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Wireless Communications: The Future


Current Position A wide range of wireless devices Mobile Fixed Short-range Broadcasting Mobile ... W-LAN for hotspot and indoor voice and ... (link scheduling, ... – PowerPoint PPT presentation

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Title: Wireless Communications: The Future

Wireless CommunicationsThe Future
  • Professor Song Chong
  • Network Systems Laboratory
  • song_at_ee.kaist.ac.kr

Current Position
  • A wide range of wireless devices
  • Mobile
  • Fixed
  • Short-range
  • Broadcasting
  • Mobile
  • Cellular
  • 2G
  • 3G
  • WiMax
  • So-called 4G
  • Mobile Mesh
  • Emerging technologies cognitive radio (CR),
    software-defined radio (SDR)

Current Position
  • Fixed
  • Point-to-point
  • Point-to-multipoint
  • Fixed mesh
  • Short-range
  • W-LANs
  • 802.11 family
  • Zigbee
  • W-PANs
  • BlueTooth
  • High-speed variants such as WiMedia/UWB
  • RFIDs
  • Broadcasting
  • Analog and digital broadcasting
  • Mobile broadcasting

3G Cellular
  • In 2006, 3G systems were starting to be widely
  • W-CDMA (European standard), Cdma2000 (US
    standard), TD-SCDMA (Chinese standard)
  • 3G will eventually take over from 2G but the
    growth may occur more because the operators
    push the new technology than the subscribers
    demand it.
  • Realistic data rates of 3G will not go beyond
    around 400 Kbps.
  • The lifetime of 3G will be around 20 years.
  • The wide use of new services 3G offers such as
    video call and streaming will take 10 years.
  • As of today, the benefits of 3G were being
    realized as increased voice call capacity.
  • 3G will face competition from other technologies.
  • W-LAN for hotspot and indoor voice and data
  • WiMax for outdoor voice and data, although this
    is not certain

4G Cellular
  • The deployment of 4G sometime around 2014-2018
    might look like a fairly certain bet.
  • Definition of 4G is still opaque, 4G is likely to
    be different from 3G (not just be a new air
    interface), and perhaps may not even emerge.

4G Cellular
  • Each generation has accepted a smaller cell size
    in return for a higher data rate.
  • Higher data rate -gt more spectrum -gt higher
    frequency -gt lower propagation range -gt smaller
  • The next step in the process, where 4G might
    logically fit, is already taken by a mix of 3G
    enhancements, WiMax and WiFi.
  • E.g., the Japanese plan for 4G (OFDM, 3-6 GHz
    band, 100 Mbps) is almost identical to the
    specification for 802.11a
  • 4G systems, if realized, can be economically
    deployed only in high-density areas.
  • A further increase in air interface data rate is
    pointless without better backhaul technologies.
  • E.g., insufficient speed of ADSL
  • There may not be sufficient economic
    justification for the development of a completely
    new standard like 4G.
  • Instead, might expect to see novel enhancements
    to the current standards making up the complete
  • E.g., WiFi-like cellular, cellular-like WiFi,
    femto-cell network

Prognosis for Cellular
  • A long period of stability, with profitable
    operation and deployment of 3G, is expected.
  • The likelihood of dramatically new or
    destabilizing technologies appears to be low.
  • There appear to be few threats to cellular
    revenues, with the exception of in-building voice
    calls transferring to W-LAN over time.

Short-range Devices
  • Potential applications for short-range devices
    are those that are not well suited to cellular.
  • Networking around the office or home
  • High-speed data transfer
  • Cable replacement
  • Machine-to-machine communications

Prognosis for Short-range Devices
  • W-LANs and BlueTooth will dominate the
    short-range devices market, providing building
    networks and device-to-device connectivity,
  • WiMedia/UWB is still a developing technology and
    it is unclear whether there are sufficient
    applications that need its very high data rates.
  • Zigbee is likely to succeed as a niche standard
    for specific applications where widespread
    interoperability is not needed but battery life
    is critical.
  • RFID is used in quite different applications from
    other short-range devices. There is little reason
    why it cannot continue to be successful.

How People React to New Technologies
How People React to New Technologies
  • A new service or product might take 4 to 10 years
    to reach mass adoption. Adding 5 years for the
    standardization and development, it might take 15
    years from conception to large-scale success.
  • It is unlikely that total communications spending
    will grow by more than 0.15 of household income
    per year.

Spectral Efficiency is Approaching Limit
  • Under some assumptions, Shannons law yields
  • where number of users that can be
  • total available bandwidth
  • user bit rate
  • closeness to the Shannon
  • For a system with 1, 1MHz and
    10 Kbps, 142 calls.
  • 3G systems with HSDPA enhancement attain 30-50
    calls per cell, delivering about a third
    of the maximum capacity achievable.
  • Efficiency of wireless systems is approaching
    fundamental limits. Reaching further these limits
    through technology is not easy.
  • A relatively easy way to drastically increase
    the capacity is to use smaller cells (micro,
    pico, femto etc.) and not better technology.
  • While there is some prospect that MIMO might
    increase capacity beyond these, this prospect
    seems relatively small, especially its benefits
    decline in a small-cell environment.

