Transcript Wireless &Mobile Communications Chapter 9: 3G Cellular

Wireless &Mobile Communications
Chapter 9: 3G Cellular
What is 3G?
The ITU’s 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?
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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:
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allow global roaming
 provide for wider bandwidths to accommodate different types
of applications
 support packet switching concepts
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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
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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
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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?
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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
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All what 2G support including:
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Registration, authentication and encryption
 SMS
 Emergency calling
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Bit rates:
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144Kbps or higher for vehicular (macro cell),
 384Kbps or higher for pedestrian (micro cell) and
 up to 2Mbps for indoor environments (pico cell)
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Billing/charging/user profiles
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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
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Support of geographic position finding services
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For the mobile
 For the network
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Support of multimedia services
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QoS
 Assymmetric links
 Fixed and variable rate
 Bit rates of up to 2Mpbs
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Support of packet services
Internet Access (wireless cellular IP)
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IMT 2000 Family Concept
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The IMT 2000 family concept defines some basic
interoperability capabilities between different IMT 2000
technologies to enable global roaming!
Different Radio Access Networks (RANs):
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CDMA2000
 W-CDMA
 UWC-136
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Different Core Network standards
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IS 41
 GSM
 ISDN
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Challenge for the Family Concept
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With IMT 2000 Standard Interfaces and Capabilities:
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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
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UIM cards should allow a subscriber to obtain:
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Any IMT 2000 service/capability basic feature set on
 Any IMT 2000 Network family member (W-CDMA, CDMA2000
and UWC-136)
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UIM Card: will be a superset of the current GSM SIM
Contains all necessary information about the user’s service
subscriptions
 Supports user identity separate from handset identity:
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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
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Physical interfaces are being standardized:
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UIM to handset interface
 Radio/Air interfaces
 RAN to Core Network
 Network to Network Interfaces (NNI) between Core Networks
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Radio independent functions are being standardized:
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UIM to handset
 Handset to Core Network
 NNI
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Key Technology Concepts for 3G
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Higher bit rates required -> 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 -> larger bandwidths
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No free lunch!!!
For a CDMA system;
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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:
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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
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Packet mode
Requires a scheduling scheme to control access to the shared channel
Generally supports a separate control channel
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CDMA Packet Mode: two main approaches
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Users share a dedicated channel (code):
Sequential access or scheduled on a need basis
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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
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Combination of the above two
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Coherent vs Non coherent demodulation
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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
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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
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All the standards naturally support FDD
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TDD can be added to allow transmission and reception in
single frequency band.
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Symmetric channels for up and down links
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
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Eb/Io vs Eb/No vs C/SIR or SNR:
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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.
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Eb =P/R
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P is the power per bit in units of energy/sec
 R is the signal bit rate in bits/sec
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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
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Maximize Frequency diversity – wider bands -> higher
processing gains
Maximize Time diversity –
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Rake receivers -> multiple signals with different delays at
receiver,
 interleaving with FEC
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Maximize Space diversity –
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diverse receive antennas at base station,
 rake receivers -> different signal paths
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Use FEC (forward error correction)
All of the above techniques come at a cost:
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Higher bandwidth
 More complex receivers (rake, multiple antennas)
 More overhead bits (FEC) per signal
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Controlling Eb
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More power is required for the transmission of bits at
higher bit rates over the same distance
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Limit the distance over which high bit rates maybe sent
Using better antennas that will focus the beam so that:
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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
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Fast power control to counteract changes in interference
due to
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Changing loads
 Changing environments
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Limit Io
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Use better antennas with focused beams in conjunction
with sectors
Use interference cancellation -> 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
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Switched beams:
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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
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Adaptive Arrays:
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Narrow beam antenna which is steered to follow the mobile(s)
 Better suited to CDMA but still have the Rake receiver problem
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