Reliable downlink transmission in unsynchronized coordinated multipoint transmission LTE networks

Abstract

The development of mobile information and communication technologies allows fast and flexible ways of receiving, exchanging and processing information over long distances and, thus, opens up new potential for the private and economic life [Rei02]. The tremendous growth of subscribers worldwide and the demand for area-wide increasing transmission rates, however, pose new challenges, particularly in the cell-edge. Schemes making use of coordinated signal transmission provide a technical solution for this challenge and can be used to mitigate the impact of inter-cell interference to improve the transmission rates in the cell-edge. The intention of this thesis is to provide a solution to maintain the advantages of coordinated signal transmission in non-ideal system conditions against conventional signal transmission. Based on the Long Term Evolution (LTE) mobile radio standard, the impact of inter-cell interference is investigated in Chapter 1 of this thesis, followed by a representation of state-of-the-art interference mitigation techniques. The main goals of this thesis are subsequently defined. Substantial physical layer aspects of LTE systems, which are necessary for achieving the defined main goals, are subsequently discussed. The necessary signal processing measures enabling interference mitigation are initially derived for ideal signal transmission in a chapter of their own. Based on the optimal coordination scheme for signal transmission, an implementation-friendly method is derived which enables to mitigate the impact of inter-cell interference in the cell-edge considerably, while keeping the computational complexity low. To consider non-ideal system conditions, the impact of varying delays among the desired receive signals is investigated analytically and, based on this, methods to compensate that impact in the receiver to the greatest possible extent are suggested. Numerical results will show that the advantages of coordinated signal transmission can be maintained in non-ideal system conditions and, as a result, the transmission rates in the cell-edge are considerably improved. The key findings will be summarized and an outlook on future work provided.