1

Multiple Access TechniquesMultiple Access Techniques

2007.102007.10

김영재 김영재 / / 연구전문그룹연구전문그룹

미래기술연구소

2

목차 Multiple Access Techniques

Contentionless Multiple Access

Contention Multiple Access

Hanging Multiple Access

MAC Issues

MAC Design Issues

MAC Layer Issues

Technology Trends

What is 4G ?

Evolution Paths to 4G

3

Multiple Access Techniques

A Multiple Access Technique is defined as a function

sharing a (limited) common transmission resource

among (distributed) terminals in a network.

4

History of Adopted Access Technique

Adopted

Not Adopted

2G systems1990

3G systems2000

4G systems2010

5G systems2020CDM

TDM

OFDM

CDMA

MIMO-SCM

OFDM

MIMO-SCM

?

?

[ Multi-carrier techniques for 4G systems]

Robust against frequency selective fading

A lot of know-how obtained through research and development of wireless LANs and digital broadcasting

Synergistic effects when combined with CDMA

5

Difference Between Multiple Access and Multiplexing

Multiple Access Multiplexing

Resource Network Link

Terminal connectivity Matrix Point-to-(multi)point

Topologies Bus, Star, Ring, Tree Path

Control Central, Distributed Terminal

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Properties of Multiple access protocols (1/2) Good properties

Shall control the allocation of channel capacity to the users

Be efficiency in terms of channel throughput and the delay of transmissions

The allocation should be fair toward individual users

Be flexible in allowing different types of traffic (e,g., voice and data)

Be stable

In equilibrium state, an increase in load should move to a new equilibrium point

Be robust with respect to equipment failure and changing conditions

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Properties of Multiple access protocols (2/2)

In the wireless mobile environment, the protocol should be able to deal

with:

The hidden terminal problem

The near-far effect

The effects of multipath fading and shadowing

The effects of cochannel interference in cellular wireless systems caused by the

use of the same frequency band in different cells

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Classification of Multiple Access Protocol

Multiple access protocols

Contentionless(scheduling)

Contention(random access)

Fixedassigned

Demandassigned

Codingconcept

Subcarrierconcept

RepeatedRandomaccess

RandomAccess

With reservation

FDMATDMA

PollingTokenpassing

CDMA OFDMA ALOHAS-ALOHA

ImplicitExplicit

Hanging(swimming)

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Contentionless Multiple Access Protocols

Fixed assignment scheduling

The available channel capacity is divided among the users

E.g., TDMA, FDMA

Demand assignment scheduling

A user is only allowed to transmit if he/she is active

Demand assignment with centralized control

Polling (e.g., IEEE 802.11 PCF), SRMA

Demand Assignment with distributed control

Implicit: PRMA

Explicit: R-TDMA

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Contention Multiple Access Protocols Features

No scheduling of transmissions

Should resolve the contention

Repeated random access protocols

P-ALOHA, S-ALOHA, CSMA

Random access with reservation

R-ALOHA

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Contention Multiple Access Protocols ALOHA

(p)ure-ALOHA : users transmit any time they desire.

(s)lotted-ALOHA : users begin their transmission only at the beginning of a slot

P P

P2

Vulnerable periodfor slotted ALOHA

Vulnerable period for pure ALOHA

Time

G

G

GeSALOHAslotted

GeSALOHApure

:

: 2

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Capacity for Contention-based protocols

PROTOCOL CAPACITY

Pure ALOHA 0.184

Slotted ALOHA 0.368

1-Persistent CSMA 0.529

Slotted 1-persistent 0.531

0.1-Persistent CSMA 0.791

Non-persistent CSMA 0.815

0.03-Persistent CSMA 0.827

Slotted non-persistent CSMA 0.857

Perfect scheduling 1.000

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Brief history of Contention MAC protocols

