Transcript Packt tx in UMTS

Outline
•
•
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•
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UMTS architecture and main features (FDD mode)
Discussion of packet performance issues
Present concepts for support of packet-switched services
UMTS evolution
Conclusions
Rev A
30-November-2001
1
On the Performance of UMTS Packet Data Services
Wolfgang Granzow
Ericsson Research Corporate Unit
Ericsson Eurolab Deutschland, Nürnberg
([email protected])
Rev A
30-November-2001
2
UMTS – enabler for the mobile Internet
• Mobile Internet: the unification of Cellular Networks and the
Internet, enabling use of Internet services when mobile
• Examples of packet data services
– Conventional internet services
• Web-browsing, electronic mail, file transfer, video/audio streaming,
e-commerce, ...
– Combination with location information and mobility
• Location based services, navigation
Rev A
30-November-2001
3
UMTS network architecture
MSC
GSN
RNC
Node B
Mobile Services Switching Center
GPRS Support Node
MSC/GSN
Radio Network controller
Base Node
RNC
RNC
Radio network
System (RNS)
Node B
Node B
Node B
Node B
Node B
Node B
Node B
Rev A
30-November-2001
4
UMTS – Main Features
• New radio access technology using new spectrum
– spectrum allocation around 2 GHz
– two radio transmission modes
• Frequency Division Duplex (FDD): 2  60 MHz
• Time Division Duplex (TDD): 15 + 20 MHz
– Wideband Code Division Multiple Access (WCDMA)
– Chip rate 3.84 Mcps  Channel bandwidth 4.4 – 5 MHz
• Built on GSM Core Network technology
• Support of user data rates 0 – 2 Mbps
• Multi-call, multimedia capability
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Radio interface architecture (simplified)
CTRL
USER
USER
RRC
CTRL
RRC
Signaling
Radio Bearers
Radio Bearers
RLC
PDCP
PDCP
RLC
RLC
RLC
Logical Channels
Control channels
Traffic channels
MAC
MAC
Transport Channels
Common
Dedicated
Shared
PHY
PHY
Physical Channels
UE
Rev A
30-November-2001
UTRAN
6
UMTS Protocol Architecture (user plane)
UTRAN
Packet switched Core Network
IP
server
UE
Application
Applic.
GGSN
TCP
Radio Access Bearers
IP
IP
TCP
IP
IP
SGSN
RNC
Radio Bearers
PDCP
RLC
Logical channels
MAC
Transport channels
PHY
PDCP
Iu UP
Iu UP
RLC
GTP-U
GTP-U
MAC
UDP
UDP
Node B
PHY
FP
FP
PHY
AAL2/
ATM
AAL2/
ATM
IP
IP
AAL5/
ATM
AAL5/
ATM
GTP-U
GTP-U
GPRS
IP backbone
UDP/
TCP
IP routing
IP
IP
IP
UDP/
TCP
IP
Physical channels
Uu
Rev A
Iub
Gn
Iu
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Gn/Gp
Gi
Data flow for packet data (UE side)
40 bytes
typ. 512 bytes (MSS = 1460 bytes)
TCP/IP
header
TCP/IP
2 or 3
bytes
L2 PDCP
PDCP
header
PDCP
header
PDCP PDU
L2 MAC
…
typ. 40
bytes
2–4
bytes
0 or 3
bytes
MAC
header
…
RLC
header
…
MAC SDU
L1
CRC
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RLC
header
MAC
header
…
Transport block (MAC PDU)
Rev A
PDCP PDU
RLC SDU
RLC SDU
L2 RLC
…
TCP/IP
header
payload (application data)
…
MAC SDU
Transport block (MAC PDU)
2 bytes
CRC
Transmission Format UTRA FDD
1 radio frame (10 ms), 15*2560 chips (3.84 Mcps)
Slot 1
Slot 2
Slot i
Uplink
frequency
Microcell
layer
5 MHz
5 MHz
5 MHz
Duplex distance, e.g. 190 MHz
• bit level QPSK (downlink) or dual-channel BPSK (uplink)
• modulation rates 15 ... 960 Ksps for spreading factors 256 ... 4
Rev A
time
Downlink
Macrocell layers
5 MHz
Slot 15
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Principal Mobile Station Transceiver Structure
Physical Layer Processing
Encoding
Interleaving
Rate matching
Multiplexing
Spreading &
Baseband
Modulation
Power control commands
Higer
layers
Code
generator
Decoding
Deterleaving
Demux
Transport
channels
Rev A
D/A
Up
conversion &
Power
Amplification
Freq.
