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1. A Single Antenna Full-Duplex Radio Using a Non-Magnetic, CMOS Circulator with In-built Isolation Tuning

   Aravind Nagulu, Tingjun Chen ( January 2019 , IEEE ICC'19 Workshop on Full-Duplex Communications for Future Wireless Networks)

   Wireless systems which can simultaneously transmit and receive (STAR) are gaining significant academic and commercial interest due to their wide range of applications such as full-duplex (FD) wireless communication and FMCW radar. FD radios, where the transmitter (TX) and the receiver (RX) operate simultaneously at the same frequency, can potentially double the data rate at the physical layer and can provide many other advantages in the higher layers. The antenna interface of an FD radio is typically built using a multi-antenna system, or a single antenna through a bulky magnetic circulator or a lossy reciprocal hybrid. However, recent advances in CMOS-integrated circulators through spatio-temporal conductivity modulation have shown promise and potential to replace traditional bulky magnetic circulators. However, unlike magnetic circulators, CMOS-integrated non-magnetic circulators will introduce some nonlinear distortion and spurious tones arising from their clock circuitry. In this work, we present an FD radio using a highly linear CMOS integrable circulator, a frequency-flat RF canceler, and a USRP software-defined radio (SDR). At TX power level of +15 dBm, the implemented FD radio achieves a self-interference cancellation (SIC) of +55 dB from the circulator and RF canceler in the RF domain, and an overall SIC of +95 dB together with SIC in the digital domain. To analyze the non-linear phenomena of the CMOS circulator, we calculated the link level data-rate gain in an FD system with imperfect SIC and then extended this calculation to count the effect of TX-RX non-linearity of the circulator. In addition, we provide a qualitative discussion on the spurious tone responses of the circulator due to the clocking imperfections and non-linearity. Index Terms—Circulator, CMOS, conductivity modulation, full-duplex, non-reciprocity, self-interference cancellation. 

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2. A Full-Duplex Radio Using a CMOS Non-Magnetic Circulator Achieving +95 dB Overall SIC

   https://doi.org/10.1109/USNC-URSI.2019.8861766  

   Nagulu, Aravind ; Chen, Tingjun ; Zussman, Gil ; Krishnaswamy, Harish ( July 2019 , in Proc. IEEE APS-URSI’19, July 2019.)

   Full-duplex (FD) wireless is an emerging wireless communication paradigm where the transmitter and the receiver operate simultaneously at the same frequency. One major challenge in realizing FD wireless is the interference of the TX signal saturating the receiver, commonly referred to self-interference (SI). Traditionally, self-interference cancellation (SIC) is achieved in the antenna, RF/analog, and digital domains. In the antenna domain, SIC can be achieved using a pair of separate TX and RX antennas, or using a single antenna shared by the TX and RX through a magnetic circulator, which is usually bulky, expensive, and not integrable with CMOS. Recent advances, however, have shown the feasibility of realizing high-performance non-reciprocal circulators in CMOS based on spatio-temporal modulation. In this work, we demonstrate a high power handling FD radio using a USRP SDR which employs SIC (i) at the antenna interface using a watt-level power-handling CMOS integrated, magnetic-free circulator, (ii) in the RF domain using a compact RF canceler, and (iii) in the digital domain. Our prototyped FD radio achieves +95 dB overall SIC at +15dBm TX power level. We analyze the effects of the circulator TX-RX non-linearity on the total achievable SIC 

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3. A Full-Duplex Radio Using a CMOS Non-Magnetic Circulator Achieving +95 dB Overall SIC

   Aravind Nagulu, Tingjun Chen ( January 2019 , URSI General Assembly and Scientific Symposium)

