Relay Selection for Two-way Relaying with Amplify-and-Forward Protocols (1111.7076v1)

Abstract: In this paper, we propose a relay selection amplify-and-forward (RS-AF) protocol in general bi-directional relay networks with two sources and $N$ relays. In the proposed scheme, the two sources first transmit to all the relays simultaneously, and then a single relay with a minimum sum symbol error rate (SER) will be selected to broadcast the received signals back to both sources. To facilitate the selection process, we propose a simple sub-optimal Min-Max criterion for relay selection, where a single relay which minimizes the maximum SER of two source nodes will be selected. Simulation results show that the proposed Min-Max selection has almost the same performance as the optimal selection with lower complexity. We also present a simple asymptotic SER expression and make comparison with the conventional all-participate amplify-and-forward (AP-AF) relaying scheme. The analytical results are verified through simulations. To improve the system performance, optimum power allocation (OPA) between the sources and the relay is determined based on the asymptotic SER. Simulation results indicate that the proposed RS-AF scheme with OPA yields considerable performance improvement over an equal power allocation (EPA) scheme, specially with large number of relay nodes.

• The paper proposes an RS-AF scheme using a Min-Max selection criterion to minimize the maximum SER between two source nodes.
• It demonstrates via simulations that RS-AF outperforms all-participate AF by effectively harnessing transmit power to achieve full spatial diversity.
• The study reveals that optimal power allocation, with half the power at the relay, further reduces SER and enhances system robustness.

Analysis of Relay Selection for Two-way Relaying with Amplify-and-Forward Protocols

The paper "Relay Selection for Two-way Relaying with Amplify-and-Forward Protocols" presents a rigorous examination of the relay selection amplify-and-forward (RS-AF) protocol in bi-directional relay networks. This is explored particularly for networks comprising two source nodes and a variable number of relay nodes denoted as $N$. The research focuses on optimizing the transmission process to facilitate efficient communication between the source nodes via relay nodes.

Key Contributions

The authors introduce an RS-AF scheme that significantly improves the system performance by leveraging an innovative approach to relay selection. The standout feature of this method is its Min-Max criterion which selects a relay that minimizes the maximum symbol error rate (SER) between the two source nodes. This sub-optimal method is proposed as a less complex alternative yet shows comparable performance to the optimal relay selection.

Numerical Findings

The research is supported by several simulation results, demonstrating that the Min-Max selection criterion achieves near-optimal performance, reducing complexity without sacrificing the system's performance. The simulations indicate a strong alignment between the proposed sub-optimal relay selection approach and the optimal selection mechanism.

1. Performance over All-Participate Amplify-and-Forward (AP-AF): The RS-AF approach consistently outperforms the traditional AP-AF methods. This is attributed to the fact that RS-AF optimally uses transmit power by deploying it at the best relay, while AP-AF splits the power among all available relays.
2. Symbol Error Rate (SER) Analysis: The derived asymptotic SER expression and its validation through simulation indicate that the RS-AF scheme achieves full spatial diversity, offering a diversity order of $N$. Further, the SER is significantly improved with optimal power allocation (OPA), enhancing system robustness particularly with larger numbers of relays.
3. Power Allocation Strategy: The paper provides a thorough analysis of power allocation between the source nodes and the selected relay, formulated in the context of minimizing SER. This power allocation illustrates that, under a fixed total transmission power, the optimal strategy involves allocating half of the power at the relay.

Implications and Future Developments

The implications of this research are impactful both theoretically and practically. The proposed RS-AF scheme and its sub-optimal selection criterion demonstrate how effective relay selection can bolster system performance in terms of SER. Practically, this insight can be expanded to improve existing relay communication systems, offering a direction towards reducing computational load while enhancing transmission reliability.

The optimal power allocation solutions also provide a pathway for further research, particularly in dynamic channel conditions or more complex relay setups, such as multi-user or multi-antenna systems. Additionally, future work may extend these approaches to various fading environments beyond the Rayleigh model, such as the Nakagami or Rician channels.

The paper makes substantial contributions to the domain of wireless relay networks by illustrating how advanced relay selection and power allocation strategies can be harmonized to achieve superior communication efficiency.