Transcript lec2-cellular1x - Computer Science, Columbia University

Cellular Networks and Mobile
Computing
COMS 6998-8, Spring 2012
Instructor: Li Erran Li
([email protected])
http://www.cs.columbia.edu/~coms6998-8/
1/30/2012: Cellular Networks: UMTS and LTE
Outline
• Wireless communications basics
– Signal propagation, fading, interference, cellular principle
• Multi-access techniques and cellular network air-interfaces
– FDMA, TDMA, CDMA, OFDM
• 3G: UMTS
– Architecture: entities and protocols
– Physical layer
– RRC state machine
• 4G: LTE
– Architecture: entities and protocols
– Physical layer
– RRC state machine
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
2
Basic Wireless Communication
Information is embedded in
electromagnetic radiation
Lossy signal and
interference
Noise
Recover
information
Transmitter
1/23/12
Receiver
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
3
Noise & Interference
• Thermal Noise
– Generated due to random motion of electrons in the
conductor and proportional to temperature
– No= KoT dBm/Hz where Ko is Boltzmann’s constant
– Receiver Noise Figure – extent to which thermal noise is
enhanced by receiver front end circuitry ~ 10 dB
• Interference – signals transmitted by other users of the
wireless network
• Signal transmitted by other wireless devices from
different wireless networks
– Example: Microwave ovens near 802.11 network
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
4
Impact of White Gaussian Noise
Shannon Capacity
10
Signal Power
Capacity (bits/sec/Hz)
9
SNR =
8
Noise Power
7
6
5
C = log (1 + SNR)
4
3
2
1
0
-10
-5
0
5
10
15
20
25
30
SNR (dB)
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
5
Scattering of Signals - Multipath Fading
Reflection
Diffraction
Absorption
s2 (t )  x(t  d / c)e
s1 (t )  x(t )e
1/23/12
j 2 f c ( t d / c )  j
j 2 f ct
Cellular Networks and Mobile Computing
(COMS 6998-8)
Multiple paths with
random phases and
gains combine
constructively and
destructively to cause
significant amplitude
variations
Courtesy: Harish Vishwanath
6
Impact of Mobility
s2 (t )  a x(t   t )e
v


