Towards 1 Gbps/UE in Cellular Systems: Understanding Ultra-Dense Small Cell Deployments (1503.03912v1)

Abstract: Todays heterogeneous networks comprised of mostly macrocells and indoor small cells will not be able to meet the upcoming traffic demands. Indeed, it is forecasted that at least a 100x network capacity increase will be required to meet the traffic demands in 2020. As a result, vendors and operators are now looking at using every tool at hand to improve network capacity. In this epic campaign, three paradigms are noteworthy, i.e., network densification, the use of higher frequency bands and spectral efficiency enhancement techniques. This paper aims at bringing further common understanding and analysing the potential gains and limitations of these three paradigms, together with the impact of idle mode capabilities at the small cells as well as the user equipment density and distribution in outdoor scenarios. Special attention is paid to network densification and its implications when transitioning to ultra-dense small cell deployments. Simulation results show that network densification with an average inter site distance of 35 m can increase the cell- edge UE throughput by up to 48x, while the use of the 10GHz band with a 500MHz bandwidth can increase the network capacity up to 5x. The use of beamforming with up to 4 antennas per small cell base station lacks behind with cell-edge throughput gains of up to 1.49x. Our study also shows how network densifications reduces multi-user diversity, and thus proportional fair alike schedulers start losing their advantages with respect to round robin ones. The energy efficiency of these ultra-dense small cell deployments is also analysed, indicating the need for energy harvesting approaches to make these deployments energy- efficient. Finally, the top ten challenges to be addressed to bring ultra-dense small cell deployments to reality are also discussed.

• The paper demonstrates that ultra-dense network deployment with a 35m inter-site distance can boost cell-edge throughput by up to 48x.
• It shows that leveraging 10GHz frequency bands with a 500MHz bandwidth enhances network performance by 5.31x despite the challenges of increased path loss.
• The study finds that while beamforming contributes modest gains (up to 1.49x), simplified scheduling and energy-efficient strategies are essential for sustainable small cell deployments.

Ultra-Dense Cellular Systems: An Analytical Perspective

The paper "Towards 1Gbps/UE in Cellular Systems: Understanding Ultra-Dense Small Cell Deployments" offers a comprehensive examination of the potential and limitations inherent in deploying ultra-dense small cell networks. This exploration considers network densification, the use of higher frequency bands, and multi-antenna strategies as key enablers for achieving 1Gbps per user equipment (UE).

Network Densification

Network densification is presented as the principal driver for increasing network capacity through enhanced spatial reuse. The paper demonstrates that reducing the inter-site distance (ISD) to 35m can yield significant gains in UE throughput, with the paper reporting a possible increase of cell-edge throughput by up to 48x. The research underscores the dual nature of densification: while it offers substantial throughput improvements, it also challenges network planning and operational expenditure. A critical insight is the identification of the "one UE per cell" scenario as the fundamental limit of spatial reuse, suggesting an optimal deployment strategy focused on user density and distribution.

Higher Frequency Bands

The exploration of higher frequency bands, specifically those around 10GHz, reveals both opportunities and challenges. Increasing the available bandwidth is intimately tied to improved throughput, as evidenced by the paper's findings where the use of a 500MHz bandwidth can enhance network performance by up to 5.31x. However, the attendant increase in path loss at higher frequencies necessitates higher transmit powers, complicating deployment logistics, especially in macrocell scenarios. Nevertheless, these challenges are somewhat mitigated within small cells due to their shorter range requirements.

Multi-Antenna Techniques

Multi-antenna techniques, particularly beamforming, yield modest gains compared to network densification and bandwidth increase. The research indicates a maximum gain of 1.49x, showing that while beamforming can contribute to performance boosts, its impact is limited. The paper suggests that spatial multiplexing, despite its implementation complexity, could better harness the potential of small cell networks, particularly if channel conditions support multiple degrees of freedom.

Scheduler Implications and Energy Efficiency

The paper further discusses the implications of reduced UE diversity on scheduling strategies, noting that Proportional Fair (PF) schedulers tend to lose their advantage in ultra-dense environments due to less fluctuating channel conditions. Simplified schedulers like Round Robin (RR) could offer comparable performance with reduced computational complexity. Energy efficiency, particularly in terms of employing advanced idle modes, is highlighted as crucial for sustainable deployment. The optimal deployment would minimize power consumption, requiring potentially novel energy-harvesting techniques and more dynamic power management practices.

Future Challenges

Attention is drawn to several unresolved challenges in achieving ultra-dense small cell deployments. These include ensuring efficient backhaul provision, managing mobility without compromising performance, and addressing the cost implications of widespread small cell deployment. Furthermore, the coexistence of small cells with existing Wi-Fi networks in unlicensed spectrum represents another frontier of research and development.

Concluding Remarks

This paper provides a detailed overview of the pathways to achieving ultra-dense cellular networks capable of delivering 1Gbps per UE. With significant numerical results, the authors cast light on both the theoretical potential and the practical challenges surrounding these deployments. Future work should focus on resolving these identified challenges, particularly concerning energy efficiency and cost-effective implementation strategies. The insights offered pave the way for continued advancement in the field of cellular network capacity expansion.