Buffer-Aided Relaying with Adaptive Link Selection (1202.5349v2)

Abstract: In this paper, we consider a simple network consisting of a source, a half-duplex decode-and-forward relay, and a destination. We propose a new relaying protocol employing adaptive link selection, i.e., in any given time slot, based on the channel state information of the source-relay and the relay-destination link a decision is made whether the source or the relay transmits. In order to avoid data loss at the relay, adaptive link selection requires the relay to be equipped with a buffer such that data can be queued until the relay-destination link is selected for transmission. We study both delay constrained and delay unconstrained transmission. For the delay unconstrained case, we characterize the optimal link selection policy, derive the corresponding throughput, and develop an optimal power allocation scheme. For the delay constrained case, we propose to starve the buffer of the relay by choosing the decision threshold of the link selection policy smaller than the optimal one and derive a corresponding upper bound on the average delay. Furthermore, we propose a modified link selection protocol which avoids buffer overflow by limiting the queue size. Our analytical and numerical results show that buffer-aided relaying with adaptive link selection achieves significant throughput gains compared to conventional relaying protocols with and without buffers where the relay employs a fixed schedule for reception and transmission.

• The paper presents a novel adaptive link selection protocol that uses instantaneous channel state information to determine whether the source or relay should transmit.
• It develops a comprehensive analytical framework validated by numerical experiments, demonstrating throughput improvements of up to 95% over fixed scheduling methods.
• The paper further proposes delay-constrained protocols and optimized power allocation strategies to balance high throughput with minimal transmission delays.

Overview of Buffer-Aided Relaying with Adaptive Link Selection

The paper proposes a novel communication protocol in a network setting comprising a source, a half-duplex decode-and-forward (DF) relay with buffering capabilities, and a destination node. The central theme is adaptive link selection, a mechanism by which the decision of whether the source or the relay should transmit data in a given time slot is made based on the channel state information (CSI) for the source-relay and relay-destination links. This adaptive approach contrasts with conventional relay protocols that adhere to a fixed transmission schedule, thereby creating opportunities for significant enhancements in throughput.

Key Contributions and Results

1. Transmission Approaches: The paper addresses two types of transmission scenarios: delay unconstrained and delay constrained. It derives optimal link selection protocols and power allocation strategies for both cases. For the delay unconstrained scenario, the research finds that optimal decisions depend solely on the instantaneous CSI of the current time slot, which simplifies implementation.
2. Analytical Framework: The paper provides a rigorous analytical treatment of the throughput performance of the proposed buffer-aided approach. It characterizes the optimal selection policy that ensures the relay buffer operates at the edge of non-absorption, which is critical for maximizing throughput. This condition means that the system is poised close to the boundary between non-absorbing (infinite capacity buffer) and absorbing (finite capacity buffer) states.
3. Delay Constrained Protocols: For delay-sensitive applications, two adaptive protocols are proposed: starving the buffer by adjusting decision thresholds and directly limiting the queue size. Both methods aim to constrain the relay-induced delay while retaining throughput advantages over static relaying schemes.
4. Numerical Experiments: The paper’s analytical predictions are supplemented by numerical confirmations using Rayleigh fading channel models. The experimental results illustrate substantial throughput improvements over conventional protocols, especially when delay considerations are relaxed. Specifically, the paper demonstrates throughput gains of nearly 95% over traditional methods once appropriate power allocation is included.
5. Power Allocation: The paper also explores the intricacies of power allocation to further optimize system performance. It determines that the joint optimization of link selection and power allocation maximizes throughput significantly beyond what each strategy achieves independently.

Implications and Future Directions

The implications of this work are manifold. This buffer-aided relaying strategy with adaptive link selection could notably enhance communication efficiency in next-generation wireless networks. By allowing for dynamic flexibility based on real-time channel conditions, these techniques promise substantial improvements in network performance metrics such as throughput and delay, which are critical to the scalability of modern communication systems.

Future research could explore extensions and applicability in multi-hop or more complex network configurations. Another avenue is the consideration of imperfect CSI, which more realistically mimics real-world scenarios. Addressing these extensions will further broaden the protocol's utility and pave the way for robust applications in wireless network infrastructure, potentially influencing standards and practices in cellular and cooperative communication systems.