Symbol Level Precoding With Low Resolution DACs for Constant Envelope OFDM MU-MIMO Systems

In Constant Envelope Symbol Level Precoding (CESLP), the transmitted signals by the antennas have constant amplitude regardless of the channel realization and the conveyed information symbols. Such solutions enable the use of power-efficient components, i.e. non-linear power amplifiers and thus, are suitable for systems based on large-scale antenna arrays. In the latter systems, the hardware complexity and power consumption can be limiting factors since each antenna element requires dedicated hardware components. These components include the Digital-to-Analog Converters (DACs) that have exponential complexity and power consumption with the number of the supported resolution. To that end, in this paper, a CESLP precoding technique is presented for Multi-User Multiple Input-Multiple Output (MU-MIMO) systems with low resolution DACs. The case of a multi-carrier system employing the well-known Orthogonal Frequency Division Multiplexing (OFDM) technique is considered. The precoding design problem is formulated as a mixed discrete-continuous least-squares optimization one which is NP-hard. An efficient low complexity solution is developed based on the Cyclic Coordinate Descent (CCD) optimization framework. Simulations show that the proposed solution is able to achieve very close performance to the one of the infinite resolution CESLP precoding counterpart even though, it is based on DACs that have resolution of only a few bits.