# Evolution and Testing of NB-IoT Solutions
## 4 NB-IoT Evolution
### D2D Communication Support in NB-IoT (Release 15)  
Release 15 studies the viability of using NB-IoT devices as remote UEs in D2D communications. Specifically, D2D communications can be used to establish short-range communications between devices which will significantly reduce energy consumption and increase the coverage in the cell edge. IoT devices with limited bandwidth represent a large category of devices which has as a main restriction the power consumption. Usually, these devices are in the proximity of more power capable devices as smartphones. A way of reducing power consumption would be to relay the transmissions through the smartphone. Thus, the smartphone would offer Proximity Services (ProSe) [16], which will enable wearables, and IoT devices in general, in order to gain global LTE connectivity using the smartphone as a relay. The relay nodes support different working modes: UL relay (DL is established directly to the network) or DL/UL relay; in both cases, the consumption is improved. It is clear that the communication through other devices with less battery constraints, such as a smartphone, can reduce power consumption significantly due to the more reduced range and also increase the coverage.  
ProSe (Proximity Services) [16] is the technology introduced in Release 12 to enable direct Device-to-Device (D2D) communication. In R12, the ProSe standard was focused on public safety and critical applications to provide communication support when cellular networks are not available. In that first release, D2D communications were only supported when the UE was out-of the network coverage. R13 also includes in/partial network coverage D2D communications and ProSe UE-Network relay functionality. The relay UE uses the direct sidelink interface (PC5) to communicate with remote UEs while connecting to the network using the standard LTE Uu interface.  
Moreover, the discussions within the 3 GPP are going a step further. 3 GPP Technical Specification Radio Access Network Working Group 1 (TSG RAN WG1) is also studying the possibility of using NB-IoT devices as relays [17; 18; 19; 20]. The use of such bandwith limited UE-to-Network relay brings a significant improvement in coverage (20 dB) in challenging scenarios such as underground or cell edge deployments. In this context, the enhancements introduced by the use of NB-IoT devices as relays are twofold: (1) to meet the extreme coverage enhancements levels; and (2) to reduce power consumption of remote UE in enhanced coverage.  
However, to support D2D communication in NB-IoT devices, with only 1 PRB (Physical Resource Block) available, different issues have to be tackled: current synchronization mechanism requires at least six PRBs, discovery message uses two PRBs, subcarrier spacing and number of tones (NB-IoT support different subcarrier spacings, some of then not supported in standard LTE), resource pool  
Figure 5: Narrowband positioning reference signal patterns.  
structure, which is the set of subframes and resources blocks assigned to the D2D communication, and synchronization accuracy (there are also separated efforts to improve the synchronization of NB-IoT devices). To deal with these issues, new formats are required for D2D channels, new discovery design options have been identified in [21], and several D2D communication enhancements for UEs with six PRBs can be applied to UEs with one PRB bandwidth limitations as proposed in the 3 GPP TSG RAN WG1 meeting held in August 2017.  
Merging these two enabling technologies, NB-IoT and D2D communication, will allow for reducing the consumption of wearables and further extending the coverage in extreme IoT deployment scenarios and will increase the value of Proximity Services (ProSe).