Overview of antenna and transmission channel model modeling methods and system simulation cases

　　For communication or radar systems, antennas play the role of transmitting and receiving electromagnetic waves. The quality of the antenna greatly affects the system's performance. Traditional theory and simulation techniques rarely include antennas in communication/radar systems for overall consideration. Antenna designers focus on indicators such as antenna directionality, efficiency, and volume, rarely considering the coordination between the antenna and the transmission channel, and even less about how antenna characteristics can affect the system.

　　This paper summarizes the differences between antenna models in two different systems and the focus of simulation by studying industry methods for modeling antenna and transmission channel models in communication terminals and radar systems, as well as system simulation cases.

　　1. Antenna model in communication terminals

　　Mobile communication channels mainly have the following characteristics: open variable parameter channels, making them easily affected by various interferences; The geographic environment of receiving points is very complex and diverse, roughly divided into three categories: urban, suburban, and rural; Users have random mobility.

　　Due to the above characteristics of mobile communication channels, compared to free space, electromagnetic wave propagation involves more diffraction and scattering waves in addition to direct waves, along with different types of loss: path propagation loss, slow fading loss, and fast fading loss [1]. To achieve optimal reception in different environments, communication terminal antennas are designed as omnidirectional as possible.

　　1.1 Single Antenna Terminal Antenna Model

　　For communication system terminals, the antenna model is usually defined by its coordinates and gain; The channel model is defined by parameters such as noise, fading, and multipath. For different types of communication systems, antenna and channel models are often combined for comprehensive consideration.

　　The diagram below references the most common antenna and channel usage patterns described in ADS software. The channel (PropGSM) is located between the base station (AntBase) and the mobile antenna (AntMobile). Mobile antenna indicators only include gain, position and height, speed, etc. The default antenna type is omnidirectional, and the main contributors to system performance are gain, multipath effects, and Doppler shift.

　　Figure 1: GSM system antenna and channel model

　　1.2 Antenna Models in MIMO Systems

　　In mobile communications, factors such as multipath fading and Doppler frequency shifts lead to a decline in received signal quality. To improve mobile signal quality, dual-antenna diversity reception technology significantly enhances signal quality at low cost and with low implementation difficulty. Using a diversity antenna is to receive two or more uncorrelated signals, so that the signal with the highest strength can be found during subsequent processing or vector signal synthesis. Therefore, the lower the correlation between antennas, the better. Since the electromagnetic environments in which antennas operate differ, wireless environments must be considered when assessing antenna correlation. The total effect of transmitters and obstacles can be described using the probability density function PDF (PDF), which characterizes the probability distribution characteristics of antennas receiving the strongest signals from different directions.

　　In addition to spatial diversity, polarization diversity also exists. Using the Cross-Polariza Discriminance (XPD) to describe the polarization of space radio waves. The larger the XPD, the greater the polarization component in the phi direction; conversely, the smaller the XPD, the smaller the polarization component in the theta direction.

　　Complex CorrelaTIon is used to describe the similarity average received by two antennas under certain electromagnetized and polarized conditions.

　　Using some commercial software, such as EMPro, it is possible to set PDF and XPD for specific diversity antenna models, considering the dual-antenna diversity reception effect [2].

　　In wireless communication system simulation software, it is possible to model the communication system's antennas and channels by importing 3D pattern maps of transmitting and receiving antennas and their relative positions, combined with typical channel models (such as WINNER), thereby simulating system specifications. The figure below shows the WINNER II channel model in the system simulation software SystemVue, which supports importing multiple antenna radiation patterns for simulation or testing and can set the two-dimensional relative positions of transmitting and receiving antenna arrays.

　　Figure 2. WINNER channel MIMO antenna model setup

　　By importing pure far-field phone radiation patterns and phone navigation maps considering SAM human head models, two channel models are created, allowing comparison of system capacity between ideal and actual working scenarios [3]. In this way, the true antenna pattern and antenna layout can be integrated into the channel model, allowing the antenna performance to affect system metrics.

　　Antenna and channel models can not only be applied in simulation software, but also serve as necessary test conditions to participate in standard testing. A typical case is Keysight's Two-Step Radiation Method (RTS).

