192620010
Mobile & Wireless Networking
Lecture 5:
Cellular Systems (UMTS / LTE) (1/2)
[Schiller, Section 4.4]
Geert Heijenk
Mobile and Wireless Networking
2013 / 2014
Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2)
q Evolution of cellular systems
q GSM
l GSM Network Architecture
l GSM radio interface
l GPRS
l EDGE
q 3G UMTS
l UMTS Network Architecture
l Wideband CDMA
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Evolution of cellular systems
Source: Agilent Technologies, 2012
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GSM Architecture
OMC, EIR,
AUC
HLR GMSC
NSS fixed network
with OSS
VLR MSC MSC
VLR
BSC
BSC
RSS
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GSM Radio Interface: TDMA/FDMA
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 guard
space tail user data S Training S user data tail space
3 bits 57 bits 1 26 bits 1 57 bits 3
546.5 µs
577 µs
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GPRS (General Packet Radio Service)
q packet switching
q using free slots only if data packets ready to send
q (~reservation Aloha)
Receiving Sending Maximum
Class
q Few changes to base station slots slots number of slots
(software) 1 1 1 2
q New core network architecture 2 2 1 3
(router-based) 3 2 2 3
5 2 2 4
8 4 1 5
10 4 2 5
12 4 4 5
Coding 1 slot 2 slots 3 slots 4 slots 5 slots 6 slots 7 slots 8 slots
scheme
CS-1 9.05 18.2 27.15 36.2 45.25 54.3 63.35 72.4
CS-2 13.4 26.8 40.2 53.6 67 80.4 93.8 107.2
CS-3 15.6 31.2 46.8 62.4 78 93.6 109.2 124.8
CS-4 21.4 42.8 64.2 85.6 107 128.4 149.8 171.2
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GPRS architecture and interfaces
SGSN
Gn
BSS SGSN GGSN PDN
MS
Um Gb Gn Gi
MSC HLR/
GR
VLR EIR
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EDGE
EDGE (Enhanced Data rates for GSM Evolution):
q New modulation technique: 8PSK instead of GMSK (bitrate x3)
q Can be combined with GPRS
q Adaptive Modulation and Coding
q Incremental Redundancy
(Hybrid ARQ)
q New BS hardware
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Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2)
q Evolution of cellular systems
q GSM
l GSM Network Architecture
l GSM radio interface
l GPRS
l EDGE
q 3G UMTS
l UMTS Network Architecture
l Wideband CDMA
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UMTS architecture (original release (R99))
UTRAN (UMTS Terrestrial Radio Access Network)
q Cell level mobility
q Radio Network Subsystem (RNS)
q Encapsulation of all radio specific tasks
UE (User Equipment)
CN (Core Network)
q Inter system handover
q Location management if there is no dedicated connection between
UE and UTRAN
Uu Iu
UE UTRAN CN
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UTRAN architecture
RNS RNC: Radio Network Controller
RNS: Radio Network Subsystem
UE1 Node B Iub
Iu
RNC CN
UE2
Node B UTRAN comprises several
RNSs
UE3 Node B can support FDD or
TDD or both
Iur
Node B RNC is responsible for handover
Iub decisions requiring
signalingto the UE
Node B
RNC Cell offers FDD or TDD
Node B
RNS
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Core network: architecture
VLR
BSS
BTS Abis Iu
BSC MSC GMSC
PSTN
Node
BTSB
IuCS
AuC
EIR HLR
GR
Node B
Iub
Node B
RNC SGSN GGSN
Gn Gi
Node B IuPS CN
RNS
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UMTS Protocol Architecture - User Plane
App App
TCP TCP
IP IP IP
PDCP PDCP GTP-U GTP-U GTP-U
RLC RLC UDP UDP UDP UDP L2 L2
MAC MAC
IP IP IP IP
FP FP
PHY L1 L1
PHY L2 L2 L2 L2 L2 L2
L1 L1 L1 L1 L1 L1
UE Node B RNC SGSN GGSN Host
Uu Iub IuPS Gn Gi
Legend: UMTS UMTS Transport Network Internet Other
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UMTS Protocol Architecture – Control Plane
UMM/SM UMM/SM
RRC RRC RANAP RANAP
RLC RLC SCCP SCCP
Signalling Signalling
MAC MAC Bearer Bearer
NBAP NBAP
PHY PHY L2 L2 L2 L2
L1 L1 L1 L1
UE Node B RNC SGSN
Uu Iub IuPS
Legend: UMTS UMTS Transport Network
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Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2)
q Evolution of cellular systems
q GSM
l GSM Network Architecture
l GSM radio interface
l GPRS
l EDGE
q 3G UMTS
l UMTS Network Architecture
l Wideband CDMA
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Wideband CDMA
Direct Sequence CDMA, also Transport channels
known as Wideband CDMA
Channel Channel
Chip rate 3.84 Mc/s coding coding
Carrier spacing 5 MHz
Multiplexing
Physical-layer
Transport-channel Mapping to physical channels procedures
processing and
measurements
Physical channels
Spreading Spreading
3.84 Mc/s
Modulation Modulation
≈5 MHz
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How do we spread the data?
