Iuwne10 S01 L05


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Iuwne10 S01 L05

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