ECE Department Florida Institute of Technology ECE 5221 Personal Communication Systems Introduction...

ECE DepartmentFlorida Institute of Technology

ECE 5221 Personal Communication Systems

Introduction to GSM

ECE DepartmentFlorida Institute of Technology Page 2

Course Outline

Part 1: Introduction

o Historical overview

o Elements of network architecture

o Elements of air interface

Part 2: Signal processing and network features

o Voice processing

o GSM Network features

Part 3: Network design

o Coverage planning

o Capacity planning

o Migration towards 3G and beyond

The GSM logo used on numerous handsets and by carries who wish to

identify a GSM product


History

Driving Factors:

• Incompatibility of the European analog cellular systems• Reaching of capacity limits• Costs of the equipment

1982, Conference of European Post and Telecommunications formed Group Speciale Mobile (GSM)

1987, 15 operators from 13 countries signed Memorandum of Understanding (MoU)

1991, Finland’s operator Radiolinia launched first GSM network in July 1991

1992, Massive deployment of GSM started

By 2000 GSM became the most popular 2G technology worldwide

GSM standard still evolving and enriched with new features and services

GSM = Global System for Mobile communications

(GSM: originally from Groupe Spécial Mobile)


Deployment worldwide

930 networks in 222 countries and regions

More than 3 billion subscribers worldwide

More than 80% worldwide market share

Worldwide map of GSM coverage (source www.gsmworld.com)


GSM in the USA

1994, US FCC auctioned large blocks of spectrum in 1900MHz

GSM started deployment in PCS band

1995, American Personal Communications launched first GSM network

In 2002, 850 band opened for GSM

Currently there are ~ 95M GSM subscribers

Largest GSM operators

ATT

T-Mobile

ATT coverage map

T-Mobile coverage map


GSM Standards

Divided into 12 series

Standardization efforts coordinated by ETSI

www.etsi.org

Specifications available online – free of charge

Standardization and public availability of specification - one of fundamental factors of GSM success

Series Specifications area

01 General

02 Service aspects

03 Network aspects

04 MS-BS interface and protocol

05 Physical layer and radio path

06 Speech coding specification

07 Terminal adapter for MS

08 BS-MSC interface

09 Network internetworking

10 Service internetworking

11 Equipment and type approval specification

12 Operation and maintenance

GSM Standard


GSM Network Layout

GSM system layout is standardized

o Standardization involves:

Elements of the network

Communication Interfaces

o Standard layout allows for the use of equipment from different suppliers

MSCArea

H LR

MSCArea

VLR

MSCTRAUBSC

BTS

BTS

BSS

MSC Area

BSS

BSSBTS

PSTN

PLMN - Public Land M obile Netw ork

Gatew ayM SC

NSS


GSM Components and Interfaces

Network has many functional components

Components are integrated through a network protocol – MAP

Standardized interfaces

Um (air interface)

A – GERAN interface

A-Bis (somewhat standardized)

BTS

BSC MSC

VLR

EIR

VLR

Gatew ayMSCM S

H LR

E

FF

B B

C

D D

G

A -

In

terf

ace

Ab

is -

In

terf

ace

Air

- I

nte

rfac

e


Mobile Station (MS)

Two functional parts

o HW and SW specific for GSM radio interface

o Subscriber Identity Module (SIM)

SIM – detaches user identity from the mobile

o Stores user information

o Without SIM – only emergency calls

Functional diagram of GSM mobile

SIM card

Most popular GSM phone Nokia 1100 – 200M+ sold

Keyboard

Control

Display

Transmit AudioSignal

Processing

Receive AudioSignal

Processing

ChannelDecoding

DeinterleavingM essage

Regenerator

ChannelEncoding

InterleavingM essage

Generator

Ciphering

Ciphering

RFProcessing

RFProcessing

SIM

Duplexer

Antenna

ANTENNAASSEM BLY

TRANSM ITTER

RECEIVER

TRANSCEIVER UNITCONTROLSECTION


Base Transceiver Station (BTS)

BTS is a set of transceivers (TX/RX).

GSM BTS can host up to 16 TX/RX.

In GSM one TX/RX is shared by 8 users.

The main role of TX/RX is to provide

conversion between traffic data on the

network side and RF communication on

the MS side.

Depending on the application, it can be

configured as macrocell, microcell, omni,

sectored, etc.

