Sunday, April 29, 2012

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.

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