**2. Background and Related Work**

In the past few years, blockchain technology has been applied to cryptocurrencies. In the blockchain concept, data are exchanged from peer to peer in a distributed and decentralised manner [33]. In principle, all blockchain technologies employ the concept of a distributed ledger, in which the data are stored on a decentralised mechanism, with a cryptographic key being distributed across the network to ensure that each transaction

matches its corresponding entity [34]. Data should be checked and validated before entering the ledger, which is why a consensus protocol is required. Although the scalability trilemma affects the development of blockchain technologies, this trilemma has no formal definition in the literature, but it has been reported in numerous studies, such as in [35,36]. Its definition was coined by Vitalik Buterin, the developer of Ethereum, a blockchain based on PoW that specifies the three characteristics a blockchain must possess if it is to expand globally: decentralisation, security and scalability. As shown in Figure 1, the scalability trilemma is symbolised by a triangle whose vertices represent three properties. The blockchain with optimal scalability is at the centre of the figure, which is not currently applicable. Brief descriptions of the three properties are presented below.

**Figure 1.** Representation of the blockchain scalability trilemma; according to the trilemma, a blockchain can be on one side of the triangle and not in the center, which represents the best.
