I. Introduction
Recently, massive multiple-input multiple-output (MIMO) systems with tens to hundreds of antennas at the base-station (BS) have gained significant attention [1]. It has been proved that massive MIMO systems can scale down transmit power as well as increase spectrum efficiency by orders of magnitude [2]. One of the challenges in massive MIMO systems is estimating the channel impulse response (CIR) for each transmit-receive link, since high data rates and energy efficiency can only be achieved when CIR is known [3], [4]. In contrast to the conventional MIMO systems with a small number of antennas, a large number of channel parameters need to be estimated in massive MIMO systems. In the meantime, the available pilot resources for channel estimation are restricted by the channel coherence time [5]. Hence, the pilot overhead may be one of the limiting factors in massive MIMO systems [1], [6]. On the other hand, the energy consumption by baseband processing grows with the number of antennas, which may obliterate the advantage of massive MIMO systems in energy efficiency. Thus, low-complexity receiver with reduced overhead is critical to massive MIMO systems.