without having to first transmit a scheduling request, and wait for an uplink grant from the eNodeB. Release 15 added a few more advanced features to reduce latency. First, for the reg- ular 1 ms TTI, shorter processing times were specified, reducing the delay between downlink data transmission and the corresponding Hybrid Automatic Repeat Request (HARQ)-acknowledgment, as well as the time between an uplink grant transmission by the eNodeB and the transmission of the corresponding uplink data by the UE, down to 3 ms compared to 4 ms, which has been assumed since the start of LTE. Second, Release 15 adds support for a shorter TTI (sTTI). The supported sTTI dura- tions are seven symbols for both Frequency Division Duplex (FDD) and TDD, and two or three symbols for FDD. Figure 16.20 shows an example with one-symbol Physical Downlink Control Channel (PDCCH) in the downlink. A similar structure applies to the uplink as well. There can be up to six subslot allocations in a single 1 ms period. Shorter TTIs resulted in major redesign of large part of LTE, especially the physical layer. For the downlink, a new control channel - sPDCCH - was defined, along with new downlink control information formats. Modifications to HARQ-operation and reference signals were also necessary. In the uplink, the control channels and reference signals were also redesigned to match sTTIs having fewer symbols. An LTE cell can operate with both regular 1 ms TTI and sTTI simultaneously. sTTI-related channels are multiplexed with ordinary LTE channels and signals in the frequency domain, and the eNodeB needs to take their presence into account in scheduling the downlink or uplink resources. For a given UE, dynamic switching between 1 ms TTI and sTTI is also supported, allowing for adaptation to instantaneous traffic needs. Note that sTTI operation is not supported on unlicensed bands. Another latency enhancement in Release 15 is the introduction of enhanced sup- port to URLLC. The standardized enhancements mainly improve the reliability of data t