Achieving full secure degrees-of-freedom for the MISO wiretap channel with an unknown eavesdropper

In this paper, we consider the achievable secure degrees-of-freedom (sdof) for the multiple-input single-output (MISO) wiretap channel with an unknown Eavesdropper's channel state information (CSI). The underlying system model comprises three nodes: a multi-antenna transmitter (Alice), a single-antenna receiver (Bob) and a single-antenna eavesdropper (Eve). Recent studies have shown that the achievable sdof in the sense of strong secrecy is zero when Eve's number of antennas is equal or more than Bob's number of antennas, which applies to the scenario considered in this paper. Moreover, using artificial noise, which deemed to be the only possible strategy to degrade Alice-Eve's channel when Eve's CSI is unknown, may no longer be efficient due to the recently introduced methods for managing interference in a one-dimensional space (e.g., real interference alignment). In this paper, we propose a novel precoding technique and a coding strategy that together achieve full sdof in the sense of strong secrecy without knowing Eve's CSI and without using artificial noise. The proposed precoding method uses the CSI of the Alice-Bob channel in a nonlinear fashion, which makes it difficult for Eve to decode even when Eve has the CSI of all channels, whereas the proposed coding scheme ensures strong secrecy.