BlockchainEmpowered Secure Spectrum Sharing for 5G Heterogeneous Networks In the future 5G paradigm, billions of machinetype devices will be deployed to enable widearea and ubiquitous data sensing, collection, and transmission. Considering the traffic characteristics of machinetomachine (M2M) communications and the spectrum shortage dilemma, a costefficient solution is to share the underutilized spectrum allocated to humantohuman (H2H) users with M2M devices in an opportunistic manner. However, the implementation of largescale spectrum sharing in 5G heterogeneous networks confronts many challenges, including lack of incentive mechanism, privacy leakage, security threats, and so on. This motivates us to develop a privacypreserved, incentivecompatible, and spectrumefficient framework based on blockchain, which is implemented in two stages. First, H2H users sign a contract with the base station for spectrum sharing, and receive dedicated payments based on their contributions. Next, the shared spectrum is allocated to M2M devices to maximize the total throughput. We elaborate the operation details of secure spectrum sharing, incentive mechanism design, and efficient spectrum allocation. A case study is presented to demonstrate the security and efficiency of the proposed framework. Finally, we outline several open issues and conclude this article.

Hash Access: Trustworthy GrantFree IoT Access Enabled by Blockchain Radio Access Networks The explosion of Internet of Things (IoT) applications makes the existing network structure insufficient because it is split by multiple noncooperative operators, does not scale with the future development of IoT, and calls for novel networking concepts and protocols. The need for multioperator IoT paradigms poses some unprecedented challenges, including trust considerations. To enable a multioperator network without shared authentication, we propose a trustworthy grantfree IoT access protocol named Hash Access by leveraging a blockchain radio access network (BRAN) architecture. BRAN reorganizes multiple individual radio access networks into a joint multioperator network based on a blockchain. BRAN can establish trust among initially trustless operators and efficiently utilize resources across networks, matching and creating values for multiple participants. Furthermore, to mitigate the loss of network efficiency caused by selfish behavior among untrustworthy IoT devices, the proposed protocol can induce IoT devices to follow an access rule and control their data traffic. The proposed framework unifies the IoT access and the underlying BRAN to reduce confirmation delays in traditional blockchains significantly. Numerical experiments illustrate the performance edge for the BRANenabled Hash Access.