j 2  fc  cos( ) ( t  t ) j



Doppler Shift =
v

s1 (t )  x(t )e
v

cos( )
j 2 f ct
Signal Amplitude
Multipath Fading
time
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
7
Flat & Frequency Selective Fading
• When the multipath delay is small compared to
symbol duration of the signal, fading is flat or
frequency non-selective
x(t   tmax )  x(t )
1
Symbol
-1
1
-1
• Happens when signal bandwidth is small
1
Bx =
 tmax
• Urban macro-cell delay spread is 10 micro seconds
• When signal bandwidth is large different bands
have different gains – frequency selective fading
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
8
Typical Pathloss
1.0
10.0
100.0
-50
-70
Free space : 20 dB/decade
A decade : transmitter and
receiver distance increase
10 times
-90
-110
Urban Macro cell
-40 dB/decade
1/23/12
Shadow fading
Log-normal with std ~ 8 dB
Cellular Networks and Mobile Computing
(COMS 6998-8)
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9
Spectrum Reuse
A
B
Ib
Ia
SINRa =
S
Sa
Sa
I a+ N
b
a and b can receive simultaneously on the same frequency band
if SINRa and SINRb are above required threshold
This happens if the respective transmitters are sufficiently
far apart
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
10
The Cellular Principle
• Base stations transmit to and receive from
mobiles at the assigned spectrum
– Multiple base stations use the same spectrum
(spectral reuse)
• The service area of each base station is called a
cell
• The wireless network consists of large number of
cells
– Example – The network in Northern NJ is about 150
base stations for a given operator
• Cells can be further divided into multiple sectors
using sectorized antennas
• Each terminal is typically served by the “closest”
base station(s)
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
11
Fixed Frequency Planning
Each base is assigned a fixed frequency band
g1
g4
g6
g5
g7
g2
g3
g1
g6
g4
g5
g2
g2
g3
g7
g1
g3
g2
Reuse of 3
g2
g1
g3
g5
g3
g6
g1
g3
g7
g2
g1
g3
g2
g6
g1
g4
Reuse of 1/3
Reuse of 7 – nearest co-channel
interferer is in the second ring
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
12
Cellular Network Evolution
CDMA 2000
IS-95
CDMA
1X-DO
ANALOG
FDMA
IS- 136
TDMA
LTE
OFDM
TDMA
GSM
1/23/12
TDMA/CDMA
EDGE
TDMA
Cellular Networks and Mobile Computing
(COMS 6998-8)
UMTS
CDMA
Courtesy: Harish Vishwanath
13
The Multiple Access problem
• The base station has to transmit to all the mobiles in its
cell (downlink or forward link)
– Signal for user a is interference for user b
– Interference is typically as strong as signal since a and b are
relatively close
– How to avoid interference?
• All mobiles in the cell transmit to the base station
(uplink or reverse link)
– Signal from a mobile near by will swamp out the signal
from a mobile farther away
– How to avoid interference?
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
14
Meeting Room Analogy
Simultaneous meetings in different rooms (FDMA)
Simultaneous
meetings in the same
room at different
times (TDMA)
Multiple meetings in the same
room at the same time (CDMA)
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
15
Frequency Division Multiple Access
Guard Band
Each mobile is assigned a separate frequency channel for the
duration of the call
Sufficient guard band is required to prevent adjacent channel
interference
Mobiles can transmit asynchronously on the uplink
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
16
Time Division Multiple Access
Time is divided into slots and only one mobile transmits
during each slot
FRAME j
SLOT 1
FRAME j+2
FRAME j + 1
SLOT 2
SLOT 3
SLOT 4
SLOT 5
SLOT 6
Guard time – Signal transmitted by mobiles at
different locations do not arrive at the base at the
same time
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
17
TDMA Characteristics
• Discontinuous transmission with information to
be transmitted buffered until transmission time
– Possible only with digital technology
– Transmission delay
• Synchronous transmission required
– Mobiles derive timing from the base station signal
• Guard time can be reduced if mobiles pre-correct
for transmission delay
– More efficient than FDMA which requires significant
guard band
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
18
Orthogonality in TDMA/FDMA
Every information signal lasts a certain duration of time and
occupies a certain bandwidth and thus corresponds to a
certain region in the time-frequency plane
frequency
Granularity is determined by
practical limitations
time
Time division and frequency division are invariant under transformation
of the channel and retain the orthogonality
Any orthogonal signaling scheme for which orthogonality is preserved
will be a useful multiple access technique
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
19
Code Division Multiple Access
• Use of orthogonal codes to separate different transmissions
• Each symbol or bit is transmitted as a larger number of bits using the user
specific code – Spreading
• Spread spectrum technology
– The bandwidth occupied by the signal is much larger than the
information transmission rate
– Example: 9.6 Kbps voice is transmitted over 1.25 MHz of bandwidth, a
bandwidth expansion of ~100
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
20
Spread Spectrum systems
frequency
time
time
code
Code orthogonality is preserved under linear
transformations and hence near orthogonality is
preserved under signal propagation
1/23/12
Cellular Networks and Mobile Computing
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Courtesy: Harish Vishwanath
21
Orthogonal Walsh Codes
Spread factor 4
Walsh Array
1
1
1
1
1 -1 1 -1
1 1 -1 -1
1 -1 -1 1
De-spreading
Information
Spreading

