Optimized self-interference cancellation based on optical dual-parallel MZM for co-frequency and co-time full duplex wireless communication under nonlinear distortion and emulated multipath effect.

Co-frequency and co-time full duplex (CCFD) technique has the potential to further improve the capacity of the fifth-generation (5G) system. However, in a CCFD-based transceiver, the self-interference-cancellation (SIC) module is essential because the transmitted signal is also captured by the receiving antenna. In this paper, an optimized SIC technique is proposed based on a dual-parallel Mach-Zehnder modulator (DP-MZM). In addition to the multipath effect between the transmitting and receiving antennas, the nonlinear distortion induced by the used optical and electrical components is considered and analyzed for the first time. In this scheme, one of the children MZMs is biased at 90°. The bias points of the other child MZM and the parent MZM are swept for SIC optimization. Compared to the traditional SIC scheme using DP-MZM, the power of the received signal is increased by 6 dB and the robustness to the bias point drift is improved. In our experiment, over 35 dB cancellation ratio is achieved for 200 MHz filtered orthogonal frequency division multiplexing (F-OFDM) interference signal with carrier frequency from 2.5 GHz to 6 GHz. To mitigate the influence of multipath effect and nonlinear distortion, the recursive least squared (RLS) linear equalizer and the RLS Volterra equalizer are designed and compared. Finally, 42 dB cancellation ratio is achieved for 200 MHz F-OFDM signal with carrier frequency of 4.5 GHz in the presence of nonlinear distortion and emulated multipath effect.

Microwave photonic RF front-end for co-frequency co-time full duplex 5G communication with integrated RF signal self-interference cancellation, optoelectronic oscillator and frequency down-conversion.

A novel RF front-end, which could simultaneously realize wideband RF signal self-interference cancellation (SIC), local oscillator (LO) generator based on optoelectronic oscillator (OEO) and frequency down-conversion has been proposed and experimentally demonstrated. In our microwave photonic RF front-end, only one single-polarization optical in-phase and quadrature-phase (IQ) modulator are required. The upper Mach-Zehnder modulator (MZM) of this optical IQ modulator works as a mixer; the lower MZM works as a reference arm; the parent Mach-Zehnder interferometer (MZI) is used to combine two output optical signals of these two child MZMs. In this way, not only self-interference signal is cancelled in optical domain but also frequency down-conversion is realized at the same time. On the other hand, the upper MZM is also shared to form an OEO by using a self-polarization-stabilization technique. By this means, no external LO signal for frequency down-conversion and electrical attenuator for SIC are needed in our scheme, contributing to compact structure and cost reduction. In our proof-of-concept experiment, a LO signal with central frequency of 10 GHz and phase noise of -108.66 dBc/Hz@10kHz is generated. By optimizing the bias points of the used optical IQ modulator, a 5×20MHz 64-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (64QAM-OFDM) LTE-A signal with central frequency of 12.6 GHz is down-converted to 2.6 GHz, and about 28 dB cancellation ratio is achieved. The proposed scheme is suitable for wideband, integrated co-frequency co-time full duplex 5G communication.

Novel dual-loop optoelectronic oscillator based on self-polarization-stabilization technique.

A novel dual-loop optoelectronic oscillator (OEO) based on self-polarization-stabilization technique has been experimentally demonstrated. The input light in each loop will retrace its path through a 45° Faraday rotator and single mode fiber with different length after reflecting off a 45° Faraday rotator mirror. By this way, the polarization state of the output signal is always rotated by 180° from the polarization state of the input signal in each loop even if the round-trip fiber is perturbed by the mechanical vibration or temperature. Moreover, the required fiber length is cut in half due to the round-trip transmission in each path, resulting in higher system stability and more compacter structure. The experimental results show that in an intensity modulator (IM)-based OEO system, this scheme has better phase noise performance than the traditional polarization multiplexing method. Furthermore, to avoid the DC bias drifting problem of IM, a simple phase modulation to intensity modulation convertor by using a polarization-dependent phase modulator and a polarizer is proposed without the need of expensive optical filters or dispersive devices. Based on this scheme, a novel dual-loop OEO using phase modulator and self-polarization-stabilization technique has been experimentally demonstrated. The phase noise of -114.1 dBc/Hz at 10 kHz away from the carrier (10 GHz) is achieved and the side mode suppression ratio is improved to 63 dB.