## Ahmed Kord

PhD Candidate, the University of Texas at Austin

## PhD Thesis

**Magnetless Non-Reciprocity Based on Linear Time-Varying Circuits for Full-Duplex Communication Systems**

The realization of high-performance magnetless circulators is crucial to enable full-duplex communication which would revolutionize the wireless industry for it doubles the capacity of the entire EM spectrum compared to currently deployed TDD and FDD systems. These three-port non-reciprocal components not only can provide the first critical 20-30 dB of the total required self-interference cancellation, thus significantly relaxing the design of the following layers of isolation based on mixed-signal and DSP techniques, but they also permit the use of a single antenna for both the TX and the RX nodes while maintaining an aggregated insertion loss less than 3 dB. Since 2015, I have been leading the efforts in Prof. Andrea Alu's research group at the University of Texas at Austin to develop magnetless non-reciprocal components including circulators based on linear time-varying circuits. Our efforts since then have led to many new discoveries gaining the interest of several companies, funding agencies, and research groups at top-tier universities, and promising to have an imprint on future communication systems. These efforts are summarized as follows:

*Single-Ended STM Circulators*

In this project, we developed two entirely-new magnetless circulator circuits from basic physics to optimal implementations at radio frequencies. These circuits are based on connecting three bandpass or bandstop resonators in either a wye or a delta topology, respectively, and modulating their natural oscillation frequencies with signals having the same frequency and amplitude but their phases increase by 120 deg in a particular direction. Such modulation scheme provides a preferred sense of precession for the rotating modes supported by the resonant network, thus lifting their degeneracy and allowing them to destructively interfere at one port and sum up at the other in a cyclic-rotating fashion, which yields the operation of a circulator. The experimental validation of the bandstop/delta topology resulted in the first watt-level magnetless circulator presented to-date with orders of magnitude improvement in several other metrics compared to previous works.

*Differential STM Circulators*

In this project, we developed new differential STM circulators based on combining two single-ended delta or wye circuits with a constant 180 deg phase difference between their modulation signals in either a voltage- or a current-mode architecture, respectively. Such differential circuits cancel out the IM products at all ports for input excitation at any frequency, thus making them the first-of-their-kind pseudo-linear time-invariant networks which perfectly mimic the operation of magnetic devices. The experimental validation of the voltage-mode architecture resulted in unprecedented performance nearly in all metrics compared to the current state of the art, including large isolation (>20 dB), low insertion loss (<2 dB), excellent matching (>20 dB), small IM products (<-30 dBc), high P1dB (>+28 dBm), high IX20dB (>+28 dBm), low noise figure (<2.7 dB), all over a sufficiently large bandwidth (>30 MHz) and a relatively small form factor (2x11x13 mm2), while using a small modulation frequency of only 10% at a center frequency of 1 GHz.

*Broadband STM Circulators*

In this project, we derived theoretical bounds on the bandwidth of STM circulators. We also proposed a technique to broaden the bandwidth of such components by combining a narrowband non-reciprocal junction with three identical bandpass filters, one at each port of the junction. The experimental validation of this technique resulted in a 20 dB IX BW of 14%, which is about 3.5 times larger than our previous results.

*CMOS STM Circulators*

The single-ended, differential, and broadband implementations of STM circulators presented above, despite achieving remarkable performance in all relevant metrics, were still based on PCB technology and commercial off-the-shelf discrete components, which limits their size and cost reduction and prohibits their large scale production. To overcome this problem, we developed in this project the first integrated-circuit implementation of STM circulators using a standard 180 nm CMOS process. The active silicon area of the fabricated chip was less than 0.2 mm2, the smallest among all magnetless circulators presented to-date, and it was mounted in a 5x5 mm2 QFN package and connected to six off-chip inductors on a PCB for testing. The total form factor of the circulator (packaged chip+inductors) was 6x6 mm2 which is at least one order of magnitude smaller than our previous PCB prototypes, and as such represents a crucial step in our quest to commercialize this technology.

*MEMS STM Circulators*

The CMOS STM circulator presented above, despite reducing the form factor and the cost of such components significantly compared to discrete implementations, was still limited in its miniaturization by the fact that it requires six high-*Q* (>50) off-chip inductors. To overcome this problem, we developed in this project an inductorless MEMS implementation of STM circulators, which replaces the *LC* tanks by compact thin-Film Bulk Acoustic Resonators (FBARs) or Aluminum Nitride Contour Mode Resonators (CMRs) and achieves the modulation through switched capacitors rather than varactors. The super high-*Q* (>1000) of the FBARs or the CMRs also permitted a drastic reduction of the modulation frequency to only 0.24% of the circulator's center frequency (6MHz/2.5GHz) resulting in a power consumption of only 6.8 uW, the lowest among all magnetless circulators reported to-date.

**Super Stealth Technology Based on Parity-Time Symmetric Satellites with Balanced Gain and Loss**

Can cloaking work? Not in our lifetime, I think! Nearly all cloaking devices presented to-date are based on passive metamaterial approaches which suffer from fundamental challenges related to their BW and the size of the object being concealed. This, in turn, casted a lot of doubt on whether cloaking can truly become a technological reality or it will remain a science fiction. A step on the right direction, however, to make invisibility *possibly* work as we envision it in the long term is to adopt an active approach. Towards this goal, we investigated a practical super stealth technology based on parity-time symmetric satellites which were realized using a combination of lossy materials that absorb the impinging signal on one side of the scattering object and active devices that re-emit the signal on the other side with the same phase and amplitude. Rigorous analysis and circuit/EM co-simulations show that such active technique can indeed overcome the limitations of passive metamaterial approaches and maintain a stable 10 dB reduction in the scattering cross-section over a fractional BW as large as 20% of the center frequency.

## M.S. Thesis

**Composite Right-/Left-Handed Metamaterials**

Composite right-/left-handed (CRLH) metamaterials are structures that exhibit simultaneous negative permittivity and permeability over a finite BW resulting in left-handed transmission within this band, in addition to the natural double-positive and right-handed characteristics far from it. These structures can be built using low-loss and high-power waveguides operating near the cutoff frequency of their dominant mode and periodically loaded with stubs having a lower cutoff. In order to optimize the design of the stubs, we developed a general form of asymptotic corrugation boundary conditions (ACBCs) that can be applied to any interface between two materials with arbitrary cross-section for each. This, in turn, permitted a systematic design of several CRLH components with enhanced performance such as backward-to-forward scanning leaky wave antennas and backward wave couplers.

**Computational Electromagnetics**

Numerical codes were developed using FDTD, FEM, and MOM methods to analyze several RF circuits and EM scattering problems. All programs were written in Matlab and were validated by comparing the results mutually and to analytical solutions of standard problems. Further improvements in terms of speed, memory utilization efficiency, and post-processing capabilities were implemented later during my PhD.

## B.S. Thesis

**Low Power Implantable Pressure Sensor IC for Non-Invasive Biomedical Applications**

This was a senior project sponsored by Si-Ware Systems and it involved six students, including myself, in fulfillment of our B.S. degree requirements. The entire chip was designed from system-level to layout using a standard 130 nm CMOS technology. I was responsible for the energy harvesting unit which included a 1.8V Low Drop-Out (LDO) regulator, a 1.2V bandgap reference, an RF rectifier, an off-chip antenna, and a backscattering communication module. I also engaged in regular discussions with the group members on the design of other analog blocks, e.g., Sigma-Delta ADC.