Younggap Kwon

Energy-Efficient Transmit Power Control for Multi-tier MIMO HetNets

In this paper, we study energy-efficient transmit power control for multi-tier multi-antenna (MIMO) heterogeneous cellular networks (HetNets), where each tier operates in closed-access policy and base stations (BSs) in each tier are distributed as a stationary Poisson point process (PPP). Each BS serves multiple users at a fixed distance away from it. We first study noncooperative energy-efficient power control, where each tier selfishly chooses its transmit power to maximize its network energy efficiency (EE). This is modeled by a noncooperative power control game. We prove the existence and the uniqueness of the Nash equilibrium of the game. Moreover, we analyze the effects of circuit power and BS densities of the tiers on their transmit power at the Nash equilibrium. Then, we investigate cooperative energy-efficient power control, where all the tiers cooperatively choose their transmit power to optimize their network EE. This cooperative power control is formulated as a multiobjective problem. To obtain Pareto-optimal solutions of the problem, we develop an algorithm that alternately updates the transmit power of the tiers. In addition to the transmit power, the circuit power consumption of the operating BSs and their active antennas affects the network EE. Due to the circuit power, activating all the BSs and turning on all the antennas may not be optimal in maximizing the network EE. Motivated by this observation, we also develop energy-efficient BS activation control and antenna activation control schemes. Finally, we extend the analysis to the highest signal-to-interference-plus-noise ratio (SINR) association, where each user connects to the BS that offers the highest SINR among the BSs in its tier. Simulation results show that the proposed energy-efficient designs significantly improve the network EE at the cost of small spectral efficiency loss compared with the spectral-efficient designs.

Root Mean Square Decomposition for EST-Based Spatial Multiplexing Systems

We consider the transceiver design for multiple-input-multiple-output (MIMO) systems when the channel state information (CSI) is available at the transmitter as well as the receiver. First, we propose an open-loop low-complexity MIMO spatial multiplexing scheme based on the energy spreading transform (EST-SM). The EST-SM can spatially multiplex multiple data streams and iteratively detect the data streams with almost negligible interstream interference at sufficiently high SNR. Then, we propose a closed-loop precoding scheme suitable for the EST-SM called root mean square decomposition (RMSD) scheme. The RMSD precoding scheme combined with the EST-SM decomposes a MIMO channel into multiple subchannels with identical SNRs. This desired property minimizes bit error rate (BER) when different bit allocations on different subchannels, which cause a significant increase in system complexity, are not used. We show that when the EST-SM is used the RMSD scheme is optimal in BER performance and it achieves full diversity. Simulation results show that the RMSD scheme outperforms other existing techniques such as the geometric mean decomposition (GMD) scheme (Jiang , IEEE Trans. Signal Process., vol. 53, no. 10, pp. 3791-3803) and the uniform channel decomposition (UCD) scheme1 (Jiang , IEEE Trans. Signal Process., vol. 53, no. 11, pp. 4283-4294) in BER performance.

Energy-Efficient Adaptation of Pilot Power, Data Power, and Transmission Rate for Downlink Multiuser MIMO Systems

This paper addresses energy-efficient link adaptation for downlink multiuser multiple-input-multiple-output (MIMO) systems. Different from the previous works on energy efficient MIMO techniques, we take into account finite pilot power and the corresponding effect of imperfect channel state information (CSI) at the users. We analyze the average throughput and define the energy efficiency (EE) of downlink multiuser MIMO systems. We show that the EE of the system is not jointly quasi-concave with respect to pilot power, data power, and transmission rate. For this reason, we propose an efficient algorithm that alternately updates pilot power, data power, and transmission rate to improve the EE of the system. Simulation results show that the proposed scheme significantly enhances the EE, and the resulting EE is comparable to the maximal EE obtained by the exhaustive search.

A game-theoretic approach for energy-efficient power control in spectrum sharing networks

In this paper, we study energy-efficient power control for spectrum-sharing networks, where multiple systems of transmitting nodes share the same spectrum. In the spectrum sharing networks, reducing the transmit power of one system is always beneficial to the energy efficiencies (EEs) of the other systems. However, reducing the transmit power may not be always optimal in maximizing the EE of a system. These conflicting interests among the systems are molded by a non-cooperative energy-efficient power control game, where each system chooses its transmit power to maximize its own EE. We prove the existence and uniqueness of the Nash equilibrium of this game. However, due to the selfish behaviors of the systems in the network, the Nash equilibrium is in general inefficient. To improve the EE of the Nash equilibrium, we incorporate pricing to the non-cooperative energy-efficient power control game. For the spectrum sharing network consisting of two systems, we find a sufficient condition on the transmit powers of the systems assuring the pricing-based game to be a supermodular game, which always admits at least one Nash equilibrium.

Energy Efficient Communication for Secure D2D Underlaid Cellular Networks

We study energy-efficient secure communication in large-scale device-to-device (D2D) underlaid cellular networks, where base stations (BSs) and D2D users send their messages occupying the same spectrum while eavesdroppers (Eve nodes) overhear their transmissions. First, we consider the secure cellular network where users operate only in the cellular mode. We propose a link adaptation of the cellular network that strikes a balance between secrecy energy efficiency (SEE) and secrecy spectral efficiency (SSE) by maximizing the weighted product of SEE and SSE. Besides, we show that SSE first increases and then decreases with the BS power when the ratio of the eavesdropper (Eve) node density to the BS density is above a certain threshold while it monotonically increases when the Eve-to-BS density ratio is below the threshold. On the other hand, SEE first increases and then decreases with the BS power regardless of the Eve-to-BS density ratio. Next, we study how to deploy the secure D2D network under the existing secure cellular network while guaranteeing an acceptable secrecy performance of the cellular network. We show that the secrecy performance of the cellular network decreases with the D2D power when the Eve nodes are sparse while it first increases and then decreases when the Eve nodes are dense. Based on the result, we propose a link adaptation for the D2D network that maximizes the weighted product of SEE and SSE under the secrecy performance constraint of the cellular network. Simulation results show that the proposed link-adaptation schemes can achieve all the points on the Pareto boundary in SSE–SEE plane.

