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Telecommunications network design involves tradeoffs between reliability, capacity, and the economics of meeting present and future customer demands. The creation of efficient networks --- be they fiber-optic, copper, or wireless --- is a daily challenge for telecom equipment manufacturers, interexchange carriers (IXCs), and regional Bell operating companies (RBOCs) alike. This research program is developing methodologies and tools for designing reliable and efficient networks that make optimal use of the associated capital investments. Models include SONET rings, channel assignment and bundling for hierchical systems, multi-period planning for staged network implementations, and value-based pricing systems for maximizing generated revenues and profits for RBOCs and IXCs.
We have developed a method for estimating the parameters of an autoregressive (AR) random process from a finite number of noisy measurements. The method uses a modified set of Yule-Walker equations that lead to a quadratic eigenvalue problem which when solved, gives estimates of the AR parameters and the measurement noise variance. This parameter estimation algorithm is demonstrated to out-perform the extended Yule_Walker method as well as the regular Yule-Walker method, which produces biased parameter estimates. The algorithm is currently being applied to linear predictive coding (LPC) of noisy speech. In noisy environments such as a car or airplane cockpit, standard LPC-based coders provide unsatisfactory performance due to the biasing of the AR model poles towards the origin. The method we have developed gives unbiased speech parameter estimates.
Recent advances in hardware technologies, such as portable computers and wireless communication networks, have led to the emergence of mobile computing systems. The status of data management in mobile computing is similar to that of distributed data management versus centralized data management in the late 60s. Namely that many of the issues are the same, but the solutions are different. In this environment the challenge is to provide access to data, located at DBMS sites on a fixed network, from a mobile computer. Data management issues include the limited speed of the wireless network, scalability, mobility, computation at the mobile host is limited by battery power, disconnection may be often, and the fact that processing should be compatible with the existing wired network. We classify current solutions to these issues within four major areas: transaction management, caching, broadcast disks, and agents. With mobile transactions, data requests are forwarded to the DBMS server itself. Broadcasts disks and caching approaches push the data down to the mobile host for access there. Agents assume that an intelligent software agent acts on behalf of the user at the mobile host. The objective of our ongoing research is to investigate ways to better address access to data from a mobile host. Unlike distributed transactions, mobile transactions do not originate and end at the same site. The implication of the movement of such transactions is that classical atomicity, concurrency and recovery solutions must be revisited to capture the movement behavior. As an effort in this direction, we have defined a model of mobile transactions by building on the concepts of split transactions and global transactions in a multidatabase environment. Our view of mobile transactions, called Kangaroo Transactions, incorporates the property that transactions in a mobile computing system hop from one base station to another as the mobile unit moves through cells. Our model is the first to capture this movement behavior as well as the data behavior which reflects the access to data located in databases throughout the static network. The mobile behavior is dynamic and is realized in our model via the use of split operations. The data access behavior is captured by using the idea of global and local transactions in a multidatabase system.
We have also begun investigating the use of agents. In our approach, mobile clients are represented by Mobile Service Agents (MSA) that run on stationary hosts, allowing mobile hosts to participate in traditional distributed applications where reliable network connectivity and sufficient communication bandwidth are assumed. Computation and data communications are pulled from mobile hosts to mobile service agents to the extent possible, reducing the power consumption at mobile hosts and the bandwidth need over the wireless link. This allows us to maintain logical network connectivity even when mobile hosts are disconnected. Data from mobile hosts may be replicated at mobile service agents to support disconnected operations at mobile service agents. All of these features of the MSA allow it to act as a proxy client on behalf of the mobile client. The MSA approach differs from earlier approaches to providing database access for mobile hosts. In fact, the MSA may exist and enhance an environment supporting either mobile transactions or caching at the mobile host.
The most recent area of our work is looking at mobile access of data as a query optimization issue. That is, when a client at a mobile host request a query, the objective is to determine the best way, out of all possible approaches, in which to access the data. The best way may be by accessing the cache at the mobile host, by submitting a transaction, or by listening to a broadcast disk. Interesting issues here are that cost factors are dynamic based on the fact that the mobile host environment is dynamic. In addition, the query optimization problem includes predicting future access so as to better determine cache contents and to hoard data for future use during possible periods of disconnection.
The emerging integrated broadband communication technologies, initially intended for optical fibre or cable transmission, will be available for wireless transmission for high bit rate radio local area networks and cellular mobile broadband systems.
It is currently recognized that multi-carrier modulation schemes, also known as OFDM schemes (Orthogonal Frequency Division Multiplexing), offer many advantages for signaling at high data rates over time dispersive channels. For this reason, they have been studied intensively in recent years, adopted in Europe for Digital Audio Broadcasting (DAB) proposed by EBU (European Broadcasting Union ), and proposed for other (namely digital TV) broadcasting systems.
The OFDM schemes depend on dividing the high bit rate stream into several interleaved low rate bit streams and using them to modulate several carriers in a spectral efficient way. Thus, low bit rate subchannels have long bit periods which reduces the sensitivity of the system due to the effects of multipath fading, specially under time dispersive channels which cause problems for conventional systems having small bit period compared to the delay spread of the channel. Also, long bit period averages the effects of fading and interference thus reducing their effects.
