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5G Primer for MIMO/Phased-Array Antennas

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5G and MIMO Design With Circuit/Antenna Co-Simulation 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 MIMO and polarization diversity antenna configurations are being rolled out. 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 with 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 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 a faster, more accurate way to attain the final result. The in-situ measurement feature in AWR Microwave Office software enables the 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 section using two phased- array examples in which the antennas are simulated in AWR AXIEM 3D planar and Analyst 3D FEM EM simulators. Patch Microstrip Array Optimized Using AWR Microwave Office Software 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. An MMIC 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 perfor- mance. The PAs are nonlinear, designed to operate at their 1dB 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 often neglected but important point 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. 5G Primer for MIMO/Phased-Array Antennas 19

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