SIGMA: Sequence Index Grouping-based Multiple Access for 6G OFDM-ISAC Systems in Multipath Environments

In this paper, we propose a novel sequence index grouping-based multiple access (SIGMA) technique for multi-user orthogonal frequency division multiplexing-based integrated sensing and communication (OFDM-ISAC) systems. The proposed technique ensures collision-free access by performing a pre-assignment of orthogonal sequences (OSs) between the base station (BS) and each user equipment (UE), where each UE modulates its communication data onto the allocated sequence index and transmits it to the BS. By exploiting the autocorrelation property of the received sequences, the BS estimates the distance to each UE, while the direction of each user is obtained using a uniform linear array (ULA) antenna equipped at the BS. Furthermore, to achieve accurate localization even in non-line-of-sight (NLoS) multipath environments, a clustering algorithm and an oversampling technique are incorporated. Extensive simulation results demonstrate that the proposed system achieves robust localization and communication performance in multi-user scenarios.