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GPRS Air Interface
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At the end of the module, the participant will be ableto:
Explain the functions of the air interface in thephysical, MAC and RLC layers
Differentiate between physical and logical GPRSchannels
List and describe the GPRS air interface logicalchannels and their functions
Explain the GPRS TDMA frame, multiframe andsuperframe structure
List and compare four different coding schemes andthe puncturing concept
Describe multiple timeslot usage
Describe briefly the process of channel allocation,in the uplink and downlink
without using any references.
Objectives
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Interface between the MS and BSS
Major bottleneck in GPRS performance
MS GSM/GPRS NetworkUm
Uplink Direction
Downlink Direct ion
What is the air interface?
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Modulation and demodulation
Framing
Channel coding
Congestion control
Medium access control Synchronisation
Multiplexing
Multi-access
Timing advance
Power control
Handover Ciphering Segmentation Interleaving Puncturing Signal measurements
Do you still
rememberthese
functionsfrom GSM?
Radio Interface Tasks
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BSC RXCDR
BTSBTS
MSC/VLR
SMS-C
EIR
AA
EE CC
DD
SMS-GMSC
SMS-IWMSC
HLR
Signalling & Data Interface
Signalling Interface
GbGb
SGSN GGSN
SGSNGGSN
PDN
Other PLMN
PCU
GsGs GrGr
GpGpGnGn
GnGn GiGi
GfGf
GbGb
GdGd
GcGc
GbGb
CG
GaGa
ETSI GPRS Reference Model
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MS BSSMS BSS SGSN GGSNSGSN GGSN
RLC /RLC /
MACMAC
RLC /RLC /
MACMAC
GSM RFGSM RF GSM RFGSM RF
LLCLLC LLCLLC
NS \ FRNS \ FR NS \ FRNS \ FR
L1 bisL1 bis L1 bisL1 bis
BSSGPBSSGP BSSGPBSSGP IPIP IPIP
L2L2 L2L2
L1L1 L1L1
TCP /TCP /
UDPUDP
TCP /TCP /
UDPUDP
GTPGTP GTPGTPSNDCPSNDCPSNDCPSNDCP
IPIPIPIP
TCP /TCP /
UDPUDP
TCP /TCP /
UDPUDP
EMAILEMAIL
FTPFTP
WEBWEB
EMAILEMAIL
FTPFTP
WEBWEB
Transmission Plane
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BSS (PCU, CCU)MS
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
GSM RF
phy. link & RF
IP / X.25
LLC
Um
RLCRadio Link
Control
MACMedium Access
Control
GSM RF
phy. link & RF
LLC segmentation/ re-assemblyacknowledged/ unacknowledgedmode
Backward Error Correction BEC
Access signalling proceduresphysical channel bundlingsub-multiplexing
physical channel organisationchannel coding
GSMK
ransm ss on anes e weenand BSS
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SNDCP PDU (SN-PDU)
LLC-PDU
RLC Block
MAC Block
Network PDU (NPDU) e.g. IP-packet
SNDCP
LLC
RLC
MAC
Phys. Link
Phys. RF
Network
LLC-PDU
RLC Block
Burst Burst Burst Burst
channel coding
PDU, radio block, and bursts
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Lowest layer of GPRS protocol stack
Primary function is to provide services for information transfer
over a physical channel What is a physical channel?
Physical Layer is split into two sublayers
Physical RF layer
Physical Link sublayer
Physical Layer
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Modulation of RF signals at the transmitter
GMSK for GPRS (1 symbol per bit)
8 PSK for EGPRS (1 symbol per 3 bits)
Demodulation of RF signals at the receiver
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
LLC
Phy. Link
Phy. RF
0 1 01
Physical RF Sublayer (RFL)
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Functions:
Framing: Placement of data into bursts, frames, radioblocks, etc.
