Blind beamforming techniques for global tracking systems

Abstract

After the development in the past 120 years since the invention of the first radio transmission, worldwide wireless communication systems are nowadays part of daily life. Behind the shining and astonishing achievement of modern communication systems, the exhaustion of existing frequency spectrum resources has been a concern. In higher frequency bands, the most advanced techniques are in development for the fifth cellular mobile communication system (5G) to meet rapid growth in its applications. The 5G system sits in the millimeter-wave band and consumes a wider bandwidth to offer a higher data transmission speed and a larger system capacity. Frequency/time/code division multiple accesses (FDMA/TDMA/CDMA) are successful techniques to reuse and to save the frequency spectrum. However, in lower frequency bands, existing communication systems face similar unprecedented demands to accommodate more users in new applications. These growing demands exceed the designed system capacity and thus call for innovative solutions while keeping compatibility to the current setup to reduce the cost of users. For example, in the automatic identification system (AIS), satellite receivers are being used for expanding the service coverage of ship tracking to the global range, and similarly in the automatic dependent surveillance-broadcast (ADS-B) system for aircraft tracking. These systems are narrowband and originally designed in the last century, but they will continue to run for at least another couples of years without major updating of the user-side equipment. The new application of AIS considered in this thesis is Satellite AIS. The satellite runs in the low-earth orbit (LEO). On the satellite, receiving AIS signals becomes much more difficult than before: one has to combat in-cell and inter-cell interfering sources from the system itself. Interference suppression is the main topic of this thesis. Narrowband spatial beamforming techniques for antenna arrays are candidate solutions to this challenge. This thesis tries to develop new beamforming techniques with a simple structure and a low computational complexity. With these techniques, this thesis establishes a framework of multiuser reception for Satellite AIS. The new beamforming techniques are proposed through three consecutive chapters associated with their foundation, evolution, and application. In Chapter~1, the background and the issues brought by Satellite AIS are introduced. Related literature is reviewed. The contribution of this thesis is shown. In Chapter~2, the beamforming problem for signals in additive white noise is discussed. As a basic tool for the proposed algorithms in this thesis, a signed URV algorithm (SURV) is proposed for the basic problem of principal subspace computation and tracking as a replacement of the singular value decomposition (SVD). The updating and downdating of SURV is direct and simple. SURV has no issue of numerical stability unlike previous algorithms in linear algebraic and shows consistent performance in both stationary and nonstationary cases. This chapter shows how SURV is derived and provides its theoretical support. In Chapter~3, the beamforming techniques for interference suppression in nonstationary scenarios are discussed. New blind beamforming techniques are proposed for separating overlapping packets in such scenarios. The connections between subspace intersection, oblique projection, the generalized SVD (GSVD), the generalized eigenvalue decomposition (GEVD), and SURV are exposed. SURV is used as one of the basic tools for the beamforming techniques. Simulation and experimental results of the proposed algorithms are shown. In Chapter~4, based on the proposed algorithms in Chapter~3, a special blind beamforming technique enabling tracking for the multi-user receiver for Satellite AIS is proposed. The proposed algorithm is based on SURV. Results of the receiver in a software simulation model and on a hardware platform are provided. In the remaining part of the thesis, the work on developing the software simulation model and constructing the hardware platform is presented. The outputs of the work are used for the verification and validation of the proposed algorithms in this thesis. In Chapter~5, the method of developing the software testbed (simulation model) is presented. This testbed is built by using several tools including SystemC-AMS and MATLAB. The software implementation of the receiver is done in MATLAB and then translated into C++. This chapter first shows a lightweight version of the testbed in the hope that readers can learn and construct their own simulation model from scratch. The chapter also shows the possibility that the lightweight version will be extended and reconfigured to a more sophisticated model for the practical global ship distribution and satellite orbit from launched satellites like what is done in Chapter~4. In Chapter~6, the structure of the hardware platform is presented. This chapter gives an example on how to build array receivers from available equipment. This platform uses an array of modified commercial RF frontends to downconvert the AIS signals to baseband. Sampled data are fed into PC and processed in MATLAB. The decoded AIS messages are analyzed and visualized on maps.