[Paper Review] Ultra-Reliable and Low-Latency Communications Using Proactive Multi-cell Association

Chun‐Hung Liu, Di-Chun Liang+2 more|arXiv (Cornell University)|Mar 23, 2019

Advanced Wireless Communication Technologies28 references4 citations

TL;DR

This paper proposes a proactive multi-cell association (PMCA) scheme in heterogeneous networks (HetNets) to enable ultra-reliable and low-latency communications (URLLC) by eliminating feedback and retransmission delays through open-loop transmission. By forming virtual cells with multiple access points (APs) and optimizing AP and user densities, PMCA achieves near-maximal reliability and sub-millisecond downlink latency, with analytical bounds showing performance gains over conventional schemes.

ABSTRACT

Attaining reliable communications traditionally relies on a closed-loop methodology but inevitably incurs a good amount of networking latency thanks to complicated feedback mechanism and signaling storm. Such a closed-loop methodology thus shackles the current cellular network with a tradeoff between high reliability and low latency. To completely avoid the latency induced by closed-loop communication, this paper aims to study how to jointly employ open-loop communication and multi-cell association in a heterogeneous network (HetNet) so as to achieve ultra-reliable and low-latency communications. We first introduce how mobile users in a HetNet adopt the proposed proactive multi-cell association (PMCA) scheme to form their virtual cell that consists of multiple access points (APs) and then analyze the communication reliability and latency performances. We show that the communication reliability can be significantly improved by the PMCA scheme and maximized by optimizing the densities of the users and the APs. The analyses of the uplink and downlink delays are also accomplished, which show that extremely low latency can be fulfilled in the virtual cell of a single user if the PMCA scheme is adopted and the radio resources of each AP are appropriately allocated.

Motivation & Objective

• To overcome the reliability-latency tradeoff in 5G cellular networks caused by closed-loop feedback mechanisms.
• To enable ultra-reliable and low-latency communications (URLLC) in heterogeneous networks (HetNets) without relying on retransmissions or real-time feedback.
• To develop a proactive multi-cell association (PMCA) scheme that forms virtual cells from multiple access points (APs) to enhance link reliability and reduce latency.
• To analyze the reliability and delay performance of PMCA in both uplink and downlink using stochastic geometry and point process theory.
• To derive analytical bounds for outage probability and latency, showing that optimal user and AP densities maximize reliability and minimize delay.

Proposed method

• Models the HetNet using a Poisson point process (PPP) for APs and users, with a virtual cell formed by associating a user with multiple APs via PMCA.
• Applies stochastic geometry to derive the downlink signal-to-interference-plus-noise ratio (SINR) outage probability by modeling interference as a shot-noise process.
• Uses the probability generating functional (PGFL) of the PPP to compute the distribution of the maximum SINR across multiple associated APs in the virtual cell.
• Derives upper bounds on the downlink outage probability for both finite and infinite numbers of associated APs, showing that reliability increases with more APs.
• Optimizes the system by deriving expressions for the optimal densities of users and APs that maximize reliability and minimize latency.
• Introduces a power control and resource allocation model that ensures low-latency transmission by minimizing control signaling and enabling grant-free access.

Experimental results

Research questions

• RQ1 Can proactive multi-cell association (PMCA) eliminate feedback-induced latency in URLLC systems while maintaining high reliability?
• RQ2 How does the number of associated APs in a virtual cell affect the outage probability and end-to-end latency in downlink transmission?
• RQ3 What is the optimal density of users and APs that maximizes reliability and minimizes latency under the PMCA scheme?
• RQ4 How does the use of open-loop communication in PMCA compare to traditional closed-loop schemes in terms of latency and reliability?
• RQ5 What analytical framework can accurately model the performance of PMCA in a heterogeneous network using stochastic geometry?

Key findings

• The PMCA scheme achieves near-maximal reliability by forming a virtual cell with multiple APs, significantly reducing outage probability compared to single-association schemes.
• The downlink outage probability is bounded above by $ 1 - \prod_{k=1}^{K} \left\{ 1 - \left(1 + \delta \ell\left(\theta, \frac{2}{\alpha}\right) \mathbb{E}[O^{2/\alpha}] \right)^{-k} \right\} $, which decreases with increasing number of associated APs.
• For an infinite number of associated APs, the outage probability converges to $ 1 - \exp\left[ -\pi \widetilde{\lambda} \int_0^\infty \exp\left( -\frac{\pi \delta \widetilde{\lambda} \theta^{2/\alpha} r}{\text{sinc}(2/\alpha)} \right) dr \right] $, showing asymptotic reliability gain.
• When all K APs in the virtual cell collaborate, the outage probability is bounded by $ 1 - \left\{ 1 - \left[ 1 + \delta \ell\left( \frac{\theta}{K^{\alpha/2 + 1}}, \frac{2}{\alpha} \right) \sum_{m=1}^2 \vartheta_m (1 - p_{m,0}) \right] \right\}^{-K} $, demonstrating that joint transmission improves reliability.
• The analysis shows that extremely low latency—on the order of 1 ms—can be achieved in the virtual cell if radio resources are properly allocated and the PMCA scheme is employed.
• Optimal user and AP densities exist that maximize reliability, with the optimal density of APs being proportional to the user density and path-loss exponent.