Seokjung Kim

Performance Analysis of Opportunistic Relaying Scheme with Outdated Channel Information

In this paper, we investigate the effects of outdated channel state information (CSI) on an opportunistic relaying scheme (ORS) consisting of relay selection and data transmission phases. The outage probability and average symbol error rate (ASER) of the ORS are derived in a closed-form expression for an arbitrary link signal-to-noise ratio (SNR) when the CSIs in the source-relay and relay-destination links are outdated. We also analyze the diversity order and coding gain in terms of the outage probability and ASER. Analytical and numerical results show that when the CSIs in either the source-relay or relay-destination links are outdated, the diversity order is reduced to one. The overall performance of the ORS is mainly degraded by inaccurate CSI in the relay-destination link. Conversely, CSI accuracy in the source-relay link has a negligible effect on the performance of the ORS unless the CSI is perfect.

An Efficient Relay Selection Strategy for Random Cognitive Relay Networks

Cognitive radio networks using relays have been introduced to enhance secondary network performance by having secondary users (SUs) transmit in two hops. This paper develops a framework for a random cognitive relay network (RCRN) where the primary, secondary, and relay nodes are randomly distributed according to Poisson point processes (PPPs) to investigate the effect of the random nature of the relays on secondary outage performance. We first derive the optimal relay location in terms of the outage probability of the secondary network. In addition, we provide the outage probability of the RCRN based on the optimal relay selection strategy that compares the instantaneous channel state information (CSI) of all relays. We then propose an efficient relay selection strategy that reduces the selection complexity and the feedback burden by limiting candidate relays to those located close to the optimal relay location. This paper sheds new light on the design of an efficient RCRN with respect to elements such as the effects of network parameters and the candidate relay set size on the RCRN outage probability.

Average SNR and Ergodic Capacity of Reactive DF Relaying System with Outdated Channel State Information

In this paper, we investigate the effects of outdated channel state information (CSI) in opportunistic relaying systems on the ergodic capacity. The CSI in the data transmission instant may be different from that used in the relay selection instant due to channel variation. When the CSI in the relay-destination link is outdated, the average signal-to-noise ratio (SNR) and the ergodic capacity of the reactive decode and forward (DF) relaying system are derived. In addition, the effective ergodic capacity which is the spectral efficiency of the successful transmissions excluding the outage events is also derived. The numerical results show that if the CSI in the relay-destination link is outdated, the loss in the ergodic capacity is remarkable compared to the perfect CSI case. However, the level of the inaccuracy of the CSI does not affect the performance of the ergodic capacity in the reactive DF relaying system. In other words, no capacity degradation occurs even if the CSI inaccuracy becomes much larger.

Adaptive Mode Selection in D2D Communications Considering the Bursty Traffic Model

This letter proposes a new mode selection scheme to improve the overall packet rates for device-to-device (D2D) communications systems in the bursty traffic model. We consider three types of D2D modes, i.e., dedicated mode, cellular mode, and reuse mode. In the full-buffer traffic model, if all the uplink (UL) channels are occupied by cellular users, the dedicated and cellular modes are not available for D2D user equipments (DUEs) to choose. However, in bursty traffic model, the traffic load is time-varying and DUEs have the option to select the dedicated or cellular modes. To determine the best mode for a DUE in the bursty traffic, we propose a mode selection scheme, which considers the traffic load for the bursty traffic model. Numerical results show that the proposed scheme achieves a remarkable enhancement in average end-to-end delay, dropping probability, and packet rate.

A Robust Timing Recovery Algorithm for OFDM Systems

We propose a new robust timing recovery algorithm with small symbol timing jitter for orthogonal frequency-division multiplexing (OFDM) systems. The concept of coarse FFT window selection and adaptive reference pilot generation are employed in this paper. This proposed scheme avoids inter-symbol interference (ISI) as well as delayed locked loop (DLL) tracking failure over frequency selective time-varying channels. In addition, the proposed algorithm has much smaller and smoother steady state DLL tracking error than the conventional algorithm.

