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EE 615 Lecture 1 Multicarrier Communications
Professor Uf Tureli
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Contents

• Syllabus http//koala.ece.stevens-tech.edu/uture
  li/EE615/home.html
• Todays lecture, motivation for Multicarrier
  Communications
• Next lecture review of random variables,
  discrete time signal processing, digital
  communications, multipath fading channels

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Books

• (Required) OFDM Wireless LANS A Theoretical and
  Practical Guide, John Terry, Juha Heiskala, Dec.
  2001, ISBN 06723221572
• (Recommended) OFDM for Wireless Multimedia
  Communications, Richard van Nee and Ramjee
  Prasad, 2000, ISBN0890065306
• (Recommended) Multicarrier Techniques for 4G
  Mobile Communications, Shinsuke Hara and Ramjee
  Prasad, 2003, ISBN1580534821

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Homework Quizzes

• MATLAB based code along w/ book.
• SystemView with Wireless LAN
• http//www.elanix.com/html/program.html
• Projects About three. Emphasis is on recent
  results. Need to have computer based component.

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Course Outline

• Propagation, Multipath and TV Channels
• Multicarrier System Model
• Implementation aspects
• Channel Estimation
• Synchronization
• Coding
• Multiuser detection/ separation

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Motivation

• Wireless and Internet trends
• WAN, WLAN, WPAN networks
• Bandwidth and Mobility Requirements
• OFDM, OFDMA, MC-CDMA technologies
• Efficient modulation, coding and diversity
  techniques

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Wireless Vision
Applications Wireless Phones Wireless
Internet Access Wireless LANs Smart
Homes/Appliances Automated Highways Video
Teleconferencing Distance Learning Sensor
Networks
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Seamless Connection
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Technical Challenges

• Hardware
• Small, lightweight, low power, multimode
  operation
• Communication Link Design
• Channel characterization
• Fast, robust, spectrally efficient communication
  techniques
• Mitigation of wireless channel impairments
• Multiple Access and Resource Allocation
• Efficient modulation, coding and smart antenna
  schemes that maximize system capacity
• Protocols and cross layer optimization

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• Networking.
• Routing and mobility management for mobile users
• Network reliability
• Network flexibility and scalability.
• Meeting application-specific requirements.
• Performance gap with wireline systems.
• Application Issues
• Adaptable to changing QOS
• Predictive caching
• Resource environment awareness

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Wireless Internet Technologies
UWB
802.11a
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Performance Gap
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Spectrum Regulation

• Spectral Allocation in US controlled by FCC
  (commercial) or OSM (defense)
• FCC allocates spectral blocks for particular
  applications.
• FCC previously gave away spectrum
• Currently holds spectral auctions
• Some spectrum set aside for universal use
• Worldwide spectrum controlled by ITU-R

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History of Mobile Comm
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Standards

• Interacting communication systems require
  standardization
• Standards determined by TIA/CTIA in US
• IEEE standards often adopted
• Worldwide standards determined by ITU-T
• In Europe, ETSI is equivalent of IEEE for
  standards development.

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Example Standards

Mobility
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Systems beyond 3G/ IMT
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Digital Audio Broadcast (DAB)

• DAB supersedes AM/FM
• Main advantage, more channels per BW
• 6 stereo (12 mono) channels per single freq,
  allows tranmission of data (as well as
  programming information)
• DAB uses single frequency network, differential
  quadrature phase shift keying (DQPSK), of BW 1.54
  MHz

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Digital Video Broadcast (DVB)

• MPEG-2 compressed 3 Mbps TV, 20 Mbps HDTV.
• 8 MHz channel, 64 QAM modulation
• Similar to DAB but intended mainly for digital
  television broadcasting.

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Wireless Broadband LANs

• Mobile Multimedia Access Communication (MMAC)
• Japanese Association of Radio Industies and
  Business (ARIB)
• OFDM, 155 Mbps, indooroutdoor
• CBR, VBR and ABR services
• USA-IEEE 802.11a, ETSI- Hiperlan II
• OFDM, 5.7 GHz, 6-54 Mbit/sec

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WPAN(Personal Area Network)

• UWB 802.15.3a proposes Multiband OFDM spread to
  500MHz, many bands envisioned. http//www.multiban
  dofdm.org/
• (Ultra-wideband communications
• Fractional bandwidth (BW/fc) gt 25
• BW gt 1.5 GHx)
• 128 Carriers, 500/1284MHz tone spacing

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Throughput Comparison
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802.15.3 UWB

• UWB Robustness to multipath and fading

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802.15.3 UWB-OFDM

• High bit rate capability proportional to power
  (from Information Theory)
• Fine time resolution (Time Bandwidth product is
  constant), e.g. localization
• Merge UWB with Multicarrier

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WMAN

• Wireless Metropolitan Area Network (WMAN)
• 802.16a
• OFDM TDMA OFDM-FDMA

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Competition

• IEEE 802.11b, DSSS or FHSS, 2.4 GHz, 1,2,11
  Mbit/sec (theoretical)
• Satellite Broadcast Systems
• HomeRF and Bluetooth
• 3G services

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802.11a vs 802.11b
Throughput comparision (Atheros)
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Propagation

• Path loss
• Multipath
• Interference
• Shadowing

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Multipath Channel

• Exponential delay profile, instantaneous and
  average.

