Dr Stefan ParkvallPrincipal ResearcherEricson Research
4G Mobile Broadband – LTEPart I
Public | © Ericsson AB 2012 | 2012-04-23 | Page 2
Data overtaking Voice
› Data is overtaking voice......but previous cellular systems designed primarily for voice
Rapid subscriber growth Rapid traffic growth
0
10
20
30
40
50
2008 2009 2010 2011 2012 2013 2014 2015
Yea
rly E
xaby
tes
Mobile PC Mobile handheld Voice
0500
100015002000250030003500
2008 2009 2010 2011 2012 2013 2014 2015
Sub
scrip
tions
(milli
on)
Embedded modules USB/PC cards Handheld
Public | © Ericsson AB 2012 | 2012-04-23 | Page 3
Mobile Broadband
› HSPA – High-Speed Packet Access (”Turbo-3G”) – Evolution of 3G/WCDMA – Data rates up to ~168 Mbit/s (DL), ~44 Mbit/s (UL)– Support for broadcast services (IMB)
› LTE (”4G”)– Very high data rates in a wide range of spectrum allocations– Data rates up to 300 Mbit/s (DL), 75 Mbit/s (UL) in frist version– Integral support for broadcast services
LTELTE
HSPAHSPA HSPA+HSPA+HSDPAHSDPA
2004 2006 20082002
WCDMAWCDMA
2005 2007 2009
Commercial operation
2003
Public | © Ericsson AB 2012 | 2012-04-23 | Page 4
The 3GPP Ecosystem
333 HSPA operators in333 HSPA operators in139 countries139 countries……
2922 HSPA devices from255 suppliers…
Source: GSA, WCIS/Informa, and Infonetics
Public | © Ericsson AB 2012 | 2012-04-23 | Page 5
Outline
I. Basic principles– Channel and traffic behavior– Link adaptation, scheduling, hybrid-ARQ– Evolving 3G, inclusion of basic principles in WCDMA
II. LTE– First step into 4G– Path towards IMT-Advanced
III. Standardization– How are HSPA and LTE created?– 3GPP, ITU, ...
Series of three seminars
Radio Channels and Packet Data – Some Properties
Public | © Ericsson AB 2012 | 2012-04-23 | Page 7
Wireless vs Wireline
› Wireless seems simple…
H
D =
B = 0
E = - Bt
= J + Dt
› …so what’s the problem?
Public | © Ericsson AB 2012 | 2012-04-23 | Page 8
Wireless vs Wireline
› Many aspects are similar……but there are some fundamental differences!
› Cable› “No” spectrum limitation
– Over-provisioning› Relatively static channels
– No fading› Congestion lost packets› No mobility
Wireline
› No cable › Spectrum is scarce
– Radio-resource management› Time-varying radio channel
– Fast fading› Fading lost packets › Mobility
Wireless
Public | © Ericsson AB 2012 | 2012-04-23 | Page 9
Radio-Channel Variations
› Transmitted power PTx received power PRx << PTx
Fast
Slo
w
r1 5.32› Path loss– Given by Tx-to-Rx distance
› Log-normal fading– Due to random variations in terrain (large scale)– Received signal strength in dB given by normal distribution
› Fast fading– Random variations in environment– Often modeled by a Rayleigh distribution
Public | © Ericsson AB 2012 | 2012-04-23 | Page 10
Radio-Channel Variations
› Transmitted signal reflected in numerous objects– Multiple delayed signal copies received– ’Large’ and ’small’ time differences between components
› ’Small’ delay difference– components add constructively...
...or destructively– Large number of components central-limit theorem Gaussian-distributed amplitude Rayleigh-distributed power
(Rayleigh-fading, fast fading)
frequency space
Power
Multi-path fading
Radio-channels – rapidly varying signal quality
Public | © Ericsson AB 2012 | 2012-04-23 | Page 11
Radio-Channel Variations
› ’Large’ delay difference– Inter-symbol interference (ISI)
n-2 n-1 n n+1
n-2 n-1 n n+1
Detect symbol n
Intersymbolinterference
Example:1 Mbit/s bit rate bit duration 1 µs – same order as time dispersion
› Handling time dispersion through......receiver-side signal processing (e.g equalizer)...transmission scheme robust to time dispersion (e.g. OFDM)
Public | © Ericsson AB 2012 | 2012-04-23 | Page 12
Interference Variations
› Transmissions in neighboring cells cause interference received signal quality affected by neighboring cell activity
Desired signalInterference
Public | © Ericsson AB 2012 | 2012-04-23 | Page 13
Traffic Variations
› Traditional voice services– Low, ~10 kbit/s data rate– Fairly constant during the call
› Packet-data services– Behavior depends on type of service– Typically rapidly and randomly varying
rate requirements(’all-or-nothing’ resource requirement)
circuit-switched ok!
