Following the article on GPRS cell reselection and NACC, I thought it was appropriate to mention neighbouring cells that the MS monitors. The list of neighbour cells is formed by a complex process in which information is gathered from different sources. As an easy reference, this post attempts to summarize this process. This post will look at:
- GSM Neighbour Cell List for GSM
- GSM Neighbour Cell List for GPRS
The term “GSM Neighbour Cell List” is so named to distinguish it from 3G Neighbour Cell List. It is applicable to GPRS as well. It is simply a list of neighbours that the MS maintains. It makes measurements on these cells and reports them to the network if configured to do so. Cell reselection depends on these measurements.
The final list that the MS maintains is the Neighbour Cell List which is a concatenation of GSM Neighbour Cell List and 3G Neighbour Cell List. For the purpose of reporting measurements, the lists are treated separately.
Every cell maintains a BCCH Allocation List or BA List which is a list of frequencies of neighbouring cells. This information is useful for the MS for performing and reporting measurements, and eventually in cell reselection and handover. The BA List is formed from information in the SI. For GPRS, BA(GPRS) exists which is formed from information in PSI. If the cell does not have a PBCCH, BA(GPRS) equates to BA(list) obtained from SI. The key difference between BA(list) and BA(GPRS) is that the former is only a frequency list and the latter is a list of frequencies paired with Base Station Identification Code (BSIC). BSIC allows for cell changes across cells that are not necessarily neighbours in a geographic sense. So an MS could do a cell reselection across cells with the same frequency but different BSIC. Such a possibility arises if the immediate neighbours are hidden (no line of sight) whereas a far away cell can provide better service due to clear LOS radio path.
Figure 1: GSM NC List (GSM)
Now let us look at the formation of GSM Neighbour Cell List for GSM (Figure 1). In idle mode, BA(list) is formed from SI2/2bis/2ter while in dedicated mode SI5/5bis/5ter are used. If the latter set is missing completely in dedicated mode, the former will be used. SI2quater enhances the BA(list) by providing BSICs. The combination of the two makes the GSM NC List. SI2quater was introduced for enhanced measurements, a feature I will describe in a separate post. In dedicated mode, Measurement Information (MI) is used instead of SI2quater but the end result is the same as in idle mode. If SI5 and MI are both not sent to the MS in dedicated mode, the GSM NC List in dedicated mode is same as in idle mode.
Figure 2: GSM NC List (GPRS)
Figure 2 illustrates the formation of GSM NC List for GPRS. In packet idle mode without PBCCH on the cell, the BA(GPRS) is only a frequency list. Combined with BSIC information in SI2quater this becomes the GSM NC List. If PBCCH is present, PSI3/3bis is used to make the BA(GPRS) which contains pairs of frequency and BSIC. This is also the GSM NC List. In packet transfer mode, the GSM NC List formed in packet idle mode is reused. Elements of this list can be deleted or new elements can be added using Packet Measurement Order (PMO) or Packet Cell Change Order (PCCO). Addition happens on the GSM NC List and is based on pairing of frequency and BSIC. Deletion happens on the BA(GPRS) and is based on frequency which implies that all cells with that frequency are removed from the GSM NC List regardless of the BSIC. Such a definition is useful if the BA(GPRS) is only a frequency list as in a cell without PBCCH.
Among the cells in the GSM NC List for GPRS, some may be cells with both Iu and A/Gb modes. There may be cells with only Iu mode. In the final count, the list can have a maximum of 96 cells and 32 unique frequencies.
is it possible to access this list from an application? guess would depend upon the hardware right.. but this could lead to get LBS apps!
Hi Rajiv,
This list is not directly accessible by applications. It is internal to the protocol. However, I believe there are standards that relate directly to LBS. LBS has quite different strategies for identifying the location of a mobile.
Locating the mobile for LBS requires that mobile be located within an accuracy of few tens of meters. A UMTS macrocell is generally of radius 2 km. GSM macrocells can possibly be as large as 35 km radius. GSM NC List can locate a mobile only to the level of cell which is generally not very useful in urban areas. There may be some use for it in rural areas. It’s worth considering.
Arvind
very interesting, but I don’t agree with you
Idetrorce
Hi Arvind,
You have mentioned “In the final count, the list can have a maximum of 96 cells and 32 unique frequencies.
”
You can have more then 32 unique frequencies . But MS will measure only 32 frequencies with te lowest indexes?
