AHPCRC Projects
Project 3-2: Robust Wireless Communications in Complex Environments |
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| Managing interference in a mobile ad hoc network. |
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| Graphics this page courtesy Arogyaswami Paulraj and George Papanicolaou (Stanford University). |
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During the last decade, the commercial wireless communications industry has made significant progress in enhancing coverage and mobility and achieving high spectral efficiency. Civilian wireless technology is typically some generations ahead of military technology because of the sheer size of the mobile communication market, which attracts huge investments in technology development, according to Stanford electrical engineering professor Arogyaswami Paulraj. However, he said, these commercial providers have not generally addressed issues of performance in complex military scenarios. Effective military wireless communications require robust networks and links that are capable of operating in complex environments, especially in dense urban neighborhoods. Such networks must resist hostile jamming while keeping low power at terminals. "These days an army marches on its batteries," Paulraj said in a Stanford press release. During the Gulf War, the Army had to bring in jet transports full of batteries to meet the demand. Smart antennas can reduce battery drain substantially, Paulraj said, because they can transmit radio energy only in the required direction. That capability also can make it more difficult for hostile forces to intercept radio signals, and it can make "spoofing"—the generation of disguised messages by enemy troops—more difficult. Designing a Better Network Collaborators include Stanford visiting scholar S.J. Thiruvengudam; postdocs Aydin Sezgin, Gokman Altay, Nicoli Czink; and Ph.D. students Mohammad Charafeddine and Stephanie Pereira, all of Stanford. The group plans to extend the simulation and optimization bench to go beyond the link level to include multiple (say 100) users simultaneously using the network in a complex urban environment. This will make the design challenge significantly more complex. However, this step will enable a much higher degree of design confidence and can better support full-scale development of such systems by industry. Additionally, the team will explore the application of HPC simulation to developing DoD imaging and sensing systems. Eventually, these methods will be optimized by large scale numerical simulations on high performance computing platforms and verified and validated in collaboration with the Army. The research team has analyzed realistic PHY (physical interface) models for battlefield networks. They have studied IM (interference management) using opportunistic scheduling, power control, and dimension orthogonalization. They have made TR (time reversal) channel measurements in indoor scenarios. In a presentation he gave last November, Paulraj explained several features of IM: Advantages offered by TR include spatial focusing, temporal focusing, and channel hardening. Spatial focusing, in which the spatial profile of the power peaks at the intended receiver and decays rapidly away from the receiver, reduces co-channel interference in a multi-cell system and increases the efficiency of bandwidth usage in the overall system. Temporal focusing produces a very short effective length of the channel impulse response at the receiver, which reduces the complexity of the equalization task. “Channel hardening” refers to the tightening of the distribution function of the effective channel impulse response. Thus, the time-reversed channel has a much smaller variance than the physical channel itself. (1) Focusing energy spatially and temporally at the intended receiver reduces the probability of signal interception. Time reversal techniques improve link reliability by increasing diversity, which reduces signal fading by generating a number of signal transmission paths, or diversity branches. Each branch carries the same information signal, but multipath fadings of each branch are not correlated. Diversity branches are recombined at the receiver to resolve the transmitted signal. The Stanford group is interested in exploring applications for the very rich diversity offered by HDB channels. One possibility is improving the spatial multiplexing in MIMO systems. Coding strategies must be developed to exploit rich-diversity channels. The research team is developing performance tools for the “interference management” schemes that they developed in 2007. They will complete a scalable PHY layer simulator for battlefield networks to explore interference and TR-enabled schemes, and they plan further developments in interference-aware scheduling techniques. The team will do experimental assessments of TR leverage in secure communications in urban scenarios, and they will study how to adapt fourth-generation (4G) mobile wireless systems such as WIMAX, LT, and UMB to MANET applications. Modern physical layer technologies such as OFDMA, MIMO and OS can enable reliable, high speed, multi media links within a large, and dynamic, mobile network of users, while meeting low power constraints. Furthermore, time reversal techniques may offer ways to exploit the multi-path to reduce co-channel interference to friendly users while reducing the probability of interception. Such goals are rarely of interest in commercial wireless communications, which are designed to operate under highly predictable and controlled environment. The design of battlefield communications system requires intense computing resources and therefore can significantly benefit from high performance computing technologies. References: |
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