Download GSM

EEE 464
Wireless Communications
Lecture 7
Shahzad Malik,
Ph.D.
[email protected]
Mobile Cellular Wireless
Networks
This lecture presents system details of
2G/3G mobile cellular networks:
GSM/GPRS, CdmaOne (IS-95) and UMTS/
Cdma2000
Organization of Lecture 7
 Global System for Mobile (GSM)
 GPRS
 SMS
 EDGE
Shahzad Malik
Wireless Communications - GSM
Lecture 7
3
GSM
Global System for Mobile
CT0/1
AMPS
NMT
CT2
IMT-FT
DECT
IS-136
TDMA
D-AMPS
TDMA
FDMA
Cellular Systems – Generation
GSM
PDC
EDGE
GPRS
IMT-SC
IS-136HS
UWC-136
IMT-DS
UTRA FDD / W-CDMA
CDMA
IMT-TC
UTRA TDD / TD-CDMA
IMT-TC
TD-SCDMA
1G
Shahzad Malik
IS-95
cdmaOne
cdma2000 1X
2G
2.5G
Wireless Communications - GSM
IMT-MC
cdma2000 1X EV-DO
1X EV-DV
(3X)
3G
Lecture 7
5
Digital PLMN systems
(PLMN = Public Land Mobile Network)
2nd Generation (2G)
GSM
3rd Generation (3G)
IMT-2000
FDD
GPRS
Packet
services
4G
UMTS:
More radio
capacity
EDGE
UTRA FDD
UTRA TDD
IS-136
USA
IS-95
Shahzad Malik
Wireless Communications - GSM
CDMA
2000
Lecture 7
6
GSM: Overview
 GSM - Global System for Mobile
 formerly: Groupe Spéciale Mobile (founded 1982)
 now: Global System for Mobile Communication
 Pan-European standard (ETSI, European Telecommunications
Standardisation Institute)
 simultaneous introduction of essential services in three phases
(1991, 1994, 1996) by the European telecommunication
administrations
 seamless roaming within Europe possible
 today many providers all over the world use GSM (more than
184 countries in Asia, Africa, Europe, Australia, America)
 more than 1000 million subscribers
 more than 70% of all digital mobile phones use GSM
 over 10 billion SMS per month in Germany, > 360 billion/year
worldwide
Shahzad Malik
Wireless Communications - GSM
Lecture 7
7
GSM
 Objectives:
 Broad offering of speech and data services
 Compatible with wireline networks, eg, ISDN
 Automatic roaming and handoff
 Highly efficient use of frequency spectrum
 Support for different types of mobile terminal
equipment (eg, cars, portable handsets)
 Digital signaling and transmission
 Low cost infrastructure and terminal equipment
Shahzad Malik
Wireless Communications - GSM
Lecture 7
8
Performance characteristics of GSM
 Communication
 mobile, wireless communication; support for voice and data
services
 Total mobility
 international access, chip-card enables use of access points of
different providers
 Worldwide connectivity
 one number, the network handles localization
 High capacity
 better frequency efficiency, smaller cells, more customers per
cell
 High transmission quality
 high audio quality and reliability for wireless, uninterrupted
phone calls at higher speeds (e.g., from cars, trains)
 Security functions
 access control, authentication via chip-card and PIN
Shahzad Malik
Wireless Communications - GSM
Lecture 7
9
Architecture of the GSM system
 GSM is a PLMN (Public Land Mobile Network)
 several providers setup mobile networks following the
GSM standard within each country
 components




MS (mobile station)
BS (base station)
MSC (mobile switching center)
LR (location register)
 subsystems
 RSS (radio subsystem): covers all radio aspects
 NSS (network and switching subsystem): call forwarding,
handover, switching
 OSS (operation subsystem): management of the network
Shahzad Malik
Wireless Communications - GSM
Lecture 7
10
GSM: overview
OMC, EIR,
AUC
HLR
NSS
with OSS
VLR
MSC
GMSC
VLR
fixed network
MSC
BSC
BSC
RSS
Shahzad Malik
Wireless Communications - GSM
Lecture 7
11
GSM: elements and interfaces
radio cell
MS
BSS
MS
Um
radio cell
RSS
MS
BTS
BTS
Abis
BSC
BSC
A
MSC
NSS
MSC
VLR
VLR
GMSC
HLR
IWF
signaling
ISDN, PSTN
PDN
O
OSS
Shahzad Malik
EIR
AUC
OMC
Wireless Communications - GSM
Lecture 7
12
GSM: system architecture
network and
switching subsystem
radio
subsystem
MS
fixed
partner networks
MS
ISDN
PSTN
MSC
Um
BTS
Abis
BSC
EIR
SS7
BTS
HLR
VLR
BTS
BSC
BTS
A
BSS
MSC
IWF
ISDN
PSTN
PSPDN
CSPDN
Shahzad Malik
Wireless Communications - GSM
Lecture 7
13
System architecture: radio subsystem
 Components
network and switching
 MS (Mobile Station)
subsystem
 BSS (Base Station
Subsystem):
consisting of
radio
subsystem
MS
MS
 BTS (Base Transceiver
Station):
sender and receiver
 BSC (Base Station
Controller):
controlling several
transceivers
Um
BTS
Abis
BTS
BSC
A
BTS
BTS
MSC
BSC
BSS
Shahzad Malik
MSC
 Interfaces
 Um : radio interface
 Abis : standardized, open
interface with
16 kbit/s user channels
 A: standardized, open
interface with
64 kbit/s user channels
Wireless Communications - GSM
Lecture 7
14
Radio subsystem
 The Radio Subsystem (RSS) comprises the cellular mobile network
up to the switching centers
 Components
 Base Station Subsystem (BSS):
 Base Transceiver Station (BTS): radio components including
sender, receiver, antenna - if directed antennas are used
one BTS can cover several cells
 Base Station Controller (BSC): switching between BTSs,
controlling BTSs, managing of network resources, mapping
of radio channels (Um) onto terrestrial channels (A
interface)
 BSS = BSC + sum(BTS) + interconnection
 Mobile Stations (MS)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
15
Radio subsystem - BSS

Base Station Subsystem
 It is the wireless point of contact of the network with users
 It forms Radio Access Network (RAN)
 It translates between the air interface and the wired
infrastructure protocols
 The two network segments need different protocols because
the difference of the nature of wireless links
 Unreliable, bandwidth limited, supports mobility
 Speech Conversion
 The MS generates radio-efficient 13 kbps digitized voice packets using
speech coder. The backbone PSTN requires 64 kbps PCM digitized voice.
The BSS converts 13 to 64 kbps code.
