Joint Design and Separation Principle for Opportunistic Spectrum Access in the Presence of Sensing Errors

TL;DR

This paper develops a joint PHY-MAC design framework for opportunistic spectrum access that accounts for sensing errors, establishing a separation principle that simplifies the complex POMDP problem and yields optimal, practical policies.

Contribution

It introduces a separation principle for joint spectrum sensing and access design in OSA, enabling closed-form solutions and reducing computational complexity.

Findings

Myopic policies are optimal for sensing and access strategies.

Decoupling sensing strategy from sensor and access design simplifies the problem.

Numerical results demonstrate robustness and tradeoffs in the proposed approach.

Abstract

We address the design of opportunistic spectrum access (OSA) strategies that allow secondary users to independently search for and exploit instantaneous spectrum availability. Integrated in the joint design are three basic components: a spectrum sensor that identifies spectrum opportunities, a sensing strategy that determines which channels in the spectrum to sense, and an access strategy that decides whether to access based on imperfect sensing outcomes. We formulate the joint PHY-MAC design of OSA as a constrained partially observable Markov decision process (POMDP). Constrained POMDPs generally require randomized policies to achieve optimality, which are often intractable. By exploiting the rich structure of the underlying problem, we establish a separation principle for the joint design of OSA. This separation principle reveals the optimality of myopic policies for the design of the spectrum sensor and the access strategy, leading to closed-form optimal solutions. Furthermore, decoupling the design of the sensing strategy from that of the spectrum sensor and the access strategy, the separation principle reduces the constrained POMDP to an unconstrained one, which admits deterministic optimal policies. Numerical examples are provided to study the design tradeoffs, the interaction between the spectrum sensor and the sensing and access strategies, and the robustness of the ensuing design to model mismatch.