[PPT] - 192620010 Mobile & Wireless Networking Lecture 5: Cellular PowerPoint Presentation

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Mobile and Wireless Networking 2013 / 2014

192620010 Mobile & Wireless Networking Lecture 5: Cellular Systems (UMTS / LTE) (1/2) [Schiller, Section 4.4]

Geert Heijenk

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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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Source: Agilent Technologies, 2012

Evolution of cellular systems

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GSM Architecture

fixed network BSC BSC MSC MSC GMSC OMC, EIR, AUC VLR HLR NSS with OSS RSS VLR

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1 2 3 4 5 6 7 8 higher GSM frame structures

935-960 MHz 124 channels (200 kHz) downlink 890-915 MHz 124 channels (200 kHz) uplink

time

GSM TDMA frame GSM time-slot (normal burst) 4.615 ms 546.5 µs 577 µs

tail user data Training S guard space S user data tail guard space

3 bits 57 bits 26 bits 57 bits 1 1 3

GSM Radio Interface: TDMA/FDMA

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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) q Few changes to base station

(software)

q New core network architecture

(router-based)

Class Receiving slots Sending slots Maximum number of slots 1 1 1 2 2 2 1 3 3 2 2 3 5 2 2 4 8 4 1 5 10 4 2 5 12 4 4 5 Coding scheme 1 slot 2 slots 3 slots 4 slots 5 slots 6 slots 7 slots 8 slots 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

MS BSS GGSN SGSN MSC Um EIR HLR/ GR VLR PDN Gb Gn Gi SGSN Gn

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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 UE CN Iu Uu

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

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UTRAN architecture

UTRAN comprises several RNSs Node B can support FDD or TDD or both RNC is responsible for handover decisions requiring signalingto the UE Cell offers FDD or TDD

RNC: Radio Network Controller RNS: Radio Network Subsystem

Node B Node B RNC

Iub

Node B UE1 RNS CN Node B Node B RNC

Iub

Node B RNS

Iur

Node B UE2 UE3

Iu

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Core network: architecture

BTS Node B BSC

Abis

BTS BSS MSC Node B Node B RNC

Iub

Node B RNS Node B SGSN GGSN GMSC HLR VLR

IuPS IuCS Iu

CN EIR

Gn Gi PSTN

AuC GR

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UMTS Protocol Architecture - User Plane

UMTS UMTS Transport Network Legend: UE IuPS Iub Gn Gi GGSN IP UDP IP GTP-U PHY MAC RLC PDCP Node B Uu UDP UDP IP SGSN GTP-U IP IP

TCP

RLC RNC IP UDP MAC PDCP GTP-U PHY L2

App

FP FP L1 L2 L1 L2 L1 L2 L1 L2 L1 L2 L1 IP L2 L1

TCP App

L2 L1 Host Internet Other

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UMTS Protocol Architecture – Control Plane

UMTS UMTS Transport Network Legend: UE IuPS Iub

Signalling Bearer

SCCP UMM/SM RANAP PHY MAC RLC RRC Node B Uu SGSN UMM/SM RLC RNC

Signalling Bearer

SCCP MAC RRC RANAP PHY L2 NBAP NBAP L1 L2 L1 L2 L1 L2 L1

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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 known as Wideband CDMA Chip rate 3.84 Mc/s Carrier spacing 5 MHz

Channel coding Transport channels Multiplexing Mapping to physical channels Spreading Spreading Physical channels Physical-layer procedures and measurements Channel coding ≈5 MHz Modulation Modulation 3.84 Mc/s Transport-channel processing

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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

rthogonal spreading codes

q precise synchronisation not necessary as the scrambling codes stay quasi-

rthogonal

data1 data2 data3 scrambling code1 spr. code3 spr. code2 spr. code1 data4 data5 scrambling code2 spr. code4 spr. code1

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 X X,X X,-X 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,-1,1,-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,-1,-1,-1,-1 SF=1 SF=2 SF=4 SF=8 SF=n SF=2n ... ... ... ...

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UMTS FDD frame structure

W-CDMA

1920-1980 MHz uplink
2110-2170 MHz downlink
chipping rate:

3.840 Mchip/s

soft handover
QPSK
complex power control

(1500 power control cycles/s)

spreading: UL: 4-256;

DL:4-512

1 2 12 13 14 ... Radio frame Pilot FBI TPC Time slot 666.7 µs 10 ms Data Data1 uplink DPDCH uplink DPCCH downlink DPCH TPCTFCI Pilot 666.7 µs 666.7 µs DPCCHDPDCH 2560 chips, 10 bits 2560 chips, 102k-1 bits (k = 1...7) TFCI 2560 chips, 102k bits (k = 0...7) Data2 DPDCH DPCCH FBI: Feedback Information TPC: Transmit Power Control TFCI: Transport Format Combination Indicator DPCCH: Dedicated Physical Control Channel DPDCH: Dedicated Physical Data Channel DPCH: Dedicated Physical Channel

Slot structure NOT for user separation but synchronisation for periodic functions!

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21 Channel bit rate [kbps] User bit rate (bef. coding) [kbps] k Spreading factor Uplink Downlink Uplink Downlink 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

Bit rates and Spreading Factors

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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 Distance (log) Short term fading

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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

PI PC Worst case: PC(dB) – PI(dB) = – 80 dB! Interferers are rejected by the processing gain: Power control with a large dynamic range is essential! G = = = 100 → 20 dB Rchip Rbit 106 104 ⇒ = – 80 + 20 = – 60 dB! C I

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Why Soft Handover?

Soft handover essential for power control Soft handover reception

q combines signals from different base stations

BS 1 BS 2 RNC

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Time Dispersion – Rake receiver – Channel Estimation

τ1 τ2 h1 h0 h2

Channel

Diversity Combination Selective Equal gain Maximum Ratio Channel Estimation Delay Delay Delay and complex amplitudes a2 a1 a0 1 1/3 1/3 1/3 h2* h1* h0*

g g g

C(n)

τ2 τ1

C(n) C(n) a2 a1 a0 r(n) Diversity Combination

a0 a1 a2

To Decoder

τ1 τ2 τ2

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Mobile Soft Handover Implementation with Rake Receiver BS 1 BS 2

g g

C2(n)

τ2 τ1

C1(n) a1 a2 Diversity Combination To Decoder C1(n) C2(n) h1 h2 τ1 τ2

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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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