Hyungjong Kim

Cognitive Radio Networks with Energy Harvesting

We consider a cognitive radio network with an energy-harvesting secondary transmitter to improve both energy efficiency and spectral efficiency. The goal of this paper is to determine an optimal spectrum sensing policy that maximizes the expected total throughput subject to an energy causality constraint and a collision constraint. The energy causality constraint comes from the fact that the total consumed energy should be equal to or less than the total harvested energy, while the collision constraint is required to protect the primary user. We first show that the system can be divided into a spectrum-limited regime and an energy-limited regime depending on where the detection threshold for the spectrum sensor lies. Assuming infinite battery capacity, we derive the optimal detection threshold that maximizes the expected total throughput subject to the energy causality constraint and the collision constraint. Analytical and numerical results show that the system is energy-limited if the energy arrival rate is lower than the expected energy consumption for a single spectrum access. They also show that a decreasing probability of accessing the occupied spectrum does not always result in decreased probability of accessing the idle spectrum in the energy-limited regime.

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.

Optimal Power Allocation and Outage Analysis for Cognitive Full Duplex Relay Systems

This paper investigates the use of full duplex relaying (FDR) in cognitive radio systems. Cognitive full duplex relay networks (CogFRNs) offer the advantage not only of increasing spectral efficiency by spectrum sharing but also of extending coverage through the use of relays. Concurrent transmissions at the source and relay in CogFRNs can overcome a loss of resource efficiency in a way that conventional half duplex relay (HDR) systems cannot. However, in CogFRNs, the primary user experiences interference from the secondary source and relay simultaneously due to the effects of full duplexing. Satisfying the interference constraint by simply reducing transmission power results in performance degradation for the secondary user. What is therefore needed is a way to optimize the transmission powers at the secondary source and relay. To address this need, we propose an optimal power allocation scheme based on minimizing the outage probability in CogFRNs. We then analyze the outage probability of the secondary user in the noise-limited and interference-limited environments. In addition, we also propose an outage-constrained power allocation scheme which does not require the instantaneous channel state information (CSI) for the link between the primary and secondary users. Simulation results show that the proposed schemes achieve performance improvement in terms of outage probability.

On the Impact of Outdated Channel Information on the Capacity of Secondary User in Spectrum Sharing Environments

Conventional investigations on the capacity of a secondary link in spectrum sharing environments have assumed that a secondary user knows perfect channel information between the secondary transmitter and primary receiver. However, this channel information may be outdated at the secondary user because of the time-varying properties or feedback latency from the primary user. If the secondary user allocates transmission power using this outdated channel information, the interference power to the primary receiver will not satisfy the predetermined interference constraint. In this paper, we investigate the impact of outdated channel information between secondary and primary users in spectrum sharing environments. We begin by deriving the ergodic capacity of secondary user along with the optimum power allocation under the average received-power constraint. We also provide a closed-form expression for the ergodic capacity without interference from the primary transmitter, and the capacity bounds with interference from the primary transmitter. Moreover, we provide the power margin required to satisfy the interference outage probability at the primary user under the peak received-power constraint. Lastly, we derive the secondary users ergodic capacity with and without interference from the primary transmitter. Comparisons done using simulations show the effects of the uncertainty of channel information and interference from the primary transmitter under both constraints.

Mean Value-Based Power Allocation without Instantaneous CSI Feedback in Spectrum Sharing Systems

In this paper, we investigate a power allocation method without feedback of the instantaneous channel gain between a secondary transmitter (ST) and a primary receiver (PR) in spectrum sharing systems where channel information between the ST and the PR is outdated. We propose a mean value-based power allocation scheme (MVPA), which uses only the mean value of the channel gain of the ST-PR link. The proposed MVPA satisfies the interference outage constraint at the PR without feedback of the instantaneous channel information, which enables MVPA to reduce feedback burden. We also derive the ergodic capacity based on MVPA in closed-form and compare the ergodic capacity based on MVPA with that of a power allocation method using the outdated channel information instantaneously. Through the comparison, we convincingly demonstrate that MVPA without feedback of instantaneous channel information provides more reliable and superior capacity performance by reducing capacity loss from which the power allocation method using the outdated channel information severely suffers.

