In the near future, I’ll post on two new topics: Time-Delay Estimation and the Cyclic Polyspectrum.
The blog is getting good traffic:
But not many comments.
So, feel free to comment on this post with your suggestions on topics that you’d like to see discussed on the CSP blog. Now is your chance to influence my direction over the next weeks and months.
Thanks for reading!
I’m interested in more on modulation classification, mainly M-PSK and M-QAM signals. I think you’ve addressed broadly classifying modulation into three subsets, and that higher order cumulants can then be used to classify further.
Another problem I’m facing is detecting tiny signals, overshadowed by a much larger carrier (sharing the same frequency band), such as in VSAT scenarios.
Thanks for the blog, it has clarified many things, and helped me get an understanding of the physical meaning of the formulas.
Mirko
I’ll increase the priority of posting on the modulation classification topic. When you say “overshadowed”, do you mean that the weak and strong signals are completely spectrally overlapping, or is it the case of a weak adjacent channel signal?
Since you are interested in MPSK/MQAM, could you look at the post on Textbook Signals at https://cyclostationary.wordpress.com/2016/01/19/textbook-signals?
Perhaps you have a response to the request at the bottom of that post.
In a VSAT system, there is a single main hub which is a large antenna, which services many smaller antenna. The result is that the main carrier is at least 10 dB stronger than the little guys. Recently, they’ve started to share the same bandwidth to become more efficient. The little guys are usually some fraction of the large carriers bandwidth, and can therefore be packed under the large carrier en mass.
Another scenario, is that of two identical carriers, with the same modulation scheme, symbol rate and center frequency. Here the cyclostationary features sum to look like that of a single carrier?
I left a reply re: Textbook Signals, not sure if its helpful.
I suppose the “little guys” packed under the main carrier can be demodulated due to the use of a directional antenna? I think I need to draw a figure here to make myself clear, but I’ll try with words for now. If the wideband powerful signal is spread across B Hz, and the “little guys” each reside in a subset of B Hz, say B/10, within the wideband signal’s band, how are they to be demodulated with such a bad SIR?
Regarding the case of two identical carriers, same modulation scheme, same everything, I’ve studied that problem too, and will have a post or two on it eventually. The cool thing is that the collection of nth-order cyclic features for the “composite signal” are distinguishable from the collection of similar features for any single signal alone.
Thanks for the replies!
So the little guys only need to receive the large hub signal which is easy.
The hub uses a new technique that you’ll find in many newer modems (see Double Talk or carrier-in-carrier), where the hub knows what it sent. So it performs an auto correlation to estimate the delay from the satellite, and removes its own signal, exposing the little carriers. This is obviously an issue from an outside point of view, as one cannot retrieve the information using this method.