Chia-Sheng Tsai

Iterative joint frequency offset and channel estimation for OFDM systems using first and second order approximation algorithms

To implement an algorithm for joint estimation of carrier frequency offset (CFO) and channel impulse response (CIR) in orthogonal frequency division multiplexing (OFDM) systems, the maximum-likelihood criterion is commonly adopted. A major difficulty arises from the highly nonlinear nature of the log-likelihood function which renders local extrema or multiple solutions for the CFO and CIR estimators. Use of an approximation method coupled with an adaptive iteration algorithm has been a popular approach to ease problem solving. The approximation used in those existing methods is usually of the first order level. Here, in addition to a new first order approximation method, we also propose a second order approximation method. Further, for the part of the adaptive iteration algorithm, we adopt a new technique which will enable performance improvement. Our first order approximation method is found to outperform the existing ones in terms of estimation accuracies, tracking range, computation complexity, and convergence speed. As expected, our second order approximation method provides an even further improvement at the expense of higher computation complication.

A Novel Energy Efficient Joint Dynamic Emissive Location-Based Routing Scheme for SOFAR Channel Underwater Sensor Networks

The Underwater Sensor Network (UWSN) could be recorded by hydrophones suspended in the channel as earthquake monitors by seismic events on or below the seafloor generated energy. In this paper, we present a novel position of sensor devices and routing protocol towards short for Sound Fixing and Ranging channel (SOFAR) for Sound Fixing and Ranging channel technologies, which could save transmission power. The sound speed also increases towards the warmer sea surface with temperature. Thus we use the oceanography to find more efficient ways of utilizing routing protocol and in to obtain more effective detection and identification capabilities for underwater sources. We present an adaptive location-based routing protocol which can overcome the location based UWSN without Global Positioning System (GPS) position. It reduced costs of deployment in long range propagation. The performances are measured according to the energy consumption per bit in which the result of simulator is better than other routing protocols such as FBR.

Frequency tracking by method of least squares combined with channel estimation for OFDM over mobile wireless channels

To track frequency offset and time-varying channel in orthogonal frequency division multiplexing (OFDM) systems over mobile wireless channels, a common technique is, based on one OFDM training block sample, to apply the maximum-likelihood (ML) algorithm to perform joint frequency tracking and channel estimation employing some adaptive iteration processes. The major drawback of such joint estimation techniques is the local extrema problem arising from the highly nonlinear nature of the log-likelihood function. This makes the joint estimation process very difficult and complicated, and many a time the results are not very satisfactory if the algorithm is not well designed. In this study, rather than using the ML algorithm, we shall apply the method of least squares (LS) for frequency tracking utilizing repeated OFDM training blocks. As will be seen, by using such an LS approach, the frequency offset estimation requires no channel knowledge. The channel state can be estimated separately after the LS frequency offset correction. This not only circumvents the local extrema complication, but also obviates the need for the lengthy adaptive iteration process of joint estimation thus greatly simplifies the entire estimation process. Most importantly, our technique can achieve excellent estimation performance as compared to the usual ML algorithms.

Enhancement of Delivery of Warning Messages for Mobile Networks

Vehicle-to-vehicle (V2V) communication systems have been studied extensively in recent years by transportation experts worldwide. Such systems enable the transmission of warning messages (alarms) between vehicles without additional roadside infrastructure. Thus, messages can be sent faster than through base stations. Because of these advantages, researchers worldwide are exploring how this technology can improve traffic safety. However, the various V2V schemes proposed thus far share the same problem: (1) unnecessary repetition of warning messages and (2) transmission to inapplicable respondents. This study proposes a new method for delivering V2V warning messages which reduces the redundant messages common in current schemes. Here, three schemes to deliver alarms are considered. One is blind flooding to broadcast alarms. Warning messages are routed by AODV protocol. Another is called Two Lanes. The broadcasting region of alarms is limited in two traffic lanes nearby a traffic accident only. And the last one is the proposed strategy where a warning message received by a car will be simply checked the prime number carried within the alarm header and decided whether the car is responsible to deliver it. If necessary, it will be forwarded to the next nodes. Otherwise, the redundant warning message shall be discarded to release system load and avoid to suspend other normal transmission proceeding. The results reveal that minimizing the delays caused by such redundant messages enables warnings to be sent to drivers more rapidly. Also, it prioritizes warning messages so that drivers are apprised of the most serious and dangerous traffic accidents. Moreover, another key finding in this paper is that the radii of the broadcasting coverage shall be adjusted carefully and adaptively to optimize the system performance.

