Double BA Lists in GSM
The Double BA Lists feature is defined per cell. This means that it is
possible to have double lists in one cell and a single list in another cell. A
single list corresponds to having the same list in idle mode and in active
mode.The Absolute Radio Frequency Channel Numbers (ARFCNs) to be measured on by
an MS in a cell shall be included in the BA list and they are defined by the
parameter MBCCHNO.
Only ARFCNs for the neighboring cells BCCH frequencies shall be defined
(serving cells BCCH frequency can also be included if separate idle mode and
active mode BA lists are used). With the parameter LISTTYPE
it is possible to determine whether the list is for idle mode or active mode,
or both.
Transfer of BA Lists to the Mobile Stations
An
MS in idle mode receives the idle mode BA list in System Information 2, 2bis
and 2ter messages that are transmitted on BCCH. The format and scheduling is
described in the 3GPP Technical Specification 44.018.
An
MS in active mode receives the active mode list in System Information 5, 5bis
and 5ter messages transmitted on the SACCH.
In
active mode the MS reports the signal strength and estimated quality of the
serving cell and the signal strength of the six strongest neighbours, for which
the BSIC is decoded. The measurement reports are sent on the SACCH every SACCH
period, The Measurement Reports contain
the downlink measurements, information about the measured frequencies and BSIC
for the reported cells and information about what list (old or new) that has
been used,
If
EMR is activated then the MS may send Enhanced Measurement Reports instead. An
Enhanced Measurement Report is a new measurement report containing additional
data, of which Bit Error Probability (BEP) and the number of correctly received
speech frames are the two most important ones.
The
active mode BA list should correspond to the defined neighbors. The BCCH
carrier of serving cell should not be in the active mode list. A short active
mode BA list gives better handover performance than a long list. If a long
active mode list is used there will be less samples taken from each cell, thus
resulting in decreased measurement accuracy. It could also be the case that the
MSs spends time decoding the BSIC of an irrelevant cell. Furthermore, if this
is not successful it will spend extra time trying to decode the BSIC of this
cell and waits longer time before it re-decodes the BSICs of the other cells.
Thus, this increases the risk that they are no longer valid. The active mode
list should, in the normal case, not exceed 15 frequencies.
The
idle mode BA list should normally correspond to all the BCCH carriers used.
Serving cell should be included in the idle mode BA list. When an MS is
switched on, the MS scans the latest received BA list. If the MS has moved, it
could be the case that the MS measures a very weak signal from one of the cells
in the old list, and camps on it. If this list is very short and does not
contain the cells which are the best ones in the current location of the MS,
the MS will continue to camp on the "wrong" cell. To set up a call
under these circumstances is not favorable. This situation can be avoided if
the idle mode list contains more frequencies. Many frequencies in the idle mode
list means that less samples are taken per frequency than what would have been
the case if the list was short. The decreased accuracy for the idle mode measurements
that this gives rise to is not considered a problem.
MBCCHNO is
the list of BCCH carriers, given as Absolute Radio Frequency Channel Numbers
(ARFCNs), of the neighboring cells (and possibly serving cell) on which the MS
shall measure. The parameter is set per cell and type of list (idle or active).
LISTTYPE is
used to specify to which list, IDLE or ACTIVE, the stated frequencies belong
when changes in the BA lists are made. If LISTTYPE is
omitted the BCCH carriers defined or changed by MBCCHNO
are valid in both lists.
Comments