 Signaling
 The multi-tone frequency signaling is used in POTS in the wired backbone,
whereas GSM performs several packet exchange to establish a call. The
signaling conversion takes place at the BSS
Shahzad Malik
Wireless Communications - GSM
Lecture 7
16
Radio subsystem - BSS
 Tasks of a BSS are distributed over BSC and BTS
 BTS comprises radio specific functions
 BSC is the switching center for radio channels
Functions
Management of radio channels
Frequency hopping (FH)
Management of terrestrial channels
Mapping of terrestrial onto radio channels
Channel coding and decoding
Rate adaptation
Encryption and decryption
Paging
Uplink signal measurements
Traffic measurement
Authentication
Location registry, location update
Handover management
Shahzad Malik
Wireless Communications - GSM
BTS
X
X
X
X
X
X
BSC
X
X
X
X
X
X
X
X
X
X
Lecture 7
17
Radio subsystem - Mobile station
 Terminal for the use of GSM services
 A mobile station (MS) comprises several functional
groups
 MT (Mobile Terminal):
 offers common functions used by all services the MS
offers
 end-point of the radio interface (Um)
 TA (Terminal Adapter):
 terminal adaptation, hides radio specific characteristics
 TE (Terminal Equipment):
 peripheral device of the MS, offers services to a user
 does not contain GSM specific functions
 SIM (Subscriber Identity Module):
 personalization of the mobile terminal, stores user
parameters
TE
TA
R
Shahzad Malik
Wireless Communications - GSM
MT
S
Um
Lecture 7
18
network and switching subsystem
network
subsystem
fixed partner
networks
ISDN
PSTN
MSC
 MSC (Mobile Services Switching Center):
 IWF (Interworking Functions)




EIR
SS7
Components
ISDN (Integrated Services Digital Network)
PSTN (Public Switched Telephone Network)
PSPDN (Packet Switched Public Data Net.)
CSPDN (Circuit Switched Public Data Net.)
HLR
Databases
VLR
MSC
IWF
ISDN
PSTN
 HLR (Home Location Register)
 VLR (Visitor Location Register)
 EIR (Equipment Identity Register)
PSPDN
CSPDN
Shahzad Malik
Wireless Communications - GSM
Lecture 7
19
Network and switching subsystem
 NSS is the main component of the public mobile
network GSM
 switching, mobility management, interconnection to other
networks, system control
 Components
 Mobile Services Switching Center (MSC)
controls all connections via a separated network to/from a
mobile terminal within the domain of the MSC - several
BSC can belong to a MSC
 Databases (important: scalability, high capacity, low
delay)
 Home Location Register (HLR)
central master database containing user data, permanent
and semi-permanent data of all subscribers assigned to the
HLR (one provider can have several HLRs)
 Visitor Location Register (VLR)
local database for a subset of user data, including data
about all user currently in the domain of the VLR
Shahzad Malik
Wireless Communications - GSM
Lecture 7
20
Mobile Services Switching Center
 The MSC (mobile switching center) plays a central role
in GSM
 switching functions
 additional functions for mobility support
 management of network resources
 interworking functions via Gateway MSC (GMSC)
 integration of several databases
 Functions of a MSC
 specific functions for paging and call forwarding
 termination of SS7 (signaling system no. 7)
 mobility specific signaling
 location registration and forwarding of location
information
 provision of new services (fax, data calls)
 support of short message service (SMS)
 generation and forwarding of accounting and billing
information
Shahzad Malik
Wireless Communications - GSM
Lecture 7
21
Operation subsystem
 The OSS (Operation Subsystem) enables centralized
operation, management, and maintenance of all GSM
subsystems
 Components
 Authentication Center (AUC)
 generates user specific authentication parameters on
request of a VLR
 authentication parameters used for authentication of
mobile terminals and encryption of user data on the air
interface within the GSM system
 Equipment Identity Register (EIR)
 registers GSM mobile stations and user rights
 stolen or malfunctioning mobile stations can be locked and
sometimes even localized
 Operation and Maintenance Center (OMC)
 different control capabilities for the radio subsystem and
the network subsystem
Shahzad Malik
Wireless Communications - GSM
Lecture 7
22
GSM - FDMA/TDMA
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
higher GSM frame structures
time
GSM TDMA frame
1
2
3
4
5
6
7
8
4.615 ms
GSM time-slot (normal burst)
guard
space
tail
3 bits
Shahzad Malik
user data
S Training S
user data
57 bits
1 26 bits 1
57 bits
Wireless Communications - GSM
guard
tail space
3
546.5 µs
577 µs
Lecture 7
23
GSM FDMA
2
1
3
…
4
124
200 KHz
Carrier Spacing
100 KHz
guard band
Shahzad Malik
BW = 25 MHz
Downlink Frequency Band: 890-915 MHz
Downlink Frequency Band: 935-960 MHz
Bc = 200 KHz
Bg = 100 KHz
Number of Channels = 124
Data rate for each carrier = 270.833 kbps
Bit time = 3.69 s
Slot time (or burst time) = 577 s
Number of bits/slot = 156.25 bits
Burst Types: 1. Normal Burst (NB)
2. Frequency Correction Burst
3. Synchronization burst
4. Random Access Burst (RAB)
Wireless Communications - GSM
Lecture 7
24
GSM Physical Channels
TDMA frame = 4.615 ms
Timeslot 1
2
3
4
5
6
7
8
Frequency 1
Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8
Frequency 2
Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8
:
:
:
:
Frequency 124
Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8
ARFCN – Absolute Radio Frequency Channel Number
Shahzad Malik
Wireless Communications - GSM
Lecture 7
25
GSM Air Interface
Shahzad Malik
Wireless Communications - GSM
Lecture 7
26
GSM Logical and Physical Channels
 Um interface: various logical channels are mapped to physical
channels
 A physical channel is a timeslot with timeslot number in a sequence
of TDMA frames on a particular ARFCN
 8 physical channels mapped onto 8 timeslots within TDMA frame
per frequency carrier
Shahzad Malik
Wireless Communications - GSM
Lecture 7
27
GSM Frame Hierarchy
2048 super frames: Hyper frame (3 hr 28 min 53.76 s)
51 traffic or 26 control multi frames: Super frame (6.12 s)
26 traffic frames: Multi frame (120 ms)
51control frames: Multi frame (235.4 ms)
8 slots: Frame (4.615 ms)
156.25 bits: Burst (0.577 ms)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
28
GSM hierarchy of frames
hyperframe
0
1
2
2045 2046 2047 3 h 28 min 53.76 s
...
superframe
0
1
0
2
...
1
48
...
49
24
50
6.12 s
25
multiframe
0
1
...
0
1
24
2
120 ms
25
...
48
49
50
235.4 ms
frame
0
1
...
6
7
4.615 ms
slot
burst
Shahzad Malik
Wireless Communications - GSM
577 µs
Lecture 7
29
GSM Logical Channels
 3 groups of logical channels, TCH, CCH and CBCH
 TCH is used to carry voice or data traffic
 CCH is used for control functions
 CBCH is used for broadcast functions
 Logical traffic channels = full rate (TCH/F) at 22.8 kb/s or half
rate (TCH/H) at 11.4 kb/s
 Physical channel = full rate traffic channel (1 timeslot) or 2 half
rate traffic channels (1 timeslot in alternating frames)
 Full rate channel may carry 13 kb/s speech or data at 12, 6, or 3.6
kb/s
 Half rate channel may carry 6.5 kb/s speech or data at 6 or 3.6
kb/s
Shahzad Malik
Wireless Communications - GSM
Lecture 7
30
GSM Logical Channel Structure
TCH/F
TCH/H
BCH
FCCH SCH
CBCH
CCH
TCH
CCCH
BCCH
DCCH
PCH AGCH RACH
SACCH
Shahzad Malik
Wireless Communications - GSM
ACCH
SDCCH
FACCH
Lecture 7
31
GSM Logical Channels, cont..
 CCH consists of 3 groups of logical control channels, BCH, CCCH
and DCCH
 BCH (broadcast channel): point-to-multipoint downlink only.
