We have now released the fifth episode of the podcast Wireless Future, with the following abstract:
What happened to millimeter wave communications? It is often described as synonymous with 5G, but barely any of the brand new 5G networks make use of it. In this episode, Erik G. Larsson and Emil Björnson discuss the basic properties of millimeter waves, whether it is the long sought “holy grail” in wireless communications, and where the technology stands today. To learn more, they recommend the articles “Antenna Count for Massive MIMO: 1.9 GHz versus 60 GHz” and “Massive MIMO in Sub-6 GHz and mmWave: Physical, Practical, and Use-Case Differences”.
You can watch the video podcast on YouTube:
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Hi Erik, Emil,
Great pod-cast as usual!
A few things on mm-wave. You mentioned the power-efficiency of the hardware a couple of times. A large part of the efficiency issue lies with the increased skin-depth of the electric currents at high carrier frequencies. This increases the resistive losses in conductive material, so for example transmission-lines, inductors, etc. has a larger loss per unit length. On top of this, you often have parasitic elements manifesting themselves as capacitive or inductive elements, for example transitions between materials in a transistor etc., which tends to limit high-frequency operation.
But on top of the issue of efficiency, there is also a limitation on the ability to actually generate power. This is commonly referred to as the Johnson-limit, and it states (in layman’s terms) that the achievable output power for a given transistor technology decreases as 1 / f^2. Johnson showed in his paper ( https://doi.org/10.1109/IRECON.1965.1147520 ) that the product between the charge carrier velocity and the maximum electric field / breakdown voltage was constant for a material. Thus, in order to make the transistor operate at higher frequencies implies lowering the breakdown voltage and thus the output power capability. Swapping to a more capable material/transistor technology is one way of increasing the output power, but that comes with a whole new set of challenges (and quite often, a bigger price-tag). There’s a database maintained by prof. Wang at Georgia tech. worth checking out. It tracks state-of-the-art of power amplifier implementations with different technlogogies (a bit like Walden’s database of ADC’s). Here, we clearly see the Johnson-limit kicking in: https://gems.ece.gatech.edu/PA_survey.html
On the receiver-side, we see similar effects due to hardware limitations. For example, achievable noise-figure in the receiver degrades in a similar manner due to losses and decreased transducer-gain in the LNA. State-of-the-art surveys shows an increased receiver noise-figure (noise-figure of the complete receiver – LNA, filters, mixer, ADC) going from the 5dB range at 2GHz to 10-12 dB at 70GHz (https://doi.org/10.23919/CLEEN.2017.8045912).
So, while there might be a lot of available bandwidth at mm-wave spectrum, making good use of it far from trivial.
Have a nice holiday!