Jane-Hwa Huang

Optimal Relay Location in Multi-Hop Cellular Systems

Relay stations (RSs) are usually used to enhance the signal strength for the users close to the cell boundary. However, transmission through a relay station needs two transmission phases, i.e., one is from the base station to the relay station and the other is from relay station to mobile stations. Thus, relay may also decrease system capacity if two-phase transmission time is considered. As a result, whether or not data are transmitted by one-hop or two-hop transmission should be determined based on both signal strength and throughput. In this paper, we investigate the optimal relay location aiming to maximize system capacity. We consider two relay selection rules for determining whether a two-hop transmission is necessary: signal strength-oriented and throughput-oriented. We find that the signal strength-oriented two-hop transmission may yield even lower system capacity than the one-hop transmission. Based on the throughput-oriented rule, we find that the throughput in the two-hop transmission can be higher than that in the one-hop transmission at some locations. We also identify the optimal relay location that can achieve the highest system capacity.

Distributed Channel Selection Principles for Femtocells with Two-Tier Interference

It goes without saying that the femtocells will be widely employed in the next generation wireless networks since the femtocells improve indoor capacity and coverage with low power and less cost. However, as the femtocells become popular, the femtocell users suffer from the complicated two-tier interference, including the macrocell-to-femtocell and femtocell-to-femtocell interference. Therefore, the femtocells pose a difficult challenge on managing the interference in a autonomous and distributed manner. In this paper, we investigate how to distributedly select the sub-channels for the OFDMA-based femtocell systems to reduce interference and to improve indoor capacity under a link reliability requirement. We develop the channel-gain oriented and interference-avoidance oriented distributed channel selection schemes. Simulation results show that the interference from the macrocell and other femtocells significantly degrades femtocell link reliability and capacity. However, by properly adjusting the number of used sub-channels, the developed channel selection schemes can improve capacity and ensure the link reliability.

Coverage and capacity of a wireless mesh network

Wireless mesh networks (WMN) play an important role in the next-generation wireless communication systems because it can support broadband services with ubiquitous coverage by low transmission power. In this paper, we propose a multi-channel ring-based wireless mesh network and develop an analytical framework to evaluate the capacity and coverage of such a network. In the analytical framework, we first establish a physical (PHY)/medium access control (MAC) cross layer throughput performance model by including the carrier sense multiple access (CSMA) MAC protocol and distance-based rate adaptation with multi-hop connections. Based on the derived throughput model, we apply the mixed-integer nonlinear programming (MINLP) optimization approach to maximize the capacity (throughput) and service coverage of a mesh cell, in which the number of rings in a mesh cell and the radius for each ring are determined.

Capacity and QoS for a Scalable Ring-Based Wireless Mesh Network

The wireless mesh network (WMN) is an economical solution to support ubiquitous broadband services. This paper investigates the tradeoffs among quality-of-service (QoS), capacity, and coverage in a scalable multichannel ring-based WMN. We suggest a simple frequency planning in the proposed ring-based WMN to improve the capacity with QoS support, and to make the system more scalable in terms of coverage. We develop a physical (PHY)/medium access control (MAC) cross-layer analytical model to evaluate the delay, jitter, and throughput of the proposed WMN, by taking account of the carrier sense multiple-access (CSMA) MAC protocol, and the impact of hop distance on transmission rate in the physical layer. Furthermore, the mixed-integer nonlinear programming optimization approach is applied to determine the optimal number of rings and the associated ring widths, aiming at maximizing the capacity and coverage of a mesh cell subject to the delay requirement

Overload Control for Machine Type Communications with Femtocells

In this paper, we propose the group-based time control mechanism to improve the network overload and delay performance in the femtocell-based machine type communications (MTC) networks. The MTC network may be congested if numerous MTC devices concurrently deliver messages to the MTC server. Femtocells can solve the radio access network (RAN) congestion, however the core network (CN) congestion becomes more difficult to handle. The proposed group-based time control method can spread the traffic load of MTC devices over the time, and thereby mitigate the RAN overload and CN overload simultaneously. Numerical results show that the proposed approach can significantly improve the network congestion and message delay compared to the existing methods.

Deployment strategies of access points for outdoor wireless local area networks

In this paper, we investigate the issues of deploying access points for wireless local area networks (WLANs) in an outdoor environment, where several adjacent access points (APs) forms a cluster and in each cluster access points are connected through wireless repeaters to ease deployment. Access points are usually placed aiming to maximize its cell coverage. Nevertheless, a larger coverage distance leads to a lower throughput for the wireless repeater. We apply the mixed-integer nonlinear programming (MINLP) optimization approach to determine the number of access points in a cluster and the best separation distance between access points. The objective is to maximize the ratio of the total carried traffic load to the total cost for a cluster of APs connected by wireless repeaters. In this paper, the uniform-spacing and the increasing-spacing placement strategies for access points are proposed and compared. Our results show that the increasing-spacing placement strategy outperforms the uniform-spacing placement strategy in terms of an objective function considering total carried traffic load, the number of access points, and the cost connecting an AP to the backbone network.