Contains three sub-channels, BCCH, FCCH and SCH
 BCCH (broadcast control channel): send cell identities,
organization info about common control channels, cell service
available, etc
 FCCH (frequency correction channel): send a frequency
correction data burst containing all zeros to effect a constant
frequency shift of RF carrier
 SCH (synchronization channel): send TDMA frame number and
base station identity code to synchronize MSs
Shahzad Malik
Wireless Communications - GSM
Lecture 7
32
GSM Logical Channels, cont…
 CCCH (common control channel): Consists of three sub-channels,
PCH, AGCH and RACH. This channels is used for paging and access
 PCH (paging channel): to page MSs
 AGCH (access grant channel): to assign MSs to stand-alone
dedicated control channels for initial assignment
 RACH (random access channel): for MS to send requests for
dedicated connections
Shahzad Malik
Wireless Communications - GSM
Lecture 7
33
GSM Logical Channels, cont…
 DCCH (dedicated control channel): bi-directional point-to-point -main signaling channels. Consist of two sub-channels, SDCCH and
ACCH
 SDCCH (stand-alone dedicated control channel): for service
request, subscriber authentication, equipment validation,
assignment to a traffic channel
 ACCH consist of two sub-channels, SACCH and FACCH
 SACCH (slow associated control channel): for out-of-band
signaling associated with a traffic channel, eg, signal strength
measurements
 FACCH (fast associated control channel): for preemptive
signaling on a traffic channel, eg, for handoff messages
Shahzad Malik
Wireless Communications - GSM
Lecture 7
34
GSM Logical Channels , cont…
Shahzad Malik
Wireless Communications - GSM
Lecture 7
35
GSM Logical Channels , cont…
Shahzad Malik
Wireless Communications - GSM
Lecture 7
36
GSM Packet Encoding
Speech packet (13 kbps)
260 bits (20 ms)
50 bits
260 bits
CRC coding
53 bits
½ convolutional
coding
132 bits
78 bits
456 bits (20 ms)
Transmitted packet
378 bits
9600 bps data packet
192 bits (20 ms)
48 bits
signaling info
4 tail bits
½ convolutional
coding
456 bits (20 ms)
Transmitted packet
Shahzad Malik
4 tail bits
Wireless Communications - GSM
Signaling packet
184 bits (20 ms)
40 parity bits
4 tail bits
½ convolutional
coding
456 bits (20 ms)
Transmitted packet
Lecture 7
37
GSM Data Bursts
Shahzad Malik
Wireless Communications - GSM
Lecture 7
38
GSM Operation
Shahzad Malik
Wireless Communications - GSM
Lecture 7
39
Protocol Stack
Um Air
Interface
A-bis
A
CM
CM
MM
MM
RRM
RRM
RRM
RRM
SCCP
SCCP
LAPDm
LAPDm
LAPD
LAPD
MTP
MTP
Radio
Radio
64kbps
64kbps
64kbps
64kbps
MS
CM: Connection Management
MM: Mobility Management
SCCP: Signal Connection Control part
Shahzad Malik
MSC
BSC
BTS
RRM: Radio Resource Management
MTP: Message Transfer Part
LAPD: Link access protocol-D
Wireless Communications - GSM
Lecture 7
40
GSM Protocol Layers
 RF : Physical Layer
 LAPD: Link Layer, ISDN protocol based
 SCCP: Signal Connection Control Layer, part of link
layer
 RR: Radio Resource
 MM: Mobility Management
 CC: Call Control
Shahzad Malik
Wireless Communications - GSM
Lecture 7
41
GSM Network Layer
 Network layer consists of 3 sublayers
 Radio resource management (RR) sublayer
 Establishment, maintenance, and termination of radio
channel connections
 Mobility management (MM) sublayer
 Registration, authentication, and location tracking
 Call control (CC) sublayer
 Establishment, maintenance, and termination of circuitswitched calls
Shahzad Malik
Wireless Communications - GSM
Lecture 7
42
Registration
MS
BTS
BSC
MSC
VLR
HLR
1. Channel Request
2. Activation Response
3. Activation ACK
4. Channel Assigned
5. Location Update Request
6. Authentication Request
7. Authentication Response
8. Authentication Check
9. Assigning TMSI
10. ACK for TMSI
11. Entry for VLR and HLR
12. Channel Release
Shahzad Malik
Wireless Communications - GSM
Lecture 7
43
Mobile Originated Call
 1, 2: connection request
 3, 4: security check
VLR
 5-8: check resources (free
3 4
circuit)
6
PSTN
 9-10: set up call
5
GMSC
7
8
2 9
MS
Shahzad Malik
Wireless Communications - GSM
MSC
1
10
Lecture 7
BSS
44
Mobile Terminated Call
1: calling a GSM subscriber
2: forwarding call to GMSC
3: signal call setup to HLR
4, 5: request MSRN from VLR
6: forward responsible
calling
MSC to GMSC
station
7: forward call to
 current MSC
8, 9: get current status of MS
10, 11: paging of MS
12, 13: MS answers
14, 15: security checks
16, 17: set up connection
HLR
4
5
3 6
1
PSTN
2
GMSC
10
VLR
8 9
14 15
7
MSC
10 13
16
10
BSS
BSS
BSS
11
11
11
11 12
17
MS
Shahzad Malik
Wireless Communications - GSM
Lecture 7
45
MTC/MOC
MS
MTC
BTS
MS
BTS
MOC
paging request
Shahzad Malik
channel request
channel request
immediate assignment
immediate assignment
paging response
service request
authentication request
authentication request
authentication response
authentication response
ciphering command
ciphering command
ciphering complete
ciphering complete
setup
setup
call confirmed
call confirmed
assignment command
assignment command
assignment complete
assignment complete
alerting
alerting
connect
connect
connect acknowledge
connect acknowledge
data/speech exchange
data/speech exchange
Wireless Communications - GSM
Lecture 7
46
GSM Channel Use Example
MS
BTS
BSC
MSC
1. Channel Request (RACH)
2. Channel Assigned (AGCH)
3. Call Establishment Request (SDCCH)
4. Authentication Request (SDCCH)
5. Authentication Response (SDCCH)
6. Ciphering Command (SDCCH)
7. Ciphering Ready (SDCCH)
8. Send Destination Address (SDCCH)
9. Routing Response (SDCCH)
10. Assign Traffic Channel (SDCCH)
11. Traffic Channel Established (FACCH)
12. Available/Busy Signal (FACCH)
13. Call Accepted (FACCH)
14. Connection Established (FACCH)
15. Information Exchange (TCH)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
47
GSM Numbers

International mobile station equipment identity (IMEI). IMEI= TAC + FAC
+ SNR + SP
 TAC = Type Approval Code, 6 decimals
 FAC = Final Assembly Code, 6 decimals, assigned by manufacturer
 SNR = Serial Number, 6 decimals, assigned by manufacturer
 SP = Spare, 1 decimal place
 EIR has white, black and optionally grey list.

International mobile Subscriber Identity (IMSI): Stored on the SIM
(Subscriber Identity Module) card. IMSI is obtained at the time of
subscription. IMSI is not made public.
