Jietao Zhang

Optimal Relay Location for Fading Relay Channels

In this paper we study the problem of relayenhanced cell (REC) coverage for which relay location is optimized to maximize the achievable REC radius. The problem is investigated for both Rayleigh and Rician relay fading channels, under a pre-determined users outage probability constraint. We propose a statistical approach to formulate the problem and develop an optimization algorithm for it. The analytical derivation is justified by numerical simulations.

Reference signal power control for load balancing in downlink LTE-A self-organizing networks

Self-organizing network (SON) is considered as a driving technology for the deployment of next generation radio access networks. This paper addresses the problem of load balancing (LB) for multi-hop cellular network (MCN) with fixed relays such as LTE-A network in the context of SON. The designed SON algorithm, namely RSPC-RL, is based on two ideas: relay node reference signal power control (RSPC) and multi-agent reinforcement learning (RL). In the proposed RSPC-RL algorithm, the relay node is modeled as an agent that learns an optimal policy of reference signal power control from its interaction with environment to balance the load distribution of the network through dynamically changing its coverage area. Numerical results show the significant performance gain brought about by the proposed algorithm RSPC-RL.

Optimized Nonuniform Constellation Rearrangement for Cooperative Relaying

The performance of cooperative relaying networks can be significantly enhanced by using constellation rearrangement (CoRe). In CoRe, the base-station and the relay-station use different constellations, each having the same number of signal points, to communicate with the user terminal. A number of CoRe schemes have been proposed in the literature based on uniform quadrature-amplitude modulation (QAM) constellations. However, it is still unclear whether nonuniform QAM constellations can further enhance the performance of CoRe. Toward this end, we investigate the problem of designing the optimum nonuniform QAM constellations for CoRe. Our motivation is that nonuniform constellations have the potential to outperform their uniform counterparts because the set of nonuniform constellations is a superset of uniform constellations. Nonuniform QAM constellations can be categorized as either decomposable or nondecomposable . Unlike nondecomposable QAM constellations, decomposable QAM constellations are generated from the Cartesian product of two pulse-amplitude modulation (PAM) constellations. We formulate an optimization problem to find the nonuniform constellations that have the minimum union bound on the uncoded symbol error rate (SER). Using convex analysis, we devise a search method to find globally optimum nonuniform decomposable constellations. We also devise a simple heuristic to find good locally optimum nonuniform nondecomposable constellations, which perform better than their decomposable counterparts.

Inter-Tier Handover in Macrocell/Relay/Femtocell Heterogeneous Networks

Handover decision is one of the technical challenges in the heterogeneous network (HetNet). Current researches on this topic concentrate mainly on the two-tier networks. In this paper, we investigates the handover decision scheme for a more complex network: three-tier macrocell/relay/femtocell network. The unique characteristics of the inter-tier handover in three-tier networks are analyzed in this paper and an effective handover algorithm is proposed to reduce the frequent and unnecessary handovers based on the ideas of dwell probability and handover priority. Simulation results show that the proposed algorithm significantly reduces the number of handovers while maintaining the call dropping rate at the same level.

Dynamic Radio Resource Management for OFDMA-Based Relay Enhanced Cellular Network

Relay technology is one of the technologies that will enhance user experience in the next generation cellular networks. One of the goals (and characteristics) of successful cellular network design is availability of services to all subscribed users irrespective of their location. Designing a good Radio Resource Management (RRM) is essential in achieving that goal. In this paper we propose a dynamic resource partitioning in the OFDMA-based Relay Enhanced Networks. We study performance of the proposed RRM scheme with fairness and maximum throughput objectives and show that it has superior performance in comparison with conventional RRM schemes.

Capacity performance of millimeter wave heterogeneous networks at 28GHz/73GHz

Current communication technologies at the microwave band are insufficient to meet the exponential increase in wireless data demands, essentially due to scarcity in bandwidth. Millimeter wave (mmW) offers a potential means to fulfill such soaring demands with wide bandwidth and beamforming capabilities. In this paper we investigate the mmW heterogeneous network (HetNet) capacity operating at 28GHz and 73GHz by system simulations, in both the 3GPP scenario and Munich urban city. Two types of control channel modes for pico cell coverage are analyzed. Simulation results show that control signaling at microwave band provides better coverage, while data communications at the mmW band enable up to multi-Tbps/km2 system capacity, which is also dependent on the pico-user association. It is found that in both scenarios the capacity gain of 73GHz over 28GHz is in the range of 7~10-fold under the same user-pico association.

A hybrid framework for capacity and coverage optimization in self-organizing LTE networks

In this paper, we address the capacity and coverage optimization (CCO) problem for LTE networks by means of self-organizing network (SON) techniques. A novel hybrid two-layer optimization framework is proposed to enhance the network capacity and coverage, where on the top layer a network entity of eCoordinator is implemented to ensure overall network coverage by optimizing the antenna tilt and capacity-coverage weight of each cell in a centralized manner, and on the bottom layer individual eNB optimizes cell-specific capacity and coverage by tuning its pilot power in a distributed manner. A heuristic algorithm is developed for the eCoordinator operation at large time granularity and the Genetic Programming (GP) approach is exploited for the eNB operation at small time granularity, for the purpose of tracking overall network performance as well as adapting to network dynamics. Our simulation results have demonstrated the usefulness of the proposed algorithms by enhancing network capacity and coverage performance under various system requirements.

Max-Min Fair Resource Allocation for Multiuser Amplify-and-Forward Relay Networks

We investigate the problem of multi-user radio resource allocation for orthogonal frequency division multiple access (OFDMA) amplify-and-forward (AF) relays. In the single-user case, the problem reduces to the well know assignment problem, which maximizes the user rate. For the multi-user case we devise a resource allocation algorithm to achieve max-min fairness. We find max-min fairness since it can provide almost flat ubiquitous coverage. We start by formulating a convex optimization, which takes a parameter that asymptotically makes the optimization produce max-min fair rates. Since the optimization is a convex problem, we are able to devise a sub-optimal gradient-based algorithm to solve it quickly. Simulations show that the algorithm achieves results very close to the optimum solutions due to its gradient origins.

An Inter-Cell Interference Coordination Scheme for Relay Based Cellular Networks

Considering the new feature after introducing the Relay Nodes (RNs) into the conventional cellular networks and the existing interference coordination techniques, a novel method of interference coordination for cell-edge users in Relay Enhanced Cells (RECs) is proposed in this paper. Firstly, all available frequency resource in each cell is divided into two parts. One part is allocated to users directly communicate with BS, and the other is especially reserved for users connected to BS through two-hop RNs. Furthermore, for the sake of taking system throughput as well as fairness into account, a dynamic score based scheduling strategy is designed for the two-hop users. The extensive simulation results show that the proposed strategy performs better on the cell throughput and the SINR of cell-edge users compared with the existing schemes, and with adaptive weights, the GoS of users can also be guaranteed.

Handoff for wireless networks with mobile relay stations

Wireless Relay networks have become very important technologies in the future wireless systems. Current research works mostly focus on scenarios where relay stations are either stationary or mobile with uniform velocity. However, in many applications, relay stations are mobile with irregular patterns. In particular, when mobile relay stations (MRSs) are deployed to complement the cellular systems, many issues such as handoff should be carefully investigated. In this paper, we focus on the handoff problem and propose a new handoff decision algorithm based on the relative velocities of user equipment (UE) to the serving access point (AP) and the target AP. We have show that the proposed handoff algorithm can significantly improve the handoff successful rate when the mobile relay station changes its moving patterns.