Experimentation with Full-Duplex Wirelessin the COSMOS Testbed

Tingjun Chen∗, Jackson Welles∗, Manav Kohli∗, Mahmood Baraani Dastjerdi∗,Jakub Kolodziejski†, Michael Sherman†, Ivan Seskar†, Harish Krishnaswamy∗, Gil Zussman∗

∗Electrical Engineering, Columbia University, New York, NY 10027, USA†WINLAB, Rutgers University, North Brunswick, NJ 08902, USA

Abstract—In order to support experimentation with full-duplex(FD) wireless, we integrated the FlexICoN Gen-2 widebandFD radio with the city-scale PAWR COSMOS testbed [1]. Inparticular, the implemented FD radio consists of an antenna,a customized Gen-2 RF self-interference (SI) canceller box, aUSRP software-defined radio (SDR), and a compute node. TheRF canceller box includes an RF SI canceller implemented usingdiscrete components on a printed circuit board (PCB), which em-ulates its RFIC canceller counterpart. The Gen-2 RF SI cancellerachieves 50 dB RF SI cancellation across 20 MHz bandwidthusing the technique of frequency-domain equalization (FDE) [2].In this abstract, we present the design and implementation of theremotely accessible Gen-2 wideband FD radio integrated withthe COSMOS sandbox at Columbia University. We also presentan example real-time wideband FD wireless link demonstrationusing the GNU Radio software.

I. INTRODUCTION

Full-duplex (FD) wireless has drawn significant atten-tion [3], [4] due to its potential to double data rate atthe physical layer and to provide many benefits at differentlayers of the network stack. One of the major challengesassociated with FD is the extremely powerful self-interference(SI) signal on top of the desired signal, which requires morethan 90 dB of self-interference cancellation (SIC) across theantenna interface, and RF and digital domains.

Within the Columbia FlexICoN project [5], we focus onthe design and experimentation with FD radios and systemsgrounded in integrated circuit (IC) implementations, which aresuitable for hand-held and form-factor-constrained devices [4].Since interfacing an RFIC canceller with software-definedradios (SDRs) presents numerous technical challenges, weimplemented the RF SI canceller on a printed circuit board(PCB) to facilitate the cross-layered experiments with an SDRplatform. In particular, we demonstrated the FlexICoN Gen-1narrowband FD radio and an FD link in [6], where 40 dBRF SIC is achieved across 5 MHz bandwidth. The Gen-1RF SI canceller implemented on a PCB emulates its RFICcounterpart that we presented in [7].

In order to allow the broader community to experiment withFD wireless, we recently integrated an improved version ofthe Gen-1 narrowband RF canceller presented in [6] with aUSRP N210 SDR in the open-access ORBIT testbed. Theimplementation of the Gen-1 RF canceller box and the de-scription of the open-access FD radio in the ORBIT testbed

Fig. 1: Block diagram of the implemented Gen-2 wideband full-duplex (FD)radio consisting of a Gen-2 RF canceller box, a USRP software-defined radio(SDR), and a compute node (e.g., a host PC/server).

(a) (b)Fig. 2: (a) The implemented FlexICoN Gen-2 wideband FD radio, and (b) itsintegration with a COSMOS sandbox located at Columbia University, wherethe FD radio can be accessed and controlled by a COSMOS compute node(e.g., a host PC/server).

can be found in [8]. We also provided example remote FDexperiments where 90 dB overall SIC is achieved across theantenna interface, and the RF and digital domains.1

More recently, in [2], we designed and experimented withthe FlexICoN Gen-2 wideband FD radio based on the tech-nique of frequency-domain equalization (FDE) [11], where50 dB RF SIC is achieved across 20 MHz bandwidth. Inthis abstract, we present an improved version of the Gen-2wideband RF SI canceller used in [2] and its integration withthe USRP SDR platform using the GNU Radio software. Wealso discuss the integration of the Gen-2 wideband FD radiowith the city-scale PAWR COSMOS testbed [1], [12] (thedetailed tutorial can be found in [13]). We envision that theimplemented FD radio and the example experiments, whichcan be further extended to more complicated communicationand networking scenarios, can facilitate further hands-on re-search in the area of FD wireless, as discussed in [8].

