With the exponential growth in mobile data traffic taking place currently and projected into the future, mobile operators need cost effective ways to manage the load of their networks. Traditionally, this has been achieved by offloading mobile traffic onto Wi-Fi networks due to their low cost and ubiquitous deployment. Recently, LTE operating in the unlicensed spectrum has drawn significant interests from mobile operators due to the availability of the unlicensed spectrum. However, the deployment of LTE networks in the unlicensed band poses significant challenges to the performance of current and future Wi-Fi networks. We discuss the LTE and Wi-Fi coexistence challenges and present analysis on performance degradation of the Wi-Fi networks at the presence of LTE.

To meet the high demand for mobile data, the Third Generation Partnership Project (3GPP) established a set of standards known as 5G New Radio (5G NR). The architecture of 5G NR includes a flexible radio access network and a core network. 3GPP has also been working on a new radio access technology, called 5G NR Unlicensed (5G NR-U), which aims at extending 5G NR to unlicensed bands. In this paper, we give an overview of the most recent 5G NR-U design elements and discuss potential concerns, including fair coexistence with other unlicensed technologies such as Wi-Fi. We use simulations to study coexistence between Wi-Fi and 5G NR-U systems. Our evaluation indicates that NR-U often achieves higher throughput and lower delay than Wi-Fi (802.11ac). The two systems experience different buffer occupancies and spectrum utilization statistics. We also discuss the improvements that NR-U offers over LTE Licensed Assisted Access (LTE-LAA). I. INTRODUCTION Next-generation wireless networks will support applications with widely diverse performance requirements. In its International Mobile Communications (IMT)-2020 recommendations, the International Telecommunications Union (ITU) specifies three use cases for next-generation wireless networks: Enhanced mobile broadband (eMBB), ultra-reliable and low latency communication (URLLC), and massive machine-type communication (mMTC). While these use cases embody different performance requirements, they all share the need for more spectrum. In its effort to extend 5G cellular operation to unlicensed spectrum, 3GPP is initially targeting the Unlicensed National Information Infrastructure (UNII) bands at 5 GHz and 6 GHz. Future specifications will address unlicensed millimeter wave (mmWave) bands at 60 GHz. Wireless systems can operate over unlicensed bands as long as they comply with spectrum regulations, which are intended to ensure harmonious coexistence of various incumbents that operate on the same band. The ubiquity of Wi-Fi networks makes achieving harmonious 5G NR-U and Wi-Fi coexistence a key objective for NR-U designers. To ensure fairness in channel access, NR-U should not impact an existing Wi-Fi system more than the impact of another Wi-Fi system [1]. Early works surveying 5G NR-U can be found in [2]-[5]. These works focused on pre-standard NR-U operation at sub-6 GHz and/or mmWave frequencies and discussed the feasibility of utilizing the channel access procedures of 'further enhanced' LTE LAA (feLAA) in 5G networks. The effectiveness of unlicensed bands for IoT applications was investigated in [6], where the authors studied challenges

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Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. For this architecture, we developed a channel model integrating both the RF and FSO channels. Based on the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link is used by itself, clouds hamper availability, due to introduction of attenuation and temporal dispersion. On the contrary, RF signals are relatively immune to the cloud effects, thus improve the availability in a hybrid of RF and FSO links, significantly. INTRODUCIONDue to the increasing demand for broadband data communications, free space optical (FSO) links are gaining more attention for their high-bandwidth and secure nature over long distances. In addition, systems using free space optics can be deployed inexpensively and rapidly, which is ideal for broadband communications. Recent advances in optical devices proved that such pointing communication links could even be used to interconnect very fast moving units such as airplanes [1].However, FSO links are well known for their susceptibility to adverse weather conditions such as cloud and fog, which pose a great challenge on availability of such links. For terrestrial FSO links, backup RF links have been proposed to ensure continuous availability during FSO downtimes such as when there is dense fog. On the other hand, in cases of heavy rain, FSO links can also serve as a backup when RF links are severely attenuated. Because of the close resemblance of cloud and fog, the same method can be applied to air-to-air FSO links to overcome communication interruptions caused by failure of FSO links. Though in this case, RF links seldom face severe signal attenuation. For an air-to-ground link, the complementary property no longer holds due to the fact that rain and thick clouds often occur, concurrently. However, a backup RF link still increases the overall system availability by enabling communications through clouds when no heavy rain is present.In order to quantitatively analyze availability of an airborne hybrid link, we utilize cloud data obtained from the International Satellite Cloud Climatology Project (ISCCP) database. Based on the cloud statistical information and the channel model we developed for this application, we evaluated availability of standalone RF and FSO links. Due to the temporal dispersion caused by thick clouds, capacity is used as the measure to indicate availability instead of attenuation. Additionally, availability of hybrid links is evaluated and compared to the standalone cases to demonstrate availability advantages. CHANNEL MODELSWe consider a hybrid wireless network shown in Figure 1. As illustrated in the figure, there are two types of links in this scenar...