Synthes.
Time
Sync.
Despreading &
Baseband
Demodulation
A/D
Physical
channels
30-November-2001
Power setting
10
Duplexer
Down
conversion
Coding, Interleaving, Rate Matching, Multiplexing
TFI1
TFI2
TFCI
TFCI
Coding
..
.
TFIN
TrCh1
CRC
attachment
TrCh2
..
.
TrChN
Coding
Inter-frame
interleaving
CRC
attachment
Coding
..
.
..
.
Inter-frame
interleaving
CRC
attachment
Coding
..
.
Inter-frame
interleaving
TF: Transport Format
TrCh: Transport Channel
TFCI: Transport Format Combination Indicator
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Rate
Matching
(repetition
and
puncturing)
Multiplexing
Intra-frame
interleaving
Principle of spectral spreading
Spreading code
Tchip
Tsymbol
Data
Baseband
modulation
Pulse
shaping
Sample-and
-hold
Frequency
Frequency
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General design objectives for packet services
• High spectral efficiency, i.e. low Eb/N0 for desired error rate (low
overhead, efficient radio link adaptation, diversity, ...)
• Low delay (interaction between TCP and RLC)
• High throughput (system and users)
• Simplicity and effectiveness of radio resource management (including
QoS management)
• Efficient usage of channelization codes on the downlink
• Efficient usage of BTS transmitter power
• Efficient usage of hardware resources (especially in the Node B)
• Low terminal power consumption
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Performance measures
• Link performance (BER/BLER vs. Eb/N0)
– Advanced coding (turbo)
– High degree of diversity (multipath, Rx antenna)
– Optional enhancements: interference cancellation, adaptive antennas, SpaceTime Transmit Diversity (STTD)
10
0
target BLERreq = 10 % 10-1
10
BLER
Example:
performance of a 144 kbps DCH
in vehicular environment (120 km/h)
with turbo coding
10
-2
-3
-4
10
-5
10
0
0.5
1
1.5
2
2.5
3
(Eb/N0)req
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3.5
4
4.5
Eb/N0
5
Packet data throughput definitions
slope: average throughput
wrt. single packet („packet bit rate“)
data volume
(e.g. # bytes)
slope:
average link
throughput Rlinktrp
e.g.
retransmissions
data arrival
in tx buffer
inititial
setup
delay
time
Variable-rate tx
on the radio link, slope: Rlink
average user throughput =
„total amount of served data“
„time to deliver data“
(average packet bit rate
for one user)
average link throughput Rlinktrp = Rlink / Ntransm = Rlink  (1 – BLER)
number of transmissions/block: Ntransm = 1/(1 – BLER)
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Performance measures
• System throughput (capacity):
– Average throughput of all users in the system
– Maximum system throughput is reached when the packet delay can grow
unbounded
– Capacity definitions:
• average number of users, or
• system throughput
when user quality drops to an unacceptable level („outage“)
– Outage can be defined in terms of a delay and/or throughput threshold
that should be met with a certain probability
– Capacity defined as system throughput is less sensitive to the traffic load
generated per user
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UTRA transport channels categories
• Common channels
– Multiplexed users (user ID in the MAC header)
• Forward Access Channel (FACH)
• Random Access Channel (RACH)
• Common Packet Channel (CPCH)
• Dedicated channels (DCH)
– Assigned to a single user (identified by the spreading code)
• Shared channels
– „Sharing“ of code resource by several users by fast re-assignment scheduling
• Downlink Shared Channel (DSCH)
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Dedicated Channel (FDD downlink)
