Secure Underwater Distributed Antenna Systems: A Multi-Agent Reinforcement Learning Approach

Dear Editor,Underwater distributed antenna systems(DAS) are stationary infrastructures consisting of multiple geographically distributed antenna elements(DAEs) which are interconnected through high-rate backbone networks [1]. Compared to centralized systems, the DAS could provide a larger coverage area and higher throughput for underwater acoustic(UWA) transmissions. In this work, exploiting the low sound speed in water, a multi-agent reinforcement learning(MARL)-based approach is proposed to secure underwater DAS against eavesdropping at the physical layer.

Close-formed bound on generalized distributed antenna system capacity

The composite channel models of the generalized distributed antenna system （GDAS） such as Rayleigh-lognormal fading are studied. Then comparisons are performed between the GDAS and the traditional multiple-input multiple-output （MIMO） system to analyze the ergodic capacity of the GDAS and make conclusions that it is impossible to achieve an analytical expression for the ergodic capacity of the GDAS. Moreover, in order to evaluate the performance of the ergodic capacity of the GDAS conveniently, the analytical lower bound and upper bound of the ergodic capacity of the GDAS are derived by using the results from multivariate statistics and matrix inequalities, under the scenarios of Rayleigh-lognormal fading and equal power allocation scheme at transmitter. Finally, the analytical bounds are verified by comparisons with the numerical results.

Downlink BER performance analysis of distributed antenna systems over shadowed Rayleigh fading channels

Due to the complexity of the composite fading channel, a new simplified channel model is proposed to analyze the bit error ratio（BER） performance of the distributed antenna system （DAS）. First, instead of the gamma-log-normal distribution, the log-normal distribution is applied to describe the output signal to noise ratio（SNR） after maximal ratio combining （MRC） at the receiver. Then, assuming that the channel state information（CSI） is available to the transmitter, by employing the Gauss-Hermite integral, an approximate analytical expression of the BER is derived for the downlink of the DAS with antenna selective transmission and MRC. Finally, the results of a Monte Carlo simulation show that the analytical results match the simulation results. Therefore, it can be concluded that the proposed approximate channel model is effective and accurate, and the derived analytical expression can be used to evaluate the real system performance.

Design and Implementation of An Active Multibeam Antenna System with 64 RF Channels and 256 Antenna Elements for Massive MIMO Application in 5G Wireless Communications

This paper focuses on the design and implementation of an active multibeam antenna system for massive MIMO applications in 5G wireless communications.The highly integrated active multibeam antenna system is designed and implemented at 5.8 GHz with 64 RF Channels and 256 antenna elements.The 64-channel highly integrated active multibeam antenna system provides a verification platform for digital beamforming algorithm and massive MIMO channel estimation for next generation wireless communications.

Discrete-Rate Adaptive Modulation with Variable Threshold for Distributed Antenna System in the Presence of Imperfect CSI

Discrete-rate adaptive modulation （AM） scheme for distributed antenna system （DAS） with imperfect channel state information （CSI） is developed, and the corresponding performance is investigated in composite Rayleigh channel. Subject to target bit error rate （BER） constraint, an improved fixed switching threshold （FST） for the AM scheme is presented by means of tightly-approximate BER expression, and it can avoid the performance loss fxom conventional FST. Based on the imperfect CSI, the variable switching threshold （VST） is derived by utilizing the maximum a posteriori method. This VST includes the improved FST as a special case, and may lower the impact of estimation error on the performance. By the switching thresholds, the spectrum efficiency （SE） and average BER of the system are respectively derived, and resulting closed- form expressions are attained. With these expressions, the system performance can be effectively evaluated. Simulation results show that the derived theoretical SE and BER can match the simulations well. Moreover, the AM with the presented FST has higher SE than that with the conventional one, and the AM with VST can tolerate the large estimation error while maintaining the target BER.