Typical BTS installation

BTS antenna system

Macrocell BTS radio cabinet hosts TX/RX

Femto-cell


BSC plays a role of a small digital exchange.

It can be connected to many BTSs and it offloads a great deal of processing from MSC

One BSC connects to several tens to couple of hundred BTS

Some of BSC responsibilities:o Handoff managemento MAHO managemento Power control o Clock distributiono Operation and maintenance

TRAU is responsible for transcoding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec.

It can physically reside on either BSC side or MSC side.

If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel.

TRAU, BSC and BTSs form Base Station Subsystem (BSS

Base Station Controller (BSC) and TRAU

Typical BSC

TRAU = Transcoding and Rate Adaptation Unit


Responsible for connecting the mobile to the

landline side

GSM MSC is commonly designed as a regular

ISDN switch with some added functionality for

mobility support

GSM Network can have more than one MSC

One of the MSC has an added functionality for

communication with public network – Gateway MSC

(GMSC)

All calls from the “outside networks” are routed

through GMSC

Mobile Switching Center (MSC)


Registry HLR/VLR

HLR – Home Location Registry

Database for permanent or semi-permanent data associated with the user

Logically, there is only one HLR per network

Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc.

HLR is usually implemented as an integral part of MSC

VLR – Visitor Location registry

Temporary database that keeps the information about the users within the service area of the MSC

Usually there is one VLR per MSC

The main task of the VLR is to reduce the number of queries to HLR. When the mobile, registers on the system its information is copied from HLR to VLR

VLR is usually integrated with the switch


AUC/EIR

AUC – Authentication center

Integral part of HLR

GSM specifies elaborate encryption

Three levels

o A5/1 USA + Europe

o A5/2 COCOM country list

o No encryption – rest of the world

EIR – Equipment Identity Registry

Responsible for tracking equipment and eligibility for service

Maintains three lists

o White list – approved mobile types

o Black list – barred mobile types

o Gray list – tracked mobile types

Over years – many other vendor specific features added to the system


GSM Air Interface - Um

Interface between the MS and the GSM network

Subject to rigorous standardization process

We examine:

o Channelization

o Multiple access scheme

o Interface organization:

On the physical level

On the logical level


Frequency allocation

For PCS-1900 band

o ARFCNul = (Fc-1850)/0.2+511; ARFCNdl = (Fc-1930)/0.2+511

For GSM-850

o ARFCNul = (Fc-824)/0.2+127; ARFCNdl = (Fc-969)/0.2+127

Mapping formulas

GSM is FDD technology


TDMA Access Scheme

Multiple users operate on the same frequency, but not at the same time.

Advantages of TDMA:

o Relatively low complexity

o MAHO

o Different user rates can be accommodated

o Easier integration with the landline

Disadvantages:

o High sync overhead

o Guard times

o Heavily affected by the multipath propagation

Uplink ( From MS to BS)

Wireless Communication Channel

Downlink ( From BS to MS )

Base Station

fu0, s1

fd0, s1, s 2, ...,s 8

S 1 S 2 S 3 .... S 8 s1

s7 s8.... s1 s2 s3

fu0, s2

fu0, s8

TDMA = Time Division Multiple Access


GSM as a TDMA system

GSM is a combination of FDMA and TDMA

TDMA supports:

o Up to 8 full rate users

o Up to 16 half rate users

GSM uses Frequency Division Duplexing BTS

USER 1 USER 2 .... USER 8

USER 6 USER 7 USER 8 USER 1

USER 1,ARFCN 1

USER 2,ARFCN 1

USER 8,ARFCN 1

USER 9,ARFCN 2

USER 10,ARFCN 2

USER 16,ARFCN 2

ARFCN 1

ARFCN 2


GSM bursts

Data sent over one time slot = burst

Five types: normal, frequency correction, synchronization, dummy, access

Format of a burst defied by its function

DL: normal, frequency correction, synchronization, dummy

UL: normal, access

Time/Frequency/Amplitude diagram for GSM normal burst


Normal Burst

Used to carry information on both control and traffic channels

Mixture of data and overhead

GSM defines 8 training sequences assigned in color code mode

Both on the forward and reverse link

• Total of 114 encoded user information bits• Total of 34 overhead bits

Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail

3 57 1 26 1 57 3

Normal burst


Frequency Correction Burst

Sometimes referred to as the F-burst

Provides mobile with precise reference to the frequency of the broadcast control channel

Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier

Only on the forward link

•Spectral characteristics of the control channel.