bit
chip
Walsh Code
Transmitter
1/23/12
Receiver
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
22
Power Control is critical
• The dynamic range of the pathloss for a typical
cell is about 80 dB
• The signal received from the closest mobile is 80
dB stronger than the farthest mobile without
power control
– Code orthogonality is not sufficient to separate the
signals - Near-far problem in CDMA
– Strict orthogonality in TDMA/FDMA makes power
control not critical
• Power Control – Mobiles adjust their transmit
power according to the distance from the base,
fade level, data rate
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
23
Why CDMA?
• Simplified frequency planning
– Universal frequency reuse with spreading gain to mitigate
interference
– Interference averaging allows designing for average
interference level instead of for worst case interference
TDMA / FDMA
1/23/12
CDMA
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
24
Why CDMA?
• Variable rate Vocoder with Power Control
– Advanced data compression technology is used to
compress data according to content
– Typical voice activity is 55% - CDMA reduces interference
by turning down transmission between talk spurts
– Reduced average transmission power increases capacity
through statistical multiplexing
– Compensate for fading through power control - transmit
more power only under deep fades avoiding big fade
margins
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
25
Why CDMA?
• Simple multipath combining to combat fading
Each signal arriving at a
different time can be
recovered separately and
combined coherently
The resulting diversity
gain reduces fading
s2 (t )  x(t  Tc )e j 2 fc (t Tc ) j
s1 (t )  x(t )e
1/23/12
j 2 f ct
Cellular Networks and Mobile Computing
(COMS 6998-8)
Spreading sequence
in x(t ) is offset by
one chip compared
to spreading
sequence in x(t  Tc )
Courtesy: Harish Vishwanath
26
Why CDMA?
Mobile can transmit and receive from multiple base stations
because all base stations use the same frequency
• Soft Handoff - Make-before-break handoff
Signals from different bases can be received separately and then
combined because each base uses a unique spreading code
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
27
What is OFDM ?
Orthogonal Frequency Division Multiplexing is block transmission of N
symbols in parallel on N orthogonal sub-carriers
1
T
Traditional Multi-carrier
Guard
Band
OFDM
Implemented
digitally
through FFTs
Frequency
Frequency
OFDM invented in Bell Labs by R.W. Chang in ~1964 and patent
awarded in 1970
Widely used: Digital audio and Video broadcasting, ADSL,
HDSL, Wireless LANs
1/23/12
Cellular Networks and Mobile Computing
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Courtesy: Harish Vishwanath
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High Spectral Efficiency in Wideband
Signaling
1
T
T large compared to
channel delay spread
 Closely spaced sub-carriers without guard
band
 Each sub-carrier undergoes (narrow band)
flat fading
- Simplified receiver processing
Frequency
Narrow Band (~10 Khz)
coding or scheduling across sub-carriers
 Dynamic power allocation across sub-carriers
allows for interference mitigation across cells
Wide Band (~ Mhz)
Sub-carriers remain orthogonal under
multipath propagation
1/23/12
 Frequency or multi-user diversity through
 Orthogonal multiple access
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Reverse link Orthogonal Frequency
Division Multiple Access
User 1
 Users are carrier
synchronized to the base
 Differential delay between
users’ signals at the base need
to be small compared to T
W
User 2
 Efficient use of spectrum by multiple
users
 Sub-carriers transmitted by different
users are orthogonal at the receiver
- No intra-cell interference
User 3
 CDMA uplink is non-orthogonal
since synchronization requirement is
~ 1/W and so difficult to achieve
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
30
Typical Multiplexing in OFDMA
Each color represents a user
Each user is assigned a frequency-time
tile which consists of pilot sub-carriers
and data sub-carriers
 Yellow color indicates pilot subcarriers
 Channel is constant in each tile
Block hopping of each user’s tile for
frequency diversity
Time
1/23/12
Typical pilot ratio: 4.8 % (1/21)
for LTE for 1 Tx antenna and
9.5% for 2 Tx antennas
Cellular Networks and Mobile Computing
(COMS 6998-8)
Courtesy: Harish Vishwanath
31
Outline
• Wireless communications basics
– Signal propagation, fading, interference, cellular principle
• Multi-access techniques and cellular network air-interfaces
– FDMA, TDMA, CDMA, OFDM
• 3G: UMTS
– Architecture: entities and protocols
– Physical layer
– RRC state machine
• 4G: LTE
– Architecture: entities and protocols
– Physical layer
– RRC state machine
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
32
UMTS System Architecture
Iu
Node B
RNC
GMSC
Node B
USIM
Cu
ME
Iub
Iur
HLR
Node B
RNC
Node B
UE
1/23/12
MSC/
VLR
External Networks
Uu
SGSN
UTRAN
Cellular Networks and Mobile Computing
(COMS 6998-8)
GGSN
CN
33
UMTS Control Plane Protocol Stacks
GMM/SM/SMS
GMM/SM/SMS
Relay
RRC
RRC
RANAP
RLC
RLC
SCCP
MAC
MAC
UMTS RF
UMTS RF
Signaling
Bearer
AAL5
ATM
UE
Uu
RNS
RANAP
SCCP
Signaling Bearer
AAL5
ATM
Iu
SGSN
UMTS User Plane Protocol Stacks
Application
Application
Relay
IP, PPP,
OSP
IP,
I PPPP,
OSP
Relay
PDCP
PDCP
Relay
GTP-U
GTP-U
GTP-U
GTP-U
UDP/
TCP
IP
IP
RLC
RLC
UDP/IP
UDP/IP
UDP/IP
MAC
MAC
AAL5
AAL5
L2
UMTS RF
ATM
ATM
L1
UMTS RF
UE
Uu
UTRAN
Iu
SGSN
UDP/IP
IPL2
IP
L1
Gn
GGSN
Gi
ISP
UTRAN
UE
UTRAN
CN
UMTS Terrestrial Radio Access Network Overview

