A study of closed loop space time block coding techniques for MIMO-OFDM

University essay from Blekinge Tekniska Högskola/Avdelningen för signalbehandling

Abstract:

In this dissertation, the STBC-OFDM technique is therefore investigated in order to improve the bit error rate performance of such wireless communication systems speci¯cally those using multiple transmitter and receiver antennas.An investigation is undertaken of STBC with two transmit and one receive antennas as proposed by Alamouti together with quasi-orthogonal STBC with four transmit and one receive antennas. The simulated performance shows that both full diversity and full code rate is achieved only in the case of two transmit and one receive antenna, whereas the diversity is decreased in the case of four transmit and one receive antennas due to the presence of coupling term. This dissertation also presents a method of minimizing such coupling terms by exploiting a phase feedback method as proposed by Cenk Toker, which forces the coupling terms to zero. Both one phase feedback and two phase feedback methods are used. The simulated performance shows that the practical performance gain between three bit feedback and the two bit feedback is only 0.43dB, so to use two bit feedback appears appropriate. Only a ¯nite number of bits is allowed for the feedback, to satisfy the quasi-static assumption on the channel. Finally, closed loop space time block code-OFDM is also investigated, since OFDM is well known to be suited for high data rate applications in fading channels, due to its high spectral efficiency. Another advantage of an OFDM system lies in its ability to transfer a frequency-selective fading channel into multiple °at fading channels on different sub-carriers. The simulation results obtained clearly show that the performance gain between three bit feedback and the two bit feedback is 0.45 dB, therefore a practical two-bit feedback scheme is feasible.

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