1) The document considers a multi-user MIMO system with M single-antenna users and N transmit antennas. It aims to transmit constant envelope signals from each antenna to facilitate the use of power-efficient RF amplifiers. 2) Analytical and numerical results show that under certain channel conditions, an array gain can be achieved even with the stringent per-antenna constant envelope constraint, such that the total transmit power can be reduced as N increases while maintaining a fixed information rate to each user. 3) Simulations for i.i.d. Rayleigh fading channels show the total transmit power can be reduced linearly with increasing N, representing an O(N) array gain. A near-optimal

1. 2020 – 2021
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Per-Antenna Constant Envelope Pre-coding for Large Multi-User
MIMO Systems
Abstract:
We consider the multi-user MIMO broadcast channel with M single-antenna users and N
transmit antennas under the constraint that each antenna emits signals having constant
envelope (CE). The motivation for this is that CE signals facilitate the use of power-efficient
RF power amplifiers. Analytical and numerical results show that, under certain mild conditions
on the channel gains, for a fixed M, an array gain is achievable even under the stringent per-
antenna CE constraint. Essentially, for a fixed M, at sufficiently large N the total transmitted
power can be reduced with increasing N while maintaining a fixed information rate to each
user. Simulations for the i.i.d. Rayleigh fading channel show that the total transmit power can
be reduced linearly with increasing N (i.e., an O(N) array gain). We also propose a precoding
scheme which finds near-optimal CE signals to be transmitted, and has O(MN) complexity.
Also, in terms of the total transmit power required to achieve a fixed desired information sum-
rate, despite the stringent per-antenna CE constraint, the proposed CE precoding scheme
performs close to the sum-capacity achieving scheme for an average-only total transmit
power constrained channel.