Two Distinct Elements :
Base Stations (Node B)
Radio Network Controllers (RNC)

1 RNC and 1+ Node Bs are group together to
form a Radio Network Sub-system (RNS)
Node B
RNC
Node B

Handles all Radio-Related Functionality


RNS
Iur
Iub
Soft Handover
Radio Resources Management Algorithms
Node B

Maximization of the commonalities of the PS
and CS data handling
RNC
Node B
RNS
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
UTRAN
36
UTRAN
UE
UTRAN
CN
Logical Roles of the RNC
Controlling RNC (CRNC)
Responsible for the load and
congestion control of its own cells
Node B
Serving RNC (SRNC)
Terminates : Iu link of user data,
Radio Resource Control Signalling
Performs : L2 processing of data
to/from the radio interface, RRM
operations (Handover, Outer Loop
Power Control)
Node B
Drift RNC (DRNC)
Performs : Macrodiversity
Combining and splitting
1/23/12
CRNC
RNC
Node B
Iu
SRNC
Node B
UE
Iur
Iu
Node B
DRNC
Node B
Iu
Node B
SRNC
Node B
Iur
Iu
Node B
UE
Cellular Networks and Mobile Computing
(COMS 6998-8)
DRNC
Node B
37
Radio Resources Management
UE
UTRAN
CN
• Network Based Functions
– Admission Control (AC)
• Handles all new incoming traffic. Check whether new connection can be admitted to
the system and generates parameters for it.
– Load Control (LC)
• Manages situation when system load exceeds the threshold and some counter
measures have to be taken to get system back to a feasible load.
– Packet Scheduler (PS): at RNC and NodeB (only for HSDPA and HSUPA)
• Handles all non real time traffic, (packet data users). It decides when a packet
transmission is initiated and the bit rate to be used.
• Connection Based Functions
– Handover Control (HC)
• Handles and makes the handover decisions.
• Controls the active set of Base Stations of MS.
– Power Control (PC)
• Maintains radio link quality.
• Minimize and control the power used in radio interface, thus maximizing the call
capacity.
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
38
Connection Based Function
Power Control
Prevent Excessive Interference
and Near-far Effect