Bandwidth allocation schemes for FTTH networks

Thanks to fiber-to-the-home (FTTH) networks offering remarkably lower cost per Mbps for multimedia communications, it has been the potential solution for the future broadband access networks. With increase of bandwidth requirement nowadays, subscribers using xDSL are changing to adopt FTTH service at a high rate for bandwidth-rich applications, such as on-line games, video-on-demand, long-distance education, or other real-time streaming reception. There are several main FTTH infrastructures, all of which are based on passive optical networks (PON) or with active nodes. And it is attractive to adopt PON architectures because there is no power consumption between optical line terminal (OLT) and the optical network units (ONU). However, in the upstream direction (from ONUs to the OLT), all ONUs using the same photonic wavelength (share the same frequency channel) need a mechanism to coordinate their transmission. In order to speed up traffic flow and upgrade the quality of service in the connections, we propose a multi-homing scheme to bundle an association to end users. In the numerical results, it reveals that with the multi-streaming approach, not only the head-of-line blocking problem was resolved, but also better total network efficiency can be achieved

A Simple Way to Compute Parameters for Load Balancer in Optical Networks

In this paper, we use the optical dynamic grooming concept to design load balancer. This paper proposes a new solution to achieve optimal load balancing for a streaming server. Dynamic traffic grooming can be utilized in combination with multipath routing to serve the bandwidth intensive applications when resource is not sufficient in the optical network. At the same time, most of the connections require bandwidth smaller than the capacity available in a wavelength, leading to low resource utilization of established light-paths. Leveraging multipath routing and dynamic grooming can efficiently utilize the residual capacity of the under-used wavelengths [.We inherit the concept of dynamic traffic grooming in optical network. We were able to develop a Windows XP load balancer. We designed some experiments to evaluate the performance of our system to determine if our system was capable of achieving the load balance. The solution will allow enterprises and service providers to reach load balancing purposes without adding additional hardware by utilizing an idle, low cost switch.

Designing Fast Identification Algorithm in the RFID System for Known Tag Sets

RFID is an automatic identification technology which exercises the radio frequency to retrieve the information saved in electronic tags. In a multi-tag environment, the collision problem caused by a number of tags simultaneously responding to a reading will jeopardize the speed of identification. Some scheme is necessary to reconcile the conflict and improve the reading time. A novel scheme is proposed in this paper to address this collision problem. In addition, our scheme can be trained to accelerate the identification of a subset of the known tags. Although there are similar schemes capable of achieving this function, our proposal accommodates rare presence of those tags outside the known set. In the simulation, we evaluate the performance of several tag anti-collision protocols. Specifically, we focus on the total reading time and the amount of messages required accomplishing the reading process. We observe that our scheme can effectively reduce both the reading time and the number of reading messages. In the best case, the proposed scheme saves 50% and 70% time when compared with the query tree algorithm (QTA) and binary tree algorithm (BTA) methods respectively. These improvements not only accelerate the reading speed but also save the on-board power in the reader.

Soft frequency reuse design to reduce inteference in femtocell systems

Femtocell is an important solution for operator to enhance the indoor coverage problem and reduce traffic within macrocells. However, there is an ICI (Inter-Cell Interference) between macrocells. Whileas femtocells are located in macrocells environments, CCI (Co-Channel Interference) may occur between femtocells and maceocells. ICI and CCI degrade the performance of femtocell and macrocell. SFR (Soft Frequency Reuse) is effective to improve interference. General SFR is reuse factor 3 but we propose a femtocell allocation method in macrocells environments based on SFR of reuse factor 4. We compare the blacking rate of SFR of reuse factor 3 and SFR of reuse factor 4. Simulation results show that the proposed femtocell deployment shceme does not increase the total blocking rate dramatically but reduce blocking rate of edge users efficiently.

Efficient Retransmission of RB-HARQ with LDPC Codes

In the paper, we study the RB-HARQ and the degree reliability using IEEE802.16e LDPC codes. The retransmission bits according to soft-decision output and degree in LDPC codes. We can select the bits with small absolute value of soft-decision output and the small degree has the priority to retransmit. To improve the efficiency of retransmission based on RB-HARQ using LDPC codes.

An Instant Messaging with Google Talk Handheld Devices Based on WEB2.0 for Tourist Call for Assistance

In recent years, rapid growth of the Internet and Instant Messaging (IM) technology makes IM a popular communication tool. In the meantime, Internet trend is evolving into the new generation, Web2.0, with emphasis on user co-constructing cybercontents. People around the world communicate with each other via on-line IM all day long. If you are stuck in an unfamiliar place and need assistance, asking assistance through IM service is a warrant that support will be delivered to you in time. Therefore, we propose to combine Google Talk (GTalk) of handheld devices with IM service over Web 2.0 forming a support system to assistance tourist travel in unfamiliar areas with confidence. When you need assistance, the content of your question message is input into a speech server by constructing an analytical model for Speech Interface Languages (SILs) system. The speech server translates your speech into text and sent out to IM Web 2.0 group for answering. Later on, when you log in the GTalk again, assistance will come to your hand at once. Because so many people are using on-line IM for 24 hours, the system should be very helpful for tourist around the world. Our system is now going on feasibility study and a large scale of experiment will conduct when we collect enough response about the system.