[Paper Review] Adaptive OFDM Index Modulation for Two-Hop Relay-Assisted Networks

Shuping Dang, Justin P. Coon+1 more|arXiv (Cornell University)|Jun 20, 2017

Advanced Wireless Communication Technologies39 references3 citations

TL;DR

This paper proposes an adaptive OFDM index modulation scheme for two-hop relay networks that dynamically selects subcarrier activation patterns based on channel state information to combat slow frequency-selective fading. By enabling mapping scheme adaptation at either the source or relay, the scheme enhances system reliability and spectral efficiency, achieving diversity gains and improved error performance compared to conventional OFDM IM.

ABSTRACT

In this paper, we propose an adaptive orthogonal frequency-division multiplexing (OFDM) index modulation (IM) scheme for two-hop relay networks. In contrast to the traditional OFDM IM scheme with a deterministic and fixed mapping scheme, in this proposed adaptive OFDM IM scheme, the mapping schemes between a bit stream and indices of active subcarriers for the first and second hops are adaptively selected by a certain criterion. As a result, the active subcarriers for the same bit stream in the first and second hops can be varied in order to combat slow frequency-selective fading. In this way, the system reliability can be enhanced. Additionally, considering the fact that a relay device is normally a simple node, which may not always be able to perform mapping scheme selection due to limited processing capability, we also propose an alternative adaptive methodology in which the mapping scheme selection is only performed at the source and the relay will simply utilize the selected mapping scheme without changing it. The analyses of average outage probability, network capacity and symbol error rate (SER) are given in closed form for decode-and-forward (DF) relaying networks and are substantiated by numerical results generated by Monte Carlo simulations.

Motivation & Objective

• To address the limitations of fixed mapping in traditional OFDM index modulation under slow frequency-selective fading.
• To enhance system reliability and spectral efficiency in two-hop relay networks by enabling dynamic subcarrier activation based on channel state information.
• To propose two practical mapping scheme selection methodologies—one centralized at the source and one decentralized at the relay—based on relay processing capability.
• To analyze key performance metrics including outage probability, network capacity, and symbol error rate in decode-and-forward relay systems.

Proposed method

• The scheme adaptively selects which subcarriers to activate based on instantaneous CSI at the source and relay, using a variable number of active subcarriers per transmission block.
• A dual-mode transmission protocol is introduced to resolve the zero-active subcarrier dilemma, where all subcarriers are inactive, by switching to amplitude and phase modulation (APM) when necessary.
• Two mapping scheme selection strategies are proposed: centralized (relay adapts) and decentralized (source adapts, relay forwards unchanged), tailored to relay processing constraints.
• Closed-form expressions are derived for average outage probability, network capacity, and symbol error rate (SER), with asymptotic analysis at high SNR to reveal diversity gain.
• The method leverages combinatorial probability to model subcarrier selection and worst-case channel gain among active subcarriers, ensuring accurate outage probability evaluation.
• Monte Carlo simulations validate the analytical results, demonstrating performance gains over conventional OFDM IM and FPSK schemes.

Experimental results

Research questions

• RQ1 How does adaptive subcarrier mapping based on CSI improve reliability in two-hop OFDM relay networks under frequency-selective fading?
• RQ2 What is the impact of relay processing capability on the design of mapping scheme selection strategies in adaptive OFDM IM?
• RQ3 How do the proposed closed-form expressions for outage probability, capacity, and SER compare to conventional OFDM IM in terms of performance and diversity gain?
• RQ4 What is the role of the dual-mode transmission protocol in resolving the zero-active subcarrier dilemma?
• RQ5 To what extent does adaptive mapping selection enhance spectral efficiency and error performance compared to fixed mapping schemes?

Key findings

• The proposed adaptive OFDM IM scheme achieves a diversity gain by selecting subcarriers based on CSI, improving reliability in slow fading environments.
• Closed-form expressions for average outage probability and network capacity are derived, with asymptotic analysis confirming the diversity order at high SNR.
• The average symbol error rate (SER) is approximated with high accuracy in closed form, especially at high SNR, validating the analytical model.
• Numerical results show that the adaptive scheme outperforms conventional OFDM IM and FPSK in terms of error performance and reliability.
• The centralized mapping scheme selection (where relay adapts) provides better error performance than the decentralized scheme, due to an extra degree of freedom in subcarrier selection.
• The dual-mode transmission protocol effectively resolves the zero-active subcarrier dilemma, ensuring reliable communication even when no subcarriers are active.