powerwaveformwireless
One disadvantage of OFDM is apparantly the high instantaneous power variations (see Wikipedia).
What causes this? I assume this is a problem of forming orthogonal waveforms since regular frequency multiplexing (FDM) doesn't suffer from these high instantaneous power variations.
Some of the losses will be associated with processing the power to step it up to a higher frequency.
Other losses will occur from inductive heating of the surrounds and energy being lost as RF
I'm going to try and answer the latter half of your question because I kept on running out of comment room:
MU-MIMO offers improvement because most mobile devices (called User Equipment (UE) in most documents I've seen) don't support as many spatial streams as the base station. In SU-MIMO, only one UE can transmit at a time. So if you have a 10x10 (10 upload x 10 download) base station, and a 2x2 client, the base station can only use two of its spatial streams. This difference is illustrated from this white paper:
So to return to your example, if both UEs and base stations are 2x2, your logic holds. However, if each UE is 1x1 OR if the base stations is 4x4 then MU-MIMO is twice as fast; 8 vs 16 ms for the former and 4 vs 8 ms for the latter.
As someone with a passing interest in MIMO, I'm not sure I'm qualified to take a stab at the mathematics of MIMO, however, I highly recommend you read white papers or application notes from manufacturers of chips or test equipment first. Although they're written to encourage you to buy their product, they often give more background than academic papers because the people buying the equipment are often not experts (yet):
Best Answer
OFDM is built from a large number of essentially independent carriers, with random phases. This gives it a probability distribution similar to noise.
As you know, noise has a potentially infinite peak to rms ratio (though you tend never to see the infinite values, with it rarely going above 12dB).
With a finite number of carrier, OFDM would not go to infinity even theoretically, but in practice rarely goes beyond 12dB.
This is in contrast to single channel comms links, which have much smaller peak to mean ratios.
As an aside, IS-95 CDMA systems also suffer from high peak to mean on the downlink for the same multiple independent carrier reason, although they are not OFDM.