understanding how single signal can carry multiple bits - Page 1
Forum section: electronics / beginners

author: m4rtin
author_rank: Regular Contributor
timestamp: November 21, 2013, 11:37:27 pm
content: As I understand, bit rate(number of bits per second) can be lot higher than the baud rate(number of signals per second) and this is achieved by modulating the digital signal. For example four-level PSK(Q-PSK) doubles the bit rate. For example if baud rate is 2400 bauds/s and Q-PSK modulation is used, then 4800bps can be transmitted. However, I'm puzzled even to understand how the Q-PSK modulation works which should be the most simple one. Does this works in a way that each baud is four 360 degree waves "long" and if waves in one baud are 90 degrees in phase, then this represents 0 and 1, if the waves are 270 degrees in phase, then this represents 1 and 1, if waves are 180 degrees in phase then this represents 1 and 0. At least that's how I understand theimage here:In addition, I always thought that one baud is one full wave(number2on image)? Last but not least, how can electronic waves stop like that as they do for example on image at the place where the number1is?

author: amyk
author_rank: Super Contributor
timestamp: November 22, 2013, 07:09:56 am
content: Quote from: Bored@Work on November 22, 2013, 06:52:43 amOne of the best ways to see this is to look at the actual signal on an oscilloscope... and realise that it only vaguely resembles the nice physically impossible diagrams in textbooks.

author: bance
author_rank: Contributor
timestamp: November 22, 2013, 07:12:00 am
content: Thank you, Bored@workA very clear explanation...

author: Unknown
author_rank: Unknown
timestamp: November 22, 2013, 08:47:09 am
content: Another way of considering it which may help - at least when it comes to digital signals - is to think of the information as being contained in thetransitionbetween states, rather than the individual states themselves.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 08, 2013, 12:28:03 pm
content: Quote from: Bored@Work on November 22, 2013, 06:52:43 amits not a bitch when the condition is to stay you out of trouble.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 08, 2013, 03:53:45 pm
content: Someone get a constellation diagram up in this bitch!I recently explained them to my software engineer boyfriend.

author: Bored@Work
author_rank: Super Contributor
timestamp: December 08, 2013, 04:07:00 pm
content: Quote from: minime72706 on December 08, 2013, 03:53:45 pmIf you don't like my explanation why don't you get your ass up and post your one?

author: minime72706
author_rank: Frequent Contributor
timestamp: December 08, 2013, 05:51:00 pm
content: Quote from: Bored@Work on December 08, 2013, 04:07:00 pmYou sure are quick to jump to conclusions by assuming that I had fault with your explanation. I just thought that they were worth mentioning when summarizing the overall concept of multi-bit symbols and general modulation schemes. I find it neat that, with a real (experimentally generated) constellation diagram, you can tell if you are likely to encounter errors if your "stars' get anywhere close to overlapping.What the hell is your problem? I'm personally known for being both kind of a bitch as well as a friendly and painfully helpful person, but I think you take it a bit too far. Maybe I'm just annoyed that, from my perspective, your reaction was completely without merit.Fine I will post one, specifically for 16 QAM, Quadrature Amplitude Modulation where each symbol can have one of 16 values, i.e. 4-bits of information per symbol.Here is an example of what digital values someone might assign to each of the 16 possible symbols used in 16 QAM systems. The axes are the real part and imaginary part for x and y respectively. (sine and cosine, really --- someone confirm that I'm not wrong here, please)Now here is a, probably simulated, example first showing when the modulation scheme fails due to noise or other signal integrity issues and then showing an implementation that MOSTLY works properly. I see some data points above the bottom-right "star" that are ambiguous. The fact is, unless you transmit at some painfully slow speed, you're going to have errors. (see Shannon-Hartley Theorem if interested)EDIT: Forgot to explain why the messy diagram on the top implies failure, or in the very least, bad performance.When you have symbols that are received and measured to be somewhere in between points on the constellation diagram, the receiver has a good chance of making an error as to which symbol was received.Although it would be an idealization to do so, look at the first image I posted that just has circles. They're equidistant, yes? Now draw, probably just with your mind, boxes around each of the circles that are all the same size and touch, making a pattern like an old-fashioned window. Since receivers are not ideal, one would not know what to do with any symbols that it detects anywhere near the intersecting lines (sides) of the squares.Think of it like a TTL gate: if a gate input receives a voltage in the cross-hatched areas, it will come to a decision as to if it's high or low, but it may easily be wrong.This is a pretty good animated gif explaining what a transmitter needs to output to choose each of the 16 possibilities. (wanted to avoid posting animated gifs here - not even sure they work)http://upload.wikimedia.org/wikipedia/commons/9/90/QAM16_Demonstration.gif