Published

Dates

Protocols

1975 ALOHA

- CSMA/CA

1975 BTMA

1976 SRMA

1990 MACA

1994 DFWMAC-DCF

1994 DFWMAC-PCF

1994 EY-NPMA

1994 MACAW

1995 FAMA

1998 GAMA

1998 PAMAS

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Hanging Multiple Access Protocols CDMA type (Spread spectrum )protocols

Direct sequence (DS) CDMA

Frequency hopping (FH) CDMA

Time hopping (TH) CDMA

Subcarrier type protocols

Multi-carrier (MC) CDMA

OFDM-FDMA

OFDM-TDMA

OFDMA

Many others

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CDMA protocols

Use coding to achieve their multiple acc

ess property

Direct sequence

Frequency hopping

Time hopping

Advantages

Low probability of signal detection and int

erception

Protection against hostile jamming

Resistance to multipath fading

Graceful performance degradation from int

erference

Frequency reuse

Frequency

Time

Direct sequence

Frequency hopping

Time hopping

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DS-CDMA Basic concepts

2( ) sinc ( )gS f T fTSpectral Density =

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DS-CDMA 의 다중화

Down Link : Walsh Code 에 의하여 확산처리 및 채널 ( 통화자 ) 구분 , Short PN Code 에 의하여 기지국 구분

Up Link : Long PN Code 에 의하여 통화자 구분

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DS-CDMA 의 다중화 Allow different users to use the channel simultaneously by assigning different

spreading code sequences to them.

Thus there is no physical separation in time or in frequency between signals from

different users.

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DS-CDMA 의 다중화 The physical channel is divided into many logical channels by the spreadin

g codes.

Unlike TDMA and FDMA, spread signals from different users do interfere

each other unless the transmissions from all users are perfectly synchroniz

ed and orthogonal codes are used.

The interference from other users is known as multiple access interference

(MAI).

Synchronous v.s. Asynchronous

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Frequency hopping (FH) CDMA Basic concepts

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Frequency hopping (FH) CDMA Example: FFH system with 2-FSK modulation, 8 hopping bins, and 2

hops per symbol (L = 2).

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Time hopped (TH) CDMA Spread the spectrum by modulating the data signal by a random

pulse-position modulated (PPM) spread signal

TH-SS is used for the conventional UWB communication.

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Subcarrier type protocols Multi-Carrier CDMA

MC-CDMA (OFDM-CDMA)

MC-DS-CDMA

OFDM

OFDM-FDMA

OFDM-TDMA

OFDM-CDMA (MC-CDMA)

OFDMA-FH

OFDMA

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Multi-Carrier CDMA Features of MC-CDMA

The advantages of DS-CDMA systems are its robustness to narrowband in

terference, multipath diversity, and capability of frequency reuse factor of

1.

But, in high-speed transmission, the increase in the number of the resolva

ble paths makes it impossible to implement the rake receiver.

The advantages of multicarrier systems are its robustness to frequency sel

ectivity and reduced complexity in equalization of the receiver.

These advantages of multicarrier modulation and flexibility offered by the

spread spectrum have motivated the combination of two techniques.

Two schemes exist:

MC-CDMA (OFDM-CDMA) and MC-DS-CDMA.

The MC-CDMA signal is generated by a serial concatenation of DS-CDM

A and OFDM. Each chip of the DS spread data symbol is mapped onto a di

fferent subcarrier.

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MC-CDMA (OFDM-CDMA) Spread the data in frequency domain and thus has inherent frequency

diversity.

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MC-DS-CDMA Signal is generated by serial-to-parallel converting the data symbols into N

substreams and applying DS-CDMA on each individual sub-stream.

MC-DS-CDMA system with one subcarrier is identical to a single carrier

DS-CDMA.

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OFDM 의 다중화 방식

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OFDM-FDMA and OFDM-TDMA OFDM-FDMA

Each user occupies a subset of subcarriers for a given time. The frequency

bands assigned to a specific user is not changed over the time.

OFDM-TDMA

Each user occupies more than one OFDM symbols, and transmits on differ

ent time slots.