Fast Close-Loop Power Control


Feedback loop with 1.5kHz cycle to
adjust uplink / downlink power to its
minimum
Even faster than the speed of
Rayleigh fading for moderate mobile
speeds
Outer Loop Power Control


1/23/12
UTRAN
CN
Outer Loop Power Control
If quality < target, increases
SIRTARGET


UE
Fast Power Control
If SIR < SIRTARGET, send
“power up” command to
MS
Adjust the target SIR setpoint in base
station according to the target BER
Commanded by RNC
Cellular Networks and Mobile Computing
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39
Connection Based Function
UE
UTRAN
CN
Handover

Softer Handover




Soft Handover





A MS is in the overlapping coverage of 2
sectors of a base station
Concurrent communication via 2 air interface
channels
2 channels are maximally combined with rake
receiver
A MS is in the overlapping coverage of 2
different base stations
Concurrent communication via 2 air interface
channels
Downlink: Maximal combining with rake
receiver
Uplink: Routed to RNC for selection combining,
according to a frame reliability indicator by the
base station
Hard handover
 HSDPA
 Inter-system and inter-frequency
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
40
HSDPA
UE
UTRAN
CN
High Speed Downlink Packet Access

Improves System Capacity and User Data Rates in the Downlink Direction
to 10Mbps in a 5MHz Channel

Adaptive Modulation and Coding (AMC)



HARQ provides Fast Retransmission with Soft Combining and Incremental
Redundancy



Replaces Fast Power Control :
User farer from Base Station utilizes a coding and modulation that requires
lower Bit Energy to Interference Ratio, leading to a lower throughput
Replaces Variable Spreading Factor :
Use of more robust coding and fast Hybrid Automatic Repeat Request (HARQ,
retransmit occurs only between UE and BS)
Soft Combining : Identical Retransmissions
Incremental Redundancy : Retransmits Parity Bits only
Fast Scheduling Function

1/23/12
which is Controlled in the Base Station rather than by the RNC
Cellular Networks and Mobile Computing
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41
Core Network
UE
UTRAN
CN
Core Network

CS Domain :




Mobile Switching Centre (MSC)
 Switching CS transactions
Visitor Location Register (VLR)
 Holds a copy of the visiting user’s
service profile, and the precise
info of the UE’s location
Gateway MSC (GMSC)
 The switch that connects to
external networks
PS Domain :


1/23/12
Serving GPRS Support Node (SGSN)
 Similar function as MSC/VLR
Gateway GPRS Support Node (GGSN)
 Similar function as GMSC
GMSC
HLR
Iu-ps

SGSN
External Networks
MSC/
VLR
Iu-cs
GGSN
Register :

