Jonghyun Bang

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.

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 new frame structure for asynchronous in-band full-duplex systems

In in-band full-duplex employing conventional frame structure, pilot contamination is inevitable due to asyn-chronism between the nodes. Asynchronism can occur for several reasons such as propagation delay and synchronization error. Pilot contamination increases channel estimation error for both the desired and self-interference channels. We propose a new frame structure that avoids interference during pilot transmission. The proposed frame structure utilizes maximum time offset information in order to cover all the possible time offsets. We then derive the mean square error (MSE) of the channel estimation when the proposed frame structure is employed. Furthermore, based on the MSE analysis, we deliver superior conditions for the proposed frame structure compared to the conventional one. Finally, we show that sacrificing pilot length to avoid interference during pilot transmission guarantees channel estimation performance when the power of the interference is large.

Performance of Full Duplex in Non-Full Buffer Traffic Model

Full-duplex (FD) systems ideally double the system capacity compared to conventional half-duplex (HD) systems. However, we can obtain that goal only when FD systems always have both downlink (DL) and uplink (UL) traffic loads. This paper sheds new light on whether FD can achieve this promising performance enhancement in more realistic traffic settings. We first provide the sum capacity of the FD system when DL and UL traffic loads are different. In addition, we define the FD gain according to traffic loads and compare FD performance with the HD system which is based on time-division duplex (TDD). We then analyze the effect of traffic loads and residual self- interference (SI) on the FD system. Simulation results and analysis reveal that FD performance decreases in a limited traffic setting.

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.