# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 1 Introduction  
The demand for data in cellular networks has increased exponentially in the past few decades. To meet this ever-increasing demand, a several-fold increase in spectrum is required [1]. However, the cost of the licensed spectrum has become exorbitantly high. Thus, cellular network operators are continuously looking for technological solutions to efficiently operate their networks with minimum cost for spectrum. This has resulted in the rapid adoption of Long-Term Evolution (LTE) based cellular networks. However, most of the available licensed spectrum in lower than 6 GHz bands has already been allocated. The co-existence of multiple access technologies in the 2.4 GHz unlicensed spectrum termed as the industrial, scientific and medical (ISM) band makes it unsuitable for operation of LTE [2]. Therefore, based on the availability across the globe, the 5 GHz unlicensed national information infrastructure (U-NII) band can be used for the operation of LTE based cellular networks with suitable modifications. The unlicensed spectrum can be used by LTE in broadly three ways. One way is to offload LTE traffic to Wi-Fi in the unlicensed spectrum [3]. A second is the integration of this unlicensed band with the licensed spectrum and utilization of LTE air-interface with minimum changes (also referred to as LTE-U) [4]. A third way is license assisted access with a listen before talk (LAA-LBT) feature potentially in LTE Release 13 (Rel-13) [5]. The benefits of using LTE-U or LAA-LBT rather than a hybrid solution of LTE and Wi-Fi are the subject of ongoing discussions within the research community [6]and is beyond the scope of this article. In this article, we focus on the second and the third, i.e., the LTE-U and LAA-LBT, respectively, as they have become promising technologies to offer increased data rates without increase in the cost of spectrum. Traditionally, the 5 GHz unlicensed band has been used for Wi-Fi, RADAR, and other applications [2]. Given the unlicensed nature, several users can simultaneously operate in this band without any coordination. Thus, the persistence of transmissions of LTE-U can significantly degrade the performance of incumbent devices in the unlicensed band which employ carrier sense multiple access (CSMA) as medium access control (MAC) protocol [7]. The motivation of this article is to explore in-depth the resource allocation techniques proposed in the literature for channel access mechanisms and channel selection schemes for LTE-U and LAA-LBT that ensure fair coexistence with existing devices. We also study the impact of coordination, traffic type, and the environment on the capacity, coverage, and latency of LTE-U and LAA-LBT using the resource allocation techniques co-existing with Wi-Fi. Furthermore, we examine in detail the key issues in LTE-U and LAA-LBT that should be addressed. We present possible future works that can help network regulators and cellular operators in adoption of best policies for LTE's operation in the unlicensed band while keeping in mind the interest of existing Wi-Fi users. For the two different potential technologies for LTE's operation in the unlicensed band, the available spectrum and the channel access mechanisms in the unlicensed spectrum are region specific. For example, the different bands and their functioning in the United States (US) is explained in detail in [2]. The unlicensed spectrum layout for some of the key cellular markets have been studied in [8], while [9] presents a detailed insight of all the sub-bands in the 5 GHz band for the major markets. The two key region specific channel access mechanisms for LTE's operation in the unlicensed band that are globally under consideration are carrier sensing adaptive transmission (CSAT) and LAA-LBT. For non-LBT markets such as South Africa and India, CSAT is proposed to be used with Rel-10 carrier aggregation protocols for co-channel existence. In case of LBT markets like Japan and Europe, Rel-13 with LAA-LBT will be used [5].