Data coding for maximising the data throughputCS1 CS4
Detection and correction of errorsdue to noise in the medium
Procedures for detecting congestionon the air interface
Procedures for synchronising MS and network
Procedures for monitoring and evaluationof radio link quality
Procedures for cell (re-)selection
Transmitter power control
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
LLC
Phy. RF
Phy. Link
tailbits
3
encrypted bits57
SB1
trainingsequenc
e26
SB1
encrypted bits57
tailbits
3
guardperiod
8,25 bits
allocated time slot
BTS
Physical Link Layer (PLL)
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Function
Uplink and downlink multiplexing of data and controlsignalling
Handling contention resolution, collision detection andrecovery for mobile originated channel access
Scheduling of access attempts, includingqueuing of packet accesses for mobileterminated channel access
Handling priority of data and controlmessages
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
LLC
Phy. RF
Phy. Link
Medium Access Layer
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Transfer of Logical Link Control layer PDUs(LLC-PDU) to the MAC layer
Segmentation and re-assembly of LLC-PDUs into RLCData Blocks
Backward Error Correction (BEC) proceduresfor selective retransmission of uncorrectable
code words in the acknowledged mode oftransmission
Transmission of code words based on channelconditions,i.e link adaptation
Storing soft values of the erroneous RLC Data Blocksand combining them with the retransmitted RLC
Data blocks
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
LLC
Phy. RF
Phy. Link
LLC-PDU
Radio Link Layer
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RLCData
BCS
SNDCP
RLCRadio Link
Control
MACMedium Access
Control
LLC
Phy. RF
Phy. Link
user data
LLC PDU
& segmentation
RLC DC Data
BCS = Block CheckSequence
RLCData
BCS
RLCData
BCS
MACHeader
radio link signalling &
control data
RLC/MAC ControlMessages
MACHeader
RLCHeader
RLCHeader
RLC/MAC GPRS blocks
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UPLINK DOWNLINK
GSM900: 890 MHz - 915 MHz 935 MHz - 960 MHz
GSM1800: 1710 MHz - 1785 MHz 1805 MHz - 1880 MHz
1 2 3 ...
Channel 1 - 124 1 - 374
200 kHz
1 2 3 ...
Duplex frequency 45 MHz / 95 MHz
guard band
GSM Radio Interface Organisation: FDD and FDMA
GSM R di I f O i i TDMA
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TDMA frame= 8 timeslots
01
23
45
7
6
01
23
4 5
76
01
23 4
5
200 kHz
Physcial channel,e.g. allocated to one
subscriber with FR voice &no frequency hopping
frequency
time
TDMA frame
GSM Radio Interface Organisation: TDMA
Wh t i b t?
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Smallest unit of transmission in GSM and GPRS
Normal Burst
Frequency Correction Burst Synchronizing Burst
Access Burst
Dummy Burst
What is a burst?
L i l Ch l i GSM
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BCCH
FCCH Frequency correction
Signallingand Control
Traffic
CCCH
DCCH
SCH Frame synchronisation + BSIC
PCH Paging mobiles
RACH Requesting dedicated channel
AGCH Allocating dedicated/traffic CH
Broadcast of cell information,e.g. channel combination
SDCCHSignalling between MS and BTSe.g. Authentication, SMS, LUP
SACCH Measurements, TA, PC, ...
FACCHExtra signalling within26 TDMA Multiframe
TCH/F full rate traffic channel
TCH/H half rate traffic channel
BCH
DL
UP
DL
DL
DL & UP
DL & UP
Logicalchannels
are usedtotransmita welldefinedcontent
Logical Channels in GSM
M i /F i
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The logical channel informationmust be transmitted on a
physical channels
Multiframes specify, at whichposition within a physicalchannel a specific logical
channel information is
transmitted
TDMA Frame
26 TDMAFrame
e.g.used for
GSMspeech
TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 7TS 6
TCH
TCH
SACCH
idle
Re-memb
erGSM
Mapping/Framing
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Radio block
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Radio
Block
TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6
TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6
TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6
TS0 TS7TS1 TS2 TS3 TS4 TS5 TS6
Frame 0
Frame 1
Frame 2
Frame 3
Radio block
What is a Packet Data Channel (PDCH)?
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Physical channel dedicated to packet data traffic is called a PDCH(Packet Data Channel)
PDCH can use spare traffic channels (TCH) in a cell Each PDCH can be shared by multiple MS and network
Each PDCH may have a number of logical channels
PDCH carry GPRS data and control signalling PDCH classified into (details later)
PCCCH (Packet Common Control Channels) PBCCH (Packet Broadcast Control Channels)
PDTCH (Packet Data Traffic Channels)
What is a Packet Data Channel (PDCH)?
Additional logical channels with GPRS
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PTCCH/D
PTCCH/U
PBCCH
Signalling
and Control
Packet
Traffic Channel
PCCCH
PPCH
PRACH MS initiates uplink transfer
PAGCH Resource assignment to an MS
PNCH Notifying PtM Packet Transfer
Broadcast of packet data
specific information
PDTCH Packet Data Transfer; (multislot)
PACCH
DL
UP
DL
DL
DL
DL & UP
PTCH
Signalling: resource allocation,
acknowledgements, PC, TA, etc.
