Multiple Access Techniques

1
Multiple Access Techniques
2007.10 ??? / ??????
???????
2
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• 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
MIMO-SCM
?
MIMO-SCM
OFDM
?
OFDM
5G systems 2020
CDMA
CDM
TDM
4G systems 2010
Adopted
3G systems 2000
Not Adopted
2G systems 1990

• 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

7
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

8
Classification of Multiple Access Protocol
Multiple access protocols
Contention (random access)
Contentionless (scheduling)
Hanging (swimming)
Fixed assigned
Demand assigned
Coding concept
Subcarrier concept
Repeated Random access
Random Access With reservation
FDMA TDMA
Polling Token passing
CDMA
OFDMA
ALOHA S-ALOHA
Implicit Explicit
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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

Vulnerable period for slotted ALOHA
Time
Vulnerable period for pure ALOHA
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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 access
  property
• Direct sequence
• Frequency hopping
• Time hopping
• Advantages
• Low probability of signal detection and
  interception
• Protection against hostile jamming
• Resistance to multipath fading
• Graceful performance degradation from
  interference
• Frequency reuse

Frequency
Direct sequence
Frequency hopping
Time hopping
Time
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DS-CDMA

• Basic concepts

Spectral Density
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DS-CDMA? ???

• Down Link Walsh Code ? ??? ???? ? ??(???) ??,
  Short PN Code ? ??? ??? ??
• Up Link Long PN Code ? ??? ??? ??

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DS-CDMA? ???

• 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? ???

• The physical channel is divided into many logical
  channels by the spreading codes.
• Unlike TDMA and FDMA, spread signals from
  different users do interfere each other unless
  the transmissions from all users are perfectly
  synchronized 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 interference, multipath
  diversity, and capability of frequency reuse
  factor of 1.
• But, in high-speed transmission, the increase in
  the number of the resolvable paths makes it
  impossible to implement the rake receiver.
• The advantages of multicarrier systems are its
  robustness to frequency selectivity 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-CDMA and OFDM. Each chip of
  the DS spread data symbol is mapped onto a
  different 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? ??? ??
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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 different time slots.

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OFDMA

• Each user occupies a subset of subcarriers for a
  given time. Users should not be overlapped in
  frequency domain at any given time. But, the
  frequency bands assigned to 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 resource varies
  from one user to another.
• Such variations make the overall system
  performance to be maximized by assigning 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
• ? The two users are independent, the channel
  response are independent,
• ? The users have perfect CSI information,
• ? There is perfect feedback channel from
  users to BS.
• ? The BS collects the channel information
  from the users and allocates subcarriers based on
  the channel measurements reports.
• For example, the figures shown below are the
  channel response for each user.

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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 one user may be good
  for another.
• For OFDM-TDMA, the SINR on each subcarrier is the
  average of two users
• For OFDMA with resource allocation, each
  subcarrier are allocated to the specific user
  that has the best channel frequency response.
  Thus the SINR for 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
  measurement/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
  useful.
• Consider an OFDMA system with a total number of N
  subcarriers and K users. Divide 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. Users 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
  feasible since both the TX and RX have the
  accurate CSI with low overhead.

35
OFDMA subchannel allocations

• Mobile case
• the channel is varying so fast that it is
  impractical for BS to allocate the channels to
  the users. Obtain the frequency diversity through
  the subcarrier spreading in the subchannel or
  frequency 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 elements, 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 different 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
• ? 5 users divide the 25 resources,
• ? There is no intra-cell interference,
• ? Since every user experiences all
  subcarrier, the frequency diversity is maximized.
• Each BS has its own hopping matrix. The design
  rule is to have minimum overlap between 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 are the
  entries from the same position in the respective
  squares, exhaust the N2 possibilities.
• 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 Re quest)

• ????? ????? ??? CRC? ???? ??
• ??ACK, ???NAK?? ????? ??
• ??? ACK ?? ?? Time out??? NACK??? ???
• ?? ????, ??, ?????, ?? ?, ?? ?? ?? ??? ??? ??
• Burst error??? ???? TCP
• Scatter Error??? ???? RLP

45
Hybrid ARQ

• ARQ? FEC? ??? ?? ????
• ?? ?? FER? ??? ?? ?? ??? ??gt ??????,
  throughput??
• Chase combining? Incremental redundancy? ???? ???
  ??
• Chase combining ??? ??? ???? ???? ?? ??? ????
  combining

Chase combining
46
Hybrid ARQ

• Incremental Redundancy ? ??? ?? ???? ??? ?? ????
  ???

