In the RX level based TCH allocation method the downlink RX level measured by the MS is used to determine whether the BTS is acceptable for TCH allocation. . TCH allocation for emergency calls is not restricted because of a too low RX level.
The minimum allowed RX level per call type is calculated as in the equation
DL_Rx_Lev(min)=C/N + DL noise level + NBL + 110.
If a downlink RX level measurement is not present, it is assumed that all call types on BCCH BTS are allowed and RX level is calculated by using the highest C/N value as in the equation
DL_Rx_Lev=C/N(max) + DL noise level + NBL + 110.