9 Cell Selection and Reselection.doc

8/11/2019 9 Cell Selection and Reselection.doc

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Cell Selection and Reselection

Note: This tutorial is intended for people that have a good basic understanding of GSM fundamentals. It may be necessary to review Introduction toGSM and Network rchitecture before reading this tutorial.

There are many factors involved in maintaining the radio link !"m interface# between the Mobile Station !MS#and the $ase Transceiver Station!$TS#. s the MS moves throughout the network the signal strength of the $TS will increase and decrease and the MS will have to continuously

monitor the signal strengths of nearby towers and update which $TS%s it camps on. This page covers all of the parameters that the MS and network willuse in order to ensure the the MS chooses the strongest tower to monitor and other network considerations.

Signal Strength

The first and arguably most important consideration in radio link management is signal strength. In GSM !and most other &' communications# the

standard measure of signal strength is d$m !decibelsin milliwatts#. The term received signal strength indicator!&SSI# is often used but in GSM theterm received-signal level !&()*+# is preferred. The distinction is that the term &SSI was generally used on analog networks and &()*+ is used on

digital networks. ,n this website &SSI will be used for general reference to signal strength and &()*+ for the actual value that is passed over thenetwork.

RXLEV

&()*+ is a number from - to / that corresponds to a d$m value range. - represents the weakest signal and / the strongest.


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&SSI below 011- d$m are generally considered unreadable in GSM. &SSI in the area of 02- d$m are rarely seen and would indicate that the MS is

right ne3t to the $TS. The main factor that affects &SSI is distance from the tower. 4owever5 other factors such as terrain5 elevation5 and large ob6ectssuch as buildings can dampen signal strength.

RXQUAL

lthough a strong &SSI is desirable5 it does not guarantee a 7uality signal. &(8") is a value that represents the 7uality of the received signal. TheMS determines the $it *rror &ate !$* of the signal and reports it back to the network. The $*& is simply a percentage of the number of bits it

receives that did not pass error checking. The bits may have been garbled along the &' path or lost due to fading or interference. The higher the $*&the lower the signal 7uality. &(8") is given as a number from - to 9 and represents a percentage range of $*&.


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Cell Selection and Reselection

ell selection refers to the initialregistration that a MS will make with a network. This normally only occurs when the phone powers up or when theMS roams from one network to another.

ell reselection refers to the process of a MS choosing a new cell to monitor once it has already registered and is camped on a cell. It is important todistinguish that selection and reselection are done by the MS itself and not governed by the network. The network would only be responsible for this

function when the MS is in a Traffic hannel !T4#. ;hen the MS reselects a new cell it will not inform the network that it has done so unless that

new cell is in a new)ocation rea !)#.

There are many parameters involved in selection and reselection of a new cell. The MS must ensure it is getting the best signal and the network mustensure that the MS does not cause unneeded strain on the network by switching cells when unnecessary or undesired.

C1

1 is the path0loss parameter that is used to determine the strongest cell for selection. The MS will calculate a 1 for each tower it can see and select

the cell tower with the highest 1. The 1 uses the following parameters for calculation:

The formula for calculating 1 is given as:1 < !# 0 Ma3!$5-#where:

< !&()*+ 0 &)M#

$ < MS Transmit =ower Ma3 4 0Ma3 &' ,utput of MS

t first this may seem complicated but if we e3amine the various parameters and how they affect the 1 score then it becomes more clear.

0 This value is merely a d$ value for the difference between what &SSI is re7uired to select that cell and what signal strength the MS sees the tower

at. If the &)M is 011-d$ and the MS sees the tower at 0>-d$ then the value of is ?-d$. The higher the value of the higher the 1 and the more

attractive this tower will be to the MS.
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$ 0 @ust because a MS can receive a tower%s signal does not mean that the MS has enough power to reach that tower. The tower tells the MS what

ma3imum power level that the MS may use to transmit to that tower. If the phone is capable of transmitting at this power than there is no problem.4owever5 what if the phone can not transmit at that power levelA The signal from the MS may not have enough power to reach the tower. ny lack in

transmitting power of the MS must be taken into account when calculating 1. $ is essentially the value of this difference. )et%s say a cell towerre7uires the MS to be able to transmit at a /-d$ power level but this MS is only capable of transmitting at ?d$. In this case the value of $ would be

Bd$. This value issubtractedfrom the value of which has the result of lowering the value of 1. If the MS is capable of transmitting at the re7uiredpower or higher then $ will be Cero and no ad6ustments to 1 will be made.

In summary5 the two main factors in determining 1 are the strength of the received signal and the transmission power the MS is capable of. 1 aloneis only used for cellselection. ;hen a MS is already camped on a cell and it wants to move to another cell it will reselectit. ell reselection uses a

different criteria C2.

C2

? is the parameter used for cell reselection. ,nce a MS is camped on a cell it will continuously monitor the strength of neighbor cells. *very $4sends out aBCCH Allocation (BA) List. This is a list of neighbor cells !&'Ns# that the MS must monitor while camped on a particular cell. The MSwill monitor these &'Ns for signal strength and only reselect a cell that is on this list. The MS will calculate a ? value for each cell on the $ list.