The operation of spreading in a CDMA system is divided into two
separate parts
q Spreading code = Scrambling code + Channelization code
Scrambling
q Separates different mobiles (in uplink) and different cells/sectors (in
downlink)
Channelization
q Separates different physical channels that are transmitted on the
same scrambling code
q The purpose of channelization is most evident in the downlink
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Spreading and scrambling of user data
Constant chipping rate of 3.84 Mchip/s
Different user data rates supported via different spreading factors
q higher data rate: less chips per bit and vice versa
User separation via unique, quasi orthogonal scrambling codes
q users are not separated via orthogonal spreading codes
q much simpler management of codes: each station can use the same
orthogonal spreading codes
q precise synchronisation not necessary as the scrambling codes stay quasi-
orthogonal
data1 data2 data3 data4 data5
spr. spr. spr. spr. spr.
code1 code2 code3 code1 code4
scrambling scrambling
code1 code2
sender1 sender2
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Orthogonal Variable Spreading Factor (OVSF) coding
1,1,1,1,1,1,1,1
1,1,1,1 ...
1,1,1,1,-1,-1,-1,-1
1,1
1,1,-1,-1,1,1,-1,-1
1,1,-1,-1 ...
X,X
1,1,-1,-1,-1,-1,1,1
X 1
1,-1,1,-1,1,-1,1,-1
X,-X 1,-1,1,-1 ...
1,-1,1,-1,-1,1,-1,1
SF=n SF=2n 1,-1
1,-1,-1,1,1,-1,-1,1
1,-1,-1,1 ...
1,-1,-1,1,-1,1,1,-1
SF=1 SF=2 SF=4 SF=8
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UMTS FDD frame structure
W-CDMA
Radio frame"
• 1920-1980 MHz uplink
• 2110-2170 MHz downlink
10 ms" 0" 1" 2" ..." 12" 13" 14" • chipping rate:
3.840 Mchip/s
Time slot" • soft handover
• QPSK
666.7 µs" Pilot" TFCI" FBI" TPC" uplink DPCCH" • complex power control
2560 chips, 10 bits"
(1500 power control
cycles/s)
666.7 µs" Data" uplink DPDCH" • spreading: UL: 4-256;
DL:4-512
2560 chips, 10*2k-1 bits (k = 1...7)"
666.7 µs" Data1" TPC"TFCI" Data2" Pilot" downlink DPCH"
FBI: Feedback Information
DPDCH" DPCCH"DPDCH"DPCCH" TPC: Transmit Power Control
TFCI: Transport Format Combination Indicator
2560 chips, 10*2k bits (k = 0...7)" DPCCH: Dedicated Physical Control Channel
DPDCH: Dedicated Physical Data Channel
Slot structure NOT for user separation DPCH: Dedicated Physical Channel
but synchronisation for periodic functions!
