MIMO-OFDM for broadband wireless communications by st1nk778

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									             III.
     OFDM/OFDMA/WLAN/WAN




                                                     May 15, 2008

                                                   Qinghai Yang

                                    School of Telecom. Engineering
                                             qhyang@xidian.edu.cn

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               Multipath Fading




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               Multipath Fading




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               Principles of MCM




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                         OFDM
• Orthogonal frequency division multiplexing (OFDM) split
  the data stream into parallel sub-channels
• Each is modulated onto one of a set of carriers.
• Modulated signals are orthogonal to one another
• Modulation/demodulation jointly use an IFFT/FFT.




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               Advantage of OFDM
• It turns frequency-selective channels to several flat
  fading channels.
• Simplify the receiver implementation as the
  equalizer is just a single-tap filter.
• OFDM may exploit the frequency diversity of
  frequency-selective channels.
• No loss to spectral efficiency (subcarrier overlap)
• Eliminating both ISI and ICI with guard interval
  or cyclic extension.


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               Current research status
• MIMO-OFDM: deployment of multiple antennas.

• Vector-OFDM: using the concept of MIMO and precoding
  technique.And now it is being developed by Cisco.

• WOFDM: wideband-OFDM, developed by Wi-Lan,
  develops spacing between channels large enough so that
  any frequency errors between transmitter and receiver have
  no effect on performance.

• Flash OFDM - Flarion (Lucent/Bell Labs spinoff) has
  developed this technology, also called fast-hopped OFDM,
  which uses multiple tones and fast hopping to spread
  signals over a given spectrum band.


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                            MIMO-OFDM
•   MIMO-OFDM takes advantage of the multipath properties of
    environments to exploit the inherent frequency diversity.
•   If M and N is # of TX and RX antenna and Q is the # of subcarrier of
    OFDM,then the wideband channel consists of MNQ narrowband
    channels.




                            IFFT                  IFFT



                  Pre-      IFFT                  IFFT          Post-
               processing                                    processing



                                                  IFFT
                            IFFT




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       Broadband Access Standards
• MIMO-OFDM has been adopted by 802.16
• For 4G? MIMO undoubtedly used, OFDM powerful
  candidate.
        Macro




                                     (MIMO-OFDM)

                                      802.16
         Micro




                 WCDMA           4G ?
                                (MIMO OFDM)
                       MIMO




                                 4
                              MIMO      Hyperlan
         Loc




                                                              Hyperaccess
                                        802.11a

                 0.2             2                       20                 200
                                     Data Rate MBPS
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               Disadvantages of OFDM
• Strict Synchronization Requirement

• Peak-to-Average Power Ratio (PAPR)

• Co-Channel Interference in Cellular OFDM




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 OFDMA / OFDM-TDMA / OFDM-CDMA




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                      WLAN
• WLAN: Wireless Local Area Network
      – IEEE 802.11: WiFiIEEE 802.11: WiFi
      – 802.11a: with OFDM 54Mb/s
      – 802.11n: with MIMO-OFDM 108Mb/s




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                                WLAN

• High data rate (except for RFID) > 1 Mb/s
      –   Packet transmission
      –   Low mobility: stationary or pedestrian
      –   Local coverage
      –   Access techniques for the Internet
      –   Local services
• Physical & MAC layers and security mechanisms
  specifiedby wireless standards, higher layers based on
  common Internet protocols

• Unlicenced Industrial, Scientific, and Medical (ISM) bands
  mostly used


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               WLAN Characteristics
• 1... 54 Mb/s data rates (currently)
• Intended for short distances, one base station
  covering, e.g.,
      – A few rooms/1 corridoor in office building
      – A small house
      – A few hundred meters distances in open air, somewhat
        longer with directive antennas
• Power consumption not always very critical
      – Except when included in a mobile terminal, which is
        becoming more and more popular these days



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               Infrastructure




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                     AP




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               WLAN Applications
• Cable-free access to internet in
      – Office environments
      – Homes (e.g. ADSL modem acting as the access
        point)
      – Hot-Spots, like airports, conference centers,
        hotels, etc.
• Also used as high-speed Internet-access
  method for homes and small offices in
      – Urban areas, instead of ADSL or cable modems


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               Wireless Bridging




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               Outdoor Application




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          Outdoor Application -Antenna




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          Outdoor Application -Antenna




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Base-station service vs. Extended station service




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               Collocated Coverage Areas




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                   WLAN Elements
• Extends Ethernet over wireless
• Two modes:
      – Infrastructure WLAN: Terminals communicate with
        Radio Access Point (‘Base-station’) which acts as
        bridge to the fixed Internet
      – Ad-Hoc Network: Terminals communicate with each
        other, without centralized control
• Each wireless station and access point has a
  wireless LAN card; Integrated solution currently,
  e.g., in Laptops, PDAs/mobile phones, game
  consoles, etc.

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               Data Rate vs. Range




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                   WMAN
• BWA, Broadband Wireless Access
• WMAN, Wireless Metropolitan Area
  Network
      – IEEE 802.16: WiMAX




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                    WMAN
• Broadband Internet connections to homes and
  small offices
• Competes with ADSL and cable modems, but is
  able to offer higher data rates
• Original idea: Fixed terminals, possibly with
  directive antennas
• Recent developements: Applicable also to mobile
  users
• Partly competing with high-data rate cellular
  system developments, like HSDPA/HSUPA/3G-
  LTE LTE

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               802.16 BWA = WiMAX
• 802.16 Fixed BWA systems between 10 and 66 MHz
• 802.16a Amendment for operation between 2 and 11 GHz
• 802.16c Enhancements including system profiles, 10-66
  GHz
• 802.16.2 Coexistence between 2 and 11
• 802.16/Conf Test and conformance specifications
• 802.16d System profiles
• 802.16e Mobile WiMAX
• etc.
• 802.20 (16m) New standard development with full
  mobility, based on WiMAX


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