bile operators may have. To cater 13 5G Modem Design Challenges for the above points, five system architectural options addressing different deployment scenarios over various transitional phases were defined. From the modem's perspec- tives, the main challenges in supporting these architectural variants lie in the sharing, and eventually the integration, of LTE and NR functionalities. In this section, some key considerations are highlighted for balancing performance and cost-effectiveness when implementing both LTE and NR modems in an SoC. 13.4.1 Dual Connectivity and Modem Architecture Option 3, also known as EN-DC, of the five system architectures is a very popular option for mobile operators because it leverages existing LTE networks to provide full coverage mobility and the newly deployed NR network to provide super-fast data transmission. Not requiring the deployment of a new core network, this option can accelerate not only the availability of 5G services but also their adoption. However, from the UE view- point, simultaneous operation of two radio technologies is required, which brings new challenges to the UE design, especially the modem architecture. One option is for a UE to be equipped with completely separated yet coordinated LTE and NR hardware modules to handle data transmission/reception with LTE and NR base stations independently. However, the resulting hardware cost would be prohibitive (e.g. the die area). The other option is hardware sharing. Hardware sharing uses a resource pooling mechanism to allocate hardware/computing capabilities dynamically based on the need for LTE/NR at any given time. Configuration messages from the network allow the dynamic allocation to operate. The capability of a UE to report its hardware capability to the network on a per LTE/NR band combination basis was introduced into the 5G specifications. For example, a UE can be designed in a way such that the lower MIMO capability is declared for low frequency bands, due to antenna placement considerations. Hardware sharin