Sooyong Choi

Optimal Duplex Mode for DF Relay in Terms of the Outage Probability

This paper deals with a full-duplex relay (FDR) system over Rayleigh fading channels. The exact outage probability of FDR is derived as a closed form to consider interferences from full duplex. Then, we obtain the conditions of the signal-to-noise ratio (SNR) and the signal to interface ratios (SIRs) for cases of FDR showing a lower outage probability than that of the half-duplex relay (HDR) system under the target outage probability. According to this condition, FDR is superior to HDR with lower SIRs in the low-SNR region rather than in the high-SNR region. In addition, the target outage probability is only satisfied when the SNR and SIRs are within the boundaries. These boundaries vary due to the target rate, the channel states of each link, and the target outage probability.

Non-orthogonal Multiple Access in a Downlink Multiuser Beamforming System

In this paper, we propose a non-orthogonal multiple access-based multiuser beamforming (NOMA-BF) system designed to enhance the sum capacity. In the proposed NOMA-BF system, a single BF vector is shared by two users, so that the number of supportable users can be increased. However, sharing a BF vector leads to interference from other beams as well as from the other user sharing the BF vector. Therefore, to reduce interference and improve the sum capacity, we additionally propose a clustering and power allocation algorithm. This clustering algorithm, which selects two users with high correlation and a large gain-difference between their channels, can reduce the interference from other beams and from the other user as well. Furthermore, power allocation ensures that each users transmit power is allocated so as to maximize the sum capacity. Numerical results verify that the proposed NOMA-BF system improves the sum capacity, compared to the conventional multiuser BF system.

Joint Mode Selection and Power Allocation Scheme for Power-Efficient Device-to-Device (D2D) Communication

This paper proposes a power-efficient mode selection and power allocation scheme in device-to-device (D2D) communication system as an underlay coexistence with cellular networks. The proposed scheme is performed based on the exhaustive search of all possible mode combinations of the devices which consist of the mode indices for all devices in the system. Specifically, the proposed scheme consists of two steps. First, we calculate the optimal power with respect to the maximum power-efficiency for all possible modes of each device. Since the power-efficiency is not a concave function for the transmission power, we obtain the suboptimal solution by using the concavity of the lower and upper bound for the power-efficiency. The powerefficiencies for all possible modes of each device are obtained by the suboptimal power allocation in the first step. In the second step, we select a mode sequence which has the maximal power-efficiency among all possible mode combinations of the devices based on the obtained power-efficiencies in the first step. Then we can jointly obtain the suboptimal transmission power and the mode maximizing the power-efficiency. The proposed suboptimal scheme for the power allocation and mode selection performs close to the upper bound with respect to the power-efficiency. The simulation results also show that the proposed scheme outperforms the conventional schemes with respect to the power-efficiency and system capacity.

Performance of an MC-CDMA system with frequency offsets in correlated fading

The performance of an MC-CDMA system with carrier frequency offsets in correlated multipath fading is mathematically analyzed. The bit error rate is obtained from such analysis for the uplink Rayleigh fading channel. The derived results show that the performance of the MC-CDMA system is sensitive to the frequency offset, and the correlation of the subcarriers. That is, the performance degradation of the MC-CDMA system is caused by both carrier frequency offset and correlation among subcarriers. From the derived results, however, the performance degradation due to correlation among subcarriers is more severe than effect of frequency offset.

A decision feedback recurrent neural equalizer as an infinite impulse response filter

An adaptive decision feedback recurrent neural equalizer (DFRNE), which models a kind of an IIR structure, is proposed. Its performance is compared with the traditional linear and nonlinear equalizers with FIR structures for various communication channels. The small size and high performance of the DFRNE makes it suitable for high-speed channel equalization.

