Linear Precoding with Friendly Jamming in Overloaded MU-MIMO Wiretap Networks

We consider the downlink of a multiuser MIMO (MU-MIMO)network in the presence of an external eavesdropper (Eve). No knowledge of Eve's location is assumed at the access point (Alice). The information signals for downlink users (Bobs)are accompanied by bogus signals (a.k.a. friendly jamming)that are generated from Alice. The network is studied in underloaded and overloaded conditions. In an underloaded (overloaded)network, the number of antennas at Alice is larger (smaller)than the total number of Bobs' antennas. In the overloaded setting, traditional methods of creating friendly jamming (FJ), such as zero-forcing-based methods, are infeasible. We propose a linear precoding scheme that relaxes such infeasibility in overloaded MU-MIMO networks. In the worst-case scenario where Eve has knowledge of the channels between Alice and Bobs, we show that our method imposes the most stringent condition on the number of antennas required at Eve to cancel out FJ signals. We verify our analysis with simulations. It turns out that choosing the number of data streams has an important role in achieving a tradeoff between security, reliability, and the achievable rate.