Jaekon Lee

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.

Radio-Over-Fiber System for TDD-Based OFDMA Wireless Communication Systems

We report on a fiber-optic transport system for next-generation wireless-communication systems utilizing 4 times 4 multiple-input multiple-output (MIMO) orthogonal-frequency-division-multiple-access (OFDMA) technology. Our system supports time-division-duplex (TDD)-based wireless signals operating at 3.775 GHz. To accommodate the TDD-based MIMO signals over a single strand of optical fiber, we utilize nine-channel coarse wavelength-division-multiplexed optical channels: one for link delay measurement and TDD control signal transmission, four for downlink, and the others for uplink. The system first measures the propagation delay between the central base station (CBS) and the remote antenna (RA) and sends the result to delay modules to compensate for the delay added by the transmission link. This procedure makes the CBS and RA emit the downstream signals simultaneously into the air and, consequently, helps avoid the performance degradation caused by the propagation delay of the radio-over-fiber system. The system then sends the MIMO signals together with TDD control signals to the RA. Our experimental demonstration is carried out with 1-Gb/s OFDMA signals having pilot, control, and data channels. For downstream, the error vector magnitudes (EVMs) are measured to be < -30 dB after a 3.9-km transmission over conventional single-mode fiber when the antenna output power is set to be 24 dBm. For upstream, the best EVMs are measured to be < -35 dB. Both the downstream and upstream performance is limited by the shot and thermal noise of the receiver when the signal power is low, whereas nonlinear distortions of electrical amplifiers start to degrade the system performance as the signal power increases. We also measure the crosstalk between channels. It is measured to be less than -42 dB for all channels, which is found to be caused by board-to-board interference at the RA.

A CQI Feedback Reduction Scheme for Multi-User MIMO/OFDMA Systems

An opportunistic scheduling is adopted to improve the capacity of the system by exploiting the multiuser diversity of multiuser MIMO (MU-MIMO)systems. However, it requires a large amount of feedback carrying the channel quality information (CQI) of each user to the transmitter. The considered per user unitary and rate control (PU2RC) needs to feedback the preferred precoding index and its CQIs, and it has a defect in scheduling the streams for the grouped user. In order to overcome these drawbacks, a novel feedback reduction scheme is proposed and is modified for OFDMA systems in this paper. It employs transmitter controlled precoding and opportunistic feedback (TCP-OFB) in conjunction with frequency scheduling. The simulation results demonstrate that TCP-OFB/OFDMA shows comparable performance to PU2RC while it only requires far reduced feedback load.

Experiment of 28 GHz Band 5G super wideband transmission using beamforming and beam tracking in high mobility environment

This paper presents some results of experimental trial in high mobility environment for the 5th generation (5G) mobile communication systems using 28 GHz band. In order to tackle rapidly increasing traffic for 2020 and beyond, new radio access network for the 5G mobile communication systems will introduce the use of higher-frequency bands such as spectra higher than 10 GHz to achieve higher capacity and super high bit rate transmission of several tens of Gbps. The target of this experimental trial is to evaluate the effectiveness of using 28 GHz band with super-wide bandwidth of 800 MHz for 5G mobile communication systems. To compensate large path-loss in higher frequency, the beamforming based on Massive multiple-input multiple-output (MIMO) is one of promising techniques and can be combined with spatial multiplexing of multiple data streams to achieve much higher capacity. In addition, to support the mobility of mobile station (MS), beam tracking technique is important. In this paper, we show some results of the outdoor experiment of Massive MIMO beamforming combined with spatial multiplexing in high mobility environment with MS speed of up to 60 km/h by using prototype system with base station (BS) having 96-element array antenna and MS having smartphone-shape antenna with 8 elements. We also show that maximum throughput of 3.77 Gbps can be achieved with MS speed of 60 km/h by using beamforming with 2-stream multiplexing and beam tracking.

A Compact CORDIC Algorithm for Synchronization of Carrier Frequency Offset in OFDM Modems

In this letter, a compact CORDIC algorithm is proposed to efficiently implement a synchronization block for carrier frequency offset (CFO) in OFDM modems. The compact CORDIC algorithm allows us to combine a CFO estimation block and a CFO compensation block into a single CFO synchronization block. It is shown by FPGA implementation results that the compact CORDIC algorithm can achieve a significant reduction in hardware complexity and latency for implementing the synchronization block in OFDM modems.