   Full-duplex (FD) wireless is an emerging wireless communication paradigm where the transmitter and the receiver operate simultaneously at the same frequency. One major challenge in realizing FD wireless is the interference of the TX signal saturating the receiver, commonly referred to self-interference (SI). Traditionally, self-interference cancellation (SIC) is achieved in the antenna, RF/analog, and digital domains. In the antenna domain, SIC can be achieved using a pair of separate TX and RX antennas, or using a single antenna shared by the TX and RX through a magnetic circulator, which is usually bulky, expensive, and not integrable with CMOS. Recent advances, however, have shown the feasibility of realizing high-performance non-reciprocal circulators in CMOS based on spatio-temporal modulation. In this work, we demonstrate a high power handling FD radio using a USRP SDR which employs SIC (i) at the antenna interface using a watt-level power-handling CMOS integrated, magnetic-free circulator, (ii) in the RF domain using a compact RF canceler, and (iii) in the digital domain. Our prototyped FD radio achieves +95 dB overall SIC at +15dBm TX power level. We analyze the effects of the circulator TX-RX non-linearity on the total achievable SIC. 

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4. Open-Access Full-Duplex Wireless in the ORBIT and COSMOS Testbeds

   https://doi.org/10.1145/3411276.3412185  

   Kohli, Manav ; Chen, Tingjun ; Dastjerdi, Mahmood Baraani ; Welles, Jackson ; Seskar, Ivan ; Krishnaswamy, Harish ; Zussman, Gil ( September 2020 , in Proc. ACM MobiCom'20 Workshop on Wireless Network Testbeds, Experimental evaluation & Characterization (WiNTECH))

   null (Ed.)

   ABSTRACT In order to support experimentation with full-duplex (FD) wireless, we recently integrated two generations of FD radios in the open-access ORBIT and COSMOS testbeds. First, we integrated a customized 1st generation (Gen-1) narrowband FD radio in the indoor ORBIT testbed. Then, we integrated two 2 nd generation (Gen-2) wideband FD radios in the city-scale PAWR COSMOS testbed. Each integrated FD radio consists of an antenna, a customized RF self-interference (SI) canceller box, a USRP software-defined radio (SDR), and a remotely accessible compute node. The Gen-1/Gen-2 RF SI canceller box includes an RF canceller printed circuit board (PCB) which emulates a customized integrated circuit (IC) RF canceller implementation. The amplitude- and phase-based Gen-1 narrowband RF canceller achieves 40 dB RF SIC across 5 MHz. The Gen-2 wideband canceller is based on the technique of frequency-domain equalization (FDE) and achieves 50 dB RF SI cancellation (SIC) across 20 MHz. In this paper, we present the design and testbed integration of the two generations of FD radios. We then present example experiments that can be remotely run and modified by experimenters. Finally, we discuss future improvements and potential FD wireless experiments that can be supported by these open-access FD radios integrated in the COSMOS testbed. 

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5. Demo: Experimentation with Wideband Real-Time Adaptive Full-Duplex Radios

   https://doi.org/10.1145/3603269.3610853  

   Levin, Alon Simon ; Kadota, Igor ; Garikapati, Sasank ; Zhang, Bo ; Jolly, Aditya ; Kohli, Manav ; Seok, Mingoo ; Krishnaswamy, Harish ; Zussman, Gil ( September 2023 , ACM SIGCOMM'23)

   We present a set of experiments utilizing wideband real-time adaptive full-duplex (FD) radios, demonstrating simultaneous transmission and reception on the same frequency channel. Each FD radio consists of a circulator-based antenna interface, a switched-capacitor delay-line-based configurable Radio-Frequency Integrated Circuit (RFIC) that implements Self-Interference Cancellation (SIC), an FPGA that optimizes the RFIC configuration in under 1.1 sec and can adapt to environmental changes in under 0.3 sec, and a Software-Defined Radio (SDR) transmitting OFDM-like packets. We demonstrate a real-time adaptive FD radio that achieves the SIC necessary to reach the noise floor across a wide bandwidth of 50 MHz. Then, we use two FD radios to create a wireless link and showcase the superior FD throughput. 

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