Shang-Juh Kao

Dynamic Orthogonal Frequency Division Multiple Access resource management for downlink interference avoidance in two-tier networks

In two-tier networks, which consist of macrocells and femtocells, femtocells can offload the traffic from macrocells thereby improving indoor signal coverage. However, the dynamic deployment feature of femtocells may result in signal interference due to limited frequency spectrum. The tradeoff between broad signal coverage and increased signal interference deserves further exploration for practical network operation. In this paper, dynamic frequency resource management is proposed to avoid both co-tier and cross-tier Orthogonal Frequency Division Multiple Access downlink interference and increase frequency channel utilization under co-channel deployment. A graph-based non-conflict group discovery algorithm is proposed to discover the disjoint interference-free groups among femtocells in order to avoid the co-tier interference. A macrocell uses the femtocell gateway for frequency resource allocation among femtocells to avoid cross-tier interference. We formulate the optimized frequency resource assignment as a fractional knapsack problem and solve the problem by using a greedy method. The simulation results show that the average data transfer rate can be increased from 21% to 60%, whereas idle rate and blocking rate are decreased in the range of 15% i¾?22% and 60% i¾?82%, respectively, as compared with conventional graph coloring and graph-based dynamic frequency reuse schemes. Copyright

An Efficient Handover Mechanism by Adopting Direction Prediction and Adaptive Time-to-Trigger in LTE Networks

In this paper, we propose an efficient handover mechanism for Long Term Evolution (LTE) networks. Both moving direction prediction and Time-to-Trigger (TTT) adjustment are taken into consideration for processing to lower the unnecessary handover. By referencing previous locations, user equipment’s (UE) moving direction is estimated by the cosine function to determine a better destined E-UTRAN NodeB (eNB). With adaptive TTT, we categorize UEs into 3 levels according to the velocity. Initially, TTT value is given for each level, and the value is decremented when the handover failure rate gets higher. In addition, we also combine both approaches of moving direction prediction and adaptive TTT into handover procedure. Simulation results show that the proposed method can effectively improve the handover efficiency. As compared with the standard handover procedure, an average reduction of 31% handovers can be obtained by applying the moving direction prediction. And, as compared with fixed TTT, up to the 36.5% of handover trigger failure rate can be expected by using the adaptive TTT handover triggering approach. The performance can be further improved when both approaches are taken in dealing with the handover procedure.

Using GI - G -1 queuing model for rtPS performance evaluation in 802.16 networks

Among the scheduling services, rtPS (real-time polling service) is designated for real-time applications. Among three packet delay intervals, performance effect on polling interval has been widely studied, but less on the intervals of scheduling and delivery. To evaluate the performance of delay-sensitive rtPS applications, instead of using continuous queueing model, a discrete-time GI-G-1 model, which considers intervals of polling, scheduling, and delivery, is proposed. By taking VoIP as a typical rtPS application, the transmission latency under different QoS settings, polling probability, and traffic load are presented. The latency is also compared among various codec schemes. The results indicate that when the codec rate is either fulfilled or dissatisfied by the promised bandwidth of service levels, the performance is highly dependent upon the polling probability, no matter what the traffic condition is. However, if the codec rate is in between the promised bandwidth of various service levels, the polling probability is a dominant factor in light traffic environment, while the settings on QoS parameters will strongly determine the performance in heavy traffic situation. In addition to the verification using simulation, the bandwidth utilization derived from the GI-G-1 model can be applied to improve the serving capacity of base stations. Copyright

A slot-based BS scheduling with maximum latency guarantee and capacity first in 802.16e networks

IEEE 802.16e standard defines a wireless broadband access network technology. The standard specifies the QoS support at the MAC level. To meet the QoS requirements, the scheduling algorithm at base station (BS) has to efficiently allocate slots for each connection request. The standard does not specify the detailed algorithm but leaves it open for implementation. In this paper, a Slot-based BS scheduling algorithm with Maximum Latency Guarantee and Capacity First (SMLG-CF) is proposed. With SMLG-CF, the connection is chosen with highest slot capacity first. With the assistance of dynamic sub-frame adjustment, the average system transmission rate can be enhanced. Through the finer slots calculation and proper allocation of transmission time and frequency, the maximum latency guarantee can be achieved for urgent requests. We simulate and compare the proposed mechanism with deficit fair priority queue (DFPQ) scheduling algorithm and Highest Urgency First (HUF) scheduling algorithm in terms of maximum latency violation rate and average transmission rate. The simulation results reveal that the applicability of the proposed mechanism.

A moving direction prediction-assisted handover scheme in LTE networks

The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) defines a wireless network standard for high packet transmission rate and low packet latency provisions. Handover is one of the important features for helping user equipments (UEs) to roam between LTE networks. However, LTE networks adapt a make-before-break handover procedure, which may cause a brief disconnection, therefore results in the packet transmission delay and packet loss problems. In this paper, we propose a moving direction prediction-assisted handover scheme for LTE networks to lower the number of handovers. We first track the location of user equipments (UEs) to predict their moving direction. By referencing previous locations, the next moving direction of UEs is estimated with the cosine function in order to determine the candidate E-UTRAN NodeBs (eNBs) for handover. Then, a target eNB is selected from the candidate eNBs through an angle-based dynamic weight adjustment scheme. By selecting a proper target eNB for handover, thus the quality of network transmission can be enhanced. Simulation results demonstrate the ability of the proposed scheme in reducing 17% average handover times, compared with the standard handover procedure, thereby reducing 12% average number of packet loss and 5% average packet delay time.

