Mobile Communication Systems

							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
                Z. Ghassemlooy
Contents


 Multiple Access
 Multiplexing
  –   SDM
  –   FDM
  –   TDM
  –   CDM
 Wideband Schemes
 Duplex Method


                     Z. Ghassemlooy
Multiple Access


 In today’s 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




                          Z. Ghassemlooy
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.




                             Z. Ghassemlooy
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)




                             Z. Ghassemlooy
SDM

                    channels ki
       k1      k2        k3        k4            k5       k6

           c
                     t                       c
                                                           t
      s1
                               f
                                        s2
                                                               f
                          c
                                         t


                     s3
                                                      f

                              Z. Ghassemlooy
  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               k1   k2   k3   k4   k5   k6

    TDM) means less susceptible        code
    to multi path ISI                                                        f
  – Requires coordination
  – works also for analog signals



                       t      Z. Ghassemlooy
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




                           Z. Ghassemlooy
Time multiplex I

 Entire spectrum is allocated for a channel some of the time
 For 2nd generation
                                         k1  k2 k3  k4  k5  k6
 Advantages:
   – Only one carrier in the medium at any given time
                                                     code
   – High throughput even for many users
                                                                                      f
   – Common TX component design,
     only one power amplifier


 Disadvantages:             t
   – precise synchronization
     necessary
   – requires terminal to support a much higher data rate than the user information
     rate
                                   Z. Ghassemlooy
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




                            Z. Ghassemlooy
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


        Downlink                                           Uplink




            288ms                    Switching point between uplink and downlink
 72ms

                                Z. Ghassemlooy
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       k1   k2   k3   k4   k5   k6
  – Higher data rates compared to code
    multiplex                        code
                                                                             f
 Disadvantages:
  – Requires precise
     coordination
                        t
                              Z. Ghassemlooy
 Code Multiplex I
 Each channel has a unique code.
 All channels use the same                        k1   k2   k3   k4   k5   k6
  spectrum at the same time.
 Advantages:                                                 coding
   – bandwidth efficient and good power control
   – no need for coordination and synchronization
   – good protection against interference and
     tapping

                                                                                 f
 Disadvantages:
   – lower user data rates
   – more complex signal regeneration

                                                        t
 Implemented using spread spectrum technology
                                  Z. Ghassemlooy
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


Frequency
                                                            Direct sequence

                                                            Frequency
                                                            hopping
                                                            Time hopping

                               Z. Ghassemlooy
                                                        Time
Direct Sequence CDMA

 Directly modulated, discrete time, discrete valued
  code signal
 Analogue or Digital
 Code bits are ‘chips’ (1)
 Rate of Code >> Rate of Data                 DS-SS Transmitter

 PSK, BPSK, D-BPSK,                          Data        Spreading
                                 Data
                                            modulator     modulation
  QPSK or MPSK



                                             Carrier          Code
                                            generator       generator
                           Z. Ghassemlooy
  DS-SS Transmitter & Receiver




Binary               Wideband
            X                                       Despreading                  Data         Binary
Data                 modulator
                                                                              demodulator     Data




                                        Code
           Code       Carrier                                       Code           Carrier
                                 Synchronisation/trac
         generator   generator                                    generator       generator
                                        king




                                   Z. Ghassemlooy
CDMA Evolution
 Early Stages
      1949     John Pierce : time hopping spread spectrum
      1949     Claude Shannon and Robert Pierce : basic ideas of CDMA
      1950     De Rosa-Rogoff : direct sequence spread spectrum
      1956     Price and Green antimultipath “RAKE” patent
                                :
      1961     Magnuski : near-far problem
      1970s    Several developments for military field and navigation systems


 Narrowband
      1978     Cooper and Nettleton : cellular application of spread spectrum
      1980s    Investigation of narrowband CDMA techniques for cellular applications
      1986     Formulation of optimum multiuser detection by Verdu
      1993     IS-95 standard

 Wideband
      1995 -   Europe     : FRAMES FMA2
               Japan       : Core-A                       WCDMA
               USA         : cdma2000
               Korea       : TTA I, TTA II
      2000s    Commercialization of wideband CDMA systems
                                       Z. Ghassemlooy
Wideband-CDMA

Framing structure

   Radio Frame (10ms)

        frame #i                 frame #i+1

   Time Slot (2560*Tc)

   timeslot #0     timeslot #1   timeslot #2           timeslot #13   timeslot #14




   Tc = chip time = 1 / 3.84 ms


                                      Z. Ghassemlooy
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




                        Z. Ghassemlooy
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.




                        Z. Ghassemlooy
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




                        Z. Ghassemlooy
  High Data Rate Wireless LAN Evolution


                                                                               Ethernet
                               Gigabit Ethernet   Fast Ethernet               （10M bit/s)
                                 （1G bit/s)       （100M bit/s)



                  ATM                                                         Ethernet (10M bit/s)

                                                                                                Conventional 2.4GHz
                                                                                               Ethernet Wireless LAN
                                                                                                      2M bit/s


     Future 5GHz                5GHz
   ATM Wireless LAN     Ethernet Wireless LAN
                            (IEEE802.11)                           36Mbit/s
      25M bit/s




                                                                                     IMT 2000
                                                                                 384kbit/s〜2Mbit/s

ARIB, Japan, 1999
                                                  Z. Ghassemlooy
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.




                        Z. Ghassemlooy
Home Link Concept

                                  Display
                                                 Personal Computer


                        5 GHz
                                                                   Display

         CATV           DVD
                        VTR



  Telephone line

                                                 5 GHz
                                                                   Personal Computer
                   Personal Computer                     Display


      Satellite
          Tuner



                                Z. Ghassemlooy
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
                             Z. Ghassemlooy
 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
                              Z. Ghassemlooy
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




                           Z. Ghassemlooy
Questions and Answers

 Tell me what you think about this lecture
  – fary@ieee.org




                     Z. Ghassemlooy