Secrecy in MIMO Networks with No Eavesdropper CSIT

TL;DR

This paper investigates the secure degrees of freedom in MIMO wiretap and multiple access channels without eavesdropper CSIT, revealing optimal s.d.o.f. regimes and the impact of CSIT absence on security performance.

Contribution

It determines the optimal sum secure degrees of freedom for MIMO wiretap and MAC channels without eavesdropper CSIT across different antenna regimes, highlighting the regimes where the channels reduce to simpler models.

Findings

Optimal s.d.o.f. characterized for regimes K ≤ N and N ≤ K ≤ 2N.

MAC-WT reduces to WTH when K ≤ N.

Loss of s.d.o.f. occurs without eavesdropper CSIT when K ≥ N.

Abstract

We consider two fundamental multi-user channel models: the multiple-input multiple-output (MIMO) wiretap channel with one helper (WTH) and the MIMO multiple access wiretap channel (MAC-WT). In each case, the eavesdropper has $K$ antennas while the remaining terminals have $N$ antennas each. We consider a fast fading channel where the channel state information (CSI) of the legitimate receiver is available at the transmitters but no channel state information at the transmitters (CSIT) is available for the eavesdropper's channel. We determine the optimal sum secure degrees of freedom (s.d.o.f.) for each channel model for the regime $K\leq N$, and show that in this regime, the MAC-WT channel reduces to the WTH in the absence of eavesdropper CSIT. For the regime $N\leq K\leq 2N$, we obtain the optimal linear s.d.o.f., and show that the MAC-WT channel and the WTH have the same optimal s.d.o.f. when restricted to linear encoding strategies. In the absence of any such restrictions, we provide an upper bound for the sum s.d.o.f. of the MAC-WT chanel in the regime $N\leq K\leq 2N$. Our results show that unlike in the single-input single-output (SISO) case, there is loss of s.d.o.f. for even the WTH due to lack of eavesdropper CSIT when $K\geq N$.