FAQ on UMTS/3G Handovers

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 17/01/2008

What is handover and how does it work?

You can learn more about Handover principles and concepts in the Handover Tutorial. Alternatively you can also read Soft Handover Tutorial if you are only interested in learning about Soft/Softer handover.

What is Blind Handover and how does it work?

If the mobile phone is leaving a UMTS cell and it cannot find a new UMTS cell then the base station can hand over an appropriately equipped mobile phone to a cell in another system. These intersystem handovers are highly complex because two technically disparate systems must be combined with each other. Basically, there are two handover options from WCDMA to GSM:

In the case of blind handover, the base station simply transmits the mobile phone with all relevant parameters to the new cell. The mobile phone changes “blindly” to the GSM cell, i.e. it has not yet received any information about the timing there. It will first contact the transmitted BCCH channel, where it tries to achieve the frequency and time synchronization within 800 ms. Next, it will switch to the handed-over physical voice channel, where it will carry out the same sequence as with the non-synchronized intercell handover.

For the second type of handover from WCDMA to GSM, the compressed mode is used within the WCDMA cell; in this mode, transmission and reception gaps occur during the transmission between base station and mobile phone. During these gaps, the mobile phone can measure and analyze the nearby GSM cells. For this purpose, the base station, similar to the GSM system, provides a neighbour cell list, and the mobile phone transfers the measurement results to the base station. The actual handover in the compressed mode is basically analogous to blind handover.

There is, of course, an intersystem handover from GSM to WCDMA. A special neighbour cell list for WCDMA cells was established in GSM to support this handover.

Reference: Handover scenarios in GSM systems - R&S

I have a small query related to IRAT HO.I just need to know what we can do to control the number of IRAT HO from 3G to 2G in some particular cells if they are at border and non border? I know that we can change some RNC parameters to change the GSM threshold,but more interested to know about Cell Level Parameters and the cells which are in the middle of a city. Give me some thoughts and idea? I want to reduce the number of IRAT HO from 3G to 2G at some of my downtown cells!!! - Arun Verma in Yahoo Groups: UMTS

Answered by Kamal Vij in Yahoo Groups: UMTS

As u might be well aware of , in UMTS UE always tries to do Intra-freq. HO. And to trigger an Inter Freq and Inter RAT there are several triggers /threshold criteria which must be fulfiulled.

For example, theCPICH EcNo coverage is very "BAD", CPICH RSCP coverage is "BAD", the UE is using very "HIGH" transmission power and is close to saturation, DL transmission power for that particular radio link is very "HIGH" and so on...

The words "BAD" and "HIGH " are defined by cell level thresholds. I cant write more precisely because it goes beyond the privacy agreements with my customer.

Please try to look for the Parameters which define Inter System THRESHOLDS .

For example , consider CPICH EcNo.

Genrally -8 is very good coverage, -12 is ok, and -15 EcNO is just close to the coverage hole.

Att -13 u may start Inter RAT. If u allow the mobiles to survive in 3G even if the coverage is -14 dB (almost close to coverage hole), u can save a lot of Inter RAT handovers.

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