[Paper Review] Adaptive Mode Selection in Bidirectional Buffer-aided Relay Networks with Fixed Transmit Powers

Vahid Jamali, Nikola Zlatanov+2 more|arXiv (Cornell University)|Mar 15, 2013

Cooperative Communication and Network Coding7 references3 citations

TL;DR

This paper proposes an adaptive mode selection protocol for buffer-aided bidirectional relay networks with fixed transmit powers, dynamically choosing among six transmission modes (four point-to-point, multiple access, and broadcast) based on instantaneous channel state information to maximize sum rate. The optimal policy selects the mode with the highest instantaneous rate, and simulations show it outperforms existing protocols by leveraging all available transmission modes, including the multiple access mode, which previous works overlooked.

ABSTRACT

We consider a bidirectional network in which two users exchange information with the help of a buffer-aided relay. In such a network without direct link between user 1 and user 2, there exist six possible transmission modes, i.e., four point-to-point modes (user 1-to-relay, user 2-to-relay, relay-to-user 1, relay-to-user 2), a multiple access mode (both users to the relay), and a broadcast mode (the relay to both users). Because of the buffering capability at the relay, the transmissions in the network are not restricted to adhere to a predefined schedule, and therefore, all the transmission modes in the bidirectional relay network can be used adaptively based on the instantaneous channel state information (CSI) of the involved links. For the considered network, assuming fixed transmit powers for both the users and the relay, we derive the optimal transmission mode selection policy which maximizes the sum rate. The proposed policy selects one out of the six possible transmission modes in each time slot based on the instantaneous CSI. Simulation results confirm the effectiveness of the proposed protocol compared to existing protocols.

Motivation & Objective

• To address the suboptimal spectral efficiency in existing bidirectional relay protocols that use fixed transmission schedules and ignore instantaneous CSI.
• To design an optimal transmission mode selection policy that maximizes the sum rate in a three-node half-duplex bidirectional relay network with buffer-aided relaying.
• To demonstrate that including the multiple access mode in the selection policy improves performance over protocols that only consider point-to-point and broadcast modes.
• To show that the optimal policy selects from all six transmission modes (four point-to-point, multiple access, broadcast) based on real-time CSI, even when transmit powers are fixed.

Proposed method

• The system models a three-node bidirectional relay network with no direct user-to-user link, using a buffer-aided relay to store and forward data.
• Six transmission modes are defined: four point-to-point (U1→R, U2→R, R→U1, R→U2), one multiple access (U1,U2→R), and one broadcast (R→U1,U2).
• The optimal mode selection policy is derived using convex optimization and duality theory, maximizing the average sum rate under fixed transmit powers.
• The policy selects the mode with the highest instantaneous rate in each time slot based on real-time CSI, using a Lagrangian dual approach to solve the optimization problem.
• Threshold regions for the dual variables μ₁ and μ₂ are derived to determine the set of candidate modes, with probabilistic selection used when dual variables are at boundaries (μ₁=0,1 or μ₂=0,1).
• The solution ensures that the optimal sum rate is achieved by selecting from all six modes, including the multiple access mode, which is not used in prior works.

Experimental results

Research questions

• RQ1 Can adaptive mode selection based on instantaneous CSI improve the sum rate in a buffer-aided bidirectional relay network with fixed transmit powers?
• RQ2 Is the multiple access mode necessary for optimal performance, or is it sufficient to use only point-to-point and broadcast modes?
• RQ3 How does the optimal mode selection policy compare to existing protocols that use fixed transmission schedules?
• RQ4 What is the impact of buffer-aided relaying on the achievable sum rate when all six transmission modes are available?

Key findings

• The proposed adaptive mode selection policy achieves a higher sum rate than existing protocols by dynamically selecting from all six transmission modes based on instantaneous CSI.
• The multiple access mode is selected with non-zero probability in the optimal policy, demonstrating its importance for performance gain.
• The optimal policy is derived using convex optimization and duality, resulting in a threshold-based selection rule for the transmission mode in each time slot.
• When dual variables μ₁ and μ₂ are at the boundaries (0 or 1), probabilistic selection between candidate modes is required to achieve the optimal sum rate.
• The proposed protocol outperforms existing schemes that restrict mode selection to only three modes (two point-to-point and broadcast), especially in fading environments.
• The protocol is optimal under fixed transmit powers and achieves the maximum possible sum rate by exploiting all available degrees of freedom in the network.