Rate performance of AF two-way relaying in low SNR region

In this letter, two common protocols in cooperative communications, namely one-way and two-way amplify-andforward (AF) half-duplex relaying, are investigated and compared in terms of rate. It is generally accepted that two-way AF half-duplex relaying is always better than its one-way counterpart in terms of rate. However, we found that depending on the definition of the total power constraint and the SNR, this is not generally true. We derived the SNR threshold, below which oneway relaying actually outperforms two-way relaying. Simulation results show that for power-limited applications operating at extremely low SNR, two-way relaying might not necessarily be a better choice over one-way relaying.