Full duplex communication has gained tremendous interest over the past few years. This is because, it can theoretically double the maximum achievable data rate in a given bandwidth. This is achieved by allowing the simultaneous transmission and reception over the same channel. Such an increase in the data rate, or spectrum efficiency in general, has the potential to be used with future cellular network in order to support applications with heavy data traffic demands, such as autonomous vehicles, virtual reality, and the internet-of-things. A bottleneck challenge facing full-duplex communication is the problem of self interference, which is caused by the leakage of the high power transmitted signal into the receiver. The self-interference can be stronger than the desired signal by 90 dB or more causing the receiver to de-sensitize. To suppress the self-interference, a circulator is needed at the antenna-air interface to provide a 20-25 dB of isolation between the transmitter and the receiver.
Circulators are non-reciprocal devices that allow only one direction of wave propagation. Traditionally, they are designed using ferromagnetic materials biased by either a magnet or a current. As a result, they are bulky in size and expensive. Moreover, ferromagnetic materials are non-compatible with IC fabrication processes, therefore it hinders the integration of the circulator with the transceiver RFICs. Consequently, circulator-based full-duplex transceivers are deemed impractical. Recently, space-time modulation of the dielectric permittivity is shown to break the reciprocal signal transmission response. Upon modulating the permittivity of conventional substrates by a sinusoidal signal that traverses the substrate in clockwise/anti-clockwise direction, a preferred sense of rotation is imparted. As a result, at a given frequency only one direction of propagation is allowed. This is analogues to the alignment of magnetic dipole moments of magnetic material upon biasing it by an external magnetic field, which breaks the bi-directional wave propagation.
Although various magnetic-free circulators based on spatial-temporal modulation are demonstrated in literature showing excellent performance in terms of low insertion loss, good impedance matching, deep isolation of more than 50 dB, and high linearity and power-handling, the 20-dB isolation bandwidth is still limited to 5-7%. This limitation is primary due to the use of first order resonant circuits, which are intrinsically narrowband.
In the first part of this dissertation, this problem is tackled through the use of higher order coupled resonators. Moreover, a generic methodology is proposed to synthesize a network to satisfy a desired 20-dB isolation bandwidth. This is achieved by utilizing microwave filter design theory and techniques. For demonstration, a magnetic-less circulator with a wideband 20-dB isolation fractional bandwidth is designed and fabricated at 500 MHz. The architecture of the proposed circulator is composed of a second order coupled resonators connected in a Y-topology. The fabricated prototype shows a 20-dB isolation fractional bandwidth of 15.4%, insertion loss of 4 dB, and is well-matched across the circulator passband.
In the second part of this dissertation, the space-time modulation is incorporated into the co-design of a magnetic-free isolating and filtering 3-dB microwave power-divider. The proposed circuit is composed of two branches each containing three coupled resonators. The circuit is analyzed using the coupling matrix for proper adjustment of the filter electrical parameters to allow the power-splitting operation. Moreover, the resonant frequency of each resonator is modulated spatially and temporally to break the reciprocal response, i.e. it allows only forward transmission. As a proof-of-concept, the resonators are implemented using folded λ/2 varactor-loaded distributed resonators at a center frequency of 900 MHz with a 3-dB fractional bandwidth of 5%. The measured results show an insertion loss of 3.37 dB, a port-to-port isolation better than 20 dB across the band, a 20-dB reverse transmission isolation bandwidth of 45 MHz, and an impedance match at all ports better than 10 dB across the operating bandwidth.
