Can you say how, specifically with regards to "receivers seeing it as white noise without knowing the timing"?
As I understand it, the same is true for e.g. DSSS (for receivers not knowing the spreading code), and the primary advantage of UWB is its precise ranging capability (due to its ability to reject multipath propagation errors and the generally high bandwidth, yielding better spatial resolution).
I studied ultra wideband decades before they started to write papers refering to UWB standard definitions [1] so I might be out of tune (pun intended) with current definitions.
A transceiver could use attosecond 10^−18, femtosecond 10^−15 picosecond 10^−12 or nanosecond 10^−9 pulses at very irregularly intervals. That seems (almost) random to any observer but not to a receiver which has pre-agreed those irregularly intervals with the transmitter (for example with quantum key distribution, with entangled particles). The receiver measures if there was a signal or not. It does not use power level, frequency, or phase (or a combination of these) of a sinusoidal wave but by generating radio energy at specific time intervals and occupying a large bandwidth, thus enabling pulse-position or time modulation.
Not just spacial distribution can be used.
You could use polarised photons, electron spin, etc.
In my wafer scale integration [2] I use very few free space photos to flip a 1 v transistor in an ultra wideband mode.
I refer you to [3] for [4] for a better explanation, even though ultra wideband is not mentioned specifically.
Contact me directly, I'll be happy to lecture for a few hours to answer your question.
The same is true for DSSS only within a relatively narrow frequency band, originally of at most a few tens of MHz, now up to a few hundred MHz.
For UWB, the energy of the signal is spread over a much wider frequency band, of at least a few GHz and up to tens of GHz. Therefore UWB behaves like a white noise with a much greater bandwidth than DSSS.
It is not difficult to jam the entire band that can be used by certain kinds of DSSS signals or to receive all of it and process it digitally to search for signals within it.
Such operations are much more difficult for the much greater bandwidth used by an UWB signal.
Ah, that makes sense, thank you! So if I understand it right, they'd be the same/very similar if we could do direct-sequence spreading over multiple GHz, but we can't, so we do UWB instead? Is the modulation just practically easier to achieve in hardware?
And regarding jamming resistance: Couldn't a jammer just transmit random pulses in the frequency band in question? Why is jamming harder than just transmitting a lot of useless data on the same channel using whatever modulation the targeted transmitter uses as well? Or is there just too much space (or rather, time) to cover with reasonable transmit power?
UWB radio transmits below the noise floor, you're going to have a hard time finding any signal there or even know if a signal is being transmitted. You can go crazy misidentifying noise combinations as signals. In that sense you need to know almost everything about it to intercept it.
That's the same for (wide) spectrum spreading though, right? Some of these signals are also below the noise floor before correlation, and you need to know the spreading code to detect them.
