Type of Document Master's Thesis Author Akkawi, Bandali K. Author's Email Address firstname.lastname@example.org URN etd-07022008-174027 Title Physical Layer Secrecy Channel Coding Degree Master of Science in Electrical Engineering (M.S.E.E.) Department Electrical & Computer Engineering Advisory Committee
Advisor Name Title Wei, Shuangqing Committee Chair Liang, Xue-Bin Committee Member Naraghi-Pour , Morteza Committee Member Keywords
- catastrophic codes
- parallel paths
- error and LLR correlation
Date of Defense 2008-06-26 Availability unrestricted AbstractWireless communications is expanding and becoming an indispensable part of our daily
life. However, due to its channel open nature, it is more vulnerable to attacks, such as
eavesdropping and jamming which jeopardize the confidentiality of wireless data, compared
to its counter-part, wireline communications. Security in wireless communication is
thus a very important factor that should be perfected to accommodate the rapid growth
of wireless communication today.
Motivated by information theoretic secrecy definitions, we adopt a simple way to define
the secrecy of a system by looking at its Bit-Error-Rate (BER) curves, the correlation
of error vectors and Log Likelihood Ratios (LLRs) of the decoded information bits. The
information bit errors and LLRs of a physical layer secure system should be uncorrelated
and the BER curve should have an acceptable sharp transition from high to low BERs
at prescribed signal to noise ratio (SNR) thresholds.
We study catastrophic codes and Serial Concatenated Convolutional Codes (SCCC)
as two candidates. For the former, we provide both detailed analytical and simulation
results, to demonstrate how we can change the encoding parameters to make the resulting
BER curves have the intended properties. For SCCC, we study two options. One is
having a catastrophic code as an inner code. The other is to use regular SCCC. Several
approaches are proposed to change the shape of the resulting BER curves.
In addition, the correlation present in their information bit errors and LLRs are investigated
to see how it can be used to detect or even correct errors. We find that regular
SCCC codes have strong correlation in their error vectors which is captured by the associated
LLRs. In low SNR regions, eavesdropper can easily make reliable decisions on
which packets to drop based on LLRs, which thus undermines the security of the main
channel data. On the other hand, by selecting proper outer codes, SCCC with catastrophic
encoder does not have such a weakness.
We conclude that Catastrophic convolutional codes, as well as serial concatenated
catastrophic codes have desired properties. Therefore, they can be considered promising
approaches to achieving practical secrecy in wireless systems.
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