eration depends on the environment and is yet proposed to be studied in 3GPP, with the study scheduled to be completed during 2015. The numbers given in [10] suggest on the average 15% improved throughput in the conditions where the situation changes dynamically and thus radio aggregation would be faster to react to changes than multipoint TCP approach would be. There is an interface needed for non-co-located cases, indicated in Figure 3.9 as well. In the UE side, the WLAN functionality needs to be also integrated tightly with the LTE functionality, most probably in the same baseband chip supporting both LTE and WLAN for ensuring low enough delay and sufficient communications between the LTE and WLAN transceivers. The LTE radio-level aggregation is more dynamic from the reaction to the environment, thus somewhat better performance could be expected, the HTTP level or Multipoint level aggregation decisions are done higher up in the network, as shown in Figure 3.10. With the radio-level interworking, LTE needs to obtain more dynamic info from the Wi-Fi conditions, otherwise the performance is not any better than multipoint TCP. The peak data rate is the same in all the alternatives. 3.6 Device-to-Device Communication with LTE In Release 12, the first wave of Device-to-Device (D2D) communication is being completed. The primary use case for Release 12 is the public safety operation. The existing narrowband public safety solutions, such as TETRA, are well suited for public safety voice communications but have a clear shortcomings due to limited data rate capabilities as a narrowband system. Also LTE Small Cell Optimization Application in the network Aggregation Aggregation at HTTP level at TCP level Wi-Fi Wi-Fi Wi-Fi Wi-Fi Aggregation at LTE level (Wi-Fi as Application in the UE the slave) Figure 3.10 Aggregation with Wi-Fi on different levels in some cases, the inter-operability is rather limited between different vendors, for example, considering the variety of systems (or vendor specific variants) in the market in 