r spacing. Subcarrier Minimum number Maximum number Minimum Maximum spacing of PRBs of PRBs bandwidth, MHz bandwidth MHz 17.28 34.56 69.12 397.44 3.5 NR Physical Layer Offset to carrier 120 kHz PRB#0 60 kHz PRB#0 PRB#1 30 kHz PRBt#0 PRB#1 PRB#2 PRB#3 15 kHz PRB#0 PRB#1 PRB#2 PRB#3 PRB#4 PRB#5 PRB#6 PRB#7 Carrier bandwidth Figure 3.5.2 PRB alignment for different numerologies. up to 14 OFDM symbols within a slot. Type A mapping typically has a relatively long trans- mission time interval, which helps to reduce the overhead from reference signals and the control channel as well as to increase coverage. Conventional slot-level-based scheduling, however, is not efficient for all deployment scenarios. For instance, for 5G NR operation in unlicensed spectrum (NR-U), it is necessary to start transmission as early as possible after Listen-Before-Talk. In the case of mmWave, high payload transmission can be real- ized within just a few OFDM symbols due to the use of large bandwidth sizes. Finally, in the case of low latency transmission required for time-critical data applications, it is beneficial to start the transmission at any OFDM symbol without constraints. To optimize the system performance for such deployment scenarios, 5G NR also supports mini-slot-based transmis- sion, denoted as Type B mapping, in addition to the slot-based scheduling. Mini-slot-based scheduling enables physical shared channel transmission to start at any OFDM symbol within a slot and to have flexible duration. To facilitate early decoding in mini-slot-based scheduling, the control channel and reference signals are located at the beginning of the transmission. The frame structure in NR supports both TDD and Frequency Division Duplexing (FDD) operations. However, in contrast to LTE where downlink or uplink assignment is performed at the subframe level, downlink or uplink assignment is performed with the finer granular- ity of one OFDM symbol in NR (3GPP TS 38.213). In particular, each OFDM symbol in a slot is classified as "downlink," "flexible,