•The peak in the spectrum allows for easier MS network acquisition

•Format of the F-burst•Fixed sequence consists of all zeros

fc fc+67.7 KHz frequency

Power Spectrum Density

BW = 200KHz

Tail Fixed Bit Sequence (All zeros) Tail

3 3142

Frequency correction burst


Synchronization Burst

Facilitates the synchronization of the MS to the network at the base band

Commonly referred to as S-burst


The same sync sequence is used in all GSM networks

Tail Synchronization Training Sequence Synchronization Tail

3 33939 64

Synchronization burst


Dummy Burst

Supports MAHO

Used to ensure constant power level of the broadcast

control channel


Tail Predefined Bit Sequence Tail

3 3142

Dummy burst


Access Burst

Used when the MS is accessing the system

Shorter in length – burst collision avoidance

Extended synchronization sequence

Used only on the reverse link

GSM mobiles use slotted ALOHA to access the system

In the case of collision – a hashing algorithm is provided

Tail Synchronization Access Bits Tail

8 41 36 3

Access burst


GSM TDMA Hierarchical Organization

0 1 2 3 4 5 6 7 21 22 23 24 25

1 TDM A Frame4.615 ms

26 M ultiframe120 ms

51 M ultiframe235.4 ms

51 x 26 Superframe or 26 x 51 Superframe6s 120 ms

Hyperframe3 h 28 min 53 s 760 ms

0 1 2 3 4 48 49 50

0 1 2 3 4 5 6 7 2043 2044 2045 2046 2047

0 1 2 3 4 5 6 7 46 47 48 49 50

0 1 2 3 4 23 24 25

0 1 2 3 4 5 6 7


GSM Time Division Duplex

Communication on the forward and reverse link does not happen simultaneously

Delay of three slots between TX and RX

Time division duplexing avoids RF duplexer at the RF stage

o Reduces the cost of mobile

o Saves battery

0

1 2 3 4 5 6 7 00

1 2 3 4 5765

Forward Link - BTS Transmits

Reverse Link - MS Transmits


GSM Logical Channels

GSM Logical Channels

TCH

TCH/F TCH/H

CCH

BCH CCCH DCCH

CBCH

ACCH SDCCH

FACCHSACCH

FCCH

SCH

BCCH

PCH

AGCH

RACH

TCH - Traffic Channel

TCH/F - Traffic Channel (Full Rate)

TCH/H - Traffic Channel (Half Rate)BCH - Broadcast Channels

FCCH - Frequency Correction Channel

SCH - Synchronization Channel

BCCH - Broadcast Control Channel

CCCH - Common Control Channels

PCH - Paging Channel

AGCH - Access Grant Channel

RACH - Random Access Channel

DCCH - Dedicated Control Channels

SDCCH - Stand-alone Dedicated Control ChannelACCH - Associated Control Channels

SACCH - Slow Associated Control Channel

FACCH - Fast Associated Control Channel

CCH - Control Channel

CBCH - Cell Broadcast Channel


Traffic channel carries speech and user data in both directions

o Full rate ~ 33.85 Kb/sec

o Half rate ~ 16.93 Kb/sec

o Full rate uses 1 slot in every frame

o Half rate uses 1 slot in every other frame

Data rates differ due to differences in Error Control Coding

Traffic Channels (TCH)

Full Rate TCH can carry:

• Voice (13 Kb/sec)

• Date at rates:

-9.6 Kb/sec

-4.8 Kb/sec

-2.4 Kb/sec

Half Rate TCH can carry:

• Voice (6.5 Kb/sec)

• Date at rates:

-4.8 Kb/sec

-2.4 Kb/sec


Control Channels

GSM Defines 3 types of Control Channels:

1. Broadcast Channels (BCH)

Broadcast information that helps mobile system acquisition, frame synchronization, etc. They advertise properties and services of the GSM network.

