## 2 Maximum Power Extrapolation Technique for 5G  
The goal of the Maximum Power Extrapolation procedure is to estimate the maximum field level (V/m) that can be obtained at the measurement point. This supposes an unrealistic condition in which a single user saturates all the resources of the communication system. As indicated in the introduction, this quantity is used as a reference for estimating the exposure to electromagnetic fields under realistic conditions using an appropriate scale factor.  
Loosely speaking, the procedure requires estimating a reference quantity that is a cell-specific signal always \"on air\" and is transmitted at maximum power. Instruments usually give the power level as it arrives at the instrument's input connector. From this value and the knowledge of the Antenna Factor [35] and the attenuation of the cable connecting the antenna to the receiver, it is possible to obtain the value of the field level.  
Finally, this value is multiplied by proper factors in order to obtain the maximum field level from the measured field level.  
In 5G, the maximum EM field level can be estimated as  
\[E_{5G}^{max}=E_{SSB}\sqrt{N_{SC}}\sqrt{F_{TDC}}\sqrt{F_{beam}} \tag{1}\]  
where \(N_{SC}\) is the number of subcarriers in the entire frequency channel, \(F_{TDC}\) is the duty cycle when TDD multiplexing is used, and \(E_{SSB}\) is the field level of a Resource Elements of the reference signal, which in 5G is an SSB-related quantity. The reference value can also be obtained by the direct measurement of the SSB using scalar analyzers [22]. SSBs are periodically transmitted using broadcast beams also in the absence of user data traffic and consequently are are always \"on air\".  
However, most radio resources are used for user traffic data, which are transmitted using traffic beams (see Figure 1). Since traffic beams have higher gain compared to broadcast beams, the REs of the user traffic data have higher power compared to the ones used in SSBs. \(F_{beam}\) is the ratio between the power per RE of the user traffic data and of the SSBs. This quantity is also the ratio between the traffic beam gain and the traffic beam gain in the measurement position.  
Indeed, the high flexibility of antennas used in 5G represents a huge challenge for \(F_{beam}\) estimation. This problem has been successfully addressed in currently deployed 5G systems.  
A first possible solution is to perform measurements over a period of time, observing traffic data by waterfall reconstruction [22]. In the case of users requiring connections in the same direction of the measurement points, it is possible to measure the field radiated by traffic beams, and hence the \(F_{beam}\) factor.  
A different solution, proposed in [24], requires a UE able to force the traffic toward the measurement position. This allows measuring both the broadcast beam level and the  
Figure 1: 5G uses two different kinds of beamsL the broadcast beams, which have lower directivity and are used to transmit the SSBs, and traffic beams, which have higher directivity and are steered toward the UE to send UE-specific information, such as traffic data [34].  
traffic beam level, obtaining the value of \(F_{beam}\). A video explaining this method is available in [36].  
It should be emphasized that forcing traffic to the measurement point requires actively interacting with the gNB. This also allows access to the rich 5G UE-specific signaling structure (as PDSCH-DMRS, NZ CSI-RS, PDSCH, and CSI-RSRP), opening up further interesting scenarios that require neither a cell reference signal nor \(F_{beam}\) estimation [26].