Home Location Register (HLR)
 Stores master copies of users
service profiles
 Stores UE location on the
level of MSC/VLR/SGSN
Cellular Networks and Mobile Computing
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42
WCDMA Air Interface
UE
UTRAN
CN
Direct Sequence Spread Spectrum
Spreading
User 1
f
Wideband
f
Spreading
f
Received
User N
f
Wideband
f
Narrowband
f
 Frequency Reuse Factor = 1
Variable Spreading Factor
(VSF)
Spreading : 256
Multipath Delay
Profile
Narrowband
Code
Gain
Despreading
t
f
User 1
Wideband
f
Spreading : 16
Wideband
t
 5 MHz Wideband Signal
Allows Multipath Diversity with
Rake Receiver
1/23/12
f
f
Wideband
 VSF Allows Bandwidth on Demand.
Lower Spreading Factor requires Higher
SNR, causing Higher Interference in
exchange.
User 2
Cellular Networks and Mobile Computing
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43
WCDMA Air Interface (Cont’d)
UE
UTRAN
Multiple Access Method
DS-CDMA
Duplexing Method
FDD/TDD
Base Station Synchronization
Asychronous Operation
Channel bandwidth
5MHz
Chip Rate
3.84 Mcps
Frame Length
10 ms
Service Multiplexing
Multiple Services with different QoS
Requirements Multiplexed on one
Connection
Multirate Concept
Variable Spreading Factor and Multicode
Detection
Coherent, using Pilot Symbols or Common
Pilot
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
CN
44
WCDMA Air Interface (Cont’d)
UE
UTRAN
CN
• Channel concepts
1/23/12
Cellular Networks and Mobile Computing
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WCDMA Air Interface (Cont’d)
UE
UTRAN
CN
Mapping of Transport Channels and Physical Channels
Broadcast Channel (BCH)
Primary Common Control Physical Channel
(PCCPCH)
Secondary Common Control Physical
Channel (SCCPCH)
Physical Random Access Channel (PRACH)
Forward Access Channel
(FACH)
Paging Channel (PCH)
Random Access Channel
(RACH)
Dedicated Channel (DCH)
Dedicated Physical Data Channel (DPDCH)
Dedicated Physical Control Channel (DPCCH)
Downlink Shared Channel
(DSCH)
Common Packet Channel
(CPCH)
Physical Downlink Shared Channel (PDSCH)
Physical Common Packet Channel (PCPCH)
Highly Differentiated Types of
Channels enable best combination
of Interference Reduction, QoS
and Energy Efficiency
1/23/12
Synchronization Channel (SCH)
Common Pilot Channel (CPICH)
Acquisition Indication Channel (AICH)
Paging Indication Channel (PICH)
CPCH Status Indication Channel (CSICH)
Collision Detection/Channel Assignment
Indicator Channel (CD/CA-ICH)
Cellular Networks and Mobile Computing
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WCDMA Air Interface (Cont’d)
UE
UTRAN
CN
• Code to channel allocation
1/23/12
Cellular Networks and Mobile Computing
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47
Codes in WCDMA
•
UE
UTRAN
CN
Channelization Codes (=short code)
– Used for
• channel separation from the single source in
downlink
• separation of data and control channels from each
other in the uplink
– Same channelization codes in every cell / mobiles
and therefore the additional scrambling code is
needed
•
Scrambling codes (=long code)
– Very long (38400 chips = 10 ms =1 radio frame),
many codes available
– Does not spread the signal
– Uplink: to separate different mobiles
– Downlink: to separate different cells
– The correlation between two codes (two
mobiles/Node Bs) is low
• Not fully orthogonal
1/23/12
Cellular Networks and Mobile Computing
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48
RRC State Machine
UE
UTRAN
CN
• IDLE: procedures based on reception rather
than transmission
– Reception of System Information messages
– PLMN selection Cell selection Registration
(requires RRC connection establishment)
– Reception of paging Type 1 messages with a DRX
cycle (may trigger RRC connection establishment)
Cell reselection
– Location and routing area updates (requires RRC
connection establishment)
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
49
RRC State Machine (Cont’d)
UE
UTRAN
CN
• CELL_FACH: need to continuously receive
(search for UE identity in messages on FACH),
data can be sent by RNC any time
– Can transfer small PS data
