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Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless Software

RF & Wireless Software Addressing 5G and MIMO Design with Circuit/Antenna In-Situ Simulations with NI AWR Software Electromagnetic (EM) simulation software is commonly used to simulate antennas with multiple feeds, including phased arrays, stacked radiators with different polarizations, and single apertures with multiple feed points. These types of antennas are popular for communication systems where multiple-in-multiple-out (MIMO) and polarization diversity antenna configurations are being used. Their use is likely to explode with the rollout of 5G wireless systems over the next several years. The beam of multiple-feed antennas is controlled by changing the phase and amplitude of the signals going into the various feeds. An accurate simulation of such a system must account for the interaction that occurs between the antenna elements and the driving feed network. The problem for simulation software is that the antenna and the driving feed network influence each other. The antenna’s pattern is changed by setting the input power and relative phasing at its various ports. At the same time, the input impedances at the ports change with the antenna pattern. Since input impedance affects the performance of the nonlinear driving circuit, the changing antenna pattern affects the overall system performance. Until now, engineers have been forced to simulate the coupled circuit/antenna effects manually using an iterative process. For example, first the antenna is driven with idealized sources with known phasing at the input ports. The impedance of Figure 1: A 4X4 patch array (left), where each patch is fed by a pin coming up from the bottom ground plane. The right picture shows the mesh of one element, and the driving pin to the ground plane the ports is then used as the load impedance for the driving circuit. The process is then iterated until convergence is reached. This procedure is awkward and time consuming. Fortunately, there is National Instruments www.ni.com/awr Figure 2: Corporate feed network for the patch array. Each element is driven by a MMIC amplifier, and controlled by a phase shifter and attenuator 50 hf-praxis 10/2016

RF & Wireless Figure 3: The picture shows one Wilkinson divider and the transmit module, which contains the phase shifter, attenuator, and a MMIC amplifier a faster, more accurate way to attain the final result. The insitu measurement feature in NI AWR Design Environment, specifically Microwave Office circuit design software, enables communication between the circuit and antenna, thus automatically accounting for the coupling between the circuit and the antenna in an easy-to-use framework. The designer identifies the antenna data source, the circuit schematic driving the antenna, and the measurement under consideration; for example, the power radiated over scan angle. This concept is illustrated in this application note using two phased-array examples in which the antennas are simulated in AXIEM 3D planar and Analyst 3D finite-element method (FEM) EM simulators respectively. Patch Microstrip Array Optimized Using Microwave Office In this example a 4X4 patch array that is driven by a corporate feed network with a phase shifter and attenuator at each element is simulated. A microwave monolithic integrated circuit (MMIC) power amplifier (PA) is placed at each element before its corresponding phase shifter. The array is only simulated once in the EM simulator. The resulting S-parameters are then used by the circuit simulator, which also includes the feed network and amplifiers. As the phase shifters are tuned over their values, the antenna’s beam is steered. At the same time, each amplifier sees the changing impedance at the antenna input it is attached to, which affects the amplifier’s performance. The PAs are nonlinear, designed to operate at their 1 dB compression point (P1dB) for maximum efficiency. They are therefore sensitive to the changing load impedances presented by the array. The combined circuit and EM simulations are necessary for a number of reasons. First, the EM simulation is necessary because the antenna elements interact with each other, which can significantly degrade the antenna’s performance. An extreme example of this is scan blindness, where the interaction between the elements causes no radiation to occur at certain scan angles. The coupling between the elements can also lead to resonances in the feed network. In order to optimize the feed network to account for deficiencies in the antenna, the entire array combined with the entire circuit must be optimized. It is critical to simulate the feed network itself since resonances can build up due to the loading at the antenna ports. Another important point, but often neglected, is that the PA driving the antenna requires a nonlinear circuit simulation. It is therefore important that the antenna’s S-parameters include a DC simulation point and values at the various harmonics used in the harmonic balance simulation. Otherwise it is possible to have unpredicted degradations in system performance due to poor matching at the harmonic frequencies or inaccurately specified DC biasing. Figure 1 shows the 4X4 patch antenna array. Each patch is fed individually by a pin going to the ground below. The port is placed at the bottom of the pin. AXIEM, which is used for the planar EM simulations, has the ability to ground a port with a metal strap, which is used as the pin. This type of simulator is ideal for planar patch arrays that may require a 3D EM simulator depending on the structure details, since the patch is not in a package and radiation effects are therefore included automatically. It should be noted that the simulation techniques described in this paper do not depend on a specific EM simulator, since third-party simulated or measured S-parameter data can be used to represent the antenna response. The corporate feed network is shown in Figure 2. The power is input from the right side. Wilkinson dividers are used to split the signal and feed the 16 patches. Figure 3 shows the feed for a typical patch. The transmit module and Wilkinson divider are shown in detail on the right side of Figure 3 and the inside of the transmit module on the left side. Each transmit module has a phase shifter, attenuator, and MMIC amplifier chip. The beam is steered by setting the phase and attenuation going into the MMIC amplifier and then sending the resulting signal to the patch. The phase and attenuation are controlled by variables in the software, which can be tuned and optimized as desired. In this manner, the beam can be scanned. Figure 4 shows the 3D view of the MMIC amplifier. It is a twostage, 8-FET amplifier designed to work at X-band. In this example, the feed network is simulated entirely in the circuit simulator. A more realis- Figure 4: 3D layout view of the designed MMIC amplifier hf-praxis 10/2016 51

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