The history of orthogonal frequency-division multiplexing (2009)
ieeexplore.ieee.org
ieeexplore.ieee.org
It looks like this is actually just a version of the document published a different way.
Posted to HN from IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference:
https://ieeexplore.ieee.org/abstract/document/4698465
Same thing but free from IEEE Communications Magazine, vol. 47, no. 11:
https://ieeexplore.ieee.org/abstract/document/5307460
Person I replied to clicked the PDF button on the freely-available version.
Anyway, I googled the title based on my possibly accurate memory.
In fairness my impression is that very few HN members ’give back’ by joining such organizations. It’s not part of today’s startup mindset.
In physics, different frequencies don’t interfere with each other—they're always independent. However, in digital operations and mathematics, this isn’t automatically true, so we need to carefully select the right frequencies.
OFDM leverages this by ensuring that, in the frequency domain, certain frequencies remain completely separate, meaning changes to one frequency (in amplitude or phase) won't affect the others.
This allows us to capture the maximum information with limited samples.
Are you overlooking harmonics? Are you overlooking the same frequency case of noise cancellation, via phase shift?
The key is that we can still analyze and manipulate frequency components independently in REAL PHYSICAL WORLD, which enables techniques like OFDM.
Here's a graphed example.
https://www.wolframalpha.com/input?i=Plot%5BSin%5Bt%5D%2CSin...
For another fun example, here's the residential power in current American homes: two 120v A/C feeds, out of phase so that you can combine them in the breaker box to get 240v A/C.
https://www.wolframalpha.com/input?i=Plot%5B120*Sin%5Bt%5D%2...
Right? One of my dumb noobie questions has always been why textbooks explain ANC as "180 phase shift" instead of "reverse amplitude". Do they really do an entire DSP pass to break apart a signal and phase-shift it just to get the output that a minus sign could get? There must be something missing from the plum-pudding model.
Another reason is that devices and materials have different frequency responses, so an external signal needs to be filtered to match the levels in the device.
The Wikipedia page for OFDM [2] says something like, if there's interference or attenuation affecting one frequency, it's better to spread that out instead of completely losing one channel. Which kinda makes sense, and would make more sense if I'd ever done RF work. I'd love to do SDR as a hobby some day.
[1] The explanation in the book was something like, "See with 100 frequencies, you can create 100 chips and each user gets 1 chip, so you can support 100 users" which sounded pointless to me.
[2] https://en.wikipedia.org/wiki/Orthogonal_frequency-division_...
CDMA has an added advantage that the bit-error-rate is inversely proportional to the number of people communicating. With just giving everybody a single channel (or timeslot for TDMA), you either have enough or you don't. When you give everybody their own code, then you end up with either a small number of very high quality channels (few people communicating) or a large number of much lower quality channels (many people communicating). This was seen as advantageous when cell networks were rapidly growing.
I don't think it's used much for terrestrial anymore, but it is still used in satellites.
"Recent Interesting and Useful Enhancements of Polyphase Filter Banks: fred harris"