A Multi-Band Full-Duplex Prototype for Integrated Sensing and Communication

A Multi-Band Full-Duplex Prototype for Integrated Sensing and Communication Bixing Yan∗, Andre Kokkeler†, Yang Miao‡ Radio Systems Group, University of Twente, Netherlands {∗b.yan, †a.b.j.kokkeler, ‡y.miao}@utwente.nl Abstract—Integrated sensing and communication (ISAC) has emerged as a key enabler for the next-generation radio network. ISAC systems aim to enable wireless sensing and data transmission functionality in one integrated system. While existing ISAC prototypes in the literature predominantly focus on single-band full-duplex operations with array antennas and spatially separated sensing targets and communication user equipment (UE) to mitigate signal interference, this paper presents a novel multi-band full-duplex ISAC prototype leveraging a software-defined radio (SDR) USRP X440. In this initial implementation, constrained by host PC performance limitations, the proposed system supports a monostatic sensing link at 24.8 GHz with 150 MHz bandwidth for range estimation alongside a bistatic communication link at 24.2 GHz with 150 MHz bandwidth, each employing distinct orthogonal frequency-division multiplexing (OFDM) waveforms. Experimental results demonstrate successful target detection with a range resolution of 1 m while maintaining robust wireless communication performance with minimal signal interference between sensing and communication functionalities. Keywords—Integrated Sensing and Communication, MultiBand, Software-Defined Radio I. INTRODUCTION Integrated sensing and communication (ISAC) systems utilize partially or fully shared radio signals and/or hardware to achieve wireless sensing and wireless data transmission functions. Using orthogonal frequency-division multiplexing (OFDM) waveforms for both sensing and communication is one solution for tightly integrated ISAC systems [1]. Building ISAC prototypes is the first step in implementing ISAC functionality. Existing ISAC prototypes introduced in [2], [3] predominantly focused on full-duplex sensing and communication within a single frequency band, eliminating signal interference through directional antennas and separately located targets and UEs. These current prototypes also required multiple FPGA modules or software-defined radios (SDR), increasing system complexity. This paper introduces an SDR-based multi-band full-duplex ISAC prototype using one USRP X440 to operate at the lower end of 5G NR FR2 or the higher end of mid-band FR3. Multi-band technology can help reduce interference more efficiently than singleband by assigning separate frequency bands for sensing and communication [1]. Full-duplex operation enables simultaneous transmission and reception, capturing target echoes in time without dead zones. As a first step, our prototype Fig. 1: Hardware architecture of proposed ISAC prototype utilizes two transmit and two receive channels: one transmitreceive pair for monostatic OFDM sensing at 24.8 GHz and another for bistatic wireless OFDM communication at 24.2 GHz. Currently, each channel operates with 150 MHz bandwidth due to host PC performance constraints while our full capacity can reach 800 MHz upon future upgration of host PC. Even with limited bandwidth in this early version, the implementation demonstrates the feasibility of multiband technologies regarding the interference level between sensing and communication signals and validates the performance capabilities for the two functionalities. II. HARDWARE AND SOFTWARE ARCHITECTURE OF THE PROPOSED PROTOTYPE The hardware structure of the proposed prototype is shown in Fig. 1. It comprises three main components: the intermediate frequency (IF) band transceiver, implemented with the SDR USRP X440; the local oscillator (LO), realized using a voltage-controlled oscillator (VCO); and the radio frequency (RF) part, where antennas are responsible for transmitting and receiving signals. The USRP X440 features eight TX/RX channels covering 1 MHz to 4 GHz, with bandwidth determined by its FPGA version. For this demonstration, the CG 1600 FPGA configuration enables four active channels across two IF bands: one TX-RX pair at 0.2 GHz and another at 0.8 GHz. A host PC with i9-14900K CPU and 100GbE network card controls the USRP X440 via GNU Radio’s U SRP Sink and USRP Source blocks for