ransmissions, by introducing additional repetitions that facilitate higher reliability of reception. 16 LTE-Advanced Evolution Release 8 with 1 ms TTI Figure 16.20 Subslot TTI structure within LTE subframe. 14 symbols Release 15 with subslot TTI Short TTIs PDCCH 2 symbols = 0.14 ms 3 symbols 16.3.6 loT Optimization IoT refers to the interconnection and the autonomous exchange of data between devices which are machines or parts of machines; often involving sensors and actuators. The future connected world will have tens of billions of IoT devices. LTE evolution will enhance LTE's capabilities to serve IoT by improving coverage, device power consump- tion, cost, and connectivity. IoT optimization is discussed in detail in Chapter 14. 16.3.7 D2D Communications 3GPP has defined direct communication between two devices with the D2D feature in Release 12. Earlier releases supported only communication between the device and the base station. D2D functionality can be utilized for multiple use cases: for public safety, for vehicle-to-vehicle (V2V) communication, for social media, and for advertisements. D2D covers critical use cases including out-of-coverage communications, relaying for extended coverage and communications in case of network failure, for example, due to earthquakes. D2D communication is also known as Proximity Services (ProSe). The principle of D2D communication is shown in Figure 16.21. The traditional com- munication has uplink and downlink connections between the device and the base sta- tion. The new D2D communication is called the sidelink and is known as the PC5 inter- face in 3GPP. D2D uses mainly uplink resources, that is, uplink spectrum in the case of FDD and uplink subframe in the case of TDD. D2D uses half-duplex operation even if the cellular network is FDD. Network synchronization is assumed for D2D communication. D2D functionality added new logical channels: ((II)) Figure 16.21 Device-to-device communication. Uplink Downlink Sidelink/PC5 16.3 LTE-Advanced Pro Technologies Sidelink discovery