# A Robust Sharding-Enabled Blockchain with Efficient Hashgraph Mechanism for MANETs
## 2 Related Works
### MANET-Based Blockchain  
The consensus mechanism of a MANET-based blockchain needs to satisfy the highly dynamic nature of nodes. According to the structure of the ledger, the MANET-based blockchains are classified into the chain and the DAG.  
Some works utilize the chain structure for MANET-based blockchains. In order to incentivize mobile devices to share their computational resources, [PERSON] et al. [11] introduced a blockchain-based credit system by using Iroha, which is a project of Hyperledger for lightweight mobile applications. It also realizes reliable data analysis against malicious devices. [PERSON] et al. [12] proposed a stability-aware PoW consensus protocol, where the node with high stability is first selected as a miner. The stability of a node indicates its mobility in the network, which is calculated by the degree of the node in certain time slots. [PERSON] et al. [10] introduced a blockchain system for security data collection in MANETs. In order to avoid forks incurred by network latency, after receiving the first valid block, a node stops mining and waits a time window for more valid blocks. When the time window expires, the winner is selected according to the specified rule, i.e., block generation time and the number of receipts within the block. [PERSON] et al. [15] optimized the block propagation in wireless blockchain networks by the partial engagement of nodes and optimal transmission routing based on a distributed Steiner tree. However, since the delay of transaction collection and block propagation is relatively high in MANETs [35], the geographical location of the miner and its connectivity in the network largely affect the blockchain performance.  
Some works utilize the DAG structure for fast MANET-based blockchains. [PERSON] et al. [36] integrated a blockgraph with RAFT to cope with network partition problems. The structure of the blockgraph contains multiple branches, and each branch corresponds to a chain of blocks generated by a network partition. When the network topology of a split, a merge or a mix occurs, the involved nodes process reconfiguration, leader elections and chain updates via the RAFT in different partitions. [PERSON] et al. [13] presented a hashgraph-based solution for common knowledge formation in decentralized swarm robots. Swarm robots within a group reach consensus via hashgraph, and the ranger robot with powerful capabilities in communication and mobility establishes the connection among groups. [PERSON] et al. [37] used hashgraph to support the dynamic changing of consensus subjects and proposed the TEE (Trusted Execution Environment)-based'single-use of self-parent' mechanism to prevent fork attacks in hashgraph consensus. However, the above blockchains are vulnerable to targeted attacks. For example, a partitioned network [36], a ranger robot [13] or the committee of consensus subjects dynamically joining or exiting [37] becomes the target.  
Our work is improved from that of [PERSON] et al. [23] and [PERSON] et al. [24], which alleviate targeted attacks incurred by nodes dynamically joining and leaving a shard. Howevaer, they are faced with the centralized problems of one miner synchronizing blocks to other nodes and a third party managing the reputation. Also, their infrastructure design cannot be adopted in MANETs.