UC Davis

Wireless communication systems have been growing at a very fast rate over the last few decades. This growth has been fueled by development of cellular technologies such as 4G LTE/5G, and machine to machine communication. Growth is expected to continue especially in the machine to machine sector with the rise of Internet of Thing (IoT) in consumer electronics. Even though the number of connected devices has grown, the Radio Frequency Spectrum has not. This has resulted in a massive overcrowding of the radio frequency spectrum. Overcrowding can create massive interference in communication links and put receivers into saturation. High levels of harmonics and intermodulation distortion are generated in saturation. In this state receivers performance degrades significantly. Some of this interference is unintentional, and some could be caused by nefarious parties. Much research has been done in order to mitigate the problems caused by interference. Transceivers that utilize interference mitigation circuits tend to be more robust in heavily congested radio frequency spectrum. In this work three different approaches are presented in dealing with in-band and out of band types of interference. The first work reports a novel design of RF signal processing module for direct sequence spread-spectrum (DSSS) communications to effectively suppress in-band radio interference. RF signal processing can flexibly achieve DSSS spreading gain without having to modify the conventional narrowband radios. Our new design leverages low power GaN switches to spread and despread RF signals efficiently while rejecting narrowband interference at the receiver. GaN devices enable the circuitry to have high 1dB compression point of 38dBm and third-order intercept point over 48dBm. The receiver correlator also spreads incoming interference, so the system is able to perform in the presence of strong blockers. The demonstrated module is shown to suppress narrow in-band interference up to 27dB. A unique design feature of this RF signal processing module is the use of an innovative non-coherent chip synchronization scheme for DSSS at RF that traditionally requires baseband signal processing. The proposed design can be used in full or half duplex system. In full duplex there is additional benefit of transmitter to receiver leakage suppression that the proposed system can achieve. Interference that fall outside the band of interest is typically taken care of by filters. Filtering can be done in digital and analog domain. Conventional analog filter have a fixed center frequency and bandwidth which can't be tuned. Modern communication system need filters that are reconfigurable in frequency and bandwidth. This work reports a tunable, high power N=4 N-Path filter. Majority of published works utilize a standard CMOS process for implantation which has power handling limitation. The N-Path filter in this work takes a heterogeneous approach to architecture design. The N-path core was designed in a GaN process, which allows for high power handling. The four phase none-overlapping clock is generated in a CMOS process. An intermediate GaN Level shifter for driving the GaN switched in N-Path core is also shown. The work also demonstrated the PCB integration methodology of the heterogeneous filter. The measurement show that the filter can be tuned from 1-1.3GHz in center frequency which simulation showing that the architecture should be able be tuned from .5GHz to 1.5GHz. Also it was demonstrated that the 3dB bandwidth is tunable from 5MHz to 20MHz. Simulation show that this heterogeneous architecture has a 1dB compression point of 18.8dBm. A full duplex system which can transmit and receive in the same frequency and time space has been of great interest since the advent of wireless communication. However, the full duplex architecture adoption is limited due to the self interference problem in where a transmitter is a strong in-band jammer to it's own receiver. In this work two independent methods for dealing with self interference are demonstrated. The first method uses tap-delays to couple off signal from the transmitter path, invert the phase with some added delay and inject it right before the LNA in the receiver path. This method was designed to have four taps. Each tap has individual amplitude and phase control. The system was designed to be modular in order to study the cancellation effects at critical points. The measured results show that an additional 20dB of isolation can be achieved over a 63MHz bandwidth, and over 30dB over 30MHz bandwidth The second method is to use orthogonal coding in time domain to achieve deep self interference cancelling by clever selection of timing offset between the transmitter and receiver code. This type of cancellation only applies to systems that use analog correlators which are shown in the first part of this work. By doing partial timing offset, energy can be redistributed in the frequency spectrum creating bands with smaller energy concentration. There are specific timing offsets between transmitter and receiver codes that will yield a nulling effect in the receive filters pass-band. This will remove more transmitters energy than what the code originally was designed for in digital domain. Also, since the spreading sequence in the transmitter and receiver are using the same clock we can use this to find a good nulling offset, and then move the transmitter and receiver codes in tandem to recover the correct timing offset. This work demonstrated that over 50dB of isolation can be achieved by the use of this method.