I did not have malfunctioning, forgetting handsets in mind,
just a handset diving through deep and multipath
fading, additionally far from the basestation, badly
needing that fast power control, but the downlink
with that control is just as faded.
The MIPS amount needed to track this for an otherwise
slow voice channel is tremendous, and data decisions
are few (8kbps vs 1 or 4MHz chip rate, wavelength of
carrier and speed of car)
If the decisions go bad there is no way one can track the
fadings, level has to be increased, affects all other
handsets,etc..
GSM, on the ohter hand, has a new training signal in
every timeslot, every timelost is a totally new
independent "universe".
Adjusting the position of the time slot is non-critical,
one cannot move so fast towards or away from a basestation
that it would cause severe overlapping.
The reason for GPRS not going for using 8 nor 7 out of 8
timeslots for receiving, but 6, is that the transmit
timeslot should not overlap with the receive timeslots,
takes time to turn off the transmitter and let things
get silent, ring out in the receive path. (DC levels, AGCs,etc)
The basestation can have much more expensive, bulkier,
power hungry filters to separate tx and rx bands.
Ilmarinen |
