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Exploiting Radio Channel Aware Physical Layer Concepts

  • In DS-CDMA, spreading sequences are allocated to users to separate different links namely, the base-station to user in the downlink or the user to base station in the uplink. These sequences are designed for optimum periodic correlation properties. Sequences with good periodic auto-correlation properties help in frame synchronisation at the receiver while sequences with good periodic cross- correlation property reduce cross-talk among users and hence reduce the interference among them. In addition, they are designed to have reduced implementation complexity so that they are easy to generate. In current systems, spreading sequences are allocated to users irrespective of their channel condition. In this thesis, the method of allocating spreading sequences based on users’ channel condition is investigated in order to improve the performance of the downlink. Different methods of dynamically allocating the sequences are investigated including; optimum allocation through a simulation model, fast sub-optimum allocation through a mathematical model, and a proof-of-concept model using real-world channel measurements. Each model is evaluated to validate, improvements in the gain achieved per link, computational complexity of the allocation scheme, and its impact on the capacity of the network. In cryptography, secret keys are used to ensure confidentiality of communication between the legitimate nodes of a network. In a wireless ad-hoc network, the broadcast nature of the channel necessitates robust key management systems for secure functioning of the network. Physical layer security is a novel method of profitably utilising the random and reciprocal variations of the wireless channel to extract secret key. By measuring the characteristics of the wireless channel within its coherence time, reciprocal variations of the channel can be observed between a pair of nodes. Using these reciprocal characteristics of common shared secret key is extracted between a pair of the nodes. The process of key extraction consists of four steps namely; channel measurement, quantisation, information reconciliation, and privacy amplification. The reciprocal channel variations are measured and quantised to obtain a preliminary key of vector bits (0; 1). Due to errors in measurement, quantisation, and additive Gaussian noise, disagreement in the bits of preliminary keys exists. These errors are corrected by using, error detection and correction methods to obtain a synchronised key at both the nodes. Further, by the method of secure hashing, the entropy of the key is enhanced in the privacy amplification stage. The efficiency of the key generation process depends on the method of channel measurement and quantisation. Instead of quantising the channel measurements directly, if their reciprocity is enhanced and then quantised appropriately, the key generation process can be made efficient and fast. In this thesis, four methods of enhancing reciprocity are presented namely; l1-norm minimisation, Hierarchical clustering, Kalman filtering, and Polynomial regression. They are appropriately quantised by binary and adaptive quantisation. Then, the entire process of key generation, from measuring the channel profile to obtaining a secure key is validated by using real-world channel measurements. The performance evaluation is done by comparing their performance in terms of bit disagreement rate, key generation rate, test of randomness, robustness test, and eavesdropper test. An architecture, KeyBunch, for effectively deploying the physical layer security in mobile and vehicular ad-hoc networks is also proposed. Finally, as an use-case, KeyBunch is deployed in a secure vehicular communication architecture, to highlight the advantages offered by physical layer security.

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Author:Abhijit Ambekar
URN (permanent link):urn:nbn:de:hbz:386-kluedo-43002
Advisor:Hans Schotten
Document Type:Doctoral Thesis
Language of publication:English
Publication Date:2016/11/02
Year of Publication:2015
Publishing Institute:Technische Universität Kaiserslautern
Granting Institute:Technische Universität Kaiserslautern
Acceptance Date of the Thesis:2015/10/05
Date of the Publication (Server):2016/03/02
Number of page:XIV,II,131
Faculties / Organisational entities:Fachbereich Elektrotechnik und Informationstechnik
DDC-Cassification:6 Technik, Medizin, angewandte Wissenschaften / 620 Ingenieurwissenschaften und Maschinenbau
Licence (German):Standard gemäß KLUEDO-Leitlinien vom 30.07.2015