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Narrowband Channel

• Multipath spread is smaller than symbol period

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Wideband Channel

• Multipath spans several symbols

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Digital Modulation in Wideband Channels

• Digital modulation encodes bit information in
  amplitude, phase, or freq. of transmitted signal
• Due to multipath, received signal consists of
  echoes.
• Frequency transfer function of the channel varies
  significantly over the BW

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Wideband Channel Defn.

• Properties of channel determine wideband or
  narrowband system
• Outdoor channel, multipath spread 5-20ms, thus
  more than 50 kbits
• Indoor channel, multipath spread 50-200ns or
  less, more than 5 Mbits are truly wideband.

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Multipath Channels

• Wideband channels
• characterized by
• scattering function.
• With multipath
• and Doppler spread
• Coherence
• bandwidth

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Equalization

• Digital Equalizer
• Criterion for coefficient choice
• Minimize BER (Hard to solve for ws)
• Eliminate ISI (Zero forcing, enhances noise)
• Minimize MSE between dn and dn

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Mitigating Multipath effects

• Channel estimation required
• Training based methods
• Tradeoffs in overhead, complexity, and delays
• Linear equalizers can completely eliminate ISI,
  but this may enhance noise.
• Decision feedback (nonlinear) equalizers can
  improve performance.

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Equalizers

• Decision Feedback Equalizer

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History

• Until 1990, analog modulation and Frequency
  Division Multiple Access (FDMA) channels of 25-30
  kHz
• After 1990, digital speech transmission
• (US) D-AMPS/ (Japan) JDC, compatible with old
  analog system, slightly wideband
• Slightly wideband, required equalizers, main
  focus simple one or two tap equalizers and fast
  time variations (slow symboling speed compared to
  rate of change in the channel)

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Digital PCS Age

• European DECT, BW 1.6 MHz, mainly indoor no
  equalizer
• GSM Time Division Multiple Access (TDMA) system,
  BW 200 kHz, outdoor and is wideband, specifies
  equalizer to counteract multipath up to 4 samples
  duration!
• Code Division Multiple Access (CDMA) IS-95
  system, uses RAKE receiver to detect echoes and
  coherently combine similar to equalization.

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Third Generation and beyond

• WCDMA (wideband CDMA)BW 5MHz, wideband even for
  indoor applications.
• New implementation issues because of the
  increased bandwidth compared to CDMA IS-95
  standard.
• Multicarrier (e.g. OFDM) is a new modulation
  technique to distribute information onto many
  subcarriers, so bits on each subcarrier are
  longer and reduce intersymbol interference due to
  multipath.

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Multicarrier Modulation

• Divide bit stream into N substreams

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Orthogonal Division Multiplexing (OFDM)

• OFDM is one of Multicarrier Modulation schemes,
  it distributes information on all carriers
• OFDM known since late 60s, and suggested for
  wireless in 80s, but progress in digital signal
  processing hardware made it feasible in the 90s,
  adopted for DAB/DVB, WLAN (IEEE 802.11a/ HiperLan
  II), in the 2000s likely adoption in Wide Area
  Networking in IEEE 802.16a as OFDM and OFDMA.

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Spectrum of OFDM

• Ex OFDM has completely separated and
  overlapping substreams
• Spectrally efficient

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Transmit Filtering

• To limit out of band emissions

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Adaptive Power Allocation

• Power Adaptation
• Spectral Efficiency
• Spectral Diversity
• Code Diversity
• Spatial Diversity

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Motivation for Spectrum and Power Efficiency

• Until late 90s, mobile radio used exclusively for
  speech telephony, bandwidth needs were
  proportional to number of users served not by
  user demand.
• Data transmission and multimedia demand high bit
  rates, upto that of a wired connection gt 1 Mbps.

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Multimedia Requirements
Delay
-
lt100ms
Packet Loss
0
lt1
BER
lt10-6
lt10-6
Data Rate
1-100Mbps
5-25 Mbps
Traffic
Bursty
Cts
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ADSL / VDSL Multitone

• Physical Layer technology is Multicarrier
• i.e. discrete Multitone (DMT) for xDSL.

• DSL 160 kbit/sec full duplex (ISDN, 2 x 64
  kbit/sec voice)
• HDSL 1.536 Mbit or 2.048 Mbit/sec
• Asym. ADSL 6.8/0.64 up/down

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MCM (DMT) and DSL

• Customer Loop insulated copper wires
• Bridge Taps are open circuit connections

http//www.iec.org/online/tutorials/xdsl_test/topi
c06.html
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Cross Talk in DSL

• Capacitive and inductive coupling between wires
• Far End Cross Talk (FEXT)
• Near End Cross Talk (NEXT)

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Next week

• Read Chapter 1 of Book
• Review of Random Variables, Discrete Time Signal
  Processing
• Digital Communications Multipath Fading
  Channels
• Comparison of MCM w/ Single Carrier
• MCM OFDM 802.11a, overview