Key Technical Observations Empirical Laws
  • Moores law
  • Best industry prediction at present suggests that
    the growth trends will slow around 2010 and may
    stop altogether around 2016.
  • Use of multiple parallel processors may allow
    some further improvement, but they are costly,
    power hungry and difficult to work with.
  • Steady progress but no key breakthrough is
    expected in areas such as processing power, hard
    disk, batteries etc.

Key Technical Observations Empirical Laws
  • Edholms law
  • Data rates for three communications categories
    (wired, wireless and nomadic) increase on similar
    exponential curves, the slower rates trailing the
    faster ones by a predictable time lag.
  • Key is its prediction that wired and wireless
    will maintain a near-constant differential in
    data rate terms, although nomadic and wired seem
    to gradually converge at around 2030.
  • The law predicts that, in 2010, 3G, Wi-Fi and
    office LAN will deliver around 1Mbps, 200 Mbps
    and 5 Gbps, respectively.

Key Technical Observations Empirical Laws
  • Coopers law
  • The number of voice calls carried over radio
    spectrum has doubled every 30 months for the past
    107 years, implying that the effectiveness of
    spectrum utilization in personal communications
    has improved a million times, i.e., , since
  • A 15 times by allocating more spectrum, a 5 times
    by frequency division, a 5 times by enhancing
    modulation techniques
  • The lions share of the improvement, a 2700
    times, was the result of effectively confining
    individual conversations to smaller and smaller
    areas by spatial division or spectrum reuse
  • Despite being close to the Shannon limit, there
    is no end in ever increasing wireless capacity if
    we are prepared to invest in an appropriately
    dense infrastructure.

Key Technical Observations Empirical Laws
  • Metcalfes law
  • The value of a network equals approximately
    (or ) where is the number of users of
    the system.
  • Unlike Moores or Edholms law, Metcalfes does
    not have a time limit to it. It will likely apply
    to a wide range of new networks in the future as
    new types of devices and networks are invented.

Technologies Lowering Cost Backhaul
  • Cells have to be connected back into the
    infrastructure via backhaul.
  • More costly as cells gets smaller.
  • Backhaul technologies
  • Cabling (copper, coaxial or fiber optic)
  • Fixed wireless
  • Wireless Mesh
  • There are no significant technological changes
    expected that can lead to reduced backhaul costs
    and availability.
  • The exception is potential advances of wireless
    mesh technology but it requires technological
    innovation to overcome its capacity and delay
    problems due to multi hopping and

Emerging Communications Techniques
  • Disruption-tolerant network (DTN)
  • Software-defined radio (SDR)
  • Cognitive radio (CR)
  • Opportunistic communications
  • Relays
  • Mesh/ad-hoc network
  • Cross-layer control

Disruption-tolerant Networks (DTN)
  • Provide useable and useful communications across
    networks that are frequently disconnected and/or
    has no stable end-to-end path due to mobility,
    density, attack, disaster or environmental
  • Use store and forward protocol and the concept of
  • Can provide increases in both availability and
  • Form an extremely important communication
    protocol, but it will be a number of years before
    operational deployment is practical.

Disruption-tolerant Networks (DTN)
  • Human mobility models
  • End-to-end delay

Brownian motion
Levy walk
Random waypoint
Software-defined Radio (SDR)
  • Many future visions of wireless communications
    involve multi-modal devices connecting to a wide
    range of different networks or devices modifying
    their behavior as they discover new types of
  • The current approach, incorporating the chipsets
    from each of the different standards into the
    device, works well but it is intrinsically
  • SDR is for communication devices to be designed
    like computers with general-purpose processing
    capabilities and different software for different
  • In the future this flexibility might enable the
    more efficient use of the spectrum through rapid
    deployment of the latest radio technologies.
  • Issues with SDR implementation, particularly at
  • Difficulties in implementing broadband antennas
  • Lack of sufficient processing power
  • Insufficient battery power
  • High cost

Software-defined Radio (SDR)
  • In practice, the benefits of SDR appear
    relatively minor compared to the issues.
  • The current approach of multi-modal devices works
    well and will likely always be less expensive
    than a general-purpose SDR radio.
  • Further, since new technologies are generally
    introduced much less frequently than users
    replace handsets, there is little need for a
    handset to download a new standard.
  • Because of this, we do not expect true SDRs
    that can reprogram their radio at handsets during
    the next two decades.
  • We do, however, expect handsets to be able to
    download a wide range of new applications.
  • We also expect SDR base stations that can modify
    their behavior as hey discover new types of
    network or standard.

