7. Conclusions
The proliferation of D2D communications in cellular networks is expected to be beneficial from a variety of perspectives. However, it will shift the current cellular communication paradigm to a more flexible and dynamic state, raising new technical challenges, such as the device discovery problem. We have studied the main categories of device discovery protocols in the literature and examined discovery design directions posed by 3GPP standardization process. Focusing on the main LTE discovery scenario, where a frequencytime block is dedicated for discovery purposes, various radio access approaches have been discussed and evaluated, while key performance aspects and useful remarks on the optimal theoretical performance have been provided. Random access approaches seem to have adequate performance, since they exploit the diversity on the radio resource utilization. However, more sophisticated approaches are needed in scenarios where the number of devices attempting discovery exceeds the number of available radio resources. Although the centralized resource allocation approach can optimize the performance even in such scenarios, further work is needed on designing distributed resource allocation schemes, since the involvement of the eNB in the resource allocation procedure requires devices in connected mode. In this view, the design of distributed proximity validation schemes, which exploit the proximity validation principles described in this paper, is an appealing approach. Additionally, the plethora of available discovery solutions already used for WLANs provides a fertile basis towards designing access schemes for LTE device discovery, especially for the less studied out-of-coverage scenario.
