Multi-user computer-aided design (CAD) is an emerging technology that promises to facilitate collaboration, enhance product quality, and reduce product development lead times by allowing multiple engineers to work on the same design at the same time. The BYU site of the NSF Center for e-Design has developed advanced multi-user CAD prototypes that have demonstrated the feasibility and value of this technology. Despite the possibilities that this software opens up for enhanced collaboration, there are now a new variety of challenges and opportunities to understand and address. For multi-user CAD to be used effectively in a modern engineering environment, it is necessary to understand and address both human and technical collaboration challenges. The purpose of this dissertation is to understand and address these challenges. Two studies were performed to better understand the human side of engineering collaboration: (1) engineers from multiple companies were interviewed to assess the collaboration challenges they experience, and (2) players of the multi-player game Minecraft were surveyed and studied to understand how a multi-user environment affects design collaboration. Methods were also developed to address two important technical challenges in multi-user CAD: (1) a method for detecting undo conflicts, and (2) additional methods for administering data access. This research addresses some of the important human and technical collaboration challenges in multi-user CAD. It enhances our understanding of collaboration challenges in engineering industry and how multi-user CAD will help address some of those challenges. It also enhances our understanding of how a multi-user design environment will affect design collaboration. The method developed for detecting conflicts that occur during local undo in multi-user CAD can be used to block conflicts from occurring and provide the user with some information about the cause of the conflict so they can collaborate to resolve it. The methods developed for administering data access in multi-user CAD will help protect against unauthorized access to sensitive data.

Virtual Reality (VR) is a highly evolving domain and is used in anincreasing number of areas in today's society. Among the technologiesassociated with VR and especially mobile VR, is hand tracking and handgesture recognition. Telemedicine is one of the elds where VR is startingto thrive, and so the concept of adding the use of hand gestures came to bein order to explore the possibilities that can come from it. This researchis conducted with the development of a prototype application that usessome of the most emerging technologies. Manomotion's hand trackingand hand gesture recognition algorithms, and Photon's servers and developerkit, which makes multi-user applications achievable, allowed theconceptual idea of the prototype to become reality. In order to test itsusability and how potential users perceive it, a user study with 24 participantswas made, 8 of which were either studying or working in themedical eld. Additional expert meetings and observations from the userstudy also contributed to ndings that helped show how hand gesturescan aect a doctor consultation in Telemedicine. Findings showed thatthe participants thought of the proposed system as a less costly and timesaving solution, and that they felt immersed in the VR. The hand gestureswere accepted and understood. The participants did not have dicultieson learning or executing them, and had control of the prototype environment.In addition, the data showed that participants considered it to beusable in the medical eld in the future.

Future wireless communications expect to experience a spectrum shortage problem. One practical solution is spectrum sharing. This thesis studies precoding strategies to allocate communication resources for spectrum sharing in multi-user wireless systems from a game-theoretic perspective. The approaches for the precoding games are developed under different constraints. It is shown that the precoding game with spectral mask constraints can be formulated as a convex optimization problem and a dual decomposition based algorithm can be exploited to solve it. However, the problem is non-convex if users also have total power constraints. This study shows that an efficient sub-optimal solution can be derived by allocating the bottleneck resource in the system. The sub-optimal solution is proved to be efficient and can even achieve an identical performance to that of the optimal solution in certain cases, but with significantly reduced complexity. / Communications

We consider the multi-user MIMO broadcast channel with M single-antenna users and N transmit antennas under the constraint that each antenna emits signals having constant envelope (CE). The motivation for this is that CE signals facilitate the use of power-efficient RF power amplifiers. Analytical and numerical results show that, under certain mild conditions on the channel gains, for a fixed M, an array gain is achievable even under the stringent per-antenna CE constraint. Essentially, for a fixed M, at sufficiently large N the total transmitted power can be reduced with increasing N while maintaining a fixed information rate to each user. Simulations for the i.i.d. Rayleigh fading channel show that the total transmit power can be reduced linearly with increasing N (i.e., an O(N) array gain). We also propose a precoding scheme which finds near-optimal CE signals to be transmitted, and has O(MN) complexity. Also, in terms of the total transmit power required to achieve a fixed desired information sum-rate, despite the stringent per-antenna CE constraint, the proposed CE precoding scheme performs close to the sum-capacity achieving scheme for an average-only total transmit power constrained channel. / <p>Funding Agencies|Swedish Foundation for Strategic Research (SSF)||ELLIIT||Knut and Alice Wallenberg Foundation||Center for Industrial Information Technology at ISY, Linkoping University (CENIIT)||</p>

