Precoding Design for Cyclic Prefix Overhead Reduction in a MISO-OFDM System

Very low latency is one of the key requirements of 5G wireless access. To reduce latency in orthogonal frequency division multiplexing (OFDM) systems while not increasing the overhead, it is of primary importance to reduce the effective delay spread of the channel and thus the length of the cyclic prefix. In this letter, we propose a precoding approach to reduce the root mean square (RMS) delay spread of the precoded channel in an OFDM system. We formulate an optimization problem with an upper bound of the RMS delay spread as the objective function and a signal-to-noise ratio for each subcarrier as constraints. A semi-definite relaxation technique is used to convert the problem into a convex problem so as to find the optimal precoding vector. Numerical results confirm that the proposed precoding design provides a significant reduction in the RMS delay spread, especially, when there are a large number of antennas at the base station.