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OFDMA Each user occupies a subset of subcarriers for a given time. Users should not be ove

rlapped in frequency domain at any given time. But, the frequency bands assigned t

o a specific user may change over the time.

Advantages of OFDMA High speed transmission

No intra-cell interference

Avoidance and averaging the inter-cell interference

Granularity

Multiuser diversity

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OFDMA Multiuser diversity

In wireless system with many users, the achievable data rate of a given resourc

e varies from one user to another.

Such variations make the overall system performance to be maximized by assig

ning each resource to the user who can exploit it best → multiuser diversity.

For example, consider a single cell with one BS and two users:

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OFDMA – multiuser diversity We have the following assumptions:

① The two users are independent, the channel response are independent,

② The users have perfect CSI information,

③ There is perfect feedback channel from users to BS.

④ The BS collects the channel information from the users and allocates su

bcarriers based on the channel measurements reports.

For example, the figures shown below are the channel response for each us

er.

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OFDMA – multiuser diversity Due to interference and noise, some of the subcarriers are in deep fading.

However, since the two users are independent, deep-faded subcarriers for o

ne user may be good for another.

For OFDM-TDMA, the SINR on each subcarrier is the average of two use

rs

For OFDMA with resource allocation, each subcarrier are allocated to the s

pecific user that has the best channel frequency response. Thus the SINR f

or OFDMA is the maximum of two users.

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OFDMA – subchannel allocations Two types of cases are defined:

Fixed case:

the channel is varying slowly and the channel estimation is accurate. The channel measure

ment/report and allocation do not have to update very often. The multiuser diversity can be

used by resource allocation.

Mobile case:

in fast fading environments, the measurement should be sent back quite often to track the

channel. Thus using the multiuser diversity is not feasible. Rather frequency diversity is use

ful.

Consider an OFDMA system with a total number of N subcarriers and K users. Divi

de the N subcarriers into L traffic channels, each with M subcarriers. Define cluster

in which C consecutive subcarriers exist. For example, (M, C) = (8, 4) is:

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OFDMA – subchannel allocations Fixed case

we can treat the channel as constant and use the multiuser diversity. User’s CSI

is periodically reported to the BS, and the BS send back the resource allocation

and the adaptive modulation and coding (AMC) scheme to the users. This is fe

asible since both the TX and RX have the accurate CSI with low overhead.

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OFDMA – subchannel allocations Mobile case:

the channel is varying so fast t

hat it is impractical for BS to a

llocate the channels to the user

s. Obtain the frequency diversi

ty through the subcarrier sprea

ding in the subchannel or freq

uency hopping.

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OFDMA – subchannel allocations Latin Square

An efficient method way of achieving the frequency diversity is the use of the Latin

square.

Def: A Latin square of order N is an N × N matrix from a set Q of N distinct element

s, say Q = {0,1,L, N −1} such that each row and column contains every element of Q

exactly once.

For example 5th degree Latin square is given below. The entries in Q represent diffe

rent users in the same cell: qij = frequency slot for user j at OFDM symbol time i

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OFDMA – subchannel allocations The user 1 is assigned frequency slots 0, 2, 4, 1, 3 respectively.

Note that

① 5 users divide the 25 resources,

② There is no intra-cell interference,

③ Since every user experiences all subcarrier, the frequency diversity is maximiz

ed.

Each BS has its own hopping matrix. The design rule is to have minimum overlap be

tween users of neighboring BSs to minimize the interference.

Two Latin squares are said to be orthogonal if the ordered pair (i, j), where i and j ar

e the entries from the same position in the respective squares, exhaust the N2 possibi

lities.

Orthogonality of Latin square corresponds to there being exactly one time/subcarrier

overlap for every pair of users in different cells.

