An OFDM Based Aeronautical Communication System (2011)

Wireless connectivity is becoming an integral part of our society. A new paradigm for aeronautical data services is beginning to take shape. The advances in signal processing, rapid prototyping, an insatiable consumer demand for Internet services, increase in aircraft traffic, aircraft safety, etc., are driving the demand for high speed data services. Programs led by the National Aeronautics and Space Administration (NASA), the Federal Aviation Administration (FAA), EUROCONTROL and Networking the Sky for Civil Aeronautical Communications (NEWSKY) are all looking into aeronautical platforms as part of their Aeronautical Data Network (ADN). The desire is to provide low delay, cost effective and high speed data connectivity for aeronautical platforms. The platforms can also be used as a relay for ground and airborne nodes. Such a capability could potentially provide data connectivity to remote areas. Most of the current high altitude platforms, i.e., aircraft, provide data connectivity through a satellite. However, satellite resources are limited and expensive, and they offer limited data throughput as compared to a terrestrial network. A potential solution is connectivity to ground stations that can provide high speed physical layers. Since the frequency spectrum is a valuable estate and needs to be used efficiently, the use of spectrum efficient techniques are evaluated. This dissertation discusses issues and challenges for developing a high speed ground based physical layer for aircraft and proposes a novel solution. A detailed analytical analysis is presented to show the issues related to aeronautical channel and its impacts to aeronautical communication system. Specifically, the impact of Doppler shifts that limit the use of efficient modulation schemes, such as Orthogonal Frequency Division Multiplexing (OFDM), is presented. OFDM is sensitive to Doppler shifts. In addition, Doppler spread and shifts in aeronautical channels depict different characteristics compared to terrestrial networks, i.e., multiple Doppler shifts and delays. Parametric techniques are investigated to accurately estimate the Doppler shifts. The results of parametric methods for estimating the Doppler shifts are presented. The simulation results of MUltiple Signal Classification (MUSIC), Eigenvector (EV) and Minimum norm methods are considered for an aeronautical channel and their performances is presented.\nOFDM, in combination with dense encoding, offers a robust communication and spectrum compression. Its use is limited to the terrestrial domain due to its sensitivity to Doppler shifts. OFDM sensitivity to frequency shifts results in Intercarrier Interference (ICI) and degrades spectral efficiency. High mobility platforms, i.e., trains and aircraft, are challenging environments for OFDM performance. OFDM ICI, caused by the high mobility of the platforms, is investigated and potential methods are proposed. A high speed aeronautical physical layer will allow ADNs to provide a critical service for various situations, such as public safety communication, Denial of Service (DoS), disaster situations, in-flight Internet, etc.\nAn aeronautical channel imposes a challenging environment for an OFDM based physical layer. This environment, two ray channel, consists of dual Doppler shifts impacting the received signal. A novel approach based on smart antenna processing is proposed, not only to mitigate the problem, but also to take advantage of the dual Doppler shifts. The Doppler shifts in the aeronautical environment can be mitigated and taken advantage of to improve the system performance. The received corrupted signal is first separated by the use of beam forming antenna processing and then combined using diversity combining techniques.\nThis research concludes with feasibility analysis that explores the system and architecture requirements for a cognitively driven and reconfigurable hardware for aeronautical platforms. The scope of such a system is to provide an intelligent configurable radio system, radio connectivity for the changing geographical locations, and political and regulatory policies with which an aircraft must comply. Such an industry could take advantage of opportunistic services available today and in the future. The global movement of the aeronautical industry can take advantage of emerging wireless services and standards to provide xii high speed seamless data connectivity. Advances in components and processing hardware can provide the configurable hardware required for such a capability. Therefore, a Cognitive Aeronautical Software Defined Radio (CASDR) system will provide an intelligent, self-configurable software and hardware solutions for the aeronautical system.