# Cognitive Interference Alignment Schemes for IoT Oriented Heterogeneous Two-Tier Networks
## 1 Introduction  
Next generation wireless networks are expected to have an \"everything, everywhere and always connected\" future, where the end users shift from individuals to things. The concept of Internet of Things (IoT) has been proposed to cater for a massive number of smart devices and provide high-speed data services. To cope with the ever-increasing wireless traffic demands induced by IoT, a hierarchical approach to network deployment with a densely populated femtocell base station has been proposed. Applications, such as cloudified mobile networks [1], virtual infrastructure [2], and industrial communication, also exploit heterogeneous architecture to support ubiquitous, flexible, and reliable connectivity [3]. Such proliferation of wireless base stations and data offloading strategies are to be expected in the near future, presumably yielding the two-tier approach to network design [4]. More specifically, a tier of IoT oriented femtocell base stations coexists with a tier of pre-existing macrocell base stations (MBSs) and shares the licensed spectrum. This two-tiered deployment aims at breaking away from the traditional cellular layout to provide very high data rates for short-range IoT devices, and reduces the load on the macrocell network [5]. In the IoT network, smart devices are a low-power plug and play ones with rather small transmission ranges. Deployed by end users, the IoT femtocell usually lacks a predefined network infrastructure. Such architecture can reduce the load and congestion on the macrocell network, and enables the resources to be allocated to the truly mobile users [6]. On the other hand, with a huge volume of IoT devices connected to the existing macrocell based network, the IoT paradigm places an overwhelming demand on the spectrum resources. It is preferred for femtocell to share the licensed frequency band with the pre-existing macrocell network. However, cell splitting and spectrum sharing will most probably cause harsh cross-tier interference, which restricts the reusability of frequency resources and becomes a key limiting factor in developing the IoT technology [7]. To ensure the peaceful coexistence of macrocell and femtocell, as well as improving the performance of cell-edge users, effective interference management scheme design has recently attracted growing interest [8]. It is indicated that the things-oriented, Internet-oriented networks are meaningless if IoT objects are not equipped with cognitive capability [4]. To mitigate the cross-tier interference, a cognitive radio (CR) has been introduced into IoT oriented heterogeneous networks. The notion of cognitive empowered IoT femtocell was proposed to enable IoT devices as femtocell users to be aware of their environment, and can quickly adapt to the network variations by changing their transmission patterns [9; 10]. In other words, the cognitive femtocell could sense the idle licensed spectrum, allowing the IoT devices to communicate with the base station over the underutilized frequency band. However, in traditional cognitive radio networks, cognitive users can utilize the licensed band only when the primary user is non-active. The capacity and accessibility will largely depend on the primary user activity [11]. Inspired by the interference management, the CR-based network can operate in an underlay paradigm. By limiting the performance degradation caused to the primary user under a predefined threshold, both primary and cognitive users can transmit their data simultaneously within the same frequency band [12]. Cognitive interference alignment (IA) is an emerging interference management technology that has received extensive research. The basic idea here is to fully exploit the precoding matrix of the cognitive transmitters, then align the cross-tier interference signals into a lower dimensional subspace.