## 2 State-of-the-Art
### Reward Systems with Monetary Value  
Incentive mechanisms, as the driving force for maintaining long-term system operation, are indispensable elements of blockchain systems. The advanced properties of blockchain can also contribute to designing effective and efficient incentive mechanisms. [PERSON] et al. [33] broadly review academic papers related to the incentive mechanisms in blockchain and blockchain-based incentive mechanisms. To systematically evaluate these papers, the authors proposed a set of requirements based on incentive properties and costs. In [34], the authors propose a general decentralized rating framework based on blockchain, supporting recommender systems and rewarding its users for their reviews. The ratings of items, the reputations, the tokens of users, and the algorithms exploited to compute the score of the items are stored on the blockchain. The system directly supports cryptocurrency payments. It also allows item owners to accept off-chain payments by manually invoking a smart contract to confirm payment execution. [PERSON] et al. [35] present a token-locking reward model that can be used to incentivize miners to accept transactions without forcing clients to sacrifice their tokens. The model can also be used to incentivize service providers. The authors' analysis showed that the model reduces volatility commonly associated with cryptocurrencies. Paying interest to token holders lowers the opportunity cost than holding fiat currencies. The authors claim that their model of rewarding service providers with interest generated from locked tokens is more profitable for clients compared to paying for services directly. An interesting aspect of the model is that the locked tokens are temporarily out of circulation. In [36], the authors developed a dynamic model of the platform economy, where tokens are used as a means of payment among users and issued by the platform to finance investment. Tokens facilitate user transactions and compensate distributed ledger-keepers, open-source developers, and crowdfunders for their contributions to platform development. Platform owners maximize their seigniorage by carefully managing the token supply. This management considers that users optimally determine token demand and form rational expectations about token price changes. By focusing on token price volatility, in [37], the authors investigate how reward uncertainty affects user contribution to a tokenized digital platform. The results reveal that, for systems that involve and reward user creativity, token price volatility facilitates the platform's short-term effect but impairs long-term user creativity. In [38], the authors examined whether blockchain can serve as the technology underpinning decentralized marketplaces to promote trust. By utilizing tokens as an incentive mechanism, the authors demonstrated that rewarding peers for reporting malicious behaviors can mitigate misconduct. Despite its simplicity, the token rewarding mechanism can be used to incentivize users to behave consistently and tackle trust issues. In [39], the authors suggest three adjustments to make a currency more stable. Firstly, they propose minimizing the difficulty of mining new blocks to prevent rapid changes in the supply of coins. Secondly, they recommend adjusting mining rewards if block intervals significantly change, thereby stabilizing the coin supply. Lastly, they introduce negative interest concepts to remove old coins from circulation. Mechanisms such as fees gradually reduce the coin's value over time. This approach encourages spending instead of hoarding. The first suggestion only applies to proof-of-work (PoW) blockchains, but the other two have a broader applicability zone and can be used in other solutions as well. For example, in [40], the authors explain that cryptocurrencies are not widely adopted online due to their high volatility and propose the use of stable coins to solve the volatility issue by securing cryptocurrency with a stable asset.