My question is about CCK modulation scheme in IEEE 802.11b standard. CCK uses 8 complex polyphase complementary codes to represent a particular 8-bit sequence and achieve 8 chips per symbol. In the receiver, via the correlation detection, the original bits are recovered. However, I am not sure that how to establish a correlation detection for the complex-valued chips.
Could you tell me the algorithm about this correlation detection ?
Many Thanks
Fast Walsh Transform is used for this purpose. You should be able to find material on this in the literature..
Hi Ganesh Ananthaswamy,
Thank you for your answer. I've found some materials about Fast Walsh Transform.
Many thanks!
Hi Jiajun, You sound like your making part of your career in detection & correlation. Here is how I think about it.
Correlation in this context should really be called cross-correlation. It could be called autocorrelation in a noise-free environment although that is never the case in real systems. You need to piecewise replicate all the possible waveforms of the transmitted signal inside the receiver. In other words, you need the universal set of those waveforms according to the design standard. The construct depends on the type of waveform generation used.
Nishant Divecha at WINLAB gives a nice summary of 802.11 CCK. Google 802.11b Modulation Scheme - Winlab,
So many modulation schemes are based on varying the amplitude and or phase of an RF carrier. For example the quadrature amplitude modulation scheme is base on (+/-)A and (+/-)Theta changes n the carrier there are 4 code symbols (+A,+90d), (+A,-90d), (-A, +90d), (-A, -90d). It can also be thought of as 4-ary phase modulation. There are several ways to think of it. There are two bits per chip and if you send 4 chips you can get 8 bits.
You have choices on how to design a correlation scheme. You can decode each chip using an I-Q coherent detector and then concatenate the bits or you can generate all 64 CCK waveforms and use a 64-channel digital correlator to detect which waveform was sent. Or some combination. Correlation helps to increase the Hamming distance and reduce the error rate.
I like the scheme proposed by Chen-Yen Huang in his US patent 7330522. you can search for it at http://patft.uspto.gov/netahtml/PTO/srchnum.htm
Here are two more to look at
http://ecet.ecs.uni-ruse.bg/cst/docs/proceedings/S1/I-9.pdf
http://dspace.mit.edu/handle/1721.1/60242 (download the PDF of the journal article) I especially like this one for the theory.
Good luck
I forgot to mention synchronization which is essential for correlation detection. You can reserve 1-4 of the cck waveforms to use as sync symbols. that way you don't need to use a preamble bit(s) scheme.
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