Energy-Efficient Power Control for TDD MISO Systems

This paper addresses energy-efficient power control for time-division-duplexing (TDD) multiple-input-single-output (MISO) systems. Assuming ideal channel reciprocity, the beamforming at the transmitter (TX) is based on the estimated channel state information (CSI) obtained from the pilot signal of the receiver (RX). We define the energy efficiencies of the TX and the RX separately and study their energy efficiency region. In addition, we seek a power control scheme that can achieve all the points on the Pareto boundary of the energy efficiency region. We propose to maximize the weighted system energy efficiency, which is the data rate divided by the weighted sum of the power consumption at the TX and the RX. A method based on the quasi-concavity of the weighted system energy efficiency is developed to solve the problem. Simulation results show that the proposed power control can achieve all the points on the Pareto boundary of the energy-efficiency region by varying the weight.

Sum-Rate Improved Interference Alignment for the M x 2 MIMO X Channel

In this letter, we propose an improved interference alignment (IA) scheme for the M × 2 constant multiple-input multiple-output (MIMO) X channel composed of M transmitters and two receivers with multiple antennas each. The proposed IA scheme enhances the sum rate as well as achieving the DOF outerbound. To enhance the sum rate, the proposed scheme designs the transmit beamformers that make the desired signal space and the interference space as close to orthogonal as possible. Simulation results show that the proposed IA scheme achieves similar sum rate as the random search scheme but with less computational complexity.

Energy-Efficient Link Adaptation for Secure D2D Underlaid Cellular Networks

We study energy-efficient secure link adaptation for large-scale device-to-device (D2D) underlaid cellular networks, where base stations (BSs), D2D transmitters, and eavesdroppers (Eve nodes) are distributed as independent homogeneous Poisson point processes. First, we analyze the impact of the underlaid D2D network on the secrecy performance, i.e., secrecy spectral efficiency (SE) and secrecy energy efficiency (EE) of the existing cellular network. We show that when the ratio of the Eve node density to the BS density is above a certain threshold, the secrecy performance of the cellular network first increases and then decreases with the D2D power. Otherwise, it decreases with the D2D power. Next, based on the results, we develop a linkadaptation scheme that controls the D2D power, the confidential message rate, and the redundancy rate to strike a balance between the D2D networks secrecy EE and secrecy SE while guaranteeing a required secrecy performance of the existing cellular network. Simulation results show that the proposed link adaptation scheme can achieve all the points on the Pareto boundary.

Energy-efficient uplink power control for multiuser SIMO systems with imperfect channel state information

This paper addresses energy-efficient design for uplink multiuser SIMO systems with imperfect channel state information (CSI) at the base station (BS). Since the CSI at the BS is always imperfect due to the channel estimation error and delay, the imperfectness of the CSI needs to be considered in practical system design. It causes interuser interference at the zero-forcing (ZF) receiver and makes it difficult to obtain the globally optimal power allocation that maximizes the energy efficiency (EE). Hence, we propose a non-cooperative energy-efficient uplink power control game, where each user selfishly updates its own uplink power. The proposed uplink power control game is shown to admit a unique Nash equilibrium. Furthermore, to improve the efficiency of the Nash equilibrium, we study a new game that utilizes a pricing mechanism. For the new game, the existence of a Nash equilibrium and the convergence of the best response dynamics are studied based on super-modularity theory. Simulation results show that the proposed schemes can significantly improve the EEs of the mobile users in uplink multiuser SIMO systems.

A Game With Randomly Distributed Eavesdroppers in Wireless Ad Hoc Networks: A Secrecy EE Perspective

We study energy-efficient secure communication using the combined approach of game theory and stochastic geometry in a large-scale wireless network, where legitimate transmitters (Alice nodes) and eavesdroppers (Eve nodes) are randomly distributed in space. We consider the following two scenarios according to the Eve tiers strategy: I) the Eve tier activates all its nodes to maximally eavesdrop the confidential messages of the Alice tier; and II) the Eve tier activates only a portion of its nodes to maximize its energy efficiency (EE) in eavesdropping according to the Alice tiers node activation. In Scenario I, we propose an alternating optimization scheme that maximizes the secrecy EE of the Alice tier by controlling the node-activation probability, the confidential message rate, the redundancy rate, and the number of active antennas. Simulation result shows that the proposed scheme can achieve the optimal secrecy EE. In Scenario II, we study an energy-efficient node activation game between the Alice tier and the Eve tier, where the former and the latter control their node-activation probabilities to maximize the secrecy EE and the eavesdropping EE, respectively. We show that the node activation game admits a unique Nash equilibrium. The node-activation probabilities of the Alice tier and the Eve tier at the Nash equilibrium can be used to estimate their network lifetimes, which are important information for the energy-efficient secure network design. Simulation result shows that the best-response dynamics converges to the Nash equilibrium within a few iterations.