In this paper, we propose a novel polar code construction scheme, suitable for the underwater acoustic (UWA) multiuser communication system using orthogonal frequency division multiplexing (OFDM). We use the polar codes because the code rate of a polar code can be arbitrarily designed. Moreover, the polar codes can also be used to create a nested code structure, divided into several subsets (codebook). We exploit this feature of the polar codes and place the information bits in the codebook to avoid superimposition of all the users. This helps to eliminate the influence of the power allocation coefficient and make full use of the power of the transducer at the transmitted node. Furthermore, the proposed scheme maximizes the utilization efficiency of the OFDM subcarrier resource for each user by avoiding null pilot tones that are usually required for channel equalization in the traditional schemes. The proposed scheme also does not depend on the channel state information (CSI) as prior information as each user can decode their own data independently by using their own codebook without successive interference cancellation (SIC). The simulation and tank experiment results demonstrate that the proposed scheme can significantly guarantee the reliability of all the users, and reduces the complexity of the receiver, thus making it most suitable for the downlink UWA multiuser communication systems. INDEX TERMS Underwater acoustic communication (UAC), orthogonal frequency division multiplexing (OFDM), multiuser detection, polar code construction.

No abstract

Radio-frequency (RF) inductively coupled negative hydrogen ion source (NHIS) has been adopted in China Fusion Engineering Test Reactor (CFETR) to generate negative hydrogen ions. By introduction the level-lumping method into a three-dimensional fluid model, the volume production and transportation of H- in the NHIS, which consists of a cylindrical driver region and a rectangular expansion chamber, are investigated self-consistently at a large input power (40 kW) and different pressures (0.3–2.0 Pa). The results indicate that with the increase of pressure, the H- density at the bottom of the expansion region first increases and then decreases. In addition, the effect of the magnetic filter is also examined. It is interesting to note that a significant increase in the H- density is observed when the magnetic filter is introduced. As the permanent magnets move towards the driver region, the H- density decreases monotonically, and the asymmetry is enhanced. This work could help to understand the H- distribution under various conditions, and thus to optimize the volume production of negative hydrogen ions in the NHIS.

No abstract