Posts Tagged ‘BA(GPRS)’

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.

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