Jihyung Kim

Cell Selection and Resource Allocation for Interference Management in a Macro-Picocell Heterogeneous Network

A heterogeneous network (HetNet) based on the long-term evolution advanced standard has been developed to improve the system performance; in this HetNet, macrocells and low-power nodes, such as picocells, coexist. Further, cell range expansion has been introduced to encourage data offloading in a HetNet. In this paper, we propose a cell selection scheme based on the signal-to-interference-plus-noise ratio to achieve an optimal offloading effect. We also propose a resource allocation scheme in the frequency and time domains, to reduce interference in a HetNet. In the frequency domain, the macrocells allocate a frequency band by using soft frequency reuse, and the picocells choose the sub-bands that are not used in the macrocell sector, to avoid interference. In addition, we found that it was difficult to allocate a limited frequency resource. Therefore, the cross-tier interference can be managed by using an almost blank subframe (ABS) with a flexible ABS ratio, to improve spectrum efficiency in the time domain. The simulation results show that the proposed schemes can improve the spectrum efficiency of both the macro-user equipment and the pico-user equipment, as well as improve the overall UE performance.

Novel Hybrid Receiver for Interference Cancellation and Suppression in Sidehaul System

Recently, the 3rd Generation Partnership Project (3GPP) has developed a sidehaul system to cope with rapidly increasing mobile data traffic. Nevertheless, numerous challenging technical problems remain. One of the major problems is interference management between small cells. In this paper, we propose a novel hybrid receiver for full suspension cancellation to reduce the interference from neighboring cells in the sidehaul system. The proposed receiver can cancel and suppress interference by integrating the interference rejection combining technique with successive interference cancellation. We perform a simulation based on the 20-MHz bandwidth of the 3GPP LTE-Advanced technology. Simulation results show that the proposed receiver can achieve a lower error rate and a higher throughput compared to conventional receivers.

Design of synchronization preambles for LTE-Advanced D2D communications

In this paper, an efficient structure of device-to-device (D2D) synchronization preamble is proposed to meet the tightened time and frequency synchronization requirements for LTE-Advanced D2D communication. The proposed preamble structure is transmitting signals only at odd subcarriers, and not transmitting signals at the other subcarriers to minimize the effect of inter-carrier interference. According to the simulation results, the proposed preamble structure provides improved time offset estimation performance compared to the repeated primary synchronization signals without degrading frequency offset estimation performance.

Gradual Time Synchronization for Wireless Mesh Networks Based on OFDMA

In this paper, we propose a gradual time synchronization scheme for wireless mesh networks (WMNs) based on orthogonal frequency division multiple access (OFDMA). For distributed time synchronization in WMNs, each node controls its transmission time and fast Fourier transform (FFT) starting time within the possible range according to the requests of neighbor nodes. The possible range is determined by the cyclic prefix (CP) and the guard interval (GI) between transmitted sub frames and received sub frames. Simulation results show that this process leads to gradual synchronization of all nodes in the network.

A Position-based Resource Allocation Scheme for V2V Communication

Recently, the 3rd Generation Partnership Project has developed vehicle-to-everything communication to manage and advance future traffic safety and mobility. In this study, we propose a position-based resource allocation scheme for vehicle-to-vehicle communication. The proposed scheme allocates a different frequency and time resources based on vehicle speed, density, direction, and position. Our performance analysis is based on urban and freeway cases involving periodic message transmission. Simulation results show that the proposed scheme can improve the cumulative distribution function of packet reception ratio (PRR) and average PRR.