Design of optimal relay location in two-hop cellular systems

Relay stations are usually used to enhance the signal strength for the users near cell boundary, thereby extending the cell coverage. However, transmission through a relay station needs two transmission phases. The first phase is from base station to relay station, and the second one is from relay station to mobile station. Thus, using relay station may decrease system capacity due to two-phase transmission time. As a result, whether or not data are transmitted by one-hop or two-hop phases should be determined according to both signal strength and throughput. In this paper, we investigate the optimal relay location aiming to maximize system capacity. We consider two relay selection rules for determining whether two-hop transmission will be used: signal strength-oriented and throughput-oriented selection rules. We find that the signal strength-oriented two-hop transmission may yield even lower system capacity than the one-hop transmission. In the throughput-oriented scheme, the two-hop transmission can achieve higher system capacity than the one-hop transmission. By simulations, we determine the optimal relay location and show the coverage enhancement by the relaying network. Extensive simulations are performed to investigate the impacts of relay transmission power and the number of relay stations on system capacity and optimal relay location. The simulation results reveal important insights into designing a relaying network with high system capacity.

High-Capacity OFDMA Femtocells by Directional Antennas and Location Awareness

In this paper, we propose a location-aware mechanism combined with a low-cost four-sector switched-beam directional antenna to enhance the spectrum efficiency of orthogonal frequency-division multiple access (OFDMA)-based femtocell systems. The considered location-awareness capability is specified in the current IEEE 802.16m WiMAX standard, but has not been applied to avoid the interference between indoor femtocells and outdoor macrocells. With the knowledge of the locations of outdoor users, the proposed four-sector switched-beam antenna in a femtocell can effectively avoid the interference among femtocells and macrocells by adjusting the number of OFDMA subcarriers used at each femtocell. Numerical results show that the proposed approach can significantly improve spectrum efficiency compared to the existing methods.

QoS Provisioning in a Scalable Wireless Mesh Network for Intelligent Transportation Systems

The wireless mesh network (WMN) is an economical low-power solution to provide wireless broadband access in intelligent transportation systems (ITSs). This paper investigates how to deploy access points (APs) to improve throughput and quality-of-service (QoS), when the multihop communication is used to extend the coverage of wireless ITS network. Frequency planning is suggested to improve the capacity with QoS provisioning and to make the system more scalable. To investigate the overall tradeoffs among QoS, throughput, and coverage, we develop an analytical model to evaluate throughput, frame delay, and jitter for the considered ITS WMN using the carrier-sense multiple-access (CSMA) medium access control (MAC) protocol. Then, we apply an optimization approach to determine the optimal separation distances between APs with the delay requirement. To provide a guideline for network planning, we compare the uniform-spacing and increasing-spacing AP deployment strategies. The uniform-spacing strategy is to make all the APs with the same separation distance. In the increasing-spacing strategy, the separation distances between APs are increased from the central AP to the outer APs. Because of the shorter separation distance, the AP closer to the central AP can deliver higher traffic. It is shown that the increasing-spacing strategy outperforms the uniform-spacing strategy in terms and a profit function, considering the capacity and the total cost for deploying APs.

Throughput-coverage tradeoff in a scalable wireless mesh network

The wireless mesh network (WMN) is an economical and low-power solution to support ubiquitous broadband services. However, mesh networks face scalability and throughput bottleneck issues as the coverage and the number of users increase. Specifically, if the coverage is extended by multiple hops, the repeatedly relayed traffic will exhaust the radio resource and degrade user throughput. Meanwhile, as the traffic increases because of more users, the throughput bottleneck will occur at the users close to the gateway. The contention collisions among these busy users near the gateway will further reduce user throughput. In this paper, a newly proposed scalable multi-channel ring-based WMN is employed. Under the ring-based cell structure, multi-channel frequency planning is used to reduce the number of contending users at each hop and overcome the throughput bottleneck issue, thereby making the system more scalable to accommodate more users and facilitate coverage extension. This paper mainly focuses on investigating the overall tradeoffs between user throughput and cell coverage in the ring-based WMN. An analytical throughput model is developed for the ring-based WMN using the carrier sense multiple access (CSMA) medium access control (MAC) protocol. In the analysis, we also develop a bulk-arrival semi-Markov queueing model to describe user behavior in a non-saturation condition. On top of the developed analytical model, a mixed-integer nonlinear optimization problem is formulated, aiming to maximize cell coverage and capacity. Applying this optimization approach, we can obtain the optimal number of rings and the associated ring widths of the ring-based WMN.