Achieving the Secrecy Capacity of the AWGN Wiretap Channel via Multilevel Coding

In this paper we study coded modulation for the AWGN wiretap channel. We propose multilevel coding in which the encoding of the message and the randomness is performed through distinct levels. Modulation dependent conditions under which the proposed multilevel coding achieves the secrecy capacity are studied. It is shown that it is always possible to achieve the secrecy capacity via this approach as the number of levels increases. While assuming maximum likelihood decoding at the eavesdropper, the proposed multilevel coding is studied under maximum likelihood decoding and multi-stage decoding at the legitimate receiver. The labeling effect on the achievable secrecy rates is also investigated. Achieving other points on the rate-equivocation region is also studied, showing that multilevel coding can approach any point on the rate-equivocation region. One of the advantages of the proposed method is to allow the use of existing BI-AWGN codes, e.g., LDPC codes, to approach the secrecy capacity. Simulations support our findings.