Novel Detection Scheme for LSAS Using Power Allocation in Multi User Scenario with LTE-A and MMB Channels

Massive MIMO (also known as the “Large-Scale Antenna System”) enables a significant reduction of latency on the air interface with the use of a large excess of service-antennas over active terminals and time division duplex operation. For large-scale MIMO, several technical issues need to be addressed (e.g., pilot pattern design and low-antenna power transmission design) and theoretically addressed (e.g., channel estimation and power allocation schemes). In this paper, we analyze the ergodic spectral efficiency upper bound of a large-scale MIMO, and the key technologies including channel uplink detection. We also present new approaches for detection and power allocation. Assuming arbitrary antenna correlation and user distributions, we derive approximations of achievable rates with linear detection techniques, namely zero forcing, maximum ratio combining, minimum mean squared error (MMSE) and eigen-value decomposition power allocation (EVD-PA). While the approximations are tight in the large system limit with an infinitely large number of antennas and user terminals, they also match our simulations for realistic system dimensions. We further show that a simple EVD-PA detection scheme can achieve the same performance as MMSE with one order of magnitude fewer antennas in both uncorrelated and correlated fading channels. Our simulation results show that our proposal is a better detection scheme than the conventional scheme for LSAS. Also, we used two channel environment channels for further analysis of our algorithm: the Long Term Evolution Advanced channel and the Millimeter wave Mobile Broadband channel.

Advanced Receiver Based on Soft Decision for Interference Suppression and Cancellation in a Sidehaul System

Recently, the 3rd Generation Partnership Project (3GPP) developed a sidehaul system to cope with rapidly increasing mobile data traffic. However, numerous challenging technical problems remain to be resolved. One of the major problems is interference management between small cells. In this paper, we propose an advanced receiver based on soft decision to reduce the interference from neighboring cells in the sidehaul system. The proposed receiver can suppress and cancel interference by integrating the interference rejection combining technique with successive interference cancellation. The interference signal is updated using soft information expressed by a log-likelihood ratio. We perform a simulation based on the 20-MHz bandwidth of the 3GPP LTE-Advanced technology. Simulation results show that the proposed receiver can achieve a lower error rate and a higher throughput compared to conventional receivers.

Design and Analysis of Novel Precoding Scheme for LSAS Using Power Allocation

Massive MIMO systems (also known as “large-scale antenna system” and “full-dimension MIMO”) significantly reduce air interface latency by using a large excess of service antennas over active terminals and time-division duplex operation. We consider the downlink of a time-division duplexing multicell multiuser MIMO system in which the base transceiver stations are equipped with a very large number of antennas. Assuming channel estimation through uplink pilots, arbitrary antenna correlations, and user distributions, we derive approximations of achievable rates with linear precoding techniques—namely, zero forcing, matched filtering, eigen-beamforming (EBF), and regularized zero-forcing (RZF). The approximations are tight in the large system limit with an infinitely large number of antennas and user terminals, but they match our simulations for realistic system dimensions. We further show that a simple EBF precoding scheme can achieve the same performance as RZF with one order of magnitude fewer antennas in both uncorrelated and correlated fading channels. Our simulation results show that our proposed precoding scheme is better than the conventional scheme. Moreover, we have used two channel environments for further analysis of our algorithm—long-term evolution advanced and millimeter wave mobile broadband channels.

Performance analysis for time synchronization of D2D communication in heterogeneous networks

In this paper, we describe time synchronization for D2D communication in heterogeneous networks. The transmission time and the detection time for D2D communication are considered in the case that each node basically has the uplink transmission time which is used for time synchronization with the base station. The analysis shows that the suitable transmission and detection times are generally the frame starting time or the uplink transmission time although they depend on the relationship among the location of received nodes, the CP length, the D2D radius, and the cell radius. In addition, the performance analysis demonstrates that the arbitrary transmission and detection times can result in serious performance degradation.

Performance analysis for channel estimation in partial co-channel environments

In this paper, we describe a guideline of pilot sequence design for multi-channel estimation using uplink DMRS of LTE-A in the partial co-channel resource allocation. In the D2D or small cell system the partial co-channel environment can be frequently occurred, and thus multi-channel estimation is required for interference mitigation. We consider three methods with uplink DMRS for channel estimation in the partial co-channel allocation: a direct application, a separated pilot sequence design, and a suitable matching between pilot sequences. The performance analysis presents that the direct application of uplink DMRS results in serious performance degradation in the partial co-channel allocation, while the separated pilot sequence design and the suitable matching between pilot sequences maintain the channel estimation performance.