LTE RF Link Budget
In any two-way radio system, the radio pathlosses and the equipment output powers and sensitivities must be taken into account for both directions. This is especially true in a mobile network (e.g. downlink: BS to UE and uplink: UE to BS), where there are different characteristics for the uplink and downlink paths.
These include, for example, the possibility of mast head amplifiers in the uplink path, the transmit power capability of the UE typically being less than that of the BS, and the sensitivity of the BS receiver typically better than that of the UE.
If the differences between the uplink and downlink are not considered, it is possible that the BS has a service area far greater than that which the UE is able to use due to the UE’s typically lower output power. Therefore, the maximum allowable pathloss in the uplink and downlink must be calculated to determine how well the links are balanced. A balanced system gain is where the maximum allowable pathloss in the uplink is equivalent to the maximum allowable pathloss in the downlink.
Some items will have an equal impact upon both the uplink and downlink. For instance, increasing the gain of the antenna (assuming it is being used for both transmission and reception) affects both the uplink and downlink, therefore maintaining system balance.
The RF link budget is typically used to determine whether the downlink or uplink is the limiting path. Thus, it is important to analyze the RF link budget in both directions.
The following sub-sections present the RF link budget as three components. The first component will be the power that is radiated out of the antennas into the surrounding environment, the second component corresponds to the level of the signal that is needed to be received, and the third component addresses the parameters that would be encountered between the transmitting antenna on one end to the receiving antenna at the other end.
The Tx EIRP value derived in above Table 1st and the Effective Rx Faded Sensitivity value derived in below Table 2nd are based on the composite resource elements (i.e. subcarrier).
These values could also be represented as a per occupied resource element.
The Rx Sensitivity and Tx EIRP can be expressed on either a per sub-carrier basis or per composite sub-carriers basis; but, to derive a valid system gain value, both parameters must share the same reference (i.e. either both are per sub-carrier or both are per composite sub-carriers).