By fully exploiting the spatial resources, unmanned aerial vehicles (UAVs) are expected to serve as an efficient complementary to terrestrial wireless communication system to provide enhanced coverage and reliable connectivity to ground users. With the growing deployment of units such as small cell base stations (BSs), however, the incurred severe interference may hinder the potential benefits of the integration of UAVs. In this paper, we first discuss the intrinsic features and potential benefits of UAVs and introduce the architecture of multi-layer heterogeneous wireless network (MHetNet), in which traditional wireless network is assisted by UAVs. Then, an explicit discussion on the factors that limit the performance of MHetNet is presented, including the UAV topology, UAV density, and spectrum sharing of UAV and terrestrial networks. We use simulation results to investigate the performance of MHetNet in terms of spatial throughput (ST). It is shown that, together with the densities of UAV and terrestrial networks, the altitude of UAV is a limiting factor that should be optimized to improve the ST of MHetNet.

Distributed cooperative networks use the cooperation among nodes to fulfill network resource sharing. However, designing an efficient Media Access Control (MAC) protocol is a key issue for the distributed cooperative network. Based on the principle of MAC-layer cooperation, this paper discusses problems and challenges for MAC protocol design in the distributed cooperative network. Through the analysis of typical cooperative MAC protocols and their performance, this paper concludes that only a reasonable MAC protocol design with the features and demands of distributed networks taken into account can make cooperative communication technology a better application in the distributed network.

Emerging as a new communication paradigm, cooperative communication is attracting attention. Relay selection is the key technology for cooperative communication, and determines whether the performance gain of cooperation can be achieved. In this paper, we first give the performance evaluation metrics for relay selection algorithms, and then discuss the corresponding categories. Finally, some classical relay selection algorithms are analyzed. Results show that the relay node should be seriously selected and configured according to system requirements in order to optimize the performance of cooperative communication.