Hybrid analog/digital MIMO architectures were recently proposed as an alternative for fully-digitalprecoding in millimeter wave (mmWave) wireless communication systems. This is motivated by the possible reduction in the number of RF chains and analog-to-digital converters. In these architectures, the analog processing network is usually based on variable phase shifters. In this paper, we propose hybrid architectures based on switching networks to reduce the complexity and the power consumption of the structures based on phase shifters. We define a power consumption model and use it to evaluate the energy efficiency of both structures. To estimate the complete MIMO channel, we propose an open loop compressive channel estimation technique which is independent of the hardware used in the analog processing stage. We analyze the performance of the new estimation algorithm for hybrid architectures based on phase shifters and switches. Using the estimated, we develop two algorithms for the design of the hybrid combiner based on switches and analyze the achieved spectral efficiency. Finally, we study the trade-offs between power consumption, hardware complexity, and spectral efficiency for hybrid architectures based on phase shifting networks and switching networks. Numerical results show that architectures based on switches obtain equal or better channel estimation performance to that obtained using phase shifters, while reducing hardware complexity and power consumption. For equal power consumption, all the hybrid architectures provide similar spectral efficiencies.

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Precodinglcombining and large antenna arra y s are essential in millimeter wave (mmWave) s y stems. In traditional MIMO s y stems, precodinglcombining is usuall y done digitall y at baseband with one radio frequenc y (RF) chain and one analog-to-digital converter (ADC) per antenna. The high cost and power consumption of RF chains and ADCs at mmWave frequencies make an all-digital processing approach prohibitive.When onl y a limited number of RF chains is available, h y brid architectures that split the precodinglcombining processing into the analog and digital domains are attractive. A previousl y proposed h y brid solution emplo y s phase shifters and mixers in the RF precodinglcombining stage. It obtains near optimal spectral efficiencies with a reduced number of RF channels. In this paper we propose a different h y brid architecture, which simplifies the hardware at the receiver b y replacing the phase shifters with switches. We present a new approach for compressed sensing based channel estimation for the h y brid architectures.Given the channel estimate, we propose a novel algorihtm that jointl y designs the antenna subsets selected and the baseband combining. Using power consumption calculations and achievable rates, we compare the performance of h y brid combining with antenna switching and phase shifting, showing that antenna selection is preferred in a range of operating conditions. I. INT RODUCTIONCommunication over millimeter wave (lmnWave) frequen cies [1] is the frontier for commercial wireless communication systems. Initial applications of mmWave to personal area networks (PAN) and local area networks (LAN) through the 60GHz unlicensed band are already standardized [2] and commercially available. The large bandwidths available at mmWave carrier frequencies also makes it interesting for 5G cellular systems [3], [4], [5], [6].MmWave communication requires very large multiple-input multiple-output (MIMO) systems to provide sufficient antenna aperture. Unfortunately, at mmWave there are additional hard ware constraints that have to imposed due to the practical limitations on the cost, complexity and power consumption with the current technology [7]. Due to mixed signal and baseband processing requirements, it may not be feasible to use one complete RF chain and one DAC or ADC per antenna, so precoding and combining can not be done entirely in the baseband. For this reason, systems like IEEE 802.11ad use analog beamforming / combining and only support single stream MIMO communication.of streams requires the use of precoding and combining, mak ing functions like low complexity and low overhead channel estimation more essential.In previous work, a hybrid architecture that accounts for hardware constraints has been proposed [8], [9], resulting in precoder/combiner design algorithms that divide the op timization process into the RF and the baseband stages, and channel estimation methods that exploit the sparse nature of the mmWave channel. This architecture is based on quantized phase shifters, as illustrate...

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