CELLULAR MOBILE COMMUNICATION SYSTEMS - · PDF fileCELLULAR MOBILE COMMUNICATION SYSTEMS May...

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CELLULAR MOBILE COMMUNICATION SYSTEMS May 16, 2006 Niclas Wiberg, Ericsson Research

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Page 1: CELLULAR MOBILE COMMUNICATION SYSTEMS -  · PDF fileCELLULAR MOBILE COMMUNICATION SYSTEMS May 16, 2006 ... GSM, GPRS (2G) WCDMA (3G) ... • power control commands

CELLULAR MOBILE COMMUNICATION SYSTEMS

May 16, 2006Niclas Wiberg, Ericsson Research

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Contents

� Introduction to cellular systems� GSM, GPRS (2G)� WCDMA (3G)� System modeling and analysis� High-Speed Downlink Packet Access (HSDPA)

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History

� Analog systems (e.g NMT)– speech

� Digital systems (e.g GSM)– speech and simple data, improved capacity

� Evolved digital systems (GPRS, EDGE)– improved data services

� Third generation (WCDMA)– high bit rates, improved flexibility, improved capacity

� Fourth generation?

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Architecture Overview

base stations

mobile stations

base station controller(s)

mobile switch(es)

fixed network

air interface

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Resource Limits

� attenuation, shadowing, and receiver noise� multipath fading & time dispersion� transmitter power� interference

� coverage vs. capacity limits

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Multiple Access:Handling Several Users Simultaneously

TDMA: Time Division Multiple Access

t

f

FDMA: Frequency Division Multiple Access

t

f

FDMA+TDMA

t

f

t

f

CDMA: Code Division Multiple Access

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The GSM Air Interface

� FDMA– 174 frequencies with 200kHz spacing (GSM 900)– separate bands for uplink and downlink– divided between operators

� TDMA– 8 time slots per frequency band (one user per time slot)

� Modulation– 271 kbit/s GMSK

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The GSM Air Interface, cont.

456 bits

244 bits

interleaving & segmentation

456 bits

244 bits

channel coding

20 ms audio frame 20 ms audio frame

speech coding

TDMA frame with 8 time slots

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The GSM Air Interface, cont.

TDMA frame ≈ 4.6 ms

3 bits tail

3 bits tail

8 bits guard

26 bits pilot

58 bitsdata

58 bitsdata

timeslot ≈ 0.58 msbitrate 271 kbit/s, GMSK modulation

payload burst

3 bits tail

3 bits tail

68 bitsguard

41 bitspilot

36 bitsdata

access burst

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GSM: Frequency Planning

� Reuse factor

� Tighter reuse:– higher capacity– interference between

cells

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GSM: Handover

activeradio link

measurement1. mobile measures

other cells

activeradio link

measurement2. better cell detected,

handover initiated

measurement activeradio link

3. handover completed

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GSM, cont.

� Power control– maintain adequate quality at minimum power– reduces interference– increases battery life

� Frequency hopping– avoids bad quality due to fading and interference

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GPRS:General Packet Radio Services

� Uses GSM modulation� Channel setup and release on demand

– “always connected” at low cost

� Timeslot scheduling– several time slots per user– several users per time slot

� Different coding schemes– bitrate depends on radio conditions

� Retransmission of erroneous packets

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EDGE:Enhanced Data rates for GSM Evolution

� Higher-order modulation – 8-ary phase-shift keying– bitrates up to 384 kbps– same spectral properties as GSM & GPRS

� Fast link adaptation– good channel conditions => high bitrates– bad channel conditions => low bitrates, better robustness

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WCDMA (UMTS, 3G) Requirements� Speech capacity & coverage better than GSM� Efficient & flexible services:

– realtime– variable bitrate– packet

� 2 Mbit/s in good conditions� 384 kbit/s everywhere

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CDMA Principle

Up sampling× SF

Encoded bit sequence (±1)

Pseudo noise spreading

sequence (±1)

ModulationTransmitted signal

Demodulation

Received noisy signalAveraging

÷ SF

Received (less) noisy bit sequence

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CDMA Interference Suppression

f

P

fc

original signal

f

P

fc

spread signal

f

P

fc

received signal

f

P

fc

despreadsignal

f

P

fc

received signal after averaging

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CDMA Bit Rate Flexibility

Power is the common shared resource

VaryingUser Bit Rate

VaryingUser Bit Rate

Translates intoVarying spreading factorVarying power levelBut, same physical resource

