DS,
The UL PDCH is not used by the MS, so the UL TBF is released. It cannot be “reused” for DL.
Look.. A ms in 4+1 will have a timeslot allocation that looks like this :
(note : the PDTCH is the logical channel used to carry Data on a PDCH)
DL PDCH allocation:
TS2=PDTCH
TS3=PDTCH
TS4=PDTCH+PACCH
TS5=PDTCH
UL PDCH allocation:
–
–
TS4=PDTCH+PACCH
–
when the UL PDCH is not used, the allocation will be :
DL PDCH allocation:
TS2=PDTCH
TS3=PDTCH
TS4=PDTCH+PACCH
TS5=PDTCH
UL PDCH allocation:
–
–
TS4=PACCH
–
However the MS will still send “Signalling” (not Data) on the UL PDCH, once in a while. This is the “UL PACCH”.
The second question : Yes, in UL, the MS will have to send RLC/MAC Acknowledgments. It is done through the UL PACCH.
Also it is probable that once in a while, after some DL download, the MS will have to send “higher protocol” acknowledgments (TCP Ack).
It is a little difficult to explain all this on a forum 🙂
Cheers,
pix