Ji-Yun Seol

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.

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.

Multi-beam transmission diversity with hybrid beamforming for MIMO-OFDM systems

The ever-increasing traffic crunch for the wireless communication has drawn attention to the large spectrum available in the millimeter-wave bands as a potential means to achieve several fold mobile data traffic increase. While the channel characteristics at the millimeter-wave bands are known to be unfavorable for the mobile wireless communication purpose, the high gain available from massive array antenna facilitated by the short wavelength makes it possible to overcome the large path-loss. In this paper, we propose a hybrid beamforming architecture that combines an analog beamforming with array antennas and a digital precoding with multiple RF chains. Furthermore, we propose a multi-beam transmission diversity scheme for single stream transmission for single user MIMO operation. It is shown through various simulation results that the proposed hybrid beamforming scheme leads to considerable performance improvements even with limited feedback.

Antenna Grouping Based Feedback Compression for FDD-Based Massive MIMO Systems

Recent works on massive multiple-input multiple-output (MIMO) have shown that a potential breakthrough in capacity gains can be achieved by deploying a very large number of antennas at the base station. In order to achieve the performance that massive MIMO systems promise, accurate transmit-side channel state information (CSI) should be available at the base station. While transmit-side CSI can be obtained by employing channel reciprocity in time division duplexing (TDD) systems, explicit feedback of CSI from the user terminal to the base station is needed for frequency division duplexing (FDD) systems. In this paper, we propose an antenna grouping based feedback reduction technique for FDD-based massive MIMO systems. The proposed algorithm, dubbed antenna group beamforming (AGB), maps multiple correlated antenna elements to a single representative value using predesigned patterns. The proposed method modifies the feedback packet by introducing the concept of a header to select a suitable group pattern and a payload to quantize the reduced dimension channel vector. Simulation results show that the proposed method achieves significant feedback overhead reduction over conventional approach performing the vector quantization of whole channel vector under the same target sum rate requirement.

A new waveform enabling enhanced QAM-FBMC systems

Multiple filter-banks enable QAM transmission and reception in filter-bank multi-carrier (FBMC) systems. In this paper, we propose a new method to design a set of simultaneously used base filters for QAM-FBMC by optimizing the trade-off between spectrum confinement and self-interference. The increased freedom in multiple base filter designs brings merits of a tailor-made spectrum confinement and desirable bit error performance simultaneously. Simulations results show the superiority of the proposed QAM-FBMC systems to other post-OFDM waveform candidates.

Antenna grouping based feedback reduction for FDD-based massive MIMO systems

Recent works on massive multiple-input multiple-output (MIMO) have shown that a potential breakthrough in capacity gains can be achieved by deploying a very large number of antennas at the basestation. Although transmit-side channel state information (CSI) can be obtained by employing channel reciprocity in time division duplexing (TDD) systems, explicit feedback of CSI from the user to the basestation is required for frequency division duplexing (FDD) systems. In this paper, we propose an antenna grouping based feedback reduction technique for FDD-based massive MIMO systems. The proposed algorithm, dubbed antenna group beamforming (AGB), groups antenna elements using pre-designed patterns. The proposed method introduces the concept of using a header of overall feedback resources to select a suitable group pattern and the payload to quantize the effective channel vector. Simulation results show that the proposed method achieves significant feedback overhead reduction over conventional approach.

QAM-FBMC: A New Multi-Carrier System for Post-OFDM Wireless Communications

Recently, as asynchronous heterogeneous network scenario becoming one of the key features for next generation wireless communications, superior spectrum confinement as well as higher spectral efficiency compared to cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been taken into consideration for future radio access technologies. In this paper, we propose a new quadrature amplitude modulation based filter-bank multi-carrier (QAM-FBMC) system that provides with an inherent spectral efficiency gain against CP-OFDM, which comes from reduction of the redundancies such as cyclic prefix (CP) and guard-band, while keeping the symbol rate same as in CP-less OFDM. We propose a new transceiver structure consisting of at least two different filter-bank bases at both transmitter and receiver sides. Practical algorithms including channel estimation and equalization to mitigate multi-path fading channel without CP are proposed. Various evaluation results show that the proposed system performs comparable to the CP-OFDM system even without CP and guard-band reduction is also available from the well-confined spectrum.

On the hybrid beamforming with shared array antenna for mmWave MIMO-OFDM systems

Along with the attention drawn to the millimeter-wave frequency bands for the drastically increased data rate support for the next generation (5G) mobile communications, the hybrid beamforming architectures combining analog beamforming and digital precoding have been investigated as potential candidates to provide good compromise between the complexity and the performance. Among the different hybrid beamforming architectures, the architecture with so-called shared array antenna is analyzed with respect to the peak-to-mean envelope power ratio over antenna elements and the total average transmission power constraint upon multi-beam transmissions. It is shown that the non-trivial normalization coefficients should be applied to the hybrid beamforming architecture under consideration to achieve the average transmission power within the total power constraint. Taking into account the total power normalization for the hybrid beamforming architecture with shared array antenna, various simulation results are provided to show the performance of the multi-beam transmission diversity against the single-beam analog beamforming.

Introduction to QAM-FBMC: From Waveform Optimization to System Design

The asynchronous heterogeneous network scenario is becoming one of the key features for 5G mobile communications. Compared to CP-OFDM, which is 4G waveform, superior spectrum confinement as well as higher spectral efficiency has been taken into consideration for 5G waveform design. In this article, we introduce a new waveform called QAM-FBMC that provides superior spectrum confinement and higher spectral efficiency simultaneously against CP-OFDM. The article explains fundamental trade-offs in waveform design and shows how they can be optimized for QAM-FBMC. We also introduce the transmission and reception procedures for QAM-FBMC including its similarities and differences compared to well-known waveforms such as CP-OFDM and OQAM-FBMC. Performance evaluation results show where the spectral efficiency gain of QAM-FBMC over CP-OFDM comes from.

Pre-processing based soft-demapper for per-tone MIMO operation in QAM-FBMC systems

As a promising candidate to replace cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM), a new quadrature amplitude modulation filter-bank multi-carrier (QAM-FBMC) has been proposed for enabling spectral efficiency increase and spectrum confinement while keeping near orthogonality. QAM transmission can combat intrinsic interference of conventional offset-QAM (OQAM)-FBMC, which is main bottleneck in channel estimator and multiple antenna transceiver design. Compared to existing single filter based QAM-FBMC, the new QAM-FBMC has more optimization freedom in multiple prototype filters to increase self signal-to-interference ratio (SIR). In this paper, the possibility of per-tone multiple input multiple output (MIMO) reception in the new QAM-FBMC system is exploited. Our MIMO receiver is constructed with per-tone MIMO equalization in oversampled signal domain and its soft-demapper in symbol domain. Not only the separate log likelihood ratio (LLR) calculation per stream but also joint LLR calculation (i.e., soft maximum likelihood (ML) detection) is implemented with pre-processing. Simulation results show that the per-tone MIMO reception in QAM-FBMC is comparable to that of CP-OFDM.