Forward link only

2. Common Control Channels (CCCH)

Facilitate establishment of the link between MS and system

Both forward and reverse link

3. Dedicated Control Channels (DCCH)

Provide for exchange the control information when the call is in progress

Both forward and reverse – in band signaling

C C H

BC H

C C C H

D C C H


Broadcast Channels (BCH)

Three types of BCH:

1. Synchronization channel (SCH)

Provides a known sequence that helps mobile synchronization

at the baseband

Communicates with S-burst

Broadcasts Base Station Identity Code (BSIC)

2. Frequency Correction channel (FCH)

Helps mobile tune its RF oscillator

Communicates with F-burst

3. Broadcast Control Channel (BCCH)

Provides mobile with various information about network, its services, access parameters, neighbor list, etc.

BC H

SC H

FC H

BC C H


Broadcast Channels (BCH) cont’d.

In general, the information sent over BCCH can be grouped into four categories:

1) Information about the network

2) Information describing control channel structure

3) Information defining the options available at the particular cell

4) Access parameters

Some BCCH messages


Common Control Channel (CCCH)

Three types of CCCH:1. Random Access Channel (RACH)

Used by mobile to initialize communication Mobiles use slotted ALOHA Reverse link only

2. Paging Channel (PCH) Used by the system to inform the mobile

about an incoming call Forward link only GSM Supports DRX

3. Access Grant Channel (AGC) Used to send the response to the mobiles

request for DCCH Forward link only

C C C H

R AC H

PC H

AG C


Dedicated Control Channels (DCCH)

Three types of DCCH:

1. Stand Alone Dedicated Control Channel (SDCCH)

Used to exchange overhead information when

the call is not in progress

2. Slow Associated Control Channel (SACCH)

Used to exchange time delay tolerant overhead

information when the call is in progress

3. Fast Associated Control Channel (FACCH)

Used to exchange time critical information

when the call is in progress

DCCH

SDCCH

SACCH

FACCH


Logical Channels - Summary

UL - Uplink DL - Downlink

Channel UL only DL only UL/DL Point to point

Broadcast Dedicated Shared

BCCH X X X

FCCH X X X

SCH X X X

RACH X X X

PCH X X X

AGCH X X X

SDDCH X X X

SACCH X X X

FACCH X X X

TCH X X X


Timing Advance

Mobiles randomly distributed in space

Timing advance prevents burst collision on the reverse link

Maximum advancement is 63 bits

BTS

SLOT 0 SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5

MS 2

MS 1

d2, Slot 2

d1, Slot 1

d1 > d 2MS 2

MS 1

T 1

T 2

Collision

T 1 - Delay of MS 1

Signal

T 2 - Delay of MS 2

Signal

SLOT 7SLOT 6

km35bit

s10693.3bit63

s

m103

2

1max 68

D


Signal Processing – From Voice to Radio Waves

Sampling,Quantization andsource encoding

ChannelEncoding

(Error CorrectionCoding)

InterleavingBurst

FormatingMapping

De-Ciphering

Modulation

De-Modulation

Ciphering

BurstFormatingMapping

De-Interleaving

ChannelDecoding

(Error Correction )

Source Decodingand Waveform

Generation

UmInterface

VoiceSignal

VoiceSignal

Transmit Side

Receive Side

As a digital TDMA technology GSM implements extensive signal processing


Sampling and Quantization

Sampling

o Sampling theorem specifies conditions for discretization of band limited analog signals

o Voice needs to be sampled at the sampling rate greater then 8000Hz

Quantization

o Discrete values assigned to continuous samples

o Quantization noise

o In GSM, voice is sampled at 8 K samples/sec and quantized with 8192 levels (13 bit words)

111 +3V110 +2V101 +1V

0V001 -1V010 -2V011 -3V

111 +3V110 +2V101 +1V

0V001 -1V010 -2V011 -3V

Analog Signal

Sampling Pulse

PAM

101 110 101 100 010 010 010 100 111 111PCM


Speech Source Encoding

Speech coder reduces the data rate needed for voice signal representation

GSM specifies operation of :

o Full rate vocoder

13Kb/sec

o Half rate vocoder

5.6Kb/sec

o Enhanced Full Rate (EFR)

12.2Kb/sec

o AMR (Adaptive multi rate)

AMR-FR (4.75-12.2Kb/sec)

AMR-HR (4.75-7.95Kb/sec)