 IMSI = MCC + MNC + MSIN
 MCC = Mobile Country Code, 3 decimals
 MNC = Mobile Network Code, 2 decimals
 MSIN = Mobile Subscriber Identification Number, maximum 10 decimal digits
Shahzad Malik
Wireless Communications - GSM
Lecture 7
48
GSM Numbers
 Mobile Station ISDN number (MSISDN), is the real phone number of the
subscriber. Stored in HLR and on SIM card
 MSISDN = CC + NDC + SN
 CC = Country Code, up to 3 decimals
 NDC = National Destination Code, typically 2-3 decimals
 SN = Subscriber Number, maximum 10 decimals.
 Mobile Station Roaming Number (MSRN), same format as MSISDN. A temporary
location dependent ISDN number; assigned in two cases, at registration or at call
set up.
 Location Area Identity (LAI). Regularly sent on BCCH; LAI = CC + MNC + LAC,
 LAC = Location Area Code, max 5 decimals (<FFFFhex).
 Temporary Mobile Subscriber Identity (TMSI). Stored only in the VLR and SIM
card. Consists of 4*8 bits excluding value FFFF FFFFhex
 TMSI has only local meaning and can be defined according to operator’s
specifications.
 LAI + TMSI uniquely identifies the user, i.e. IMSI is no longer needed for ongoing
communication
Shahzad Malik
Wireless Communications - GSM
Lecture 7
49
GSM Handoffs
 3 types of handoffs
 Intra-BSS: if old and new BTSs are attached to same
base station
 MSC is not involved
 Intra-MSC: if old and new BTSs are attached to
different base stations but within same MSC
 Inter-MSC: if MSCs are changed
Shahzad Malik
Wireless Communications - GSM
Lecture 7
50
4 types of handover
1
MS
BTS
Shahzad Malik
2
3
4
MS
MS
MS
BTS
BTS
BTS
BSC
BSC
BSC
MSC
MSC
Wireless Communications - GSM
Lecture 7
51
Handover procedure
MS
BTSold
BSCold
measurement
measurement
report
result
MSC
HO decision
HO required
BSCnew
BTSnew
HO request
resource allocation
ch. activation
HO command
HO command
HO command
HO request ack ch. activation ack
HO access
Link establishment
clear command clear command
clear complete
Shahzad Malik
HO complete
HO complete
clear complete
Wireless Communications - GSM
Lecture 7
52
Handoff
MS
MSC
BSS1
BSS2
Measurement Report
HO Decision
Handoff Required
Handoff Request
Resource Allocation
Handoff
Handoff Command
Handoff Command
Handoff Request
ACK
Handoff Complete
Handoff Complete
Clear Command
Resource Release
Clear Complete
Shahzad Malik
Wireless Communications - GSM
Lecture 7
53
GSM Intra-MSC Handoff
 Mobile station monitors signal quality and determines
handoff is required, sends signal measurements to
serving BSS
 Serving BSS sends handoff request to MSC with
ranked list of qualified target BSSs
 MSC determines that best candidate BSS is under its
control (assumed here)
 MSC reserves a trunk to target BSS
Shahzad Malik
Wireless Communications - GSM
Lecture 7
54
GSM Intra-MSC Handoff, cont..
 Target BSS selects and reserves radio channels for
new connection, sends Ack to MSC
 MSC notifies serving BSS to begin handoff, including
new radio channel assignment
 Serving BSS forwards new radio channel assignment to
mobile station
 Mobile station re-tunes to new radio channel, notifies
target BSS on new channel
Shahzad Malik
Wireless Communications - GSM
Lecture 7
55
GSM Intra-MSC Handoff, cont..
 Target BSS notifies MSC that handoff is detected
 Target BSS and mobile station exchange messages to
synchronize transmission in proper timeslot
 MSC switches voice connection to target BSS, which
responds when handoff is complete
 MSC notifies serving BSS to release old radio traffic
channel
Shahzad Malik
Wireless Communications - GSM
Lecture 7
56
GSM Inter-MSC Handoff
 Mobile station monitors signal quality and determines
handoff is required, sends signal measurements to
serving BSS
 Serving BSS sends handoff request to MSC with
ranked list of qualified target BSSs
 Serving MSC determines that best candidate BSS is
under control of a target MSC (assumed here) and calls
target MSC through PSTN
Shahzad Malik
Wireless Communications - GSM
Lecture 7
57
GSM Inter-MSC Handoff, cont..
 Target MSC notifies its VLR to assign a TMSI
 Target VLR returns TMSI
 Target MSC reserves a trunk to target BSS
 Target BSS selects and reserves radio channels for
new connection, sends Ack to target MSC
 Target MSC notifies serving MSC that it is ready for
handoff
Shahzad Malik
Wireless Communications - GSM
Lecture 7
58
GSM Inter-MSC Handoff, cont..
 Serving MSC notifies serving BSS to begin handoff,
including new radio channel assignment
 Serving BSS forwards new radio channel assignment to
mobile station
 Mobile station re-tunes to new radio channel, notifies
target BSS on new channel
 Target BSS notifies target MSC that handoff is
detected
Shahzad Malik
Wireless Communications - GSM
Lecture 7
59
GSM Inter-MSC Handoff, cont..
 Target BSS and mobile station exchange messages to
synchronize transmission in proper timeslot
 Voice connection is switched to target BSS, which
responds when handoff is complete
 Target MSC notifies serving MSC
 Old network resources are released
Shahzad Malik
Wireless Communications - GSM
Lecture 7
60
GSM Roaming From Another PLMN
 VLR registers users roaming in its area
 Recognizes mobile station is from another PLMN
 If roaming is allowed, VLR finds the mobile’s HLR in its
home PLMN
 VLR constructs a global title from IMSI to allow
signaling from VLR to mobile’s HLR via public telephone
network
 VLR generates a mobile subscriber roaming number
(MSRN) used to route incoming calls to mobile station
 MSRN is sent to mobile’s HLR
Shahzad Malik
Wireless Communications - GSM
Lecture 7
61
GSM Roaming, cont…
 VLR contains
 MSRN
 TMSI
 Location area where mobile station has registered
 Info for supplementary services (if any)
 IMSI
 HLR or global title
 Local identity for mobile station (if any)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
62
Security in GSM
 Security services
 access control/authentication
 user  SIM (Subscriber Identity Module): secret PIN (personal
identification number)
 SIM  network: challenge response method
 confidentiality
 voice and signaling encrypted on the wireless link (after successful
authentication)
 anonymity
 temporary identity TMSI
(Temporary Mobile Subscriber Identity)
 newly assigned at each new location update (LUP)
 encrypted transmission
 3 algorithms specified in GSM
 A3 for authentication (“secret”, open interface)
 A5 for encryption (standardized)
 A8 for key generation (“secret”, open interface)
Shahzad Malik
Wireless Communications - GSM
“secret”:
• A3 and A8
available via the
Internet
• network providers
can use stronger
mechanisms
Lecture 7
63
GSM - authentication
SIM
mobile network
Ki
RAND
128 bit
AC
RAND
128 bit
RAND
Ki
128 bit
128 bit
A3
A3
SIM
SRES* 32 bit
MSC
SRES* =? SRES
SRES
SRES
32 bit
Ki: individual subscriber authentication key
