Wideband Beamspace Channel Estimation for Millimeter-Wave MIMO Systems Relying on Lens Antenna Arrays

Beamspace channel estimation is indispensable for millimeter-wave MIMO systems relying on lens antenna arrays for achieving substantially increased data rates, despite using a small number of radio-frequency chains. However, most of the existing beamspace channel estimation schemes have been designed for narrowband systems, while the rather scarce wideband solutions tend to assume that the sparse beamspace channel exhibits a common support in the frequency domain, which has a limited validity owing to the effect of beam squint caused by the wide bandwidth in practice. In this paper, we investigate the wideband beamspace channel estimation problem without the common support assumption. Specifically, by exploiting the effect of beam squint, we first prove that each path component of the wideband beamspace channel exhibits a unique frequency-dependent sparse structure. Inspired by this structure, we then propose a successive support detection (SSD) based beamspace channel estimation scheme, which successively estimates all the sparse path components following the classical idea of successive interference cancellation. For each path component, its support at different frequencies is jointly estimated to improve the accuracy by utilizing the proved sparse structure, and its influence is removed to estimate the remaining path components. The performance analysis shows that the proposed SSD-based scheme can accurately estimate the wideband beamspace channel at a low complexity. Simulation results verify that the proposed SSD-based scheme enjoys a reduced pilot overhead, and yet achieves an improved channel estimation accuracy.

I. Introduction

Millimeter-wave (mmWave) multiple-input multiple-output (MIMO) working at 30–300 GHz has been recently recognized as a promising technique to substantially increase the data rates of wireless communications [1], [2], since it can provide a very wide bandwidth (e.g., 2–5 GHz) [3]. However, in the conventional MIMO architecture working at sub-6 GHz cellular frequencies, each antenna requires a dedicated radio-frequency (RF) chain (including the digital-to-analog/analog-to-digital converter, mixer, and so on) [4], [5]. Employing this architecture in mmWave MIMO will lead to unaffordable hardware cost and power consumption due to the following two reasons [6]: 1) the number of antennas is usually very large to compensate for the severe path loss (e.g., 256 antennas may be used at mmWave frequencies instead of 8 antennas at cellular frequencies) [7]; 2) the power consumption of the RF chain is high due to the increased sampling rate (e.g., 250 mW/RF chain at mmWave frequencies, compared to 30 mW/RF chain at cellular frequencies) [8]. To solve this problem, mmWave MIMO relying on lens antenna array has been proposed [9]. By employing the lens antenna array (an electromagnetic lens with power focusing capability and a matching antenna array with elements located on the focal surface of the lens [10]), we can focus the signal power arriving from different directions on different antennas [11], and transform the mmWave MIMO channel from the spatial domain to its sparse beamspace representation (i.e., beamspace channel) [12]. This allows us to select a small number of power-focused beams for significantly reducing the effective MIMO dimension and the associated number of RF chains. Consequently, the high power consumption and hardware cost of mmWave MIMO systems can be mitigated [13]–[15].