Hi Jackson,
You are right. I was not precise in my wording. MS will handle only 32 frequencies with the lowest indices. Specifications ref: TS 44.060, 5.6.3.2.
Arvind
Hi Aravind,
The information you have shared is very useful.
I have few doubts in the neighbour cell formation.
In case of meas_info(on dedicated channel) sent to the mobile, based on this meas_info the mobile will uniquely prepare its NCL, in that case is there a necessity that a list should be maintained per mobile.
Or is it just like 5/5bis sent to all mobiles over dedicated channels?
The indexing the mobile does at its part while creating the ncl is very obscure to me.
In case of enh_mr the MS sends, it just provides that index and not the bsic/bcch_freqncell unlike normal MR. can you please comment on this?
itsmejiji@yahoo.com
thanks, jiji
Hi Jiji,
Each MS will have its own NCL. The contents of Measurement Information sent to an MS could be different from what is sent to another MS. At the BS, I would think that a similar list must be maintained for each MS. This is necessary because some parts of the report carry only indices as you rightly point out.
For making the indices, the rules in the standards are very precise. The problem is that the standard has evolved over a long time with additions along the way. Sections related to measurements are many. You will have to read them all, unfortunately.
Arvind
Hi Arvind,
Thanks first.
I read through 5.08 and 4.18 but both these standards doesn’t extensively talk about how the UE creates this NCL.
Can you give me some pointers in the 3GPP standards where i can find information about neighbour cell list creation for 2G and 3G (with/without Meas_Info).
Also I need another suggestion from you.
Actually we need to implement inter-RAT HO from 2G to 3G.
The preprocessing happens at BTS and we send to BSC the best cells to which the UE can be handed over and the cause (say rxlev or pbdget etc.)
We process the measurements that is brought in per sacch period in a way explained in annex A of 5.08. We make use of the HreqAve and HreqT parameters to average and maintain the results of averaging.
Now I also need to process the 3G measurements brought in by the MS/UE.
Do i follow the same processing (as done for rxlev of 2G neighbour cell measurement preprocessing) or do I need to handle the measurements (ec/no or rscp or rssi of 3G) of 3G in a different way?
Can you give me some pointers in standards/clauses in the standards that will be of very great help.
Regards,
Jiji
itsmejiji@yahoo.com
Arvind,
In your first post you have mentioned this:
“SI2quater was introduced for enhanced measurements, a feature I will describe in a separate post.”
Did you post that? Can you get me the link please….
-jiji
Hi,
Do you think I can handover the mobile to a cell in reselection neig. cell list (and not in handover neig cell list)?
If the mobile brings measurement reports based on si2 (res.list), should i do a handover to a cell that is present in reselection?
nive
Why not? There is no constraint on the network about which target cell to use. Network implementations should take decisions to make the best of radio resources and serve mobiles effectively. Remember that the standards are written largely as requirements on the mobile, not on the network.
Jiji, I haven’t written about enhanced measurements. Check back in a couple of weeks. I have been really busy.
The standard 4.18 says:
“For the GSM neighbour cell list the MS shall combine the BA (list) received in SI5/SI5bis/SI5ter with the BSIC list received in one or more instances of the MEASUREMENT INFORMATION message with the same BA_IND value as the BA (list). When the BA_IND is changed the MS shall rebuild the combined list (the BSIC list shall also be rebuilt).”
Can you share your insights on this… How this combining of BA list and BSIC list actually happen?
hi…
how about dedicated mode and idle mode that occur in 3G?
may you explain it?thanks..
Elva,
NC List is a concept used only in GSM/GPRS. It is not used when connectivity is on UMTS. When you say dedicated mode and idle mode in 3G, the concept of NC List is not applicable. 3G has its own way of collecting measurement from the mobile and reporting those back to the network.
Hi all,
i have some question, please help me!
1. GSM neighbor cell requirement standards?
2. How to check GSM neighbor list?
Thanks,
anhtt,
The two best places to read about GSM NC List are:
TS 44.018 (4.18 is old releases)
TS 44.060 (4.60 is old releases)
Checking GSM NC list is internal to the protocol. If you are developer, you can see them with your debugging tools. If you are a end-user, the list is not accessible.
Hi all,
I wanted to know that in case of GSM Neighbor cell definition.
all the same operator cell around that one particular cell of same operator.
please elaborate!!!!!