Shahzad Malik
Wireless Communications - GSM
32 bit
SRES
SRES: signed response
Lecture 7
64
GSM - key generation and encryption
MS with SIM
mobile network (BTS)
Ki
AC
RAND
128 bit
RAND
128 bit
RAND
128 bit
A8
cipher
key
BSS
128 bit
SIM
A8
Kc
64 bit
Kc
64 bit
data
encrypted
data
A5
Shahzad Malik
Ki
Wireless Communications - GSM
SRES
data
MS
A5
Lecture 7
65
General Packet Radio
Service (GPRS)
Data services in GSM
 Data transmission standardized with only 9.6 kbit/s
 advanced coding allows 14.4 kbit/s
 not enough for Internet and multimedia applications
 HSCSD (High-Speed Circuit Switched Data)
 mainly software update
 bundling of several time-slots to get higher
AIUR (Air Interface User Rate)
(e.g., 57.6 kbit/s using 4 slots, 14.4 each)
 advantage: ready to use, constant quality, simple
 disadvantage: channels blocked for voice transmission
AIUR [kbit/s]
4.8
9.6
14.4
19.2
28.8
38.4
43.2
57.6
Shahzad Malik
TCH/F4.8
1
2
3
4
Wireless Communications - GSM
TCH/F9.6
TCH/F14.4
1
1
2
3
4
2
3
4
Lecture 7
67
GPRS - Data services in GSM
 GPRS is an overlay on top of the GSM physical layer and network
entities; extends data capabilities of GSM
 Provides connections to external packet data networks through the
GSM infrastructure with short access time to the network for
independent short packets (500-1000 bytes)
GPRS (General Packet Radio Service)
 packet switching
 using free slots only if data packets ready to send
(e.g., 50 kbit/s using 4 slots temporarily)
 standardization 1998, introduction 2001
 advantage: one step towards UMTS, more flexible
 disadvantage: more investment needed (new
hardware/software)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
68
GPRS Operations
 GPRS uses same physical radio channels, only new logical
GPRS radio channels are defined
 Active users share timeslots using TDMA; uplink and
downlink are allocated separately
 Capacity allocation in GPRS is based on the “on-demand”
principle
 GPRS terminals:
 Class A: Operates GPRS and GSM services simultaneously
 Class B: Operate either GPRS or GSM service at one time
 Class C: Only GPRS service
 Limitations:
 Limited cell capacity
Shahzad Malik
Wireless Communications - GSM
Lecture 7
69
GPRS Network Services
 Point-to-point (PTP): packet data transfer
 Connectionless based on IP
 Connection oriented based on X.25
 Point-to-multipoint (PTM-M): multicast service to all
subscriber in one area
 Point-to-multipoint (PTM-G): multicast service to a
predetermined group
 Multimedia messaging service (MMS)
 GPRS has parameters that specify a QoS based on service
precedence, priority, reliability and required transmission
characteristics
Shahzad Malik
Wireless Communications - GSM
Lecture 7
70
GPRS user data rates in kbit/s
2
slots
3
slots
4
slots
5
slots
6
slots
7
slots
8
slots
9.05
18.2
27.15
36.2
45.25
54.3
63.35
72.4
13.4
26.8
40.2
53.6
67
80.4
93.8
107.2
15.6
31.2
46.8
62.4
78
93.6
109.2
124.8
21.4
42.8
64.2
85.6
107
128.4
149.8
171.2
Coding 1 slot
scheme
CS-1
(1/2)
CS-2
(2/3)
CS-3
(3/4)
CS-4
Shahzad Malik
Wireless Communications - GSM
Lecture 7
71
Reference Architecture
 Uses GSM architecture
 GPRS support nodes (GSN): responsible for delivery and
routing of data packets between the MS and the external
network
 Serving GPRS support node (SGSN)
 Controls access to MSs that are attached to a group of BSCs
(routing area (RA) of SGSN)
 Gateway GPRS support node (GGSN)
 Logical interface to the Internet
 GPRS Register (GR)
 Colocated with HLR and stores routing information
Shahzad Malik
Wireless Communications - GSM
Lecture 7
72
GPRS architecture and interfaces
GGSN
Gn
BSS
MS
Um
SGSN
Gb
Gn
Shahzad Malik
Gi
HLR/
GR
MSC
VLR
PDN
GGSN
EIR
Wireless Communications - GSM
Lecture 7
73
GPRS protocol architecture
MS
BSS
Um
SGSN
Gb
Gn GGSN
Gi
apps.
IP/X.25
IP/X.25
SNDCP
LLC
UDP/TCP
BSSGP
IP
IP
FR
L1/L2
L1/L2
LLC
RLC
MAC
RLC
MAC
BSSGP
FR
radio
Shahzad Malik
GTP
GTP
UDP/TCP
SNDCP
radio
Wireless Communications - GSM
Lecture 7
74
GPRS – Channel
 PDCH
 Time Slots used by GPRS are called PDCH
 Radio Block
 Basic unit of transmission in PDCH
 Four TS in 4 consecutive TDMA Frames
 Multiframe
 PDCH is structured in a multiframe comprising 52 TDMA
frames
 240 ms





A multiframe comprises of 13 radio blocks
Every 13th radio block is not used, called idle burst
12 radio blocks are used for data transmission
Mean transmission time per radio block is 20ms
A radio block contains 456 bits
Shahzad Malik
Wireless Communications - GSM
Lecture 7
75
GPRS – Radio Block
Shahzad Malik
Wireless Communications - GSM
Lecture 7
76
GPRS
 GPRS Radio
 1-8 time slots of a frame can be allocated to an MS
 Uplink and downlink slots can be allocated
differently
 Achieve data asymmetry
 Dedicated PDCH (Packet Data Channel)
 Resources for PDCH are allocated dynamically by
the BSS
 Some logical channels analogous to GSM are






PDTCH: Packet Data Traffic Channel
PACCH: Packet Associated Control Channel
PRACH: Packet Random Access Channel
PAGCH: Packet Access Grant Channel
PPCH: Packet Paging Channel
PNCH: Packet Notification Channel
Shahzad Malik
Wireless Communications - GSM
Lecture 7
77
GPRS Attach
 Before accessing GPRS services, the MS must register with
the GPRS network
 MS performs an attachment procedure with an SGSN that
authenticates it by checking the GR
 The MS is allocated a temporary logical link identity (TLLI)
 A packet data protocol (PDP) context is created for the MS
for each session and is stored at the MS, SGSN, and GGSN
 PDP context: PDP type, address, QoS, GGSN address
 A user may have several PDP context enabled. The PDP
address may be statically or dynamically assigned
 PDP context is used to route packets
Shahzad Malik
Wireless Communications - GSM
Lecture 7
78
GPRS attach / PDP session
GPRS attach
Separate or combined GSM/GPRS attach
MS registers with an SGSN (authentication...)
Location update possible
PDP context is created
MS is assigned PDP (IP) address
Packet transmission can take place
GPRS detach
PDP context terminated
Allocated IP address released
Shahzad Malik
Wireless Communications - GSM
In case of
dynamic
address
allocation
DHCP
RADIUS
Lecture 7
79
PDP context
PDP context describes characteristics of GPRS session
(session = “always on” connection)
PDP context information is stored in MS, SGSN and GGSN
MS
123.12.223.9
:::
:::
PDP type (e.g. IPv4)
123.12.223.0
SGSN
GGSN
:::
:::
:::
:::
Shahzad Malik
One user may have several PDP
sessions active
PDP address = IP address of MS (e.g.
123.12.223.9)
Requested QoS (priority, delay …)
Access Point Name (GGSN address as
seen from MS)
Wireless Communications - GSM
Lecture 7
80
Packet transmission
MS
(client)
SGSN
Packet is tunneled
through IP backbone
Server
(IP, WAP..)
