Friday, 2 May 2008

All about F-DPCH

Fractional DPCH was added in Rel-6 to optimise the consumption of downlink channelization codes. When using HS-DSCH (High Speed Downlink Shared Channel), the main use for DL DPCH (also known as A-DPCH where A stands for Associated) is to carry power control commands (TPC bits) to the UE in order to adjust the uplink transmission. If all RBs (Radio Bearers) including SRBs (Signalling Radio Bearers) are mapped on to HS-DSCH then the DL codes are being wasted. SF 256 is used for A-DPCH and so every code being used by a user is seriously depleting the codes available for other UE's. To overcome this F-DPCH is used so that multiple UE's can share a single DL channelisation code. The limitation is 10 UEs in Rel-6.
For several users, the network configures each user having the same code but different frame timing and, thus, users can be transmitted on the single code source. The original timing is thus retained which avoids the need to adjust timings based on Release 99 power control loop implementation.

During slots where the DPCCH is not transmitted, the NodeB cannot estimate the uplink signal-to-interference ratio for power-control purposes and there is no reason for transmitting a power control bit in the downlink. Consequently, the UE shall not receive any power control commands on the F-DPCH in downlink slots corresponding to inactive uplink DPCCH slots.

There are some restrictions for FDPCH. It is not usable with services requiring data to be mapped to the DCH, such as AMR speech calls and CS video. Also, the lack of pilot information means that a method like feedback-based transmit diversity (closed loop mode) is not usable. The use of closed loop diversity is based on user-specific phase modification, wherein pilot symbols would be needed for verification of the phase rotation applied. On the other hand, when utilizing the F-DPCH, SRBs can benefit from high data rates of HSDPA and reduce service setup times remarkably

Finally, as you may have already figured out, by using F-DPCH the cell capacity has been improved and at the same time for same number of users, the interference has gone down significantly.

In Release 7, Rel-6 limitation has been removed. In R6, for a given UE in soft handover the TPC from all F-DPCH had to have the same offset timing. In R7, F-DPCH (TPC bits) can have different timing from different cells. This is possible due to introduction of 9 new F-DPCH slot formats (slot format 0 is the legacy F-DPCH slot format). The RRC signalling is done seperately for slot formats from the RNC to each of the cells.

You may also be interested in this Ericsson paper titled "The effect of F-DPCH on VoIP over HSDPA Capacity". Available here.


Martin said...

Hi Zahid,

In addition, I think the feature will also be very beneficial together with other Continuous Packet Connectivity (CPC) enhancements to increase the number of always on and notebook users, who create continuous "keep alive" traffic, i.e. one packet every couple of seconds, which is even less traffic than VoIP but requires a constant bearer nevertheless. Some networks push such traffic to the FACH, but the objective of CPC is clearly to have them on the shared channel as well.

Cool blog entry, thanks!


Anonymous said...

I was looking for F-DPCH, I came across this post. It gave me a good idea what the purpose of F-DPCH is. Thanks for the blog entry.

Anonymous said...

I want to know how the Pilot bits can be replaced. Is it not necessary to have Pilot bits in F-DPCH.

Anonymous said...

The closed loop power control relies on the common pilot bits instead.

Anonymous said...

this gives clear view of the F-DPCH
thanks for this post...

Anonymous said...

Since FDPCH does not have pilot bits, how the channel estimation is done ?

Devendra Sharma said...

F-DPCH only has TPC bit to control the uplink power transmission for the UE's. As far as downlink is concerned the channel estimation is done based on CPICH value and hence the CQI value is calculated. This CQI value is then sent on HS-DPCCH which assists in Adaptive Modulation and Coding technique for the downlink data transfer. You can also refer to the following blog regarding CQI

Sunny said...

Is there any commercialized mobile/NW starting to support the F-DPCH?


Vikas said...

There is a requirement that E-DPCCH can be transmitted only if DPCCH is also transmitted in the same slot [TS 25.211,]. With SRBs on HSPA and using F-DPCH, will there still be any dedicated DL/UL DPCCH? What is the need for one?

~ Vikas

Zahid Ghadialy said...

Hi Vikas,

If I am not mistaken then DPCCH is needed for its Pilot bits and TPC commands as E-DPCCH does not carry these. The E-DPCCH can use 2ms or 10ms TTI and the Pilot, TPC uses the old R99 approach so it had to be on a 10ms basis.


Anonymous said...

Good post. Just to comment on DL power control, it's little different for F-DPCH. unlike DPCH there are no pilot bits so SIR estimation is also done on the basis of TPC bits. As far as quality target is concern, spec just talks about BER (bit error rate) which is used to set SIR target at the time of F-DPCH channel establishment. Wondering F-DPCH behavior in fading channel conditions as OLPC is not mandated.

Anonymous said...

Hello, I want to change the F_DPCH offset in the radioBearerSetup message to that other then "0", 2, 4, 6 or 8

On doing so the RadioBearerSetup fails with invalid configuration, what are the other dependents on this, D-DPCH is not configured

Abhay Tripathi said...

hi Zahid

How are you doing ?

A quick question on F-DPCH , are you aware of few UEs or Qualcomm chipset which supports FDPCH.?

I have a few R7 UE (QC based) but i dont see the F-DPCH capcbility in their RRC connections.


Abhay Tripathi

Zahid Ghadialy said...

Hi Abhay, I know that 9200/9600 chipset supports it but its Rel8/Rel9 LTE based dongles.

Shadab said...

can you please explain the difference between F-DPCH (Rel 6)and enhance F-DPCH (rel 7).

kk said...

Great document indeed .. Can somebody tell me why F_DPCH information goes in common e_dch information when the UE state is CELL_FACH in case of SIB5 ? What's the dependency of HS_UPA on F_DPCH ? and how will F_DPCH be used in case common UL transport channel E_DCH is being used ? Yours answers would be of great help to me.

Anonymous said...

I implemented FDPC and CPC, but now see degredation in HSDPA SCC success rate. :(
Looks like this is caused by some UEs not working properly with these features.

Anonymous said...

I have tested many time F DPCH in Nokia RU40-RU50-WCDMA16 and always i got the same degradation in SCC SR and HSPA Drop.
Does any one tested before within Nokia ?