How UE identify various radio links and usage of frame offset and chip offset… _ Mukesh Acharya's Blog

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How UE identify various radio links and usage of frame offset and chip offsetPosted on July 15, 2010

The RNC maintains CFN(Connection Frame Number) for the timing relation between UE and UTRAN .A

similar concept is maintained known as SFN (System Frame Number) at the cell level.These two parameters

are necessary for the overall synchronization process in the UTRAN.

The SFN has a range from 0 to 4095, whereas the CFN has a range from 0 to 255, i.e. 16 cycles of CFN is equal

to 1 cycle of SFN , so modulo 256 arithmetic is applied to derived the CFN from SFN.

Now the question is how these two are inter related ?

Actually the CFN is used for L2 synchronization between UE and UTRAN , but it is not transmitted to the air

interface, both , UE and Node-B derive it from the SFN.

Now the next question is arrived , that how UE will know about the SFN value and what will happen when UE

have more than 1 RL, how UE will find the timing between these different RLS, the answer of this , lies in the

concept of frame offset and chip offset.Lets first check about the SFN value then we will see how these two

offsets are used.

UE gets the value of SFN with the help of System information Blocks, which are transmitted in a cell via BCH

channel or sayPCCPCH channel.A UE has to read System information Block when it enters into a cell, and

from this UE gets the information about the various parameters of the cells including the SFN value, which itfinds in the header portion of the message.Once the UE gets the SFN value , it can derived the CFN value withe

the help of the formula, CFN = (SFN (DOFF div 38400)) mod 256

Where DOFF is the default DPCH offset , specified in the RRC message(RRC connection setup, in case of soft

handover , RL specific frame offset is sent instead of default one), while the Node-B derives this value, with

the help of frame offset and chip offset, so in case of Node-B this DOFF is nothing but the frame offset + chip

offset.

Now lets explore the Frame offset and Chip offset funda, the frame offset and chip offset values are nothing but

the timing relation between downlink DPCH relative to P-CCPCH.

The frame offset and chip offset value is given to the Node-B during the RL setup(or rl addition) procedure and

the same values are also sent to UE in the RRC message(rrc connection setup or through Active set update

message). So that both UE and the Node-B know from where to start listening the new RL.

In case of UE , the radio link specific DPCH frame offset becomes more important when UE involves in the soft

hand overs, this frame offset will tell the UE ,the new radio frame timing relative to the P-CCPCH radio frame

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About Mukesh Ac harya

Myself is Mukesh , I am working as a Software Professional in a Telecom Software Company in Gurgaon, New Delhi , India. My

interest includes playing cricket, listening to music, searching in the internet on various topics, blogging etc. Current interest in

the wireless domain, 3G networks, UMTS ,LTE... You can find more about me at my personal page at

http://freesmile.in/mukesh.html In case if you want to develop a site for NGO or some social issue, you can contact me at

[email protected] for free website creation. Also if y ou have any query regarding the Right To Information Act

(RTI) , please do send the mail to me.

View all posts by Mukesh Achary a

timing of the new cell.So now

UE knows from where to start

listening the new radio link

frames in the new cell.See the

diagram below,

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4 Responses to How UE identify various radio links and usage of frame offset and chip offset

Muke sh Acharya's Blog

Navneetsays:

September 12, 2011 at 10:37 pm

Hello Mukesh,

i read your article, its pretty good. But ive some questions,

Please have a look.

The questions are about the topic How ue identifies v arious Radio Links.

1) Why cant CFN be just equal to SFN, whats the necessasity or using another variable?

2) Can we CFN just equal begin 0 for every Radio Link?

3) Why do we need a Frame Offset, i kind of understand why we need a Chip Offset. but requiring a Frame offset. i couldnt

understand it.

Let me know if im not clear somewhere.

We can discuss further.

Navneet

Reply

Girishsays:

December 29, 2011 at 6:02 am

Good article Mukesh !

Reply

Taofiksays:

February 7, 2012 at 10:22 am

Thanks Mukesh.

I think you can update this timing for the F-DPCH transmission case in the enhanced cell_fach feature.

Taofik

Reply

Varunsays:

October 6, 2012 at 10:56 am

CFN & SFN explanation is really good .Thanks MukeshReply

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How UE identify various radio links and usage of frame offset and chip offset… _ Mukesh Acharya's Blog