ed in future releases of 5G NR. Section 13.3.2 addresses system flexibility, namely, flexible slot format indication and flexible scheduling, alongside the challenges they mean for the design of the modem, following a similar analysis as for low latency. 13.3.1 Low Latency Aspects Versus Modem Processing Capability To improve performance and enable new use cases, low latency is one of the key per- formance indicators for the design of the 5G system. In a cellular system, the latency of a packet transmission results from its transit through the various system entities (each with its own processing delay) and interfaces on the data path and varies as a function of the system's topology on that path. This part focuses on the latency caused by the physical layer at the access network, which includes the following: (1) Queueing delay due to scheduling (2) Time to a transmit instance (e.g. the start of a slot) (3) Duration of the packet transmission (4) Processing delays in the base station (gNB) and in the device (5) Latency due to scheduling request, retransmission, device feedback, and SO on. To achieve low latency, 5G NR features various enhancements compared to LTE, as summarized in Table 13.4. These include shorter slot duration, mini-slot trans- mission, multiple PDCCH monitoring occasions per slot, shorter PDSCH/PUSCH processing time, preemption indication, front-loaded DMRS, and Orthogonal Fre- quency Domain Multiplexing (OFDM) symbol-based Physical Uplink Control Channel (PUCCH). In the following, the details of these enhancements and the associated impact on the modem processing capability, if any, are presented. 13 5G Modem Design Challenges Table 13.4 Overview of NR physical layer techniques for low latency. Enhancement Approach Description Shorter slot duration Reduction of (2), (3) Compared with LTE, larger SCS is supported in NR. Durations of an OFDM symbol and of a slot are reduced. Mini-slot Reduction of (2), (3) Transmission over a fraction of a slot is allowed. transmission Multiple PDCCH Reduction 