Date of Award

Summer 2020

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

David W. Matolak

Abstract

Many wireless communication systems will need to accommodate a larger number of users in the future. One application in particular in which this is critical is low data rate, long range communication links with very large numbers of nodes, such as the internet of things (IoT), possibly the internet of flying things (IoFT), etc. These systems demand advanced multi-access techniques with minimal multiple access interference (MAI). They should also be robust to multiple impairments, including multipath channel distortions, Doppler spreading, and interference. Chirp waveforms are one type of waveform set that can satisfy future system demands in the presence of these impairments. When the constant amplitude variety of chirp is used, this exhibits a desirable very low peak to average power ratio (PAPR). The ridge-shaped ambiguity function of chirp signals can also be useful for radar and channel modeling (sounding) applications. Hence chirps are promising candidates for many such applications. Chirps are specified in the IEEE 802.15.4a standard as chirp spread spectrum (CSS). Another growing application area requiring advanced communications is aviation. In particular, unmanned aircraft systems (UAS), also known as unmanned aerial vehicles (UAVs), and “drones,” will in the future operate within airspace along with commercial, cargo, and other piloted aircraft. The command and control (C2), or control and non-payload communications (CNPC) link must provide highly reliable safety critical information for the control of the UAV both in terrestrial-based line of sight conditions and in satellite communication links. Chirp signaling features make chirp signal sets good candidates to meet CNPC link requirements.

In this dissertation, we investigate multi-user chirp signaling for future aviation communication and channel sensing systems. We describe the basics of chirp signaling, chirp sounding, and investigate via mathematical analysis, computer simulations, and some experiments, the effects of aviation channel-induced non-idealities such as Doppler and asynchronism on the chirp signaling schemes. We also describe a hybrid design where the system is not only a communication entity but also does channel estimation (sounding). We describe methods to increase spectral efficiency and how to avoid multiple access interference among users (and intersymbol interference for a given user). We also conducted experiments on chirp channel sounding using a small drone and software defined radios, and provide some channel characterization results.

The majority of this work, and our major contributions, pertain to detailed evaluation of performance of multi-user chirp spread spectrum systems under a variety of conditions. We find, analytically, new expressions for bit error rate performance of binary coherent and noncoherent chirp spread spectrum signals, and we compare and validate numerical and analytical results with simulations. These error probability expressions are general, and can be used for any multi-user chirp signaling set. We also design more practical sets of chirp signals that out-perform existing chirp signal sets when synchronization is imperfect, a condition we term quasi-synchronous. These new practical chirp designs employ nonlinear trajectories in the time-frequency plane. Our new chirp designs also outperform existing schemes in the presence of Doppler shifts. We provide examples of air to ground link performance with empirical channel models to illustrate the superior performance of our proposed designs.

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