On Peak versus Average Interference Power Constraints for Protecting Primary Users in Cognitive Radio Networks (0806.0676v2)

Abstract: This paper considers spectrum sharing for wireless communication between a cognitive radio (CR) link and a primary radio (PR) link. It is assumed that the CR protects the PR transmission by applying the so-called interference-temperature constraint, whereby the CR is allowed to transmit regardless of the PR's on/off status provided that the resultant interference power level at the PR receiver is kept below some predefined threshold. For the fading PR and CR channels, the interference-power constraint at the PR receiver is usually one of the following two types: One is to regulate the average interference power (AIP) over all the fading states, while the other is to limit the peak interference power (PIP) at each fading state. From the CR's perspective, given the same average and peak power threshold, the AIP constraint is more favorable than the PIP counterpart because of its more flexibility for dynamically allocating transmit powers over the fading states. On the contrary, from the perspective of protecting the PR, the more restrictive PIP constraint appears at a first glance to be a better option than the AIP. Some surprisingly, this paper shows that in terms of various forms of capacity limits achievable for the PR fading channel, e.g., the ergodic and outage capacities, the AIP constraint is also superior over the PIP. This result is based upon an interesting interference diversity phenomenon, i.e., randomized interference powers over the fading states in the AIP case are more advantageous over deterministic ones in the PIP case for minimizing the resultant PR capacity losses. Therefore, the AIP constraint results in larger fading channel capacities than the PIP for both the CR and PR transmissions.

• The paper demonstrates that average interference power (AIP) improves ergodic and outage capacities compared to peak interference power (PIP) under fading conditions.
• It introduces the concept of interference diversity, where randomized interference under AIP yields better transmission performance.
• Simulation results validate that AIP enables dynamic power allocation, optimizing cognitive radio networks while safeguarding primary user quality.

Analysis of Interference Power Constraints in Cognitive Radio Networks

The paper "On Peak versus Average Interference Power Constraints for Protecting Primary Users in Cognitive Radio Networks" by Rui Zhang investigates an essential aspect of cognitive radio (CR) networks—spectrum sharing with primary radio (PR) users. The crux of this paper is the exploration of constraints on interference power which cognitive radios must adhere to while operating in shared frequency bands with primary users. The author specifically compares average interference power (AIP) and peak interference power (PIP) constraints under fading channel conditions, discovering a counterintuitive conclusion favoring the AIP constraint over the PIP.

Key Contributions

1. Spectrum Sharing Mechanism: The paper examines the interference-temperature constraint under two conditions: AIP, where interference levels are averaged across fading states, and PIP, where interference is limited at each fading state. AIP provides cognitive radios with flexibility in dynamic power allocation, leading to improved ergodic and outage capacities for both CR and PR links.
2. Interference Diversity Phenomenon: An interesting finding presented is the role of interference diversity in enhancing transmission capacities. For PR, the randomized nature of interference under the AIP constraint, compared to the deterministic interference under PIP, leads to a constructive diversification effect that maximizes ergodic and outage capacities, highlighting a fundamental property of fading channels and their response to interference.
3. Fading Channel Capacity: The paper argues that despite PIP's rigorous interference limit, it is less beneficial compared to AIP when considering the ergodic and outage capacities achievable by PR receivers. Under AIP, PR fading channels benefit from interference diversity, experiencing minimally adverse effects due to convexity in capacity functions.
4. Practical Implications: The findings suggest that when designing CR networks, implementing an AIP constraint might better balance the interference mitigation obligation toward PR users and the throughput optimization for CR users. The results challenge the traditional notion that peak constraints offer better protection and pave the way for more nuanced design strategies in interference management.

Numerical Results and Simulation

The paper includes simulations validating theoretical results, indicating how varying channel attenuation and interference constraints affect CR and PR capacities. Notably, CR's ergodic capacity under AIP converges to that under PIP with growing channel attenuation, an observation aligned with the premise of maximizing CR throughput whilst adhering to interference constraints.

Implications and Future Work

The AIP constraint's superiority suggests that CR systems could optimize their power control strategies under this constraint to enhance overall network performance without compromising PR quality of service. This discovery opens up discussions for future research, focusing on extending these results to multipurpose antenna systems and more complex networks accounting for realistic channel state information presence.

In conclusion, this paper offers a significant reevaluation of interference power constraints in cognitive radio networks. It provides a robust theoretical framework substantiated by empirical validation, indicating AIP's practical advantages over PIP. This work enhances the understanding of interference dynamics in cognitive radio networks and lays the groundwork for advanced spectrum sharing strategies.