Index modulation based designs, error performance and physical layer security analyses for unmanned aerial vehicle networks

Abstract

Current 5G networks will not be able to meet emerging communications demands. As a result, research has begun on 6G wireless communication networks, which are expected to be deployed after 2030. 6G wireless communication networks will further improve mobile broadband, extend coverage and enable networks to include more and more smart devices. Reconfigurable front-ends for dynamic spectrum access, the Internet of Space Things enabled by CubeSats and Unmanned Aerial Vehicles (UAVs), cell-free massive multiple-input multiple-output (MIMO) communication networks, and intelligent communication environments that enable a wireless propagation environment with active signal transmission and reception are key technology advances to meet the requirements of 6G networks. Therefore, 6G will introduce new technical requirements and performance metrics driven by new application needs. 6G networks using terahertz and optical radio bands could reach $1-10$ terabytes per second. Moreover, high-frequencies can provide data rates that can reach gigabytes per second for user experience. Spectrum efficiency can be increased by a factor of $3-5$, while energy efficiency can be increased by a factor of about $10$ through the use of artificial intelligence for better network management compared to 5G. Other key performance indicators, such as cost-effectiveness, security capacity, coverage, intelligence levels, etc., should also be established to provide a more complete assessment of 6G networks. To provide global coverage, 6G wireless communication networks will expand from terrestrial communication networks in 1G-5G to integrated space-air-ground-sea networks, including satellites, UAVs, terrestrial networks, and marine communications. Over the past few years, a wide range of applications for UAVs has been established due to the advantages of their flexible design, rapid deployment, and low cost. A UAV can be used as aerial base stations (BS), user equipment (UE), or relay terminal in the 6G network because of their flexible design. UAVs can also be used in satellite networks which are another potential communication platform for 6G. In spite of the significant progress made in UAV technology, there are still several challenges. To enable UAV-based communication systems, extensive research is needed to accurately model the channel as UAV channels are unique due to their 3D deployment, high mobility, spatial and temporal instability, and aircraft shadowing. In addition to channel modeling challenges, UAV-based communications face several challenges, including security and regulatory issues, limited battery life, and seamless integration with existing networks. Index modulation (IM) can be considered as a potential technique to increase the spectral efficiency of UAV-based communications. IM uses information about the main building blocks of the wireless communication network to increase spectral efficiency and due to its advantages it has attracted considerable interest from the academia over the past decade. One of the common IM techniques, spatial modulation (SM), maps the information bits to the antenna indices. Similar to the concept of SM, distributed spatial modulation (DSM) allows the transmission of information bits using relay indices in a cooperative system. The DSM technique increases the aggregate throughput of the system and improves source reliability through distributed diversity. Another common IM technique, media-based modulation (MBM), embeds information in the selection of a particular transmission channel from a variety of channel states created by integrating parasitic elements such as radio frequency (RF) mirrors and PIN diodes in a reconfigurable antenna (RA). Similar to the external parasitic elements in MBMs, intelligent reflective elements in reconfigurable intelligent surfaces (RIS) enrich the propagation environment and perform proper phase shifts to modify the channel. This improves overall signal-to-noise ratio (SNR) quality by utilizing low-cost PIN diodes or varactors. Although MBM and RIS are based on similar structures, the MBM technique is designed to transmit additional information bits, while RIS increases the overall system reliability. 