1The detailed instructions and code of the open-access Gen-1 FD radiointegrated in the ORBIT testbed are available at [9], [10].978-1-7281-2700-2/19/$31.00 2019 c© IEEE

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(b) Practical case w/ a real antenna.Fig. 3: Measured TX/RX isolation of the Gen-2 RF canceller without turningon the canceller, and with the narrowband (Gen-1)/wideband (FDE) path on,when the circulator antenna port is: (a) terminated by 50Ω, and (b) connectedto an antenna.

II. THE GEN-2 WIDEBAND FULL-DUPLEX RADIO

Figs. 1 and 2 show the diagram and implementation ofthe Gen-2 wideband FD radio, which is integrated with theCOSMOS testbed sandbox at Columbia University. The imple-mented FD radio consists of an antenna, a Gen-2 RF cancellerbox, a USRP SDR, and a compute node (e.g., a laptop/PC ora server). The Gen-2 RF canceller box (see Fig. 2(a)) includesa customized antenna tuner, a coaxial circulator, a Gen-2 RFSI canceller, and a SUB-20 controller.The Gen-2 RF SI Canceller. To alleviate the requirements onthe RX front-end linearity and the analog-to-digital converter(ADC) dynamic range, an FD radio needs to achieve sufficientSI isolation and cancellation in the RF/analog domain beforeinvolving SIC in the digital domain. In particular, the Gen-2RF canceller is implemented using discrete components on aPCB and is optimized around 900 MHz operating frequency.2

To achieve RF SIC, a reference signal coupled from theRF canceller TX input is passed to the cancellation path.The implemented Gen-2 canceller includes two cancellationpaths that can be selected through an RF switch: the Gen-1(narrowband) path and the FDE (wideband) path. Then, RFSIC is performed at the RF canceller RX output. In particular,the implementation of the narrowband path is identical to theGen-1 RF canceller implementation described in [8], and thedetailed implementation of the FDE path can be found in [2].Measured RF SIC. Fig. 3 shows two example measurementsof the TX/RX isolation achieved by the Gen-2 RF canceller(measured between the TX input and RX output ports of thecanceller using a vector network analyzer) when the circulatorantenna port is terminated by 50Ω (ideal), and is connectedto a real antenna (practical), respectively.

The results show that in the ideal case, the circulatorprovides around −25 dB TX/RX isolation without turning onthe Gen-2 RF canceller. When the canceller is turned on, theFDE path can achieve 40/50 dB RF SIC across 28/20 MHzbandwidth, which is a 2.5/5× improvement in bandwidthcompared to that achieved by the Gen-1 path. In the practicalcase, due to the worse TX/RX isolation provided by theantenna interface, the Gen-1 path only achieves 40 dB RF SIC

2We select 900 MHz around the Region 2 902–928 MHz frequency but thedesign can be easily extended to other frequency bands (e.g., 2.4/5 GHz).

across 4 MHz, while the FDE path can still achieve 40/50 dBRF SIC across 28/22 MHz bandwidth.

III. A REAL-TIME FD LINK DEMONSTRATION

The demonstration setup contains two Gen-2 wideband FDradios that simultaneously transmit and receive OFDM packetsat around 900 MHz carrier frequency with up to 20 MHzsampling rate. A laptop is used to remotely login into theCOSMOS testbed to execute the example FD link experiment,where both FD radios are controlled by a COSMOS computenode (server). In order to achieve optimized performance, weimplemented the RF canceller control and the digital SICalgorithm as customized GNU Radio out-of-tree modules inC++. Overall, the Gen-2 FD radio achieves a total amount of90–95 dB SIC across up to 20 MHz bandwidth, where 50 dBand 40-45 dB SIC is achieved in the RF and digital domains,respectively. Through the GNU Radio graphical user interface(GUI), demo participants can observe: (i) the real-time signalreceived at the FD nodes after RF and digital SIC, (ii) thecoefficients returned by the packet-level digital SIC algorithm,and (iii) the decoded OFDM packets at the FD radios and thecorresponding packet error rate.

ACKNOWLEDGMENTS

This work was supported in part by NSF grants ECCS-1547406, CNS-1650685, and CNS-1827923, and the DARPASPAR program.

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