Ang-Hsun Tsai

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.

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.

High Capacity Femtocells with Directional Antennas

Femtocells are low-power, low-cost and user-deployed home base stations that can improve poor indoor coverage and enhance system capacity. However, as femtocells are deployed extensively, the two-tier interference will become serious, including the interference from macrocells and that from femtocells. Thanks to the low side lobe, the E-plane horns based reconfigurable antenna can be used to reduce the interference and enhance the capacity. In this paper, we investigate the effects of applying E-plane horns based reconfigurable directional antenna on link reliability and capacity of the orthogonal frequency-division multiple access (OFDMA) based femtocell systems. We compare our newly designed E-plane horns based reconfigurable antenna,and the sector antenna provided in the IEEE 802.16m, as well the omni-directional antenna. It is shown that E-plane horns based reconfigurable antenna system has higher femtocell capacity than the IEEE 802.16m sector antenna system due to the advantages of low side lobe and high main-lobe gain.

Stable Subchannel Allocation for OFDMA Femtocells with Switched Multi-Beam Directional Antennas

In this paper, we propose a low-complexity distributed stable subchannel allocation algorithm combined with a low-cost reconfigurable switched multi-beam directional antenna to improve the capacity and the fairness performance for the orthogonal frequencydivision multiple access (OFDMA)-based femtocell systems. With the proposed deferred acceptance algorithm, the stable subchannel allocation scheme can achieve a better tradeoff between capacity and fairness. Numerical results show that the proposed stable subchannel allocation scheme with the switched multi-beam antenna can significantly improve femtocell capacity compared to the traditional user-oriented subchannel allocation scheme and ensure the fairness as compared to the traditional channel-oriented subchannel allocation scheme.

Capacity comparison for CSG and OSG OFDMA femtocells

The low-power and low-cost femtocells can improve indoor coverage and capacity. However, when femtocells are deployed extensively, the serious two-tier interference problem occurs. The method to access the femtocell is the key to manage the interference in the femtocell systems. In this paper, we investigate the impacts of access methods and directional antenna on link reliability and capacity for the orthogonal frequency-division multiple access (OFDMA)-based femtocell systems. Simulation results show that the open access method can improve the femtocell capacity over the traditional closed access method. Nevertheless, under the link reliability requirement of indoor and outdoor users, the improvement in femtocell capacity due to open access method is minor. Thanks to the narrow-beam pattern, the switched-beam directional antenna can be used to further reduce the interference and enhance the capacity. It is shown that the open access method using the switched-beam antenna can achieve higher femtocell capacity than that using the omnidirectional antenna.

Network-assisted device-decided channel selection and power control for multi-pair device-to-device (D2D) communications in heterogeneous networks

The device-to-device (D2D) communications can improve the spectrum efficiency with low power and effectively offload the traffic from the macro-eNB. However, the macro/femto/D2D heterogeneous networks face a complicated three-tier interference issue. The completely centralized resource management is not applicable in such three-tier heterogeneous networks. Therefore, we propose a network-assisted device-decided (NADD) scheme to jointly select the channels and adjust the transmission power for D2D devices. In the NADD method, the macro-eNB broadcasts resource allocation instruction to D2D devices. According to the resource allocation instruction and the devices communication quality, D2D devices autonomously and dynamically select the suitable channels and adjust the transmission power. Simulation results show that NADD scheme can significantly improve the total throughput and ensure the link reliability of macrocell users, comparing to both the fixed power scheme and the network-decided (ND) power control scheme.

Bi-SON: Big-Data Self Organizing Network for Energy Efficient Ultra-Dense Small Cells

In this paper, we present a big-data self organizing network (Bi-SON) framework aiming to optimize energy efficiency of ultra-dense small cells. Although small cell can enhance the capacity of cellular mobile networks, ultra-dense small cells suffer from severe interference and poor energy efficiency. The self organizing network (SON) can automatically manage and optimize the system performance. However, current SON-enable mechanisms mostly focus on indoor femtocells. Our proposed Bi-SON suggests a data flow framework from data collection, analysis and optimization to reconfiguration. We adopt the statistics analysis approach to determine the optimal system parameters to improve the energy efficiency of a huge number of outdoor small cells. The Bi-SON mechanism periodically collects the management data of small cells, e.g. transmission power, reference signal receiving power and the number of users per cell. We find that simple sorting and filtering data analysis from huge number of small cells can already effectively find the almost optimal solution. Our simulation results show that Bi-SON can improve throughput and energy efficiency by 50% and 135% respectively, compared to the scheme without energy saving approach.

Data-driven power control of ultra-dense femtocells: A clustering based approach

In this paper we present a data-driven power control (DDPC) approach to improve total cell throughput and energy efficiency of ultra-dense femtocells. Although femtocells can increase the capacity and coverage in an indoor environment, ultra-dense femtocells may consume a lot of energy and generate severe interference. We investigate a data-driven clustering approach to reduce co-tier interference among femtocells in a dense deployment scenario. The proposed DDPC approach periodically collects the operation data of dense femtocells, including reference signal received power (RSRP) from each user, the transmission power and the number of users per femtocell, and so on. The collected data are processed via the affinity propagation (AP) clustering algorithm to determine the cluster centers to perform power control. The AP clustering algorithm can automatically determine appropriate the number of clusters and the corresponding cluster centers for various femtocell densities. Simulation results show that the proposed DDPC approach can increase 41% higher total cell throughput and 64% higher energy efficiency respectively, compared to the approach without power control in the ultra-dense femtocells.

Channel-Oriented Channel Allocation Scheme (COCAS) for Multi-Antenna OFDMA Femtocells

In this paper, we propose a low-complexity distributed channel-oriented channel allocation scheme (COCAS) to improve the link reliability of users and the spectrum efficiency of femtocells for multi-antenna orthogonal frequency-division multiple access (OFDMA)-based femtocell systems. By two steps of comparisons on channel gains of resource blocks and antennas, the COCAS can jointly allocate suitable antennas and resource blocks for transmission. We also investigate the impacts of channel allocation and multiple antennas on link reliability of users and spectrum efficiency of femtocells. Simulation results show that the proposed COCAS can achieve higher spectrum efficiency for femtocells than the random channel allocation scheme with a given number of antennas equipped on the femtocell base station, under the link reliability requirement.