Spatial Path Index Modulation in mmWave/THz-Band Integrated Sensing and Communications

TL;DR

This work introduces SPIM-ISAC, a spatial path index modulation framework for integrated sensing and communications in upper mmWave and THz bands. It develops beam-split-aware parameter estimation (BSA-MUSIC for radar, BSA-OMP for communications) and proposes hybrid, SI-AO, SD-AO, and BSA-hybrid beamformers to exploit SPIM while mitigating beam-split without extra hardware. The SE analysis combines SPIM mutual information with lower-bounded baseband terms, and simulations show SPIM-ISAC outperforms fully digital and conventional MIMO-ISAC, with significant gains despite beam-split. The proposed approach offers a hardware-efficient, high-SE solution for future ISAC systems operating in wideband mmWave/THz regimes.

Abstract

As the demand for wireless connectivity continues to soar, the fifth generation and beyond wireless networks are exploring new ways to efficiently utilize the wireless spectrum and reduce hardware costs. One such approach is the integration of sensing and communications (ISAC) paradigms to jointly access the spectrum. Recent ISAC studies have focused on upper millimeter-wave and low terahertz bands to exploit ultrawide bandwidths. At these frequencies, hybrid beamformers that employ fewer radio-frequency chains are employed to offset expensive hardware but at the cost of lower multiplexing gains. Wideband hybrid beamforming also suffers from the beam-split effect arising from the subcarrier-independent (SI) analog beamformers. To overcome these limitations, this paper introduces a spatial path index modulation (SPIM) ISAC architecture, which transmits additional information bits via modulating the spatial paths between the base station and communications users. We design the SPIM-ISAC beamformers by first estimating both radar and communications parameters by developing beam-split-aware algorithms. Then, we propose to employ a family of hybrid beamforming techniques such as hybrid, SI, and subcarrier-dependent analog-only, and beam-split-aware beamformers. Numerical experiments demonstrate that the proposed SPIM-ISAC approach exhibits significantly improved spectral efficiency performance in the presence of beam-split than that of even fully digital non-SPIM beamformers.

Figures (12)

• Figure 1: Normalized array gain with respect to spatial direction at the low, center, and high-end subcarriers for (left) $f_{\mathrm{CENTER}}=3.5$ GHz, $B=0.1$ GHz; (middle) $f_{\mathrm{CENTER}}=28$ GHz, $B=2$ GHz; and (right) $f_{\mathrm{CENTER}}=300$ GHz, $B=30$ GHz, respectively.
• Figure 2: IM over subcarriers (left), antennas (middle), and spatial path indices (right).
• Figure 3: The SPIM-ISAC architecture processes the incoming data streams and employs spatial path index information $s_0$ in a switching network, which connects $N_\mathrm{RF}$ RF chains to $P= L + K$ taps on the analog beamformers to exploit $K$ paths for the radar targets and $L_\mathrm{S}$ out of $L$ spatial paths for the communications user.
• Figure 4: Array gain for (a) the beam-split-free and (b) beam-split-corrupted beams generated at $65^\circ$ and $60^\circ$, respectively. (c) The IBI computed for these two beams, and (d) the total IBI across the subcarriers.
• Figure 5: SE versus SNR when the radar-communications trade-off parameter $\varepsilon = 0.5$.

Theorems & Definitions (7)

• Remark 1
• Remark 2
• Remark 3
• Lemma 1
• proof
• Remark 4
• Remark 5