Communication at millimeter wave (mmWave) frequencies 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 wearables. 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.

An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems

Hybrid analog/digital multiple-input multiple-output architectures were recently proposed as an alternative for fully digital-precoding in millimeter wave 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 that 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 estimate, we develop two algorithms for the design of the hybrid combiner based on switches and analyze the achieved spectral efficiency. Finally, we study the tradeoffs 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.

Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches?

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.

Before the emergence of ultra-wideband (UWB) radios, widely used wireless communications were based on sinusoidal carriers, and impulse technologies were employed only in specific applications (e.g. radar). In 2002, the Federal Communication Commission (FCC) allowed unlicensed operation between 3.1–10.6 GHz for UWB communication, using a wideband signal format with a low EIRP level (−41.3dBm/MHz). UWB communication systems then emerged as an alternative to narrowband systems and significant effort in this area has been invested at the regulatory, commercial, and research levels.

IEEE Transactions on Microwave Theory and Techniques and Antennas and Propagation Announce a Joint Special Issue on Ultra-Wideband (UWB) Technology

For the first time to the best of our knowledge, this paper provides an overview of millimeter-wave (mmWave) 5G antennas for cellular handsets. Practical design considerations and solutions related to the integration of mmWave phased-array antennas with beam switching capabilities are investigated in detail. To experimentally examine the proposed methodologies, two types of mesh-grid phased-array antennas featuring reconfigurable horizontal and vertical polarizations are designed, fabricated, and measured at the 60 GHz spectrum. Afterward the antennas are integrated with the rest of the 60 GHz RF and digital architecture to create integrated mmWave antenna modules and implemented within fully operating cellular handsets under plausible user scenarios. The effectiveness, current limitations, and required future research areas regarding the presented mmWave 5G antenna design technologies are studied using mmWave 5G system benchmarks.

Millimeter-Wave 5G Antennas for Smartphones: Overview and Experimental Demonstration

This paper presents the design and analysis of an improved wideband in-phase/quadrature (I/Q) network and its implementation in a wideband phased-array front-end. It is found that the addition of two resistors (Rs) in the all-pass I/Q network results in improved amplitude and phase performance versus capacitance loading and frequency, which is essential for wideband millimeter-wave applications. A prototype 60-80-GHz phased-array front-end based on 0.13-μm SiGe BiCMOS is demonstrated using the improved quadrature all-pass filter and with 4-bit phase-shifting performance at 55-80 GHz. Application areas are in wideband millimeter-wave systems.

An Improved Wideband All-Pass I/Q Network for Millimeter-Wave Phase Shifters

This paper presents a 16-element phased-array receiver for 76-84-GHz applications with built-in self-test (BIST) capabilities. The chip contains an in-phase/quadrature (I/Q) mixer suitable for automotive frequency-modulation continuous-wave radar applications, which is also used as part of the BIST system. The chip achieves 4-bit RF amplitude and phase control, an RF to IF gain of 30-35 dB at 77-84 GHz, I/Q balance of and at 76-84 GHz, and a system noise figure of 18 dB. The on-chip BIST covers the 76-84-GHz range and determines, without any calibration, the amplitude and phase of each channel, a normalized frequency response, and can measure the gain control using RF gain control. System-level considerations are discussed together with extensive results showing the effectiveness of the on-chip BIST as compared with standard S-parameter measurements.

A 76–84-GHz 16-Element Phased-Array Receiver With a Chip-Level Built-In Self-Test System

A 16-element phased array receiver with built-in-self test (BIST) is demonstrated at 76–84 GHz. The BIST technique employs a miniature capacitive coupler located at the input port of each phased-array channel, and uses the receiver I/Q down-converter to measure the amplitude and phase of each channel. This allows for measuring the response of individual channels if one channel is turned on at a time, and an on-chip array factor if several channels are turned on and the phase between them is varied. BIST measurements done at 76–84 GHz agree very well with S-parameter measurements with a matched load and an open circuit load at each port, and show that this technique can be used to greatly lower the testing cost and improve the self-calibration of mm-wave phased-array RFICs.

A 76–84 GHz 16-element phased array receiver with a chip-level built-in-self-test system

This paper presents a 76-84 GHz low-power 4- element phased array receiver built using a 0.13 μm BiCMOS process. The power consumption is reduced by using a single-ended design and alternating the amplifiers and phase shifter cells to result in a low noise figure at a low power consumption. A variable gain amplifier and an 11° trim bit are used to correct for the rms gain and phase errors at different operating frequencies. The phased array consumes 32 mW per channel and results in a gain of 10-19 dB at 76-84 GHz, a noise figure of 10.5 ±0.5 dB at 80 GHz and an rms gain and phase error <;0.8 dB and <;7.2 °, respectively, up to 81 GHz, and <;1.1 dB and 10.4° up to 84 GHz. The phased array also shows a channel to channel coupling of <; - 30 dB up to 84 GHz. To our knowledge, this work presents state-of-the-art on-chip performance at W-band frequencies.

A Low-Power BiCMOS 4-Element Phased Array Receiver for 76–84 GHz Radars and Communication Systems

This talk will present our latest work on silicon RFICs for phased-array applications with emphasis on very large chips with built-in-self-test capabilities for 5G systems. SiGe is shown to be ideal for mm-wave applications due to its high temperature performance (automotive radars, base-stations, defense systems, etc.) and lower power consumption. These chips drastically reduce the cost of microwave and millimeter-wave phased arrays by combining many elements on the same chip, together with digital control and some cases, high-efficiency antennas. The phased-array chips also result in an easier packaging scheme using either a multi-layer PCB or wafer-level packages. We believe that this family of chips will be essential for millimeter-wave 5G communication systems.

Sang Young Kim

Papers

An Improved Wideband All-Pass I/Q Network for Millimeter-Wave Phase Shifters

A 76–84-GHz 16-Element Phased-Array Receiver With a Chip-Level Built-In Self-Test System

A 76–84 GHz 16-element phased array receiver with a chip-level built-in-self-test system

A Low-Power BiCMOS 4-Element Phased Array Receiver for 76–84 GHz Radars and Communication Systems

Millimeter-wave large-scale phased-arrays for 5G systems