Joint Rate and SINR Coverage Analysis for Decoupled Uplink-Downlink Biased Cell Associations in HetNets (1412.1898v2)

Abstract: Load balancing by proactively offloading users onto small and otherwise lightly-loaded cells is critical for tapping the potential of dense heterogeneous cellular networks (HCNs). Offloading has mostly been studied for the downlink, where it is generally assumed that a user offloaded to a small cell will communicate with it on the uplink as well. The impact of coupled downlink-uplink offloading is not well understood. Uplink power control and spatial interference correlation further complicate the mathematical analysis as compared to the downlink. We propose an accurate and tractable model to characterize the uplink SINR and rate distribution in a multi-tier HCN as a function of the association rules and power control parameters. Joint uplink-downlink rate coverage is also characterized. Using the developed analysis, it is shown that the optimal degree of channel inversion (for uplink power control) increases with load imbalance in the network. In sharp contrast to the downlink, minimum path loss association is shown to be optimal for uplink rate. Moreover, with minimum path loss association and full channel inversion, uplink SIR is shown to be invariant of infrastructure density. It is further shown that a decoupled association---employing differing association strategies for uplink and downlink---leads to significant improvement in joint uplink-downlink rate coverage over the standard coupled association in HCNs.

• The paper introduces decoupled uplink and downlink associations that significantly enhance joint rate and SINR coverage in heterogeneous networks.
• The paper employs an inhomogeneous Poisson point process framework and fractional power control to accurately model uplink interference and user dynamics.
• The paper validates its analytical findings through extensive simulations, emphasizing improved load balancing and QoS in dense urban environments.

Joint Rate and SINR Coverage Analysis for Decoupled Uplink-Downlink Biased Cell Associations in HetNets

The paper "Joint Rate and SINR Coverage Analysis for Decoupled Uplink-Downlink Biased Cell Associations in HetNets" presents a comprehensive analytical model addressing the challenges in heterogeneous cellular networks (HCNs) by factoring in asymmetrical cell associations between uplink and downlink. The paper hinges on the necessity of load balancing and the peculiar nature of uplink interference vis-a-vis downlink interference.

Key Contributions and Analytical Approaches

1. Decoupled Uplink-Downlink Association: The paper deviates from the assumption of coupled uplink and downlink associations, where traditionally users connect to the same base station for both directions. It suggests decoupled associations, promoting distinct strategies for uplink and downlink associations that cater to their respective optimal conditions. The paper offers analytical insights demonstrating how decoupled associations can significantly enhance joint uplink-downlink rate coverage, juxtaposed against traditional methods.
2. Uplink Interference Modeling: A novel aspect of this work is the accurate modeling of uplink interference in HCNs. Utilizing a Poisson point process (PPP), the authors approximate the distribution of interfering UEs as an inhomogeneous PPP, contingent on association parameters. This assumption refines prior approaches by addressing the correlation of uplink transmissions, a nontrivial aspect given that uplink interference stems from one user per cell transmitting on a given resource block.
3. Power Control and Load Balancing: Exploration into fractional power control (FPC) and its impact on uplink rate coverage reveals that minimum path loss association is optimal for uplink rate coverage, a significant departure from the downlink trends. The paper concludes that full channel inversion renders uplink signal-to-interference ratio (SIR) invariant to infrastructure density, akin to macrocellular network trends in the downlink.
4. Joint Uplink-Downlink Rate Coverage: The paper explores joint rate analysis, demonstrating that different optimal association weights for uplink and downlink as part of decoupled associations lead to improved performance in satisfying quality of service (QoS) thresholds. This section is augmented by an analysis of load distribution across base stations considering user association, highlighting different strategies for different network tiers.

Numerical Results and Validation

The authors validate their analytical results through extensive simulations, focusing on both SIR and rate coverage under varying system parameters. A particularly notable validation involves decoupled association strategies which outperform coupled associations in scenarios necessitating symmetric uplink-downlink performance, such as in applications requiring balanced QoS.

Implications and Future Directions

The paper's findings hold substantive implications for the design and deployment of future cellular networks, particularly as they grow denser and more heterogeneous. Practically, operators striving to enhance network throughput and efficiency can leverage decoupled associations, particularly in dense urban environments.

From a theoretical standpoint, the approach calls for further examination into the bounds of uplink interference modeling and the interplay with other emerging technologies, such as massive MIMO and multi-tier network architectures. Exploring adaptive power control mechanisms and further validating the model under diverse realistic conditions could form the trajectory of subsequent research work.

In summary, the paper advances the understanding of uplink and downlink dynamics in HCNs, offering a robust framework that challenges conventional assumptions and posits decoupled associations as a forward-looking network strategy. The work is significant for both practitioners and theoreticians aiming to optimize network operations in the burgeoning field of heterogeneous networks.