# Ultra-Reliable Link Adaptation for Downlink MISO Transmission in 5G Cellular Networks
## 1 Introduction  
The evolution of mobile communication systems has culminated in the dawn of a 5G vision, which envisages providing services, both human and machine centric, with diverse quality of service (QoS) requirements and applications [1]. In order to address these challenges, a number of targets have been identified for 5G, to be achieved in 2020 time-frame. The foremost is enhancing existing mobile data volume by a factor of 1000\(\times\), which is to be attained by addition of new spectrum, enhancement of spectral efficiency of existing systems, greater number of antennas, and smaller cells. In contrast, new and significantly more challenging targets stem from the application of various wireless technologies to machine-centric domains such as machine type communications (MTC) or machine to machine communications. It comprises of two main paradigms--massive MTC and mission-critical MTC. In massive MTC, the idea is to connect a large number of machines such as sensors, actuators, and other devices to a common platform, paving the way for Internet of Things. Most of these machines will likely be low-cost, requiring low-data volumes and energy consumption, but long deployment periods. On the other hand, mission critical MTC scenarios are usually characterized by very low latency, high reliability, and high availability. This has led to an emergence of new use cases, which often consist of very stringent requirements in terms of latency, reliability, and availability -- collectively referred to as ultra-reliable communications (URC) in 5G systems. URC is primarily envisioned to be applied at short time-scales, where it has a multitude of applications such as vehicle-to-vehicle communication and smart grid control. A target envisioned for URC in 5G is 99.999% reliability with 2 ms latency [2]. Thispresents novel challenges at multiple levels, and requires complete rethinking of system design approaches. URC networks differ from conventional mobile broadband systems, in that the focus is not on the peak and median performance, but on the situation of lower 0.00001 percentile of users. URC network design is essentially based on the analysis of worst case scenarios, and the preventive measures one can take to avert them. The concept of reliability pertaining to 5G systems can be defined in many ways, depending on the use case at hand. In order to develop a systematic framework inclusive of all use cases, ultra-reliability can essentially be understood as a network property that constitutes three dimensions [3; 4]: availability, reliability, and latency. The network is unavailable when it is undergoing an unscheduled downtime. The main factors that impact availability include hardware/software faults, human errors, natural disasters, depletion of network resources due to overloading, _etc._ On the other hand, latency is an end-to-end network design issue, and its enhancement may require changes in architecture and procedures at different layers. It can be improved by reducing delays emanating from propagation, processing, large packet sizes, and queuing. Finally, reliability can be considered as the probability of successful transmission across a wireless link, given that the system is available, and is able to meet the minimum latency requirements. A multitude of factors contribute to reliability at a network level. In particular, mobility related changes in the channels of users and existence of multiple nodes in the network competing for the same resources, may lead to a significant increase in problem complexity. Accordingly, impairments such as interference, inaccuracy in channel state information (CSI), and link adaptation errors cause loss in reliability. These issues can be addressed via an efficient management of network radio resources across multiple degrees of freedom (e.g., time, space, frequency). For instance, in [5], resource allocation is considered to mitigate interference, and guarantee a network-wide target data rate to the users.