– UE and network resource required low
– Cell re-selections when UE mobile
– Inter-system and inter-frequency handoff possible
– Can receive paging Type 2 messages without a
DRX cycle
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
50
RRC State Machine (Cont’d)
UE
UTRAN
CN
• CELL_DCH: need to continuously receive, and sent
whenever there is data
– Possible to transfer large quantities of uplink and downlink
data
– Dedicated channels can be used for both CS and PS
connections
– HSDPA and HSUPA can be used for PS connections
– UE and network resource requirement is relatively high
– Soft handover possible for dedicated channels and HSUPA
Inter-system and inter-frequency handover possible
– Paging Type 2 messages without a DRX cycle are used for
paging purposes
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
51
RRC State Machine (Cont’d)
UE
UTRAN
CN
• State promotions have promotion delay
• State demotions incur tail times
Courtesy: Feng Qian
Tail Time
Delay: 2s
Delay: 1.5s
Tail Time
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
Channel
Radio
Power
IDLE
Not
allocated
Almost
zero
CELL_FACH
Shared,
Low Speed
Low
CELL_DCH
Dedicated,
High Speed
High
Outline
• Wireless communications basics
– Signal propagation, fading, interference, cellular principle
• Multi-access techniques and cellular network air-interfaces
– FDMA, TDMA, CDMA, OFDM
• 3G: UMTS
– Architecture: entities and protocols
– Physical layer
– RRC state machine
• 4G: LTE
– Architecture: entities and protocols
– Physical layer
– RRC state machine
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
53
LTE Technical Objectives and
Architecture
• User throughput [/MHz]:
– Downlink: 3 to 4 times Release 6 HSDPA
– Uplink: 2 to 3 times Release 6 Enhanced Uplink
• Downlink Capacity: Peak data rate of 100 Mbps in 20 MHz
maximum bandwidth
• Uplink capacity: Peak data rate of 50 Mbps in 20 MHz
maximum bandwidth
• Latency: Transition time less than 5 ms in ideal conditions
(user plane), 100 ms control plane (fast connection setup)
• Cell range: 5 km - optimal size, 30km sizes with reasonable
performance, up to 100 km cell sizes supported with
acceptable performance
Cellular Networks and Mobile
1/23/12
Computing (COMS 6998-8)
54
LTE Technical Objectives and
Architecture (Cont’d)
• Mobility: Optimised for low speed but
supporting 120 km/h
– Most data users are less mobile!
• Simplified architecture: Simpler E-UTRAN
architecture: no RNC, no CS domain, no DCH
• Scalable bandwidth: 1.25MHz to 20MHz:
Deployment possible in GSM bands.
Cellular Networks and Mobile
1/23/12
Computing (COMS 6998-8)
55
LTE Architecture
• Entities and functionalities
NAS security
Idle state mobility handling
EPS bearer control
UE IP address allocation
Radio bearer control
Mobility
Packet filtering
Inter-cell RRM
anchoring
Connection mobility Control
Cellular Networks and Mobile Computing
1/23/12 Radio admission control
(COMS 6998-8)
56
LTE Control Plane Protocol Stack
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
57
LTE Data Plane Protocol Stack
1/23/12
Cellular Networks and Mobile Computing
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Functions of eNodeB
UE
eNodeB
CN
• Terminates RRC, RLC and MAC protocols and takes care of Radio
Resource Management functions
–
–
–
–
–
Controls radio bearers
Controls radio admissions
Controls mobility connections
Allocates radio resources dynamically (scheduling)
Receives measurement reports from UE
• Selects MME at UE attachment
• Schedules and transmits paging messages coming from MME
• Schedules and transmits broadcast information coming from MME
& O&M
• Decides measurement report configuration for mobility and
scheduling
• Does IP header compression and encryption of user data streams
Cellular Networks and Mobile
1/23/12
Computing (COMS 6998-8)
59
Functions of MME
UE
eNodeB
CN
• Mobility Management Entity (MME) functions
– Manages and stores UE context
– Generates temporary identities and allocates
them to UEs
– Checks authorization
– Distributes paging messages to eNBs
– Takes care of security protocol
