SAR/ISAR Imaging in 6G Network

Abstract: Imaging is a crucial sensing function that finds wide applications in environmental reconstruction, autonomous driving, etc. However, the signal processing methods for existing radio imaging techniques, such as millimeter wave (mmWave) imaging, require high-resolution range estimation enabled by Gigahertz-level or even Terahertz-level bandwidth, and cannot be applied in 6G integrated sensing and communication (ISAC) network with Megahertz-level bandwidth. This paper proposes two novel high-resolution radio imaging schemes that can work on the 6G signals with limited bandwidth - bandwidth-independent synthetic aperture radar (BI-SAR), where the movable base station (BS) revolves along the static targets by 360 degrees; as well as bandwidth-independent inverse synthetic aperture radar (BI-ISAR), where the BS is static and the targets revolve along an axis by 360 degrees. Different from conventional SAR and ISAR counterparts that rely on range estimation, our proposed imaging schemes solely utilize Doppler information to perform imaging without any range information. The main technical challenge of our schemes lies in the anisotropic scattering functions over different directions, which hinder the coherent synthesis of the backscattered signals from all directions. We design an iterative adaptive approach-based Doppler association (IAA-DA) algorithm to tackle the above issue. Moreover, we also derive the imaging resolution to characterize the reconstruction quality. Real-world experiments are provided to show the feasibility and the effectiveness of our proposed 6G imaging schemes.

• The paper introduces BI-SAR and BI-ISAR frameworks that decouple image resolution from signal bandwidth by exploiting accumulated Doppler signatures.
• It employs the IAA-DA algorithm to map Doppler shifts over a 360° span, enabling super-resolution imaging even under anisotropic scattering conditions.
• Experimental validation on a mmWave testbed demonstrates centimeter-level resolution, offering a practical solution for high-resolution ISAC in 6G networks.

High-Resolution Doppler-Based SAR/ISAR Imaging in 6G Networks

Introduction

Integrated Sensing and Communication (ISAC) is a central paradigm for sixth-generation (6G) wireless networks, aiming to unify high-capacity communications and radio sensing functionalities within the same infrastructure. This work addresses the challenge of high-resolution radio imaging using cellular signals with limited bandwidth—overcoming a major limitation of classical Synthetic Aperture Radar (SAR) and Inverse Synthetic Aperture Radar (ISAR). Traditional SAR/ISAR require gigahertz-scale bandwidth for acceptable range resolution, which is infeasible in most practical cellular deployments that operate with megahertz-level bandwidths. The paper introduces bandwidth-independent SAR (BI-SAR) and ISAR (BI-ISAR) schemes that leverage purely the Doppler domain for image formation in 6G ISAC systems (2604.00583).

Conventional Limitations and Motivation

Millimeter-wave (mmWave) imaging and radar modalities depend fundamentally on large instantaneous bandwidths to resolve targets in range, dictating the ultimate image resolution. With bandwidths in 5G/6G cellular infrastructure limited to a few hundreds of megahertz (e.g., 100–400 MHz), the range resolution collapses to meter-scale, insufficient for many environmental and industrial sensing applications. Recent multi-view, multi-anchor imaging schemes have shown theoretical possibility for bandwidth-independent radio images, such as radio tomographic imaging (RTI) [5374407, 10618967, 10367810, 10947014], but demand dense instrumentation and are impractical for urban-scale deployments.

This work posits a single-anchor architecture—either a moving base station (BI-SAR) or rotating targets with a fixed BS (BI-ISAR)—employing only Doppler signatures accumulated over a 360° angular span to synthesize effective two-dimensional virtual apertures. The method is robust to anisotropic scattering, a property ubiquitous in realistic imaging scenarios.

BI-SAR and BI-ISAR Modalities

System Model

Both scenarios involve a single bistatic configuration (one transmitter/receiver), with either:

• BI-SAR: The BS (e.g., on a UAV or robot) revolves in a horizontal plane about static targets.
• BI-ISAR: The BS is static, targets undergo controlled rotation around an axis.

A sequence of orthogonal-frequency-division multiplexing (OFDM) waveforms, compatible with 6G air interfaces, is transmitted and echoes are collected for multiple azimuthal angles. The crucial regime is when the maximum inter-point range within the region of interest is below the range resolvable by the signal bandwidth, thus making per-frequency-bin delay distinction impossible; all backscattering occurs in a single delay bin.

Signal Processing Formulation

The received signal at each observation angle and OFDM symbol can be described as a weighted integral over the target region, where the weights encode the local scattering (anisotropic), propagation loss, and a phase term embodying the Doppler shift resulting from spatial movement—either of the BS or the target. Under a far-field approximation, the three-dimensional problem reduces to integrating over the projection in the plane orthogonal to the rotation axis.