The cell tower with the highest ? wins and the MS will move to that cell and camp on it. Deep in mind the ? is calculated by the MS and the MSdecides which cell tower to camp on. The cell that the MS camps on is known as the serving cell. s long as the losing cell and the gaining cell are

both in the same)ocation reathe MS will not notify the network that is is selecting a new cell. The MS only needs to notify the network if it is

reselecting the cell that is in a new location area in which case it will do a location update.
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The C2 is calculated using the following ara!eters"

The formula for calculating ? is:

? < 1 E &, 0 !TempF,ffset 4#

4 < 1 if the MS has been monitoring a particular cell for less than thepenalty time.

4 < - if the MS has been monitoring the particular cell for longer than the penalty time.

4 < - if the particular cell is the serving cell !the one the MS is currently camped on#.

)et%s look at an e3ample to see how the temporary offset works. The following chart shows two e3ample cell towers and values for 1 and ?

parameters. The time progresses as the MS moves away from cell and towards cell $. 'or sake of simplicity5 we are assuming that the MS can

transmit at the ma3 power allowed and that neither cell is using &,.


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- seconds 0 The MS is camped on cell . The MS calculates the ? value as /H. Since the &()*+ for cell $ is not above the &)M the 1 !and ?#are below -. MS will not select a cell with a 1 below - and it will not reselect a cell with a ? below -.

1- seconds 0 The &()*+ for cell $ meets the minimum threshold !&)M#. The MS starts a timer as soon as it puts it on its strongest neighbor list.The penalty time for cell $ is B- seconds5 so for the first B- seconds that cell $ is on the strongest neighbor list it will apply the temporary offset to the

? value. fter including the offset5 the ? for cell $ is 0?- d$m.

?- seconds 0 The ? for cell continues to drop as the ? for cell $ continues to rise. ;ith a ? of ?25 cell is still the most attractive.

/- seconds 0 ell drops to a ? of ?1 and cell $ has a ? of 02.

B- seconds 0 ell drops to a ? of 1H. ell $ rises to a ? of /. Notice here that if it were not for the temporary offset5 the ? for cell $ would be at?/. t this point the MS would normally reselect cell $. 4owever5 due to the temporary offset5 cell is still the most attractive.

2- seconds 0 t this point the penalty time for cell $ has e3pired and the temporary offset is no longer applied. The ? for cell $ raises from / to ?9.

The ? for cell $ wins over the ? for cell and the MS reselects cell $.

The temporary offset would be used if the network wanted to discourage mobile stations from reselecting a cell as soon as the MS saw it. This iscommonly found in pico0cells. This forces a MS to be in the area of the cell for a certain period before reselecting it. It prevents mobile stations that

6ust happen to be passing by from reselecting the cell. In order to reselect the cell5 the MS must be in the area for a certain period of time or be close

enough that the &()*+ overcomes the negative offset value.

Cell Reselection #ffset $CR#%0 &, is a value from - to /. *ach step represents a ? d$m step !- to 1? d$m#. This value is added to 1. higher&, value will make the cell tower more attractive to the MS. The higher the &,5 the more attractive the cell will be. The network might assign a

&, value to a cell if the network wanted to encourage mobile stations to utiliCe that cell. The network might want to do this in order to reduce the

load on other cells during times of high traffic volume or to force MS%s to a certain band.


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&eigh'or List ( The neighbor list is a list of the strongest cells that the MS can see. The &()*+ for these cells is transmitted in a measurement

report from the MS to the $TS on the S4 whenever the MS has been allocated an S4 or a T4. The $S and MS use these measurementsto determine if the MS needs to move to a different cell. ;henever a cell is in an active S4 or T4 the network will always manage the handoff.

The MS will only move from one cell to another by itself when it is in idle mode.

Cell Reselection )*steresis $CR)%

;hen a MS reselects a new cell it does not need to notify the network unless that new cell is in a different )ocation rea. ;hen a MS moves into a

new location area it must do alocation updatewhich generates signal messaging between the $TS5 $S5 MS5 +)&5 and 4)&. If a MS is locatedalong the border of two location areas then it will see cells in both location areas. MS along the borderline might reselect a cell in one location area

and then a few minutes later reselect a cell in the other location area and continuously bounce back and forth between location areas generating toomuch signaling overhead and putting strain on the network.

In order to mitigate this problem the &4 is used. It is a value that is similar to the temporary offset value of ?. &4 is applied to ? when the

desired cell is in a different location area. This results in making the cell in this different location area less desirable to the MS. The MS must move

close enough to the new location area to overcome the offset thus ensuring the MS is truly close enough to the new location area to warrant a locationupdate. ,nce the MS reselects the cell in the new location area it will perform a location update. The MS will then apply the &4 value to all cells it

sees in the old location area which will make them less attractive to the MS and ensure the MS does not continually bounce back and forth between two

location areas.

Cell +ar Access $C+A%ell $ar ccess is a single bit !- or 1# value sent down on the $4. If $ is set to 1 then MS%s are not allowed to select

that cell. If the value is set to - then MS%s may select it. $ would be used on umbrella cells in order to prevent MS%s from selecting it. The umbrellacell would be reserved for when the network needs to manage high levels of traffic. This gives the network total control of access to the umbrella cell.

Cell +ar Qualifier $C+Q%

This value is similar to the $ but it applies to reselection. cell that has $8 set to 1 does not allow MS%s to reselect it. $8 set to - allows normalaccess to that cell.
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9 Cell Selection and Reselection.doc

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