Exploiting Multi-Antennas for Opportunistic Spectrum Sharing in Cognitive Radio Networks (0711.4414v1)

Abstract: In cognitive radio (CR) networks, there are scenarios where the secondary (lower priority) users intend to communicate with each other by opportunistically utilizing the transmit spectrum originally allocated to the existing primary (higher priority) users. For such a scenario, a secondary user usually has to trade off between two conflicting goals at the same time: one is to maximize its own transmit throughput; and the other is to minimize the amount of interference it produces at each primary receiver. In this paper, we study this fundamental tradeoff from an information-theoretic perspective by characterizing the secondary user's channel capacity under both its own transmit-power constraint as well as a set of interference-power constraints each imposed at one of the primary receivers. In particular, this paper exploits multi-antennas at the secondary transmitter to effectively balance between spatial multiplexing for the secondary transmission and interference avoidance at the primary receivers. Convex optimization techniques are used to design algorithms for the optimal secondary transmit spatial spectrum that achieves the capacity of the secondary transmission. Suboptimal solutions for ease of implementation are also presented and their performances are compared with the optimal solution. Furthermore, algorithms developed for the single-channel transmission are also extended to the case of multi-channel transmission whereby the secondary user is able to achieve opportunistic spectrum sharing via transmit adaptations not only in space, but in time and frequency domains as well.

• The paper introduces a convex optimization formulation to maximize secondary capacity while adhering to interference constraints.
• It demonstrates that beamforming optimally balances spatial multiplexing and interference management in both MISO and MIMO scenarios.
• The study validates through simulations that hybrid D-SVD/P-SVD algorithms yield significant capacity gains across varied SNR regimes.

Exploiting Multi-Antennas for Opportunistic Spectrum Sharing in Cognitive Radio Networks

The paper "Exploiting Multi-Antennas for Opportunistic Spectrum Sharing in Cognitive Radio Networks" by Rui Zhang and Ying-Chang Liang presents a comprehensive paper on the potential benefits of using multi-antennas in Cognitive Radio (CR) networks for spectrum sharing. It explores the fundamental problem of balancing the throughput maximization for secondary users while minimizing the interference to primary users, using an information-theoretic perspective and advanced optimization techniques.

Key Contributions and Methodologies

The authors address several crucial aspects of spectrum sharing in CR networks:

1. Convex Optimization Formulation: The core of the paper is the formulation of the secondary user's capacity optimization as a sequence of convex optimization problems. This is achieved by considering both the transmit power constraints of the secondary user and interference power constraints at multiple primary receivers.
2. Beamforming and Spatial Multiplexing: For the case where the secondary transmission involves a Multiple-Input Single-Output (MISO) channel, the paper proves that beamforming is the optimal strategy. It then generalizes to the Multiple-Input Multiple-Output (MIMO) case and presents two suboptimal algorithms — Direct-Channel Singular Value Decomposition (D-SVD) and Projected-Channel Singular Value Decomposition (P-SVD). These algorithms aim to balance spatial multiplexing and interference aversion.
3. Interference Diversity: The paper introduces the concept of interference diversity, particularly when the number of primary receivers exceeds the number of secondary transmit antennas. This phenomenon can be exploited to enhance the capacity of the secondary transmission.
4. Multi-Channel Transmission: Extending the single-channel paper, the authors also address multi-channel transmissions (such as OFDM systems) where the secondary transmitter can adapt its resources across space, time, and frequency domains for optimal spectrum sharing.

Numerical Results and Key Findings

Several simulation scenarios were carried out to illustrate the effectiveness of the proposed solutions:

• Capacity Gains: The results indicate substantial capacity gains for the secondary transmission, even under stringent interference constraints. This is particularly pronounced when multi-antennas are employed at the secondary transmitter.
• Algorithm Performance: The D-SVD algorithm performs close to optimal at low SNR regimes, while the P-SVD is more effective at high SNR regimes. The hybrid D-SVD/P-SVD algorithm is proposed to dynamically choose between these extremes, showing significant performance improvements by effectively exploiting interference diversity.
• Comparison Across Primary Users: The paper shows that the hybrid algorithm, which projects the secondary user's channel into a null space of a selected subspace of the primary receivers' channel, performs superiorly across a range of interference-power constraints.

Implications and Future Directions

The findings have significant practical and theoretical implications:

• Practical Applications: The proposed algorithms can be directly applied to real-world CR networks to enhance spectrum utilization, particularly in scenarios involving dense deployments of primary and secondary users.
• Theoretical Extensions: Future work could explore more generalized interference constraints at primary receivers, potentially incorporating aspects of primary users’ channel capacities. Additionally, incorporating game-theoretic approaches could enable decentralized and cooperative spectrum sharing strategies in a more dynamic environment.
• Multiuser Scenarios: The methods and results presented can be extended to scenarios involving multiple secondary users. This extension would explore new channels like the MIMO multiple-access and broadcast channels, or even the distributed MIMO interference channel.

In conclusion, this paper provides a thorough exploration of multi-antenna techniques for opportunistic spectrum sharing in CR networks, presenting innovative algorithms and insightful findings that are poised to impact future developments in the field.