Packet-data systems – rapidly varying data rates
packet-switched NW
Public | © Ericsson AB 2012 | 2012-04-23 | Page 14
TCP Basics
› TCP – Internet’s end-to-end transport layer protocol (non-real time)
› Main responsibilities of TCP:– provide reliable data transport– avoid congestion in the network
TCP
Appl.
IP
TCP
Appl.
IP
Phy. Layer
Link Layer
Phy. Layer
Link Layer
IPIP
Phy. Layer
Link Layer
Phy. Layer
Link Layer
Client ServerRouterRouter
Phy. Layer
Link Layer
IPIP
Phy. Layer
Link Layer
Interaction with wireless links requires attention!
Public | © Ericsson AB 2012 | 2012-04-23 | Page 15
TCP Basics
› Error recovery and congestion control are intertwined– lost packets are used as congestion signal by TCP radio-link errors should be ‘hidden’ from TCP
– Lost packets timeout slow start
0
10000
20000
30000
40000
50000
60000
0 100 200 300 400 500 600 700 800
Time [ms]Se
q N
o [b
ytes
]
RTT=30 ms
RTT=60 ms
Slow start phase
Congestion avoidance
phase
Slow start phase
Congestion avoidance phase
› TCP congestion management– Window = not-yet-ACKed packets in transmission– Phase 1: Slow start
› Increase window by one on each received ACK› window grows exponentially
– Phase 2: Congestion avoidance› Increse window by 1/window_size on each ACK› window grows linearly
Public | © Ericsson AB 2012 | 2012-04-23 | Page 16
TCP Basics
› TCP performance determined by data rate and latency– High data rate alone not sufficient – need low latency as well– Delay-bandwidth product
Length of the pipe: Latency
Width of the pipe: Data Rate
High data rate and low latency
Public | © Ericsson AB 2012 | 2012-04-23 | Page 17
Radio Channels and Packet Data
› Radio-channel quality varies......distance to base station...random environmental variations...interference variations
› Traffic pattern varies......user behavior...server load
Adapt to and exploit channel and traffic variations!
Basic Principles Used By HSPA and LTE
Public | © Ericsson AB 2012 | 2012-04-23 | Page 19
Rate Control
› Eb/N0 – fundamental quantity in communications– Eb received energy per information bit [J]– N0 noise power spectral density [W/Hz]
› Block-Error Rate vs Eb/N0– Practical schemes – BLER decreases with increasing Eb
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Eb/N0 [dB]
Bit-
erro
r pr
obab
ility
QPSK
16QAM 64QAM
Public | © Ericsson AB 2012 | 2012-04-23 | Page 20
Rate Control
› N0 is given– Noise etc
› How to control Eb despite varying radio-channel quality?– Eb = P•T = P / R
Cha
nnel
Q
ualit
y
Tx P
ower
Dat
a R
ate
Power Control
Dat
a R
ate
Cha
nnel
Q
ualit
y
Tx P
ower
Rate Control
Public | © Ericsson AB 2012 | 2012-04-23 | Page 21
Rate Control
› Packet-data services typically accept (short-term) data-rate variations– Internet has unpredictable data rates– Short-term variations acceptable even for most services with strict QoS
requirements – only cares about average data rate
› Rate control more efficient than power control– Power amplifier runs at ’full power all the time’
Public | © Ericsson AB 2012 | 2012-04-23 | Page 22
Rate Control
› Data rate controlled through...
