Age of Information for Short-Packet Covert Communication with Time Modulated Retrodirective Array

In this paper,the covert age of information(CAoI),which characterizes the timeliness and covertness performance of communication,is first investigated in the short-packet covert communication with time modulated retrodirective array(TMRDA).Specifically,the TMRDA is designed to maximize the antenna gain in the target direction while the side lobe is sufficiently suppressed.On this basis,the covertness constraint and CAoI are derived in closed form.To facilitate the covert transmission design,the transmit power and block-length are jointly optimized to minimize the CAoI,which demonstrates the trade-off between covertness and timelessness.Our results illustrate that there exists an optimal block-length that yields the minimum CAoI,and the presented optimization results can achieve enhanced performance compared with the fixed block-length case.Additionally,we observe that smaller beam pointing error at Bob leads to improvements in CAoI.

Temporal Characteristics of GaAs NEA and Alkali Metal Photocathodes

The temporal characteristics of GaAs NEA and alkali metal photocathodes are studied using Monte Carlo simulation method. The electron transit time and its distribution functions in the photocathodes are calculated. Based on the results, the time modulation transfer functions and temporal resolutions of the photocathodes are obtained. The results show that the response time and temporal resolution of alkali metal photocathode is in femitosecond order and those of GaAs NEA photocathode are in picosecond order.

A Review of the Four Dimension Antenna Arrays

The four dimensional （4D） antenna arrays introduce a fourth dimension, time, into conventional antenna arrays to offer greater flexibility in the design of high performance antenna arrays. This paper presents the tutorial on the study of 4D antenna arrays and the review of the recent research findings on 4D antenna arrays. Issues considered include the theory of 4D antenna arrays, different time modulation schemes, numerical simulation results, and some experimental results on their applications to low sidelobe designs. Throughout the discussion, some challenging issues on the study of 4D antenna arrays arc highlighted.

On the Time Fractional Modulation for Electron Acoustic Shock Waves

Nonlinear features of electron-acoustic shock waves are studied. The Burgers equation is derived and converted to the time fractional Burgers equation by Agrawal＇s method. Using the Adomian decomposition method, the shock wave solutions of the time fractional Burgers equation are constructed. The effect of time fractional parameter on the shock wave properties in auroral plasma ＆ investigated.

Time Modulated Array Antennas: A Review

Time-modulated array(TMA)antennas,introduce the dimension of time into antenna design to control the radiation patterns and frequency spectral characteristics,thus improve the reconfigurability of array antennas and provide multiple functional-ities.They have great application potential in military and civilian fields,such as precision guidance and mobile communication,and are currently a hot spot of academic research.This article provides a review on the fundamentals and applications of TMAs.First,the basic theory and mathematical formulations of TMAs are introduced.Second,the most important applications of TMAs,namely time-modulated phased arrays(TMPA),are discussed from the perspectives of harmonic suppression and harmonic utiliza-tion,which are used for single-beam and multibeam radiation.Then,we survey the combination of TMA with various types of novel antenna arrays,such as single-channel digital beamforming(DBF)arrays,frequency diverse arrays(FDAs),and retrodirective arrays,to create new hardware implementation methods and enhance their performance.Next,recent advances in dedicated integrated chips for TMA,which have played a significant role in driving the progress of TMAs from academic research to practical applications,are presented.Finally,the challenges and prospects for TMAs are discussed,including new research directions and emerging applica-tion scenarios.

Shaped patterns synthesis in time-modulated antenna arrays with static uniform amplitude and phase excitations

Abstract A novel approach for the synthesis of shaped beam patterns in time-modulated antenna arrays(TMAAs)with static uniform amplitude and phase excitations is proposed in this paper.Based on the sideband radiation in TMAAs,shaped beam patterns can be realized by only controlling the switch-on time sequences of the TMAAs.Differential evolution(DE)algorithm is adopted to optimize the time modulation parameters to obtain the desired flat-top and cosecant-squared beams and to suppress the sidelobe levels(SLLs).Simulation results of a time-modulated linear array(TMLA)and a timemodulated semicircular array(TMSA)demonstrate the effectiveness of the proposed approach for the synthesis of shaped beam patterns from TMAAs.