Paging MSs for packet data
and circuit switched services
Used by MS to send randomburst to BSS for timing advance
Used to send timing advance
Information to MSs of one PDCH
UP & DL
UL
DL
PDCCH
Based
onGSMRel.99
Additional logical channels with GPRS
GPRS channels in a cell
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A GPRS cell uses one or more TCH available in a cell as PDCH
The allocation of TCH for PDCH is done dynamically according tothe demand of GPRS traffic
Common control signalling on GPRS at initial phase of packettransfer can be either on
Dedicated Channel (PCCCH):
All GPRS attached MSs camp on the PCCCH
PCCCH are permanently or dynamically allocated when demandrises
If the network releases PCCCH then MSs move to CCCH
GSM CCCH (when PCCCH is not allocated)
All GPRS attached MSs camp on the CCCH as they do in Idle state
Existence of PDCH does not imply existence of PCCCH
GPRS channels in a cell
Broadcast channels
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GSM
FCCH Frequency Correction ChannelSCH Synchronisation Channel
BCCH Broadcast Control Channel
GPRSPBCCH Packet Broadcast Control Channel
Broadcasts packet data specific System Information messages
MS continuously monitors this
GSM BCCH can also be used
Broadcast channels
Common Control channels
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GSMPCH (Paging Channel) RACH (Random Access Channel) AGCH (Access Grant Channel)
GPRS PPCH (Packet Paging Channel)
Can be used for paging both CS & PS services GSM PCH can also be used
PRACH (Packet Random Access Channel)Used for uplink channel reservation & to obtain
TA GSM RACH can also be used
PAGCH (Packet Access Grant Channel)Used for resource assignment during packettransfer establishment phase
GSM AGCH can also be used
PNCH (Packet Notification Channel)Downlink only channel used for PTM-M
notifications to a group of MS before PTM-Mpacket transfer
Only in GPRS Phase 2
Common Control channels
Dedicated channels
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GSM
SDCCH
(Standalone DedicatedControl Channel)
SACCH(Slow AssociatedControl Channel)
FACCH(Fast Associated
Control Channel) TCH (EFR/FR/HR
Traffic Channel)
GPRS
PACCH (Packet Associated Control Channel) Bi-directional dedicated channel for transferring
ack./power control or resource assignment/re-assignment messages
PDTCH (Packet Data Traffic Channel)
Bi-directional
Corresponds to the resource allocated to a single MS
on one physical channel for user data transmission
PTCCH (Packet Timing advance Control Ch.)
Uplink dedicated (for transmission of random accessbursts)
Downlink common (for transmission of timingadvance information to several MSs).
Dedicated channels
GPRS: 52 TDMA Frame
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TS 0 TS 1 TS 2 TS 3 (Frame 0) TS 4 TS 5 TS 6 TS 7
Radio Block 0
PTCCH
Radio Block 3
Radio Block 4
Radio Block 5
PTCCH
Radio Block 9
Radio Block 10
Radio Block 11
Radio Block 1
Radio Block 2
IDLE
Radio Block 6
Radio Block 7
Radio Block 8
IDLE TS 3 (Frame 51)
TDMA Frame
1 Radio Block= 4 Frames= 456 info. bits
PDCH multiframe= 52 TRMA Frames
GPRS: 52-TDMA-Frame
GPRS: 52-TDMA-Frame for PDCHs
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B0 B1 B2 T B3 B4 B5 i B6 B7 B8 T B9 B10 B11 i
Radio Block
= 4 TS in consecutive
TDMA frames
idle frame
= 1 frame
52 TDMA Frame = PDCH Multiframe
Uplink on one PDCH:
Multiplexing of
PDTCH & PACCH, or PDTCH, PACCH & PRACH
Downlink on one PDCH:
Multiplexing of
PDTCH, PACCH PDTCH, PACCH & PCCCH, incl.
PBCCH (indicated by BCCH) PDTCH, PACCH and PCCCH
(indicated by (P)BCCH)
PTCCH
GPRS: 52 TDMA Frame for PDCHs
GPRS multiframe structure
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GSM uses a 51 & 26-multiframe structure
GPRS uses a 51 & 52-multiframe structure
Important concepts: Timeslots (TS), in which 114 bit bursts are sent (duration)
Frames = Eight timeslots (8 * 114 bits)
Multiframe = 52 frames (52 * 8 * 114 bits)
Radio block is 4 timeslots (or bursts) from 4 consecutiveframes in the same carrier and in the same TS (456 bits)
Each multiframe has 12 blocks + 2 idle frames + 2 PTCCHframes
12 *4 + 2 +2 = 52 frames
Each uplink TS can be used by several MSs for data.
