HSUPA [Under LTE > 3G till Rel 6]
» FDD Enhanced uplink (HSUPA) overall description - 25.309, Rel 6 «
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Success (and essence) of HSDPA is in fast adaptation of shared downlink channels (channelisation codes and TTIs) to instantaneous radio conditions. How about uplink, how can we achieve similar high speed performance in uplink too ?
In uplink, channelisation code limitation is not present (UMTS article of CDMA codes). So it seems UE can put in more power to achieve higher data rates. But more power would mean more interference (both intra-cell and inter-cell). Of course, Node B can avoid interference going above limit by appropriate (TFC) allocations (done at the beginning of radio bearer establishment, reconfiguration etc.). But this can not be fastly adapted to varing interferences.
What we rather need is: "fast" adaptation of dedicated uplink channel data rates as per varying radio conditions (interference). This can not be done by individual UEs alone, but need to be done with the help of Node B. Similar to HSDPA, we can in fact use smaller TTIs for which data rates can be adapted. One way to do it is UE to send "request" to go for higher data rates and Node B to "grant" such requests based on its own calculations. If the UE request contain more detailed information - like amount of data that is to be sent, priority of data - Node B can make better decisions overall.
This is the brief idea behind "enhanced uplink" enhancement - also known as HSUPA - done in Rel 6.
HSUPA introduced new dedicated uplink channel E-DCH, use of which - even though dedicated - is ruled by allocations done by Node B in terms of TTIs. Node B allocations are conveyed to UEs in separate (dedicated and/or shared) channel, E-GCH.
We will see more details of HSUPA in next article.
References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming and Release 6 document at 3gpp.
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