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					Elements of a wireless network

                               wireless hosts
                                laptop, PDA, IP phone
                                run applications
                                may be stationary
                                  (non-mobile) or mobile
                 network             wireless does not
              infrastructure          always mean mobility




                                                        6-1
Elements of a wireless network
                               base station
                                typically connected to
                                 wired network
                                relay - responsible
                                 for sending packets
                                 between wired
                 network         network and wireless
              infrastructure     host(s) in its “area”
                                   e.g., cell towers,
                                     802.11 access
                                     points




                                                      6-2
Elements of a wireless network
                               wireless link
                                typically used to
                                 connect mobile(s) to
                                 base station
                                also used as backbone
                                 link
                 network        multiple access
              infrastructure     protocol coordinates
                                 link access
                                various data rates,
                                 transmission distance




                                                     6-3
Elements of a wireless network
                               infrastructure mode
                                base station connects
                                 mobiles into wired
                                 network
                                handoff: mobile
                                 changes base station
                 network         providing connection
              infrastructure     into wired network




                                                     6-4
Elements of a wireless network
                         ad hoc mode
                          no base stations
                          nodes can only
                            transmit to other
                            nodes within link
                            coverage
                          nodes organize
                            themselves into a
                            network: route among
                            themselves




                                               6-5
Wireless network taxonomy
                     single hop            multiple hops

                   host connects to        host may have to
infrastructure    base station (WiFi,    relay through several
   (e.g., APs)    WiMAX, cellular)         wireless nodes to
                  which connects to        connect to larger
                    larger Internet       Internet: mesh net

                                           no base station, no
                                          connection to larger
       no          no base station, no
 infrastructure                          Internet. May have to
                  connection to larger
                                         relay to reach other
                  Internet (Bluetooth,
                                          a given wireless node
                      ad hoc nets)
                                            MANET, VANET




                                                                  6-6
Wireless Link Characteristics (1)
Differences from wired link ….

    decreased signal strength: radio signal
     attenuates as it propagates through matter
     (path loss)
    interference from other sources: standardized
     wireless network frequencies (e.g., 2.4 GHz)
     shared by other devices (e.g., phone); devices
     (motors) interfere as well
    multipath propagation: radio signal reflects off
     objects ground, arriving ad destination at
     slightly different times

…. make communication across (even a point to point)
   wireless link much more “difficult”
                                                        6-7
Wireless Link Characteristics (2)
 SNR: signal-to-noise ratio                   10-1

       larger SNR – easier to                 10-2
        extract signal from noise (a
        “good thing”)                          10-3


   SNR versus BER tradeoffs




                                         BER
                                               10-4

      given physical layer:
                                               10-5
        increase power -> increase
        SNR->decrease BER                      10-6

       given SNR: choose physical             10-7
        layer that meets BER                          10     20        30   40

        requirement, giving highest
                                                           SNR(dB)

        thruput                                        QAM256 (8 Mbps)
         • SNR may change with
                                                       QAM16 (4 Mbps)
           mobility: dynamically adapt
           physical layer (modulation                  BPSK (1 Mbps)
           technique, rate)
                                                                                 6-8
Cellular networks: the first hop
Two techniques for sharing
  mobile-to-BS radio
  spectrum
 combined FDMA/TDMA:
  divide spectrum in                 time slots
  frequency channels, divide
  each channel into time
  slots                  frequency
 CDMA: code division
                           bands

  multiple access



                                                  6-9
6-10
Cellular standards: brief survey
2G systems: voice channels
 IS-136 TDMA: combined FDMA/TDMA (north
  america)
 GSM (global system for mobile communications):
  combined FDMA/TDMA
      most widely deployed
 IS-95 CDMA: code division multiple access



                              Don’t drown in a bowl
                 GSM          of alphabet soup: use this
                              for reference only


                                                           6-11
6-12
Cellular standards: brief survey
2.5 G systems: voice and data channels
 for those who can’t wait for 3G service: 2G extensions
 general packet radio service (GPRS)
    evolved from GSM
    data sent on multiple channels (if available)

 enhanced data rates for global evolution (EDGE)
    also evolved from GSM, using enhanced modulation
    data rates up to 384K

 CDMA-2000 (phase 1)
    data rates up to 144K
    evolved from IS-95


                                                        6-13
Cellular standards: brief survey
3G systems: voice/data
 Universal Mobile Telecommunications Service (UMTS)
    data service: High Speed Uplink/Downlink packet
     Access (HSDPA/HSUPA): 3 Mbps
 CDMA-2000: CDMA in TDMA slots
    data service: 1xEvlution Data Optimized (1xEVDO)
     up to 14 Mbps




  ….. more (and more interesting) cellular topics due to mobility (stay
  tuned for details)

