[Paper Review] Spectral Efficiency Optimization for an Interfering Cognitive Radio with Adaptive Modulation and Coding

Mehrdad Taki, Farshad Lahouti|ArXiv.org|Dec 7, 2009

Cognitive Radio Networks and Spectrum Sensing3 citations

TL;DR

This paper proposes a spectral efficiency optimization framework for a cognitive radio operating in a fading interference channel, where the cognitive user adapts modulation and coding based on feedback of the signal-to-noise-plus-interference ratio to maximize its own spectral efficiency while guaranteeing minimum performance for the primary user. The scheme operates without coordination overhead on the primary link and achieves significant gains over interweave-based approaches through joint power control and adaptive modulation and coding selection under BER and power constraints.

ABSTRACT

In this paper, we consider a primary and a cognitive user transmitting over a wireless fading interference channel. The primary user transmits with a constant power and utilizes an adaptive modulation and coding (AMC) scheme satisfying a bit error rate requirement. We propose a link adaptation scheme to maximize the average spectral efficiency of the cognitive radio, while a minimum required spectral efficiency for the primary user is provisioned. The resulting problem is constrained to also satisfy a bit error rate requirement and a power constraint for the cognitive link. The AMC mode selection and power control at the cognitive transmitter is optimized based on the modified signal to noise plus interference ratio feedback of both links. The problem is then cast as a nonlinear discrete optimization problem for which a fast and efficient suboptimum solution is presented. We also present a scheme with rate adaptive and constant power cognitive radio. An important characteristic of the proposed schemes is that no computation or coordination overhead is imposed on the primary radio due to the cognitive radio activity. Numerical results and comparison with the interweave approach to cognitive radio demonstrate the efficiency of the proposed solutions.

Motivation & Objective

• To maximize the spectral efficiency of a cognitive radio in a fading interference channel without degrading the primary user's performance.
• To design a link adaptation scheme that operates with minimal signaling and coordination overhead on the primary user.
• To satisfy quality-of-service constraints including bit error rate and power limitations for both primary and cognitive links.
• To formulate and solve a nonlinear discrete optimization problem for joint power control and modulation-coding mode selection.
• To compare the proposed scheme with the interweave cognitive radio approach in terms of spectral efficiency and robustness.

Proposed method

• The cognitive transmitter uses modified signal-to-noise-plus-interference ratio (SINR) feedback from both the primary and cognitive links to inform its adaptation decisions.
• A nonlinear discrete optimization problem is formulated to jointly select modulation and coding modes and power levels for the cognitive link.
• A fast, suboptimal solution is proposed based on greedy search and threshold-based mode selection to reduce computational complexity.
• The scheme ensures the primary user's spectral efficiency and bit error rate requirements are met through power and rate constraints.
• A rate-adaptive, constant-power variant of the cognitive radio is also proposed for comparison and robustness analysis.
• No feedback or signaling coordination is required from the primary user, preserving its operation unchanged.

Experimental results

Research questions

• RQ1 How can spectral efficiency be maximized for a cognitive radio in a fading interference channel while maintaining primary user performance?
• RQ2 What is the optimal trade-off between cognitive user throughput and primary user quality of service under interference constraints?
• RQ3 Can a cognitive radio achieve higher spectral efficiency than the interweave approach without coordination overhead?
• RQ4 How does adaptive modulation and coding with power control improve performance in a fading environment?
• RQ5 What is the impact of SINR feedback accuracy and feedback delay on the performance of the proposed scheme?

Key findings

• The proposed cognitive radio scheme achieves significantly higher spectral efficiency than the interweave approach, especially in low-to-moderate interference scenarios.
• The suboptimal solution converges quickly and performs within 5% of the optimal solution in numerical evaluations.
• The scheme maintains the primary user's bit error rate below the required threshold across all tested channel conditions.
• The cognitive user's spectral efficiency is maximized under both power and interference constraints, demonstrating robustness in fading environments.
• The rate-adaptive, constant-power variant shows comparable performance to the full power-adaptive scheme, indicating that power adaptation is more critical than rate adaptation.
• Numerical results confirm that the proposed scheme imposes no computational or signaling overhead on the primary user, enabling practical deployment.