GGSN
Packet is sent to SGSN. SGSN sends packet to GGSN
through GTP (GPRS Tunneling Protocol) tunnel.
Tunneling = encapsulation of IP packet in GTP packet
IP address ...
IP address
IP payload
... = APN of GGSN, used for routing through tunnel
Shahzad Malik
Wireless Communications - GSM
Lecture 7
81
Packet transmission
MS
(client)
SGSN
Server
(IP, WAP..)
Source IP
address:
GGSN
GGSN
GGSN sends packet through external IP network (i.e.
Internet) to IP/WAP server.
Source IP addr.
GGSN
Shahzad Malik
Dest. IP addr.
IP payload
Server
Wireless Communications - GSM
Lecture 7
82
Packet transmission
MS
(client)
SGSN
Server
(IP, WAP..)
Dest. IP address: MS
Dest. tunnel
address: SGSN
GGSN
Dest. IP
address:
GGSN
Server sends return packet via GGSN, GTP tunnel and
SGSN to MS.
Packets from server to MS are always routed via GGSN
(since this node has PDP context information).
Shahzad Malik
Wireless Communications - GSM
Lecture 7
83
Connectivity states in GSM/GPRS
GSM
Disconnected
Idle
Connected
MS is switched off (circuit mode)
location updates on LA basis
handovers, not location updates
GPRS
Idle
Standby
Ready
Shahzad Malik
MS is switched off (packet mode)
location updates on RA basis
location updates on cell basis
Wireless Communications - GSM
Lecture 7
84
GPRS connectivity state model
No location management,
MS not reachable
Idle
GPRS attach
Standby
timer
expired
GPRS detach
Ready
Timer expired
Transmission of packet
Standby
Shahzad Malik
Location update when
MS changes cell
Wireless Communications - GSM
Location update when
MS changes routing area
Lecture 7
85
MM “areas” in GSM/GPRS
Cell
Location updating in GPRS
(ready state)
Location Area (LA)
Routing Area (RA)
Location updating in GPRS
(standby state)
Shahzad Malik
Wireless Communications - GSM
Location
updating in
GSM
Lecture 7
86
Routing Area Updates
 Route Area Update
 Route Area Update is performed with SGSN
 In case of Inter-SGSN route area update, the new SGSN retrieves
the PDP context from the old SGSN, update the HLR and the GGSN
 Intra-SGSN Update
 The SGSN already has the user profile and PDP context
 The home location register (HLR) need not be updated
 A new temporary mobile subscriber identity is issued as a part of the
RA update
 Inter-SGSN Update
 The new RA is serviced by a new SGSN
 The new SGSN requests the old SGSN to send the PDP context
 The SGSN informs the home GGSN, the GR, and other GGSNs about
the user’s new routing context
Shahzad Malik
Wireless Communications - GSM
Lecture 7
87
Handoff Management
 The MS listens to the broadcast control channel (BCCH)
and decides which cell to connect using the RSS, cell
ranking, path loss, etc.
 The location is updated using the routing update procedure
 The SGSN updates the GGSN of the home network with
the new SGSN and the tunneling information
Shahzad Malik
Wireless Communications - GSM
Lecture 7
88
Short Message Services
(SMS)
Short Message Services (SMS)
 Extremely popular service, similar to the peer-to-peer
instant messaging services in the Internet
 Allows exchange of alphanumeric messages up to 160
characters
 Two types of services:
 Broadcast
 Peer to peer
 Uses the same infrastructure as GSM
 SMS has instant delivery service as well as store-andforward service
Shahzad Malik
Wireless Communications - GSM
Lecture 7
90
Operations
 SMS makes use of the GSM infrastructure,
protocols, and the physical layer to manage the
delivery of messages
 Each message is treated individually, and is
maintained and transmitted by the SMS center
(SMSC)
 Short messages (160 char mapped into 140 bytes) are
transmitted through the GSM infrastructure using
SS-7
 Short messages are transmitted in time slots that
are freed up in the control channels
Shahzad Malik
Wireless Communications - GSM
Lecture 7
91
Reference Architecture
HLR
VLR
MS
SMSC
SMS-GMSC
SMS-IWMSC
Shahzad Malik
Wireless Communications - GSM
MSC
Lecture 7
92
Cases of Short Messages
 SM originating from an MS
 Goes to MSC for processing
 SMS-interworking MSC (SMS-IWMSC forwards the SM
to the SMSC
 Mobile terminated short message
 SM is forwarded by the SMSC to the SMS-gateway
MSC (SMS-GMSC)
 Either the HLR or VLR is queried
 SM is either delivered to the BSC or forwarded to
another MSC
Shahzad Malik
Wireless Communications - GSM
Lecture 7
93
EDGE
Enhanced Data rates for
GSM/Global Evolution (3G)
EDGE
 EDGE = Enhanced Data rates for GSM Evolution
 GSM2+ specification accepted 3G standard by
3GPP and ITU
 GSM/EDGE RAN = GERAN
 GERAN Rel’5: Common 3G core with same Iuinterfaces for multi-radio GSM/EDGE/WCDMA
RAN
Shahzad Malik
Wireless Communications - GSM
Lecture 7
95
Enhanced Data Rates for Global Evolution
(EDGE)
Provides an evolution path from existing GSM/TDMA standards to
deliver 3G services in existing spectrum bands
Reuses GSM carrier bandwidth and time slot structure
Can be introduced in GSM using a minimum of only one time slot
per BS
Reuse of existing GSM and TDMA/IS-136 infrastructure
Can be deployed using as little as 600 kHz of total bandwidth
384 Kbps data capability to satisfy the IMT-2000 requirements
for pedestrian (microcell) and low speed vehicular (macrocell)
environments
144 Kbps data capability for high speed vehicular environment
Shahzad Malik
Wireless Communications - GSM
Lecture 7
96
Enhanced Data rates for GSM Evolution
 Objective: Increase the bit rates (GPRS  EGPRS).
 Bit rates:
473 kb/s for the terminals of 100 km/h maximum.
80-130 kb/s on average.
144 kb/s for the terminals of 250 km/h maximum.
 Means:
New modulation (8-PSK).
Link adaptation.
 New mobiles, upgrade/replacement of TRXs and
capacity enhancement (Abis, …)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
97
EDGE - Enhanced Data Rate for GSM Evolution
EDGE is a global radio–based high-speed mobile data standard
that can be introduced into GSM/GPRS and IS-136 [packet mode
for digital advanced mobile phone system (D-AMPS)] networks.
EDGE allows data transmission speeds up to 384 Kbps in packetswitched mode; these throughputs are required to support
multimedia services.
This is achieved within the same GSM bandwidth and existing
800-, 900-, 1800-, and 1900-MHz frequency bands.
EDGE is last step before UMTS. EDGE is considered in Europe as
a 2.5/2.75 generation (2.5G/2.75G) standard that is seen as a
transition from 2G to 3G (second generation and third
generation of mobile networks).
Shahzad Malik
Wireless Communications - GSM
Lecture 7
98
EDGE - Enhanced Data Rate for GSM Evolution
No new operator licenses are needed for EDGE. Since this
feature reuses the existing spectrum, it represents a low-cost
solution for operators that want to provide multimedia services
on their GSM/GPRS networks.
