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5 6 9 , 6 5 9
another knowledge site

HSPA Evolution - CPC

HSPA Evolution - CPC [Under LTE > HSPA Evolution]

3GPP Rel 7 is considered as HSPA Evolution or HSPA+. We will take a cursory look at following important features:

— MIMO
— 64QAM for HSDPA (FDD)
— 16QAM for HSUPA (FDD)
— CPC (FDD)
— Various other optimisations and performance requirements for protocol and hardware

Please refer release 7 document at 3GPP for complete list of Rel 7 features.


CPC [Under LTE > HSPA Evolution ]
» Continuous connectivity for packet data - 25.903, Rel 7 «
» Physical channels and mapping onto transport channels - 25.211, Rel 7 «
» RRC - 25.331, Rel 7 «
» MAC - 25.321, Rel 7 «

One of the important features that takes us from 3G to 4G is "Always ON" experience. PS connection, being shared, need not end after use (like the CS connection). PS connection can continue to be there for some more time so that it can be immediately re-activated and used if user application asks for it. Though it looks optional, bursty nature of data applications makes it mandatory to achieve better usage of power (more from UE point of view) and wireless medium (more from Node B point of view).

How we can better this ... something like an "always on" or close to an "always on" experience ?!

First step in this direction was already taken during UMTS design. Rather than just two plain (RRC) states, "connected" and "not connected", we rather have two modes, Idle & Connected and intermediate (or additional) states (as seen in article on RRC states). Having additional states mean, UE remain in various stages of readiness

Next step would be to remain in Connected mode/CELL_DCH state as long as possible without much sacrificing power and RF usage.

CPC (Continuous Packet Connectivity) tries to achieve this with following features:

— New UL DPCCH slot format
— UL and DL discontinuous transmission
— HS-SCCH-less operation

These features are required to be configurable per UE, so the network remain backward compatible. Please do refer 3GPP TS 25.903 (Rel 7) for good insight into CPC.

New UL DPCCH slot format

Below is UL DPCCH slot (2560 chips/10 bits) format:

Pilot TFCI FBI TPC

Rel 6 specifies different slot formats (different bit strctures). CPC introduce new UL DPCCH slot format in which TPC bits are increased from 1 or 2 bits to 4 bits and Pilots bits set to 6 bits; no TFCI/FBI bits. The idea is to reduce power overhead required for pilot (called pilot energy) without sacrificing on channel estimate and give more space for power control. We could have gone for 5 pilot bits and 5 TPC bits (or other combinations of it), but above combination was found out to be sufficiently optimised (3GPP TS 25.903, sec 4). It keeps UE power control ready, channel (quality) estimate is in place sufficiently, and at the same time, power consumption is reduced.

Note that this slot format is meant for period of (data transmission) inactivity, so to be used with UL DTX (next CPC item).

Discontinuous transmission is not new, but having that in dedicated state (for WCDMA) ensures that UE remain connected with lesser power and noise impact. Note that aim here is to have these improvement over HSDPA, HSUPA, and HARQ features to reap maximum benefits.

UL DTX

In UL, interference is relatively more important (HSUPA article). UL DTX helps in two ways, it reduces UL interference and also saves UE power. UL DTX (applicable to CELL_DCH) works on simple principle of having two cycles for UL (DPCCH) transmision. Cycle 1 (UE_DTX_cycle_1) is normal cycle with usual periods of activity and inactivity. Cycle 2 (UE_DTX_cycle_2) is of longer periods of inactivity.

After certain configurable period of inactivity on E-DCH(Inactivity_Threshold_for_UE_DTX_cycle_2), UE goes for Cycle 2. RRC signaling is used for passing UL DTX parameters. Cycle 2 is integer multiple of Cycle 1.

An important note here is that: above UL DTX does not stop HARQ acknowledgements as HARQ concept is based on signal quality and so its accuracy/performance is not to be compromised by delaying UL transmissions.

For HSDPA CQI reports (refer HSDPA article for CQI): during CQI_DTX_TIMER, CQI transmission follow Rel 6 cycle and after that it follows UL DTX cycle till next HS-DSCH activity (in which case the process repeats). 

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, UMTS by Sanchez and Thioune, and release 7 document at 3GPP.

Copyright © Samir Amberkar 2010-11§

3G till Rel 6 - MBMS cont. « LTE Index » HSPA Evolution - CPC cont.