Iuwne10 S01 L05
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Iuwne10 S01 L05






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    Iuwne10 S01 L05 Iuwne10 S01 L05 Presentation Transcript

    • Wireless Fundamentals Understanding Spread Spectrum Technologies
    • Spread Spectrum
        • Narrowband and spread spectrum are the two main ways of sending a signal.
        • Spread spectrum uses less energy at peak.
        • The bandwidth required depends on the amount of information to be sent.
        • FHSS is a time-based narrowband hopping of frequencies.
        • DSSS is a broadband use of frequencies.
      FHSS Versus DSSS
    • DSSS: Encoding
        • Each bit is transformed into a sequence, called “chip” or “symbol.”
        • In this example, the chipping code is called Barker 11.
        • Up to 9 bits can be lost.
        • When using DBPSK, the phase shifts with 180 ° angles; each shift represents 1 bit.
        • When using DQPSK, shifts are 90 ° ; each shift represents 2 bits.
        • DBPSK allows 1 Mb/s.
        • DQPSK allows 2 Mb/s.
      DSSS Modulations: DBPSK and DQPSK
        • With CCK, each symbol of 6 bits is associated to a unique code sequence as shown on the example here .
        • Coding 4 bits per symbol allows 5.5 Mb / s, coding 8 bits per symbol allows 11 Mb / s .
      DSSS Modulation: CCK
    • Orthogonal Frequency-Division Multiplexing
        • Of 64 subcarriers:
        • 12 zero subcarriers (in black) on sides and in center
        • Sides function as frequency guard band, leaving 16.5-MHz occupied bandwidth
        • Center subcarrier zero for DC offset/carrier leak rejection
        • 48 data subcarriers (in green) per symbol
        • 4 pilot subcarriers (in red) per symbol for synchronization and tracking
        • Uses the same principles as DBPSK and DQPSK: BPSK shifts 180 º; QPSK shifts 90º.
        • Speed depends on density of signal per tone.
      OFDM Modulations: BPSK and QPSK 18 375 QPSK 12 250 QPSK 9 187.5 BPSK 6 125 BPSK Total Data Rate (Mb/s) Data Rate per Subchannel (kb/s) Modulation
        • With QAM, 90 º shifts are associated with amplitude modulation.
        • With four amplitude positions, 16 values are possible.
        • OFDM for wireless uses 16-QAM and 64-QAM, with speeds up to 54 Mbps.
      OFDM Modulation: QAM
    • Channels and Overlap Issues
        • With channels built for 5-MHz interchannel space, each DSSS channel uses more than one channel.
        • Only three or four nonoverlapping channels are available in the 2.4-GHz ISM band.
        • Channel overlap can be co-channel interference or adjacent channel interference.
    • Summary
        • Spread spectrum technologies offer better resistance to narrowband interferences.
        • Wireless networks use DSSS.
        • DBPSK allows 1 Mb/s, DQPSK 2 Mb/s.
        • Using CCK increases the speed to 11 Mb/s.
        • OFDM uses subcarriers to carry the signal.
        • BPSK allows 9 Mb/s, QPSK 18 Mb/s.
        • Using QAM increases the speed to 54 Mb/s.
        • Larger channels imply interference and channel collocation planning.