Wireless

1
Wireless Mobile Communications Chapter 9 3G
Cellular

• What is 3G?
• The ITUs International Vision
• The need/motivation for 3G
• The Major Players
• 3G Architecture and Services
• W-CDMA and CDMA2000 technologies

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What is 3G?

• The current cellular system is referred to as 2G
  cellular.
• It differs from the the first generation cellular
  in that the system is fully digital and provides
  roaming on a national or regional basis
• The next generation cellular, 3G, is envisioned
  to enable communication at any time, in any
  place, with any form, as such, it will
• allow global roaming
• provide for wider bandwidths to accommodate
  different types of applications
• support packet switching concepts
• The ITU named this vision IMT-2000
  (International Mobile Telecommunications 2000)
  with the hope of having it operational by the
  year 2000 in the 2000MHz range.

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IMT 2000 Vision

• Common spectrum worldwide (2.8 2.2 GHz band)
• Multiple environments, not only confined to
  cellular, encompasses cellular, cordless,
  satellite, LANs, wireless local loop (WLL)
• Wide range of telecommunications services (data,
  voice, multimedia, etc.)
• Flexible radio bearers for increased spectrum
  efficiency
• Data rates of 9.6Kbps or higher for global (mega
  cell), 144Kbps or higher for vehicular (macro
  cell), 384Kbps or higher for pedestrian (micro
  cell) and up to 2Mbps for indoor environments
  (pico cell)
• Global seamless roaming
• Enhanced security and performance
• Full integration of wireless and wireline

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Major 3G Technologies Proposed for IMT 2000

• W-CDMA backward compatible with GSM (called UMTS
  by the ETSI)
• The IS-95 standard (CDMAOne) is evolving its own
  vision of 3G CDMA2000
• The IS-136 standard is evolving its own migration
  to 3G, Universal Wireless Communications, UWC-136
  or IS-136 HS

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Who will be first to offer IMT 2000?

• The Japanese are leading the pack with their
  W-CDMA implementation. It is being rolled out in
  the year 2001.
• The Koreans plan to have CDMA2000 up an running
  before the world cup in 2002.
• The Europeans are pushing hard to UMTS up soon
  but the current push is fro 2.5G, a middle of the
  road to protect current infrastructure
  investments.
• In the US no major push yet, some service
  providers are following in the footsteps of the
  Europeans by pushing a 2.5G solution.

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IMT 2000 Services/Capabilities 1/2

• All what 2G support including
• Registration, authentication and encryption
• SMS
• Emergency calling
• Bit rates
• 144Kbps or higher for vehicular (macro cell),
• 384Kbps or higher for pedestrian (micro cell) and
  
• up to 2Mbps for indoor environments (pico cell)
• Billing/charging/user profiles
• Sharing of usage/rate information between service
  providers
• Standardized call detail recording
• Standardized user profiles

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IMT 2000 Services/Capabilities 2/2

• Support of geographic position finding services
• For the mobile
• For the network
• Support of multimedia services
• QoS
• Assymmetric links
• Fixed and variable rate
• Bit rates of up to 2Mpbs
• Support of packet services
• Internet Access (wireless cellular IP)

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IMT 2000 Family Concept

• The IMT 2000 family concept defines some basic
  interoperability capabilities between different
  IMT 2000 technologies to enable global roaming!
• Different Radio Access Networks (RANs)
• CDMA2000
• W-CDMA
• UWC-136
• Different Core Network standards
• IS 41
• GSM
• ISDN

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Challenge for the Family Concept

• With IMT 2000 Standard Interfaces and
  Capabilities
• Any Family RAN could interface with any Family
  Core Network for some minimum set of features.
• More advanced features are possible in limited
  regions where the Family RAN and the Family Core
  Network are optimally matched
• The Core Network functionality should be kept
  independent of the Radio technology.
• By maintaining independence, each can evolve
  separately based on needs
• User Identity Modules (UIM) Plug-In modules could
  be used in locally rented handsets for Global
  Roaming with at least the minimum feature set.
  (similar to GSM SIMs)

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UIM Roaming

• UIM cards should allow a subscriber to obtain
• Any IMT 2000 service/capability basic feature set
  on
• Any IMT 2000 Network family member (W-CDMA,
  CDMA2000 and UWC-136)
• UIM Card will be a superset of the current GSM
  SIM
• Contains all necessary information about the
  users service subscriptions
• Supports user identity separate from handset
  identity
• Allows a user to use different handsets, with all
  usage billed to the single user
• Allows a user to rent a handset in a foreign
  country/network and obtain instant service

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To reach the IMT 2000 vision

• Physical interfaces are being standardized
• UIM to handset interface
• Radio/Air interfaces
• RAN to Core Network
• Network to Network Interfaces (NNI) between Core
  Networks
• Radio independent functions are being
  standardized
• UIM to handset
• Handset to Core Network
• NNI

