Asymptotic MIMO artificial-noise secrecy rates with eigenmode partitioning

In a multiple-input multiple-output (MIMO) wiretap channel system, it has been shown that artificial noise can be transmitted in the null space of the main channel to guarantee the secrecy at the intended receiver. Previous formulas for MIMO asymptotic capacity assume that all channel eigenmodes will be utilized. However, optimizing over possible antenna configurations requires partitioning the available eigenmodes. With only some eigenmodes used for signal transmission, finding an exact closed-form asymptotic solution is, in general, intractable. We present a large-scale MIMO approximation with eigenmode partitioning, accurate for realistic numbers of antennas, and with greatly reduced computational complexity.