# Detecting unipolar and repeating preamble on wired communications? **URL:** \ **Category:** GNU Radio\ **Created:** [May 5, 2015, 8:32am UTC](https://www.ruby-forum.com/t/detecting-unipolar-and-repeating-preamble-on-wired-communications/241837 "2015-05-05T08:32:28Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [May 5, 2015, 8:32am UTC](https://www.ruby-forum.com/t/detecting-unipolar-and-repeating-preamble-on-wired-communications/241837/1 "2015-05-05T08:32:28Z")
I am trying to implement a preamble detection. A couple of things that, I’m not using wireless communication but wired communication. And also, I’m not using a bipolar signal, but a unipolar signal. (Bipolar: a signal swings between +A volts (or amperes) and -A volts; Ethernet, USB, Wi-Fi, … are using bipolar signals, Unipolar: a signal swings between +A volts and 0 volts) Using a unipolar signal, I am having a difficulties on determining whether a signal has a repetition. A preamble has a typical form of 1, 0, 1, 0, 1, 0, 1, 0, (and data payload follows) and it is mapped into a real signal and received at a receiver: +A+n, n, +A+n, n, +A+n, n, +A+n, n, (and data payload follows) (n is very small noise and fluctuation, n can be either negative or positive) I think an optical fiber communication might use unipolar signalling, but I’m not sure. Is there anyone already implemented unipolar preamble detection succesffully, or could anyone give me some hints on mathematical approach or something? Regards, Jeon. ---
**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\ **Post date:** [May 5, 2015, 9:28am UTC](https://www.ruby-forum.com/t/detecting-unipolar-and-repeating-preamble-on-wired-communications/241837/2 "2015-05-05T09:28:33Z")
Hi, > (Bipolar: a signal swings between +A volts (or amperes) and -A volts; > Ethernet, USB, Wi-Fi, … are using bipolar signals, > Unipolar: a signal swings between +A volts and 0 volts) There is no difference … “0 volt” is just an arbitrary reference point. 0 ↔ +A swing is the same as -A/2 ↔ A/2 swing if you shift your reference … Ethernet and USB are _differential_ but that’s just to improve signal integrity / noise immunity and such. At the receiver end, the differential input buffer essentially compares both and see which one is higher. (a bit simplified, but that’s the gist of it). > Is there anyone already implemented unipolar preamble detection > succesffully, or could anyone give me some hints on mathematical approach or > something? If ‘A’ is known, just remove -A/2 from your input signal. If it is unknown,you can use a very long time constant low pass to find it. (assuming your signal doesn’t have long runs of 0 or 1). Cheers, Sylvain ---
**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\ **Post date:** [May 8, 2015, 7:00am UTC](https://www.ruby-forum.com/t/detecting-unipolar-and-repeating-preamble-on-wired-communications/241837/3 "2015-05-08T07:00:52Z")
Dear, Sylvain M., Thanks.I got a hint from your answer. What about using moving average instead LPF? I think moving average (MA) is much simpler than LPF if computational cost is considered. But, another issue is, precision on estimation A and A/2. In that case, LPF would be better. Anyway, I will do applying either LPF and MA on my flow graph. Thanks. Regards, Jeon. 2015-05-05 16:27 GMT+09:00 Sylvain M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):