Applied Physical Layer Orthogonal frequency division multiplexing Encryption

"Mobile cellular telephony has been the most rapidly adopted technology in history.Today it is the most popular and widespread personal technology on theplanet, with an estimated 4.6 billion subscriptions globally by the end of 2009. Mobile broadband subscriptions overtook fixed broadband subscribers in2008, highlighting the huge potential for the mobile Internet." (Abstract from [1].)

Fig. 1: A decade of Information and Communication Technology (ICT) growth driven by mobile technologies (Source: The World in 2009: ICT Facts and Figures, ITU, [1].)

Shannon described encryption as a set of reversible (nonsingular) transformations from one space to another space, so that a potential cryptanalyst would not be able to decipher the message unless they had access to the encryption key. Encryption has since been developed along two distinct routes. The prominent approach is based on the generation of coding schemes that are practically unbreakable because of their computational complexity, thus combining encryption with complexity theory. Along the second path lie information theoretic approaches, allowing for theoretic perfect secrecy. Analyses for the wireless fading channel and MIMO systems have established non-zero secrecy capacities for such systems, although on average the eavesdropper’s channel can be better than the legitimate user’s channel. Furthermore, helping interferer or jamming approaches have recently been proposed.

In this project, we investigate physical layer encryption, specifically for OFDM signals. We propose a scheme termed Masked-OFDM, according to which OFDM signals are masked through faster than Nyquist signalling. Secrecy capacity that approximates the underlying OFDM capacity is attained, due to the ill-conditioning of the overall system linear statistical model. The proposed scheme compromises neither the bandwidth efficiency nor the error performance of the underlying OFDM in AWGN and slow fading channels, at the cost of increased transmission power. The main motivation behind the proposed approach stems from the fact that problems modeled by ill-posed operators prevent the extraction of accurate estimates as a result of their instability. The mutual information between the transmitter and any potential unlicensed receiver is small, irrespective of the signal dimensions, so that almost perfect secrecy is attained.

The APLOE research project aims at enabling wireless systems design accommodating the requirements of fourth generation (4G) wireless products, in two aspects. Firstly, because it proposes a practical approach to achieve perfect secrecy (unconditional security) in OFDM systems. Secondly, because it complies with the requirement of preserving the system’s bandwidth efficiency - avoiding overheads that reduce throughput. The importance of both aspects is evident from the fact that the International Telecommunications Union (ITU)) has set up study groups relevant to the aforementioned issues; Study Group 17 looking at "Security" and Study Group 13 looking at “Future Networks”. Correspondingly, all the major telecommunications companies have invested - and continue to invest - heavily on Long Term Evolution (LTE) products, a potential immediate application of the present project. It is worth noting that a number of important standardization issues for LTE - security amongst them - remain open.