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