Robust precoder design for massive MIMO with peak total power constrained single‐RF‐chain transmitters

Massive multiple‐input multiple‐output (MIMO) transmission/reception is a very promising enablingtechnique for future cellular systems. The performance of massive MIMO systemsrelies on the availability of channel state information (CSI) at thetransmitter. However, due to estimation errors and delay this CSI is imperfect. Additionally, the use of many radio frequency (RF) chains to drive a largenumber of antennas at the transmitter quickly becomes impractical when thatnumber increases. Thus, reducing the number of RF chains in massive MIMO systemsis essential in order to reduce the system complexity and cost. Considering amassive MIMO system with a single‐RF‐chain transmitter, in this study, theauthors design a precoding technique that is robust to the channel uncertainty. To reflect realistic restrictions in the authors' design, they consider the peaktotal transmitted power rather than the average power constraint. Also, theyconsider imperfect CSI and model the uncertainty region as a bounded one, whichis a reasonable assumption. In this transmitter structure, there is only onepower amplifier and load modulation rather than voltage modulation is used togenerate the desired signals on the antenna elements. They demonstrate that whena very simple fixed equaliser is used at all user terminals, the problem ofminimising the mean‐square error of the received signals at user terminals underthe worst‐case channel uncertainty can be transformed into a convex optimisationproblem. They provide simulation results and demonstrate that the proposedrobust precoding technique outperforms non‐robust techniques in terms of powerefficiency and signal‐to‐interference‐plus‐noise ratios.