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Depending on phase matching criteria of lambda's on a given core, I would mostly agree that various wavelengths are not significantly coupled. I also agree there are a different power budget for LH modems vs. DCI, but power on LH modems is not something that often gets publicly disclosed. I am not too concerned with the overall power, more the power density (and component density) that 19 channel MIMO would require.

The main point I was trying to make is the impracticality of MIMO SDM. The topic has been discussed to death (see the endless papers from Nokia) and has yet to be deployed because the spatial gain is never worth the real world implementation issues.

As others have mentioned, this is mostly a proof of concept for a high core count weakly-coupled fibre from Sumitomo. I also want to highlight the use of a 19 channels MIMO receiver structure which is completely impractical. The linked article also fails to mention a figure for MIMO gain.

I can think of a number of cases where using a hard decision decoder can be a better choice. Power would be one factor and I would strongly disagree that the power delta between soft and hard codes at these data rates is small. Unfortunately, I can not find any public data to back up that claim for what appear (based on coding gain and overhead) an HD-FEC using RS and a SD-FEC using braided BCH or LDPC.

Other factors can include reduced routing complexity and area requirements of HD since you have to shuffle around soft information. Extra die space is expensive so you want to avoid it if possible.

However, I think the most likely is the latency reduction you get when using HD-FEC. I know that some applications of microwave links are extremely latency sensitive, could be that this research is targeting one of those applications.

Something not totally clear from the title, but it seems the claimed rate was actually achieve with a transmitter structure similar to what you would find in coherent optics (see figure 4). Instead of coupling to fiber, they couple to a high speed photodiode that radiates at the ~140GHz laser wavelength.

EDIT: Noticed that after a closer reading of the paper, the real goal was to assess the LO phase noise improvement when moving from a RF synthesizer to a SBS laser and PD based LO.

Quick correction - not repeaters but amplifiers. The term repeater is commonly used to refer to a device that demodulates and remodulates a signal.

Why so many taps? That's north of 2 seconds.

What is "reliable" though? All digital systems will undergo some rate of correctable/uncorrectable errors, it just depends on what you consider to be an acceptable rate of failure for your device.

That may hold for a trivial device or a perfectly spec compliant device. However, the former is not interesting and the later does not exist. I agree that more test coverage would be beneficial, but I think your heavily downplaying the difficulty of writing realistic mock hardware.

I would argue the FFT algorithm is much more important in communication systems than the Viterbi algorithm. There are plenty of powerful Forward Error Correction schemes that do not use Convolutional codes (and thus cannot use Viterbi algorithms). However, every single system with a reasonably long channel equalizer makes use of a real time FFT algorithm.

Fun fact, Golay codes are the only perfect code outside of Hamming codes. Perfect codes are interesting in that the Hamming bound becomes an equality, effectively saying that the distance between all valid codewords is exactly the minimum distance "d".

Looks like Intel 4nm will only start shipping 2023H2. I'm not sure I see Intel 2nm shipping less than 6 months later, especially given Intel's recent history of stumbling on execution. How firm do you believe the 2024H2 delivery date of N2 to be?

I mean yes and no? Kalman filters fuses information about known system behaviour along with observations in an optimal predictor (for certain types of noise). It uses a matrix in the same way that almost every operation on a computer uses a matrix.

Edit: I missed the quip about the Kalman comment being in jest, my reply probably wasn’t necessary but I’ll leave it up.