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Re: TDMA - Reading the mail



>In a TDMA system or FDMA with digital modulation when more than one station
>transmits on a channel using the result is the same as multiple stations
>competing for an FM repeater -- the strongest signal wins if it is
>significantly stronger than the rest. If there is more than one at about

Right. How much stronger you have to be to capture the channel depends
on the modulation and coding method. With FM it has to be well over 10
dB.  Even without spreading, a good digital modulation and coding
scheme might have a capture ratio as low as (say) 2 dB. This improves
the chances that *somebody* will get through in a collision.

This suggests that on the transponder TDMA timeslots used for
unreserved random access (i.e., slotted Aloha), stations randomly vary
their uplink power from one transmission to the next, always keeping
the power high enough to overcome noise, of course.

When spread spectrum (CDMA) is used, the "capture ratio" is actually
negative -- which is why it lets you have several stations all
transmitting at once.

Phil
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