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OFDMA – subchannel allocations When N is prime, there are N – 1 mutually orthogonal Latin squares. For a =1,L, N −

1, we define an N × N matrix Qa with entry: (i, j = 0, 1, L, N −1)

For example, N = 5 can support four cells. Each user has interference from one user

per cell. User 1 in 1st cell receives interference from user 3 in 2nd cell, user 5 in 3rd

cell, and user 2 in 4th cell.

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MAC Design Issues Many factors

FDD & TDD for MAC

AMC (Adaptive Modulation & Coding)

FEC (Forward Error Correction)

ARQ (Automatic Repeat reQuest)

Hybrid-ARQ

Burst Packet transmission

…

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FDD & TDD for MAC

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AMC (Adaptive Modulation & Coding) QPSK to 64QAM

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AMC (Adaptive Modulation & Coding)

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FEC (Forward Error Correction

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ARQ (Automatic Repeat Request) 수신단에서 전송에러의 유무를 CRC 를 이용하여 점검

정상 :ACK, 비정상 :NAK 으로 수신단에서 회신

송신측 : ACK 신호 수신 Time out 되거나 NACK 수신시 재전송

사용 프로토콜 , 지연 , 패킷사이즈 , 패킷 수 , 버퍼 크기 등에 의하여 성능에 영향

Burst error 형태의 유선환경 : TCP

Scatter Error 형태의 무선환경 : RLP

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Hybrid ARQ ARQ 와 FEC 의 조합에 의한 에러복구

높은 초기 FER 값 설정에 의한 전력 효율성 증대 => 통화용량증대 , throughput 증대 Chase combining 과 Incremental redundancy 를 사용하여 효율성 제고

Chase combining: 에러가 발생한 프레임을 폐기하지 않고 재전송 프레임과 combining

Chase combining

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Hybrid ARQ Incremental Redundancy: 재 전송시 마다 채널코딩 이득을 점차 증가시켜 재전송

Incremental Redundancy

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Burst Packet transmission

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Burst Packet transmission

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MAC Layer Issue List (1/4)Legacy support Interoperability for legacy equipment

Operating frequencies and bandwidth Bandwidth scalability

Duplex Schemes Shall be designed to support both TDD and FDD

operational modes

FDD; full duplex/half duplex, UL/DL bandwidth

configurable

TDD: DL/UL ratio should be adjustable

State transition latency Minimize the time to take IDLE_STATEACTIVE

STATE

User throughput enhancement Average user throughput, Cell edge user throughput

Sector capacity enhancement Total unidirectional sustained throughput (DL/UL),

excluding MAC & PHY layer overheads, across all users

scheduled on the same RF channel

MIMO & beam forming support MIMO-mode feedback – optimize the feedback

mechanism by minimizing the feedback periods and

information.

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MAC Layer Issue List (2/4)Interference mgmt. requirements Support advanced interference mitigation schemes-> to

improve the Cell-edge performance

Support enhanced flexible frequency re-use schemes->

to improve the system capacity in the interference limited

situation

Reduction of overhead requirements System Overhead Reduction

- MAP overhead reduction, frame structure enhancem

ents, etc.

- Efficient feedback channel, efficient MIMO feedback

information elements design

User Overhead Reduction

- RoHC over RTP/UDP/IP, TCP/IP, etc

Handover Optimized handover support

Support inter-RAT Handover (vertical HO)

Support IEEE 802.21 MIH (Media Independent

Handover)

Support IEEE 802.16g NCMS (Network Control and

Management Services)

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MAC Layer Issue List (3/4)Enhanced MBS Provide enhanced multicast and broadcast spectral effi

ciency

Support optimized switching between broadcast and u

nicast services

QoS support Support QoS Classes, enabling an optimal matching of

service, application and protocol requirements to RAN

resources and radio characteristics

- applying different Physical Error Rate per Service Flow

Enhancement of Power Saving Requirements Provide enhanced power saving functionality