Varying spreading factorVarying power levelBut, same physical resource

Bit rate

Spreading factor

Power level

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CDMA Near/Far Effect

Power control needed to keep interference down

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WCDMA Air Interface Basics

� 3.84 MHz chip rate� QPSK modulation, 5 MHz bandwidth� Spreading factor from 4 to 256� Bitrates up to

– 2 Mbit/s with multicode– 1 Mbit/s with single code

� 1500 Hz closed-loop power control� Frame length 10-80 ms

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WCDMA Physical Channels and Power Control

uplink physical control channel

uplink physical data channel

downlink physical control channel

downlink physical data channel

Physical control channel:• pilot for channel estimation• power control commands• data format information

Physical data channel:• user data

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WCDMA Air Interface Example:Speech and Data Multiplexed

244 bit speech frame

320 bit data640 bit data

244 bit speech frame

320 bit data(no data)

coding, multiplexing, and interleaving

1200 bit2400 bit

300 bit 1200 bit

10 ms

slot segmentation

15 slots

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WCDMA: Frequency Reuse 1

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WCDMA: Soft Handover

� Enhances quality� Reduces interference due to

fast fading� Transmit power determined

by best link

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Simple Quality Model (Uplink)

� Received signal power user i:

gi pi

� Total received power, “interference”

I = N0 + g1 p1 + g2 p2 + … + gM pM

� Signal-to-noise ratio user i

γi = gi pi / (I − gi pi) ≈ gi pi / I

p1

p2

pM

g1 g2

gM

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Simple Capacity Calculation (Uplink)� Power control: assume pi is

controlled such that the signal-to-noise ratio is constant:

gi pi / I = Γ

� Interference

I = N0 + g1 p1 +…+ gM pM = N0 + M I Γ

� Solving for I results in

I = N0 / (1 − M Γ)0 20 40 60 80 100

0

2

4

6

8

10

12

14

16

18

20

Number of users

Noi

se r

ise

[dB

]

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High-Speed Downlink Packet Access (HSDPA)

� Fast Link Adaptation– Data rate adapted to radio conditions– 2 ms time bases– Higher order modulation

� Fast Retransmissions– Roundtrip time of 10-20 ms possible– Soft combination of multiple attempts

� Shared Channel Transmission– Dynamically shared in code and time

� Fast Radio-Dependent Scheduling– Transmit to users with good radio

conditions– 2 ms time basis 2 ms

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Fast Link Adaptation

� Adjust transmission parameters to match instantaneousradio channel conditions

– Path loss and shadowing– Interference variations– Fast multi-path fading

� Rate controlled (not power controlled)– Encoding rate, number of channelization codes & modulation

type adapted based on available power

– Adaptation on 2 ms TTI basis ⇒ 500 times/sec!

High data rate

Low data rate

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Fast Link Adaptation Power Utilization

Dedicated channels (power controlled)

Common channels

Power usage with dedicated channels channels

t

Unused power

Power

HSDPA with dynamic power allocationt

Dedicated channels (power controlled)

Common channels

HSDPA (rate controlled)

To

tal c

ell p

ow

er

Power

To

tal c

ell p

ow

er

– No need for extra spectrum/carrier– Voice and data on same carrier

Original WCDMA WCDMA HSDPA

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Scheduling

� Scheduling = which UE to transmit to at a given time instant

� Basic idea: – Transmit to users based on radio channel quality, targeting

fading peaks– May lead to large variations in data rate between users– Tradeoff: fairness vs. cell throughput

high data rate

low data rate

Time

#2#1 #2 #2#1 #1 #1

User 2

User 1

Scheduled user

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Performance Example

� 2-3 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

Normalized throughput 1 equals approx. 600 kbit/s/cell.

Web browsing, PedA channel model

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Latency Radio-access-network contribution of Round Trip Time (RTT)

Latency

≈≈≈≈150 ms Initial WCDMA (R99)

HSDPA<100 ms

30-50 msEnhanced uplink

UL 384 kbpsDL 384 kbps

UL 384 kbpsDL 14400 kbps

UL 4700 kbpsDL 14400 kbps

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