The 4G signal from the local base station is captured using an external logarithmic Yagi antenna mounted high up on a pole. The external antenna is pointed directly towards the base station and needs to have 'line of sight' which means that there are no major obstacles such as large buildings, hills and trees in the way. 'Line of sight' does not mean that we have to actually see the base station, it's more of a theoretical term and really just means 'no major obstacles'. 4G signals generally have good penetrating ability but do not like going around big objects.
The signal then travels through a low loss coaxial cable and is routed into the Rx (791 to 821 MHz for my network) port on the duplex band pass filter (BPF) on the left hand side of the diagram above.
Following the red arrows in the diagram, the signal is routed into a low noise amplifier (LNA) which can be configured using the Arduino in three possible ways: 'Off', 'One LNA' and 'Both LNAs'. Selecting both LNAs can provide 37.5 dB of gain with a noise figure of just 0.55 dB.
Next in the signal path is the variable gain amplifier (VGA) which is controlled by the Arduino using the SPI bus. This component actually contains two amps cascaded together giving a total possibility of four cascaded amps in the whole Rx part of the circuit. The VGA is capable of adding another 32 dB of gain to the signal and can handle up to 1/2 watt of power with a noise figure of 1.5 dB.
Since the circuit can pick up some unwanted interference, the signal is now routed into a second identical BPF which then connects to a plate antenna located inside the building. The BPF filters are 'duplexed' which allows the same antenna to be used for Rx and Tx simultaneously.
The strong signal transmitted from the cell phone is typically about 1/4 watt when far away from the base station and weaker when close to the station. The circuit has a VGA for transmitting the cell phone's signal back to the base station but this is not currently used as transmitting noisy or very strong signals to the base station can damage the network. Instead, the duplexer is used to monitor the strength of the cell phone's signal via a 'Received Signal Strength Indicator' (RSSI) chip which can be used to check that network compatible phones are present. Compatibility is determined by the actual BPFs selected as they have different frequency bands for each network.