- Optimized sleep to scan and scan to sleep mode switching

- Automatic sleep mode reactivation by BS

- Optimized sleep mode deactivation/reactivation by MS

- Optimized paging message indication and decoding

Security Protection of the integrity of the system

Protection and confidentiality of user-generated tarffic

and user-related data

Secure access to, secure provisioning and availability of

service provided by the system

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MAC Layer Issue List (4/4)

LBS Support location-based service requirements

- location determination latency

- position accuracy

Support of Multi-hop Relay Coverage extension

Throughput Enhancement

- e.g., consideration of 16m-MMR support

Regulatory Issues Support regional regulatory requirements

- Emergency Service (E911)

- CALEA (Communication Assistance for Law

Enforcement Act)

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Technology Trends What is 4G ?

Evolution Paths to 4G

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What is 4G ?

The 4G is defined as a completely new fully IP-based integrated SYSTEM

of systems and NETWORK of networks achieved after

CONVERGENCE of wired and wireless networks as well as

computers, consumer electronics, and communication technology and

several other convergences that will be capable of provide 100Mbps

and 1Gbps, respectively in outdoor and indoor environments, with

end-to-end QoS and high security, offering any kind of services at any

time as per user requirements, anywhere with seamless

interoperability, always on, affordable cost, one billing and fully

personalized.

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Convergence is what 4G is about

4G Mobile Comm. Systems4G Mobile Comm. Systems

Fixed

Cellular PhoneSystems, such as2G, 3G, and 3.5G

WPANs, WLAN suchas IEEE 802.11a/n,HiPERLAN/2 and

MMAC

BroadcastingSatellite

Communication

What is 4G?

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4G Technology Status What is 4G?

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4G Technology in search of a business case What is 4G?

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Software-The Next Big Wireless Challenge

New wireless bearersSoftware-defined radioCognitive radio

Haptics, motion,Touch sensitivityLocationImage and gestureRecognitionSociable interfaces

New hardware archi.Low-power processorsSolid-state storageBatteries and fuel cells

Nano platformsMobile middlewareService discoveryDistributed architectureAnd algorithmsDistirbuted node andApplication managementMobile agentsSecurity

Flexible displaysMirco projectorsPassive displays

What is 4G?

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From SDR to Cognitive Radio

Base stations 2006

Mobile equipmentStarting 2008

2012 onwards

Software-Controlled Radio

Software-Defined Radio

Cognitive Radio

•Software control over wireless parameters•Limited or no control over frequency band and modulation techniques

•Software control over wireless parameters•Limited or no control over frequency band and modulation techniques

•Flexible control over all radio parameters•All-digital processing•Frequency agile•Multiple protocols and modulation techniques•Reduces device cost and component count

•Flexible control over all radio parameters•All-digital processing•Frequency agile•Multiple protocols and modulation techniques•Reduces device cost and component count

•Intelligent spectrum sharing•Dynamic selection of frequency bands and modulation techniques•Aware of other radios• Requires regulatory changes

•Intelligent spectrum sharing•Dynamic selection of frequency bands and modulation techniques•Aware of other radios• Requires regulatory changes

What is 4G?

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Evolution Paths to 4G Technology Evolution Path to 4G

Migrating to Wireless Convergence

Evolution of Mobile Communication

OFDM in Evolution of Mobile Communication

Cooper’s Law

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Migrating to Wireless Convergence

3G Release ‘99

3G Release ‘99

WiFi 80211a/b/g

WiFi 80211a/b/g

Nomadic Wireless

Broadband

Nomadic Wireless

Broadband

Fully Mobile withHandoff

Portable

Stationary

3GReleases

4,5,6HSPA

3GReleases

4,5,6HSPA

3G LTE/SAEIPV6, OFDM3G LTE/SAE

IPV6, OFDM

WiMAX MobileWiMAX Mobile

<1 Bps/Hz<200 Kph

2005–2006 2007–2010

<5 Bps/Hz>0 Kph

<2 Bps/Hz<200 Kph

<3.8 Bps/Hz<120 Kph

IPv6

MIMO/OFDMA

Handoff between 3G, WiMAX, WiFi

700 MHZ – 3.5 GHZ

SDR/CR>5 Bps/Hz

<120 Kph

COEXISTENCE COMPETITION CONVERGENCE

4G WIRELESS

WiFi 802.11n

WiFi 802.11n

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Evolution of Mobile Comm.