Need for a mechanism to identify
GPRS multiframe structure
GPRS multislot capabilities
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GPRS multislot capabilities
1-slot
2-slot
3-slot
0 1 2 3 4 5 6 7 0 1
0 2 3 4 5 65 6 7 1
0 1 2 3 4 5 6 7 0 1
0 2 3 4 5 65 6 7 1
0 1 2 3 4 5 6 7 0 1
0 2 3 4 5 65 6 7 1
Downlink
Uplink
Monitor
Downlink
Uplink
Monitor
Downlink
Uplink
Monitor
Channel coding
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GPRS: CS 1-4. (Trade-off of throughput vs. error performance)
EGPRS: MCS 1-9
CS-1 CS-2 CS-3 CS-4
Increasing data throughput rates
Increasing protection against errors
Channel coding
Block diagram
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RLC Data Block+ MAC header
Convolutional Code
In: 228bitsOut: 456 bits
Cyclic Coding +
Tail
16 + 4 bits
Fire Code + Tail
In: 184 bits
Out: 228 bits
Reordering,
Partioning,
Adding StealingFlags
Interleaving
CS-1
CS-2, 3, 4 Convolutional Code
And Puncturing
In: x bits
Out: 456 bits
CS-4
Information bits Interleaved bits
Block diagram
Stages in processing
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Rate 1/2 Convolution Coding Stage
Puncturing Stage
456 bits
USF BCS
40/16bits
MAC Header
Tail
(4 bits)
Precoded USF
3/6/12 bits
USF
(3 bits)
MAC
(5 bits)
RLC Data/Control Block
(176/288/307 bits)
Cyclic or Fire Coding
Stages in processing
Summary of coding schemes
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Scheme Code
rate
USF Pre-
coded
USF
Radio Block
excl. USF
and BCS
BCS Tail Coded bits Punctured
bits
Data rate
kb/s
CS-1 1/2 3 3 181 40 4 456=2*(3+181+40+4) 0 9.05
CS-2
2/3 3 6 268 16 4 588=2*(6+268+16+4) 132 13.4
CS-3
3/4 3 6 312 16 4 676=2*(6+312+16+4) 220 15.6
CS-4 1 3 12 428 16 - 456=428+12+16 0 21.4
y g
Coding schemes and multiple timeslots
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Increased data
transmission ratesvia new
Coding Schemes
CS-19.05kbps
CS-213.4
kbps
CS-315.6kbps
CS-4
21.4kbps
Bundlingof up to8 physical channels
Net transmission rate up to171.2kbps
g p
Coding schemes and multiple timeslots
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Increased data
transmission ratesvia new
Coding Schemes
CS-19.05kbps
CS-213.4
kbps
CS-315.6kbps
CS-4
21.4kbps
Bundlingof up to8 physical channels
Net transmission rate up to171.2kbps
g p
Coding schemes and multiple timeslots
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Increased data
transmission ratesvia new
Coding Schemes
CS-19.05kbps
CS-213.4
kbps
CS-315.6kbps
CS-4
21.4kbps
Bundlingof up to8 physical channels
Net transmission rate up to171.2kbps
g p
Coding schemes and multiple timeslots
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Channel CodingScheme
CS-1 CS-2 CS-3 CS-4
Single TSL Data
Rate
9.05 kbit/s 13.4 kbit/s 15.6 kbit/s 21.4 kbit/s
3-TSL Data Rate 27.15 kbit/s 40.2 kbit/s 46.8 kbit/s 64.2 kbit/s
8-TSL Data Rate 72.0 kbit/s 107.2 kbit/s 124.8 kbit/s 171.2kbit/s
g p
Simulated throughput of user data
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0
2
4
6
810
12
14
16
0 5 10 15 20 25
C/I
Kbit/s
CS-1
CS-2
CS-3
CS-4
0
10
20
30
40
50
0 5 10 15 20 25
C/I
Kbit/s
CS-1
CS-2
CS-3
CS-
Minimum Average
Typical NW C/I
Min imum Average
Typical NW C/I
1 Tim eslot 3 Tim eslots
g
Radio block allocation to MS
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How does a MS know which PDCH and which radio block on that PDCH it may use?
Assigning radio blocks of a PDCH to different MSs is done by MAC layer
Allocation may be static or dynamic
This process is referred to as content arbitration
Content arbitration occurs only in the uplink direction. Why?
