Hsiao-Chen Lu

Relay Station Placement Strategy in IEEE 802.16j WiMAX Networks

In this paper, we study the relay station (RS) placement strategy in IEEE 802.16j WiMAX networks. Specifically, the impact of RS placement on IEEE 802.16j network performance is analyzed. A throughput maximization RS placement problem is mathematically formulated as a binary integer programming problem. We prove the NP-hardness of the formulated problem. To find the sub-optimal solution to the problem with huge input size, we propose an efficient near-optimal placement solution for IEEE 802.16j WiMAX networks. Simulations on the IEEE 802.16j network performance with our RS placement strategy are conducted. The throughput performance shows that with the deployment strategy we proposed, the IEEE 802.16j network capacity can be tremendously enhanced, especially when hotspots are present in the network.

Joint Base Station and Relay Station Placement for IEEE 802.16j Networks

In this paper, the network deployment problem for IEEE 802.16j networks is studied. We consider jointly deploying a number of base stations and relay stations to serve mobile stations distributed arbitrarily in a given geographic area such that the cost is within the budget and the system capacity is maximized. This problem is formulated as an Integer Linear Programming (ILP) model. We analyze the complexity of the problem and design an efficient two-stage network deployment algorithm which tackles both user fairness and capacity enhancement. Computational experiments are conducted to show the effect of network deployments in different traffic distribution scenarios. We also show the impact of different deployment profiles on network capacity and fairness and discuss the cost-efficiency tradeoffs between base stations and relay stations. Our numerical results indicate that deploying relay stations can enhance system capacity and fairness under the condition that the number of base stations deployed is not too small. The degree of enhancement is higher when the traffic demand distribution is concentrated in hotspots.

Cooperative Strategies in Wireless Relay Networks

In this paper, we study the cooperative strategies of relay stations in wireless relay networks. As pure relay stations distributed across the network and centrally controlled by the base station can be exploited to form cooperative antenna arrays, we determine which relay stations should cooperate with one another such that the performance of wireless relay networks can be optimized. Specifically, we define and formulate the throughput maximization problem for wireless relay networks under practical model of relay station cooperation. We design an iterative search algorithm to solve the relay station cooperation problem. The proposed algorithm is proved via simulations to yield near-optimal solutions with only linear-time complexity. The simulation results also show that cooperative transmissions of relay station can significantly improve system throughput and reduce the handover probability of mobile stations. More importantly, the placement of relay stations is crucial to the throughput gain obtained by relay station cooperation.

On cooperative strategies in wireless relay networks

In this paper, we study the cooperative strategies of relay stations in wireless relay networks. In such networks, relay stations distributed across the network and centrally controlled by the base station can be exploited to form cooperative antenna arrays. We show that via relay station cooperation, the achievable downstream rate of mobile stations can be enhanced. Thus, we determine which relay stations should cooperate with one another and which mobile stations should receive data from the cooperative relays such that the performance of the entire network can be optimized. The utility maximization relay station cooperation problem is formulated with the goal of maximizing system capacity and balancing user traffic demand. We analyze the complexity of this problem and show that it can be decomposed into two NP-hard sub-problems. To tackle this problem, we propose two algorithms with different degrees of efficiency and complexity. The simulation results show that relay station cooperation can not only enhance the throughput of the network, but also improve fairness among users. More importantly, we find that proper relay station placement can further enhance the throughput gain.

Cooperative multicasting in network-coding enabled multi-rate wireless relay networks

Network coding has been broadly applied to improve the efficiency of wireless multicast. In this paper, we consider the multicast process in modern relay-assisted wireless communication systems such as the IEEE 802.16j and the LTE-advanced networks, where the relay stations can cooperatively forward network-coded packets to the subscriber stations using different transmission rates. We show that under such multi-rate environments, previous solutions which seek to minimize the packet forwarding counts may lead to longer multicast delay. To solve this problem, in this work, we aim at finding a minimal delay transmission schedule of the relay stations under multi-rate considerations. We first show that this problem is NP-hard. Then we use a Markov decision process to model the relay station re-transmission process. Via this model, we derive the formulations for optimal re-transmission strategies as well as optimal re-transmission delays. Moreover, based on the recursive structure of the re-transmission delays derived from the model, we propose a dynamic programming algorithm which can solve optimal re-transmission strategies for the system. For complexity considerations, we also propose two light-weight on-line re-transmission heuristics. Simulation results show that the Markov decision process can accurately characterize the relay re-transmission process in network-coding-enabled wireless relay networks, and that minimal multicast delay can be achieved by dynamic programming-based relay re-transmissions. Moreover, simulation results suggest that the two heuristics may be suited to different scenarios, and both can achieve near-optimal performances efficiently.

On Relay Selection in Wireless Relay Networks with Cooperative Network Coding

In this paper, we study the cooperative network coding problem in relay-assisted wireless access networks. In such a network, each mobile station can choose different transmission schemes: directly transmitting to the base station, pure relaying by a relay station, or via cooperative network coding. We analyze the impact of the different schemes on the network throughput. We find the performance of pure relaying and cooperative network coding is affected by the selection of the relay station, making the relay selection problem inseparable from the cooperative network coding problem. We formulate the joint relay selection and resource allocation problem, and find this problem equivalent to the multiple 0/1 knapsack problem. To tackle this problem, we propose an efficient network-coding-based resource allocation algorithm, and define a coding gain metric. The simulation results show our algorithm significantly outperform other schemes. More importantly, the performance improvement by cooperative network coding is significantly affected by the number of relay stations and the distribution of mobile stations.

Multi-Carrier Admission Control and Carrier Assignment for IEEE 802.16m Wireless Networks

WiMAX is a promising solution for last-mile broadband Internet access. 802.16m is a new IEEE standard featuring multi-carrier technology to meet the requirement of IMT-Advanced for next generation mobile WiMAX. The design goal of IEEE 802.16m is to achieve 1Gbps over-the-air data rate with the help of multi-carrier operation. However, multi-carrier operation brings many challenges. In this paper, we study the call admission control problem together with multi-carrier assignment in IEEE 802.16m networks. We formulate an optimization problem and propose a two-stage carrier assignment algorithm to solve it. The simulation results show that the proposed algorithm achieves satisfactory aggregate throughput and revenue, while maintaining fairness among users.