This gives the motivation for using OFDM in mobile communications whether for indoor or outdoor environments. For indoor environment, OFDM can be used in a broadband system that carry audio, video, fax and data at high bit rate which constitutes the idea of a complete wireless office. For outdoor environment, OFDM can be used in US licensed/unlicensed PCS applications.
It should be understood that OFDM schemes have their own problems which need to be addressed. These are the effect of nonlinearity of the channel (mainly caused by the power amplifiers), Inter-Channel Interference (ICI) due to the doppler effect, and frequency offsets. These problems need to be studied intensively for the wireless environment to be able to implement the OFDM system and make use of its many advantages.
The above paper presents a new model for the spare capacity allocation problem in a self-healing, mesh-ring hybrid, SONET telecommunications network. This model is know as the spare capacity network flow model and has not, we believe, previously appeared in the literature. The spare capacity network flow model uses a node-arc formulation while others have used an arc-path approach. The node-arc formulation is much more compact, and its special structure can be exploited in devising solution techniques.
An original algorithm, the network cut algorithm, is developed to solve the spare capacity network flow model. Application of decomposition principles results in an algorithm that iterates between a pure integer master problem and a set of minimum cost network flow subproblems. The algorithm incorporates a branch-and-bound procedure to solve the master problem, and pure network primal and dual simplex optimizers to solve the subproblems. The network cut algorithm is much more efficient than directly solving either the continuous relaxation or the mixed integer version of the spare capacity network flow model. (This research was supported in part by the Texas Higher Education Coordinating Board Grant Number 003613-002.)
The CDMA technology requires an optimal power control mechanism to reduce the interference and to improve the available capacity. Currently, the CDMA power control mechanism relies on the feedback on bit error rate and signal strength. We have derived a power control scheme which is based on the signal strength control only. The main advantage with the approach is to reduce the feedback time for power control. To provide both data and voice channel via CDMA technology, an adaptive power control scheme can be employed to reduce the bit error rate for data channel. Also, it is necessary to have the low latency power control for high throughput data channel. Furthermore, to increase the capacity, the measure of voice quality can be provided to reduce the transmission power requirement with acceptable voice quality.
The research focus is on the system and protocol architectures necessary for the seamless interconnection of wired and wireless networks. Some of the specific problems that are addressed include development and evaluation of distributed restoration protocols for SONET networks, restoration protocols for ATM networks, wireless broadband networks, PCS systems, and reliability and availability analysis of networks.
The goal is to develop mechanisms that are implementable at the application layer to detect and recover from hardware and software failures. Methods have been developed to integrate high-level checks to detect control-flow errors and data value errors caused by hardware faults. The concept of resilient objects (ROs) has been developed to address software faults in a distributed object computing environment such as CORBA.
The objective here is to develop methodologies to design and analyze human machine interfaces. An integrated approach involving formal and experimental techniques is being developed for the design of highly usable and reliable interfaces. This project is in collaboration with the psychology department in SMU, the Computer Engineering Research center at the University of Texas at Austin, the UT Southwestern Medical Center, and the E-systems Inc. in Greenville.
Recent interest has centered on the use of "Smart Antennas" in cellular as well as DCS 1800/PCS 1900 bands with a view to obtaining significantly improved spectral efficiencies, better quality of service, higher capacities, and improved coverage, while at the same time achieving significant savings in base station costs. The wide variety of digital signal processors and complex and innovative algorithms has made it possible to conceive of antennas which can perform with a high degree of intelligence.
Currently two approaches have been adopted for developing smart antennas: switched beam and adaptive array. The basic principle is to increase the gain of the base station antenna in the direction of reception from the mobile. This is achieved by forming multiple lobes in the 360 degree azimuth and the desired angle of coverage in the vertical. The smart antenna directs its main lobe with the enhanced gain towards the preferred user, with the side lobes being directed towards the interfering users, thereby improving the C/I ratio. This is typically achieved in adaptive arrays, by the use of space diversity in combination with an appropriate beamforming algorithm.
We have investigated the use of adaptive filtering techniques using a version of the conjugate gradient method for adaptive beamforming applications. The method does not involve matrix inversions and is hence computationally attractive. The required conjugate directions can be obtained by time averaging the data over a suitable number of snapshots so that the algorithm essentially uses time-diversity to obtain improved performance. We have also developed a modification of this algorithm which uses subaperture sampling to generate the conjugate direction vectors. This new implementation uses spatial smoothing instead of temporal smoothing and as such may be attractive for use in applications where the time delay and/or storage requirements associated with temporal averaging may not be acceptable. It is expected that a combination of temporal and spatial smoothing will have potential application in areas such as cellular communication systems.
Recent results have shown that if the antenna array is inclined with reference to a mobile collinear antenna, significant degradation in signal level can occur. It has been shown that use of polarization diversity using patterns in two directions can greatly reduce this degradation. We have proposed the use of the conjugate gradient algorithm to design an antenna array for use with an omnidirectional antenna in a polarization diversity scheme to provide enhanced performance at the base station.
For more information see Research and Sponsored Projects at SMU
Dr. Larry Smith
Office of Research Administration
Southern Methodist University
PO Box 750302
Dallas TX 75275-0302
(214) 768-4306
FAX: (214) 768-1079
Location:
Room G03 on the Garden Level of Perkins
Administration Building, the SE corner of University Dr. and
Hillcrest Blvd., University Park, Texas.
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