6G communication networks are designed for full connectivity with high operational flexibility and autonomy. Despite these advantages, the heterogeneity of the 6G network with UAVs and satellites makes it more vulnerable to security threats. For this reason, physical layer security (PLS) can act as an additional layer of security to enhance the trustworthiness of the radio access network. Traditional PLS solutions, like using active relays or friendly jammers (FJs) which use artificial noise (AN) to provide security, can result in increased hardware costs and power consumption. In this thesis, first the UAV channels are investigated with measurements. Then, in order to meet the high reliability requirements of future generation networks, integrated UAV systems are considered and novelties with solid theoretical foundations are proposed using DSM, and MBM. After reliability analysis for UAV systems, security problems are considered and novel system designs with non-orthogonal multiple access (NOMA), SSI-based UAV relay selection, joint transmit-receive pattern selection (JTRPS), and RIS are analytically investigated. In the first part of this thesis, we have measured the air-to-ground (AtG) channel by exploiting its statistics in realistic outdoor channel conditions for the UAV. In this study, the path loss exponent is found with curve fitting and fading statistics are estimated using the maximum likelihood (ML) decision rule. Practical measurements showed that the AtG channel is likely to be modeled with Nakagami and Rician distributions. In the second part of this thesis, DSM technique, one of the IM techniques, is considered for both ground-to-ground (GtG) communication with UAV relays and UAV BS included AtG communication from error performance perspective due to the increased throughput advantage of DSM. By considering inherited characteristics of UAVs such as limited power, we proposed a cyclic redundancy check (CRC) aided UAV-relays. In this way erroneous UAV activation, error propagation and futile power consumption are prevented. Furthermore, DSM is generalized by using relay indices and modulated symbols for UAV BS to transmit information. As a continuation study of IM techniques for UAVs, MBM technique is realized by using RAs with mirror activation patterns (MAPs), which depend on the different on-off situations of RF mirrors. By this way the higher capacity gains can be achieved since the channel coefficients received from multiple paths are independent and identically distributed (i.i.d.). Therefore, a novel RA-embedded UAV relay-aided dual-hop communication system is proposed, combining SSI-based MAP selection at the first hop with the MBM technique at the second hop. As only one RF chain is required in this system, RA-embedded UAVs are cost-effective. In addition, SSI-based MAP selection improves spectral efficiency by eliminating a high data rate feedback channel carrying fast channel state information (CSI). For the purpose of simplifying the theoretical analysis and taking into account the standardization parameters, the AtG channel is modeled with a double Nakagami distribution. Theoretical bit error probability (BEP) analysis and asymptotic expressions are obtained and validated with simulation results. Besides the spectrum efficiency and high reliability {\color{Green}concerns}, PLS is also a significant concept for 6G networks. Especially, the intrinsic broadcast nature of UAVs and satellites makes them more susceptible to security threats. In particular, UAV eavesdroppers or UAV jammers have a physical channel advantage because of the high line-of-sight (LOS) probability with ground users. Motivating from this point, security threats are investigated for UAV networks. In the first part of the PLS analysis, NOMA based UAV BS aided terrestrial networks are investigated with secrecy analysis. In the second part of the PLS analysis, passive and active eavesdropper (PE/AE) {\color{Green}UAVs are} considered in space-air-ground-integrated network (SAGIN) that includes full-duplex (FD) UAV relays. Furthermore, the received SNR is increased with SSI-based relay selection, which improves the outage probability (OP). One of the PLS improving methods, FJ, is deployed in SAGIN by selecting from the FD-UAV relays. The proposed system operates in one transmission slot, unlike its half-duplex (HD) counterparts. Transmission secrecy outage probability (TSOP) expressions are derived to comprehensively evaluate the reliability and security performance of the SAGIN. In the third part of the PLS analysis, RIS, {\color{Green}which} favorably adapts the propagation environment by using low-cost reflective elements (REs), is considered for aerial communication in the existence of UAV eavesdropper to enhance security performance. To improve the received SNR both JTRPS and ideal phase shifting at RIS are proposed. Moreover, capacity-based secrecy outage probability (SOP) and TSOP expressions are derived and theoretical results are validated by simulations. In summary, this thesis presents novel UAV-included communication systems for future-generation networks by considering realistic channel models and channel parameters in standardization studies. Through this process, we deployed DSM, relay and pattern selection, MBM, NOMA techniques to UAV-based systems. Moreover, we initially investigated BEP, symbol error probability (SEP) to evaluate the reliability of the UAV systems. To investigate the secrecy performance of UAV systems, SOP, and TSOP expressions are studied with detailed comprehensive analysis.