### Our Contributions  
Many excellent tutorials [8; 9] and surveys [6; 10] have been published in physical layer key generation, which provide comprehensive overview and insightful comments to understand the fundamental principles, procedures, applications and security issues in this field. The focused issues and the main contents of some published surveys and tutorials are highlighted in Table 1.  
In contrast to existing surveys and and tutorials, our work aims to clarify the feasibility and scalability of CRKG to support new air interface technologies, e.g., full duplex, massive MIMO, mmWave, and heterogeneous IoT devices in the 5G and beyond. These new air interface technologies make it more convenient to extract security elements from wireless resources. Thereby, this paper brings CRKG closer to practical dissemination and products applications, as illustrated in Figure 2. The main contributions of the article are summarized as follows:  
* **CRKG with Various Duplex Modes**: We are the first to investigate the fundamental performance and challenges of key generation with various relevant duplex modes, including Time Division Duplex (TDD), Frequency Division Duplex (FDD) and In-band Full-Duplex (IBFD). In TDD systems, the key generation rate is limited by the velocity of users. Under FDD operation, due to the summation of different propagation paths, the channel responses are generally not similar. To cope with this problem, we provide a solution by separating different propagation paths. Although IBFD enables wireless users to transmit and receive simultaneously over the same frequency band, it brings new challenges due to the large self-interference. Finally, we compare the secret key rates of these three duplex modes under the influence of the velocity, frequency interval and residual self-interference. \begin{table}
\begin{tabular}{p{85.4 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Surveys/Tutorials** & **Publications** & **Focused Issues** \\ \hline [PERSON] et al. [8] & IEEE Wireless Communications & Fundamental and general framework of CRKG, protocols and performance comparison of received signal strength-based and channel phase-based CRKG. \\ \hline [PERSON] et al. [9] & IEEE Communication Magazine & Security issues of CRKG under practical passive and active attacks and countermeasures. \\ \hline [PERSON] et al. [10] & IEEE Access & Comprehensive overview on the principles, performance metrics, procedures and applications of CRKG. \\ \hline [PERSON] et al. [6] & Entropy & Physical layer key generation and physical layer encryption in the IoT. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Brief summaries on existing surveys/tutorials. * **CRKG With Massive MIMO and mmWave**: We provide a new study on how to increase the secret key rate of CRKG by leveraging the advantages of massive MIMO and mmWave. The impact of number of antennas on secret key rates is analyzed and demonstrated through simulations. In addition, we point out some essential problems, including the channel state information (CSI) acquisition and pilot contamination attacks in massive MIMO and severe path loss and penetration loss in mmWave communications. We also provide potential solutions by employing some specific channel characteristics, e.g., sparsity in massive MIMO and non-stationary in mmWave. * **CRKG Prototypes in the IoT**: We review some existing practical explorations and verify the scalability of CRKG among heterogeneous IoT devices. IoT is expected to play an essential role within future 5G and beyond systems. We select four typical wireless communication technologies that are common in IoT, including ZigBee, ultra-wideband and WiFi for short-range applications and LoRa for long-range applications. Furthermore, we discuss several open research challenges and provide some new research directions of CRKG in the future IoT networks. The rest of the article is organized as follows. The performance and challenges of key generation with three typical duplex modes are discussed in Section 2.