　　The two-step radiation method divides MIMO OTA testing into two steps: the first stage involves measuring the terminal radiation pattern in a dark chamber and using the terminal's reporting function to obtain the radiation pattern of the DUT; In the second stage, the radiation pattern information measured in the first stage is loaded into the channel simulator, simulating a wireless channel that includes the antenna characteristics of the object under test. The downlink signal output from the base station simulator first loads the direction map information of the DUT

　　Figure 3: Diagram of the two-step radiation test method

　　The wireless channel is convoluted and transmitted by the measuring antenna to conduct performance tests of the receiver.

　　The consistency between the two-step radiation method and the multi-probe method (MPAC) measurements, which has become the CTIA MIMO OTA measurement standard, has been recognized by 3GPP. A formal conclusion was approved at the 3GPP RAN4 meeting concluded in May 2017[4].

　　2, antenna models in radar systems

　　Unlike omnidirectional antennas on mobile terminals, radar systems generally have antenna beamwidths ranging from several to over ten degrees. Radar systems operate in both search and tracking modes, requiring precise modeling of beam direction [5].

　　Traditional simulation systems mainly focus on simulating the signal flow level of the radar system, considering the signal transmission path and signal processing results, without considering the influence of antenna pattern patterns and directivity on the radar system. For example, in VSS, considering the distance and speed of the target, the transceiver antenna is simplified to a gain model, which only affects the signal level received by the receiver. Under this system simulation architecture, only some parameters of the antenna (such as reflection coefficient, impedance, etc.) can be associated with the cascaded RF system.

　　For complex application scenarios, it is necessary to consider the position information of the dynamic platform (such as ships, aircraft, or combat vehicles) and antennas. System simulation software SystemVue offers a hierarchical design solution, which, in addition to signal analysis, can also incorporate the position of the moving platform (such as the geocentric inertial coordinate system), velocity information, and antenna position information of the phased array radar system for analysis. This platform can configure multi-target and multi-station radars, as well as multi-antenna configuration.

　　Figure 4. Schematic of three-layer simulation setup for radar system

　　In the signal layer, set the antenna's operating mode (search or tracking), antenna radiation pattern, and other basic indicators; In the antenna layer, the radar target position is set, as well as the tilt angle, pitch angle, and yaw angle of the radar platform. The antenna's tilt, pitch, and yaw angles within the radar platform are set; In the trajectory layer, information such as the position (longitude, dimension, altitude), direction, speed, acceleration, and movement trajectory of the radar transceiver platform and target is set separately. By converting under different coordinate systems, the antenna's radiation pattern, radar platform, and target movement trajectory information are comprehensively considered.

　　The example of EW receiver testing can be used to illustrate the complex application scenarios of radar. In the scene, the EW receiver (EW Rx) is used to monitor four radar stations in space. The task of the EW receiver is to detect all these signals, identify each signal, and organize the position, velocity, time waveform, and frequency content of each radar station.

　　Figure 5: EW receiver test scenario

　　To test an EW receiver, a test signal must be generated, which does not simply mean overlaying multiple time waveforms. Since EW receivers may be installed on airplanes, cars, or warships, the tools used to generate these test signals must allow users to specify the location, speed, movement trajectory, and other details of the EW Rx station. Additionally, for each radar station, the tool must allow users to specify its position, speed, time waveform, frequency, antenna operating mode, and so on [6].

　　If precise environmental modeling is required, signal layer simulation software like Simulink and SystemVue cannot handle this. Using professional scenario simulation software such as STK to model target postures and environments can achieve more realistic target characteristics.

　　As shown in Figure 6, SystemVue is used to generate a linear FM pulse signal source, and factors such as nonlinearity and noise from RF devices are added through the RF transmission path. The time-domain signal enters the STK software via interface. STK pre-defines outdoor terrain and landforms, as well as indicators such as aircraft movement trajectories and flight attitudes. The radar is in tracking mode, aiming to illuminate the aircraft with beams as much as possible, while the plane performs various maneuvers to evade radar detection. The time-domain signals for the entire scene are returned to SystemVue software, where the radar detection probability is obtained through post-processing programs.

　　Figure 6: Schematic diagram of STK joint simulation

　　3, Conclusion

　　It is evident that in communication or radar systems, antennas no longer appear in isolation but are closely integrated with communication channels and radar usage scenarios, working together. If antenna designers and communication/radar system designers can leverage existing commercial software and mature theories to integrate antenna characteristics into system design, they can significantly reduce the risk of joint debugging and speed up product design.