Modulation
. . . .
Modulation
FHSS Modulation
DSSS Modulation
OFDM Modulation
FHSS Modulation
 FHSS uses Gaussian frequency shift keying (GFSK) to encode the data.
 Twolevel GFSK (2GFSK) uses two frequencies to represent a 0 or a 1 bit.
 Fourlevel 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 16QAM and 64QAM modulation.
 Quadrature amplitude modulation (QAM) is a hybrid of phase and amplitude modulation.
 The 802.11ac amendment also introduced the use of 256QAM.
 A constellation diagram, also known as a constellation map, is a twodimensional 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 16QAM 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 CWNA106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA106 Wiley.