Joint Antenna Selection and Robust Beamforming Design in Multi-cell Distributed Antenna System

Most of studies on Distributed Antenna System(DAS) focus on maximizing the sum capacity and perfect channel state information at transmitter(CSIT).However,CSI is inevitable imperfect in practical wireless networks.Based on the sources of error,there are two models.One assumes error lies in a bounded region,the other assumes random error.Accordingly,we propose two joint antenna selection(AS) and robustbeamforming schemes aiming to minimize the total transmit power at antenna nodes subject to quality of service(QoS) guarantee for all the mobile users(MUs) in multicell DAS.This problem is mathematically intractable.For the bounded error model,we cast it into a semidefinite program(SDP) using semidefinite relaxation(SDR) and S-procedure.For the second,we first design outage constrained robust beamforming and then formulate it as an SDP based on the Bernstein-type inequality,which we generalize it to the multi-cell DAS.Simulation results verify the effectiveness of the proposed methods.

Performance Analysis of WLAN Medium Access Control Protocols in Simulcast Radio-Over-Fiber-Based Distributed Antenna Systems

The performance of three wireless local-area network(WLAN) media access control(MAC) protocols is investigated and compared in the context of simulcast radioover-fiber-based distributed antenna systems(RoF-DASs) where multiple remote antenna units(RAUs) are connected to one access point(AP) with different-length fiber links.The three WLAN MAC protocols under investigation are distributed coordination function(DCF) in basic access mode,DCF in request/clear to send(RTS/CTS) exchange mode,and point coordination function(PCF).In the analysis,the inter-RAU hidden nodes problems and fiber-length difference effect are both taken into account.Results show that adaptive PCF mechanism has better throughput performances than the other two DCF modes,especially when the inserted fiber length is short.

Capacity Analysis on Distributed Antenna System with Imperfect CSI over Rayleigh Fading Channel

Considering that perfect channel state information(CSI)is hard to obtain in practice,the capacity of downlink distributed antennas system(DAS)with imperfect CSI is analyzed over Rayleigh fading channel.Based on the performance analysis,using the probability density function and numerical calculation,an accurate closedform expression of ergodic capacity of downlink DAS under imperfect CSI is derived.It includes the one under perfect CSI as a special case.This theoretical expression can provide good performance evaluation for downlink DAS for both perfect and imperfect CSI due to its accuracy.Simulation results indicate that the theoretical analysis agrees well with the corresponding simulation,and the capacity can be increased effectively by decreasing the estimation error and/or path loss.

Capacity and outage probability analysis of downlink MRT-SC in distributed antenna system

This paper investigates the downlink capacity distribution and the outage probability of the interested area of maximum ratio transmission-selection combining(MRT-SC) scheme in the distributed antenna system(DAS).Composite fading channels are assumed,which include path loss,lognormal shadowing and multi-path Rayleigh fading.Analytical approximations of the capacity's cumulative distribution function(CDF),the outage capacity,the mean capacity,and the outage probability of the interested area are derived by means of moment generation function(MGF) and Gauss-Hermite series expansion based approaches.The influence of antenna number,path loss exponent,and shadowing standard deviation on the capacity distribution are investigated.The simulation results agree with the analytical approximations well,and thus the analytical approximations are able to substitute the time-intensive Monte Carlo simulation for further investigation.

Ergodic Capacity Analysis and Adaptive Transsmion Scheme in Multi-user Distributed Antenna Systems

This paper focuses on analyzing the ergodic capacity performance of limited feedback (LFB) beamforming in multi-user distributed antenna system (DAS). In such a system, multi-user interference (MUI) is inevitably due to the channel uncertainties caused by quantization error. Considering this, we propose a parameter named effective ergodic capacity rate (EECR), which denotes the capacity offset between finite rate feedback and perfect channel state information (CSI). The simulation results show that the derived approximated EECR is very tight to actual EECR. Based on the approximated EECR, an adaptive minimum bit feedback scheme is proposed, which can effectively reduce the overhead of feedback channel and the complexity of the system. The simulation results verify the effectiveness of the proposed scheme.