Outage probability of cognitive amplify-and-forward relay networks under interference constraints

This paper presents the outage performance in cognitive relay networks. The closed-form expression for the outage probability is derived in cognitive amplify-and-forward (AF) relay networks under the interference constraint. Through the Monte Carlo simulation, it is shown that our analytic result is a tight bound especially at high average signal to noise ratio (SNR). Additionally, the numerical result shows the feasible region of AF relay in cognitive relay networks. This result also shows that the relaying transmission in the cognitive networks is inferior to the direct transmission in aspect of the outage performance, if the location of relay is too close to the primary receiver.

Power allocation and outage probability analysis for secondary users in cognitive full duplex relay systems

This paper investigates the use of full duplex relaying (FDR) in cognitive radio systems. Cognitive FDR systems offer the advantage not only of increasing spectral efficiency by spectrum sharing but also of extending the coverage through the use of relays. Concurrent transmissions at the source and relay in FDR systems can overcome a loss of resource efficiency in a way that conventional half duplex relay (HDR) systems cannot. However, in cognitive FDR systems, the primary user experiences interference from the secondary source and relay simultaneously due to the full duplexing. Satisfying the interference constraint by simply reducing transmission power results in performance degradation for the secondary user. What is therefore needed is a way to optimize the transmission powers at the secondary source and relay. Therefore, we propose an optimal power allocation scheme based on minimizing the outage probability in cognitive FDR systems. We then analyze the outage probability of the secondary user in the noise-limited and interference-limited environments. Simulation results show that the proposed schemes achieve performance improvement in terms of outage probability.

On the design of ZF and MMSE Tomlinson-Harashima precoding in multiuser MIMO amplify-and-forward relay system

In this paper, we design a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) and a minimum mean square error (MMSE) THP for multiuser MIMO relay system with direct link. Based on these precoding structures, we propose the optimal combining weights of the ZF THP and the MMSE THP in half duplex relay system, and the sub-optimal weights of MMSE THP to reduce feedback channel state information (CSI). Through the Monte Carlo simulation, we verify that the optimal weights combining method outperforms the constant weights combining method. In addition, we confirm that the performance of the sub-optimal weights combining method is close to the optimal method, while reducing the feedback CSI simultaneously.

Performance Analysis of Wireless Dual-Hop Systems With Multirelay and Multiuser

This paper presents closed-form expressions for the end-to-end performance of wireless dual-hop systems with multiple amplify-and-forward relays and multiple users over Rayleigh flat fading channels. These results are based on the statistics of the harmonic mean of two random variables related to exponential distribution. Two generalized scheduling policies are considered in the systems for selection diversity from multirelay and multiuser: the centralized scheduling (CS) and the distributed scheduling (DS). Numerical results show that the performance of DS can be degraded when large number of relays are used relative to the number of users. Additionally, it is shown that DS is quite competitive with CS at small number of relays that can be considered to be desirable.

Optimal Tone Space Selection Scheme for OFDMA-VTS in Carrier Aggregation

In this paper, we propose an optimal tone space selection scheme for orthogonal frequency division multiple access with variable tone spaces (OFDMA-VTS) in carrier aggregation based on user-specific channels. We begin by analyzing inter-symbol interference (ISI) and inter-carrier interference (ICI) in non-contiguous carrier aggregation, which differ according to the delay and Doppler spreads associated with each user, as well as the tone space that each user utilizes. Based on that, we propose an optimal tone space selection scheme as a way to minimize both ISI and ICI. We also derive an upper bound on OFDMA-VTS capacity when it is applied to the proposed scheme. Furthermore, we investigate the effect of inter-user interference (IUI) in contiguous carrier aggregation and show that the proposed scheme can be applied to OFDMA-VTS in contiguous carrier aggregation as a suboptimal solution. Numerical results demonstrate that the proposed scheme maximizes OFDMA-VTS capacity by minimizing the total ISI and ICI interference, thereby enhancing the system capacity over what is possible with conventional OFDMA.