On the Impact of Channel Aging and Doppler-Affected Clutter on OFDM ISAC Systems

TL;DR

The paper addresses the joint impact of channel aging and Doppler-afflicted clutter on OFDM-based ISAC in a MU-MIMO, monostatic sensing setup. It introduces a spatial-temporal AR aging model for UE channels and a Kronecker-separable clutter model to capture Doppler and delay spreads, paired with an aging-aware MMSE channel estimator and a clutter-covariance estimation-plus-whitening radar pipeline. The authors demonstrate substantial gains over block fading in low-to-moderate mobility and show effective clutter suppression enabling accurate range-angle-velocity estimation, albeit requiring dedicated sensing beams for adequate range resolution. The work provides a practical framework for ISAC performance evaluation in urban clutter and mobility, with implications for design choices in precoding and sensing resource allocation.

Abstract

The temporal evolution of the propagation environment plays a central role in integrated sensing and communication (ISAC) systems. A slow-time evolution manifests as channel aging in communication links, while a fast-time one is associated with structured clutter with non-zero Doppler. Nevertheless, the joint impact of these two phenomena on ISAC performance has been largely overlooked. This addresses this research gap in a network utilizing orthogonal frequency division multiplexing waveforms. Here, a base station simultaneously serves multiple user equipment (UE) devices and performs monostatic sensing. Channel aging is captured through an autoregressive model with exponential correlation decay. In contrast, clutter is modeled as a collection of uncorrelated, coherent patches with non-zero Doppler, resulting in a Kronecker-separable covariance structure. We propose an aging-aware channel estimator that uses prior pilot observations to estimate the time-varying UE channels, characterized by a non-isotropic multipath fading structure. The clutter's structure enables a novel low-complexity sensing pipeline: clutter statistics are estimated from raw data and subsequently used to suppress the clutter's action, after which target parameters are extracted through range-angle and range-velocity maps. We evaluate the influence of frame length and pilot history on channel estimation accuracy and demonstrate substantial performance gains over block fading in low-to-moderate mobility regimes. The sensing pipeline is implemented in a clutter-dominated environment, demonstrating that effective clutter suppression can be achieved under practical configurations. Furthermore, our results show that dedicated sensing streams are required, as communication beams provide insufficient range resolution.

Figures (21)

• Figure 1: A single-cell network, where the BS uses multi-antenna transmission to provide ISAC services. This downlink signal is used to transmit pilots for channel estimation daei2024improved and as a radar probe for monostatic sensing liyanaarachchi2023joint. The blue arrows represent target echoes, whereas the red arrows represent ground clutter echoes.
• Figure 2: Representation of the pilot and data symbols transmission structure in the time-frequency domain. This transmission occurs on the communication channel, which is estimated at the start of each pilot slot.
• Figure 3: Clustered multipath transmission geometry of the channel between the BS and UE $k$. Both covariances $\mathbf{C}_{{\textrm{Tx}},k}$ and $\mathbf{C}_{{\textrm{Rx}},k}$ originate from the same set of clusters, albeit seen from different angles.
• Figure 4: Visual representation of the clutter environment. Clutter is formed by a discrete number of clutter patches, both static and dynamic, which can be approximated with a median angle, Doppler shift, and delay.
• Figure 5: Simulated scenario with $L=2$ radar targets, $N=4$ clutter patches and a single communication UE.