1 Multiple Access Techniques Multiple Access Techniques 2007.10 2007.10 김김김 김김김 / / 김김김김김김 김김김김김김 미미미미미미미
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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
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Multiple Access Techniques
A Multiple Access Technique is defined as a function
sharing a (limited) common transmission resource
among (distributed) terminals in a network.
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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
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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.
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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
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-EV
PHS ++
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'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
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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!