A vertical handover scheme from WMAN to WLAN by taking into account the maximum available resource

In heterogeneous wireless networks, the end users are expected to get more bandwidth from WLAN, other than from WMAN, for real time applications. However, adopting traditional Received Signal Strength (RSS) based approach could be inadequate for vertical handover. When WLAN is within the coverage of a WMAN Base Station, an associated Mobile Node (MN) can hardly connect to a WLAN Access Point (AP) due to the default high handover triggering threshold. On the other hand, without knowing the utilization information of scanned APs, once the MN handover to a busy AP by detecting a better signal, it may suffer from bandwidth contention. In this paper, we propose an efficient handover scheme, from WMAN to WLAN, to make use of the received signal to interference and noise ratio and the information about back-haul bandwidth for the criteria to trigger a handover. The time to start a handover is determined by considering the variations of data rate as the MN moves and the most suitable target AP is selected according to its maximum available bandwidth. The simulation results show that the MN consistently has a better throughput with the proposed scheme, as compared with the RSS-based vertical handover scheme. And among all candidates APs, the MN has the minimum packet delay with the selected AP association.

Handoff optimization in 802.11 wireless networks

In 802.11 wireless networks, a complete handoff procedure for a mobile node requires access point (AP) selection, AP switch, call admission control (CAC), IP address re-allocation, and network re-configuration. Most current handoff schemes deal only with either AP selection or IP address re-allocation. In this paper, an integrated handoff procedure is proposed. First, AP selection is accomplished by choosing an AP with the lowest channel utilization and smaller number of associated users. The information about load of each AP is reported through modified beacon frames. In the case of adopting load-based AP selection, the average throughput can be increased up to 56%, as opposed to pure SNR-based AP selection. Next, both CAC and IP address pre-fetch are performed simultaneously through the simplified DHCP procedure. Specifically, efficient limited fractional guard channel policy (ELFGCP) is proposed for the CAC phase. By adopting ELFGCP, the failure probability can be reduced as much as 45% from conventional LFGCP. Finally, the simulation results demonstrate the applicability of the integrated approach, and the overall disconnection time due to handoff can be reduced from 2.9 to 0.004 s using traditional handoff procedures.

A beacon-based handover scanning mechanism with QoS support in WiMAX FemtoCell architecture

Deploying FemtoCells in WiMAX network, also called WiMAX FemtoCell architecture, gains a lot of attention due to better indoor services can be provided and WiMAX network traffic can be shared by the FemtoCell. However, since the signal of WiMAX base station (BS) is stronger than FemtoCell Base Station (fBS), the handover procedure may not be triggered even though fBS is within the BS coverage. Besides, since the coverage of FemtoCell is small, it is possible that a huge number of fBSs are deployed in a WiMAX BS coverage, which causes a large volume of power consumption to scan all indoor fBSs for a handover. In this paper, we propose a beacon-based handover scanning mechanism with Quality of Service (QoS) support. In the proposed mechanism, through the beacon and modified MOB_NBR-ADV messages, a neighbor cell list with QoS parameters is generated automatically at mobile station (MS). The decision criteria for triggering the handover procedure and selecting an appropriate target indoor cell from many candidate fBSs are developed. Through the adoption of the proposed mechanism, the system throughput can be improved and the traffic load through the WiMAX BS can be reduced effectively. A simulation was conducted using QualNet simulator. The simulation results demonstrate that along with the number of FemtoCells increases, both the system throughput and the utilization of FemtoCells increase significantly, as compared with conventional handover mechanisms.

An efficient uplink scheduling mechanism with enabling Multi-device Transmission and Maximum Latency Fulfillment in IEEE 802.16j networks

IEEE 802.16j spreads out the coverage of the WiMAX network via extending the signal transmission by using the relay technology. To take the advantage of relaying in IEEE 802.16j network, an efficient schedule with quality of service (QoS) provision for multiple link transmissions is necessary, especially when link interference exists. In this paper, we propose an uplink scheduling mechanism with enabling Multi-device Transmission and Maximum Latency Fulfillment (MT-MLF). In the mechanism of MT-MLF, an interference detection mechanism is first carried out, then a resource distribution algorithm and a dynamic frame adjustment method are developed. Through the adoption of the proposed scheduler, both the uplink resource utilization and the requirement of real time applications can be better enhanced. Two simulation experiments were conducted with different levels of interference. The results demonstrate that under a fixed QoS type of connections, when the total number of connections goes up to 360 and 420 and the maximum latency violation rate approaches 20%, the average uplink transmission rate of MT-MLF can achieve 6.67Mbps and 7.92Mbps, which apparently outperform 4Mbps and 3.91Mbps as compared with regular relay scheduling schemes.

A beacon-based handover scanning mechanism with QoS support in WiMAX femtocell architecture

WiMAX femtocell architecture has gained a lot of attention because of its better indoor service provision through traffic sharing between WiMAX and femtocell. However, since the signal of a WiMAX base station (BS) is usually stronger than that of a femtocell base station (fBS), the handover procedure of mobile stations may not be triggered due to the stronger signal association. And, a huge number of fBSs deployed within a WiMAX BS coverage area will require extra power to scan all fBSs for a handover determination. In this article, we propose a beacon-based handover scanning mechanism with Quality of Service (QoS) support. Through the modified beacons and MOB_NBR-ADV messages, a neighbor cell list with QoS parameters is generated automatically at each mobile station. The decision criteria for handover triggering and an appropriate target cell selection are also presented. The QualNet simulation results demonstrate that along with increased fBSs, the proposed mechanism can improve the total system throughput from 37% to 51% and obtain higher bandwidth utilization in the case of increasing number of fBSs, as compared with Full Scan and Neighbor Cell Information Mechanisms.