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shallowdeep -> RE: Why is the laptop faster once on the train? (7/19/2011 10:43:21 PM)
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Could have been either Hillwilliam's first serious explanation or TreasureKY's.
If a signal is poor, cellular systems will fall back to less efficient, but more robust, modulation and coding schemes to keep bit error rates at acceptable levels. The result is a drop in your effective data rates. In urban environments, signals can fade out due to shadowing and multipath interference pretty easily, even if you're within what might otherwise be reasonable range of the cellular base station.
However, if changing your location didn't help any and your phone showed a decent signal, it was quite possibly an issue with too many users, as Hillwilliam suggested. Even with ideal signal propagation, a base station only has a finite amount of spectrum to serve the area it's covering. Since all users in that area must share the same channel, things slow down if too many of them try to use it at once, as might easily happen in a crowded train station. To share a channel among users, a base station breaks the channel's resources into smaller chunks of frequency and time (or, equivalently, orthogonal codes) which can then be individually assigned to different users. As the number of users increases, the fraction of time, frequency, or code allocations on the channel that are granted to a given user will decrease, meaning reduced data rates.
quote:
ORIGINAL: Muttling
I'm not sure how vectors… play into… complex numbers, but it sounds cool.
That part's easy, at least: phasors. They're even quite applicable to the subject here.
Also, as wireless communications fall into the electrical engineering realm, I feel slightly obligated to point out that all calculations (even wholly nonsensical ones) with the imaginary unit should properly use a "j" for notation, not the confusing "i" employed by mathematicians. :)
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