› ...different channel coding rates– Advantageous channel conditions high code rate– Code rates from 1/3 to ~1
› ...different modulation schemes– Advantageous channel conditions higher-order modulation
› ...different multi-antenna schemes
QPSK 16QAM 64QAM
Public | © Ericsson AB 2012 | 2012-04-23 | Page 23
Shared-ChannelTransmission
› Dedicated channel– Resources assigned at ”call setup”– Independent of instantaneous traffic– ”Circuit-switched”
› Shared channel– Dynamic sharing of common resource– Adapts to instantaneous traffic situation– ”Packet-switched”
Shared channel – dynamic resource management
Public | © Ericsson AB 2012 | 2012-04-23 | Page 24
Channel-dependentScheduling
› Scheduling determines at each time instant…– …to whom to assign the shared channel– …which data rate to use (rate adaptation)
› Basic idea: transmit at fading peaks– Known as multi-user diversity
Cha
nnel
Qua
lity User #1
User #2
User #3
Time#1 #3 #2 #3 #1
Effective channel variations seen by the base station
Public | © Ericsson AB 2012 | 2012-04-23 | Page 25
Channel-dependent Scheduling
› Round Robin (RR)– Cyclically assign the channel to users without taking quality
conditions into account– Simple but poor performance R
adio
Lin
k Q
ualit
y
Time
Rad
io L
ink
Qua
lity
Time
› Max C/I– Assign the channel to the user with the best absolute quality– High system throughput but not fair
Rad
io L
ink
Qua
lity
Time
› Proportional Fair (PF)– Assign the channel to the user with the best relative quality– High throughput, fair
Public | © Ericsson AB 2012 | 2012-04-23 | Page 26
Channel-dependent Scheduling
› Good schedulers take radio and traffic variations into account
› Radio-channel variations– Schedule at fading peaks
› Traffic variations– Schedule when user has data– May take priorities into account
› Example: VoIP has higher priority than file download
Public | © Ericsson AB 2012 | 2012-04-23 | Page 27
Channel-dependent Scheduling
› The larger the unfairness, the higher the system throughput......true for full buffers but realistic traffic complicates the picture
User throughput
Full buffer
PFMax C/I
RR
CD
F of
use
r thr
ough
put
1
User throughput
Web browsing
PFMax C/I
RR
CD
F of
use
r thr
ough
put
1
Public | © Ericsson AB 2012 | 2012-04-23 | Page 28
Hybrid ARQ with
Soft Combining
› Retransmission of erroneously received packets– Fast no disturbance of TCP behavior
› Soft combining of multiple transmission attempts– Soft combining improved performance
NAK
ACK
NACK
ACK
ACK
+ +
Transmitter
Receiver
P1,1 P1,2 P2,1 P2,2 P3,1
P1,1 P1,1 P2,1 P2,2 P3,1
P1,2 P2,2
Public | © Ericsson AB 2012 | 2012-04-23 | Page 29
Hybrid ARQ with
Soft Combining
› Coding– Add redundancy at transmitter– Exploit redundancy at receiver to correct (most) transmission errors– Code rate R = k/n, code rate fine tuned by puncturing– The lower the code rate R, the lower the error rate but the higher the overhead
› Hybrid-ARQ– Correct most errors with coding– Detect uncorrectable transmission errors, request retransmissions
k information bits
n’ coded bits
n transmitted bits
Puncturing
Coding
Public | © Ericsson AB 2012 | 2012-04-23 | Page 30
R=3/4
redundancy version 1
Initial transmission
Transmitted bits
Bits input to decoder
Resulting code rate
Accumulated energy per bit
Eb
CRC insertion,Turbo coding
Data
Puncturing to• generate different redundancy versions
• match the number of coded bits to the channel
redundancy version 2
R=3/8
First retransmission
2Eb
redundancy version 3
R=1/4
Second retransmission
3Eb
Hybrid ARQ with
Soft Combining
› Incremental redundancy
R=1/4
Third retransmission
redundancy version 1
4Eb
HSPAHigh-Speed Packet Access
LTELTE
HSPAHSPAWCDMAWCDMA HSPA+HSPA+HSDPAHSDPA
Public | © Ericsson AB 2012 | 2012-04-23 | Page 32
Rel5R99 Rel7Rel6Rel4 Rel8 Rel9
WCDMA Background
› WCDMA (”3G”)– Basics developed mid-90’s, standard ready -99– Circuit-switched voice– ”ISDN-like” packet data (typically up to 384 kbit/s)
› HSPA (”Turbo-3G”)– Packet-data improvement add-on to WCDMA– First version ~2002, still evolving – Data rates up to 168 Mbit/s (downlink), 44 Mbit/s (uplink)
› HSPA is an evolution of WCDMA– Incorporating the basic principles in an existing 3G network
HSPAHSPAWCDMAWCDMA HSPA evolutionHSPA evolutionHSDPAHSDPA
Public | © Ericsson AB 2012 | 2012-04-23 | Page 33
ArchitectureWCDMA
Admission control, call set-up, radio-resource management, retransmission handling...