A notable insight is that, after appropriate range compression and angular association, the problem is analogous to an anisotropic computed tomography (CT) model—except that, unlike CT or isotropic multi-view radio imaging [10475383], the scattering function is highly direction-dependent and non-coherent.

Iterative Adaptive Approach-Based Doppler Association (IAA-DA) Algorithm

The primary technical challenge is the fusion over angles in the presence of anisotropic scattering. Classical back-projection or filtered back-projection (FBP) algorithms break down due to non-coherence between observations at differing azimuths. The proposed solution utilizes an Iterative Adaptive Approach for Doppler Association (IAA-DA):

1. Range Compression: Exploits uniform bandwidth-limited responses to improve SNR while discarding non-resolvable inter-point range information.
2. Angle-Doppler Map Formation: At each angular step, computes the Doppler spectrum of the received signal (using super-resolution IAA), capturing how different spatial locations manifest as angularly modulated sinusoidal frequency patterns.
3. Doppler Association: Establishes a one-to-one mapping between image coordinates and their corresponding sinusoidal Doppler trajectories as a function of rotation angle, enabling back-projection of energy onto the $(x, y)$ plane.
4. Incoherent Fusion: Incoherent integration is necessary due to the random (anisotropic) phase relationships between returns at different vantage points.

The approach exploits the uniqueness of the Doppler signature for each spatial coordinate, circumventing the need for bandwidth-based range estimation and enabling high-fidelity 2D power imaging.

Theoretical Resolution Analysis

The classical resolution bound for radar imaging is dictated by bandwidth in range and aperture in cross-range. In the Doppler-only paradigm, the synthesized aperture is determined instead by angular coverage and central carrier frequency. The theoretical point spread function (PSF) is derived, and resolution is shown to scale as:

$$\delta_{\text{IAA-DA}} = \kappa_I \kappa_S C_R \frac{\lambda}{4 \delta_{SA} \sqrt{1+H^2/R^2}}$$

where $\lambda$ is the carrier wavelength, $C_R$ a constant, $\delta_{SA}$ the angular coherence interval, and $\kappa_I$, $\kappa_S$ encode degradation due to anisotropy and super-resolution gain respectively. Importantly, resolution does not depend on the absolute distance between the anchor and the target, only on system geometry and carrier frequency.

The equivalent effective synthesized bandwidth arises from the total observation time and angular velocity, indicating that centimeter-level resolution (in current mmWave bands of 28–30 GHz) can be achieved with conventional communication-grade signals.

Experimental Validation

A mmWave testbed operating at 29 GHz, with OFDM waveforms and 100 MHz bandwidth (yielding >1 m range resolution in classical schemes), validates the theory.

• Centimeter-level resolutions are achieved, with the method able to resolve targets separated by as little as 7.6 cm.
• Imaging robustness is demonstrated in scenarios with single/multiple scatterers and in penetration (concealed objects inside a box).
• Compared to algorithms relying on isotropic scattering (SRDI [10475383]) or conventional CT back-projection, the proposed method exhibits superior localization accuracy, suppressed sidelobes, and robust target delineation under strongly anisotropic scattering.
• Super-resolution techniques (IAA) provide further gains over DFT-based Doppler mapping, sometimes exceeding a 1.36× improvement.

Implications and Outlook

This work fundamentally alters the resource requirements for high-resolution radar imaging within 6G networks. By decoupling imaging performance from bandwidth and exploiting Doppler and motion diversity, the method enables high-resolution, practical ISAC imaging at the system/network edge without specialized ultra-wideband hardware. The paradigm is applicable to scenarios where angular aperture synthesis is possible (e.g., UAV, robotic, or vehicular BSs, or objects with natural rotation), and provides an effective approach for non-contact, internal, and through-barrier imaging.

From a theoretical perspective, it shifts the resolution limiting factor from instantaneous bandwidth to carrier frequency and aperture diversity, which implies that advances in carrier frequency (e.g., THz ISAC) and long-duration continuous observations will directly translate to further gains.

Practical deployments may leverage this scheme for environmental mapping, robotics, automotive safety, infrastructure monitoring, and search-and-rescue under constrained hardware or spectrum. The direction-dependent, Doppler-based imaging concept is extendable to more complex ISAC topologies, passive radar, and multi-static cooperative settings, paving the way for general-purpose, communications-grade high-resolution radar imaging in 6G and beyond.

Conclusion

The BI-SAR and BI-ISAR frameworks represent a rigorous, experimentally validated advance in the integration of sensing and communications. Through sophisticated Doppler-only processing, a physically principled algorithm (IAA-DA), and careful consideration of inherent anisotropy and system constraints, the research demonstrates centimeter-scale, bandwidth-independent imaging on a cellular waveform. This establishes a foundational methodology for deploying high-resolution ISAC in future wireless networks (2604.00583).