  In heterogeneous dense networks where spectrum is shared, users privacy
remains one of the major challenges. On a multi-antenna relay-assisted
multi-carrier interference channel, each user shares the frequency and spatial
resources with all other users. When the receivers are not only interested in
their own signals but also in eavesdropping other users' signals, the cross
talk on the frequency and spatial channels becomes information leakage. In this
paper, we propose a novel secrecy rate enhancing relay strategy that utilizes
both frequency and spatial resources, termed as information leakage
neutralization. To this end, the relay matrix is chosen such that the effective
channel from the transmitter to the colluding eavesdropper is equal to the
negative of the effective channel over the relay to the colluding eavesdropper
and thus the information leakage to zero. Interestingly, the optimal relay
matrix in general is not block-diagonal which encourages users' encoding over
the frequency channels. We proposed two information leakage neutralization
strategies, namely efficient information leakage neutralization (EFFIN) and
optimized information leakage neutralization (OPTIN). EFFIN provides a simple
and efficient design of relay processing matrix and precoding matrices at the
transmitters in the scenario of limited power and computational resources.
OPTIN, despite its higher complexity, provides a better sum secrecy rate
performance by optimizing the relay processing matrix and the precoding
matrices jointly. The proposed methods are shown to improve the sum secrecy
rates over several state-of-the-art baseline methods.
