Adam Goad

In reconfigurable array transmissions, a phased-array transmitter changes its characteristics, yet must still be able to control its transmission while optimizing its performance. To enable full reconfiguration while transmitting predictably, performing accurate, real-time measurements within the transmit chain is useful. This recently developed in-situ measurement approach, shown in multiple previous contributions, is summarized in this paper. It serves two purposes: (1) informing the real-time optimization algorithm whether changes in transmitter characteristics improve or worsen performance, and (2) updating the array calibration to obtain the desired transmit array pattern. This will enable real-time, “on the fly” optimizations of transmitters to coexist with other wireless devices in an increasingly congested spectral environment. A four-port coupler, with two monitoring outputs, is used to monitor the total voltage and current between a reconfigurable impedance tuner and the antenna in each element of a transmit array chain. Experimental work from the different prior contributions shows the overall trajectory, reliability, and proposed applications of this in-situ measurement technique. Less than 1 mV of error vector magnitude is shown in vector network analyzer methods compared with simulations using the in-situ coupler approach. The integration and calibration of a software-defined radio to perform antenna input current in-situ measurements has been implemented, with an average current error vector magnitude of 258 µA when comparing the software-defined radio measurements with simulations. Simulation results have shown that in-situ measurements can successfully correct input voltage waveforms for accurate directionally modulated transmissions, lessening reliance on fixed transmitter array pre-calibrations.

Electronic design automation software is used to streamline the iterative design process and automate many tedious tasks. Optimizers often modify design parameters to minimize or maximize a user-defined cost function. Simulators are returning to a need to use user-imported code to assist simulation-based design. This paper describes examples of how user code can be integrated into the microwave design process in standard computer-aided design software tools.

Nonlinear amplifier designers benefit from fast, accurate performance assessment and optimization techniques for computer-aided design. We demonstrate the synthesis of missing measurement dataset portions, even when extrapolating data beyond the boundary of the partial dataset’s domain. A novel method is presented to optimize device load impedance in conjunction with a third parameter. We demonstrate that this method can optimize settings for generic devices where an a priori estimate of the data range is unknown. A modified neural network architecture is used for improved data quality, and Bayesian optimization is applied to infer proper neural network normalization parameters for an unknown device. After applying the learned normalization parameters, the extrapolated dataset realizes a predicted optimal reflection coefficient per input power within a vector distance of about 0.1 on the Smith chart, with a mean output power error of less than 0.5 dB. This was accomplished using only 32 measurements—about 0.1% of the full set of 24192 measurements. If an a priori estimate of the normalization parameters is available, similar performance can be achieved in as few as 16 measurements. This is expected to be a useful tool for reducing design time and improving optimization accuracy in the computer-aided design of transistor amplifiers.

Continued spectral crowding can potentially affect the operation of critical passive devices, such as radiometers and radio telescopes. Proliferation of fifth generation (5G) wireless communication systems in the 24–30 GHz band could cause massive interference with satellite-based radiometers that operate in the 23.6–24.0 GHz (from out-of-band spurious emissions) and the 50–58 GHz bands (from spurious harmonic operation of 5G systems). A brokering system is presented to protect crucial passive devices from unwanted interference by coordinating with active systems and limiting both in-band and out-of-band electromagnetic emissions from the active systems. Based on interference criteria presented by the passive systems to the broker, a spatial-spectral mask is created to limit the transmission of the active device in both the spatial and frequency domains.

Spectral efficiency has become increasingly important in the modern congested spectral environment. Directional modulation multiplexing helps by using spatial diversity to transmit different messages in different directions at the same frequency. This approach can be used to provide dual-function radar-communication transmissions that are direction-specific. In these transmissions, the vector current excitations of different array elements are calculated to provide the directional transmission, resulting in different output power levels for the different elements. This approach results in varying levels of compression for the element amplifiers, leading to varying distortion and performance. Mutual coupling between elements can change the antenna reflection coefficients, resulting in distortion of the directionally transmitted symbols and further alter element performance. Real-time reconfigurable matching circuitry, placed between each element amplifier device and antenna, ensures that each amplifier is optimized to maximize gain while minimizing distortion when the frequency, intended directions, or directional transmission symbol combinations are changed. An algorithm is proposed and demonstrated in simulation which uses high-speed plasma-switched impedance tuners, in-situ RF current measurements, and amplifier pre-characterization to quickly optimize array circuitry. These optimizations may be completed in microseconds, allowing reconfigurability commensurate with pulse-to-pulse adaptivity relative to the radar function of the transmission.

In the increasingly congested wireless spectrum, phased-array transmitters can be used to provide spatial diversity as an additional measure of coexistence. Reconfigurable matching networks placed between the power amplifier device and the antenna can allow the amplifiers in the transmitter elements to maximize output power while maintaining the desired beam fidelity. A challenge with the use of reconfigurable matching networks in array transmitters is that they alter the system calibration. This article provides measurement validation of an in situ measurement approach to assess antenna input current using a dual-directional coupler placed in an array element. Measurement validation was performed using a two-port vector network analyzer (VNA) with a load—pull tuner emulating changing antenna impedance. This approach is expected to ease the process of maintaining the pattern and is expected to ease issues stemming from changing array calibration with reconfigurable array element circuits.

As use of the radio frequency spectrum continues to grow, risk of interference to passive RF systems becomes greater. A cooperative spectral broker is proposed to dynamically coordinate frequency allocations amongst devices, offering protection for incumbents without compromising their performance and providing opportunistic access to frequencies outside original assignments and allocations. Policy measures necessary to support systems of this nature are suggested, and a case study for identifying bands and applications that could benefit from such a system and policy is discussed.

Alternating projection onto convex sets (POCS) provides an iterative procedure to find a signal that satisfies two or more convex constraints when the sets intersect. For nonintersecting constraints, the method of simultaneous projections produces a minimum mean square error (MMSE) solution. In certain cases, a minimax solution is more desirable. Generating a minimax solution is possible using dilated POCS. The minimax solution uses morphological dilation of nonintersecting signal convex constraints. The sets are progressively dilated to the point where there is intersection at a minimax solution. Examples are given contrasting the MMSE and minimax solutions in problems of tomographic reconstruction of images. Dilated POCS adds a new imaging modality for image synthesis. Lastly, morphological erosion of signal sets is suggested as a method to shrink the overlap when sets intersect at more than one point.

A significant challenge for dual-beam, codesigned systems, such as for simultaneous radar and communications transmission, has been identified as the spurious beams produced by spatial intermodulation from nonlinearities in the transmitter power amplifier. We demonstrate that tuning the load impedance of the power amplifiers can reverse the elementwise power amplifier nonlinear distortion, reducing the magnitude of the unwanted beam transmissions and restoring the integrity of the dual-beam antenna pattern for application to an active electronically scanned array. A comparison to a recently documented predistortion approach for correcting spurious transmissions is provided, and it is shown that impedance tuning can address both element output power and linearity, whereas predistortion addresses only the linearity issue and causes the amplifier gain in many of the array elements to suffer. For multiple dual-beam angle scenarios, simulation results show that impedance tuning is effective at maximizing beam power in both beam directions, maximizing current gain in the element power amplifiers, and minimizing the size of the unwanted spurious beams.