Analysis of out-of-band interference from saturated power amplifiers in Massive MIMO

Nonlinear power amplifiers distort the transmitted signal and out-of-band (OOB) radiation becomes a source of interference for users operating in adjacent channels. This paper studies the effect of Massive Multiple Input Multiple Output (MIMO) system on the OOB interference. Massive MIMO relies on channel based precoding which ensures the signal is added constructively at the receiver user equipment. However, the effect of the precoding on the OOB radiation should be investigated given that a random user could experience an increased interference. Assuming a third order polynomial model to describe the behavior of the power amplifier, we model the received power spectrum density. Mathematical analysis and system level simulation confirm that OOB does not recombine constructively avoiding large interference. Massive MIMO allows to increase the in-band received power for the target users without increasing the interference in adjacent bands. In other words, less stringent requirements are demanded for power amplifier design, confirming that simpler hardware with respect to conventional transmission scheme is sufficient. We show that in Massive MIMO, even power amplifiers with efficiency of 65.4% satisfy 3GPP OOB constraints.