Availability of direct path in half‐duplex‐based cooperative relay networks

In this paper, we validate the availability of direct path in half‐duplex‐based cooperative relay networks from a practical point of view. Cooperative relaying is a low‐complexity technique, which schedules orthogonal transmissions through the divided time slots. By doing so, transmission impairments due to multi‐path fading and path loss are mitigated by obtaining a diversity gain. In conventional approaches, most researchers have focused on a role of relay and assumed that the received signal‐to‐noise ratio in source‐to‐destination link is doubled when source transmits the same signal twice during the two transmission phases. However, in practical wireless environments, a wireless channel is not static but varies with time. Thus, although the source retransmits the same signal during the second transmission phase instead of forwarding by a relay, the (time) diversity gain may be obtained. As a result, the performance of relaying‐aided cooperative communication is not always better than that of the repeated transmission (RT), but the RT scheme may be a better option than a cooperative relaying scheme. To this end, we first show that the RT scheme is comparable to conventional cooperative relaying schemes. We then propose a selection decode‐and‐forward (DF) relaying scheme, which combines the DF relaying and RT schemes. The proposed selection DF relaying scheme has better outage performance than comparable relaying schemes in time‐varying channels. Lastly, all the theoretical results are validated through numerical evaluations and Monte Carlo simulations.