An Uplink Status Flag (USF) is used for content arbitration
USF is transmitted downlink and it tells which MS may use a radio block
In initial PAGCH (AGCH), MS is assigned a USF value for each PDCH
MS monitors USF values in downlink transmission on assigned PDCH MS may transmit in radio blocks that have the same USF value as was allocated to it
in the PAGCH message
USF has 3 bits at the beginning of each radio block on the downlink
So one PDCH can be used by up to 8 MSs at one time (or up to 7 MSs plus 1 PRACH)
Usage of Uplink State Flag (USF)
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USF=
1
USF=
3
USF=
2
USF=1:B0- B4
USF=2:B5- B9
B0 B
1B2
B3
B4
B5
B6
B7
B8B9B1
0B1
1
B9
B8B7
B6
B5
B3
B2
B1
B0
B4
Radio block identification
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Downward multiplexing of radio blocks is done using Temporary Flow Identifier (TFI)
What is TFI? TFI is assigned in a resource assignment message prior to transmission LLC layer frames
between MS and BSS TFI is unique among concurrent processes
TFI is preferable to MS identity which is a very long number
TFI is included in every RLC frame header
Help!Whichonesare
mine?
Radio blocks, but
several subscribers served on on physical channel
up to 8 channels can be used by one phone
Radio resource management
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Mobile originated packet transfer
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MS Network
Packet Channel Request
Packet Immediate Assignment
Packet Resource Request
Packet Resource Assignment
PRACH or RACH
PAGCH or AGCH
PACCH
PACCH
(Optional)
(Optional)
Access & allocation
Mobile terminated packet transfer
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MS Network
Packet Resource AssignmentPACCH orPAGCH orAGCH
Downlink PDTCH assignment in Ready state
Mobile terminated packet transfer
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With resource reassignment
Data Block
temporary Packet Ack/Nack
Access and Assignment
MS Network
PDTCH
PACCH
Packet Resource Reassignment
Packet Resource Reassignment Ack
PACCH
PACCH
PACCHfinal Packet Ack/Nack
Data BlockPDTCH
Data BlockPDTCH
Data Block (polling)PDTCH
Data BlockPDTCH
Data BlockPDTCH
Data BlockPDTCH
Data BlockPDTCH
Data BlockPDTCH
Data Block (last, polling)PACCH
Modulation
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MODULATORMODULATOR
Digital databits/sec
Modulated
datasymbols/sec
PSK modulation scheme
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Digital bits Symbol Phase
(1,1,1) 0 0
(0,1,1) 1 /4
(0,1,0) 2 /2
(0,0,0) 3 3/4
(0,0,1) 4
(1,0,1) 5 -3/4
(1,0,0) 6 -/2
(1,1,0) 7 -/4
(0,0,1)
(1,0,1)
(0,0,0)(0,1,0)
(0,1,1)
(1,1,1)
(1,1,0)
(1,0,0)
I
Q
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Review Questions to
GPRS Air Interface
Review
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1. Which fields are used for medium access control and multiplexingmultiple users on the uplink and downlink PDCH?
2. How many users can share the same Packet Data Channel (PDCH)timeslot in the uplink direction?
For medium access control, the USF field is used
for uplink. There is no need for MAC in the
downlink direction since there is only one
source in that direction.
The TFI is used to identify the data flow in theuplink and downlink direction.
The USF field has 3 bits; consequently, up to 8 subscriberscan share one PDCH physical channel. 111 can be
reserved for PRACH, and then only up seven users can
share a PDCH with USF, using the values 000 to 110.
Review
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3. How many frames, radio blocks, and bursts are there in aPDCH multiframe?
4. What is the purpose of PTCCH?
One PDCH multiframe that is transmitted in a GPRS
timeslot consists of 52 TDMA frames organised into 12
radio blocks of 4 bursts each, 2 idle TDMA frames and 2
PTCCH TDMA frames.
UL: for transmission of RACH bursts to estimate timing
advance.
(up to 16 MSs)
DL: for transmitting timing advance information for up to
16 MSs.
Review
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5. Which layer is responsible for segmentation and reassembly of LLCPDUs and Backward Error Correction (BEC) procedures?
6. Which coding scheme has adopted the same coding as used forSDCCH?
7. Which layer uses the functionality of USF?
8. Which coding scheme does not use FEC?
9. Which logical channels can be used for resource assignment?
RLC layer (Radio Link Control)
CS-1 (9.05 kb/s)
MAC layer (Medium Access Control)
CS-4 (21.4 kbps, but no redundancy added)
AGCH (PAGCH), PACCH