Cell Reselect Penalty Time PT

Value range: 0~31, the corresponding time is 20~620s. 31 is to change the function direction of CRO on C2.

Unit: None

Content: Cell Reselect Penalty Time.

Recommendation: 0

If the communication in a cell is affected due to very heavy traffic or some other reasons, this cell should be the last where MS works in (there is a repulse for this cell). In this case, PT can be set as 31, which causes TO invalid. C2 value is the difference of C1 and CRO, therefore, C2 value of this cell is decreased manually. MS will reselect this cell with little possibility. Besides, the network operator can set CRO according to the repulse degree to this cell. The higher the repulse degree, the larger CRO.

For the cell with very low traffic, MS should prefer to work in this cell. In this case, CRO is recommended to be between 0-20dB. It can be set according to the preference degree to this cell. The higher the preference degree and the larger CRO. Generally TO is recommended to be the same or a little more than CRO. PT is mainly used to prevent MS’s too frequent cell reselections. Generally it is recommended to be 0 (20s) or 1 (40s).

For the cell with medium traffic, generally CRO is recommended to be 0 and PT be 31 as a result of C2=C1, that is, no manual modification on the cell.

Setting of PT can effectively prevent the fast moving MS from accessing the micro-cell. This parameter can be set according to the size of micro-cell. And it is recommended to be 20s for the ordinary micro-cells. When PT is set as 31, it is used to change the direction of action to CRO.

 

Cell Reselect Temporary Offset TO

Value range: 0~7 , the corresponding value is 0~60dB, 7 corresponds to “Infinite”

Unit:  None

Content: Cell Reselect Temporary Offset. It is a parameter in C2 criterion to give a temporary modification within PT time.

Recommendation: 0

 

TO indicates the temporary modification on C2. Temporary means that it functions for C2 only within a duration which is determined by PT parameter. Setting of this parameter only affects MS of GSM Phase II.

 

 

Cell Reselect Offset CRO

Value range: 0~63, the corresponding level value: 0~126dB, stepped every 2dB.

Unit: None

Content: Cell Reselect Offset. It is a parameter in C2 calculation to give a manual modification in MS cell reselection.

Recommendation: 0

 

After cell selection, MS will reselect another better cell in idle mode. It is C2 parameter that determines cell reselection. The principle for MS reselection is: select the cell with the maximum C2 as the serving cell. C2 is determined by the following factors:

  C2=C1+CRO-TO*H(PT-T)(PT <31)

  C2=C1-CRO                   (PT = 31)

         H(x)=0    if   PT-T<0；

         H(x)=1     if   PT-T>0

As shown above, C1 indicates the quality of radio channel. The larger C1 is, the better the channel is. C2 value is based on C1, through CRO, C2 of various cells can be adjusted. Thus C2 value can be calculated according to CRO, TO and PT in order to prefer selecting the cell in reselection process. That is, in dual-band network,several parameters that influence C2 value can be set to make C2 value of GSM1800 larger than that of GSM900. Therefore, even though the signal strength of GSM1800 cell is weaker than that of GSM900 cell, MS can still reside in GSM1800 by the aid of  these parameters. Besides CRO, there are another two parameters influencing C2:TEMPORARY_OFFSET(TO) and PENALTY_TIME(PT).

CRO is a manual modification on C2. Reasonable setting of this parameter can reduce handover times and realize assignment to a better cell. Usually it is not set as larger than 25dB. Generally the cells with the same priority in the network have basically the same CRO. Setting of this parameter only affects MS of GSM Phase II.

 

Cell Bar Qualify CBQ

Value range:   Yes(1) , No(0)

Unit: None

Content: Cell Bar Qualify. CBQ only affects the selection of cells but is not related to cell reselection. It works with CBA to define the access priorities of cells.

Recommendation:  No.

For the area overlapped by cells, the operator often wants MS to preferably select certain cell during cell selection according to the cell capacity, traffic and cell functions, i.e. setting the cell priority. This function can be implemented by setting parameter “cell bar quality”. It works with parameters “cell bar access” to determine the priority of the cell.

CBQ   CBA     Cell selection priority  Cell reselection priority 

 No  Yes          Normal            Normal 

 No  No          Disabled           Disabled 

 Yes  Yes             Low             Normal 

 Yes  No             Low             Normal 

Usually the priorities of all cells should be set as “Normal”. But in some special cases such as micro-cell and dual-band network, the operator may expect MS to preferably enter the cell of a certain type. In this case, the network operator can set the priority of this type of cell as “Normal” while setting the other cells as “Low”. MS will select the cell with lower priority only when there is no appropriate cell with the priority as “Normal During the network optimization by means of cell priority, it is necessary to note that CBQ only influences the cell selection. Therefore, in order to achieve the target, C2 (cell reselection parameter) must be taken into consideration.


