Due to the proliferation of mobile internet access, the cellu-lar 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 effi-ciency and reliability. At the base station side, available re-sources (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 possibili-ty of using multiple terminals to cooperatively achieve spa-tial 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 fur-ther pushes the collaboration of terminals from the theory to the practice. The utilization of D2D-based cooperative relay-ing is promising in the era of 5G. In this paper, we compara-tively study several cooperative multi-relay schemes, includ-ing the proposed opportunistic space-time coding, in the presence of imperfect channel state information. The numeri-cal results reveal that the proposed scheme is the best coop-erative solution until now from the perspective of multiplex-ing-diversity tradeoff.
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