As a type of virtual learning community, multi-user virtual environments (MUVEs) are not only sources of entertainment but are also places where learning opportunities and community development can be created and fostered. Some multi-user virtual environments that have emerged have been designed to serve students and teachers in the K-12 sector. Although learning is a goal in these contexts, this study focused on some of the community building and social networking components. The purpose of this study was to examine whether, to what degree and how nine elementary aged students projected themselves socially through this medium. The results could provide insight into the integration of such environments into K-12 educational contexts and could serve as a launching point for further research into the learning and community aspects of MUVEs. A case study approach was used in this research study. The researcher chose a class of nine students who were enrolled in an educational MUVE as part of their regular studies. Although these students completed assignments in the MUVE, only their social interactions were analyzed. The data was collected from student communication logs in the educational MUVE Quest Atlantis and from interviews with participants. Document analysis was used to analyze transcripts of student communications in Quest Atlantis as well as transcripts from text-based interviews. The results obtained demonstrate the types of communication and tool selection patterns of elementary aged students when using text to communicate in a MUVE and provide insight that can be used by teachers to inform the integration of MUVEs in their unique learning contexts. Findings indicated that frequency of communication varied substantially between participants but message content was similar and content volume varied depending on the communication tool. Gender differences were pronounced. Results also revealed that all participants were comfortable and enjoyed their involvement in the MUVE.

This thesis contributes toward understanding fundamental limits of multi-user fading channels and random wireless networks. Specifically, considering different samples of channel gains corresponding to different users/nodes in a multi-user wireless system, the maximum number of channel gains supporting a minimum rate is asymptotically obtained. First, the user capacity of fading multi-user channels with minimum rates is analyzed. Three commonly used fading models, namely, Rayleigh, Rician and Nakagami are considered. For broadcast channels, a power allocation scheme is proposed to maximize the number of active receivers, for each of which, a minimum rate Rmin>0 can be achieved. Under the assumption of independent Rayleigh fading channels for different receivers, as the total number of receivers n goes to infinity, the maximum number of active receivers is shown to be arbitrarily close to ln(P.ln(n))/Rmin with probability approaching one, where P is the total transmit power. The results obtained for Rayleigh fading are extended to the cases of Rician and Nakagami fading models. Under the assumption of independent Rician fading channels for different receivers, as the total number of receivers n goes to infinity, the maximum number of active receivers is shown to be equal to ln(2P.ln(n))/Rmin with probability approaching one. For broadcast channels with Nakagami fading, the maximum number of active receivers is shown to be equal to ln(ω/μ.P.ln(n))/Rmin with probability approaching one, where ω and μ are the Nakagami distribution parameters. A by-product of the results is to also provide a power allocation strategy that maximizes the total throughput subject to the rate constraints. In multiple-access channels, the maximum number of simultaneous active transmitters (i.e. user capacity) is obtained in the many user case in which a minimum rate must be maintained for all active users. The results are presented in the form of scaling laws as the number of transmitters increases. It is shown that for all three fading distributions, the user capacity scales double logarithmically in the number of users and differs only by constants depending on the distributions. We also show that a scheduling policy that maximizes the number of simultaneous active transmitters can be implemented in a distributed fashion. Second, the maximum number of active links supporting a minimum rate is asymptotically obtained in a wireless network with an arbitrary topology. It is assumed that each source-destination pair communicates through a fading channel and destinations receive interference from all other active sources. Two scenarios are considered: 1) Small networks with multi-path fading, 2) Large Random networks with multi-path fading and path loss. In the first case, under the assumption of independent Rayleigh fading channels for different source-destination pairs, it is shown that the optimal number of active links is of the order log(N) with probability approaching one as the total number of nodes, N, tends to infinity. The achievable total throughput also scales logarithmically with the total number of links/nodes in the network. In the second case, a two-dimensional large wireless network is considered and it is assumed that nodes are Poisson distributed with a finite intensity. Under the assumption of independent multi-path fading for different source-destination pairs, it is shown that the optimal number of active links is of the order N with probability approaching one. As a result, the achievable per-node throughput obtained by multi-hop routing scales with Θ(1/√N).

