Analytical Framework for Effective Degrees of Freedom in Near-Field XL-MIMO
Published in arXiv, 2024
Abstract: Extremely large-scale multiple-input-multiple-output (XL-MIMO) is an emerging transceiver technology for enabling next-generation communication systems, due to its potential for substantial enhancement in both the spectral efficiency and spatial resolution. However, the achievable performance limits of various promising XL-MIMO configurations have yet to be fully evaluated, compared, and discussed. In this paper, we develop an effective degrees of freedom (EDoF) performance analysis framework specifically tailored for near-field XL-MIMO systems. We explore five representative distinct XL-MIMO hardware designs, including uniform planar array (UPA)-based with point antennas, two-dimensional (2D) continuous aperture (CAP) plane-based, UPA-based with patch antennas, uniform linear array (ULA)-based, and one-dimensional (1D) CAP line segment-based XL-MIMO systems. Our analysis encompasses two near-field channel models: the scalar and dyadic Green’s function-based channel models. More importantly, when applying the scalar Green’s function-based channel, we derive EDoF expressions in the closed-form, characterizing the impacts of the physical size of the transceiver, the transmitting distance, and the carrier frequency. In our numerical results, we evaluate and compare the EDoF performance across all examined XL-MIMO designs, confirming the accuracy of our proposed closed-form expressions. Furthermore, we observe that with an increasing number of antennas, the EDoF performance for both UPA-based and ULA-based systems approaches that of 2D CAP plane and 1D CAP line segment-based systems, respectively. Moreover, we unveil that the EDoF performance for near-field XL-MIMO systems is predominantly determined by the array aperture size rather than the sheer number of antennas.
Index Terms: XL-MIMO, effective degrees of freedom, near-field communication, polarization effect
Fig. 1: A UPA-based XL-MIMO system with each antenna element being a sizeless point antenna.
Fig. 2: A 2D CAP plane-based XL-MIMO system.
Fig. 3: An illustration of the transmitter equipped with UPA-based XL-MIMO with patch antennas.
Recommended citation: Zhe Wang, Jiayi Zhang, Wenhui Yi, Hongyang Du, Dusit Niyato, Bo Ai, and Derrick Wing Kwan Ng, "Analytical Framework for Effective Degrees of Freedom in Near-Field XL-MIMO," arXiv:2401.15280, 2024. https://arxiv.org/pdf/2401.15280.pdf