AMR rate - function of C/I

BPF A/Dconverter

SPEECH

ENCODER

CHANNEL

CODING

TO

MODULATORMICROPHONE

BAND-PASS300 Hz-3.4 kHz

SPEECH

DECODER

CHANNEL

DECODERLP

LOW-PASS4 kHz

D/Aconverter

Vocoders enable efficient channel utilization


Performance comparison of some commercial vocoders

Mean Opinion Scores (MOS) - Voice Qualitysource IIR. The First Annual CDMA Congress

London, Oct. 29-30, 1997

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Clean Speech 20dB SNRBabble

20dB SNRCar

15dB SNRStreet

Mu-PCM

8Kb/s EVRC(CDMA)

13Kb/s CELP(CDMA)

IS-136 ACELP

GSM EFR


Error control coding (ECC) increases the robustness of the signal

ECC increases the overhead and reduces the efficiency of the communication

In GSM, the ECC increases the overhead per user by 57%

Channel Encoding

TYPE IaBITS

TYPE IIBITS

TYPE IbBITS

CONVOLUTIONAL

ENCODER

r=1/2

K=5

MU

XERROR DETECTING CODE

50

132

78

3

4

189

189

378

456

0TO

INTERLEAVER

FRO

M V

OC

OD

ER


Interleaving

In mobile

communications, the

errors are “bursty”

Optimal performance

from ECC is obtained

for uniform error

distribution

Interleaving increases

the performance of ECC

in mobile environment

252015105

24191494

23181383

22171272

21161161

bbbbb

bbbbb

bbbbb

bbbbb

bbbbb

Data is writtencolumn-wise

Data is readrow-wise

Interleaver

b1 b 2 b 3 b 4 b5 b 6 b 7 b 8 b 9 b 10 b 11 b 12 b 13 b 14 b 25 b 16 b 17 b 18 b 19 b 20 ...

b1 b 6 b 11 b 16 b21 b 2 b 7 b 12 b 17 b 22 b3 b 8 b 13 b 18 b 23 b 4 b 9 b 14 b 19 b 24 ..

Burst ErrorCaused by

Rayleigh Fading

Errors are spread over the bit stream


Modulation: GMSK (Gaussian MSK)

GMSK has excellent spectral characteristics

o Low sidelobes

o Robust to non- linearities

Price paid is in the increased Inter Symbol Interference (ISI)

Simplified GMSK block diagram

MSK

Filtered MSKGMSK

(f-f o) / Rb0 1 2 3

-80

-60

-40

-20

0

POWER SPECTRALDENSITY

dBSpectral characteristics of GMSK


Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail

3 57 1 26 1 57 3

Sequence used for equalizer training

Equalization

Necessary due to the multipath propagation

Needs to have :

o Fast convergence

o Low complexity

Two modes of operation

1. Training

2. Equalization

GSM equalizer capable of equalizing for two equal multi paths separated by 16 microseconds

Introduces overhead of about 18%

RFProcessing

AdaptiveEqualizer

EqualizationAlgorithmExtraction of

SynchronizationBits

UnequalizedData

EqualizedData


GSM Network Features

Mobile Assist Handoff (MAHO)

Discontinuous Transmission (DTX)

Dynamic Power Control (DPC)

Frequency Hopping (FH)

Intercell Handoff


Mobile Assisted Handoff (MAHO)

GSM Implements MAHO

In the process of evaluating handoff candidates, GSM systems evaluate measurements performed by both the MS and BTS

There are three types of measurements:

