Nerd Rage

Here is some­thing that recently piqued my inter­est (and kept me busy for the bet­ter part of last week). It could be (mar­gin­ally) inter­est­ing for peo­ple which work on OFDM or Nyquist WDM and – as far as I know – no one has looked at this in more detail…

After what seems (to me) to have been sev­eral eons worth of exper­i­ment­ing, writ­ing, re-writing, re-experimenting, more re-writing and then some more re-writing (all of which mostly by oth­ers), the all-optical FFT idea that was used at the IPQ depart­ment at KIT finally made it into Nature Pho­ton­ics. When I got there in late ’09, …

This is prob­a­bly gonna be the last of the OFDM posts around here, since I’m no longer work­ing on OFDM-related top­ics. The next post was sup­posed to be about recep­tion of opti­cal OFDM sig­nals, and the one after that maybe about the var­i­ous imped­i­ments that can occur in spe­cific types of receivers (e.g. opti­cal beat …

Due to the time-distributed nature of the sub­chan­nel sym­bols and the unique algo­rithm used to demul­ti­plex the com­pound sig­nal, the influ­ence of var­i­ous trans­mis­sion impair­ments on the sub­chan­nel sig­nals is very dif­fer­ent from what it is in single-channel sys­tems. As we will see, this makes OFDM much eas­ier to ana­lyze and pre­dict its per­for­mance, but …

Before we go fur­ther into the sig­nal pro­cess­ing top­ics that started with the cyclic pre­fix, we shall take a look at how we can gen­er­ate OFDM sig­nals ready for trans­mis­sion and then, next, how to get them into the fiber. By now you should have a firm grasp on what an OFDM sig­nal is. If …

One of the most com­mon mis­un­der­stand­ings about OFDM that keeps com­ing in most dis­cus­sions on the topic is if sub­chan­nels are still orthog­o­nal when there is a cyclic pre­fix involved. But let’s start at the begin­ning… Dis­per­sion We have seen in this post that the DFT can be used at the receiver to demul­ti­plex the …

We have seen in this post that the DFT spec­trum of a sin­gle OFDM sym­bol is a line spec­trum with one line for each sub­chan­nel $k$ which car­ries the mod­u­lated data $c_{k}$. Within a sin­gle sym­bol there is no spec­tral over­lap, which is the rea­son why OFDM works. What’s also inter­est­ing is the long-time (more …

This post con­tin­ues the intro­duc­tion to opti­cal OFDM that I started here. In this post, we’ll dis­cuss what an OFDM sym­bol looks like and show, start­ing from the orthog­o­nal­ity con­di­tion, that the dis­crete Fourier trans­form can be used to demul­ti­plex an OFDM chan­nel into its sub­chan­nels. Using the Fourier trans­form, we also take a look …

And now for some­thing com­pletely dif­fer­ent… This will be the first of (hope­fully) a series of posts that will be an intro­duc­tion to opti­cal OFDM (or what I have learned in the past half year or so about it). Opti­cal OFDM has been some­what of a hot topic in recent years in the com­mu­nity, but …