Cognitive Radio (CR)
  • Three approaches to spectrum scarcity
  • Unlicensed bands e.g., ISM band
  • Underlay must operate below the FCC Part 15
    noise limit and must use a very broadband carrier
    (at least 500 MHz), e.g, UWB
  • Overlay dynamic usage of previously allocated
    spectrum when non-allocated users can prove that
    they will not disrupt the incumbent, e.g., CR
  • CR has been defined by ITU as a radio or system
    that senses, and is aware of, its operational
    environment and can dynamically and autonomously
    adjust its radio operating parameters
  • The premise for CR is the observation that
  • Effectively all the spectrum of interest has been
    allocated, thereby firmly establishing spectral
  • Most of the spectrum, in most of the places, most
    of the time is underutilized
  • CR is sometimes described as frequency-agile

Cognitive Radio (CR)
  • Spectrum utilization for two of the USAs busiest
  • The net spectrum utilization is 17.4 for Chicago
    and only 13.1 for New York.
  • This suggests considerable opportunity for the
    deployment of overlay solutions based on CR.

Cognitive Radio (CR)
  • Will CR work? It may not work well.
  • One of the key challenges is to overcome the
    hidden terminal problem.
  • The problem can be solved by the base station
    transmitting beacon, indicating the spectrum
    band is free.
  • Such an approach requires central management by
    the owner of the band including a choice as to
    whether they wish to allow secondary access and
    if so under what conditions.
  • Is the spectrum needed? There is little need.
  • 3G operators in 2005 were still typically only
    using 50 of their spectrum allocation.
  • Additional 3G spectrum was promised at 2.5-2.7
    GHz and at UHF after analog TV switch-off.
  • Cellular demand may eventually fall as more
    traffic flows to W-LANs.

Opportunistic Communications
  • Opportunistic scheduling

User 1
Fading channel
User M
Opportunistic Communications
  • Opportunistic routing
  • Source broadcasts each packet without intended
  • Learn the set of nodes which actually received
    the packet.
  • A receiver in the set that is closest to the
    destination is selected to forward the packet.
  • This continues until the destination receives the
  • Opportunistic routing provides more throughput
    than conventional routing
  • Each transmission has more independent chances of
    being received and forwarded.
  • Take advantage of transmissions that reach
    unexpectedly far.
  • TX counts In opportunistic routing,
    (1-(1-0.25)4)-11 2.46
  • In conventional routing, 41

  • New generation of cellular requires dense BS
    deployment for the following reasons.
  • Higher data rates can be attained by a smaller
    cell and a higher carrier frequency.
  • Transmission at high carrier frequency (gt 2GHz)
    is vulnerable to non-line-of-sight environment
    such as metropolitan area.
  • However, it is unacceptable due to its high
    deployment and maintenance cost.
  • A cost-effective alternative is multi-hop
    relaying approach.
  • Dense deployment of cheap relay stations (RS)
    with low transmit power
  • Multi-hop wireless connection to BS, forming
    wireless mesh

  • Benefits of relays
  • Low cost compared to BS deployment
  • Coverage and fairness enhancement
  • Unknowns and challenges
  • If RS-RS and RS-BS transmissions use the same
    radio with MS-BS and MS-RS transmissions, total
    system throughput may decrease.
  • RSs act as additional interference sources to
    neighboring cells so that ICI becomes more severe
    and total system throughput may decrease unless
    ICI is tightly managed.
  • Cross-layer control of wireless mesh network is a
    big challenge.

Wireless Mesh Networks (WMN)
  • A wireless inter-network of various sub-networks
    including Wi-Fi networks, cellular networks,
    WiMax networks, sensor networks etc.
  • A wireless backhaul network for Wi-Fi networks,
    cellular networks, WiMax networks etc.
  • Many other application areas including community
    networking, enterprise networking, home
    networking etc.

Wireless Mesh Networks (WMN)
  • The current 802.11-based mesh technology cannot
    meet the promise.
  • Insufficient capacity even with multiple channels
  • Unfairness depending on path length
  • No proven wireless multi-hop protocol stack
  • TCP from wired Internet, routing protocols (AODV,
    OLSR, DSR etc.) from MANET and MAC from Wi-Fi
  • A clean slate protocol stack that can squeeze
    most performance out is necessary to meet the
    promise. Its design involves
  • Understanding of optimal interaction between
    transport, routing, MAC (link scheduling, power
    control) and PHY layers
  • Finding distributed algorithms and protocols that
    can most closely approximate the optimality
  • Understanding of multi-link interference and
    finding maximal independent link sets in a
    distributed manner
  • Understanding of optimal interaction between mesh
    links and access links if they share the same

Cross-layer Control
  • Wired multi-hop networks
  • Network utility maximization
  • Link capacity is given and constant
  • Flow control problem at transport layer

Cross-layer Control
  • Lagrangian function
  • Dual problem
  • Dual decomposition
  • Flow control at source
  • Congestion price at link
  • TCP is an approximation of this dual decomposition

Cross-layer Control
  • Wireless multi-hop networks
  • Long-term network utility maximization
  • Link capacity is time-varying and a function of
    resource control
  • Joint rate, power allocation and link scheduling

Cross-layer Control
  • Lagrangian function
  • Dual problem
  • Dual decomposition
  • Flow control at source
  • Scheduling/power control at link
  • Congestion price at link
  • Joint MAC and transport problem
  • Distributed scheduling/power control is a
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