10 ms
1-rate
1/2-rate
fixed
spreading
factor
0-rate
Variable
rate
R=1
R=0
R = 1/2
R=1
: user data (Dedicated Physical Data Channel, DPDCH)
: physical control info (Dedicated Physical Control Channel, DPCCH)
(Pilot+TPC+TFCI)
DL DCH Features:
Rev A
•
•
•
Fast closed-loop power control
Macro diversitity
potential blocking due to insufficient spreading codes
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Dedicated Channel (FDD uplink)
10 ms
1-rate
1/2-rate
variable
spreading
factor on
DPDCH
1/4-rate
0-rate
Variable
rate
R=1
R = 1/2
R=0
R=0
R = 1/2
: DPDCH (Data)
: DPCCH (Pilot+TFCI+FBI+TPC), fixed spreading factor 256
UL DCH Features:
Rev A
•
•
Fast closed-loop power control
Macro diversitity
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DCH characteristics
• Lowest delay among all transport channels
• Large overhead in Eb/N0 at low data duty cycle due to physical control :
12
11
10
[dB]
overhead
DPCCH
DPCCH
overhead (dB)
Example:
UL RDPCCH = 15 kbps,
independent of RDPDCH (w = 1)
( Eb / N 0 ) total
RDPCCH

 1 w
( Eb / N 0 ) DPDCH
Rs
data
( E / N 0 ) DPCCH
w s
( E s / N 0 ) DPDCH
Rev A
9
8
7
6
5
4
3
2
1
0
0
20
40
60
80
100
120
mean modulation rate
140
160
mean modulator data rate [kbits]
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20
180
200
Impact of overhead on capacity
400
Example:
K max 
350
M G
F  ( E s / N 0 ) req α  w  R DPCCH / R data 
300
no overhead
capacity,
Kmax
Capacity,
K
(Es/N0)req = 3 dB
M = 0.6 (load margin)
F = 1.5 (ratio of inter-cell to intra-cell interference)
RDPCCH = 15 kbps
Rdata = RDPDCH(UL) =120 kbps
max
250
200
min overhead
150
100
full overhead
50
0
-2
10
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-1
10
activity
activityfactor,
factor, 

0
10
Random Access Channel (FDD uplink)
Message
38400 chips (10 ms) or 20 ms
Preamble
4096 chips
“Access slot”
5120 chips
DPDCH
DPCCH
Physical
Random Access
Channel (PRACH)
-Uplink -
Timing
offset
Acquisition Indicator
Channel (AICH)
- Downlink Acquisition Indicator (AI)
4096 chips
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RACH characteristics
• Slotted-ALOHA type of contention-based channel
• No power control during message transmission
– reasonable initial message power derived via preamble ramping
• Access delay due to ramping 5 – 50 ms (mean  15 ms)
• Increased delay in case of message collisions
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RACH throughput performance
• Example (derived with a specific choice of parameter settings, throughput S and
offered load G normalized to 1 message per access slot):
6
S=G
S = G eG/16 (K = 16 signatures),
no interference
5
Throughput
S
4
simulation for 10 ms messages, SF = 128, K = 16
3
2
no load control
S = G eG (K = 1)
1
0
0
5
10
15
20
25
30
35
40
Offered Traffic G
max normalized throughput S = 3.3 corresponds to 2475 messages/s
(80 user data bits/message, 198 kbps)
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45
50
Forward Access Channel (FDD downlink)
•
•
•
•
Several FACHs with different transport format can be multiplexed on the physical layer
Mapped to Secondary Common Control Physical Channel (S-CCPCH)
No fast power control, no macro diversity (transmitted at broadcast power level, i.e. on
average rather high energy per data bit Eb spent)
Scheduling delay
S-CCPCH
physical control
Rev A
data of other users
data of desired users
TTI 2
TTI 1
TTI 1
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TTI 3
Downlink Shared Channel (DSCH)
•
•
Format indicator (TFCI) on associated DPCH includes assignment of PDSCH