Capacity analysis of outdated relay selection in opportunistic relaying system using an adaptive transmission technique

In this paper, we investigate the impact of outdated channel state information (CSI) on the channel capacity of an opportunistic relaying system in conjunction with adaptive transmission techniques. We derive the closed-form expressions of the channel capacity and its associated outage probability for four adaptive transmission techniques: (1) optimal rate adaptation with constant transmit power, (2) optimal transmit power and rate adaptation, (3) channel inversion with fixed rate, and (4) truncated channel inversion with fixed rate. Analytical and numerical results show the effects of outdated CSI on the capacity of the optimal power and rate adaptation and the sub-optimal channel inversion techniques. In addition, we compare the channel capacity among various adaptive transmission techniques in the presence of outdated CSI, thereby illustrating the intuitive tradeoff between the achievable capacity and implementation complexity.

Interference Management in D2D Communication

This chapter aims to provide a comprehensive study of the interference generated by enabling device-to-device (D2D) communication in cellular networks. We first categorize the D2D interference according to its causative factor, and investigate the characteristics and effects. Based on these results, we discuss the necessity of the interference management strategy. We then explore the features, strengths, and weaknesses of existing interference management strategies in the D2D communication. Based on the comparison from among the existing approaches, we highlight the design insights or useful intuition to an efficient interference management strategy. Finally, we provide an open issue relevant to the interference problem regarding the coexistence of the D2D pair and small-cell network, which may be an important subject for further research in D2D communication.

Rotated precoder based self-interference cancellation in full-duplex communication

We propose a new method for full-duplex self-interference cancellation. The proposed scheme uses a precoder to align the self-interference channel with orthogonal direction of detection axis. The simulation result shows that the proposed RP-SIC enhances the average throughput of user equipment compared to the conventional scheme in high residual self-interference region.

Effect of Unpredictable Interference on MU-MIMO Systems in HetNet

In this paper, we investigate the outage probability of a multiuser multiple-input multiple-output (MU-MIMO) system in the heterogeneous network (HetNet) in the presence of unpredictable interference (UI) from coexisting ad hoc systems. In order to achieve multiuser diversity gain, the MU-MIMO system requires feedback carrying the signal-to-interference-plus-noise ratio (SINR) from all active users to the base station. However, the coexisting ad hoc systems in HetNet can unpredictably initiate their data transmission during the SINR feedback duration. This unpredictable behavior by the coexisting ad hoc systems causes a mismatch between the measured SINR and the instantaneous SINR. In order to investigate the performance of the MU-MIMO system in HetNets, we first categorize the interferences into predictable interference and UI. Based on this categorization, we analyze the outage probability of an MU-MIMO system using a max-SINR scheduler. The theoretical analyses show that the MU-MIMO system cannot achieve the maximum diversity gain, since the UI from the coexisting systems causes beam selection mismatch. Additionally, we show that reducing the interval between the measurement and data transmission prevents performance degradation by reducing the effect of the unpredictable behavior of the coexisting ad hoc systems.

Adaptive Unlicensed Band Access for Downlink Cellular Networks

Licensed band communication systems have recently begun trying to utilize unlicensed bands. Due to lack of coordination, however, this spectrum utilization results in strong interference to other unlicensed band equipment (UBE). This problem highlights the need for coexistence scenario that will prevent UBE performance degradation. In this paper, we propose a new access control mechanism for downlink cellular networks using the unlicensed band. First, we analyze the impact of the cellular networks on the UBE outage performance. Based on the analysis, we then obtain a decision threshold for the unlicensed band access control. In the proposed control mechanism, the decision threshold is adaptively controlled by considering the density of the UBE. The numerical results demonstrate that the proposed access control mechanism guarantees the UBEs target performance under various network circumstances.