Won-Il Roh

Millimeter-wave beamforming as an enabling technology for 5G cellular communications: theoretical feasibility and prototype results

The ever growing traffic explosion in mobile communications has recently drawn increased attention to the large amount of underutilized spectrum in the millimeter-wave frequency bands as a potentially viable solution for achieving tens to hundreds of times more capacity compared to current 4G cellular networks. Historically, mmWave bands were ruled out for cellular usage mainly due to concerns regarding short-range and non-line-of-sight coverage issues. In this article, we present recent results from channel measurement campaigns and the development of advanced algorithms and a prototype, which clearly demonstrate that the mmWave band may indeed be a worthy candidate for next generation (5G) cellular systems. The results of channel measurements carried out in both the United States and Korea are summarized along with the actual free space propagation measurements in an anechoic chamber. Then a novel hybrid beamforming scheme and its link- and system-level simulation results are presented. Finally, recent results from our mmWave prototyping efforts along with indoor and outdoor test results are described to assert the feasibility of mmWave bands for cellular usage.

An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems

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.

Tens of Gbps support with mmWave beamforming systems for next generation communications

The recent strong demands for higher data rate support to cope with the explosive mobile data crunch has initiated research on the next generation (5G) wireless mobile communication technologies that could provide with drastic capacity increase. One of the promising candidates is to use wide spectrum in the mmWave bands, where a breakthrough to overcome the unfavorable channel properties needs to be preceded. In this paper, we propose a novel hybrid beamforming scheme that jointly combines RF beamforming and baseband precoding as a key enabling technique to make efficient use of mmWave channel. The link level simulation results show that the proposed scheme can almost achieve the optimal performance with noticeably reduced implementation complexity. Furthermore, system level simulation results are provided pointing out the possibility of tens of Giga-bits/sec data rate support with the proposed scheme.

Advancement of MIMO technology in WiMAX: from IEEE 802.16d/e/j to 802.16m

WiMAX is the first cellular standard that employs OFDMA technology and provides true integrated services for both fixed and mobile broadband access. Among the many new technologies adopted in WiMAX, MIMO antenna technology plays an essential role in delivering fast, rich-content, mobile broadband service reliably over extended coverage areas. In this article we provide a survey on the state of art of MIMO technologies in current WiMAX standards with an emphasis on practical engineering considerations. Moreover, we also briefly discuss the ongoing MIMO technologies in the evolution toward the next-generation WiMAX network.

Mobile's millimeter-wave makeover

People in real estate joke that there are only three things that matter in the business of buying and selling property: location, location, location. The same could be said for radio spectrum. The frequencies used for cellular communications have acquired the status of waterfront lots-highly coveted and woefully scarce. And like beach-home buyers in a bidding war, mobile operators must constantly jockey for these prime parcels, sometimes shelling out as much as tens of billions of dollars for just a small sliver of the electromagnetic pie.

Characterization of Pt-Cu-Fe ternary electrocatalysts supported on carbon black

Ternary Pt-Cu-Fe alloy catalysts, useful for low temperature fuel cells, were prepared from aqueous media, followed by heat treatment at 900 °C for various heating periods. Supported metal crystallites were characterized with various techniques including XRD, XPS, TEM and ICP-AES. XRD patterns indicate that the lattice structure of platinum changes from a face-centred cubic to a contracted facecentred tetragonal structure as it forms an alloy. As the heating period increases, the extent of formation of an ordered alloy increases and the formation is completed in 2.5 h, as confirmed by the intensity of superlattice diffraction lines. The presence of different oxidation states is confirmed by XPS and the amount of higher oxidation state is reduced by heat-treatment, but there is no evidence of development of a new photoelectron peak or shift in binding energy by alloy formation. For the electrochemical reduction reaction of oxygen in fuel cell operation, ordered alloys have shown improved catalytic activity compared to platinum alone. After the stability test in hot phosphoric acid, the ordered structure is preserved even though a significant amount of transition metal is dissolved, and some increase in particle size in the heat-treated catalysts is observed.

Interference Level Control in Mobile WiMAX Uplink System

Interference over thermal noise (IoT) level control is crucial to system performance and link quality in interference limited scenarios, especially for cell edge users. This paper proposes a simple and novel technique for IoT control by adopting the concept of load control where load is defined as the total interference impacting on neighbor sectors. The proposed technique is able to control IoT level accurately and is very robust against impairments in practical implementations, while minimizing the required signaling overhead.

Introduction to the Special Issue on Signal Processing for Millimeter Wave Wireless Communications

The eleven papers in this special section focus signal processing for millimeter wave (mmWave) wireless communications. These mmWave frequencies are defining a new era of wireless communication. The mmWave band relieves spectral gridlock at lower frequencies by offering much higher bandwidth communication channels than presently used in commercial wireless systems. The next generation of wireless local area networks is exploiting the mmWave unlicensed band at 60 GHz to provide multigigabit-per-second data rates. There is also growing interest in using mmWave licensed spectrum for 5G cellular systems. MmWave communication could also provide important benefits in other application scenarios like wearable networks, vehicular communications, or autonomous robots. The potential for mmWave is immense. Signal processing is critical for enabling the next generation of mmWave communication. Because of the wide bandwidth, overall complexity and mixed signal power consumption are significant concerns. The papers in this section bring together contributions from researchers and practitioners in the area of signal processing for wireless communications with an emphasis on communication at millimeter wave frequencies. I