This thesis contributes toward understanding fundamental limits of multi-user fading channels and random wireless networks. Specifically, considering different samples of channel gains corresponding to different users/nodes in a multi-user wireless system, the maximum number of channel gains supporting a minimum rate is asymptotically obtained. First, the user capacity of fading multi-user channels with minimum rates is analyzed. Three commonly used fading models, namely, Rayleigh, Rician and Nakagami are considered. For broadcast channels, a power allocation scheme is proposed to maximize the number of active receivers, for each of which, a minimum rate Rmin>0 can be achieved. Under the assumption of independent Rayleigh fading channels for different receivers, as the total number of receivers n goes to infinity, the maximum number of active receivers is shown to be arbitrarily close to ln(P.ln(n))/Rmin with probability approaching one, where P is the total transmit power. The results obtained for Rayleigh fading are extended to the cases of Rician and Nakagami fading models. Under the assumption of independent Rician fading channels for different receivers, as the total number of receivers n goes to infinity, the maximum number of active receivers is shown to be equal to ln(2P.ln(n))/Rmin with probability approaching one. For broadcast channels with Nakagami fading, the maximum number of active receivers is shown to be equal to ln(ω/μ.P.ln(n))/Rmin with probability approaching one, where ω and μ are the Nakagami distribution parameters. A by-product of the results is to also provide a power allocation strategy that maximizes the total throughput subject to the rate constraints. In multiple-access channels, the maximum number of simultaneous active transmitters (i.e. user capacity) is obtained in the many user case in which a minimum rate must be maintained for all active users. The results are presented in the form of scaling laws as the number of transmitters increases. It is shown that for all three fading distributions, the user capacity scales double logarithmically in the number of users and differs only by constants depending on the distributions. We also show that a scheduling policy that maximizes the number of simultaneous active transmitters can be implemented in a distributed fashion. Second, the maximum number of active links supporting a minimum rate is asymptotically obtained in a wireless network with an arbitrary topology. It is assumed that each source-destination pair communicates through a fading channel and destinations receive interference from all other active sources. Two scenarios are considered: 1) Small networks with multi-path fading, 2) Large Random networks with multi-path fading and path loss. In the first case, under the assumption of independent Rayleigh fading channels for different source-destination pairs, it is shown that the optimal number of active links is of the order log(N) with probability approaching one as the total number of nodes, N, tends to infinity. The achievable total throughput also scales logarithmically with the total number of links/nodes in the network. In the second case, a two-dimensional large wireless network is considered and it is assumed that nodes are Poisson distributed with a finite intensity. Under the assumption of independent multi-path fading for different source-destination pairs, it is shown that the optimal number of active links is of the order N with probability approaching one. As a result, the achievable per-node throughput obtained by multi-hop routing scales with Θ(1/√N).

As a type of virtual learning community, multi-user virtual environments (MUVEs) are not only sources of entertainment but are also places where learning opportunities and community development can be created and fostered. Some multi-user virtual environments that have emerged have been designed to serve students and teachers in the K-12 sector. Although learning is a goal in these contexts, this study focused on some of the community building and social networking components. The purpose of this study was to examine whether, to what degree and how nine elementary aged students projected themselves socially through this medium. The results could provide insight into the integration of such environments into K-12 educational contexts and could serve as a launching point for further research into the learning and community aspects of MUVEs. A case study approach was used in this research study. The researcher chose a class of nine students who were enrolled in an educational MUVE as part of their regular studies. Although these students completed assignments in the MUVE, only their social interactions were analyzed. The data was collected from student communication logs in the educational MUVE Quest Atlantis and from interviews with participants. Document analysis was used to analyze transcripts of student communications in Quest Atlantis as well as transcripts from text-based interviews. The results obtained demonstrate the types of communication and tool selection patterns of elementary aged students when using text to communicate in a MUVE and provide insight that can be used by teachers to inform the integration of MUVEs in their unique learning contexts. Findings indicated that frequency of communication varied substantially between participants but message content was similar and content volume varied depending on the communication tool. Gender differences were pronounced. Results also revealed that all participants were comfortable and enjoyed their involvement in the MUVE.

A non-coherent PN code tracking loop with multi-user detection and simplified multi-path interference cancellation (MPIC) is proposed for direct sequence spread spectrum communications system. A decorrelator decision-feedback detector (DDFD) is first applied on the incoming signal to mitigate the multi-user interference. Then, a simplified multi-path interference cancellation (MPIC) is further used to increase signal quality. Finally, a modified code tracking loop (MCTL) is adopted for non-coherent PN code tracking. Mathematical expressions of the S-curve and tracking jitter are derived. Mean time to lose lock is also compared with traditional tracking loops. From the numerical results, we know the proposed PN code tracking loop can efficiently mitigate the interference from multi-user and multi-path and improve the performance of code tracking loop.

In this thesis, One of the difficulties in the orthogonal frequency division multiple access (OFDMA) systems is the multi-user interference (MUI) induced by the carrier frequency offset (CFO). In sub-band OFDMA systems, each user occupies a consecutive and non-overlapping frequency sub-band. The pilots are usually placed at the edges of a sub-band for frequency synchronization and channel estimation. However, the both frequency synchronization and channel estimation performance are influenced seriously by the multi-user interference (MUI) induced by CFO of other users. The MUI can be reduced by inserting the guard sub-carrier between adjacent users. In this paper, a novel pilot architecture is investigated, which has the same bandwidth efficiency as the conventional guard sub-carrier insertion scheme. In addition, the proposed pilot architecture provides better carrier frequency synchronization and channel estimation performance than conventional pilot assignment. Moreover, this new pilot architecture also has MUI reduction utility.