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Key Technology Concepts for 3G

• Higher bit rates required -gt more bandwidth
• Packet and circuit switched services
• Coherent demodulation
• TDD
• Architecting for minimum required Eb/Io
• Control Eb
• Limit/Cancel Io
• Smart antennas

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Higher bit rates -gt larger bandwidths

• No free lunch!!!
• For a CDMA system
• For 2-4Mbps you need around 20MHz channel
• For 1-2Mbps you need around 10MHz channel
• For 256Kbps-1Mbps you need around 5MHz channel

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Packet and Circuit Switched Services

• CS channels 32 384 Kbps
• PS channels 64Kbps to 2Mbps
• Circuit mode versus packet mode for data
  services
• Circuit mode
• provides a dedicated channel for the duration of
  the call
• Can mux control with data in same channel, can be
  a problem for data if bit stealing is used
• Packet mode
• Requires a scheduling scheme to control access to
  the shared channel
• Generally supports a separate control channel
• CDMA Packet Mode two main approaches
• Users share a dedicated channel (code)
• Sequential access or scheduled on a need basis
• Users share the allowable total interference for
  the carrier
• Each user gets a unique code
• Users must be scheduled and transmissions
  controlled to limit the load in the system
• Combination of the above two

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Coherent vs Non coherent demodulation

• Non coherent demodulation where the receiver
  has no reference phase with which to compare the
  received signal
• Coherent demodulation where the receiver does
  have a reference pahse, supplied by the
  transmitted
• A continuous Pilot ( or Reference) channel
  transmitted along with the signal (e.g. pilot
  channel in IS-95 for downlink)
• A known sequence of Pilot (or Reference) symbols
  (or bits) embedded, periodically, in the signal
  bit stream (e.g. proposed for W-CDMA in both
  uplink and downlink channels, also CDMA2000
  incorporates a pilot channel in reverse direction)

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TDD

• All the standards naturally support FDD
• Symmetric channels for up and down links
• TDD can be added to allow transmission and
  reception in single frequency band.
• Japanese W-CMDA supports an asymmetric TDD
  channel in addition to the FDD support
• TDD allows for flexible spectrum usage, does not
  require paired frequency bands
• Simpler, lower cost handsets no need for
  duplex filters
• More complex synchronization, the channel flips
  back and forth between uplink and downlink.

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Architecting for Minimum Required Eb/Io

• Eb/Io vs Eb/No vs C/SIR or SNR
• The former two refer to the energy per bit and
  are therefore more applicable to digital systems.
  The latter two are generally used to refer to
  analog systems.
• Using I vs N basically has to do with what the
  noise source is, in cellular systems it is
  primarily due to interference so I is the
  preferred term.
• Eb P/R
• P is the power per bit in units of energy/sec
• R is the signal bit rate in bits/sec
• Eb is the received energy per bit of the signal,
  Io is the interference power density
• Eb is directionally proportional to the received
  power of the signal
• For CDMA Eb/Io (Pm/Itot) x (W/R) SIR x
  Processing Gain
• Eb/Io is the key parameter in determining the
  probability of receiving a bit correctly (I.e.,
  the BER)

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Techniques to keep Eb/Io low with higher bit rates

• Maximize Frequency diversity wider bands -gt
  higher processing gains
• Maximize Time diversity
• Rake receivers -gt multiple signals with different
  delays at receiver,
• interleaving with FEC
• Maximize Space diversity
• diverse receive antennas at base station,
• rake receivers -gt different signal paths
• Use FEC (forward error correction)
• All of the above techniques come at a cost
• Higher bandwidth
• More complex receivers (rake, multiple antennas)
• More overhead bits (FEC) per signal

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Controlling Eb

• More power is required for the transmission of
  bits at higher bit rates over the same distance
• Limit the distance over which high bit rates
  maybe sent
• Using better antennas that will focus the beam so
  that
• The transmitter aims at the target without
  wasting energy in all directions
• The receiver captures more of the signal as it is
  focused on a narrow beam
• Fast power control to counteract changes in
  interference due to
• Changing loads
• Changing environments

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Limit Io

• Use better antennas with focused beams in
  conjunction with sectors
• Use interference cancellation -gt receive all
  signals and subtract all but the desired one from
  the total
• Use more accurate and fasted power control
  techniques
• To not transmit signals when there is a silence
  in the signal

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Smart Antennas

• Switched beams
• Several antenna beams used to receive the signal
• Use the antenna that receives the strongest
  signal
• Not well suited to CDMA
• Switching will cause chip errors
• Switching could disturb synchronization and
  demodulation
• Works against the concept of the Rake receiver
• Adaptive Arrays
• Narrow beam antenna which is steered to follow
  the mobile(s)
• Better suited to CDMA but still have the Rake
  receiver problem