spreading code
Jointly power controlled with the associated DCH
10 ms
Data for the
considered user
Data for
other users
PDSCH 2
Data for the
considered user
PDSCH 1
DL-DPCH
10 ms
DPCCHs
UL-DPCH
Rev A
30-November-2001
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DPCCHs
DSCH characteristics
• No macrodiversity
– mostly suitable in the inner cell area
– then approximately same spectral efficiency as a DCH with the same rate
• Avoids capacity limitation due to non-availability of codes
• Scheduling delay
– Aimed to be compensated by higher link data rate
SF = 256
SF = 128
SF = 64
SF = 32
SF = 16
SF = 8
SF = 4
SF = 2
SF = 1
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30-November-2001
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OVSF codes allocated
to PDSCHs (example)
UE modes and RRC States („activity levels“)
UTRAN Connected Mode
–
–
UE listens to PCH in DRX mode
location known on URA level
–
–
DPCH allocated
location known on cell level
Dedicated (DCH) or Shared
(DSCH) Transport Ch. can
be used
URA_PCH
CELL_PCH
CELL_DCH
Release RRC
Connection
CELL_FACH
Establish RRC Release RRC
Connection
Connection
Idle Mode
Rev A
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–
–
–
–
–
UE listens to PCH in DRX mode
location known on cell level
UE continuously monitors FACH on downlink
RACH (and/or CPCH) can be used anytime
location known on cell level
Establish RRC
Connection
–
–
UE not registered to the network
Cell broadcast info can be received
UTRAN
Service example:
e.g. email download, WAP browsing
Cell_FACH:
UE
S-CCPCH
data of other users
data of desired users
TTI 2
TTI 1
TTI 1
PRACH
DPDCH
DPCCH
TTI 3
physical control
DPDCH
DPCCH
AICH
TCP acks
RLC acks
RRC measurement reports
Rev A
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4.5 - 6.5 kbps
average load
UTRAN
Service example:
ftp or email download, Web browsing
Cell_DCH:
UE
DL-DPCH
TTI 2
TTI 1
UL-DPCH
DPCCH
Rev A
DPCCH
DPCCH
DPDCH
DPCCH
DPCCH
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DPCCH
DPCCH
DPCCH
DPCCH
DPDCH
DPCCH
UTRAN
Service example:
ftp or email upload
Cell_DCH:
UE
DL-DPCH
UL-DPCH
DPDCH
DPCCH
Rev A
DPDCH
DPCCH
DPDCH
DPDCH
DPDCH
DPDCH
DPCCH
DPCCH
DPCCH
DPCCH
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DPCCH
DPCCH
DPDCH
DPCCH
DPDCH
DPCCH
UTRAN
Service example:
e.g. file download, Web browsing
Cell_DCH:
UE
Data for the
considered
user
Data for
other users
PDSCH 2
Data for the
considered
user
10 ms
PDSCH 1
DL-DPCH
10 ms
DPCCHs
UL-DPCH
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DPCCHs
Channel Switching
• Dynamic switching between common and dedicated channels, i.e.
common channel RRC states (Cell_FACH and Cell_PCH) and dedicated
channel RRC state (Cell_DCH)
• provides radio link adaptation to different levels of data transmission
activity, in order to achieve at low transmission activity following
objectives:
• efficient utilisation of BTS TRX hardware resources dedicated to each cell
• high utilisation of the downlink channelization codes available in a cell
• low terminal power consumption
• reasonable high spectral efficiency and reasonable delay compared to dedicated
channel transmissions
• channel type switching is a special type of intra-cell handover
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Up-switching (Cell_FACH  Cell_DCH)
time
switching
decision (RRC/SRNC)
Power
PDSCH
assigned to
the given user
DPCCH
downlink PDSCH
and DPCH
DPCH
switching
command
SCCPCH
measurement
report
PRACH
ramping
confirm
uplink DPCH
Cell_FACH
Rev A
transport and DPCH synchr.