 Application of CBQ


 

Each circle in the diagram indicates a cell. For some causes, the traffic in cell A and that of cell B are obviously higher than those of the adjacent cells. To make the traffic of the entire area distributed on average, set the priorities of cell A and cell B as “Low” and those of other cells as “Normal”. In this way, the services in the shadow areas in the diagram will be shared by the adjacent cells. It must be pointed out that this setting will reduce the actual coverage areas of cell  A and cell B, which is different from decreasing the transmitting powers of cell A and cell B.

 

 

RXLEV_ACCESS_MIN

Value range: 0~63

Unit:  Level

Content: It means the minimum received signal level required for MS to access a cell.

Recommendation: 8

 

To prevent MS from accessing the system when the receiving signal level is very low that makes the satisfying communication quality unavailable and wastes the radio resources of the network, GSM specifications require that when MS needs to register in the network, its receiving level must be greater than a threshold, i.e. the minimum receiving level of MS permitted to access; otherwise MS will fail to access.

For some cells with high traffic, this parameter can be increased appropriately to lower the values of C1 and C2 of this cell. Accordingly the effective coverage range of the cell will be reduced. But the value of RXLEV_ACCESS_MIN should not be too large, otherwise coverage hole  (with regard to idle mode MS) will be created at the edge of the cell. When this method is used to balance the traffic, the value of RXLEV_ACCESS_MIN is recommended to be no more than 20.

Except for the areas with densely distributed BTS and good coverage, generally it is not recommended to use RXLEV_ACCESS_MIN to adjust the traffic of the cell. For isolated BTS or BTS with poor coverage, this value can be increased properly; otherwise the call drop rate will increase and QoS will be affected.

 

MS_TXPWR_MAX_CCH

Value range: 0~31

Unit:  level

Content: This parameter determines the max. allowed output power of the MS when it begins to access a cell and has not yet received power command.

Recommendation: 5(900MHz), 0(1800MHz)

During MS’s communication with BTS, its transmitting power is controlled by the network via power command. This command is transmit in SACCH (There are two head bytes. One is power control byte, the other is time advance). MS must get the power control head from the downlink SACCH, and use its stipulated transmitting power as the output power. If power level of the MS cannot output that power, the transmitting power will use the nearest value.

Since SACCH is associated with signaling, it must be used after combining with other channels, such as SDCCH, TCH. Therefore, the network’s control of MS power begins after MS receives SACCH.While the power used by MS before it receives SACCH (power used when RACH is sent) is determined by the maximum power level of control channel.

List is the corresponding dBm values related to the 32 maximum transmitting power control levels for GSM900:

{39, 39, 39, 37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13, 11, 9, 7, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5}

For GSM1800:

{30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 10, 8, 6, 4, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 36, 34, 32}.

This parameter will affect cell selection and cell reselection.

C1 = RLA_C - RXLEV_ACCESS_MIN- MAX((MS_TXPWR_MAX_CCH- P),  0)

   RLA_C: mean receiving level of MS

   RXLEV_ACCESS_MIN: minimum receiving level of MS permitted to access

   P: maximum physical transmitting power level of MS.

Cell Reselection Process

Cell reselection for cells in same location area

If the C2 value of the target cell is higher than that of the serving cell and last for longer than 5 seconds, a cell reselection process will be performed and the MS tunes to the new cell.

Cell reselection for cells in different location areas

If the C2 value of the target cell is higher than that of the serving cell by at least the value of CRH and last for longer than 5 seconds, a location updating process and the cell reselection process will be performed.

Cell reselections caused by C2 have a time interval of 15 seconds

If current serving cell is prohibited, or down link fails, or C1 is less than 0 continuously for 5s, cell reselection will also be triggered.

MS starts a cell reselection if the access times exceed the  MAX retrans . 

 

 

C2=C1+CRO-TO*H(PT-T)

When PT is not equal to 31:

C2=C1-CRO

When PT is equal to 31.

Wherein:

1. Function H(x)=0, when x<0; H(x)=1, when x>0.

2. T is a timer with initial value 0. When a certain cell becomes of one of the six neighbor cells with highest signal level, T corresponding to this cell begins to count with accuracy of one TDMA frame (about 4.62ms). When it is out of the six strongest neighbor cells, the corresponding timer is reset.

3. CRO is used for manually correct the cell reselection parameter C2.

4. The function of TO is to reduce the value of C2 from the time when timer begins to count to the time when the value of timer reaches the stipulated PENALTY_TIME.

5. PT is the time TO acts on C2. But PT=31 is reserved as the symbol to change the CRO’s effect on C2.