Optimal antenna placement in distributed antenna systems

To minimize the outage probability of the cell （OPC） in downlink distributed antenna systems with selection transmission, a complex-encoding genetic algorithm （GA） is proposed to find the optimal locations of the antenna elements （AEs）. First, the outage probability at a fixed location in the cell is investigated. Next, an analytical expression of the OPC is derived, which is a function of the AE locations. Then the OPC is used as the objective function of the antenna placement optimization problem, and the complex- encoding GA is used to find the optimal AE locations in the cell. Numerical results show that the optimal AE locations are symmetric about the cell center, and the outage probability contours are also given with the optimal antenna placement. The algorithm has a good convergence and can also be used to determine the number of AEs which should be installed in order to satisfy the certain OPC value. Lastly, verification of the OPC＇s analytical expression is carried out by Monte Carlo simulations. The OPC with optimal AE locations is about 10% lower than the values with completely random located AEs.

RESEARCH ON INDOOR ELECTROMAGNETIC RADIATION FIELD OF MULTIPLE ANTENNA SYSTEM

The complexity of the indoor environment brings great challenges to predict the electromagnetic radiation field of multiple antenna systems. Based on the Finite Difference Time Domain (FDTD) algorithm, using the mobile phone shielding device as the multiple antenna systems example, the mobile phone shielding device's indoor electromagnetic radiation field is researched by measurment method and simulation method. The effectivity of prediction method is verified by comparing the prediciton results with the measurment results. About 80% of the error can be controlled less than dB. The quantitative research has certain guiding significance to the prediction of the multiple antenna systems radio wave propagation.

Asymptotic sum rate of MMSE receivers in multi-user distributed antenna systems

In this paper, the asymptotic sum rate of a multi-user distributed antenna system （DAS） is analyzed. To mitigate inter-user interference, minimum mean squared error （MMSE） receivers are utilized to cooperatively process received signals in the uplink. It shows that inter-user interference is efficiently mitigated and the uplink sum rate of a multi-user DAS is greatly improved by adopting MMSE receivers. For very large number of users and remote antennas, the asymptotic uplink sum rate of MMSE receivers is derived by using virtue of the random matrix theory, which can be The approximation is verified to be quite accurate by Monte Carlo simply calculated in an iterative way simulations.

MI-NLMS adaptive beamforming algorithm for smart antenna system applications

A Matrix Inversion Normalized Least Mean Square (MI-NLMS) adaptive beamforming algorithm was developed for smart antenna application. The MI-NLMS which combined the individual good aspects of Sample Matrix Inversion (SMI) and the Normalized Least Mean Square (NLMS) algorithms is described. Simulation results showed that the less complexity MI-NLMS yields 15 dB improvements in interference suppression and 5 dB gain enhancement over LMS algorithm, converges from the initial iteration and achieves 24% BER improvements at cochannel interference equal to 5. For the case of 4-element uniform linear array antenna, MI-NLMS achieved 76% BER reduction over LMS algorithm.

Energy Efficient Power Allocation Scheme for Distributed Antenna Systems with Target BER Constraint

The energy efficiency(EE) of distributed antenna system with quality of service(Qo S) requirement is investigated over composite Rayleigh fading channel,where the shadow fading,path loss and Rayleigh fading are all considered. Our aim is to maximize the EE which is defined as the ratio of the transmission rate to the total consumed power subject to the maximum transmit power of each remote antenna constraint and Qo S(target BER) requirement. According to the definition of EE and using the upper bound of average EE,the optimized objective function is provided. Based on this,utilizing Karush-KuhnTucker conditions and numerical calculation,a suboptimal energy efficient power allocation(PA) scheme is developed,and the closedform expression of PA coefficients is derived. The scheme may obtain the EE performance close to the existing optimal scheme. Moreover,it has relatively lower complexity than the existing scheme because only the statistic channel information and less iteration are required. Simulation results show the presented scheme is valid and can meet the target BER requirement,and the EE can be increased as target BER requirement decreases.