1. Signal Strength Measurements

2. Signal Quality Measurements

3. Timing Advance Measurements

Measurement type

Link Cell DTX Measurement Source

RSL Downlink Serving Cell Full Set Mobile

RSL Downlink Serving Cell Subset Mobile

RSL Downlink Neighbors N/A Mobile

Quality Downlink Serving Cell Full Set Mobile

Quality Downlink Serving Cell Subset Mobile

RSL Uplink Serving Cell Full Set BTS

RSL Uplink Serving Cell Subset BTS

RSL Uplink Neighbors Full Set BTS

RSL Uplink Neighbors Subset BTS

Quality Uplink Serving Cell Full Set BTS

Quality Uplink Serving Cell Subset BTS

Timing Advance Uplink Serving Cell N/A BTS

Measurement type

Link Cell DTX Measurement Source

RSL Downlink Serving Cell Full Set Mobile

RSL Downlink Serving Cell Subset Mobile

RSL Downlink Neighbors N/A Mobile

Quality Downlink Serving Cell Full Set Mobile

Quality Downlink Serving Cell Subset Mobile

RSL Uplink Serving Cell Full Set BTS

RSL Uplink Serving Cell Subset BTS

RSL Uplink Neighbors Full Set BTS

RSL Uplink Neighbors Subset BTS

Quality Uplink Serving Cell Full Set BTS

Quality Uplink Serving Cell Subset BTS

Timing Advance Uplink Serving Cell N/A BTS

Measurement typeMeasurement type

LinkLink CellCell DTXDTX Measurement SourceMeasurement Source

RSLRSL DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile

RSLRSL DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile

RSLRSL DownlinkDownlink NeighborsNeighbors N/AN/A MobileMobile

QualityQuality DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile

QualityQuality DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile

RSLRSL UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS

RSLRSL UplinkUplink Serving CellServing Cell SubsetSubset BTSBTS

RSLRSL UplinkUplink NeighborsNeighbors Full SetFull Set BTSBTS

RSL RSL Uplink Uplink NeighborsNeighbors SubsetSubset BTSBTS

QualityQuality UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS

QualityQuality UplinkUplink Serving CellServing Cell SubsetSubset BTSBTS

Timing AdvanceTiming Advance UplinkUplink Serving CellServing Cell N/AN/A BTSBTS


MAHO - Signal Strength Measurements

Performed on uplink and downlink

Reported as a quantized value RXLEV:

RXLEV = RSL[dBm] + 110

Minimum RXLEV:

-110, MAX RXLEV = -47

On the downlink, measurement performed for both serving cell and up to 32 neighbors

Up to 6 strongest neighbors are reported back to BTS through SACHH

Example measurement report


MAHO - Signal Strength Measurements

Measurements of the neighbors are performed on the BCCH channels – not affected by the DTX

Measurements on the serving channel – affected by the DTX.

Perform over a subset of SACCH that guarantees transmission even in the case of active DTX

Before processing, the RXLEV measurements are filtered to prevent unnecessary handoffs

-100

-90

-80

-70

-60

-50

-40

0 500 1000 1500 2000

Measurement

RX

LE

V (

dB

m)

510

520

530

540

550

560

570

580

BC

CH

AR

FC

N

RX LEV (dBm) BCCH

Example RSL measurement


MAHO – Signal Quality Measurements

Performed on uplink and downlink Only on the serving channel Reported as a quantized value RXQUAL For a good quality call RXQUAL < 3 Measurements are averaged before the

handoff processing If DTX is active, the measurements are

performed over the subset of SACCH that guarantees transmission

RXQUAL BER

0 Less than 0.1

1 0.26 to 0.30

2 0.51 to 0.64

3 1.0 to 1.3

4 1.9 to 2.7

5 3.8 to 5.4

6 7.6 to 11.0

7 Above 15

RXQUAL BER

0 Less than 0.1

1 0.26 to 0.30

2 0.51 to 0.64

3 1.0 to 1.3

4 1.9 to 2.7

5 3.8 to 5.4

6 7.6 to 11.0

7 Above 15

RXQUALRXQUAL BERBER

00 Less than 0.1Less than 0.1

11 0.26 to 0.300.26 to 0.30

22 0.51 to 0.640.51 to 0.64

33 1.0 to 1.31.0 to 1.3

44 1.9 to 2.71.9 to 2.7

55 3.8 to 5.43.8 to 5.4

66 7.6 to 11.07.6 to 11.0

77 Above 15Above 15

RXQUAL mapping table

RXQUAL measurements

Measurement report


Performed on uplink (BTS)

Only on the serving channel

Used by the BTS to estimate distance to the MS

Expressed in number of bits of TX advancement

Can be between 0 and 63

TA

MAHO – Time Alignment Measurement


Discontinuous Transmission (DTX)

Typical voice activity is around 60% DTX discontinues transmission during

silent periods Benefits of DTX

o Uplink: System interference reduction Lower battery consumption

o Downlink System interference reduction Reduction of the intermodulation

products Lower power consumptions

Downsides of DTX usage:

o MAHO measurements are less accurate

o Voice quality is degraded due to slowness of VAD

Mobile station Environment Typical voice

activityHandset Quiet location 55%

Handset Moderate office noise with voice interference

60%

Handset Strong voice interference (ex. airport,

railway station)

