## 2 Advances within 3 GPP Standardization on D2D
### IoT and Wearables  
IoT devices with a very long expected battery lifetime and wearables with other cellular-connected devices in their proximity would especially benefit from short D2D links. Motivated by this, 3 GPP opened a release 15 study item \"Study on Further Enhancements to LTE Device to Device, UE to Network Relays for IoT and Wearables\" [42]. The primary objective of the study was to improve the power efficiency of the remote UEs (IoT devices and wearables) by allowing them to form a D2D connection with a UE who is willing to act as a relay [40]. Enhancements were planned to release 13 UE-to-network relaying to support end-to-end security and QoS as well as efficient path switching  
Figure 1: Architecture and logical interfaces for proximity services (ProSe); PC: Interface between ProSe components; S1: interface between base station and the core network; Uu: Air interface. between conventional and D2D air interfaces. In addition, the needed changes for sidelink were studied to provide a reliable D2D communication link for low cost and low power IoT devices. The study considered a diverse group of scenarios that could benefit from UE-to-network relaying. From the coverage point of view, the remote UE could be located within the cell, out of cell, or can be operating in the coverage-enhanced mode [40]. As cellular IoT devices mainly reach enhanced coverage by a high number (up to 2048) of repeated transmissions [43], the power efficiency gain of using short D2D links with minimal repetitions is obvious in this scenario. Relaying using the sidelink can be bi- or uni-directional, as shown in Figure 2. Bidirectional relaying is more straightforward to implement with minimal signaling from the eNB. However, bidirectional relaying over sidelink requires UL waveform reception capabilities for the remote UEs. This would mean implementing a UL receiver for low-cost IoT devices, which may not be feasible from the device cost point of view. Thus, many of the open issues in D2D relaying for IoT are related to the question, how to efficiently implement mandatory functionalities, such as discovery, for unidirectional relaying. As a result of the 3 GPP study, a relaying architecture was proposed. Relaying is done above the radio link control (RLC) layer, i.e., the RLC and lower layers are terminated at the D2D link and higher layers at the remote UE and the eNB [33]. Several solutions for paging and system information transfer for remote UEs as well as path switch and group handover enhancements were also proposed. These layer 2 studies mostly assumed the feasibility of bidirectional relaying; the impact of unidirectional relaying was not fully analyzed in the study item. For example, the discovery procedure for the unidirectional relaying case with remote UEs only capable of receiving downlink (DL) signals was still left open. Another aspect in the 3 GPP study was to study the required enhancements to sidelink physical layer operation. The target was to also enable the sidelink support for low-cost UEs with a limited bandwidth of one (narrowband IoT) or six (LTE-M) physical resource blocks (PRBs) and potentially with no sidelink reception capabilities [33]. Enhancements were proposed to the synchronization procedure such that the relay UE can act as a synchronization source for the remote UEs. Also, the needed enhancements for the support of unicast communications over the  
Figure 2: Device-to-device (D2D) relaying variants for cellular Internet of Things (IoT) devices; UE: user equipment. sidelink were identified and proposed for resource allocation, semi-persistent scheduling, power control, measurements and feedback for link adaptation. Based on the performance evaluation results presented in [33], especially the adaptive modulation and coding together with the adaptive number of sidelink transmissions provided a significant energy efficiency gain for the remote UEs. There are still several open issues regarding D2D and UE relaying for cellular IoT.