63

OFDM in Evolution of Mobile Comm.

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3GPP Evolution Path to 4G

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3GPP2 Evolution Path to 4G

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Cooper’s Law (1/3) Area Spectral Efficiency (ASE) – Bps/Hertz/Sector

Area Spectral Efficiency

0.00.5

1.01.5

2.02.5

3.03.5

4.04.5

PHS

GPRS

IS-9

5 cd

ma

cdm

a 1X

WCDM

A

cdm

a 1X

-DO

cdm

a 1X

-EV

PHS ++

cdm

a ++

iArra

ycom

's I-B

urst

++ means smart antennasB

its/

HZ

/Sq

. Km

1 Million Times Improvement Between 1955-2000

Frequency Division:5X improvement between 1955-2000

Modulation Techniques: 5X from FM, SSB, and TDM

Widening of the Usable Radio Spectrum:25X improvement

Spatial Division Multiplexing and Spectrum Re-Use: 1,600X improvement

1 million times improvement between 1955–2000

Frequency Division: 5X improvement between 1955–2000-

Modulation Techniques : 5X from FM, SSB, and TDM

Widening of the Usable Radio Spectrum:25X improvement

Spatial Division Multiplexing and Spectrum Re-use: 1,600X improvement

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Cooper’s Law (2/3) Comparison of Wireless Technologies

TDD: PHS, GSM, PDC200KHz channels

TDD: PHS, GSM, PDC200KHz channels

0.2 Bps/Hz/Sector1.2 (with AAS)

0.2 Bps/Hz/Sector1.2 (with AAS)

CDMA1.25MHz channels

CDMA1.25MHz channels

0.3Bps/Hz/Sector0.3Bps/Hz/Sector

Wideband CDMA5MHz Channels

Wideband CDMA5MHz Channels

0.4Bps/Hz/Sector0.4Bps/Hz/Sector

13.4–32Kbps13.4–32Kbps

64Kbps64Kbps

144–384Kbps144–384Kbps

CDMA2000 EV-DOSeparate 1.25MHz

data channel

CDMA2000 EV-DOSeparate 1.25MHz

data channel

0.5Bps/Hz/Sector0.5Bps/Hz/Sector384–600Kbps384–600Kbps

HSDPA, HSUPA5 MHz Separate & MixedHSDPA, HSUPA

5 MHz Separate & Mixed0.6–1.03Bps/Hz/Sector0.6–1.03Bps/Hz/Sector512Kbps–1Mbps512Kbps–1Mbps

Mobile WiMAX1.25-20MHz

Mobile WiMAX1.25-20MHz

0.8–3.8Bps/Hz/Sector0.8–3.8Bps/Hz/Sector512Kbps–1Mbps512Kbps–1Mbps

Average User Throughput Aggregate Spectral Efficiency

68

Cooper’s Law (3/3) MIMO Will Be Needed for > 4Bps/Hz

2004 2005 2006

20

50

100

200

40Mbps

100Mbps

160Mbps

20Mbps >18Mbps

>36Mbps

>90Mbps

Mbps

• OFDM

• MIMO-OFDM• MAC layer

• Larger bandwidth• MAC& MIMO joint optimization• Robust MIMO to correlation and Doppler

• MIMO-OFDM optimization: higher order modulation, improved channel coding• Advanced MIMO detector

2Bps/Hz

4Bps/Hz

5Bps/Hz

8bps/Hz

Test bed development

Air interface design and verification by

simulation

2007

10 MHz10 MHz10 MHz10 MHz

20 MHz20 MHz20 MHz20 MHz

Source: Alcatel

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Thank you for your kind attention!