[Paper Review] Resource Allocation for Mixed RF and Hybrid RF/FSO Relaying

Vahid Jamali, Diomidis S. Michalopoulos+2 more|arXiv (Cornell University)|Jun 16, 2015

Optical Wireless Communication Technologies14 references3 citations

TL;DR

This paper proposes an optimal resource allocation strategy for a mixed RF and hybrid RF/FSO backhaul network, where users communicate via RF to a relay, which forwards data via a hybrid RF/FSO link to a destination. By adaptively allocating RF bandwidth between the user-relay and relay-destination links based on CSI, the scheme maximizes spectral efficiency and ensures reliable connectivity, outperforming systems using only RF or FSO backhauls.

ABSTRACT

In this paper, we consider a mixed RF and hybrid RF/FSO system where several mobile users transmit their data over an RF link to a relay node (e.g. a small cell base station) and the relay forwards the information to a destination (e.g. a macro cell base station) over a hybrid RF/FSO backhaul link. The relay and the destination employ multiple antennas for transmission and reception over the RF links while each mobile user has a single antenna. The RF links are full-duplex with respect to the FSO link and half-duplex with respect to each other, i.e., either the user-relay RF link or the relay-destination RF link is active. For this communication setup, we derive the optimal resource allocation policy for sharing the RF bandwidth resource between the RF links. Our numerical results show the effectiveness of the proposed communication architecture and resource allocation policy, and their superiority compared to existing schemes which employ only one type of backhaul link.

Motivation & Objective

• To address the reliability and capacity limitations of pure FSO backhaul in mobile networks due to atmospheric fading and blockages.
• To design a hybrid RF/FSO backhaul system that leverages the diversity of RF and FSO link impairments to improve connectivity.
• To develop an optimal resource allocation policy that dynamically shares RF bandwidth between the user-relay and relay-destination RF links based on real-time CSI.
• To ensure reliable data forwarding via a buffer at the relay, enabling half-duplex operation on RF links while maintaining throughput.

Proposed method

• Formulates a dual-hop mixed RF and hybrid RF/FSO relay system with multiple-antenna relays and destinations, and single-antenna users.
• Models the user-relay and relay-destination links as full-duplex RF and half-duplex hybrid RF/FSO links, respectively.
• Derives a dual decomposition-based optimization framework to jointly maximize spectral efficiency by allocating RF bandwidth between the two RF links.
• Uses a gradient-based algorithm to solve the dual problem and find the optimal bandwidth split, with convergence guaranteed under small step sizes.
• Applies a queueing-theoretic upper bound to model the effective data rate at the relay, accounting for buffer constraints and variable channel conditions.
• Introduces a hybrid transmission protocol that adaptively switches between RF uplink and downlink modes based on CSI, ensuring continuous data forwarding.

Experimental results

Research questions

• RQ1 How can RF bandwidth be optimally shared between the user-relay and relay-destination RF links in a mixed RF/hybrid RF/FSO relay system?
• RQ2 What is the impact of channel state information (CSI) on the performance of hybrid RF/FSO backhaul systems with multiple-antenna relays and destinations?
• RQ3 How does the proposed resource allocation policy improve spectral efficiency and reliability compared to systems using only RF or FSO backhauls?
• RQ4 What is the optimal trade-off between RF and FSO link utilization under varying atmospheric and propagation conditions?
• RQ5 How does buffer-aided half-duplex operation at the relay affect end-to-end throughput and system stability?

Key findings

• The proposed resource allocation policy achieves higher spectral efficiency than systems using only RF or FSO backhauls, especially under deep fading conditions.
• When the expected RF capacity exceeds the FSO capacity, the optimal bandwidth allocation sets λ* = 1, prioritizing the RF link for the relay-destination hop.
• When the expected RF capacity is lower than the FSO capacity, the optimal λ* lies in (0,1), balancing both RF links to maximize throughput.
• The system achieves a throughput upper bound τ^upp = N × min{C̄₁^RF, C̄₂^RF + C̄^FSO}, where C̄₁^RF increases with λ and C̄₂^RF decreases with λ.
• Numerical results confirm that the hybrid architecture maintains connectivity and outperforms pure FSO or RF backhaul systems under severe atmospheric fading.
• The gradient-based dual optimization converges to the optimal solution, ensuring practical implementation with guaranteed performance.