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LTE (3G till Rel 6 - HSUPA cont.) - 11

HSUPA continued [Under LTE > 3G till Rel 6 > HSUPA]
» FDD Enhanced uplink (HSUPA) overall description - 25.309, Rel 6 «
» Phy channels and their mapping mapping on transport channels - 25.211, Rel 6 «

Basic HSUPA channels

HSUPA introduced dedicated physical data channel E-DPDCH (Enhanced-Dedicated Physical Data Channel) and control channel E-DPCCH (Enhanced-Dedicated Physical Control Channel) mapped to new transport channel E-DCH (Enhanced-Dedicated CHannel). There could be one or more E-DPDCHs. E-DCH support TTI of 2ms and10ms (10ms TTI help in where overhead for 2ms is more, this is likely to happen for relatively bigger cell sizes).

Scheduling grants

As seen in earlier article, UEs are to be conveyed about the maximum data rates they can go for particular TTI(s). This is done via shared grant channel, E-AGCH (E-DCH Absolute Grant Channel) and relative grant channel, E-RGCH (E-DCH Relative Grant Channel). Both are physical channels, mapped to E-DCH.

Now through grant channels, UEs are *not* told about "maximum data rates" that they can go for, but rather UEs are told about "maxium E-DPDCH:DPCCH ratio" they can go for ! This does make sense as Node B is concerned about interference rather than actual data rates. UEs has freedom to choose appropriate TFC (from predefined tables), called as E-TFC, provided E-DPDCH:DPCCH ratio is within the grant given. From UE point of view, it may not have full power at its disposal as some power may be required for dedicated channels. So based on amount of (next set of) data to be sent, power available, and grant given, UE is supposed to choose E-TFC.

Needless to say, the E-TFC information is sent over control channel E-DPCCH.

Scheduling requests

One point which is missed above is: how do UEs to send "scheduling requests" to Node B ?

Best method is UE to send detailed information about amount of data to be sent, priority of data, power available etc. to Node B. But this is "heavy". Lighter method would be: Node B to give certain scheduling grant to UE and UE to inform Node B - whether it is ok with it or not - through a single "happy" bit (set if happy, reset if not). Node B can then give increased scheduling grants (relative or absolute) to UE if it is not happy.

HSUPA use both. If it is detailed information, UE sends it over E-DPDCH. For informing "happy" bit, E-DPCCH is sufficient. Note that E-DPCCH is always with E-DPDCH. But that makes sense as "happy" bit always signify that there is certain scheduling grant already present for UE. Detailed scheduling request over E-DPDCH can be sent along with data or alone (non-scheduled).

Further HSUPA channels

Like HSDPA, HSUPA also support (fast) HARQ with Incremental Redundancy. A new downlink dedicated physical channel, E-HICH (E-DCH HARQ Indicator CHannel), is introduced to send ACK/NACK required for retransmissions.

Impact on RAN (protocol stack)

Like HSDPA, HSUPA introduced new entity MAC-e in Node B to take care of HARQ, scheduling etc. for E-DCH.

Last but not least, unlike HSDPA channels, E-DPDCH can go for soft handover (it makes sense uplink data to be received by more than one cell and send +ve ACK from whichever cell received correct data chunk. UE do not have to then retransmit if it passed in at least one cell). To take care of reordering (combining) data from different Node Bs during soft handover, a new MAC entity (MAC-es) was introduced in SRNC.

This complete a brief on HSUPA.

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming and Release 6 document at 3gpp.

Copyright © Samir Amberkar 2010-11§

3G till Rel 6 - HSUPA « LTE Index » 3G till Rel 6 - Channel mapping diagram