TSG-RAN Working Group 2 (Radio layer 2 and Radio layer 3) 	TSGR2#11(00)0491
Turin, Italy, February 28 to March 3, 2000	

Agenda Item:	7.1

Source: 	Ericsson	

Title: 		Mapping between GSM RXLEV and CPICH Ec/No

Document for:	Discussion, and proposal

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1	Introduction
At the RAN ad hoc meeting on RRM issues, 9-11 February, in Turin, Italy, it was concluded that a mapping function should be used for translating and comparing GSM RXLEV with CPICH Ec/No for cell-reselection. It is possible that a mapping function “Ax + B” as suggested by Nokia in RPA-000055 may be sufficient but it is proposed that further investigations is done to verify that. 
This paper discusses how a mapping function may be investigated/created and some of the assumptions and investigations/simulations needed. 
2	Discussion
To be able to map between GSM RSSI and CPICH Ec/No a common quantity that is possible to estimate and defines the “goodness” of the cells regardless the Radio Access technology must be defined.
In this paper the “Probability for block errors when receiving BCCH” is proposed to be the common quantity used for representing the "goodness" of a cell.
In order for the mapping to work well the following key assumptions must be true.
1.	Any errors in the estimated BCCH BLER probability due to different UE speeds and channel conditions need to be correlated for the estimated GSM BCCH BLER and the estimated UTRAN BCCH BLER. I.e. the relative “goodness” of cells will be maintained regardless UE speed and channel conditions.
2.	The translation between CPICH Ec/No and UTRAN BCCH Ec/No must be possible to approximate with a simple offset regardless the UE speed and channel conditions.
3.	The translation between GSM BCCH carrier RSSI and GSM BCCH C/I must be consistent within a cell.
If 1 is not true, cell reselection between GSM and UTRAN may be falsely triggered due to insignificant differences in channel conditions or UE speeds and not on the actual relative quality of the cells.
If 2 is not true, it may be problematic to compensate for differences in power setting between BCCH mapped on P-CCPCH and the CPICH.
If 3 is not true or at least that a maximum interference level within a GSM cell can be assumed there may be a problem to get knowledge that a GSM cell is good enough.
Note that the BCCH carrier is frequency planned in a GSM system so the interference will not be smoothed out as the case is for the traffic channels when frequency hopping is used. The RSSI levels that provide good enough quality may differ 30 dB between noise limited cells and interference-limited GSM cells. 
Below is schematic graphical view on the mapping presented using the BCCH BLER estimate as representing the “goodness” of a cell for relative comparison between UTRAN cells and a noise limited GSM cells. The numbers and the curve-form used in the graphs should only be viewed as examples.

As seen from the graphical example:
For channel condition A CPICH Ec/No 19 dB is mapped to -105 dBm GSM carrier BCCH RSSI and CPICH Ec/No -15 dB is mapped to -100 dBm assuming Imax=0. 
The mapping need to be adjusted for channel condition B. Note that the adjustments may only be performed per cell and for all UE’s in that cell if the adjustment parameters are broadcasted. 
The UTRAN cell needs typically an offset to compensate the difference in the output power between BCCH and CPICH.
The GSM cell needs also an offset to compensate for the anticipated maximum interference level Imax in the cell.
The BLER curves are quite steep (within 5 dB it goes from very good to very bad cell) and the mapping consequently becomes very sensitive to errors introduced by different measurement accuracy and BCCH BLER performance of different UE's. 
The mapping is probably best defined by a default lookup table and the other differences (power offset, Interference) is compensated for by using cell specific offsets when cells of different Radio Access technologies are compared.
The factor B in Nokias proposed formula is essentially this offset.
The need for factor A to be able to adjust the anticipated look up table based on the concept of comparing BCCH BLER should be investigated. It is expected that it is the offset that in practice will control the cell reselection. Furthermore the amount of bits that is available to be used in the GSM BCCH are very few.
It is also proposed that when BLER essentially becomes better than e.g.10E-6 (out of range for the look up table) the "goodness" of the cell is increased as the RSSI or Ec/No is increasing dB by dB.
It is also proposed that there should be a minimum level when the cell is not considered for cell reselection.
3	Proposal
It is proposed that a look up table based mapping based on equal BCCH BLER is further investigated.