  Symbol-level precoding (SLP) based on the concept of constructive
interference (CI) is shown to be superior to traditional block-level precoding
(BLP), however at the cost of a symbol-by-symbol optimization during the
precoding design. In this paper, we propose a CI-based block-level precoding
(CI-BLP) scheme for the downlink transmission of a multi-user multiple-input
single-output (MU-MISO) communication system, where we design a constant
precoding matrix to a block of symbol slots to exploit CI for each symbol slot
simultaneously. A single optimization problem is formulated to maximize the
minimum CI effect over the entire block, thus reducing the computational cost
of traditional SLP as the optimization problem only needs to be solved once per
block. By leveraging the Karush-Kuhn-Tucker (KKT) conditions and the dual
problem formulation, the original optimization problem is finally shown to be
equivalent to a quadratic programming (QP) over a simplex. Numerical results
validate our derivations and exhibit superior performance for the proposed
CI-BLP scheme over traditional BLP and SLP methods, thanks to the relaxed
block-level power constraint.
