## 1 Introduction
For example, in [22; 23], rateless codes were used in 5G networks and can meet the requirement of ultra-reliable low-latency communications (uRLLC); in [24], a fountain-code-based forward error correction (FEC) technique was used in wireless sensor networks and can extend the network lifetime while decreasing the energy consumption and frame error rates as compared to the automatic repeat request (ARQ) technique; in [25], a rateless-code-based layer-aligned overlapping structure was proposed to transmit video streaming and was verified for good performance on streaming layered videos. In recent years, cooperative relay communications with rateless coding have attracted more and more attention since they can enhance communication reliability and increase the capacity of networks [26; 27; 28]. The authors in [29] investigated a cooperative communications scheme in multiple-relay networks using fountain codes, which can reduce transmission time and energy consumption. In [30], a dynamic rateless coded cooperation scheme with incremental redundancy was proposed, and the strict carrier synchronization limitation can be relaxed. In [31], a secure transmission strategy by employing rateless codes at the physical layer in a cooperative system was proposed for Industrialist Internet-of-Things (IIoT) applications, which can provide significant benefits relative to fixed-rate codes in terms of achieving secrecy. The authors in [32] developed a fountain-coded cooperative scheme and proposed a rateless-coded-incremental-relay selection algorithm in clustered machine-to-machine networks. The proposed scheme can significantly improve the end-to-end performance than that of non-cooperative direct transmissions. A hybrid-DLT-assisted cooperative relay communication scheme was designed and analyzed in [33], which can significantly reduce transmission latency and energy consumption. In a typical multiple-access relay network, multiple source nodes transmit information to a destination node via a common relay node. It requires that all source nodes are connected to the same relay node. However, when there are obstacles blocking the links between the relay and some of the sources, or when an emergency event destroys the connections of some source-relay links, the channel conditions will deteriorate dramatically, and connection interruptions may even occur. In this situation, traditional distributed networks can no longer provide reliable and efficient information transmission. Recently, in some literature, unmanned aerial vehicles (UAVs) have been employed as mobile relaysto improve the flexibility and reliability of networks [34; 35; 36]. In these UAV-assisted relay networks, the line-of-sight (LoS) path is important for air-to-ground (A2G) communications [37]. However, it is difficult to predict the existence of LoS paths because of the random appearance of obstacles. A feasible approach is to model and simulate the channel [38] in order to obtain the channel characteristics for the adjustment and optimization of the transmission scheme, such as symmetry-based spectrum mapping methods and ray tracing methods [39]. Another promising approach is using rateless codes on erasure channel models for higher application layers since rateless codes can adapt to varying channel conditions. Considering that conventional distributed fountain coding in poor channels requires more coded symbols to be sent, which results in high overhead and unacceptable latency, this paper proposes a novel enhanced distributed fountain coding scheme employing a cooperative relay. The main contributions of this paper are summarized as follows:  
1. A novel multiple-access network with a cooperative relay is proposed. By introducing an enhanced cooperative relay in a distributed network, the coverage of the relays can be extended, and the transmission efficiency and reliability of the network can be improved. Based on this network model, a novel enhanced distributed fountain coding scheme is proposed. 2.