Hey guys!
I have a question that I thought maybe some folks here might be able to help shed some light on:
Q: What is the maximum spectral efficiency that can be reasonably accomplished by modern technology/engineering/science/magic today?
I have a basic understanding of Shannon's Channel Capacity Limit. Ideally, infinite transmit power leads to infinite signal-to-noise, which leads to log2(1+ infinite) spectral efficiency

. Now, as we do not live on the planet Ideal, there will be a number of factors which will prevent us from simply jamming more and more power into larger and larger Tx and Rx antennas, ad-infinitum. Eventually, non-linearities in the transmitter and receiver are going to prevent you from obtaining ever more channel capacity, regardless of the amount of power or antenna gain you attempt to dump into the link.
Unfortunately, I can't find a good source to help me determine where this limit might be.
Assumptions for a random thought experiment I've been conducting with a friend of mine:
Frequency: X band, 8.0 GHz
LOS link: 16 km (10 mi)
Transmitter: 15 W of Tx power, through a 0.1 sq. meter antenna.
Receiver system has a larger number of identical 0.1 sq. meter antennas attached to dedicated ADC's, somewhere between 1 and 200 separate antennas. The system might periodically use some sort of pilot signal to synchronize all receivers, allowing for coherent summation of signal power across all receive antennas.
The SNR you obtain from the above scenario is somewhat ridiculous, somewhere between approximately 45 and 65 dB, corresponding to a spectral efficiency of around 15-22 bit/Hz. Of course, it would be relatively easy to image even larger numbers. If we used 150 W of Tx power through 1 meter antennas, we can obtain 100 dB of SNR, good for 32 bit/Hz!
Now, I'm sure that someone somewhere using infinite funding could make a 30 bit/Hz datalink out of unobtainum if they really, really wanted to, but what I'm not sure about is the practicality. I'm assuming that most components going into typical RF devices don't have anywhere near the linearity or phase noise required to achieve this kind of performance.
So, assuming a reasonable cost (<$50k per Rx or Tx module), and a reasonable amount of volume (say, 1,000 - 10,000 modules), and a reasonable development budget (say, well less than $100 million dollars), what kind of spectral efficiency could reasonably be obtained with today's technology?
In addition to any answers folks might come up with, I would be supremely grateful if anyone has any books they think might be helpful on this topic.
Specifically, I would love to be able to calculate the expected link performance given actual device datasheets and see where they stack up in this example.
Thanks very much in advance!
Best regards,
Andrew