Why to Decouple the Uplink and Downlink in Cellular Networks and How To Do It (1503.06746v1)

Abstract: Ever since the inception of mobile telephony, the downlink and uplink of cellular networks have been coupled, i.e. mobile terminals have been constrained to associate with the same base station (BS) in both the downlink and uplink directions. New trends in network densification and mobile data usage increase the drawbacks of this constraint, and suggest that it should be revisited. In this paper we identify and explain five key arguments in favor of Downlink/Uplink Decoupling (DUDe) based on a blend of theoretical, experimental, and logical arguments. We then overview the changes needed in current (LTE-A) mobile systems to enable this decoupling, and then look ahead to fifth generation (5G) cellular standards. We believe the introduced paradigm will lead to significant gains in network throughput, outage and power consumption at a much lower cost compared to other solutions providing comparable or lower gains.

• The paper introduces DUDe by decoupling UL and DL connections to improve signal quality and reduce transmission power and interference.
• The paper demonstrates that decoupling uplink and downlink enhances spectral efficiency and can double uplink throughput as shown in simulation studies.
• The paper outlines practical implementations through centralized RAN, shared cell IDs, and dual connectivity to achieve efficient load balancing in evolving 5G networks.

Decoupling Uplink and Downlink in Cellular Networks: A Critical Evaluation of DUDe

This paper addresses the constraints of traditional coupled uplink (UL) and downlink (DL) associations in cellular networks, specifically within the context of heterogeneous networks (HetNets) and dense deployments anticipated for 5G. The authors propose a novel approach referred to as Downlink/Uplink Decoupling (DUDe), which allows the DL and UL transmissions to connect to different base stations (BSs), potentially enhancing performance metrics such as throughput, reliability, and power efficiency.

Key Arguments for Decoupling

The paper elucidates five primary arguments supporting DUDe:

1. Enhanced Uplink Signal-to-Noise Ratio (SNR) and Reduced Transmission Power: DUDe enables mobile terminals to connect to the most advantageous base station for either direction. A higher available SNR occurs when devices connect to proximal, low-power small cells in the uplink, leading to reduced path loss and enhanced signal reception.
2. Improved Uplink Interference Conditions: By independently choosing the best BS for the UL and DL independently, DUDe allows for a reduction in uplink interference. The reduction in power transmission inherently decreases interference levels, also benefiting device-to-device (D2D) communication scenarios.
3. Elevated Uplink Data Rates: Decoupling facilitates better signal quality, resulting in higher spectral efficiency and data rates. The improvements in channel conditions and power management strategies lead to a twofold increase in throughput for certain UE configurations as evidenced by both analytical and simulation models.
4. Efficient Load Balancing: With DUDe, the load across the network can be more fairly distributed as users can connect differently for UL and DL, aiding in optimal resource utilization.
5. Cost-effective Implementation with Centralized RAN Architectures: DUDe does not demand extensive new infrastructure, leveraging existing trends towards centralized or cloud-based RANs to facilitate low latency and rapid communication between BSs.

Implementation Considerations and Changes

The implementation of DUDe within current LTE-A systems, and proposed adaptations for 5G architecture, is discussed extensively. Three deployment architectures are presented: centralized processing units, shared cell-IDs, and dual connectivity mechanisms. These offer varying degrees of latency and operational complexity but allow practical implementation within existing and future network standards. The analysis suggests that necessary architectural modifications remain feasible without significant disruption.

Implications for Future Network Standards

The authors highlight the relevance of DUDe in evolving networks, particularly with the hyper-densification expected in 5G. By decoupling UL and DL connections, DUDe potentially offers solutions to achieve the aggressive capacity and reliability targets of 5G. Importantly, DUDe is posited to seamlessly integrate with performance-enhancing technologies such as massive MIMO and millimeter-wave communications.

Concluding Remarks

Through extensive theoretical and simulation-based evaluations, the paper establishes DUDe as a promising approach for future cellular networks. The minimal infrastructural hindrance combined with the substantial performance benefits positions DUDe as an attractive strategy for operators aiming to optimize resource utilization and enhance user experience in increasingly complex network environments. The findings encourage further exploration of DUDe's applications, particularly in conjunction with advanced computational techniques and dynamic resource management strategies anticipated in 5G and beyond.