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Bit rates and Spreading Factors
Channel bit rate [kbps] User bit rate (bef. coding) [kbps]
k Spreading
factor Uplink Downlink Uplink Downlink
0 512 N/A 15 kbps N/A 6 kbps
1 256 15 kbps 30 kbps 15 kbps 24 kbps
2 128 30 kbps 60 kbps 30 kbps 51 kbps
3 64 60 kbps 120 kbps 60 kbps 90 kbps
4 32 120 kbps 240 kbps 120 kbps 210 kbps
5 16 240 kbps 480 kbps 240 kbps 432 kbps
6 8 480 kbps 960 kbps 480 kbps 912 kbps
7 4 960 kbps 1920 kbps 960 kbps 1872 kbps
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Fading
Path loss – fading due to distance
q 1/distanceα (α between 3 and 4)
Long term (slow) fading – caused by shadowing
q Log-normal
Short term (fast) fading – caused by multipath propagation
q Rayleigh fading amplitude
Signal level (dB)
Path loss
Long term fading
Short term fading
Distance (log)
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Purpose of Power Control
Goal
q mobile station transmitted power is controlled such that all users in
the cell experience the same SIR (Signal to Interference Ratio) at
the base station receiver
Open Loop (initial power setting)
q compensate for pathloss and slow fading
q uses downlink pilot channel
Closed Loop (fast power control)
q compensates also for fast fading
q needs dedicated downlink control channel for power control
commands
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Dynamic Range of Power Control
PC
PI
Worst case: PC(dB) – PI(dB) = – 80 dB!
Interferers are rejected by the processing gain:
Rchip 106
G= = = 100 → 20 dB
Rbit 104
C
⇒ = – 80 + 20 = – 60 dB!
I
Power control with a large dynamic range is essential!
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Why Soft Handover?
Soft handover essential for power control
Soft handover reception
q combines signals from different base stations
RNC
BS 1 BS 2
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Time Dispersion – Rake receiver – Channel Estimation
r(n)
τ2
h0 Channel τ1
C(n) C(n) C(n)
h2
h1
g g g
a2 a1 a0
τ1 τ2
Diversity Combination To
Decoder
Diversity Channel a0
a2 a1 a0
Combination Estimation
τ1 τ2
Selective Delay 0 0 1
a1
τ2
Equal gain Delay 1/3 1/3 1/3
a2
Maximum Delay and h* h1* h0*
Ratio complex amplitudes 2
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Mobile Soft Handover Implementation with Rake
Receiver
h1 h2
τ1 τ2
τ1 τ2
BS 1 BS 2
C1(n) C1(n) C2(n) C2(n)
g g
a1 a2
Diversity Combination To
Decoder
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Softer Handover
Softer handover reception
q combines signals from one base station
BS
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One cell reuse is typical for CDMA
In CDMA, all cells use the same carrier frequency
(frequency reuse = 1)
q makes soft handover possible
q requires efficient power control
q makes system load control more complex
FDMA/TDMA (reuse > 1) CDMA (reuse = 1)
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Capacity
WCDMA capacity limited by
q Amount of interference that can be tolerated
q Amount of interference generated by each user
q Amount of downlink orthogonal codes
Any reduction in generated interference directly improves capacity
q Voice activity
q Bursty transmission (packet-like services)
q Narrow-beam antennas
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Resource Planning versus Power Planning
GSM (TDMA)
q Frequency planning
q Slot assignment
CDMA
q Increased output power ⇒ increased interference ⇒ lower capacity
q Power planning!
Reducing interference (by any means)
⇒ direct increase of capacity
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Cellbreathing
GSM
q Users have their own dedicated time(/frequency) slot
q Number of users in cell does not directly influence cell size
UMTS
q Cellsize is closely related to cell capacity
q Capacity is determined by signal to noise ratio
q Interference adds to the noise:
l other cells
l other users in the same cell
q If there is a lot of noise, users at the cell border cannot increase
their signal any further à cannot communicate
q So: cell size decreases as number of active users increases: Cell
breathing
q Number of active users should be limited
q This complicates cell planning
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Cell breathing: example
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