– Controls idle state mobility
– Ciphers & integrity protects NAS signaling
Cellular Networks and Mobile
1/23/12
Computing (COMS 6998-8)
60
Session Establishment Message Flow
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
61
Session States
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
62
LTE vs UMTS
• Functional changes compared to the current
UMTS Architecture
PGW
SGW
GGSN
SGSN
(not user plane
functions)
PDN GateWay
Serving GateWay
MME
Mobility Management Entity
RNC
Node B
RNC functions moved to eNodeB.
• No central radio controller node
• OFDM radio, no soft handover
• Operator demand to simplify
1/23/12
eNodeB
PGW/SGW
• Deployed according to traffic demand
• Only 2 user plane nodes (non-roaming
case)
Control plane/user plane split for better
scalability
• MME control plane only
• Typically centralized and pooled
Cellular Networks and Mobile Computing
(COMS 6998-8)
63
LTE PHY Basics
UE
eNodeB
CN
• Six bandwidths
– 1.4, 3, 5, 10, 15, and 20 MHz
• Two modes
– FDD and TDD
• 100 Mbps DL (SISO) and 50 Mbps UL
• Transmission technology
– OFDM for multipath resistance
– DL OFDMA for multiple access in frequency/time
– UL SC-FDMA to deal with PAPR ratio problem
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
64
Frame Structure
UE
eNodeB
CN
Frame Structure Type 1 (FDD)
Frame Structure Type 2 (TDD)
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
65
Resource Grid
UE
eNodeB
CN
One downlink slot, Tslot
6 or 7 OFDM symbols
Resource block
:
Transmission BW
Resource element
12 subcarriers
• 6 or 7 OFDM symbols in
1 slot
• Subcarrier spacing = 15
kHz
• Block of 12 SCs in 1 slot =
1 RB
– 0.5 ms x 180 kHz
– Smallest unit of allocation
:
l=0
1/23/12
l=6
Cellular Networks and Mobile Computing
(COMS 6998-8)
66
2-D time and Frequency Grid
UE
eNodeB
CN
#19
#18
#17
#16
#5
#4
#3
#2
NscRB subcarriers (=12)
#1
Power
#0
Frequency
NBWDL subcarriers
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
67
DL PHY Channels and Signals
UE
eNodeB
CN
• Signals: generated in PHY layers
– P-SS: used for initial sync
– S-SS: frame boundary determination
– RS: pilots for channel estimation and tracking
• Channels: carry data from higher layers
– PBCH: broadcast cell-specific info
– PDCCH: channel allocation and control info
– PCFICH: info on size of PDCCH
– PHICH: Ack/Nack for UL blocks
– PDSCH: Dynamically allocated user data
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
68
DL Channel Mapping
UE
eNodeB
CN
P-SCH - Primary Synchronization Signal
S-SCH - Secondary Synchronization Signal
PBCH - Physical Broadcast Channel
PDCCH -Physical Downlink Control Channel
PDSCH - Physical Downlink Shared Channel
16QAM
Reference Signal – (Pilot)
64QAM
QPSK
Time
Frequency
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
69
UL PHY Signals and Channels
UE
eNodeB
CN
• Signals: generated in the PHY layer
– Demodulation RS : sync and channel estimation
– SRS: Channel quality estimation
• Channels: carry data from higher layers
– PUSCH: Uplink data
– PUCCH: UL control info
– PRACH: Random access for connection
establishment
1/23/12
Cellular Networks and Mobile Computing
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70
UL Channel Mapping
64QAM
16QAM
QPSK
PUSCH
Demodulation Reference Signal
(for PUSCH)
UE
eNodeB
CN
QPSK
BPSK
PUCCH
Demodulation Reference Signal
(for PUCCH format 0 or 1, Normal CP)
Time
Frequency
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
71
RRC State Machine
UE
eNodeB
CN
• Much simpler than
UMTS
1/23/12
Cellular Networks and Mobile Computing
(COMS 6998-8)
72
Summary
• Cellular networks are very different from WiFi
– Cannot be based on carrier sensing due to large coverage
area
– Path must be setup dynamically due to mobility
– Need to handle charging functions and QoS
• Different physical layer technologies have very
different overhead during inactivity
– Dedicated channels prevent others from using the channel
• Frequent RRC state transitions in UE can result in high
network overhead and UE battery power consumption
1/23/12
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73
Questions?