6 ms·
DSSS [1] is really interesting and worth reading up on if you’re already familiar with LFSRs. To summarize, you start by generating a deterministic, pseudorando
by brian-armstrong 8y ago
DSSS [1] is really interesting and worth reading up on if you’re already familiar with LFSRs. To summarize, you start by generating a deterministic, pseudorandom, periodic sequence. You then take your message and modulate (bitwise XOR) each bit of the message by the entire sequence e.g. if my sequence is 31 bits long, then my message modulated is now 31x long.
The receiver just correlates the received message by the pseudorandom sequence. The correlation gives me what each bit is and a belief/probability for that bit.
This works really well because the sequence is orthogonal to just about every other sequence (it’s white noise!). It’s even orthogonal to time shifted versions of itself, meaning that it does well at rejecting multipath distortion.
1: https://en.m.wikipedia.org/wiki/Direct-sequence_spread_spectrum https://en.m.wikipedia.org/wiki/Direct-sequence_spread_spect...
- madengr 8y agoI wonder how many orthogonal codes exist? The Walsh codes in CDMA have a pretty low correlation, as long as they all stay synchronized.
- lpmay 8y agoAnother cool thing, that was counter intuitive to me is that more than just rejecting multipath, spread spectrum can actually make use of multipath to improve the link (although I doubt it is done much in practice). Conceptually, what you do is correlate on the incoming signal like normal, this first lock is the dominant path. You take the component that correlated and subtract it from the incoming signal and correlate again on what remains. This lock is the second multipath. You can do this to recover as many multipaths as you want or have dynamic range for. Finally, the relative offsets in the correlations tell you how to re-align all the multipaths in time. Once they are aligned you can sum them together and get more SNR than if there had been no multipath at all.
- brian-armstrong 8y agoYes, this is really cool! If anyone’s curious, this is called a rake receiver https://en.m.wikipedia.org/wiki/Rake_receiver https://en.m.wikipedia.org/wiki/Rake_receiver
- closeparen 8y agoThis seems like a common sort of trick in electrical engineering. XLR cables used in professional audio have three conductors to carry a single signal. Why? One of them exists purely to collect interference - the same interference that affects the primary transmission. The equipment on the other side subtracts it from the primary signal to get a clean result. This is why hundreds of feet of unshielded cable can still sound fine.
- friendzis 8y agoThis is called balanced input/output. TRS connectors are also quite common, especially for interconnects. I find TRS/XLR combi sockets [0] used for this purpose fascinating :) Third wire does not collect interference in a balanced line. Instead, voltage signalling balanced line uses one ground and two signal lines of opposite polarity and input is difference between these two signal lines. Induced interference is more or less the same on both signal lines, therefore cancels out. At a quick glance this article [1] seems to explain this without delving into electronics. Some differential lines (e.g. RS-485, popular in industrial automation) use current signalling thus requiring two wires per signal. [0]: http://www.cliffuk.co.uk/products/combijacks/combijacks.jpg http://www.cliffuk.co.uk/products/combijacks/combijacks.jpg [1]: http://www.aviom.com/blog/balanced-vs-unbalanced/ http://www.aviom.com/blog/balanced-vs-unbalanced/
- tomn 8y agoIt gets even stranger: https://en.wikipedia.org/wiki/Star_quad_cable https://en.wikipedia.org/wiki/Star_quad_cable
- blattimwind 8y ago> Some differential lines (e.g. RS-485, popular in industrial automation) use current signalling thus requiring two wires per signal. Virtually all high-speed digital communication uses differential signalling.
- friendzis 8y agoAgree on that, but all high speed interconnects I know use voltage signalling.