                                                                   6-14
6-15
               Characteristics of selected wireless link
               standards

                   200       802.11n

                    54       802.11a,g               802.11a,g point-to-point                   data
Data rate (Mbps)




                   5-11      802.11b                           802.16 (WiMAX)

                     4                   UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO                    3G cellular
                                                                                              enhanced
                     1    802.15


                   .384                        UMTS/WCDMA, CDMA2000                           3G

                   .056                          IS-95, CDMA, GSM                             2G


                             Indoor        Outdoor         Mid-range            Long-range
                             10-30m        50-200m          outdoor               outdoor
                                                           200m – 4 Km          5Km – 20 Km

                                                                                                    6-16
Cellular Concept
 Given a propagation environment, increasing
  transmitted power will increase the service
  coverage area.
      The coverage area can be controlled by using a proper
       transmitted power level.
 In cellular systems, the total service area is
  divided into a number of smaller areas, each of
  which is a radio cell.
      Advantages:
        • Low transmitted power
        • Frequency reuse possible.
 Regular polygons may be used to represent the cell
  coverage.


                                                               6-17
Cellular Concept
 Hexagonal cells are popular because
    closest to a circle
    tight cellular packing
    perfect partitioning of the service area.
 Frequency reuse is limited by co-channel
  interference. Cells which use the same frequency
  channels are called co-channel cells.
 Frequency is reused from cell cluster to cell
  cluster. No frequency channel is reused among
  cells in the same cell cluster.
      Cells in each cell cluster use unique frequency channels.




                                                                   6-18
Multiple Access Techniques
 Radio cell: a geographical coverage area in
  which the services of mobile stations
  (MSs) are supported by a single base
  station (BS)
 Forward link (downlink): BS → multiples
  MSs (one to many broadcasting)
 Reverse link (uplink): MSs →BS (many to
  one multiple access)



                                                6-19
 Multiple Access Techniques
 Multiple MSs want to access the common BS
  simultaneously
 If two or more user signals arrive at the BS at
  the same time, there will be interferences,
  unless the signals are orthogonal
 How can we achieve the orthogonality?




                                                6-20
FDMA
 The total bandwidth is divided into
  nonoverlapping frequency bands (channels)
 Each user occupies a channel for the
  duration of the connection
     waste of resources
 Narrowband transmission
 Forward and reverse links use FDD




                                              6-21
  TDMA
 Time is partitioned into frames
 Each frame consists of Nslot data slots plus a header
  and a trailer
 Each slot is for transmission of one information unit
 A user continues to use the same slot in every frame
  during call connection
      waste of resources
 TDMA systems require strict time synchronization.




                                                          6-22
TDMA




       6-23
TDMA
 W-TDMA: Each user occupies the total
  frequency bandwidth during its slots
 N-TDMA: The total frequency spectrum is
  divided into frequency subbands (channels);
  within each frequency channel, TDMA is
  used. −→Both time and frequency are
  partitioned.




                                                6-24
Code Division Multiple Access (CDMA)
 used in several wireless broadcast channels
    (cellular, satellite, etc) standards
   unique “code” assigned to each user; i.e., code set
    partitioning
   all users share same frequency, but each user has
    own “chipping” sequence (i.e., code) to encode data
   encoded signal = (original data) X (chipping
    sequence)
   decoding: inner-product of encoded signal and
    chipping sequence
   allows multiple users to “coexist” and transmit
    simultaneously with minimal interference (if codes
    are “orthogonal”)


                                                          6-25
CDMA




       6-26
 CDMA Encode/Decode
                                                                                                          channel output Zi,m
                                                                              Zi,m      = d .c i    m
           data                                       d0 = 1
                                                                                                                   1        1 1 1 1 1 1        1
                    d1 = -1
           bits
sender
                                                                                                        -1 -1 -1       -1                 -1       -1 -1 -1

                  1 1 1        1              1 1 1            1                                            slot 1                  slot 0
           code                                                                                            channel                 channel
                          -1       -1 -1 -1               -1       -1 -1 -1
                                                                                                            output                  output
                      slot 1                         slot 0

                                                                                                          M
                                                                                                   Di = S Zi,m.cm
                                                                                                         m=1
                                                                                                                   M
         received                               1         1 1 1 1 1 1               1
                                                                                                                                                              d0 = 1
                                     -1 -1 -1        -1                        -1       -1 -1 -1
         input                                                                                                                   d1 = -1

                                      1 1 1          1                1 1 1         1                                            slot 1                   slot 0
                     code                                                                                                       channel                  channel
                                                -1        -1 -1 -1             -1       -1 -1 -1

         receiver                              slot 1                         slot 0
                                                                                                                                 output                   output



                                                                                                                                                                       6-27
CDMA: two-sender interference




                                6-28

				
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posted:8/15/2011
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