AWR Application Notes

AWR Software for the Design of MIMO and Phased Array Antenna Systems 8 www.cadence.com/go/awr RF Link Modeling AWR software products include the simulation and modeling technology to capture these impairments accurately and incor- porate the results into the AWR VSS phased array assembly model. This is an important functionality, since RF links are not ideal and can cause the array behavior to deviate significantly. The phased array assembly can operate in either the RX or TX mode, supporting the configuration of the array-element geometry, each element's antenna characteristics, the RF link characteristics, and the common linear characteristics of the combiner/splitter used to join the elements together. The configuration is performed primarily through a text data file, with commonly swept settings either specified directly via block parameters (such as steering angles), or specified in the data file but capable of being overridden via block parameters (such as individual element gain and phase adjustments). The configuration of the phased array assembly may be divided into several sections: f Array geometry – Defines the number of elements, their placement, and any geometry-related gain and phase tapers f Antenna characteristics – Defines antenna gain, internal loss, polarization loss, mismatch loss, and radiation patterns for both receive and transmit configurations f RF link characteristics – Defines links for individual elements including gain, noise, and P1dB. Supports two-port RF nonlinear amplifiers using large-signal nonlinear characterization data typically consisting of rows of input power or voltage levels and corresponding output fundamental, harmonic, and/or intermodulation product levels. Frequency-dependent data is also supported f Assignment of antenna and RF link characteristics to individual elements f Power splitter characteristics – Splits the incoming signal into n-connected output ports f Mutual coupling characteristics (previously discussed) One common challenge is that not all RF links should be equal. For example, gain tapers are commonly used in phased arrays; however, when identical RF links are used for all antenna elements, elements with higher gains may operate well into compression while others operate in a purely linear region, causing undesired array performance. To avoid this problem, designers often use different RF link designs for different elements. While this is a more complicated task, AWR VSS phased array modeling enables them to achieve this, resulting in more efficient phased arrays. To assist the design team creating the feed network and providing the RF link to the systems team, AWR VSS software includes the capability to automatically generate the characteristics of the phased array element link defined by the data tables. The designer starts by creating a schematic-based link design per the system requirements. A "measurement" extracts the design characteristics, which can include circuit-level design details (nonlinearities), through AWR Microwave Office co-simulation and saves a properly-formatted data file for use with the phased array assembly model. In-Situ Nonlinear Simulations An accurate simulation must also account for the interactions that occur 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 overall system performance. In this case, the input impedance of each element in the array must be characterized for all beam-steering positions. 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 to which it is attached, which affects the amplifier's performance.

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