http://ieeexplore.ieee.org/Xplore An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems Robert W. Heath Jr., Nuria Gonz´ alez-Prelcic, Sundeep Rangan, Wonil Roh, and Akbar Sayeed Abstract—Communication at millimeter wave (mmWave) fre- quencies is defining a new era of wireless communication. The mmWave band offers higher bandwidth communication channels versus those presently used in commercial wireless systems. The applications of mmWave are immense: wireless local and personal area networks in the unlicensed band, 5G cellular systems, not to mention vehicular area networks, ad hoc networks, and wear- ables. Signal processing is critical for enabling the next generation of mmWave communication. Due to the use of large antenna arrays at the transmitter and receiver, combined with radio frequency and mixed signal power constraints, new multiple- input multiple-output (MIMO) communication signal processing techniques are needed. Because of the wide bandwidths, low complexity transceiver algorithms become important. There are opportunities to exploit techniques like compressed sensing for channel estimation and beamforming. This article provides an overview of signal processing challenges in mmWave wireless systems, with an emphasis on those faced by using MIMO communication at higher carrier frequencies. I. I NTRODUCTION T HE millimeter wave (mmWave) band is the frontier for commercial – high volume consumer – wireless commu- nication systems [1]. MmWave makes use of spectrum from 30 GHz to 300 GHz whereas most consumer wireless systems operate at carrier frequencies below 6 GHz. The main benefit of going to mmWave carrier frequencies is the larger spectral channels. For example, channels with 2 GHz of bandwidth are common for systems operating in the 60 GHz unlicensed mmWave band. Larger bandwidth channels mean higher data Copyright (c) 2016 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org R. W. Heath Jr. is with The University of Texas at Austin, Austin, TX, USA (email: rheath@utexas.edu). Nuria Gonzalez-Prelcic is with the University of Vigo, Spain, (email: nuria@gts.uvigo.es). Sundeep Rangan is with New York University, USA, (email: srangan@nyu.edu). Wonil Roh, is with Samsung Electronics, South Korea, (email: wonil.roh@samsung.com). Akbar Sayeed, is with the University of Wisconsin-Madison, USA, (email: akbar@engr.wisc.edu). R. Heath would like to acknowledge support from the National Science Foundation under grant numbers NSF-CCF-1319556, NSF-CCF-1514275, and NSF-CCF-1527079, the U.S. Department of Trans- portation through the Data-Supported Transportation Operations and Planning (D-STOP) Tier 1 University Transportation Center, the Intel / Verizon 5G program, MERL, Nokia, Huawei, and Toyota. N. Gonz´ alez-Prelcic would like to acknowledge support from the Spanish Government and the European Re- gional Development Fund (ERDF) under projects TACTICA and COMPASS (TEC2013-47020-C2-1-R). A. Sayeed would like to acknowledge support from the National Science Foundation under grant numbers ECCS-1247583 and IIP-1444962, and the Wisconsin Alumni Research Foundation. Manuscript received June 01, 2015; revised October 30, 2015. Digital Object Identifier: 10.1109/JSTSP.2016.2523924 rates. Despite the recent interest in mmWave, the study of mmWave is in fact as old as wireless itself. Some of the first experiments like those of Bose and Lebedev [2] were performed in the 1890s in the mmWave band. The first standardized consumer radios were in the 60 GHz unlicensed band. WirelessHD [3] is the name for the successful personal area network (PAN) technology developed by a consortium of companies. It is used primarily to replace cables that carry uncompressed high definition video. IEEE 802.11ad [4] is a wireless local area network (WLAN) standard. It was essentially developed in the former WiGig consortium that was later absorbed into the WiFi Alliance. The development of wireless communication in the 60 GHz unlicensed band was the topic of tremendous amounts of research [5]–[13]. The aforementioned PAN and LAN standards use about 2 GHz of bandwidth and support OFDM (orthogonal frequency division multiplexing) or SC-FDE (single-carrier frequency- domain equalization) type modulations to provide data rates up to 6 Gbps. Beamforming through several (up to four) small antenna arrays is also supported. Evolutions of these standards are expected to support more sophisticated forms of multiple- input multiple-output (MIMO) communication for higher data rates. Products based on WirelessHD have been available for several years while those based on IEEE 802.11ad are starting to ship in higher volumes. It seems that WLAN and PAN devices operating at 60 GHz will be the first widely deployed consumer wireless devices at mmWave. MmWave is also receiving tremendous interest by academia, industry, and government for 5G cellular systems [14]–[19]. The main reason is that spectrum available in sub-6 GHz bands is limited. Though signal processing approaches like cognitive radio [20], [21] free more spectrum, it still is not enough if gigabit-per-second data rates are required. Initial work has established the viability of 5G cellular through propagation studies and later through system capacity analysis. Surprisingly, there is much earlier work on mmWave cellular which proposes the integration of voice/data communication at 60 GHz [22]. The Federal Communication Commission in the USA is among the first to back enthusiasm behind 5G with spectrum for mobile cellular applications [19]. MmWave is already a significant footprint wireless back- haul. Traditional physical layer designs for 60 GHz back- haul assume expensive directional antennas, reducing cost advantages over wired solutions [1]. Low cost mmWave technologies with adaptive arrays, however, are actively being 1932-4553 c  2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.