Research On Downlink Performance And Capacity Of Distributed Antennas System

With the ever-increasing requirements of high data rate and quality of service in wireless communication, many technologies have been proposed to improve the signal quality and the channel capacity, and to reduce the bit error rate (BER). As the effective technology, Distributed antennas system (DAS) has been concerned for a long time.In DAS, antenna modules are geographically distributed to reduce access distance instead of centralizing at a location and each distributed antenna module is connected to a base station via fiber optics or RF links. The base station controls the signals to be transmitted and processes the signals received in ever antenna modules. Therefore, the DAS can be considered as a special multi-input multi-output (MIMO) system.The channel characteristics and performance indicators of DAS are studied in this dissertation. It introduces the fading types and several commonly used channel models in detail. And it highlights the compositions of the channel model used in the DAS and also the mathematical derivation of composite shadowing-fading channel. Meanwhile, it briefly introduces the commonly used performance indicators in wireless communication such as BER and channel capacity.As the DAS can be considered as a special MIMO system and in order to have a more comprehensive and in-depth understanding about the DAS, the dissertation introduces a variety of MIMO system models, including the single-input single-output (SISO) system, the single-input multiple-output (SIMO) system, the multiple-input single-output (MISO) system and MIMO system. At the same time, it gives an in-depth analysis of BER and channel capacity of these systems.For the existing distributed antennas system (EDAS) cannot provide sufficiently low BER and high channel capacity at the border regions, the dissertation proposes an improving distributed antennas system (IDAS), which adopts directional antennas and lies them reasonably. The IDAS, EDAS and traditional configuration (TC) system are analyzed and simulated in single cell and multi-cell environments. The results show that compared with EDAS and TC, IDAS can acquire lower symbol error probability and larger channel capacity especially in the adverse channel conditions. And compared with EDAS, IDAS can largely reduce the complexity of network planning and the cost. Meanwhile, the dissertation also demonstrates that the correlation between shadow fading and the difference of shadow fading influence the BER and channel capacity in the communication system.