# Improving Transaction Speed and Scalability of Blockchain Systems via Parallel Proof of Work +
## 2 Related Work  
The process of coordinator selection is extremely useful for improving the performance of a distributed system. In this approach, which was first implemented by [PERSON] [13], a consensus protocol is proposed with a coordinator election for a partially synchronous processor [14]. The coordinator divides and distributes the portion of work to peers in a network, where the final decision is taken by using a consensus protocol.  
A similar type of work for leader election in the Bitcoin platform was conducted in Bitcoin-NG [15]. This accomplishes an execution change by decoupling Bitcoin's blockchain task into two planes: leader selection and exchange serialization. It also partitions time into the period, where every period has a solitary leader.  
In Bitcoin-NG, there are two types of blocks: the key block and the microblock. The key block contains the leader information as well as information about the previous block. The microblock contains the transaction information. Thus, to generate the key block, a proof of work needs to be performed. Once elected, a leader is able to issue microblocks using his/her private key which contains the transaction information. The amount of microblock issued to the leader is dependent on signing speed and delay network propagation. The microblocks have no proof of work; therefore do not affect the chain weight.  
A framework for parallel mining has been proposed by [PERSON], [PERSON], and [PERSON] [16]. Here, each transaction is connected to at least two other verified transactions and miners verify all new transactions in parallel. The network is graph-structured rather than linear structured, similar to Bitcoin and to the Tangle network used by IOTA [17].  
Proof of stake, which is an alternative to proof of work, is used to create a new block in the blockchain network [18]. In proof of stake, a validator is chosen for each block based on the amount and duration of the stake. The validator is responsible for validating a block in this system, in which there is no need to solve a puzzle. Moreover, the validator cannot mine any currency, but instead receives only a transaction fee as a reward.  
In the Bitcoin pool framework (mining pools), many miners work together in parallel within a pool and use their hash energy to identify a solution for the block. The result is that a considerable amount of hash power is used to solve the mathematical puzzle which is found by a combination of all the miners' computational energy within the pool. This platform increases the possibility of solving the hash problem. If a block is solved, the block reward is distributed to all the miners who contributed to creating that block. Block awards are provided to the miners depending on their effort to create the block. Several methods, such as Shared Maximum Pay Per Share (SMPPS), Capped Pay Per Share with Recent Backpay (CPPSRB), and Equalized Shared Maximum Pay Per Share (ESMPPS), exist for distributing rewards [19]. However, the process of solving the proof of work, mining and rewards are different in mining pools.  
To summarize, Bitcoin miners work separately to process transactions and create the next block in the network. As a result, for every block, the efforts of all miners except the successful miner, become useless and hence the need for massive amounts of energy. The existing mechanism where miners and/or validators cooperate with each other (Practical Byzantine Fault Tolerance or Pool Mining) to create a block brings centralization to the network. The proposed parallel Proof of Work motivates the miners to solve the puzzle by distributing the amount of work. Also, it maintains the decentralization and anonymity in the network, and along the way reduces the amount of energy required.