65-70%

Hands free / handset

Variable vehicle noise 60%



There are three reasons for DPC:1. Reduction of battery consumption

2. Elimination of “near-far” problem

3. Reduction of system interference

Power Class GSM (900MHz)[W]

PCS-1900 / GSM – 1800[W]

1 20(1) 1

2 8 0.24

3 5 Not Defined

4 2 Not Defined

5 0.8 Not Defined

Power Class GSM (900MHz)[W]

PCS-1900 / GSM – 1800[W]

1 20(1) 1

2 8 0.24

3 5 Not Defined

4 2 Not Defined

5 0.8 Not Defined

Power ClassPower Class GSM (900MHz)[W]

GSM (900MHz)[W]

PCS-1900 / GSM – 1800[W]

PCS-1900 / GSM – 1800[W]

11 20(1)20(1) 11

22 88 0.240.24

33 55 Not DefinedNot Defined

44 22 Not DefinedNot Defined

55 0.80.8 Not DefinedNot Defined

(1) Not available commercially



DPC for MSo Depending on its power class, MS can adjust its power between the max and min

value in 2dB steps

o MS can perform 13 adjustments every SACCH period, i.e., 480ms

o Large adjustments > 24 dB will not be completed before the arrival of new command

o Commonly implemented as BSC feature. Many vendors are moving it at the BTS level

DPC for BTSo Vendor specific

o Based on MAHO reports


Hierarchical Cell Structure (HCS)

Incorporates various cell sizes into layers of RF coverage

Three common layers:

1. Umbrella cells (HL = 0)

2. Macrocells (HL = 1)

3. Microcell (HL = 2)

HCS provides a way to assign preference levels between the cells

Very effective way for capacity and interference management

SignalS trength

ReselectionPoints

Select M icro-Cell

SS_SUFF

M acrocel

PreferredM icro-Cell

D istance

HL = 1

HL = 2


Handling of Fast Moving Mobiles

If the mobile is moving at a high speed, it will

spend a short time in the coverage area of the

microcell

To prevent excessive handoffs, a temporal

GSM introduces temporal penalty – prevents

immediate handoff initialization

If the duration of mobile stay within the

coverage area is shorter than the temporal

penalty, it will never initialize handoff


Frequency Hopping (FH)

FH - multiple carriers used over the course of radio transmission

o There are two kinds of FH:

1. Slow Hopping – change of carrier frequency happens at the rate slower than the symbol rate

2. Fast Hoping – carrier frequency changes faster than the symbol rate

o GSM implements slow FH Scheme

o Carrier frequency is changed once per time slot

o There are two reasons for frequency hopping

1. Frequency Diversity

2. Interference avoidance


Frequency Diversity of FH

Mobile environment is characterized with small scale fading

The depth of signal fade is a function frequency

If two signals are sufficiently separated in frequency domain they fade independently

Frequency diversity gain diminishes for fast moving mobiles


Interference Avoidance of FH

FH averages interference

Allows for tighter reuse of frequencies

Increases the capacity of the system

User 1

User 2

User 3

User 4

User 5

f1

f4

f1

f1 f1

f1

f1

f2

f2

f2

f2

f3

f4

f1

f1

f2

f3

f3f4

f1

f4 f3

f1

f3

f4

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T


Baseband FH in GSM

Each radio operates on a fixed frequency

The bursts are routed to individual radios in accordance to their hopping sequence

Advantages of baseband hopping No need to “real time” retune – simpler

radios More efficient combiners

Disadvantage of baseband hopping Number of hopping frequencies limited

by the number of radios

TX/RX

TX/RX

TX/RX

CarrierFreuqnacy

f1

CombinerCarrier

Freuqnacyf2

CarrierFreuqnacy

fn

1

2

n

Bus for Routingand Switchning


Synthesized FH in GSM

Each radio is hopping in an independent way

Radios retune – “real time”

Advantages of synthesized hopping:Set of the hopping frequencies can be

assigned in an arbitrary way

Disadvantage of synthesized hopping:Need for expensive and lossy combiners

TX/RX

TX/RX

TX/RX

CarrierFreuqnacyf0,f 1,...,f m

BroadbandCombiner

1

2

n




FH Algorithms

• Random Hopping• Implemented in a pseudo – random way• Uses one of 63 available pseudorandom sequences• The actual frequency is obtained as a modulo

operation with number of available frequencies in allocation list (FH group)