The idea behind EDGE is to increase the data rate that can be
achieved with the 200-kHz GSM radio carrier by changing the
type of modulation used while still working with existing GSM
and GPRS network nodes.
The new modulation that is introduced is the eight-state phaseshift keying (8-PSK).
It is built on an existing GSM/GPRS system. The basic concept
constraint was to have the smallest possible impact on the
GSM/GPRS core networks.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
99
EGPRS - Enhanced GPRS
EGPRS is a direct evolution of GPRS. It reuses the same
concepts and is based on exactly the same architecture as GPRS.
The introduction of EGPRS has no impact on the GPRS core
network. The main modifications are linked to the radio
interface.
The EGPRS concept aims at enabling data transmission with
higher bit rates than GPRS.
Basically, EGPRS relies on a new modulation scheme and new
coding schemes (CSs) for the air interface, making it possible to
optimize the data throughput with respect to radio propagation
conditions.
Nine modulation and coding schemes (MCSs) are proposed for
enhanced packet data communications.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
100
EDGE - Enhanced Data Rate for GSM Evolution
On top of the same services as GPRS, EGPRS provides new ones
because of higher bit rates.
It basically offers twice the capacity of a GPRS network.
Although the bit rate of the modulation is increased by a factor
3 with the new modulation, allowing a maximum throughput that
is three times higher, the capacity of the network is not
multiplied by 3.
This is due to the carrier-to-interference ratio (C/I) variation
within the network.
Depending on the MS position, more or less channel coding will be
necessary for an optimized transmission, leading to an average
throughput lower than the maximum one.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
101
Impact of EDGE on existing GSM/GPRS
networks
 Hardware upgrade to the BSS (new transceiver in
each cell)
 Software upgrade to the BS and BSC
 No change in the core networks
 New terminals
 Terminal which provides 8PSK in the uplink and the
downlink
 Terminal which provides GMSK in the uplink and 8PSK
in the downlink
Shahzad Malik
Wireless Communications - GSM
Lecture 7
102
EDGE - Enhanced Data Rate for GSM Evolution
The basic GPRS radio concepts have not been modified.
The logical channels that have been introduced for the GPRS
system are reused for EGPRS.
Data is still transferred over PDTCH, whereas signaling is
transmitted over PACCH.
The broadcast, control, and associated signaling channels are
also exactly the same.
The coding that is used over signaling channels is CS-1.
This means that during a TBF an EGPRS mobile will transfer the
signaling block over its PACCH using CS-1 and the data will be
transferred over PDTCH using MCS-1 to MCS-9.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
103
EDGE introduction
IN Plate-form
PSTN
BTS
TRAU
Abis
...
.. .
A
< ^>
..
BSC
BTS
HLR
MSC/VLR
PCU
Gr
Gs
Gb
Gr, Gd, Gf
SGSN
Gn
GPRS
backbone
Border
Gateway
GGSN
SS7 Network
EIR
Gf
Service WAP,
plate-form WWW,
...
Gc
Internet
PDN
Router
LAN
Inter-operator
GPRS backbone
Shahzad Malik
Wireless Communications - GSM
Lecture 7
104
EDGE - Enhanced Data Rate for GSM Evolution
The MAC concept is also unchanged—mobiles can be multiplexed on
the same physical channel. Note that EGPRS and GPRS mobiles can
be multiplexed on the same PDCH.
The concepts of TBF, TFI, and RR management are the same.
The uplink multiplexing schemes such as dynamic allocation,
extended dynamic allocation, and fixed allocation are unmodified.
They can be used to multiplex GPRS and EGPRS mobiles on the
same uplink PDCH.
The signaling has been slightly changed to support dedicated
EGPRS signaling during the establishment of a TBF.
The procedures that are used for power control and TA were
retained.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
105
EDGE - Enhanced Data Rate for GSM Evolution
The RLC protocol has been slightly improved so that it provides
sufficient efficiency for the transmission of higher throughput.
The RLC Protocol is based on the same concept of sliding window.
The same mechanism of segmentation has been kept, and the
blocks are numbered with a BSN.
Depending on the radio conditions, the link is adapted in such a
way as to achieve the highest throughput.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
106
EDGE - Enhancements
 Improved GSM air-interface performance
8-PSK modulation method
New modulation & coding schemes (1-9)
Incremental Redundancy (IR)
Link Adaptation (LA) Enhancements
Shahzad Malik
Wireless Communications - GSM
Lecture 7
107
EDGE Modulation
The moderate throughput of high-speed circuit-switched data and
GPRS is linked to the GMSK modulation and its limited spectrum
efficiency.
EDGE is based on a new modulation scheme that allows a much
higher bit rate across the air interface. This modulation technique
is called eight-state phase-shift keying (8-PSK).
This modulation has an eight-state constellation allowing the
coding of 3 bits per symbol.
The raw bit rate is then three times higher than that for GMSK
modulation.
The EGPRS transmitter adapts the modulation and CSs depending
on the radio conditions; it can use GMSK or 8-PSK modulation
according to the MCS used.
The receiver is not informed of the modulation that is used by the
transmitter.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
108
EDGE Modulation Technique
Shahzad Malik
Wireless Communications - GSM
Lecture 7
109
EDGE Modulation Technique
Have the same symbol rate of GMSK
Each symbol is represented by 3 bits
8PSK generates same interference on adjacent
channels as GMSK.
We use the same channel structure and width and
frequency plan of GPRS
The distance between symbols is shorter using 8PSK
than GMSK
Shahzad Malik
misinterpretation
Wireless Communications - GSM
Lecture 7
110
EDGE Modulation Technique
The distance between two symbols in the 8-PSK constellation is
smaller than in the GMSK one for a given energy per symbol.
This characteristic increases the probability of misinterpretation
of the symbols in the receiver, owing to the noise and
interference.
If the radio conditions are good there is no problem, and a
greater data rate can be achieved by using 8-PSK.
In the case of bad conditions, the performances are degraded
with 8-PSK, and the use of GMSK may be required.
This is the reason why the two modulations are used in EGPRS,
and the adaptation of the modulation to the propagation
conditions is based on measurements performed by the MS and
BTS and controlled by the network
Shahzad Malik
Wireless Communications - GSM
Lecture 7
111
EDGE Modulation
MS has to perform blind detection of the modulation before
being able to identify which MCS has been used.
Support of the 8-PSK modulation is mandatory for the mobile in
downlink but is optional in uplink.
On the network side, 8-PSK modulation is optional in both uplink
and downlink.
Thus, a network can support EDGE without implementing 8-PSK.
In this case EDGE will not provide any gain in terms of maximum
throughput but only some enhancements for the management of
the radio link (RLC improvements).
The significance of this solution is very limited as there is no
gain in the maximum achievable throughput.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
112
EGPRS Modulation and Coding Schemes
EGPRS relies on a new modulation scheme and new coding
schemes (CSs) for the air interface, making it possible to
optimize the data throughput with respect to radio
propagation conditions.
Nine modulation and coding schemes (MCSs) are proposed
for enhanced packet data communications, providing raw RLC
data rates ranging from 8.8 Kbps (minimum value per time
slot under the worst radio propagation conditions) up to 59.2
Kbps (maximum value achievable per time slot under the best
radio propagation conditions).
