- This topic has 13 replies, 1 voice, and was last updated 12 years, 5 months ago by pix.
10th September 2008 at 06:17 #53824Dangerous MindsGuest
Hello ALL >> Can anybody please explain how is the neighbour information scanned by a MS? Is it done on the Synchronization Channel (Layer 3 message: Sync Channel Information)? And how does the MS scan the RxLev of the BCCH of the serving cell, in case the call is not on the BCCH TRX?10th September 2008 at 09:59 #53825pixGuest
In Idle mode, the MS listens only to the BCCH of its serving cell. It will decode all System Information messages sent on this channel. Some of these SI msg (SI5 and SI5bis, as far as i recall) lists all the neighbors cells declared for this serving cells (actually, it justs lists down the neighbors’ BCCHs).
The MS measures the RXLEV of the serving’s BCCH and the RXLEV of all neighbors declared in the SI msg.
In Dedicated mode, the MS does not measure the BCCH RXLEV of its serving cell. It measures the TCH RXLEV of the serving only. Plus, of course, the RXLEV neighbors. It will report back those measures to the BTS, but because there might be a lot of neighbors, it will limit the report to the serving’s RxLev and the 6 strongest neighbors’ RxLev.
I don’t know if that answers your question..
FYI, Synch. channel has nothing to do with all this.
Pix11th September 2008 at 07:10 #53826Dangerous MindsGuest
Thanks a lot for the reply. You have almost answered my question 🙂 Now there is a slight confusion. How and when does the MS scan the RxLev of the BCCH of the serving cell? And also, all the System Information messages are transmitted on the SACCH, so why does the drop call rate increase for a cell if it is experiencing co channel interference? If a cell has 4 TRXs for example, most of the calls would not be established on BCCH TRX, so why does the call drop rate increase?
THanks/11th September 2008 at 07:42 #53827PixGuest
Hi Dangerous Mind,
Quote: “How and when does the MS scan the RxLev of the BCCH of the serving cell?”
I don’t understand.. the MS does not scan a RxLev. RxLev is a measurement: the MS measures the received power of the BCCH downlink signal.
Quote: “And also, all the System Information messages are transmitted on the SACCH, so why does the drop call rate increase for a cell if it is experiencing co channel interference? If a cell has 4 TRXs for example, most of the calls would not be established on BCCH TRX, so why does the call drop rate increase?”
No, the SI messages are transmitted on the BCCH timeslot, on the BCCH logical channel. So all MS’s can listen to them.
As soon as a MS gets a SDCCH and/or a TCH there will be a SACCH interleaved with this “dedicated” channel. The SACCH and the TCH are located on the same timeslot. And here, yes, the SI msg are sent on the downlink SACCH. But this SACCH belongs to only one user, nobody else can listen to it.
Now, what’s the relation with cacll drop ? A call drop radio is detected when the BTS or the MS cannot decode the received SACCH. We do the assumption that if the SACCH is not decoded, then the TCH is not either. And that’s correct : they both are on the same timeslot, on the same frequency. So they suffer exactly the same radio conditions.
This assumption however is not working in AMR anymore ; the TCH bursts are highly protected, while the SACCH remains sensitive to interference. Therefore the call drop is detected (no SACCH decoded), while the actual voice (the TCH) could still be decoded by the MS. In this situation the drop occurs, but the subscriber feels like there was no need for a drop.
Pix13th September 2008 at 08:57 #53828Dangerous MindsGuest
OK Thanks again but let me rephrase my question:
Firstly, yes you are right that the MS measures the received power level of the BCCH.
But in dedicated mode, the MS is locked on to the TCH, so how does it measure the received power level of the BCCH? Do we need to look at any layer 3 message that the MS received from the BCCH timelsot?
In our network, a cell usually runs on 4 TRXs –
1 BCCH TRX 900 band
1 TCH TRX 900 band
2 TRXs 1800 band
And the SDCCH and GPRS timeslots are also located on the BCCH TRX. So logically, most of the calls are established on the non BCCH TRXs. However, whenever there are cases of co BCCH, the call drop rate of a cell shoots up. My question is: most of the calls, once established, move to the TCH, so why are there so many drops just because the BCCH TRX is experiencing interference?