,,,,,,, 3214321 fffffff

,,,,,,, 3234421 fffffff

Cyclic Hopping

o Frequencies are used in the consecutive order

o If the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN


Intracell Handoff

High Interference

Measurement indicates:

o Poor RXQUAL

o Good RXLEV

There is high probability that the call will improve with the handoff to different carrier within the same cell

To avoid unnecessary handoffs, system introduces maximum number of intercell handoffs


GSM RF Planning / Design

Link Budget and Nominal Cell Radius Calculation

Receiver Sensitivity

Required C/I ratio

Mobile Transmit Power

Examples of Link Budget

Calculation of a Nominal Cell Radius

Frequency Planning and Reuse Strategies

Frequency Planning Using Regular Schemes

Automatic Frequency Planning

Capacity of GSM Networks


Migration:

1. High speed circuits switched data (HSCSD)

2. Packet switched data (GPRS,EDGE)

3. Integrated packet services – possibly under different access scheme (UMTS)

GSM Migration Towards 3G

G SM 2+9.6 Kb/sec

H S C S D64 K b/sec

G P R S114 K b/sec

E D G E384 K b/sec

U M TS2M b/sec

19991Q

20002Q

20003Q

20014Q

2002

T im eline

D ata R ate

H S C S D - H igh S peed C ircu it S w itched D ataG P R S - G enera l P acket R ad io S ystemE D G E - E nhanced D ata G S M E nvironm entU M TS - U n iversa l M ob ile T e lephone S ervice


GSM 2+ Data Services

GSM’s traffic channel can support the data transfer of a bit rate up to 9.6Kb/sec

o This data rate can be used for:

Short messages

Fax services

E-mail, etc.

o Circuit switched data services

o Not suitable for Internet

Too slow

Too costly (user would pay for the “circuit” even if there is no traffic exchanged


High Speed Circuit Switched Data (HSCSD)

HSCSD is using existing GSM organization to provide data services of a somewhat higher data rates

It can combine several existing traffic channels into a single connection, i.e., it allows for mobiles multislot operation

HSCSD can be implemented through software upgrades on existing networks and no hardware upgrades are needed

Seems to be less accepted by the service providers


GPRS is another new transmission capability for GSM that will be especially developed to accommodate for high-bandwidth data traffic

GPRS will handle rates from 14.4Kbps using just one TDMA slot, and up to 115Kbps and higher using all eight time slots

It introduces packet switching - can accommodate the data traffic characteristics

General Packed Radio Data (GPRS)


GPRS Network architecture

New type of node:

GPRS Service Node (GSN)

BSC

BSC

MSC

VLR

HLR

AUC

EIR

BTS

BTS

BTS

BTS

BTS - Base StationBSC - Base Station ContollerMSC - Mobile Switching CenterVLR - Visitor Location RegisterHLR - Home Location RegisterAUC - Authentification CenterEIR - Equipment Identity Register

Um Interface

A-Bis Interface

AInterface

D

C

PSTNB

B,C,D,E,F - MAPInterfaces

SGSN

GGSN

SGSN - Service GPRS Support Node

GGSN - Gateway GPRS Support Node

GnInterface

Gr

OutsidePacket

Network


GPRS Call routing

SGSN

GGSN

GGSN

SGSN

BTS

BTS

GPRS - PDN

GPRS - PDN

Routing is performed “parallel” to the GSM network


Packet switched

Upgrades the modulation scheme

o From GMSK to 8-PSK

o Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots

o Variable data rate – depending on the channel conditions

Defines several different classes of service and mobile terminals

Enhanced Data GSM Environment (EDGE)

EDGE enabled data mobile


Practically achievable data rates

Theoretical rates are constrained by mobile power and processing capabilities

Most mobiles support less than the maximum allowed by standard

Practically achievable data rates


UMTS – 3G cellular service

Provides data rates up to 2Mb/sec

Possibly standardized as W-CDMA

Universal Mobile Telephone Service (UMTS)

Outline of UMTS (WCDMA) network

Date post:	23-Dec-2015
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ECE Department Florida Institute of Technology ECE 5221 Personal Communication Systems Introduction...

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