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_STATE??ACTIVE 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-gt to improve the Cell-edge performance Support enhanced flexible frequency re-use schemes-gt to improve the system capacity in the interference limited situation
Reduction of overhead requirements System Overhead Reduction - MAP overhead reduction, frame structure enhancements, 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 efficiency Support optimized switching between broadcast and unicast 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)
53
Technology Trends

• What is 4G ?
• Evolution Paths to 4G

54
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.

55
Convergence is what 4G is about
What is 4G?
Broadcasting Satellite Communication
Fixed
4G Mobile Comm. Systems
Cellular Phone Systems, such as 2G, 3G, and 3.5G
WPANs, WLAN such as IEEE 802.11a/n, HiPERLAN/2
and MMAC
56
4G Technology Status
What is 4G?
57
4G Technology in search of a business case
What is 4G?
58
Software-The Next Big Wireless Challenge
What is 4G?
Nano platforms Mobile middleware Service
discovery Distributed architecture And
algorithms Distirbuted node and Application
management Mobile agents Security
New wireless bearers Software-defined
radio Cognitive radio
Haptics, motion, Touch sensitivity Location Image
and gesture Recognition Sociable interfaces
New hardware archi. Low-power processors Solid-sta
te storage Batteries and fuel cells
Flexible displays Mirco projectors Passive
displays
59
From SDR to Cognitive Radio
What is 4G?

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

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

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

Software-Controlled Radio
Base stations 2006
Mobile equipment Starting 2008
Software-Defined Radio
Cognitive Radio
2012 onwards
60
Evolution Paths to 4G

• Technology Evolution Path to 4G
• Migrating to Wireless Convergence
• Evolution of Mobile Communication
• OFDM in Evolution of Mobile Communication
• Coopers Law

61
Migrating to Wireless Convergence
lt2 Bps/Hz lt200 Kph
3G LTE/SAE IPV6, OFDM
lt1 Bps/Hz lt200 Kph
3G Releases 4,5,6 HSPA
3G Release 99
Fully Mobile with Handoff
4G WIRELESS
WiMAX Mobile

• IPv6
• MIMO/OFDMA
• Handoff between 3G, WiMAX, WiFi
• 700 MHZ 3.5 GHZ
• SDR/CR
• gt5 Bps/Hz
• lt120 Kph

lt3.8 Bps/Hz lt120 Kph
Nomadic Wireless Broadband
Portable
WiFi 802.11n
Stationary
WiFi 80211a/b/g
lt5 Bps/Hz gt0 Kph
COEXISTENCE
COMPETITION
CONVERGENCE
20052006
20072010

62
Evolution of Mobile Comm.
63
OFDM in Evolution of Mobile Comm.
64
3GPP Evolution Path to 4G
65
3GPP2 Evolution Path to 4G
66
Coopers Law (1/3)

• Area Spectral Efficiency (ASE) Bps/Hertz/Sector

1 million times improvement between
1 Million Times Improvement Between
19552000
1955
-
2000
Frequency Division
5X improvement
Frequency Division
5X improvement
between 19552000
-
between 1955
-
2000
Modulation Techniques
5X from FM,
Modulation Techniques
5X from FM,
SSB, and TDM
SSB, and TDM
Widening of the Usable Radio
Widening of the Usable Radio
Spectrum
25X improvement
Spectrum
25X improvement
Spatial Division Multiplexing and
Spatial Division Multiplexing and
Spectrum Re
-
use
1,600X
Spectrum Re
-
Use
1,600X
improvement
improvement
67
Coopers Law (2/3)

• Comparison of Wireless Technologies

TDD PHS, GSM, PDC 200KHz channels
0.2 Bps/Hz/Sector 1.2 (with AAS)
13.432Kbps
Average User Throughput
Aggregate Spectral Efficiency
CDMA 1.25MHz channels
0.3Bps/Hz/Sector
64Kbps
Wideband CDMA 5MHz Channels
0.4Bps/Hz/Sector
144384Kbps
CDMA2000 EV-DO Separate 1.25MHz data channel
0.5Bps/Hz/Sector
384600Kbps
HSDPA, HSUPA 5 MHz Separate Mixed
0.61.03Bps/Hz/Sector
512Kbps1Mbps
Mobile WiMAX 1.25-20MHz
0.83.8Bps/Hz/Sector
512Kbps1Mbps
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Coopers Law (3/3)

• MIMO Will Be Needed for gt 4Bps/Hz

Mbps
Air interface design and verification by
simulation
20 MHz
8bps/Hz
200
Test bed development

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

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

10 MHz
5Bps/Hz

• MIMO-OFDM
• MAC layer

100
4Bps/Hz

• OFDM

50
2Bps/Hz
20
2004
2006
2007
2005
Source Alcatel
69

• Thank you for your kind attention!