 time is 0.5 ms, and the decoding time is a multiple of 1 ms. If we want to make the round trip time shorter, we need to define a shorter transmission time. The minimum transmission time in High Speed Packet Access (HSPA) is 2 ms, in LTE 1 ms, and in 5G 0.125 ms, which enables a 1-2 ms round trip time in 5G. A shorter frame size also requires different structures for the control channels and faster processing times in UEs and base stations. Figure 3.9 shows transmission times and the corresponding round trip times. The 5G subframe is 1 ms long and has 1, 2, 4, or more slots depending on the subcarrier spacing. The slot length equals 14 symbols. The typical scheduling interval is one slot. In addition it is possible to schedule data using a mini-slot with a length of typically 2, 4, or 7 symbols. The purpose of the mini-slot is to enable low latency communication for all subcarrier spacings. Mini-slot brings in the flexibility that allows a low latency service to use a very short transmission time while other services can use a longer transmission time. The mini-slot concept is illustrated in Figure 3.10 with a 1 ms slot and a 0.14 ms mini-slot of two symbols. 3.4.3 Self-Contained Subframe The self-contained subframe in 5G includes both downlink and uplink control parts. It enables low latency operation with acknowledgment/negative acknowledgment (ACK/NACK) feedback in the same TDD subframe. A single subframe contains everything related to the data transmission: downlink grant, downlink data, and uplink ACK. The modularity of the self-contained subframe is handy when introducing new services later in 5G networks. Since 5G with the self-contained subframe supports blank sub-frames, it enables forward compatibility with future services on the same carrier. The self-contained subframe is divided into the downlink control part, data trans- mission part, and uplink control part. The data part can be utilized for downlink or for uplink in the case of dynamic TDD operation. Since the control channels are at the 3 Technol