The Electromagnetic Compatibility and Distribution of Antenna System

The mathematical model of electromagnetic compatibility and the distribution of aircraft antenna system have been investigated. The solutions of the antenna gain and electromagnetic interference margin in the regions of low frequency, resonance and high frequency were discussed. By using the basic analytical method of the EMI margin the distributed antenna system can be determined. The main program flow chart of distributed antenna design were given, and illustrated with examples of the microstrip antennas.

Antenna system characteristics and solar radio burst observations

The Chinese Spectral Radio Heliograph（CSRH） is an advanced aperture synthesis solar radio heliograph, independently developed by National Astronomical Observatories, Chinese Academy of Sciences. It consists of 100 reflector antennas,which are grouped into two antenna arrays（CSRH-I and CSRH-II） for low and high frequency bands respectively. The frequency band of CSRH-I is 0.4–2 GHz and that for CSRH-II is 2–15 GHz. In the antenna and feed system, CSRH uses eleven feeds to receive signals coming from the Sun. The radiation pattern has a lower side lobe and the back lobe of the feed is well illuminated. The characteristics of gain G and antenna noise temperature T affect the quality of solar radio imaging. For CSRH, the measured G is larger than 60 d Bi and T is less than 120 K. After CSRH-I was established, we successfully captured a solar radio burst between 1.2–1.6 GHz on 2010 November12 using this instrument and this event was confirmed through observations with the Solar Broadband Radio Spectrometer at 2.84 GHz and the Geostationary Operational Environmental Satellite. In addition, an image obtained from CSRH-I clearly revealed the profile of the solar radio burst. The other observational work involved the imaging the Fengyun-2E geosynchronous satellite which is assumed to be a point source.Results indicate that the data processing method applied in this study for deleting errors in a noisy image could be used for processing images from other sources.

Green Distributed Antenna Systems for Smart Communities: A Comprehensive Survey

Smart communities are an emerging communication means in which humans and smart devices will interact with each other and deliver ubiquitous services by exploiting social intelligence.Distributed antenna system(DAS),one of the key technologies to realize smart decisions in smart communities,can settle network smart coverage problem and improve system energy/spectrum efficiency significantly.Considering that energy consumption is an important element for community communications,in this paper,we survey the existing green DAS research for smart communities.In particular,our investigation covers antenna distribution,system capacity,spectrum efficiency,energy efficiency,and green access issues.Moreover,we analyze the existing application opportunities and challenges.This survey contributes to better understanding of the challenges and approaches for green DAS in existing smart community networks and further shed novel light on some future research directions.

Outage Analysis of Distributed Antenna Systems over Shadowed Nakagami-m Fading Channels

This paper presents an outage analysis of distributed antennas system(DAS) suffering from shadowed Nakagami-m fading environment where the desired signal also suffers from co-channel interference. The desired signal and interfering signal are subjected to path loss,multipath and shadowing fading. Based on Wilkinson's method,the signal to interference ratio(SIR) probability density function(PDF) of fixed DAS is obtained. Some numerical results of outage probability with different parameters are analyzed. The analysis results can provide sufficient precision for evaluating the outage performance of DAS.

An Optimized Design of Antenna Arrays for the Smart Antenna Systems

In recent years,there has been an increasing demand to improve cellular communication services in several aspects.The aspect that received the most attention is improving the quality of coverage through using smart antennas which consist of array antennas.this paper investigates the main characteristics and design of the three types of array antennas of the base station for better coverage through simulation(MATLAB)which provides field and strength patterns measured in polar and rectangular coordinates for a variety of conditions including broadsides,ordinary End-fire,and increasing directivity End-fire which is typically used in smart antennas.The method of analysis was applied to twenty experiments of process design to each antenna type separately,so sixty results were obtained from the radiation pattern indicating the parameters for each radiation pattern.Moreover,nineteen design experiments were described in this section.It is hoped that the results obtained from this study will help engineers solve coverage problems as well as improve the quality of cellular communication networks.