**1. Introduction**

Thanks to rapid advancements in artificial intelligence (AI), big data, information fusion, and Internet of Things (IoT) technologies, it has become realistic for the concept of smart cities to provide seamless, intelligent, and safe services for communities [1,2]. As a class of robotic vehicles in the IoT, unmanned aerial vehicles (UAV), commonly known as drones, are widely adopted in smart city scenarios for sensing data, carrying payloads, and performing specific missions guided either by remote control centers or in autonomous ways [3]. Thanks to fifth-generation (5G) communication networks and mobile edge computing (MEC) technology, UAVs demonstrate higher mobility than other robotic vehicles, and they can provide on-the-fly communication capabilities in a remote area where terrestrial infrastructure is under-developed or disaster-struck areas where physical or technology has infrastructure been destroyed [4]. Moreover, drones equipped with different types of sensors, such as environmental sensors or cameras, can form UAV networks to guarantee better quality-of-service (QoS) or quality-of-experience (QoE) for users who demand a large number of network-based intelligent services in smart cities, such as video surveillance [5], disaster management, smart transportation, medical suppliers, and public safety [6,7].

**Citation:** Xu, R.; Wei, S.; Chen, Y.; Chen, G.; Pham, K. LightMAN: A Lightweight Microchained Fabric for Assurance- and Resilience-Oriented Urban Air Mobility Networks. *Drones* **2022**, *6*, 421. https:// doi.org/10.3390/drones6120421

Academic Editors: Ivana Semanjski, Antonio Pratelli, Massimiliano Pieraccini, Silvio Semanjski, Massimiliano Petri and Sidharta Gautama

Received: 31 October 2022 Accepted: 13 December 2022 Published: 16 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

With an ever-increasing presence of UAVs in urban air mobility (UAM) networks, the highly connected internet of drones (IoD) also raises new concerns on performance, security, and privacy. On an architectural level, conventional UAV-enabled applications rely on a centralized framework, which is prone to a single point of failure (SPF). As centralized servers coordinate flying drones and perform decision-making tasks, the entire UAV system may be paralyzed if control centers experience malfunctions or are under attacks such as denial of service (DoS) attacks. In addition, complete centralized frameworks that swarm a large number of distributed drones are prone to performance bottlenecks (PBN). As a result, increasing end-to-end network latency degrades QoS or QoE in real-time applications. Moreover, the dynamicity of UAV networks including resource-constrained drones also meets security and privacy challenges within a distributed network environment. Security threats that can severely affect UAV networks can be categorized as firmware attacks (e.g., false code injection, firmware modification, malware infection, etc.) and network attacks (e.g., spoofing, jamming, command injection, network isolation, etc.) [8]. Owing to encrypted data transmission between drones and unauthorized access to data stored on servers, privacy breaches lead to revealing sensitive information such as location, flying path, or other identity-related data.

Thanks to multiple attractive features, such as decentralization, immutability, transparency, and traceability, blockchain has demonstrated great potential to revolutionize centralized UAV systems. By utilizing a cryptographic consensus mechanism and peer-to-peer (P2P) networking infrastructure for message propagation and data transmission, blockchain allows all participants to maintain a transparent and immutable public distributed ledger. The decentralization provided by blockchain is promising for the mitigation of the impact of SPF and PBN by reducing the overhead of the central server in UAV networks. In addition, encryption algorithms, consensus protocols, and tamper-proof distributed ledgers of blockchain enhance the privacy and security of UAV networks. As a result, blockchain provides a "trust-free" network to guarantee the integrity, accountability, and traceability of UAV data. Furthermore, smart contracts (SC) introduce programmability into a blockchain to support a variety of customized business logic rather than classic P2P cryptocurrency transactions [9]. Therefore, blockchain is promising to enhance governance, regulation, and assurance in UAM networks with the help of decentralized security services, such as identification authentication [10], access control [11], and data validation [12].

The shift from centralized UAV networks to decentralized blockchain-assisted UAV systems improves the efficiency of system operations and ensures security and privacy guarantees. Existing blockchain-based UAV solutions mainly consider blockchain as a trusted network and immutable storage to improve the efficiency of communications [13,14], incentive mechanisms [15], security of access authentication [16,17], and data sharing processes [18,19]. However, directly adopting conventional blockchains to build decentralized UAV networks still meets tremendous challenges in IoD scenarios. The current solutions based on permissionless blockchains (e.g., Bitcoin [20] or Ethereum [21]) demand high computation resources in proof-of-work (PoW) mining processes such that they are not affordable to resource-constrained drones. While using permissionless blockchains such as Hyperledger [22] can achieve low energy consumption and high throughout, they are highly limited in terms of scalability and communication complexity.

To address the aforementioned limitations of integrating blockchain into UAV networks, this paper proposes LightMAN, a lightweight microchained fabric for data assurance and operation resilience-oriented UAM networks. Unlike existing works [6,8,18,19] that rely on computation-intensive PoW blockchains, LightMAN adopts microchain [23], a lightweight-designed blockchain, to achieve efficiency and security guarantees for a small-scale permissioned UAV network. As drone information and flight logs are securely and accurately stored on the immutable distributed ledger of the microchain, participants within a UAM network can verify the authenticity of drones and verify tamper-proof data sent to/from drones without relying on a third-party agency. Compared with blockchainbased UAV networks that either directly save raw data on the distributed ledger [18] or

outsource raw data to a cloud server [19], our LightMAN allows encrypted data to be stored on a distributed data storage (DDS), while the microchain only records references of data as checkpoints. Such a hybrid on-chain and off-chain storage strategy not only improves performance (e.g., latency and throughput) but also ensures privacy preservation for sensitive information in UAM networks.

In brief, the key contributions of this paper are highlighted as follows:


The remainder of the paper is organized as follows: Section 2 provides background knowledge of UAV and blockchain technologies and reviews existing state-of-the-art blockchain-based UAV systems. Section 3 introduces the rationale and system architecture of LightMAN. Section 4 presents the prototype implementation, experimental setup, and performance evaluation. Finally, Section 5 summarizes this paper with a brief discussion on current limitations and future directions.

#### **2. Background and Related Work**

This section describes the fundamentals of the UAV concept, explains blockchain technology, and introduces the state-of-the-art decentralized solutions to secure UAM networks.
