With the recent advances in unmanned aerial vehicles (UAVs), the development of energy-efficient networking technology for mission-oriented multiple cooperative UAVs has become crucial. Routing in flying ad-hoc networks (FANETs) with UAVs is a challenging issue because of the high speed and sudden changes in direction of UAVs. Traditional routing protocols in FANETs periodically send hello messages for the establishment and maintenance of the routes. However, sending hello messages periodically after a fixed interval increases bandwidth wastage when the hello interval is excessively short or causes long delays in neighbour discovery when the hello interval is overly long. Moreover, several disconnected UAV groups have been observed in which the group members are connected among themselves but detached from the main network. By exchanging excessive hello messages inside the group, the UAVs maintain an unnecessary neighbourhood, causing wastage of energy. However, FANETs have certain advantages, such as knowledge about mission-related information. To solve the problem of unnecessary energy drain, we propose a novel adaptive hello interval scheme-energy efficient hello (EE-Hello)-based on available mission-related information, such as the volume of the allowed airspace, number of UAVs, UAV transmission range, and UAV speed. We present a method to decide the distance that a UAV needs to travel before sending a hello message. We also specify a technique to determine the number of UAVs necessary to achieve specific network requirements, such as packet delivery ratio or throughput, with the expenditure of minimum energy. We show that the proposed EE-Hello can save about 25% of the energy currently used, by suppressing unnecessary hello messages without degrading the overall network throughput. INDEX TERMS FANETs, UAV networks, green UAV networks, energy efficient routing, adaptive hello interval. I. INTRODUCTION Recently, Unmanned Aerial Vehicles (UAVs) have become very popular because of their wide range of applications for which they can be used [1], [2]. In particular, their capability to work as a group with minimum human intervention has led to a productive area of research. However, energy efficiency is a major concern in today's UAVs [3]. Generally, small UAVs can fly for a maximum of 30 minutes depending upon available energy. Therefore, research has focused on producing energy-efficient green UAVs that can fly for longer. In addition, in a multi-UAV system, UAVs need to maintain communication links between themselves in order to accomplish their mission cooperatively. However, owing to the rapid The associate editor coordinating the review of this manuscript and approving it for publication was Onur Alparslan.

In late 2016, Google proposed the Bottleneck Bandwidth and Round-trip propagation time (BBR) congestion control algorithm to achieve high bandwidth and low latency for Internet traffic. Unlike loss-based congestion control algorithms, BBR works without filling the bottleneck buffer. Consequently, BBR can reduce packet loss and minimize end-to-end packet delay, which has attracted the attention of many researchers in recent years. However, some studies have reported the creation of persistent queues that cause unintended problems, resulting in a serious fairness issue between TCP BBR flows with different round-trip times (RTTs). Although existing congestion control algorithms also exhibit fairness issue between different RTT flows, BBR has a more serious problem in that the imbalance is considerable even with small RTT difference between the two flows, and the long RTT flow uses most of the bandwidth. The preponderance of long RTT flows is a serious problem because a particular user may cause imbalance by maliciously increasing the delay. Therefore, we propose a Delay-Aware BBR (DA-BBR) congestion control algorithm to mitigate the RTT fairness issue between BBR flows. In a network emulation experiment using the Mininet, the DA-BBR increased the fairness index by 1.6 times that of the original BBR, and the retransmission was greatly reduced. DA-BBR flow with short RTT demonstrated fair throughput even in competition with DA-BBR flows where RTT is five times higher.INDEX TERMS BBR, congestion control, fairness, round-trip time, TCP.

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The recent rapid proliferation of mobile devices has significantly increased the importance of network mobility (NEMO) technology for Internet access, which enables all the nodes within a mobile network to provide session continuity when the mobile network moves. This paper proposes an efficient network mobility support scheme with direct home network prefix (HNP) assignment to reduce the location update cost and packet tunneling cost. In the proposed scheme, the moving mobile access gateway (mMAG), instead of the local mobility anchor (LMA), assigns a mobile node (MN)'s HNP using a sub-prefix of its own HNP when an MN is attached to the mMAG. An IP address swapping mechanism is used to deliver the packets destined for MNs moved to the mMAG without an additional tunnel header. In the proposed scheme, the mMAG also performs a location update on behalf of the MNs. Therefore, the proposed scheme can reduce both the location update cost and the packet tunneling cost. Numerical analysis and simulation results show that the proposed scheme outperforms the existing schemes in terms of the total cost, throughput, and signaling overhead.