Due to the proliferation of mobile internet access, the cellular traffic is envisaged to experience a 1000⁃fold growth in the second decade of the 21st century. To meet such a huge traffic demand, the Fifth Generation (5G) network have to adopt new techniques to substantially increase spectral efficiency and reliability. At the base station side, available resources (power supply, equipment size, processing capability, etc.) are far more sufficient than that of the terminal side, which imposes a high challenge on the uplink transmission. The concept of cooperative communications opens a possibility of using multiple terminals to cooperatively achieve spatial diversity that is typically obtained by means of multiple antennas in the base station. The application of Device⁃to⁃Device (D2D) communications in the 3GPP LTE system further pushes the collaboration of terminals from the theory to the practice. The utilization of D2D⁃based cooperative relaying is promising in the era of 5G. In this paper, we comparatively study several cooperative multi⁃relay schemes, including the proposed opportunistic space⁃time coding, in the presence of imperfect channel state information. The numerical results reveal that the proposed scheme is the best cooperative solution until now from the perspective of multiplexing⁃diversity tradeoff.
cooperative communications; Device⁃to⁃Device (D2D); Distributed Space⁃Time Coding (DSTC); outdated chanel state information; opportunistic relaying