Dedicated channels
RNC RNC
NodeB
Core Network
PSTN Internet
to other Node Bs to other Node Bs
UE
SGSN
GGSN
SGSN
Fast, dynamic adaptation to radio-channel variations new NodeB functionalityscheduling, rate adaptation, hybrid-ARQ
WCDMA/HSPA
Public | © Ericsson AB 2012 | 2012-04-23 | Page 34
HSPA Basics – Downlink
› WCDMA– At call set-up, each user is assigned an orthogonal spreading code– Spreading factor data rate
› HSPA – evolution of WCDMA– Shared set of channelization codes– Multi-code transmission
SF=16
SF=8
SF=4
SF=2
SF=1
HSPA
Common channels (not power controlled)
Dedicated channels (power controlled)To
tal a
vaila
ble
cell
pow
er
Public | © Ericsson AB 2012 | 2012-04-23 | Page 35
HSPA Basics – Downlink
› Channel-dependent scheduling– 2 ms basis
› Rate control– 2 ms basis
› Hybrid-ARQ with soft combining– Roundtrip time of 12 ms possible
› Short (2 ms) TTI– Reduced delays 2 ms
› Shared channel transmission– Dynamically shared code resource
Public | © Ericsson AB 2012 | 2012-04-23 | Page 36
Downlink Scheduling
› Each code covers full bandwidth channel-dependent scheduling in time-domain only
– No access to frequency domain
› Downlink scheduler controls...– ...to which user to transmit– ...the set of codes to use– ...the modulation scheme to use (QPSK, 16QAM, 64QAM)– ...the code rate to use
...for each 2 ms transmission interval
› Scheduling decision informed to terminals on a shared control channel– All terminals monitor shared control channels for scheduling decisions
Public | © Ericsson AB 2012 | 2012-04-23 | Page 37
Performance Example
› ~3 times capacity increase– for web browsing– less for streaming
› ~3 times lower download time– for web browsing– large TCP objects (file transfer) can
show even larger performance gains
› Scheduling strategy has a large impact on the performance.
0 0.5 1 1.5 2 2.5 3 3.50.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Normalized System Throughput
Nor
mal
ized
Del
ay
HS-DSCH, max C/I HS-DSCH, RR64 kbps dedicated
Fast Scheduling
Fast Link Adaptation
Reduced hybrid ARQ round-trip
delay
Web browsing, PedA channel model
Public | © Ericsson AB 2012 | 2012-04-23 | Page 38
Summary
› Adapt to and exploit…– variations in the radio channel quality– variations in the traffic pattern
…instead of combating them!
› Traffic pattern is time varying
› Radio channel quality is time varying
Public | © Ericsson AB 2012 | 2012-04-23 | Page 39
Summary
Multi-antenna support
Hybrid ARQ
Channel-dependent schedulingShared channel transmission
f5 MHz
10 MHz5 MHz
Multi-carrier transmission
Rate control
› Packet-data add-on to WCDMA› First version ~2002, still evolving
HSPA (“Turbo-3G”)
Public | © Ericsson AB 2012 | 2012-04-23 | Page 40
Summary – 3GPP Ecosystem
333 HSPA operators in333 HSPA operators in139 countries139 countries……
2922 HSPA devices from255 suppliers…
Source: GSA, WCIS/Informa, and Infonetics
Public | © Ericsson AB 2012 | 2012-04-23 | Page 41
For Further information…
Available in English, Chinese, Korean and Japanese.
...or read The Book!
Open the 3GPP specifications...