How do I design the RF front end for a multi-band 5G smartphone?

How do I design the RF front end for a multi-band 5G smartphone covering multiple FR1 and FR2 bands? A modern 5G smartphone RF front end is one of the most complex RF systems ever integrated into a consumer device, supporting 10-15 frequency bands with carrier aggregation: (1) FR1 front end architecture: the RFFE (RF Front-End) contains: PA modules: 2-4 PA modules covering low-band (600-960 MHz), mid-band (1.7-2.7 GHz), and high-band (3.3-4.2 GHz). Each PA module integrates: GaAs HBT or CMOS PA, output matching network, power detector, and bias control. LNA modules: 1-2 modules with multiple LNA paths (one per band). GaAs pHEMT or SOI CMOS for NF < 1.5 dB. Filters: BAW (Bulk Acoustic Wave) and SAW (Surface Acoustic Wave) filters for band selection. BAW for bands > 1.5 GHz (higher Q, better power handling). SAW for bands < 1.5 GHz (lower cost). FBAR (Film Bulk Acoustic Resonator) for the tightest band spacing. Antenna switches: SOI CMOS SP10T or higher (10+ throws for 10+ bands). Antenna tuners: tunable capacitors or switches to match the antenna impedance across all bands. (2) FR2 front end: antenna-in-package (AiP) modules: each module contains a phased array antenna (4×4 or 8×2 patch array), SiGe BiCMOS or CMOS beamforming IC (with phase shifters, attenuators, PA, and LNA per element), and power management and control. A typical smartphone has 2-4 AiP modules located at different positions (top, bottom, sides) for body blockage mitigation. EIRP: 23-43 dBm (depending on the power class and number of elements). (3) Carrier aggregation: the RF front end must support simultaneous transmission and reception on multiple bands. Example: 2 FR1 uplink carriers + 4 FR1 downlink carriers simultaneously. This requires: multiple PA and LNA chains active at the same time, inter-band isolation of > 50 dB (to prevent desensitization), and careful frequency planning to avoid intermodulation products falling in active receive bands.