author: Electro Fan
author_rank: Super Contributor
timestamp: December 08, 2013, 08:27:26 pm
content: Quote from: minime72706 on December 08, 2013, 05:51:00 pmVery nice/helfpul animated gif!

author: Bored@Work
author_rank: Super Contributor
timestamp: December 08, 2013, 09:42:36 pm
content: EDIT: Comment deleted by Moderator. Play nice.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 09, 2013, 12:20:19 am
content: EDIT: personal attack response removed by moderator.Quote from: Electro Fan on December 08, 2013, 08:27:26 pmI'm glad you found it useful. You'd hope that, by now, that most technical articles on Wikipedia would have useful illustrations ...

author: Bored@Work
author_rank: Super Contributor
timestamp: December 09, 2013, 10:18:57 pm
content: Man, you are really an asshole. First you call me a bitch then you run complaining to the moderators. But don't worry, I have deleted my explanation, so you can show of with the stuff you copied from somewhere.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 14, 2013, 12:23:33 am
content: I called myself a bitch youdecrepit old mandeaf/blind but unfortunately not dumb old man. (dumb in the Helen Keller sense) This is exactly what I'm talking about. You appear to go out of your way to cause conflict; however, maybe you just skim text worse than I do and misinterpret statements that should otherwise be clear. By the way, I wouldn't call the person who acted on this thread a "moderator", if you know what I mean.I said that I consider myself somewhat of a "bitch", per se, because I am known for dry humor and abrasive language. It generally gets favorable reviews in person, but doesn't always transfer over online.I used the word "bitch" earlier in the thread, in my first post, referring to the thread itself. i.e. my post involving the phrase "get some constellation diagrams up in this bitch" was meant to mean "I like your explanation, but I think someone should discuss constellation diagrams because they're a good visual representation of this overall idea." I didn't do it myself because I wasn't in the mood at the time and figured you or someone else (specifically you since you were already apt to discuss the topic) should make a mention of constellation diagrams with a simple example. If I were to have done the explanation, I would have ended up doing exactly what I did ... quick research before I make myself look stupid. Maybe you, being more experienced than me, would have done a better job. That's all I was getting at ... despite what you may think, I'm not trying to get on your bad side and start fights.I never called you a bitch and stop trying to pretend that I did. I never go out of my way to start arguments with you, but when you go after me for some false reason, I won't back down until you just re-read the text you were reacting to and realize you misinterpreted it. If I was in the wrong, I may not admit it immediately (because I might be pissy or something), but I'll eventually concede that I was wrong.As for "The stuff I copied from somewhere", you're right, I did; however, I did put meaningful effort toward choosing the images that I did. I spent at least 10 minutes sorting through the junk that appears when you search for "constellation diagram" or "16 QAM constellation diagram" on Google. I specifically wanted an image showing how digital codes are associated with each symbol and how the symbols might actually appear in a real system (noisy) as well as something that quickly explains the math associated with my 16 QAM example (the gif with amplitudes and angles). I am not an expert, but I think my content was low enough level that I didn't throw any fallacies at the OP or anyone else, at least I hope not.