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  • By chrisspencer59 - edited: October 6, 2015

    I.m sure this is an easy one... in fact I'm 99.9% sure I know the answer and that the CWNA book is incorrect!

    So..

    40MHz HT channel has 114 Subcarriers in total, 108 Data Subcarriers and 6 Pilot subcarriers ( CWNA E-Learning)

    OR

    40MHz HT channel has 128 Subcarriers in total, 108 Data Subcarriers and 6 Pilot subcarriers (Sybex CWNA Official study guide page 636)

    I have submit this as an errata... but thought it might be useful for learners here such as myself.

    Chris

  • By Howard - edited: October 6, 2015

    128 is the correct number.

    You must also count the 14 unused subcarriers which are an integral part of the OFDM modulation.

    Among other things, these empty subcariers help reduce the Peak to Average Power Ratio (PAPR) of the RF amplifiers output for more easily (and clearly) understood signals.  Typically the improvement is from 2 to 4 dB.

    SISO OFDM has these unused subcarriers, but it's even more important for MIMO.

  • Thanks Howard,

    so...apart from having a D'oh moment...

    20MHz OFDM has 64 Subcarriers, 48 for data, 4 Pilot , 12 unused, non HT 802.11 a/g

    20MHz HT has 64  Subcarriers ,  52 for data. 4 Pilot, 8 unused.

    40MHz HT has 114 Subcarriers, 108 for data, 6 Pilot + the 14 unused = 128

    (52 data subcarriers+ 4 pilot = 56 x2 =112 + additional 2 "spare/unused from top of lower and bottom of higher 20 MHz channel =114)

    Maybe I wasn't grasping the concept in the e-learning that the 114 subcarriers was  the number of "usable or used" subcarriers and not the TOTAL number of subcarriers that there are, whilst the book gives a complete breakdown of Total/Data/Pilot/unused carriers.

    Thanks for the clarification...perhaps I should have used our friend Google :-)

  • By Howard - edited: October 6, 2015

    Pilots are an important feature.   Not only do they help with channel losses, they can also help with a poor transmitted signal.

    The attached files show both good and bad Spectral Flatness Plots for different radios.   These are the raw measurements of the Transmitters output.

    Note that the problems were caused by firmware errors.

    As usual, change the file suffix to "png" to view the files.

  • Thank you Howard, much appreciated.

    Chris

  • Hi Guys,

    The number of total sub carriers are (in HT and VHT):

    20 MHz = 64 subcarriers 

    40 MHz = 128 subcarriers (=2x64 or 2 x 20MHz channel)

    80 MHz = 256 subcarriers (=2x128 or 2 x 40MHz channel)

    160 MHz = 512 subcarriers (=2x256 or 2 x 80MHz channel)

    Then don't you think the answer to question 18 on page 695 (CWNA-106 book) should be BDF in case of total subcarriers?

    In the book i think the author is assuming total subcarriers(64 in case of 20MHz).

    Actually on practice test the explanation is different then in the book. In pdf version of the same chapter review questions, the explanation is for "used" subcarriers rather then total as the explanation is below. In this case the answer should be ACF.

    "A 20 MHz channel uses 52 subcarriers. A 40 MHz channel uses 114 subcarriers. An 80 MHz channel uses 242 subcarriers. A 160 MHz channel is made of two 80 MHz channels that can be either side by side or separated from each other. The number of subcarriers in a 160 MHz channel is exactly two times the number of 80 MHz subcarriers, 484 subcarriers."

    Is there a know errata?

    Please reply asap as I have CWNA test this Saturday :)

    Regards,

    Mohsin

  • By Howard - edited: April 11, 2018

    I don't have the book edition in question, but it's obvious you understand the difference.  So I would not worry about this issue for your test.   As usual, just read the question very carefully.

    If you remember that there are also the unused sub-carriers (with the same spacing) you'll never miscalculate the total number.   They are still part of the total.

    BTW, you may not notice the unused carriers at the extreme channel edges in a spectrum plot, but the central unused carrier is very obvious in any "real" plot.   This center subcarrier is often referred to as the "null" center subcarrier, and helps demodulation by removing any DC component in the signal.

    That leaves an uneven number of unused subcarriers, which are placed at either end of the burst with one more unused one on the left, or lower frequency side.   Both of these help with meeting the spectral mask requirements of the OFDM signals.

    The CWDP book covering PWO-250 has a great descriptions on unused subcarriers.   Matthew Gast's "Definitive Guide" is another indeipensible book to have for this type of detail.

  • Thanks Howard

    Yes there are unused subcarriers and i do count them in "total" subcarriers.

    But the answer in book and pdf were not matching so i thought there should be an errata.

    Anyway, just wish me good luck :)

    Regards,

    Mohsin

  • By Howard - edited: January 6, 2016

    Of course.

    Best of luck  to you !!!

    Errata have sometimes been produced, and other times not.   When different publishers have produced the books they have also occaisionally moved the pages from their original locations.    Brad and friends seem to be managing this kind of things better than in the old days. 

  • Thanks Howard

    Out of the context, but I am getting above high 90% on the sybex practice exams and assessment test. Do you think i am ready for the exam? do you recommend reading some other material?

    I really want to do this on first attempt :) and i need more then 80% AT LEAST!

    Regards,

    Mohsin

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