Ultra-Reliable Multi-Connectivity With Negatively Dependent Fading Channels

Ultra-reliable wireless communications for critical industrial infrastructure communications is challenging because of the random fading channel. In order to improve the outage performance, diversity techniques are studied extensively and are currently standardized in 5G and beyond. The dependencies of fading channels originate from the random propagation environments, local scatterers, multipath conditions, and shadowing effects. It is well known that spatial and spectral correlation deteriorates the outage performance and well accepted that independent fading channels provide the highest reliability gains. In this work, we consider a negative dependence between the fading channels and show that the outage performance can be significantly improved by negative dependencies. To this end, we construct an arbitrarily dependent bivariate exponential model and show that it supports the whole range of dependency. Based on this construction, we show that the zero-outage capacity is positive for the completely negatively dependent case even under short-term power constraints, i.e., both link selection and link combining achieve zero-outage performance. Furthermore, we show the monotonicity of the outage probability for the relevant operating regime. Finally, we completely characterize the impact of correlation on the average performance in a bivariate binary channel model. Numerical assessments confirm the closed-form expressions of the outage probability and motivates to deploy network nodes in order to create negatively dependent fading channel gains.

I. Introduction

The International Telecommunications Union (ITU) initiative has defined the three classes of 5G use cases: 1) enhanced Mobile Broadband (eMBB), 2) massive MachineType Communication (mMTC), and 3) Ultra-Reliable LowLatency Communication (URLLC) [1]. In order to understand the trade-offs and efficient operating points for ultra-reliable communications, [2] develops a framework by listing enabling technologies and methods as well as their application in use cases. Different forms of diversity are exploited to achieve high reliability of fading channels with interfering links. Furthermore, multi-connectivity approaches, where one node is simultaneously connected to multiple access points, are studied and standardized [3]. It is differentiated between interand intra-frequency multi-connectivity: COoperative MultiPoint [4], LTE Dual Connectivity as specified by 3GPP as Small Cell Enhancements in LTE Rel.12 [5] and multi-RAT DC (MR-DC) [6] in [7]. Two main research trends can be observed: Offering high data rates by utilizing millimeter wave (mmWave) frequencies provided by multiple 5G secondary nodes, see for example [8], and facilitating highly available transmission by combining multiple links, for example in [9].