Why We Need The UL/DL Spectrum Split in 5G NR?

We can match the #LTE 1800MHz downlink coverage using #5G NR 3.5Ghz Spectrum. But How? 

In general, the coverage area of the highest frequency (in this case 3.5GHz) will have less coverage area than the lowest frequency (1800MHz). 
Utilizing advanced antenna techniques such as massive MIMO and beamforming can help to solve this issue. 

However, we have another issue in the Uplink direction. Why? 
In the uplink direction, MIMO and beamforming are unpractical due to the limited real estate in the device, therefore if the uplink was to use the same frequencies as downlink, the size of the cell would extremely decrease to the maximum in the uplink.

To overcome this problem it has been proposed to utilize lower band spectrum such as the 1800 MHz spectrum for the uplink data (i.e. supplementary uplink). 

The main difference between using carrier aggregation for uplink and using uplink/downlink split is that carrier aggregation attempts to enhance the bandwidth of the uplink while uplink/downlink split is utilized to expand the coverage of the UE.

While carrier aggregation can increase the capacity by utilizing multiple component carriers (e.g., 20Mhz band in 1.8GHz and 20Mhz band in 800Mhz), uplink/downlink split makes the UE use different component carrier for uplink  from that of the downlink. That is 20MHz band in 1.8GHz for uplink and 20MHz band in 3.5GHz for downlink.