4-Assignment to Other Cell

Slide 1Assignment to Another Cell
call set-up, when a traffic channel is to be assigned.
The feature makes it possible to assign a traffic
channel in another (preferred) cell than the one
currently serving the connection (the serving cell at
call set-up is selected by the MS in idle mode).
Call set-up
GSM Radio Network Features
The feature Assignment to another cell operates at call set-up, when a traffic channel is to be assigned. The feature makes it possible to select a traffic channel in another cell than that currently serving the connection. The result, in terms of effects on the entire network, depends on the radio conditions in the target cell. The target cell is either ranked better than the serving cell in the locating evaluations, or worse.
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Assignment to a Better Cell
When locating starts at immediate assignment, it may find a better cell than the serving one. This is because the MS selected the serving cell only according to what was the strongest server (the C2 criterion). However locating has more information for example if the L criterion is used or if there are preferred lower layer cells. Also, if the MS is moving, the radio environment may have changed since the MS made its last idle mode measurement. Assignment to better cell will cater for this.
Achievements:
Unnecessary handovers will be prevented (let locating have its way at once).
The overall interference is decreased, based on the assumption that the locating algorithm decreases the interference.
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Assignment to a Worse Cell
If a mobile can not connect to the selected cell due to congestion or if locating detects an urgency condition during call set-up, a cell worse than the serving one is allowed to be suggested by the locating function. In both cases, this will lead to an increase in the probability of connecting the call successfully, thus a more flexible dimensioning of the network can be made. The channel utilization increases without actually increasing the GOS, due to truncation gains.
Assignment to worse cell is also called Directed Retry.
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Parameter Summary
The recommended values of ASSOC=ON and AW=ON (if moderate interference). The recommended value of IBHOASS=ON but it may well depend on the compatibilities between different makes of BTSs.
(At excessive TA urgency AW is always allowed independent of the parameter AW)
Note: IBHOASS, which is a BSC parameter, determines whether assignment (allocation of TCH) to another BSC in the SAME or in an OTHER MSC service area is allowed (1) or not allowed (0). Note that the MSC parameter HNDSDCCHTCH which determines whether assignment to another BSC in the same MSC service is allowed (1) or not (0) MUST BE SET (=1) to be able to use the parameter IBHOASS. If IBHOASS is not set assignment to another BSC is NOT permitted.
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Parameter Summary
The recommended value of AWOFFSET depends if there is risk for interference or not (check STS for not too many urgencies). AWOFFSET can be as large as 10 (dB) (or even 20) but if adjacent channel interference is likely (because adjacent carriers in the neighboring cell) then maybe a value of AWWOFFSET=7 is more appropriate. With a re-use of around 10 maybe AWOFFSET=5 is good and even lower if re-use is tighter. If f-hop is used AWOFFSET can probably be larger because of interference averaging.
CAND can be used to stop assignment to cells which are too far away.
(TA is not known by locating for non-cosited neighbors.)
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Parameter Summary
The ramping of locating filters means that SSRAMPSI must be so short so that n-cells are not discriminated too long, if assignment to better cell should have a chance to happen. The assignment takes about 2.8 seconds.
SSRAMPSI should probably be around 1-2.
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Value Ranges and Default Values
ASSOC is a parameter that turns the feature Assignment to Other Cell on or off. The parameter is set per BSC.
AW determines if an assignment to a worse cell is allowed from that cell. However, to allow assignment to a worse cell, the parameter ASSOC has to be set to ON as well. The parameter is set per cell.
IBHOASS determines whether an assignment to another BSC can be performed. The parameter is set per BSC.
HNDSDCCHTCH controls whether an inter-MSC/intra-MSC inter-BSC handover from a SDCCH to a TCH is allowed (= 1) or not allowed (= 0). This parameter is only defined for Ericsson MSCs. MAP version 2 is required as well as HNDBEFOREBANSW = 1 for inter-MSC assignments.
HOMAPVERSION determines the primary MAP version to be used when establishing outgoing inter-MSC handover dialogue towards non-anchor MSCs.
HNDBEFOREBANSW determines if inter-MSC handover and inter-MSC assignment before answer from B-subscriber are allowed (=1) or not allowed (=0).
AWOFFSET is a signal strength offset parameter. It defines the maximum range of the signalstrength corridor between a congested serving cell and its neighbour, in where the neighbour cells are eligible as candidates for Assignment to Worse Cell.
CAND is set per neighbour cell relation and specifies if the neighbour cell shall be treated as a possible candidate at assignment and at handover. The parameter can take the following values:
AWN, the cell is a possible candidate at assignment to a better or worse cell, but not at normal handover.
NHN, the cell is a possible candidate at assignment to better cell and at normal handover.
BOTH, the cell is a possible candidate at assignment to a better or worse cell and at normal handover.
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algorithm. The assignment is done to that cell.
•
•
If a mobile can not connect to a cell due to congestion
or if locating detects an urgency condition during call
set-up assignment to a worse cell is possible.
Achievements:
successfully.
•
Assignment to another cell ON or OFF. The
parameter is set per BSC.
•
is set per cell and determines if an assignment to
a worse cell is allowed from that cell. However, to
allow assignment to a worse cell, the parameter
ASSOC
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BSC can be performed during assignment
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is a signal strength offset parameter used to define the
range around the original cell border where an assignment to a worse
cell is allowed. It is set per neighbor relation.
•
CAND
is set per neighbor relation and specifies if the neighbor cell
shall be treated as a possible candidate at assignment and at
handover. The parameter can take the values:
*
AWN
, the cell is only a possible candidate at assignment to a
worse cell. Note: The cell is not a candidate for assignment
to better cell or handover.
*
NHN
, the cell is a possible handover candidate in case of an
assignment to a better cell and normal handover.
*
, the cell is always a possible candidate, at assignment as
well as at handover.
TINIT
is a timer used to inhibit locating for a time interval after
Immediate assignment, Assignment or Handover in order to stabilize
the ranking values. The timer is set per BSC.
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is used when there is an assignment failure due to
congestion or due to bad radio conditions as a result of the first
candidate list. A new list is sent after the timer has expired. The timer
is set per BSC.

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4-Assignment to Other Cell