The IEEE 802.11a specifications are used by many to understand a wireless communication link built using OFDM. In this post, I have put together a set of 10 multiple choice questions based on 802.11a specifications. The questions are on the building blocks in 802.11a specifications, preamble structure and so on. Upon completion of the quiz, you will be lead to a page showing the correct answers and their explanations.

Click here to download IEEE 802.11a specifications.

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Good luck!

[QUIZZIN 4]

Krishna,

In OFDM, why is the CRC8 or CRC16 or CRC32 is passed through an inverter ? I mean, after the CRC is calculated, before appending it to data the CRC’s one’s complement is calculated. Why not the CRC used as is ??

Thanks

Mohan

@mohan: That was a nice question. I did some quick googling and found the following link

http://en.wikipedia.org/wiki/Computation_of_CRC#Post-invert

http://en.wikipedia.org/wiki/Mathematics_of_CRC#Variations

Quoting from wiki entry:

“Appending 0 bits to a message is equivalent to multiplying its polynomial by x, and if it was previously a multiple of the CRC polynomial, the result of that multiplication will be, as well. A similar solution can be applied at the end of the message, inverting the CRC register before it is appended to the message. Again, any non-zero change will do; inverting all the bits is simply the most common.”I have not done the math to verify it myself. Will do that in another post.

please tell how to calculate all the parameters of IEEE 802.11a specification,

@preet: Plz use the IEEE specifications @ http://standards.ieee.org/getieee802/download/802.11a-1999.pdf

can you sujjest any method for solving sunchronization problem in ofdm,,,,,

@preet: Synchronization is quite an involved issue. Need to fidn algorithms for frequency/phase and time synchronization. Have brief post on frequency offset estimation in OFDM

https://dsplog.com/2008/03/03/frequency-offset-estimation-using-80211a-short-preamble/

Regarding frequency offset tolerance, does it mean for 802.11g, this figure needs to be only around 100 KHz ~(2.4*40)? Or does the tolerance become +/- 40ppm in that case?

@Sudeep: For the 802.11g (ERP-PHY, see clause 19.4.7.2 in IEEE 802.11-2007 specifications), the tolerance specified is +/-25ppm. So the maximum frequency offset comes around to 2.5GHz * 50ppm = 125kHz.

Regarding the question on symbol duration, the checkbox answer shows it as 3.2 us, where as in the explanation it is mentioned as 4 us including the GI. I think the check box should also show 4 us as the correct option to avoid confusion.

@Sudeep: Thanks for noticing it. I corrected.