Regards/13th September 2008 at 16:15 #53829pixGuest
1) no the MS in dedicated mode does not listen to its BCCH. All necessary layer 3 msg are sent on the TCH (via the SACCH)
2 it depends on your TRX PREF MARK. Perhaps the calls are setup on the BCCH because there is a higher priority there.
Normally, if the calls are sent to TRX2,3 and 4 rather than 1, then you should at least see some call setup failures, because BCCH and SDCCH are interfered.
And finally, not all calls are sent to the TRXs 2/3/4. When all other TRXs are full (or if the 1800 TRXs cannot welcome this call due to poor radio conditions), then the BCCH TRX will welcome TCH as well.
Example… If 10% of calls are sent on BCCH trx, and if all of them are dropped, then there will be a drop rate of about 10% 🙂 Even more, because there will be a faster tch turnover on the first trx.14th September 2008 at 20:02 #53830Alex_RGuest
i think your question’s answer is related to frequency planning
what i mean is if u have your carriers from 1 to 20 and u have from 1 to 10 as BCCH carriers so from 11 to 20 are TCH carriers and when u make TCH frequency plan follow the BCCH frequency group u can find call drops even when u use TCH carrier because u use the same plan for BCCH carriers and TCH carriers
Best regards15th September 2008 at 09:28 #53831hasanGuest
It doesn’t make sense that all call drops are related only to the interference in the BCCH.
When calls are in non BCCH TRX(u can either use single TCH or Hopping for that TRX),then drops due to interference on that is because that/those TCH frequency/frequencies are getting interferences.
And calls when are in BCCH,interference due to co or adjacent BCCH.
It depends calls are where….in the BCCH TRX or in the non-BCCH TRX.
But BCCH should be more priority..as according to u..SD are on BCCH..so it should be keep clean…to reduce other drop like SD drop…in your network.16th September 2008 at 05:53 #53832Dangerous MindsGuest
Problem solved 🙂
but there are a couple of related questions:
1) What is the function of the Synchronization Channel layer 3 messages that we can see in TEMS?
2) Again in TEMS, if my call is established on the 1800 TRX, I can see the RxLevels of both the BCCH as well as the 1800 TCH TRX. Does the MS decode the Synchronization channel from the BCCH TRX and read the received power?
Thanks/16th September 2008 at 08:38 #53833pixGuest
hello dangerous minds,
could you share with what was the problem and how it was solved , please?
the MS does NOT read the received power from any message. It measures it.
TEMS is a very particular mobile, which can show the RXLEV of the TCH and the RXLEV of the BCCH.
“Standard” mobiles does not report such BCCH measurements in dedicated mode.
Synchronization channel is a pure sinusoid which allows mobiles to “oscillate” at the same frequency as the BTS. So they can hear what the BTS is saying.
If MS is a little “out”-sync, then it can’t decode BTS signals.23rd September 2008 at 03:32 #53834Dangerous MindsGuest
Yes, when you said: “If 10% of calls are sent on BCCH trx, and if all of them are dropped, then there will be a drop rate of about 10% 🙂 Even more, because there will be a faster tch turnover on the first trx”
Next question 🙂
If my call is on the 1800 TRX, and power control is enabled, how are the HO decisions taken? since the Received Levels are not of the BCCH plus power control has also reduced the transmitted power.Do Level based and Power Budget Handovers take into account the fact that RxLevels being reported by the MS are less than what they should be because of Power Control?
Thanks yet again/23rd September 2008 at 10:04 #53835PixGuest
first, MY question:
how did you solve the previous problem ?25th September 2008 at 05:17 #53836Dangerous MindsGuest
Sorry, I could not understand your query. Could you elaborate what you are asking?25th September 2008 at 07:54 #53837pixGuest
hello dangerous minds. You have written, previously:
Dangerous Minds – 16 Sep 2008
Problem solved 🙂
but there are a couple of related questions:
I’m just asking : how did you solve the problem ?