Effective Delay Spread Reduction for Low Latency MISO-OFDM Systems

Abstract

These days, many issues are being actively discussed for the next generation wireless communication system. One of the important issues is latency reduction to support latency-sensitive applications such as virtual reality games, remote controlled robots, emergency alerts in vehicle communications. To meet the tight latency requirements for such applications, the symbol duration should be remarkably reduced than that of current orthogonal frequency division multiplexing (OFDM) systems which make use of cyclic prefix (CP) to mitigate inter-symbol-interference (ISI). However, as symbol duration decreases, the CP overhead occupies a larger portion of resources, and impairs the performance of the OFDM system. On the other hand, if we decrease CP duration to reduce overhead, ISI due to delay spread would also degrade the system performance. Hence, we not only need to reduce CP duration for efficient low latency communications, but also need to mitigate ISI induced from the reduced CP duration. In this thesis, we propose a novel precoding method in MISO-OFDM systems to reduce effective delay spread of channels for CP overhead reduction. We formulate the precoding vector selection problem which aims to minimize effective delay spread as well as signal to noise ratio (SNR) impairment. In addition, to reduce the computational complexity of the problem, we suggest an efficient precoding vector selection algorithm which make use of the gradient descent method. Simulation results show that the proposed scheme remarkably reduces the effective delay spread of channels while maintaining tolerable SNR degradation. We show that the performance of the proposed scheme improves as the base station (BS) has more antennas. Future work may include the extension of the proposed scheme to massive MIMO systems in which the BS has a massive number of antennas. In massive MIMO systems, we can make use of the excess degree of freedom (DoF) to reduce effective delay spread of channels while multiuser interference (MUI) is eliminated.