ases the complexity of software decision making significantly in comparison with LTE, where event timing is fairly static. 13.3.2.2 Flexible Scheduling The flexibility of NR scheduling is realized by configuring the following: Time domain resource allocation Time gap from PDCCH reception to PDSCH/PUSCH Timing for HARQ-ACK transmission, and SO on In LTE, the corresponding configurations for the above assignments are static. The PDCCH is located at the beginning of a subframe, and the associated PDSCH/PUSCH has a fixed starting OFDM symbol until the end of the subframe. The HARQ-ACK back is then provided with a constant timing, for example, 4 ms later for Frequency Division Duplex (FDD). In NR, the arrival time of these tasks can occur at any OFDM symbol in a slot and may change in a per-slot basis. The flexibility in scheduling requires a UE to tolerate different levels of timing require- ments and the associated UE behaviors configured by the network. At the same time, the UE should distribute its computing resources and timing to various tasks and maintain its power consumption as low as possible. Moreover, planning for the resource such as the amounts of memory, hardware, and SO on, becomes difficult because the usage sce- narios to be supported by the device are unclear at the planning stage. Furthermore, the testing activities become involved because abundant test cases are needed to cover all target scenarios. 13.4 Multi-RAT Coexistence and Modem Architecture The deployment of a new mobile communication system generation can take several years, whether as a new greenfield deployment or as a complement to an existing, older, generation. The latter scenario requires a smooth migration from the older to the newer generation, which implies both graceful coexistence and continuity of service between the two systems. The definition of a new 5G radio technology, that is, NR, and of a new 5G System was made with full consideration of the widespread use of LTE sys- tems worldwide and of the various migration plans mo