signal transmission and reception. Fig. 2: ISAC prototype’s signal processing architecture A VCO generates the 24 GHz LO signal in the LO stage, amplified by an amplifier (P A1in Fig. 1) before a 4-way splitter distributes it to all channels for frequency conversion. I/Q mixers perform up/down conversion in the RF band for 5G NR FR2 compatibility. The system employs two colocated 3×12-element array antennas (15 dBi gain each) for transmission (one for sensing with beams facing towards sensing targets, one for communication with beams facing the user equipment (UE)), with a nearby horn antenna (20 dBi gain) receiving monostatic sensing signals. A separate horn antenna connected via a 6 m cable serves as a downlink UE for bistatic communication reception. The prototype operates in two RF bands: 24.2 GHz for wireless communication and 24.8 GHz for monostatic sensing. The proposed prototype’s software architecture consists of three components: signal generation for sensing and/or communication, communication receiver algorithms, and sensing algorithms. The signal generation and communication receiver are managed through GNU Radio with opensource blocks. The sensing algorithms are implemented in MATLAB as shown in Fig. 2 due to the lack of relevant signal processing units required in GNU Radio. For the first demonstration of the prototype, the sensing algorithms focus on the target range estimation through the standard FFTbased method and peak finding with synchronized received and transmit signals. Decent velocity estimation requires longer symbol duration and more subcarriers and will be involved in future updated systems with better parallel computation ability. Currently, separate files generate distinct OFDM signals for sensing and communication, respectively. III. PROOF-OF-CONCEPT TRIAL AND EXAMPLE RESULTS Indoor measurements evaluated the prototype’s performance using a 30cm ×30cm metal plate as a stable sensing target positioned at 1 m and 2 m from the sensing transceiver. The scenario is shown in Fig. 3. Two different text files generated distinct OFDM signals for sensing and communication, respectively. The OFDM signals both had 64 subcarriers and 12000 symbols with 150 MHz bandwidth, providing a range resolution of 1 m and a maximum unambiguous range of 64 m. Fig. 3: Indoor measurement scenario of the ISAC prototype The bit error rate (BER) for wireless communication was 0, indicating error-free communication with minimal interference from sensing signals. This low BER was achieved under controlled ideal laboratory conditions, and a larger BER would be expected in real-world deployments with multipath, interference, and mobility. The range profile estimations accurately detected the metal plate at both 1 m and 2 m, as shown in Fig. 4, confirming the sensing capabilities of the prototype. 0 5 10 Range (m) 80 90 100 110 120 130 140 150 Magnitude (dB) 1.00 m (a) Target at 1 m 0 5 10 Range (m) 60 70 80 90 100 Magnitude (dB) 2.00 m (b) Target at 2 m Fig. 4: Range profile estimation for the metal plate IV. CONCLUSION This paper introduces a full-duplex ISAC prototype based on USRP X440, which currently operates at two frequency bands (24.2 and 24.8 GHz) with two transmit and two receive channels. Each channel utilizes 150 MHz bandwidth, achieving a range resolution of 1 m for monostatic OFDM sensing while maintaining error-free bistatic wireless communication. Future work will enhance the host PC’s performance to increase bandwidth (up to 800 MHz per channel) and explore advanced signal synchronization, angle estimation, and velocity estimation algorithms. ACKNOWLEDGMENT This work was supported by the University of Twente sectorplan. REFERENCES [1] C. Sturm et al., ”An OFDM System Concept for Joint Radar and Communications Operations,” VTC Spring 2009 - IEEE 69th Vehicular Technology Conference, Barcelona, Spain, 2009, pp. 1-5. [2] Q. Zhang et al., ”Sensing and Communication Integrated System for Autonomous Driving Vehicles,” IEEE INFOCOM 2020 - IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS), Toronto, ON, Canada, 2020, pp. 1278-12793. [3] C. D. Ozkaptan et al., ”Software-Defined MIMO OFDM Joint RadarCommunication Platform with Fully Digital mmWave Architecture,” 2023 IEEE 3rd International Symposium on Joint Communications & Sensing (JC&S), Seefeld, Austria, 2023, pp. 1-6.