On Multi-antenna Systems for Wireless Transmission Mediums

Current and emergent services are demanding higher data rates, improved spectral e-ciency and increased network capacity. To face this new requirements, it is important to flnd schemes able to reduce the efiects of fading and explore new types of diversity. The current work performs a comparison between difierent multi-antenna techniques that can be employed to achieve the requirements of the Fourth Generation Cellular Systems (4G). The described schemes are studied at the link and system level using Bit Error Rate as the performance index. 1. INTRODUCTION The use of multiple antennas at both the transmitter and receiver aims to improve performance or to increase symbol rate of systems, but it usually requires a higher implementation complexity. The antenna spacing must be larger than the coherence distance to ensure independent fading across difierent antennas elements (1,2). Alternatively, difierent antennas should use orthogonal polarizations to ensure independent fading across difierent antennas. Multi-antenna systems are used in order to push the performance or capacity/throughput limits as high as possible without an increase of the spectrum bandwidth, although at the cost of an obvious increase of complexity. In the case of frequency selective fading channel, difierent symbols sufier from interference from each other, whose efiect is usually known as Intersymbol Interference (ISI). This efiect tends to increase with the used bandwidth. By exploiting diversity, multi-antenna systems can be employed to mitigate the efiects of ISI. The various multi-antenna conflgurations are referred to as Single Input Single Output (SISO), Multiple Input Single Output (MISO), Single Input Multiple Output (SIMO) or Multiple Input Multiple Output (MIMO). The SIMO and MISO architectures are forms of receive and transmit diversity schemes, respectively. MIMO architectures can be used for combined transmit and receive diversity, for the parallel transmission of data or for spatial multiplexing. When used for spatial multiplexing, MIMO technology promises high bit rates in a narrow bandwidth. Therefore, it is of high signiflcance to spectrum users. In this case, MIMO system considers the transmission of difierent signals from each transmit antenna element so that the receiving antenna array receives a superposition of all transmitted signals.