nlicensed bands, for example, to start transmission directly after a successful listen-before-talk procedure without waiting for the slot boundary. Mini-slots can consist of two, four, or seven OFDM symbols, where the first symbol includes (uplink or downlink) control information (see Fig. 3.42). For low-latency applications, the HARQ protocol can be configured either on a slot or a mini-slot basis. For the regular frame structure used by delay-tolerant applications, slot bundling as in LTE is also possible. Mini-slots may also be used for fast flexible scheduling of services (pre- emption of URLLC over eMBB). However, mini-slots are likely to be supported by few UE categories. A major difference between LTE and NR in terms of scheduling granularity is that LTE transmission time interval is fixed at 1 ms whereas NR transmission interval is a slot or a fraction of slot whose length is a function of the subcarrier spacing. For given subcarrier spacing parameter 11, the slots are numbered as ascending order within a subframe. There are N symbol slot conse- cutive OFDM symbols in a slot where N slot symbol depends on the CP. The start of slot nslot in a subframe is aligned in time with the start of OFDM symbol NslotNsymbol in the same subframe [12]. In the TDD mode, the OFDM symbols in a slot can be classified as downlink, flexible, or uplink. Table 3.13 shows possible slot formats. In a downlink frame slot, the UE assumes that downlink transmissions only occur in downlink or flexible symbols, whereas in an uplink frame slot, the UE only transmits in uplink or flexible symbols. In LTE TDD, there are a number of predefined patterns for uplink/downlink OFDM symbol allocation in a radio frame, while NR does not define any preset uplink/downlink patterns (see Fig. 3.45). A slot format indication (SFI) parameter informs the UE whether an OFDM symbol is downlink, uplink or flexible. The SFI can indicate link direction over one or many slots when configured through RRC. The SFI carries an index to Table 3.13 (pre-con- figur