Mimo ofdm

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MIMO techniques ( Multiple-Input/Multiple-Output ) for OFDM modulations
Miguel Onofre, Technical University of Lisbon, Portugal

In the future wireless communication systems, the use of multiple antennas at the ends of a wireless connection, through the MIMO technology, will dramatically increase spectral efficiency and reliability of the connection. MIMO technology combined withtechnical Orthogonal Frequency Division Multiplexing (OFDM) appears to be forefront of the solution for the future mobile systems and in the next generation of fixed networks systems architecture. In this report, the fundamental concepts of MIMO OFDM V-BLAST technology will be covered.



Our present social, economic and professional life style leads to an explosion of Internetservices. As a result, new applications and new multimedia communication services arise each day. However, the increase in new communication services requires higher data rates of the digital transmission systems. The request for higher rates of data translates into a requirement for greater bandwidth, but the electromagnetic spectrum and the maximum radiated power have physical limitations andregulations. Thus the only way to increase the data rate of the transmission systems with high reliability, meeting today’s demands of companies, within these limitations, is possible only through new and sophisticated techniques of digital signal processes and techniques of the encoding channel. Also high data rate wireless systems, consequently present very small periods, and in strong fadingchannels face Inter-Symbol Interferences (ISI) due to multipath propagation and their inherent delay spread, as a result of which the Bit-Error Rate (BER) increases. OFDM is a technique capable of mitigating the ISI and efficient in the frequency selective channel. It provides an optimal spectral efficiency due to its orthogonal carriers and their spacing. On other hand MIMO systems exploit the richlyscattered multipath, increasing capacity

and obtaining better performance according with the number of antennas. The objectives of the report are twofold. Firstly, we present the main features of the aforementioned technologies, the combining of MIMO systems with OFDM technology. Secondly, we describe the detection of VBLAST algorithm at the receiver to reduce Cancel-Channel Interference(CCI), increasing the system performance. Moreover, we will demonstrate the simulation results of MIMO-OFDM V-BLAST, its performance through the BER, and its capacity for different number and configurations of antennas at both the transmitter and receiver side. This report shows MIMO-OFDM systems with V-BLAST to be a solution for high-speed wireless systems, through improvement of the spectralefficiency, range and reliability of the connection.



A. Orthogonal Frequency Division Multiplexing (OFDM) As a result of multipath, many reflected signals reach the receiver with different times, i.e., echoes can cause ISI and combined can cause attenuation. In the situation of frequency selective fading in radio propagation, some paths of the multipath vary withtime resulting in attenuation. For a narrowband signal, the distortion is usually minimized if the bandwidth is smaller than the coherence bandwidth of the channel, because all the frequencies in the band are usually attenuated in the same way. A signal which has a high bandwidth, greater than the coherence bandwidth, will be subject to greater distortion, but will have less variation in totalreceived power, even when subject to different multipath levels. Considering a spread delay τrms , the coherence bandwidth Bc is determinate by: 1 ≈ (1) This situation can be overcome in 2 ways. The first method is by transmitting the signal in 1

broadband, spreading its spectrum, as in CDMA, where a deep fading or a null in the spectrum result only in a small, rather than a complete loss of...
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