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Modulation
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Modulation


FHSS Modulation

  • FHSS uses Gaussian frequency shift keying (GFSK) to encode the data.
  • Two-level GFSK (2GFSK) uses two frequencies to represent a 0 or a 1 bit.
  • Four-level GFSK (4GFSK) uses four frequencies, with each frequency representing 2 bits (00, 01, 10, or 11).
  • Because it takes transmission cycles before the frequency can be determined, the symbol rate (the rate that the data is sent) is only about 1 or 2 million symbols per second, a fraction of the 2.4 GHz carrier frequency.

DSSS Modulation

  • After the data has been encoded using a chipping method, the transmitter needs to modulate the signal to create a carrier signal containing the chips.
  • Differential binary phase shift keying (DBPSK) utilizes two phase shifts, one that represents a 0 chip and another that represents a 1 chip.
  • To provide faster throughput, differential quadrature phase shift keying (DQPSK) utilizes four phase shifts, allowing each of the four phase shifts to modulate 2 chips (00, 01, 10, 11) instead of just 1 chip, doubling the speed.

OFDM Modulation

  • OFDM uses binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) phase modulation for the lower ODFM data rates.
  • The higher OFDM data rates use 16-QAM and 64-QAM modulation.
  • Quadrature amplitude modulation (QAM) is a hybrid of phase and amplitude modulation.
  • The 802.11ac amendment also introduced the use of 256-QAM.
  • A constellation diagram, also known as a constellation map, is a two-dimensional diagram often used to represent QAM modulation.
  • A constellation diagram is divided into four quadrants, and different locations in each quadrant can be used to represent data bits.
  • Areas on the quadrant relative to the horizontal axis can be used to represent various phase shifts.
  • Areas relative to the vertical axis are used to represent amplitude shifts.
  • A 16-QAM constellation diagram uses sixteen different combinations of phase and amplitude shifts.
  • Each of the sixteen different points within the four quadrants can be used to represent four data bits.

Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.

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