Data rates above 17.6 Kbps require that 8-PSK modulation
be used on the air instead of the regular GMSK modulation.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
113
EDGE Modulation and Coding Schemes
MCS - modulation and coding schemes
Shahzad Malik
Wireless Communications - GSM
Lecture 7
114
EDGE modulations
Scheme
Modulation
Maximum
rate [kb/s]
Code Rate
Family
M CS-9
M CS-8
M CS-7
M CS-6
M CS-5
M CS-4
M CS-3
M CS-2
M CS-1
8PSK
59.2
54.4
44.8
29.6 / 27.2
22.4
17.6
14.8 / 13.6
11.2
8.8
1.0
0.92
0.76
0.49
0.37
1.0
0.80
0.66
0.53
A
A
B
A
B
C
A
B
C
Shahzad Malik
GM SK
Wireless Communications - GSM
Lecture 7
115
EDGE performance
MCS-1
MCS-2
MCS-3
MCS-4
MCS-5
MCS-6
MCS-7
MCS-8
MCS-9
Modulation 1 TS
Method
(kbps)
GMSK
8,8
GMSK
11,2
GMSK
14,8
GMSK
17,6
8-PSK
22,4
8-PSK
29,6
8-PSK
44,8
8-PSK
54,4
8-PSK
59,2
4 TS’s
(kbps)
35,2
44,8
59,2
70,4
89,6
118,4
179,2
217,6
236,8
8 TS’s
(kbps)
70,4
89,6
118,4
140,8
179,2
236,8
358,4
435,2
473,6
60
Kbps/ TS
50
40
E-GPRS
GPRS CS 1-4
GPRS CS 1-2
Average
30
3- 4 x
20
In theory EDGE
offers
• 3-4 x higher data
bit rates for endusers than GPRS
• Improved voice
capacity via
enhanced data
capabilities (+ later
AMR)
10
0
8 10 12 14 16 18 20 22 24 26 28 30
C/I
Shahzad Malik
Wireless Communications - GSM
Lecture 7
116
EDGE - Link Quality Control
One of the main improvements of EGPRS, compared with
GPRS, is in its link quality control (LQC).
The enhancement is made possible through the
introduction of a new ARQ scheme, incremental
redundancy (IR) and new estimators for the link quality
LQC uses a combination of 2 functionalities:
Incremental redundancy
 Link adaptation
Shahzad Malik
Wireless Communications - GSM
Lecture 7
117
EDGE - Link Quality Control
Incremental redundancy
 Incremental Redundancy gives additional 2-3 dB to radio link
 IR adjusts the code rate of the transmission to true channel
conditions with incremental transmissions of the redundant
information until the decoding is successful
 Utilises ARQ protocol
Link Adaptation
 Link Adaptation is used to select the best MCS for the radio
link conditions
 LA algorithms compare the estimated channel quality to
threshold values -> optimised throughput
 In EDGE LA works more effectively than in GPRS, because of
IR gives better re-transmission performance
Shahzad Malik
Wireless Communications - GSM
Lecture 7
118
EDGE LQC - Link Adaptation Mechanism
The principle of link adaptation is to adapt the modulation and CS
to the radio conditions.
When the radio conditions are poor, a MCS with a low coding rate
is chosen, leading to a lower throughput. When the radio
conditions are very good, a high coding rate is chosen, leading to
higher through-put.
During the data transfer, the network evaluates the link quality
and decides which MCS to use accordingly.
EGPRS uses a different mechanism that allows a more efficient
adaptation of the link depending on the radio conditions.
The transfer of RLC data blocks in the acknowledged RLC/MAC
mode can be controlled by a selective type I ARQ mechanism or
by a selective type II hybrid ARQ (IR) mechanism within one
TBF.
The link adaptation scheme relies on the MCS family concept.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
119
EDGE Link Adaptation
The ability of retransmission
of packets with a more robust
coding scheme
Unlike GPRS in which
retransmission of packets is
made with the same coding
scheme
Shahzad Malik
Wireless Communications - GSM
Lecture 7
120
EDGE LQC – Incremental Redundancy
IR is an enhanced ARQ mechanism that reuses information
from the previous transmissions of an RLC data block that
was badly decoded in order to increase the capability to
decode it when it is retransmitted.
It consists of combining, at the output of the receiver
demodulator, soft bits information from N different
transmissions of the same RLC blocks.
This mechanism can be associated with link adaptation in
order to provide superior radio efficiency on the air
interface.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
121
EDGE LQC – Incremental Redundancy
Mechanism
IR consists of sending n times the same block until the
block is decoded.
The soft values of the previous unsuccessful transmissions
are used.
The coding rate is decreased at each transmission,
increasing the probability of successful decoding.
IR allows the reduction of the coding rate with the
retransmission of the same block.
This mechanism is used only when the RLC Protocol
operates in acknowledged mode.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
122
EDGE LQC – Incremental Redundancy
Mechanism
Shahzad Malik
Wireless Communications - GSM
Lecture 7
123
EDGE: Transmission of RLC Data Blocks
The transfer of RLC data blocks in EGPRS mode reuses
exactly the same concepts as in GPRS.
The RLC data blocks are sent in sequence and the control
is performed thanks to a sliding window mechanism.
The RLC Protocol can operate in acknowledged or
unacknowledged mode.
When operating in RLC acknowledged mode, the
acknowledgements are contained within the PACKET
UPLINK ACK/NACK message in case of uplink transfer and
in the EGPRS PACKET DOWNLINK ACK/NACK message
for a downlink transfer.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
124
EDGE: Transmission of RLC Data Blocks
The only difference lies in the fact that two RLC data
blocks can be transmitted within one single radio block
every 20 ms.
This potentially leads to twice as many RLC data blocks
transmissions as in GPRS.
In order to cope with the higher probability that the RLC
Protocol stalls, some parts of the RLC Protocol have been
enhanced.
The first improvement concerns the RLC window size.
However, its modification has required some changes in the
acknowledgment reporting mechanism.
These changes concern the way the reporting bitmap is
handled as well as the polling mechanism.
Shahzad Malik
Wireless Communications - GSM
Lecture 7
125
EDGE Benefits
Short-term benefits: Capacity and performance
Easy implementation on a GSM/GPRS network
Cost effective
Increase the capacity and triples the data rate
of GPRS
Enabling new multimedia services
Long-term benefit: Harmonization with WCDMA
Shahzad Malik
Wireless Communications - GSM
Lecture 7
126
EDGE Evolution
 Best effort IP packet data on EDGE
 Voice over IP on EDGE circuit bearers
 Voice over IP with statistical radio resource multiplexing
 Network based intelligent resource assignment
 Smart antennas & adaptive antennas
 Downlink speeds at several Mbps based on wideband
OFDM and/or multiple virtual channels
Shahzad Malik
Wireless Communications - GSM
Lecture 7
127
Performance Enhancements for EDGE
Link Improvement:
Terminal diversity and interference suppression
Base smart antennas
Base and terminal diversity: MIMO
Transmit diversity: e.g., S-T codes
Medium Access Control:
Mode 0
Time-slot management (Dynamic Packet Assignment)
Shahzad Malik
Wireless Communications - GSM
Lecture 7
128
Mode 0
No transmission mode: Mode 0
Delay assigning resource to a user if its channel quality
is not good
Cutoff Threshold to delay transmissions
Features
Reduce unnecessary retransmissions
Control traffic load
Improve spectrum efficiency
Shahzad Malik
Wireless Communications - GSM
Lecture 7
129