gh frequency bands, the use of the mini-slot allows also avoids having the transmission "locked" to one direction for the duration of 14 symbols when 6.4 Basic Structures for 5G Frame Structure 1 ms Subframe 0.5 ms Slot 30 kHz PDCCH PDSCH Figure 6.10 5G downlink with PDCCH and PDSCH in 5G sub-frame structure. 1 ms Subframe 1 ms Slot 15 kHz Mini-slot PDCCH Mini-slot PDSCH Figure 6.11 5G. Mini-slot example. using hybrid beamforming. More details of the beamforming operation are explained later in this chapter. An example of the mini-slot structure is illustrated in Figure 6.11. There is a further special frame structure called the self-contained sub-frame. The idea with the self-contained sub-frame is to minimize the latency by reserving space for the feedback signaling in the sub-frame itself. As shown in Figure 6.12, there are a few empty symbols after the downlink data transmission has finalized and then space for a small amount of uplink control information (UCI) transmission. This would enable (if the UE is fast enough in the decoding) faster feedback to be provided than in a regular TDD setup, where one has to wait for the next free available uplink resource. The first-phase networks and devices are not foreseen to support the self-contained sub-frame, as Release-15-based TDD deployments assume that the TDD network is synchronized between operators for the uplink and downlink split. If other operators in the same band would not be using this feature, the uplink/downlink interference likely would not enable this, except in deployments which are isolated or otherwise have a large enough minimum coupling loss between the base stations of different operators. Also, the use case for the self-contained sub-frame is rather the Industrial Internet of Things (IoT) type of operation. See Chapter 15 on Release 16 developments. 1 ms Subframe 1 ms Slot 15 kHz Downlink PDSCH Guard Period Uplink PDCCH Figure 6.12 Self-contained sub-frame. 6 5G Physical Layer 5G Channel Structures and Beamforming Basics One of the key enable