Mobile Communication Systems

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Mobile Communication Systems
Part 7- Multiplexing
Professor Z Ghassemlooy Scholl of Computing,
Engineering and Information Sciences University
of Northumbria U.K. http//soe.ac.uk/ocr
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Contents

• Multiple Access
• Multiplexing
• SDM
• FDM
• TDM
• CDM
• Wideband Schemes
• Duplex Method

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Multiple Access

• In todays data communications systems there is a
  need for several users to share a common channel
  resource at the same time.
• The resource could be
• high speed optical fibre links between continents
• frequency spectrum in a cellular telephone system
• twisted pair ethernet cable in the office

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Multiple Access

• For multiple users to be able to share a common
  resource in a managed and effective way, it
  requires
• Some form of access protocol
• Defines how or when the sharing is to take place
  and the means for identifying individual
  messages. Process is known as multiplexing in
  wired networks and multiple access in wireless
  digital communications.

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Multiplexing/Multiple Access

• There four possible ways to divide the frequency
  spectrum among many channels
• Space-division multiplexing (SDM)
• Frequency-division multiplexing (FDM) /
  Frequency Division Multiple Access (FDMA)
• Time-division multiplexing (TDM) / Time Division
  Multiple Access (TDMA)
• Code-division multiplexing (CDM) / Code Division
  Multiple Access (CDMA)

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SDM
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Frequency Multiplex I

• Dividing the entire frequency spectrum into
  smaller bands
• A frequency band is allocated per channel for the
  entire transmission time
• FDM, used in 1st generation systems, wastes
  spectrum
• Advantages
• lower channel bit rate (than
• TDM) means less susceptible
• to multi path ISI
• Requires coordination
• works also for analog signals

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Frequency Multiplex II

• Disadvantages
• In-efficient use of bandwidth if the traffic is
  distributed unevenly
• Requires guard band between channels
• Cannot readily support variable user data rates,
  fixed channel width means fixed bit rate

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Time multiplex I

• Entire spectrum is allocated for a channel some
  of the time
• For 2nd generation
• Advantages
• Only one carrier in the medium at any given time
• High throughput even for many users
• Common TX component design,
• only one power amplifier
• Disadvantages
• precise synchronization necessary
• requires terminal to support a much higher data
  rate than the user information rate

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Example TDMA System

• GSM is a good example of a TDMA system
• GSM handsets transmit data at a rate of 270
  kbit/s in a 200 kHz channel using GMSK
  modulation.
• each frequency channel is assigned 8 users, each
  having a basic data rate of around 13 kbit/s

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TDMA Frame

• TDMA used for the 3G air interface
• A frame length 4.615 ms and it consist of
• 64 1/64 time slots of length 72
• 16 1/16 time slots of length 288

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Time and Frequency Multiplex I

• Combination of both methods
• A certain frequency band for a given amount of
  time is allocated per channel
• Example GSM
• Advantages
• Improved protection against tapping
• and frequency selective interference
• Higher data rates compared to code
• multiplex
• Disadvantages
• Requires precise
• coordination

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Code Multiplex I

• Each channel has a unique code.
• All channels use the same
• spectrum at the same time.
• Advantages
• bandwidth efficient and good power control
• no need for coordination and synchronization
• good protection against interference and
• tapping
• Disadvantages
• lower user data rates
• more complex signal regeneration
• Implemented using spread spectrum technology

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CDMA Classification

• CDMA direct sequence (DS)
• CDMA frequency hopping (FH)
• Carrier frequency changes periodically, after T
  secs
• Hopping pattern determined by spread code
• CDMA time hopping (TH)
• Data transmitted in rapid bursts
• Time intervals determined by code

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Direct Sequence CDMA

• Directly modulated, discrete time, discrete
  valued code signal
• Analogue or Digital
• Code bits are chips (?1)
• Rate of Code gtgt Rate of Data
• PSK, BPSK, D-BPSK,
  QPSK or MPSK

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DS-SS Transmitter Receiver
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CDMA Evolution

• Early Stages
• Narrowband
• Wideband

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Wideband-CDMA
Framing structure
Tc chip time 1 / 3.84 ms
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High Speed Wireless Access

• Mobile communication system
• Up to 30 Mbps
• Using the SHF and other band (3-60 GHz)
• Used for mobile video telephone conversations

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Ultra High Speed Wireless LAN

• Wireless LAN
• Up to 156 Mbps
• Using the millimeter wave radio band
• (30-300 GHz)
• Used for high quality TV conferences.

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5GHz Band Mobile Access

• Two types
• ATM type Wireless Access
• Ethernet type Wireless LAN
• Using 5GHz band
• Each system can transmit at up to 20-25Mbps
• Used for multimedia information

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High Data Rate Wireless LAN Evolution
ARIB, Japan, 1999
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Wireless Home-Link

• Wireless Home-Link
• Up to 100Mbps
• Using the SHF and other band(3-60GHz)
• Between PCs and Audio Visual equipments
• Multimedia information.

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Home Link Concept
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Duplex Methods

• Separating the send and receive signals (remember
  full duplex). Two approaches
• Frequency Division Duplex (FDD)
• Uses a pair of frequency bands one for uplink
  and another for downlink
• used in all second generation cellular systems
• requires good frequency separation filters -
  diplexer
• Time Division Duplex (TDD)
• Uses a single frequency band for both uplink and
  downlink  sharing the transmission time
• propagation delay limits cell size
• very efficient for asymmetric traffic, e.g.
  internet download
• used in cordless systems (DECT) and wireless LANs

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What is Universal Mobile Telecommunication System
?

• European name for third generation (3G) radio
  system
• (1G analog, 2G digital voice and low speed
  data (GSM))
• Key features with respect to 2G
• Integration of fixed and mobile networks
• Expanded range of services (Packet, Internet,
  Multimedia)
• Bit rates
• Rural outdoor 144 kb/s, 500 km/h
• Suburban outdoor 384 kb/s, 120 km/h
• Indoor, low range outdoor 2Mb/s, 10 km/h
• Flexibility
• Variable bit rates
• Circuit switched and packet oriented bearers
• Negotiation of bearer service
  attributes (bearer type, bit rate, delay BER,
  up/down symmetry, protection)
• Adaptability to quality, traffic, network load
  radio conditions

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Summary

• Multiple Access - sharing resources
• Frequency Division Multiple Access - FDMA
• Time Division Multiple Access - TDMA
• Code Division Multiple Access CDMA
• Duplex Methods
• Frequency Division Duplex - FDD
• Time Division Duplex - TDD

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Questions and Answers

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