A Beamforming Codebook Restriction for Cross-Tier Interference Coordination in Two-Tier Femtocell Networks

This paper examines a novel multichannel multi-antenna-based cross-tier interference coordination strategy for two-tier femtocell networks. To reduce the cross-tier interference and improve the aggregate throughput of two-tier femtocell networks without exchanging feedback information by backhauling, we propose a beamforming codebook restriction strategy. The beamforming codebook restriction strategy enables the femtocell users to select the best channel that is robust to cross-tier interference before the transmission by reducing the size of the macrocell beamforming codebook. Although restricting the beamforming codebook increases the quantization error for macrocell users, the opportunistic channel selection strategy and the proportional fair scheduler compensate for the increased quantization error by exploiting the channel selection diversity gain and the multiuser diversity gain. Both analytical and numerical results demonstrate that the proposed strategies collaboratively improve the aggregate throughput of two-tier femtocell networks.

Performance of MC-CDMA systems in non-independent Rayleigh fading

In this paper, the average bit error probability of an uplink multi-carrier code division multiple access (MC-CDMA) system using maximal-ratio combiner (MRC) is derived for a frequency-selective slow fading channel. In contrast to a conventional multicarrier channel model in which the subcarriers experience independent subchannels, the subcarriers in MC-CDMA system undergo the correlated fading channel in a general tapped-delay line (TDL) model with variable channel parameters. Numerical results are shown based on the derived result, to show that conventional multicarrier channels tend to overemphasize the performance of an MC-CDMA system.

Network massive MIMO for cell-boundary users: From a precoding normalization perspective

In this paper, we propose network massive multiple-input multiple-output (MIMO) systems, where three radio units (RUs) connected via one digital unit (DU) support multiple user equipments (UEs) at a cell-boundary through the same radio resource, i.e., the same frequency/time band. For precoding designs, zero-forcing (ZF) and matched filter (MF) with vector or matrix normalization are considered. We also derive the formulae of the lower and upper bounds of the achievable sum rate for each precoding. Based on our analytical results, we observe that vector normalization is better for ZF while matrix normalization is better for MF. Given antenna configurations, we also derive the optimal switching point as a function of the number of active users in a network. Numerical simulations confirm our analytical results.

New OFDM Channel Estimation with Dual-ICI Cancellation in Highly Mobile Channel

The number of coefficients for estimating time-varying channel is generally much larger than the maximum number of observable data. Linearly time-varying channel (LTV) models have been widely used as a way of reducing the number of these channel parameters. This approach allows estimation using the channel slope and channel frequency response between symbols. A fatal problem with this approach, however, is that before it is even initiated, the estimated channel in the frequency domain has already been degraded by inter-carrier interference (ICI). We demonstrate this by analyzing the channel estimation mean square error (MSE) and signal to interference and noise ratio (SINR) after ICI cancellation with respect to the LTV model. We then propose a new channel estimation technique using dual-ICI cancellation which resolves this problem by pre-canceling ICI for the channel estimation using the channel of the previous symbol and post-canceling ICI for symbol detection with the more accurate channel information. The process is initialized using the first symbol as a preamble to estimate the channel in the time domain. Performance comparisons of MSE and SINR show that the proposed method is better suited to fast-fading channels than the conventional method, where ICI dominates the interference and noise power.

A new filter-bank multicarrier system for QAM signal transmission and reception

A filter-bank multicarrier - quadrature amplitude modulation (FBMC-QAM) system with two prototype filters for transmitting QAM signal is proposed. The proposed transmitter performs separate filtering for the even-numbered sub-carrier symbols and the odd-numbered sub-carrier symbols. We derive the orthogonality conditions for a FBMC-QAM system without the intrinsic interference. In order to satisfy the suggested orthogonality conditions, we perform a kind of block interleaving for the odd-numbered sub-carrier filtering. The receiver structure is the counterpart to the transmitter. Numerical results showed that the proposed FBMC-QAM system has almost the same bit error rate (BER) performance compared to the FBMC-OQAM and the orthogonal frequency division multiplexing (OFDM) system. with the proposed FBMC-QAM, multiple-input multiple-output transmission schemes and channel estimation schemes can be utilized similarly as in OFDM.