig. 5). In the uplink, both open-and closed-loop transmit-antenna selection aresupported as optional features.

LTE transmit diversity is based on so-calledspace-frequency block coding (SFBC), comple-mented with frequency-switched transmit diversi-ty (FSTD) in the case of four transmit antennas[1]. Transmit diversity is primarily intended forcommon downlink channels to provide addition-al diversity for transmissions for which channel-dependent scheduling is not possible. However,transmit diversity also can be applied to user-data transmission, for example, to voice-over-IP(VoIP), where the relatively low user-data ratesmay not justify the additional overhead associat-ed with channel-dependent scheduling.

In case of spatial multiplexing, multiple anten-nas at both the transmitter (base station) andthe receiver (terminal) side are used to providesimultaneous transmission of multiple, paralleldata streams, also known as layers, over a singleradio link, thereby significantly increasing thepeak data rates that can be provided over theradio link. As an example, with four base-stationtransmit antennas and a corresponding set of (atleast) four receive antennas at the terminal side,up to four data streams can be transmitted inparallel over the same radio link, effectivelyincreasing the data rate by a factor of four.

LTE multi-stream transmission is pre-coderbased. A number of transmission layers aremapped to up to four antennas by means of aprecoder matrix of size NA×NL, where the num-ber of layers NL, also known as the transmissionrank, is less than or equal to the number ofantennas NA. The transmission rank, as well asthe exact precoder matrix, can be selected by thenetwork, based on channel-status measurementsperformed and reported by the terminal, alsoknown as closed-loop spatial multiplexing.

In the case of spatial multiplexing, by select-ing rank-1 transmission, the precoder matrix,which then becomes an NA×1 precoder vector,performs a (single-layer) beamforming function.More specifically, this type of beamforming canbe referred to as codebook-based beamforming

as the beamforming can be done only accordingto a limited set of pre-defined beamforming(precoder) vectors.

In addition to the codebook-based beam-forming as a special case of the LTE spatial mul-tiplexing, LTE also supports more generalnon-codebook-based beamforming. In contrast tocodebook-based beamforming, in the case ofnon-codebook-based beamforming, the terminalmust make an estimate of the overall beam-formed channel. To enable this, LTE providesthe possibility for the transmission of user equip-ment (UE)-specific reference symbols, transmit-ted using the same beamforming as the userdata, and enabled for the terminal to estimatethe overall beamformed channel.

POWER CONTROL ANDINTER-CELL INTERFERENCE COORDINATION

LTE provides (intra-cell) orthogonality betweenusers in both uplink and downlink, that is, atleast in the ideal case, no interference betweentransmissions within the same cell but only inter-ference between cells. Hence, LTE performancein terms of spectrum efficiency and availabledata rates is, relatively speaking, more limited byinterference from other cells (inter-cell interfer-ence) compared to WCDMA/HSPA, especiallyfor users at the cell edge. Therefore, the meansto reduce or control the inter-cell interferencepotentially can provide substantial benefits toLTE performance, especially in terms of the ser-vice (data rates, etc.) that can be provided tousers at the cell edge.

Uplink power control is one of the mecha-nisms in LTE used for this purpose. It is used tocontrol not only the received signal strength inthe intended cell, but also to control the amountof interference in neighboring cells. LTE uplink-power control supports fractional path-loss com-pensation, implying that users close to the cellborder use relatively less transmit power, andthus generate relatively less interference toneighbor cells. However, LTE provides moreadvanced interference-handling schemes as well.

Inter-cell interference coordination (ICIC) isin essence a scheduling strategy used to limit theinter-cell interference. A simple method toimprove cell-edge data rates is to restrict theusage of parts of the bandwidth statically, forexample, through a reuse larger than one. Suchschemes improve the signal-to-interference ratiosof the used frequencies. However, the loss due toreduced bandwidth availability is typically largerthan the corresponding gain due to higher signal-to-interference ratio, leading to an overall loss ofefficiency. Therefore, the LTE standard providestools for dynamic inter-cell-interference coordina-tion of the scheduling in neighbor cells such thatcell-edge users in different cells preferably arescheduled on complementary parts of the spec-trum when required. Note that a major differencefrom static reuse schemes is that LTE still allowsfor the total available spectrum to be used in allcells. Bandwidth restrictions are applied onlywhen motivated by traffic and radio conditions.

Interference coordination can be applied toboth uplink and downlink, although with somefundamental differences between the two links. In

!!

Figure 5. Multiple-antenna techniques in LTE.

Diversity for improvedsystem performance

Beam-forming for improved coverage(less cells to cover a given area)

Spatial-division multiple access(”MU-MIMO”) for improved capacity

(more users per cell)

Multi-layer transmission(”SU-MIMO”) for higher data rates

in a given bandwidth

PARKVALL LAYOUT 3/25/09 2:17 PM Page 48

36



Scheduling and Resource Allocation

u Fast schedulingu Scheduled, shared channel on both uplink and downlink

=> all transmissions in UL and DL must be explicitly scheduled

u Support for "semi-persistent" (periodical) allocation of resources,e.g. for VoIP

37



Opportunistic SchedulingPrefer mobiles with good channel quality=> maximizes system throughput

Ensure fairness for users with continuously poor channel quality

NodeB

Sample Flow

Flow #1

Flow #3

UE 3

UE 2

UE 1

Flow #2

38



Interference Coordination

39



Self-Organization in LTE

Goal: minimize OPEX by automation of planning, optimization and repair

Use Cases for Self-organized Network Managementu Physical cell-ID automatic configuration (PCI)u Automatic Neighbor Relation (ANR)u Coverage and capacity optimization (CCO)u Inter-cell interference coordination (ICIC)u Random Access Channel (RACH) optimizationu Mobility load balancing optimization (MLB)u Mobility robust optimization (MRO)u Energy saving (power on/off)

40



References

LTE/SAEu A. Toskala et al, “UTRAN Long-Term Evolution,” Chapter 16 in Holma/ Toskala: WCDMA for UMTS, Wiley 2007u E. Dahlman et al, “3G Evolution, HSPA and LTE for Mobile Broadband,” Elsevier Journal, 2007u Special Issue on LTE/ WIMAX, Nachrichtentechnische Zeitung, pp. 12–24, 1/2007u 3rd Generation Partnership Project Long Term Evolution (LTE), official website:

http://www.3gpp.org/Highlights/LTE/LTE.htmu Technical Paper, “UTRA-UTRAN Long Term Evolution (LTE) and 3GPP System Architecture Evolution (SAE)”,

last update October 2006, available at: ftp://ftp.3gpp.org/Inbox/2008_web_files/LTA_Paper.pdf

Standardsu TS 36.xxx series, RAN Aspectsu TS 36.300, “E-UTRAN; Overall description; Stage 2”u TR 25.912, “Feasibility study for evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial

Radio Access Network (UTRAN)”u TR 25.814, “Physical layer aspect for evolved UTRA”u TR 23.882, “3GPP System Architecture Evolution: Report on Technical Options and Conclusions”

Self-organizing networks and LTEu Self-organizing networks and LTE, http://www.lightreading.com/document.asp?doc_id=158441u NGMN Recommendation on SON and O&M Requirements, Dec. 5, 2008, NGMN,

http://www.ngmn.org/uploads/media/NGMN_Recommendation_on_SON_and_O_M_Requirements.pdf

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