radio interface delay
delay
30-November-2001
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Cell_DCH (DCH + DSCH)
Down-switching (Cell_DCH  Cell_FACH  Cell_PCH)
PDSCH
assigned to
the given user
Power
downlink
PDSCH
and DPCH
time
switching
decision
(RRC/SRNC)
DPCCH
switching
command
Paging indicator
On PICH
RLC ack
SCCPCH
confirm
PRACH
measurement
report
ramping
RLC ack
uplink
DPCH
Cell_DCH (DCH + DSCH)
Rev A
inactivity
interval
transport and
radio interface
delay
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Cell_FACH
Cell_PCH
Throughput illustration for Web page download
600
completed
generated
arrived at RLC
delivered
Downlink bits [kbit]
Downlink Traffic Volume [kbits]
500
400
50 kBytes data packet
300
200
slope: Rlinktrp = Rlink (1-BLER)
Rlinkmax = 64 kbps
100
0
Rev A
1
2
3
30-November-2001
4
36
6
5
Time [s]
7
8
9
10
System throughput vs. user throughput
50
10% best packets
median
10% worst packets
Example
configuration:
64 kbps DCH,
30 codes
available per cell
with SF= 32
(kbps/cell)
ratethroughput
bit user
packet
average
(kbps/cell)
45
20
30 35
40
50
35
30
25
20
30
35
20
45
15
20
10
0
50
30
users per cell
35
45
5
Note:
45
50
0
100
200
300
400
500
system throughput (kbps/cell)
600
700
This example shall just illustrate the principal characteristics of throughput
performance which were obtained for some specific assumptions not discussed here; absolute
capacity figures depend strongly on the choice of simulation parameters and assumptions.
Rev A
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Summary on packet data performance
• Results from system-level simulations:
– Data on dedicated channels
• throughput very much dependent on configuration and traffic characteristics,
• can be very low if only a few channels for very high rate are configured due to
code limitation effects
– Data on common channels
• inefficient for large amount of data due to lack of tight power control
(especially FACH)
– Data on shared channels
• can reach approx. same system throughput as a configuration with low-rate
dedicated channels, at much higher peak data rate per user
• scheduling policy affects performance („fairness“ reduces system throughput)
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UMTS Evolution (Release 4 and 5)
• Main future new features (affecting packet services):
– All-IP transport in the Radio Access and Core Networks
– Enhancements of services and service management
– High-speed Downlink Packet Access (HSDPA)
• Introduces additional downlink channels:
– High-Speed Downlink Shared Channel (HS-DSCH)
– Shared Control Channels for HS-DSCH
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HS-DSCH characteristics
•
Provision of 8 –10 Mbps peak user data rate by
– Fast selection of modulation and coding scheme depending on channel conditions
(no fast power control)
– Short transmission time interval (2 ms)
– Fast hybrid ARQ (incremental redundancy and/or Chase combining)
– Fast scheduling
– Fast cell selection/handover
dedicated
channels
dedicated
channels
Rev A
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HS-DSCH physical layer processing chain
Adaptation Algorithm
Ch. Code #1
Mapping on
Code-Tree
Turbo
Encoding
CRC
Puncturing and
Repetition
gain
Modulation
Interleaving
Ch. Code #N
Coding Rate:
1/3 - 1
Rev A
scrambling
QPSK
16QAM
(64QAM opt.)
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Example:
12 out of 16 codes
with SF=16
other
channels
Adaptive Modulation and Coding
Modulation and
Coding Schemes
(Example)
C/I
64QAM,
64QAM,
16QAM,
16QAM,
QPSK,
QPSK,
C/I
time
Rev A
1
30-November-2001
0.1
42
0.01 FER
R=0.75 (12.96 Mbps)
R=0.50 (8.64 Mbps)
R=0.63 (7.20 Mbps)
R=0.38 (4.32 Mbps)
R=0.50 (2.88 Mbps)
R=0.25 (1.44 Mbps)
for 12 codes
(of 16)
Scheduling Strategies
Transmission time interval
3 slots (2 ms)
Example:
Max C/I scheduling
C/I
time
C/I
served
mobile
time
C/I
time
Rev A
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Conclusions
• UMTS provides the presently most advanced cellular radio
access technology
• Mature technology, already proven to work
• Prepared for future evolution
• Fine-tuning of parameters in order to optimize end-to-end user
and overall system performance still remains a challenging
task
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Conclusions (cont.)
• But ...
the market success will primarily not depend on technology
–
–
–
–
–
–
Marketing strategy of network operators and service providers
Charging policy and tariffing
Availabilty of handsets in large volumes at low price at UMTS introduction
Early provision of useful and inventive new services
Simplicity to apply the offered services
Readiness of the subscribers to get used to new services
• Social factors: openess to modern technology (provision of high level of
security to resolve all doubts on potential hazards, EMC, confidentiallity)
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