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• Kitchen, J
• Moran, B
• Howard, SD.
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• Moran, B
• Howard, SD.
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• Kitchen, J
• Moran, B
• Howard, SD.

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Entropy 2008, 10(4), 722-735; doi:10.3390/e10040722

Article

Intercept Capacity: Unknown Unitary Transformation

John Kitchen ^1,* , Bill Moran ¹  and Stephen D. Howard ²

¹ Department of Electrical and Electronic Engineering, University of Melbourne, Australia ² EWRD, DSTO, PO Box 1500, Edinburgh, Australia

* Author to whom correspondence should be addressed.

Received: 22 May 2008 / Accepted: 10 November 2008 / Published: 20 November 2008

(This article belongs to the Special Issue Concepts of Entropy and Their Applications - Papers presented at the Meeting at University of Melbourne, 26 November - 11 December 2007)

Download PDF Full-Text [1471 KB, uploaded 20 November 2008 15:42 CET]

Abstract: We consider the problem of intercepting communications signals between Multiple-Input Multiple-Output (MIMO) communication systems. To correctly detect a transmitted message it is necessary to know the gain matrix that represents the channel between the transmitter and the receiver. However, even if the receiver has knowledge of the message symbol set, it may not be possible to estimate the channel matrix. Blind Source Separation (BSS) techniques, such as Independent Component Analysis (ICA) can go some way to extracting independent signals from individual transmission antennae but these may have been preprocessed in a manner unknown to the receiver. In this paper we consider the situation where a communications interception system has prior knowledge of the message symbol set, the channel matrix between the transmission system and the interception system and is able to resolve the transmissionss from independent antennae. The question then becomes: what is the mutual information available to the interceptor when an unknown unitary transformation matrix is employed by the transmitter.

Keywords: Communications; Unitary; BSS

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