So much of what is talked about surrounding 5G connectivity has been focused on the download side of the network. While it is true that there are ever-evolving improvements to pump data from the cell tower to the devices faster such as carrier aggregation, higher order modulation and 4x4 MIMO, little attention has been paid to improving the uplink channel [from device to network]. Within the 3GPP Release 15 [which marked the official launch of 5G New Radio (NR)], a key design requirement of the new wireless standard calls for the implementation of MIMO on both network and devices. For example, 5G smartphones using the globally common TDD band N78 (3.5GHz), will be required to have 4x4 MIMO connectivity and fast antenna switches to perform SRS (Sounding Reference Signal) built within the 5G RFFE. The reliance of wider bandwidths of higher frequencies in 5G NR has forced the design decision to implement download enhancements [as RF propogation becomes more challenged as radio wavelengths decrease]. This inherent physical limitation of path loss dictates the use of massive MIMO antennas on the network infrastructure side as well as 4x4 MIMO on the device. In this article, we will discuss how this new 5G RFFE design challenge means for the RF components ecosystem; specifically, the additional use of PAMiFs (power amplifier module with integrated filters).
Source: Real World Performance of 5G; A Technical Paper prepared for SCTE•ISBE by Dave Morley, Director, 5G and Regulatory Shaw Communications Inc., Calgary, Alberta Canada
Figure 1 – coverage differences in Sub 6GHz 5G operating at 3.5 GHz and the use of SUL to enhance uplink coverage
 4x4 MIMO was an option for 4G LTE