A Distortion Nullforming Precoder in Massive MIMO Systems With Nonlinear Hardware

In this letter, we propose a novel nullforming precoding scheme to cancel the distortion from multiple-input–multiple-output (MIMO) base stations with nonlinear hardware towards victims that operate either in the same or adjacent frequency bands. The proposed precoder is based first on the design of a nullforming scheme within the same band which steers nulls towards the direction of victims. Then, its power allocation is updated to achieve per-antenna constant envelope (CE) precoding without destructing the nullforming of the desired signal. It is shown analytically that in line-of-sight channels, single-user transmission, CE precoders have the property that the radiation pattern of nonlinear distortion has the same spatial characteristics as that of the in-band desired signal. As a result, the designed nulls are towards the desired directions for both in-band and out-of-band frequencies. Finally, numerical results corroborate the effectiveness of the proposed precoder.